Table Of ContentContributors
Numbers in parentheses indicate the pages on which the Alexis Brice (85, 133) Neurologie et Therapeutique
authors' contributions begin. Experimentale, INSERM U289, Departement Genetique,
Cytogenetique et Embryologie, Groupe Hospitaller Pitie-
Rim Amouri (179) Department of Neurology, Institut
Salpetriere, 75651 Paris Cedex, France
National de Neurologie, La Rabta, Tunis, Tunisia
Tomoki Chiba (305) Department of Molecular Oncology,
Shiuchi Asakawa (305) Department of Molecular Biology,
The Tokyo Metropolitan Institute of Medical Science,
Keio University School of Medicine, Tokyo 160-8582,
Tokyo, Japan
Japan
Shweta Choudhry (121) Functional Genomics Unit, Center
Tetsuo Ashizawa (103) Department of Neurology, The
for Biochemical Technology, CSIR Delhi, India
University of Texas Medical Branch, Galveston, Texas
77555 J.W. Day (75) Department of Neurology, Institute of
Robert W. Baloh (205) Department of Neurology, UCLA Human Genetics, University of Minnesota, Minneapolis,
School of Medicine, University of California, Los Minnesota 55455
Angeles, CaHfomia 90095
Katherine A. Dick (75) Department of Genetics, Cell
Carrolee Barlow (195) The Laboratory of Genetics, The Biology, and Development, Institute of Human Genetics,
Salk Institute for Biological Studies, La JoUa, California University of Minnesota, Minneapolis, Minnesota 55455
92037
Carlo Dionisi-Vici (231) Department of Neurosciences,
Samir Belal (179) Department of Neurology, Institut Division of Metabolic Disorders, Bambino Gesu'
National de Neurologie, La Rabta, Tunis, Tunisia Children Research Hospital, Rome, Italy
Enrico Bertini (231) Department of Neurosciences, Alexandra Diirr (133) INSERM U289 and Departement
Laboratory of Molecular Medicine, Bambino Gesu' Genetique, Cytogenetique et Embryologie, Groupe
Children Research Hospital, Rome, Italy Hospitaller Pitie-Salpetriere, 75651 Paris Cedex, France
Kailash P. Bhatia (385) University Department of Clinical Moneef Feki (179) Hospital La Rabta, Servie Biochimie,
Neurology, Institute of Neurology, Queen Square, Tunis, Tunisia
University College London, London WCIN, United
Hiroto Fujigasaki (133) INSERM U289, Groupe
Kingdom
Hospitaller Pitie-Salpetriere, 75651 Paris Cedex, France
Thomas D. Bird (541) Department of Medicine and and Department of Neurology and Neurological Science,
Neurology, University of Washington, Seattle, Graduate School, Tokyo Medical and Dental University,
Washington 98108 Tokyo, Japan
Samir K. Brahmachari (121) Functional Genomics Unit, Sana Gabsi-Gherairi (179) Department of Neurology,
Center for Biochemical Technology, CSIR Delhi, India Institut National de Neurologie, La Rabta, Tunis, Tunisia
Susan B. Bressman (407) Department for Neurology, Beth Daniel H. Geschwind (443) Department of Neurology,
Israel Medical Center, New York, New York 10003 University of California, Los Angeles, California 90095
XV
XVI Contributors
Sid Gilman (213) Department of Neurology, University of Surgeons, Columbia University, New York, New York
Michigan, Ann Arbor, Michigan 48109 10032
Lawrence I. Golbe (287) UMDNJ-Robert Wood Johnson Russell L. Margolis (121) Department of Psychiatry,
Medical School, New Brunswick, New Jersey 08901 Division of Neurobiology, Johns Hopkins University
School of Medicine, Baltimore, Maryland 21287
Lev G. Goldfarb (151) National Institute of Neurological
Disorders and Stroke, National Institutes of Health, Connie Marras (273) The Parkinson's Institute, Sunnyvale,
Bethesda, Maryland 20892 California 94089
Nobutaka Hattori (305) Department of Neurology, Tohru Matsuura (103) Department of Neurology, Baylor
Juntendo University School of Medicine, Tokyo 113- College of Medicine, Houston, Texas 77030
8421, Japan
John H. Menkes (341) Division of Pediatric Neurology,
Susan J. Hayflick (429) Molecular and Medical Genetics, Cedars-Sinai Medical Center, Los Angeles, California
Pediatrics, and Neurology, Oregon Health & Science 90048
University, Portland, Oregon 97201
Shinsei Minoshima (305) Department of Molecular
Faycal Hentati (179) Department of Neurology, Institut Biology, Keio University School of Medicine, Tokyo
National de Neurologic, La Rabta, Tunis, Tunisia 160-8582, Japan
Joseph J. Higgins (325) Center for Human Genetics and Yoshikuni Mizuno (305) Department of Neurology,
Child Neurology, Mid-Hudson Family Health Institute, Juntendo University School of Medicine, Tokyo 113-
New Paltz, New York 12561 8421, Japan
Susan E. Holmes (121) Department of Psychiatry, Division Hidehiro Mizusawa (71) Department of Neurology and
of Neurobiology, Johns Hopkins University School of Neurological Science, Graduate School, Tokyo Medical
Medicine, Baltimore, Maryland 21287 and Dental University, Tokyo, 113-8519 Japan
Hiroshi Ichinose (419) Institute for Comprehensive Ulrich Miiller (395) Institut fUr Humangenetik, Justus-
Medical Science, Fujita Health University, Toyoake, Japan Liebig-Universitut, D35392 Giessen, Germany
Satish Jain (121) Department of Neurology, Neurosciences Toshiharu Nagatsu (419) Institute for Comprehensive
Center, All India Institute of Medical Sciences, New Medical Science, Fujita Health University, Toyoake,
Delhi, India Japan
Joanna C. Jen (81, 205) Department of Neurology, UCLA Martha A. Nance (541) Park Nicollet Clinic, St. Louis
School of Medicine, The University of California, Los Park, Minnesota 55426
Angeles, California 90095
Takahide Nomura (419) Department of Pharmacology,
School of Medicine, Fujita Health University, Toyoake,
Christine Klein (451) Department of Neurology, Medical
Japan
University of Liibeck, 23538 Lubeck, Germany
Min-Kyu Oh (117) Division of Neurology, Cedars-Sinai
Michael D. Koob (95) Institute of Human Genetics,
Medical Center, Los Angeles, California 90048
University of Minnesota, Minneapolis, Minnesota 55455
Elizabeth O'Hearn (121) Departments of Neurology and
Rejko Kriiger (315) Department of Neurology, University
Neuroscience, Johns Hopkins University School of
of Tiibingen, Tubingen, Germany
Medicine, Baltimore, Maryland 21287
J. William Langston (287) Parkinson's Institute,
Harry T. Orr (35) Departments of Laboratory Medicine
Sunnyvale, California 94089
and Pathology, and Genetics, Cell Biology and
Anne-Sophie Lebre (85) Neurologic et Therapeutique Development, Institute of Human Genetics, University of
Experimentale, INSERM U289, Departement Genetique, Minnesota, Minneapolis, Minnesota 55455
Cytogenetique et Embryologie, Groupe Hospitalier Pitie-
Ruth Ottman (353) G.H. Sergievsky Center, College of
Salpetriere, 75651 Paris Cedex, France
Physicians and Surgeons, Columbia University, New
Seung-Jae Lee (287) Parkinson's Institute, Sunnyvale, York, New York 10032
California 94089
Laurie J. Ozelius (407) Department of Molecular Genetics,
Elan D. Louis (353) G.H. Sergievsky Center and Albert Einstein College of Medicine, Bronx, New York
Department of Neurology, College of Physicians and 10461
Contributors xvu
Massimo Pandolfo (165) Universite Libre de Bruxelles- Maria-Jesus Sobrido (443) Department of Neurology,
Hopital Erasme, B-1070 Brussels, Belgium Hospital Universitario de Salamanca, 37000 Salamanca,
Spain
David L. Pauls (491) Department of Psychiatry, Massachusetts
General Hospital, Harvard Medical School, Charlestown, Achal K. Srivastava (121) Department of Neurology,
Massachusetts 02129 Neurosciences Center, All India Institute of Medical
Sciences, New Delhi, India
Henry Paulson (57) Department of Neurology, University
of Iowa Roy J. and Lucille A. Carver College of Matthew W. State (491) Child Study Center, Yale
Medicine, Iowa City, Iowa 52242 University School of Medicine, New Haven Connecticut
06520
Andy Peiffer (503) Department of Pediatrics, Division of
Medical Genetics, University of Utah, Salt Lake City, Giovanni Stevanin (85, 133) Neurologic et Therapeutique
Utah 84112 Experimentale, INSERM U289, Groupe Hospitaller
Pitie-Salpetriere, 75651 Paris Cedex 13, France
Susan L. Perlman (253) Department of Neurology, UCLA
School of Medicine, University of California, Los S. H. Subramony (57) Department of Neurology,
Angeles, California 90095 University of Mississippi Medical Center, Jackson,
Mississippi 39216
Stefan-M. Pulst (1, 19, 45, 117, 491, 541) Division of
Neurology, Cedars-Sinai Medical Center, Departments of Chiho Sumi-Ichinose (419) Department of Pharmacology,
Medicine and Neurobiology, UCLA School of Medicine, School of Medicine, Fujita Health University, Toyoake,
Los Angeles, California 90048 Japan
Laura P.W. Ranum (75) Department of Genetics, Cell James P. Sutton (511) California Neuroscience Institute,
Biology, and Development, Institute of Human Genetics, Oxnard, California 93030
University of Minnesota, Minneapolis, Minnesota 55455
Toshiaki Suzuki (305) Department of Molecular Oncology,
Andrea Richter (189) Service de Genetique Medicale, The Tokyo Metropolitan Institute of Medical Science,
Hopital Sainte-Justine, Departement de Pediatric, Tokyo,Japan
Universite de Montreal, Montreal, Quebec, Canada
Keiji Tanaka (305) Department of Molecular Oncology,
Olaf Riess (315) Department of Neurology, University of The Tokyo Metropolitan Institute of Medical Science,
Tubingen, Tubingen, Germany Tokyo,Japan
Christopher A. Ross (121) Departments of Psychiatry and
Caroline Tanner (273) The Parkinson's Institute,
Neuroscience, Johns Hopkins University School of
Sunnyvale, California 94089
Medicine, Baltimore, Maryland 21205
Kai Treuner (195) The Laboratory of Genetics, The Salk
David C. Rubinsztein (365) Department of Medical
Institute for Biological Studies, La Jolla, California 92037
Genetics, Cambridge Institute for Medical Research,
Addenbrooke's Hospital, Cambridge, CB2 2XY, United Shoji Tsuji (139, 143) Department of Neurology, Brain
Kingdom Research Institute, Niigata University, Niigata 951, Japan
and Department of Neurology, University of Tokyo,
Nobuyoshi Shimizu (305) Department of Molecular
Tokyo 113-8655, Japan
Biology, Keio University School of Medicine, Tokyo
160-8582, Japan Hema Vakharia (117) Division of Neurology, Cedars-Sinai
Medical Center, Los Angeles, California 90048
Cliff Shults (213) Department of Neurosciences, University
of California San Diego and Neurology Service, VA San Hiroyo Yoshino (305) Department of Neurology, Juntendo
Diego Healthcare System, La Jolla, California 92093 University School of Medicine, Tokyo 113-8421, Japan
David K. Simon (473) Department of Neurology, Beth Massimo Zeviani (231) Division of Biochemistry and
Israel Deaconess Medical Center and Harvard Medical Genetics, and Division of Child Neurology, Istituto
School, Boston, Massachusetts 02115 Nazionale Neurologico "C. Besta," Milano, Italy
Foreword
The field of movement disorders has witnessed an or spasticity. Disorders in the excess movement category are
explosion of research activity in the past decade, with better commonly referred to as hyperkinesias (excessive
understanding of nosology, physiology, pharmacology, and movements), dyskinesias (unnatural movements), and
treatment. But perhaps the most explosive growth has been abnormal involuntary movements). Even though the terms
the identification of genes and the mapping of gene are interchangeable, dyskinesias is the one most often used.
locations for many of these disorders. The availability of The five major categories of dyskinesias, in alphabetical
simple diagnostic tests for the genes involved saves the order, are: chorea, dystonia, myoclonus, tics, and tremor.
physician countless hours and saves patients money. Equally Within each category, there are many entities and etiologies.
important is our newly gained knowledge of the clinical Movement disorders due to a paucity of movement are
phenomenology of various movement disorders because we referred to as hypokinesia (decreased amplitude of
can now identify them using gene tests. For example, the movement), but bradykinesia (slowness of movement) and
identification of the gene for Machado-Joseph disease and akinesia (loss of movement) could be reasonable alternative
its subsequent labeling as SCA-3 has provided a fuller under names. The parkinsonian syndromes are the most common
standing of how common this disorder, previously thought cause of such paucity of movement; other hypokinetic
to be rare, actually is. The research has also revealed the disorders represent only a small group of patients. Basically,
extensive clinical spectrum by which the abnormal gene movement disorders can be conveniently divided into
manifests itself. parkinsonism and all other types; there is about an equal
The editor of this book, Stefan-M. Pulst, is a neurologist number of patients in each of these two groups.
and neuroscientist who is equally at home in the clinical Thus, the time is ripe for Genetics of Movement
phenomenology of movement disorders and their Disorders, in which experts gathered by the editor
molecular-genetic analysis. This is reflected in the structure summarize the genetic information that has threatened to
of the chapters that take the reader from phenotype to overwhelm us. We are just at the beginning of this rich field,
genotype including descriptions of neuroimaging, neuro with many more genetic discoveries to come. At an exciting
pathology, and neurophysiology when appropriate. The time both in movement disorders and neurogenetics, this
neuroscientist will find the discussions of normal and book superbly brings the two fields together. Genetics of
abnormal gene function of interest, while clinical neuro Movement Disorders presents a unique new breed of
logists and geneticists will use this volume to clarify issues neurology text, a successful synthesis of molecular
of genotype/phenotype correlations. The editor has chosen neuroscience and clinical medicine. Future researchers and
contributors who are both leaders in the field of neuro current clinicians will value this volume as an ideal entry
genetics and experienced physicians who use genetic testing point.
in their clinical practice and who are familiar with the
promises and pitfalls of molecular tests.
Movement disorders are neurological syndromes in Stanley Fahn, M.D.
which there is either an excess of movement or a paucity of Columbia University College of Physicians & Surgeons
voluntary and automatic movements unrelated to weakness New York City, NY
XIX
Preface
Movement disorders have shaped neurologic thinking for traits. Reduced penetrance or variable expressivity of
more than a century. They provide visible and striking specific alleles can be traced back to interaction with other
afflictions of the nervous system often combining symptoms alleles or environmental factors that ameliorate or worsen
attributable to hyper- or hypoexcitability of specific the phenotype. The chapters on Parkinson disease and
neuronal groups with features of neurodegeneration. Progressive Supranuclear Palsy serve as examples for this
Nosologic classification of these disorders has thus relied on group of diseases encompassing both rare single gene
the phenotype, in recent years aided by neuroimaging and to defects and examples of more common predisposing alleles
a lesser degree by neurophysiologic investigations. in the general population.
Positional or candidate gene-based identification of disease Molecular investigations have provided insights into
genes has now led to new genotypically based classifica movement disorders that might have seemed audacious or
tions. DNA tests increasingly substitute for clinical, mor even preposterous just a few years ago. A case in point is the
phological, or biochemical definitions as the gold standard development of disease models in rodents or invertebrates
for diagnosis. But this century has already brought with it a based on the knockout of genes or the expression of mutated
classification based on changes at the protein level. Terms transgenes. Although these models may not be perfect
such as tauopathy or polyglutamine disease indicate that replicas of their respective human diseases, they recapitulate
classifications based on protein abnormalities may unite salient pathological, biochemical, or functional features.
what the genotype has divided. Neuroscientists and geneticists are now using these models
This book is written for neurologists, neuroscientists, to investigate pathways, to identify modifiers of disease
geneticists, and genetic counselors. For those not familiar pathogenesis, and to test novel therapies. All chapters
with genetic terminology and methods, the first chapter address animal models and in vitro studies which will make
provides an introduction. The following chapters are divided this book of interest to the molecular biologist working on
according to classic definitions of movement disorders taking fundamental issues of human CNS diseases.
into account that a patient does not present with a genotype My thanks go to all the contributors who did not tire of
written on his or her forehead but with symptoms and signs up-dating chapters to include recently identified disease
of disease. All authors are familiar with the differential genes and disease mechanisms. Without Cindy Minor's
diagnosis and treatment of the movement disorders covered excellent editorial assistance and kind insistence the book
in their chapters. The use of DNA-based testing in the would have seen a much later publication date. Hilary Rowe
evaluation of the patient with a movement disorder has served as a knowledgeable Publishing Editor and Paul
received particular emphasis. The genotype is correlated Gottehrer competently supervised production management.
with the phenotype to aid the physician in counseling and to Thanks also go to members of my laboratory, past and
provide the basic scientist with a model for the effect of present, for daring experiments and stimulating discussions
mutant alleles at the organismal level. and to Carmen Warschaw and Teddi Winograd for their
The focus of genetics in the first decade of the new support of academic neurology. Finally, in the name of all
millennium is moving from the identification of alleles the contributors I wish to acknowledge patients and families
causing mendelian diseases to the analysis of common traits. whose participation in genetic studies made the discovery of
Single gene defects in addition to causing a specific disease disease genes possible.
with high probability may be understood as identifying Stefan-M. Pulst, M.D., Dr. med.
genes that may have other alleles that modify more common Los Angeles, June 2002
XXI
C H A P T ER
Introduction to Medical Genetics and
Methods of DNA Testing
STEFAN-M. PULST
Division of Neurology, Cedars-Sinai Medical Center
Departments of Medicine and Neurobiology, UCLA School of Medicine
Los Angeles, California 90048
I. Concepts and Terminology part current methodology. Thus, mutation types are covered
II. Patterns of Inheritance in the first part and their detection in the second part of this
A. Phenotype chapter.
B. Dominance and Recessiveness For centuries neurology and the classification of neuro
C. Inheritance Patterns logic illnesses have mainly relied on the neurologic pheno
D. The Structure of a Gene type. This was particularly true for the movement disorders
E. Genetic Linkage Analysis
leading to keen observations on subtle variations of dis
R Types of Mutations
orders affecting movement control. Effective symptomatic
III. Molecular Genetic Tools
treatments were developed based on the increasing
A. Polymerase Chain Reaction
understanding of physiology and pathology of movement
B. Southern Blot Analysis
disorders. In the last decades neuroimaging has played an
C. DNA Sequencing
D. Analysis of RNA Transcripts and Proteins ever-increasing role in the understanding of these disorders.
E. Detection of DNA Polymorphisms Grouping disorders based on inheritance patterns did not
IV. Molecular Genetic Testing necessarily result in an improved classification. Diseases
A. Direct Mutation Detection like the dominant ataxias shared a similar inheritance
B. Indirect Analysis Using Linked Genetic Markers pattern, but phenotypes did not "breed true" within families
V. Animal Models and overlap between families was significant. We now
A. Expression of Foreign Transgenes know that mutations in several different genes can cause
B. Gene Targeting (Knock-Out, Knock-In) ataxia with an added twist conferred by the presence of
VI. Web-Based Information for Genetic Diagnosis and dynamic mutations. A truly novel understanding and group
Testing
ing of disorders was only possible with the advent of
Acknowledgments
linkage analysis and eventually isolation of the causative
References
disease genes. Recently, new concepts have emerged that
are based on protein structure or function such as that of the
polyglutamine disorders.
Movement disorders have been at the forefront of
This chapter will provide some of the necessary neurologic diseases that were investigated using genetic
basics for those clinicians not familiar with medical genetic methods to map disease genes and ultimately to identify the
terminology and molecular-genetic methods. For the neuro- genes themselves. A movement disorder was actually the
scientist it may provide a brief overview of the benefits first neurologic illness mapped to a specific chromosome.
and problems associated with an approach to the nervous This disorder, now known as spinocerebellar ataxia type 1,
system based on the identification of disease genes. The was linked to the HLA region on human chromosome 6
first part will review concepts and terminology, the second using polymorphic protein markers (Jackson et al., 1977).
Genetics of Movement Disorders Copyright 2003, Elsevier Science (USA). All rights reserved.
Introduction to Medical Genetics and Methods of DNA Testing
This predated by three years the suggestion that restriction by physical examination. The genotype describes the nature
fragment length polymorphisms (RFLPs) could be used to of the two alleles (for autosomal genes) for a specific gene.
establish a genetic map of the human genome (Botstein Allele is the term for different forms of a gene based on
et al., 1980). Another movement disorder, Huntington's different DNA sequences at this locus (for more details see
disease (HD), was the first neurologic illness that was Fig. 1.7).
genetically mapped using this new class of polymorphic There may not be a defined relationship between geno
DNA markers (Gusella et al, 1983). type at a locus and phenotype. Mutations in different genes
may produce a similar phenotype (nonallelic heterogeneity).
For example, mutations in different genes on different
L CONCEPTS AND TERMINOLOGY chromosomes may cause familial Parkinson disease (see
Chapter 26). These observations suggest, but do not prove
The following summarizes some of the basic knowledge that the proteins encoded by these genes may be involved in
concerning molecular and medical genetics. For more the same cellular pathway as has recently been suggested
detailed discussions the reader is referred to other textbooks for the interaction of parkin and asynuclein (Shimura et al.,
(Jorde et al, 2000; Pulst, 2000, Strachan and Read, 1999; 2001).
Watson et al.\ 1992). Additional information and figures On the other hand, different types of mutations in the
can be found on the Web (Human Genome web site, 2002; same gene may result in different phenotypes (allelic
Genetics science learning center web site, 2002). Instead of heterogeneity). Examples are mutations in the CACNAIA
a systematic review, preference was given to terms and gene that can result in episodic ataxia, familial hemiplegic
methods that are used in subsequent chapters. migraine, or a progressive degenerative ataxia, designated
as SCA6 (see Chapters 8 and 22).
Finally, the same mutation can result in very different
11. PATTERNS OF INHERITANCE phenotypes or in significantly different ages of onset or
disease progression. Phenotypic variability associated with
Mendelian or unifactorial inheritance refers to a pattern a specific mutation may be caused by several genetic and
of inheritance that can be explained on the basis of nongenetic factors. The phenotype is determined not only
mutation in a single gene. Thus, the presence or absence by the genotype at the disease locus, but also by genotypes
of a genetic character depends on the genotype at a single at other loci. These factors are often referred to as modify
locus. Monogenic traits are also referred to as Mendelian ing loci, modifying genes, or modifying alleles. Stochastic
traits, because they follow the well-delineated patterns of events, such as mitotic loss of alleles or the partition of
inheritance first described by Mendel in 1865. In contrast to mitochondria into daughter cells are important in deter
previous animal and plant breeders, Mendel did not stop at mining phenotype (see Chapters 24 and 41). In addition,
the descriptive level, but was able to conclude that there the phenotype may be influenced by the interaction of
must be dominant and recessive traits by analysis of the environmental factors with the genotype, a hypothesis
ratios of observed traits in the offspring. Mendel's original recently favored to explain the majority of Parkinson
publications can now be viewed on the Web in the original disease cases (see Chapter 26).
German or translated into English (Blumberg, 2002).
In contrast to monogenic disorders, complex genetic
B. Dominance and Recessiveness
traits cannot be explained on the basis of mutations in a
single specific gene, and a phenotype is only observed Fundamental to the understanding of Mendelian inheri
when mutations in several genes have occurred with or tance are the concepts of dominance and recessiveness.
without a significant contribution from environmental Dominance and recessiveness are properties of traits, not of
factors. After a focus on single gene disorders, movement genes, since different mutations in the same gene on the
disorder genetics is now also beginning to analyze complex autosomes may show dominant or recessive inheritance.
diseases where multiple genes and environmental factors Dominance is not a property intrinsic to a particular
must act in concert to cause disease (see Chapter 26). allele, but describes the relationship between it and the
corresponding allele on the homologous chromosome with
regard to a particular trait. A trait is dominant, if it is
A. Phenotype
manifest in the heterozygote. Dominant alleles exert their
Whereas genotype refers to a person's DNA sequences phenotypic effect despite the presence of a normal (wild-
at a specific chromosomal locus, the term phenotype type) allele on the second homologous chromosome. Thus,
describes what can be observed clinically. In addition to the if the phenotypes associated with genotypes AA and AB are
"clinical" phenotype, one can also examine cellular or bio identical but are different from the BB phenotype, the A
chemical phenotypes which may not be directly observable allele is dominant to allele B. Conversely, the B allele is
Introduction to Medical Genetics and Methods of DNA Testing
recessive to allele A. Recessive mutations lead to pheno- or involve regulatory genes or structural genes that are
typic consequences only when both alleles contain muta sensitive to gene dosage effects. The gene encoding guano-
tions. If the mutations on both alleles are different, this is sine triphosphate cyclohydrolase I (GCH) is mutated in
referred to as compound heterozygosity. An example would Dopa-responsive dystonia (see Chapter 37). Although
be a patient with Friedreich's ataxia whose mutation is an mutations in genes encoding enzymes are usually recessive,
expansion of the intronic GAA repeat on one allele, whereas mutations in this enzyme are associated with a phenotype,
the mutation in the other allele is a missense mutation (see probably because enzyme levels above 50% of normal are
Chapter 18). necessary in critical neuronal populations.
The same mutation can lead to recessive or dominant Dominant-negative mutations result in a mutant protein
traits. Sickle cell anemia is a recessive disease. The trait that interferes with the action of the normal protein. For
"sickle cell anemia" is manifested in homozygotes with example, in a homodimeric protein (a protein complex made
the Hemoglobins (HbS) mutation. However, the trait up of two identical proteins), the mutation of one allele will
"sickling," the aggregation of red blood cells at low oxygen result in only 25% of the resulting dimers having a normal
tensions, is a dominant trait, since it is seen in hetero- composition.
zygotes that carry one allele with the HbS mutation and one Certain mutations may combine gains and losses of
wild-type allele. function. Examples are the polyglutamine diseases such as
Most human dominant syndromes occur only in hetero- HD. Expansion of the polyglutamine tract results in a gain
zygotes. Some geneticists refer to dominant mutations that of toxic function. At the same time studies of transcription
have the same phenotype in heterozygotes and homozy factors that interact with huntingtin have suggested that
gotes as "true" dominant. This is distinguished from semi- huntingtin with expanded polyQ domains sequesters spe
dominant when the heterozygote AB has an intermediate cific transcription factors thus resulting in a loss of function
phenotype between the phenotypes of AA and BB. This (see Chapter 33).
is probably the case for most polyglutamine diseases (and
the respective mouse models), where individuals with two
C. Inheritance Patterns
mutant alleles have a more severe phenotype. If the pheno
types of AB, AA, and BB are identical, alleles A and B are The relationship between members of a family are
said to be co-dominant. conveniently indicated by a pedigree notation (Fig. 1.1). In
addition to their blood relationship, phenotypic and geno-
typic information can be included in one graphic display.
1. Mechanisms of Dominance
The first individual ascertained in a pedigree (proband) is
The majority of mutations result in an inactive gene indicated by an arrow.
product (see Section II.F). The function of the remaining
normal allele, however, is sufficient in most cases to
1. The Multigeneration Pedigree
guarantee normal cellular function. Therefore, most mutant
alleles are recessive. When the function of the remaining One of the most frequent misconceptions encountered
allele is not sufficient to maintain normal function, this is in the routine genetic assessment of movement disorder
referred to as haploinsufficiency. patients is the fact that physicians are content with obtain
ing a family history restricted to siblings or parents. When
a. Gain of Function
determining inheritance patterns it is crucial to obtain a
Most dominant mutations lead to a gain of function. three-generation pedigree including all first and second
This may be due to the loss of negative regulatory domains, degree relatives. Without the history and examination of
loss of normal protein degradation, or abnormal protein many at-risk individuals, it may be impossible to dis
processing or protein-protein interaction. A toxic gain of tinguish autosomal recessive inheritance from dominant
function has been suggested for mutant proteins containing inheritance with reduced penetrance, dominant inheritance
extended polyglutamine stretches (see Chapter 2). Although with anticipation, or from other forms of inheritance.
mutations that lead to truncation of the protein can result X-linked recessive inheritance requires the absence of
in dominant alleles (see EA-2, Chapter 22), dominant transmission through males, but this may only be apparent
mutations are more conmionly missense mutations leading when a larger number of male transmissions can be studied.
to amino acid substitution. The following may serve as an example. We published a
pedigree of Portuguese ancestry with atypical Parkinsonism
b. Loss of Function Mutations Associated with Dominant
and an inheritance pattern suggestive of autosomal domi
Traits
nant inheritance. On account of the Portuguese background
Loss of function mutations may be dominant, when they and phenotype we assumed that this pedigree likely
involve a critical rate-limiting step in a metabolic pathway. segregated a mutation in the SCA3 gene, although on a
Introduction to Medical Genetics and Metliods of DNA Testing
Symbol Comments
m
Assign gender by phenotype.
4
2a. Affected individual Key/legend used to defme shading or other fill, (e.g., hatches, dots, etc.)
(Female)
r
With >2 conditions, the individuars symbol should be partitioned accordingly,
2b. Affected individual
each segment shaded with a different fill and defined in legend.
k (Male)
3. Multiple individuals Number of siblings written inside symbol. 1
(Number known for female) (Affected individuals should not be grouped). |
5^ 4. Deceased individual If death is known, write **d." with age at death below symbol.
(Sex Unknown)
k
d.35v
5a, Proband First affected family member coming to medical attention.
L6
5b. Consultand Individual(s) seeking genetic counseling/testing.
6. Spontaneous If due to an ectopic pregnancy, write ECT below symbol.
maie abortion (SAB) Also note sex below symbol.
7. Affected SAB If gestational age known, write below symbol.
female Key/legend used to defme shading.
8. Termination of No abbreviations used for sake of consistency.
sex pregnancy (TOP)
unknown
(i) 9. Obligate carrier Normal phenotype and negative test result, (e.g., Woman with
r (will not manifest disease) normal physical exam and carrier of a mutation in the frataxin gene)
10. Asymptomatic/ Clinically unaffected at this time but could later exhibit symptoms.
Presymptomatic carrier
Rf 11. Genetic line of Biologic parents shown connected by horizontal line.
descent (vertical or diagonal) Offspring are connected to parents by a vertical line.
Monozygotic Dizygotic
12, Twins A horizontal line between the symbols
implies a relationship line.
D—O If degree of relationship is not obvious from the pedigree,
13. Consanguinity
it should be stated above the relationship line, (e.g., third cousins) _\
FIGURE 1.1 Pedigree symbols according to Bennett et al. (1995). For use in pedigrees see also Figure 1.2.
formal basis maternal inheritance could not be excluded develop the disease nor pass it on to their offspring. The
(Sutton and Pulst, 1997). Subsequent molecular analysis disease appears over multiple generations which appears as
determined that the family did not carry expansions in "vertical transmission" in the pedigree notation. Males and
the SCA3 CAG repeat nor was there linkage to the SCA3 females are evenly affected, and the disease is passed on
locus, but the family segregated a mutation in the mito from affected fathers or mothers to male and female
chondrial genome (see also Fig. 41.2). Maternal inheritance offspring with equal probability.
was not striking due to lack of affected males having a Penetrance in an individual who carries a disease allele
significant number of offspring (Simon et ai, 1999). may not show any manifestations of the disease phenotype.
However, this individual may transmit the disease to the
next generation. Studies have shown that in pedigrees with
2. Autosomal Dominant
familial torsion dystonia approximately 30% of individuals
In autosomal dominant disorders, a mutation in a single who are known to carry the DYTl mutation do not show
gene on any of the 22 autosomes produces clinical symp signs of a movement disorder (Pauls and Korczyn, 1990).
toms or signs. The disease or mutant allele is dominant to Thus, the penetrance of mutations in the DYTl gene is
the normal (wild-type) allele, and the disease phenotype is considered to be 30%. Penetrance is also related to the
seen in heterozygotes. Offspring are at 50% risk to inherit thoroughness of the clinical examination and the use of
the disease. Offspring who inherit the normal allele will not ancillary studies (see Fig. 1.2).
Introduction to Medical Genetics and Metliods of DNA Testing
o
A 2
B 1
C 2
II Haplotype ^ Genotype
A 1 A 3 E ll 4
I mm—I
B 2 ^ ^ ll I "^
C 4 C 3 [IB 1
in i i
P
A 1 A 2 A 2 A 1 A 2
B 2 B 1 B 2 B 1 B 2 B 1
C 2 C 41 C 4 c 4HU C 2 C 2
FIGURE 1.2 Pedigree segregating an autosomal dominant trait; individual 11:1 is non-penetrant. The segregation of
alleles for three marker loci (A, B, and C) is shown. Vertical boxes provide a depiction of chromosome segments
allowing easy visualization of recombination events. A double recombination is shown for the maternally inherited
chromosome in individual 111:2. The haplotype for markers A,B,C is boxed in individual 11:1 (al-b2-c-4), the genotype
for marker A is boxed in individual 11:2 (a3/4).
The terms penetrance and expressivity of a mutation may pass on an abnormal allele to their offspring, they are
need to be clearly differentiated. Penetrance is an all-or- called carriers. Parents of affected individuals are usually
none phenomenon. Signs of a given phenotype (clinical, carriers of the disease gene, and each parent contributes one
biochemical, imaging, etc.) are either present or not present. abnormal copy to the offspring. Disease risk to siblings
Variable expressivity describes the extent and variability of is 25%, and 50% of siblings are at risk to be carriers. In
the expression of the phenotype. Variable expressivity of a contrast to autosomal dominant inheritance, where vertical
mutation can refer to variable severity of disease symptoms transmission is observed, horizontal aggregation is typical
or variable age of onset, but also to expression of com for recessive disorders, where multiple individuals in one
pletely different symptoms in carriers of the mutation. The generation are affected (Fig. 1.3).
typical mutation in the DYTl gene, i.e., deletion of a GAG The likelihood of encountering recessive disorders is
codon, causes generalized dystonia in most individuals. increased in specific populations that have a high frequency
However, in one family, individuals with this mutation of mutant alleles. For example, Tay-Sachs disease is
developed writer"s cramp. This is an example of variable common in Ashkenazi Jews with a heterozygote frequency
expressivity of the mutation (Gasser et al, 1998). of 1 in 30, whereas the frequency in Caucasians is only 1 in
3000. Consanguinity may be present in pedigrees with
autosomal recessive inheritance. The probability that a
3. Autosomal Recessive
mating was consanguineous increases when the mutant
When both copies of a gene need to be mutated in order allele is very rare. For many of the rare neurologic recessive
to produce a phenotype, the disorder is inherited as an diseases, consanguinity becomes a factor. For example, at
autosomal recessive trait. Only individuals homozygous the low mutant gene frequency of 0.008 for Wilson disease,
for the mutant allele will develop the disease, whereas half of the cases are the result of consanguineous matings
heterozygous individuals (one normal copy and one (Saito, 1988). Consanguinity is seen in many cultures of
mutated copy) are clinically normal. Since heterozygotes the Mideast.
