Molecular genetic analysis of autosomal dominant cerebellar ataxia with retinal degeneration (ADCA type II) caused by CAG triplet repeat expansion.

Autosomal dominant cerebellar ataxia with retinal degeneration (ADCAII) was previously mapped by linkage analysis studies to chromosome 3p12-p21.1 (SCA7). Positional cloning efforts have recently identified a novel gene, SCA7 , containing a translated CAG repeat, expanded in SCA7 patients. We cloned the SCA7 gene from a yeast artificial chromosome (YAC) clone contig spanning the SCA7 candidate region. Using a combination of genomic sequencing and cosmid-based exon trapping, two expressed sequence tags were identified. Sequencing of the corresponding cDNA clones and RT-PCR analysis identified the full-length SCA7 cDNA. Together, our sequence data defined the intron/exon boundaries of the first two coding exons of the SCA7 gene, with the first exon containing the expanded CAG repeat. Further, sequence comparison with the published SCA7 cDNA identified one additional putative exon in the 5'-UTR region of the SCA7 gene. The SCA7 gene was mapped on the YAC contig in the 2.5 cM interval between D3S1600 and D3S1287. In one extended Belgian SCA7 pedigree the expanded alleles ranged from 38 to at least 55 repeats with allele lengths being inversely correlated with onset age of ADCAII symptoms. The SCA7 repeats increased in length in successive generations. Normal alleles had from four to 18 repeats, with 10 repeats being the most common allele.