A study of the Huntington's disease associated trinucleotide repeat in the Scottish population

The Contribution of Somatic Expansion of the CAG Repeat to Symptomatic Development in Huntington's Disease: A Historical Perspective

The discovery in the early 1990s of the expansion of unstable simple sequence repeats as the causative mutation for a number of inherited human disorders, including Huntington's disease (HD), opened up a new era of human genetics and provided explanations for some old problems. In particular, an inverse association between the number of repeats inherited and age at onset, and unprecedented levels of germline instability, biased toward further expansion, provided an explanation for the wide symptomatic variability and anticipation observed in HD and many of these disorders. The repeats were also revealed to be somatically unstable in a process that is expansion-biased, age-dependent and tissue-specific, features that are now increasingly recognised as contributory to the age-dependence, progressive nature and tissue specificity of the symptoms of HD, and at least some related disorders. With much of the data deriving from affected individuals, and model systems, somatic expansions have been revealed to arise in a cell division-independent manner in critical target tissues via a mechanism involving key components of the DNA mismatch repair pathway. These insights have opened new approaches to thinking about how the disease could be treated by suppressing somatic expansion and revealed novel protein targets for intervention. Exciting times lie ahead in turning these insights into novel therapies for HD and related disorders.

Risk factors for the onset and progression of Huntington disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by chorea, behavioural and psychiatric manifestations, and dementia, caused by a CAG triplet repeat expansion in the huntingtin gene. Systematic review of the literature was conducted to determine the risk factors for the onset and progression of HD. Multiple databases were searched, using terms specific to Huntington disease and to studies of aetiology, risk, prevention and genetics, limited to studies on human subjects published in English or French between 1950 and 2010. Two reviewers independently screened the abstracts and identified potentially relevant articles for full-text review using predetermined inclusion criteria. Three major categories of risk factors for onset of HD were identified: CAG repeat length in the huntingtin gene, CAG instability, and genetic modifiers. Of these, CAG repeat length in the huntingtin gene is the most important risk factor. For the progression of HD: genetic, demographic, past medical/clinical and environmental risk factors have been studied. Of these factors, genetic factors appear to play the most important role in the progression of HD. Among the potential risk factors, CAG repeat length in the mutant allele was found to be a relatively consistent and significant risk factor for the progression of HD, especially in motor, cognitive, and other neurological symptom deterioration. In addition, there were many consistent results in the literature indicating that a higher number of CAG repeats was associated with shorter survival, faster institutionalization, and earlier percutaneous endoscopic gastrostomy.

Examination of Huntington's disease with atypical clinical features in a Bangladeshi family tree

Atypical manifestation of Huntington's disease (HD) could inform ongoing research into HD genetic modifiers not present in the primarily European populations studied to date. This work demonstrates that expanding HD genetic testing into under‐resourced healthcare settings can benefit both local communities and ongoing research into HD etiology and new therapies.

A Drosophila model of Huntington disease-like 2 exhibits nuclear toxicity and distinct pathogenic mechanisms from Huntington disease

Huntington disease-like 2 (HDL2) and Huntington disease (HD) are adult-onset neurodegenerative diseases characterized by movement disorders, psychiatric disturbances and cognitive decline. Brain tissue from HD and HDL2 patients shows degeneration of the striatum and ubiquitinated inclusions immunoreactive for polyglutamine (polyQ) antibodies. Despite these similarities, the diseases result from different genetic mutations. HD is caused by a CAG repeat expansion in the huntingtin (HTT) gene, while HDL2 results from an expansion at the junctophilin 3 (JPH3) locus. Recent evidence indicates that the HDL2 expansion may give rise to a toxic polyQ protein translated from an antisense mRNA derived from the JPH3 locus. To investigate this hypothesis, we generated and characterized a Drosophila HDL2 model and compared it with a previously established HD model. We find that neuronal expression of HDL2-Q15 is not toxic, while the expression of an expanded HDL2-Q138 protein is lethal. HDL2-Q138 forms large nuclear aggregates, with only smaller puncta observed in the cytoplasm. This is in contrast to what is observed in a Drosophila model of HD, where polyQ aggregates localize exclusively to the cytoplasm. Altering localization of HLD2 with the addition of a nuclear localization or nuclear export sequence demonstrates that nuclear accumulation is required for toxicity in the Drosophila HDL2 model. Directing HDL2-Q138 to the nucleus exacerbates toxicity in multiple tissue types, while confining HDL2-Q138 to the cytoplasm restores viability to control levels. We conclude that while HD and HDL2 have similar clinical profiles, distinct pathogenic mechanisms are likely to drive toxicity in Drosophila models of these disorders.

Advances in Huntington’s disease diagnostics: development of a standard reference material

Huntington's disease (HD) is a neurodegenerative disease that affects four to seven individuals per 100,000. The onset of symptoms usually begins in middle age, although approximately 5% become symptomatic as juveniles. Death occurs approximately 15 years following the onset of symptoms, which include choreic movements, cognitive decline and psychiatric changes. HD is an autosomal dominant inherited disease that is associated with an expansion of a trinucleotide (CAG) repeat located on chromosome 4. Physicians rely on a positive family history, and diagnostic and genetic tests to detect the expansion in the number of CAG trinucleotide repeats in the HD gene to confirm the diagnosis. More than 99% of HD patients have 40 or more CAG triplet repeats and, therefore, targeted mutational analysis is greater than 99% sensitive. Individuals with 26 triplet repeats or less are normal, and while those with 27-35 repeats may not demonstrate symptoms themselves, their offspring may have the disease. Individuals with 36-39 repeats may or may not exhibit symptoms. The College of American Pathology/American College of Medical Genetics Biochemical and Molecular Genetics Resource Committee has emphasized the need to standardize the methodology for the determination of the accurate number of CAG repeats. This will prevent false-positive or -negative results when conducting predictive or prenatal testing of at-risk individuals. The National Institute of Standards and Technology is developing a standard reference material to provide these positive and negative controls needed by clinical testing laboratories. The use of a HD standard reference material will provide the quality control and assurance that data from different laboratories are both comparable and accurate.

Disease-associated polyglutamine stretches in monomeric huntingtin adopt a compact structure

Abnormal polyglutamine (polyQ) tracts are the only common feature in nine proteins that each cause a dominant neurodegenerative disorder. In Huntington's disease, tracts longer than 36 glutamines in the protein huntingtin (htt) cause degeneration. In situ, monoclonal antibody 3B5H10 binds to different htt fragments in neurons in proportion to their toxicity. Here, we determined the structure of 3B5H10 Fab to 1.9 Å resolution by X-ray crystallography. Modeling demonstrates that the paratope forms a groove suitable for binding two β-rich polyQ strands. Using small-angle X-ray scattering, we confirmed that the polyQ epitope recognized by 3B5H10 is a compact two-stranded hairpin within monomeric htt and is abundant in htt fragments unbound to antibody. Thus, disease-associated polyQ stretches preferentially adopt compact conformations. Since 3B5H10 binding predicts degeneration, this compact polyQ structure may be neurotoxic.

Huntington CAG repeat size does not modify onset age in familial Parkinson's disease: the GenePD study

The ATP/ADP ratio reflects mitochondrial function and has been reported to be influenced by the size of the Huntington disease gene (HD) repeat. Impaired mitochondrial function has long been implicated in the pathogenesis of Parkinson's disease (PD), and therefore, we evaluated the relationship of the HD CAG repeat size to PD onset age in a large sample of familial PD cases. PD affected siblings (n = 495), with known onset ages from 248 families, were genotyped for the HD CAG repeat. Genotyping failed in 11 cases leaving 484 for analysis, including 35 LRRK2 carriers. All cases had HD CAG repeats (range, 15-34) below the clinical range for HD, although 5.2% of the sample (n = 25) had repeats in the intermediate range (the intermediate range lower limit = 27; upper limit = 35 repeats), suggesting that the prevalence of intermediate allele carriers in the general population is significant. No relation between the HD CAG repeat size and the age at onset for PD was found in this sample of familial PD.

Factors associated with HD CAG repeat instability in Huntington disease

BACKGROUND

The Huntington disease (HD) CAG repeat exhibits dramatic instability when transmitted to subsequent generations. The instability of the HD disease allele in male intergenerational transmissions is reflected in the variability of the CAG repeat in DNA from the sperm of male carriers of the HD gene.

RESULTS

In this study, we used a collection of 112 sperm DNAs from male HD gene-positive members of a large Venezuelan cohort to investigate the factors associated with repeat instability. We confirm previous observations that CAG repeat length is the strongest predictor of repeat-length variability in sperm, but we did not find any correlation between CAG repeat instability and either age at the time of sperm donation or affectedness status. We also investigated transmission instability for 184 father-offspring and 311 mother-offspring pairs in this Venezuelan pedigree. Repeat-length changes were dependent upon the sex of the transmitting parent and parental CAG repeat length but not parental age or birth order. Unexpectedly, in maternal transmissions, repeat-length changes were also dependent upon the sex of the offspring, with a tendency for expansion in male offspring and contraction in female offspring.

CONCLUSION

Significant sibling-sibling correlation for repeat instability suggests that genetic factors play a role in intergenerational CAG repeat instability.

Evidence for a modifier of onset age in Huntington disease linked to the HD gene in 4p16

Huntington disease (HD) is a neurodegenerative disorder caused by the abnormal expansion of CAG repeats in the HD gene on chromosome 4p16.3. A recent genome scan for genetic modifiers of age at onset of motor symptoms (AO) in HD suggests that one modifier may reside in the region close to the HD gene itself. We used data from 535 HD participants of the New England Huntington cohort and the HD MAPS cohort to assess whether AO was influenced by any of the three markers in the 4p16 region: MSX1 (Drosophila homeo box homologue 1, formerly known as homeo box 7, HOX7), Delta2642 (within the HD coding sequence), and BJ56 ( D4S127). Suggestive evidence for an association was seen between MSX1 alleles and AO, after adjustment for normal CAG repeat, expanded repeat, and their product term (model P value 0.079). Of the variance of AO that was not accounted for by HD and normal CAG repeats, 0.8% could be attributed to the MSX1 genotype. Individuals with MSX1 genotype 3/3 tended to have younger AO. No association was found between Delta2642 (P=0.44) and BJ56 (P=0.73) and AO. This study supports previous studies suggesting that there may be a significant genetic modifier for AO in HD in the 4p16 region. Furthermore, the modifier may be present on both HD and normal chromosomes bearing the 3 allele of the MSX1 marker.

Interaction of normal and expanded CAG repeat sizes influences age at onset of Huntington disease

Huntington disease (HD) is a neurodegenerative disorder caused by the abnormal expansion of CAG repeats in the HD gene on chromosome 4p16.3. Past studies have shown that the size of expanded CAG repeat is inversely associated with age at onset (AO) of HD. It is not known whether the normal Huntington allele size influences the relation between the expanded repeat and AO of HD. Data collected from two independent cohorts were used to test the hypothesis that the unexpanded CAG repeat interacts with the expanded CAG repeat to influence AO of HD. In the New England Huntington Disease Center Without Walls (NEHD) cohort of 221 HD affected persons and in the HD-MAPS cohort of 533 HD affected persons, we found evidence supporting an interaction between the expanded and unexpanded CAG repeat sizes which influences AO of HD (P = 0.08 and 0.07, respectively). The association was statistically significant when both cohorts were combined (P = 0.012). The estimated heritability of the AO residual was 0.56 after adjustment for normal and expanded repeats and their interaction. An analysis of tertiles of repeats sizes revealed that the effect of the normal allele is seen among persons with large HD repeat sizes (47-83). These findings suggest that an increase in the size of the normal repeat may mitigate the expression of the disease among HD affected persons with large expanded CAG repeats.

Participation of par-4 in the degeneration of striatal neurons induced by metabolic compromise with 3-nitropropionic acid

Huntington's disease (HD) is a progressive neurodegenerative disorder characterized by chorea, psychiatric disturbances, and dementia. It is caused by a polyglutamine repeat expansion in the huntingtin protein. The striatum is a major site of neuronal loss in HD, but the mechanisms underlying the neurodegenerative process have not been established. Systemic administration of the succinate dehydrogenase inhibitor 3-nitropropionic acid (3NP) to rodents results in motor dysfunction and degeneration of striatal neurons with features similar to those of HD. Here we report that levels of prostate apoptosis response-4 (Par-4; a protein recently linked to neuronal apoptosis) increase in striatum, and to a lesser extent in cortex and hippocampus, after systemic administration of 3NP to adult rats. The increase in Par-4 levels occurred within 6 h of 3NP administration and was followed by an increase in caspase activation which preceded neuronal loss. Exposure of cultured primary striatal neurons to 3NP induced a rapid increase of Par-4 levels and caspase activation. Treatment of striatal neurons with a Par-4 antisense oligonucleotide blocked Par-4 induction by 3NP, suppressed caspase activation, and attenuated neuronal apoptosis. The caspase-3 inhibitor DEVD suppressed 3NP-induced apoptosis of striatal neurons, but did not prevent induction of Par-4, indicating that Par-4 acts upstream of caspase-3 activation in the cell death pathway. Our results suggest that Par-4 plays an important role in the degeneration of striatal neurons in an experimental model of HD.

Family history and DNA analysis in patients with suspected Huntington's disease

OBJECTIVES

Until recently a definite diagnosis of Huntington's disease could be made by a combination of clinical findings, a positive family history, and pathological confirmation. Prevalence data are based on these criteria. After finding the gene and its pathogenic mutation direct diagnostic confirmation became available. The aim of this study was to determine to what extent the direct assessment of CAG repeat length has allowed the diagnoses of additional patients, with atypical psychiatric or neurological disease, or those without a family history, that could otherwise not be diagnosed using traditional criteria.

PATIENTS AND METHODS

From all 191 referred patients suspected of having Huntington's disease between July 1993 and January 1996 CAG repeat length was determined and the family history was reviewed in the Leiden roster. After a retrospective search the patients were subdivided in positive, negative, suspect, and unknown family histories. Patients with an expanded repeat (>35) were finally diagnosed as having Huntington's disease. The family history was compared with the repeat length and the clinical features.

RESULTS

Clinical information was obtained for 172 patients. Of these, 126 patients had an expanded repeat, 77 had a positive, eight a negative, 40 a suspect, and one an unknown family history. Of the 44 patients with a normal repeat length four had a positive family history. Of the two patients with an intermediate repeat (between 30-36 repeats), one with a negative family history received a clinical diagnosis of Gilles de la Tourette's syndrome. The other had an unknown family history.

CONCLUSION

Despite verification of the family history through the Leiden roster, many more patients and families could be diagnosed with the new approach than would have been possible with the traditional criteria. Because prevalence studies have been based on this type of information, the data suggest an underestimation of the prevalence of Huntington's disease in the community of 14%.

Poly-L-glutamine forms cation channels: relevance to the pathogenesis of the polyglutamine diseases

We report that long-chain poly-L-glutamine forms cation-selective channels when incorporated into artificial planar lipid bilayer membranes. The channel was permeable to alkali cations and H(+) ions and virtually impermeable to anions; the selectivity sequence based on the single-channel conductance was H(+) >> Cs(+) > K(+) > Na(+). The cation channel was characterized by long-lived open states (often lasting for several minutes to tens of minutes) interrupted by brief closings. The appearance of the channel depended critically on the length of polyglutamine chains; ion channels were observed with 40-residue stretches, whereas no significant conductance changes were detected with 29-residue tracts. The channel-forming threshold length of poly-L-glutamine was thus between 29 and 40 residues. A molecular mechanics calculation suggests a mu-helix (. Biophys. J. 69:1130-1141) as a candidate molecular structure of the channel. The channel-forming nature of long-chain poly-L-glutamine may provide a clue to the elucidation of the pathogenetic mechanism of the polyglutamine diseases, a group of inherited neurodegenerative disorders including Huntington's disease.

Molecular diagnosis of Huntington disease in Brazilian patients

Huntington disease (HD) is a progressive neurodegenerative disorder with autosomal dominant inheritance, characterized by choreiform movements and cognitive impairment. Onset of symptoms is around 40 years of age and progression to death occurs in approximately 10 to 15 years from the time of disease onset. HD is associated with an unstable CAG repeat expansion at the 5' and of the IT15 gene. We have genotyped the CAG repeat in the IT15 gene in 44 Brazilian individuals (42 patients and 2 unaffected family members) belonging to 34 unrelated families thought to segregate HD. We found one expanded CAG allele in 32 individuals (76%) belonging to 25 unrelated families. In these HD patients, expanded alleles varied from 43 to 73 CAG units and normal alleles varied from 18 to 26 CAGs. A significant negative correlation between age at onset of symptoms and size of the expanded CAG allele was found (r=0.6; p=0.0001); however, the size of the expanded CAG repeat could explain only about 40% of the variability in age at onset (r2=0.4). In addition, we genotyped 25 unrelated control individuals (total of 50 alleles) and found normal CAG repeats varying from 16 to 33 units. The percentage of heterozigocity of the normal allele in the control population was 88%. In conclusion, our results showed that not all patients with the "HD" phenotype carried the expansion at the IT15 gene. Furthermore, molecular diagnosis was possible in all individuals, since no alleles of intermediate size were found. Therefore, molecular confirmation of the clinical diagnosis in HD should be sought in all suspected patients, making it possible for adequate genetic counseling.

Triplet repeat variability in the signal peptide sequence of the Xmrk receptor tyrosine kinase gene in Xiphophorus fish

Trinucleotide repeats in several human genes have been found to undergo spontaneous variation in repeat numbers in succeeding generations. Expansion of the repeat beyond a certain length causes specific pathological disorders. So far, a naturally occurring triplet repeat instability of transcribed sequences has been reported only from humans. However, the signal peptide encoding region of the receptor tyrosine kinase gene Xmrk from fish of the genus Xiphophorus contains a CTG repeat that differs in length even between closely related individuals. The consequence of this variability is signal peptides with shorter or longer hydrophobic core regions reaching, in some individuals, the critical maximum length for functional protein export or even exceeding it. In one stock, animals that are homozygous for such an allele were extremely rare, indicating that the triplet repeat length variability of the Xmrk gene of Xiphophorus may indeed have an influence on the function of the gene product and, under certain conditions, may affect the fitness of the individual.

Juvenile Huntington disease in the Netherlands

Juvenile Huntington disease (JHD) patients are distinguished from adult patients by an age at onset of less than 20 years. Investigating patients in our own database, we examined the proposition derived from studies in world literature that JHD should not be viewed as a separate clinical entity but rather as a manifestation of the rigid variant of the disease. Of 53 patients with JHD recorded in the Leiden Roster for Huntington Disease, relationships between sex, age at onset, duration of illness, maternal or paternal inheritance, motor symptom, first clinical features, and characteristics during the disease course, were obtained from the patients' files, and investigated. Although chorea is present in JHD, patients more often developed rigidity. Paternal inheritance, early dementia, epilepsy/myoclonus, and tremor during the disease course are confined for the most part to the rigid cases. A shorter duration of illness was evident in male patients with rigid JHD who inherited the disease from their father and developed their first disease feature at a younger age. The recognition of JHD as a distinct clinical entity does not appear to be warranted. Therefore, we propose, in accordance with other investigators, that rigid JHD be considered a clinical variant with special features.

Toward understanding the molecular pathology of Huntington's disease

Huntington's Disease (HD) is caused by expansion of a CAG trinucleotide beyond 35 repeats within the coding region of a novel gene. Recently, new insights into the relationship between CAG expansion in the HD gene and pathological mechanisms have emerged. Survival analysis of a large cohort of affected and at-risk individuals with CAG sizes between 39 and 50 repeats have yielded probability curves of developing HD symptoms and dying of HD by a certain age. Animals transgenic for the first exon of huntingtin with large CAG repeats lengths have been reported to have a complex neurological phenotype that bears interesting similarities and differences to HD. The repertoire of huntingtin-interacting proteins continues to expand with the identification of HIP1, a protein whose yeast homologues have known functions in regulating events associated with the cytoskeleton. The ability of huntingtin to interact with two of its four known protein partners appears to be influenced by CAG length. Caspase 3 (apopain), a key cysteine protease known to play a seminal role in neural apoptosis, has also been demonstrated to specifically cleave huntingtin in a CAG length-dependent manner. Many of these features are combined in a model suggesting mechanisms by which the pathogenesis of HD may be initiated. The development of appropriate in vitro and animal models for HD will allow the validity of these models to be tested.

Reduced penetrance of the Huntington's disease mutation

Controversy persists concerning the significance of Huntington disease (HD) alleles in the 36-39 repeat range. Although some clinically affected persons have been documented with repeats in this range, elderly unaffected individuals have also been reported. We examined 10 paternal transmissions of HD alleles of 37-39 repeats in collateral branches of families with de novo HD. All 10 descendants, including many who are elderly, are without symptoms of HD. Forty percent of the transmissions were unstable, although none varied by more than one repeat. The observation that individuals with alleles of 37-39 repeats may survive unaffected beyond common life expectancy supports the presence of reduced penetrance for HD among some persons with repeat sizes which overlap the clinical range. Non-penetrance may be increased in the collateral branches of de novo mutation families when compared to penetrance estimates from patient series. There was no CAA-->CAG mutation for the penultimate glutamine in either a de novo expanded 42 repeat allele or the corresponding non-penetrant 38 repeat allele in a family with fresh mutation to HD.

The likelihood of being affected with Huntington disease by a particular age, for a specific CAG size

Prior studies describing the relationship between CAG size and the age at onset of Huntington disease (HD) have focused on affected persons. To further define the relationship between CAG repeat size and age at onset of HD, we now have analyzed a large cohort of affected and asymptomatic at-risk persons with CAG expansion. This cohort numbered 1,049 persons, including 321 at-risk and 728 affected individuals with a CAG size of 29-121 repeats. Kaplan-Meier analysis has provided curves for determining the likelihood of onset at a given age, for each CAG repeat length in the 39-50 range. The curves were significantly different (P < .0005), with relatively narrow 95% confidence intervals (95% CI) (+/-10%). Penetrance of the mutation for HD also was examined. Although complete penetrance of HD was observed for CAG sizes of > or = 42, only a proportion of those with a CAG repeat length of 36-41 showed signs or symptoms of HD within a normal life span. These data provide information concerning the likelihood of being affected, by a specific age, with a particular CAG size, and they may be useful in predictive-testing programs and for the design of clinical trials for persons at increased risk for HD.

The genetic defect causing Huntington's disease: repeated in other contexts?

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Analysis of the (CAG)n repeat causing Huntington's disease in a Mexican population

We investigated the allele distribution of the polymorphic (CAG)n repeat in the IT15 gene in 96 normal subjects from the Mexican population and 83 unrelated patients with Huntington's disease. Our results show that the size distributions of normal and affected alleles do not overlap. Normal alleles range from 13 to 32 triplets, with 18 being the most frequent allele, while HD alleles contain 37 to 76 repeats with 42 being the most frequent. One allele in the range of intermediate alleles was found (32 repeats) in a normal subject. The juvenile onset cases in this study are associated with an expansion greater than 49 repeats. In the available parent-offspring pairs, paternal alleles show instability with an expansion of 28 repeats in one case.

HIP-I: a huntingtin interacting protein isolated by the yeast two-hybrid system

We report the discovery of the huntingtin interacting protein I (HIP-I) which binds specifically to the N-terminus of human huntingtin, both in the two-hybrid screen and in in vitro binding experiments. For the interaction in vivo, a protein region downstream of the polyglutamine stretch in huntingtin is essential. The HIP1 cDNA isolated by the two-hybrid screen encodes a 55 kDa fragment of a novel protein. Using an affinity-purified polyclonal antibody raised against recombinant HIP-I, a protein of 116 kDa was detected in brain extracts by Western blot analysis. The predicted amino acid sequence of the HIP-I fragment exhibits significant similarity to cytoskeleton proteins, suggesting that HIP-I and huntingtin play a functional role in the cell filament networks. The HIP1 gene is ubiquitously expressed in different brain regions at low level. HIP-I is enriched in human brain but can also be detected in other human tissues as well as in mouse brain. HIP-I and huntingtin behave almost identically during subcellular fractionation and both proteins are enriched in the membrane containing fractions.

Instability of highly expanded CAG repeats in mice transgenic for the Huntington's disease mutation

Six inherited neurodegenerative diseases are caused by a CAG/polyglutamine expansion, including spinal and bulbar muscular atrophy (SBMA), Huntington's disease (HD), spinocerebellar ataxia type 1 (SCA1), dentatorubral pallidoluysian atrophy (DRPLA) Machado-Joseph disease (MJD or SCA3) and SCA2. Normal and expanded HD allele sizes of 6-39 and 35-121 repeats have been reported, and the allele distributions for the other diseases are comparable. Intergenerational instability has been described in all cases, and repeats tend to be more unstable on paternal transmission. This may present as larger increases on paternal inheritance as in HD, or as a tendency to increase on male and decrease on female transmission as in SCA1 (ref. 15). Somatic repeat instability is also apparent and appears most pronounced in the CNS. The major exception is the cerebellum, which in HD, DRPLA, SCA1 and MJD has a smaller repeat relative to the other brain regions tested. Of non-CNS tissues, instability was observed in blood, liver, kidney and colon. A mouse model of CAG repeat instability would be helpful in unravelling its molecular basis although an absence of CAG repeat instability in transgenic mice has so far been reported. These studies include (CAG) in the androgen receptor cDNA, (CAG) in the HD cDNA, (CAG) in the SCA1 cDNA, (CAG) in the SCA3 cDNA and as an isolated (CAG) tract.

Huntington disease--another chapter rewritten

To those of us who began life when humans had 48 chromosomes and who began working in genetics when the (by then 46) chromosomes had no bands and chromosome 4 could not reliably be distinguished from chromosome 5, the mere ability to diagnose and correlate the clinical phenotypes of genetic disorders with their molecular genotypes is a source of continuing astonishment and pleasure. Indeed, molecular genetic analysis of neurogenetic disorders such as Huntington disease (HD) has provided a steady stream of challenges and surprises to all who believe the genetic principles that they were taught about these disorders. The paper by Rubinsztein et al. in this issue of the journal highlights yet another surprise, which was adumbrated even in the initial paper announcing the discovery of the HD gene: incomplete penetrance of HD gene mutations.

Phenotypic characterization of individuals with 30-40 CAG repeats in the Huntington disease (HD) gene reveals HD cases with 36 repeats and apparently normal elderly individuals with 36-39 repeats

Abnormal CAG expansions in the IT-15 gene are associated with Huntington disease (HD). In the diagnostic setting it is necessary to define the limits of the CAG size ranges on normal and HD-associated chromosomes. Most large analyses that defined the limits of the normal and pathological size ranges employed PCR assays, which included the CAG repeats and a CCG repeat tract that was thought to be invariant. Many of these experiments found an overlap between the normal and disease size ranges. Subsequent findings that the CCG repeats vary by 8 trinucleotide lengths suggested that the limits of the normal and disease size ranges should be reevaluated with assays that exclude the CCG polymorphism. Since patients with between 30 and 40 repeats are rare, a consortium was assembled to collect such individuals. All 178 samples were reanalyzed in Cambridge by using assays specific for the CAG repeats. We have optimized methods for reliable sizing of CAG repeats and show cases that demonstrate the dangers of using PCR assays that include both the CAG and CCG polymorphisms. Seven HD patients had 36 repeats, which confirms that this allele is associated with disease. Individuals without apparent symptoms or signs of HD were found at 36 repeats (aged 74, 78, 79, and 87 years), 37 repeats (aged 69 years), 38 repeats (aged 69 and 90 years), and 39 repeats (aged 67, 90, and 95 years). The detailed case histories of an exceptional case from this series will be presented: a 95-year-old man with 39 repeats who did not have classical features of HD. The apparently healthy survival into old age of some individuals with 36-39 repeats suggests that the HD mutation may not always be fully penetrant.

Expanded glutamines and neurodegeneration--a gain of insight

Glutamine repeat expansion has been established as the mutation underlying five inherited neurodegenerative diseases. The mechanism by which this apparently universal mutation, in ubiquitously expressed proteins, causes highly selective neurodegeneration is unknown. The proteins containing the glutamine expansions are otherwise unrelated and likely to have different functions. Two recently published papers provide evidence of a conformational change occurring in polyglutamine expansions, which may allow novel interactions and is consistent with a toxic gain-of-function hypothesis. HAP1, a protein that interacts with huntingtin (Huntington's disease protein), has an expression profile that intriguingly mirrors the selective neurodegeneration seen in Huntington's disease.

Psychiatric symptoms and CAG expansion in Huntington's disease

The mutation responsible for Huntington's disease (HD) is an elongated CAG repeat in the coding region of the IT15 gene. A PCR-based test with high sensitivity and accuracy is now available to identify asymptomatic gene carriers and patients. An inverse correlation between CAG copy number and age at disease onset has been found in a large number of affected individuals. The influence of the CAG repeat expansion on other phenotypic manifestations, especially specific psychiatric symptoms has not been studied intensively. In order to elucidate this situation we investigated the relation between CAG copy number and distinct psychiatric phenotypes found in 79 HD-patients. None of the four differentiated categories (personality change, psychosis, depression, and nonspecific alterations) showed significant differences in respect to size of the CAG expansion. In addition, no influence of individual sex on psychiatric presentation could be found. On the other hand in patients with personality changes maternal transmission was significantly more frequent compared with all other groups. Therefore we suggest that clinical severity of psychiatric features in HD is not directly dependent on the size of the dynamic mutation involved. The complex pathogenetic mechanisms leading to psychiatric alterations are still unknown and thus genotyping does not provide information about expected psychiatric symptoms in HD gene carriers.

Huntington's disease: CAG genetics expands neurobiology

Huntington's disease, with its progressive uncontrolled movements and characteristic selective neuropathology, has represented a baffling enigma to geneticists and neurobiologists alike. Discovery of the HD gene and its defect has demystified the genetic aspects of the disorder, but has not yet explained its pathogenesis. Attempts to explore this issue suggest that the defect acts as a gain of function, conferring a new deleterious property on the huntingtin protein, and that the gene's normal function may be irrelevant to the disease process.

Analysis of triplet repeats in the huntingtin gene in Japanese families affected with Huntington's disease

Huntington's disease (HD) is associated with the expansion of a CAG repeat in the huntingtin gene. Molecular analysis of the repeat in Japanese HD patients and normal controls was performed. The size of the CAG repeat ranged from 37 to 95 repeats in affected subjects and from seven to 29 in normal controls. A significant correlation was found between the age of onset and the CAG expansion. The length of the expanded repeat is unstable in meiotic transmission and large increases occur in paternal transmission. At the same time the CCG repeat polymorphism adjacent to the CAG repeat was analysed and haplotypes of HD chromosomes were identified. Strong linkage disequilibrium was found between the CAG repeat expansion and an allele of (CCG)10 in Japanese HD chromosomes. It is distinct from that described previously in western populations. Western HD chromosomes strongly associate with an allele of (CCG)7. Possible mechanisms underlying the disequilibrium in Japan are discussed.

Comparative sequence analysis of the human and pufferfish Huntington's disease genes

The Huntington's disease (HD) gene encodes a novel protein with as yet no known function. In order to identify the functionally important domains of this protein, we have cloned and sequenced the homologue of the HD gene in the pufferfish, Fugu rubripes. The Fugu HD gene spans only 23 kb of genomic DNA, compared to the 170 kb human gene, and yet all 67 exons are conserved. The first coding exon, the site of the disease-causing triplet repeat, is highly conserved. However, the glutamine repeat in Fugu consists of just four residues. We also show that gene order may be conserved over longer stretches of the two genomes. Our work describes a detailed example of sequence comparison between human and Fugu, and illustrates the power of the pufferfish genome as a model system in the analysis of human genes.

Normal CAG and CCG repeats in the Huntington's disease genes of Parkinson's disease patients

The clinical features of Parkinson's disease, particularly rigidity and bradykinesia and occasionally tremor, are seen in juvenile-onset Huntington's disease. Therefore, the CAG and CCG repeats in the Huntington's disease gene were investigated in 45 Parkinson's disease patients and compared to 40 control individuals. All of the Parkinson's disease chromosomes fell within the normal size ranges. In addition, the distributions of the two repeats in the Parkinson's disease patients did not differ significantly from those of the control population. Therefore, abnormalities of these trinucleotide repeats in the Huntington's disease gene are not likely to contribute to the pathogenesis of Parkinson's disease.

Huntington's disease

Early in 1993, an unstable, expanded trinucleotide repeat in a novel gene of unknown function was identified on HD chromosomes. This discovery unleased a flurry of experimentation that has established the expanded CAG repeat the almost universal cause of the characteristic neurologic symptoms and pathology of this neurodegenerative disorder of midlife onset. The biochemical basis for the specific neuronal loss of HD remains uncertain, but the genetic lesion probably acts via its consequent polyglutamine segment in the protein product, huntingtin. This review will describe the basic parameters of the HD repeat's behavior and the knowledge that has accumulated concerning its potential mechanisms of action.

Trinucleotide (CAG) repeat expansion in chromosomes of Spanish patients with Huntington's disease

Huntington's disease (HD) is a neurodegenerative and hereditary disease characterized by progressive movement disorders and mental and behavioral abnormalities. The HD gene is an expanding and unstable trinucleotide repeat (CAG repeat sequences). We studied 77 individuals from 38 families with HD in an attempt to obtain information for genetic counselling and differential diagnosis. Our results indicate that individuals with more than 40 repeats will be affected by the disease, whereas those with fewer than 30 will be healthy. There can be some overlap between 30 and 40 repeats, and one should be careful when interpreting these results.

Mutation analysis in patients with possible but apparently sporadic Huntington's disease

Until the advent of mutation analysis it was impossible to make a certain diagnosis of Huntington's disease (HD) in the absence of a positive family history, and sporadic cases of possible HD presented a substantial diagnostic dilemma. We have looked for the characteristic expanded trinucleotide (CAG) repeat sequence in the HD gene in 44 patients with probable or possible HD who did not have similarly affected relatives. We used two methods, the traditional widely used method, which estimates both the CAG repeat and the flanking CCG repeat and gives the CAG length by subtraction, and the more precise CAG method, which estimates the repeat length directly. With the CAG method, the HD mutation was detected in 25 (89%) of 28 patients with the typical clinical features of HD and 5 (31%) of 16 in whom the diagnosis was more doubtful. The CAG-CCG method gave results in the borderline abnormal range of repeats for 13 of the 33 patients eventually shown to have an unequivocal repeat expansion by the CAG method. Most of these patients had late onset of symptoms. There was evidence of expansion of an intermediate-length paternal allele in 1 patient and of non-paternity in another. The identification of the mutation causing HD means that it is now possible to confirm or exclude the diagnosis with confidence, even in the absence of a family history, by analysis of DNA from a blood sample. The precise method of measuring the CAG repeat, which is technically more difficult than the traditional method, may be needed to clarify results in a substantial proportion of such patients.

Study of the Huntington's disease (HD) gene CAG repeats in schizophrenic patients shows overlap of the normal and HD affected ranges but absence of correlation with schizophrenia

The CAG repeats in the Huntington's disease gene were investigated in chromosomes from 71 unrelated schizophrenic persons and 18 patients with schizoaffective disorder in order to determine if any of these patients had abnormal expansions. All of the probands had repeat sizes in the normal range (< 35 repeats) and there was no significant difference between the allele distributions of these patients and the normal controls. The families of two patients with 32 repeats and one patient with 34 repeats were investigated further and showed no uniform segregation of the disease with the large repeat alleles. The proband with 34 repeats inherited a chromosome that originally had 36 repeats in her father. The presence of 36 CAG repeats in members of her family and in HD patients suggests that there is an overlap between the normal and Huntington's disease CAG repeat size ranges. The more recently described CCG polymorphism in this gene was also examined in the schizophrenic and schizoaffective persons. All patients had alleles in the normal range.

Molecular basis of the common electrophoretic polymorphism (Fu1/Fu2) in human alpha-L-fucosidase

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Mutational bias provides a model for the evolution of Huntington's disease and predicts a general increase in disease prevalence

Huntington's disease (HD) correlates with abnormal expansion in a block of CAG repeats in the Huntington's disease gene. We have investigated HD evolution by typing CAG alleles in several human populations and in a variety of primates. We find that human alleles have expanded from a shorter ancestral state and exhibit unusual asymmetric length distributions. Computer simulations are used to show that the human state can be derived readily from a primate ancestor, without the need to invoke natural selection. The key element is a simple length-dependent mutational bias towards longer alleles. Our model can explain a number of empirical observations, and predicts an ever-increasing incidence of HD.

Trinucleotide repeat expansions and human genetic disease

Trinucleotide repeat expansions are now a well-established mutational mechanism in human genetic disease. An unstable CAG repeat is known to be responsible for three neurodegenerative disorders: Huntington's disease, spinal and bulbar muscular atrophy and spinocerebellar ataxia type 1. Similarities in the genetics of these diseases, the size of the repeat expansions and the position of the unstable repeat within the gene (when known) suggest a common basis to the observed phenotypes. The cloning of two regions at which chromosome breakage can be induced (FRAXA and FRAXE) has in each case uncovered an unstable CG-rich triplet repeat which becomes methylated when fully expanded. In addition to these two classes of mutation, the presence of an expanded CTG repeat in the 3' untranslated region of a protein kinase causes myotonic dystrophy. The size of the respective expansions, repeat stability, mutational origins and possible mechanisms of action are discussed.