The Crosstalk Between DNA Mismatch Repair and Replication




Zhang, Junqiu

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DNA replication fidelity relies on DNA mismatch repair (MMR) and the proofreading nuclease activity of DNA polymerases. Normally, biosynthetic errors can be removed by the polymerase's proofreading nuclease activity upon their incorporation, and those errors that have escaped the proofreading nuclease are corrected by MMR. However, this model is challenged by the fact that cells expressing a proofreading-deficient P286R polymerase ɛ (Polɛ-P286R) display a hypermutable phenotype usually seen in MMR-deficient cells, implying the blockage of MMR function by Polɛ-P286R. We show here that consistent with frequent misincorporation by Polɛ-P286R, elevated levels of MMR proteins were found in replicating DNA/chromatin in Polɛ-P286R cells, but this does not result in a reduced mutation frequency, suggesting that cluster binding of MMR proteins at the replication fork inhibits MMR. Instead, the high-level binding of MMR proteins blocks the recruitment of fork protection factors FANCD2 and BRCA1 to replication forks, and promotes MRE11-catalyzed nascent strand degradation. This MMR-dependent degradation causes DNA breaks and chromosome abnormalities, thereby promoting an ultramutator phenotype. Therefore, our findings identify a novel MMR function in triggering replication stress response to promote genome instability when replication forks are filled with biosynthetic errors. The importance of MMR in maintaining genome stability prompts us to further study the mechanism of MMR in vitro, particularly how the MMR initiation complex is formed in response to misincorporation. Using purified recombinant proteins, we assembled MMR initiation complex in vitro and visualized protein-protein and protein-DNA interactions under transmission electron microscopy. These analyses allowed us to gain molecular insights into the mechanism of MMR initiation.

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