COMPETENCE IN MISMATCH REPAIR PROHIBITS CLONAL EXPANSION OF CANCER-CELLS TREATED WITH N-METHYL-N'-NITRO-N-NITROSOGUANIDINE

The phenomenon of alkylaton tolerance has been observed in cells that are deficient in some component of the DNA mismatch repair (MMR) syste m. An alkylation-induced cell cycle arrest had been reported previousl y in one MMR-proficient cell line, whereas a MMR-defective clone deriv ed from this line escapes from this arrest. We examined human cancer c ell lines to determine if the cell cycle arrest were dependent upon th e MMR system, Growth characteristics and cell cycle analysis after MNN G treatment were ascertained in seven MMR-deficient and proficient cel l lines, with and without confirmed mutations in hMLH1 or hMSH2 by an in vitro transcription/translation assay. MMR-proficient cells underwe nt growth arrest in the G2 phase of the cell cycle after the first S p hase, whereas MMR-deficient cells escaped an initial G2 delay and resu med a normal growth pattern. In the HCT116 line corrected for defectiv e MMR by chromosome 3 transfer, the G2 phase arrest lasted more than f ive days. In another MMR-proficient colon cancer cell line, SW480, cel l death occurred five days after MNNG treatment. A competent MMR syste m appears to be necessary for G2 arrest or cell death after alkylation damage, and this cell cycle checkpoint may allow the cell to repair d amaged DNA, or prevent the replication of mutated DNA by prohibiting c lonal expansion.