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As opposed to factors that control sensitivity to the acute cytotoxic effect of cisplatin (DDP), little is known about the factors that determine the rate at which resistance develops. This study examined how loss of p53 or DNA mismatch repair (MMR) function affected the rate of development of resistance to DDP in human colon carcinoma cells during sequential cycles of DDP exposure that mimic the way the drug is used in the clinic. We used a panel of 4 sublines molecularly engineered to express either the MMR- and p53-proficient phenotype or singly or doubly-deficient phenotypes. Loss of MMR alone increased the rate of development of resistance to DDP by 1.6-fold; however, loss of both MMR and p53 increased the rate by 3.0-fold. In studies directed at identifying the mechanism of this effect, loss of p53 or MMR was found to increase the steady-state level of REV3, the catalytic subunit of Pol ζ, and of REV1 mRNA; loss of both functions increased these levels much further by a factor of 20.2-fold for REV3 and 10.3-fold for REV1. The basal level of homologous recombination measured using a reporter vector was 1.3 - 1.7-fold higher in cells that had lost either p53 or MMR function, and 2.6-fold higher in cells that had lost both. DDP induced a 17-fold increase in homologous recombination between sequences that did not contain DDP adducts in the p53 and MMR-proficient cells; the magnitude of induction was even greater in cells that had lost either one or both functions. We conclude that, separate from effects on sensitivity to the cytotoxic effect of DDP, loss of MMR, especially when combined with loss of 53, results in rapid evolution of DDP resistance during sequential rounds of drug exposure. This is likely mediated by enhanced mutagenic translesion synthesis. DDP also activates homologous recombination between adduct-free sequences in a p53- and MMR-dependent manner whose time course is similar to the effect of DDP on cell cycle progression.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]