| At least 15 human diseases are caused by the instability of gene-specific (CTG)·(CAG) repeats. The mechanism of instability remains unknown, though model systems suggest a role for aberrant double-strand break (DSB) repair. Using a primate DSB repair system, I have investigated the fidelity of repair of a DSB within a (CTG)·(CAG) tract. DSBs were introduced into repeat-containing plasmids using a BsmI site, such that the repeat tract comprised one free end of the linear plasmid. Substrates with 17, 47 and 79 repeats, in linear duplex form or containing slipped-structures, were transfected into primate cells. The presence of repeats and slipped structures reduced repair efficiency. Repair products were characterized for alterations of the repeat tract and flanking sequence. DSB repair induced predominantly repeat deletions, and notably showed the preferential removal of the repetitive end of the DSB. This was enhanced when slipped structures were present within the repeat tract. |
