Role of DNA repair in carcinogen-induced ras mutation

In this contribution we discuss the gene- and cell type-specific repair of miscoding DNA alkylation products as a risk parameter in both mutation induction and malignant transformation by N-nitroso carcinogens. Upon exposure to N-nitroso compounds such as N-methyl-N-nitrosourea (MeNU) or N-ethyl-N-nitrosourea (EtNC), about a dozen different alkylation products are formed in cellular DNA. Among these are O-6-methylguanine (O-6-MeGua) and O-6-ethylguanine (O-6-EtGua), respectively, which differ only by one CH, group in their alkyl residue and, when unrepaired, cause G:C --> A:T transition mutations by anomalous base pairing during DNA replication. We have analyzed the global and gene-specific repair of O-6-MeGua and O-6-EtGua in target cell DNA, ras gene mutation frequencies, and tumor incidence, in the model of mammary carcinogenesis induced in 50-day-old female Sprague-Dawley rats by a single application of MeNU or EtNU. Both carcinogens induce histologically indistinguishable mammary adenocarcinomas at high yield. In the target mammary epithelia, O-6-MeGua is repaired at similar slow rates in both transcriptionally active genes (Ha-ras, beta-actin), silent genes (lgE heavy chain), and in bulk DNA, by the one-step repair protein o(6)-alkylguanine-DNA alkyltransferase (MGMT; low level of expression in the target cells). The slow repair of O-6-MeGua translates into a high frequency of mutations at the central position of Ha-ras codon 12 (G (G) under bar A) in MeNU-induced tumors. O-6-EtGua, however, is removed similar to 20 times faster than O-6-MeGua selectively from transcribed genes via an MGMT independent, as yet uncharacterized excision mechanism. Accordingly, no Ha-uas codon 12 mutations are found in the EtNU-induced mammary tumors. Neither MeNU- nor EtNU-induced tumors exhibit mutations at codons 13 and 61 of Ha-ras or at codons 12, 13 and 61 of Ki-ras. While a moderate surplus MGMT activity of the target cells - contributed by a bacterial MGMT transgene (ada) - significantly counteracts mammary tumorigenesis in MeNU-exposed rats, this is not the case in the EtNU-treated animals. Differential repair of structurally distinct DNA lesions in transcribed or (temporarily) silent genes thus determines the probability of mutation and, together with cell type-specific and interindividual differences in DNA repair capacity, influences carcinogenic risk. (C) 2000 Elsevier Science B.V. All rights reserved.