Huntington's Disease Chromosomes Research Articles

Huntington's Disease Gene Hunters: An Expanding Tale.

Huntington's Disease Gene Hunters: An Expanding Tale.

Haplotype analysis encompassing HTT gene in Chinese patients with Huntington's disease.

Huntington's disease (HD) is a dominantly inherited neurodegenerative disorder with varied prevalence in different populations, which may be associated with specific haplotypes. This study aimed to explore the haplotypes encompassing the HTT gene in the Chinese population. A total of 406 individuals with HD and 59 normal relatives from 253 families with HD were enrolled. A total of 29 tagsingle nucleotide polymorphisms (tSNPs) were selected and genotyped for the haplotype analysis. In stage one, we used 18 tSNPs to replicate the distribution of three major haplogroups (A, B, C). We found that risk-associated haplogroup variants A1 and A2, enriched on Caucasian HD chromosomes, were totally absent from both Chinese HD and control chromosomes, and the distributions of haplogroups between HD and control chromosomes were similar. Therefore, in stage two, we used 29 tSNPs (including the18 tSNPs) to define new haplogroups (I, II, III) and found that haplogroup I accounted for 61.4% on HD chromosomes and 34.4% on control chromosomes, indicating that haplogroup I was enriched on Chinese HD chromosomes. This is the first haplotype analysis encompassing HTT in the Chinese population. The results contribute to explaining the low prevalence of HD in China and provide a better understanding of genetic diversity in the HTT region.

Haplotype analysis of the 4p16.3 region in Portuguese families with Huntington's disease.

Huntington's disease (HD) is a neurodegenerative disorder characterized by involuntary choreic movements, cognitive impairment, and behavioral changes, caused by the expansion of an unstable CAG repeat in HTT. We characterized the genetic diversity of the HD mutation by performing an extensive haplotype analysis of ∼1Mb region flanking HTT in over 300 HD families of Portuguese origin. We observed that haplotype A, marked by HTT delta2642, was enriched in HD chromosomes and carried the two largest expansions reported in the Portuguese population. However, the most frequent HD haplotype B carried one of the largest (+12 CAGs) expansions, which resulted in an allele class change to full penetrance. Despite having a normal CAG distribution skewed to the higher end of the range, these two core haplotypes had similar expanded CAG repeat sizes compared to the other major core haplotypes (C and D) and there was no statistical difference in transmitted repeat instability across haplotypes. We observed a diversity of HTT region haplotypes in both normal and expanded chromosomes, representative of more than one ancestral chromosome underlying HD in Portugal, where multiple independent events on distinct chromosome 4 haplotypes have given rise to expansion into the pathogenic range. © 2015 Wiley Periodicals, Inc.

Stable DNA Methylation Boundaries and Expanded Trinucleotide Repeats: Role of DNA Insertions

The human genome segment upstream of the FMR1 (fragile X mental retardation 1) gene (Xq27.3) contains several genetic signals, among them is a DNA methylation boundary that is located 65–70 CpGs upstream of the CGG repeat. In fragile X syndrome (FXS), the boundary is lost, and the promoter is inactivated by methylation spreading. Here we document boundary stability in spite of critical expansions of the CGG trinucleotide repeat in male or female premutation carriers and in high functioning males (HFMs). HFMs carry a full CGG repeat expansion but exhibit an unmethylated promoter and lack the FXS phenotype. The boundary is also stable in Turner (45, X) females. A CTCF-binding site is located slightly upstream of the methylation boundary and carries a unique G-to-A polymorphism (single nucleotide polymorphism), which occurs 3.6 times more frequently in genomes with CGG expansions. The increased frequency of this single nucleotide polymorphism might have functional significance. In CGG expansions, the CTCF region does not harbor additional mutations. In FXS individuals and often in cells transgenomic for EBV (Epstein Barr Virus) DNA or for the telomerase gene, the large number of normally methylated CpGs in the far-upstream region of the boundary is decreased about 4-fold. A methylation boundary is also present in the human genome segment upstream of the HTT (huntingtin) promoter (4p16.3) and is stable both in normal and Huntington disease chromosomes. Hence, the vicinity of an expanded repeat does not per se compromise methylation boundaries. Methylation boundaries exert an important function as promoter safeguards.

CAG-Expansion Haplotype Analysis in a Population with a Low Prevalence of Huntington's Disease

Background and PurposeThe prevalence of Huntington's disease (HD) among East Asians is less than one-tenth of that among Caucasians. Such a low prevalence may be attributable to a lack of carriers of specific predisposing haplogroups associated with the high instability of the Huntingtin gene (HTT). The aim of this study was to evaluate the association between specific HTT haplogroups and the occurrence of HD in a Thai population.MethodsCAG-repeat sizes and HTT haplotypes were analyzed in 18 Thai HD patients and 215 control subjects. Twenty-two tagging single-nucleotide polymorphisms (tSNPs) were genotyped.ResultsOnly 18 patients from 15 unrelated families were identified over the last 17 years. Pathological CAG-repeat alleles ranged from 39 to 48 repeats (43.5±3.0, mean±SD), and normal alleles ranged from 9 to 24 repeats (16.49±1.74). Only two of the chromosomes studied comprised intermediate alleles. Unlike the Caucasian data, all but 1 of the 22 tSNPs were not associated with the occurrence of HD. The predisposing haplogroups for Caucasian HD (haplogroups A1 and A2) are very rare in Thai patients (<4%). Both HD and normal chromosomes are commonly haplogroups A5 and C, in contrast to the case for Chinese and Japanese patients, in whom only haplogroup C was common in HD chromosomes. The frequency of CAG-repeat sizes of haplogroup A5 and C were also similarly distributed.ConclusionsHD chromosomes of Thai patients may arise randomly from each haplogroup, with a similar mutation rate. This rate is much lower than the CAG expansions from Caucasian HD haplogroups. These data suggest that the different mechanisms underlie CAG expansion in Thai and Caucasian patients.

Haplotype analysis of the CAG and CCG repeats in 21 Brazilian families with Huntington’s disease

We studied the allelic profile of CAG and CCG repeats in 61 Brazilian individuals in 21 independent families affected by Huntington's disease (HD). Thirteen individuals had two normal alleles for HD, two had one mutable normal allele and no HD phenotype, and forty-six patients carried at least one expanded CAG repeat allele. Forty-five of these individuals had one expanded allele and one individual had one mutable normal allele (27 CAG repeats) and one expanded allele (48 CAG repeats). Eleven of these forty-five subjects had a mutant allele with reduced penetrance, and thirty-four patients had a mutant allele with complete penetrance. Inter- and intragenerational investigations of CAG repeats were also performed. We found a negative correlation between the number of CAG repeats and the age of disease onset (r=-0.84; P<0.001) and no correlation between the number of CCG repeats and the age of disease onset (r=0.06). We found 40 different haplotypes and the analysis showed that (CCG)(10) was linked to a CAG normal allele in 19 haplotypes and to expanded alleles in two haplotypes. We found that (CCG)(7) was linked to expanded CAG repeats in 40 haplotypes (95.24%) and (CCG)(10) was linked to expanded CAG repeats in only two haplotypes (4.76%). Therefore, (CCG)(7) was the most common allele in HD chromosomes in this Brazilian sample. It was also observed that there was a significant association of (CCG)(7) with the expanded CAG alleles (χ(2)=6.97, P=0.0084). Worldwide, the most common CCG alleles have 7 or 10 repeats. In Western Europe, (CCG)(7) is the most frequent allele, similarly to our findings.

Common SNP-Based Haplotype Analysis of the 4p16.3 Huntington Disease Gene Region

Age at the onset of motor symptoms in Huntington disease (HD) is determined largely by the length of a CAG repeat expansion in HTT but is also influenced by other genetic factors. We tested whether common genetic variation near the mutation site is associated with differences in the distribution of expanded CAG alleles or age at the onset of motor symptoms. To define disease-associated single-nucleotide polymorphisms (SNPs), we compared 4p16.3 SNPs in HD subjects with population controls in a case:control strategy, which revealed that the strongest signals occurred at a great distance from the HD mutation as a result of "synthetic association" with SNP alleles that are of low frequency in population controls. Detailed analysis delineated a prominent ancestral haplotype that accounted for ∼50% of HD chromosomes and extended to at least 938 kb on about half of these. Together, the seven most abundant haplotypes accounted for ∼83% of HD chromosomes. Neither the extended shared haplotype nor the individual local HTT haplotypes were associated with altered CAG-repeat length distribution or residual age at the onset of motor symptoms, arguing against modification of these disease features by common cis-regulatory elements. Similarly, the 11 most frequent control haplotypes showed no trans-modifier effect on age at the onset of motor symptoms. Our results argue against common local regulatory variation as a factor influencing HD pathogenesis, suggesting that genetic modifiers be sought elsewhere in the genome. They also indicate that genome-wide association analysis with a small number of cases can be effective for regional localization of genetic defects, even when a founder effect accounts for only a fraction of the disorder.

HTT haplotypes contribute to differences in Huntington disease prevalence between Europe and East Asia

Huntington disease (HD) results from CAG expansion in the huntingtin (HTT) gene. Although HD occurs worldwide, there are large geographic differences in its prevalence. The prevalence in populations derived from Europe is 10-100 times greater than in East Asia. The European general population chromosomes can be grouped into three major haplogroups (group of similar haplotypes): A, B and C. The majority of HD chromosomes in Europe are found on haplogroup A. However, in the East-Asian populations of China and Japan, we find the majority of HD chromosomes are associated with haplogroup C. The highest risk HD haplotypes (A1 and A2), are absent from the general and HD populations of China and Japan, and therefore provide an explanation for why HD prevalence is low in East Asia. Interestingly, both East-Asian and European populations share a similar low level of HD on haplogroup C. Our data are consistent with the hypothesis that different HTT haplotypes have different mutation rates, and geographic differences in HTT haplotypes explain the difference in HD prevalence. Further, the bias for expansion on haplogroup C in the East-Asian population cannot be explained by a higher average CAG size, as haplogroup C has a lower average CAG size in the general East-Asian population compared with other haplogroups. This finding suggests that CAG-tract size is not the only factor important for CAG instability. Instead, the expansion bias may be because of genetic cis-elements within the haplotype that influence CAG instability in HTT, possibly through different mutational mechanisms for the different haplogroups.

An ovine transgenic Huntington's disease model.

Huntington's disease (HD) is an inherited autosomal dominant neurodegenerative disorder caused by an expansion of a CAG trinucleotide repeat in the huntingtin (HTT) gene [Huntington's Disease Collaborative Research Group (1993) A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington's disease chromosomes. The Huntington's Disease Collaborative Research Group. Cell, 72, 971-983]. Despite identification of the gene in 1993, the underlying life-long disease process and effective treatments to prevent or delay it remain elusive. In an effort to fast-track treatment strategies for HD into clinical trials, we have developed a new large-animal HD transgenic ovine model. Sheep, Ovis aries L., were selected because the developmental pattern of the ovine basal ganglia and cortex (the regions primarily affected in HD) is similar to the analogous regions of the human brain. Microinjection of a full-length human HTT cDNA containing 73 polyglutamine repeats under the control of the human promotor resulted in six transgenic founders varying in copy number of the transgene. Analysis of offspring (at 1 and 7 months of age) from one of the founders showed robust expression of the full-length human HTT protein in both CNS and non-CNS tissue. Further, preliminary immunohistochemical analysis demonstrated the organization of the caudate nucleus and putamen and revealed decreased expression of medium size spiny neuron marker DARPP-32 at 7 months of age. It is anticipated that this novel transgenic animal will represent a practical model for drug/clinical trials and surgical interventions especially aimed at delaying or preventing HD initiation. New sequence accession number for ovine HTT mRNA: FJ457100.

4p16.3 haplotype modifying age at onset of Huntington disease

Huntington disease (HD) is caused by an expanded CAG repeat sequence in the HD gene. Although the age at onset is correlated to the CAG repeat length, this correlation only explains approximately half of the variation in onset age. Less variation between siblings indicates that the variation is, in part, explained by genetic modifiers. We analyzed polymorphic loci within or close to the HD gene on the HD chromosome in Danish HD patients. We found one specific haplotype segregating with later age at onset, compared with patients with similar CAG repeat length and another haplotype. The nine Danish families in the study carrying this haplotype most likely have a common founder. Several of the polymorphic loci displayed alleles that may be specific to the late-onset haplotype, implicating that from this study we cannot determine which of the loci tested (or other polymorphic loci in this chromosomal area) do in fact contain genetic modifiers of age at onset.

The genetic analysis of Turkish patients with Huntington's disease.

Exploring the (CAG)n expansion within IT 15 gene in Turkish Huntington's disease (HD) patients and its relation to downstream (CCG)n repeat polymorphism to elucidate population specific haplotypic heterogeneity. Twenty-seven patients with clinical diagnosis of HD from 19 families were sampled. The triplet repeats were evaluated by sizing the fluorescent PCR products on an ABI 310 capillary gel electrophoresis unit. The number of (CAG)n repeat expansions (range: 40-76, mean: 45.6+/-7) were inversely correlated with age of onset (r=-0.81, P<0.0001). The (CCG)n polymorphism in HD chromosomes was confined to (CCG)7 in all patients. In normal chromosomes CAG polymorphism was accumulated at a relatively higher range (mean: 19.3+/-2.9) together with the common occurrence of(CCG)7 and (CCG)10 alleles. The distribution range of the CAG and CCG repeat polymorphism in normal chromosomes and strong linkage disequilibrium between HD mutation and (CCG)7 haplotype provided a striking similarity to populations of western European descent.

Analysis of CCG Repeats in Huntingtin Gene among HD Patients and Normal Populations in Japan

Huntington's disease (HD) is a hereditary autosomal dominant neurodegenerative disease characterized by motor, cognitive, and psychiatric symptoms. The molecular basis of the disease is the expansion of the trinucleotide CAG in the first exon of a gene on chromosome four (4p 16.3). There is another triplet sequence, a CCG repeat, immediately 3' adjacent to the CAG repeat in Huntingtin. This triplet sequence is also polymorphic, alleles of 7 or 10 repeats are predominant in populations, and strong linkage disequilibrium between the CCG (7) allele and HD has been shown in western HD chromosomes, whereas Japanese HD chromosomes strongly associate with an allele of (CCG)10. Distribution of CAG and the CCG repeats in Huntingtin in 15 patients with HD living in southern Japan were selected to evaluate the regional difference in the CCG repeat number in Japan. Among our 15 HD patients, only 4 patients had the (CCG)7 allele, and the (CCG)10 alleles were found in the remaining 11 patients. In this study, a linkage disequilibrium was found between Japanese HD chromosomes and (CCG)10, whereas western HD chromosomes are strongly associated with (CCG)7. These data suggest that (CCG)10 allele is dominant in southern Japan.

No evidence of association between BDNF gene variants and age-at-onset of Huntington's disease

Huntington's disease (HD) is a late-onset, autosomal dominant neurodegenerative disease caused by a CAG trinucleotide expansion. The number of repeats on the HD chromosome explains most of the variability in age of onset, but genetic factors other than the HD gene are responsible for part of the residual variance. Based on the role played by the brain derived neurotrophic factor (BDNF) in neurodysfunction and neurodegeneration in HD, we searched for novel polymorphisms in the neuron restrictive silencer element located in the BDNF promoter. Then, the effect of the Val66Met variant in determining age of onset was tested in a large sample of HD carriers by using a multivariate regression approach. The CAG repeat number accounted for 62% of the variance. After correction for the predominant effect of the CAG expansion, no multiple regression model provided evidence of association between the Val66Met genotype and variation in age-at-onset. Additional studies are warranted to further investigate BDNF as genetic modifier of the HD phenotype.

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.

Risk of neurodegenerative diseases in children conceived by intracytoplasmic sperm injection?

Excessive amplifications of the trinucleotide repeat (CAG)n in the translated part of a series of human genes have been associated with various neurodegenerative diseases: spinobulbar muscular atrophy (SBMA) or Kennedy's disease, 1 La Spada AR Wilson EM Lubahn DB Harding AE Fischbeck KH Androgen receptor gene mutations in X-linked spinal and bulbar muscular atrophy.. Nature. 1991; 352: 77-79 Crossref PubMed Scopus (2395) Google Scholar Huntington's disease, 2 Huntington's Disease Collaborative Research GroupA novel gene containing a trinucleotide repeat that is unstable on Huntington's disease chromosomes.. Cell. 1993; 72: 971-983 Summary Full Text PDF PubMed Scopus (6995) Google Scholar spinocerebellar ataxia type 1 (SCA1), 3 Orr HT Chung MY Banfi S et al. Expansion of an unstable trinucleotide CAG repeat in spinocerebellar ataxia type 1.. Nat Genet. 1993; 4: 221-226 Crossref PubMed Scopus (1470) Google Scholar spinocerebellar ataxia type 3 (SCA3) or Machado-Joseph disease, 4 Kawaguchi Y Okamoto T Taniwaki M et al. CAG expansion in a novel gene for Machado-Joseph disease at chromosome 14q 32.1.. Nat Genet. 1994; 8: 221-227 Crossref PubMed Scopus (1545) Google Scholar spinocerebellar ataxia type 6 (SCA6), 5 Zhuchenko O Bailey J Bonnen P et al. Autosomal dominant cerebellat ataxia (SCA6) associated with small polyglutamine expansions in the alphal A-voltage-dependent calcium channel.. Nat Genet. 1997; 15: 62-69 Crossref PubMed Scopus (1424) Google Scholar spinocerebellar ataxia type 7 (SCA7), 6 David G Durr A Stevanin G et al. Molecular and clinical correlations in autosomal dominant cerebellar ataxia with progressive macular dystrophy (SC A7).. Hum Mol Genet. 1998; 7: 165-170 Crossref PubMed Scopus (232) Google Scholar and dentatorubral pallidoluysian atrophy (DRPLA) disease. 7 Koide R Ikeuchi T Onoder O et al. Unstable expansion of CAG repeat in hereditary dentatorubral-pallidoluysian atrophy (DRPLA).. Nat Genet. 1994; 6: 9-13 Crossref PubMed Scopus (1035) Google Scholar The trinucleotide repeat expansion in each of these diseases is associated with “anticipation”, which means that the age at which the disease becomes manifest declines with successive generations. This trend can be explained by the instability of abnormally expanded repeats, which causes them to increase in size by unequal crossing-over or single-strand slippage events during meiotic DNA replication in the male or female germ-line. These mutations are then passed on to the next generation. However, CAG repeats can remain stable in the same genes, although often with different numbers in each gene copy. 8 Andrew SE Goldberg YP Hayden MR Rethinking genotype and phenotype correlations in polyglutamine expansion disorders.. Hum Mol Genet. 1997; 6: 2005-2010 Crossref PubMed Scopus (120) Google Scholar Moreover, the (CAG)n-encoded aminoacid, polyglutamine, most probably functions as a polar zipper (ie, it links specific transcription factors bound to separate DNA segments), 9 Perutz ME Johnson T Suzuki M et al. Glutamine repeats as polar zippers: their possible role in inherited neurodegenerative diseases.. Proc Natl Acad Sd USA. 1994; 91: 5355-5358 Crossref PubMed Scopus (941) Google Scholar or fine-tunes the transcriptional activation process of downstream genes. 10 Gerber HP Seipel K Georgiev O et al. Transcriptional activation modulated by homopolymeric glutamine and pro line stretches.. Science. 1994; 263: 808-811 Crossref PubMed Scopus (520) Google Scholar Linkage between male infertility and trinucleotide repeat expansion in the androgen-receptor geneOur results indicate a relation between CAG repeat length in the androgen-receptor gene and the risk of defective spermatogenesis. With the use of intracytoplasmic sperm injection, this mutation could be inherited, possibly leading to an increase in male infertility in future generations. Should further elongation of the CAG repeat occur in these future generations, there is an added risk of increased severity of male infertility, and potentially an increased incidence of neurodegenerative disease. Full-Text PDF

Improved polymerase chain reaction conditions for quick diagnostics of Huntington disease

Huntington disease (HD) belongs to a growing list of neurodegenerative disorders (fragile X syndrome [6], myotonic dystrophy [1], spino-bulbar muscular atrophy [2]etc.) characterized by unstable expanded trinucleotide repeats (so-called `dynamic mutations'). The dynamic mutation causing HD represents the expansion of CAG triplets in the first exon of a gene IT15 (chromosome 4) coding for huntingtin. This trinucleotide stretch is varying in the range of 11–34 in normal chromosomes and 39–121 in HD chromosomes. The most direct diagnostic approach is to amplify the proximal region of IT15 gene (from patients genomic DNA) by polymerase chain reaction (PCR) and estimate the number of CAG triplets. All protocols published to date are difficult to reproduce because amplification is inefficient giving additional non-specific products. The strategy of our experiment is shown in Fig. 1. We designed one new primer, primer No. 2 (another primer was primer No. 1) and novel PCR conditions. Primer No. 2 is located closer to CAG triplets and its extension is not including the GC rich region. PCR amplified products, using primer Nos. 1 and 2, thus do not include the GC rich region and, therefore, are much more efficiently amplified (compared to the products of amplification with primer Nos. 1 and 3).

Homozygotes and heterozygotes for ciliary neurotrophic factor null alleles do not show earlier onset of Huntington's disease.

The CAG repeat number on Huntington's disease (HD) chromosomes accounts for about 60% of the variance in the age at onset of HD. However, distinct familial factors may also influence the age at onset. HD is associated with loss of medium-sized GABA-ergic striatal output neurons. Intracerebral administration of human ciliary neurotrophic factor (CNTF) protects striatal output neurons in excitotoxic rodent and primate models of HD induced by intrastriatal quinolinic acid injection. We have examined the effect of a common null mutation in the human CNTF gene on the age of onset of HD using a multiple regression approach that takes into account the CAG repeat number on HD chromosomes. We failed to detect an earlier onset of HD in nine homozygotes and 71 heterozygotes with this CNTF mutation compared with 203 homozygotes with wild-type alleles.

Huntington's disease: confirmation of diagnosis and presymptomatic testing in Spanish families by genetic analysis.

Huntington's disease is a neuropsychiatric disorder with late age of onset, caused by an elongation of a (CAG)n repeat in the IT15 gene. This trinucleotide repeat has been studied by polymerase chain reaction amplification in 86 members of 43 Spanish families with Huntington's disease and in 60 unrelated subjects from the general population. The number of (CAG)n repeats in Huntington's disease chromosomes varied from 40 to 85, with 49 and 52 repeats being the most common, whereas in normal chromosomes it ranged from 12 to 32 with 20 (CAG)n repeats being the most frequent allele. In four patients with juvenile onset the number of (CAG)n repeats was greater than 50 and only one was of maternal transmission. There was a clear inverse correlation between the number of repeats and the age of onset of the disease. The study contributed to the diagnosis of 10 patients in whom the clinical diagnosis was uncertain, and identified 41 "at risk" patients after a previous psychological-psychiatric evaluation.

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.

Origins and evolution of huntington disease chromosomes

Huntington disease (HD) is one of five neurodegenerative disorders resulting from an expansion of a CAG repeat located within the coding portion of a novel gene. CAG repeat expansion beyond a particular repeat size has been shown to be a specific and sensitive marker for the disease. A strong inverse correlation is evident between CAG length and age of onset. Sporadic cases of HD have been shown to arise from intermediate sized alleles in the unaffected parent. The biochemical pathways underlying the relationship between CAG repeat length and specific cell death are not yet known. However, there is an increasing understanding of how and why specific chromosomes and not others expand into the disease range. Haplotype analysis has demonstrated that certain normal chromosomes, with CAG lengths at the high range of normal, are prone to further expansion and eventually result in HD chromosomes. New mutations preferentially occur on normal chromosomes with these same haplotypes associated with higher CAG lengths. The distribution of different haplotypes on control chromosomes in different populations is thus one indication of the frequency of new mutations for HD within that population. Analysis of normal chromosomes in different populations suggests that genetic factors contribute to expansion and account for the variation in prevalence rates for HD worldwide.