Repair of DNA double-strand breaks within the (GAA○TTC)n sequence results in frequent deletion of the triplet-repeat sequence

Friedreich ataxia is caused by an expanded (GAA circle TTC)(n) sequence, which is unstable during intergenerational transmission and in most patient tissues, where it frequently undergoes large deletions. We investigated the effect of DSB repair on instability of the (GAA circle TTC)(n) sequence. Linear plasmids were transformed into Escherichia coli so that each colony represented an individual DSB repair event. Repair of a DSB within the repeat resulted in a dramatic increase in deletions compared with circular templates, but DSB repair outside the repeat tract did not affect instability. Repair-mediated deletions were independent of the orientation and length of the repeat, the location of the break within the repeat or the RecA status of the strain. Repair at the center of the repeat resulted in deletion of approximately half of the repeat tract, and repair at an off-center location produced deletions that were equivalent in length to the shorter of the two repeats flanking the DSB. This is consistent with a single-strand annealing mechanism of DSB repair, and implicates erroneous DSB repair as a mechanism for genetic instability of the (GAA circle TTC)(n) sequence. Our data contrast significantly with DSB repair within (CTG circle CAG)(n) repeats, indicating that repair-mediated instability is dependent on the sequence of the triplet repeat.