Human Mismatch Repair Protein hMutLα Is Required to Repair Short Slipped-DNAs of Trinucleotide Repeats

Mismatch repair (MMR) is required for proper maintenance of the genome by protecting against mutations. The mismatch repair system has also been implicated as a driver of certain mutations, including disease-associated trinucleotide repeat instability. We recently revealed a requirement of hMutS beta in the repair of short slip-outs containing a single CTG repeat unit (1). The involvement of other MMR proteins in short trinucleotide repeat slip-out repair is unknown. Here we show that hMutL alpha is required for the highly efficient in vitro repair of single CTG repeat slip-outs, to the same degree as hMutS beta. HEK293T cell extracts, deficient in hMLH1, are unable to process single-repeat slip-outs, but are functional when complemented with hMutL alpha. The MMR-deficient hMLH1 mutant, T117M, which has a point mutation proximal to the ATP-binding domain, is defective in slip-out repair, further supporting a requirement for hMLH1 in the processing of short slip-outs and possibly the involvement of hMHL1 ATPase activity. Extracts of hPMS2-deficient HEC-1-A cells, which express hMLH1, hMLH3, and hPMS1, are only functional when complemented with hMutL alpha, indicating that neither hMutL beta nor hMutL gamma is sufficient to repair short slip-outs. The resolution of clustered short slip-outs, which are poorly repaired, was partially dependent upon a functional hMutL alpha. The joint involvement of hMutS beta and hMutL alpha suggests that repeat instability may be the result of aberrant outcomes of repair attempts.