Identification of the Spinocerebellar Ataxia Type 2 Gene Using a Direct Identification of Repeat Expansion and Cloning Technique, DIRECT.

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominant, neurodegenerative disorder that affects the cerebellum and other areas of the central nervous system, where unstable expansion of CAG repeats has been strongly suggested as the causative mutation. We have devised a novel strategy, the direct identification of repeat expansion and cloning technique (DIRECT), which allows selective detection of expanded CAG repeats on genomic Southern blot and cloning of the causative genes. By applying DIRECT, we identified an expanded CAG repeat of the gene for SCA2. CAG repeats of normal alleles range in size from 15 to 24 repeat units, while those of SCA2 chromosomes are expanded to 35 to 59 repeat units. The SCA2 cDNA is 4351 bp in length and predicted to code for 1313 amino acids with the CAG repeats coding for a polyglutamine tract. Thus DIRECT is a robust strategy for identification of pathologically expanded trinucleotide repeats and will dramatically accelerate the search for causative genes of neuropsychiatric diseases caused by trinucleotide repeat expansions