Studies Of DNA Mismatch Repair

National Institute Of Environmental Health Sciences

Investigators

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Abstract

This year, we made substantial progress on a key components of the system that repairs nuclear DNA replication errors, DNA Mismatch Repair. First, we investigated the role of the endonuclease activity of the yeast Pms1 protein in generating the signal for strand discrimination during mismatch repair. This signal is created by the endonuclease activity of Pms1, which directs mismatch repair by generating a nick in the newly replicated DNA strand. We used whole genome sequencing to show that loss of this endonuclease activity, via a pms1 variant, results in strong mutator effects throughout the Saccharomyces cerevisiae genome. Rates for these events are increased in strains combining pms1-DE with mutator variants of each of the three major leading and lagging strand replicases. In all cases, mutation rates, spectra, biases, and context preferences are statistically indistinguishable from strains with equivalent polymerases but lacking initial mismatch recognition due to deletion of MSH2. This implies that, across the nuclear genome, strand discrimination via the Pms1 endonuclease is as important for MMR as is initial mismatch recognition by MutS heterodimers. Second, in a mismatch repair-defective strain, we investigated a novel mechanism for generating mutations that we call transient misalignment. We provided evidence for four variants of the mechanism, all of which likely operate in yeast cells during nuclear DNA replication. The data suggest that transient misalignment also occurs in humans and may possibly explain some of the mutations observed in human tumors.