Mismatch G-T binding activity and MSH2 expression is quantitatively related to sensitivity of cells to methylating agents.

To elucidate mechanisms involved in alkylating drug resistance, Chinese hamster cells resistant to methylating agents have been generated upon transfection with human DNA. Here it is shown that these Chinese hamster ovary (CHO) variants exhibit the tolerance phenotype: they are alkyltransferase deficient (Mex-), cross-resistant to 6-thioguanine, exhibit reduced G-T binding (MutS alpha) activity and express the mismatch repair protein MSH2 at a significantly lower level than the corresponding control. By comparing wild-type cells with different tolerant strains that show gradual differences in resistance to methylating agents, it was shown that both the G-T binding activity and the amount of MSH2 protein inversely correlates with the level of methylating drug resistance. Although the tolerant cell variants analysed express MSH2 at a significantly lower level than the wild-type, MSH2 mRNA expression was not impaired. Furthermore, MSH2 promoter activity was not reduced upon transient transfection into tolerant cells. The results indicate that quantitative differences in expression of components of mismatch repair do exist in mammalian cells that affect cell survival upon methylation. It appears that post-transcriptional mechanisms are involved in regulation of MSH2 expression.