Nancy S. Wexler

A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington's disease chromosomes

The Huntingtons disease (HD) gene has been mapped in 4p16.3 but has eluded identification. We have used haplotype analysis of linkage disequilibrium to spotlight a small segment of 4p16.3 as the likely location of the defect. A new gene, IT15, isolated using cloned trapped exons from the target area contains a polymorphic trinucleotide repeat that is expanded and unstable on HD chromosomes. A (CAG)n repeat longer than the normal range was observed on HD chromosomes from all 75 disease families examined, comprising a variety of ethnic backgrounds and 4p16.3 haplotypes. The (CAG)n repeat appears to be located within the coding sequence of a predicted approximately 348 kd protein that is widely expressed but unrelated to any known gene. Thus, the HD mutation involves an unstable DNA segment, similar to those described in fragile X syndrome, spino-bulbar muscular atrophy, and myotonic dystrophy, acting in the context of a novel 4p16.3 gene to produce a dominant phenotype.

Huntington's disease in Venezuela: Neurologic features and functional decline

We studied 65 Huntingtons disease patients and 225 at-risk individuals over the past 4 years. The rate of decline of these untreated patients from Venezuela was similar to that seen in US patients who had received neuroleptic drugs. Chorea, oculomotor dysfunction, and dysdiadochokinesis were early symptoms; parkinsonian features and dystonia came later. Juvenile patients declined nearly twice as fast as adult-onset patients. No distinctive neurologic phenotypes were seen in children of two affected parents.

Huntington disease expansion mutations in humans can occur before meiosis is completed

Single-molecule DNA analysis of testicular germ cells isolated by laser capture microdissection from two Huntington disease patients showed that trinucleotide repeat expansion mutations were present before the end of the first meiotic division, and some mutations were present even before meiosis began. Most of the larger Huntington disease mutations were found in the postmeiotic cell population, suggesting that expansions may continue to occur during meiosis and/or after meiosis is complete. Defining the germ-line cell compartments where the trinucleotide repeat expansions occur could help to elucidate the underlying mechanisms of instability.

Localization of the Huntington's disease gene to a small segment of chromosome 4 flanked by D4S10 and the telomere.

Huntingtons disease (HD) is an autosomal dominant neurodegenerative disorder of late onset, characterized by progressive motor disturbance, psychological manifestations, and intellectual deterioration. The HD gene has been genetically mapped by linkage to the DNA marker D4S10, but the exact physical location of the HD defect has remained uncertain. To delineate critical recombination events revealing the physical position of the HD gene, we have identified restriction fragment length polymorphisms for two recently mapped chromosome 4 loci, RAF2 and D4S62, and determined the pattern of segregation of these markers in both reference and HD pedigrees. Multipoint linkage analysis of the new markers with D4S10 and HD establishes that the HD gene is located in a very small physical region at the tip of the chromosome, bordered by D4S10 and the telomere. A crossover within the D4S10 locus orients this segment on the chromosome, providing the necessary information for efficient application of directional cloning strategies for progressing toward, and eventually isolating, the HD gene.

Replication of twelve association studies for Huntington’s disease residual age of onset in large Venezuelan kindreds

Background: The major determinant of age of onset in Huntington’s disease is the length of the causative triplet CAG repeat. Significant variance remains, however, in residual age of onset even after repeat length is factored out. Many genetic polymorphisms have previously shown evidence of association with age of onset of Huntington’s disease in several different populations. Objective: To replicate these genetic association tests in 443 affected people from a large set of kindreds from Venezuela. Methods: Previously tested polymorphisms were analysed in the HD gene itself (HD), the GluR6 kainate glutamate receptor (GRIK2), apolipoprotein E (APOE), the transcriptional coactivator CA150 (TCERG1), the ubiquitin carboxy-terminal hydrolase L1 (UCHL1), p53 (TP53), caspase-activated DNase (DFFB), and the NR2A and NR2B glutamate receptor subunits (GRIN2A, GRIN2B). Results: The GRIN2A single-nucleotide polymorphism explains a small but considerable amount of additional variance in residual age of onset in our sample. The TCERG1 microsatellite shows a trend towards association but does not reach statistical significance, perhaps because of the uninformative nature of the polymorphism caused by extreme allele frequencies. We did not replicate the genetic association of any of the other genes. Conclusions:GRIN2A and TCERG1 may show true association with residual age of onset for Huntington’s disease. The most surprising negative result is for the GRIK2 (TAA)n polymorphism, which has previously shown association with age of onset in four independent populations with Huntington’s disease. The lack of association in the Venezuelan kindreds may be due to the extremely low frequency of the key (TAA)16 allele in this population.

The Relationship Between CAG Repeat Length and Age of Onset Differs for Huntington's Disease Patients with Juvenile Onset or Adult Onset

Age of onset for Huntingtons disease (HD) varies inversely with the length of the disease‐causing CAG repeat expansion in the HD gene. A simple exponential regression model yielded adjusted R‐squared values of 0.728 in a large set of Venezuelan kindreds and 0.642 in a North American, European, and Australian sample (the HD MAPS cohort). We present evidence that a two‐segment exponential regression curve provides a significantly better fit than the simple exponential regression. A plot of natural log‐transformed age of onset against CAG repeat length reveals this segmental relationship. This two‐segment exponential regression on age of onset data increases the adjusted R‐squared values by 0.012 in the Venezuelan kindreds and by 0.035 in the HD MAPS cohort. Although the amount of additional variance explained by the segmental regression approach is modest, the two slopes of the two‐segment regression are significantly different from each other in both the Venezuelan kindreds [F(2, 439) = 11.13, P= 2 × 10−5] and in the HD MAPS cohort [F(2, 688) = 38.27, P= 2 × 10−16]. In both populations, the influence of each CAG repeat on age of onset appears to be stronger in the adult‐onset range of CAG repeats than in the juvenile‐onset range.

Prevalence of adult Huntington's disease in the UK based on diagnoses recorded in general practice records

Background and purpose The prevalence of Huntingtons disease (HD) in the UK is uncertain. Recently, it has been suggested that the prevalence may be substantially greater than previously reported. This study was undertaken to estimate the overall UK prevalence in adults diagnosed with HD, using data from primary care. Methods The electronic medical records of patients aged 21 years or more, with recorded diagnoses of HD, were retrieved from the UKs General Practice Research Database. Prevalence was estimated from the number of persons with recorded diagnoses of HD, on 1 July each year, between 1990 and 2010. This number was divided by the total number of persons registered with participating general practices on that same date. These data were also used to estimate both age specific prevalence and prevalence in various regions of the UK. Results A total of 1136 patients diagnosed with HD, aged 21 years or more, were identified from the database. The estimated prevalence (expressed per 100 000 population) rose from 5.4 (95% CI 3.8 to 7.5) in 1990 to 12.3 (95% CI 11.2 to 13.5) in 2010. Although an increased prevalence was observed within every age group, the most dramatic was in older patients. Age specific prevalence was highest in the 51–60 year age range (15.8 95% CI 9.0 to 22.3). The prevalence of adult HD was lowest in the London region (5.4 (95% CI 3.0 to 8.9)) and highest in the North East of England (18.3 (95% CI 8.6 to 34.6)) and Scotland (16.1 (95% CI 10.8 to 22.9)). Conclusions The prevalence of diagnosed HD is clearly substantially higher in the UK than suggested from previous studies. By extrapolation to the UK as a whole, it is estimated that there are more than 5700 people, aged 21 years or more, with HD. There has also been a surprising doubling of the HD population between 1990 and 2010. Many factors may have caused this increase, including more accurate diagnoses, better and more available therapies and an improved life expectancy, even with HD. There also appears to be a greater willingness to register a diagnosis of HD in patients’ electronic medical records. Such a high prevalence of HD requires more ingenuity and responsiveness in its care. How to appropriately care for, and respond to, so many individuals and families coping with the exigencies of HD demands our greatest resolve and imagination.

Recombination Events Suggest Potential Sites for the Huntington's Disease Gene

The Huntingtons disease gene (HD) maps distal to the D4S10 marker in the terminal 4p16.3 subband of chromosome 4. Directed cloning has provided several DNA segments that have been grouped into three clusters on a physical map of approximately 5 X 10(6) bp in 4p16.3. We have typed RFLPs in both reference and HD pedigrees to produce a fine-structure genetic map that establishes the relative order of the clusters and further narrows the target area containing the HD gene. Despite the large number of meiotic events examined, the HD gene cannot be positioned relative to the most distal cluster. One recombination event with HD suggests that the terminal-most markers flank the disease gene; two others favor a telomeric location for the defect. Efforts to isolate the HD gene must be divided between these two distinct intervals until additional genetic data resolve the apparent contradiction in localization.

A genetic linkage map of the long arm of human chromosome 22

We have used a recombinant phage library enriched for chromosome 22 sequences to isolate and characterize eight anonymous DNA probes detecting restriction fragment length polymorphisms on this autosome. These were used in conjunction with eight previously reported loci, including the genes BCR, IGLV, and PDGFB, four anonymous DNA markers, and the P1 blood group antigen, to construct a linkage map for chromosome 22. The linkage group is surprisingly large, spanning 97 cM on the long arm of the chromosome. There are no large gaps in the map; the largest intermarker interval is 14 cM. Unlike several other chromosomes, little overall difference was observed for sex-specific recombination rates on chromosome 22. The availability of a genetic map will facilitate investigation of chromosome 22 rearrangements in such disorders as cat eye syndrome and DiGeorge syndrome, deletions in acoustic neuroma and meningioma, and translocations in Ewing sarcoma. This defined set of linked markers will also permit testing chromosome 22 for the presence of particular disease genes by family studies and should immediately support more precise mapping and identification of flanking markers for NF2, the defective gene causing bilateral acoustic neurofibromatosis.

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.