Relationship between DNA methylation and mutational patterns induced by a sequence selective minor groove methylating agent

Me-lex, a methyl sulfonate ester appended to a neutral N-methylpyrrolecarbo xamide-based dipeptide, was synthesized to preferentially generate N-3-meth yladenine (3-MeA) adducts which are expected to be cytotoxic rather than mu tagenic DNA lesions. In the present study, the sequence specificity for DNA alkylation by Me-lex was determined in the p53 cDNA through the conversion of the adducted sites into single strand breaks and sequencing gel analysi s. In order to establish the mutagenic and lethal properties of Me-lex lesi ons, a yeast expression vector harboring the human wild-type p53 cDNA was t reated in vitro with Me-lex, and transfected into a yeast strain containing the ADE2 gene regulated by a p53-responsive promoter. The results showed t hat: 1) more than 99% of the lesions induced by Me-lex are 3-MeA; 2) the co -addition of distamycin quantitatively inhibited methylation at all minor g roove sites; 3) Me-lex selectively methylated A's that are in, or immediate ly adjacent to, the lex equilibrium binding sites; 4) all but 6 of the 33 i ndependent mutations were base pair substitutions, the majority of which (1 7/33; 52%) were AT-targeted; 5) AT --> TA transversions were the predominan t mutations observed (13/33; 39%); 6) 13 out of 38 (39%) independent mutati ons involved a single lex-binding site encompassing positions A(600-602), a nd 9 occurred at position 602 which is a real Me-lex mutation hotspot (n = 9, p < 10(-6), Poisson's normal distribution). A hypothetical model for the interpretation of mutational events at this site is proposed. The present work is the first report on mutational properties of Me-lex. Our results su ggest that 3-MeA is not only a cytotoxic but also a premutagenic lesion whi ch exerts this unexpected property in a strict sequence-dependent manner.