Stability of the human fragile X (CGG)(n) triplet repeat array in Saccharomyces cerevisiae deficient in aspects of DNA metabolism

Expanded trinucleotide repeats underlie a growing number of human diseases. The human FMR1 (CGG)(n) array can exhibit genetic instability characterize d by progressive expansion over several generations leading to gene silenci ng and the development of the fragile X syndrome. While expansion is depend ent upon the length of uninterrupted (CGG)(n), instability occurs in a limi ted germ Line and early developmental window, suggesting that lineage-speci fic expression of other factors determines the cellular environment permiss ive for expansion. To identify these factors, we have established normal- a nd premutation-length human FMR1 (CGG), arrays in the yeast Saccharomyces c erevisiae and assessed the frequency of length changes greater than 5 tripl ets in cells deficient in various DNA repair and replication functions. In contrast to previous studies with Escherichia coli, we observed a low frequ ency of orientation-dependent large expansions in arrays carrying long unin terrupted (CGG)(n) arrays in a wild-type background. This frequency was una ffected by deletion of several DNA mismatch repair genes or deletion of the EXO1 and DIN7 genes and was not enhanced through meiosis in a wild-type ba ckground. Array contraction occurred in an orientation-dependent manner in most mutant backgrounds, but loss of the Sgs1p resulted in a generalized in crease in array stability in both orientations. In contrast, FMR1 arrays ha d a 10-fold-elevated frequency of expansion in a rad27 background, providin g evidence for a role in lagging-strand Okazaki fragment processing in (CGG )(n) triplet repeat expansion.