DNA mismatch repair protein Mlh1 is required for tetravalent chromium intermediate-induced DNA damage

Hexavalent chromium (Cr[VI]) is associated with occupational lung cancer and poses a significant public health concern. When exposed to Cr[VI], cells rapidly internalize this compound and metabolize it to Cr[III]. Byproducts of Cr[VI] metabolism include unstable Cr[V] and Cr[IV] intermediates that are believed to be directly responsible for the genotoxicity and carcinogenicity caused by Cr[VI] exposure; however, the carcinogenic potential of the Cr intermediates and the mechanisms of Cr-induced carcinogenesis remain to be further defined. Utilizing synthetic Cr[IV] and Cr[V] compounds, we demonstrate here that Cr[IV] or Cr[V] exposure induces DNA double-strand breaks; however, of the two compounds, mammalian cells only respond to Cr[V]-induced DNA damage. Exposure to Cr[V], but not Cr[IV], results in initiation of cell cycle checkpoints and activates the ATM kinase, a critical regulator of the DNA damage response. Furthermore, cells exposed to Cr[IV] have significantly increased mutation frequencies in the HPRT gene compared to cells exposed to Cr[V], indicating that Cr[IV] possesses a higher mutagenic potential than Cr[V]. We also find that MLH1, a critical mismatch repair (MMR) protein, is required for activation of the G2/M cell cycle checkpoint in response to Cr[VI] exposure and to limit Cr-induced mutagenesis. Our results provide evidence for Cr[IV] as the ultimate mutagenic intermediate produced during Cr[VI] metabolism and indicate that functional MMR is crucial in the cellular response to chromium exposure.