Mismatch Repair (MMR)

The primary source of spontaneous alterations in DNA occurs through the generation of mismatched bases during DNA replication. Normally, in E. coli spontaneous mutations arise at a frequency of approximately one in every 10 ¹⁰ bases synthesized. This is an incredible achievement analogous to not misspelling a single word in 8 million pages of text. This faithful copying of DNA is dependent on at least four factors; the energetics of proper base pairing during DNA synthesis, certain protein requirements such as single-stranded DNA binding protein and the proof-reading activity of the polymerases 3’ to 5’ exonuclease. Importantly, cells lacking a mismatch repair (MMR) system have a spontaneous mutation frequency approximately 1000 times greater than normal cells. The human MMR pathway generated tremendous attention recently when it was discovered that germline mutations in MMR genes are present in many families with hereditary cancers, including nonpolyposis colorectal cancer; forever documenting how important this repair pathway is to cells.

Our fundamental understanding of MMR comes from years of work performed on the methyl-directed MMR system in E. coli.. The basic mechanism of MMR in all systems studied to date involves three steps; recognition of the mismatch, excision of the misincorporated base and DNA surrounding the mismatch, and as a last step, repair synthesis to replace the excised DNA . In methyl-directed MMR, the mismatched base is recognized by the MutS protein. In a reaction requiring ATP hydrolysis, MutL, together with the MutS-mismatched DNA complex stimulate strand scission by MutH, opposite a dam methylated GATC parental DNA sequence, ensuring that the DNA excised is the newly replicated unmethylated daughter DNA. In the excision step, ATP hydrolysis fuels the unwinding and degradation of ssDNA containing the mismatched base from the MutH nick site through the mismatched base. The unwinding reaction that displaces the daughter strand is catalyzed by DNA helicase II, and the ssDNA exonuclease reaction is catalyzed by either ExoVII or RecJ from one side or Exonuclease I from the other side of the mismatch, as dictated by which methylated GATC site is recognized and cleaved. In the final step of the reaction, DNA polymerase III and SSB proteins perform repair synthesis to replace the excised DNA.

MMR in mammalian cells resembles the pathway described above for E. coli.. In humans, the MutH and MutS analogs are heterodimers. hMutSa recognizes the eight mismatches generated from misincorporation during DNA synthesis and most single base loops resulting from single insertion/deletion events. hMutSb binds efficiently to 2-5 bp loops. Each of these heterodimers is proposed to work with MutLa, the human analog of E. coli MutL. As in the E. coli system, after recognition of the mismatched base or loop, an excision reaction removes the mismatch or loop, and the DNA surrounding it. Based on genetic studies in yeast and the interaction reported between the yeast Msh2 and Exonuclease 1 (Exo 1) proteins, the human homolog to Exo 1, Hex1/hExo1, may act as the excision nuclease in this process. Consistently, recent data has shown an interaction between the human Exo1-like protein and hMsh2. As a final step in the reaction, repair synthesis replaces the missing DNA. To date, the signals specifying the daughter strand with the incorrect base, the proteins involved in exonuclease digestion, and the proteins contributing to repair synthesis are not clearly defined in humans.

by Jim George