Characterization of MLH1 and MSH2 DNA mismatch repair proteins in cell lines of the NCI anticancer drug screen

Purpose and methods: The lack of a functional DNA mismatch repair (MMR) pat hway has been recognized as a common characteristic of several different ty pes of human cancers due to mutation affecting one of the MMR genes or due to promoter methylation gene silencing. These MMR-deficient cancers are fre quently resistant to alkylating agent chemotherapy such as DNA-methylating or platinum-containing, compounds. To correlate drug resistance with MMR st atus in a large panel of human tumor cell lines, we evaluated by Western bl ot the cellular levels of the two MMR proteins most commonly mutated in hum an cancers, MLH1 and MSH2, in the NCI human tumor cell line panel. This pan el consists of 60 cell lines distributed among nine different neoplastic di seases. Results: We found that in most of these cell lines both MLH1 and MS H2 were expressed, although at variable levels. Five cell lines (leukemia C CRF-CEM, colon HCT 116 and KM12 and ovarian cancers SK-OV-3 and IGROV-1) sh owed complete deficiency in MLH1 protein. MSH2 protein was detected in all 57 cell lines studied. Absence of MLH1 protein was always linked to resista nce to the methylating chemotherapeutic agent temozolomide. This resistance was independent of cellular levels of O-6-alkylguanine DNA alkyltransferas e. Based on data available for review in the NCI COMPARE database, cellular levels of MLH1 and MSH2 did not correlate significantly with sensitivity t o any standard anticancer drug or with any characterized molecular target a lready tested against the same panel of cell lines. Conclusion: Based on ev aluation of 60 tumor cell lines in the NCI anticancer drug screen, MLH1 def iciency was more common than MSH2 deficiency and was always associated with a high degree of temozolomide resistance. These data will enable correlati ons with other drug sensitivities and molecular targets in the COMPARE data base to evaluate linked processes in tumor drug resistance.