Trinucleotide repeat (TNR) tract expansions in specific genes are known to cause numerous neurological and neuromuscular diseases. The mechanism of DNA instability is believed to involve the formation of slipped-repeat structures, and ongoing expansion requires the presence of functional mismatch repair (MMR) proteins. It is known that MMR efficiency is affected by the structure of the DNA lesion, leading to outcomes such as G-T mismatches being repaired more readily than C-C mismatches. It has also been demonstrated that the structure of slipped TNRs can affect their repair, with CAG slip-outs being repaired with higher efficiency than CTG slip-outs. Here I will show that the length of the slip-out, the number of slip-outs, and the slip-out junction all affect repair; additionally, I show that their formation/processing can occur during DNA replication. While isolated short slip-outs are efficiently repaired (much more efficiently than long slip-outs), clustered slip-outs are poorly repaired. The repair of short TNR slip-outs involves the MMR protein MutSβ, but not MutSα. MutSβ is known to contribute to TNR instability, and my results indicate a potential mechanism: attempts to repair short slip-outs meet with interference due to the presence of numerous adjacent slip-outs, and this leads to errors in the repair process causing mutagenesis. Expansion products can also occur during the processing of longer slip-outs. Long CAG and CTG slip-outs can vary widely at the slip-out junction, and I show that these variations can lead to differences in correct repair, even occasionally leading to repair of the wrong strand and thus the retention of excess repeats. Finally, I show that the processing of slipped-repeats by MMR proteins can occur during DNA replication. Altogether, this thesis reveals the role of mismatch repair proteins in instability during the repair and replication of trinucleotide repeat tracts, and how features of DNA structures in the repetitive tracts affect their processing.Ph
