Repair of cytotoxic lesions introduced into DNA by methylating agents.

We have investigated the biological role of O6-methylguanine and methylphosphotriesters in the DNA of mammalian cells. Our approach has been to express Escherichia coli (E. coli) DNA repair activities of well-defined specificity in Chinese hamster cells. Expression of O6-methylguanine-DNA methyltransferase is necessary and sufficient to confer resistance to the cytotoxic action of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) indicating the potential involvement of O6-methylguanine in cell killing by this compound. We present evidence that methylphosphotriesters in DNA do not constitute a cytotoxic threat. Despite this, cell lines resistant to MNNG may display elevated expression of a methylphosphotriester repair function. This may be the result of fortuitous co-amplification of transfected sequences and indicates that care should be exercised in correlating resistance to methylating agents with particular DNA repair enzymes.