Mutations in the human mismatch repair (MMR) genes are associated with hereditary non-polyposis colorectal cancer as well as other sporadic cancers. MMR gene mutations have been implicated in the resistance of human tumours to cisplatin and several tumour-derived MMR-deficient cells show cisplatin resistance in vitro. In addition, hypoxia, a common feature of the tumour microenvironment, has been shown to influence tumour responses to conventional cancer treatments. We have examined the role of the mMSH2 MMR protein on repair of cisplatin-damaged DNA and cisplatin sensitivity in mMSH2-deficient murine fibroblasts and mMSH2-proficient controls under conditions of normoxia and hypoxia. Sensitivity to cisplatin was measured using the MTT assay and clonogenic survival. Repair of cisplatin-damaged DNA was measured using a host cell reactivation (HCR) assay employing a non-replicating recombinant virus expressing the β-galactosidase reporter gene. Sensitivity to cisplatin was significantly less and HCR of the cisplatin-damaged reporter gene was significantly greater in SV40-transformed mMSH2-deficient cells (MS5-7) compared to mMSH2-proficient controls (BC1-6) under both normoxic and hypoxic conditions. In contrast, sensitivity to cisplatin was significantly greater and HCR was similar in primary mMSH2-deficient compared to mMSH2-proficient murine fibroblasts under both normoxic and hypoxic conditions. Sensitivity to cisplatin was also significantly greater and HCR was similar in primary mMSH2-deficient compared to mMSH2-proficient murine fibroblasts transfected with a control plasmid under both normoxic and hypoxic conditions. In contrast, sensitivity to cisplatin was less and HCR was similar in primary mMSH2-deficient compared to mMSH2-proficient murine fibroblasts transfected with a plasmid expressing SV40 large T antigen under both normoxic and hypoxic conditions. These results suggest that loss of MMR alone does not result in increased resistance to cisplatin in murine fibroblasts and that additional concomitant alterations in cells expressing the SV40 large T antigen are responsible for cisplatin resistance through a modulation of DNA repair capacity and/or apoptosis.

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