Repeat instability in the 27-39 CAG range of the HD gene in the Venezuelan kindreds: Counseling implications.

The instability of the CAG repeat size of the HD gene when transmitted intergenerationally has critical implications for genetic counseling practices. In particular, CAG repeats between 27 and 35 have been the subject of debate based on small samples. To address this issue, we analyzed allelic instability in the Venezuelan HD kindreds, the largest and most informative families ascertained for HD. We identified 647 transmissions. Our results indicate that repeats in the 27-35 CAG range are highly stable. Out of 69 transmitted alleles in this range, none expand into any penetrant ranges. Contrastingly, 14% of alleles transmitted from the incompletely penetrant range (36-39 CAGs) expand into the completely penetrant range, characterized by alleles with 40 or more CAG repeats. At least 12 of the 534 transmissions from the completely penetrant range contract into the incompletely penetrant range of 36-39 CAG repeats. In these kindreds, none of the individuals with 27-39 CAGs were symptomatic, even though they ranged in age from 11 to 82 years. We expect these findings to be helpful in updating genetic counseling practices.

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.

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.

Analysis of germline mutation spectra at the Huntington's disease locus supports a mitotic mutation mechanism.

Trinucleotide repeat disease alleles can undergo 'dynamic' mutations in which repeat number may change when a gene is transmitted from parent to offspring. By typing >3500 sperm, we determined the size distribution of Huntington's disease (HD) germline mutations produced by 26 individuals from the Venezuelan cohort with CAG/CTG repeat numbers ranging from 37 to 62. Both the mutation frequency and mean change in allele size increased with increasing somatic repeat number. The mutation frequencies averaged 82% and, for individuals with at least 50 repeats, 98%. The extraordinarily high mutation frequency levels are most consistent with a mutation process that occurs throughout germline mitotic divisions, rather than resulting from a single meiotic event. In several cases, the mean change in repeat number differed significantly among individuals with similar somatic allele sizes. This individual variation could not be attributed to age in a simple way or to ' cis ' sequences, suggesting the influence of genetic background or other factors. A familial effect is suggested in one family where both the father and son gave highly unusual spectra compared with other individuals matched for age and repeat number. A statistical model based on incomplete processing of Okazaki fragments during DNA replication was found to provide an excellent fit to the data but variation in parameter values among individuals suggests that the molecular mechanism might be more complex.

Single sperm analysis of the trinucleotide repeats in the Huntington's disease gene: quantification of the mutation frequency spectrum.

The CAG triplet repeat region of the Huntington's disease gene was amplified in 923 single sperm from three affected and two normal individuals. Average-size alleles (15-18 repeats) showed only three contraction mutations among 475 sperm (0.6%). A 30 repeat normal allele showed an 11% mutation frequency. The mutation frequency of a 36 repeat intermediate allele was 53% with 8% of all gametes having expansions which brought the allele size into the HD disease range (> or = 38 repeats). Disease alleles (38-51 repeats) showed a very high mutation frequency (92-99%). As repeat number increased there was a marked elevation in the frequency of expansions, in the mean number of repeats added per expansion and the size of the largest observed expansion. Contraction frequencies also appeared to increase with allele size but decreased as repeat number exceeded 36. Our sperm typing data are of a discrete nature rather than consisting of smears of PCR product from pooled sperm. This allowed the observed mutation frequency spectra to be compared to the distribution calculated using discrete stochastic models based on current molecular ideas of the expansion process. An excellent fit was found when the model specified that a random number of repeats are added during the progression of the polymerase through the repeated region.

A genetic linkage map of the chromosome 4 short arm.

We have generated an 18-interval contiguous genetic linkage map of human chromosome 4 spanning the entire short arm and proximal long arm. Fifty-seven polymorphisms, representing 42 loci, were analyzed in the Venezuelan reference pedigree. The markers included seven genes (ADRA2C, ALB, GABRB1, GC, HOX7, IDUA, QDPR), one pseudogene (RAF1P1), and 34 anonymous DNA loci. Four loci were represented by microsatellite polymorphisms and one (GC) was expressed as a protein polymorphism. The remainder were genotyped based on restriction fragment length polymorphism. The sex-averaged map covered 123 cM. Significant differences in sex-specific rates of recombination were observed only in the pericentromeric and proximal long arm regions, but these contributed to different overall map lengths of 115 cM in males and 138 cM in females. This map provides 19 reference points along chromosome 4 that will be particularly useful in anchoring and seeding physical mapping studies and in aiding in disease studies.

A genetic linkage map of human chromosome 9q.

A genetic linkage map of human chromosome 9q, spanning a sex-equal distance of 125 cM, has been developed by genotyping 26 loci in the Venezuelan Reference Pedigree. The loci include 12 anonymous microsatellite markers reported by Kwiatkowski et al. (1992), several classical systems previously assigned to chromosome 9q, and polymorphisms for the genes tenacin (HXB), gelsolin (GSN), adenylate kinase 1 (AK1), arginosuccinate synthetase (ASS), ABL oncogene (ABL1), ABO blood group (ABO), and dopamine beta-hydroxylase (DBH). Only a marginally significant sex difference is found along the entire length of the map and results from one interval, between D9S58 and D9S59, that displays an excess of female recombination. A comparison of the genetic map to the existing physical data suggests that there is increased recombination in the 9q34 region with a recombination event occurring every 125-400 kb. This map should be useful in further characterizing the relationship between physical distance and genetic distance, as well as for genetic linkage studies of diseases that map to chromosome 9q, including multiple self-healing squamous epithelioma (MSSE), Gorlin syndrome (NBCCS), xeroderma pigmentosum (XPA), nail-patella syndrome (NPS1), torsion dystonia (DYT1), and tuberous sclerosis (TSC1).

The Tiresias complex: Huntington's disease as a paradigm of testing for late-onset disorders.

Huntington's disease represents the first disorder for which positional cloning techniques successfully localized an autosomal gene--in 1983. Events since that time have proved the gene recalcitrant to identification and characterization. Since 1986, presymptomatic and prenatal testing for Huntington's disease has been available internationally, although on a limited basis. Testing for Huntington's disease provides an excellent model for designing service programs for genetic testing for late-onset, fatal disorders, particularly when the gene is not yet in hand and no therapeutic intervention is possible. Special training and precautions must be in place before presymptomatic genetic testing should be offered.

A genetic linkage map of human chromosome 21: analysis of recombination as a function of sex and age.

A genetic linkage map of human chromosome 21 has been constructed using 22 anonymous DNA markers and five complementary DNAs (cDNAs) encoding the amyloid beta protein precursor (APP), superoxide dismutase 1 (SOD1), the ets-2 proto-oncogene (ETS2), the estrogen inducible breast cancer locus (BCEI), and the leukocyte antigen, CD18 (CD18). Segregation of RFLPs detected by these DNA markers was traced in the Venezuelan Reference Pedigree (VRP). A comprehensive genetic linkage map consisting of the 27 DNA markers spans 102 cM on the long arm of chromosome 21. We have confirmed our initial findings of a dramatically increased rate of recombination at the telomere in both females and males and of significantly higher recombination in females in the pericentromeric region. By comparing patterns of recombination in specific regions of chromosome 21 with regard to both parental sex and age, we have now identified a statistically significant downward trend in the frequency of crossovers in the most telomeric portion of chromosome 21 with increasing maternal age. A less significant decrease in recombination with increasing maternal age was observed in the pericentromeric region of the chromosome. These results may help in ultimately understanding the physical relationship between recombination and nondisjunction in the occurrence of trisomy 21.

Disease gene identification: ethical considerations.

How can we offer genetic testing and screening with adequate protection? Who will have access to the results? How do we balance a person's desire to know with the pain that knowledge may bring? We can examine these issues in the light of our experience with two fatal autosomal diseases: cystic fibrosis, in which the gene is known, and Huntington's disease, in which it is not.

Increased recombination adjacent to the Huntington disease-linked D4S10 marker.

Huntington disease (HD) is caused by a genetic defect distal to the anonymous DNA marker D4S10 in the terminal cytogenetic subband of the short arm of chromosome 4 (4p16.3). The effort to identify new markers linked to HD has concentrated on the use of somatic cell hybrid panels that split 4p16.3 into proximal and distal portions. Here we report two new polymorphic markers in the proximal portion of 4p16.3, distal to D4S10. Both loci, D4S126 and D4S127, are defined by cosmids isolated from a library enriched for sequences in the 4pter-4p15.1 region. Physical mapping by pulsed-field gel electrophoresis places D4S126 200 kb telomeric to D4S10, while D4S127 is located near the more distal marker D4S95. Typing of a reference pedigree for D4S126 and D4S127 and for the recently described VNTR marker D4S125 has firmly placed these loci on the existing linkage map of 4p16.3. This genetic analysis has revealed that the region immediately distal to D4S10 shows a dramatically higher rate of recombination than would be expected based on its physical size. D4S10-D4S126-D4S125 span 3.5 cM, but only 300-400 kb of DNA. Consequently, this small region accounts for most of the reported genetic distance between D4S10 and HD. By contrast, it was not possible to connect D4S127 to D4S125 by physical mapping, although they are only 0.3 cM apart. A more detailed analysis of recombination sites within the immediate vicinity of D4S10 could potentially reveal the molecular basis for this phenomenon; however, it is clear that the rate of recombination is not continuously increased with progress toward the telomere of 4p.

Software support for Huntingtons disease research.

Huntingtons disease (HD) is a hereditary disorder involving the central nervous system. Its effects are devastating, to the affected person as well as his family. The Department of Medical and Molecular Genetics at Indiana University (IU) plays an integral part in Huntingtons research by providing computerized repositories of HD family information for researchers and families. The National Huntingtons Disease Research Roster, founded in 1979 at IU, and the Huntingtons Disease in Venezuela Project database contain information that has proven to be invaluable in the worldwide field of HD research. This paper addresses the types of information stored in each database, the pedigree database program (MEGADATS) used to manage the data, and significant findings that have resulted from access to the data.

Huntington disease: no evidence for locus heterogeneity.

A total of 63 families with Huntington disease (HD) were examined for linkage between HD and G8 (D4S10). The families included 57 Caucasian, four Black American, and two Japanese. The combined maximum lod score was 87.69 at theta = 0.04 (99% confidence interval 0.018-0.071). The maximum frequency of recombination was 0.03 in males and 0.05 in females. Fifty-seven families gave positive lod scores; five small families gave mildly negative lod scores. The maximum likelihood estimate of alpha, the proportion of linked loci, was 1.0 with a lower 99% confidence interval of 0.88. These data suggest that there is only one HD locus, although a second rare locus cannot be ruled out.

The role of mitochondrial DNA in Huntington's disease.

Huntington's disease is generally considered to be a late-onset neurodegenerative disorder, which follows a protracted course of deteriorating motor control and cognitive impairment. However, in a minority of cases, the onset of symptoms occurs early in life. A preponderance of the juvenile-onset HD victims have inherited the genetic defect from their fathers. This variation in age of onset, based on the sex of the affected parent, has suggested that maternally inherited genes may influence expression of the disorder. We describe a portion of a large Venezuelan HD pedigree in which both the mother and father of three juvenile-onset HD patients share a common maternal lineage. Scanning of mtDNA from members of this family with 43 restriction endonucleases failed to reveal any differences in the mitochondrial genotype that could account for the difference in age of onset between the affected father and his progeny. Members of a related family with an affected father but no juvenile-onset progeny also appeared to share the same mitochondrial genotype. In addition, the mitochondrial gene products from lymphoblast cell lines of these family members were analyzed on polyacrylamide gels after incubation of cells with [35S]methionine, but no detectable alterations were seen. Taken together, these data suggest that the maternally inherited mitochondrial genome does not play a crucial role in determining in age of onset in HD.

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.

Genetic linkage map of human chromosome 21.

Two of the most common disorders affecting the human nervous system, Down syndrome and Alzheimer's disease, involve genes residing on human chromosome 21. A genetic linkage map of human chromosome 21 has been constructed using 13 anonymous DNA markers and cDNAs encoding the genes for superoxide dismutase 1 (SOD1) and the precursor of Alzheimer's amyloid beta peptide (APP). Segregation of restriction fragment length polymorphisms (RFLPs) for these genes and DNA markers was traced in a large Venezuelan kindred established as a "reference" pedigree for human linkage analysis. The 15 loci form a single linkage group spanning 81 cM on the long arm of chromosome 21, with a markedly increased frequency of recombination occurring toward the telomere. Consequently, 40% of the genetic length of the long arm corresponds to less than 10% of its cytogenetic length, represented by the terminal half of 21q22.3. Females displayed greater recombination than males throughout the linkage group, with the difference being most striking for markers just below the centromere. Definition of the linkage relationships for these chromosome 21 markers will help refine the map position of the familial Alzheimer's disease gene and facilitate investigation of the role of recombination in nondisjunction associated with Down syndrome.