Evolutionary scenario for acquisition of CAG repeats in human SCA1 gene.

We investigated the CAG repeat sequence of the spinocerebellar ataxia type 1 (SCA1) gene in various species of primates to reveal how human has acquired the repeat structure with interruptions. Our results demonstrate no repetitive structure in the region corresponding to the human CAG repeats in prosimians and New World monkeys like in rodents, perfect (uninterrupted) CAG repeats in Old World monkeys, and interrupted CAG repeats in hominoids. Comparative analysis on the secondary structures of the primate SCA1 transcripts suggests the human prototype was built in the common ancestor of simians. We show an evolutionary scenario for acquisition of CAG repeats with interruptions in the human SCA1 gene.