Redundant mispair excision mechanisms in eukaryotic DNA mismatch repair

DNA mismatch repair (MMR) corrects DNA replication errors, some forms of chemical damage to DNA bases, and acts in processes such as heteroduplex rejection, triplet repeat expansion, antibody maturation and cytotoxicity of alkylating agents. During MMR, the newly synthesized DNA strand containing the error must be removed and resynthesized to prevent mutations. In bacteria, this strand is displaced by the UvrD DNA helicase. In eukaryotes, this strand is excised by redundant pathways. In the first excision pathway identified, Saccharomyces cerevisiae Msh2-Msh6 (or Msh2-Msh3) and/or Mlh1-Pms1 (human MLH1-PMS2) recruit Exo1 to 5’ nicks, and 5’-to-3’ excision by Exo1 excises the daughter strand containing the mispair to generate a single-stranded gap. The Mlh1-Pms1 endonuclease also mediates excision through extensive nicking of the daughter strand to generate single-stranded gaps. In coupled strand-displacement synthesis coordinated by Msh2-Msh6 (or Msh2-Msh3), DNA polymerase delta and Rad27 (human FEN1)-mediated 5’ flap cleavage excise the mispair and resynthesize the daughter strand. Resynthesis in the Exo1- and Mlh1-Pms1-mediated pathways can be performed by both DNA polymerase delta and epsilon, and in all excision pathways the nicked product can be sealed by DNA ligase. Genetic analysis in S. cerevisiae suggests that these three pathways correspond to all or at least the major excision pathways; however, other factors such as Fan1, Artemis, Mre11, Dna2, Fun30, and the RSC complex have been implicated in eukaryotic MMR but are less well understood. The roles of excision and its redundant pathways have also been less well characterized for other processes involving MMR proteins.