Leslie Manace Brenman
Spinocerebellar ataxia type 6 (SCA6) is one of multiple autosomal dominant progressive ataxias due to an expanded trinucleotide repeat gene sequence. Characterization of normal and pathogenic trinucleotide (or triplet) repeat sizes in the gene associated with SCA6, CACNA1A, is ongoing. Normal alleles contain 18 or fewer CAG repeats and full-penetrance alleles contain 20 to 33 triplet repeats; 19 CAG repeats seem to have variable effect. Meiotic expansion from the 19-CAG repeat allele to full pathogenic range has been reported. Affected individuals homozygous for this intermediate 19-CAG repeat allele and asymptomatic individuals up to late age-heterozygous for this intermediate allele have also been reported. We present a case report of a 44-year-old woman heterozygous for 19 CAG repeats with normal remaining gene sequence in CACNA1A presenting with an SCA6 phenotype, adding to evidence for pathogenicity of this trinucleotide repeat size.