The Devil is in the details for DNA mismatch repair

Abstract

Ensuring high-fidelity DNA replication is essential for maintaining genome stability in organisms from Escherichia coli to humans. This task requires an intricate network of cellular components that carries out replication and postreplication DNA repair (1). In eukaryotes, replication is carried out by Pol e and Pol δ that synthesize the leading and lagging strands, respectively, using short RNA–DNA primers synthesized by Pol α. Although replication gives rise to a low spontaneous mutation rate of ∼10−10 mutations per base pair per generation, errors in the nucleotide incorporation step occur about once every 104 to 105 insertions on average, although the frequency varies considerably depending on a number of parameters including the particular mismatch, chromosomal context, and the DNA polymerase. In addition to stringent selectivity at each step of nucleotide incorporation, high-fidelity DNA polymerases possess 3′-endonuclease activities that act as proofreaders, removing errant bases whose abnormal geometries act as speed bumps, allowing excision of the incorrect base and insertion of the correct one. Working closely with the replication machinery is a postreplication DNA repair pathway: DNA mismatch repair (MMR). MMR targets replication errors that have escaped proofreading by excising a region that contains the mismatched base(s) on the newly synthesized strand and giving a high-fidelity DNA polymerase a second chance. In cells whose MMR function is compromised by mutation or epigenetic silencing a hypermutator phenotype ensues. Loss of MMR results in inherited cancer susceptibility (e.g., Lynch syndrome), as well as an increased incidence of sporadic cancers (2). MMR has been reconstituted with a heteroduplex (mismatched) DNA substrate and purified proteins from E. coli and subsequently with yeast and human proteins (3⇓–5). Until now, eukaryotic systems have all used Pol δ. In PNAS, Bowen and Kolodner report the first reconstitution with purified Saccharomyces cerevisiae proteins of 5′ … [↵][1]1To whom correspondence should be addressed. Email: peggyh{at}niddk.nih.gov. [1]: #xref-corresp-1-1