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O(6)-Methylguanine and O(6)-chloroethylguanine, which are the primary cytotoxic DNA lesions produced by 5-(3,3-dimethyl-1-triazeno)imidazole-4-carboxamide (dacarbazine) and 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU), respectively, can be repaired by O(6)-methylguanine-DNA methyltransferase (MGMT), coded by the MGMT gene. However, the two types of drugs exhibit different effects on cells defective in both MGMT and MLH1 functions, the latter being related to the cellular activity to recognize mismatched bases of DNA for inducing apoptosis. Human cells deficient in both MGMT and MLH1 are resistant to the killing effect of dacarbazine and exhibit an increased mutant frequency after treatment with dacarbazine. On the other hand, these doubly deficient cells are sensitive to the killing action of ACNU and there is no significant increase in ACNU-induced mutant frequency. A mismatch recognition complex, composed of MSH2, MSH6, MLH1, PMS2 and PCNA, is formed after exposing MGMT-deficient cells to dacarbazine, but not in cells treated with ACNU. In contrast, the phosphorylation of Chk1 efficiently occurs in cells treated with dacarbazine as well as with ACNU, the former being in MLH1-dependent manner, whereas the latter in MLH1-independent manner. Therefore, the signals delivered from different sources would merge at the step of Chk1 activation or at an earlier step, and the subsequent process leading to apoptosis appears to be common.

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