Base-pair mismatches in DNA that arise from DNA replication errors can contribute to cancer. Eukaryotic DNA Mismatch Repair (MMR), which is essential to suppressing cancer, involves redundant pathways that either employ a DNA snipping enzyme, exonuclease 1 (Exo1), to remove mismatches or involve Exo1-independent pathways to do so. The latter are poorly understood. In a September paper in Nature Communications, Ludwig San Diego’s Richard Kolodner reported that in Saccharomyces cerevisiae (Baker’s yeast) the enzyme Rad27 defines an Exo1-independent pathway that removes mismatches during MMR. Though deletion of Rad27 was known to lead to the rapid accumulation of mutations across the yeast genome, its role in MMR has been controversial. One way to resolve the controversy would be to delete both Exo1 and Rad27 and examine the outcome, but such double mutations proved to be lethal. Richard and his team got around this problem by engineering yeast to encode an Exo1 enzyme that lacks segments—known as SHIP and MIP boxes—essential to MMR but retain parts required for its other life-sustaining functions when Rad27 is also inactive. This double mutant had much stronger MMR defects than either single mutant, demonstrating that Exo1 and Rad27 define redundant MMR pathways. The researchers also reconstituted the reaction involving Rad27 in vitro, offering insight into the mechanism of Rad27-dependent MMR. These findings will guide the search for similar Exo-1 independent MMR pathways in human cells.
This article appeared in the February 2022 issue of Ludwig Link. Click here to download a PDF (1 MB).