Killing of chromium-damaged cells by mismatch repair and its relevance to carcinogenesis.

Hexavalent chromium compounds are widespread environmental contaminants that are well recognized as human carcinogens and potent respiratory toxicants. Intracellular metabolism of chromium(VI) leads to the production of numerous chromium-DNA adducts that are primarily formed at the phosphate groups. The mechanism of toxicity of these DNA modifications in human cells has been uncertain for a long time because chromium and other phosphate-based adducts did not block DNA replication with purified polymerases. Our recent studies identified mismatch repair proteins as activators of toxic responses to chromium-DNA damage, which resolved an apparent discrepancy in genotoxic activity of chromium adducts in cells and in vitro. The discovered mechanism of toxicity provided the basis for a novel model of chromium carcinogenesis based on the selection of resistant clones that lack mismatch repair and progress to cancer due to high levels of spontaneous mutagenesis.