1 Triplet Repeat Diseases Stephan J. Guyenet and Albert R. La Spada University of Washington, Seattle, WA 98195 1 A Novel Mechanism of Genetic Mutation Emerges 3 1.1 Repeat Sequences of All Types and Sizes 3 1.2 Trinucleotide Repeat Expansion as a Cause of Disease: Unique Features Explain Unusual Genetics 4 2 Repeat Diseases and Their Classification 5 2.1 Summary of Repeat Diseases 5 2.2 Differences in Repeat Sequence Composition and Location within Gene 6 2.3 Classification Based upon Mechanism of Pathogenesis and Nature of Mutation 6 3 Type 1: The CAG/Polyglutamine Repeat Diseases 9 3.1 Spinal and Bulbar Muscular Atrophy 9 3.2 Huntington’s Disease 12 3.3 Dentatorubral Pallidoluysian Atrophy 15 3.4 Spinocerebellar Ataxia Type 1 16 3.5 Spinocerebellar Ataxia Type 2 18 3.6 Spinocerebellar Ataxia Type 3/Machado–Joseph Disease 19 3.7 Spinocerebellar Ataxia Type 6 21 3.8 Spinocerebellar Ataxia Type 7 22 3.9 Spinocerebellar Ataxia Type 17 25 3.10 Role of Aggregation in Polyglutamine Disease Pathogenesis 25 3.11 Protein Context 29 3.12 Transcriptional Dysregulation 30 3.13 Proteolytic Cleavage 30 4 Type 2: the Loss-of-function Repeat Diseases 33 4.1 Fragile X Syndrome 33 Encyclopedia of Molecular Cell Biology and Molecular Medicine, 2nd Edition. Volume 15 Edited by Robert A. Meyers. Copyright 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-30652-8 2 Triplet Repeat Diseases 4.2 Fragile XE Mental Retardation 34 4.3 Friedreich’s Ataxia 36 4.4 Progressive Myoclonus Epilepsy Type 1 39 5 Type 3: the RNA Gain-of-function Repeat Diseases 40 5.1 Myotonic Dystrophy Type 1 40 5.2 Myotonic Dystrophy Type 2 42 5.3 Spinocerebellar Ataxia Type 8 43 5.4 The Fragile X Tremor – Ataxia Syndrome (FXTAS) 44 6 Type 4: The Polyalanine Diseases 46 6.1 Overview 46 6.2 Oculopharyngeal Muscular dystrophy 46 6.3 Synpolydactyly (Syndactyly Type II) 48 6.4 Cleidocranial Dysplasia 48 6.5 Holoprosencephaly 48 6.6 Hand-foot-genital Syndrome 49 6.7 Blepharophimosis-ptosis-epicanthus Inversus Syndrome 49 6.8 Syndromic and Nonsyndromic X-linked Mental Retardation 49 6.9 Congenital Central Hypoventilation Syndrome 50 7 Unclassified Repeat Diseases Lacking Mechanistic Explanations 50 7.1 Spinocerebellar Ataxia Type 10 50 7.2 Spinocerebellar Ataxia Type 12 51 7.3 Huntington’s Disease Like 2 (HDL2) 52 Bibliography 54 Books and Reviews 54 Primary Literature 54 Keywords Aggregate Accumulation of proteinaceous and/or ribonucleic acid material into a structure that is visible in a cell at the light microscope level. Anticipation Worsening severity of a disease phenotype as the causal (typically dominant) genetic mutation is transmitted from one generation to the next in a family segregating the disease of interest. Triplet Repeat Diseases 3 Gain of function Refers to a type of mutation that imparts a novel activity or action to the gene product containing the mutation. Loss of function Refers to a type of mutation that eliminates the action of the gene product encoded by the gene within which the mutation resides. Repeat Expansion An elongation of a repeat to a larger size that no longer falls within the size distribution range typically seen in the normal population; this process is now recognized as a mechanism of human genetic mutation. Trinucleotide Three DNA base pairs of specific sequence composition (e.g. cytosine-adenine- guanine). The repeat expansion disorders are a group of human diseases that are caused by the elongation of a DNA repeat sequence. In this chapter, we provide an overview of the discovery of repeat expansion as an important cause of human disease, and we summarize the molecular genetics and mechanistic basis of 27 microsatellite repeat disorders. Comparison of the many repeat expansion disorders reveals distinct categories of repeat diseases, allowing us to propose a classification of the repeat expansion disorders based upon mutation sequence and pathogenic mechanism. The four types of repeat expansion disorders defined by this approach are the CAG/polyglutamine repeat diseases; the loss-of-function repeat diseases; the RNA gain-of-function repeat diseases; and the polyalanine diseases. Although the genetic basis for most of these diseases was determined less than a decade or so ago, considerable advances have been made in our understanding of how ‘‘dynamic mutations’’ produce molecular pathology and human disease. 1 cleaved them, yielded a new methodology A Novel Mechanism of Genetic Mutation for differentiating individuals (‘‘molec- Emerges ular fingerprinting’’). This methodology 1.1 also found application in the mapping of Repeat Sequences of All Types and Sizes inherited genetic diseases, taking advan- tage of human variation in the form of Long before the sequencing of the human so-called restriction fragment length poly- genome was undertaken, the discovery of morphisms (RFLP’s). In the search for bacterial enzymes that recognize specific even more informative genetic markers, DNA sequences (‘‘recognition sites’’) and investigators uncovered a variety of very 4 Triplet Repeat Diseases short repeat sequences (‘‘short tandem independently made paradigm-shifting repeats’’ – STR’s; ‘‘simple sequence length discoveries. In one case, a CAG trinu- polymorphisms’’ – SSLP’s) that were cleotide repeat expansion within the first highly dispersed throughout the human coding exon of the androgen receptor (AR) genome. These latter repeat sequences gene was found to be the cause of an came to be known as ‘‘microsatellites’’ to X-linked neuromuscular disorder known differentiate them from the ‘‘minisatel- as spinal and bulbar muscular atrophy lites’’ that were being used for molec- (SBMA or Kennedy’s disease). CAG en- ular fingerprinting. Minisatellites were codes the amino acid glutamine; thus, originally defined as tandem arrays of elongation of a polyglutamine tract within 14–100 bp repeating sequences. In the the AR protein was hypothesized to be case of minisatellites, the repeat sequence the molecular basis for the motor neu- was in essence a ‘‘consensus’’ as devi- ron degeneration in SBMA. In the other ations from the exact repeat sequence case, a disorder known as the fragile X were common. Microsatellites, however, syndrome of mental retardation (FRAXA), were typically pure perfect repeats of also X-linked but much more common, less than 13 bp, the most common mi- was reported to result from expansion of crosatellites being dinucleotide repeats, a CGG repeat. In the latter case, although trinucleotide repeats, or tetranucleotide re- originally envisioned to encode a polyargi- peats. The discovery of minisatellites and nine tract, the CGG repeat turned out microsatellites yielded a virtual bonanza to be in the 5 untranslated region of a of reagents for molecular fingerprinting novel gene, the so-called FMR-1 gene (for and genetic linkage mapping, while also ‘‘fragile X mental retardation-1’’). providing evolutionary biologists and pop- The identification of triplet repeat ex- ulation geneticists with intriguing material pansions as the cause of two inherited to attempt to reconstruct evolutionary diseases was an exciting turning point in relationships and inter- or intraspecies re- the field of human molecular genetics not lationships. Although human geneticists only because of the novel nature of these and evolutionary biologists were applying findings, but also because of the unusual microsatellites in different ways for their genetic characteristics of this new type of own studies, they shared the commonly mutation. Analysis of families segregat- held belief that such repeats were neutral ing FRAXA revealed the existence of three and therefore unlikely to be of much func- distinct allele categories defined by the tional consequence, let alone play a role in length of the pure CGG repeat: a normal causing human disease. Of course, all that size range; a disease size range; and an would soon change in the last decade of intermediate, ‘‘premutation’’ size range. the twentieth century. As discussed below, individuals carrying premutation-sized CGG repeats never de- 1.2 velop the mental retardation phenotype, Trinucleotide Repeat Expansion as a but instead are at risk for passing on even Cause of Disease: Unique Features Explain larger CGG repeats to their children or Unusual Genetics grandchildren who then display the men- tal retardation phenotype. An important In 1991, two groups working on seem- tenet that emerged from these early stud- ingly unrelated inherited genetic diseases ies was that expanded CGG repeats (larger Triplet Repeat Diseases 5 than the normal size range) displayed an defining feature of DM in 1918, a num- exceptionally high rate of further mutation. ber of leading geneticists, among them This observation reversed a commonly Penrose, L.S., dismissed its authenticity, held view of the genetic material – the claiming that it was a product of ascertain- notion that any single nucleotide in the hu- ment bias due to better clinically defining man genome displayed a mutation rate of the profound variable expressitivity in this − ∼1 × 10 5. In the case of expanded CGG disorder. This view persisted from its pro- repeats in premutation carriers, the rate mulgation in the 1950s, reinforced by approached unity (100)! Besides displaying the concept that the genetic material is this high mutation rate, there were other seldom subject to alteration or modifi- unusual features: (1) the CGG repeats cation that could be heritable. However, showed a marked tendency to further ex- with the discovery that expanded trinu- pansion, suggesting that repeat mutation cleotide repeats could further expand, and was a polar process; and (2) the sex of the that indeed the expansion process was a individual transmitting the premutation- prerequisite for the FRAXA disease pheno- sized CGG allele determined whether a type – accounting for maternal inheritance large expansion into the disease range and greater percentages of affected individ- would be possible. For FRAXA, expansion uals in more recent generations – a role into the disease range could only occur if for repeats in diseases displaying antici- the premutation allele was transmitted by a pation was entertained.