DNA mismatch repair (MMR) maintains genome stability by primarily correcting DNA replication errors in the newly synthesized strand. Defects in MMR leads to cancer and other human diseases. The MMR reaction has been reconstituted, which involves mismatch recognition by MutS family proteins (MutS in prokaryotes and MSH2•MSH6-formed MutSα or MSH2•MSH3-formed MutSβ in eukaryotes), removal of the mispaired base by nucleases in a manner dependent on MutS- and MutL-family proteins, and repair DNA synthesis by a replicative DNA polymerase in concert with DNA replication factors. Despite extensive studies, many fundamental questions in MMR are still unknown.
1. How is MMR specifically targeted to the newly-synthesized strand?
Unlike the obvious DNA lesions for other DNA repair pathways, both bases in a mismatch (e.g., mispaired G•T or C•A) are normal DNA components. Thus, to remove the incorrect base, the MMR system has to know the wrong information-containing daughter strand. Previous studies have revealed that DNA strand breaks in the newly synthesized daughter strand serve as a strand discrimination signal, ensuring the repair specifically targeted to the daughter strand. However, a strand break is usually several hundred base pairs away from a mismatch, how these two distal sites communicate with each other during MMR has been a standing puzzle in the field. Our lab has reconstituted the human MMR reaction in a defined system. Using this reconstituted system, we aim to resolve this fundamental but important problem in MMR.
2. How does MMR occur in vivo?
Our knowledge about the mechanism of MMR essentially came from the in vitro studies using naked DNA heteroduplexes. However, DNA is packed into nucleosome-consisting chromatin in vivo. How MMR occur in vivo is largely unknown. Recently, we have shown that MutSα is recruited to replicating chromatin through its physical interaction with H3K36me3 (histone H3 lysine 36 trimethylation). Consistent with this observation, disrupting the MutS-H3K36me3 interaction leads to a mutator phenotype similar to that of cells defective in MMR genes. How are other MMR proteins recruited to chromatin? How do they interact with each other in vivo? Understanding these questions will provide critical information for clinical practice, including cancer diagnosis and therapy. Using cutting-edge technologies, we are studying the in vivo MMR reaction.
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