CGG/CCG repeats exhibit orientation-dependent instability and orientation-independent fragility in Saccharomyces cerevisiae

An expansion to > 200 CGG/CCG repeats thereafter called CGG) in the 5' region of the FMR1 gene causes fragile X syndrome, and this locus becomes a folate-sensitive fragile site. We used Saccharomyces cerevisiae as a model system to study the stability and fragility of CGG repeats. Tracts of (CGG)₈₁ and (CGG)₁₆₀ were integrated onto a yeast chromosome in both orientations relative to the nearest replication origin. Tracts of this length are pre-mutation alleles in humans, with a high probability of expansion in future generations. The CGG tracts in yeast colonies showed a length-dependent instability with longer tracts being more prone to contraction than shorter tracts. In addition, there was an orientation bias for tract stability with tracts having fewer contractions when the CCG strand was the template for lagging strand synthesis. Expansions of the CGG tracts also occurred in an orientation-dependent manner, although at a lower frequency than contractions. To determine whether CGG tracts are fragile sites in yeast, the CGG tracts were flanked by direct repeats, and the rate of recombination between the repeats determined. Strains carrying the (CGG)₁₆₀ tract in either orientation had a large increase in their rate of recombination compared with a no-tract control strain. Because this increase was dependent on genes involved in double-strand break repair, recombination was likely to be initiated by CGG tract-induced breakage between the direct repeats. The observation of orientation-dependent instability and orientation-independent fragility suggests that at least some aspects of their underlying mechanisms are different.