Human MutLγ, the MLH1-MLH3 heterodimer, is an endonuclease that promotes DNA expansion

MutL proteins are ubiquitous and play important roles in DNA metabolism. MutL gamma (MLH1-MLH3 heterodimer) is a poorly understood member of the eukaryotic family of MutL proteins that has been implicated in triplet repeat expansion, but its action in this deleterious process has remained unknown. In humans, triplet repeat expansion is the molecular basis for similar to 40 neurological disorders. In addition to MutL gamma, triplet repeat expansion involves the mismatch recognition factor MutS beta (MSH2-MSH3 heterodimer). We show here that human MutL. is an endonuclease that nicks DNA. Strikingly, incision of covalently closed, relaxed loop-containing DNA by human MutL gamma is promoted by MutS beta and targeted to the strand opposite the loop. The resulting strand break licenses downstream events that lead to a DNA expansion event in human cell extracts. Our data imply that the mammalian MutL gamma is a unique endonuclease that can initiate triplet repeat DNA expansions.