DNA MISMATCH BINDING DEFECTS, DNA-DAMAGE TOLERANCE, AND MUTATOR PHENOTYPES IN HUMAN COLORECTAL-CARCINOMA CELL-LINES

DNA mismatch binding ill vitro, resistance to DNA methylation damage, and spontaneous mutation rates were examined in human colorectal adeno carcinoma cell lines. Of 11 cell lines, 3 (DLD1, HCT15, and LoVo) were defective in mismatch binding. All three lines had a mutator phenotyp e. These properties indicate that DLD1 and HCT15 may, like LoVo, carry mutations in the mismatch recognition protein hMSH2. Mismatch binding was normal in the remaining eight lines, including HCT116 in which a second mismatch repair protein, hMLH1, is defective. Two lines, SW620 and SW48, did not express detectable levels of the DNA repair enzyme O -6-methylguanine-DNA methyltransferase. SW620 exhibited the expected s ensitivity to N-methyl-N-nitrosourea. In contrast, SW48 cells were hig hly resistant to N-methyl-N-nitrosourea and also slightly to methyl me thanesulfonate, indicating that they are tolerant to DNA methylation d amage. SW48 exhibited the spontaneous mutator phenotype and microsatel lite instability that are hallmarks of a defect in mismatch repair. Th is cell line provides evidence for the association between methylation tolerance and defective mismatch correction in human colorectal carci noma cells. The properties of methylation-tolerant, mismatch repair-de fective cells identify possible selective pressures that might facilit ate the natural selection of mismatch repair-defective tumors.