Mismatch repair and differential sensitivity of mouse and human cells to methylating agents

The long-patch mismatch repair pathway contributes to the cytotoxic effect of methylating agents and loss of this pathway confers tolerance to DNA met hylation damage. Two methylation-tolerant mouse cell lines were identified and were shown to be defective in the MSH2 protein by in vitro mismatch rep air assay. A normal copy of the human MSH2 gene, introduced by transfer of human chromosome 2, reversed the methylation tolerance. These mismatch repa ir defective mouse cells together with a fibroblast cell line derived from an MSH2(-/-) mouse, were all as resistant to N-methyl-N-nitrosourea as repa ir-defective human cells. Although long-patch mismatch repair-defective hum an cells were 50- to 100-fold more resistant to methylating agents than rep air-proficient cells, loss of the same pathway from mouse cells conferred o nly a 3-fold increase. This discrepancy was accounted for by the intrinsic N-methyl-N-nitrosourea resistance of normal or transformed mouse cells comp ared with human cells, The >20-fold differential resistance between mouse a nd human cells could not be explained by the levels of either DNA methylati on damage or the repair enzyme O-6-methylguanine-DNA methyltransferase, The resistance of mouse cells to N-methyl-N-nitrosourea was selective and no c ross-resistance to unrelated DNA damaging agents was observed. Pathways of apoptosis were apparently intact and functional after exposure to either N- methyl-N-nitrosourea or ultraviolet light. Extracts of mouse cells were fou nd to perform 2-fold less long-patch mismatch repair. The reduced level of mismatch repair may contribute to their lack of sensitivity to DNA methylat ion damage.