Fast repair of O(6)-ethylguanine, but not O(6)-methylguanine, in transcribed genes prevents mutation of H-ras in rat mammary tumorigenesis induced by ethylnitrosourea in place of methylnitrosourea

Differential repair of structurally distinct mutagenic lesions in critical genes may influence the cellular risk of malignant conversion. We have investigated rat mammary tumorigenesis induced by N-ethyl-N-nitrosourea (EtNU) versus N-methyl-N-nitrosourea (MeNU) with respect to tumor incidence, ras gene mutation, and gene-specific repair. Both carcinogens induced mammary adenocarcinomas at high yield. In mammary epithelia (very low expression of O(6)-alkylguanine-DNA alkyltransferase, MGMT), O(6)-methylguanine (O(6)-MeGua) was eliminated from transcribed (H-ras and β-actin) and inactive genes (IgE heavy chain) at the same slow rate as determined for bulk genomic DNA. The persistence of O(6)-MeGua in DNA correlated with a high frequency of G:C → A:T transition mutations at codon 12 of the H-ras gene in MeNU-induced tumors. Repair of O(6)-ethylguanine (O(6)-EtGua), too, was slow in the IgE heavy chain gene as in bulk DNA. Contrasting with O(6)-MeGua, however, O(6)-EtGua was removed ≈20 times faster from the active H-ras and β-actin genes via MGMT-independent mechanism(s). Accordingly, no H-ras codon 12 mutations were found in EtNU-induced tumors, and 5- to 8-fold surplus alkyltransferase activity of the mammary epithelia—via a bacterial ada transgene—did not significantly counteract tumorigenesis in EtNU-exposed contrary to MeNU-treated animals. Neither MeNU- nor EtNU-induced tumors exhibited mutations at codons 13 and 61 of H-ras or codons 12, 13, and 61 of K-ras. Fast repair of O(6)-EtGua, but not O(6)-MeGua, in transcribed genes thus prevents mutational activation of H-ras when rat mammary carcinogenesis is initiated by EtNU in place of MeNU.