Molecular analysis of late onset Huntington's disease

Late onset Huntington's disease is characterised by onset of symptoms after the age of 50 and is usually associated with a milder course. We have analysed the CAG trinucleotide repeat within the HD gene in 133 late onset patients from 107 extended families. The median upper allele size for the CAG repeat was 42 with a range of 38 to 48 repeats. A significant negative correlation (r = -0.29, p = 0.001) was found between the length of repeat and age of onset for the total cohort. However, for persons with age of onset greater than 60, no significant correlation was found. In addition, no significant correlation was found between age of onset and size of the lower allele and the sex of the affected parent or grandparent. There was no preponderance of maternal descent for late onset cases in this series. This study shows that variation in repeat length only accounts for approximately 7% of the variation in age of onset for persons beyond the age of 50 and clearly shows how with increasing onset age the effect of the repeat length on this onset age seems to diminish.

Familial predisposition to recurrent mutations causing Huntington's disease: genetic risk to sibs of sporadic cases

Huntington's disease (HD) is associated with expansion of a CAG repeat in a new gene. We have recently defined a premutation in a paternal allele of 30 to 38 CAG repeats in the HD gene which is greater than that seen in the general population (< 30 repeats) but below the range seen in patients with HD (> 38). These intermediate alleles are unstable during transmission through the germline and in sporadic cases expand to the full mutation associated with the clinical phenotype of HD. Here we have analysed three new mutation families where, in each, the proband and at least one sib have CAG sizes in the HD range. In one of these families, two sibs with expanded CAG repeats are both clinically affected with HD, thus presenting a pseudorecessive pattern of inheritance. In all three families the parental intermediate allele has expanded in more than one offspring, thus showing a previously unrecognised risk of inheriting HD to sibs of sporadic cases of HD.

Molecular analysis of new mutations for Huntington's disease: intermediate alleles and sex of origin effects

Huntington's disease (HD) is associated with expansion of a CAG repeat in a novel gene. We have assessed 21 sporadic cases of HD to investigate sequential events underlying HD. We show the existence of an intermediate allele (IA) in parental alleles of 30-38 CAG repeats in the HD gene which is greater than usually seen in the general population but below the range seen in patients with HD. These IAs are meiotically unstable and in the sporadic cases, expand to the full mutation associated with the phenotype of HD. This expansion has been shown to occur only during transmission through the male germline and is associated with advanced paternal age. These findings suggest that new mutations for HD are more frequent than prior estimates and indicate a previously unrecognized risk of inheriting HD to siblings of sporadic cases of HD and their children.

Molecular analysis of juvenile Huntington disease: the major influence on (CAG)n repeat length is the sex of the affected parent

Juvenile Huntington disease (HD), characterised by onset of symptoms before the age of 20 with rigidity and intellectual decline, is associated with a predominance of affected fathers. In order to investigate the molecular basis for the observed parental effect, we have analysed the CAG trinucleotide repeat within the HD gene in 42 juvenile onset cases from 34 families. A highly significant correlation was found between the repeat length and age of onset (r = -0.86, p < 10(-7) and it was determined that the sex of transmitting parent was the major influence on CAG expansion leading to earlier onset. Neither the size of the parental upper allele, the age of parent at conception of juvenile onset child, nor the grandparental sex conferred a significant effect upon expansion. Affected sib pair analysis of CAG repeat length, however, revealed a high correlation (r = 0.91, p < 10(-7). Furthermore, analysis of nuclear and extended families showed a familial predisposition to juvenile onset disease. This study demonstrates that the sex of transmitting parent is the major influence on trinucleotide expansion and clinical features in juvenile Huntington disease.

The relationship between trinucleotide (CAG) repeat length and clinical features of Huntington's disease

Huntington's disease (HD) is associated with the expansion of a CAG trinucleotide repeat in a novel gene. We have assessed 360 HD individuals from 259 unrelated families and found a highly significant correlation (r = 0.70, p = 10(-7)) between the age of onset and the repeat length, which accounts for approximately 50% of the variation in the age of onset. Significant associations were also found between repeat length and age of death and onset of other clinical features. Sib pair and parent-child analysis revealed that the CAG repeat demonstrates only mild instability. Affected HD siblings had significant correlations for trinucleotide expansion (r = 0.66, p < 0.001) which was not apparent for affected parent-child pairs.

An Alu element retroposition in two families with Huntington disease defines a new active Alu subfamily

Alu repetitive elements represent the most common short interspersed elements (SINEs) found in primates, with an estimated 500,000 members in the haploid human genome. Considerable evidence has accumulated that these elements have dispersed in the genome by active transcription followed by retroposition, and that this process is ongoing. Sequence variation between the individual elements has lead to the hierarchical classification of Alu repeats into families and subfamilies. Young subfamilies that are still being actively transposed are of considerable interest, and the identification of one such subfamily (designated 'PV') has lead to the hypothesis that the most recent retroposition events are due to a single master Alu source gene. In the course of our search for the gene causing Huntington disease, we have detected an Alu retroposition event in two families. Sequence analysis demonstrates that this Alu element is not a member of the PV subfamily, but is similar to 5 other Alu elements in the GenBank database. Together, these Alu elements, all of which contain a 7 base-pair internal duplication, define a distinct subfamily, designated as the Sb2 subfamily, providing evidence for a second actively retroposing Alu source gene. These data provide support for multiple source genes for Alu retroposition in the human genome.

A transcription map of the region containing the Huntington disease gene

A transcription map of the Huntington disease gene region was generated by a direct cDNA selection strategy using genomic DNA from the 4p16.3 region surrounding the D4S95 and D4S127 loci. A total of 58 cDNA fragments were obtained from cDNAs derived from fetal brain, frontal cortex, liver and bone marrow following hybridization to overlapping YACs from this region. These cDNA clones were aligned into transcription units by hybridization to specific mRNAs, by sequence overlap and by physical mapping onto overlapping YAC clones. Nine separate transcription units spanning approximately one megabase were detected by RNA hybridization. They represent a minimum number of genes in this region and do not include those genes expressed specifically in tissues not used for the hybridization. The transcription map that is provided by the cDNA segments will lead to the generation of a detailed gene map of this region.

Identification of an Alu retrotransposition event in close proximity to a strong candidate gene for Huntington's disease

Huntington's disease (HD) is a late-onset autosomal dominant neuropsychiatric disorder presenting in mid-adult life with personality disturbance and involuntary movements, cognitive and affective disturbance, and inexorable progression to death. The underlying genetic defect has been mapped to chromosomal band 4p16.3 (refs 2, 3). Analysis of specific recombination events in some families with HD has further refined the location of the HD defect to a 2.2 megabase DNA interval. Using a direct complementary DNA selection strategy we have identified at least seven transcriptional units within the minimal region believed to contain the HD gene. Screening with one of the cDNA clones identified an Alu insertion in genomic DNA from two persons with HD which showed complete cosegregation with the disease in these families but was not found in 1,000 control chromosomes. Two genes including the previously identified alpha-adducin gene and another that encodes for a 12-kilobase transcript, map in close proximity to the Alu insertion site. The 12-kilobase transcript should be regarded as a strong candidate for the HD gene.

Cloning and mapping of the alpha-adducin gene close to D4S95 and assessment of its relationship to Huntington disease

The genetic defect underlying Huntington's disease (HD) has been mapped to 4p16.3. Refined localization using recombinant HD chromosome analysis and allelic association analyses have identified two distinct candidate regions. Using a cDNA hybrid selection procedure we have cloned the gene for alpha-adducin, a subunit of a cytoskeletal protein crucial for spectrin-actin membrane plasticity. This gene maps to the proximal 2.2 Mb candidate region within 20 kb of D4S95. Alleles of markers at this locus have been shown to exhibit significant linkage disequilibrium with HD. A 4 kb alpha-adducin transcript was identified which is abundantly expressed in the caudate nucleus, the site of major neuronal loss in HD. Sequencing of the brain alpha-adducin cDNA from two HD patients and an age-matched control did not detect any sequence alterations specific to HD. However, we identified in brain cDNA of both patients and control samples, two alternately spliced brain exons, not previously described in the erythrocyte cDNA. A 93 bp exon is inserted in frame between codon 471 and 472 while a 34 bp exon inserted within codon 621 disrupts the frame and introduces a stop codon after 11 novel amino acids. The mapping of the adducin gene adjacent to D4S95 and its pattern of expression, as well as its potential for distinct alternately spliced variants, reinforces the necessity to accurately assess the role of the expression of this gene in the pathogenesis of HD.