Expansions, contractions, and fragility of the spinocerebellar ataxia type 10 pentanucleotide repeat in yeast

摘要

Spinocerebellar ataxia 10 (SCA10) is an autosomal dominant disease caused by large-scale expansions of the (ATTCT)_n repeat within an intron of the human ATXN10 gene. In contrast to other expandable repeats, this pentanucleotide repeat does not form stable intra- or interstranded DNA structures, being a DNA unwinding element instead. We analyzed the instability of the (ATTCT)_n repeat in a yeast experimental system, where its expansions led to inactivation of the URA3 reporter gene. The inactivation was due to a dramatic decrease in the mRNA levels owing to premature tran- scription termination and RNA polyadenylation at the repeat. The rates of expansions strongly increased with the repeat's length, mimicking genetic anticipation in human pedigrees. A first round of genetic analysjs showed that a functional TOF1 gene precludes, whereas a functional RAD5 gene promotes, expansions of the (ATTCT)_n repeat. We hypothesize that repeat expansions could occur upon fortuitous template switching during DNA replication. The rate of repeat contractions was elevated in the Tof 1 knockout strain, but it was not affected by the RADS gene. Supporting the notion of replication irregularities, we found that (ATTCT)_n repeats also cause length-dependent chromosomal fragility in yeast. Repeat-mediated fragility was also affected by the Tof 1 and Rad5 proteins, being reduced in their absence.