Genetics for Pediatricians: The Molecular Genetic Basis of Pediatric Disorders

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Derek Johnston Children’s Department, University Hospital, Queen’s Medical Centre, Nottingham, UK

“Pediatricians will find this easy-to-read book a major step forward in their clinical practice. It should be of interest to working pediatricians who need help in diagnosing syndromes and in understanding the molecular tests that are needed for diagnosis. It wisely does not attempt to discuss every single genetic condition that exists, but confines itself to the important conditions.”

Genetics for Mohnish Suri, Ian D Young

“This text is designed to be readily accessible, and effectively blends clinical features with molecular and clinical genetics. It will provide a valuable bridge between standard pediatric sources and Internet-provided databases. Suri and Young are highly respected clinical geneticists with vast experience in the pediatric applications of their speciality. They are also accomplished communicators – they recognize the challenges of clinical syndrome identification, and the necessity to balance diagnostic enthusiasm with restraint when it comes to selecting from an ever-expanding repertoire of investigations, many of which generate both personal and financial pressures.”

Genetics for Pediatricians

Genetic testing plays an important role in the investigation of almost every child who presents with one of the many common inherited disorders. It can be difficult for even the most conscientious practitioner to keep abreast of developments and to appreciate both the significance and the relevance of some of the major discoveries of recent years. So, it is with the busy general pediatrician in mind that this contemporary account of the molecular aspects of pediatric disorders has been prepared.

Series Editor Eli Hatchwell

ISBN 1-901346-63-3

Remedica

346633

Pediatricians Mohnish Suri Ian D Young

Jo Sibert Head of Department and Professor of Child Health, Department of Child Health, University of Wales School of Medicine, Cardiff, UK

9 781901

Remedica genetics series

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Genetics for Pediatricians

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The Remedica Genetics for… Series Genetics for Cardiologists Genetics for Dermatologists Genetics for Endocrinologists Genetics for Hematologists Genetics for Oncologists Genetics for Ophthalmologists Genetics for Orthopedic Surgeons Genetics for Pediatricians Genetics for Pulmonologists Genetics for Rheumatologists

Published by Remedica Publishing 32–38 Osnaburgh Street, London, NW1 3ND, UK 20 N Wacker Drive, Suite 1642, Chicago, IL 60606, USA E-mail: [email protected] www.remedica.com Publisher: Andrew Ward In-house editors: Thomas Moberly and James Griffin © 2004 Remedica Publishing While every effort is made by the publishers and authors to see that no inaccurate or misleading data, opinions, or statements appear in this book, they wish to make it clear that the material contained in the publication represents a summary of the independent evaluations and opinions of the authors and contributors. As a consequence, the authors, publishers, and any sponsoring company accept no responsibility for the consequences of any such inaccurate or misleading data, opinions, or statements. Neither do they endorse the content of the publication or the use of any drug or device in a way that lies outside its current licensed application in any territory. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher. ISBN 1 901346 63 3 ISSN 1472 4618 British Library Cataloguing-in Publication Data A catalogue record for this book is available from the British Library.

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Genetics for Pediatricians Mohnish Suri Department of Clinical Genetics City Hospital Nottingham UK Ian D Young Department of Clinical Genetics Leicester Royal Infirmary Leicester UK Series Editor Eli Hatchwell Investigator Cold Spring Harbor Laboratory USA

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To our wives and parents.

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Introduction to the Genetics for… series Medicine is changing. The revolution in molecular genetics has fundamentally altered our notions of disease etiology and classification, and promises novel therapeutic interventions. Standard diagnostic approaches to disease focused entirely on clinical features and relatively crude clinical diagnostic tests. Little account was traditionally taken of possible familial influences in disease. The rapidity of the genetics revolution has left many physicians behind, particularly those whose medical education largely preceded its birth. Even for those who might have been aware of molecular genetics and its possible impact, the field was often viewed as highly specialist and not necessarily relevant to everyday clinical practice. Furthermore, while genetic disorders were viewed as representing a small minority of the total clinical load, it is now becoming clear that the opposite is true: few clinical conditions are totally without some genetic influence. The physician will soon need to be as familiar with genetic testing as he/she is with routine hematology and biochemistry analysis. While rapid and routine testing in molecular genetics is still an evolving field, in many situations such tests are already routine and represent essential adjuncts to clinical diagnosis (a good example is cystic fibrosis). This series of monographs is intended to bring specialists up to date in molecular genetics, both generally and also in very specific ways that are relevant to the given specialty. The aims are generally two-fold: (i)

to set the relevant specialty in the context of the new genetics in general and more specifically

(ii)

to allow the specialist, with little experience of genetics or its nomenclature, an entry into the world of genetic testing as it pertains to his/her specialty

These monographs are not intended as comprehensive accounts of each specialty — such reference texts are already available. Emphasis has been placed on those disorders with a strong genetic etiology and, in particular, those for which diagnostic testing is available.

Genetics for Pediatricians

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The glossary is designed as a general introduction to molecular genetics and its language. The revolution in genetics has been paralleled in recent years by the information revolution. The two complement each other, and the World Wide Web is a rich source of information about genetics. The following sites are highly recommended as sources of information: 1.

PubMed. Free on-line database of medical literature. http://www.ncbi.nlm.nih.gov/PubMed/

2.

NCBI. Main entry to genome databases and other information about the human genome project. http://www.ncbi.nlm.nih.gov/

3.

OMIM. Online Mendelian Inheritance in Man. The Online version of McKusick’s catalogue of Mendelian disorders. Excellent links to PubMed and other databases. http://www.ncbi.nlm.nih.gov/omim/

4.

Mutation database, Cardiff. http://www.uwcm.ac.uk/uwcm/mg/hgmd0.html

5.

National Coalition for Health Professional Education in Genetics. An organization designed to prepare health professionals for the genomics revolution. http://www.nchpeg.org/

Finally, a series of articles from the New England Journal of Medicine, entitled Genomic Medicine, has been made available free of charge at http://www.nejm.org. Eli Hatchwell Cold Spring Harbor Laboratory

Introduction

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Preface There can be very few areas of medicine in which progress has been achieved at such a rapid pace as molecular genetics. Almost every common single-gene disorder has succumbed to the march of scientific progress to the extent that genetic testing now plays an important role in the investigation of almost every child who presents with one of the many common inherited disorders which make a major contribution to pediatric morbidity and mortality throughout the world. The rate of progress is such that it can be difficult for even the most conscientious practitioner to keep abreast of developments and to appreciate both the significance and the relevance of some of the major discoveries of recent years. It is with the busy general pediatrician in mind that this contemporary account of the molecular aspects of pediatric disorders has been prepared. The number of conditions which have been mapped or in which the causative gene has been isolated is vast. Thus in order to ensure that this text is of manageable proportions, coverage has been restricted to the more common single-gene disorders which are likely to be encountered in general pediatric practice. “Small print” obscurities and the many inborn errors for which comprehensive biochemical testing is available have generally been omitted. Instead attention has been focused on the more common conditions in which molecular analysis can play an important role in diagnosis or in the management of a child and his or her family. In some instances, notably with eye and skin disorders, we have also omitted rare disorders which fall within the remit of other specialties, particularly when these have received detailed coverage in other books in this series. In addition to providing a unique insight into the cause of so many previously unexplained disorders, recent advances in molecular genetics have also demonstrated that, far from being straightforward, Mendelian inheritance and its contribution to genetic disease can be remarkably complex. Thus a “simple” disorder such as cystic fibrosis has proved to be extremely heterogeneous both clinically and at the molecular level, with over 1,000 different mutations reported at the main disease locus. Indeed, for many conditions such as cystic fibrosis and β-thalassemia, mutational heterogeneity has proved to be the norm. Entities such as nonsyndromal sensorineural hearing loss illustrate Genetics for Pediatricians

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that locus heterogeneity can also be extremely important. Further examples of etiologic complexity are provided by the Bardet–Biedl syndrome, which shows not only locus heterogeneity but also the curious phenomenon of triallelic inheritance, and by Hirschsprung disease, for which the concept of synergistic heterozygosity has started to shed light on how genes at several loci can interact to contribute to what is commonly referred to as oligogenic or polygenic inheritance. And if this was not enough, research on pediatric disorders such as the fragile X syndrome and the Angelman/Prader–Willi syndromes has identified “new” genetic mechanisms such as triplet repeat instability with anticipation, and imprinting/uniparental disomy, respectively. So as well as providing a useful up-to-date account of molecular pathogenesis, we hope that this text will also help readers become better acquainted with some of the new and exciting developments that have characterized molecular genetic research over the last few years. In writing this book we would like to offer our thanks to colleagues who have provided photographs, and to Mrs Diane Castledine for secretarial assistance. Above all we would like to express our gratitude to, and admiration for, the many children and families who, over the years, have taught us so much more than they could possibly have learned from us. Mohnish Suri Ian D Young

Preface

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Contents 1. Progressive Ataxias and Neurologic Disorders Ataxia–Telangiectasia Duchenne Muscular Dystrophy Facioscapulohumeral Muscular Dystrophy Friedreich Ataxia Hereditary Motor and Sensory Neuropathy Limb-girdle Muscular Dystrophy Myotonic Dystrophy Spinal Muscular Atrophy

1 2 4 7 8 10 18 23 27

2. Cerebral Malformations and Mental Retardation Syndromes Angelman Syndrome Fragile X Syndrome Holoprosencephaly Hunter Syndrome Huntington Disease Lesch–Nyhan Syndrome Lissencephaly Lowe Syndrome Neuronal Ceroid Lipofuscinosis Pelizaeus–Merzbacher Syndrome Prader–Willi Syndrome Rett Syndrome X-linked Adrenoleukodystrophy X-linked α-Thalassemia and Mental Retardation Syndrome X-linked Hydrocephalus

29 30 34 36 40 41 43 45 52 53 57 59 61 62 64 66

3. Disorders of Vision

69 70 72 74 75 79 80

Aniridia Bardet–Biedl Syndrome Juvenile Retinoschisis Leber Congenital Amaurosis Norrie Disease Rieger Syndrome

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4. Hearing Disorders Nonsyndromal Hearing Loss Hearing Loss due to Connexin 26 Gene Defect Pendred Syndrome Usher Syndrome Waardenburg Syndrome

83 84 85 86 87 90

5. Neurocutaneous Disorders and Childhood Cancer Multiple Endocrine Neoplasia Type 2 Neurofibromatosis Type 1 Retinoblastoma Tuberous Sclerosis von Hippel–Lindau Disease

93 94 96 98 101 103

6. Connective Tissue and Skeletal Disorders Achondroplasia Ehlers–Danlos Syndrome Hereditary Multiple Exostoses Marfan Syndrome Osteogenesis Imperfecta Pseudoachondroplasia Stickler Syndrome

107 108 110 115 117 119 124 125

7. Cardio-respiratory Disorders Barth Syndrome Cystic Fibrosis DiGeorge/Shprintzen Syndrome Holt–Oram Syndrome Laterality Defects Noonan Syndrome Primary Ciliary Dyskinesia Williams Syndrome

129 130 131 133 135 137 138 139 141

Contents

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8. Craniofacial Disorders Apert Syndrome Crouzon Syndrome Greig Syndrome Pfeiffer Syndrome Rubinstein–Taybi Syndrome Saethre–Chotzen Syndrome Sotos Syndrome Treacher Collins Syndrome Van der Woude Syndrome

143 144 146 148 149 151 152 153 154 155

9. Endocrine Disorders Androgen Insensitivity Syndrome Congenital Adrenal Hyperplasia Diabetes Insipidus Growth Hormone Deficiency Growth Hormone Receptor Defects Panhypopituitarism Pseudohypoparathyroidism

157 158 160 163 164 166 167 169

10. Gastrointestinal and Hepatic Diseases Alagille Syndrome α1-Antitrypsin Deficiency Hirschsprung Disease

173 174 175 177

11. Hematologic Disorders Fanconi Anemia Glucose-6-Phosphate Dehydrogenase Deficiency Hemophilia A Hemophilia B Hereditary Elliptocytosis Hereditary Spherocytosis Sickle Cell Anemia α-Thalassemia β-Thalassemia von Willebrand Disease

181 182 183 185 187 189 190 193 194 197 198

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12. Immunologic Disorders Bruton Agammaglobulinemia Chronic Granulomatous Disease Severe Combined Immunodeficiency Wiskott–Aldrich Syndrome

201 202 203 205 207

13. Metabolic Disorders Medium Chain Acyl-CoA Dehydrogenase Deficiency Menkes Disease Ornithine Transcarbamylase Deficiency Phenylketonuria Wilson Disease

209 210 211 212 214 215

14. Renal Disorders

217 218 220 224 225 226

Alport Syndrome Beckwith–Wiedemann Syndrome Cystinosis Orofaciodigital Syndrome Type I Polycystic Kidney Disease 15. Abbreviations

229

16. Glossary

235

17. Index

285

Contents

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1 1. Progressive Ataxias and Neurologic Disorders

Ataxia–Telangiectasia 2 Duchenne Muscular Dystrophy 4 Facioscapulohumeral Muscular Dystrophy 7 Friedreich Ataxia 8 Hereditary Motor and Sensory Neuropathy 10 Limb-girdle Muscular Dystrophy 18 Myotonic Dystrophy 23 Spinal Muscular Atrophy 27

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Ataxia–Telangiectasia (also known as: AT; Louis-Bar syndrome) MIM

208900

Clinical features

AT is a neurocutaneous syndrome. Patients present with progressive truncal and gait ataxia, unusual head movements, choreoathetosis, and oculomotor apraxia in both horizontal and vertical gaze. Other features include motor developmental delay, dysarthria, and mask-like facies. Telangiectasia appears over the bulbar conjunctiva, face, and ears from the age of 3–4 years (see Figure 1). Many children have a history of recurrent respiratory infections, and 30%–40% of patients develop a malignancy. These include T-cell leukemias and B-cell lymphomas in children and epithelial tumors (such as breast and ovarian cancer) in adults. Patients with AT usually survive into their twenties, although longer survival periods have been documented. Investigations show elevated levels of α-fetoprotein and carcinoembryonic antigen and reduced levels of immunoglobulin (Ig)G2, IgA, and IgE. Chromosome analysis can show reciprocal balanced translocations involving the short arm of chromosome 7 and the long arm of chromosome 14, or the short arm of chromosome 2 and the long arm of chromosome 22.

Age of onset

Most children present with ataxia between the ages of 1 and 3 years.

Epidemiology

The population incidence is estimated to be about 1 in 40,000 to 1 in 100,000 live births. About 1% of the general population are believed to be carriers (heterozygotes).

Inheritance

Autosomal recessive

Chromosomal location

11q22.3

Gene

ATM (ataxia–telangiectasia mutated)

Mutational spectrum

Over 400 mutations have been described. These include small and large deletions and insertions, as well as nonsense, missense, and splice-site mutations. About 65%–70% of mutations result in protein truncation, and these mutations produce no detectable protein. The remaining

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Ataxia–Telangiectasia

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Figure 1. Telangiectasia over the bulbar conjunctiva in a child with ataxia–telangiectasia.

mutations result in the production of a normal-sized protein that is nonfunctional. Almost all nonconsanguineous patients are compound heterozygotes, ie, they have different mutations in their two ATM alleles. Molecular pathogenesis

ATM has 66 exons and encodes a protein with 3,056 amino acids. The ATM protein is ubiquitously expressed and has homology to yeast and mammalian phosphatidylinositol-3 kinases, which are involved in signal transduction, cell cycle control, and DNA repair. It is believed that the ATM protein phosphorylates several other proteins, including p53, ABL, BRCA1, TERF1, RAD9, and nibrin (the protein product of the gene for Nijmegen breakage syndrome, MIM 251260), after cell exposure to ionizing radiation. This delays the progression of the cell through the cell cycle at the G1/S checkpoint, allowing the cell to repair DNA damage before entering the S phase. Without ATM protein the cell would be able to progress to the S phase without repairing the DNA damage sustained by radiation exposure, which could predispose to the development of cancer. The molecular pathogenesis of the neurocutaneous phenotype of AT is unknown.

Genetic diagnosis and counseling

The diagnosis can be confirmed by demonstrating increased chromosomal breakage in cultured lymphocytes after X-irradiation and reduced or absent expression of ATM protein in lymphocytes. Genetic testing is only available on a research basis.

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Counseling is on the basis of autosomal recessive inheritance. Carrier females (particularly those who carry a missense mutation) are at increased risk of developing breast cancer. Missense mutations in ATM are believed to be associated with an increased cancer risk as a result of a dominant-negative effect. Prenatal diagnosis is possible by linkage analysis or by ATM mutation analysis if mutations have been identified previously in an affected child from the family. Prenatal diagnosis has been attempted by amniocentesis followed by X-irradiation of cultured amniocytes to look for chromosomal breakage, but this method of prenatal diagnosis is unreliable.

Duchenne Muscular Dystrophy (also known as: DMD) MIM

310200

Clinical features

This condition mainly affects males, who present with delayed motor-developmental milestones, proximal muscle weakness with pseudohypertrophy of some muscles, particularly the calves (see Figure 2), and cardiomyopathy. The muscle weakness is progressive. In classical cases, loss of ambulation occurs before the age of 12 years and death occurs in the twenties. Intermediate forms of DMD exist in which progression is slower, with loss of ambulation occurring between 11 and 16 years of age. Learning difficulties are seen in approximately 60% of patients. Death is usually due to respiratory infection or cardiomyopathy. About 2.5% of female carriers are symptomatic (manifesting carriers).

Figure 2. Calf hypertrophy in a boy with Duchenne muscular dystrophy.

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Duchenne Muscular Dystrophy

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Age of onset

Usually in the first year of life, although diagnosis is often delayed.

Epidemiology

This is the most common form of muscular dystrophy, affecting 1 in 3,500 live-born males. The prevalence of symptomatic carriers in the female population is estimated to be 1 in 100,000.

Inheritance

X-linked recessive

Chromosomal location

Xp21.2

Gene

DMD (dystrophin)

Mutational spectrum

An intragenic deletion that involves one or more exons is present in 65%–70% of patients. There are two deletion hotspots, one between exons 2 and 20 and the other between exons 44 and 53. Intragenic duplications are seen in 5%–6% of cases. The remainder of cases involve point mutations (nonsense, missense, and splice-site mutations), which are distributed across the whole gene.

Molecular pathogenesis

DMD is the largest known gene in the human genome. It is 2.4 Mb in size and composed of 79 exons. It encodes a large, rod-shaped cytoskeletal protein made up of 3,685 amino acids. The dystrophin protein has an actin-binding domain, two calpain-homology domains, 22 spectrin repeats, one WW domain (a short domain of about 40 amino acids that contains two tryptophan residues that are spaced 20–23 amino acids apart – the term WW derives from the two tryptophan residues, as the single letter code for tryptophan is W) and one ZZ-type zinc finger domain. The gene is subject to alternative splicing, and there are at least four isoforms of dystrophin. These include a muscle (M) isoform, a brain (B) isoform, and a cerebellar Purkinje (CP) isoform. Dystrophin is expressed in several tissues and plays an important role in anchoring the cytoskeleton to the plasma membrane. In muscle, dystrophin links the sarcolemmal cytoskeleton to the extracellular matrix. It is thought to protect the sarcolemma during muscular contractions. Mutations that result in the DMD phenotype are associated with protein truncation or loss of the translational reading frame. These mutations result in the absence of dystrophin. Mutations that maintain the translational reading frame result in the phenotype of Becker muscular dystrophy (MIM 300376). These

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mutations result in the production of a shortened and only partially functional protein. Patients with Becker muscular dystrophy have clinical features similar to those of DMD, but the condition is milder, progression is slower, and survival is prolonged. Mutations in the 5´ end of DMD and in-frame deletions in exons 48 and 49 can also cause X-linked dilated cardiomyopathy (MIM 302045). Mutations in the 5´ end of DMD result in failure to transcribe the M isoform in skeletal and cardiac muscle. However, the absence of this isoform in skeletal muscle can be compensated for by up-regulation of the B and CP isoforms. This does not appear to be the case in cardiac muscle, where the lack of dystrophin expression results in cardiomyopathy. The mechanism by which in-frame deletions in exons 48 and 49 cause X-linked dilated cardiomyopathy is not understood, but it has been suggested that intron 48 might contain sequences that are necessary for the expression of dystrophin in cardiac muscle. Genetic diagnosis and counseling

The diagnosis of DMD is based on clinical features, markedly elevated plasma creatine kinase (CK) levels, muscle biopsy (with immunohistochemistry using monoclonal antibodies to dystrophin), and mutation testing. Routine genetic testing can only detect intragenic deletions and duplications. Testing for point mutations in DMD is only undertaken in a few specialized research laboratories and is best performed on dystrophin mRNA extracted from a fresh or frozen muscle biopsy. Genetic counseling is on an X-linked recessive basis. Female relatives of affected males who have an intragenic deletion or duplication can be offered carrier testing. Carrier females have a 50% chance of having an affected son, and can be offered a reliable genetic prenatal test for this condition by chorionic villus sampling. There is a two-thirds chance that the mother of a sporadic case (single affected male with no family history) is a carrier. The mother of a sporadic case can also have somatic or gonadal mosaicism for the DMD mutation. Therefore, there is a 10%–15% recurrence risk of DMD in a subsequent pregnancy for the mother of a sporadic case. In DMD families in which the DMD mutation cannot be identified, carrier testing involves linkage analysis and serial plasma CK assays. Linkage analysis using intragenic and flanking markers can also be used for prenatal diagnosis in these families.

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Duchenne Muscular Dystrophy

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Facioscapulohumeral Muscular Dystrophy (also known as: FSHMD) MIM

158900 (type 1A) 158901 (type 1B)

Clinical features

This is a slowly progressive muscular dystrophy. The affected patient usually presents with facial weakness, shoulder-girdle weakness and wasting, and scapular winging. Later, there is involvement of feet and hip-girdle dorsiflexors. There is often striking wasting of the neck muscles and the muscles of the upper arm. Retinal vasculopathy and high-frequency sensorineural hearing loss are also recognized features.

Age of onset

Late childhood or adolescence.

Epidemiology

The incidence of FSHMD is approximately 1 in 20,000.

Inheritance

Autosomal dominant

Chromosomal location

Type 1A: 4q35 Type 1B: unknown

Gene

Unknown (both types)

Mutational spectrum

Most cases of FSHMD are type 1A. Although the gene for this condition has not yet been identified, almost all patients have a chromosomal rearrangement in the subtelomeric region of the long arm of chromosome 4 (4q35). This region contains a polymorphic 3.3-kb repeat element termed D4Z4. In the general population, the number of D4Z4 repeats varies from 10 to more than 100. Affected individuals have a deletion in this region that reduces the number of D4Z4 repeats to less than 10. This reduction is the basis of a diagnostic molecular genetic test for FSHMD type 1A.

Molecular pathogenesis

Unknown. It has been suggested that deletion of the D4Z4 repeat sequences could interfere with the expression of a gene located some distance away on the long arm of chromosome 4 by a “position effect”. Recent work suggests that an element within the D4Z4 repeat sequence specifically binds a multiprotein complex that mediates transcriptional repression of genes at 4q35. Deletion of D4Z4 sequences below a certain number could result in a reduction in the number of repressor complexes.

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This could decrease or abolish the transcriptional repression of 4q35 genes, with overexpression of one or more of these genes resulting in the FSHMD phenotype. Genetic diagnosis and counseling

Genetic testing is routinely available and enables a diagnosis to be made in most cases. Counseling is on the basis of autosomal dominant inheritance. About 30% of patients represent new mutations. The condition demonstrates 95% penetrance by the age of 20 years, although penetrance is lower in females. Anticipation has been described in some families. Prenatal testing can be done by genetic testing or, in suitable families, by linkage analysis.

Friedreich Ataxia MIM

229300 (Friedreich ataxia 1) 601992 (Friedreich ataxia 2)

Clinical features

This is the most common cause of cerebellar ataxia in childhood. Affected children present with dysarthria and progressive ataxia of their gait. Neurologic examination demonstrates weakness of the lower limbs, absent knee and ankle jerks, extensor plantar reflexes, decreased position and vibration sense in legs, positive Romberg sign, and pes cavus. Other features include scoliosis, diabetes mellitus, optic atrophy, and deafness. Nerve conduction studies show reduced or absent sensory action potentials, but normal motor-nerve conduction velocities. Echocardiograms show features of hypertrophic cardiomyopathy in 70% of patients.

Age of onset

Usually between 5 and 15 years of age. Almost all cases present before the age of 25 years, although onset after this age has also been described (late-onset form).

Epidemiology

The estimated population prevalence is 1–2 per 50,000. The carrier (heterozygote) frequency is between 1 in 60 and 1 in 110.

Inheritance

Autosomal recessive

Chromosomal location

Friedreich ataxia 1: 9q13 Friedreich ataxia 2: 9p11–p23

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Friedreich Ataxia

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Gene

Friedreich ataxia 1: FRDA (frataxin) Friedreich ataxia 2: unknown

Mutational spectrum

FRDA has seven exons that are subject to alternative splicing. The major protein product of this gene is the 210-amino-acid protein frataxin. This is encoded by exons 1–4 spliced to exon 5A. The majority of patients (~96%) are homozygous for an expansion of a GAA triplet repeat motif in the first intron of the gene. The normal number of GAA triplet repeats is 9–22. In affected individuals the size range is 66–1,700 repeats, with most patients having 600–1,200 repeats. The remaining patients are compound heterozygotes for a pathogenic GAA repeat expansion in one FRDA allele and an inactivating mutation (nonsense or frame-shift) in the other allele.

Molecular pathogenesis

Frataxin is located in the inner mitochondrial membrane, where it plays an important role in oxidative phosphorylation and iron homeostasis. The GAA repeat expansion interferes with the transcription of FRDA, resulting in frataxin deficiency. This is associated with a defect of mitochondrial oxidative phosphorylation and accumulation of iron within the mitochondria. Thus, Friedreich ataxia is essentially a mitochondrial disorder and this is reflected in its clinical features.

Genetic diagnosis and counseling

Genetic testing is available from diagnostic laboratories. However, it is limited to testing for the pathogenic GAA repeat expansion. Counseling is on the basis of autosomal recessive inheritance. The sibling recurrence risk is 25%, but there can be marked variability in the expression of the condition in members of the same family. This can manifest as a different age of onset and/or a difference in the rate of progression. Carrier testing and prenatal diagnosis are available in families where molecular genetic analysis has confirmed that the affected individual is homozygous for a GAA repeat expansion.

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Hereditary Motor and Sensory Neuropathy (also known as: HMSN; Charcot–Marie–Tooth disease; peroneal muscular atrophy) The hereditary motor and sensory neuropathies are a clinically and genetically heterogeneous group of disorders. Four main clinical phenotypes can be recognized: classical HMSN, Dejerine–Sottas syndrome, congenital hypomyelinating neuropathy (CHN), and hereditary neuropathy with liability to pressure palsies (HNPP). Each of these phenotypes is discussed in turn. Table 1 summarizes the classification, distinguishing clinical features, inheritance patterns, and molecular genetics of the various forms of HMSN. MIM

See Table 1.

Clinical Features

Classical HMSN/HMSN I & II Patients with classical HMSN present with distal weakness and wasting of the legs, often associated with pes cavus and loss of ankle jerks. Sensory symptoms are usually mild and include paresthesia of the hands and feet. The condition progresses at a variable rate to involve the small muscles of the hands and proximal parts of the lower limbs. Classical HMSN patients can be divided into two groups based on their nerve conduction velocities (NCVs). Patients with HMSN I have a demyelinating neuropathy with reduced NCVs (patients over the age of 2 years have a motor NCV of <38 m/s in the median nerve). Patients with HMSN II have an axonal form of neuropathy, with normal or only slightly reduced NCVs (patients over the age of 2 years have a motor NCV of >38 m/s in the median nerve). Dejerine–Sottas syndrome/HMSN III The Dejerine–Sottas syndrome phenotype is more severe than that of classical HMSN, and patients with this condition present with hypotonia, generalized muscle weakness, motor developmental delay, ataxia, and areflexia. They often have palpable peripheral nerves. Muscle weakness tends to progress more rapidly than in classical HMSN, and patients are often nonambulatory by adolescence. However, the condition is quite variable in its severity and progression. Nerve conduction studies show very low NCVs (often <10 m/s), in association with demyelination with onion-bulb formation or hypomyelination on sural nerve biopsy. HMSN IV Autosomal recessive forms of HMSN I are designated HMSN IV. The phenotype is similar to that of HMSN I, but HMSN IV tends to present earlier and progress more rapidly. NCVs are usually <20 m/s.

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Congenital hypomyelinating neuropathy This is the most severe form of HMSN. Affected children present in infancy with severe hypotonia, generalized weakness, and areflexia. The condition can mimic spinal muscular atrophy, though nerve conduction studies show very slow or unrecordable NCVs and sural nerve biopsy shows amyelination or hypomyelination of nerve fibers. Affected children can have respiratory and swallowing difficulties, and an arthrogryposis-like presentation has also been described. CHN can take a lethal course, causing early death, though improvement has been described in some children. Hereditary neuropathy with liability to pressure palsies This is the mildest form of HMSN. Affected individuals present with recurrent peroneal- and ulnar-nerve pressure palsies, from which they often make a complete recovery. Nerve conduction studies show slightly reduced NCVs, with prolonged distal motor latencies of median and peroneal nerves. Sural nerve biopsy shows sausage-shaped swellings of the myelin sheath of nerve fibers. These swellings are called tomaculae. Age of onset

HMSN I and II usually present in the first decade of life, but onset can also occur in adult life. HMSN III usually presents in the first 2 years of life. HMSN IV usually presents in the first decade of life. CHN usually presents at birth or during early infancy. HNPP usually presents in adult life.

Epidemiology

The population prevalence of all forms of HMSN is about 1 in 2,500.

Inheritance, See Table 1. chromosomal location, gene, and mutational spectrum Molecular pathogenesis

PMP22 is composed of four exons. It is expressed in Schwann cells and encodes a 160-amino-acid integral membrane protein called peripheral myelin protein 22. This protein is involved in the formation and compaction of myelin in peripheral nerves. Mutations in PMP22 cause HMSN I, HMSN III, and HNPP. Duplication of PMP22 is believed to cause HMSN IA by a dosage effect. It has been suggested that overexpression of PMP22 could cause the protein to accumulate

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in the late Golgi-cell membrane compartment of Schwann cells, which could interfere with normal myelin assembly. Deletions of PMP22 cause HNPP as a result of haploinsufficiency. Point mutations in PMP22 are associated with a more severe phenotype than duplications of this gene, and are believed to cause HMSN by a dominant-negative effect (ie, the mutant protein interferes with the function of the normal protein produced by the normal allele of this gene). MPZ has seven exons and encodes a 248-amino-acid integral membrane protein called myelin protein zero. This protein has a large extracellular domain containing an immunoglobulin V-type fold, a single transmembrane segment, and a short cytoplasmic C-terminal end. Myelin protein zero is a major structural component of the myelin of peripheral nerves and is involved in the formation and compaction of myelin. Mutations in this gene can cause HMSN I, II, III, and CHN by interfering with the function of the protein in the myelin sheath of peripheral nerves. LITAF is a widely expressed gene with four exons that encodes a 161-amino-acid protein called lipopolysaccharide-induced tumor necrosis factor-α factor. This protein plays an important role in the regulation of tumor necrosis factor-α and could play a role in protein degradation pathways. Missense mutations in this gene cause HMSN IC, but the precise molecular mechanism is unknown. EGR2 has two exons and encodes a transcription factor protein (early growth response 2) with 476 amino acids. This protein is involved in the differentiation and maintenance of Schwann cells by regulating transcription of MPZ and PRX. Mutations in EGR2 can cause HMSN I, HMSN III, and CHN. NEFL contains four exons. It encodes the neurofilament protein (light polypeptide) that has 543 amino acids. This protein is one of three components of neurofilaments. Neurofilaments are cytoplasmic intermediate filaments of neurons. They are believed to play a role in the maturation of regenerating myelinated nerve fibers, and mutations in NEFL could lead to HMSN IF and IIE by interfering with this function. CX32 (or GJB1) is a small gene with only two exons. It is expressed in myelinated peripheral nerves and codes for connexin 32, a gap-junction protein with 283 amino acids. Gap junctions are involved in cell–cell communication. Mutations in CX32 result in an X-linked dominant form

12

Hereditary Motor and Sensory Neuropathy

MIM

118220

118200

601098

607678

607734

302800

118210

600882

605588

606071

601472

Classification

HMSN IA

HMSN IB

HMSN IC

HMSN ID

HMSN IF

HMSN X

Progressive Ataxias and Neurologic Disorders

HMSN IIA

HMSN IIB

HMSN IIB1

HMSN IIC

HMSN IID

Weakness of vocal cord and intercostal muscles, normal NCVs Weakness and wasting of hand at onset, normal NCVs

Normal or slightly reduced NCVs Ulcero-mutilating features, normal or slightly reduced NCVs Slow NCVs

Autosomal dominant

Autosomal recessive Autosomal dominant

Autosomal dominant Autosomal dominant

7p15

Unknown

1q21.2

3q21

1p36.2

Xq13.1

8p21

10q21.1–q22.1

16p12–p13.3

1q22

17p11.2

Chromosomal location

GARS

Unknown

LMNA

RAB7

KIF1B

CX32

NEFL

EGR2

LITAF

MPZ

PMP22

Gene

Glycyl-tRNA synthetase

Unknown

Lamin A/C

Kinesin family member 1B RAS-related GTPbinding protein 7

Lipopolysaccharideinduced tumor necrosis factor-α factor Early growth response 2 Neurofilament protein, light polypeptide Connexin 32

Myelin protein zero

Peripheral myelin protein 22

Product

Missense mutations

Missense mutation (Arg298Cys) Unknown

Missense mutations

Missense mutation (Arg409Trp) In-frame deletion or missense mutation (Pro8Arg) Missense mutations account for ~75% of all mutations. Nonsense and frame-shift mutations, as well as in-frame deletions and insertions have also been identified Missense mutations

Missense mutations

A 1.5-Mb duplication of 17p11.2 including PMP22 is the most common cause of HMSN IA. Point mutations in this gene have also been identified, including missense and frame-shift mutations Missense mutations

Mutational spectrum

16:04

X-linked dominant

Autosomal dominant Autosomal dominant

Autosomal dominant Autosomal dominant

Autosomal dominant

Inheritance

7/9/04

Slow NCVs, onset in infancy or early childhood Males are more severely affected than females. Males: slow NCVs. Females: normal or slow NCVs

Very slow NCVs

Slow NCVs

Slow NCVs

Slow NCVs

Distinguishing clinical features

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13

14

607684

606595

607706

607731

607677

607736

607831

HMSN IIE

MSN IIF

HMSN IIG

HMSN IIH

HMSN II I

HMSN IIJ

HMSN IIK

See text

17p11.2

1q22 10q21.1–q22.1 17p11.2

19q13.1–q13.2

Autosomal dominant Autosomal dominant Autosomal recessive

Autosomal recessive

8q13–q21.1

1q22

1q22

8q21.3

8q13–q21.1

7q11–q21

8p21

Chromosomal location

Autosomal dominant

Autosomal recessive

Autosomal dominant

Autosomal dominant

Autosomal recessive

Autosomal recessive

Autosomal dominant

Autosomal dominant

Inheritance

PRX

PMP22

EGR2

MPZ

PMP22

GDAP1

MPZ

MPZ

Unknown

GDAP1

Unknown

NEFL

Gene

Periaxin

Peripheral myelin protein 22

Early growth response 2

Myelin protein zero

Peripheral myelin protein 22

Ganglioside-induced differentiationassociated protein 1

Myelin protein zero

Myelin protein zero

Unknown

Ganglioside-induced differentiationassociated protein 1

Unknown

Neurofilament protein, light polypeptide

Product

Nonsense and frame-shift mutations

1.5-Mb duplication of 17p11.2, including PMP22 in both alleles or duplication of one allele and a point mutation (usually a missense mutation) in the other allele

Missense mutation (Arg359Trp)

Missense mutations

Missense and frame-shift mutation

Homozygosity for Ser194Stop mutation

Missense mutations (Thr124Met or Asp75Val)

Missense mutations (two patients had three different missense mutations in the same allele)

Unknown

Nonsense mutations

Unknown

Missense mutations

Mutational spectrum

16:04

Slightly reduced NCVs with onset in early childhood

Normal or slightly reduced NCVs with papillary abnormalities and deafness

Normal or slightly reduced NCVs

–

Normal or slightly reduced NCVs with vocal cord paresis

Normal NCVs

Normal or slightly reduced NCVs

Distinguishing clinical features

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HMSN III/ 145900 Dejerine–Sottas syndrome

MIM

Classification

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Hereditary Motor and Sensory Neuropathy

214400

601382

604563

601596

601455

162500

605253

HMSN IVA

HMSN IVB1

HMSN IVB2

HMSN IVC

HMSN IVD/ HMSN L (see text)

HNPP

Progressive Ataxias and Neurologic Disorders

CHN

See text

1q22 10q21.1–q22.1 10q21.1–q22.1

Autosomal dominant Autosomal recessive

17p11.2

8q24.3

5q32

11p15

11q22

8q13–q21.1

Chromosomal location

Autosomal dominant

Autosomal dominant

Autosomal recessive

Autosomal recessive

Autosomal recessive

Autosomal recessive

Autosomal recessive

Inheritance

EGR2

EGR2

MPZ

PMP22

NDRG1

Unknown

SBF2

MTMR2

GDAP1

Gene

Early growth response 2

Early growth response 2

Myelin protein zero

Peripheral myelin protein 22

N-myc downstreamregulated gene 1 protein

Unknown

SET-binding factor 2

Myotubularin-related protein 2

Ganglioside-induced differentiationassociated protein 1

Product

Missense mutation

Double missense mutation (on same allele)

Nonsense mutation

Over 85% of patients have a 1.5-Mb deletion of 17p11.2, including PMP22. The remainder have frame-shift or splicesite mutations that result in loss of function of the gene

Nonsense mutation (Arg148Stop)

Unknown

Nonsense mutations and in-frame deletion

Nonsense, frame-shift, and splice-site mutations

Demyelinating type: nonsense and missense mutations Axonal type: nonsense, missense, and frame-shift mutations

Mutational spectrum

16:04

See text

Onset in first decade, early-onset deafness, slow NCVs

Slow NCVs

Slow NCVs

Slow NCVs

Both a demyelinating and an axonal form are recognized. Axonal type: patients can have vocal cord paresis

Distinguishing clinical features

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Table 1. Hereditary motor and sensory neuropathies (HMSNs): classifications, inheritance patterns, and molecular genetics. CHN: congenital hypomyelinating neuropathy; HNPP: hereditary neuropathy with liability to pressure palsies; NCV: nerve conduction velocity.

MIM

Classification

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of HMSN. Mutant protein may have an increased tendency to form conducting hemichannels compared with normal protein. This could prevent the normal functioning of Schwann cells and neurons by increasing their membrane permeability. KIF1B has 47 exons and encodes an N-terminal-type motor protein with 1,816 amino acids. It acts as a motor for the anterograde transport of mitochondria. A mutation in this gene could result in the production of a mutant protein without any motor activity. The precise mechanism by which a mutation in this gene causes HMSN IIA is unknown. RAB7 has six exons and encodes a ubiquitously expressed protein with 207 amino acids called RAS-associated protein 7. This is a small GTPase, which is a member of the RAS-related GTP-binding protein family. It is believed to be involved in vesicular transport of proteins. It is not understood how mutations in this gene result in HMSN IIB. LMNA has 10 exons and codes for two proteins by alternative splicing of its exons. The gene products include lamin A and lamin C. Both proteins are components of the nuclear lamina. The mechanisms by which mutations in this gene cause HMSN IIB1 are not understood. Mutations in LMNA can cause several other conditions (see LGMD entry, p.18). GARS has 17 exons and encodes glycyl-tRNA synthetase. This is an enzyme with 685 amino acids that catalyses the esterification of glycine to its cognate tRNA during protein synthesis. Missense mutations in GARS cause HMSN IID and an autosomal dominant form of distal spinal muscular atrophy (type V, MIM 600794) by an unknown mechanism. GDAP1 has six exons and codes for ganglioside-induced differentiationassociated protein. This has 358 amino acids and may be involved in the signal transduction pathway in neuronal development. The precise mechanism by which mutations in GDAP1 cause HMSN IIG, IIK, and IVA is not understood. PRX contains six exons and produces two mRNA transcripts. One transcript produces L-periaxin and the other S-periaxin. Both proteins are expressed in Schwann cells and interact with the C-termini of plasma membrane proteins and with cytoskeletal proteins, and are required for the maintenance of peripheral nerve myelin. Mutations in this gene cause a form of HMSN III.

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MTMR2 is composed of 15 exons. Its protein product, myotubularinrelated protein 2, has 643 amino acids and is ubiquitously expressed. It is a dual-specificity phosphatase with homology to myotubularin. It is not understood how mutations in this gene cause HMSN IVB1. SBF2 is a large gene with 43 exons. Its protein product, SET-binding factor 2, has 1,849 amino acids and is a member of the pseudophosphatase branch of myotubularin-related proteins. It is expressed in fetal brain, spinal cord, and peripheral nerves and is involved in the differentiation of Schwann cells during myelination. Mutations in SBF2 cause HMSN IVB2. NDRG1 has 16 exons. Its protein product has 394 amino acids and is ubiquitously expressed. It appears to be expressed at particularly high levels in Schwann cells. NDRG1 protein is involved in growth arrest and cell differentiation, and it appears to have a role in Schwann cell signaling that is necessary for axonal survival. Mutations in NDRG1 cause HMSN IVD (this condition is also called HMSN, Lom type, or HMSN L because it only affects members of the Gypsy community of Lom in Bulgaria). Genetic diagnosis and counseling

PMP22, MPZ, and CX32 mutation analysis is available from several diagnostic laboratories. However, testing for mutations in the other genes is not routinely available at this time. All autosomal dominant and sporadic cases of HMSN I should be tested for mutations in PMP22 and MPZ. Patients from X-linked dominant HMSN families, sporadic male cases with HMSN I, and sporadic female cases with HMSN II should also be tested for mutations in CX32. Detailed pedigree analysis can often establish the mode of inheritance of HMSN in a family and allow accurate genetic advice to be given to other family members. HMSN I can show remarkable interfamilial and intrafamilial variability of expression. Therefore, parents of an apparently sporadic case should be carefully examined and offered nerve conduction studies to determine whether one parent is mildly affected. Predictive testing can be offered to at-risk members of families in which a mutation has been identified. Counseling in HNPP families is carried out on an autosomaldominant basis.

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Limb-girdle Muscular Dystrophy (also known as: LGMD) The LGMDs are a group of hereditary muscle disorders that predominantly affect the shoulder and pelvic girdles. There are several autosomal dominant (LGMD1) and autosomal recessive (LGMD2) forms with remarkable locus heterogeneity. Table 2 summarizes the classification, distinguishing clinical features, inheritance pattern, and molecular genetics of the various forms of LGMD. MIM

See Table 2.

Clinical features

The LGMDs are a clinically and genetically heterogeneous group of disorders. Affected individuals present with proximal weakness of the upper and lower limbs.

Age of onset

See Table 2.

Epidemiology

LGMDs affect all populations, but their incidence varies in different populations. Autosomal dominant forms only account for about 10% of cases. Mutations in one of the sarcoglycan genes (sarcoglycanopathies) can be seen in 8%–25% of patients with LGMD. In most populations the most frequently seen form of LGMD is LGMD2A, which accounts for 40%–45% of cases. However, LGMD2I is probably the most common form of LGMD in the UK.

Inheritance, chromosomal location, and gene

See Table 2.

Molecular pathogenesis

TTID is composed of 10 exons and codes for a structural muscle protein with 498 amino acids called titin immunoglobulin domain protein or myotilin. This is a thin, filament-associated, Z-disc protein that binds to α-actinin, F-actin, and filamin c. It cross-links actin filaments and controls sarcomere assembly, and is believed to play an important role in the stabilization and anchorage of thin filaments. Mutations in TTID probably cause LGMD by interfering with the proper organization of Z-discs. LMNA contains 10 exons and encodes two proteins as a result of alternative splicing of its exons. These proteins include lamin A (664 amino acids) and lamin C (572 amino acids). Both lamins

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are nuclear envelope proteins. How mutations in LMNA cause LGMD is not known. Mutations in LMNA have also been described in several other conditions, including an autosomal dominant form of dilated cardiomyopathy (type 1A, MIM 115200), an autosomal dominant form of Emery–Dreifuss muscular dystrophy (MIM 181350), Dunnigan type partial lipodystrophy (MIM 151660), an autosomal recessive form of HMSN (HMSN IIB1, MIM 605588 – see p.13), and two dysmorphic syndromes associated with premature ageing: Hutchinson-Gilford syndrome or progeria (MIM 176670) and mandibuloacral dysplasia (MIM 248370). CAV3 contains three exons and encodes caveolin 3, which has 131 amino acids. Caveolin 3 is the muscle-specific form of the caveolin protein family. Caveolins are the main protein components of caveolae (50–100 nm invaginations of plasma membranes). Mutations in CAV3 act in a dominant-negative manner by interfering with oligomerization of caveolin 3. This disrupts caveolae formation in the sarcolemmal membrane. Caveolin 3 interacts with dysferlin at the surface of the sarcolemmal membrane, and is also involved in normal expression of α-dystroglycan at the sarcolemmal surface. Caveolin 3 deficiency could therefore result in muscular dystrophy by interfering with the normal expression of dysferlin and α-dystroglycan. CAPN3 has 24 exons and encodes an 821-amino-acid protein called calpain 3. This is a muscle-specific, calcium-dependent protease. It appears to have a role in controlling the levels of muscle-specific transcription factors, though the precise role of calpain 3 in muscle and the mechanism by which a deficiency of this protein causes muscular dystrophy are unknown. DYSF is a large gene with 55 exons. It encodes dysferlin, a 2,080 amino-acid protein that localizes to the sarcolemmal membrane and co-immunoprecipitates with caveolin 3 in skeletal muscle. It is expressed very early in human development. Studies in mice have shown that dysferlin has an essential role in the resealing of the sarcolemma in response to injury. Therefore, mutations in DYSF probably cause muscular degeneration by disrupting the muscle membrane repair machinery. Mutations in DYSF have also been identified in Miyoshi myopathy, an autosomal recessive distal myopathy (MIM 254130). SGCA has eight exons and encodes α-sarcoglycan (also called 50-kDa dystrophin-associated glycoprotein [DAG]), which has 387 amino acids. Progressive Ataxias and Neurologic Disorders

19

20

253700

600119

LGMD2C

LGMD2D

Toe-walking, muscle cramps, scapular winging, calf hypertrophy

Calf hypertrophy 3–15 years

Childhood

Late teens

8–15 years

15–20 years

Adulthood

~5 years

Autosomal recessive

Autosomal recessive

Autosomal recessive

Autosomal recessive

Autosomal dominant

Autosomal dominant

Autosomal dominant

Autosomal dominant

Autosomal dominant

Caveolin 3

Lamin A/C

SGCG

DYSF

Small deletions Nonsense, missense, and splice-site mutations, duplications. Exon 3 mutational hotspot. Arg77Cys mutation accounts for ~40% of all mutations

α-Sarcoglycan

Missense, frame-shift, and splice-site mutations

Missense and splice-site mutations, small deletions and insertions

Unknown

Unknown

Missense mutations and 9-bp deletion

Missense and splice-site mutations, 3-bp deletion

γ-Sarcoglycan

Dysferlin

Calpain 3

Unknown Unknown

17q12–q21.33 SGCA

13q12

2p13.1–p13.3

Mutational spectrum

Titin immunoglobulin- Missense mutations domain protein or myotilin

Protein product

Unknown Unknown

CAV3

LMNA

TTID

Gene

15q15.1–q21.1 CAPN3

6q23

7q

3p25

1q21.2

5q31

Inheritance Chromosomal location

16:04

Inability to walk on tiptoe, calf atrophy, markedly elevated CK levels

603009

LGMD2B

None

Contractures of tendo-Achilles and other sites, scapular winging, hip abductors spared

603511

LGMD1D

Muscle cramps, calf hypertrophy, moderately elevated CK levels

253600

601253

LGMD1C

18–35 years

Age of onset

Cardiac involvement, 4–38 years particularly cardiac conduction defects

LGMD2A

159001

LGMD1B

Dysarthria

Dilated cardiomyopathy with cardiac conduction defects

159000

LGMD1A

Distinctive clinical features

7/9/04

LGMD with 602067 dilated cardiomyopathy

MIM

Classification

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Limb-girdle Muscular Dystrophy

604286

601287

601954

254110

607155

LGMD2E

LGMD2F

LGMD2G

LGMD2H

LGMD2I

9–15 years

4–10 years

Childhood

Age of onset

Progressive Ataxias and Neurologic Disorders None

6 months to 40 years

Autosomal recessive

Autosomal recessive

Autosomal recessive

Autosomal recessive

Autosomal recessive

19q13.3

9q31–q34.1

17q12

5q33

4q12

Inheritance Chromosomal location

FKRP

TRIM32

TCAP

SGCD

SGCB

Gene

Frame-shift, missense, and nonsense mutations, 3-bp deletion

δ-Sarcoglycan

Fukutin-related protein

Tripartite motifcontaining protein 32

Most affected individuals are homozygous for the missense mutation Leu276Ile. The remainder are compound heterozygotes for this mutation and a 4-bp deletion that results in premature protein truncation

All patients are homozygous for the missense mutation Asp487Asn

Nonsense and frame-shift mutations

Missense and proteintruncating mutations

β-Sarcoglycan

Telethonin

Mutational spectrum

Protein product

16:04

Weakness of facial, 8–27 years trapezius and deltoid muscles late in disease course, mild to moderate elevation of CK levels

Foot drop, proximal and distal lower limb weakness present at onset, mild to moderate elevation of CK levels, rimmed vacuoles on muscle biopsy

Cardiomyopathy, very severe clinical course with loss of ambulation between 9–16 years and death between 9–19 years

None

Distinctive clinical features

7/9/04

Table 2. Limb-girdle muscular dystrophies (LGMDs): classification, clinical features, and molecular genetics. CK: creatine kinase.

MIM

Classification

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SGCB has only six exons and encodes a protein with 318 amino acids, which is called β-sarcoglycan (43-kDa DAG). SGCD has nine exons and encodes δ-sarcoglycan (35-kDa DAG), which has 290 amino acids. SGCG is composed of eight exons and codes for γ-sarcoglycan (35-kDa DAG). This protein also has 290 amino acids. The sarcoglycans are transmembrane proteins that are an important component of the dystrophin–glycoprotein complex at the sarcolemmal membrane. The components of this complex link dystrophin inside the sarcolemma to the laminin α2 chain of merosin and other proteins in the extracellular matrix. The dystrophin–glycoprotein complex is believed to play a critical role in maintaining the integrity of the sarcolemmal membrane, particularly during muscle contraction. Therefore, absence or deficiency of the critical components of this complex can result in the phenotype of muscular dystrophy. Heterozygous mutations in SGCD can also cause one form of dilated cardiomyopathy type 1L (MIM 606685). TCAP is a small gene with only two exons. It encodes a structural sarcomeric protein called titin cap or telethonin. This protein has 167 amino acids and localizes to the Z-disc of adult skeletal muscle. TRIM32 has two exons and encodes a protein with 653 amino acids. Its protein product, TRIM 32 protein, is thought to be an E3 ubiquitin ligase. The mechanism by which mutations in this gene result in the LGMD phenotype is unknown. FKRP is composed of four exons and encodes fukutin-related protein, which has 495 amino acids. Fukutin-related protein is probably a Golgi-resident glycosyltransferase that is involved in the glycosylation of α-dystroglycan. This protein links the dystrophin–glycoprotein complex to various extracellular proteins, including the laminin α2 chain of merosin, neurexin, and agrin. Deficiency of fukutin-related protein probably results in muscular dystrophy due to aberrant glycosylation of α-dystroglycan. Genetic diagnosis and counseling

22

The diagnosis of LGMD is made by the combination of clinical features, immunohistochemistry on a muscle biopsy sample, and molecular genetic analysis. Immunohistochemistry and genetic testing are only available from a few specialized laboratories. Interpretation of the results of muscle immunohistochemistry is difficult and should only be carried out by laboratories experienced in the use of this technique. It is important to rule out facioscapulohumeral muscular dystropy and Emery–Dreifuss

Limb-girdle Muscular Dystrophy

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muscular dystrophy in families that appear to have an autosomal dominant form of LGMD. In large families with an autosomal recessive form of LGMD, linkage analysis might allow the precise form of LGMD to be identified. However, this should always be confirmed by muscle immunohistochemistry or mutation analysis of the relevant gene. Because of the genetic heterogeneity of LGMD, genetic counseling is difficult. Accurate genetic counseling and prenatal diagnosis are only possible in families where a definitive diagnosis can be made by muscle immunohistochemistry and genetic testing. Accurate counseling is also possible in large families with several affected members, where it is possible to determine the precise mode of inheritance (autosomal recessive or dominant). Isolated cases of LGMD could represent an autosomal recessive form of LGMD or a new autosomal dominant mutation.

Myotonic Dystrophy (also known as: MD; dystrophia myotonica; Steinert disease. Includes proximal myotonic myopathy [PROMM]) MIM

160900 (MD1) 602668 (MD2) 600109 (PROMM)

Clinical features

Four forms of MD1 can be recognized based on age of onset and clinical features. These include a mild form, an adult or classical form, a congenital form, and a childhood or juvenile form. Patients with mild MD usually present with presenile cataracts. The classical form of MD is a multisystem disorder. Symptoms include: muscle weakness and wasting, grip and percussion myotonia, cardiac arrhythmias that can present as syncope or sudden death, gastrointestinal problems, cataracts, an increased incidence of diabetes mellitus, and testicular atrophy in males. The distribution of muscle weakness and wasting is characteristic and is responsible for the well-recognized facial features of this condition. These include frontal balding, ptosis, facial weakness, bitemporal narrowing (due to wasting of the temporalis muscles), wasting of the jaw muscles, and a slender neck due to wasting of the sternomastoids. Early appearance and progression of male pattern baldness is also a feature. Distal limb muscles tend to be affected earlier than proximal muscles.

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Congenital MD is the most severe form of this disease and is the result of anticipation. It can present antenatally as reduced fetal movements and polyhydramnios, or in the neonatal period as severe hypotonia, respiratory distress (often requiring ventilation), feeding difficulties, facial weakness, cardiac problems (cardiomyopathy or arrhythmias), and talipes or arthrogryposis. A chest X-ray will often show thin ribs. Many patients with congenital MD die in childhood. Survivors show delayed development and have learning difficulties and characteristic facies (facial diplegia, an open-mouthed appearance with a tented upper lip, and a prominent, everted lower lip). A childhood or juvenile form of this condition has also been described. These patients usually present between 1 and 10 years of age with speech and language delay and learning difficulties, although some patients present with muscle weakness and myotonia at school age. MD2 and PROMM are probably a single entity as they have similar clinical features and are allelic or have very closely linked genes. Patients with these conditions present with slowly progressive proximal muscle weakness, mild myotonia, cardiac arrhythmias, and late-onset cataracts. White matter changes have been described in some families. Features that help to distinguish these conditions from the classical form of MD1 include the absence of facial weakness and the characteristic facial features that are seen in patients with classical MD, absent or minimal distal limb weakness, and the presence of myalgia. Age of onset

The mild form of MD1 presents in late adult life, the classical form presents in late adolescence or early adult life, the congenital form presents antenatally or in the neonatal period, and the childhood or juvenile form presents in early childhood. MD2 and PROMM present in adulthood.

Epidemiology

MD1 has an estimated incidence of 1 in 8,000. It appears to be particularly prevalent in the Saguenay-Lac-St-Jean region of Canada, where its prevalence is 1 in 475. MD2 and PROMM are rare disorders. Their population incidence and prevalence are unknown.

Inheritance

24

MD1: autosomal dominant MD2/PROMM: autosomal dominant

Myotonic Dystrophy

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Chromosome location

MD1: 19q13.3 MD2/PROMM: 3q13.3–q24

Genes

MD1: DMPK (dystrophia myotonica protein kinase) MD2/PROMM: ZNF9 (zinc finger protein 9)

Mutational spectrum

MD1 is caused by the expansion of a CTG triplet repeat motif in the 3´-untranslated region of the last exon of DMPK. In the general population, the number of CTG repeats varies from 5 to 37. Affected individuals have more than 50 repeats and there appears to be a correlation between the number of repeats and the severity of the phenotype. Repeat sizes of 50–100 are associated with the mild form of MD, whereas repeat sizes of between 500 and 1,500 result in the congenital MD phenotype. Expansions of between 100 and 500 are usually associated with classical MD, but it is not possible to predict the age of onset or the severity of disease in this group of patients. The CTG repeat shows meiotic instability and its size tends to increase in successive generations. This is responsible for the phenomenon of anticipation, in which the phenotype of a disease increases in severity in successive generations. Maternal transmission can be associated with a large expansion in the CTG repeat number, whereas paternal transmission is usually associated with a modest expansion of the repeat or, in some cases, a decrease in the number of repeats. Thus, congenital MD, which is caused by very large CTG repeat expansions, is almost always maternally transmitted. In contrast, the childhood or juvenile form of MD is more frequently paternally transmitted. Patients with MD2/PROMM have an expansion of a CCTG repeat motif in the first intron of ZNF9. Affected individuals have between 75 and 11,000 repeats, with an average of 5,000.

Molecular pathogenesis

DMPK has 15 exons and produces two main protein isoforms of 71 kDa and 80 kDa as a result of alternative splicing. Both of these isoforms are predominantly expressed in skeletal and cardiac muscle. The precise mechanism by which the CTG repeat expansion causes MD1 is unknown. Interest has focused on the possibility that the allele with the CTG repeat expansion produces mRNA that inappropriately binds to proteins via CUG repeats (thymidine is replaced by uridine in RNA). One particular protein (CUG-binding protein) is involved in processing mRNA from

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several other genes, including cardiac troponin T. Binding of CUG-binding protein to the mRNA product of the DMPK allele with the CTG repeat expansion could interfere with its ability to process the mRNAs of several other genes, and altered expression of these genes could result in the MD1 phenotype. Thus, the CTG repeat expansion in DMPK would appear to be a gain-of-function mutation. ZNF9 has five exons and encodes a protein with 177 amino acids. This protein has seven zinc-finger domains and is believed to be an RNA-binding protein. Mutant ZNF9 mRNA accumulates in the nucleus and probably results in the MD2/PROMM phenotype in a manner analogous to the expansions in the DMPK gene that cause MD1. Genetic diagnosis and counseling

Genetic testing for MD1 is widely available, so all patients with MD should have genetic testing to confirm the diagnosis. Patients whose clinical features are suggestive of MD but who test negative for the CTG repeat expansion in DMPK are likely to have MD2/PROMM or an alternative myotonic disorder. Counseling is on the basis of autosomal dominant inheritance. Women with MD1 should be told that their children could be affected with congenital MD as a result of anticipation. Women who have neuromuscular disease or who have previously had an affected child with congenital MD are particularly at risk of having a baby with congenital MD. Patients with MD should be told that they are at risk of developing cardiac arrhythmias, cataracts, and diabetes mellitus, and they should be under the care of a physician. They should also be told about the complications of general anesthesia, including malignant hyperthermia and postanesthetic apnea. Patients should be asked to carry an alert card or bracelet. Presymptomatic/predictive genetic testing can be offered to those from families where a CTG repeat expansion has been previously documented in an affected individual (and who therefore have a 50% risk of being affected). Prenatal diagnosis is also available by testing DNA extracted from either a chorionic villus sample or cultured amniocytes for the CTG repeat expansion. Genetic testing for MD2/PROMM is only available on a research basis. Counseling is as for autosomal dominant inheritance. Anticipation does occur, but is milder than that seen in MD1.

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Myotonic Dystrophy

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Spinal Muscular Atrophy (also known as: SMA) MIM

253300 (SMA type I/Werdnig–Hoffmann disease) 253550 (SMA type II) 253400 (SMA type III/Kugelberg–Welander syndrome)

Clinical features

Children with SMA present with generalized muscle weakness and wasting, hypotonia, and areflexia. The muscle weakness begins proximally, and characteristically involves the intercostal muscles and later the diaphragm, but spares the extraocular muscles and facial muscles. Fasciculation of the tongue and other muscles is a helpful diagnostic clue. Childhood SMA is classified into three types based on age of onset, extent of motor development, and prognosis. The most severe form is SMA type I. Children with this condition never learn to sit and usually die by the age of 2 years. Patients with SMA type II learn to sit without support, but never learn to walk unaided. The prognosis is variable, with some patients dying in childhood and others surviving to adulthood. Patients with SMA type III are able to walk independently. They have slowly progressive muscle weakness, and survive into adulthood. Diagnosis can be confirmed by electromyography (which shows a neurogenic pattern) and by muscle biopsy (which shows grouped atrophy of both type I and II fibers, with hypertrophy of type I fibers).

Age of onset

SMA type I: before 3 months SMA type II: 3–18 months SMA type III: after 18 months

Epidemiology

The incidence of all forms of SMA is about 1 in 10,000 live births. SMA has been described in all ethnic groups. The heterozygote (carrier) frequency is about 1 in 50.

Inheritance

All forms of childhood SMA are inherited in an autosomal recessive manner. However, a small proportion (~2%) of those with type II or III may have a form inherited in an autosomal dominant manner.

Chromosomal location

5q12.2–q13.3

Gene

SMN1 (survival of motor neuron 1)

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Mutational spectrum

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There are two copies of the SMN gene: a telomeric copy (SMN1) and a centromeric copy (SMN2). There are only minor differences in the coding sequence of these two genes. Both genes are expressed, but SMN1 produces much higher levels of the functional full-length transcript than SMN2. Because of the homology between these two genes, gene conversion events are frequent, resulting in the conversion of SMN1 to SMN2. The vast majority of patients with SMA (~96%) are homozygous for a deletion or gene conversion of SMN1. A small number of patients (~4%) are compound heterozygotes with a deletion or gene conversion affecting one SMN1 allele and a different mutation in the other allele. Other mutations in SMN1 are rare, but can include point mutations (mostly missense mutations and splice-site mutations) and frame-shift mutations. The presence of multiple copies of SMN2 in patients homozygous for a deletion or gene conversion of SMN1 can modify the phenotype and lead to less severe disease (SMA types II or III). Other genetic modifiers of the phenotype have also been described (eg, splicing mechanisms of the SMN2 gene and deletion of the H4F5 gene that lies upstream of SMN1).

Molecular pathogenesis

The protein product of the SMN1 and SMN2 genes is expressed in several areas, including the central nervous system, skeletal muscle, heart, liver, kidneys, lungs, thymus, and pancreas. Within the central nervous system it is expressed in the anterior horn cells of the spinal cord. The SMN protein is localized to the cytoplasm and nucleus. In the nucleus it is localized in small, discrete, dot-like structures called “gems”. It interacts with several small nuclear ribonucleoproteins and appears to have an important role in the generation of the pre-mRNA splicing machinery, and, therefore, in mRNA processing in the cell. Although SMN1 is ubiquitously expressed, loss of function of this gene only results in degeneration of spinal motor neurons because these cells are believed to need high levels of SMN protein to survive.

Genetic diagnosis and counseling

Genetic testing for SMA is routinely available. Carrier testing for SMA is also available from diagnostic laboratories. Counseling is on an autosomal recessive basis. Prenatal diagnosis can be offered to parents of children with SMA in whom the diagnosis has been confirmed by genetic testing.

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2 2. Cerebral Malformations and Mental Retardation Syndromes Angelman Syndrome 30 Fragile X Syndrome 34 Holoprosencephaly 36 Hunter Syndrome 40 Huntington Disease 41 Lesch–Nyhan Syndrome 43 Lissencephaly 45 Lowe Syndrome 52 Neuronal Ceroid Lipofuscinosis 53 Pelizaeus–Merzbacher Syndrome 57 Prader–Willi Syndrome 59 Rett Syndrome 61 X-linked Adrenoleukodystrophy 62 X-linked α-Thalassemia and Mental Retardation Syndrome 64 X-linked Hydrocephalus 66

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Angelman Syndrome (also known as: AS; happy puppet syndrome) MIM

105830

Clinical features

Affected children show severe developmental delay with very limited speech, ataxia, and easily provoked laughter; they have a happy demeanor and excitable personality. Convulsions occur in 80%, usually with onset in early childhood. Other common features include microcephaly, drooling, prognathism, hypopigmentation, and a scoliosis (this can progress and require surgical correction). Life expectancy is relatively normal.

Age of onset

The diagnosis is usually made in early childhood. With the benefit of hindsight, it often becomes apparent that problems were first evident in infancy.

Epidemiology

The incidence is estimated to be between 1 in 10,000 and 1 in 40,000.

Inheritance

The mode of inheritance is complex, as four different causes of AS are recognized: class I (maternally derived chromosome 15q11–q13 interstitial deletion), class II (paternal uniparental disomy [UPD] for chromosome 15), class III (imprinting defect involving the Prader–Willi syndrome [PWS]/AS critical region), and class IV (mutation in UBE3A).

Chromosomal location

15q11–q13

Gene

UBE3A (ubiquitin protein ligase E3A)

Mutational spectrum

Missense, nonsense, splice-site, and frame-shift mutations.

Molecular pathogenesis

AS is caused by abnormal expression of the maternally imprinted UBE3A gene, which contains 16 exons and encodes a ubiquitin protein ligase that is thought to play a role in the localization of proteins in the brain. UBE3A is chiefly expressed in the hippocampus and cerebellum and is expressed only from the maternal allele in brain (ie, it shows tissue-specific imprinting).

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Class I. There is an interstitial deletion involving 15q11–q13 in the maternally derived chromosome 15. This accounts for approximately 70% of all cases. Most such deletions occur de novo. Rarely, a more complex chromosome rearrangement involving deletion of 15q11–q13 is identified, possibly as a de novo finding or as a result of malsegregation of a maternally balanced rearrangement. Class II. Paternal UPD for chromosome 15 accounts for around 2% of all cases. In this situation, both number 15 chromosomes are of paternal origin. This may be due to nondisjunction in paternal meiosis, resulting in a disomic sperm. The disomic sperm may then have fertilized a monosomic ovum, resulting in transient trisomy 15 in the zygote with subsequent loss of the maternally derived chromosome 15 (“trisomy rescue”). Class III. Imprinting defects account for approximately 4% of all cases. Roughly half of these are caused by very small deletions involving the PWS/AS imprinting box/center (see Figure 1). The cause of the remaining 50% is uncertain. Class IV. Mutations in UBE3A are found in 5%–10% of cases. Most of these arise de novo, but the mother is a carrier in up to 20% of cases through either inheriting a “silent” mutation from her father or showing germ-line mosaicism. In around 10%–15% of cases no chromosomal or molecular abnormality can be identified. In these situations there may be an alternative diagnosis, such as Rett syndrome (p.61–2), or there may be a mutation in an as yet unidentified UBE3A-related gene. M

P

AS IC

PWS IC

P

P M

Centromere UBE3A

antisense UBE3A

SNRPN

qter

Figure 1. Simplified diagram of the PWS/AS (Prader–Willi syndrome/Angelman syndrome) critical region on chromosome 15. In the maternal chromosome the AS imprinting center (IC) is active and methylates (silences) the SNRPN promoter. UBE3A is expressed. In the paternal chromosome the PWS IC activates SNRPN and other adjacent genes, including antisense UBE3A, which silences UBE3A. M and P refer to maternally and paternally derived patterns of expression, respectively.

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Genotype–phenotype Children with class I microdeletions are the most severely affected, correlation with the highest incidence of microcephaly and seizures and the most severe learning disability, with absent speech. They alone show hypopigmentation, almost certainly because of haploinsufficiency for the P gene, which is located at 15q11–q13 and deficiency of which causes type II oculocutaneous albinism (MIM 203200). The next most severely affected children are those with class IV UBE3A mutations. These children often have microcephaly and seizures, but demonstrate better developmental progress than children in class I. Children with UPD (class II) and imprinting mutations (class III) are the least severely affected, with a low incidence of microcephaly and seizures, but they still show severe learning disability with only very limited speech. Genetic diagnosis and counseling

32

Diagnosis in classes I–III can be made by methylation analysis using a methylation-sensitive restriction enzyme and a probe from the PWS/AS critical region (see Figure 2). This reveals the presence of only a paternal band. Microdeletion analysis by fluorescence in situ hybridization (FISH) identifies all class I cases (see Figure 3). If a chromosomal abnormality is identified, parental chromosome studies should be undertaken. Paternal UPD (class II) is identified by restriction fragment length polymorphism or microsatellite analysis. Class IV cases (UBE3A mutations) can only be detected by mutation analysis (usually singlestrand conformation polymorphism screening followed by direct sequencing). Most microdeletions identified by FISH are de novo and have a low risk of recurrence of less than 1%, which is attributable to maternal germ-line mosaicism. The recurrence risk for paternal UPD cases (class II) is negligible. If an imprinting center microdeletion or UBE3A mutation is present in the affected child’s mother then the recurrence risk is 50%; otherwise, the recurrence risk is low but not negligible because of possible maternal germ-line mosaicism.

Angelman Syndrome

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Paternal chromosome 15

A

B

Probe

A

Maternal chromosome 15

Maternal chromosome

Southern blot

Paternal chromosome

Normal

Prader—Willi Angelman syndrome syndrome

Figure 2. The principle underlying the methylation test for the Angelman and Prader–Willi syndromes. “A” represents sites cleaved by a nonmethylation-sensitive restriction enzyme. “B” represents a restriction site cleaved by a methylation-sensitive restriction enzyme such as HpaII. Normally, the maternal chromosome 15 is imprinted (methylated) so that cleavage at site “B” does not occur. In Angelman syndrome there is absence of a normally imprinted maternal chromosome. The opposite applies in Prader–Willi syndrome.

Figure 3. Deletion of the proximal long arm of one number 15 chromosome (15q11–q13) in a child with Angelman syndrome demonstrated by fluorescence in situ hybridization analysis. Image courtesy of Mrs Karen Marshall, Cytogenetics Laboratory, Leicester Royal Infirmary, UK.

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Fragile X Syndrome MIM

309550

Clinical features

This is the most common inherited cause of mental retardation. Both sexes can be affected, but facial dysmorphism is usually seen in males. Presenting features include global developmental delay, moderate to severe learning difficulties, autistic features, attention deficit hyperactivity disorder, and seizures. Dysmorphic features include macrocephaly, a long face with broad forehead, anteverted ears, and prominent chin. Additional features include hyperextensible metacarpophalangeal joints, pes planus, hypotonia, and macroorchidism in postpubertal males.

Age of onset

The condition can be diagnosed in infancy, but diagnosis is often delayed until childhood.

Epidemiology

Prevalence is about 1 in 5,000 in males and 1 in 8,000 in females. Studies indicate that about 0.6% of patients (male and female) with mental retardation have fragile X syndrome.

Inheritance

X-linked dominant

Chromosomal location

Xq27.3

Gene

FMR1 (fragile X mental retardation 1)

Mutational spectrum

The FMR1 gene has a polymorphic CGG repeat motif in the 5´ untranslated region of the first exon. In the vast majority of patients (~99%), this motif is expanded to more than 200 repeats. An expansion of this size is called a “full mutation”. Normal individuals have 6–54 CGG repeats. Fragile X carriers have 55–200 CGG repeats. An expansion of this size is called a “premutation”. Premutations are unstable in meiosis and can expand to a full mutation on maternal transmission. Individuals with 45–54 repeats are said to have an “intermediate” allele. These alleles can demonstrate instability, either increasing or decreasing in size during meiosis. Intermediate alleles do not usually expand to a full mutation in a single generation. About 1% of patients with fragile X syndrome have other FMR1 mutations.

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These include partial or complete gene deletions as well as missense, frame-shift, and splice-site mutations. Molecular pathogenesis

FMR1 has 17 exons and encodes FMR protein (FMRP), which has 632 amino acids. Expression levels of FMRP are highest in the brain, testes, lymphocytes, and placenta. Expansion of CGG repeat numbers to more than 200 results in hypermethylation of the repeats and surrounding sequences. This results in transcriptional silencing of the FMR1 gene, which is then unable to produce FMRP. Other mutations in this gene result in the production of a truncated protein that is rapidly degraded or the production of nonfunctional protein. Alleles of FMR1 with normal or premutation-sized CGG repeats produce normal amounts of FMRP. There is good evidence to suggest that FMRP is a translational regulator. It is able to bind RNA, including its own mRNA and about 4% of neuronal mRNA transcripts. The absence of FMRP could alter the transcriptional profile of many of these mRNAs, some of which probably encode important neuronal proteins.

Genetic diagnosis and counseling

Molecular genetic testing for fragile X syndrome is widely available. However, this is confined to testing for the CGG expansion mutation. Almost all patients with nonspecific developmental delay will undergo genetic testing for fragile X syndrome, but the diagnostic yield of such testing is poor. More focused testing (such as testing of males with moderate to severe developmental delay and autistic features or some of the facial features of fragile X syndrome, or testing males or females with a family history of mental retardation) improves the diagnostic yield. Chromosome analysis to look for the fragile site at Xq27.3 (FRAXA) is not a reliable test for fragile X syndrome because it does not detect all females. Also, it is not a reliable method of carrier detection. In addition, other fragile sites located close by (FRAXE and FRAXF) can cause diagnostic confusion. Genetic counseling in fragile X syndrome is difficult. Males with a premutation are called “normal transmitting males”. They will transmit their X chromosome, with the premutation, to all of their daughters and their Y chromosome to all of their sons. The premutation usually undergoes slight expansion when paternally transmitted, but expansion of a premutation to a full mutation is not seen. Therefore, all daughters of a normal transmitting male are fragile X carriers.

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Females with a premutation are called carriers. They will transmit their X chromosome with the CGG expansion to half their sons and half their daughters. A premutation has a high risk of expanding to a full mutation when maternally transmitted, and the risk of expansion depends on the size of the premutation. Premutations with more than 70 CGG repeats have an 80% chance of expanding to a full mutation. The smallest known premutation that has expanded to a full mutation in a single generation had 59 CGG repeats. All males who inherit a full mutation from their mother will be affected with fragile X syndrome. Between 50% and 80% of females with a full mutation have some degree of learning difficulties. Prenatal diagnosis can be offered to female carriers.

Holoprosencephaly (also known as: HPE) MIM

See Table 1. Type

MIM

Chromosomal location

Gene

HPE1

236100

21q22.3

Unknown

HPE2

157170

2p21

SIX3 (sine oculis homeobox, Drosophila homolog 3)

HPE3

142945

7q36

SHH (Sonic hedgehog)

HPE4

142946

18p11.3

TGIF (transforming growth factor β-induced factor)

HPE5

603073

13q32

ZIC2 (zinc finger protein of cerebellum 2)

HPE6

605934

2q37.1–q37.3

Unknown

HPE7

601309

9q22.3

PTCH (homolog of Drosophila segment polarity gene patched)

Table 1. Holoprosencephaly (HPE): MIM numbers, chromosomal locations, and genes.

Clinical features

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HPE has a very variable phenotype. Severe forms (alobar or semilobar HPE) present with characteristic facial dysmorphism (such as cebocephaly or premaxillary agenesis), microcephaly, profound developmental delay, spastic quadriparesis, seizures, and failure to thrive. Mild forms (lobar HPE) can present with normal facial features or mild facial dysmorphism (such as ocular hypotelorism, abnormal superior labial frenulum, single median maxillary incisor, and high-arched palate), together with microcephaly, developmental delay, and subtle neurologic abnormalities such as anosmia.

Holoprosencephaly

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HPE can be an isolated problem or part of a multiple malformation syndrome. Isolated HPE is usually caused by a single gene mutation. Syndromic forms of HPE can be caused by chromosomal aberrations or single gene mutations. Trisomy 13 is an important cause of HPE. Mutations in the genes that are discussed here result in isolated, nonsyndromic HPE. Age of onset

Severe forms are usually apparent at birth or in the neonatal period. Diagnosis of the milder forms is often delayed, sometimes until late childhood.

Epidemiology

HPE has been reported in all populations and has an incidence of 1 in 16,000 live births.

Inheritance

Usually autosomal dominant with reduced penetrance. Most cases are sporadic.

Chromosomal location and gene

See Table 1.

Mutational spectrum

SIX3 mutations (causing HPE2) are a rare cause of familial and sporadic cases of isolated HPE. Most reported mutations have been missense mutations that are predicted to result in functional inactivation of the gene. One family had a frame-shift mutation that resulted in protein truncation. SHH mutations (causing HPE3) are seen in 6%–7% of sporadic cases with isolated HPE. However, mutations in this gene can be seen in 35%–40% of families with autosomal dominant HPE. The most frequently seen mutations are nonsense and missense mutations, but deletions and insertions have also been identified. Mutations are predicted to have a loss of function effect. There is no genotype–phenotype correlation. TGIF mutations (causing HPE4) are seen in only 1%–2% of patients with isolated HPE. All mutations identified so far have been missense. These mutations are predicted to have a loss of function effect. ZIC2 mutations (causing HPE5) account for only 2%–3% of isolated HPE cases. Patients with ZIC2 mutations have severe or mild HPE with relatively normal facial features. Most mutations in this gene have been frame-shift mutations, usually short insertions that result

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in loss of function. In one family, a 30-bp insertion was identified in exon 3. This resulted in expansion of a polyalanine tract from its normal length of 15–25 residues by 10 residues. PTCH mutations (causing HPE7) are a rare cause of sporadic and familial HPE. Mutations in PTCH also cause nevoid basal cell carcinoma syndrome (Gorlin syndrome, MIM 109400). All HPE-causing mutations in PTCH have been missense mutations, which can show nonpenetrance. Molecular pathogenesis

SIX3 is a homologue of the Drosophila “sine oculis” gene, which encodes a nuclear protein that is involved in eye development. SIX3 has two exons and is expressed in fetal and adult retinal tissue. Its protein product has 332 amino acids and is believed to be a transcription factor essential for the development of the eyes and anterior neural plate in humans. Mutations in this gene are a rare cause of autosomal dominant and sporadic HPE. Mutations are most likely to cause HPE by interfering with normal ventral induction. SHH is a small gene with only three exons that encodes a signaling protein called Sonic hedgehog (SHH), which has 462 amino acids. SHH has an N-terminal signaling domain and a C-terminal catalytic domain and is believed to have an important role in patterning of the ventral neural tube. Therefore, mutations in SHH cause HPE by disrupting ventral induction in early embryogenesis. TGIF has three exons and encodes a protein with 272 amino acids called transforming growth factor β-induced factor. This has several putative functions. It competitively inhibits binding of the retinoic acid receptor to a retinoid-responsive promoter. Reduced TGIF levels could down-regulate SHH expression by enhancing the binding of retinoic acid receptors. It also interacts with a SMAD2/SMAD4 complex in the nucleus forming a transcriptional repressor. Finally, it is also believed to link the NODAL signaling pathway to normal development of the human forebrain. The ZIC2 gene is a homolog of the Drosophila “odd-paired” gene. It has three exons and is only expressed in the fetal brain in humans. Unlike SHH (which is expressed in the ventral neural tube), ZIC2 is expressed in the dorsal neural tube. The protein product of this gene has 533 amino acids and is known to be involved in normal neural development. The mechanism by which mutations in this gene cause HPE is not understood.

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PTCH has 23 exons and encodes a 1,447-amino-acid protein, with 12 transmembrane spanning segments, that is a transmembrane receptor for SHH. PTCH normally represses SHH signaling by binding to another transmembrane protein called Smoothened. When SHH binds to PTCH it releases Smoothened from repression allowing intracellular signaling to proceed. Mutations in PTCH probably cause HPE by enhancing the repressive activity of PTCH on SHH signaling. Genetic diagnosis and counseling

The diagnosis of HPE can be confirmed by neuroimaging. All affected patients should be examined carefully for other congenital abnormalities and chromosome analysis should be performed to rule out a syndromic form of HPE. Parents of patients with isolated nonsyndromic HPE should be carefully examined for microcephaly, iris coloboma, anosmia or hyposmia, absent or abnormal superior labial frenulum, single median maxillary incisor, and high-arched palate. These clinical findings are called HPE “microforms”. They can be seen in individuals who carry a mutation in one of the HPE genes, but have no evidence of HPE on neuroimaging. If HPE microforms are seen in the parent of an affected child then that parent is likely to be a carrier of a mutation in one of the HPE genes and the family should be counseled on an autosomal dominant basis. It is important to remember that autosomal dominant HPE can show remarkable intrafamilial variability of expression and another affected child could be severely affected with alobar HPE or could present with only HPE microforms. If neither parent has any HPE microforms but there is parental consanguinity, the family should be counseled on an autosomal recessive basis. If there is no parental consanguinity then the affected child is likely to have a sporadic form of HPE. The sibling recurrence risk of HPE in this situation is ~6%. Genetic testing is only available on a research basis. It is not very helpful in sporadic cases because of the degree of genetic heterogeneity and because mutations can only be identified in a very small number of patients. It is helpful to look for mutations in SHH in families in which HPE is clearly segregating in an autosomal dominant manner.

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Hunter Syndrome (also known as: HS; mucopolysaccharidosis type II [MPS II]) MIM

309900

Clinical features

This condition occurs in a severe (MPSIIA) and mild form (MPSIIB). The severe form is three times more frequent. Affected individuals present with coarsening of the facial features, short stature with dysostosis multiplex on skeletal survey, hepatosplenomegaly, progressive sensorineural hearing loss, cardiac valvular disease, and retinitis pigmentosa. The severe form is associated with progressive mental retardation; death occurs in most cases by the age of 15 years. In contrast, patients with the mild form of the disease are not intellectually impaired and have prolonged survival.

Age of onset

In severe cases onset is in the first year of life. In mild cases onset can be delayed until mid-childhood or even later.

Epidemiology

The condition affects all races, but is particularly prevalent in Israeli Jews, where its incidence is 1 in 34,000 male births. In the British population the incidence is estimated to be 1 in 132,000 male births.

Inheritance

X-linked recessive

Chromosomal location

Xq28

Gene

IDS (iduronate 2-sulfatase)

Mutational spectrum

Over 150 different mutations have been identified in IDS. Approximately 20% of patients have complete deletion of or a gross structural alteration in IDS. These patients usually present with the severe form of this condition. Another 20% of patients have small intragenic deletions and the remainder have point mutations in IDS. The latter include nonsense, missense, frame-shift, and splice-site mutations. Missense mutations can result in a severe or intermediate phenotype.

Molecular pathogenesis

IDS has nine exons and encodes the 550-amino-acid lysosomal enzyme iduronate 2-sulfatase. Mutations in IDS result in deficiency of this enzyme. This enzyme acts on dermatan and heparan sulfate and

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catalyzes the first step in degradation of these glycosaminoglycans in the lysosome. Deficiency of iduronate 2-sulfatase therefore results in accumulation of dermatan and heparan sulfate in tissues and excretion of these glycosaminoglycans in urine. The clinical features of HS are the result of tissue accumulation of these glycosaminoglycans. Genetic diagnosis and counseling

The diagnosis of HS can be made by iduronate 2-sulfatase assay in white cells or plasma. Counseling is on an X-linked recessive basis. Plasma assay of iduronate 2-sulfatase can be used for carrier detection (levels of this enzyme are approximately 50% lower than normal in carriers). However, IDS mutation analysis is the most reliable method of carrier detection. Prenatal diagnosis is possible by measuring iduronate 2-sulfatase levels in uncultured or cultured chorionic villi or cultured amniocytes.

Huntington Disease (also known as: HD) MIM

143100

Clinical features

This is an adult-onset neurodegenerative disorder that presents with the triad of personality change, chorea, and dementia. Psychiatric problems can occur, including depression and social withdrawal. Dementia is a late feature of the disease, but social functioning may be impaired at an early stage. The condition is slowly progressive with a typical duration of about 15 years. A more severe, juvenile-onset form also exists. This presents with rigidity, dystonia, seizures, ataxia, and cognitive decline. This form tends to progress more rapidly than the adult-onset form, with death in the 20s. The juvenile form is usually paternally inherited and is the result of anticipation.

Epidemiology

The population prevalence of HD is 4–7 per 100,000. The condition affects all races although it appears to have a lower incidence in Japanese, Chinese, Finnish, and African–American populations.

Age of onset

The usual age of onset is 35–40 years. However, onset has been described in the mid-70s and in childhood. The juvenile form presents in late childhood or adolescence.

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Inheritance

Autosomal dominant

Chromosomal location

4p16.3

Gene

IT15 (important transcript 15)

Mutational spectrum

All patients with HD have a pathogenic expansion of a CAG repeat motif in the first exon of IT15. In the normal population, the number of these repeats varies between nine and 35. Patients with HD have ≥ 36 CAG repeats. Individuals with 36–39 repeats usually develop HD, but may remain asymptomatic or develop HD late in life. Individuals with ≥ 40 repeats will almost invariably develop HD. There appears to be some correlation between increasing size of the CAG repeat expansion and earlier age of onset, but there are not enough data presently available to use this information for counseling purposes.

Molecular pathogenesis

IT15 is a large gene with 67 exons. It encodes a widely expressed protein with 3,144 amino acids called huntingtin. The CAG-repeat expansion in IT15 is incorporated into this protein and results in the production of mutant huntingtin with an expanded polyglutamine tract, which accumulates in the nucleus. In transgenic mice, cells expressing intranuclear huntingtin undergo apoptosis. Expression of caspase 1 triggers apoptosis by activating caspase 3. This may also be the mechanism of neuronal injury in patients with HD. The CAG repeat expansion therefore appears to be a toxic gain of function mutation.

Genetic diagnosis and counseling

Genetic testing for HD is available as a diagnostic service from most molecular genetic laboratories. A clinical diagnosis of HD should always be confirmed by genetic testing. Counseling is on an autosomal dominant basis. Adults at 50% (and 25%) risk of inheriting this condition can be offered predictive (presymptomatic) testing after appropriate counseling. Predictive testing for children is not recommended because it confers no medical benefit and removes the opportunity for that child to make the decision about predictive testing in adulthood. This is important because almost half of all adults who seek predictive testing for HD elect not to proceed after genetic counseling. Predictive testing of children also results in loss of confidentiality as the result of the test will be disclosed to their parents and often their general practitioner. Children who are shown to have

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inherited a pathogenic CAG repeat expansion on testing could be treated differently from their siblings. Other important reasons for not offering predictive testing to children are the adverse implications that a “bad” result could have for future employment and life insurance. Juvenile HD is almost always paternally inherited. If this diagnosis is suspected in a child or adolescent then genetic testing can be undertaken to confirm the diagnosis. Prenatal diagnosis of HD by direct mutation testing can be offered to individuals who have been shown to carry a pathogenic CAG repeat expansion. Individuals at 50% risk of inheriting HD who have not had predictive testing and who do not want this test can be offered prenatal diagnosis by exclusion testing (a form of linkage analysis that is used to determine whether the fetus has inherited a marker linked to the IT15 gene from the affected or unaffected grandparent).

Lesch–Nyhan Syndrome (also known as: HGPRT [hypoxanthine-guanine phosphoribosyl transferase] deficiency) MIM

300322

Clinical features

This condition, which almost exclusively affects males, presents with developmental delay, learning difficulties, self-injurious behavior (selfmutilation and biting of lips, cheeks, hands, and fingers), and involuntary movements (chorea, athetosis, and dystonia). Affected children are hypotonic initially, but later develop spasticity. Other features include the formation of renal urate calculi, and gout can develop late in the course of the disease. The diagnosis is suggested by elevated plasma urate levels and an elevated urinary urate to creatinine ratio in males with developmental delay, and can be confirmed by demonstration of reduced or undetectable levels of HGPRT in red blood cells.

Age of onset

First year of life.

Epidemiology

Lesch–Nyhan syndrome is seen in all ethnic groups. It is a rare disorder with a population prevalence of about 1 in 380,000.

Inheritance

X-linked recessive

Chromosomal location

Xq26–q27.2

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Gene

HPRT1 (hypoxanthine-guanine phosphoribosyl transferase 1)

Mutational spectrum

Over 200 mutations have been identified in HPRT1. Most mutations are unique and there are no mutational hot-spots in the gene. Missense, nonsense, and frame-shift mutations are seen in about 70% of patients. A large deletion or insertion is seen in 10%–12% of patients. Splice-site mutations are seen in about 12%–13% of patients.

Molecular pathogenesis

HPRT1 has nine exons and encodes the HGPRT enzyme, which has 657 amino acids. This enzyme converts hypoxanthine to inosine monophosphate and guanine to guanine monophosphate. Most mutations in HPRT1 result in reduced production of HGPRT. Deficiency of HGPRT interferes with the normal reutilization of hypoxanthine, which is converted to xanthine and uric acid. The precise mechanism by which HGPRT deficiency causes neurologic problems is not understood. Patients with Lesch–Nyhan syndrome are thought to have very few dopaminergic neurons in their brain, and this could contribute to the neurologic features of this condition.

Genetic diagnosis and counseling

HPRT1 mutation analysis is not routinely available. Counseling is on the basis of X-linked recessive inheritance. Reliable carrier testing is available for the female relatives of affected males in whom a mutation has been identified in HPRT1. Prenatal diagnosis is available by HGPRT assay on cultured chorionic villi or cultured amniocytes, or by mutation analysis in families in which an HPRT1 mutation has been identified in an affected male. Prenatal diagnosis should be offered to all women who have had an affected child (whether or not they are carriers) because of the possibility of gonadal mosaicism.

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Lissencephaly (also known as: agyria spectrum; pachygyria spectrum) Lissencephaly is a congenital malformation of the brain that is characterized by a completely or relatively smooth brain surface. It includes a spectrum of abnormalities of cortical sulcation ranging from agyria (completely smooth brain due to absent gyri) to pachygyria (areas of brain with reduced number of broad gyri). Histologically there are two main types of lissencephaly. In classical or type I lissencephaly the cerebral cortex is thick with only four layers (normal cerebral cortex has six layers) due to undermigration of neuronal precursors. In cobblestone or type II lissencephaly the cerebral cortex is completely disorganized with clusters of cortical neurons separated by glio-mesenchymal tissue. The surface of the cerebral cortex has a warty appearance due to the presence of clusters of neurons that have overmigrated during cortical development. Lissencephaly can be an isolated abnormality or part of a wider syndrome. Several syndromic forms of lissencephaly have been described. Table 2 lists the distinguishing clinical features, inheritance pattern, and molecular genetics of isolated and syndromic forms of classical and cobblestone lissencephaly. MIM

See Table 2.

Clinical features

Classical (Type I) Lissencephaly This is usually an isolated malformation (isolated lissencephaly sequence). Children with this condition usually present with global developmental delay, microcephaly, seizures, and spasticity. Neuroimaging shows agyria or pachygyria, a thick cortical plate with shallow Sylvian fissures, a hypoplastic corpus callosum, dilated posterior horns of the lateral ventricles, and a normal cerebellum. There is also an X-linked form of isolated classical lissencephaly. This presents as classical lissencephaly in males, but as subcortical band heterotopia in females, who present with mild to moderate learning difficulties and epilepsy. The condition can be diagnosed by an MRI scan which shows the characteristic “double cortex” sign. This consists of a band of heterotopic gray matter lying parallel to and just beneath the cerebral cortex and separated from it by a band of white matter. Miller–Dieker syndrome is a syndromic form of classical lissencephaly. Children with this syndrome have characteristic facial features with a high, square forehead, bitemporal narrowing, vertical furrowing of the forehead, epicanthic folds, small nose with anteverted nares, thin upper lip, and micrognathia.

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Another rare syndromic form of classical lissencephaly is X-linked lissencephaly with ambiguous genitalia. This condition affects karyotypic males (ie, children with the chromosome pattern 46,XY) who present with ambiguous genitalia, profound developmental delay, seizures with onset soon after birth, hypotonia or spasticity with brisk deep tendon reflexes, feeding difficulties, and lissencephaly that is more severe in the posterior regions of the cerebral cortex. Cortical thickness in these children is only moderately increased (5–7 mm compared with the normal cortical thickness of 2–3 mm) and all children also have agenesis of the corpus callosum. The prognosis is poor; most affected children die in the first year of life. Cobblestone (Type II) Lissencephaly This is usually seen as a component of several rare syndromes. Isolated cobblestone lissencephaly has been described, but is extremely rare. Patients with cobblestone lissencephaly usually present with congenital hydrocephalus or microcephaly, seizures, hypotonia, severe developmental delay, muscular weakness, and hypotonia. Ocular abnormalities and encephalocele are important presenting features of Walker–Warburg syndrome. Neuroimaging shows agyria or pachygyria, thick cortex with a granular surface, hydrocephalus, and a small cerebellum with vermis aplasia or hypoplasia. The distinguishing clinical features, inheritance pattern, and molecular genetics of syndromic forms of cobblestone lissencephaly are listed in Table 2. Age of onset

Children with classical and cobblestone lissencephaly are usually diagnosed in the first year of life. The diagnosis of SCBH is often delayed until late childhood or adolescence.

Epidemiology

The prevalence of classical lissencephaly is 11.7 per 1,000,000 births. The prevalence of cobblestone lissencephaly is unknown.

Genes and molecular LIS1 (PAFAH1B1) has 11 exons and encodes the 409-amino-acidpathogenesis containing α subunit of the platelet-activating factor acetylhydrolase isoform 1B. This protein interacts with several microtubule-associated proteins, including doublecortin, dynein, and dynactin. Therefore, the protein product of LIS1 is probably involved in cellular division of neuronal progenitor cells as well as neuronal migration. Deletions of LIS1 result in haploinsufficiency of this gene and ~90% of intragenic mutations in this gene result in the production of a truncated protein.

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These mutations are associated with a severe phenotype. Missense mutations in LIS1 are associated with a milder phenotype (such as pachygyria with more severe involvement of the posterior parietal and occipital lobes or SCBH). Somatic mosaicism for LIS1 mutations can result in the phenotype of SCBH in males and females. Deletions and intragenic mutations in LIS1 most probably cause lissencephaly by preventing normal neuronal migration. 14-3-3ε is composed of six exons and codes for a 255-amino-acid protein that is the epsilon subunit of tyrosine 3-monooxygenase/ tryptophan 5-monooxygenase activation protein. The protein product of 14-3-3ε is required for cytoplasmic dynein function and neuronal migration. It lies 40 kb telomeric to LIS1 at 17p13.3, and deletion of both 14-3-3ε and LIS1 appears to cause the more severe lissencephaly phenotype of Miller–Dieker syndrome. DCX is made up of seven exons. It encodes doublecortin, which has 402 amino acids. Doublecortin is almost exclusively expressed in the frontal lobes of the fetal brain. It is believed to direct normal neuronal migration by regulating the organization and stability of microtubules in fetal neurons by interacting with the protein product of LIS1. Mutations in DCX cause lissencephaly in males by preventing normal neuronal migration. Females with DCX mutations have two populations of cortical neurons due to X inactivation. In one group of neurons, the X chromosome that carries the mutated DCX gene is inactivated. These neurons migrate normally to form a cerebral cortex of normal appearance. In the other group of neurons, the X chromosome with the normal DCX gene is inactivated. These neurons undergo migrational arrest. This results in the formation of a second cortical layer that lies deep in the normal cortex (subcortical band heterotopia or double cortex). Rarely, males with DCX mutations can present with SCBH. Some of these males exhibit somatic mosaicism for DCX mutations and the others are thought to have a mild mutation that results in some residual function of doublecortin. ARX is homologous to the Drosophila aristaless gene. It has five exons and encodes a protein with 562 amino acids. It is expressed in the embryonic and fetal forebrain and testes. The ARX protein is involved in the differentiation, radial and tangential migration and maintenance of specific neuronal subtypes in the cerebral cortex. It is also involved in differentiation of the testes. Loss of function mutations in ARX cause XLAG by interfering with normal neuronal migration (particularly Cerebral Malformations and Mental Retardation Syndromes

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247200

MDS

XLAG

300215

Lissencephaly with X-linked moderately thick cortex recessive (5–10 mm) and anterior to posterior gradient, agenesis of corpus callosum, ambiguous genitalia in 46,XY males (also see p.64, 158)

Females: pachygyria with posterior to anterior gradient, subcortical band heterotopia

X-linked dominant

Sporadic

Sporadic

Inheritance

Xp22.13

Xq22.2–q23

17p13.3

17p13.3

ARX

DCX

14-3-3ε

LIS1 (PAFAH1B1)

LIS1 (PAFAH1B1)

Chromosomal Gene location

Aristaless-related homeobox protein

Doublecortin

Lissencephaly 1 (platelet-activating factor acetylhydrolase isoform 1B, α subunit) Epsilon subunit of tyrosine 3-mono-oxygenase/tryptophan 5-mono-oxygenase activation protein

Lissencephaly 1 (platelet-activating factor acetylhydrolase isoform 1B, α subunit)

Protein product

Frame-shift, missense, and nonsense mutations that result in loss of function

Missense mutations are seen in 65%–70% of patients. Small and large deletions and nonsense and splice-site mutations have also been described

Microdeletions of 17p13.3 that involve both LIS1 and 14-3-3ε

Large deletions of LIS1 are seen in ~60% of patients. Intragenic mutations include small deletions and nonsense, splice-site, and missense mutations

Mutational spectrum

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Males: classical lissencephaly with very thick cortex (10–20 mm) and posterior to anterior gradient (and rarely subcortical band heterotopia)

Lissencephaly with very thick cortex (10–20 mm) and anterior to posterior gradient and characteristic facial dysmorphism

Lissencephaly or pachygyria with very thick cortex (10–20 mm) and anterior to posterior gradient (more severe posterior involvement)

Clinical features

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XLIS with 300067 SCBH

607432

ILS

Classical lissencephaly

Condition MIM

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Lissencephaly

MIM

236670

253280

253800

WWS

MEBD

FCMD

Cerebral Malformations and Mental Retardation Syndromes Congenital muscular dystrophy with elevated CPK levels, mental retardation, myopia, optic atrophy

Autosomal recessive

Autosomal recessive

Autosomal recessive

Autosomal recessive

Inheritance

9q31

1p33–p34

9q34.1

7q22

FCMD

POMGnT1

POMT1

RELN

Chromosomal Gene location

Fukutin

Protein O-mannose β-1,2-N-acetylglucosaminyltransferase 1

Protein O-mannosyl transferase 1

Reelin

Protein product

75% of patients are homozygous for a 3-kb insertion of a retrotransposon sequence in the 3´ untranslated region of the gene. Other mutations that have been identified include nonsense, missense, and frame-shift mutations, as well as a 1.2-kb L1 insertion

Missense, splice-site, and frame-shift mutations

Nonsense, missense, and frame-shift mutations have been identified. Mutations in this gene account for only 20% of all cases of WWS

Splice-site mutations

Mutational spectrum

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Ocular anomalies (myopia, glaucoma, cataracts, retinal dystrophy), congenital muscular dystrophy with elevated CPK levels

Congenital hydrocephalus, encephalocele, ocular abnormalities (anterior chamber abnormalities, cataracts, persistent primary hyperplasic vitreous, retinal dysplasia and detachment), congenital muscular dystrophy with elevated CPK levels, death in infancy

Lissencephaly with moderately thick cortex (5–10 mm) and posterior to anterior gradient, abnormal hippocampus, cerebellar hypoplasia, lymphoedema, chylous ascites

Clinical features

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Table 2. Isolated and syndromic forms of classical and cobblestone lissencephaly: classification, distinguishing clinical features, inheritance pattern, and molecular genetics. CPK: creatine phosphokinase; FCMD: Fukuyama muscular dystrophy; ILS: isolated lissencephaly sequence; MDS: Miller–Dieker syndrome; MEBD: muscle–eye–brain disease; SCBH: subcortical band heterotopia; WWS: Walker–Warburg syndrome; XLAG: X-linked lissencephaly with ambiguous genitalia; XLIS: X-linked form of isolated classical lissencephaly.

600514

Lissencephaly with cerebellar hypoplasia and lymphoedema

Cobblestone lissencephaly

Condition

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tangential migration in the developing cortex) and loss of normal testicular differentiation. Mutations in ARX can also be seen in patients with X-linked infantile spasms (MIM 308350), X-linked mental retardation (MIM 300419 and 300430), X-linked myoclonic epilepsy with mental retardation and spasticity (MIM 300432), and X-linked mental retardation with dystonic movements, ataxia, and seizures (Partington syndrome, MIM 309510). POMT1 contains 20 exons and encodes the protein O-mannosyltransferase 1. This is a ubiquitously expressed protein with 725 amino acids that is thought to catalyze the first step in O-mannosylation (a form of glycosylation) of target proteins in the brain, nerves, and skeletal muscle. An important protein that undergoes O-mannosylation is α-dystroglycan. Mutations in POMT1 result in reduced or absent O-mannosylation of α-dystroglycan. Hypoglycosylation of α-dystroglycan, which links the sarcolemmal dystrophin–glycoprotein complex to various extracellular proteins (such as the laminin α2 chain of merosin, neurexin, and agrin), is believed to be an important factor in the pathogenesis of the congenital muscular dystrophy of Walker–Warburg syndrome. Glycosylated α-dystroglycan is also believed to play an important role in normal neuronal migration. Therefore, absent or reduced glycosylation of α-dystroglycan could cause cobblestone lissencephaly by allowing overmigration of neurons in the developing brain. POMGnT1 has 22 exons and encodes the protein O-mannose β-1,2-N-acetylglucosaminyltransferase-1. This enzyme is also involved in the O-mannosylation of target proteins (such as α-dystroglycan). Mutations in this gene result in hypoglycosylation of target proteins, particularly α-dystroglycan. This is thought to be responsible for the brain abnormalities and congenital muscular dystrophy of muscle–eye–brain disease. FCMD is composed of 10 exons and encodes fukutin, which has 461 amino acids. The precise function of this protein is unknown, but it is probably a glycosyl transferase that is involved in the glycosylation of cell surface molecules, such as α-dystroglycan, like the protein products of POMT1 and POMGnT1 (see above). Hypoglycosylation of α-dystroglycan in patients with fukutin deficiency is probably responsible for the muscle and brain abnormalities seen in Fukuyama congenital muscular dystrophy. Compound heterozygosity for the common 3-kb 50

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retrotransposon insertion in one allele and a point mutation in the other allele is associated with a more severe phenotype compared with homozygosity for the common 3-kb retrotransposon insertion. RELN is a very large gene with 64 exons. It encodes reelin, which has 3,641 amino acids. Reelin is expressed in fetal and postnatal brain and liver. Reelin is thought to play an important role in normal lamination of the cerebral and cerebellar cortex by arresting normal neuronal migration. It probably does this by modulating integrin-mediated cell–cell adhesion. Genetic diagnosis and counseling

All children with classical lissencephaly should have chromosome analysis to look for a rearrangement involving 17p13.3 and fluorescence in situ hybridization (FISH) analysis to look for a deletion of this region. Genetic testing for LIS1 mutations (large deletions and intragenic mutations) is only available on a research basis. Parents of children with classical lissencephaly in whom a 17p13.3 rearrangement or deletion is identified should also be tested for a rearrangement of 17p13 by chromosome and FISH analysis. If parental karyotypes are normal the recurrence risk of classical lissencephaly in another child is likely to be low. Families with lissencephaly in males and SCBH in females, or with more than one affected male with lissencephaly, should be tested for mutations in DCX, which is available from specialized laboratories. Mutation testing should be offered to mothers of male patients in whom a DCX mutation is identified. Prenatal testing should be offered to the mothers of all male patients with classical lissencephaly in whom a DCX mutation is identified, even if the mother has tested negative for the mutation. There is likely to be a significant recurrence risk for classical lissencephaly in a son or for SCBH in a daughter due to the possibility of low-level somatic or gonadal mosaicism for this mutation in the mother. Mothers of male patients with classical lissencephaly should be investigated for SCBH by cranial MRI scan if they have learning difficulties or epilepsy. If SCBH is identified in the mother, counseling should be on an X-linked basis: sons have a 50% risk of being affected with classical lissencephaly and daughters have a 50% risk of being affected with SCBH. Genetic testing for syndromic forms of cobblestone lissencephaly is only available on a research basis. All forms of cobblestone lissencephaly are inherited in an autosomal recessive manner and this is the basis for genetic counseling of these families. Prenatal diagnosis of Walker–Warburg syndrome may be possible by antenatal ultrasound scanning.

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Lowe Syndrome (also known as: oculo-cerebro-renal syndrome) MIM

309000

Clinical features

This condition almost always affects males, who present with ocular anomalies, mental retardation, and renal Fanconi syndrome. The ocular anomalies include congenital or early-onset cataracts and early-onset glaucoma that results in visual impairment or blindness. Other features include feeding difficulties, failure to thrive, epilepsy, hypotonia, scoliosis, behavioral problems, and short stature. Fanconi syndrome can progress to renal failure. Teenagers and adults with Lowe syndrome can develop arthropathy. Most affected males die by the age of 40 years.

Age of onset

The ocular features are present at birth or develop in early infancy. Fanconi syndrome is usually present by the age of 1 year.

Epidemiology

A rare disorder involving all ethnic groups.

Inheritance

X-linked recessive

Chromosomal location

Xq26.1

Gene

OCRL1 (oculo-cerebro-renal syndrome, Lowe 1)

Mutational spectrum

Mutations can be identified in OCRL1 in approximately 95% of affected males. They include nonsense, missense, frame-shift, and splice-site mutations. Most mutations are unique and involve exons 10, 12–15, 18, 19, 21, and 22. OCRL1 is partially or wholly deleted in ~7% of patients.

Molecular pathogenesis

OCRL1 is a large gene with 24 exons. One small exon (18a) is subject subject to alternative splicing. The isoform of OCRL1 that includes this exon is expressed primarily in neural tissue. The gene codes for phosphatidylinositol-4,5-bisphosphate-5-phosphatase, which is present in the trans-Golgi network of several cell types. This enzyme is thought to regulate intracellular levels of phosphatidylinositol-4,5-bisphosphate. Mutations in OCRL1 result in reduced or absent activity of phosphatidylinositol-4,5-bisphosphate-5-phosphatase. This results in elevated intracellular levels of phosphatidylinositol-4,5-bisphosphate which is thought to interfere with the normal function of the Golgi network.

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This could result in abnormal cell migration and cell differentiation in the brain, lens, and kidneys (by a mechanism that is not understood), or could involve changes in the composition of the cell membrane. Genetic diagnosis and counseling

OCRL1 mutation analysis is available from a limited number of diagnostic laboratories. The diagnosis can be confirmed in affected males by demonstrating reduced activity of the enzyme phosphatidylinositol4,5-bisphosphate-5-phosphatase in cultured skin fibroblasts. Counseling is on the basis of X-linked recessive inheritance. There is a 70% chance that the mother of an isolated case of Lowe syndrome is a carrier. Virtually all female carriers have cortical lenticular opacities or posterior lenticonus on slit-lamp examination. Prenatal diagnosis has been established by identifying cataracts on antenatal scans in the late second and third trimester. Earlier prenatal diagnosis is available by phosphatidylinositol-4,5-bisphosphate-5-phosphatase assay on cultured chorionic villi or cultured amniocytes or by mutation analysis in families in which an OCRL1 mutation has been identified in an affected male. Prenatal diagnosis should be offered to all women who have had an affected son (whether or not they are carriers) because of the possibility of gonadal mosaicism.

Neuronal Ceroid Lipofuscinosis (also known as: NCL; Batten disease) MIM

See Table 3

Clinical features

The NCLs are a group of neurodegenerative disorders that are characterized by the deposition of autofluorescent material (with similarities to ceroid and lipofuscin) in various cells, including neurons. All the NCLs present with psychomotor deterioration, epilepsy, and visual impairment, and affected patients die prematurely. The various types are differentiated by the age of onset, the presenting clinical features, and the appearance of the accumulated material on electron microscopic analysis. Infantile NCL This condition presents with early psychomotor deterioration, ataxia, autistic features, and repetitive hand movements reminiscent of Rett syndrome. Myoclonic jerks, optic atrophy, and acquired microcephaly are evident later in the course of the condition. The characteristic

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Type

MIM

Age of onset

Chromosomal location

Gene

Infantile NCL or Santavuori–Haltia–Hagberg disease

256730

6–18 months

1p32

CLN1

Late infantile NCL or Jansky–Bielschowski disease

204500

18 months to 4 years

11p15.5

CLN2

Juvenile NCL or Spielmeyer–Vogt–Sjögren disease

204200

5–10 years

16p12

CLN3

Adult NCL or Kufs disease

204300

Late 20s or early 30s

Unknown

Unknown

Late infantile NCL, Finnish variant

256731

4–7 years

13q21.1–q32

CLN5

Table 3. Neuronal ceroid lipofuscinosis (NCL): types, MIM numbers, ages of onset, chromosomal locations, and genes. CLN: ceroid lipofuscinosis, neuronal.

electrophysiologic findings include an extinguished electroretinogram (ERG) and visual evoked potential (VEP) and “vanishing” electroencephalograph (EEG). Ultrastructural examination of a conjunctival, skin, or rectal biopsy shows granular osmiophilic deposits (GRODs) in several different cell types. Most children die before the age of 5 years. Late infantile NCL This form of NCL presents with psychomotor deterioration and seizures. Visual failure occurs late in the course of the illness. Electrophysiologic findings include an abnormal EEG with multifocal spikes and slow waves, and a characteristic response to photic stimulation at a slow rate with each flash producing a spike over the occipital region. The ERG is extinguished, but the VEP shows giant waves. Ultrastructural examination of conjunctival, skin, or rectal biopsy shows inclusions with curvilinear profiles. Most children die between 6 and 15 years of age. Juvenile NCL This condition presents with progressive visual impairment leading to blindness by the age of 8 years. Retinal examination shows an absent macular reflex and sometimes bull’s-eye maculopathy. Behavioral problems and loss of scholastic skills are other early features, but neurologic problems are only evident late in the course of the disease. Electrophysiologic investigations show an absent ERG and VEP early in the course of the disease. Vacuolated lymphocytes on peripheral smear examination are a characteristic finding. Electron microscopic examination of peripheral blood lymphocytes, skin, conjunctival, or rectal biopsy shows fingerprint inclusions. The condition is slowly progressive, with death occurring between the ages of 15 and 30 years. 54

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Adult NCL This condition presents with cognitive problems and behavioral disturbances. Extrapyramidal signs and myoclonic seizures can be seen later, but visual impairment is not a feature of this form of NCL. EEG may show spike-wave complexes and a positive response to photic stimulation in some cases, but ERG and VEP are normal. Ultrastructural examination of rectal biopsy is usually required to make the diagnosis. This can show GRODs or deposits with fingerprint or rectilinear profiles predominantly in cells of neural origin. The condition is very slowly progressive, with prolonged survival (death occurs more than 20 years after onset). Late infantile NCL, Finnish variant This condition has a similar presentation to classical late infantile NCL, but the age of onset is similar to that of juvenile NCL. The electrophysiologic findings are also similar to classic late infantile NCL. However, electron microscopic examination of tissue biopsies shows deposits with fingerprint or rectilinear profiles. Disease progress can be slower than in classical late infantile NCL, with death occurring between the ages of 13 and 30 years. Epidemiology

The NCLs affect all races with a population incidence of 1 per 100,000–1,000,000. Infantile NCL is seen most frequently in Finland. Juvenile NCL has an incidence of about 1 in 25,000 live births and is one of the most common neurodegenerative diseases of childhood. The Finnish variant of late infantile NCL is only seen in families originating from the west coast of Finland.

Age of onset, chromosomal location, and gene

See Table 3.

Inheritance

All forms of NCL are inherited in an autosomal recessive manner. Autosomal dominant inheritance has been documented in one family with adult NCL.

Mutational spectrum CLN1: approximately 60% of mutations identified in this gene are missense mutations. One missense mutation, Arg122Trp, accounts for 98% of all mutations in the Finnish population. Other mutations include nonsense, frame-shift, and splice-site mutations. Two nonsense mutations (Arg151Stop and Leu10Stop) account for ~40% of mutations in non-Finnish patients.

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CLN2: half of all mutations identified in this gene are missense. Other mutations include splice-site and nonsense mutations as well as deletions. There are two common mutations in CLN2, an intronic splice-site mutation (IVS5-1G>C), and a nonsense mutation (Arg208Stop), which together account for almost 60% of all mutations. CLN3: mutations identified in this gene include missense, nonsense, and splice-site mutations, as well as small and large deletions and one intronic mutation. A 1.02-kb deletion that removes exons 7 and 8 is the most frequently identified mutation. About 80% of Finnish patients with juvenile NCL are homozygous for this mutation. This is also the most common mutation in other populations. Those patients who are not homozygous for the 1.02-kb deletion are often compound heterozygotes, with this deletion in one allele and a different mutation in their other allele. CLN5: only three mutations have been identified in this gene: a 2-bp deletion in exon 4, one nonsense, and one missense mutation. Approximately 90% of patients are homozygous for the 2-bp deletion. Molecular pathogenesis

CLN1 has 7 exons and encodes a 306-amino-acid protein called palmitoyl-protein thioesterase (PPT). This is a lysosomal enzyme that removes palmitate groups from cysteine residues in S-acylated proteins. Mutations in CLN1 result in loss of activity of PPT, which presumably results in the accumulation of S-acylated proteins in lysosomes. CLN2 has 13 exons. It encodes a novel 562-amino-acid lysosomal protease. This lysosomal protease is identical to tripeptidyl peptidase I, an enzyme that removes three amino acids from the N-terminal regions of proteins undergoing lysosomal degradation. Mutations in CLN2 result in a deficiency of this enzyme and the accumulation of proteins in lysosomes. CLN3 encodes a novel transmembrane protein of unknown function. The CLN3 protein has been localized to lysosomes, mitochondria, and Golgi. It may be responsible for regulating lysosomal pH by an unknown mechanism. Nearly all mutations in CLN3 are predicted to result in loss of function of the CLN3 protein, which could result in an abnormal lysosomal pH. This could interfere with protein degradation in lysosomes, resulting in the abnormal accumulation of these proteins. CLN5 contains four exons and is believed to encode a novel protein with 407 amino acids. This protein has no homology to any other protein.

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Recent work suggests it is a soluble lysosomal glycoprotein. The common mutation in CLN5 blocks the lysosomal targeting of this protein. This implies that the pathogenesis of the Finnish variant of late infantile NCL could be the result of defective lysosomal trafficking of the protein product of CLN5 interfering with its normal biologic function. Genetic diagnosis and counseling

Counseling for all types of NCL is on an autosomal recessive basis. Genetic testing for infantile, late infantile, and juvenile NCL is available from only a few specialized laboratories. Prenatal diagnosis is possible by mutation analysis if the precise mutations have been identified in an affected child, or by linkage analysis if the mutations have not been identified. However, before undertaking linkage analysis it is important to confirm the exact type of NCL in the affected child. In infantile NCL, prenatal diagnosis by genetic testing can be combined with PPT assay on chorionic villi. In late infantile NCL, prenatal testing has been performed by electron microscopic examination of uncultured amniocytes for deposits with the typical curvilinear profile.

Pelizaeus–Merzbacher Syndrome MIM

See Table 4.

Clinical features

Affected children present with rotary or roving nystagmus, developmental delay, seizures, and optic atrophy. Hypotonia is initially present; spasticity develops later, mainly in the lower limbs. Other clinical features include laryngeal stridor, extrapyramidal signs, and neuroregression. Brainstem-evoked potentials are abnormal and MRI scans of the brain show delayed or poor myelination. The classical type mainly affects males, but the connatal variant affects both sexes. The clinical picture is more severe in the connatal variant, with acquisition of very few developmental milestones, very rapid progression, and early death. Compared with the classical type, there is almost no myelination seen on an MRI scan of the brain in the connatal variant.

Age of onset

Both forms present in early infancy, although the connatal type can present in the neonatal period.

Epidemiology

Extremely rare

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See Table 4. Type

MIM

Inheritance

Chromosomal location

Gene

Mutational spectrum

Classical type

312080

X-linked recessive

Xq22

PLP1 (proteolipid protein-1)

Deletions, duplications, and point mutations. Duplications are seen in 50% of familial cases

Connatal variant

260600

Autosomal recessive

Unknown

Unknown

Unknown

Table 4. Pelizaeus–Merzbacher syndrome: MIM numbers, inheritances, chromosomal locations, genes, and mutational spectra.

Molecular pathogenesis

Classical type: PLP1 has seven exons and encodes proteolipid protein 1. There are two isoforms of this protein. One isoform has 276 amino acids while the other isoform (called DM20) has only 241 amino acids. Proteolipid protein 1 is an integral membrane protein with four transmembrane domains. It is one of the main components of myelin in the central nervous system. Mutations in the PL1P gene result in reduced production of protein, which results in absent or delayed myelination in the brain. Missense mutations in the PLP1 gene (Gly220Cys, Ala242Gln, and Ala242Val) can be associated with a very early presentation of Pelizaeus–Merzbacher syndrome in males with similarities to the connatal form of this condition. Mutations in PLP1 also cause one form of X-linked spastic paraplegia (SPG2, MIM 312920). Connatal variant: unknown

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Classical type: the diagnosis can be made by an MRI scan of the brain and abnormal brainstem-evoked responses. PLP1 mutation analysis is not available as a routine service. Counseling is on the basis of X-linked recessive inheritance. Carrier females can show white matter abnormalities on an MRI brain scan. Reliable carrier testing and prenatal diagnosis is available if the PLP1 mutation is identified in the affected male. Prenatal diagnosis is possible using linkage analysis, or mutation analysis in families in which a PLP1 mutation has been identified in an affected male.

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Connatal type: counseling is on the basis of autosomal recessive inheritance. Some affected males and females have been shown to have mutations in PLP. In these families, counseling would be on the basis of X-linked inheritance.

Prader–Willi Syndrome (also known as: PWS; Prader–Labhardt–Willi syndrome) MIM

176270

Clinical features

Presentation is usually in the neonatal period with hypotonia and poor feeding, followed (in early childhood) by the onset of life-long hyperphagia leading to obesity. Other features include mild mental retardation, short stature, small hands and feet, hypogonadotrophic hypogonadism, hypopigmentation, and challenging behavior (particularly in later childhood and adolescence). High pain threshold, poor temperature control, skin picking, and sleep disturbance can also occur.

Age of onset

Hypotonia can manifest before birth as reduced fetal movement.

Epidemiology

The incidence is between 1 in 10,000 and 1 in 15,000. All races are affected.

Inheritance

This is complex. Four different mechanisms have been identified: paternally derived chromosome 15q11–q13 interstitial deletion/ microdeletion, maternal uniparental disomy (UPD) for chromosome 15, chromosome abnormality (such as an unbalanced translocation involving loss of 15q11–q13 on the paternally derived chromosome), an imprinting defect (involving the PWS/Angelman syndrome [see p.31] critical region).

Chromosomal location

15q11–q13

Gene

SNRPN (small nucleoribonucleoprotein N) ZNF127 (zinc finger protein 127) NDN (necdin)

Mutational spectrum Deletions in SNRPN, involving the promoter region and exon 1

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PWS represents the opposing phenotype to AS in that it is caused by the absence of paternally expressed genes at chromosome 15q11–q13, where there is a cluster of genes that show parent-specific imprinting (see Figure 1, p.31). SNRPN is primarily implicated. This consists of at least 10 exons and is abundantly expressed (but only from the paternally derived allele) in the brain, heart, and striated muscle, where it encodes a protein involved in pre-mRNA splicing and processing. Other paternally expressed genes in the PWS critical region include ZNF127 (which encodes a factor involved in protein–protein interaction) and NDN (which encodes the protein necdin, which is expressed in neurons in the developing nervous system). Deficiency of all of these genes, and probably of others that are as yet unidentified, contributes to the PWS phenotype. In approximately 70%–75% of cases there is a 4-Mb deletion in the paternally derived chromosome 15, which results from misalignment with unequal recombination between regions of flanking homologous END repeats. These repeats are derived from large genomic duplications of a gene called HERC2. Around 25% of cases result from maternal UPD for chromosome 15. In approximately 1%–3% of cases an “imprinting mutation” impairs the setting of the normal paternal imprint (which normally arises close to the SNRPN promoter). In many cases, this involves a tiny deletion of 6–200 kb extending into the promoter and first exon of SNRPN.

Genotype–phenotype Children with maternal UPD15 show a lower incidence of skin picking correlation and hypopigmentation than deletion cases. The lower incidence of hypopigmentation is explained by the presence of the nonimprinted P or OCA2 (type 2 oculo-cutaneous albinism) gene in the deletion region. Genetic diagnosis and counseling

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Diagnosis is best made by methylation analysis using a methylationsensitive restriction enzyme and a probe such as SNRPN (see Figure 2, p.33). A microdeletion can sometimes be seen on conventional chromosome analysis but is much more reliably identified by fluorescence in situ hybridization (see Figure 3, p.33). UPD can be detected using informative microsatellite markers. The recurrence risk for the common de novo microdeletion is <1% (attributable to paternal germ line mosaicism) and is negligible for UPD. Imprinting errors can arise de novo (recurrence risk <1%) or be silently transmitted (eg, from a paternal grandmother), with a potential recurrence risk of 50%.

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Rett Syndrome MIM

312750

Clinical features

Almost all affected cases are female. Patients show normal development and head growth until 6 months of age. After this age they show psychomotor regression and deceleration of head growth. An early feature is loss of acquired hand skills, associated with the development of repetitive hand movements or hand stereotypies (wringing, clapping, and mouthing movements). Patients develop autistic features with severe mental retardation and little or no useful speech. Additional features include episodic hyperventilation or apnea, epilepsy, spasticity, scoliosis, poor growth, and small feet with peripheral vasomotor disturbances. Older patients can develop cardiac arrhythmias.

Age of onset

From 6 months to 3 years

Epidemiology

1 in 10,000 to 1 in 15,000 births

Inheritance

X-linked dominant with male lethality. Most cases represent new mutations.

Chromosomal location

Xq28

Gene

MECP2 (methyl-CpG-binding protein 2)

Mutational spectrum

Over 170 mutations have been described. These include missense, nonsense, and splice-site mutations, as well as gross deletions, small insertions and deletions, and complex rearrangements.

Molecular pathogenesis

MECP2 has four exons and encodes a ubiquitously expressed protein called methyl-CpG-binding protein 2. This protein has 486 amino acids and two structural domains: the methyl-CpG binding domain (MBD) and the transcriptional repression domain (TRD). MBD recognizes a methylated CpG dinucleotide while TRD interacts with other proteins such as Sin3A and histone deacetylases to selectively repress gene transcription. MECP2 mutations may allow transcription of genes that are not normally transcribed in early embryonic development. Normal brain development could be particularly sensitive to the transcription of these genes.

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Mutations in MECP2 have been identified in males with an X-linked form of severe mental retardation associated with progressive spasticity (MIM 300279), X-linked mental retardation with psychosis, pyramidal signs, and macro-orchidism (MIM 300055), X-linked nonspecific mental retardation, and neonatal-onset nonprogressive encephalopathy. Mutations in this gene have also been identified in females with features suggestive of Angelman syndrome (see p.30–3). Genetic diagnosis and counseling

Although the majority of cases of Rett syndrome are sporadic in origin, there have been a few instances of recurrence in another female child, probably as a result of gonadal mosaicism. If a girl with Rett syndrome has no male or female sibling or maternal relative with mental retardation then the risk of recurrence is likely to be very small. If there is a male or female sibling or a maternal relative with mental retardation then the recurrence risk could be as high as 50%. If a MECP2 mutation is identified in the affected child then the parents should be offered a prenatal test for Rett syndrome.

X-linked Adrenoleukodystrophy (also known as: X-ALD) MIM

300100

Clinical features

The childhood form of X-ALD usually presents with gradual intellectual decline and progressive gait abnormalities. Additional problems include focal seizures, cortical blindness, extrapyramidal signs, and cerebellar ataxia. Affected children develop dementia and severe, terminal, spastic quadriplegia. Some children also develop features of adrenal insufficiency. About 25% of patients present with progressive spastic paraparesis and features of adrenal insufficiency. This phenotype is called adrenomyeloneuropathy (AMN). X-ALD presents as isolated Addison disease in 10% of patients.

Age of onset

62

The childhood form of X-ALD usually presents between the ages of 5 and 10 years. AMN usually presents in adult life.

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Epidemiology

This condition affects all ethnic groups. Its incidence is estimated to be between 1 in 20,000 and 1 in 100,000. A recent study from the USA suggested a minimum incidence of 1 in 42,000.

Inheritance

X-linked recessive

Chromosomal location

Xq28

Gene

ABCD1 (ATP-binding cassette, subfamily D, member 1)

Mutational spectrum

Over 400 mutations have been identified in ABCD1, of which most are unique to a particular family. Missense mutations account for approximately 50% of the total. Other mutations include frame-shift, splice-site, and nonsense mutations. Large deletions are seen in about 5%–6% of patients. Most mutations result in complete absence of the protein product. There appears to be no correlation between genotype and phenotype. There is interfamilial phenotypic variability.

Molecular pathogenesis

X-ALD is a peroxisomal disorder that is associated with elevated levels of saturated very long-chain fatty acids (VLCFAs), particularly hexacosanoate (C26:0), in all body tissues. This is due to the inability of VLCFAs to be degraded in peroxisomes. The first step in the degradation of VLCFAs is catalyzed by the enzyme VLCFA-CoA synthetase (also called lignoceroyl-CoA ligase). ABCD1 is composed of 10 exons and its protein product (ALD protein) has 745 amino acids. ALD protein is a peroxisomal membrane protein that is involved in the import or anchoring of VLCFA-CoA synthetase into the peroxisomal membrane. Deficiency of ALD protein results in a deficiency of VLCFA-CoA synthetase in the peroxisomal membrane and decreased peroxisomal degradation of VLCFAs. Accumulation of VLCFAs and their disruptive effects on the cell membrane structure and function could explain the neurologic manifestations of X-ALD. Adrenal dysfunction is due to accumulation of cholesterol esters of VLCFAs in the cells of the zona fasciculata and reticularis.

Genetic diagnosis and counseling

The diagnosis of X-ALD can be made by neuroimaging (contrastenhanced CT or MRI scans) and by measuring the levels of VLCFAs (C26:0 and C26:C22 ratio) in plasma, red cells, or cultured fibroblasts. All affected patients should be tested for adrenal insufficiency as this

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is often subclinical. Molecular genetic analysis is only available from a few specialized laboratories and is helpful in carrier detection and prenatal diagnosis. Counseling is on the basis of X-linked recessive inheritance. Only 5% of patients are affected as the result of a new mutation. Carriers can be identified by elevated levels of VLCFAs in plasma or red cells, but false negative results can be obtained in about 15% of cases. Therefore, genetic testing is the most reliable method of carrier detection. Almost 20% of carriers develop a mild AMN-like phenotype between the ages of 25 and 50 years. Prenatal diagnosis is possible by measuring levels of VLCFAs in cultured chorionic villi or cultured amniocytes. However, this should only be undertaken by a laboratory familiar with this analysis in order to reduce the possibility of a false negative result. Where possible, biochemical analysis should be complemented by genetic testing (mutation or linkage analysis).

X-linked α-Thalassemia and Mental Retardation Syndrome (also known as: ATR-X syndrome) MIM

301040

Clinical features

This condition mainly affects males, who present with characteristic facial features, severe to profound mental retardation, genital abnormalities, and α-thalassemia. The facial features include short palpebral fissures with telecanthus, epicanthic folds, a small, triangular, and anteverted nose with short columella (with alae nasi extending below the level of the columella), inverted V-shaped upper lip, and a full, everted lower lip. Some patients are never able to walk and most have no speech. Genital abnormalities can range from cryptorchidism to complete sex reversal in a patient with a 46,XY karyotype. The α-thalassemia is mild and difficult to detect by hemoglobin (Hb) electrophoresis. It is best identified by looking for HbH inclusion bodies in red cells in a fresh blood sample. Additional features include neonatal hypotonia, epilepsy, microcephaly, and short stature.

Age of onset

Neonates present with hypotonia and feeding difficulties. Developmental delay is usually evident in infancy, and the characteristic facial features can be recognized in early childhood.

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Epidemiology

Rare

Inheritance

X-linked recessive

Chromosomal location

Xq13

Gene

XNP (X-linked nuclear protein)

Mutational spectrum

Over 50 mutations have been identified. These include missense, nonsense, and frame-shift mutations, as well as intragenic deletions. An XNP mutation has also been described in a large family with Juberg–Marsidi syndrome (MIM 309590). This is an X-linked recessive form of mental retardation. Affected males have severe mental retardation, failure to thrive, short stature, deafness, genital abnormalities (small penis, poorly formed scrotum, and cryptorchidism), and delayed bone age. These patients do not have HbH inclusions in their red cells, while patients with ATR-X syndrome do.

Molecular pathogenesis

XNP contains 36 exons and codes for ATRX protein, which has 2,375 amino acids. This is thought to be a chromatin-mediated transcription regulator. It is associated with pericentric heterochromatin during interphase and with centromeres of many chromosomes and the stalks of acrocentric chromosomes in metaphase. This suggests that ATRX protein is also involved in establishing or maintaining the pattern of methylation in the genome. Mutations in XNP are associated with a decrease in the levels of ATRX protein. This down-regulates expression of the α-globin gene, which results in the α-thalassemia of ATR-X syndrome. The other features of ATR-X syndrome are thought to be the result of reduced levels of ATRX protein interfering with the expression of other genes. These genes have yet to be identified. Mutations that result in a truncated protein product that lacks the C-terminal conserved domains are associated with the most severe genital abnormalities.

Genetic diagnosis and counseling

The diagnosis can be established by demonstrating HbH inclusions in red cells from a fresh blood sample in a male patient with the characteristic clinical features. However, HbH inclusions are not seen in all cases. If this diagnosis is strongly suspected, then additional tests should be performed, including methylation studies and sequencing of exons 8–10 of XNP (these code for the zinc finger motif at the N-terminal

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region of the ATRX protein). The latter test is able to identify about 60% of mutations in the gene. These tests are only available from specialized laboratories. Counseling is on the basis of X-linked recessive inheritance. There is an 85% chance that the mother of a sporadic case is a carrier. Carriers are asymptomatic and do not have HbH inclusions in their red cells. This is due to skewed X-chromosome inactivation in which the X chromosome that carries the mutated XNP gene is preferentially inactivated. Carriers can only be reliably identified by XNP mutation analysis. Gonadal and gonosomal mosaicism for XNP mutations has been demonstrated in females. Therefore, prenatal testing should be offered to all families who have an affected son with ATR-X syndrome, even if the mother has been shown not to carry the XNP mutation identified in the affected child. Prenatal diagnosis is possible by XNP mutation analysis or by linkage analysis in families where an XNP mutation has not been identified in the proband.

X-linked Hydrocephalus (also known as: X-linked aqueduct stenosis) MIM

307000

Clinical features

This condition almost exclusively affects males, who present with macrocephaly, severe developmental delay, adducted thumbs, and spastic paraplegia. An MRI scan of the brain may show an absent or dysplastic corpus callosum, aqueduct stenosis, and an absence of pyramids. It is important to remember that most cases of hydrocephalus are nongenetic in origin. Hydrocephalus can be seen in children with a chromosomal abnormality (mosaic trisomy 8 or diploid/triploid mosaicism) and it can be a feature of a multiple malformation syndrome (eg, hydrolethalus syndrome or Walker–Warburg syndrome).

Age of onset

66

At or soon after birth. Hydrocephalus is often identified on antenatal scans.

X-linked Hydrocephalus

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Epidemiology

This is the most common inherited cause of hydrocephalus. Its incidence is estimated to be 1 in 30,000 live male births. Between 10% and 30% of males with congenital hydrocephalus may have the X-linked form.

Inheritance

X-linked recessive

Chromosomal location

Xq28

Gene

L1CAM (L1 cell-adhesion molecule)

Mutational spectrum

Over 90 mutations have been identified in L1CAM. These include nonsense, missense, splice-site, and frame-shift mutations. Intragenic deletions and duplications have also been described. Mutations in L1CAM can cause: X-linked hydrocephalus; the syndrome of mental retardation, aphasia, shuffling gait, and adducted thumbs (MASA syndrome; MIM 303350); X-linked complicated spastic paraparesis (MIM 303350); and X-linked agenesis of the corpus callosum. All of these phenotypes can be seen in the same family. Truncating mutations in the extracellular region of the gene result in a severe phenotype, whereas missense mutations in the extracellular domain and mutations in the cytoplasm domain are associated with a milder phenotype.

Molecular pathogenesis

L1CAM is a large gene with 28 exons. It encodes the L1-cell adhesion molecule, which has 1,257 amino acids. This is a surface glycoprotein belonging to the immunoglobulin superfamily that is expressed in neurons and Schwann cells. It has six immunoglobulin-like C2-type domains and five fibronectin type III-like domains. In the developing brain it is involved in cell–cell and cell–substrate adhesion, neuronal migration, growth, and development, and myelination of axons. It is also involved in the establishment of long-term memory. The precise mechanisms by which mutations in L1CAM cause disease are unknown.

Genetic diagnosis and counseling

L1CAM mutation analysis should be performed in males with congenital or early-onset hydrocephalus and the clinical features described above. It should also be performed in patients with MASA syndrome, families with complicated spastic paraparesis affecting males, and X-linked agenesis of the corpus callosum. Genetic testing is available from diagnostic laboratories. Counseling is on the basis of X-linked recessive inheritance. Carriers can only be reliably identified

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by L1CAM mutation analysis. The condition can show marked interfamilial and intrafamilial variability of expression. Prenatal diagnosis is possible by direct mutation analysis (in families where an L1CAM mutation has been identified in an affected male) or by linkage analysis. Prenatal diagnosis is also possible by serial antenatal ultrasonography to look for ventriculomegaly.

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3 3. Disorders of Vision

Aniridia 70 Bardet–Biedl Syndrome 72 Juvenile Retinoschisis 74 Leber Congenital Amaurosis 75 Norrie Disease 79 Rieger Syndrome 80

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Aniridia (including: WAGR [Wilms’ tumor, aniridia, genitourinary anomalies, mental retardation] syndrome) MIM

106210

Clinical features

Children with this condition present with complete or partial absence of the iris. Associated findings include photophobia, impaired vision, nystagmus, corneal dystrophy, glaucoma, cataracts, and dislocated lenses.

Age of onset

The diagnosis is usually obvious at birth.

Epidemiology

Aniridia is a rare ocular malformation with an incidence of 1 in 56,000 live births.

Inheritance

Autosomal dominant. About 25%–30% of cases are sporadic in origin.

Chromosomal location

11p13

Gene

PAX6 (paired box gene 6)

Mutational spectrum

Cytogenetically visible interstitial deletions of 11p13 or a cryptic deletion of this region can be identified in a significant proportion of patients with both familial and sporadic aniridia. Some patients with a cryptic 11p13 deletion have been found to be mosaic for the deletion. Chromosomal rearrangements (translocations, inversions, and insertions) involving 11p13 have also been seen in familial and sporadic cases of aniridia. In most of these cases the 11p13 breakpoint was a considerable distance from PAX6. These rearrangements are believed to interfere with the expression of PAX6 by a “position effect”. Interstitial deletions of 11p13 can be associated with the WAGR syndrome phenotype (MIM 194070). Patients with familial and sporadic aniridia who do not have a deletion or rearrangement of 11p13 have intragenic mutations in PAX6. These include nonsense, missense, and splice-site mutations as well as small deletions and insertions. The paired homeodomain of the gene is a mutational hotspot. Almost all mutations are believed to result in a loss of function effect.

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Mutations in PAX6 can also give rise to ocular phenotypes other than aniridia. These include Peters’ anomaly (MIM 604229), autosomal dominant keratitis (MIM 148190), isolated foveal hypoplasia (MIM 136520), congenital cataracts, ectopic pupils, and multiple ocular anomalies (including Peters’ anomaly, Axenfeld anomaly, congenital cataract, and foveal hypoplasia). Mutations in PAX6 have also been associated with subtle central nervous system (CNS) malformations, such as agenesis or hypoplasia of the anterior commissure and cerebellar anomalies in patients with aniridia. Molecular pathogenesis

PAX6 contains 14 exons and is expressed in the eye, forebrain, cerebellum, and olfactory bulbs of the fetus. It encodes a 422-aminoacid transcriptional regulator protein involved in ocular, CNS, pituitary, and pancreatic development. Aniridia is believed to be the result of haploinsufficiency or a loss of function mutation of PAX6. The WAGR syndrome phenotype is a contiguous gene syndrome. The interstitial deletion of 11p13 in these patients includes the PAX6 gene and the adjacent WT1 gene. Haploinsufficiency of PAX6 causes aniridia while haploinsufficiency of the WT1 gene causes genitourinary abnormalities (such as small penis, hypospadias, cryptorchidism, and ambiguous genitalia) in patients with a male karyotype and predisposition to Wilms’ tumor. Deletion of other, as yet unidentified, genes is believed to cause mental retardation.

Genetic diagnosis and counseling

Disorders of Vision

All patients with aniridia should have chromosome analysis and fluorescence in situ hybridization (FISH) analysis with probes for PAX6, WT1, and flanking markers. Patients with aniridia who have a deletion of WT1 on FISH analysis are at high risk of developing Wilms’ tumor and should be offered regular screening with renal ultrasound scans. PAX6 mutation analysis is only available on a research basis. In familial forms of aniridia, counseling is on an autosomal dominant basis. Parents of sporadic cases in whom a cytogenetically visible or cryptic 11p13 deletion has been identified should be tested for a rearrangement of 11p13 by chromosome and FISH analysis. Sibling recurrence risk would depend on whether one of the parents carries an 11p13 rearrangement. Parents of a sporadic case in whom no 11p13 deletion or rearrangement can be identified should be carefully examined for anterior chamber and iris abnormalities and early-onset cataracts. If no ocular abnormality is identified in either parent then the sibling recurrence risk is low.

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Prenatal diagnosis can be offered to the parents of patients with an 11p13 deletion or rearrangement and to parents of patients in whom a PAX6 mutation has been identified.

Bardet–Biedl Syndrome (also known as: BBS) MIM

See Table 1.

Clinical features

Typically, the clinical features consist of obesity, postaxial polydactyly, hypogonadism, learning disabilities, rod–cone retinal dystrophy, and structural renal abnormalities (including hypoplasia and cystic dysplasia). Obesity and retinal dystrophy tend to be progressive. Renal function can also deteriorate leading to renal failure in adult life. Other less common features include cataracts, brachydactyly and/or syndactyly, cardiac defects, and diabetes mellitus. Learning disabilities are usually relatively mild, and in the absence of severe renal involvement life expectancy is usually normal.

Age of onset

Polydactyly is apparent at birth. Obesity becomes apparent in childhood. Retinal dystrophy usually presents in childhood or the teenage years.

Epidemiology

The prevalence in populations of European origin is approximately 1 in 150,000. Much higher incidences have been observed in Newfoundland and in parts of the Middle East, notably in Kuwait.

Inheritance

Autosomal recessive (although molecular analysis has demonstrated triallelic inheritance in some families). Type

MIM

Chromosomal location

Gene

BBS1

209901

11q13

BBS1

BBS2

606151

16q21

BBS2

BBS3

600151

3p13–p12

Unknown

BBS4

600374

15q22.3–q23

BBS4

BBS5

603650

2q31

Unknown

BBS6

604896

20p12

MKKS (Mckusick–Kaufman syndrome)

Table 1. Bardet–Biedl syndrome (BBS): types, MIM numbers, chromosomal locations, and genes.

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Chromosomal location and gene

See Table 1.

Mutational spectrum

BBS1: missense, nonsense, and splice-site mutations with a presumed loss of function effect. One particular mutation (Met390Arg) appears to be a common cause of BBS. BBS2: missense, nonsense, and frame-shift mutations with a loss of function effect. BBS4: deletions, insertions, and splice-site mutations with a loss of function effect. BBS6: missense and nonsense point mutations and frame-shift deletions with a loss of function effect.

Molecular pathogenesis

BBS1 is composed of 17 exons and spans approximately 23 kb. It shows ubiquitous expression and, like BBS2, encodes a protein of unknown function. BBS2 contains 17 exons and shows strong evolutionary conservation with widespread tissue expression. The structure of the protein product does not resemble that of any other known protein and its function is unknown. BBS4 contains 16 exons with several Alu repeat sequences.These repeat sequences predispose to the formation of deletion mutations by unequal homologous recombination. It shows ubiquitous expression with highest levels in the kidneys. The predicted protein sequence suggests that BBS4 encodes a protein that mediates protein–protein interactions and plays a role in regulating cell signaling. BBS6 is caused by mutations in MKKS. Mutations in this gene also cause the McKusick–Kaufman syndrome (MIM 236700), which is characterized by hydrometrocolpos, postaxial polydactyly, and congenital cardiac defects. MKKS contains six exons and encodes a protein that shows similarity to type II chaperonins. These facilitate ATP-dependent protein folding. The relationship between MKKS genotype and phenotype is not clear-cut. There is some evidence that mutations with a milder effect on protein structure and function (ie, missense as opposed to nonsense) give rise to the McKusick–Kaufman syndrome. Alternatively the Bardet–Biedl phenotype may require the presence of a third BBS mutation as discussed below.

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Mutation analysis for BBS1, BBS2, BBS4, and MKKS is available in a few specialist centers on a research basis only. Mutations in BBS1 are the most frequent cause of BBS. Traditionally, BBS has been assumed to show straightforward autosomal recessive inheritance. However, recent studies have shown that some affected individuals have mutations in not two but three BBS genes (eg, two BBS2 mutations and one BBS6 mutation). This has been described as “triallelic inheritance” and suggests that disease expression requires both a dominantly inherited susceptibility mutation at one BBS locus and recessive homologous mutations at another BBS locus. Such a mechanism would imply a recurrence risk for siblings of 1 in 8. In practice, a recurrence risk of 1 in 4 is usually quoted.

Juvenile Retinoschisis (also known as: X-linked retinoschisis) MIM

312700

Clinical features

This condition presents with reduced visual acuity in males. There is no history of preceding night blindness and color vision is usually unaffected. Most affected individuals have moderate visual impairment in childhood and teenage years. Slowly progressive macular dystrophy develops in adulthood and can progress to blindness. The condition can be diagnosed by retinal examination and confirmed by dark-adapted flash electroretinography (ERG). The characteristic retinal abnormality is the presence of intraretinal cysts that extend from the fovea in a spoke-wheel pattern. These cysts can also involve the peripheral retina. The ERG shows an electronegative pattern with normal amplitude of the a-wave and reduced amplitude of the b-wave. Female carriers are usually asymptomatic and only rarely show abnormalities on retinal examination.

Age of onset

Visual loss presents in early childhood, but the age of onset and the rate of progression can show great variability in affected members of the same family.

Epidemiology

This is a rare cause of visual impairment in males. It is particularly prevalent in Finland.

Chromosomal location

Xp22.1–p22.2

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Gene

RS1 (retinoschisis 1)

Mutational spectrum

Missense mutations account for approximately 75% of mutations in the RS1 gene. Most of these mutations localize to exons 4–6, which code for the highly conserved discoidin domain of the gene. One particular missense mutation (Glu72Lys) has been identified in almost 15% of affected patients. Other mutations include nonsense, splice-site, and frame-shift mutations. There is no genotype–phenotype correlation.

Molecular pathogenesis

RS1 has six exons and encodes a 224-amino-acid protein called retinoschisin. This contains a highly conserved discoidin domain that is believed to be involved in cell–cell adhesion and phospholipid binding. Retinoschisin is a secreted protein and is predicted to have a globular conformation. It appears to be released by the photoreceptor cells of the retina and has functions within the inner retinal layers. The precise function of retinoschisin is unknown, but it may be involved in cell adhesion processes during retinal development. There is evidence to suggest that missense mutations interfere with normal protein folding and result in the production of a protein with abnormal conformation that cannot be secreted.

Genetic diagnosis and counseling

Genetic testing is only available on a research basis. Counseling is on an X-linked recessive basis. Genetic testing is the only reliable method of identifying female carriers. Prenatal diagnosis can be undertaken by linkage analysis or mutation analysis (if the familial mutation is known).

Leber Congenital Amaurosis (also known as: LCA) MIM

See Table 2.

Clinical features

All forms of LCA present with congenital blindness or early onset visual loss, roving eye movements, and a severely attenuated or extinguished electroretinogram. Some children demonstrate the oculodigital (Franceschetti’s) sign (affected children poke or put pressure on their eyes) and others have severe photophobia. Other ocular findings can include refractive error (high myopia or hypermetropia), cataracts, and keratoconus. The retina appears normal initially. Later retinal findings

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can resemble retinitis pigmentosa, although other retinal appearances have also been reported (eg, chorioretinal atrophy or macular colobomas). LCA can be an isolated problem (uncomplicated LCA) or part of a syndrome (syndromic or complicated LCA). Patients with syndromic forms of LCA can have developmental delay, neurologic abnormalities (such as hypotonia and cerebellar vermis hypoplasia), cardiomyopathy, hepatic fibrosis, and renal abnormalities (cystic renal dysplasia, juvenile nephronophthisis). LCA is a recognized feature of several well-known syndromes such as infantile Refsum disease, Joubert syndrome, Zellweger syndrome, and Senior–Loken syndrome (the combination of LCA and juvenile nephronophthisis). Only the molecular genetics of the isolated forms of LCA are discussed here as these forms comprise 80%–90% of all patients with LCA. Age of onset

At birth or in early infancy (before the age of 6 months)

Epidemiology

LCA affects all ethnic groups, with a population prevalence of approximately 3 per 100,000. Almost 70% of patients with LCA1 are of Mediterranean origin.

Inheritance

Almost all forms of LCA are inherited in an autosomal recessive manner. Mutations in the CRX gene are thought to result in an autosomal dominant form of LCA.

Chromosomal location and gene

See Table 2.

Type

MIM

Chromosomal location

Gene

LCA1

204000

17p13.1

GUCY2D (guanylate cyclase 2D)

LCA2

204100

1p31

RPE65 (retinal pigment epithelium-specific protein 65 kDa)

LCA3

604232

14q24

Unknown

LCA4

604393

17p13.1

AIPL1 (aryl hydrocarbon-interacting receptor protein-like 1)

LCA5

604537

6q11–q16

Unknown

LCA6

605446

14q11

RPGRIP1 (retinitis pigmentosa GTPase regulator-interacting protein 1)

LCA due to mutations in CRX gene

602225

19q13.3

CRX1 (cone–rod homeobox-containing gene)

LCA due to mutations in CRB1 gene

604210

1q31–q32.1

CRB1 (crumbs homolog 1)

Table 2. Leber congenital amaurosis (LCA): types, MIM numbers, chromosomal locations, and genes.

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Mutations in the six genes identified for isolated LCA account for only 50% of all cases. Mutations in most of these genes can also result in other types of retinal dystrophies such as retinitis pigmentosa, cone–rod dystrophy, and juvenile retinal dystrophy. GUCY2D Mutations in this gene have included frame-shift, nonsense, splice-site, and missense mutations. Several patients carry protein-truncating mutations in both alleles. Mutations in this gene can be seen in 6%–20% of patients with LCA. RPE65 Nonsense, missense, and splice-site mutations have been identified in this gene. Between 3% and 16% of patients with LCA have mutations in RPE65. AIPL1 About 70% of mutations in this gene are null (nonsense, frame-shift, and splice-site) mutations. One nonsense mutation (Trp278Stop) accounts for approximately 50% of mutations in this gene. Missense mutations have also been identified. Mutations in this gene are seen in ~6% of patients with LCA. RPGRIP1 Most mutations in this gene are predicted to result in protein truncation. These include nonsense and splice-site mutations. Two missense mutations and one in-frame deletion have also been identified. Mutations in RPGRIP1 are seen in 5%–6% of LCA patients. CRX Mutations in only a single allele of CRX can cause LCA. Frame-shift and missense mutations have been identified in this gene. Only one family has been reported in which LCA resulted from a homozygous missense CRX mutation. Most LCA patients with CRX mutations have been sporadic cases. Therefore, their LCA could be the result of a new autosomal dominant mutation in this gene or there could be an unidentified mutation in the other allele of this gene or in another gene (digenic inheritance) in these patients. Mutations in CRX are seen in only 2%–3% of LCA patients. CRB1 Nonsense, frame-shift, and missense mutations have been identified in this gene. One missense mutation (C948Y) accounts for approximately

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25% of all mutations. About 9%–14% of patients with LCA have mutations in CRB1. Molecular pathogenesis

GUCY2D has 20 exons. Its protein product is a 1,103- amino-acid photoreceptor guanylate cyclase involved in the phototransduction cascade. In the dark state cGMP levels are restored by guanylate cyclase. Loss of function mutations in GUCY2D interfere with the restoration of the basal cGMP levels in photoreceptors. The effects of this are similar to constant light exposure which results in impairment of photoreceptor function. RPE65 is composed of 14 exons. It encodes a 533-amino-acid retinalpigment-specific protein that is involved in the metabolism of all transretinyl esters to 11-cis-retinol. Mutations in this gene probably result in retinal dystrophy by interfering with the production of 11-cis-retinol. AIPL1 has six exons and encodes a 384-amino-acid protein that is expressed in the rod photoreceptor cells of the peripheral and central retina. Its precise function is unknown, but it may be essential for the maintenance of rod photoreceptor function. RPGRIP1 contains 24 exons and encodes a protein that interacts with the protein product of the RPGR (retinitis pigmentosa GTPase regulator) gene. The RPGRIP protein has 1,267 amino acids. It is localized in the connecting cilia of rod and cone photoreceptors and is believed to be a structural component of the ciliary axoneme. CRX is a small gene. It has only three exons and it encodes a 299-amino-acid photoreceptor-specific transcription factor that controls the expression of several photoreceptor-specific genes. It is believed to play an important role in the differentiation of photoreceptor cells. CRB1 contains 11 exons. It encodes a 1,376-amino-acid extracellular protein that has homology to a Drosophila protein called “crumbs”. This protein is probably involved in cell–cell interaction and maintaining cell polarity in the retina.

Genetic diagnosis and counseling

Molecular genetic analysis in children with LCA is difficult because of the extent of genetic heterogeneity. Genetic testing is only available on a research basis from a few specialized laboratories. Genetic counseling is on an autosomal recessive basis. The sibling recurrence risk is 25% and offspring recurrence risks for affected individuals are likely to be relatively low. The only exception is LCA

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patients with heterozygous CRX mutations – in these cases the sibling recurrence risk is probably less than 25% while the offspring recurrence risk could be as high as 50%.

Norrie Disease MIM

310600

Clinical features

The condition shows both interfamilial and intrafamilial variability of expression. Affected males present with bilateral pseudoglioma and blindness. The characteristic ocular findings include iris atrophy and synechiae, retrolental mass, and retinal folds or detachment. Cataracts and phthisis bulbi can develop later. Mental retardation is seen in two thirds of patients. Behavioral or psychiatric problems are seen in about 25% of patients. One third of affected males have late-onset, progressive, high-frequency sensorineural hearing loss. Female carriers are usually asymptomatic, although retinal abnormalities such as retinal detachment have been identified in a small number of carriers.

Age of onset

The ocular features are apparent at birth in almost all affected males.

Epidemiology

No reliable epidemiologic data are available.

Inheritance

X-linked recessive

Chromosomal location

Xp11.4

Gene

NDP (Norrie disease protein)

Mutational spectrum

Whole gene deletions, intragenic deletions, and point mutations have all been described. Point mutations include both missense and nonsense mutations. All mutations are believed to result in a loss of function effect. Mutations in NDP have also been identified in Coat’s disease (MIM 300216), X-linked exudative vitreoretinopathy (MIM 305390), and in a small proportion (~3%) of patients with advanced retinopathy of prematurity.

Molecular pathogenesis Disorders of Vision

NDP has three exons and encodes a 133-amino-acid protein known as norrin. Norrin is a member of a superfamily of growth factors 79

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containing a cysteine knot motif. It is expressed in the outer nuclear, inner nuclear, and ganglion cell layers of the retina, the cerebellar granular layer, hippocampus, olfactory bulb, and cerebral cortex. The exact function of this protein is unknown, but it could be involved in cell–cell interaction and neurodevelopment. It is likely to play a critical role in differentiation of the retina and retinal vasculogenesis. Genetic diagnosis and counseling

Mutation analysis is available from a few diagnostic laboratories. This is very useful for confirmation of diagnosis, identification of carriers, and prenatal diagnosis. Counseling is on an X-linked recessive basis.

Rieger Syndrome (also known as: iridogoniodysgenesis type II) MIM

See Table 3.

Clinical features

Rieger syndrome is characterized by the combination of anterior segment dysgenesis, facial dysmorphism, dental anomalies, and redundant umbilicus. Anterior chamber abnormalities can include posterior embryotoxon, Axenfeld anomaly, Rieger anomaly, pupillary abnormalities (such as dyscoria and polycoria), and iris strands extending to Schwalbe’s line. Glaucoma develops later in most patients. Dental anomalies include conical crowns of anterior teeth and hypodontia of primary and permanent dentition. Other findings include cleft palate, hypospadias, and anal stenosis. Umbilical anomalies have not been described in Rieger syndrome type II.

Age of onset

The ocular and umbilical anomalies are apparent at birth.

Epidemiology

Rieger syndrome is rare; it has a population prevalence of 1 in 200,000.

Inheritance

Autosomal dominant

Chromosomal location and gene

See Table 3.

Type

MIM

Chromosomal location

Gene

Type I

180500

4q25–q26

PITX2 (paired-like homeodomain transcription factor 2)

Type II

601499

13q14

Unknown

Table 3. Rieger syndrome: types, MIM numbers, chromosomal locations, and genes.

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Mutational spectrum

Missense, splice-site, and nonsense mutations have all been identified in PITX2. In-frame and frame-shift duplications have also been identified. Mutations in PITX2 can also cause autosomal dominant iris hypoplasia (MIM 137600) and Peters’ anomaly (MIM 604229).

Molecular pathogenesis

PITX2 has five exons. It encodes a 317-amino-acid transcription factor that is involved in development of the anterior pituitary and teeth. Mutations in PITX2 are thought to result in functional haploinsufficiency of this gene.

Genetic diagnosis and counseling

PITX2 mutation analysis is only available on a research basis. Counseling is on an autosomal dominant basis. The condition can show variability of expression both between and within families. All affected individuals require life-long screening for glaucoma.

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4 4. Hearing Disorders

Nonsyndromal Hearing Loss 84 Hearing Loss due to Connexin 26 Gene Defect 85 Pendred Syndrome 86 Usher Syndrome 87 Waardenburg Syndrome 90

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Nonsyndromal Hearing Loss At the time of writing over 70 loci for nonsyndromal hearing loss have been identified and approximately 20 of the relevant genes have been isolated. These are summarized in Table 1. Mutations in each of these genes make only a small contribution to inherited hearing loss. The exception is the connexin 26 gene (CX26 or GJB2), which accounts for up to 40% of all childhood hearing loss in some populations. Disorder

MIM

Locus

Chromosomal location

Gene

DFNA

602121 603324 121011 600994

1 2 3 5

5q31 1p34 13q12 7p15

606201 602574 603196 603550 276903 120290

6 (=14) 8 (=12) 9 10 11 13

4p16.3 11q22–q24 14q12–q13 6q22–q23 11q13.5 6p21

HDIA8 (diaphanous) CX31 (connexin 31) CX26/30 (connexin 26/30) DFNA5 (deafness, autosomal dominant nonsyndromic sensorineural 5) WFS1 (Wolfram syndrome) TECTA (tectorin) COCH (cochlin) EYA4 (eyes absent) MYO7A Myosin VIIA) COL11A2 (collagen 11α2)

602460 160775 600970

15 17 22 28 1 2 3 4 6 7 (=11) 8 (=10) 9 12 16 18 21 22 29 30 1 3

5q31 22q 6q13 8q22 13q12 11q13.5 17p11.2 7q31 3p14–p21 9q13–q21 21q22 2p22–p23 10q21–q22 15q21–q22 11p14–p15.1 11q22–q24 16p12.2 21q22 10p11.1 Xq22 Xq21.1

POU4F3 (POU domain 4F3) MYH9 (myosin heavy chain 9) MYO6 (myosin VI) TFCP2L3 (transcription factor CP2L3) CX26/30 (connexin 26/30) MYO7A (myosin VIIA) MYO15 (myosin XV) PDS (pendrin) TMIE (transmembrane inner-ear-expressed gene) TMCI (transmembrane cochlear-expressed gene) TMPRSS3 (transmembrane protease) OTOF (otoferlin) CDH23 (cadherin 23) STRC (stereocilin) USHIC (usher IC) TECTA (tectorin) OTOA (otoancorin) CLDN14 (claudin 14) MYO3A (myosin IIIA) DDP (deafness/dystonia peptide) POU3F4 (POU domain 3F4)

DFNB

DFN

220290 276903 602666 605646 607237 606706 605511 603681 605516 606440 276904 602574 607038 605608 606808 300356 300039

Table 1. Nonsyndromal sensorineural hearing loss: disorders, MIM numbers, loci, chromosomal locations, and genes. Loci are indicated as DFNA (autosomal dominant), DFNB (autosomal recessive), and DFN (X-linked). A full list of hearing loss loci is maintained at the hereditary hearing loss homepage (http://dnalab-www.uia.ac.be/dnalab/hhh/).

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Hearing Loss due to Connexin 26 Gene Defect MIM

220290

Clinical features

Hearing loss caused by homozygous or compound heterozygous mutations in the connexin 26 gene is usually severe to profound. Audiology indicates that the hearing loss is sensorineural in origin with a sloping or flat audiometric curve. Radiologic studies of the inner ear are normal. Variation in severity within families and sibships is well recognized. Heterozygous mutations in connexin 26 can cause a rarer autosomal dominant nonsyndromal form of hearing loss as well as a very rare syndromal form in association with palmo-plantar keratoderma (MIM 148350).

Age on onset

Usually the hearing loss is congenital and nonprogressive.

Epidemiology

The most common mutation (see “35delG”, below) has an estimated carrier frequency of approximately 1 in 50 in most European, Mediterranean, and North American populations. Another mutation (167delT) shows a carrier frequency of 3%–4% in the Ashkenazi Jewish population.

Inheritance

Usually autosomal recessive. Some mutations are manifest in the heterozygous state (ie, autosomal-dominant inheritance).

Chromosomal location

13q11–q12

Gene

CX26 (connexin 26), also known as GJB2 (gap junction β-2)

Mutational spectrum The most common mutation is a deletion of one of a series of six guanine residues (35delG). Other common mutations include 167delT in the Ashkenazi Jewish population and 235delC in the Japanese. The full mutational spectrum includes missense and nonsense point mutations, splice-site mutations, and frame-shift deletions and insertions. Most of these mutations are believed to exert a loss of function effect. Molecular pathogenesis

Hearing Disorders

Connexin genes encode the subunits of gap junction proteins which form intercellular channels. These facilitate the transport of small molecules and ions between adjacent cells. Each gap junction consists of two connexons, or hemichannels, made up of six connexin subunits. Normal gap junction formation and function fails in individuals who 85

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are homozygous for the 35delG mutation. This leads to an inability to recycle the potassium ions needed for the initiation of action potentials in the cochlear hair cells. Mutations in CX26 account for up to 50% of all autosomal recessive nonsyndromal sensorineural hearing loss in populations of European and Mediterranean origin, with 35delG representing around 70% of all CX26 mutations. (The 35delG mutation is sometimes denoted as 30delG, ie deletion of the first rather than the last of the six contiguous guanine residues.) In many individuals only a single CX26 mutation can be identified, a finding which has been difficult to interpret. Recent studies have shown that many such individuals have a second mutation consisting of a deletion in CX30, which encodes connexin 30 and is contiguous with CX26 at chromosome 13q11–12. This combined CX26–CX30 locus constitutes the DFNB1 (deafness-recessive 1) locus and has been cited as an example of digenic inheritance. Genetic diagnosis and counseling

Mutation analysis for the common 35delG mutation is readily available. When homozygous mutations are identified a sibling recurrence risk of 1 in 4 can be given with confidence. In theory, prenatal diagnosis could be offered, but this raises very difficult and contentious ethical issues. Analysis for other mutations in CX26 and in other nonsyndromal hearing loss genes is available on a limited research basis.

Pendred Syndrome (also known as: PDS; deafness with goiter) MIM

274600

Clinical features

These consist chiefly of progressive hearing loss and goiter. The hearing loss is sensorineural in origin and may be present at birth or become apparent in early childhood. Disturbance of vestibular function is variable. Radiology of the inner ear often reveals the presence of a Mondini deformity (deficiency of the interscalar septum in the inner coils of the cochlea) and all patients show enlargement of the endolymphatic sac and duct in association with a dilated vestibular aqueduct. Goiter develops in approximately 80% of cases, usually in early adult life, and hypothyroidism occurs in around 50% of all cases.

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Age of onset

The hearing loss is often congenital in onset or develops in early childhood. Rarely a goiter can be present at birth. Hypothyroidism has been detected on neonatal screening.

Epidemiology

The estimated incidence is 7.5–10 per 100,000. PDS is the most common cause of syndromal hearing loss and accounts for around 5%–8% of all childhood-onset hearing loss.

Inheritance

Autosomal recessive

Chromosomal location

7q31

Gene

SLC26A4 (solute carrier family 26, member 4; also known as pendrin)

Mutational spectrum

Missense point mutations, splice-site mutations, and single base deletions. All have a loss of function effect.

Molecular pathogenesis

The PDS gene encodes a 780-amino-acid protein, known as pendrin, which is expressed in the inner ear, the thyroid gland, and renal cortical collecting ducts. Pendrin acts as a transporter of chloride and iodide anions. Its expression pattern in the cochlea suggests an important role in maintaining homeostasis of the endolymphatic fluid. Its precise role in the organification of iodide in the thyroid gland remains unclear, but is thought to involve the transport of iodide across the apical membrane of the thyrocyte into the colloid space.

Genetic diagnosis and counseling

Mutation analysis is available only on a limited research basis. Thus the diagnosis is usually based on the combined clinical, biochemical, and radiologic findings. Counseling is as for autosomal recessive inheritance.

Usher Syndrome MIM

See Table 2.

Clinical features

These consist essentially of variable sensorineural hearing loss and retinitis pigmentosa. In type I hearing loss is severe to profound, visual loss begins in early childhood, and vestibular function is impaired. In type II hearing loss is moderate to severe, vestibular function is normal, and the retinopathy manifests in late childhood or early teens. Type III is

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the mildest form with childhood onset, slowly progressive hearing loss, variable vestibular involvement, and teenage-onset night blindness. This progresses slowly and is of variable severity. Age of onset

In types I and II the hearing loss is congenital. In type III onset of hearing loss is in childhood, after the acquisition of speech (ie, “postlingual”).

Epidemiology

The overall incidence has been estimated to be around 3–4 per 100,000. Usher syndrome is believed to account for 3%–6% of serious hearing loss in children. Types IC and IIIA show increased incidences in the Acadian population of Louisiana and in the Finnish population, respectively.

Inheritance

Autosomal recessive

Chromosomal See Table 2. location, gene, and mutational spectrum Molecular pathogenesis

Type IB is caused by mutations in MYO7A and accounts for 75% of all type I cases. MYO7A contains 49 exons. It encodes myosin VIIa, a member of the myosin family of proteins that interact with actin filaments to convert energy from ATP into mechanical force. MYO7A is expressed in the apical stereocilia and cytoplasm of hair cells, the cochlea and vestibular system, both the apical processes of pigment epithelium cells, and the connecting cilia of rod and cone photoreceptor cells in the retina. Normally, stereocilia bend in response to vibrations resulting in the opening of ion channels which leads to the conversion of mechanical to electrical energy in hair cells. Mutant myosin VIIa impedes this process and also prevents the normal distribution of melanosomes in the retinal pigment epithelium. MYO7A mutations also account for the human nonsyndromal DFNB2 and DFNA11 forms of hearing loss (see Table 2). Mutations in the mouse ortholog cause the deafness syndrome known as “shaker”. Type IC results from mutations in the USHIC gene, which encodes a PDZ-domain-containing protein known as harmonin. PDZ proteins organize multiprotein complexes in areas such as synaptic junctions and anchor transmembrane proteins, such as receptors and ion channels. USHIC and MYO7A have similar expression patterns in the USHIC and MYO7A have similar expression patterns in the inner

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Type

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Chromosomal Gene location

Mutational spectrum

Type IA

276900

14q32

Unknown

Unknown

Type IB

276903

11q13.5

MYO7A (myosin VIIA)

Missense, nonsense, and splice-site mutations. Also frame-shift deletions. All with a loss of function effect

Type IC

276904

11p15.1

USH1C (harmonin)

Splice-site mutations and deletions. Also expansion of an intronic VNTR. All with a loss of function effect

Type ID

601067

10q21–q22

CDH23 (cadherin 23)

Missense, nonsense, and splice-site mutations. Also deletions and insertions

Type IE

602097

21q21

Unknown

Unknown

Type IF

602083

10q21–q22

PCDH15 (protocadherin 15)

Nonsense mutations and insertions with a probable loss of function effect

Type IG

606943

17q24–q25

Unknown

Unknown

Type IIA

276901

1q41

USH2A (usherin)

A single mutation, 2299delG, accounts for 16%–44% of all mutant alleles. Also missense, nonsense, splice-site, and frame-shift mutations

Type IIB

276905

3p24.2–p23

Unknown

Unknown

Type IIC

605472

5q14–q21

Unknown

Unknown

Type IIIA 276902

3q21–q25

USH3A Missense, nonsense, (Usher syndrome and deletion mutations type III)

Table 2. Usher syndrome: types, MIM numbers, chromosomal locations, genes, and mutational spectra. VNTR: variable number of tandem repeats.

ear and are components of the same multiprotein complex. A contiguous gene deletion syndrome that includes the USHIC locus has been described resulting in hearing loss, hyperinsulinism, renal tubular dysfunction, and enteropathy. Type ID is caused by mutations in CDH23, which encodes a large transmembrane protein entitled otocadherin. This is an important component of hair bundle formation. Mutations in the mouse ortholog cause the condition known as “waltzer” in which the organization of stereocilia is disrupted. Mutations in CDH23 also cause the DFNB12 form of nonsyndromal hearing loss (see Table 1, p.84).

Hearing Disorders

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Type IF is caused by mutations in PCDH15, which encodes a protocadherin protein expressed in the inner ear and retina. Type IIA results from mutations in USH2A, which contains 21 exons and encodes a protein given the name of usherin. This contains a single laminin type VI domain, 10 laminin-like epidermal growth factor domains and four fibronectin type III domains. Mutations are scattered throughout the gene; to date, no clear genotype–phenotype correlation has emerged. The gene that causes type IIIA is known as USH3A and contains four exons that encode a 120-amino-acid transmembrane protein of unknown function. Genetic diagnosis and counseling

Specific mutation analysis is only available at specialized researchbased laboratories. Counseling is as for autosomal recessive inheritance.

Waardenburg Syndrome (also known as: WS. Includes Klein–Waardenburg and Waardenburg–Shah syndromes) MIM

See Table 3.

Clinical features

Hearing loss and variable depigmentation occur in all forms of WS. The hearing loss is sensorineural, shows congenital onset, and is usually nonprogressive. It can be unilateral or bilateral and varies from mild to profound. The incidence of significant hearing loss in types I and II is approximately 70% and 90%, respectively. Absence of normal pigmentation manifests in the eyes with hypoplastic blue irides or complete or partial iris heterochromia, in the hair with a white forelock, and in the skin with areas of hypopigmentation. Types I and II are distinguished by the presence of dystopia canthorum (lateral displacement of the inner canthi) in type I, but not in type II. Types III and IV are both rare and are characterized by the presence of upper limb abnormalities such as contractures in type III and Hirschsprung disease in type IV.

Age of onset

Hearing loss is congenital. Areas of hypopigmentation and iris heterochromia become apparent in infancy or early childhood.

Epidemiology

The overall incidence has been estimated to be approximately 1 in 40,000. WS has been reported in all ethnic groups.

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Type

MIM

Inheritance Chromosomal location

Gene

Mutational spectrum

Type I

193500

Autosomal dominant

2q35

PAX3 (paired-box 3)

Missense, nonsense, and splice-site mutations. Also frame-shift insertions and deletions. All with a loss of function effect

Type IIA

193510

Autosomal dominant

3p14.1–p12.3

MITF (microphthalmiaassociated transcription factor)

Missense, nonsense, and splice-site mutations

Type IIB

600193

Autosomal dominant

1p21–p13.3

Unknown

Unknown

Type IIC

606662

Autosomal dominant

8p23

Unknown

Unknown

Type III (Klein–Waardenburg syndrome)

148820

Autosomal 2q35 dominant or autosomal recessive

PAX3

Missense and nonsense mutations

Type IV (Waardenburg–Shah syndrome)

277580

Autosomal 13q22 recessive or autosomal dominant

EDNRB (endothelin receptor, type B)

Nonsense mutations with a loss of function effect

20q13.2–q13.3 EDN3 (endothelin 3) 22q13

SOX10 (SRY-box 10)

Table 3. Waardenburg syndrome (WS): types, MIM numbers, inheritances, chromosomal locations, genes, and mutational spectra.

Inheritance, See Table 3. chromosomal location, gene, and mutational spectrum Molecular pathogenesis

Hearing Disorders

WS types I and III represent neurocristopathies in that they result from abnormal development, migration, or differentiation of neural crest cells that originate in the neural groove and neural tube. Studies in mice suggest that PAX3 is expressed in neural crest-derived cells during embryogenesis as well as in segmental mesoderm and the developing limb buds. PAX3 encodes a DNA binding transcription factor that contains a highly conserved 130-amino-acid paired-box (hence “PAX”) domain. Recognized PAX3 mutations act either as null alleles or impair normal DNA binding. Thus at a simplistic level, mutations in PAX3 that exert a loss of function effect appear to result in dose-dependent

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impairment of normal neural crest development. The very rare WS type III represents the severe end of the PAX3 neurocristopathy phenotype and has been described in association with both heterozygous and homozygous mutations. MITF also encodes a transcription factor. This transactivates the gene for tyrosinase, an enzyme that is essential for normal melanocyte differentiation. Thus the clinical features in WS type IIA are attributable to an abnormality of melanocytes rather than neural crest cells. Interestingly, it has recently emerged that MITF expression is regulated by PAX3 and SOX10 acting synergistically, indicating that the clinical features of WS types I and III result from a combination of both neural crest cell and melanocyte dysfunction. WS type IV is an extremely rare form of neurocristopathy. It is associated with absence of melanocytes and inner ear cells, giving rise to the features of WS, together with absence of parasympathetic enteric neurons of the terminal hindgut, resulting in Hirschsprung disease. The type IV phenotype can be caused by homozygous mutations in either EDNRB or in the gene that encodes its ligand, EDN3. Heterozygous mutations in either of these genes can result in Hirschsprung disease. Type IV can also be caused by heterozygous loss of function mutations in SOX10, a member of the SOX family of transcription regulators. A few type IV patients with SOX10 mutations also show progressive central nervous system involvement with mental retardation, nystagmus, cerebellar ataxia, and spasticity – findings consistent with abnormal SOX10 expression in glial cells. Genetic diagnosis and counseling

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Mutation analysis for PAX3 and MITF is available at a few specialist laboratories. Mutation analysis for other WS genes is only offered on a research basis. Counseling in most cases of WS is on the basis of autosomal dominant inheritance with variable expression, but caution should be exercised when counseling for types III and IV, both of which can show either autosomal dominant or autosomal recessive inheritance.

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5 5. Neurocutaneous Disorders and Childhood Cancer Multiple Endocrine Neoplasia Type 2 94 Neurofibromatosis Type 1 96 Retinoblastoma 98 Tuberous Sclerosis 101 von Hippel–Lindau Disease 103

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Multiple Endocrine Neoplasia Type 2 (also known as: MEN2) MIM

171400 (MEN2A or Sipple syndrome) 162300 (MEN2B or Wagenmann–Froboese syndrome) 155240 (familial medullary thyroid carcinoma [FMTC])

Clinical features

MEN2A This is the most common form of MEN2. It presents with medullary thyroid carcinoma (MTC). About 50% of all patients develop pheochromocytoma and 15%–30% develop hyperparathyroidism (due to parathyroid hyperplasia or adenoma). Hirschsprung disease has also been described in affected patients. MEN2B This is a rare form of MEN2. Affected individuals have a Marfanoid habitus, thickened everted margins of the upper eyelid, neuromas of the lip and tongue, and proximal muscle weakness with wasting. Medullated corneal nerve fibers can be seen on slit-lamp examination. Almost 90% of patients develop MTC and 50% develop pheochromocytoma. FMTC This diagnosis is made in families with four or more cases of MTC without any of the features of MEN2A or MEN2B.

Age of onset

Variable presentation in childhood has been described, particularly in MEN2B.

Epidemiology

This is a rare cancer predisposition syndrome.

Inheritance

Autosomal dominant

Chromosomal location

10q11.2

Gene

RET (rearranged during transfection) proto-oncogene

Mutational spectrum

MEN2A Almost all cases of this condition are caused by missense mutations involving six codons in exons 10 and 11. These are codons 609, 611, 618, 620, 630, and 634, all of which code for cysteine. Mutations in

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codon 634 are seen in 87% of patients. Small frame-shift insertions in exon 11 have also been described in two families. MEN2B Two missense mutations (Met918Thr and Ala883Phe) in exon 15 account for ~99% of cases. The former mutation is seen in 95% of patients and the latter accounts for 4% of cases. FMTC Mutations that cause MEN2A can also cause FMTC. Missense mutations involving codons 618, 620, and 634 account for 77% of cases. Mutations that have only been identified in patients with FMTC include Asp768Asn, Tyr791Phe, Ser891Ala, and a 2-bp insertion in exon 8. Molecular pathogenesis

RET spans over 55 kb of genomic DNA and contains 21 exons. It encodes a receptor tyrosine kinase that is primarily expressed in neural crest and urogenital precursor cells. It is involved in cell survival, proliferation, and differentiation. The receptor is activated by a complex comprising its ligand and a cell-surface bound coreceptor for the ligand. The ligand first binds to the coreceptor, which in turn presents the ligand to RET. The binding of the ligand to RET results in its dimerization. This causes autophosphorylation of tyrosine residues in its intracellular tyrosine kinase domain. Interaction of adaptor proteins with sequences adjacent to the phosphorylated tyrosine residues causes activation of several downstream pathways, including the RAS–MAP (mitogen-activated protein) kinase pathway that is needed for neuronal growth and differentiation. Activation of phosphatidylinositol-3 phosphate kinase (PI3-K) by RET is associated with cell proliferation and cellular motility. Activation of other downstream pathways can result in a neoplastic phenotype. Ligands for RET are members of the glial cell-line derived neurotrophic factor (GDNF) family and include GDNF, neurturin, persephin, and artemin. Mutations in the extracellular domain that cause MEN2A and FMTC result in activation of the receptor by ligand-independent dimerization. Mutations in the intracellular tyrosine kinase domain that cause MEN2B result in activation of the kinase activity of this domain in the absence of ligand binding and dimerization of the receptor. Activating mutations in RET cause a neoplastic phenotype by persistent downstream signaling. Inactivating mutations of RET are associated with Hirschsprung disease (see p.177–9) and Haddad syndrome (MIM 209880). The latter condition is the combination of Hirschsprung disease and congenital central hypoventilation syndrome, also known as Ondine’s curse.

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Counseling in MEN2 is on an autosomal dominant basis. About 5% of cases of MEN2A and almost 50% of cases of MEN2B are the result of a new mutation in RET. Genetic testing should be performed in all cases and is available from diagnostic laboratories. Identification of a mutation in RET allows confirmation of diagnosis and enables predictive testing to be offered to at-risk family members. Because MTC can occur at an early age, predictive testing should be offered to at-risk individuals by the age of 5 years in MEN2A and FMTC families and before the age of 5 years in MEN2B families. Children and older individuals in whom a RET mutation is identified should be offered prophylactic total thyroidectomy. This should be carried out by 5 years of age in MEN2A and FMTC families and before the age of 5 years in MEN2B families (some experts recommend prophylactic thyroidectomy before 6 months of age). The presence of an unsuspected pheochromocytoma should always be ruled out in these patients prior to surgery as this tumor can cause sudden death as a result of an anesthesia-induced hypertensive crisis. Affected and at-risk individuals from MEN2A and FMTC families should be screened for pheochromocytoma and hyperparathyroidism by measuring catecholamine levels in a 24-hour urine sample and plasma calcium levels on an annual basis. At-risk individuals from MEN2 families (all types) who cannot be offered predictive testing should also be screened for MTC by an annual pentagastrin stimulation test until the age of 35 years.

Neurofibromatosis Type 1 (also known as: NF1; Von Recklinghausen’s disease) MIM

162200

Clinical features

Multiple café-au-lait (CAL) patches are the usual presenting feature. Affected children have six or more CAL patches greater than 5 mm in diameter (in postpubertal children these patches should be more than 15 mm in diameter). Other cutaneous manifestations of NF1 include axillary (see Figure 1) and inguinal freckling and cutaneous neurofibromas (see Figure 2). Other diagnostic features include plexiform neurofibromas, Lisch nodules (iris hamartomas), pseudoarthrosis of the tibia, sphenoid wing dysplasia, and optic nerve glioma. Affected children often have macrocephaly and short stature. Complications include learning difficulties, epilepsy, scoliosis, hypertension, and plexiform neurofibromas of the head and neck.

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Figure 1. Axillary freckling.

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Figure 2. Multiple café-au-lait patches and cutaneous neurofibromas.

Age of onset

First year of life

Epidemiology

The condition affects individuals of all races. It is one of the most common autosomal dominant disorders with a population prevalence of 1 in 3,000.

Inheritance

Autosomal dominant

Chromosomal location

17q11.2

Gene

NF1

Mutational spectrum

The NF1 gene is very large, spanning over 350 kb of genomic DNA with 60 exons. Over 500 mutations have been identified. Most mutations are “private” (ie, restricted to a particular family). All types of mutations have been described (including nonsense, missense, frame-shift, and splice-site mutations) as well as small and large intragenic deletions and other rearrangements. Around 80% of mutations result in the production of a truncated protein and these mutations are evenly distributed over the entire coding sequence of the gene. The entire NF1 gene is deleted in about 5% of patients. These patients have a distinct phenotype with severe learning difficulties, facial dysmorphism, relatively large hands and feet, overgrowth, and numerous neurofibromas. Whole gene deletions can be detected by fluorescence in situ hybridization analysis.

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Molecular pathogenesis

The NF1 gene encodes a protein called neurofibromin, which is widely expressed in several tissues. Neurofibromin has a domain that demonstrates homology to GTPase-activating protein (GAP). The GAP-related domain down-regulates the activity of RAS, which is a major regulator of cellular growth and differentiation. The precise cellular functions of neurofibromin are unknown. There is evidence to suggest that NF1 is a tumor suppressor gene. NF1 is thought to result from haploinsufficiency.

Genetic diagnosis and counseling

NF1 is a clinical diagnosis. Genetic testing is difficult because of the large size of the gene, the vast number of mutations that have been identified, and the high proportion of “novel” mutations. Mutation testing is only available on a research basis at the present time. Counseling is on an autosomal dominant basis. About 50% of cases represent new mutations. NF1 shows a lot of variability in expression in affected members of the same family. Parents of such cases should be carefully examined clinically to look for the cutaneous features of NF1 and by slit-lamp examination to look for Lisch nodules. If neither parent fulfils the diagnostic criteria for NF1 there is a 1%–2% recurrence risk for this condition in their next pregnancy because of the possibility of gonadal mosaicism. In large families with affected individuals in two or more generations, prenatal diagnosis can usually be offered by linkage analysis using intragenic and flanking markers. However, many families elect not to have a prenatal test as this does not provide any information about the severity or complications of NF1 in an affected child.

Retinoblastoma MIM

180200

Clinical features

The usual presenting features of retinoblastoma are leucocoria (whiteeye or cat’s-eye reflex) or strabismus. Atypical presentations include glaucoma, uveitis, hyphema, and vitreous hemorrhage.

Age of onset

Most retinoblastomas present before the age of 5 years. Bilateral disease presents earlier (mean age at diagnosis 12 months) than unilateral disease (mean age at diagnosis 24 months).

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Epidemiology

Retinoblastoma is the most common ocular tumor of childhood with a global incidence of about 1 in 15,000–20,000 live births.

Inheritance

Autosomal dominant

Chromosomal location

13q14.1–q14.2

Gene

RB1 (retinoblastoma 1)

Mutational spectrum

A small number of patients with retinoblastoma (2%–3%) have an interstitial deletion or a translocation involving 13q14. Patients with an interstitial deletion of this region usually have other clinical features such as microcephaly, developmental delay, and dysmorphic features. Over 350 mutations have been identified in RB1. These include large rearrangements, small intragenic deletions and duplications, and point mutations. Point mutations include nonsense, missense, splice-site, and frame-shift mutations. Most mutations result in protein truncation.

Molecular pathogenesis

RB1 is a tumor suppressor gene. It contains 27 exons and it encodes a 724-amino-acid protein. Its protein product (Rb) is a nuclear phosphoprotein that inhibits cellular proliferation by inhibiting progress of cells from the G1 to the S phase of the cell cycle. It does this by interacting with the E2F family of transcription factors. The Rb–E2F complex arrests cells in the G1 phase of the cell cycle by transcriptional repression of other genes such as TGFB and CDKN2A. Both alleles of RB1 have to be inactivated before uncontrolled cellular proliferation can occur.

Genetic diagnosis and counseling

Only 10% of patients with retinoblastoma have a family history of this tumor. These patients have an inherited germ line RB1 mutation and develop bilateral disease. About 30% of patients have bilateral disease but no family history of retinoblastoma. These patients also have germ line RB1 mutations, but these are assumed to be “new” mutations. The remaining patients have unilateral disease and no family history of retinoblastoma. Many patients with unilateral multifocal tumors are also likely to have a “new” germ line RB1 mutation, although somatic mosaicism for an RB1 mutation is also possible. Approximately 85% of patients with a unilateral unifocal tumor have sporadic disease as a result of chance inactivation of both alleles of RB1 in the tumor. About 10% of patients with a unilateral unifocal tumor have a “new” germ line RB1 mutation and 5% are somatic mosaic for an RB1 mutation.

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Family history

Tumor type

Probability of germline mutation

Risk to offspring

Risk to siblings

Positive

Bilateral retinoblastoma

100%

50%

–

Negative

Bilateral retinoblastoma

95%

Assumed to be 50%

Around 3%–5% (due to germline mosaicism)

Negative

Multifocal, unilateral retinoblastoma

Uncertain

Difficult to determine

Difficult to determine

Negative

Unifocal, unilateral retinoblastoma

5%–10%

2%–5%

1%

Table 1. Family history, tumor-type, probability of germline mutation, and risks to offspring and siblings.

All patients with retinoblastoma (familial and sporadic) should be offered genetic testing, which is available from a few specialized laboratories. In cases with unilateral disease both blood and fresh or archived tumor tissue will be needed for RB1 mutation analysis. Genetic counseling is guided by family history, the number and distribution of tumors, and the results of genetic testing (see Table 1). Siblings and offspring of patients should be offered regular screening for retinoblastoma by retinal examination (under anesthesia until the age of 3 years) from birth to the age of 11 years. This can be stopped if genetic testing shows that the individual being screened has not inherited the RB1 mutation identified in the affected proband. Long-term survivors of retinoblastoma with a germ line RB1 mutation are at increased risk of developing second non-ocular malignant tumors. These include osteosarcoma, soft tissue sarcomas (such as fibrosarcoma), malignant melanoma, and brain tumors. The risk of a second non-ocular tumor is much higher in patients who received radiotherapy for the treatment of their retinoblastoma.

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Tuberous Sclerosis (also known as: TS; TS complex; epiloia; Bourneville–Pringle syndrome) MIM

191100

Clinical features

TS is a neurocutaneous disorder. The neurologic features include epilepsy, developmental delay, learning difficulties, and behavioral problems (including autistic features). Only 40%–50% of affected individuals have mental retardation. The most commonly seen seizures are infantile spasms. Cutaneous manifestations include fibrous forehead plaques, hypomelanotic or ash-leaf macules (see Figures 3 and 4), café-au-lait patches, confetti hypopigmentation, shagreen patch, facial angiofibromas, and periungual fibromas (see Figure 5). Renal lesions include cysts and angiomyolipomas. Other manifestations include cardiac rhabdomyomas, retinal hamartomas (phakomas), dental pits, phalangeal cysts, and pulmonary lymphangioleiomyomatosis (in adult females). Characteristic neuroradiologic findings include calcified subependymal nodules, cortical tubers or hamartomas, and giant cell astrocytomas (see Figure 6).

Age of onset

First year of life

Epidemiology

TS affects all populations with a prevalence of 1 in 6,000–10,000.

Inheritance

Autosomal dominant

Figure 3. Hypomelanotic (ash-leaf) macule in patient with tuberous sclerosis.

Neurocutaneous Disorders and Childhood Cancer

Figure 4. Hypomelanotic macule seen under Wood’s light.

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Figure 5. Multiple periungual fibromas in tuberous sclerosis.

Figure 6. Calcified subependymal nodules and calcified cortical tuber on computed tomography scan in a patient with tuberous sclerosis.

Chromosomal location

9q34.3, 16p13.3

Gene

TSC1 (tuberous sclerosis complex 1), TSC2 (tuberous sclerosis complex 2)

Mutational spectrum TSC1 has 23 exons and codes for a 1,164-amino-acid protein called hamartin. Mutations in TSC1 are seen in about 50% of familial cases, but are only identified in about 10%–20% of sporadic cases. These include small deletions, nonsense mutations, frame-shift mutations, and splicesite mutations. About 50% of mutations are single base substitutions, 82% of which are nonsense mutations. Virtually all TSC1 mutations are inactivating as they result in the production of a truncated protein. TSC2 contains 41 exons and encodes a 1,807-amino-acid protein called tuberin. Mutations in TSC2 can be identified in 50% of familial cases and about 80% of sporadic cases. All types of mutations have been described. Deletions, insertions, nonsense, frame-shift, and splice-site mutations result in protein truncation. Missense mutations have also been described. These usually involve the GTPase-activating protein (GAP)-related domain of the protein and result in the production of a protein with reduced activity. Molecular pathogenesis 102

Both hamartin and tuberin are believed to be GAPs. Both proteins have been shown to associate in vivo and probably act through the same

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pathway. Hamartin exhibits GAP activity towards RAP1 and RAB5, whereas tuberin only exhibits GAP activity towards RAB5. RAP1 is a p21 protein that is capable of suppressing cellular transformation. RAB5 is involved in the regulation of the endocytic pathway. Precisely how mutations in TSC1 and TSC2 give rise to the phenotype of TS is unknown. There is evidence to suggest that both TSC1 and TSC2 are tumor suppressor genes. Loss of both alleles of either TSC1 or TSC2 can result in the formation of TS-related hamartomas. Genetic diagnosis and counseling

The diagnosis of TS requires both clinical and radiologic examination. Genetic testing is available from a small number of diagnostic laboratories. Counseling is on the basis of autosomal dominant inheritance. The condition shows high penetrance, but instances of nonpenetrance and somatic mosaicism have been described. About 60% of cases are the result of a new mutation. Both parents of an affected child should be carefully examined for the cutaneous and retinal findings of TS. Some experts also advocate testing both parents of an affected child with cranial CT or MRI scans and renal ultrasound scans. If the parents do not have TS then the risk of recurrence in another child is small (2%–3%) and probably due to gonadal mosaicism. Prenatal diagnosis can be offered to families in which a mutation in TSC1 or TSC2 has been identified in an index case.

von Hippel–Lindau Disease (also known as: VHL) MIM

193300

Clinical features

VHL disease is a tumor predisposition syndrome. Affected individuals are at risk of developing: cerebellar, brain-stem, and spinal hemangioblastomas; retinal angiomas; clear cell renal cell carcinoma; and pheochromocytomas. Central nervous system (CNS) hemangioblastomas, retinal angiomas, and pheochromocytomas are usually benign, but clear cell renal cell carcinoma is a malignant tumor and is the most frequent cause of death in patients with VHL. Less common tumors include pancreatic islet cell tumors, endolymphatic sac tumors, epididymal tumors, and paragangliomas (pheochromocytomas

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located in extra-adrenal locations). Renal and pancreatic cysts can also be seen in this condition. Age of onset

Onset before the age of 5 years is exceptional. Children usually present with either retinal angioma or CNS hemangioblastoma.

Epidemiology

This condition affects all ethnic groups, with an incidence of 1 in 36,000 live births.

Inheritance

Autosomal dominant with age-dependent penetrance: the condition has 2% penetrance by the age of 5 years, 19% penetrance by the age of 15 years, and 99% penetrance by the age of 65 years.

Chromosomal location

3p25–p26

Gene

VHL

Mutational spectrum

More than 300 mutations have been described in VHL. Approximately 70% of patients have point mutations whereas the remainder have partial or complete deletions of the gene. Point mutations include nonsense, missense, and splice-site mutations. Mutations are spread across all three exons of the gene, but codon 167 appears to be a mutational hotspot. There appears to be genotype–phenotype correlation. Most patients with truncating or null mutations in VHL have VHL without pheochromocytoma (VHL type 1). Most patients with missense mutations have VHL with pheochromocytoma or isolated pheochromocytoma (VHL type 2).

Molecular pathogenesis

VHL spans about 10 kb of genomic DNA. It has three exons and produces two transcripts as a result of alternative splicing of exon 2. One transcript codes for a 213-amino-acid protein (isoform 1) and the other for a 172-amino-acid protein (isoform 2). The protein product of VHL (pVHL) forms a complex with several cellular proteins including elongin C, elongin B, and cullin 2. This complex has E3 ubiquitin ligase activity and is involved in degradation of hypoxiainducible factor 1α (HIF-1α). Loss of function of pVHL leads to the stabilization of HIF-1α, which activates transcription of several hypoxiainducible genes such as vascular endothelial growth factor (VEGF), glucose transporter 1 (GLUT-1), and platelet-derived growth factor-B

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(PDGF-B). Overexpression of these genes could result in the production of the vascular tumors of VHL. There is evidence to suggest that pVHL also interacts with other proteins such as fibronectin, protein kinase C, and probably other (as yet unidentified) proteins. Through these interactions it could promote correct formation of the fibronectin extracellular matrix and could also inhibit cell signaling. VHL is a tumor suppressor gene. The normal allele of this gene is inactivated in VHL-related tumors in individuals who inherit a mutation in one allele of this gene (ie, individuals with a germ line mutation). The reason for the genotype–phenotype correlation is unknown. Germ line mutations that result in the production of pVHL with some residual function (some missense mutations) are associated with a predisposition to pheochromocytoma, whereas mutations that completely inactivate the gene (deletions and protein-truncating mutations) are not associated with predisposition to these tumors. Genetic diagnosis and counseling

Counseling is on an autosomal dominant basis. Genetic testing is available on a routine basis. All patients with VHL should be offered genetic testing so that predictive testing can be offered by direct mutation analysis to other family members who are at risk of inheriting this condition. Relatives of affected individuals in whom a VHL mutation cannot be identified can be offered predictive testing by linkage analysis (using markers closely linked to VHL). Affected patients should be offered regular screening for other VHL-related tumors. The recommended screening protocol for affected individuals includes physical examination, retinal examination (by direct and indirect ophthalmoscopy and fluorescein angiography), abdominal ultrasound scan, and 24-hour urine collection for catecholamines on an annual basis. In addition, affected individuals should be offered 3-yearly CT or MRI brain scans until the age of 50 years and 5-yearly scans thereafter. Individuals at 50% risk of inheriting VHL should also be offered screening until their risk status can be clarified by predictive testing (by mutation or linkage analysis). The recommended screening protocol for at-risk relatives includes annual retinal examination by direct and indirect ophthalmoscopy from the age of 5 years, annual 24-hour urine collection for catecholamines from the age of 10 years, 3-yearly CT or MRI scan of the brain, and annual abdominal scan from the age of 15 years. If the results of predictive testing show that an at-risk relative has not inherited

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the familial VHL mutation then screening can be stopped. An at-risk relative shown to have inherited the VHL mutation (or shown, by linkage analysis, to have inherited the high-risk VHL allele) should be screened using the same protocol that is used for affected individuals.

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6 6. Connective Tissue and Skeletal Disorders

Achondroplasia 108 Ehlers–Danlos Syndrome 110 Hereditary Multiple Exostoses 115 Marfan Syndrome 117 Osteogenesis Imperfecta 119 Pseudoachondroplasia 124 Stickler Syndrome 125

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Achondroplasia MIM

100800

Clinical features

The clinical features are characteristic and distinctive (see Figure 1). Facially, there is macrocephaly with frontal bossing and a flat nasal bridge. The trunk is relatively normal in length with a lumbar lordosis. The limbs show rhizomelic shortening with short fingers giving a “trident” appearance to the hand. Average adult height is 130 cm in males and 125 cm in females. In the absence of central nervous system complications, both intelligence and life expectancy are normal. Complications can include hydrocephalus (rare, seen in <5%), cervical cord compression due to a small foramen magnum (seen in 5%–10% of cases), spinal stenosis (seen in >50% of cases by the age of 60 years), and premature osteoarthritis, which is common in middle age.

Age of onset

The diagnosis is usually evident at birth and can be suspected on the basis of limb shortening during the third trimester of pregnancy.

Epidemiology

The incidence in neonates is approximately 1 in 10,000–20,000. The condition shows no ethnic predilection.

Inheritance

Autosomal dominant with full penetrance and consistent expression

Figure 1. A young child with achondroplasia.

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Chromosomal location

4p16.3

Gene

FGFR3 (fibroblast growth factor receptor 3)

Mutational spectrum

Point mutations with a gain of function effect

Molecular pathogenesis

FGFR3 encodes a receptor with an extracellular region containing three immunoglobulin-like domains, a single membrane-spanning domain, and an intracellular split tyrosine kinase domain (see Figure 2). On binding with the specific growth factor ligand, the receptor molecules dimerize with subsequent phosphorylation of the tyrosine residues in the intracellular domain. This activation of the receptor leads to suppression of endochondral growth through the process of signal transduction. Almost all cases of achondroplasia are caused by either a G→A or G→C mutation (Gly380Arg) at nucleotide 1138 in the transmembrane domain of FGFR3. Other mutations in FGFR3 result in a spectrum of skeletal involvement causing short stature ranging from the severe and invariably lethal forms of thanatophoric dysplasia to the much milder condition of hypochondroplasia (see Table 1). The mutations that cause achondroplasia

Ig I

Ig III

Ig II

TDI 742 C→T

Arg248Cys

TDI

TM

TK1

TK2

ACH

HCH

TDII

1138 G→A or G→C

1620 C→A

1948 A→G

Asn540Lys

Lys650Glu

Tyr373Cys Gly380Arg

Figure 2. Diagrammatic representation of FGFR3 (fibroblast growth factor receptor 3) with arrows showing the sites of common mutations. ACH: achondroplasia; HCH: hypochondroplasia; Ig: immunoglobulin loop; TDI/TDII: thanatophoric dysplasia types 1 and 2; TK1/2: tyrosine kinase domains 1 and 2; TM: transmembrane domain.

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Disorder

MIM

Achondroplasia

100800

Gly380Arg

Hypochondroplasia

146000

Asn540Lys

SADDAN

Not listed

Lys650Met

Thanatophoric dysplasia • type I (curved femora)

187600

Arg248Cys,Tyr373Cys

187601

Lys650Glu

• type II (straight femora with cloverleaf skull)

Amino acid residue affected

Table 1. Skeletal disorders caused by mutations in FGFR3 (fibroblast growth factor receptor 3). SADDAN: severe achondroplasia with developmental delay and acanthosis nigricans.

result in ligand-dependent receptor activity. In contrast, the mutations that are associated with thanatophoric dysplasia result in receptor activation that is independent of ligand binding. Genetic diagnosis and counseling

Detection of the common 1,138 G→A and 1,138 G→C mutations is readily available. Counseling is as for autosomal dominant inheritance. Approximately 80% of all cases result from new mutations almost all of which have occurred in the paternally derived allele, as suggested by the long-observed paternal age effect. If both parents have achondroplasia, each of their children has a 1 in 4 chance of being homozygous affected, which almost always results in death in the perinatal period.

Ehlers–Danlos Syndrome MIM

See Table 2.

Clinical features

The term “Ehlers–Danlos syndrome” embraces a group of disorders that share the common features of increased joint laxity and skin hyperextensibility (see Figures 3 and 4) with other variable manifestations, such as skin friability and easy bruising, periodontal disease, mitral valve prolapse, and vascular rupture. Clinical features characteristic of the different traditional subtypes are listed in Table 2. Marked clinical and genetic heterogeneity exist and many patients are difficult to classify. In the classic case the skin is soft and velvety in texture with increased extensibility but normal recoil elasticity. The latter feature distinguishes Ehlers–Danlos syndrome from cutis laxa. In severe cases the skin tears easily and heals with thin “cigarette paper” scars. Joint involvement can

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Figure 3. Skin laxity in Ehlers–Danlos syndrome.

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Figure 4. Joint laxity in Ehlers–Danlos syndrome.

vary from mildly increased laxity to severe instability with recurrent dislocation. Vascular manifestations are also variable and can range from a mild bruising tendency to catastrophic arterial rupture. Age of onset

Prematurity, possibly the result of deficient collagen in the amniotic membranes, has been noted in some cases. More often, presentation is in early childhood with bruising, skin fragility, and mild delay in achieving motor milestones because of increased joint laxity.

Epidemiology

Severe types are rare, but mild forms are relatively common.

Inheritance, chromosomal location, and gene

See Table 2.

Mutational spectrum Types I and II and molecular Most cases are caused by mutations in either COL5A1 or COL5A2. pathogenesis In COL5A1 there are usually splice-site mutations or small deletions or insertions resulting in frame-shifts, all with a loss of function effect as manifested by “nonsense-mediated” decay of the mRNA. In COL5A2 both splicing and missense mutations have been identified. These mutations are thought to interfere with posttranslational modification of type V collagen and thereby prevent normal processing of collagen in skin and connective tissue.

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Type

Name

MIM

Inheritance

Clinical features

Chromosomal location

Gene

I

Classical or gravis

130000

AD

Moderate to severe skin involvement, mild joint laxity

2q31

COL5A2 (collagen, type V, α-2) COL5A1, (collagen, type V, α-1) COL1A1 (collagen, type I, α-I)

9q34.2–q34.3 17q21.32–q22

II

Classical or mitis

130010

AD

Mild skin and joint involvement

2q31

COL5A2 (collagen, type V, α-2) COL5A1 (collagen, type V, α-1)

9q34.2–q34.3 III

Familial hypermobility

130020

AD

Marked joint laxity with recurrent dislocation

2q31

COL3A1 (collagen, type III, α-1)

IV

Arterial or vascular 130050

AD

Bruising; “acrogeric” 2q31 extremities; rupture of arteries, bowel, and uterus

COL3A1 (collagen, type III, α-1)

V*

“X-linked”

305200

XR

Mild skin and joint involvement

Unknown

Unknown

VI

Ocular-scoliotic

225400

AR

Moderate skin and joint involvement, ocular fragility and rupture, progressive scoliosis

1p36.3–p36.2

PLOD1 (procollagen-lysine, 2-oxoglutarate 5-dioxygenase)

VIIA

Arthrochalasis multiplex congenita

130060

AD

Congenital hip dislocation and severe generalized joint laxity

17q21.31–q22

COL1A1 (collagen, type I, α-1)

VIIB

Arthrochalasis multiplex congenita

130060

AD

Congenital hip dislocation and severe generalized joint laxity

7q22.1

COL1A2 (collagen, type I, α-2)

VIIC

Dermatosparaxis

225410

AR

Severe skin and joint involvement

5q23

ADAMTS2

VIII

Periodontal

130080

AD

Mild skin involvement with generalized periodontitis

Unknown

Unknown

IX†

Occipital horn syndrome

304150

XR

Soft skin, bladder diverticula, occipital exostoses (“horns”)

Xq12–q13

ATP7A (ATPase, Cu[2+]-transporting, α polypeptide)

Table 2. Ehlers–Danlos syndrome: clinical classification. AD: autosomal dominant; AR: autosomal recessive; ATP7A: ATP-ase, Cu(2+)-transporting, α polypeptide; XR: X-linked recessive. *Only a single family reported. Existence of this form has not been confirmed. †Has been reclassified as a disorder of copper transport. ADAMTS2 is a disintegrin-like and metalloproteinase with thrombospondin type 1 motif 2 (ie, procollagen I N-proteinase).

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Mutations in COL1A1 have been identified in a few patients with classical type I features. These are missense point mutations resulting in substitution of a conserved arginine by cysteine in the triple helical domain, which in turn leads to the synthesis of a structurally deficient collagen molecule. Type III A missense point mutation in COL3A1 resulting in a glycine to serine substitution has been identified in a single family. In most families the basic defect is unknown. Type IV Most if not all cases are caused by mutations in COL3A1 leading to abnormal synthesis and secretion of type III procollagen. These consist mainly of missense point mutations resulting in substitution of glycine residues in the triple helical domain of the pro-α1(III) chain (ie, a dominant-negative effect). Splice-site mutations (resulting in exonskipping) and small deletions have also been observed. On electron microscopy, skin fibroblasts show intracellular storage of the abnormal type III procollagen molecules in the dilated rough endoplasmic reticulum. Type V Unknown Type VI This is caused by loss of function mutations in PLOD1, which encodes lysyl hydroxylase, the enzyme that converts lysine to hydroxylysine in collagen types I and III. Reduced hydroxylation results in reduced intermolecular cross-linkage with loss of tissue integrity. Approximately 20 mutations have been identified, most of which are unique. However, two recurrent mutations, a duplication of exons 10–16 and a nonsense Tyr511Stop mutation in exon 14, account for approximately half of all reported cases. Type VII Types VIIA and VIIB are caused by mutations that result in loss of exon 6 of COL1A1 or COL1A2, respectively. These can involve splice-site mutations or deletions. Exon 6 in COL1A1 and COL1A2 encodes the N-proteinase site which enables cleavage of the secreted procollagen by procollagen I N-proteinase to form the mature collagen molecule. Exon 6 also encodes a lysine residue which, after hydroxylation, is involved in cross-linking. The integration of immature procollagen

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with mature collagen chains (together with defective cross-linking) results in reduced tensile strength in ligaments and occasionally in bone. In type VIIC the basic defect lies in reduced activity of the procollagen I N-proteinase which normally cleaves pro-α1(I) and pro-α2(I) chains to form the mature type I collagen molecule. Most of the small number of reported cases have been caused by homozygosity for a nonsense point mutation resulting in a premature termination codon. Type VIII Unknown. Type IX This can be considered a mild allelic variant of Menkes disease (see p.211–2). In seven of eight reported cases, a splice-site mutation was identified in ATP7A (which encodes a Golgi-membrane-bound copper transport protein). These represent mild loss of function mutations resulting in relatively normal mRNA processing with some residual functional protein activity. Genetic diagnosis and counseling

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In most cases, the diagnosis and counseling are based on clinical features and family history. Specific mutation analysis for most forms of Ehlers–Danlos syndrome is available at only a few research laboratories. Electron microscopy of collagen fibrils in skin can reveal characteristic findings such as increased diameter, irregular borders, and “cauliflower” fibers in types I/II and reduced diameter in type IV, in association with intracellular storage of abnormal type III procollagen. The diagnosis of type IV can be confirmed by the demonstration of reduced type III procollagen production using cultured fibroblasts. Type VI can be diagnosed biochemically by an assay of lysyl hydroxylase activity in cultured cells or by demonstrating an altered ratio of hydroxylated to unhydroxylated cross-links in urine. Cultured fibroblasts from children with type VII show failure of cleavage of type I procollagen to collagen. In type VIIC, “hieroglyphic” fibers are seen on electron microscopy.

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Hereditary Multiple Exostoses (also known as: HME; diaphyseal aclasis; multiple cartilaginous exostoses; multiple osteochondromatosis) Clinical features

These consist primarily of bony lumps, which first appear in early childhood and then increase in size and number until adult life when growth ceases. The lumps are usually not painful but can cause problems by pressing on adjacent nerves or tendons. They can also lead to deformity by causing disproportionate growth, particularly in the forearm with bowing of the radius and ulna. The incidence of malignant change to chondrosarcoma or osteosarcoma was thought to be high. However, several contemporary studies have concluded that the true incidence is of the order of 0.5%–2.0%. Malignancy shows a mean age of onset of 31 years; it is rare before 10 years and after 50 years. The exostoses occur most commonly at the ends of the long bones pointing away from the epiphyses (see Figure 5). This location gives the impression that they move down the diaphyses with the passage of time. This can result in a Madelung deformity at the wrist as well as bowing of the radius and ulna. Different sized exostoses also occur on the ribs and on both the pectoral and pelvic girdles.

Age of onset

The bony lumps are usually first noted in early childhood.

Epidemiology

The overall incidence has been estimated to be around 1 in 50,000. All ethnic groups are affected.

Figure 5. Typical appearance of exostoses in hereditary multiple exostoses.

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Autosomal dominant

MIM, chromosomal See Table 3. location, gene, and mutational spectrum Type

MIM

Chromosomal location

Gene

Mutational spectrum

Type I

133700

8q24.11–q24.13

EXT1 (exostoses, multiple, type I)

Missense, nonsense, frame-shift, and splicesite point mutations, deletions, insertions, and in-frame deletions. All with a loss of function effect. These account for around 70% of all cases of HME.

Type II 133701

11p11.2–p12

EXT2 (exostoses, multiple, type II)

Missense, nonsense, and splice-site point mutations, and frame-shift deletions and insertions. All with a loss of function effect.

Type III 600209

19p

EXT3 (exostoses, multiple, type III) (unknown)

Unknown

Table 3. Hereditary multiple exostoses (HME): types, MIM numbers, chromosomal locations, genes, and mutational spectra.

Molecular pathogenesis

EXT1 and EXT2 consist of 11 and 15 exons, respectively. They encode transmembrane glycoproteins localized to the endoplasmic reticulum. Together these proteins form an oligomeric complex which acts as a glycosyltransferase in the polymerization of heparan sulfate. Heparan sulfate is an important constituent of glycosaminoglycans (GAGs – formerly known as mucopolysaccharides). GAGs are known to function as cofactors in signal transduction (the process whereby cells receive instructions [“signals”], such as to grow, differentiate, or migrate). The EXT1/EXT2 heterocomplex product has much higher glycosyltransferase activity than EXT1 or EXT2 homocomplexes alone. EXT1 and EXT2 thus act as tumor suppressors in that their combined protein product has a negative regulatory role on cell turnover. Studies in osteochondromas (exostoses) from patients with HME have revealed loss of heterozygosity for EXT1. This implies loss of the wild-type allele so that the cell line giving rise to the clonal osteochondroma contains

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only the inherited mutant allele. Further malignant transformation to an osteochondrosarcoma almost certainly involves additional somatically acquired mutational events consistent with the multistep concept of carcinogenesis. As well as being responsible for HME types I and II, EXT1 and EXT2 are also involved in two contiguous gene deletion syndromes involving 8q24.11–q24.33 and 11p11.2–p12, respectively. Deletion of 8q24.11–q24.33 results in Langer–Giedion syndrome, in which multiple exostoses are associated with a characteristic facies, sparse scalp hair, short angulated digits, short stature, and variable mental retardation. Deletion of 11p11.2–p12 also leads to the development of multiple exostoses and mental retardation: additional findings include subtle facial anomalies and biparietal foramina. Genetic diagnosis and counseling

Specific mutation analysis is not routinely available. Chromosome analysis looking for a deletion is indicated when multiple exostoses are associated with other findings such as mental retardation. Counseling is on the basis of autosomal dominant inheritance with variable expression.

Marfan Syndrome MIM

154700

Clinical features

Primarily involving the musculoskeletal, cardiovascular, and ocular systems. Musculoskeletal involvement is typified by the Marfanoid habitus (tall stature with long limbs, together with pectus deformity, scoliosis, pes planus, and hyperextensible joints). Cardiovascular features include mitral valve prolapse, aortic enlargement, and aortic dissection. The main ocular features consist of myopia and ectopia lentis (see Figure 6). Other findings and complications can include spontaneous pneumothorax with apical blebs on chest X-ray, lumbosacral dural ectasia (as revealed by computed tomography and magnetic resonance imaging scans), herniae, and skin striae. Contemporary photographs indicate that Abraham Lincoln had a Marfanoid habitus, prompting suggestions that he may have had Marfan syndrome. To date, efforts to analyze his DNA extracted from archival tissues have been resisted, chiefly because of the complexity of mutation detection in FBN1.

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Figure 6. Lens subluxation in Marfan syndrome.

Age of onset

In the severe neonatal form, aortic dilatation and valvular insufficiency are present at birth. More commonly, the diagnosis becomes apparent in mid childhood.

Epidemiology

Marfan syndrome occurs in all ethnic groups, with an estimated prevalence of at least 1 in 14,000.

Inheritance

Autosomal dominant with very variable expression and a few reports of nonpenetrance (ie, complete failure of expression). Approximately 25% of cases result from new mutations.

Chromosomal location

15q21.1

Gene

FBN1 (fibrillin 1)

Mutational spectrum

Mainly missense mutations with a dominant-negative effect. Also small frame-shift deletions and insertions.

Molecular pathogenesis

Fibrillin 1 is an important constituent of microfibrils in the extracellular matrix. Microfibrils act as a template for the deposition of elastin and facilitate the linkage of elastin fibers both to each other and to adjacent components of the extracellular matrix. They also play an important anchoring role in skin and in the ocular zonule where they hold the lens in place. FBN1 is a large gene with 65 exons and encodes a protein with five distinct structural and functional domains (A–E). Domains B and D contain motifs that show homology to epidermal growth factor (EGF) and contain a consensus sequence for calcium-binding. Over 100 mutations have been identified in FBN1 in Marfan syndrome patients,

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most of which are unique (or “private”) to each individual family or sporadic case. In around 20%–25% of cases that fulfill the strict diagnostic criteria no mutation can be identified. These cases are probably caused by mutations in regulatory regions of FBN1 as there is no firm evidence for a second disease locus at present. Approximately 70% of all mutations are missense and most of these involve the calcium-binding EGF-like motifs. Some genotype–phenotype correlations have emerged. The severe neonatal form of Marfan syndrome is associated with missense mutations in exons 24–27 and exon-skipping mutations in exons 31 and 32. A few specific mutations have been found in families segregating either isolated aortic aneurysms or ectopia lentis (dislocation or subluxation of the lens). Most mutations are believed to exert a dominant-negative effect by encoding abnormal fibrillin 1 monomers that interact with wild-type monomers to prevent the formation of normal microfibrillar aggregates. Mutations in exons 24–34 in FBN2 on chromosome 5q23–31 cause the rare condition, congenital contractural arachnodactyly, also known as Beals’ syndrome (MIM 121050). This condition shows phenotypic overlap with the musculoskeletal features of Marfan syndrome but without the ocular or cardiovascular complications. Genetic diagnosis and counseling

Specific mutation analysis is difficult because of the large size of FBN1. Linkage analysis can be offered in informative families for diagnostic purposes. Fibrillin immunofluorescence in skin samples or fibroblast cultures is not sufficiently reliable to be used for diagnostic purposes. Counseling is on the basis of autosomal dominant inheritance with marked interfamilial and intrafamilial variation in severity.

Osteogenesis Imperfecta (also known as: OI. Includes: osteogenesis imperfecta congenita [OIC]; osteogenesis imperfecta tarda [OIT]) MIM

166200 (type I) 166210 (type II) 166220 (type IV) 259420 (type III)

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Figure 7. Appearance of the teeth in a child with osteogenesis imperfecta.

Clinical features

Increased bone fragility occurs in all forms of OI. Variable findings include blue sclerae, deafness, joint laxity, and dentinogenesis imperfecta (see Figure 7). Clinical involvement can vary from mild (with a history of only a few fractures and no deformity) to profoundly severe with death in utero. Four clinical and radiologic groupings are recognized (see Table 4). Not all patients fall precisely within this classification, but the clinical, radiologic, and molecular findings within these four categories are generally consistent.

Age of onset

This is very variable and can range from before birth (with limb shortening as a result of multiple fractures) to adult life.

Epidemiology

The combined incidence of all forms of OI is approximately 1 in 5,000. Type I is by far the most common.

Inheritance

Most cases show autosomal dominant inheritance. Autosomal recessive inheritance has been confirmed biochemically and by molecular analysis in only a few families.

Chromosomal location and gene

17q21.31–q22 (COL1A1 [collagen type I α1 chain]) 7q22.1 (COL1A2 [collagen type I α2 chain])

Mutational spectrum Point mutations and deletions with a loss of function effect (null alleles). Missense mutations, exon-skipping mutations, insertions and deletions with a dominant-negative effect. Molecular pathogenesis

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Almost all cases of OI are caused by heterozygous mutations in one of the type I collagen genes. Type I collagen is a trimer made up of two pro-α1(I) chains and one pro-α2(I) chain encoded by COL1A1 and COL1A2, respectively. Mutations that result in failure of synthesis of a chain or in its inability to be incorporated into a type I collagen trimer

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Type

Clinical

Radiologic

I

Osseous fragility (mild to moderate) Blue sclerae Mixed hearing loss in adult life (50%) Mitral valve prolapse (15%)

Osteoporosis (mild) Platyspondyly (mild) Wormian bones

II

Osseous fragility (very severe) Soft calvarium Short trunk and limbs Small chest and protuberant abdomen Early lethality

Gross underossification Crumpled long bones Beaded ribs Platyspondyly (severe)

III

Osseous fragility (severe) Progressive deformity Short stature Normal sclerae

Osteoporosis (severe) Thin bowed long bones Biconcave vertebrae Wormian bones

IV

Osseous fragility (variable) Normal sclerae Dentinogenesis imperfecta Occasional severe deformity

Osteoporosis

Table 4. Osteogenesis imperfecta: clinical and radiologic features.

result in mild OI in which type I collagen is reduced in quantity but is relatively normal qualitatively. Such mutations generally result in null alleles with a loss of function effect. In contrast, mutations that lead to the production of a mutant chain that is processed into the mature collagen molecule cause severe OI by acting in a dominant-negative manner. These mutations, which include insertions, deletions, and exon-skipping mutations, lead to malalignment of the pro-α1 and pro-α2 chains. Similarly, missense point mutations that cause substitution of one of the glycine residues in the repetitive Gly–X–Y structure of a collagen chain lead to abnormal configuration of any triple helix into which the mutant collagen chain is incorporated. As each mature triple helical collagen molecule contains two pro-α1 chains, a mutation with a dominant-negative effect will result in 75% of all mature type I collagen molecules being abnormal (see Figure 8). In general, glycine substitutions in carboxy-terminal sites produce lethal outcomes (as in type II OIC, see Figure 9), whereas glycine substitutions nearer the amino-terminal result in milder phenotypes. Mutations in the large family of collagen genes account for a diverse group of inherited disorders, many of which are extremely rare. These are summarized in Table 5.

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MIM

Gene

Locus

Gene product

Disorder

120150

COL1A1

17q21.31–q22

α1(I)

Osteogenesis imperfecta EDSVIII Osteoporosis

120160

COL1A2

7q22.1

α2(I)

Osteogenesis imperfecta EDSVII Osteoporosis

120140

COL2A1

12q13.11–q13.2 α1(II)

120180

COL3A1

2q31

α1(III)

EDSIV

120070

COL4A3

2q36–q37

α3(IV)

Alport syndrome

120131

COL4A4

2q36–q37

α4(IV)

Alport syndrome

303630

COL4A5

Xq22.3

α5(IV)

Alport syndrome

303631

COL4A6

Xq22.3

α6(IV)

Leiomyomatosis

120215

COL5A1

9q34.2–q34.3

α1(V)

EDSI and EDSII

120190

COL5A2

2q31

α2(V)

EDSI and EDSII

120220

COL6A1

21q22.3

α1(VI)

Bethlem myopathy

120240

COL6A2

21q22.3

α2(VI)

Bethlem myopathy

120250

COL6A3

2q37

α3(VI)

Bethlem myopathy

120120

COL7A1

3p21.3

α1(VII)

Epidermolysis bullosa (dystrophic types)

120210

COL9A1

6q13

α1(IX)

MED1

120260

COL9A2

1p33–p32.2

α2(IX)

MED2

120270

COL9A3

20q13.3

α3(IX)

MED3

120110

COL10A1

6q21–q22.3

α1(X)

Metaphyseal chondrodysplasia (Schmid)

Achondrogenesis II Hypochondrogenesis SEDC SEMD Kniest dysplasia Stickler syndrome I Osteoarthritis

120280

COL11A1

1p21

α1(XI)

Stickler syndrome II

120290

COL11A2

6p21.3

α2(XI)

Stickler syndrome III Weissenbacher–Zweymuller syndrome

113811

COL17A1

10q24.3

α1(XVII)

Junctional epidermolysis bullosa

120328

COL18A1

21q22.3

α1(XVIII)

Knobloch syndrome (myopia and encephalocele)

Table 5. Collagen gene disorders. Collagen types are indicated by Roman numerals. The constituent chains are designated using Arabic numerals followed in brackets by the collagen type. Collagen genes are identified by the collagen type, written in Arabic numerals, followed by a capital A, followed by the number of the pro-α chain they encode; for example, COL2A1 encodes the α1(II) chain of type II collagen. EDS: Ehlers–Danlos syndrome; MED: multiple epiphyseal dysplasia; SEDC: spondyloepiphyseal dysplasia congenita; SEMD: spondyloepimetaphyseal dysplasia.

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Normal synthesis Collagen trimers

Chains

AAC pro-α1 (A) pro-α1 (B) pro-α2 (C)

}

ABC BAC BBC

}

Outcome

Normal type I collagen fiber

Mutation with a loss of function effect Collagen trimers

Chains *pro-α1 (A*) pro-α1 (B) pro-α2 (C)

}

BBC BBC

}

Outcome

Reduced amount of type I collagen causing mild OI

Mutation with a dominant-negative effect Chains

Collagen trimers

A*A*C *pro-α1 (A*) pro-α1 (B) proα2 (C)

}

A*BC BA*C BBC

}

Outcome

Degraded or abnormally branched type I collagen fibers causing severe OI

Figure 8. Different mutational mechanisms in osteogenesis imperfecta (OI).

Figure 9. X-ray findings in a baby with type II osteogenesis imperfecta.

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In theory, the diagnosis of type I OI could be established by demonstrating reduced synthesis of procollagen I by dermal fibroblasts, but in practice this is rarely available. Similarly, collagen gene mutation analysis is not readily available, since it would be a very expensive exercise because collagen type I genes are very large and each family harbors a unique mutation. Thus in almost all instances the diagnosis is based on clinical and radiologic features. Counseling is as for autosomal dominant inheritance. For type II OI the observed recurrence risk in siblings is around 5%–6%, reflecting the fact that many apparently new mutations stem from parental germ-line mosaicism.

Pseudoachondroplasia (also known as: pseudoachondroplastic spondyloepiphyseal dysplasia) MIM

177170

Clinical features

Both trunk and limb length are reduced, sometimes severely. Older children develop lordosis/scoliosis with a waddling gait. The lower limbs can show genu valgum or genu varum. Ligamentous laxity is particularly severe in the hands with short soft hypermobile fingers. Adult height is between 80 and 130 cm. Early osteoarthritis is a frequent complication and hip replacement is required in approximately 33% of affected individuals by the age of 33 years. The tubular bones show shortening with irregular expanded metaphyses and small fragmented epiphyses. The vertebral bodies are flattened with anterior tonguing.

Age of onset

Usually presents in the second year of life with short stature.

Epidemiology

Pseudoachondroplasia is rare. No precise incidence figures are available.

Inheritance

Autosomal dominant. A very rare autosomal recessive form may exist.

Chromosomal location

19p13.1

Gene

COMP (cartilage oligomeric matrix protein)

Mutational spectrum

Point mutations and deletions with a dominant-negative effect. Also expansion of a short GAC repeat.

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Molecular pathogenesis

COMP is an extracellular calcium binding protein involved in chondrocyte migration and proliferation. It is expressed at high levels in chondrocytes in developing bone and tendon. Reported mutations occur in one of the calcium binding domains. These have an adverse effect, as calcium binding to COMP is a cooperative process involving all of the seven calcium binding regions. COMP is a pentamer, so incorporation of a single mutant chain can disrupt protein function (hence the dominant-negative effect). In pseudoachondroplasia, chondrocytes show abnormal inclusions in the rough endoplasmic reticulum. These probably represent proteoglycan accumulation resulting from defective calcium-dependent proteoglycan binding. Mutations in COMP result in a phenotypic spectrum varying from pseudoachondroplasia (at the severe end) to various forms of multiple epiphyseal dysplasia (at the mild end).

Genetic diagnosis and counseling

Limited mutation detection is available in a few specialist laboratories. Counseling is as for autosomal dominant inheritance. One case of possible autosomal recessive inheritance has been shown to be due to dominant transmission of a parental mosaic germline mutation.

Stickler Syndrome (also known as: hereditary arthro-ophthalmopathy. Includes Marshall syndrome and Weissenbacher–Zweymuller syndrome) MIM

See Table 6.

Clinical features

The different forms of Stickler syndrome share a common phenotype which differs mainly in the degree of ocular involvement. Patients typically have a flat facial profile with depressed nasal bridge, anteverted nares, and micrognathia. Approximately 25% have a cleft palate which together with severe micrognathia can contribute to a diagnosis of Pierre Robin anomaly. In childhood there may be generalized joint hypermobility whereas a degenerative arthropathy often develops in middle age. Mitral valve prolapse has been reported in some studies but not in others. Early onset hearing loss of mixed conductive and sensorineural origin occurs in around 40% of cases and is reported as being more common in Stickler syndrome type II than in type I. Most patients with Stickler syndrome show congenital onset nonprogressive severe high myopia with a high risk of subsequent retinal

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Type

MIM

Chromosomal location

Gene

Mutational spectrum

Type I

108300

12q13.11–q13.2

COL2A1 (collagen, type II, α1)

Nonsense point mutations introducing a premature stop codon with a probable loss of function (“null allele”) effect

Type II

604841

1p21

COL11A1 (collagen, type XI, α1)

Point mutations affecting splicing consensus sequences of 54-bp exons or deletions causing loss of 54-bp exons in COL11A1

Type III

184840

6p21.3

COL11A2 (collagen, type XI, α2)

Missense and splice site point mutations. Also a 27-bp deletion. All with a probable dominantnegative effect

Table 6. Stickler syndrome: types, MIM numbers, chromosomal locations, genes, and mutational spectra.

detachment. Cataracts can also be present and may be congenital. In type I (also known as the membranous vitreous type) vestigial vitreous gel occupies the immediate retrolental space surrounded by a distinct folded membrane. In type II (also known as the beaded vitreous type), there are irregularly thickened bundles of fibers sparsely distributed throughout the vitreous cavity. There is no ocular involvement in type III. Age of onset

The facial features and myopia are present from birth.

Epidemiology

Stickler syndrome is relatively common, although many cases are undiagnosed.

Inheritance

Autosomal dominant

Chromosome See Table 6. location, gene, and mutational spectrum Molecular pathogenesis

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Type I Patients with this type of Stickler syndrome fall at the mild end of the phenotypic spectrum caused by mutations in COL2A1 (see Table 5, p.122). Type II collagen is present in hyaline cartilage, in the ocular vitreous, and in the nucleus pulposus of the intervertebral discs. The nature of the identified mutations, which almost invariably result in the generation of a stop codon and a severely truncated type II collagen chain, indicates that this phenotype results from a quantitative defect in type II collagen synthesis in contrast to the other skeletal disorders associated with mutations in COL2A1 (see Table 5, p.122). Stickler Syndrome

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Type II Type XI collagen is a heterotrimer made up of three distinct chains: α1(XI), α2(XI), and α3(XI). The α1(XI) and α2(XI) chains are encoded by COL11A1 and COL11A2, respectively, whereas the α3(XI) chain is a posttranslationally modified variant of the COL2A1 gene product. In vivo, type XI collagen associates with type II collagen. Thus it is predictable that mutations involving either type II or type XI collagen result in a similar phenotype. Almost all of the mutations identified in COL11A1 result in deletion of 54-bp exons in the major triple-helical domain. This is consistent with a dominant-negative effect resulting from integration of shortened chains into the type XI collagen helical heterotrimer. Similar mutations have been identified in patients with Marshall syndrome, a condition known to show clinical overlap with Stickler syndrome but with more pronounced facial features, shorter stature, and a lower incidence of retinal detachment. Studies at the molecular level have confirmed the clinical suspicion that Marshall syndrome and Stickler syndrome represent slightly different manifestations of a single syndrome. Type III This non-ocular form of Stickler syndrome is caused by loss of function mutations in COL11A2, which encodes the α2(XI) chain of type XI collagen. Unlike COL11A1, COL11A2 is not expressed in the vitreous, which accounts for the absence of ocular involvement. Mutations in COL11A1 have also been identified in patients with Weissenbacher– Zweymuller syndrome which is characterized at birth by the Pierre Robin anomaly, nasal hypoplasia, short “dumb-bell” shaped humeri and femora, and coronal vertebral clefting. Affected children go on to develop deafness and large epiphyses (giving rise to the term otospondylomegaepiphyseal dysplasia). Thus, as with type II Stickler syndrome, molecular studies have provided support for nosologic “lumping” rather than splitting of these overlapping clinical entities. Genetic diagnosis and counseling

Specific mutation analysis is not routinely available. The diagnosis is based on clinical, ocular, and radiologic findings and can be very difficult in mild cases. Counseling is on the basis of autosomal dominant inheritance with very variable expression.

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7 7. Cardio-respiratory Disorders

Barth Syndrome 130 Cystic Fibrosis 131 DiGeorge/Shprintzen Syndrome 133 Holt–Oram Syndrome 135 Laterality Defects 137 Noonan Syndrome 138 Primary Ciliary Dyskinesia 139 Williams Syndrome 141

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Barth Syndrome α-methylglutaconic aciduria II) (also known as: 3α MIM

302060

Clinical features

Dilated cardiomyopathy, skeletal myopathy, neutropenia, short stature, and abnormal mitochondria with tightly packed cristae and inclusion bodies. Death often occurs in infancy or early childhood as a result of heart failure and/or sepsis associated with agranulocytosis. The heart may show features of endocardial fibroelastosis and there may be increased levels of 3α-methylglutaconic acid in urine.

Age of onset

Either congenital or in early infancy

Epidemiology

The condition appears to be rare, but may be under diagnosed.

Inheritance

X-linked recessive

Chromosomal location

Xq28

Gene

G4.5 or TAZ (tafazzin)

Mutational spectrum

Heterogeneous with missense, nonsense, and splice-site mutations plus deletion/insertion frame-shifts.

Molecular pathogenesis

G4.5 contains 11 exons and produces several different mRNA molecules due to alternative splicing of exons 5–7. Variable hydrophobic and hydrophilic regions exist in the protein at the N-terminal and central regions, respectively. The precise role of the protein, named tafazzin (after an Italian television personality), is unknown although a possible homology to acyltransferases has been suggested. No clear genotype–phenotype correlation has emerged.

Genetic diagnosis and counseling

Mutation analysis is available on a limited basis. This should be considered in families showing X-linked recessive inheritance of isolated dilated cardiomyopathy as this can also be due to mutations in G4.5. Counseling is as for X-linked recessive inheritance.

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Cystic Fibrosis (also known as: CF; mucoviscidosis) MIM

219700

Clinical features

Classic CF is characterized by recurrent pulmonary and/or gastrointestinal disease commencing in infancy or early childhood. Pulmonary involvement manifests as recurrent infection and inflammation leading to chronic bronchitis, bronchiectasis, fibrosis, and respiratory failure, and culminating eventually in cor pulmonale and death. Gastrointestinal problems can include meconium ileus (10%–15%), malabsorption due to pancreatic insufficiency (90%), recurrent distal intestinal obstruction (20%), and rectal prolapse (20%). Other complications include insulin-dependent diabetes mellitus (5%), cirrhosis (2%–5%), gallstones (10%), and male infertility (96%). Life expectancy has improved dramatically from 5 years in 1955 to around 30 years at present. In nonclassic CF, the features are much less severe with the mildest presentation being that of congenital bilateral absence of the vas deferens (CBAVD, MIM 277180) in otherwise healthy adult males.

Age of onset

Gastrointestinal involvement may present in the second trimester as increased bowel echogenicity on ultrasound or as meconium ileus soon after birth. Sixty percent of all cases are diagnosed by the age of 1 year.

Epidemiology

The incidence of CF shows marked variation amongst different races ranging from 1 in 2,500–3,000 in individuals of European origin to 1 in 15,000 in African Americans.

Inheritance

Autosomal recessive

Chromosomal location

7q31.2

Gene

CFTR (cystic fibrosis transmembrane conductance regulator)

Mutational spectrum

Over 1,000 mutations have been reported to the CF mutational database (www.genet.sickkids.on.ca). These include missense (40%), nonsense (20%), and splice-site (10%) point mutations, as well as frame-shift deletions and insertions, promoter deletions, and intronic point mutations (which activate cryptic splice donor sites). One specific

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mutation, ∆F508 (deletion of phenylalanine residue at position 508), accounts for 60%–80% of all CF mutant alleles in Europe. Molecular pathogenesis

CFTR encodes a 1,480-amino-acid protein known as the CF transmembrane conductance regulator (CFTR). This contains two transmembrane domains (which anchor it to the cell membrane), two nucleotide-binding folds (NBF) (which interact with ATP), and a single regulatory (R) domain (which is phosphorylated by protein kinase A). CFTR functions as a chloride channel which is activated by phosphorylation of the R domain and ATP interaction with the NBF domains. Activation also results in opening of adjacent outwardly rectifying chloride channels (ORCC) and closure of epithelial sodium channels. Defective CFTR activity in epithelial cells lining the airways of the lungs results in low volumes of airway-surface liquid with increased viscosity of pulmonary secretions. Mutations in CFTR can be classified either on the basis of their effect on CFTR function or on their phenotypic outcome. Five classes of functional mutation are recognized. These are: 1) reduced synthesis (eg, nonsense, frame-shift, and splice-site mutations resulting in reduced mRNA) 2) defective maturation (eg, missense mutations and ∆F508). These mutations prevent normal processing of CFTR to the cell membrane 3) abnormal activation (eg, missense mutations, such as Gly551Asp, involving an ATP-binding domain) 4) altered conductance (eg, missense mutations, such as Arg117His, involving the CFTR chloride channel) 5) defective regulation (eg, missense mutations which impair regulation of the ORCC and epithelial sodium channel) Phenotypically, CFTR mutations are classified on the basis of whether they cause classic CF with or without pancreatic insufficiency, or much milder nonclassic forms of CF such as CBAVD. Homozygosity for ∆F508 results in classic CF with chronic lung disease and pancreatic insufficiency. Most nonsense mutations also result in classic CF. Some missense mutations, such as Arg117His, result in a milder phenotype with pancreatic sufficiency. Isolated CBAVD is associated with “mild” mutations such as Arg117His (which allows partial CFTR function).

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The complexity of the genotype–phenotype relationship is illustrated by the observation that when Arg117His is in cis (ie, in the same allele) with a 5T polythymidine polymorphism in intron 8 (which influences the splicing efficiency of exon 9) it causes severe CF when another CF mutation is present on the other allele. However, when Arg117His is in cis with a 7T polymorphism it causes only CBAVD in combination with another CF allele. The difficulty of predicting phenotype from genotype is increased by the demonstration of at least one CF modifier locus (CFM1, MIM 603855) on chromosome 19. Genetic diagnosis and counseling

CFTR analysis for common mutations is widely available and usually involves testing for ∆F508 and up to 30 other known mutations, which account for 85%–90% of all CF alleles in a particular population. Rare mutations can be investigated at specific reference laboratories. Counseling is as for autosomal recessive inheritance with carrier frequencies in Caucasian populations of 1 in 20–30 depending on the disease incidence. Carrier detection by “cascade screening” is offered in families with an affected child. Prenatal diagnosis can be offered either directly by mutation analysis or indirectly by linkage analysis using intragenic polymorphisms in informative families.

DiGeorge/Shprintzen Syndrome (includes: CATCH 22; conotruncal anomaly face syndrome; DiGeorge syndrome; Shprintzen syndrome; velocardiofacial syndrome [VCFS]) MIM

188400

Clinical features

These are extremely variable and embrace a wide spectrum of clinical involvement, ranging from the classical severe DiGeorge syndrome to the much milder Shprintzen/VCFS phenotype. The DiGeorge syndrome is characterized by thymic aplasia or hypoplasia, hypoparathyroidism, and cardiac malformations (notably interrupted aortic arch with a ventricular septal defect and persistent truncus arteriosus). Central nervous system abnormalities occur in one third of cases and many affected children die in early infancy as a consequence of the severe cardiac defect. In the much milder Shprintzen syndrome/VCFS, affected children usually have a characteristic facies (long face with short palpebral fissures and a broad bulbous nasal tip) with short stature, cleft palate (possibly submucous),

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and a cardiac anomaly (most commonly Fallot’s tetralogy or a ventricular septal defect). Average IQ is approximately 80 and many children have particular problems with language and speech. Some also manifest behavioral problems and affected adults show an increased incidence of schizophrenia and bipolar depression. Age of onset

The abnormalities are present at birth. Presentation can be in infancy with hypocalcemia, failure to thrive, and cardiac failure or with learning difficulties and velopharyngeal incompetence (in later childhood).

Epidemiology

All populations are affected with an estimated incidence of 1 in 4,000.

Inheritance

Autosomal dominant/chromosomal microdeletion

Chromosomal location

22q11

Gene

TBX1 (T-BOX1) (although this is proven in mice, it has not been proven in humans; this is discussed below)

Mutational spectrum

Unknown

Molecular pathogenesis

Approximately 90% of all cases of the DiGeorge/Shprintzen syndrome have a microdeletion involving the proximal region of the long arm of one chromosome 22. This is readily identifiable using fluorescence in situ hybridization (FISH) (see Figure 1). In 90% of these cases the deletion is 3 Mb in size and encompasses an estimated 30 genes. In the remaining 10%, the deletion is 1.5 Mb in size and contains 24 genes. The 22q11 region contains eight low-copy repeat sequences which flank the deletions and probably account for their generation through unequal crossing over. Despite extensive research, no specific gene has been found to account for the associated clinical abnormalities. In mice, however, it has been established that haploinsufficiency for Tbx1 (the human homolog of which is located in the 22q11 deletion region) contributes to or causes the cardiovascular abnormalities. Understanding of the genotype–phenotype relationship is complicated by reports of the DiGeorge/Shprintzen phenotype in individuals with non-overlapping deletions, implying either that no specific gene is causal or that the

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Figure 1. Chromosome 22 microdeletion in a child with DiGeorge/Shprintzen syndrome demonstrated by fluorescence in situ hybridization. Image courtesy of Mrs Karen Marshall, Cytogenetics Laboratory, Leicester Royal Infirmary, UK.

various deletions have long-range negative effects on the expression of neighboring genes. TBX1 is a particularly attractive candidate for the causal gene as it is a putative transcription regulatory gene and defects in the closely related TBX5 account for the Holt–Oram syndrome in which cardiac defects are common (see next entry). Genetic diagnosis and counseling

Detection of a microdeletion on chromosome 22q11 is widely available by FISH. More specific molecular analysis is only undertaken on a research basis. De novo mutations account for 80%–90% of cases and convey a low (<1%) recurrence risk for siblings. The parents of all cases should be offered microdeletion analysis since 10%–20% of cases will have an “asymptomatic” deletion carrier parent. Intrafamilial variation can be marked and can extend to affected monozygotic twins who have been reported to show discordant phenotypes.

Holt–Oram Syndrome (also known as: HOS; heart–hand syndrome) MIM

142900

Clinical features

These involve primarily the upper limbs and the heart. Findings in the upper limbs are variable, usually asymmetrical, and most commonly present as absence or hypoplasia of the thumbs and radii. Other findings can vary from minimal involvement (such as clinodactyly or limitation of supination) to hypoplasia of the ulnae and humeri, or even phocomelia. Cardiac defects occur in over 90% of cases with atrial septal defect (35%) and ventricular septal defect (25%) being the most common abnormalities.

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Age of onset

The limb and heart abnormalities are present at birth and in severe cases can be detected on ultrasound during the second trimester.

Epidemiology

HOS is rare, with an estimated incidence of approximately 1 in 100,000.

Inheritance

Autosomal dominant

Chromosomal location

12q24.1

Gene

TBX5 (T-BOX 5)

Mutational spectrum

Missense, nonsense, and frame-shift deletion mutations all result in haploinsufficiency.

Molecular pathogenesis

TBX5 is one of a series of developmental regulatory genes which share a common conserved motif known as the T-BOX. T-BOX genes act as transcription factors through binding of the T-BOX domain to DNA. TBX5 is expressed not only in embryonic forelimbs and heart, but also in the lungs, pharynx, and retina, which are not involved in the HOS phenotype. Expression studies have shown that TBX5 associates with other regulatory genes to promote cardiomyocyte differentiation. Truncating mutations tend to result in severe heart and limb malformations, whereas missense mutations result in severe heart or limb abnormalities with only mild involvement of the other system. Correlation of the phenotype associated with specific mutations indicates that organ-specific gene activation by TBX5 is determined by binding to different target DNA sequences.

Genetic diagnosis and counseling

TBX5 mutation analysis is undertaken at a few specialized laboratories. Mutations are identified in only ~50% of familial cases. Counseling is as for autosomal dominant inheritance with close to complete penetrance, but marked intrafamilial variability in expression.

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Laterality Defects (also known as: asplenia with cardiovascular anomalies; heterotaxy; isomerism; Ivemark syndrome; polysplenia syndrome; situs ambiguus) MIM

208530, 306955, 601086

Clinical features

Situs solitus refers to normal orientation of the heart and abdominal organs. Complete reversal of normal lateralization is referred to as situs inversus, as seen in Kartagener syndrome (see p.139–41). Any other disturbance of lateralization is known as situs ambiguus or heterotaxy. Isomerism is a form of situs ambiguus in which organs such as the lungs and heart, which normally have distinguishable right and left forms, develop so that their left and right sides are mirror images of one other. Thus both lungs may be trilobed in right isomerism or bilobed in left isomerism. Asplenia and polysplenia occur in right and left isomerism, respectively. Severe cardiac abnormalities are common in both forms of isomerism, notably atrioventricular septal defect, single ventricle, double outlet right ventricle, and pulmonary stenosis in right isomerism; and atrioventricular septal defect, interruption of the inferior vena cava, and pulmonary stenosis/atresia in left isomerism. Intestinal malrotation and urogenital defects can also occur.

Age of onset

The malformations are present at birth and can often be detected in pregnancy by ultrasonography.

Epidemiology

The incidence has been estimated at around 1 in 24,000.

Inheritance

Most cases are sporadic. Families showing autosomal dominant, autosomal recessive, and X-linked inheritance have been reported.

Chromosomal location

6p21 (autosomal-dominant form), unknown (autosomal-recessive form), and Xq26.2 (X-linked form), respectively.

Gene

Unknown (autosomal-dominant and autosomal-recessive forms); ZIC3 (zinc finger protein of cerebellum, 3) (X-linked form).

Mutational spectrum

ZIC3: missense, nonsense, and frame-shift mutations with a loss of function effect.

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Molecular pathogenesis

ZIC3 encodes a zinc finger transcription regulator which is thought to be expressed at an early stage in the determination of left–right asymmetry in the human embryo. Mutations in ZIC3 have been identified in a small number of families in which hemizygous males showed situs ambiguus with variable cardiac, splenic, and gastrointestinal abnormalities. Heterozygous females in one of these families showed situs inversus. Females in the other families were normal. Affected males in these families also showed an increased incidence of midline defects such as cerebellar hypoplasia and sacral agenesis. The explanation for these observations is not known.

Genetic diagnosis and counseling

Mutation analysis is not available. Most cases are sporadic with an empiric recurrence risk for siblings of around 5%.

Noonan Syndrome MIM

163950

Clinical features

Typically, these consist of mild short stature, neck webbing, congenital heart disease, characteristic facies, and undescended testes in boys. The most common cardiac anomalies are pulmonary stenosis, atrial and ventricular septal defects, patent ductus arteriosus, and hypertrophic cardiomyopathy. Facial features include hypertelorism, downward sloping palpebral fissures, ptosis, and low-set posteriorly rotated ears. Approximately one third of all cases have mild learning difficulties and around 50% have a bleeding diathesis with prolonged activated partial thromboplastin time. Other features can include hydrops and polyhydramnios in pregnancy, and major feeding difficulties in early childhood.

Age of onset

The clinical features are often apparent at birth although the diagnosis is usually not made until mid-childhood.

Epidemiology

The incidence has been estimated to be between 1 in 1,000–2,500.

Inheritance

Autosomal dominant

Chromosomal location

12q24.1

Gene

PTPN11 (protein-tyrosine phosphatase, nonreceptor-type, 11)

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Mutational spectrum

Missense mutations with a gain of function effect. Mutations in exons 3 and 7 account for approximately 80% of all identified mutations.

Molecular pathogenesis

PTPN11 encodes a nonreceptor type protein tyrosine phosphatase known as SHP-2, which is ubiquitously expressed and is involved in mesodermal patterning, limb development, and hematopoietic cell differentiation. SHP-2 plays a key role in cell signaling, thereby mediating the cellular response to growth factors, hormones, and cytokines. Activation of SHP-2 results from interaction of an aminoterminal switch domain with a tyrosine phosphatase domain resulting in modulation of phosphatase activity. Almost all identified mutations involve either the switch or the tyrosine phosphatase domain and result in stabilization of phosphatase activity by a gain of function effect. One particular mutation, Asn308Asp, accounts for approximately one third of all reported cases. It is not clear exactly how increased tyrosine phosphatase activity results in the clinical phenotype, although SHP-2 is known to be expressed in the developing pulmonary and aortic valves. This probably accounts for the observation that pulmonary stenosis is more common in cases caused by PTPN11 mutations than in cases from families which are not linked to this locus.

Genetic diagnosis and counseling

Mutation analysis for PTPN11 is only available on a limited basis. Only 50% of families are linked to this locus. Genetic counseling is as for autosomal dominant inheritance with variable expression and almost complete penetrance. Note that features of Noonan syndrome have been observed in a few individuals with neurofibromatosis type I. In some of these individuals, rearrangements have been identified in NF1 (see p.96–8).

Primary Ciliary Dyskinesia (also known as: PCD; immotile cilia syndrome. Includes Kartagener syndrome.) MIM

242650 (primary ciliary dyskinesia) 244400 (Kartagener syndrome)

Clinical features

PCD is a clinically and genetically heterogeneous group of disorders characterized by ciliary dysfunction leading to chronic bronchiectasis, otitis, and sinusitis. Most affected males are infertile due to sperm immotility. Approximately 50% of patients show situs inversus

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Chromosomal location

Gene

Mutational spectrum

5p15–p14

DNAH5 (dynein, axonemal, heavy chain 5)

Missense, nonsense, splice-site, and frame-shift mutations

9p21–p13

DNAI1 (dynein, axonemal, intermediate chain 1)

Missense mutations, insertions, and deletions. A common 1-bp splice-site insertion mutation in intron 1 has been reported

Table 1. Primary ciliary dyskinesia: chromosomal locations, genes, and mutational spectra.

(ie, complete reversal of normal left–right asymmetry, with dextrocardia). The combination of PCD with situs inversus constitutes the condition known as Kartagener syndrome. Age of onset

Presentation is usually in early childhood with recurrent respiratory infection.

Epidemiology

The estimated incidence worldwide is approximately 1 in 16,000. An increased incidence has been noted in Polynesians in New Zealand and Samoa.

Inheritance

Autosomal recessive

Chromosomal See Table 1. location, gene, and mutational spectrum Molecular pathogenesis

The basic defect in PCD lies in abnormal cilia motility with loss of the dynein arms being the most common finding on electron microscopy. Dyneins consist of a large family of proteins involved in microtubuledependent cell motility. Inner and outer dynein arms are bound to each microtubule doublet of the ciliary axoneme where they generate motility through ATP-dependent cycles of attachment and detachment. Dynein arms are composed of a mixture of heavy, intermediate, and light chains each encoded by a different gene. Thus PCD can be expected to show marked locus heterogeneity. To date, mutations have been identified in two dynein assembly genes, DNAH5 and DNAI1 (encoding heavy chain 5 and intermediate chain 1, respectively). These mutations are predicted to result in truncated nonfunctional proteins which prevent normal dynein arm formation resulting in immotile cilia and subsequent chronic respiratory disease.

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Specific mutation analysis is not generally available. Counseling is on the basis of autosomal recessive inheritance with recurrence risks of 1 in 4 for PCD and 1 in 8 for Kartagener syndrome. This risk of 1 in 8 is a consequence of the randomization of left–right axis asymmetry which ensues from abnormal cilia motility.

Williams Syndrome (also known as: WS; infantile hypercalcemia; Williams–Beuren syndrome) MIM

194050

Clinical features

These consist primarily of a characteristic facial appearance, mild mental retardation with an engaging extrovert (“cocktail party”) personality, cardiac anomalies, and variable renal involvement.The facial features consist of blue stellate irides, anteverted nostrils, long philtrum, full lips, and wide mouth. Supravalvular aortic stenosis (SVAS) and supravalvular pulmonary stenosis occur in around 65% and 25% of cases, respectively. Renal involvement can include renal artery stenosis, nephrocalcinosis secondary to hypercalcemia, aplasia, hypoplasia, and duplication. Average IQ is approximately 50 with relatively good language and verbal skills in contrast to poor mathematical abilities.

Age of onset

The clinical features are apparent at birth.

Epidemiology

The estimated incidence is 1 in 10,000–20,000.

Inheritance

Chromosomal microdeletion.

Chromosomal location

7q11.2

Gene

ELN (elastin) and up to 16 other genes.

Mutational spectrum

Microdeletion in 95% of cases. A broad spectrum of mutations (including missense, nonsense, frame-shift, and splice-site) have been identified in ELN in isolated nonsyndromal SVAS.

Molecular pathogenesis

The common microdeletion in WS patients is approximately 1.5 Mb and contains at least 17 genes. This microdeletion is generated by unequal meiotic recombination mediated by regions of flanking homologous DNA. Smaller microdeletions have been noted in a few patients. An inversion

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involving the WS region has been noted in some nondeleted patients and curiously this inversion has also been found in asymptomatic parents of some children with a deletion, implying that it may predispose to microdeletion formation. Mosaicism for the deletion has been observed in a small proportion of children. In some WS patients no molecular abnormality can be identified. There is no evidence for a significant parent-of-origin effect. Haploinsufficiency for elastin (encoded by ELN within the deleted region) accounts for the SVAS seen in WS patients and for nonsyndromal SVAS (MIM 185500). Insoluble elastin is believed to be an important regulator of cellular proliferation in arterial smooth muscle. Hence reduced levels of elastin lead to increased proliferation of arterial smooth muscle cells. Hemizygosity for LIMK1 (lim domain kinase 1) is thought to contribute to impaired visuospatial cognition in WS children. Other genotype–phenotype correlations have not been established. Genetic diagnosis and counseling

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The common microdeletion can be readily identified using fluorescence in situ hybridization. Molecular (cytogenetic) diagnosis in nondeletion cases is only undertaken on a research basis. The recurrence risk for a de novo microdeletion is low (<1%).

Williams Syndrome

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8 8. Craniofacial Disorders

Apert Syndrome 144 Crouzon Syndrome 146 Greig Syndrome 148 Pfeiffer Syndrome 149 Rubinstein–Taybi Syndrome 151 Saethre–Chotzen Syndrome 152 Sotos Syndrome 153 Treacher Collins Syndrome 154 Van der Woude Syndrome 155

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Apert Syndrome (also known as: acrocephalosyndactyly type I) MIM

101200

Clinical features

These comprise a combination of acrocephaly and brachycephaly due to multiple suture synostosis, particularly involving the coronal sutures, together with osseous and/or cutaneous syndactyly involving the second to fifth fingers and all toes (see Figure 1). The facies is usually flat with a high broad forehead, shallow orbits, and hypertelorism. Approximately 50% of cases show mild to moderate mental retardation possibly because of associated cerebral malformations. Some studies suggest that early craniectomy improves the intellectual outcome but this remains uncertain.

Age of onset

The features are obvious at birth and can be suspected on ultrasound examination in the second trimester.

Epidemiology

The incidence at birth varies between 1 in 50,000–100,000 in different populations.

Inheritance

Autosomal dominant

Chromosomal location

10q26

Gene

FGFR2 (fibroblast growth factor receptor 2)

Mutational spectrum

Two specific missense substitutions, which have a gain of function effect.

Figure 1. Appearance of a hand and foot in a child with Apert syndrome.

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Almost all cases of Apert syndrome are caused by one of two missense point mutations, Ser252Phe (934 C→G) or Pro253Arg (937 C→G) in FGFR2. This encodes a membrane-bound receptor with an extracellular region containing three immunoglobulin (Ig)-like domains, a single membrane-spanning domain and an intracellular split tyrosine kinase domain (see Figure 2). On binding with the specific FGF ligand (22 distinct FGFs have been identified), the receptor molecules dimerize with subsequent phosphorylation and activation of the tyrosine residues in the intracellular domain. An alternative splicing event generates two forms of the IgIII domain with different binding characteristics. The alternative forms consist of IgIIIa with either IgIIIb or IgIIIc. IgIIIa/IIIb is known as FGFR2b and encodes a keratinocyte growth factor receptor which is expressed in epithelium, whereas IgIIIa/IIIc, also known as FGFR2c, encodes a receptor known as BEK (bacterially expressed kinase) which is expressed in mesenchyme. Thus FGFR2 expression accounts for two distinct receptor activities which helps explain the diverse phenotypic features seen in affected children – see below.

IgIII

IgII

TM

TK1

TK2

FGFR1 P Pro252Arg

FGFR2 C C+P C A Ser252Phe Pro253Arg

FGFR3 M Pro250Arg

C* Ala391Glu

Figure 2. Diagrammatic representation of fibroblast growth factor receptors (FGFRs) with arrows showing locations of the common craniosynostosis syndrome mutations. A: Apert syndrome; C: Crouzon syndrome; C*: Crouzon syndrome with acanthosis nigricans; IgI, IgII, and IgIII: immunoglobulin like-domains; M: Muenke syndrome (isolated coronal synostosis); P: Pfeiffer syndrome; TK1 and TK2: tyrosine kinase domains; TM: transmembrane domains.

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In Apert syndrome, the two common mutations occur in the ligand-binding linker region between the second and third immunoglobulin-like domains, resulting in loss of ligand specificity with ectopic ligand-dependent activation. The Pro253Arg mutation is thought to be associated with more severe syndactyly because of inappropriate FGF7/10 binding by FGFR2c in epithelium, whereas the Ser252Phe mutation is associated with a higher incidence of cleft palate because of enhanced FGF2/FGFR2c signaling in mesenchyme. Both of these mutations show a positive association with advanced paternal age and occur almost exclusively in spermatogenesis as opposed to oogenesis. Genetic diagnosis and counseling

Analysis for the two common recurring mutations is readily available. Most cases result from new dominant mutations. The risk to each offspring of an affected individual is 1 in 2.

Crouzon Syndrome (also known as: craniofacial dysostosis) MIM

123500

Clinical features

These consist of variable craniosynostosis, maxillary hypoplasia, shallow orbits, and proptosis. Craniosynostosis usually develops in infancy and can involve any or all of the coronal, sagittal, and lambdoid sutures. Inconsistent features include optic atrophy, cleft lip/palate, iris coloboma, and acanthosis nigricans. Progressive hydrocephalus and conductive hearing loss are potential complications. Intelligence is usually normal and involvement of the extremities is minimal.

Age of onset

The facial features are usually apparent by the age of 1 year.

Epidemiology

The birth prevalence is estimated to be approximately 1 in 60,000. All races are affected.

Inheritance

Autosomal dominant. Approximately one third to one half of all cases result from new mutations.

Chromosomal See Table 1. location, gene, and mutational spectrum

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Chromosomal location

Gene

Mutational spectrum

10q26

FGFR2 (fibroblast growth factor receptor 2)

Mainly missense, also splice-site mutations

4p16.3

FGFR3 (fibroblast growth factor receptor 3)

Missense mutation

Table 1. Crouzon syndrome: chromosomal locations, genes, and mutational spectra.

Molecular pathogenesis

Most cases of Crouzon syndrome are caused by missense mutations in exons IIIa or IIIc of FGFR2, which encodes the third extracellular immunoglobulin domain of the membrane spanning FGFR2 tyrosine kinase receptor (see p.145). New mutations have been shown to arise almost exclusively in the paternal germ line. FGFR2 encodes two alternative products, keratinocyte growth factor receptor (KGFR) and BEK (bacterially expressed kinase), which have different ligand-binding characteristics and different patterns of expression. KGFR is involved in skin development whereas BEK is active in osteogenesis, with BEK transcripts concentrated in the bones of the skull including the ossicles in the middle ear. The gain of function mutations in the BEK form of FGFR2 that cause Crouzon syndrome result in cross-linking of unpaired cysteine residues leading to covalent dimerization and activation of receptor subunits. Note that mutations in FGFR2 also cause Apert syndrome (see previous entry) and many cases of Pfeiffer syndrome (see p.149–50). They also account for other much rarer craniosynostosis syndromes, including the Antley–Bixler syndrome (MIM 207410 – main clinical features: craniosynostosis, midface hypoplasia, humeroradial synostosis, bowed femora), the Beare–Stevenson cutis gyrata syndrome (MIM 123790 – main clinical features: craniosynostosis, cutis gyrata, acanthosis nigricans, anogenital anomalies) and the Jackson–Weiss syndrome (MIM 123150 – main clinical features: craniosynostosis with variable foot abnormalities). Rather than discrete pathologic entities, these disorders are now viewed as representing overlapping and somewhat variable clinical phenotypes resulting from the effects of particular mutations on ligand-binding, splice-form expression, and FGFR2b/2c receptor activation (see p.145). The occurrence of Crouzon syndrome with acanthosis nigricans is caused by a specific Ala391Glu (1172 C→A) missense mutation in the transmembrane domain of FGFR3. This mutation occurs very close to

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the site of the recurrent mutation which accounts for most cases of achondroplasia (nucleotide 1138 – see p.108–10). Genetic diagnosis and counseling

Screening for the common mutations is undertaken on a limited service basis. Counseling is as for autosomal dominant inheritance with variable expression.

Greig Syndrome (also known as: Greig cephalopolysyndactyly syndrome) MIM

175700

Clinical features

The head shows frontal bossing with a broad forehead, hypertelorism, and occasional craniosynostosis (craniosynostosis is seen in 5% of cases). The thumbs and big toes are broad with postaxial polydactyly in the hands and preaxial polydactyly or polysyndactyly in the feet (see Figure 3). Variable soft tissue syndactyly occurs in the fingers and toes. Intelligence is normal and there are few significant medical problems.

Figure 3. The feet of a child with Greig syndrome.

Age of onset

The features are apparent at birth.

Epidemiology

The condition is rare. Precise incidence figures are not available.

Inheritance

Autosomal dominant

Chromosomal location

7p13

Gene

GLI3 (glioma-associated oncogene 3 or GLI–Kruppel family member 3)

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Mutational spectrum

Missense, nonsense, and frame-shift mutations; all have a loss of function (haploinsufficiency) effect.

Molecular pathogenesis

GLI3 encodes a transcription factor with a central DNA-binding domain composed of five zinc finger motifs together with transcription and repression domains. In Drosophila, and probably also in humans, the GLI3 homolog (known as cubitus interruptus) is actively involved in positive and negative regulation of the Sonic hedgehog developmental pathway in limb development. In humans with Greig syndrome, mutations map not only to the zinc finger domain but also throughout the coding regions of GLI3. Curiously, mutations in GLI3 also account for preaxial polydactyly type IV (MIM 174700), postaxial polydactyly type A1 (MIM 174200), and the Pallister–Hall syndrome (MIM 146510) in which meso-axial or postaxial polydactyly is associated with a hypothalamic hamartoma and an imperforate anus.

Genetic counseling and diagnosis

Mutation analysis of GLI3 is available on a limited research basis. Counseling is as for autosomal dominant inheritance with very variable expression.

Pfeiffer Syndrome (also known as: acrocephalosyndactyly, type V) MIM

101600

Clinical features

These consist of variable craniosynostosis with broad thumbs and big toes in valgus position and mild soft-tissue syndactyly. In severe cases there is a cloverleaf skull with ocular proptosis and hydrocephalus. Mild cases may only show turricephaly and/or brachycephaly with maxillary hypoplasia and down-slanting palpebral fissures. Occasional findings include brachydactyly, radioulnar synostosis, cerebellar herniation, and hearing loss.

Age of onset

The clinical features are usually apparent at birth.

Epidemiology

Pfeiffer syndrome is less common than Apert and Crouzon syndromes. Precise figures are not available.

Inheritance

Autosomal dominant

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Chromosomal See Table 2. location, gene, and mutational spectrum Chromosomal location

Gene

Mutational spectrum

8p11.2–p11.2

FGFR1 (fibroblast growth factor receptor 1)

A single common missense mutation

10q26

FGFR2 (fibroblast growth factor receptor 2)

Mainly missense, also splice-site mutations

Table 2. Pfeiffer syndrome: chromosomal locations, genes, and mutational spectra.

Molecular pathogenesis

Pfeiffer syndrome can be caused by a specific single mutation (involving the linker region between the second and third immunoglobulin loops in FGFR1) or by multiple mutations (involving the same region or the third immunoglobulin domain of FGFR2). The recurrent mutation in FGFR1 involves a C→G transversion resulting in a Pro252Arg substitution. Mutations at the same region in FGFR2 (ie, Ser252Phe and Pro253Arg) result in Apert syndrome (see p.145). An identical mutation in FGFR3 (Pro250Arg) results in nonsyndromal coronal craniosynostosis or the combination of coronal craniosynostosis, with carpal and tarsal synostosis and cone-shaped epiphyses of the phalanges, a condition also known as Muenke syndrome (MIM 602849). The spectrum of mutations in FGFR2 which causes Pfeiffer syndrome is very similar to that which causes Crouzon syndrome and there have been several reports of identical mutations causing both syndromes. These observations support the hypothesis that Pfeiffer and Crouzon syndromes represent overlapping phenotypes. New mutations which cause these syndromes show a paternal age effect and have been demonstrated to arise almost exclusively in the paternal germ line.

Genetic diagnosis and counseling

150

Screening for the common mutations is available on a limited service basis. Counseling is as for autosomal dominant inheritance with full penetrance but very variable expression. The parents of a child with an apparent new mutation should be examined carefully to exclude mild involvement.

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Rubinstein–Taybi Syndrome MIM

180849

Clinical features

Typically, these consist of a characteristic facial appearance with microcephaly, down-slanting palpebral fissures, beaked nose, long nasal septum, broad angulated thumbs, and big toes. Intelligence is impaired, with an average IQ of 50. Other findings and complications include congenital heart defects (33%), patellar dislocation, retinal dystrophy, and keloid scar formation.

Age of onset

The features are usually apparent in infancy or early childhood.

Epidemiology

The estimated incidence at birth is 1 in 275,000–300,000.

Inheritance

Autosomal dominant

Chromosomal location

16p13.3

Gene

CBP or CREBBP (cyclic AMP regulated enhancer binding protein)

Mutational spectrum

Mainly microdeletions and truncating nonsense mutations; these probably have a loss of function effect.

Molecular pathogenesis

CBP encodes a large nuclear protein that is involved in transcription regulation and the integration of several different transduction pathways. Studies in Drosophila indicate that CBP may interact with the GLI3 and TWIST pathways, which would account for the degree of phenotypic overlap seen in the Rubinstein–Taybi, Greig (see p.148–9), and Saethre–Chotzen (see p.152–3) syndromes. CBP probably acts by remodeling the structure of chromatin thereby allowing transcription factors to the nuclear DNA.

Genetic diagnosis and counseling

Mutation analysis is not readily available and even in a research setting mutations can be found in only ~45%–50% of cases. The 10% of cases caused by microdeletions can be detected by fluorescence in situ hybridization. Most cases are sporadic, probably because of a very reduced reproductive capacity. A few examples of affected parent and child have been reported with marked intrafamilial variation. Thus, the parents of apparent isolated cases should be examined carefully.

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Saethre–Chotzen Syndrome (also known as: SCS; acrocephalosyndactyly type III) MIM

101400

Clinical features

SCS is characterized by variable craniosynostosis with subtle facial dysmorphism and digital anomalies. Coronal suture synostosis results in brachycephaly and, if asymmetrical, plagiocephaly. Facial features include asymmetry, a broad sloping forehead, ptosis, and small, low-set ears with a prominent crus. The hands and feet show brachydactyly with syndactyly, most commonly between the second and third fingers and between the second and third toes, and fifth finger clinodactyly. The big toes may be broad and bifid. Intelligence is usually normal or, occasionally, mildly impaired.

Age of onset

Craniosynostosis may be present at birth or develop in early childhood.

Epidemiology

SCS is one of the most common craniosynostosis syndromes with an estimated incidence of 1 in 25,000–50,000 live births.

Inheritance

Autosomal dominant

Chromosomal location

7p21

Gene

TWIST

Mutational spectrum

Missense and nonsense mutations, insertions, and both small and large deletions; they all have a probable loss of function effect.

Molecular pathogenesis

Approximately 50% of all cases have a mutation in, or deletion of, TWIST, which encodes a transcription factor with a basic helix–loop–helix motif consisting of a DNA-binding domain followed by two helices and an intervening loop domain. The gene is so named because of the twisted appearance of the body seen in Drosophila with a recessive lethal mutation. Mutations in SCS patients are evenly distributed amongst the DNA-binding, helix, and loop domains. Large megabase deletions embracing the TWIST locus have been identified in a few patients who have significant learning difficulties. Studies in Drosophila indicate that TWIST regulates expression of the FGFR gene family. This would be consistent with the observation that

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some patients with SCS-like features have been shown to have either a mutation in FGFR2 (located at 10q26) or, more often, the common Pro250Arg mutation in FGFR3 (see p.145). In one study of 32 unrelated SCS patients, 12 were found to have a mutation in TWIST, seven had the Pro250Arg mutation in FGFR3 and one had a 6-bp deletion in FGFR2. No mutation could be identified in the remaining 12 cases. Genetic diagnosis and counseling

Mutation screening for TWIST is undertaken at a small number of specialist laboratories as is detection of the common Pro250Arg FGFR3 mutation. Counseling is on the basis of autosomal dominant inheritance with marked intrafamilial variation.

Sotos Syndrome (also known as: cerebral gigantism) MIM

117550

Clinical features

Overgrowth predates delivery (with mean birth weights of 4.2 kg in boys and 4.0 kg in girls) and is marked for the first 4 years, followed by a gradual fall to the 97th centile during later childhood. Affected children show macrocephaly with frontal bossing, hypertelorism, and a prominent jaw. The primary dentition erupts prematurely and bone age is advanced. Neurodevelopmental findings include hypotonia in infancy, nonprogressive ataxia, and mild developmental delay. Increased susceptibility to develop childhood neoplasm has been reported but the overall risk is thought to be low (<5%).

Age of onset

The clinical features are apparent at birth.

Epidemiology

All ethnic groups are affected and several hundred cases have been reported although no accurate incidence figures are available.

Inheritance

Autosomal dominant

Chromosomal location

5q35

Gene

NSD1 (nuclear receptor SET-domain protein 1)

Mutational spectrum

Missense, nonsense, and frame-shift mutations, there is also a common 2.2-Mb microdeletion.

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Molecular pathogenesis

NSD1 contains 23 exons and encodes a multiple-domain protein that shows homology to a family of proteins which act as coregulators of androgen/steroid receptors. The mechanism by which mutations in NSD1 cause Sotos syndrome is unknown. The fact that mutations identified to date are predicted to result in haploinsufficiency suggests a regulatory role in growth suppression. Mutations in NSD1 also cause another overgrowth disorder known as Weaver syndrome (MIM 277590).

Genetic diagnosis and counseling

Mutation analysis is available only on a very restricted research basis. Thus the diagnosis is made on the clinical features and radiologic evidence of advanced bone age, which may be disharmonic with phalangeal age in advance of carpal age. Counseling is as for autosomal dominant inheritance with very variable expression.

Treacher Collins Syndrome (also known as: mandibulofacial dysostosis; Treacher Collins–Franceschetti syndrome) MIM

154500

Clinical features

These are limited to abnormalities of craniofacial development. Maxillary and mandibular hypoplasia result in down-slanting palpebral fissures with underdevelopment of the lateral third of the lower eyelids, small cheekbones, and microretrognathia. The pinnae are small and/or misplaced in 80% of cases. Bilateral conductive hearing loss and cleft palate each occur in approximately 30% of cases. Intelligence is normal.

Age of onset

The features are evident at birth.

Epidemiology

The incidence is approximately 1 in 50,000 live births. All ethnic groups are affected.

Inheritance

Autosomal dominant

Chromosomal location

5q32–q33.1

Gene

TCOF1 (treacle)

Mutational spectrum

Mainly nonsense and frame-shift deletion/insertion mutations resulting in premature termination and haploinsufficiency.

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Molecular pathogenesis

TCOF1 contains 25 exons and shows peak expression at the edges of the developing neural folds and in the developing branchial arches. The encoded protein, known as treacle, shows homology to a family of nucleolar phosphoproteins with nuclear and nucleolar localization signals. A role as a chaperone in nuclear–cytoplasmic transport through nuclear–nucleolar shuttling is predicted. Specific genotype–phenotype correlations have not been identified.

Genetic diagnosis and counseling

Mutation analysis is available on a limited research basis. Counseling is as for autosomal dominant inheritance with marked inter- and intrafamilial variation.

Van der Woude Syndrome (also known as: lip-pit syndrome) MIM

119500

Clinical features

This is the most common syndromal form of cleft lip/palate. Features consist of cleft lip and/or cleft palate in association with characteristic pits closely adjacent to the midline in the lower lip. The orofacial clefting shows marked variation between and within families so that both cleft lip and cleft palate can occur in affected members of the same kindred. This is in contrast to nonsyndromal cleft lip/palate, which shows polygenic/multifactorial inheritance.

Age of onset

The features are apparent at birth.

Epidemiology

The estimated incidence is 1 in 35,000–100,000.

Inheritance

Autosomal dominant

Chromosomal location

1q32–q41

Gene

IRF6 (interferon regulatory factor 6)

Mutational spectrum

Missense, nonsense, and frame-shift insertions and deletions with either a loss of function or dominant-negative effect.

Molecular pathogenesis

IRF6 is one of a family of nine transcription factors which share conserved DNA-binding and protein-binding domains. In mice,

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it is expressed along the medial edge of the fusing palate, as well as in tooth buds, genitalia, and skin. The precise function of IRF6 in humans is unknown although a role in the transforming growth factor-β signaling pathway is suspected. Van der Woude syndrome is associated with mutations in IRF6 which have a loss of function effect. Truncating mutations are distributed throughout the gene whereas missense mutations occur mainly in one of the conserved DNA-binding or protein-binding domains. Missense mutations that have a dominant-negative effect (by allowing a mutated IRF6 protein to bind to other proteins) result in a different condition, the popliteal pterygium syndrome (MIM 119500). In the popliteal pterygium syndrome, orofacial clefting and lip pits occur in association with webbing of the skin (particularly in the popliteal fossa) and genital abnormalities. Genetic diagnosis and counseling

156

Mutation analysis is available only on a research basis. Counseling is as for autosomal dominant inheritance with slightly reduced penetrance (95%) and variable expression.

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9 9. Endocrine Disorders

Androgen Insensitivity Syndrome 158 Congenital Adrenal Hyperplasia 160 Diabetes Insipidus 163 Growth Hormone Deficiency 164 Growth Hormone Receptor Defects 166 Panhypopituitarism 167 Pseudohypoparathyroidism 169

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Androgen Insensitivity Syndrome (also known as: AIS; androgen receptor deficiency; testicular feminization syndrome. Includes Reifenstein syndrome) MIM

300068, 312300

Clinical findings

Complete and partial androgen insensitivity are amongst the more common causes of male pseudohermaphroditism, in which individuals have a male karyotype (46,XY) but ambiguous or female external genitalia. Other rarer single-gene causes of male pseudohermaphroditism are summarized in Table 1. In complete AIS there is full sex-reversal, with female external genitalia, female breast development, a blind vagina, absent uterus, and abdominal or inguinal testes. In partial or incomplete AIS (also known as Reifenstein syndrome) there is variable genital ambiguity ranging from hypospadias and oligospermia (with gynecomastia) to an almost complete female phenotype with partial labio-scrotal fusion. It has been suggested that Joan of Arc may have been affected by this syndrome.

Age of onset

The findings are present at birth and date from late embryogenesis when sex development is complete.

Epidemiology

The estimated incidence is 1 in 20,000 to 1 in 50,000.

Inheritance

X-linked recessive, only chromosomal males are affected

Chromosomal location

Xq11–q12

Gene

AR (androgen receptor)

Mutational spectrum

Mainly missense and nonsense point mutations. Also splice-site mutations, deletions, and insertions, all with a loss of function effect.

Molecular pathogenesis

The androgen receptor gene contains eight exons and encodes a protein with regulatory, DNA-binding, nuclear localization, and androgen-binding domains. Mutations are found throughout the gene, with clustering in the DNA- and androgen-binding domains. There is a weak genotype– phenotype correlation, with large deletions and truncating (nonsense) mutations resulting in complete absence of functional receptor. Impaired receptor activity results in insensitivity to circulating androgen in late embryogenesis and early fetal life (when genital development is determined).

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Disorder

MIM

Inheritance

Basic defect

Clinical features in males

Androgen insensitivity

300068

XR

Absence of functional androgen receptors

See text

5α-Reductase deficiency

264600

AR

Conversion of testosterone to dihydrotestosterone

Ambiguous genitalia with masculinization at puberty

Leydig cell hypoplasia

152790

AR

Absence of functional luteinizing hormone receptors

Female external genitalia, absent uterus, testes with no Leydig cells

1) Lipoid 1) congenital 1) adrenal 1) hyperplasia

201710

AR

Conversion of cholesterol to pregnenolone

Ambiguous or female external genitalia with severe salt loss

2) 17α-Hydroxylase/ 1) 17,20-desmolase 1) deficiency

202110

AR

Conversion of pregnenolone to DHEA

Ambiguous external genitalia with incomplete Wolffian duct development

3) 3β-Hydroxysteroid 1) dehydrogenase 1) deficiency

201810

AR

Conversion of DHEA to androstenedione

Ambiguous external genitalia with normal Wolffian ducts and severe salt loss

4) 17β-Hydroxysteroid 264300 1) dehydrogenase 1) deficiency

AR

Conversion of androstenedione to testosterone

Ambiguous genitalia with Wolffian ducts and pubertal virilization

Testosterone biosynthesis defects

Table 1. Single-gene causes of male pseudohermaphroditism. AR: autosomal recessive; DHEA: dehydroepiandrosterone; XR: X-linked recessive.

In complete AIS, failure of Wolffian duct stimulation results in the absence of internal male genitalia. Normal production of anti-Müllerian hormone by the testes results in failure of uterine development. The testes continue to produce both testosterone and small quantities of estrogen, which result in breast development and pubertal feminization in the complete syndrome. Note that expansion of a CAG triplet repeat in the first exon of AR causes an X-linked form of spino-bulbar muscular atrophy known as Kennedy disease (MIM 313200). This progressive neurologic disorder presents in adult life and is associated with oligospermia and gynecomastia. Genetic diagnosis and counseling

Endocrine Disorders

Diagnosis is made on the basis of clinical and cytogenetic findings, supported by androgen receptor assay using fibroblasts cultured from genital skin. Specific mutation analysis is available at a small number of specialized laboratories. Counseling is as for X-linked recessive inheritance. The testes should be removed, usually after puberty to allow for spontaneous pubertal feminization, in view of the associated 5% risk of gonadal neoplasia in adult life. 159

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Congenital Adrenal Hyperplasia (also known as: CAH; 21-hydroxylase deficiency; CAH type 1; CYP21 deficiency) MIM

201910

Clinical features

The clinical features are a direct consequence of impaired cortisol and aldosterone synthesis, leading to increased pituitary adrenocorticotrophic hormone secretion, which in turn leads to elevated adrenal production of cortisol precursors and androgens. Three forms of presentation are recognized: the classic virilizing and salt-losing forms, and a nonclassic attenuated form. Children with the classic virilizing form show progressive virilization which can manifest in female neonates as genital ambiguity with clitoromegaly and variable fusion of the labio-scrotal folds. In older children, excess androgen production results in precocious sexual hair with accelerated skeletal maturation, and ultimately short stature. In the salt-losing form, affected females are virilized and infants of both sexes develop hyponatremic circulatory collapse soon after birth due to deficient aldosterone synthesis, which (if untreated) is life-threatening. Presentation is variable in the nonclassic late-onset form, and can include premature development of pubic hair, severe acne, delayed menarche, hirsutism, and oligomenorrhea.

Age of onset

In the classic forms, presentation is at birth or in early infancy.

Epidemiology

CAH type I accounts for 90%–95% of all forms of CAH, with an estimated worldwide frequency of 1 in 15,000 live births. A particularly high incidence has been noted in the Eskimo population of southwest Alaska.

Inheritance

Autosomal recessive

Chromosomal location

6p21.3

Gene

CYP21 (cytochrome P450, subfamily XXIA)

Mutational spectrum

Mainly deletions and gene conversions resulting from nonreciprocal transfer of inactivating mutations from the closely adjacent pseudogene (see below). Also, a wide range of missense, nonsense, splice-site, and frame-shift mutations.

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Cholesterol

DHEA-2

Pregnenolone

17 -OH-Pregnenolone

Progesterone

17-OH-Progesterone

11-Deoxycorticosterone

11-Deoxycortisol

Corticosterone

Cortisol

DHEA DHEA

Androstenedione

18-OH-Corticosterone

Aldosterone

Figure 1. Adrenal steroidogenesis. The major enzymatic steps are shown. Steroids with mineralocorticoid activity are shown with a gray background. Androgenic steroids are shown with a purple background. Cortisol is the major glucocorticoid produced in man.

Molecular pathogenesis

Endocrine Disorders

CYP21 is situated within the HLA complex. It contains 10 exons that encode the adrenal steroid 21-hydroxylase, which is a microsomal cytochrome P450. This enzyme is responsible for the conversion of progesterone to deoxycorticosterone and of 17-OH progesterone to 11-deoxycortisol (see Figure 1). CYP21 lies in a tandem paired arrangement with its pseudogene CYP21P and two isoforms of the C4 complement gene (ie, C4A–CYP21P–C4B–CYP21). This arrangement probably arose as a result of a recombination or duplication event in early evolution. The nature of this arrangement predisposes to two forms of mutation: deletions (due to unequal crossing-over in meiosis following misalignment) and conversions (due to transfer of silencing DNA sequences from CYP21P to CYP21). These mutations account for most cases of classic CAH type I. Missense mutations with a milder effect on enzyme activity tend to be associated with the nonclassic phenotype, although the relationship between genotype and phenotype is not consistent, as illustrated by reports of discordant disease presentations in individuals with apparently identical mutations. 161

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Disorder

MIM

Gene

Mutational spectrum

11β-Hydroxylase deficiency

202010 8q21

CYP11B1

Missense, nonsense, As in CAH type I, plus hypertension splicing, and frame-shift with hypokalemic alkalosis deletions/insertions

Clinical features

Aldosterone synthase 203400 8q21 deficiency

CYP11B2

Missense, nonsense, and deletions

Salt loss leading to dehydration Ambiguous external genitalia in males and females, plus salt loss in some cases

3β-Hydroxysteroid dehydrogenase deficiency

201810 1p13.1 HSD3B2

Mainly missense point mutations

17α-Hyroxylase/ 17,20-lyase deficiency

202110 10q24.3 CYP17

Missense, nonsense, Ambiguous genitalia in males, splice-site, and frameprimary amenorrhea in females, shift deletions/insertions hypertension

Congenital lipoid adrenal hyperplasia

201710 8p11.2 StAR

Mainly nonsense and frame-shift deletions/ insertions

Ambiguous or female external genitalia, with severe salt loss and susceptibility to infection

Table 2. Rare causes of congenital adrenal hyperplasia (CAH) (all show autosomal recessive inheritance).

Genetic diagnosis and counseling

Diagnosis is based on the combination of clinical and biochemical findings, notably elevated 17-OH progesterone levels, as measured in serum by radioimmunoassay. Mutation detection is undertaken at specialist designated laboratories, but only about 70% of all mutations can be identified. Linkage analysis, formerly based on human leukocyte antigen haplotype studies and now using intragenic markers, can be utilized for carrier detection and prenatal diagnosis when specific mutations cannot be identified. Prenatal treatment with maternal dexamethasone commencing before 6–7 weeks gestation prevents virilization in a high proportion of affected female fetuses. For this to be considered it is necessary that informative molecular analysis has been undertaken in an older affected sibling, so that appropriate mutation detection or linkage analysis can be carried out on chorionic villi from the new pregnancy, with a view to continuing the dexamethasone only if the fetus is both female and affected. Other, much rarer, causes of congenital adrenal hyperplasia are summarized in Table 2.

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Diabetes Insipidus (also known as: DI. Includes neurohypophyseal and nephrogenic DI) MIM

See Table 3.

Clinical findings

Excessive thirst and polyuria occur in all forms of DI. In the nephrogenic forms, severe fluid loss in infancy can result in hypernatremic dehydration, leading to convulsions and death or residual mental retardation. Bladder distension and chronic renal disease are possible later complications. Some women with neurohypophyseal DI encounter problems with labor and lactation due to associated oxytocin deficiency.

Age of onset

Soon after birth or in early infancy

Epidemiology

Many families have been reported with both neurohypophyseal and nephrogenic forms, but accurate incidence figures are not available.

Inheritance, See Table 3. chromosomal location, gene, and mutational spectrum Molecular pathogenesis

Type

Neurohypophyseal DI This is caused by mutations in the antidiuretic gene AVP, which contains three exons and encodes a complex protein consisting of a signal peptide, the active hormone vasopressin (from exon 1), and its carrier protein neurophysin (from exons 2 and 3). Synthesis occurs in the neurons of the supraoptic and paraventricular nuclei MIM

Inheritance

Chromosomal location

Gene

Mutational spectrum

Neurohypophyseal DI 125700

Autosomal dominant (rarely autosomal recessive)

20p13

AVP (arginine vasopressin)

Mainly missense point mutations. Also nonsense mutations and deletions

Nephrogenic DI type I

304800

X-linked recessive

Xq28

AVPR2 (arginine Mainly missense point vasopressin mutations. Also nonsense receptor 2) and deletion or insertion frame-shift mutations

Nephrogenic DI type II

125800

Autosomal dominant

12q13

AQP2 (aquaporin-2)

Mainly missense point mutations

Table 3. Diabetes insipidus: MIM numbers, types, inheritance, chromosomal locations, genes, and mutational spectra.

Endocrine Disorders

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of the hypothalamus, followed by transport down neuronal axons to the posterior pituitary for storage and subsequent release. A large number of mainly missense mutations have been reported across the gene. These disturb various functions, including cleavage of the signal peptide, which results in accumulation of precursors in the endoplasmic reticulum, and failure of neurophysin transport. Nephrogenic DI Most cases are caused by mutations in AVPR2, which encodes the antidiuretic hormone (ADH) V2 receptor, a member of the superfamily of G protein-coupled receptors with seven membrane-spanning domains. Normally, the V2 receptor activates adenyl cyclase. Patients with X-linked nephrogenic DI do not show an increase in urinary cyclic AMP excretion following ADH administration and are therefore described as being ADH insensitive. Mutations have been reported across the gene in evolutionarily conserved positions of the receptor with no clear genotype–phenotype correlation. Approximately 5% of cases are caused by mutations in the aquaporin-2 gene, which encodes the water channel of the collecting duct. Individuals with this autosomal dominant form of DI are therapeutically unresponsive to ADH but show a normal urinary cyclic AMP response to ADH administration. Genetic diagnosis and counseling

Diagnosis is based on biochemical studies of plasma and urine osmolality, and the patient’s response to water deprivation and to exogenous vasopressin administration. Mutation analysis is not readily available, so genetic counseling should be based on the family history and biochemical findings.

Growth Hormone Deficiency (also known as: GH deficiency; pituitary dwarfism; primordial dwarfism) MIM

See Table 4.

Clinical features

Type I patients have proportionate short stature with increased subcutaneous fat, wrinkled skin, and a high-pitched voice. Puberty starts spontaneously, but may be delayed. Similar features are seen in some individuals with type II, whereas others simply show short stature. Bone age is retarded in both groups. Types I and II can sometimes be distinguished on the basis of a decreased (type I) or increased (type II)

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Type

MIM

Inheritance

Chromosomal location

Gene

Mutational spectrum

I

262400

Autosomal recessive

17q22–q24

GH1 (growth hormone)

Mainly large deletions (type IA) and splice-site frame-shift mutations (type IB), all with a loss of function effect

II

173100

Autosomal dominant

17q22–q24

GH1 (growth hormone)

Splice-site mutations with a dominant-negative effect

III

307200

X-linked recessive

Xq21.3–q22

BTK (Bruton tyrosine kinase)

Splice-site mutation leading to exon skipping

Table 4. Growth hormone deficiency: types, MIM numbers, inheritance, chromosomal locations, and genes.

insulin response to glucose ingestion. In type III there is proportionate short stature, retarded bone age, delayed onset of puberty, and associated hypogammaglobulinemia. Age on onset

Growth deceleration usually becomes apparent in early childhood. In severe type I cases (designated type IA), short stature may be apparent at birth.

Epidemiology

GH deficiency affects approximately 1 in 5,000 to 1 in 10,000 children. Estimates of the proportion of familial cases range from 3% to 30%.

Inheritance, See Table 4. chromosomal location, gene, and mutational spectrum Molecular pathogenesis

GH1 is located within a cluster of five GH genes thought to have arisen from a series of ancestral gene duplication events. It contains five exons plus control sequences that bind to the product of PIT1, a pituitaryspecific transcription factor, which regulates pituitary development and hormone expression (see the entry on panhypopituitarism p.167–9). GH1 encodes circulating growth hormone which, at the cellular level, binds with two GH receptor molecules to initiate signal transduction. Mutations in GH1 account for both types I and II GH deficiency. In type IA, the most severe form, homozygosity or compound heterozygosity for large deletions or truncating mutations results in complete absence of GH secretion. Many of these patients develop antibodies to exogenous GH. In type IB, the most common form, small quantities of GH are produced and the production of antibodies to GH is not a problem. Some of these patients are homozygous for intronic donor splice-site

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mutations, whereas in other type IB familial cases, no GH mutations have been identified. In these patients the defects are suspected to lie in the GH-releasing hormone gene or in the gene that encodes its receptor (MIM 139191). Mutations in type II GH deficiency involve splicing transcripts for intron 3, leading to skipping of exon 3 and loss of 40 amino acids from the GH protein. It has been proposed that this shortened GH protein inactivates the normal GH protein through disruption of normal intracellular protein transport or by the formation of abnormal heterodimers. Genetic diagnosis and counseling

GH1 mutation analysis is available on a limited basis and should be pursued if possible, particularly when the family history points to a genetic cause (eg, consanguinity or the existence of an affected relative). Counseling of isolated cases is difficult as many cases are caused by factors that convey a low recurrence risk (eg, trauma, neoplasia, and infection) and there is no reliable method for identifying those cases with a Mendelian etiology.

Growth Hormone Receptor Defects (also known as: GHR defects; growth hormone insensitivity syndrome; Laron dwarfism) MIM

262500

Clinical features

The clinical features are similar to those seen in isolated severe growth hormone (GH) deficiency, but with the important distinction that plasma levels of GH are high. Affected individuals show marked proportionate short stature with small facies, blue sclerae, obesity, high-pitched voice, and delayed bone age. The onset of puberty is also delayed. Biochemically, these individuals have high levels of circulating GH and low levels of insulin-like growth factor 1 (IGF1). Exogenous GH does not induce an IGF1 response. Hypoglycemia is common in infancy and there is a poor insulin response to glucose ingestion.

Age of onset

Severe growth retardation becomes apparent in early childhood.

Epidemiology

The condition is very rare. Accurate incidence figures are not available.

Inheritance

Autosomal recessive

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Chromosomal location

5p13–p12

Gene

GHR (growth hormone receptor)

Mutational spectrum

Large deletions plus missense, nonsense, and splice-site mutations, all with a loss of function effect.

Molecular pathogenesis

GHR contains nine exons (numbered 2–10) that encode a receptor protein with extracellular, transmembrane, and cytoplasmic domains. The extracellular domain is encoded by exons 2–7 and occurs freely in plasma as GHBP (GH-binding protein) due to either alternative splicing of GHR mRNA or proteolysis of the mature peptide. Thus assay of GHBP in plasma can assist in diagnosis. Activation of the cytoplasmic domain by ligand (GH) binding promotes signal transduction, leading to the production of IGF1. Mutations in GHR are scattered throughout the gene. Most of the reported deletions are in the extracellular domain, resulting in undetectable levels of GHBP. There is evidence that point mutations tend to be associated with less severe short stature than deletions and splicing abnormalities. Note that some individuals with Laron dwarfism (Type II, MIM 245590) have postreceptor defects involving IGF1 or its receptor. There is some evidence that nonresponsiveness to IGF1 may account for the short stature seen in African pygmies.

Genetic diagnosis and counseling

Diagnosis is based on the combination of clinical and hormonal findings. Specific mutation analysis is available only on a limited research basis. Counseling is as for autosomal recessive inheritance.

Panhypopituitarism (also known as: combined pituitary hormone deficiency) MIM

262600

Clinical features

These are variable and depend on which of the pituitary hormones are deficient. Deficiency of growth hormone results in the clinical features of isolated growth hormone deficiency (p.164–6). Thyroid-stimulating hormone (TSH) deficiency results in variable hypothyroidism. Lack of gonadotropins, the most common tropic hormone deficiency, results

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in sexual immaturity and primary amenorrhea in the female and hypogonadism in the male. Adrenocorticotrophic hormone deficiency may cause severe hypoglycemia. Age of onset

Infancy to late childhood

Epidemiology

Genetic forms of panhypopituitarism are rare. Accurate figures are not available.

Inheritance

Autosomal recessive and autosomal dominant

Chromosomal See Table 5. location, gene, and mutational spectrum Chromosomal location

Gene

Mutational spectrum

3p11

PIT1 (pituitary-specific transcription factor 1)

Inactivating missense and nonsense point mutations and deletions. Also missense point mutations with a dominant-negative effect

5q

PROP1 (prophet of PIT1)

A common 2-bp frame-shift deletion. Also missense point mutations and other frame-shift deletions

9q34.4

LHX3 (lim homeobox 3)

Missense mutation and intragenic deletion

Table 5. Panhypopituitarism: chromosomal locations, genes, and mutational spectra.

Molecular pathogenesis

168

PROP1, PIT1, and LHX3 all encode transcription factors involved in pituitary development and hormonal synthesis. PROP1 regulates expression of PIT1, which in turn binds to and activates the promoters of the growth hormone and prolactin genes. Inactivating mutations in PROP1 result in deficiencies of luteinizing hormone, follicle-stimulating hormone, growth hormone, TSH, and prolactin, and account for a large proportion of familial panhypopituitarism cases. Mutations in LHX3 also result in multiple hormone deficiency and rigidity of the cervical spine.

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Mutation analysis is undertaken only on a very limited research basis. Most cases of multiple pituitary hormone deficiency are not genetic in origin, but no method other than mutation analysis is available for distinguishing these from the rare familial forms. Thus, accurate genetic counseling for an isolated/sporadic case cannot be achieved.

Pseudohypoparathyroidism (also known as: PHP; Albright’s hereditary osteodystrophy [AHO]; pseudopseudohypoparathyroidism [PPHP]) MIM

See Table 6.

Clinical features

The clinical phenotype associated with PHPIA was described by Albright and colleagues in 1942 and is referred to as Albright’s hereditary osteodystrophy (AHO). Typically, patients have a round face, short stature, obesity, brachydactyly, and ectopic calcification. Mental retardation is present in over 50% of cases and can vary from mild to severe. Other occasional findings include cataracts and abnormal dentition, with thin enamel and small crowns. Biochemical changes usually include hypocalcemia and hyperphosphatemia in the presence of a raised level of parathyroid hormone (hence the designation “PHP”). The diagnosis can be confirmed by demonstrating absence of the normal rise in plasma and urinary cAMP in response to an intravenous injection of parathyroid hormone. Hypothyroidism occurs in over 50% of cases. Patients with PPHP have the phenotype of AHO with hypocalcemia, hyperphosphatemia, and raised parathyroid hormone levels. However, they show a normal increase in urinary cAMP excretion in response to parathyroid hormone administration. The physical features of AHO are not present in patients with PHP types IB and II. These patients exhibit parathyroid-hormone resistant hypocalcaemia and hyperphosphatemia, with an abnormal cAMP response in type IB and a relatively normal response in type II.

Age of onset

Ectopic calcification may be noted at birth. Other features become apparent in early childhood.

Epidemiology

All forms of PHP are rare, although no precise incidence figures are available.

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Inheritance

Chromosomal Gene location

PHP 103580 Autosomal dominant 20q13.2 type IA with a possible parent-of-origin (imprinting) effect

GNAS1 (guanine nucleotide-binding protein alpha-stimulating activity polypeptide 1)

PHP 603233 Autosomal dominant 20q13.3 type IB

Unknown

PHP type II

203330 Autosomal recessive

Unknown

PPHP

300800 Autosomal dominant 20q13.2 with a possible parent-of-origin (imprinting) effect

Unknown

GNAS1

Table 6. Pseudohypoparathyroidism: MIM numbers, types, inheritance, chromosomal locations, and genes.

Inheritance, chromosomal location, and gene

See Table 6.

Mutational spectrum Missense and nonsense point mutations, and deletions with a loss of function effect. Molecular pathogenesis

GNAS1encodes the alpha subunit of the guanine nucleotide-binding (Gs) protein. This constitutes a component of the signaling system whereby parathyroid hormone and other hormones stimulate adenylate cyclase to produce intracellular cAMP. In PHP type IA and PPHP, Gsα activity is reduced by 50% in erythrocyte membranes. Thus, these conditions are both caused by heterozygous loss of function mutations in GNAS1. Curiously, both phenotypes can be found within the same kindred, but not within the same sibship. One possible explanation for this unusual example of an identical mutation causing diverse phenotypes is that the GNAS1 gene shows differential tissue expression depending on whether it was inherited from the father or the mother. Support for this hypothesis of parental imprinting comes from animal studies indicating that the homologous region in mice is imprinted, and from the observation in humans that maternal transmission of an inactivating mutation usually results in PHP type IA whereas paternal transmission results in PPHP. It has been postulated that there is paternal silencing of GNAS1 in the renal cortex.

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The precise cause of PHP type IB is not known, although the locus has been mapped to a region of chromosome 20 close to the imprinted GNAS1 locus. However, Gsα levels are normal in PHP type IB. The basic defect is likely to involve regulatory regions of the maternal GNAS1, which are involved in establishing the parent-specific pattern of imprinting. Support for this hypothesis comes from the report of a boy with paternal uniparental disomy for chromosome 20q. This boy showed parathyroid hormone-resistant hypocalcemia and hyperphosphatemia with normal levels of Gsα protein, but absence of the normal maternal imprinting pattern of GNAS1. Note that the other condition associated eponymously with Albright, McCune–Albright syndrome, is caused by somatic mosaicism for activating mutations in GNAS1. This is in contrast to PHP type IA, which results from inactivating mutations. Genetic diagnosis and counseling

Endocrine Disorders

Specific mutation analysis is available at only a few specialist laboratories. Counseling for types 1A, IB, and PPHP is on the basis of autosomal dominant inheritance, with variable expression possibly due to tissue-specific imprinting. PHP type II is extremely rare and has been described in only a few families.

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10 10. Gastrointestinal and Hepatic Diseases

Alagille Syndrome 174 α1-Antitrypsin Deficiency 175 Hirschsprung Disease 177

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Alagille Syndrome (also known as: arterio-hepatic dysplasia) MIM

118450

Clinical features

These are variable in both degree and age of onset. Patients with the full syndrome have characteristic facies with a broad forehead, deep-set eyes, and pointed chin (see Figure 1). Typically, they also have neonatal jaundice due to paucity of intralobular bile ducts (90%), posterior embryotoxon in the eyes (80%), valvular or peripheral pulmonary stenosis (70%), and butterfly vertebrae (50%). Growth retardation is common and 40% of affected children have renal abnormalities. Approximately 25% develop potentially life-threatening liver failure.

Age of onset

Usually neonatal

Epidemiology

The estimated incidence is 1 in 100,000 live births.

Inheritance

Autosomal dominant

Chromosomal location

20p12

Gene

JAG1 (jagged 1)

Figure 1. A young child with Alagille syndrome.

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Mutational spectrum

This is very heterogeneous. Small frame-shift deletions and insertions account for over 50% of mutations. The remainder consist of missense, nonsense, and splice-site mutations, with large deletions constituting approximately 5%.

Molecular pathogenesis

JAG1 contains 26 exons and produces a 5.9-kb mRNA transcript that encodes a ligand for NOTCH1, a member of the NOTCH family of transmembrane receptors. These play an important role in determining cell fate. The protein product contains a ligand domain known as DSL (delta/serrate/lag2), a NOTCH region, and a transmembrane domain. It shows an expression pattern consistent with the clinical phenotype. Mutations predominate in the extracellular region of the protein and most are predicted to have a truncating effect. Mutations that result in deletion of the DSL domain show an association with early liver failure.

Genetic diagnosis and counseling

Mutation analysis is not widely available, so diagnosis is usually based on a combination of clinical and histologic findings. Expression is very variable. The parents of an apparently isolated case should be offered full assessment (including liver function tests, ocular examination, spinal X-rays, and echocardiography). Both somatic and germ line mosaicism have been reported in apparently unaffected parents of severely affected children.

α1-Antitrypsin Deficiency MIM

107400

Clinical features

These include liver disease (which can present in the neonatal period) and chronic progressive obstructive lung disease (which usually develops in middle age). Approximately 10% of neonates with severe α1-antitrypsin (α1AT) deficiency develop neonatal hepatitis syndrome, with prolonged conjugated hyperbilirubinemia, disturbed liver function tests, and hepatosplenomegaly. Over two thirds of these children show spontaneous recovery, but 10%–15% go on to develop irreversible cirrhosis. Other childhood problems are rare. Most affected adults develop obstructive lung disease in middle or old age, with an earlier age of onset in cigarette smokers.

Age of onset

Liver disease can present in the neonatal period.

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Epidemiology

The severe PI*Z deficiency allele (see below) shows the highest incidence in Scandinavia, with an allele frequency of close to 1 in 40, giving a population prevalence of homozygotes of just over 1 in 1,600. The prevalence in other Caucasian populations is approximately 1 in 2,500. The mild PI*S deficiency allele shows an allele frequency of 1 in 30 to 1 in 40 in Europeans. α1AT deficiency is very rare in Asian and Afro-Caribbean populations.

Inheritance

Autosomal recessive

Chromosomal location

14q32.1

Gene

PI (protease inhibitor 1)

Mutational spectrum

Very heterogeneous, but mainly missense point mutations. Also frame-shift insertions and deletions.

PI variants and nomenclature

Over 75 PI variants have been identified, initially by using starch gel electrophoresis and more recently by techniques such as isoelectric focusing and agarose electrophoresis. These are named on the basis of their mobility (F [fast], M [medium], S [slow], and Z [very slow]); subsequently, alleles were designated as PI*F, PI*M, PI*S, PI*Z. Phenotypes are described as PI MZ, PI SZ, etc. Alleles that produce no detectable α1AT in serum are known as null alleles and are designated as PI*Q0. The common mild PI*S and severe PI*Z deficiency alleles have been shown to be due to Glu264Val and Glu342Lys substitutions respectively.

Molecular pathogenesis

PI contains six introns and seven exons that encode the 394-amino-acid α1AT protein, a member of the serpin family of protease inhibitors. α1AT is the major plasma inhibitor of leukocyte elastase and also has inhibitory activity against proteinase 3, cathepsin G, chymotrypsin, trypsin, plasmin, and thrombin. Protease inhibition is mediated by a reactive loop of 16-amino-acid residues, which protrude from the molecule and bind with the target protease to prevent further enzyme activity. Mutant alleles are classified into three groups, depending on their effects on α1AT activity. Deficiency alleles, such as PI*S and PI*Z, result in reduced levels of serum α1AT activity. Null alleles result in complete absence of serum α1AT activity. Finally, dysfunctional alleles, such

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as PI*Pittsburgh, result in altered protein function. In the case of PI*Pittsburgh, this manifests as a bleeding disorder because of antithrombin 3 activity. Pulmonary disease is a direct consequence of reduced antielastase activity. Hepatic disease ensues from the accumulation of mutant protein aggregates in hepatocytes, which, in the case of the Z protein, occurs because the mobile reactive center loop of one molecule can become inserted into that of another molecule. Susceptibility to the development of hepatic disease is probably determined by polymorphic variation in the endoplasmic reticulum pathways responsible for the degradation of the mutant Z-protein aggregates. Genetic diagnosis and counseling

Diagnosis is based on conventional assay of serum α1AT activity and the establishment of the PI phenotype by isoelectric focusing. This is increasingly being supplemented by direct mutation analysis. Therapeutic approaches include the regular infusion of purified human α1AT to prevent progressive lung disease, and liver transplantation for advanced liver disease.

Hirschsprung Disease (also known as: HSCR; aganglionic megacolon; congenital intestinal aganglionosis) MIM

142623

Clinical features

HSCR is characterized histopathologically by congenital absence of ganglion cells in the myenteric and submucosal plexuses of the colon and rectum. It is divided into long and short segment types (L-HSCR and S-HSCR), depending on the presence or absence of disease involvement proximally beyond the sigmoid colon. Around 70% of cases are isolated, or “nonsyndromal”; the remaining 30% are associated with other abnormalities or conditions such as Down’s syndrome and Waardenburg syndrome type IV (see p.90–2). Affected children present with either acute intestinal obstruction and abdominal distension or with chronic constipation and failure to thrive. The diagnosis is made by anorectal manometry and rectal biopsy.

Age of onset

Presentation is usually at birth or in early infancy.

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Chromosomal location

Gene

Mutational spectrum

1p36.1

ECE1 (endothelin-converting enzyme 1)

Missense mutation

2q22

SIP1 (survival of motor-neurons Large deletions and interacting protein 1) truncating mutations

3p21

Unknown

Unknown

5p13.1–p12

GDNF (glial cell line-derived neurotropic factor)

Missense mutations

10q11.2

RET (receptor tyrosine kinase)

Mainly missense mutations. Also nonsense, splice-site, and frame-shift mutations. All have probable loss of function effect

13q22

EDNRB (endothelin receptor, type B)

Mainly missense and nonsense mutations

19q12

Unknown

Unknown

20q13.2–q13.3 EDN3 (endothelin 3)

Missense and nonsense mutations

22q13

Nonsense mutations

SOX-10 (sex-determiningfactor-related box 10)

Table 1. Hirschsprung disease: chromosomal locations, genes, and mutational spectra.

Epidemiology

The estimated incidence is 1 in 4,000. S-HSCR accounts for around 80% of cases. The male to female ratio is 4:1.

Inheritance

Oligogenic

Chromosomal See Table 1. location, gene, and mutational spectrum Molecular pathogenesis

The underlying genetic contribution to HSCR is complex. The prevailing view is that RET is the major susceptibility gene, with several other genes making a smaller contribution. This has given rise to the concept of “synergistic heterozygosity”. Heterozygous mutations in RET have been noted in around 50% of all familial cases and in up to 75% of children with L-HSCR. L-HSCR is known to convey greater familial risk than S-HSCR (see below) and this is attributed to the consequences of adverse interaction between the products of different genes involved in the RET (ie, RET and its ligand GDNF), endothelin (ie, EDNRB and its ligand EDN3), and SOX-10 mediated pathways. RET encodes a transmembrane tyrosine kinase receptor that mediates cell signaling in the embryonic enteric nervous system. Similarly, the

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endothelin-induced signaling pathway is active in the development and migration of colonic neural crest cells. Mutations in one or more of these genes are sufficient to cause disease. The relationship between genotype and phenotypic expression remains unclear. RET is also implicated in S-HSCR but with a lower incidence of detected germ line mutations. As yet unknown genes at the other loci (3p21 and 19q12) appear to act as modifiers of RET expression in causing S-HSCR. Similarly, an intragenic polymorphism (c135G/A) in RET also influences disease expression. Note that mutations in RET are also responsible for multiple endocrine adenomatosis type 2 (multiple endocrine neoplasia [MEN]2; see p.94–6). Both HSCR and MEN2 have been described in the same individual or the same family. HSCR is a common feature in children with an interstitial deletion of chromosome 2q22. This is due to haploinsufficiency for SIP1, which encodes a transcriptional repressor involved in the patterning of neural crest cells and the central nervous system. Children with this condition, sometimes referred to as Mowat–Wilson syndrome, usually show mental retardation with microcephaly and facial dysmorphism. Genetic diagnosis and counseling

Specific mutation analysis is available on a very limited basis, mainly in a research setting. Counseling still relies heavily on empiric risk figures, which are higher for L-HSCR than for S-HSCR and are influenced by the sex of both the proband and the relative at risk. For example, the recurrence risk for the brother of a boy with L-HSCR is 16%, whereas the risk for the sister of a girl with S-HSCR is 3%. Note that L-HSCR and S-HSCR can occur within the same family.

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11 11. Hematologic disorders

Fanconi Anemia 182 Glucose-6-Phosphate Dehydrogenase Deficiency 183 Hemophilia A 185 Hemophilia B 187 Hereditary Elliptocytosis 189 Hereditary Spherocytosis 190 Sickle Cell Anemia 193 α-Thalassemia 194 β-Thalassemia 197 von Willebrand Disease 198

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Fanconi Anemia (also known as: FA; Fanconi pancytopenia) MIM

See Table 1.

Clinical features

The most consistent and characteristic feature is progressive irreversible bone marrow failure, usually with childhood onset, resulting in anemia, leucopenia, and thrombocytopenia. Affected individuals also show an increased incidence of malignancy, notably acute myeloid leukemia and squamous cell carcinoma. Approximately two thirds of cases have one or more congenital malformations including radial-ray and cardiac defects, microcephaly, microphthalmia, and genital anomalies. Other findings can include short stature and café-au-lait pigmentation. Average life expectancy is approximately 20 years.

Age of onset

Malformations may be apparent at birth. Pancytopenia usually develops by 10 years of age.

Epidemiology

The estimated worldwide incidence is approximately 1–5 in 1,000,000. There is a particularly high incidence (1 in 22,000) in white Afrikaansspeaking South Africans.

Inheritance

Autosomal recessive

Chromosomal See Table 1. location, gene, and mutational spectrum Molecular pathogenesis

182

The letters A to G represent separate complementation groups identified on the basis of cell fusion studies looking at the correction of hypersensitivity to cross-linking agents. Each group corresponds to a distinct FA disease gene, which produces a specific FA protein. These proteins form a nuclear complex comprised of proteins A, B, C, E, F, and G, which is activated by DNA damage. This in turn activates the D2 protein, which is targeted to nuclear foci where it interacts with the BRCA1 protein, among others, to repair DNA damage and particularly interstrand cross-links. Group A accounts for around 60% of all cases; it may be more common than the other forms because the high incidence of homopolymeric tracts in FANCA predisposes to microdeletion formation through misalignment. A specific mutation in FANCC shows a carrier

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MIM

Group

Chromosomal location

Gene

Mutation

227650

A

16q24.3

FANCA (Fanconi anemia complementation group A)

Mainly microdeletions and insertions

227660

B

13q12.3

BRCA2 (breast cancer 2, early onset)

Nonsense and frame-shift mutations

227645

C

9q22.3

FANCC (Fanconi anemia complementation group C)

Missense, nonsense, and splice-site mutations. Also deletions and insertions

605724

D1

13q12.3

BRCA2 (breast cancer 2, early onset)

Missense and frame-shift mutations

227646

D2

3p25.3

FANCD2 (Fanconi anemia complementation group D2)

Missense and nonsense mutations Also deletions and insertions

600901

E

6p22–p21

FANCE (Fanconi anemia complementation group E)

Missense, nonsense, and splice-site mutations

603467

F

11p15

FANCF (Fanconi anemia complementation group F)

Deletions and nonsense mutations

602956

G

9p13

FANCG = XRCC9 (X-ray repair complementing defective, in Chinese hamster, 9)

Nonsense and splice-site mutations and deletions

Table 1. Fanconi anemia: MIM numbers, groups, chromosomal locations, genes, and mutational spectra.

frequency of approximately 1 in 150 in Ashkenazi Jews and is associated in homozygous form with a high incidence of malformations and early onset of anemia. Genetic diagnosis and counseling

Specific mutation analysis is available on a very limited basis and usually in a research setting. The diagnosis is usually made by demonstrating hypersensitivity to agents such as mitomycin C and diepoxybutane, which induce cross-links, resulting in increased breakage in metaphase chromosomes. Counseling is as for autosomal recessive inheritance.

Glucose-6-Phosphate Dehydrogenase Deficiency (also known as: G6PD deficiency; favism) MIM

305900

Clinical features

These are variable and show a close correlation with the underlying mutation and its effect on G6PD activity and stability (see Table 2). Generally, hemolysis only occurs after exposure to drugs with a direct oxidant action (eg, primaquine, dapsone, nitrofurantoin) or during severe intercurrent illness, such as infection or diabetic ketoacidosis.

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Class G6PD activity

Clinical features

Examples

I

Very low

Chronic nonspherocytic hemolytic anemia

G6PDChicago

II

<10%

Some prolonged, intermittent hemolysis G6PDMediterranean

III

10%–60%

Self-limited, moderate, intermittent hemolysis

G6PDA–

IV

Normal

None

G6PDB, G6PDA+

Table 2. Functional classification of glucose-6-phosphate dehydrogenase (G6PD) deficiency.

Neonatal jaundice leading to kernicterus can occur with class II variants. Ingestion of the fava bean causes hemolysis (favism) in G6PD-deficient Caucasians and Asians, but only rarely in those of African origin. Age of onset

From birth onwards in severe cases

Epidemiology

The worldwide distribution is similar to that of malaria with incidences of 10%–20% in Africans and African Americans, 2%–20% in Mediterranean regions, and 10%–15% in the Middle East and Southeast Asia.

Inheritance

X-linked recessive

Chromosomal location

Xq28

Gene

G6PD

Mutational spectrum

Very heterogeneous. Almost all mutations are missense, with a few small in-frame deletions.

Molecular pathogenesis

G6PD is 18-kb in length and contains 13 exons that encode the 515-amino-acid G6PD enzyme. G6PD oxidizes glucose-6-phosphate to 6-phosphogluconolactone to generate the only source of NADPH available for red blood cells to prevent damage by oxidation. Normally, as red blood cells age the intracellular activity of G6PD decreases. This decline in activity is exacerbated by reduced stability of the G6PDA– variant, which is common in African Americans, and results from the presence of two substitutions: Asn126Asp and either Val68Met or Arg227Leu. (Presence of only the Asn126Asp substitution converts the normal G6PDB to G6PDA+ and is of no clinical significance.) This decline in G6PDA– stability, and hence activity, in older red blood

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cells results in a population of cells susceptible to oxidant damage and explains why attacks of hemolysis are relatively mild and self-limiting. In contrast, the defect in G6PDMediterranean (caused by a Ser188Phe substitution) results in G6PD instability in red blood cells of all ages, so that spontaneous recovery is much slower. The fact that mutations that would result in complete loss of function (such as frame-shifts) are never observed, indicates that zero enzyme activity is not compatible with survival. Genetic diagnosis and counseling

Specific mutation analysis is not routinely available. Diagnosis is usually made on the basis of the clinical and family history, the presence of Heinz bodies during hemolytic episodes, and specific enzyme assay using commercially available kits. Carrier detection based on enzyme assay is unreliable because of random X-chromosome inactivation. Microscopic examination of individual cells on a blood film is preferable. Counseling is on the basis of X-linked recessive inheritance, with emphasis on the importance of avoiding drugs and other agents that can induce hemolysis.

Hemophilia A (also known as: classic hemophilia; factor VIII deficiency) MIM

306700

Clinical features

There is a wide spectrum of severity depending on the level of factor VIII activity in plasma. Factor VIII levels less than 1% of normal are seen in the 50% of patients with severe disease who present with spontaneous hemorrhage into joints, muscles, and internal organs. Levels of between 1% and 5% result in moderate disease, with excessive bleeding after minor trauma. Levels of between 5% and 25% cause mild disease with abnormal bleeding only after major trauma or surgery.

Age of onset

Children with severe disease present soon after birth with bleeding from the umbilicus or in infancy with spontaneous hemarthrosis.

Epidemiology

The incidence in males is between 1 in 5,000 and 1 in 10,000. All ethnic groups are affected.

Inheritance

X-linked recessive

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Gene A

Centromere

Telomere

Normal X chromosome

Factor VIII gene

Centromere

“Flip” inversion in meiosis I

Crossover site Centromere

Telomere

Recombinant X chromosome

Figure 1. How the common hemophilia A mutation is generated by a crossover within a loop or “flip” inversion. Gene A represents homologous sequences within the factor VIII gene and at a subtelomeric position on the same arm of the X chromosome.

Chromosomal location

Xq28

Gene

F8 (also known as F8C [factor VIII])

Mutational spectrum

A common “flip inversion” accounts for 40%–50% of all serious cases (see Figure 1). The remaining cases show marked heterogeneity, with missense, nonsense, and splice-site point mutations, together with large and small deletions and insertions. Missense mutations are more common in patients with mild disease and in those who are cross-reacting material (CRM) positive (ie, those who produce normal levels of a nonfunctional protein).

Molecular pathogenesis

F8C is a large gene with 26 exons and a 9-kb mRNA transcript. Factor VIII is synthesized in the liver and circulates in the plasma bound to von Willebrand factor for stability. The factor VIII protein contains six structural domains (A1–A2–B–A3–C1–C2), the largest of which, the B domain, is excised during activation to form heavy and light chains. Cleavage at specific sites (Arg372 and Arg1689) by thrombin is necessary

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for activation. This allows factor VIII to bind to a phospholipid surface and interact with factor IXa to form the X-ase complex necessary for the activation of factor X. All mutations interfere with the production, stability, or function of factor VIII coagulation activity. Approximately 5% of patients are CRM positive, and around half of these have a CpG missense point mutation. Most patients with severe disease are CRM negative, in that they do not produce a protein with either antigenic or coagulation activity. Such individuals are at risk of developing antibodies to factor VIII following replacement therapy. Genetic diagnosis and counseling

Diagnosis of the common “flip” inversion and general mutation screening are widely offered as a service. Linkage analysis using intragenic polymorphisms can be used when a specific mutation cannot be identified. Carrier detection by molecular analysis is more reliable than older methods based on assay of antigenic and coagulation activity. Around 30% of cases are isolated. As most point mutations and almost all “flip” inversions originate in a male meiosis, it is prudent to assume that the mother of an isolated case with one of these mutations is a carrier until proven otherwise.

Hemophilia B (also known as: Christmas disease; factor IX deficiency) MIM

306900

Clinical features

The clinical features are indistinguishable from those seen in hemophilia A, with levels of factor IX activity less than 1% causing severe disease and levels of 1%–5% causing moderate disease.

Age of onset

Severe disease can present soon after birth or later in infancy.

Epidemiology

The incidence in males is approximately 1 in 30,000 to 1 in 50,000 in all ethnic groups.

Inheritance

X-linked recessive

Chromosomal location

Xq27.1–q27.2

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Gene

F9 (factor IX)

Mutational spectrum

Numerous point mutations (>1,500) include missense, nonsense, splice-site, and CpG dinucleotide changes. There are also deletions and insertions.

Molecular pathogenesis

The factor IX gene spans 34 kb and contains eight exons that encode a 461-amino-acid precursor factor IX protein. This contains a signal peptide followed by a propeptide domain, which undergoes vitamin K-dependent posttranslational modification. The mature factor IX glycoprotein circulates in blood as a zymogen and contains an amino-terminal domain with α-carboxyglutamic acid residues and two epidermal growth factor domains. Factor IX is activated to factor IXa by cleavage at Arg145/Arg146. Factor IXa assembles with activated factor VIII to form the X-ase complex responsible for the cleavage of factor X to its active form, factor Xa. Most missense mutations interfere with an important step in factor IX protein formation or function such as posttranslational modification, zymogen activation, or protein assembly. Point mutations in the promoter region close to the major transcription start site are associated with the unique factor IX Leyden form of hemophilia B. This presents as severe disease in childhood with spontaneous remission at puberty. Patients with deletions resulting in severe disease with no factor IX protein production are at increased risk of developing “inhibitor” antibodies following replacement therapy.

Genetic diagnosis and counseling

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Mutation detection by direct DNA sequence analysis is offered at several clinical reference laboratories. When a specific mutation cannot be identified, linkage analysis using intragenic polymorphisms can be used for carrier detection and, if appropriate, prenatal diagnosis. Molecular analysis for carrier detection is much more reliable than assay of antigen to coagulation ratio or coagulation activity as the latter is influenced by random X-chromosome inactivation.

Hemophilia B

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Hereditary Elliptocytosis (also known as: HE) MIM

130500

Clinical features

HE constitutes a group of inherited disorders characterized by oval-shaped red blood cells (elliptocytes) in peripheral blood. In “common HE”, many affected individuals are asymptomatic, while a minority have a mild hemolytic anemia with splenomegaly and occasional gall stones. In “homozygous HE”, there is moderate to severe hemolytic anemia with elliptocytes and poikilocytes in the blood film. In “hereditary pyropoikilocytosis” (HPP, MIM 266140), there is severe hemolytic anemia with splenomegaly and increased osmotic fragility. Individuals with homozygous HE and HPP usually benefit from splenectomy.

Age of onset

Hematologic abnormalities are present at birth.

Epidemiology

Common HE affects approximately 1 in 200 individuals in parts of Africa and Southeast Asia where malaria is endemic. The incidence in Caucasians is 1 in 2,000 to 1 in 4,000. Homozygous HE and HPP are rare.

Inheritance

Autosomal dominant (common HE), autosomal recessive (homozygous HE and HPP)

Chromosomal See Table 3. location, gene, and mutational spectrum Chromosomal location

Gene

Mutational spectrum

1p36.2–p34

EPB41 (erythrocyte membrane protein band 4.1 = protein 4.1)

Missense mutations, deletions, and duplications

1q21

SPTA1 (α-spectrin)

Mainly missense mutations

14q22–q23.2

SPTB (β-spectrin)

Missense mutations and deletions

17q21–q22

AE1 (anion exchanger member 1 = band 3)

Common 27-bp deletion

Table 3. Hereditary elliptocytosis: chromosomal locations, genes, and mutational spectra.

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All forms of HE are caused by defects in the red blood cell cytoskeleton (see next entry on hereditary spherocytosis), in which actin and protein 4.1 interact with spectrin at the junction of the spectrin heterotetramers to maintain both the shape and the deformability of the cell. Common HE is caused by mutations in EPB41 (40%) or SPTA1 (60%). EPB41 is linked to the Rh locus on chromosome 1, and HE used to be subdivided into types 1 and 2 on the basis of linkage or nonlinkage to this locus. Codon 28 is a hotspot for mutations in SPTA1, with Arg28 being critical for spectrin heterodimer stability. Individuals with homozygous HE are usually homozygotes or compound heterozygotes for missense mutations in SPTA1, resulting in severely impaired spectrin assembly. HPP can also be caused by homozygosity or compound heterozygosity for SPTA1 mutations or by a single mutation in SPTA1, together with another mutation influencing spectrin stability. A specific form of elliptocytosis (known as Southeast Asian ovalocytosis) occurs at a high frequency in Malaysia, the Philippines, and Papua New Guinea. This is caused by a deletion of nine amino acids in the boundary of cytoplasmic and membrane domains of the band 3 protein, which becomes entangled in the cytoskeleton, leading to increased cell rigidity. The relatively high incidence of this particular deletion is explained by the observation that it conveys protection to cerebral malaria.

Genetic diagnosis and counseling

Mutation analysis is available only in a research setting. The diagnosis is made on the presence of elliptocytes with or without poikilocytes in a peripheral blood film, supplemented by hematologic indices and osmotic fragility studies. Counseling is on the basis of autosomal dominant (HE) or autosomal recessive (homozygous HE and HPP) inheritance.

Hereditary Spherocytosis (also known as: HS) MIM

182900

Clinical features

HS constitutes a group of disorders in which the red blood cell cytoskeleton is defective, resulting in impaired cell membrane stability. Presentation can be with jaundice in neonates or with gall stones, episodic anemia, or splenomegaly in childhood or adult life. Most patients are only mildly to moderately affected, with minimal disability. Splenectomy, normally

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delayed until after the age of 5 years to reduce the risk of pneumococcal infection, is usually curative. Age of onset

Spherocytes are present in peripheral blood at birth.

Epidemiology

The incidence in individuals of Northern European origin is approximately 1 in 5,000. HS is much less common in other ethnic groups.

Inheritance

Usually autosomal dominant. Up to 25% of cases result from autosomal recessive inheritance.

Chromosomal See Table 4. location, gene, and mutational spectrum Chromosomal Gene location

Mutational spectrum

1q21

SPTA1 (α-spectrin)

Mainly missense mutations

8p11.2

ANK1 (ankyrin 1)

Frame-shift and nonsense null mutations in autosomal dominant HS; missense and promoter mutations in autosomal recessive HS

14q22–q23.2 SPTB (β-spectrin) 17q21–q22

Frame-shift and large deletions

AE1 (anion exchanger Missense and nonsense point mutations, member 1, also known also duplications as solute carrier family 4 and band 3)

Table 4. Hereditary spherocytosis (HS): chromosomal locations, genes, and mutational spectra.

Molecular pathogenesis

Hematologic Disorders

The basic defect in HS lies in the red blood cell cytoskeleton. This consists of a submembranous network composed mainly of actin and a complex of α- and β-spectrin. This cytoskeleton is anchored to the cytoplasmic surface of the red cell lipid bilayer plasma membrane by ankyrin, which binds to β-spectrin in the cytoskeleton and band 3 in the cell membrane. Band 3 is a membrane-spanning protein that is involved in the transport of inorganic anions. Defects in the cytoskeleton lead to inadequate support for the lipid bilayer leading to a loss of membrane surface area which converts the red cells from biconcave discs to spherocytes (see Figure 2). Spherocytes become trapped in the spleen (because of their reduced flexibility) where they are removed by macrophages. This in turn leads to hemolysis and anemia.

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Figure 2. Blood film from a patient with spherocytosis (courtesy of Dr Claire Chapman, Department of Haematology, Leicester Royal Infirmary, UK).

The precise relationship between genotype and phenotype is not fully understood. Inheritance is autosomal dominant in about 75% of cases. These autosomal dominant cases are caused mainly by mutations in ANK1 and SPTB, with AE1 mutations accounting for around 10% of cases. Autosomal recessive inheritance is associated with mutations in ANK1, SPTA1, and AE1. The severity of HS appears to be modified by factors other than the primary gene defect. Note that mutations in AE1 are also responsible for some forms of hereditary renal tubular acidosis and Southern Asian ovalocytosis (MIM 166900). Genetic diagnosis and counseling

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Specific mutation analysis is only available on a very limited research basis. The diagnosis is based on the presence of spherocytes on a blood film (see Figure 2), an elevated mean corpuscular hemoglobin concentration (>36%), and increased red blood cell osmotic fragility. Counseling is usually on the basis of autosomal dominant inheritance with variable expression. Autosomal recessive inheritance should be suspected in severely affected patients.

Hereditary Spherocytosis

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Sickle Cell Anemia (also known as: sickle cell disease) MIM

603903

Clinical features

Affected children present with any or all of the classical triad of anemia, vaso-occlusive crises, and infection. The anemia results from sickling (see Figure 3) and hemolysis of red blood cells when oxygen tension is reduced. Clumping of sickled red blood cells causes painful vaso-occlusive crises, which can result in ischemic damage to the limb bones, lungs, brain, kidneys, heart, and spleen. Functional asplenia contributes to increased susceptibility to bacterial infection, particularly Streptococcus pneumoniae and Hemophilus influenza. In the worst cases, both quality of life and overall life expectancy are severely compromised.

Figure 3. Blood film showing sickle-shaped red cells (courtesy of Dr Claire Chapman, Department of Haematology, Leicester Royal Infirmary, UK).

Age of onset

Presentation is usually between the ages of 6 and 12 months.

Epidemiology

The frequency of the hemoglobin (Hb)S carrier state, known as the sickle cell trait, is high in all populations that originate from areas in which malaria is, or has been, endemic. Typical frequencies are 20% in Africa, 8% in African Americans, and 5% in the Middle East and Mediterranean regions. HbC and HbD can also cause sickle cell anemia in the compound heterozygous state. In Africa HbC carrier rates are similar to those for HbS. HbD has a carrier frequency of around 3% in parts of India.

Inheritance

Hematologic Disorders

Sickle cell disease shows autosomal recessive inheritance. Sickle cell trait (ie, the heterozygous state) can be considered to show dominant inheritance as there is evidence that the carrier state can occasionally be symptomatic. 193

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Chromosomal location

11p15.5

Gene

HBB (hemoglobin β-globin gene)

Mutational spectrum Missense mutations, ie: HbS GAG (Glu6) → GTG (Val6) HbC GAG (Glu6) → AAG (Lys6) HbD GAA (Glu121) → CAA (Gln121) Molecular pathogenesis

The substitution of value for glutamic acid at the β6 residue (the sixth amino acid residue in the β chain) results in the formation of an α2β2 tetramer which is unstable in the deoxygenated form. When the oxygen saturation falls below 85%, α2β2 tetrameric polymers form rod-like structures that cause red cells to become sickle-shaped. This, in turn, leads to increased blood viscosity (with clumping of sickled red blood cells causing a risk of vaso-occlusion) and hemolysis of the deformed sickle cells (resulting in rapid red blood cell turnover and anemia).

Genetic diagnosis and counseling

Specific mutation analysis for HbS is readily available by either polymerase chain reaction (using allele-specific oligonucleotides) or Southern blotting (utilizing the fact that the mutation alters an MstII cleavage site). Counseling is as for autosomal recessive inheritance. Population screening programs have been introduced with varying success in areas with a high incidence of carriers. The prediction of phenotype in compound heterozygotes for different HBB mutations can be very difficult. Generally, compound heterozygosity for HbS with either HbD or βº thalassemia results in moderate to severe sickle cell disease. HbS with HbC tends to result in milder sickle cell disease.

α-Thalassemia (includes hemoglobin H [HbH] disease) MIM

141800

Clinical features

These depend on how many of the four normal α-globin genes are intact. If all four are nonfunctional then there is profound intrauterine anemia, which manifests as hydrops fetalis with ascites and generalized

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edema. This results in spontaneous pregnancy loss or neonatal death. Most of the hemoglobin (Hb) in these infants is Hb Barts (γ4 tetramers). Loss of three functional α-globin genes results in moderately severe anemia (Hb 8–10 g/100 mL) known as HbH (β4 tetramers) disease. Loss of two functional α-globin genes (α-thalassemia-1 or α-thalassemia trait) leads to mild anemia (Hb 10–12 g/100 mL), which may be asymptomatic. Loss of a single functional α-globin gene is totally asymptomatic (“silent carrier”). Age of onset

Hydrops fetalis presents in the second or third trimester. HbH disease usually presents in childhood with anemia and hemolysis.

Epidemiology

Carrier frequencies are as high as 25% in parts of Africa, 20% in Southeast Asia, and 5%–10% in Mediterranean regions.

Inheritance

Hydrops fetalis and HbH disease show autosomal recessive inheritance. Mild disease associated with loss of two functional alleles can show autosomal dominant or recessive inheritance.

Chromosomal location

16pter–p13.3

Gene

HBA (hemoglobin α-globin gene cluster)

Mutational spectrum

Mainly deletions involving one or both pairs of α-globin genes. Missense, nonsense, splice-site, and frame-shift mutations have also been found. These result in abnormal RNA processing or translation, or posttranslational instability.

Molecular pathogenesis

Normally there are two closely contiguous α-globin genes on the short arm of each chromosome 16. Most cases of α-thalassemia are caused by deletions resulting in loss of two (α-thalassemia-1), three (HbH disease), or all four (hydrops fetalis) of these genes. The homology in and around all four α-globin genes predisposes to misalignment in meiosis I, with unequal crossing-over generating chromosomes with one and three genes instead of the normal two (see Figure 4). Selection for the clinically silent heterozygous state (ie, three α-globin genes) through relative immunity to malaria is thought to account for the high frequency of carriers in areas where malaria is endemic.

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Figure 4. Diagrammatic representation of the α-globin gene cluster on chromosome 16. (a) shows normal alignment in meiosis I. (b) shows misalignment with a crossover, which results in the recombinant deletion and triplication chromosomes shown in (c).

Subsequent unequal crossover events or larger deletions result in loss of both α-globin genes from a single chromosome. Thus, the heterozygous state of two normal genes, also referred to as α-thalassemia-1 or α-thalassemia trait, can result from either an α–/α– genotype (deletions in trans) or an αα/– – genotype (deletions in cis). The αα/– – genotype is mainly seen in Southeast Asia, which explains why hydrops fetalis, caused by a – –/– – genotype, is much more common in this population than in Africa, where heterozygotes usually have an α–/α– genotype. The clinical consequences of anoxia, hemolysis, and anemia are attributable to an imbalance in the ratio of α-globin to β-globin chain production, as illustrated by the presence of HbH (β4 tetramers) inclusions in red blood cells. 196

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Specific α-globin gene deletion or point mutation analysis is undertaken at a few specialized reference laboratories. Screening programs detect carriers by measuring red blood cell indices and carrying out Hb electrophoresis.

β-Thalassemia (also known as: Cooley’s anemia. Includes thalassemia intermedia and thalassemia major) MIM

141900

Clinical features

Children with severe β-thalassemia (also known as thalassemia major or βº-thalassemia – mutations can result in either partial [β+] or complete [βº] absence of β-globin) usually present with hypochromic microcytic anemia and require regular life-long transfusions to maintain a hemoglobin (Hb) level above 10 g/dL. Extramedullary expansion results in hepatosplenomegaly with mild facial coarsening. Regular transfusion results in iron overload, leading to potentially irreversible damage to the heart, liver, pancreas, and endocrine glands. These complications can be largely prevented by strict adherence to a rigorous regimen of iron chelation therapy. Children with β+-thalassemia tend to have milder anemia, which is not always transfusion dependent (thalassemia intermedia). Heterozygotes (β-thalassemia trait) are asymptomatic.

Age of onset

Usually after 3 months of age when the β-globin gene becomes fully expressed.

Epidemiology

The carrier frequency is 10%–20% in the Mediterranean region and approximately 5% in the Indian subcontinent. Lower carrier frequencies are observed in China, Southeast Asia, and the Middle East.

Inheritance

Autosomal recessive

Chromosomal locus

11p15.5

Gene

HBB (hemoglobin β-globin gene)

Mutational spectrum The mutational spectrum is very heterogeneous, with over 170 different mutations identified. These include missense and nonsense point mutations, splice-site mutations, insertions, and deletions. Common

Hematologic Disorders

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mutations exist specific for different ethnic groups. Many mutations reduce the quantity of mRNA (eg, promoter, RNA splicing and mRNA capping or tailing mutations). Molecular pathogenesis

The reduction in β-globin chain synthesis results in an α to β chain imbalance, with excess α chains forming insoluble aggregates in erythroid precursors in the bone marrow. This results in intramedullary hemolysis and ineffective erythropoiesis. The large numbers of erythroid precursors cause expansion of the bone marrow cavities and bone deformation. δβ-thalassemia and hereditary persistence of fetal Hb are caused by large deletions involving both of the closely adjacent β- and δ-globin gene loci. Individuals who are homozygous for these deletions are only mildly anemic because of compensatory increases in HbF production. Clinically, they are either asymptomatic or have mild thalassemia intermedia.

Genetic diagnosis and counseling

Specific HBB mutation analysis is available on a limited basis because of the marked mutational heterogeneity. Linkage analysis using highly polymorphic intragenic markers can be utilized for prenatal diagnosis in informative families. Carrier detection is achieved by measurement of red blood cell indices and Hb electrophoresis. Therapy with a compatible bone marrow transplant is potentially curative.

von Willebrand Disease (also known as: vWD; pseudohemophilia) MIM

193400

Clinical features

vWD is the most common inherited bleeding disorder and is classified into three types on the basis of a quantitative defect (type I), a qualitative defect (type II), or complete absence (type III) of von Willebrand factor (vWF). Type I (accounting for 70%–80% of all cases) is characterized by spontaneous bruising and mucosal bleeding from the nose and gastrointestinal tract, hemorrhage after surgery, and menorrhagia in women. Types IIA and IIB have a similar presentation. In types IIN and III, the clinical manifestations more closely resemble those seen in hemophilia A and B.

Age of onset

Early childhood in severe forms (mainly type III)

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Epidemiology

Some studies have suggested incidence/prevalence figures as high as 1%, but the true figure is probably nearer 1 in 1,000.

Inheritance

Autosomal dominant (types I, IIA and IIB); autosomal recessive (types IIN and III)

Chromosomal location

12p13.3

Gene

VWF (von Willebrand factor)

Mutational spectrum

Types I and II. Marked mutational heterogeneity with missense and nonsense point mutations, together with frame-shifts and deletions. Type III. Mainly deletions and nonsense or frame-shift mutations.

Molecular pathogenesis

VWF is composed of 52 exons spanning 178 kb. It encodes a 2,813-amino-acid protein, which contains four repeated domains (designated A–D) that are present in multiple copies. Specific domains are responsible for specific interactions. A1–A3 interact with collagen, heparin, and platelets; C1–C2 interact with platelets; D1–D3 interacts with heparin and factor VIII. VWF acts as a stabilizer for factor VIII and as an adhesive link between platelets and the blood vessel wall at sites of vascular injury. Types I and II vWD are distinguishable on the basis of a quantitative or a qualitative defect in the VWF. In types IIA, IIB, and IIN, mutations involve the A2, A1, and D1–3 domains, respectively. Thus, collagen and platelet binding are affected in types IIA and IIB, whereas in type IIN, the phenotype resembles that of mild hemophilia A. In type III vWD, which can be considered as the homozygous state for type I, VWF is absent in the blood, resulting in the clinical features of both vWD and hemophilia A.

Genetic diagnosis and counseling

Hematologic Disorders

Mutation analysis is available at a small number of specialized laboratories, although often a specific mutation cannot be identified. Prenatal diagnosis has been achieved in a few families with severe type III disease using linkage analysis. The diagnosis of vWD is usually made using standard coagulation assays (factor VIII activity, vWF antigen level, ristocetin cofactor assay of vWF functional activity). Counseling in types I, IIA, and IIB is on the basis of autosomal dominant inheritance with incomplete penetrance.

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12 12. Immunologic Disorders

Bruton Agammaglobulinemia 202 Chronic Granulomatous Disease 203 Severe Combined Immunodeficiency 205 Wiskott–Aldrich Syndrome 207

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Bruton Agammaglobulinemia (also known as: X-linked agammaglobulinemia) MIM

300300

Clinical features

These reflect an isolated B-cell defect that manifests as multiple recurrent bacterial infections such as pneumonia and meningitis. Without treatment affected boys develop chronic lung disease. They are also more susceptible to viral infections such as hepatitis, and to the development of rheumatoid-like arthritis. Treatment with regular injections of gammaglobulin significantly reduces the incidence of pyogenic infection and dramatically improves the long-term prognosis.

Age of onset

Infection usually first occurs after the age of 3 months as the level of maternally acquired immunoglobulin G (IgG) declines.

Epidemiology

The incidence in males is approximately 1 in 100,000.

Inheritance

X-linked recessive

Chromosomal location

Xq21.1–q22

Gene

BTK (Bruton tyrosine kinase)

Mutational spectrum

Over 300 different mutations have been reported. These include missense and nonsense point mutations, as well as frame-shift insertions and deletions. A mutational database is maintained at http://www.uta.fi/imt/bioinfo.

Molecular pathogenesis

BTK consists of 19 exons. These encode a 659-amino-acid cytoplasmic tyrosine kinase, known as Btk, which is expressed throughout B-cell and myeloid differentiation. Btk is one of a family of tyrosine kinases that are activated by growth factors and which contain several domains. Most missense mutations involve the kinase domain. Other mutations are distributed throughout the gene. No clear correlation has been established between the specific mutation and the severity of the phenotype. The precise role of Btk in B-cell differentiation and proliferation is unknown.

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Note that a specific splice-site mutation in BTK causes a rare form of Bruton agammaglobulinemia associated with growth hormone deficiency (see p.165, MIM 307200). Genetic diagnosis and counseling

Mutation detection is offered by several laboratories, with 90%–95% of all mutations being detectable by single-strand conformation polymorphism screening of genomic DNA. A small proportion of mutations can only be detected by complementary DNA analysis or Southern blotting. When a specific mutation has been identified the most desirable approach is to use mutation analysis for female carrier detection. Alternatively, an attempt can be made to demonstrate nonrandom X-chromosome inactivation in B cells using a double digest with a methylation-sensitive restrictive enzyme. Life-long treatment with gammaglobulin should be instituted in males as soon as the diagnosis is made.

Chronic Granulomatous Disease (also known as: CGD) MIM

233690 (cytochrome-b α-subunit deficiency) 233700 (neutrophil cytosolic factor 1 deficiency) 233710 (neutrophil cytosolic factor 2 deficiency) 306400 (cytochrome-b β-subunit deficiency)

Clinical features

CGD represents a group of disorders characterized by an inability of neutrophils to kill bacteria, resulting in recurrent bacterial infection. Phagocytosis proceeds normally, but a defect in the cell’s ability to generate activated oxygen radicals means that bacteria that do not generate hydrogen peroxide cannot be killed. Affected children present with recurrent infection, including abscesses, dermatitis, and pneumonia. Untreated CGD results in severe failure to thrive and ultimately death.

Age of onset

Onset is in early infancy in typical severe cases. Milder cases can present in childhood or adult life.

Epidemiology

The estimated worldwide incidence is approximately 1 in 250,000.

Inheritance

Approximately 70% of cases show X-linked recessive inheritance. The remaining 30% are autosomal recessive.

Immunologic Disorders

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Chromosomal See Table 1. location, gene, and mutational spectrum Chromosomal location

Gene

Mutational spectrum

1q25

NCF2 (neutrophil cytosolic factor 2)

Missense, nonsense and splice-site mutations. Also insertions and deletions

7q11.23

NCF1 (neutrophil cytosolic factor 1)

Mainly a 2-bp deletion

16q24

CYBA (cytochrome-b α-subunit)

Mainly missense mutations

Xp21.1

CYBB (cytochrome-b β-subunit)

Very heterogeneous, with all forms of mutations distributed throughout the gene and its regulatory regions

Table 1. Chronic granulomatous disease: chromosomal locations, genes, and mutational spectra.

Molecular pathogenesis

CGD results from a defect in the neutrophils’ ability to convert oxygen to superoxide. This process is mediated by NADPH oxidase, which involves a small transmembrane electron-transport system. This results in the oxidation of NADPH and the generation of superoxide. The superoxide becomes intracellular when invagination occurs during phagocytosis. Activation of oxidase requires an assembly of components, including cytochrome-b (made up of α and β chains, encoded by CYBA and CYBB, respectively) and two neutrophil cytosolic proteins (p47-phox encoded by NCF1 and p67-phox encoded by NCF2). Mutations in any of these genes can result in the CGD phenotype. Most cases (70%) of CGD are caused by mutations in CYBB. This holds a position of distinction in human genetics as the first disease gene to be identified by positional cloning (in 1986). The mutational spectrum is wide and includes “large” contiguous gene deletions, embracing the closely adjacent loci for Duchenne muscular dystrophy (see p.4–6, MIM 310200) and the McLeod phenotype (MIM 314850), which involves absence of a specific red blood cell protein and reduced levels of Kell blood group antigens. Approximately two thirds of autosomal recessive cases of CGD are caused by homozygosity or compound heterozygosity for a dinucleotide deletion in NCF1. This deletion results from misalignment with recombination between NCF1 and its highly homologous pseudogenes, which carry the deletion.

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Specific mutation analysis and immunoblot assay for the relevant proteins are undertaken at a small number of specialist reference laboratories. The diagnosis is usually made using the nitroblue tetrazolium test, which identifies neutrophils that cannot generate superoxide. Carrier detection for the X-linked form can be undertaken by examination of peripheral blood neutrophils for a subpopulation of abnormal cells or, more reliably, by direct mutation analysis. In a large series of 131 kindreds, the mother was found to be a carrier in 89% of cases.

Severe Combined Immunodeficiency (also known as: SCID; Swiss-type agammaglobulinemia) MIM

102700 (adenosine deaminase [ADA] deficiency) 164050 (purine nucleoside phosphorylase [PNP] deficiency) 300400 (X-linked form)

Clinical features

Severe defects in both cellular and humoral immunity result in early susceptibility to bacterial, viral, and fungal infections. Infants usually present with recurrent and persistent diarrhea, pneumonitis, and dermatitis. Common infectious agents include candida and Pneumocystis carinii. Immunization with live agents (such as BCG [bacille Calmette–Guérin] and polio) can be life-threatening. Without treatment, the disease course is rapidly progressive with death by the age of 1–2 years. Milder forms present later and have a much better prognosis.

Age of onset

Usually around the age of 2–3 months with recurrent infection

Epidemiology

At the age of 1 year the estimated prevalence of the autosomal recessive and X-linked recessive forms is approximately 3 in 1,000,000. Much higher figures have been noted in genetic isolates, such as the Mennonites in Manitoba and in some native North American Indians. The X-linked form accounts for around 50% of all cases.

Inheritance, See Table 2. chromosomal location, gene, and mutational spectrum

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Form

Inheritance Chromosomal Gene location

Adenosine deaminase deficiency

Autosomal recessive

Mutational spectrum

20q13.11

ADA (adenosine Mainly missense deaminase) point mutations. Also small deletions and splice-site mutations

Purine Autosomal nucleoside recessive phosphorylase deficiency

14q13.1

PNP (purine nucleoside phosphorylase)

X-linked form

Xq13

IL2RG Mainly missense (interleukin-2 and nonsense receptor, γ chain) mutations

X-linked recessive

Missense and splice-site mutations and small deletions

Table 2. Severe combined immunodeficiency: inheritance, chromosomal locations, genes, and mutational spectra.

Molecular pathogenesis

In the USA it has been estimated that approximately 50% of SCID cases are of the X-linked form. The remainder show autosomal recessive inheritance, with ADA and PNP deficiency accounting for around 15% of all cases. The remaining 35% of cases can be caused by any one of several individually rare autosomal recessive immune defects, including major histocompatibility complex class I and II deficiency (bare lymphocyte syndrome [MIM 209920]) and interleukin (IL)-2 receptor abnormalities. ADA consists of 12 exons and 11 introns, with a large number of Alu repetitive elements that predispose to the formation of tiny deletions through homologous recombination. The enzyme is present in all tissues, with highest activity in the thymus and other lymphoid tissues. Enzyme activity is usually reduced to less than 1% of normal in the cell lines of affected patients. PNP consists of six exons and five introns and is ubiquitously expressed, with its highest levels in erythrocytes and the kidneys. Affected patients show extremely low levels of enzyme activity. With both ADA and PNP deficiency there is a correlation between the level of enzyme activity and clinical outcome, with levels greater than 1% being associated with later onset of disease. Such levels are usually caused by intronic single base-pair substitutions, which cause exon skipping or activation of cryptic splice sites. The precise mechanism by which ADA and PNP deficiency cause SCID remains unclear. Normally, these enzymes catalyze sequential steps in the metabolism of purine ribonucleosides and deoxyribonucleosides.

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The prevailing view is that substrate accumulation or alternative metabolites have a toxic effect on lymphocyte differentiation and function, possibly by inhibition of ribonucleotide reductase, which is required for DNA replication in dividing cells. In reality, it is likely that several different toxic effects contribute to the severe lymphocytopenia. The relatively common X-linked form of SCID is caused by mutations in IL2RG, which encodes the γ chain of the IL-2 receptor. This subunit is shared with four other IL receptors, (IL-4, -7, -9, and -15), which probably accounts for the very severe phenotypic consequences of IL2RG mutations. These include impaired growth and differentiation of T and B cells, and of cells of monocyte lineage. IL2RG consists of eight exons with a wide spectrum of pathogenic missense and nonsense mutations that result in defective γ chains necessary for formation of the high and intermediate affinity IL cytokine family receptor. Genetic diagnosis and counseling

Diagnosis is usually made on the basis of reduced numbers of T cells and very low levels of immunoglobulins. Specific mutation analysis for ADA, PNP, and IL2RG, together with other known rarer genetic defects, is available at a small number of specialized reference laboratories. The mother is shown to be a carrier in around 50% of isolated cases of the X-linked form. If specific mutation analysis is not available for female carrier detection, this can be achieved by analysis of X-chromosome inactivation in T lymphocytes: carriers show a nonrandom pattern. Hair root DNA has been used for analysis in children who have undergone successful bone marrow transplantation from a histocompatible sibling or other relative. Gene therapy using autologous transduced hematopoietic stem cells has been applied successfully for ADA deficiency and the X-linked form.

Wiskott–Aldrich Syndrome (also known as: WAS) MIM

301000

Clinical features

These include the classic triad of thrombocytopenia, infection, and eczema, as well as a tendency to develop lymphoid malignancy. The phenotype varies from severe infection with early death in childhood to a mild thrombocytopenia. Thrombocytopenia is the earliest feature, with platelets that are reduced in size and have a diminished half-life.

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Eczema and infection with pyogenic bacteria develop in infancy in association with variable defects in humoral and cellular immunity. Other features can include nephropathy and hemolytic anemia. Bone marrow transplantation restores normal platelet features and radically improves the long-term outcome. Age of onset

Thrombocytopenia can present in the neonatal period.

Epidemiology

WAS is rare, with an estimated incidence in the USA of 1 in 250,000 male births.

Inheritance

X-linked recessive

Chromosomal location

Xp11.22–p11.23

Gene

WAS, also known as WASP (WAS protein)

Mutational spectrum Missense mutations in exons 1–4. Nonsense, frame-shift, and splice-site mutations in exons 6–11. Molecular pathogenesis

WAS contains 12 exons and encodes a protein that is involved in both intracellular signaling and regulation of the actin cytoskeleton. Its role in cell signaling, and hence growth and differentiation of early hematopoietic precursors, is mediated by interaction of the WAS protein with tyrosine kinases. Regulation of the cytoskeleton and cell movement involves interaction of the WAS protein with actin. Both intrafamilial and interfamilial variations have been demonstrated at the phenotypic level. Missense mutations resulting in reduced levels of a normally sized WAS protein have been noted in mild X-linked thrombocytopenia (MIM 313900), whereas nonsense and frame-shift mutations tend to result in the much more severe WAS phenotype. However, no consistent genotype–phenotype correlation has emerged. Curiously, it has been shown that an activating mutation in WAS can cause a rare X-linked form of severe congenital neutropenia (MIM 300299).

Genetic diagnosis and counseling

Mutation detection is available on a limited basis at designated specialized laboratories. This can also be utilized for prenatal diagnosis and carrier detection. Carrier detection can be achieved by demonstrating nonrandom X-chromosome inactivation in B cells and T cells from peripheral blood. Counseling is on the basis of X-linked recessive inheritance.

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13 13. Metabolic Disorders

Medium Chain Acyl-CoA Dehydrogenase Deficiency 210 Menkes Disease 211 Ornithine Transcarbamylase Deficiency 212 Phenylketonuria 214 Wilson Disease 215

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Medium Chain Acyl-CoA Dehydrogenase Deficiency (also known as: MCAD deficiency) MIM

201450

Clinical features

Presentation is usually with acute onset of hypoglycemia and vomiting leading to convulsions and coma, often following a period of fasting. Blood levels of ammonia are elevated and liver function tests are abnormal. The clinical presentation falls within the differential diagnosis of Reye’s syndrome and can be misdiagnosed as sudden infant death syndrome because of the high associated mortality.

Age of onset

Usually between 3 months and 2 years. Some affected individuals first present in later childhood or adolescence and some remain asymptomatic throughout life.

Epidemiology

The incidence in the UK and USA is approximately 1 in 15,000 to 1 in 20,000 live births. The carrier frequency is approximately 1 in 65.

Inheritance

Autosomal recessive

Chromosomal location

1p31

Gene

ACADM (acyl-CoA dehydrogenase, medium-chain)

Mutational spectrum Most mutations (90%) involve a 985A→G transition resulting in a Lys304Glu substitution (this is also referred to as Lys329Glu based on the precursor protein structure). The remaining 10% are mainly missense, with small numbers of deletion and duplication frame-shifts. Molecular pathogenesis

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ACADM contains 12 exons and encodes a 421-amino-acid protein that catalyzes the initial reaction in the beta-oxidation of C4 to C12 straight-chain acyl-CoAs. This is one step in the complex sequence of events which releases energy through mitochondrial fatty acid oxidation. The common Lys304Glu mutation interferes with the assembly of the mature homotetrameric enzyme and with its stability in mitochondria. Linkage disequilibrium studies are consistent with a common founder effect. No straightforward genotype–phenotype correlation has emerged.

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Analysis of serum acylcarnitines by tandem mass spectrometry provides a rapid and reliable preclinical diagnosis and has been proposed as a means of neonatal screening using dried blood spots. DNA analysis is widely available for the common mutation. Rare mutation detection by sequencing is undertaken on a very limited basis. Prenatal diagnosis can be achieved by either DNA analysis or enzyme assay.

Menkes Disease (also known as: kinky hair disease) MIM

309400

Clinical features

The clinical features are a direct consequence of copper deficiency. Affected infants present with lethargy, failure to thrive, convulsions, and spasticity. Growth and development are severely delayed and death occurs in early childhood. Other characteristic features include hypothermia, Wormian bones, and fragile, steely, depigmented hair (see Figure 1), which on microscopy shows pili torti (see Figure 2). Some affected children have a less severe phenotype with longer survival.

Figure 1. Appearance of the hair in a boy with Menkes disease.

Figure 2. Pili torti in a boy with Menkes disease.

Age of onset

Features are apparent at birth or develop in early infancy.

Epidemiology

The incidence in live births has been estimated to be between 1 in 100,000 and 1 in 250,000.

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Inheritance

X-linked recessive

Chromosomal location

Xq12–q13

Gene

ATP7A (Cu2+-transporting ATPase, α polypeptide) also known as MNK

Mutational spectrum Mainly insertions, deletions, nonsense, and splice-site mutations, resulting in protein truncation. Molecular pathogenesis

ATP7A contains 23 exons and encodes a 1,500-amino-acid copper-transporting ATPase with copper-binding, phosphorylation, transduction, and ATP-binding domains. The protein is localized in the membrane of the trans-Golgi network. It cycles between the trans-Golgi and plasma membranes, facilitating the trafficking and cellular efflux of copper. Mutations result in accumulation of copper in the intestinal mucosa and kidney, with deficiency in other tissues, such as the central nervous system. The mutational spectrum is very heterogeneous, with 40% of patients in one series having truncating insertions or deletions. Splice-site mutations that allow a degree of normal mRNA processing result in milder phenotypes, such as the occipital horn syndrome (MIM 304150, see p.112).

Genetic diagnosis and counseling

The diagnosis is made by assay of serum copper and ceruloplasmin, both of which are low. Mutation and linkage analysis are available in specific laboratories and are preferred to copper uptake studies for carrier detection and prenatal diagnosis because of the technical difficulties associated with copper uptake studies.

Ornithine Transcarbamylase Deficiency (also known as: OTC deficiency; ornithine carbamoyltransferase [OCT] deficiency) MIM

311250

Clinical features

In the classic severe form, affected male infants present soon after birth with lethargy, convulsions, coma, and severe hyperammonemia, leading to a rapidly fatal outcome. More mildly affected boys and some heterozygous females present later in childhood or in adult life with behavioral disturbance and alterations of consciousness, particularly after a heavy protein load. Older patients often develop an aversion to

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protein. These individuals usually have a good prognosis, but are at risk of developing hyperammonemia during intercurrent illness or episodes of fasting. Age of onset

Onset typically occurs after protein ingestion soon after birth.

Epidemiology

The overall incidence has been estimated to be 1 in 80,000 live births.

Inheritance

X-linked, with many female carriers showing mild manifestations

Chromosomal location

Xp21.1

Gene

OTC (ornithine transcarbamylase)

Mutational spectrum This is very heterogeneous, with over 200 unique mutations reported. Large deletions account for 10%–15% of all mutations. The rest consist mainly of missense point mutations with smaller numbers of nonsense, splice-site, and small deletion mutations. Molecular pathogenesis

OTC contains 10 exons and encodes the OTC enzyme, which catalyzes the conversion of ornithine and carbamoyl phosphate to citrulline. OTC is a homotrimeric mitochondrial enzyme that is expressed almost exclusively in liver. It is synthesized as a subunit in cytoplasm as a precursor with an NH2 extension. The NH2 extension is required for mitochondrial uptake prior to cleavage in the mitochondria. Mutations are distributed throughout the gene and most are unique to individual families. An exception is a recurring Arg109Gln substitution, which accounts for 10% of all missense mutations. Those mutations that result in severe neonatal disease are clustered in important functional or structural domains in the interior of the protein at sites of enzyme activity or at the interchain surface. Mutations associated with late-onset disease are located on the surface of the protein.

Genetic diagnosis and counseling

Biochemical methods are unreliable for carrier detection, so mutation analysis is the preferred method. This is undertaken at several specialist laboratories. Prenatal diagnosis can only be achieved by liver biopsy, which is hazardous, or by molecular analysis based on specific mutation detection or linkage. The male to female mutation rate has been shown to be very high (ie, most mutations originate in males), indicating that

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the mothers of most isolated affected males are carriers. At present, only approximately 80% of all mutations can be identified suggesting that some mutations probably occur in introns or control sequences.

Phenylketonuria (also known as: PKU; phenylalanine hydroxylase deficiency) MIM

261600

Clinical features

Children develop severe mental retardation with eczema and hypopigmentation of hair and skin. Neurologic sequelae include spasticity, hyperactivity, convulsions, and occasional autistic behavior. Careful dietary restriction of phenylalanine up to the age of 12 years results in normal intellectual development. Most children with elevated blood phenylalanine levels (detected through neonatal screening programs) have either transient or mild hyperphenylalaninemia (enzyme level >5%). This usually conveys a good prognosis and does not require treatment.

Age of onset

Congenital

Epidemiology

The average incidence in western and northern European populations is 1 in 10,000 live births (the carrier frequency is 1 in 50). Lower incidences of 1 in 50,000 and 1 in 143,000 have been noted in black Americans and the Japanese, respectively.

Inheritance

Autosomal recessive

Chromosomal location

12q24.1

Gene

PAH (phenylalanine hydroxylase)

Mutational spectrum

This is very heterogeneous with over 350 separate mutations reported (www.mcgill.ca/pahdb). Most of these are missense, with small numbers of splice-site and nonsense mutations.

Molecular pathogenesis

PAH is 90 kb long and is transcribed into a 2.4-kb mRNA. It encodes the enzyme responsible for the conversion of phenylalanine to tyrosine. The active enzyme exists as a trimer or tetramer – most mutations

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impair enzyme activity by causing protein instability or abnormal aggregation. Mutations in the N-terminal domain show clustering in residues 46–48 and affect the ability of the enzyme to bind to phenylalanine. Although many mutations have been reported, they tend to be associated with a small number of haplotypes. There is some evidence to support heterozygote advantage, possibly resulting from protection against spontaneous miscarriage. Two of the most common northern European mutations are an Arg408Trp substitution and a GT→AT substitution in the 5´ splice site of intron 12, which results in skipping of exon 12 during RNA splicing. Genetic diagnosis and counseling

The diagnosis of PKU is based on conventional biochemical assay. However, the enzyme is only expressed in the liver so prenatal diagnosis can be achieved only by molecular analysis (either specific mutation detection or linkage analysis). Mutation detection is undertaken at a small number of laboratories. In 1%–3% of children with severe hyperphenylalaninemia, the basic defect lies not in PAH but in one of the enzymes involved in the synthesis or recycling of tetrahydrobiopterin (BH4), the cofactor for PAH in the conversion of phenylalanine to tyrosine. These enzymes include dihydropteridine reductase (MIM 261630), pterin 4-α-carbinolamine dehydratase (MIM 264070), and 6-pyruvoyltetrahydropterin synthase (MIM 261640). Treatment for these enzyme deficiencies, all of which show autosomal recessive inheritance, involves both a low phenylalanine diet and supplementation with neurotransmitters, which are deficient as a result of lack of BH4 (which acts as a cofactor for two other enzymes, tyrosine hydroxylase and tryptophan hydroxylase).

Wilson Disease (also known as: hepato-lenticular degeneration) MIM

277900

Clinical features

Increased copper deposition in the brain, cornea, and liver, results in progressive neurologic and liver disease. Neurologic involvement can include behavioral disturbance, dystonia, tremor, and spasticity, with occasional frank psychiatric illness. Copper deposition in the cornea

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results in a characteristic brown–green collarette of discoloration known as a Kayser–Fleischer ring. Hepatic involvement culminates in cirrhosis and liver failure. Other findings can include hemolytic anemia and renal Fanconi syndrome. Age of onset

Late childhood or early adult life

Epidemiology

The incidence worldwide is estimated to be between 1 in 30,000 and 1 in 55,000. Wilson disease is particularly common in Sardinia.

Inheritance

Autosomal recessive

Chromosomal location

13q14.3–q21.1

Gene

ATP7B (Cu2+-transporting ATPase β polypeptide), also known as WND

Mutational spectrum

Mainly missense mutations with a common His1069Gln mutation in European populations. Also nonsense mutations, deletions, and insertions.

Molecular pathogenesis

ATP7B encodes a 1,411-amino-acid copper-transporting ATPase with close homology to the Menkes syndrome copper-transporting protein (see p.211–2). In the liver, ATPase is localized to the trans-Golgi network, where it is responsible for the transport of copper into the hepatocyte secretory pathways for incorporation into ceruloplasmin and for excretion of copper into bile. As with the Menkes syndrome protein, the WND ATPase contains multiple domains including copper-binding and ATP-binding domains. The common European His1069Gln mutation disrupts ATP-binding. Truncating mutations tend to be associated with both a more severe phenotype and the onset of liver disease in childhood.

Genetic diagnosis and counseling

Serum copper and ceruloplasmin levels are usually low, while urinary copper excretion is raised, as is the level of copper in liver tissue. Mutation analysis can be undertaken at a small number of centers and can be used for prenatal and preclinical diagnosis. Counseling is as for autosomal recessive inheritance with a carrier frequency of approximately 1 in 100.

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14 14. Renal Disorders

Alport Syndrome 218 Beckwith–Wiedemann Syndrome 220 Cystinosis 224 Orofaciodigital Syndrome Type I 225 Polycystic Kidney Disease 226

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Alport Syndrome (also known as: AS; nephropathy and deafness) MIM

104200 (autosomal dominant form [ADAS]) 203780 (autosomal recessive form [ARAS]) 301050 (X-linked form [XLAS]) 308940 (Alport syndrome and diffuse leiomyomatosis [ASDL])

Clinical features

AS is characterized by renal, cochlear, and ocular involvement. Presentation is usually with persistent microhematuria, or occasional gross macroscopic hematuria in childhood followed by proteinuria and eventual end-stage renal disease in middle age. Sensorineural hearing loss develops in early adult life and is progressive. Anterior lenticonus (ie, protrusion of the central part of the lens into the anterior chamber) becomes apparent in early adulthood and is sometimes associated with lens opacities and recurrent corneal erosion. In the very rare ASDL form, leiomyomata develop in childhood in the upper airways, esophagus, and female genital tract.

Age of onset

Presentation is usually in late childhood or early adult life with hematuria and/or sensorineural hearing loss.

Epidemiology

AS has an estimated incidence of approximately 1 in 50,000 live births.

Inheritance

Around 80% of cases show X-linked inheritance with heterozygous females showing only asymptomatic microhematuria, although a few develop progressive renal disease. Of the remaining cases, 15% show autosomal recessive and 5% autosomal dominant inheritance.

Chromosomal See Table 1. location, gene, and mutational spectrum Molecular pathogenesis

218

In most, if not all, forms of AS the basic defect is in abnormal expression of type IV collagen genes in the basement membrane (where type IV collagen is the main structural component). Six type IV collagen genes have been cloned and are located pairwise on chromosomes 13q34 (COL4A1 and COL4A2), 2q36–q37 (COL4A3 and COL4A4), and Xq22.3 (COL4A5 and COL4A6). They encode type IV α collagen

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Form

Chromosomal Gene location

Mutational Spectrum

ADAS (autosomal dominant)

2q36–q37

Unknown

Unknown

ARAS (autosomal recessive)

2q36–q37

COL4A3, COL4A4

Nonsense mutations and deletions with a loss of function effect

XLAS (X-linked)

Xq22.3

COL4A5

Large deletions (20%), missense (35%–40%), splice-site (15%), and nonsense mutations or small frame-shift deletions/insertions (25%–30%), with either loss of function or dominant-negative effects

ASDL (Alport syndrome and diffuse leiomyomatosis

Xq22.3

COL4A5, COL4A6

Large deletions, including the 5′ exons of both COL4A5 and COL4A6

Table 1. Alport syndrome: forms, chromosomal locations, genes, and mutational spectra.

chains; these share a similar structure with a carboxy-terminal noncollagenous domain (NC1), a long collagenous domain (with a repetitive glycine-X-Y triplet sequence), and a short noncollagenous amino-terminal sequence. These chains form heterotrimers, which in turn form a nonfibrillar network in the basement membrane. This acts as a scaffold for the deposition of other matrix components. In normal renal development, there is a switch from COL4A1 and COL4A2 expression in early childhood to COL4A3, COL4A4, and COL4A5 expression as glomerular maturation proceeds. This switch usually does not occur in AS, resulting in the accumulation of type IV α1 and α2 chains together with types V and VII collagen in the glomerular basement membrane, possibly as a result of compensatory activation of the relevant genes. This accumulation of inappropriate collagen may account for the observed electron microscopic changes of thickening and splitting of the glomerular basement membrane, with progressive glomerulosclerosis. It is not clear why a mutation in one of the genes encoding the α3–α4–α5 network should prevent expression of the other genes. The explanation may lie in a proposed dominant-negative effect resulting from integration of a single abnormal chain in a trimeric molecule, as observed in osteogenesis imperfecta (see p.119–24).

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There is only a weak correlation between genotype and phenotype, with some evidence that glycine substitutions and splice-site mutations tend to be associated with a later onset of end-stage renal failure. Antiglomerular basement membrane nephritis develops in 10%–20% of patients receiving a renal transplant due to the development of antibodies to the type IV α3 chain. This is most likely to occur when a large deletion or nonsense mutation in COL4A3 or COL4A5 leads to absence of the NC1 domain of the type IV α3 chain in the patient’s basement membrane, with immunologic intolerance for this domain in transplanted kidneys. The rare ASLD is caused by large deletions that remove the 5′ ends of the COL4A5 and COL4A6 genes, which are located close together in a head-to-head orientation. The deletion breakpoint in COL4A6 is always in the second intron. It is not known how these deletions predispose to the widespread development of leiomyomata. Genetic diagnosis and counseling

Mutation analysis is available on a limited basis for XLAS. If a specific mutation cannot be identified, then linkage analysis can be utilized for carrier detection and prenatal diagnosis. Approximately 90% of XLAS carriers show microhematuria. Mutation analysis is available on a research basis only for ARAS. Up to 50% of ARAS carriers show microhematuria, so it can sometimes be difficult to distinguish ADAS from ARAS. ADAS is rare and shows clinical overlap with the Epstein (MIM 153650) and Fechtner (MIM 153640) syndromes, in which hereditary nephritis and deafness are associated with thrombocytopenia and large platelets. These very rare autosomal dominant disorders are caused by mutations in MYH9 (which encodes the nonmuscle myosin heavy chain 9).

Beckwith–Wiedemann Syndrome (also known as: BWS; exomphalos-macroglossia-gigantism [EMG] syndrome) MIM

130650

Clinical features

As reported by Beckwith in 1963 and Wiedemann in 1964, BWS is characterized by variable overgrowth with macroglossia, ear pits or creases, visceromegaly, hemihypertrophy, and abdominal wall defects, including exomphalos and umbilical hernia. Potentially serious complications include neonatal hypoglycemia (over 50% of cases)

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and childhood-onset tumors (7%–20% of cases). Of these, the most common is Wilms tumor, with an estimated incidence of 7%–10% by age 4 years. Other reported tumors include hepatoblastoma, neuroblastoma, adrenal carcinoma, and rhabdomyosarcoma. Age of onset

The diagnosis is usually apparent at birth and can be suspected on the basis of ultrasound findings of visceromegaly and exomphalos in the second trimester.

Epidemiology

The incidence in live-born infants is approximately 1 in 13,000. All ethnic groups are affected.

Inheritance

Autosomal dominant, with exclusively maternal transmission in 15% of cases. The remaining 85% of cases are sporadic and are caused by several different epigenetic mechanisms, as discussed in the following sections.

Chromosomal location

11p15

Genes

CDKN1C (cyclin-dependent kinase inhibitor 1C) H19 (also know as ASM1 – adult skeletal muscle) IGF2 (insulin-like growth factor 2) KVLQT1 (potassium channel, voltage-gated KQT-like subfamily, member 1), KVLQT1-AS (KVLQT1 antisense)

Mutational spectrum The genetic mechanisms involved in BWS are complex and have not and molecular been fully elucidated. They involve disturbance in the expression of pathogenesis a cluster of genes located in the BWS critical region at 11p15. Most of these genes are imprinted through differential methylation, depending on the parent of origin. The BWS critical region is thought to contain at least two imprinting centers plus at least 12 other genes, including the five listed above (see Figure 1). Numerous chromosomal and molecular abnormalities have been identified in patients with BWS. Mechanistically, these can be considered under the following headings. Paternal uniparental disomy (UPD) This is present in approximately 10%–20% of sporadic cases, usually in the form of mosaicism for a normal cell line and a cell line showing segmental 11p15 paternal UPD, probably arising as a result of somatic

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Chromosome II

11p15

Centromere

BWSCR

pter

Maternal chromosome

Expressed

H19

IGF2

KVLQTI-AS

KVLQTI

CDKNIC

Paternal chromosome

Not expressed

Figure 1. Schematic representation of the Beckwith–Wiedemann syndrome critical region (BWSCR) on the short arm of chromosome 11.

(mitotic) recombination involving homologous nonsister chromatids. The demonstration of paternal UPD implies loss of the normal maternal imprint. Chromosome abnormalities A visible chromosome abnormality can be identified in around 1% of all cases. This usually involves a paternally derived 11p15 duplication, a maternally derived 11p15 deletion, or rearrangement with an 11p15 breakpoint. It is hypothesized that any such breakpoint will disturb the setting of the normal maternal imprint. Germline mutations in CDKN1C Missense and nonsense substitutions, and deletions and insertions have been identified in CDKN1C in approximately 40% of familial and 5% of sporadic cases. CDKN1C is expressed almost exclusively from the maternally derived chromosome 11. It encodes a regulatory kinase that inhibits cell proliferation in the G1 phase of the cell cycle. Errors of imprinting and methylation Based on expression studies of several imprinted genes in the BWS gene cluster, defects in at least two imprinting centers are suspected. KVLQT1-AS (also known as LIT1) is normally expressed from the

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paternal allele only, but KVLQT1-AS shows loss of imprinting in 40%–50% of sporadic cases, with expression from the maternal allele also. This gene encodes an antisense transcript for KVLQT1, which is normally only expressed from the maternal allele. However, in many BWS cases it is also found to be expressed from the paternal allele. The role of these two genes in causing the features of BWS is not known. (Note that mutations in KVLQT1 can cause Jervell and Lange-Nielsen syndrome, MIM 220400.) Evidence for the involvement of a second imprinting center stems from expression studies of H19 and IGF2, two closely linked genes telomeric of KVLQT1. These show reciprocal imprinting with paternal expression of IGF2 and maternal expression of H19. IGF2 encodes a fetal growth factor, whereas H19 encodes an mRNA that suppresses tumor growth. In approximately 10%–15% of BWS patients, this IGF2/H19 domain shows an abnormal imprinting pattern that is independent of the imprinting status at the KVLQT1 locus. The precise causal mechanisms in BWS are not well understood. As a gross oversimplification, but a useful aide mémoire, it can be helpful to consider the overgrowth in BWS as a consequence of either overexpression of paternally expressed growth promoting genes such as IGF2 (as a result of chromosomal duplication, paternal UPD or aberrant imprinting), or underexpression of maternally derived growth suppressing genes such as CDKN1C (as a result of chromosome deletion or disruption or an imprinting error). Genetic diagnosis and counseling

Renal Disorders

Detailed chromosome analysis should be undertaken in all cases, focusing specifically on chromosome 11p15. Specific mutation and imprinting studies are available at only a few specialized research laboratories. Regarding genotype–phenotype correlation, there is a suggestion that hemihypertrophy shows a positive association with paternal UPD and that the risk of tumor formation is greatest when there is abnormal methylation of H19. The recurrence risk is very low for cases due to paternal UPD and 50% for maternally inherited mutations in CDKN1C. A recurrence risk of 5% has been suggested for isolated cases of unknown cause.

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Cystinosis MIM

219800 (infantile nephropathic cystinosis) 219900 (juvenile or adolescent nephropathic cystinosis) 219750 (adult nonnephropathic cystinosis)

Clinical features

Affected children present with polyuria, polydipsia, dehydration, failure to thrive, and rickets, all as a result of renal Fanconi syndrome. Without treatment, end-stage renal failure develops between 8 and 12 years of age. Other characteristic features include photophobia due to the deposition of cystine crystals in the cornea, hypothyroidism, myopathy, and blindness.

Age of onset

Features of Fanconi syndrome are evident from 6–12 months of age.

Epidemiology

Cystinosis affects all ethnic groups with an incidence of 1 in 100,000 to 1 in 200,000 live births.

Inheritance

Autosomal recessive

Chromosomal location

17p13

Gene

CTNS

Mutational spectrum

Over 50 mutations have been described, most of which result in loss of function. About 50% of patients of northern European ancestry are homozygous for a 57-kb intragenic deletion that results in a loss of the first 10 exons of the gene. Smaller deletions, insertions, missense, nonsense, and splice-site mutations have also been described. Patients with nephropathic cystinosis have two severe mutations that result in almost complete lack of protein expression. Patients with nonnephropathic cystinosis have one severe mutation and one mild mutation, so that some protein function is retained.

Molecular pathogenesis

CTNS has 12 exons and encodes a 367-amino-acid protein called cystinosin. Cystinosin is an integral lysosomal membrane protein with seven transmembrane domains that is expressed in fetal and adult kidneys, pancreas, and skeletal muscle. Cystine produced from protein degradation is transported by cystosin from the lysosome to the cytoplasm, where it can be reutilized in protein synthesis. Lack of

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cystinosin causes cystine to accumulate in lysosomes. Accumulation of cystine in various tissues is responsible for the clinical features of cystinosis. Cystinosis is therefore a lysosomal storage disorder. Genetic diagnosis and counseling

The diagnosis of cystinosis can be confirmed by white-cell cystine assay. Another useful diagnostic test is slit-lamp examination to look for the characteristic corneal crystals, although crystal deposition might not be evident in infancy. CTNS mutation analysis is available from a few specialized laboratories and is useful for carrier detection. Counseling is on an autosomal recessive basis. Prenatal diagnosis is available by cystine assay using cultured chorionic villi or cultured amniocytes.

Orofaciodigital Syndrome Type I (also known as: OFD1; oral-facial-digital syndrome type I) MIM

311200

Clinical features

The orofaciodigital syndromes consist of a heterogeneous group of nine discrete entities with overlapping clinical features. The oral findings in OFD1 include a midline cleft lip, tongue clefts or nodules, multiple frenula, and a high-arched or cleft palate. Facial features include a prominent forehead with hypertelorism and a broad nasal bridge with alar hypoplasia. The hands show variable brachydactyly, clinodactyly, and syndactyly, while preaxial polysyndactyly is the most characteristic finding in the feet. Mild mental retardation is reported in up to 40% of cases, often in association with a central nervous system malformation such as hydrocephalus or partial agenesis of the corpus callosum. Adult-onset polycystic kidney disease, with the cysts being of glomerular origin, is a common complication.

Age of onset

The dysmorphic features are present at birth (they can usually also be detected using ultrasound during the second trimester).

Epidemiology

OFD1 is by far the most common of the OFD syndromes, all of which are rare.

Inheritance

X-linked dominant, with intrauterine lethality in males

Chromosomal location

Xp22.3–p22.2

Renal Disorders

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Gene

CXORF5 (chromosome X open reading frame 5), also known as OFDI

Mutational spectrum

Missense, nonsense, splice-site, and frame-shift mutations

Molecular pathogenesis

CXORF5 was one of the first transcripts to be assigned to the X chromosome. It contains 23 exons with two transcripts subject to alternative splicing, which are widely expressed. The locus is not subject to normal X-chromosome inactivation and the identity of the protein product is unknown. This is predicted to contain many coiled-coil α-helical domains, similar to those in the protein products of genes that cause adult polycystic kidney disease types 1 and 2 (see next entry). This observation may explain the relatively high incidence of polycystic kidney disease in OFD1.

Genetic diagnosis and counseling

The diagnosis is based on the pattern of clinical findings rather than molecular analysis, which is only available on a research basis. Counseling is as for X-linked dominant inheritance with male lethality.

Polycystic Kidney Disease (also known as: PKD. Includes adult polycystic kidney disease [APKD], infantile or autosomal recessive polycystic kidney disease [ARPKD], polycystic kidneys, and hepatic disease) MIM

See Table 2.

Clinical findings

Both forms of APKD usually present in the third to fifth decades with hematuria, abdominal pain or swelling, and uremia, although the onset of hypertension is usually earlier (second or third decade). Ultrasonography shows enlargement of the kidneys, with multiple large discrete cysts distributed throughout both kidneys. Approximately 75% of affected individuals develop end-stage renal failure by the age of 70 years. Known associations include polycystic liver disease and intracerebral aneurysms. Cysts may also be present in the pancreas and spleen. The disease runs a milder course in APKD2 than in APKD1, with end-stage renal disease occurring at average ages of 69 and 53 years, respectively. ARPKD usually presents at birth with massive bilateral nephromegaly and respiratory problems due to associated oligohydramnios.

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Form

MIM

Inheritance

Chromosomal location

Gene

Mutational spectrum

APKD1 (adult polycystic kidney disease type 1)

173900

Autosomal dominant

16p13.3

PKD1 Mainly nonsense or frame-shift (polycystic kidney mutations, with some missense disease 1) point mutations and in-frame deletions. All have a loss of function effect

APKD2 (adult polycystic kidney disease type 2)

173910

Autosomal dominant

4q12–22

PKD2 Nonsense and splice-site (polycystic kidney mutations, frame-shift deletions, disease 2) and insertions. All have a loss of function effect

ARPKD (autosomal recessive polycystic kidney disease)

263200

Autosomal recessive

6p21.1–p12

PKHD1 (fibrocystin)

Missense and nonsense point mutations. Also small deletion and insertion frame-shifts

Table 2. Polycystic kidney disease: forms, MIM numbers, inheritance chromosomal locations, genes, and mutational spectra.

Approximately 30% of affected children die in infancy. Fifty percent of those who survive develop end-stage renal failure by the age of 10 years. Congenital hepatic fibrosis is a constant finding and leads to liver dysfunction in later childhood. Age of onset

APKD can present in the neonatal period or in childhood, but usually presents in middle age. ARPKD is often first suspected on ultrasound scanning during pregnancy or at delivery because of dystocia caused by the enlarged kidneys.

Epidemiology

APKD affects approximately 1 in 1,000 individuals worldwide. ARPKD has an estimated incidence of approximately 1 in 20,000 live births with a somewhat higher frequency in the Afrikaner population of South Africa.

Inheritance, See Table 2. chromosomal location, gene, and mutational spectrum Molecular pathogenesis

Renal Disorders

Mutations in PKD1 account for 85% of APKD families. PKD1 contains 46 exons and encodes a large protein known as polycystin 1. This associates with the cell membrane at points of cell–cell contact and contains 11 transmembrane domains, with a coiled-coil domain in the cytoplasmic tail. PKD2 contains 15 exons and encodes a smaller (968-amino-acid) protein known as polycystin 2. This contains six transmembrane domains, with cytoplasmic N- and C-termini. 227

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Polycystin 1 and 2 are believed to interact by binding of the coiled-coil domain of polycystin 1 with the C-terminal tail of polycystin 2 to produce calcium-permeable cation channels in the renal tubular cell membrane. Renal cyst formation has been shown to involve a “two-hit” mechanism involving both a germ-line and a somatic mutation in PKD1 and/or PKD2. When mutations in both APKD genes are involved, this is referred to as a “transheterozygous” state. Rare examples of individuals with both APKD and tuberous sclerosis are the result of contiguous gene deletions involving the closely adjacent PKD1 and TSC2 loci on chromosome 16. The renal disease in these individuals is usually much more severe than in typical APKD1. ARPKD is caused by mutations in PKHD1, which contains 66 exons and encodes a large protein, known as fibrocystin; this is expressed in adult kidney, liver, and pancreas. It has been proposed that fibrocystin is a receptor protein that acts in renal tubule and biliary duct differentiation. Genetic diagnosis and counseling

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Specific mutation analysis for PKD1 is difficult because of the presence of multiple silent copies of part of the gene elsewhere on chromosome 16. Linkage analysis for both APKD1 and APKD2 is readily available for preclinical and prenatal diagnosis. However, demand for prenatal diagnosis is very limited because very few families wish to pursue termination of pregnancy for adult PKD as they don’t perceive it to be a serious enough problem. Linkage analysis can also be used for the prenatal diagnosis of ARPKD. Clinically, APKD shows full penetrance by age 30 years, with all heterozygotes showing cysts on renal ultrasonography.

Polycystic Kidney Disease

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ADA ADAS AHO AIS AMN APKD ARAS AS ASDL AT ATM ATR-X BBS BCG BEK BWS CAH CAL CBAVD CF CFTR CGD CHN CK CNS CRM DI DMD EEG EGF EMG ERG FA FISH FMRP FMTC FSHMD G6PD

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adenosine deaminase autosomal dominant form of Alport syndrome Albright’s hereditary osteodystrophy androgen insensitivity syndrome adrenomyeloneuropathy adult polycystic kidney disease autosomal recessive form of Alport syndrome Alport or Angelman syndrome Alport syndrome and diffuse leiomyomatosis ataxia–telangiectasia ataxia–telangiectasia mutated X-linked α-thalassemia and mental retardation Bardet–Biedl syndrome bacille Calmette–Guérin bacterially expressed kinase Beckwith–Wiedemann syndrome congenital adrenal hyperplasia café-au-lait congenital bilateral absence of the vas deferens cystic fibrosis cystic fibrosis transmembrane conductance regulator chronic granulomatous disease congenital hypomyelinating neuropathy creatine kinase central nervous system cross-reacting material diabetes insipidus Duchenne muscular dystrophy electroencephalograph epidermal growth factor exomphalos-macroglossia-gigantism electroretinogram Fanconi anemia fluorescence in situ hybridization FMR protein familial medullary thyroid carcinoma facioscapulohumeral muscular dystrophy glucose-6-phosphate dehydrogenase

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GAG GAP GDNF GH GHR GROD Hb HD HE HGPRT HME HMSN HNPP HOS HPE HPP HS HSCR Ig IGF IL KGFR LCA LGMD MASA MBD MCAD MD MDS MEBD MEN2 MIM MPS MTC NBF NCL NCV

Abbreviations

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glycosaminoglycan GTPase-activating protein glial cell-line derived neurotrophic factor growth hormone growth hormone receptor granular osmiophilic deposit hemoglobin Huntington disease hereditary elliptocytosis hypoxanthine-guanine phosphoribosyl transferase hereditary multiple exostoses hereditary motor and sensory neuropathy hereditary neuropathy with liability to pressure palsies Holt–Oram syndrome holoprosencephaly hereditary pyropoikilocytosis hereditary spherocytosis or Hunter syndrome Hirschsprung disease immunoglobulin insulin-like growth factor interleukin keratinocyte growth factor receptor Leber congenital amaurosis limb-girdle muscular dystrophy mental retardation, aphasia, shuffling gait, and adducted thumbs methyl-CpG binding domain medium chain acyl-CoA dehydrogenase myotonic dystrophy Miller–Dieker syndrome muscle–eye–brain disease multiple endocrine neoplasia type 2 Mendelian inheritance in man mucopolysaccharidosis medullary thyroid carcinoma nucleotide-binding folds neuronal ceroid lipofuscinosis nerve conduction velocity

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NF1 OCT OFD1 OI OIC OIT ORCC OTC PCD PDS PHP PKD PKU PNP PPHP PPT PROMM pVHL PWS SCBH SCID SCS SMA SVAS TRD TS TSH UPD VCFS VEP VHL VLCFA vWD vWF WAGR WAS WS

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neurofibromatosis type 1 ornithine carbamoyltransferase orofaciodigital syndrome type I osteogenesis imperfecta osteogenesis imperfecta congenita osteogenesis imperfecta tarda outwardly rectifying chloride channels ornithine transcarbamylase primary ciliary dyskinesia Pendred syndrome pseudohypoparathyroidism polycystic kidney disease phenylketonuria purine nucleoside phosphorylase pseudopseudohypoparathyroidism palmitoyl-protein thioesterase proximal myotonic myopathy protein product of the VHL gene Prader–Willi syndrome subcortical band heterotopia severe combined immunodeficiency Saethre–Chotzen syndrome spinal muscular atrophy supravalvular aortic stenosis transcriptional repression domain tuberous sclerosis thyroid-stimulating hormone uniparental disomy velocardiofacial syndrome visual evoked potential Von Hippel–Lindau disease very long-chain fatty acid von Willebrand disease von Willebrand factor Wilms’ tumor, aniridia, genitourinary anomalies, mental retardation Wiskott–Aldrich Syndrome Waardenburg syndrome

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WS X-ALD XLAG XLAS XLIS

Abbreviations

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Williams syndrome X-linked adrenoleukodystrophy X-linked lissencephaly with ambiguous genitalia X-linked form of Alport syndrome X-linked form of isolated classical lissencephaly

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A Adenine (A)

One of the bases making up DNA and RNA (pairs with thymine in DNA and uracil in RNA).

Agarose gel electrophoresis

See electrophoresis.

Allele

One of two or more alternative forms of a gene at a given location (locus). A single allele for each locus is inherited separately from each parent. In normal human beings there are two alleles for each locus (diploidy). If the two alleles are identical, the individual is said to be homozygous for that allele; if different, the individual is heterozygous. For example, the normal DNA sequence at codon 6 in the beta-globin gene is GAG (coding for glutamic acid), whereas in sickle cell disease the sequence is GTG (coding for valine). An individual is said to be heterozygous for the glutamic acid → valine mutation if he/she possesses one normal (GAG) and one mutated (GTG) allele. Such individuals are carriers of the sickle cell gene and do not manifest classical sickle cell disease (which is autosomal recessive).

Allelic heterogeneity Similar/identical phenotypes caused by different mutations within a gene. For example, many different mutations in the same gene are now known to be associated with Marfan’s syndrome (FBN1 gene at 15q21.1). Amniocentesis

Withdrawal of amniotic fluid, usually carried out during the second trimester, for the purpose of prenatal diagnosis.

Amplification

The production of increased numbers of a DNA sequence. 1. In vitro In the early days of recombinant DNA techniques, the only way to amplify a sequence of interest (so that large amounts were available for detailed study) was to clone the fragment in a vector (plasmid or phage) and transform bacteria with the recombinant vector. The transformation technique generally results in the “acceptance” of a single vector molecule by each bacterial cell. The vector is able to exist autonomously within the bacterial cell, sometimes at very high copy numbers (eg, 500 vector copies per cell). Growth of the bacteria containing the vector, coupled

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with a method to recover the vector sequence from the bacterial culture, allows for almost unlimited production of a sequence of interest. Cloning and bacterial propagation are still used for applications requiring either large quantities of material or else exceptionally pure material. However, the advent of the polymerase chain reaction (PCR) has meant that amplification of desired DNA sequences can now be performed more rapidly than was the case with cloning (a few hours cf. days), and it is now routine to amplify DNA sequences 10 million-fold. 2. In vivo Amplification may also refer to an increase in the number of DNA sequences within the genome. For example, the genomes of many tumors are now known to contain regions that have been amplified many fold compared to their nontumor counterparts (ie, a sequence or region of DNA that normally occurs once at a particular chromosomal location may be present in hundreds of copies in some tumors). It is believed that many such regions harbor oncogenes, which, when present in high copy number, predispose to development of the malignant phenotype. Aneuploid

Possessing an incorrect number (abnormal complement) of chromosomes. The normal human complement is 46 chromosomes, any cell that deviates from this number is said to be aneuploid.

Aneuploidy

The chromosomal condition of a cell or organism with an incorrect number of chromosomes. Individuals with Down’s syndrome are described as having aneuploidy, because they possess an extra copy of chromosome 21 (trisomy 21), making a total of 47 chromosomes.

Anticipation

A general phenomenon that refers to the observation of an increase in severity, and/or decrease in age of onset, of a condition in successive generations of a family (see Figure 1). Anticipation is now known, in many cases, to result directly from the presence of a dynamic mutation in a family. In the absence of a dynamic mutation, anticipation may be explained by “ascertainment bias”. Thus, before the first dynamic mutations were described (in Fragile X and myotonic dystrophy), it was believed that ascertainment bias was the complete explanation for anticipation. There are two main reasons for ascertainment bias: 1. Identical mutations in different individuals often result in variable expressions of the associated phenotype. Thus, individuals within a

Glossary

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Figure 1. Autosomal dominant inheritance with anticipation. In many disorders that exhibit anticipation, the age of onset decreases in subsequent generations. It may happen that the transmitting parent (grandparent in this case) is unaffected at the time of presentation of the proband (see arrow). A good example is Huntington’s disease, caused by the expansion of a CAG repeat in the coding region of the huntingtin gene. Note that this pedigree would also be consistent with either gonadal mosaicism or reduced penetrance (in the carrier grandparent).

family, all of whom harbor an identical mutation, may have variation in the severity of their condition. 2. Individuals with a severe phenotype are more likely to present to the medical profession. Moreover, such individuals are more likely to fail to reproduce (ie, they are genetic lethals), often for social, rather than direct physical reasons. For both reasons, it is much more likely that a mildly affected parent will be ascertained with a severely affected child, than the reverse. Therefore, the severity of a condition appears to increase through generations. Anticodon

The 3-base sequence on a transfer RNA (tRNA) molecule that is complementary to the 3-base codon of a messenger RNA (mRNA) molecule.

Ascertainment bias

See anticipation.

Autosomal disorder

A disorder associated with a mutation in an autosomal gene.

Autosomal dominant An autosomal disorder in which the phenotype is expressed in (AD) inheritance the heterozygous state. These disorders are not sex-specific. Fifty percent of offspring (when only one parent is affected) will usually manifest the disorder (see Figure 2). Marfan syndrome is a good example of an AD disorder; affected individuals possess one wild-type (normal) and one mutated allele at the FBN1 gene.

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Figure 2. Autosomal dominant (AD) inheritance.

Autosomal recessive An autosomal disorder in which the phenotype is manifest in the (AR) inheritance homozygous state. This pattern of inheritance is not sex-specific and is difficult to trace through generations because both parents must contribute the abnormal gene, but may not necessarily display the disorder. The children of two heterozygous AR parents have a 25% chance of manifesting the disorder (see Figure 3). Cystic fibrosis (CF) is a good example of an AR disorder; affected individuals possess two mutations, one at each allele.

Figure 3. Autosomal recessive (AR) inheritance.

Autosome

Any chromosome, other than the sex chromosomes (X or Y), that occurs in pairs in diploid cells.

B Barr body

An inactive X chromosome, visible in the somatic cells of individuals with more than one X chromosome (ie, all normal females and all males with Klinefelter’s syndrome). For individuals with nX chromosomes, n–1 Barr bodies are seen. The presence of a Barr body in cells obtained by amniocentesis or chorionic villus sampling used to be used as an indication of the sex of a baby before birth.

Base pair (bp)

Two nucleotides held together by hydrogen bonds. In DNA, guanine always pairs with cytosine, and thymine with adenine. A base pair is also the basic unit for measuring DNA length.

Glossary

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C Carrier

An individual who is heterozygous for a mutant allele (ie, carries one wild-type [normal copy] and one mutated copy of the gene under consideration).

CentiMorgan (cM)

Unit of genetic distance. If the chance of recombination between two loci is 1%, the loci are said to be 1 cM apart. On average, 1 cM implies a physical distance of 1 Mb (1,000,000 base pairs) but significant deviations from this rule of thumb occur because recombination frequencies vary throughout the genome. Thus if recombination in a certain region is less likely than average, 1 cM may be equivalent to 5 Mb (5,000,000 base pairs) in that region.

Centromere

Central constriction of the chromosome where daughter chromatids are joined together, separating the short (p) from the long (q) arms (see Figure 4).

Chorionic villus sampling (CVS)

Prenatal diagnostic procedure for obtaining fetal tissue at an earlier stage of gestation than amniocentesis. Generally performed after 10 weeks, ultrasound is used to guide aspiration of tissue from the villus area of the chorion.

Chromatid

One of the two parallel identical strands of a chromosome, connected at the centromere during mitosis and meiosis (see Figure 4). Before replication, each chromosome consists of only one chromatid. After replication, two identical sister chromatids are present. At the end of mitosis or meiosis, the two sisters separate and move to opposite poles before the cell splits.

Chromatin

A readily stained substance in the nucleus of a cell consisting of DNA and proteins. During cell division it coils and folds to form the metaphase chromosomes.

Chromosome

One of the threadlike “packages” of genes and other DNA in the nucleus of a cell (see Figure 4). Humans have 23 pairs of chromosomes, 46 in total: 44 autosomes and two sex chromosomes. Each parent contributes one chromosome to each pair.

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Chromosome p arm Centromere q arm DNA Double helix

Nucleus Chromatid Telomere

AT A T T

Figure 4. Chromosome structure.

A G A

C T

Chromosomal

A disorder that results from gross changes in chromosome dose.

disorder

May result from addition or loss of entire chromosomes or just portions of chromosomes.

Clone

A group of genetically identical cells with a common ancestor.

Codon

A 3-base coding unit of DNA that specifies the function of a corresponding unit (anticodon) of transfer RNA (tRNA).

Complementary DNA (cDNA)

DNA synthesized from messenger RNA (mRNA) using reverse transcriptase. Differs from genomic DNA because it lacks introns.

Complementation

The wild-type allele of a gene compensates for a mutant allele of the same gene so that the heterozygote’s phenotype is wild-type.

Complementation analysis

A genetic test (usually performed in vitro) that determines whether or not two mutations that produce the same phenotype are allelic. It enables the geneticist to determine how many distinct genes are involved when confronted with a number of mutations that have similar phenotypes.

Glossary

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Occasionally it can be observed clinically. Two parents who both suffer from recessive deafness (ie, both are homozygous for a mutation resulting in deafness) may have offspring that have normal hearing. If A and B refer to the wild-type (normal) forms of the genes, and a and b the mutated forms, one parent could be aa,BB and the other AA,bb. If alleles A and B are distinct, each child will have the genotype aA,bB and will have normal hearing. If A and B are allelic, the child will be homozygous at this locus and will also suffer from deafness. Compound heterozygote

An individual with two different mutant alleles at the same locus.

Concordant Consanguinity

A pair of twins who manifest the same phenotype as each other. Sharing a common ancestor, and thus genetically related. Recessive disorders are seen with increased frequency in consanguineous families.

Consultand

An individual seeking genetic advice.

Contiguous gene syndrome

A syndrome resulting from the simultaneous functional imbalance of a group of genes (see Figure 5). The nomenclature for this group of disorders is somewhat confused, largely as a result of the history of their elucidation. The terms submicroscopic rearrangement/deletion/ duplication and microrearrangement/deletion/duplication are often used interchangeably. Micro or submicroscopic refer to the fact that such lesions are not detectable with standard cytogenetic approaches (where the limit of resolution is usually 10 Mb, and 5 Mb in only the most fortuitous of circumstances). A newer, and perhaps more comprehensive, term that is currently applied to this group of disorders is segmental aneusomy syndromes (SASs). This term embraces the possibility not only of loss or gain of a chromosomal region that harbors many genes (leading to imbalance of all those genes), but also of functional imbalance in a group of genes, as a result of an abnormality of the machinery involved in their silencing/transcription (ie, methylationbased mechanisms that depend on a master control gene). In practice, most contiguous gene syndromes result from the heterozygous deletion of a segment of DNA that is large in molecular terms but not detectable cytogenetically. The size of such deletions is usually 1.5–3.0 Mb. It is common for one to two dozen genes to be involved in such deletions, and the resultant phenotypes are often

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22

21

15.3 15.2 15.1 14

p

13 12

7

11.2 11.1 11.1 11.21 11.22

11.23

Williams’ syndrome region: 1.5–2.5 Mb in size.

21.1 21.2 21.3

22.1

q

31.1 31.2

31.3

32 33 34 35

36

Figure 5. Schematic demonstrating the common deletion found in Williams’ syndrome, at 7q11.23. The common deletion is not detectable using standard cytogenetic analysis (even high resolution), despite the fact that the deletion is at least 1.5 Mb in size. In practice, only genomic rearrangements that affect at least 5–10 Mb are detectable, either by standard cytogenetic analysis or, in fact, any technique whose endpoint involves analysis at the chromosomal level. Such deletions are termed microdeletions or submicroscopic deletions. Approximately 20 genes are known to be involved in the 7q11.23 microdeletion, and work is underway to determine which genes contribute to which aspects of the Williams’ syndrome phenotype.

complex, involving multiple organ systems and, almost invariably, learning difficulties. A good example of a contiguous gene syndrome is Williams’ syndrome, a sporadic disorder that is due to a heterozygous deletion at chromosome 7q11.23. Affected individuals have characteristic phenotypes, including recognizable facial appearance and typical behavioral traits (including moderate learning difficulties). Velocardiofacial syndrome is currently the most common microdeletion known, and is caused by deletions of 3 Mb at chromosome 22q11. Crossing over

Reciprocal exchange of genetic material between homologous chromosomes at meiosis (see Figure 6).

Cytogenetics

The study of the structure of chromosomes.

Cytosine (C)

One of the bases making up DNA and RNA (pairs with guanine).

Cytotrophoblast

Cells obtained from fetal chorionic villi by chorionic villus sampling (CVS). Used for DNA and chromosome analysis.

Glossary

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A

A´

A

A´

B C

B´ C´

B´ C´

B C

Figure 6. Schematic demonstrating the principle of recombination (crossing over). On average, 50 recombinations occur per meiotic division (1–2 per chromosome). Loci that are far apart on the chromosome are more likely to be separated during recombination than those that are physically close to each other (they are said to be linked, see linkage), ie, A and B are less likely to cosegregate than B and C. Note that the two homologues of a sequence have been differentially labeled according to their chromosome of origin.

D Deletion

244

A particular kind of mutation that involves the loss of a segment of DNA from a chromosome with subsequent re-joining of the two extant ends. It can refer to the removal of one or more bases within a gene or to a much larger aberration involving millions of bases. The term deletion is not totally specific, and differentiation must be made between heterozygous and homozygous deletions. Large heterozygous deletions are a common cause of complex phenotypes (see contiguous gene syndrome); large germ-line homozygous deletions are extremely rare, but have been described. Homozygous deletions are frequently described in somatic cells, in association with the manifestation of the malignant phenotype. The two deletions in a homozygous deletion need not be identical, but must result in the complete absence of DNA sequences that occupy the “overlap” region.

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Denature

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Broadly used to describe two general phenomena: 1. The “melting” or separation of double-stranded DNA (dsDNA) into its constituent single strands, which may be achieved using heat or chemical approaches. 2. The denaturation of proteins. The specificity of proteins is a result of their 3-dimensional conformation, which is a function of their (linear) amino acid sequence. Heat and/or chemical approaches may result in denaturation of a protein – the protein loses its 3-dimensional conformation (usually irreversibly) and, with it, its specific activity.

Diploid

Having two sets of chromosomes. The number of chromosomes in most human somatic cells is 46. This is double the number found in gametes (23, the haploid number).

Discordant

A pair of twins who differ in their manifestation of a phenotype.

Dizygotic

The fertilization of two separate eggs by two separate sperm resulting in a pair of genetically nonidentical twins.

DNA (deoxyribonucleic acid)

The molecule of heredity. DNA normally exists as a double-stranded (ds) molecule; one strand is the complement (in sequence) of the other. The two strands are joined together by hydrogen bonding, a noncovalent mechanism that is easily reversible using heat or chemical means. DNA consists of four distinct bases: guanine (G), cytosine (C), thymine (T), and adenine (A). The convention is that DNA sequences are written in a 5´ to 3´ direction, where 5´ and 3´ refer to the numbering of carbons on the deoxyribose ring. A guanine on one strand will always pair with a cytosine on the other strand, while thymine pairs with adenine. Thus, given the sequence of bases on one strand, the sequence on the other is immediately determined: 5´–AGTGTGACTGATCTTGGTG–3´ 3´–TCACACTGACTAGAACCAC–5´ The complexity (informational content) of a DNA molecule resides almost completely in the particular sequence of its bases. For a sequence of length “n” base pairs, there are 4n possible sequences. Even for relatively small n, this number is astronomical (4n = 1.6 x 1060 for n = 100).

Glossary

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The complementarity of the two strands of a dsDNA molecule is a very important feature and one that is exploited in almost all molecular genetic techniques. If dsDNA is denatured, either by heat or by chemical means, the two strands become separated from each other. If the conditions are subsequently altered (eg, by reducing heat), the two strands eventually “find” each other in solution and re-anneal to form dsDNA once again. The specificity of this reaction is quite high, under the right circumstances – strands that are not highly complementary are much less likely to re-anneal compared to perfect or near perfect matches. The process by which the two strands “find” each other depends on random molecular collisions, and a “zippering” mechanism, which is initiated from a short stretch of complementarity. This property of DNA is vital for the polymerase chain reaction (PCR), Southern blotting, and any method that relies on the use of a DNA/RNA probe to detect its counterpart in a complex mix of molecules. DNA chip

A “chip” or microarray of multiple DNA sequences immobilized on a solid surface (see Figure 7). The term chip refers more often to semiconductor-based DNA arrays, in which short DNA sequences (oligos) are synthesized in situ, using a photolithographic process akin to that used in the manufacture of semiconductor devices for the electronics industry. The term microarray is much more general and includes any collection of DNA sequences immobilized onto a solid surface, whether by a photolithographic process, or by simple “spotting” of DNA sequences onto glass slides. The power of DNA microarrays is based on the parallel analysis that they allow for. In conventional hybridization analysis (ie, Southern blotting), a single DNA sequence is usually used to interrogate a small number of different individuals. In DNA microarray analysis, this approach is reversed – an individual’s DNA is hybridized to an array that may contain 30,000 distinct spots. This allows for direct information to be obtained about all DNA sequences on the array in one experiment. DNA microarrays have been used successfully to directly uncover point mutations in single genes, as well as detect alterations in gene expression associated with certain disease states/cellular differentiation. It is likely that certain types of array will be useful in the determination of subtle copy number alterations, as occurs in microdeletion/microduplication syndromes.

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Figure 7. DNA chip. DNA arrays (or “chips”) are composed of thousands of “spots” of DNA, attached to a solid surface (normally glass). Each spot contains a different DNA sequence. The arrays allow for massively parallel experiments to be performed on samples. In practice, two samples are applied to the array. One sample is a control (from a “normal” sample) and one is the test sample. Each sample is labeled with fluorescent tags, control with green and test with red. The two labeled samples are cohybridized to the array and the results read by a laser scanner. Spots on the array whose DNA content is equally represented in the test and control samples yield equal intensities in the red and green channels, resulting in a yellow signal. Spots appearing as red represent DNA sequences that are present at higher concentration in the test sample compared to the control sample and vice versa.

DNA methylation

Addition of a methyl group (–CH3) to DNA nucleotides (often cytosine). Methylation is often associated with reduced levels of expression of a given gene and is important in imprinting.

DNA replication

Use of existing DNA as a template for the synthesis of new DNA strands. In humans and other eukaryotes, replication takes place in the cell nucleus. DNA replication is semiconservative – each new doublestranded molecule is composed of a newly synthesized strand and a pre-existing strand.

Glossary

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Dominant (traits/diseases)

Manifesting a phenotype in the heterozygous state. Individuals with Huntington’s disease, a dominant condition, are affected even though they possess one normal copy of the gene.

Dynamic/ nonstable mutation

The vast majority of mutations known to be associated with human genetic disease are intergenerationally stable (no alteration in the mutation is observed when transmitted from parent to child). However, a recently described and growing class of disorders result from the presence of mutations that are unstable intergenerationally. These disorders result from the presence of tandem repeats of short DNA sequences (eg, the sequence CAG may be repeated many times in tandem), see Table 1. For reasons that are not completely clear, the copy number of such repeats may vary from parent to child (usually resulting in a copy number increase) and within the somatic cells of a given individual. Abnormal phenotypes result when the number of repeats reaches a given threshold. Furthermore, when this threshold has been reached, the risk of even greater expansion of copy number in subsequent generations increases.

E Electrophoresis

The separation of molecules according to size and ionic charge by an electrical current. Agarose gel electrophoresis Separation, based on size, of DNA/RNA molecules through agarose. Conventional agarose gel electrophoresis generally refers to electrophoresis carried out under standard conditions, allowing the resolution of molecules that vary in size from a few hundred to a few thousand base pairs. Polyacrylamide gel electrophoresis Allows resolution of proteins or DNA molecules differing in size by only 1 base pair. Pulsed field gel electrophoresis (Also performed using agarose) refers to a specialist technique that allows resolution of much larger DNA molecules, in some cases up to a few Mb in size.

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Disorder

Protein/location

Repeat

Repeat location

Normal range

Pre-mutation

Full mutation

Type

MIM

Progressive myoclonus epilepsy of UnverrichtLundborg type (EPM1)

cystatin B 21q22.3

C4GC4G CG

Promoter

2–3

12–17

30–75

AR

254800

Fragile X type A (FRAXA)

FMR1 Xq27.3

CGG

5’UTR

6–52

~60–200

~200–>2,000

XLR

309550

Fragile X type E (FRAXE)

FMR2 Xq28

CGG 5

C’UTR

6–25

–

>200

XLR

309548

Friedreich’s ataxia (FRDA)

frataxin 9q13

GAA

intron

1 7–22

–

200–>900

AR

229300

Huntington’s disease (HD)

huntingtin 4p16.3

CAG

ORF

6–34

–

36–180

AD

143100

Dentatorubal-pallidoluysian atrophy (DRPLA)

atrophin 12p12

CAG

ORF

7–25

–

49–88

AD

125370

Spinal and bulbar muscular atrophy (SBMA – Kennedy syndrome)

androgen receptor CAG Xq11-12

ORF

11–24

–

40–62

XLR

313200

Spinocerebellar ataxia type 1 (SCA1)

ataxin-1 6p23

CAG

ORF

6–39

–

39–83

AD

164400

Spinocerebellar ataxia type 2 (SCA2)

ataxin-2 12q24

CAG

ORF

15–29

–

34–59

AD

183090

Spinocerebellar ataxia type 3 (SCA3)

ataxin-3 14q24.3-q31

CAG

ORF

13–36

–

55–84

AD

109150

Spinocerebellar ataxia type 6 (SCA6)

PQ calcium channel 19p13

CAG

ORF

4–16

–

21–30

AD

183086

Spinocerebellar ataxia type 7 (SCA7)

ataxin-7 3p21.1-p12

CAG

ORF

4–35

28–35

34–>300

AD

164500

Spinocerebellar ataxia type 8 (SCA8)

SCA8 13q21

CTG

3’UTR

6–37

–

~107–2501

AD

603680

Spinocerebellar ataxia type 10 (SCA10)

SCA10 22q13-qter

ATTCT

intron 9

10–22

–

500–4,500

AD

603516

Spinocerebellar ataxia type 12 (SCA12)

PP2R2B 5q31-33

CAG

5’UTR

7–28

–

66–78

AD

604326

Myotonic dystrophy (DM)

DMPK 19q13.3

CTG

3’UTR

5–37

~50–180

~200–>2,000

AD

160900

Table 1. “Classical” repeat expansion disorders. 1Longer alleles exist but are not associated with disease. AD: autosomal dominant; AR: autosomal recessive; ORF: open reading frame (coding region); 3´ UTR: 3´ untranslated region (downstream of gene); 5´ UTR: 5´ untranslated region (upstream of gene); XLR: X-linked recessive.

Empirical Based on observation, rather than detailed knowledge of, eg, modes recurrence of inheritance or environmental factors. risk – recurrence risk Endonuclease

An enzyme that cleaves DNA at an internal site (see also restriction enzyme).

Euchromatin

Chromatin that stains lightly with trypsin G banding and contains active/potentially active genes.

Euploidy

Having a normal chromosome complement.

Glossary

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Exon

Coding part of a gene. Historically, it was believed that all of a DNA sequence is mirrored exactly on the messenger RNA (mRNA) molecule (except for the presence of uracil in mRNA compared to thymine in DNA). It was a surprise to discover that this is generally not the case. The genomic sequence of a gene has two components: exons and introns. The exons are found in both the genomic sequence and the mRNA, whereas the introns are found only in the genomic sequence. The mRNA for dystrophin, an X-linked gene associated with Duchenne muscular dystrophy (DMD), is 14,000 base pairs long but the genomic sequence is spread over a distance of 1.5 million base pairs, because of the presence of very long intronic sequences. After the genomic sequence is initially transcribed to RNA, a complex system ensures specific removal of introns. This system is known as splicing.

Expressivity

Degree of expression of a disease. In some disorders, individuals carrying the same mutation may manifest wide variability in severity of the disorder. Autosomal dominant disorders are often associated with variable expressivity, a good example being Marfan’s syndrome. Variable expressivity is to be differentiated from incomplete penetrance, an all or none phenomenon that refers to the complete absence of a phenotype in some obligate carriers.

F Familial

Any trait that has a higher frequency in relatives of an affected individual than the general population.

FISH

Fluorescence in situ hybridization (see In situ hybridization).

Founder effect

The high frequency of a mutant allele in a population as a result of its presence in a founder (ancestor). Founder effects are particularly noticeable in relative genetic isolates, such as the Finnish or Amish.

Frame-shift mutation

Deletion/insertion of a DNA sequence that is not an exact multiple of 3 base pairs. The result is an alteration of the reading frame of the gene such that all sequence that lies beyond the mutation is missence (ie, codes for the wrong amino acids) (see Figure 8). A premature stop codon is usually encountered shortly after the frame shift.

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T

T

T

C

C

C

C

C

A

C

C

C

A

PITX2 sequence

Mutant (protein)

ATG Met

TTT Phe

TCC Ser

CCC Pro

ACC Thr

CAA Gln

Normal (protein)

ATG Met

TTT Phe

TCC Ser

CCA Pro

CCC Pro

AAC Asn

Figure 8. Frame-shift mutation. This example shows a sequence of PITX2 in a patient with Rieger’s syndrome, an autosomal dominant condition. The sequence graph shows only the abnormal sequence. The arrow indicates the insertion of a single cytosine (C) residue. When translated, the triplet code is now out of frame by 1 base pair. This totally alters the translated protein’s amino acid sequence. This leads to a premature stop codon later in the protein and results in Rieger’s syndrome.

G Gamete (germ cell)

The mature male or female reproductive cells, which contain a haploid set of chromosomes.

Gene

An ordered, specific sequence of nucleotides that controls the transmission and expression of one or more traits by specifying the sequence and structure of a particular protein or RNA molecule. Mendel defined a gene as the basic physical and functional unit of all heredity.

Gene expression

The process of converting a gene’s coded information into the existing, operating structures in the cell.

Gene mapping

Determines the relative positions of genes on a DNA molecule and plots the genetic distance in linkage units (centiMorgans) or physical distance (base pairs) between them.

Genetic code

Relationship between the sequence of bases in a nucleic acid and the order of amino acids in the polypeptide synthesized from it

Glossary

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2nd

2nd

2nd

2nd

T

C

A

G

TTT Phe [F]

TCT Ser [S]

TAT Tyr [Y]

TGT Cys [C]

T

TTC Phe [F]

TCC Ser [S]

TAC Tyr [Y]

TGC Cys [C]

C

TTA Leu [L]

TCA Ser [S]

TAA Ter [end]

TGA Ter [end]

A

TTG Leu [L]

TCG Ser [S]

TAG Ter [end]

TGG Trp [W]

G

CTT Leu [L]

CCT Pro [P]

CAT His [H]

CGT Arg [R]

T

CTC Leu [L]

CCC Pro [P]

CAC His [H]

CGC Arg [R]

C

CTA Leu [L]

CCA Pro [P]

CAA Gln [Q]

CGA Arg [R]

A

CTG Leu [L]

CCG Pro [P]

CAG Gln [Q]

CGG Arg [R]

G

ATT Ile [I]

ACT Thr [T]

AAT Asn [N]

AGT Ser [S]

T

ATC Ile [I]

ACC Thr [T]

AAC Asn [N]

AGC Ser [S]

C A

ATA Ile [I]

ACA Thr [T]

AAA Lys [K]

AGA Arg [R]

ATG Met [M]

ACG Thr [T]

AAG Lys [K]

AGG Arg [R]

G

GTT Val [V]

GCT Ala [A]

GAT Asp [D]

GGT Gly [G]

T

GTC Val [V]

GCC Ala [A]

GAC Asp [D]

GGC Gly [G]

C

GTA Val [V]

GCA Ala [A]

GAA Glu [E]

GGA Gly [G]

A

GTG Val [V]

GCG Ala [A]

GAG Glu [E]

GGG Gly [G]

G

3rd

3rd

3rd

3rd

Table 2. The genetic code. To locate a particular codon (eg, TAG, marked in bold) locate the first base (T) in the left hand column, then the second base (A) by looking at the top row, and finally the third (G) in the right hand column (TAG is a stop codon). Note the redundancy of the genetic code – for example, three different codons specify a stop signal, and threonine (Thr) is specified by any of ACT, ACC, ACA, and ACG.

(see Table 2). A sequence of three nucleic acid bases (a triplet) acts as a codeword (codon) for one amino acid or instruction (start/stop). Genetic counseling

Information/advice given to families with, or at risk of, genetic disease. Genetic counseling is a complex discipline that requires accurate diagnostic approaches, up-to-date knowledge of the genetics of the condition, an insight into the beliefs/anxieties/wishes of the individual seeking advice, intelligent risk estimation, and, above all, skill in communicating relevant information to individuals from a wide variety of educational backgrounds. Genetic counseling is most often carried out by trained medical geneticists or, in some countries, specialist genetic counselors or nurses.

Genetic heterogeneity Association of a specific phenotype with mutations at different loci. The broader the phenotypic criteria, the greater the heterogeneity (eg, mental retardation). However, even very specific phenotypes may be genetically heterogeneous. Isolated central hypothyroidism is a good example: this autosomal recessive condition is now known to be associated (in different individuals) with mutations in the TSH β 252

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chain at 1p13, the TRH receptor at 8q23, or TRH itself at 3q13.3–q21. There is no obvious distinction between the clinical phenotypes associated with these two genes. Genetic heterogeneity should not be confused with allelic heterogeneity, which refers to the presence of different mutations at the same locus. Genetic locus

A specific location on a chromosome.

Genetic map

A map of genetic landmarks deduced from linkage (recombination) analysis. Aims to determine the linear order of a set of genetic markers along a chromosome. Genetic maps differ significantly from physical maps, in that recombination frequencies are not identical across different genomic regions, resulting occasionally in large discrepancies.

Genetic marker

A gene that has an easily identifiable phenotype so that one can distinguish between those cells or individuals that do or do not have the gene. Such a gene can also be used as a probe to mark cell nuclei or chromosomes, so that they can be isolated easily or identified from other nuclei or chromosomes later.

Genetic screening

Population analysis designed to ascertain individuals at risk of either suffering or transmitting a genetic disease.

Genetically lethal

Preventing reproduction of the individual, either by causing death prior to reproductive age, or as a result of social factors making it highly unlikely (although not impossible) that the individual concerned will reproduce.

Genome

The complete DNA sequence of an individual, including the sex chromosomes and mitochondrial DNA (mtDNA). The genome of humans is estimated to have a complexity of 3.3 x 109 base pairs (per haploid genome).

Genomic

Pertaining to the genome. Genomic DNA differs from complementary DNA (cDNA) in that it contains noncoding as well as coding DNA.

Genotype

Genetic constitution of an individual, distinct from expressed features (phenotype).

Germ line

Germ cells (those cells that produce haploid gametes) and the cells from which they arise. The germ line is formed very early in embryonic development. Germ line mutations are those present constitutionally

Glossary

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in an individual (ie, in all cells of the body) as opposed to somatic mutations, which affect only a proportion of cells. Giemsa banding

Light/dark bar code obtained by staining chromosomes with Giemsa stain. Results in a unique bar code for each chromosome.

Guanine (G)

One of the bases making up DNA and RNA (pairs with cytosine).

H Haploid

The chromosome number of a normal gamete, containing one each of every individual chromosome (23 in humans).

Haploinsufficiency

The presence of one active copy of a gene/region is insufficient to compensate for the absence of the other copy. Most genes are not “haploinsufficient” – 50% reduction of gene activity does not lead to an abnormal phenotype. However, for some genes, most often those involved in early development, reduction to 50% often correlates with an abnormal phenotype. Haploinsufficiency is an important component of most contiguous gene disorders (eg, in Williams’ syndrome, heterozygous deletion of a number of genes results in the mutant phenotype, despite the presence of normal copies of all affected genes).

Hemizygous

Having only one copy of a gene or DNA sequence in diploid cells. Males are hemizygous for most genes on the sex chromosomes, as they possess only one X chromosome and one Y chromosome (the exceptions being those genes with counterparts on both sex chromosomes). Deletions on autosomes produce hemizygosity in both males and females.

Heterochromatin

Contains few active genes, but is rich in highly repeated simple sequence DNA, sometimes known as satellite DNA. Heterochromatin refers to inactive regions of the genome, as opposed to euchromatin, which refers to active, gene expressing regions. Heterochromatin stains darkly with Giemsa.

Heterozygous

Presence of two different alleles at a given locus.

Histones

Simple proteins bound to DNA in chromosomes. They help to maintain chromatin structure and play an important role in regulating gene expression.

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Holandric

Pattern of inheritance displayed by mutations in genes located only on the Y chromosome. Such mutations are transmitted only from father to son.

Homologue or homologous gene

Two or more genes whose sequences manifest significant similarity because of a close evolutionary relationship. May be between species (orthologues) or within a species (paralogues).

Homologous chromosomes

Chromosomes that pair during meiosis. These chromosomes contain the same linear gene sequences as one another and derive from one parent.

Homology

Similarity in DNA or protein sequences between individuals of the same species or among different species.

Homozygous

Presence of identical alleles at a given locus.

Human gene therapy The study of approaches to treatment of human genetic disease, using the methods of modern molecular genetics. Many trials are under way studying a variety of disorders, including cystic fibrosis. Some disorders are likely to be more treatable than others – it is probably going to be easier to replace defective or absent gene sequences rather than deal with genes whose aberrant expression results in an actively toxic effect. Human genome project

Worldwide collaboration aimed at obtaining a complete sequence of the human genome. Most sequencing has been carried out in the USA, although the Sanger Centre in Cambridge, UK has sequenced one third of the genome, and centers in Japan and Europe have also contributed significantly. The first draft of the human genome was released in the summer of 2000 to much acclaim. Celera, a privately funded venture, headed by Dr Craig Ventner, also published its first draft at the same time.

Hybridization

Pairing of complementary strands of nucleic acid. Also known as re-annealing. May refer to re-annealing of DNA in solution, on a membrane (Southern blotting) or on a DNA microarray. May also be used to refer to fusion of two somatic cells, resulting in a hybrid that contains genetic information from both donors.

Glossary

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I Imprinting

A general term used to describe the phenomenon whereby a DNA sequence (coding or otherwise) carries a signal or imprint that indicates its parent of origin. For most DNA sequences, no distinction can be made between those arising paternally and those arising maternally (apart from subtle sequence variations); for imprinted sequences this is not the case. The mechanistic basis of imprinting is almost always methylation – for certain genes, the copy that has been inherited from the father is methylated, while the maternal copy is not. The situation may be reversed for other imprinted genes. Note that imprinting of a gene refers to the general phenomenon, not which parental copy is methylated (and, therefore, usually inactive). Thus, formally speaking, it is incorrect to say that a gene undergoes paternal imprinting. It is correct to say that the gene undergoes imprinting and that the inactive (methylated) copy is always the paternal one. However, in common genetics parlance, paternal imprinting is usually understood to mean the same thing.

In situ hybridization Annealing of DNA sequences to immobilized chromosomes/cells/ (ISH) tissues. Historically done using radioactively labeled probes, this is currently most often performed with fluorescently tagged molecules (fluorescent in situ hybridization – FISH, see Figure 9). ISH/FISH allows for the rapid detection of a DNA sequence within the genome. Incomplete penetrance

Complete absence of expression of the abnormal phenotype in a proportion of individuals known to be obligate carriers. To be distinguished from variable expressivity, in which the phenotype always manifests in obligate carriers, but with widely varying degrees of severity.

Index case – proband The individual through which a family medically comes to light. For example, the index case may be a baby with Down’s syndrome. Can be termed propositus (if male) or proposita (if female). Insertion

256

Interruption of a chromosomal sequence as a result of insertion of material from elsewhere in the genome (either a different chromosome, or elsewhere from the same chromosome). Such insertions may result in abnormal phenotypes either because of direct interruption of a gene (uncommon), or because of the resulting imbalance (ie, increased dosage) when the chromosomes that contain the normal counterparts of the inserted sequence are also present. Genetics for Pediatricians

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Figure 9. Fluorescence in situ hybridization. FISH analysis of a patient with a complex syndrome, using a clone containing DNA from the region 8q24.3. In addition to that clone, a control from 8pter was used. The 8pter clone has yielded a signal on both homologues of chromosome 8, while the “test” clone from 8q24.3 has yielded a signal on only one homologue, demonstrating a (heterozygous) deletion in that region.

Intron

A noncoding DNA sequence that “interrupts” the protein-coding sequences of a gene; intron sequences are transcribed into messenger RNA (mRNA), but are cut out before the mRNA is translated into a protein (this process is known as splicing). Introns may contain sequences involved in regulating expression of a gene. Unlike the exon, the intron is the nucleotide sequence in a gene that is not represented in the amino acid sequence of the final gene product.

Inversion

A structural abnormality of a chromosome in which a segment is reversed, as compared to the normal orientation of the segment. An inversion may result in the reversal of a segment that lies entirely on one chromosome arm (paracentric) or one that spans (ie, contains) the centromere (pericentric). While individuals who possess an inversion are likely to be genetically balanced (and therefore usually phenotypically normal), they are at increased risk of producing unbalanced offspring because of problems at meiosis with pairing of the inversion chromosome with its normal homologue. Both deletions and duplications may result, with concomitant congenital abnormalities related to genomic imbalance, or miscarriage if the imbalance is lethal.

Glossary

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K Karyotype

A photomicrograph of an individual’s chromosomes arranged in a standard format showing the number, size, and shape of each chromosome type, and any abnormalities of chromosome number or morphology (see Figure 10).

Kilobase (kb)

1000 base pairs of DNA.

Knudson hypothesis See tumor suppressor gene

L Linkage

Coinheritance of DNA sequences/phenotypes as a result of physical proximity on a chromosome. Before the advent of molecular genetics, linkage was often studied with regard to proteins, enzymes, or cellular characteristics. An early study demonstrated linkage between the Duffy blood group and a form of autosomal dominant congenital cataract (both are now known to reside at 1q21.1). Phenotypes may also be linked in this manner (ie, families manifesting two distinct Mendelian disorders). During the recombination phase of meiosis, genetic material is exchanged (equally) between two homologous chromosomes. Genes/ DNA sequences that are located physically close to each other are unlikely to be separated during recombination. Sequences that lie far apart on the same chromosome are more likely to be separated. For sequences that reside on different chromosomes, segregation will always be random, so that there will be a 50% chance of two markers being coinherited.

Linkage analysis

258

An algorithm designed to map (ie, physically locate) an unknown gene (associated with the phenotype of interest) to a chromosomal region. Linkage analysis has been the mainstay of disease-associated gene identification for some years. The general availability of large numbers of DNA markers that are variable in the population (polymorphisms), and which therefore permit allele discrimination, has made linkage analysis a relatively rapid and dependable approach (see Figure 11). However, the method relies on the ascertainment of large families manifesting Mendelian disorders. Relatively little phenotypic Genetics for Pediatricians

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36.3 36.2 36.1 35 34.3 34.2 34.1 33 32.3 32.2 32.1

25.3 25.2 25.1

25

23

31.3 31.2 22 31.1 22.3 22.2 22.1

13.3

12

23 24

21.2 21.1

15

14.3 14.2

14

14.1

13

13

12

12

11.2 11.1 11.1 11.2 12

14.1 14.2 14.3 21.1 21.2 21.3

11.2 11.1 11.1 11.2 12 13.1 13.2 13.3 21

31

32.2 32.3

21

23.3 23.2 23.1

24

21.3

15.3 15.2 15.1

22

22

22.3 22.2 22.1

14

21.3 21.2 21.1

21

21.1

13

12

13 12 11.1 11

11.2

12

12

13

11.2 11.1 11.1 11.21 11.22 11.23 12

12

11.1 11.1

14

11.2 11.2

11.2 11.1 11.1 11.21 11.22 11.23

15 16.1

21.1

21.1

21.2 21.3

21.2

14

16.2 16.3

21.3 15

25.1 25.2 25.3

26

22

26.1

27

26.2

28

23.1 23.2 23.3

21

21 22.1 22.2

31.1 31.1 31.2

29

31.3

33

32

34

33 34

36.1

31.2 31.3 32 33.1 33.2 33.3

25.1 25.2 25.3 26

13 21.1 21.2 21.3 22.1 22.2

31.1 31.2

22.1

22.3

22.2 22.3

31

23 31.3

23.1 23.2 23.3

12

22.1

22.3

24

26.3

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Figure 10. Schematic of a normal human (male) karyotype. (ISCN 550 ideogram produced by the MRC Human Genetics Unit, Edinburgh, reproduced with permission.)

heterogeneity is tolerated, as a single misassigned individual (believed to be unaffected despite being a gene carrier) in a pedigree may completely invalidate the results. Genetic heterogeneity is another problem, not within families (usually) but between families. Thus, conditions that result in identical phenotypes despite being associated with mutations within different genes (eg, tuberous sclerosis) are often hard to study. Linkage analysis typically follows a standard algorithm: 1. Large families with a given disorder are ascertained. Detailed clinical evaluation results in assignment of affected vs. unaffected individuals. 2. Large numbers of polymorphic DNA markers that span the genome are analyzed in all individuals (affected and unaffected). 3. The results are analyzed statistically, in the hope that one of the markers used will have demonstrably been coinherited with the phenotype in question more often than would be predicted by chance.

Glossary

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5kb 2kb

2kb

5kb 2kb

5kb 2kb

5kb 2kb

2kb

5kb 2kb

2kb

2kb

2kb

In the example above, note that the (affected) mother has a 5-kb band in addition to a 2-kb band. All the unaffected individuals have the small band only, all those who are affected have the large band. The unaffected individuals must have the mother’s 2-kb fragment rather than her 5-kb fragment, and the affected individuals must have inherited the 5-kb band from the mother (as the father does not have one) – note that those individuals who only show the 2-kb band still have two alleles (one from each parent), they are just the same size and so cannot be differentiated. Thus, it appears that the 5-kb band is segregating with the disorder. The results in a family such as this are suggestive but further similar results in other families would be required for a sufficiently high LOD score.

X

2kb

X

3kb

X

Probe

The probe recognizes a DNA sequence adjacent to a restriction site (see arrow) that is polymorphic (present on some chromosomes but not others). When such a site is present, the DNA is cleaved at that point and the probe detects a 2-kb fragment. When absent, the DNA is not cleaved and the probe detects a fragment of size (2 + 3) kb = 5 kb. X refers to the points at which the restriction enzyme will cleave the DNA. The recognition sequence for most restriction enzymes is very stringent – change in just one nucleotide will result in failure of cleavage. Most RFLPs result from the presence of a single nucleotide polymorphism that has altered the restriction site. Figure 11. Schematic demonstrating the use of restriction fragment length polymorphisms (RFLPs) in linkage analysis.

The LOD score (logarithm of the odds) gives an indication of the likelihood of the result being significant (and not having occurred simply as a result of chance coinheritance of the given marker with the condition). Linkage disequilibrium

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Association of particular DNA sequences with each other, more often than is likely by chance alone (see Figure 12). Of particular relevance to inbred populations (eg, Finland), where specific disease mutations are found to reside in close proximity to specific variants of DNA markers, as a result of the founder effect. Genetics for Pediatricians

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Marker A

Marker B

–

+

–

–

+

+

+

–

Mutant allele

Many generations

+

–

–

–

Mutant allele

Mutant allele

Figure 12. Schematic demonstrating the concept of linkage disequilibrium. A gene is physically very close to marker B and further from marker A. Markers A and B, both on the same chromosome, can exist in one of two forms : +/–. Thus there are four possible haplotypes, as shown. If the founder mutation in the gene occurred as shown, then it is likely that even after many generations the mutant allele will segregate with the – form of marker B, as recombination is unlikely to have occurred between the two. However, since marker A is further away, the gene will now often segregate with the – form of marker A, which was not present on the original chromosome. The likelihood of recombination between the gene and marker A will depend on the physical distance between them, and on rates of recombination. It is possible that the gene would show a lesser but still significant degree of linkage disequilibrium with marker A.

Linkage map

A map of genetic markers as determined by genetic analysis (ie, recombination analysis). May differ markedly from a map determined by actual physical relationships of genetic markers, because of the variability of recombination.

Locus

The position of a gene/DNA sequence on the genetic map. Allelic genes/sequences are situated at identical loci in homologous chromosomes.

Locus heterogeneity Mutations at different loci cause similar phenotypes.

Glossary

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LOD (Logarithm of the Odds) score

A statistical test of linkage. Used to determine whether a result is likely to have occurred by chance or to truly reflect linkage. The LOD score is the logarithm (base 10) of the likelihood that the linkage is meaningful. A LOD score of 3 implies that there is only a 1:1,000 chance that the results have occurred by chance (ie, the result would be likely to occur once by chance in 1,000 simultaneous studies addressing the same question). This is taken as proof of linkage (see Figure 11).

Lyonization

The inactivation of n–1 X chromosomes on a random basis in an individual with n X chromosomes. Named after Mary Lyon, this mechanism ensures dosage compensation of genes encoded by the X chromosome. X chromosome inactivation does not occur in normal males who possess only one X chromosome, but does occur in one of the two X chromosomes of normal females. In males who possess more than one X chromosome (ie, XXY, XXXY, etc.), the rule is the same and only one X chromosome remains active. X-inactivation occurs in early embryonic development and is random in each cell. The inactivation pattern in each cell is faithfully maintained in all daughter cells. Therefore, females are genetic mosaics, in that they possess two populations of cells with respect to the X chromosome: one population has one X active, while in the other population the other X is active. This is relevant to the expression of X-linked disease in females.

M Meiosis

The process of cell division by which male and female gametes (germ cells) are produced. Meiosis has two main roles. The first is recombination (during meiosis I). The second is reduction division. Human beings have 46 chromosomes, and each is conceived as a result of the union of two germ cells; therefore, it is reasonable to suppose that each germ cell will contain only 23 chromosomes (ie, the haploid number). If not, then the first generation would have 92 chromosomes, the second 184, etc. Thus, at meiosis I, the number of chromosomes is reduced from 46 to 23.

Mendelian inheritance

Refers to a particular pattern of inheritance, obeying simple rules: each somatic cell contains two genes for every characteristic and each pair of genes divides independently of all other pairs at meiosis.

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A catalogue of human Mendelian disorders, initiated in book form by Dr Victor McKusick of Johns Hopkins Hospital in Baltimore, USA. The original catalogue (produced in the mid-1960s) listed approximately 1500 conditions. By December 1998, this number had risen to 10,000 and by November 2003 the figure had reached 14,897. With the advent of the Internet, MIM is now available as an online resource, free of charge (OMIM – Online Mendelian Inheritance in Man). The URL for this site is: http://www.ncbi.nlm.nih.gov/omim/. The online version is updated frequently, far faster than is possible for the print version; therefore, new gene discoveries are quickly assimilated into the database. OMIM lists disorders according to their mode of inheritance: 1 - - - - (100000– ) Autosomal dominant (entries created before May 15, 1994) 2 - - - - (200000– ) Autosomal recessive (entries created before May 15, 1994) 3 - - - - (300000– ) X-linked loci or phenotypes 4 - - - - (400000– ) Y-linked loci or phenotypes 5 - - - - (500000– ) Mitochondrial loci or phenotypes 6 - - - - (600000– ) Autosomal loci/phenotypes (entries created after May 15, 1994). Full explanations of the best way to search the catalogue are available at the home page for OMIM.

Messenger RNA (mRNA)

The template for protein synthesis, carries genetic information from the nucleus to the ribosomes where the code is translated into protein. Genetic information flows: DNA → RNA → protein.

Methylation

See DNA methylation.

Microdeletion

Structural chromosome abnormality involving the loss of a segment that is not detectable using conventional (even high resolution) cytogenetic analysis. Microdeletions usually involve 1–3 Mb of sequence (the resolution of cytogenetic analysis rarely is better than 10 Mb). Most microdeletions are heterozygous, although some individuals/families have been described with homozygous microdeletions. See also contiguous gene syndrome.

Glossary

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Microduplication

Structural chromosome abnormality involving the gain of a segment that may involve long sequences (commonly 1–3 Mb), which are, nevertheless, undetectable using conventional cytogenetic analysis. Patients with microduplications have three copies of all sequences within the duplicated segment, as compared to two copies in normal individuals. See also contiguous gene syndrome.

Microsatellites

DNA sequences composed of short tandem repeats (STRs), such as di- and trinucleotide repeats, distributed widely throughout the genome with varying numbers of copies of the repeating units. Microsatellites are very valuable as genetic markers for mapping human genes.

Missense mutation

Single base substitution resulting in a codon that specifies a different amino acid than the wild-type.

Mitochondrial disease/disorder

Ambiguous term referring to disorders resulting from abnormalities of mitochondrial function. Two separate possibilities should be considered. 1. Mutations in the mitochondrial genome (see Figure 13). Such disorders will manifest an inheritance pattern that mirrors the manner in which mitochondria are inherited. Therefore, a mother will transmit a mitochondrial mutation to all her offspring (all of whom will be affected, albeit to a variable degree). A father will not transmit the disorder to any of his offspring. 2. Mutations in nuclear encoded genes that adversely affect mitochondrial function. The mitochondrial genome does not code for all the genes required for its maintenance; many are encoded in the nuclear genome. However, the inheritance patterns will differ markedly from the category described in the first option, and will be indistinguishable from standard Mendelian disorders. Each mitochondrion possesses between 2–10 copies of its genome, and there are approximately 100 mitochondria in each cell. Therefore, each cell possesses 200–1,000 copies of the mitochondrial genome. Heteroplasmy refers to the variability in sequence of this large number of genomes – even individuals with mitochondrial genome mutations are likely to have wild-type alleles. Variability in the proportion of molecules that are wild-type may have some bearing on the clinical variability often seen in such disorders.

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Figure 13. Mitochondrial inheritance. This pedigree relates to mutations in the mitochondrial genome.

Mitochondrial DNA

The DNA in the circular chromosome of mitochondria. Mitochondrial DNA is present in multiple copies per cell and mutates more rapidly than genomic (nuclear) DNA.

Mitosis

Cell division occurring in somatic cells, resulting in two daughter cells that are genetically identical to the parent cell.

Monogenic trait

Causally associated with a single gene.

Monosomy

Absence of one of a pair of chromosomes.

Monozygotic

Arising from a single zygote or fertilized egg. Monozygotic twins are genetically identical.

Mosaicism or mosaic Refers to the presence of two or more distinct cell lines, all derived from the same zygote. Such cell lines differ from each other as a result of DNA content/sequence. Mosaicism arises when the genetic alteration occurs postfertilization (postzygotic). The important features that need to be considered in mosaicism are: The proportion of cells that are “abnormal”. In general, the greater the proportion of cells that are abnormal, the greater the severity of the associated phenotype. The specific tissues that contain high levels of the abnormal cell line(s). This variable will clearly also be relevant to the manifestation of any phenotype. An individual may have a mutation bearing cell line in a tissue where the mutation is largely irrelevant to the normal functioning of that tissue, with a concomitant reduction in phenotypic sequelae. Mosaicism may be functional, as in normal females who are mosaic for activity of the two X chromosomes (see Lyonization).

Glossary

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Mosaicism may occasionally be observed directly. X-linked skin disorders, such as incontinentia pigmenti, often manifest mosaic changes in the skin of a female, such that abnormal skin is observed alternately with normal skin, often in streaks (Blaschko’s lines), which delineate developmental histories of cells. Multifactorial inheritance

A type of hereditary pattern resulting from a complex interplay of genetic and environmental factors.

Mutation

Any heritable change in DNA sequence.

N Nondisjunction

Failure of two homologous chromosomes to pull apart during meiosis I, or two chromatids of a chromosome to separate in meiosis II or mitosis. The result is that both are transmitted to one daughter cell, while the other daughter cell receives neither.

Nondynamic (stable) mutations

Stably inherited mutations, in contradistinction to dynamic mutations, which display variability from generation to generation. Includes all types of stable mutation (single base substitution, small deletions/ insertions, microduplications, and microdeletions).

Nonpenetrance

Failure of expression of a phenotype in the presence of the relevant genotype.

Nonsense mutation

A single base substitution resulting in the creation of a stop codon (see Figure 14).

Northern blot

Hybridization of a radiolabeled RNA/DNA probe to an immobilized RNA sequence. So called in order to differentiate it from Southern blotting, which was described first. Neither has any relationship to points on the compass. Southern blotting was named after its inventor Ed Southern

Nucleotide

A basic unit of DNA or RNA consisting of a nitrogenous base – adenine, guanine, thymine, or cytosine in DNA, and adenine, guanine, uracil, or cytosine in RNA. A nucleotide is composed of a phosphate molecule and a sugar molecule – deoxyribose in DNA and ribose in RNA. Many thousands or millions of nucleotides link to form a DNA or RNA molecule.

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G T C

C T C

T G A

G

Collagen IIα1 sequence

Mutant (protein)

ACT Thr

GTC Val

CTC Leu

TGA STOP

Normal (protein)

ACT Thr

GTC Val

CTC Leu

TGC Cys

Figure 14. Nonsense mutation. This example shows a sequence graph of collagen II (α1) in a patient with Stickler syndrome, an autosomal dominant condition. The sequence is of genomic DNA and shows both normal and abnormal sequences (the patient is heterozygous for the mutation). The base marked with an arrow has been changed from C to A. When translated the codon is changed from TGC (cysteine) to TGA (stop). The premature stop codon in the collagen gene results in Stickler syndrome.

O Obligate carrier

See obligate heterozygote.

Obligate heterozygote (obligate carrier)

An individual who, on the basis of pedigree analysis, must carry the mutant allele.

Oncogene

A gene that, when over expressed, causes neoplasia. This contrasts with tumor suppressor genes, which result in tumorigenesis when their activity is reduced.

P p

Glossary

Short arm of a chromosome (from the French petit) (see Figure 4).

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Palindromic sequence

A DNA sequence that contains the same 5´ to 3´ sequence on both strands. Most restriction enzymes recognize palindromic sequences. An example is 5´–AGATCT–3´, which would read 3´–TCTAGA–5´ on the complementary strand. This is the recognition site of BglII.

Pedigree

A schematic for a family indicating relationships to the proband and how a particular disease or trait has been inherited (see Figure 15).

Penetrance

An all-or-none phenomenon related to the proportion of individuals with the relevant genotype for a disease who actually manifest the phenotype. Note the difference between penetrance and variable expressivity.

Phenotype

Observed disease/abnormality/trait. An all-embracing term that does not necessarily imply pathology. A particular phenotype may be the result of genotype, the environment or both.

Physical map

A map of the locations of identifiable landmarks on DNA, such as specific DNA sequences or genes, where distance is measured in base pairs. For any genome, the highest resolution map is the complete nucleotide sequence of the chromosomes. A physical map should be distinguished from a genetic map, which depends on recombination frequencies.

Plasmid

Found largely in bacterial and protozoan cells, plasmids are autonomously replicating, extrachromosomal, circular DNA molecules that are distinct from the normal bacterial genome and are often used as vectors in recombinant DNA technologies. They are not essential for cell survival under nonselective conditions, but can be incorporated into the genome and are transferred between cells if they encode a protein that would enhance survival under selective conditions (eg, an enzyme that breaks down a specific antibiotic).

Pleiotropy

Diverse effects of a single gene on many organ systems (eg, the mutation in Marfan’s syndrome results in lens dislocation, aortic root dilatation, and other pathologies).

Ploidy

The number of sets of chromosomes in a cell. Human cells may be haploid (23 chromosomes, as in mature sperm or ova), diploid (46 chromosomes, seen in normal somatic cells), or triploid (69 chromosomes, seen in abnormal somatic cells, which results in severe congenital abnormalities).

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Male, female - unaffected

Abortion/stillbirth

Sex not known

Twins

Male, female – affected

Monozygotic twins

4 unaffected females

Heterozygote (AR)

Deceased, affected female

Heterozygote (X-linked)

Consanguineous marriage

Propositus/proband

Figure 15. Symbols commonly used in pedigree drawing.

Point mutation

Single base substitution.

Polygenic disease

Disease (or trait) that results from the simultaneous interaction of multiple gene mutations, each of which contributes to the eventual phenotype. Generally, each mutation in isolation is likely to have a relatively minor effect on the phenotype. Such disorders are not inherited in a Mendelian fashion. Examples include hypertension, obesity, and diabetes.

Polymerase chain reaction (PCR)

A molecular technique for amplifying DNA sequences in vitro (see Figure 16). The DNA to be copied is denatured to its single strand form and two synthetic oligonucleotide primers are annealed to complementary regions of the target DNA in the presence of excess deoxynucleotides and a heat-stable DNA polymerase. The power of PCR lies in the exponential nature of amplification, which results from repeated cycling of the “copying” process. Thus, a single molecule will be copied in the first cycle, resulting in two molecules. In the second cycle, each of these will also be copied, resulting in four copies. In theory, after n cycles, there will be 2n molecules for each starting molecule. In practice, this theoretical limit is rarely reached, mainly for technical reasons. PCR has become a standard technique in molecular biology research as well as routine diagnostics.

Glossary

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Polymorphism

May be applied to phenotype or genotype. The presence in a population of two or more distinct variants, such that the frequency of the rarest is at least 1% (more than can be explained by recurrent mutation alone). A genetic locus is polymorphic if its sequence exists in at least two forms in the population.

Premutation

Any DNA mutation that has little, if any, phenotypic consequence but predisposes future generations to the development of full mutations with phenotypic sequelae. Particularly relevant in the analysis of diseases associated with dynamic mutations.

Proband (propositus) The first individual to present with a disorder through which a pedigree – index case can be ascertained. Probe

General term for a molecule used to make a measurement. In molecular genetics, a probe is a piece of DNA or RNA that is labeled and used to detect its complementary sequence (eg, Southern blotting).

Promoter region

The noncoding sequence upstream (5´) of a gene where RNA polymerase binds. Gene expression is controlled by the promoter region both in terms of level and tissue specificity.

Protease

An enzyme that digests other proteins by cleaving them into small fragments. Proteases may have broad specificity or only cleave a particular site on a protein or set of proteins.

Protease inhibitor

A chemical that can inhibit the activity of a protease. Most proteases have a corresponding specific protease inhibitor.

Proto-oncogene

A misleading term that refers to genes that are usually involved in signaling and cell development, and are often expressed in actively dividing cells. Certain mutations in such genes may result in malignant transformation, with the mutated genes being described as oncogenes. The term proto-oncogene is misleading because it implies that such genes were selected for by evolution in order that, upon mutation, cancers would result because of oncogenic activation. A similar problem arises with the term tumor suppressor gene.

Pseudogene

Near copies of true genes. Pseudogenes share sequence homology with true genes, but are inactive as a result of multiple mutations over a long period of time.

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3'

5'

5'

3' 95°C

DENATURATION

3'

5' P1

P2

1st Cycle

5'

3'

3'

5'

5'

3'

3'

5'

5'

3'

2nd Cycle

Genomic doublestranded DNA

P2

P1

Temperature is lowered to ~50°C to permit annealing of primers to their complementary DNA sequence Temperature is elevated to the optimal heat (~72°C) for the thermophilic polymerase, resulting in primer extension

Denaturation and annealing of primers

3'

Figure 16. Schematic illustrating the technique of polymerase chain reaction (PCR).

Purine

A nitrogen-containing, double-ring, basic compound occurring in nucleic acids. The purines in DNA and RNA are adenine and guanine.

Pyrimidine

A nitrogen-containing, single-ring, basic compound that occurs in nucleic acids. The pyrimidines in DNA are cytosine and thymine, and cytosine and uracil in RNA.

Q q

Long arm of a chromosome (see Figure 4).

R Re-annealing

See hybridization

Recessive (traits, diseases)

Manifest only in homozygotes. For the X chromosome, recessivity applies to males who carry only one (mutant) allele. Females who carry X-linked mutations are generally heterozygotes and, barring unfortunate X-inactivation, do not manifest X-linked recessive phenotypes.

Glossary

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Reciprocal translocation

The exchange of material between two non-homologous chromosomes.

Recombination

The creation of new combinations of linked genes as a result of crossing over at meiosis (see Figure 6).

Recurrence risk

The chance that a genetic disease, already present in a member of a family, will recur in that family and affect another individual.

Restriction enzyme

Endonuclease that cleaves double-stranded (ds)DNA at specific sequences. For example, the enzyme BglII recognizes the sequence AGATCT, and cleaves after the first A on both strands. Most restriction endonucleases recognize sequences that are palindromic – the complementary sequence to AGATCT, read in the same orientation, is also AGATCT. The term “restriction” refers to the function of these enzymes in nature. The organism that synthesizes a given restriction enzyme (eg, BglII) does so in order to “kill” foreign DNA – ”restricting” the potential of foreign DNA that has become integrated to adversely affect the cell. The organism protects its own DNA from the restriction enzyme by simultaneously synthesizing a specific methylase that recognizes the same sequence and modifies one of the bases, such that the restriction enzyme is no longer able to cleave. Thus, for every restriction enzyme, it is likely that a corresponding methylase exists, although in practice only a relatively small number of these have been isolated.

Restriction fragment A restriction fragment is the length of DNA generated when DNA is length polymorphism cleaved by a restriction enzyme. Restriction fragment length varies (RFLP) when a mutation occurs within a restriction enzyme sequence. Most commonly the polymorphism is a single base substitution, but it may also be a variation in length of a DNA sequence due to variable number tandem repeats (VNTRs). The analysis of the fragment lengths after DNA is cut by restriction enzymes is a valuable tool for establishing familial relationships and is often used in forensic analysis of blood, hair, or semen (see Figure 11). Restriction map

A DNA sequence map, indicating the position of restriction sites.

Reverse genetics

Identification of the causative gene for a disorder, based purely on molecular genetic techniques, when no knowledge of the function of the gene exists (the case for most genetic disorders).

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Reverse transcriptase Catalyses the synthesis of DNA from a single-stranded RNA template. Contradicted the central dogma of genetics (DNA → RNA → protein) and earned its discoverers the Nobel Prize in 1975. RNA (ribonucleic acid)

RNA molecules differ from DNA molecules in that they contain a ribose sugar instead of deoxyribose. There are a variety of types of RNA (including messenger RNA, transfer RNA, and ribosomal RNA) and they work together to transfer information from DNA to the protein-forming units of the cell.

Robertsonian translocation

A translocation between two acrocentric chromosomes, resulting from centric fusion. The short arms and satellites (chromosome segments separated from the main body of the chromosome by a constriction and containing highly repetitive DNA) are lost.

S Second hit hypothesis See tumor suppressor gene Segmental aneusomy A general term designed to encompass microdeletion/microduplication syndrome (SAS) syndrome, contiguous gene syndrome, and any situation that results in loss of function of a group of genes at a particular chromosome location, irrespective of genomic copy number (ie, loss of function may be related to mutations in master control regions, which affect the expression of many genes). See also contiguous gene syndrome. Sex chromosomes

Refers to the X and Y chromosomes. All normal individuals possess 46 chromosomes, of which 44 are autosomes and two are sex chromosomes. An individual’s sex is determined by his/her complement of sex chromosomes. Essentially, the presence of a Y chromosome results in the male phenotype. Males have an X and a Y chromosome, while females possess two X chromosomes. The Y chromosome is small and contains relatively few genes, concerned almost exclusively with sex determination and/or sperm formation. By contrast, the X chromosome is a large chromosome that possesses many hundreds of genes.

Sex-limited trait

A trait/disorder that is almost exclusively limited to one sex and often results from mutations in autosomal genes. A good example of a sex-limited trait is breast cancer. While males are affected by breast cancer, it is much less common (~1%) than in women. Females

Glossary

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are more prone to breast cancer than males, not only because they possess significantly more breast tissue, but also because their hormonal milieu is significantly different. In many cases, early onset bilateral breast cancer is associated with mutations either in BRCA1 or BRCA2, both autosomal genes. An example of a sex-limited trait in males is male pattern baldness, which is extremely rare in premenopausal women. The inheritance of male pattern baldness is consistent with autosomal dominant, not sex-linked dominant, inheritance. Sex-linked dominant See X-linked dominant Sex-linked recessive See X-linked recessive Sibship

The relationship between the siblings in a family.

Silent mutation

One that has no (apparent) phenotypic effect.

Single gene disorder A disorder resulting from a mutation on one gene. Somatic cell

Any cell of a multicellular organism not involved in the production of gametes.

Southern blot

Hybridization with a radiolabeled RNA/DNA probe to an immobilized DNA sequence (see Figure 17). Named after Ed Southern (currently Professor of Biochemistry at Oxford University, UK), the technique has spawned the nomenclature for other types of blot (Northern blots for RNA and Western blots for proteins).

Splicing

Removal of introns from precursor RNA to produce messenger RNA (mRNA). The process involves recognition of intron–exon junctions and specific removal of intronic sequences, coupled with reconnection of the two strands of DNA that formerly flanked the intron.

Start codon

The AUG codon of messenger RNA recognized by the ribosome to begin protein production.

Stop codon

The codons UAA, UGA, or UAG on messenger RNA (mRNA) (see Table 2). Since no transfer RNA (tRNA) molecules exist that possess anticodons to these sequences, they cannot be translated. When they occur in frame on an mRNA molecule, protein synthesis stops and the ribosome releases the mRNA and the protein.

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A

B

Figure 17. Southern blotting.

Synergistic heterozygosity

This refers to the phenomenon whereby the manifestation of a phenotype normally associated with complete loss of function of a single gene (ie, that gene has two mutations) may be associated with heterozygous mutations in two distinct genes that inhabit the same or related pathways.

T Telomere

End of a chromosome. The telomere is a specialized structure involved in replicating and stabilizing linear DNA molecules.

Teratogen

Any external agent/factor that increases the probability of congenital malformations. A teratogen may be a drug, whether prescribed or illicit, or an environmental effect, such as high temperature. The classical example is thalidomide, a drug originally prescribed for morning sickness, which resulted in very high rates of congenital malformation in exposed fetuses (especially limb defects).

Termination codon

See stop codon.

Glossary

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RNA polymerase

CTC

Sense strand

DNA 3' CUC GAG

Antisense strand

5' RNA

Figure 18. Schematic demonstrating the process of transcription. The sense strand has the sequence CTC (coding for leucine). RNA is generated by pairing with the antisense strand, which has the sequence GAG (the complement of CTC). The RNA produced is the complement of GAG, CUC (essentially the same as CTC, uracil replaces thymine in RNA).

Thymine (T)

One of the bases making up DNA and RNA (pairs with adenine).

Transcription

Synthesis of single-stranded RNA from a double-stranded DNA template (see Figure 18).

Transfer RNA (tRNA)

An RNA molecule that possesses an anticodon sequence (complementary to the codon in mRNA) and the amino acid which that codon specifies. When the ribosome “reads” the mRNA codon, the tRNA with the corresponding anticodon and amino acid is recruited for protein synthesis. The tRNA “gives up” its amino acid to the production of the protein.

Translation

Protein synthesis directed by a specific messenger RNA (mRNA), (see Figure 19). The information in mature mRNA is converted at the ribosome into the linear arrangement of amino acids that constitutes a protein. The mRNA consists of a series of trinucleotide sequences, known as codons. The start codon is AUG, which specifies that methionine should be inserted. For each codon, except for the stop codons that specify the end of translation, a transfer RNA (tRNA) molecule exists that possesses an anticodon sequence (complementary to the codon in mRNA) and the amino acid which that codon specifies. The process of translation results in the sequential addition of amino acids to the growing polypeptide chain. When translation is complete, the protein is released from the ribosome/mRNA complex and may

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LEU tRNA with anticodon GAG, charged with Leucine

Amino (NH2) terminus of protein

GAG CUCGUC 5'

3' mRNA

Ribosome

Ribosome moves to next codon

Figure 19. Schematic of the process of translation. Messenger RNA (mRNA) is translated at the ribosome into a growing polypeptide chain. For each codon, there is a transfer RNA (tRNA) molecule with the anticodon and the appropriate amino acid. Here, the amino acid leucine is shown being added to the polypeptide. The next codon is GUC, specifying valine. Translation happens in a 5´ to 3´ direction along the mRNA molecule. When the stop codon is reached, the polypeptide chain is released from the ribosome.

then undergo posttranslational modification, in addition to folding into its final, active, conformational shape. Translocation

Glossary

Exchange of chromosomal material between two or more nonhomologous chromosomes. Translocations may be balanced or unbalanced. Unbalanced translocations are those that are observed in association with either a loss of genetic material, a gain, or both. As with other causes of genomic imbalance, there are usually phenotypic consequences, in particular mental retardation. Balanced translocations are usually associated with a normal phenotype, but increase the risk of genomic imbalance in offspring, with expected consequences (either severe phenotypes or lethality). Translocations are described by incorporating information about the chromosomes involved (usually but not always two) and the positions on the chromosomes at which the breaks have occurred. Thus t(11;X)(p13;q27.3) refers to an apparently balanced translocation involving chromosome 11 and X, in which the break on 11 is at 11p13 and the break on the X is at Xq27.3

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Triallelic inheritance The association of a phenotype with three mutations. The classical example is Bardet–Biedl syndrome, in which some individuals only manifest the phenotype when three independent mutations are present (two on one gene and another on one of several genes implicated in this disorder). Triallelic inheritance has been trumpeted as providing an insight into the no-man’s land that lies between Mendelian and polygenic disorders. Triplet repeats

Tandem repeats in DNA that comprise many copies of a basic trinucleotide sequence. Of particular relevance to disorders associated with dynamic mutations, such as Huntington’s chorea (HC). HC is associated with a pathological expansion of a CAG repeat within the coding region of the huntingtin gene. This repeat codes for a tract of polyglutamines in the resultant protein, and it is believed that the increase in length of the polyglutamine tract in affected individuals is toxic to cells, resulting in specific neuronal damage.

Trisomy

Possessing three copies of a particular chromosome instead of two.

Tumor suppressor genes

Genes that act to inhibit/control unrestrained growth as part of normal development. The terminology is misleading, implying that these genes function to inhibit tumor formation. The classical tumor suppressor gene is the Rb gene, which is inactivated in retinoblastoma. Unlike oncogenes, where a mutation at one allele is sufficient for malignant transformation in a cell (since mutations in oncogenes result in increased activity, which is unmitigated by the normal allele), both copies of a tumor suppressor gene must be inactivated in a cell for malignant transformation to proceed. Therefore, at the cellular level, tumor suppressor genes behave recessively. However, at the organismal level they behave as dominants, and an individual who possesses a mutation in only one Rb allele still has an extremely high probability of developing bilateral retinoblastomas. The explanation for this phenomenon was first put forward by Knudson and has come to be known as the Knudson hypothesis (also known as the second hit hypothesis). An individual who has a germ-line mutation in one Rb allele (and the same argument may be applied to any tumor suppressor gene) will have the mutation in every cell in his/her body. It is believed that the rate of spontaneous somatic mutation (defined functionally, in terms of loss of function of that gene by whatever mechanism) is of the order of one in a million per gene per cell division.

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Given that there are many more than one million retinal cells in each eye, and many cell divisions involved in retinal development, the chance that the second (wild-type) Rb allele will suffer a somatic mutation is extremely high. In a cell that has acquired a “second hit”, there will now be no functional copies of the Rb gene, as the other allele is already mutated (germ-line mutation). Such a cell will have completely lost its ability to control cell growth and will eventually manifest as a retinoblastoma. The same mechanism occurs in many other tumors, the tissue affected being related to the tissue specificity of expression of the relevant tumor suppressor gene.

U Unequal crossing over

Occurs between similar sequences on chromosomes that are not properly aligned. It is common where specific repeats are found and is the basis of many microdeletion/microduplication syndromes (see Figure 20).

Uniparental disomy (UPD)

In the vast majority of individuals, each chromosome of a pair is derived from a different parent. However, UPD occurs when an offspring receives both copies of a particular chromosome from only one of its parents. UPD of some chromosomes results in recognizable phenotypes whereas for other chromosomes there do not appear to be any phenotypic sequelae. One example of UPD is Prader–Willi syndrome (PWS), which can occur if an individual inherits both copies of chromosome 15 from their mother.

Uniparental heterodisomy

Uniparental disomy in which the two homologues inherited from the same parent are not identical. If the parent has chromosomes A,B the child will also have A,B.

Uniparental isodisomy

Uniparental disomy in which the two homologues inherited from the same parent are identical (ie, duplicates). So, if the parent has chromosomes A,B then the child will have either A,A or B,B.

Uracil (U)

A nitrogenous base found in RNA but not in DNA, uracil is capable of forming a base pair with adenine.

Glossary

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B1

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B2

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C1

Repeats 1 and 2 represent identical repeated sequences in different positions on the chromosome. These are likely to have no function.

C2 Equal (normal) recombination at meiosis

B1

A1 A2

A1 A2

B1

B2

C2

Product 1 Duplication of region B and all genes within it A2

C2

B2 C1 Meiotic exchange (crossing over)

C1 Unequal (abnormal) recombination at meiosis B2

B1

C1

Product 2 Deletion of region B and all genes within it A1

C2

Figure 20. Schematic demonstrating (i) normal homologous recombination and (ii) homologous unequal recombination, resulting in a deletion and a duplication chromosome.

V Variable expressivity Variable expression of a phenotype: not all-or-none (as is the case with penetrance). Individuals with identical mutations may manifest variable severity of symptoms, or symptoms that appear in one organ and not in another. Variable number of tandem repeats (VNTR)

Certain DNA sequences possess tandem arrays of repeated sequences. Generally, the longer the array (ie, the greater the number of copies of a given repeat), the more unstable the sequence, with a consequent wide variability between alleles (both within an individual and between individuals). Because of their variability, VNTRs are extremely useful for genetic studies as they allow for different alleles to be distinguished.

W Western blot

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Like a Southern or Northern blot but for proteins, using a labeled antibody as a probe.

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X X-autosome translocation

Translocation between the X chromosome and an autosome.

X chromosome

See sex chromosomes.

X-chromosome inactivation

See lyonization.

X-linked

Relating to the X chromosome/associated with genes on the X chromosome.

X-linked recessive (XLR)

X-linked disorder in which the phenotype is manifest in homozygous/hemizygous individuals (see Figure 21). In practice, it is hemizygous males that are affected by X-linked recessive disorders, such as Duchenne’s muscular dystrophy (DMD). Females are rarely affected by XLR disorders, although a number of mechanisms have been described that predispose females to being affected, despite being heterozygous.

X-linked dominant

X-linked disorder that manifests in the heterozygote. XLD disorders

(XLD)

result in manifestation of the phenotype in females and males (see Figure 22). However, because males are hemizygous, they are more severely affected as a rule. In some cases, the XLD disorder results in male lethality.

Y Y chromosome

See sex chromosomes.

Z Zippering

Glossary

A process by which complementary DNA (cDNA) strands that have annealed over a short length undergo rapid full annealing along their whole length. DNA annealing is believed to occur in two main stages. A chance encounter of two strands that are complementary results in a short region of double-stranded DNA (dsDNA), which if perfectly matched, stabilizes the two single strands so that further re-annealing

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Figure 21a. X-linked recessive inheritance – A. Most X-linked disorders manifest recessively, in that heterozygous females (carriers) are unaffected and males, who are hemizygous (possess only one X chromosome) are affected. In this example, a carrier mother has transmitted the disorder to three of her sons. One of her daughters is also a carrier. On average, 50% of the male offspring of a carrier mother will be affected (having inherited the mutated X chromosome), and 50% will be unaffected. Similarly, 50% of daughters will be carriers and 50% will not be carriers. None of the female offspring will be affected but the carriers will carry the same risks to their offspring as their mother. The classical example of this type of inheritance is Duchenne muscular dystrophy.

Figure 21b. X-linked recessive inheritance – B. In this example the father is affected. Because all his sons must have inherited their Y chromosome from him and their X chromosome from their normal mother, none will be affected. Since all his daughters must have inherited his X chromosome, all will be carriers but none affected. For this type of inheritance, it is clearly necessary that males reach reproductive age and are fertile – this is not the case with Duchenne’s muscular dystrophy, which is usually fatal by the teenage years in boys. Emery-Dreifuss muscular dystrophy is a good example of this form of inheritance, as males are likely to live long enough to reproduce.

Figure 22. X-linked dominant inheritance. In X-linked dominant inheritance, the heterozygous female and hemizygous male are affected, however, the males are usually more severely affected than the females. In many cases, X-linked dominant disorders are lethal in males, resulting either in miscarriage or neonatal/infantile death. On average, 50% of all males of an affected mother will inherit the gene and be severely affected; 50% of males will be completely normal. Fifty percent of female offspring will have the same phenotype as their affected mother and the other 50% will be normal and carry no extra risk for their offspring. An example of this type of inheritance is incontinentia pigmenti, a disorder that is almost always lethal in males (males are usually lost during pregnancy).

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of their specific sequences proceeds extremely rapidly. The initial stage is known as nucleation, while the second stage is called zippering. Zygote

Glossary

Diploid cell resulting from the union of male and female haploid gametes.

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17 Index

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Page numbers in italics indicate tables. Page numbers in bold indicate figures. vs indicates a comparison or differential diagnosis. ABCD1 63–4 ACADM 210 acetylhydrolase 1B 46–7 achondrogenesis II 122 achondroplasia 108–10 acrocephalosyndactyly type I see Apert syndrome type III see Saethre–Chotzen syndrome (SCS) type V see Pfeiffer syndrome acrocephaly 144 acylcarnitines 211 ADA 206, 206–7 ADAMTS2 112 Addison disease 62 adenosine deaminase (ADA) deficiency 205–7 adrenal carcinoma 221 adrenal steroid 21-hydroxylase 161–2 adrenoleukodystrophy, X-linked 62–4 adrenomyeloneuropathy (AMN) 62 adult nonnephropathic cystinosis 223–5 adult polycystic kidney disease type 1 (APKD1) 226–7 type 2 (APKD2) 226–8 AE1 hereditary elliptocytosis 189 hereditary renal tubular acidosis 192 hereditary spherocytosis 191 Southeast Asian ovalocytosis 192 agammaglobulinemia, X-linked see Bruton agammaglobulinemia aganglionic megacolon see Hirschsprung disease agenesis of the corpus callosum, X-linked 67 agyria spectrum see lissencephaly AIPL1 76, 77, 78 AIS (androgen insensitivity syndrome) 158–9 Alagille syndrome 174–5 Albright’s hereditary osteodystrophy (AHO) (pseudohypoparathyroidism) 169–71 aldosterone synthesis 161 congenital adrenal hyperplasia 160 aldosterone synthetase deficiency 162 α1-antitrypsin deficiency 175–7 α-fetoprotein (AFP) 2 Alport syndrome (AS) 218–20 genes 122 androgen insensitivity syndrome (AIS) 158–9 androgen receptor androgen insensitivity syndrome 158–9 Sotos syndrome 154 anemia 193 Angelman syndrome (AS) 30–3 Rett syndrome vs 31, 62 anion exchange member 1 hereditary elliptocytosis 189 hereditary spherocytosis 191 aniridia 70–2 ANK1 191 ankyrin 1 191 α1-antitrypsin deficiency 175–7 Antly–Bixler syndrome 147 Apert syndrome 144–6 fibroblast growth factor receptors 145 AQP2 163 aquaporin-2 163, 164

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aqueduct stenosis, X-linked (X-linked hydrocephalus) 66–8 AR androgen insensitivity syndrome 158–9 CAG repeat 159 areflexia 10, 11, 27 arginine vasopressin (AVP) 163, 163–4 arginine vasopressin receptor 2 163, 164 aristaless-related homeobox protein 48 artemin 95 arterio-hepatic disorder (Alagille syndrome) 174–5 arthrochalasis multiplex congenita 112 arthro-ophthalmopathy, hereditary (Stickler syndrome) 122, 125–7 ARX 47, 48, 50 AS see Alport syndrome (AS); Angelman syndrome (AS) ASM1 (H19) 221 ASM1(H19) 223 asplenia with cardiovascular anomalies (laterality defects) 137–8 ataxia–telangiectasia (AT) 2–4 ATB7B 216 ATM 2–4 ATP7A Ehlers–Danlos syndrome 112 Menkes disease 212 atrial septal defects (ASDs) Holt–Oram syndrome 136 laterality defects 137 Noonan syndrome 138 ATR-X syndrome 64–6 autosomal dominant hyperplasia 81 autosomal dominant keratitis 71 autosomal recessive polycystic kidney disease (ARKPD) 226–8 AVP 163, 163–4 AVPR2 163, 164 bacterially expressed kinase (BEK) 145, 147 Bardet–Biedl syndrome (BBS) 72–4 bare lymphocyte syndrome 206 Barth syndrome 130 Batten disease see neuronal ceroid lipofucinosis (NCL) BBS (Bardet–Biedl syndrome) 72–4 BBS1 72, 73 BBS2 72, 73 BBS4 72, 73 BBS6 73 B-cell lymphomas 2 Beals’ syndrome 119 Beare–Stevenson cutis gyrata syndrome 147 Becker muscular dystrophy 5–6 Beckwith–Wiedemann syndrome (BWS) 220–3 Bethlem myopathy 122 bilateral pseudoglioma 79 birth weights, Sotos syndrome 153 bone marrow failure, Fanconi anemia 182 Bourneville–Pringle syndrome (tuberous sclerosis) 101–3 brachycephaly Apert syndrome 144 Saethre–Chotzen syndrome 151 brain tumors, retinoblastoma 100 BRCA2 183 bronchiectasis, primary ciliary dyskinesia 139 Bruton agammaglobulinemia 202–3 associated with growth hormone deficiency 203 Bruton tyrosine kinase 202–3 BTK Bruton agammaglobulinemia 202–3 growth hormone deficiency 165

Index

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butterfly vertebrae, Alagille syndrome 174 BWS (Beckwith-Wiedemann syndrome) 220–3 cadherin 23 nonsyndromal hearing loss 84 Usher syndrome 89 café-au-lait macules 96 CAG repeat, AR 159 CAH (congenital adrenal hyperplasia) 160–2, 162 calpain 3 19, 20 cancer 93–105 CAPN3 19, 20 carcinoembryonic antigen 2 cardiomyopathy hypertrophic, Noonan syndrome 138 X-linked 6 cardio-respiratory disorders 129–42 cartilage oligomeric matrix protein 124–5 “cascade screening”, cystic fibrosis 133 cataracts, congenital 71 catecholamines 96 CAV3 19, 20 caveolin 3 19, 20 CBP/CREBBP 151 CDH23 nonsyndromal hearing loss 84 Usher syndrome 89, 89–90 CDKN1C 221, 222 cerebral gigantism (Sotos syndrome) 153–4 cerebral malformations 29–68 ceruloplasmin Menkes disease 212 Wilson disease 216 CF (cystic fibrosis) 131–3 CFTR 131–3 CGD (chronic granulomatous disease) 203–5 CGG repeats, fragile X syndrome 34–6 Charcot–Marie–Tooth disease see hereditary motor and sensory neuropathy (HMSN) chondrosarcomas, hereditary multiple exostoses 115 chorea, Huntington disease 41 Christmas disease (hemophilia B) 187–8 chromosome 1 chronic granulomatous disease 204 cobblestone lissencephaly 49 collagen gene disorders 122 congenital adrenal hyperplasia 162 congenital hypomyelinating neuropathy 15 Dejerine–Sottas syndrome 14 hereditary elliptocytosis 189 hereditary motor and sensory neuropathy 13, 14, 15 hereditary spherocytosis 191 Hirschsprung disease association 179 infantile neuronal ceroid lipofucinosis 54 Leber congenital amaurosis 76 limb-girdle muscular dystrophy 20 medium chain acyl-CoA dehydrogenase deficiency 210 nonsyndromal hearing loss 84 ocular-scoliotic Ehlers–Danlos syndrome 112 Stickler syndrome II 126 Usher syndrome 89 van der Woude syndrome 155 Waardenburg syndrome 91 chromosome 2 Alport syndrome 219 arterial/vascular Ehlers–Danlos syndrome 112 Bardet–Biedl syndrome 72

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classical Ehlers–Danlos syndrome 112 collagen gene disorders 122 familial hypermobility Ehlers–Danlos syndrome 112 Hirschsprung disease association 179 holoproscencephaly 36 limb-girdle muscular dystrophy 20 nonsyndromal hearing loss 84 Waardenburg syndrome 91 chromosome 3 Bardet–Biedl syndrome 72 collagen gene disorders 122 Fanconi anemia 183 hereditary motor and sensory neuropathy 13 Hirschsprung disease association 179 limb-girdle muscular dystrophy 20 nonsyndromal hearing loss 84 panhypopituitarism 168 proximal myotonic myopathy 25 Usher syndrome 89 von Hippel–Lindau disease 104 Waardenburg syndrome 91 chromosome 4 achondroplasia 109 adult polycystic kidney disease type 2 227 Crouzon syndrome 147 fascioscapulohumeral muscular dystrophy 7 Huntington disease 42 limb-girdle muscular dystrophy 21 nonsyndromal hearing loss 84 polycystic kidney disease 227 Rieger syndrome 80 chromosome 5 dermatosparaxis Ehlers–Danlos syndrome 112 growth hormone receptor defects 167 hereditary motor and sensory neuropathy 15 Hirschsprung disease association 179 limb-girdle muscular dystrophy 20, 21 nonsyndromal hearing loss 84 panhypopituitarism 168 primary ciliary dyskinesia 140 Sotos syndrome 153 spinal muscular atrophy 27 Treacher Collins syndrome 154 Usher syndrome 89 chromosome 6 autosomal recessive polycystic kidney disease 227 collagen gene disorders 122 congenital adrenal hyperplasia 160 Fanconi anemia 183 laterality defects 137 Leber congenital amaurosis 76 limb-girdle muscular dystrophy 20 nonsyndromal hearing loss 84 polycystic kidney disease 227 Stickler syndrome III 126 chromosome 7 arthrochalasis multiplex congenita 112 chronic granulomatous disease 204 cobblestone lissencephaly 49 collagen gene disorders 122 cystic fibrosis 131 Greig syndrome 148 hereditary motor and sensory neuropathy 13 holoproscencephaly 36 limb-girdle muscular dystrophy 20 nonsyndromal hearing loss 84

Index

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osteogenesis imperfecta 120 Pendred syndrome 87 Saethre–Chotzen syndrome 151 Williams syndrome 141 chromosome 8 congenital adrenal hyperplasia 162 hereditary motor and sensory neuropathy 13, 14, 15 hereditary multiple exostoses 116 hereditary spherocytosis 191 nonsyndromal hearing loss 84 Pfeiffer syndrome 150 Waardenburg syndrome 91 chromosome 9 classical Ehlers–Danlos syndrome 112 cobblestone lissencephaly 49 collagen gene disorders 122 Fanconi anemia 183 Friedreich Ataxia 8 Fukuyama muscular dystrophy 49 holoproscencephaly 36 limb-girdle muscular dystrophy 21 nonsyndromal hearing loss 84 panhypopituitarism 168 primary ciliary dyskinesia 140 tuberous sclerosis 102 Walker–Warburg syndrome 49 chromosome 10 Apert syndrome 144 collagen gene disorders 122 congenital adrenal hyperplasia 162 congenital hypomyelinating neuropathy 15 Crouzon syndrome 147 Dejerine–Sottas syndrome 14 hereditary motor and sensory neuropathy 13, 14, 15 Hirschsprung disease association 179 multiple endocrine neoplasia type 2 94 nonsyndromal hearing loss 84 Pfeiffer syndrome 150 Usher syndrome 89 chromosome 11 aniridia 70 ataxia–telangiectasia 2 Bardet–Biedl syndrome 72 Beckwith–Wiedemann syndrome 221, 222 Fanconi anemia 183 hereditary motor and sensory neuropathy 15 hereditary multiple exostoses 116 late infantile neuronal ceroid lipofucinosis 54 nonsyndromal hearing loss 84 sickle cell anemia 194 β-thalassemia 197 Usher syndrome 89 chromosome 12 collagen gene disorders 122 Holt–Oram syndrome 136 nephrogenic diabetes insipidus 163 Noonan syndrome 138 phenylketonuria 214 Stickler syndrome I 126 von Willebrand disease 199 chromosome 13 connexin 26 gene defect 84, 85 Fanconi anemia 183 Hirschsprung disease association 179 holoproscencephaly 36 late infantile neuronal ceroid lipofucinosis 54

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limb-girdle muscular dystrophy 20 nonsyndromal hearing loss 84 retinoblastoma 99 Rieger syndrome 80 Waardenburg syndrome 91 Wilson disease 216 chromosome 14 α1-antitrypsin deficiency 176 hereditary elliptocytosis 189 hereditary spherocytosis 191 Leber congenital amaurosis 76 nonsyndromal hearing loss 84 severe combined immunodeficiency 206 Usher syndrome 89 chromosome 15 Angelman syndrome 30, 31 Bardet–Biedl syndrome 72 limb-girdle muscular dystrophy 20 Marfan syndrome 118 nonsyndromal hearing loss 84 Prader–Willi syndrome 31, 59 chromosome 16 adult polycystic kidney disease type 1 227 Bardet–Biedl syndrome 72 chronic granulomatous disease 204 Fanconi anemia 183 hereditary motor and sensory neuropathy 13 nonsyndromal hearing loss 84 polycystic kidney disease 227 α-thalassemia 195 chromosome 17 arthrochalasis multiplex congenita 112 classical Ehlers–Danlos syndrome 112 collagen gene disorders 122 cystinosis 224 Dejerine–Sottas syndrome 14 growth hormone deficiency 165 hereditary elliptocytosis 189 hereditary motor and sensory neuropathy 13, 14, 15 hereditary neuropathy with liability to pressure palsies 15 hereditary spherocytosis 191 Leber congenital amaurosis 76 limb-girdle muscular dystrophy 20, 21 lissencephaly 48 Miller–Dieker syndrome 48 neurofibromatosis type 1 97 nonsyndromal hearing loss 84 osteogenesis imperfecta 120 Usher syndrome 89 chromosome 18, holoproscencephaly 36 chromosome 19 Dejerine–Sottas syndrome 14 hereditary motor and sensory neuropathy 14 hereditary multiple exostoses 116 Hirschsprung disease association 179 Leber congenital amaurosis 76 limb-girdle muscular dystrophy 21 myotonic dystrophy 25 pseudoachondrodysplasia 124 chromosome 20 adenosine deaminase deficiency 206 Alagille syndrome 174 Bardet–Biedl syndrome 72 collagen gene disorders 122 Hirschsprung disease association 179 neurohypophyseal diabetes insipidus 163

Index

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pseudohypoparathyroidism 171 severe combined immunodeficiency 206 Waardenburg syndrome 91 chromosome 21 collagen gene disorders 122 holoproscencephaly 36 nonsyndromal hearing loss 84 Usher syndrome 89 chromosome 22 DiGeorge/Shprintzen syndrome 134 Hirschsprung disease association 179 nonsyndromal hearing loss 84 Waardenburg syndrome 91 chromosome analysis aniridia 71 lissencephaly 51 chronic granulomatous disease (CGD) 203–5 classic hemophilia (hemophilia A) 185–7 claudin 14 84 CLDN14 84 clear cell renal cell carcinomas, von Hippel–Lindau disease 103 cleft lip/palate, van der Woude syndrome 155 clinical examination Ehlers–Danlos syndrome 114 tuberous sclerosis 103 CLN1 54, 55 CLN2 54, 56 CLN3 54, 56 CLN5 54, 56–7 Coat’s disease 79 cobblestone lissencephaly 46 COCH 84 cochlin 84 COL1A1 Ehlers–Danlos syndrome 112, 113–4, 122 osteogenesis imperfecta 120–1, 122 osteoporosis 122 COL1A2 Ehlers–Danlos syndrome 112, 113–4, 122 osteogenesis imperfecta 120–1, 122 osteoporosis 122 COL2A1 122, 126–7 Stickler syndrome I 122, 126, 126 COL3A1 112, 113, 122 COL4A3 122, 218–20, 219 COL4A4 122, 218–9, 219 COL4A5 122, 218–20, 219 COL4A6 Alport syndrome 219 leiomyomatosis 122 COL5A1 111, 112, 113–4, 122 COL5A2 111, 112, 113–4, 122 COL6A1 122 COL6A2 122 COL6A3 122 COL7A1 122 COL9A1 122 COL9A2 122 COL9A3 122 COL10A1 122 COL11A1 122, 126, 127 COL11A2 nonsyndromal hearing loss 84 Stickler syndrome III 122, 126, 127 Weissenbacher–Zweymuller syndrome 122 COL17A1 122

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COL18A1 122 collagen type IIα2 84 type IV 218–20 combined pituitary hormone deficiency (panhypopituitarism) 167–9 COMP 124–5 congenital adrenal hyperplasia (CAH) 160–2, 162 congenital cataracts 71 congenital central hyperventilation syndrome 95 congenital hypomyelinating neuropathy (CHN) 11, 15 congenital intestinal aganglionosis see Hirschsprung disease congenital lipoid adrenal hyperplasia 162 connective tissue disorders 107–27 connexin 26/30 84 26 gene defect 85–6 31 84 conotruncal anomaly facial syndrome (DiGeorge/Shprintzen syndrome) 133–5 convulsions, Angelman syndrome 30 Cooley’s anemia (β-thalassemia) 197–8 copper deficiency, Menkes disease 211 copper deposition, Wilson disease 215–6 copper-transporting ATPase Menkes disease 212 Wilson disease 216 cortisol synthesis 161 congenital adrenal hyperplasia 160 craniofacial disorders 143–56 craniofacial dysostosis see Crouzon syndrome craniosynostosis Crouzon syndrome 146 Saethre–Chotzen syndrome 151 CRB1 76, 77–8 creatine kinase (CK) 6 CREBBP/CBP 151 cross-reacting material (CRM) positive, hemophilia A 186 Crouzon syndrome 146–8 FGFR2 150 fibroblast growth factor receptors 145 Crouzon syndrome with acanthosis nigricans 147–8 fibroblast growth factor receptors 145 CRX1 76, 77, 78 CTNS 224 cutaneous neurofibromas 96 cutis laxa, Ehlers–Danlos syndrome vs 110–1 CX26/30 connexin 26 gene defect 85–6 nonsyndromal hearing loss 84 CX31 84 CX32 (GLB1) 12, 13, 16, 17 CXORF5(OFDI) 225–6 CYBA 204, 204 CYBB 204, 204–5 cyclin-dependent kinase inhibitor 1C 221 CYP11B1 162 CYP11B2 162 CYP17 162 CYP21 160–2 cystic fibrosis (CF) 131–3 cystic fibrosis transmembrane conductance regulator (CFTCR) 131–3 cystinoin 224 cystinosis 223–5 cytochrome-b α-subunit 204 β-subunit 204

Index

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DCX 47, 48 DDP 84 deafness with goiter (Pendred syndrome) 86–7 Dejerine–Sottas syndrome 10 dementia, Huntington disease 41 depigmentation, Waardenburg syndrome 90 dermatosparaxis Ehlers–Danlos syndrome 112 17, 20-desmolase deficiency 159 dexamethasone, congenital adrenal hyperplasia 162 DFNA5 84 diabetes insipidus (DI) 163–4 nephrogenic 163, 164 neurohypophyseal 163, 163–4 diaphyseal aclasis (hereditary multiple exostoses) 115–7 DiGeorge/Shprintzen syndrome 133–5 dihydropteridine reductase 215 dilated cardiomyopathy Barth syndrome 130 LMNA 19 DMD 4–6 DMD (Duchenne muscular dystrophy) 4–6 DMPK 25–6 DNAH5 140 DNAI1 140 DNA methylation, Beckwith–Wiedemann syndrome 222–3 doublecortin 46–47, 48 Down’s syndrome, Hirschsprung disease association 177 Duchenne muscular dystrophy (DMD) 4–6 Dunnigan type partial lipodystrophy 19 dynein 46–47, 140, 140–1 dysarthria, Friedreich ataxia 8 DYSF 19, 20 dysferlin 19, 20 dystonia canthorum 90 dystrophia myotonica protein kinase 25–6 dystrophin 5–6 ECE1 179 eczema phenylketonuria 214 Wiskott–Aldrich syndrome 207–8 EDN3 Hirschsprung disease association 179 Waardenburg syndrome 91, 92 EDNRB Hirschsprung disease association 179 Waardenburg syndrome 91, 92 EGR2 12, 13, 14, 15 Ehlers–Danlos syndrome 110–4 cutis laxa vs 110–1 genes/chromosomal locations 112, 122 elastin 141–2 electroencephalography (EEG) adult neuronal ceroid lipofucinosis 55 infantile neuronal ceroid lipofucinosis 54 electromyography 27 electron microscopy, Ehlers–Danlos syndrome 114 electroretinography (ERG) infantile neuronal ceroid lipofucinosis 54 juvenile retinoschisis 74 elliptocytes 189–90 elliptocytosis, hereditary 189–90 ELN 141–2 Emery–Dreifuss muscular dystrophy limb-girdle muscular dystrophy vs 22–3 LMNA 19

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endocrine disorders 157–71 endothelin 3 179 endothelin-converting enzyme-1 179 endothelin receptor, type B 179 EPB41 189 epidermolysis bullosa 122 junctional 122 Epstein syndrome, Alport syndrome overlap 220 erythrocyte cytoskeleton hereditary elliptocytosis 190 hereditary spherocytosis 191–2 exomphalos-macroglossia-gigantism (EMG) syndrome (Beckwith-Wiedemann syndrome) 220–3 EXT1 hereditary multiple exostoses 116, 116–7 Langer–Giedion syndrome 117 EXT2 116, 116–7 EXT3 116 exudative vitreoretinopathy, X-linked 79 EYA4 84 F8 186–7 F9 188 facial features achondroplasia 108 Alagille syndrome 174 DiGeorge/Shprintzen syndrome 133–4 holoproscencephaly 36 Miller–Dieker syndrome 45 pseudohypoparathyroidism 169 Rubenstein–Taybi syndrome 151 Stickler syndrome 125 Treacher Collins syndrome 154 Williams syndrome 141 X-linked α-thalassemia and mental retardation syndrome 64 facioscapulohumeral muscular dystrophy (FSHMD) 7–8 limb-girdle muscular dystrophy vs 22–3 factor VIII deficiency (hemophilia A) 185–7 factor IX deficiency (hemophilia B) 187–8 Fallot’s tetralogy, DiGeorge/Shprintzen syndrome 134 familial medullary thyroid carcinoma see multiple endocrine neoplasia type 2 (MEN2) family history, Ehlers–Danlos syndrome 114 FANCA 182, 183 FANCC 182, 183 FANCD2 183 FANCE 183 FANCF 183 FANCG 183 Fanconi anemia 182–3 Fanconi pancytopenia (Fanconi anemia) 182–3 favism (glucose-6-phosphate dehydrogenase deficiency) 183–5 FBN1 118–9 FBN2 119 FCMD cobblestone lissencephaly 49 Fukuyama muscular dystrophy 49, 50–1 Fechtner syndrome, Alport syndrome overlap 220 α-fetoprotein (AFP) 2 FGFR1 150, 150 FGFR2 Antly–Bixler syndrome 147 Apert syndrome 144–6, 145, 147 Beare–Stevenson cutis gyrata syndrome 147 Crouzon syndrome 147, 147–8, 150 Jackson–Weiss syndrome 147 Pfeiffer syndrome 147, 150, 150

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FGFR3 achondroplasia 109, 109–10, 110 Crouzon syndrome 147, 147–8 hypochondroplasia 109, 110 severe achondroplasia with developmental delay and acanthosis nigricans 110 thanatophoric dysplasia 109, 110 fibrillin 1 118–9 fibroblast growth factor receptors achondroplasia 109, 109–10 Apert syndrome 145 Crouzon syndrome 145 with acanthosis nigricans 145 Muenke syndrome 145 Pfeiffer syndrome 145 fibrocystin 227, 228 FKRP 21, 22 “flip inversions”, hemophilia A 186, 186, 187 fluorescence in situ hybridization (FISH) Angelman syndrome 32, 33 aniridia 71 DiGeorge/Shprintzen syndrome 134–5, 135 lissencephaly 51 neurofibromatosis type 1 97 Prader–Willi syndrome 60 Rubenstein–Taybi syndrome overlap 151 Williams syndrome 142 FMR1 34–5 14-3-3ε 47, 48 fragile X syndrome 34–6 Franceschetti’s sign, Leber congenital amaurosis 75 frataxin 9 FRDA 9 freckling, neurofibromatosis type 1 96 Friedreich Ataxia 8–9 FSHMD see facioscapulohumeral muscular dystrophy fukutin cobblestone lissencephaly 49 Fukuyama muscular dystrophy 49, 50–1 fukutin-related protein 21, 22 Fukuyama muscular dystrophy 49 G4.5 (TAZ) 130 G6PD 184–5 gait abnormalities, X-linked adrenoleukodystrophy 62 GARS 13, 16 gastrointestinal disorders 173–9 GDAP1 14, 15, 16 GDNF 179 gel electrophoresis, α1-antitrypsin deficiency 176 GH1 165, 165–6 GHR 167 GLI3 148–9 glial cell-line derived neurotrophic factor (GDNF) 95 Hirschsprung disease association 179 glucose-6-phosphate dehydrogenase deficiency 183–5 GNAS1 170, 171 Gorlin syndrome 38 granular osmophilic deposits (GRODs) 54 Greig cephalopolysyndactyly syndrome see Greig syndrome Greig syndrome 148–9 Rubenstein–Taybi syndrome overlap 151 growth hormone deficiency 164–6 receptor defects 166–7 guanine nucleotide-binding (Gs) protein 170, 171 GUCY2d 76, 77, 78

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H19 (ASM1) 221, 223 Haddad syndrome 95 happy puppet syndrome see Angelman syndrome (AS) harmonin 88, 89 HBA 195–6 HBB sickle cell anemia 194 β-thalassemia 197–8 HbH inclusions, X-linked α-thalassemia and mental retardation syndrome 65–6 HD (Huntington disease) 41–3 HD1A8 84 hearing disorders 83–92 Alport syndrome 218 heart–hand syndrome (Holt–Oram syndrome) 135–6 Heinz bodies 185 hemangioblastomas, von Hippel–Lindau disease 103 hematologic disorders 181–99 hematuria Alport syndrome 218 polycystic kidney disease 226 hemoglobin α-globin gene 196 α-thalassemia 195–7 hemoglobin β-globin gene sickle cell anemia 194 β-thalassemia 197–8 hemoglobin H (HbH) disease 194–7 hemophilia A 185–7 hemophilia B 187–8 hepatic disorders 173–9 hepatoblastoma, Beckwith–Wiedemann syndrome 221 hepato-lenticular degeneration (Wilson disease) 215–6 HERC2 60 hereditary arthro-ophthalmopathy (Stickler syndrome) 122, 125–7 hereditary elliptocytosis 189–90 hereditary motor and sensory neuropathy (HMSN) 10–7 clinical features 9–10 molecular pathogenesis 11–2, 16–7 hereditary multiple exostoses (HME) 115–7 hereditary neuropathy with liability to pressure palsies (HNPP) 11 hereditary pyropoikilocytosis 189 hereditary renal tubular acidosis 192 hereditary spherocytosis 190–2 heterotaxy (laterality defects) 137–8 hexacosanoate 64 HGPRT deficiency (Lesch–Nyhan syndrome) 43–4 Hirschsprung disease 177–9 disease association 177 RET 95 Waardenburg syndrome 92 HMSN see hereditary motor and sensory neuropathy holoproscencephaly (HPE) 36–9 Holt–Oram syndrome (HOS) 135–6 HRPT 44 HSCR see Hirschsprung disease HSD3B2 162 Hunter syndrome (HS) 40–1 huntingtin 42 Huntington disease (HD) 41–3 Hutchinson–Gilford syndrome 19 hydrocephalus, X-linked 66–8 hydrops fetalis 195–6 11β-hydroxylase deficiency 162 17α-hydroxylase deficiency 159 congenital adrenal hyperplasia 162 21-hydroxylase deficiency (congenital adrenal hyperplasia) 160–2, 162

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3β-hydroxysteroid dehydrogenase deficiency 159 congenital adrenal hyperplasia 162 17β-hydroxysteroid dehydrogenase deficiency 159 hyperammonemia, ornithine transcarbamylase deficiency 212–3 hyperphosphatemia, pseudohypoparathyroidism 169 hypertrophic cardiomyopathy, Noonan syndrome 138 hypocalcemia, pseudohypoparathyroidism 169 hypochondrogenesis 122 hypochondroplasia 109, 110 hypoglycemia, medium chain acyl-CoA dehydrogenase deficiency 210 hypopigmentation Angelman syndrome 32 phenylketonuria 214 Prader–Willi syndrome 60 hypothyroidism, pseudohypoparathyroidism 169 hypotonia Pelizaeus–Merzbacher syndrome 57 Prader–Willi syndrome 59 spinal muscular atrophy 27 hypoxanthine-guanine phosphoribosyl transferase deficiency (Lesch–Nyhan syndrome) 43–4 IDS 40–1 iduronate 2-sulfatase 40–1 IGF2 221, 223 IL2RG 206, 207 immotile cilia syndrome see primary ciliary dyskinesia immunoglobulin levels, ataxia–telangiectasia 2 immunohistochemistry Duchenne muscular dystrophy 6 limb-girdle muscular dystrophy 22 immunologic disorders 201–8 imprinting Beckwith–Wiedemann syndrome 222–3 Prader–Willi syndrome 60 pseudohypoparathyroidism 170 infantile hypercalcemia (Williams syndrome [WS]) 141–2 infantile nephropathic cystinosis 223–5 infantile Refsum disease 76 infantile spasms, X-linked 50 infections sickle cell anemia 193 Wiskott–Aldrich syndrome 207–8 insulin-like growth factor (IGF)1 166 insulin-like growth factor (IGF)2 221 interferon regulatory factor 6 155–6 interleukin-2 receptor, γ chain 206, 207 intestinal aganglionosis, congenital see Hirschsprung disease IRF6 155–6 iridogoniodysgenesis type II (Rieger syndrome) 80–1 isoelectric focusing, α1-antitrypsin deficiency 176 isolated fovea hypoplasia 71 isomerism (laterality defects) 137–8 IT15 42–3 Ivemark syndrome (laterality defects) 137–8 Jackson–Weiss syndrome 147 JAG1 174–5 jagged 1 174–5 Jansky–Bielschowski disease see neuronal ceroid lipofucinosis (NCL) jaundice Alagille syndrome 174 glucose-6-phosphate dehydrogenase deficiency 184 hereditary spherocytosis 190 Jervell syndrome 223 joint laxity, Ehlers–Danlos syndrome 110 Joubert syndrome 76

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Juberg–Marsidi syndrome 65 junctional epidermolysis bullosa 122 juvenile nephropathic cystinosis 223–5 juvenile retinoschisis 74–5 Kartagener syndrome see primary ciliary dyskinesia Kayser–Fleischer ring, Wilson disease 216 keratinocyte growth factor receptor (KGFR) Apert syndrome 145–6 Crouzon syndrome 147 KIF1B 13, 16 kinky hair disease see Menkes disease Klein–Waardenburg syndrome see Waardenburg syndrome Kneist dysplasia 122 Knobloch syndrome 122 Kufs disease see neuronal ceroid lipofucinosis (NCL) Kugelberg–Welander syndrome 27–8 KVLQT1 Beckwith–Wiedemann syndrome 221, 222–3 Jervell syndrome 223 Lange–Nielsen syndrome 223 L1CAM 67–8 L1 cell-adhesion molecule 67–8 lamin A hereditary motor and sensory neuropathy 13, 16 limb-girdle muscular dystrophy 18, 20 lamin B 16 lamin C hereditary motor and sensory neuropathy 13 limb-girdle muscular dystrophy 18 Lange–Nielsen syndrome 223 Langer–Giedion syndrome 117 Laron dwarfism (growth hormone receptor defects) 166–7 laterality defects 137–8 laughter, Angelman syndrome 30 LCA see Leber congenital amaurosis Leber congenital amaurosis (LCA) 75–9 clinical features 75–6 genes/chromosomal location 76 molecular pathogenesis 78 leiomyomatosis 122 Lesch–Nyhan syndrome 43–4 leucocoria, retinoblastoma 98 Leyden hemophilia B 188 Leydig cell hyperplasia 159 LGMD see limb-girdle muscular dystrophy LHX3 168, 168 limb-girdle muscular dystrophy (LGMD) 18–23 Emery–Dreifuss muscular dystrophy vs 22–3 fascioscapulohumeral muscular dystrophy vs 22–3 molecular pathogenesis 18–9, 22 lim homeobox 3 168 linkage analysis ataxia–telangiectasia 4 congenital adrenal hyperplasia 162 neuronal ceroid lipofucinosis 55 lipoid adrenal hyperplasia, congenital 162 lipoid congenital adrenal hyperplasia 159 lip-pit syndrome (van der Woude syndrome) 155–6 LIS1 (PAFAH1B1) lissencephaly 46–7, 48 Miller–Dieker syndrome 48 Lisch nodules, neurofibromatosis type 1 98 lissencephaly 45–51 clinical features 45–6

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genes/molecular pathogenesis 46–7, 48–9, 50–1 X-linked 45 LITAF 12, 13 LMNA hereditary motor and sensory neuropathy 13, 16 limb-girdle muscular dystrophy 18–9, 20 Louis-Bar syndrome (ataxia–telangiectasia [AT]) 2–4 Lowe syndrome 52–3 17, 20-lyase deficiency 162 lysyl hydroxylase 113–4 Madelung deformity 115 magnetic resonance imaging (MRI) Pelizaeus–Merzbacher syndrome 57, 58–9 subcortical band heterotopia 45 X-linked adrenoleukodystrophy 63 X-linked hydrocephalus 66 major histocompatibility complex (MHC) deficiency 206 male pseudohermaphroditism 159 androgen insensitivity syndrome 158 malignant melanomas, retinoblastoma 100 mandibuloacral dysplasia 19 mandibulofacial dystosis (Treacher Collins syndrome) 154–5 O-mannose β-1,2-N-acetylglucosaminyltransferase-1 50 O-mannosyl transferase 1 49, 50 Marfanoid habitus, multiple endocrine neoplasia type 2 94 Marfan syndrome 117–9 Marshall syndrome 125–7 MASA syndrome 67 McCune–Albright syndrome 170 Mckusick–Kaufman syndrome 72, 73 MD (myotonic dystrophy) 23–6 MECP2 61–2 medium chain acyl-CoA dehydrogenase deficiency 210–1 melanomas, malignant, retinoblastoma 100 MEN2 see multiple endocrine neoplasia type 2 Menkes disease 211–2 Ehlers–Danlos syndrome 114 mental retardation 29–68 phenylketonuria 214 X-linked 50 metabolic disorders 209–16 metaphyseal chondrodysplasia 122 methylation analysis Angelman syndrome 32, 33 Prader–Willi syndrome 60 methyl-CpG-binding protein 2 61–2 3α-methylglutaconic aciduria II (Barth syndrome) 130 microhematuria, Alport syndrome 220 Miller–Dieker syndrome (MDS) 45, 47, 48 MITF 91, 92 MKKS 72, 73 Mowat–Wilson syndrome 179 MPZ 12, 13, 14, 15, 17 MTMR2 15, 16–7 mucopolysaccharidosis type II (MPS II) (Hunter syndrome [HS]) 40–1 mucoviscidosis (cystic fibrosis) 131–3 Muenke syndrome 150 fibroblast growth factor receptors 145 multiple cartilogenous exostoses (hereditary multiple exostoses [HME]) 115–7 multiple endocrine neoplasia type 2 (MEN2) 94–6 RET 178 multiple epiphyseal dysplasia (MED) 122, 125 multiple osteochondromatosis (hereditary multiple exostoses [HME]) 115–7 muscle weakness Duchenne muscular dystrophy 4

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myotonic dystrophy 23–4 muscular dystrophy of muscle–eye–brain disease (MEBD) 50 MYH9 84, 220 MYO3A 84 MYO6 84 MYO7A nonsyndromal hearing loss 84 Usher syndrome 88–9, 89 MYO15 84 myosin heavy chain 9 84 myosin IIIA 84 V 84 VI 84 VIIA 84, 88, 89 myotilin 18 myotonic dystrophy (MD) 23–6 myotubularin-related protein 2 16–7 NADPH oxidase 204 NCF1 204, 204 NCF2 204, 204 NCL see neuronal ceroid lipofucinosis (NCL) NDN 59–60 NDP 79–80 NDRG1 15, 17 necdin 59–60 NEFL 12, 13, 14T nephrogenic diabetes insipidus 163, 164 nephropathy and deafness see Alport syndrome (AS) nerve conduction velocities (NCVs) 10–1, 13–5 neuroblastoma, Beckwith–Wiedemann syndrome 221 neurocutaneous disorders 93–105 neurofibromatosis type 1 96–8 Noonan syndrome 139 neurofibromin 97–8 neurohypophyseal diabetes insipidus 163, 163–4 neuroimaging holoproscencephaly 39 X-linked adrenoleukodystrophy 63 neurologic disorders 1–28 neuronal ceroid lipofucinosis (NCL) 53–7 adult 54, 55 infantile 53–4, 54 juvenile 54, 54 late infantile 54, 54 neurturin 95 neutropenia, Barth syndrome 130 neutrophil cystolic factor 1 204 neutrophil cystolic factor 2 204 NF1 97–8 Nijmegen breakage syndrome 3 nonsyndromal hearing loss 84 Noonan syndrome 138–9 neurofibromatosis type 1 139 “normal transmitting males”, fragile X syndrome 35 Norrie disease 79–80 NSD1 153–4 nuclear receptor SET-domain protein I 153–4 nystagmus, Pelizaeus–Merzbacher syndrome 57 obesity, Bardet–Biedl syndrome 72 occipital horn syndrome 112 OCRL1 52–3 octocadherin 89–99 oculo-cerebro-renal syndrome (Lowe syndrome) 52–3

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oculodigital sign, Leber congenital amaurosis 75 OFD1 (orofaciodigital syndrome type I [OFD1]) 225–6 OI see osteogenesis imperfecta Ondine’s curse 95 oral-facial-digital syndrome type I (orofaciodigital syndrome type I [OFD1]) 225–6 ornithine transcarbamylase deficiency 212–4 orofaciodigital syndrome type I (OFD1) 225–6 osteoarthritis 122 osteochondrosarcomas, hereditary multiple exostoses 116–7 osteogenesis imperfecta (OI) 119–24 clinical features 120, 121 mutational mechanisms 123 radiologic features 121, 123 osteogenesis imperfecta congenita (OIC) see osteogenesis imperfecta (OI) osteogenesis imperfecta tarda (OIT) see osteogenesis imperfecta (OI) osteoporosis 122 osteosarcoma hereditary multiple exostoses 115 retinoblastoma 100 OTC 213–4 OTOA 84 otoancorin 84 OTOF 84 otoferlin 84 outwardly rectifying chloride channels 132 P1 176–7 pachygyria spectrum see lissencephaly PAFH1B1 see LIS1 (PAFAH1B1) PAH 214–5 Pallister–Hall syndrome 149 palmitoyl-protein thioesterase (PPT) 55 panhypopituitarism 167–9 Partington syndrome 50 patent ductus arteriosus, Noonan syndrome 138 paternal uniparental disomy (UPD), Beckwith–Wiedemann syndrome 221 PAX3 91, 91–2 PAX6 70–2 PCDH15 89, 90 PDS 84 PDS (Pendred syndrome) 86–7 pedigree analysis, hereditary motor and sensory neuropathy 17 Pelizaeus–Merzbacher syndrome 57–9 Pendred syndrome (PDS) 86–7 pendrin 84 periaxin 14, 16 peroneal muscular atrophy see hereditary motor and sensory neuropathy (HMSN) persephin 95 personality changes Huntington disease 41 Williams syndrome 141 Peter’s anomaly PAX6 71 PITX2 81 Pfeiffer syndrome 149–50 FGFR2 147 fibroblast growth factor receptors 145 phagocytosis, chronic granulomatous disease 203 phenylalanine decarboxylase deficiency (phenylketonuria [PKU]) 214–5 phenylketonuria (PKU) 214–5 pheochromocytomas multiple endocrine neoplasia type 2 96 von Hippel–Lindau disease 103 phosphatidylinositol-4,5-bisphosphatase 52–3 PHP (pseudohypoparathyroidism) 169–71

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Pierre Robin anomaly Stickler syndrome 125 Weissenbacher–Zweymuller syndrome 127 PIT1 168, 168 pituitary dwarfism (growth hormone deficiency) 164–6 pituitary-specific transcription factor 1 168 PITX2 80, 81 PKD1 227, 227–8 PKD2 227, 227–8 PKHD1 227, 228 PKU (phenylketonuria) 214–5 plagiocephaly, Saethre–Chotzen syndrome 151 PLOD1 112, 113 PLP1 58, 58 PMP22 11–2, 13, 14, 15, 17 PNP 206, 206–7 polycystic kidney disease (PKD) 226–8 polycystin 1 227 polycystin 2 227 polydipsia, cystinosis 224 polyhydramnios, myotonic dystrophy 24 polysplenia syndrome (laterality defects) 137–8 polyuria cystinosis 224 diabetes insipidus 163 POMGnT1 cobblestone lissencephaly 49, 50 muscular dystrophy of muscle–eye–brain disease 50 POMT1 49, 50 popliteal pterygium syndrome 156 postaxial polydactyly type A1 149 posterior embryotoxon 174 POU3F4 84 POU4F3 84 Prader–Labhardt–Willi syndrome see Prader–Willi syndrome (PWS) Prader–Willi syndrome (PWS) 30, 59–60 chromosome 15 31, 59 preaxial polydactyly type IV 149 presenile cataracts, myotonic dystrophy 23 primary ciliary dyskinesia 139–41 situs inversus 137, 140 primordial dwarfism (growth hormone deficiency) 164–6 progeria 19 progressive ataxias 1–28 PROMM (proximal myotonic myopathy) 23–6 PROP1 168, 168 prophet of POT1 168 protease inhibitor 1 176–7 protein-tyrosine phosphatase, nonreceptor-type 11 138–9 proteolipid protein 1 58 protocadherin 15 89 proximal myotonic myopathy (PROMM) 23–6 PRX 14, 16 pseudoachondrodysplasia 124–5 pseudoachondrodysplastic spondyloepiphyseal dysplasia 124–5 pseudohemophilia (von Willebrand disease) 198–9 pseudohypoparathyroidism (PHP) 169–71 pseudopseudohypoparathyroidism (PPHP) 169–71 psychomotor regression, Rett syndrome 61 PTCH Gorlin syndrome 38 holoproscencephaly 36, 38, 39 PTPN11 138–9 pulmonary stenosis Alagille syndrome 174 Noonan syndrome 138

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purine nucleoside phosphorylase (PNP) deficiency 205–7 PWS see Prader–Willi syndrome pyropoikilocytosis, hereditary 189 6-pyruvoyltetrahydropterin synthase 215 RAB7 16 radiologic examination, tuberous sclerosis 103 RAS-associated protein-7 16 RAS-MAP kinase pathway 95 RB1 99–100 receptor tyrosine kinase 179 5α-reductase deficiency 159 reelin cobblestone lissencephaly 49 lissencephaly 51 Refsum disease, infantile 76 RELN cobblestone lissencephaly 49 lissencephaly 51 renal disorders 217–26 Bardet–Biedl syndrome 72 renal Fanconi syndrome cystinosis 224 Lowe syndrome 52 renal tubular acidosis, hereditary 192 restriction fragment length polymorphism analysis (RFLP), Angelman syndrome 32 RET Haddad syndrome 95 Hirschsprung disease 95 association 178–9, 179 multiple endocrine neoplasia type 2 94–6, 178 retinal angiomas, von Hippel–Lindau disease 103 retinal dystrophy, Bardet–Biedl syndrome 72 retinal examination, juvenile neuronal ceroid lipofucinosis 54 retinitis pigmentosa Leber congenital amaurosis vs 76 Usher syndrome 87–8 retinoblastoma 98–100 retinoschisin 75 retinoschisis, X-linked (juvenile retinoschisis) 74–5 Rett syndrome 61–2 Angelman syndrome vs 31, 62 infantile neuronal ceroid lipofucinosis vs 53 rhabdomyosarcoma, Beckwith–Wiedemann syndrome 221 Rieger syndrome 80–1 RPE65 76, 77, 78 RPGRIP1 76, 77, 78 RS1 75 Rubenstein–Taybi syndrome 151 Saethre–Chotzen syndrome (SCS) 152–3 Rubenstein–Taybi syndrome overlap 151 Santavuori–Haltia–Hagberg disease see neuronal ceroid lipofucinosis (NCL) α-sarcoglycan 19, 20, 22 β-sarcoglycan 21, 22 δ-sarcoglycan 21, 22 γ-sarcoglycan 20, 22 sarcoglycanopathies 18 sarcomas, retinoblastoma 100 SBF2 15, 17 screening, von Hippel–Lindau disease 105 SCS see Saethre–Chotzen syndrome Senior–Loken syndrome 76 SET-binding factor 2 17 severe achondroplasia with developmental delay and acanthosis nigricans (SADDAN) 110 severe combined immunodeficiency (SCID) 205–7

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severe congenital neutropenia, WAS 208 sex-determining factor-related box 10 179 SGCA 19, 20 SGCB 21, 22 SGCD 21 SGCG 20 “shaker” 88 SHH 36, 37, 38 Shprintzen syndrome (DiGeorge/Shprintzen syndrome) 133–5 sickle cell anemia 193–4 sickle cell trait 193 SIP1 179 Sipple syndrome see multiple endocrine neoplasia type 2 (MEN2) situs ambiguus (laterality defects) 137–8 situs inversus Kartagener syndrome 140 primary ciliary dyskinesia 139–40 SIX3 36, 37, 38 skeletal disorders 107–27 skin hyperextensibility, Ehlers–Danlos syndrome 110 SLC26A4 87 SMA (spinal muscular atrophy) 27–8 small nucleoribonucleoprotein N 59–60 SMN1 27–8 SNRPN 59–60, 60 Sotos syndrome 153–4 Southeast Asian ovalocytosis 190, 192 SOX-10 91, 179 spastic paraparesis, X-linked 67 spastic paraplegia, X-linked 58 spectrins hereditary elliptocytosis 189 hereditary spherocytosis 191 spherocytosis, hereditary 190–2 Spielmeyer–Vogt–Sjögren disease see neuronal ceroid lipofucinosis (NCL) spinal muscular atrophy (SMA) 27–8 spondyloepimetaphyseal dysplasia (SEMD) 122 spondylopepiphyseal dysplasia congenita (SEDC) 122 SPTA1 hereditary elliptocytosis 189 hereditary spherocytosis 191 SPTB hereditary elliptocytosis 189 hereditary spherocytosis 191 StAR 162 stature, growth hormone deficiency 164 Steinert disease (myotonic dystrophy [MD]) 23–6 stereocilin 84 Stickler syndrome 122, 125–7 strabismus, retinoblastoma 98 STRC 84 subcortical band heterotopia (SCBH) 45 supravalvular aortic stenosis (SVAS), Williams syndrome 141 supravalvular pulmonary stenosis (SVPS), Williams syndrome 141 survival of motor neurons interacting protein 179 Swiss-type agammaglobulinemia (severe combined immunodeficiency [SCID]) 205–7 tafazzin 130 TAZ (G4.5) 130 T-BOX DiGeorge/Shprintzen syndrome 134–5 Holt–Oram syndrome 136 TBX1 134–5 TBX5 135, 136 TCAP 21, 22 T-cell leukemias, ataxia–telangiectasia 2

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TCOF1 154–5 TECTA 84 tectorin 84 telethonin 21, 22 testicular feminization syndrome (androgen insensitivity syndrome [AIS]) 158–9 testosterone biosynthesis defects 159 tetrahydrobiotin (BH4) 215 TFCP2L3 84 TGIF 36, 37, 38 α-thalassemia 194–7 and mental retardation syndrome, X-linked 64–6 β-thalassemia 197–8 thanatophoric dysplasia 109, 109–10, 110 thirst, diabetes insipidus 163 thrombocytopenia Wiskott–Aldrich syndrome 207–8 X-linked 208 thymic aplasia, DiGeorge/Shprintzen syndrome 133 thyroidectomy, multiple endocrine neoplasia type 2 96 thyroid stimulating hormone (TSH) deficiency 168 titin cap 22 titin immunoglobulin domain protein 18, 20 TMCI 84 TMIE 84 TMPRSS3 84 transforming growth factor β-induced factor 38 Treacher Collins syndrome 154–5 Treacher Collins–Franceschetti syndrome 154–5 treacle (TCOF1) 154–5 triallelic inheritance, Bardet–Biedl syndrome 74 TRIM32 21, 22 trisomy 8, X-linked hydrocephalus 66 “trisomy rescue”, Angelman syndrome 31 TSC1 102–3 TSC2 102–3 TS complex (tuberous sclerosis) 101–3 TTID 18, 20 tuberin 102–3 tuberous sclerosis 101–3 tumor suppressor genes EXT1 116–7 EXT2 116–7 RB1 99–100 VHL 104–5 TWIST 151–2 UBE3A 30–1 ultrasonography, polycystic kidney disease 226 ureate levels, Lesch–Nyhan syndrome 43 uremia, polycystic kidney disease 226 USH2A 89, 90 USH3A 89, 90 usher IC 84 usherin 89, 90 Usher syndrome 87–90 USHIC nonsyndromal hearing loss 84 Usher syndrome 88–9, 89 valvular stenosis, Alagille syndrome 174 van der Woude syndrome 155–6 vaso-occlusive crisis, sickle cell anemia 193 velocardiofacial syndrome (VCFS) (DiGeorge/Shprintzen syndrome) 133–5 ventricular septal defects (VSDs) DiGeorge/Shprintzen syndrome 134 Holt–Oram syndrome 136

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laterality defects 137 Noonan syndrome 138 very-long chain fatty acids (VLCFAs) 63–4 VHL 104–5 visual disorders 69–81 Lowe syndrome 52 Pelizaeus–Merzbacher syndrome 57 visual evoked potentials (VEPs) 54 von Hippel–Lindau disease (VHL) 103–5 von Reckinghausen’s disease see neurofibromatosis type 1 von Willebrand disease 198–9 VWF 199 Waardenburg–Shah syndrome see Waardenburg syndrome Waardenburg syndrome 90–2 Hirschsprung disease 92, 177 Wagenmann–Froboese syndrome see multiple endocrine neoplasia type 2 (MEN2) WAGR syndrome (aniridia) 70–2 Walker–Warburg syndrome 46, 51 “waltzer” mice 89–90 WAS 208 Weissenbacher–Zweymuller syndrome 125–7 COL11A2 122 Pierre Robin anomaly 127 Stickler syndrome 127 Werdnig–Hoffmann disease 27–8 WFS1 84 Williams–Beuren syndrome (Williams syndrome [WS]) 141–2 Williams syndrome (WS) 141–2 Wilm’s tumor, aniridia, genitourinary anomalies mental retardation syndrome 70–2 Wilms tumor, Beckwith–Wiedemann syndrome 220–1 Wilson disease 215–6 Wiskott–Aldrich syndrome (WAS) 207–8 Wolfram syndrome 84 WS (Williams syndrome) 141–2 WT1 71 X-chromosome Alport syndrome 219 androgen insensitivity syndrome 158 Barth syndrome 130 Bruton agammaglobulinemia 202 chronic granulomatous disease 204 collagen gene disorders 122 Duchenne muscular dystrophy 4 fragile X syndrome 34 glucose-6-phosphate dehydrogenase deficiency 184 growth hormone deficiency 165 hemophilia A 186 hemophilia B 187 hereditary motor and sensory neuropathy 13 Hunter syndrome 40 juvenile retinoschisis 74 laterality defects 137 Lesch–Nyhan syndrome 43 Lowe syndrome 52 Menkes disease 212 nephrogenic diabetes insipidus 163 nonsyndromal hearing loss 84 Norrie disease 79 occipital horn syndrome 112 ornithine transcarbamylase deficiency 213 orofaciodigital syndrome type I 225 Pelizaeus–Merzbacher syndrome 58 Rett syndrome 61 severe combined immunodeficiency 206

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Wiskott–Aldrich syndrome 208 X-linked lissencephaly 48 X-linked lissencephaly with ambiguous genitalia 48 X-linked disorders adrenoleukodystrophy 62–4 agammaglobulinemia see Bruton agammaglobulinemia agenesis of the corpus callosum 67 aqueduct stenosis 66–8 α-thalassemia and mental retardation syndrome 64–6 cardiomyopathy 6 complicated spastic paraparesis 67 exudative vitreoretinopathy 79 hydrocephalus 66–8 infantile spasms 50 lissencephaly (XLIS) 45 lissencephaly with ambiguous genitalia (XLAG) 46 mental retardation 50 myoclonic epilepsy with mental retardation and 50 nuclear protein 65 retinoschisis (juvenile retinoschisis) 74–5 spastic paraplegia 58 thrombocytopenia 208 XNP 65–6 Zellweger syndrome, Leber congenital amaurosis 76 ZIC2 36, 36, 38 ZIC3 137–8 zinc finger protein 3 137–8 zinc finger protein 9 25, 26 zinc finger protein 127 59–60 ZNF9 25, 26 ZNF127 59–60

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