Role of ras protooncogene activation in the formation of spontaneous and nitrosamine-induced lung tumors in the resistant C3H mouse.

The role of ras activation in the formation of spontaneous and chemically induced tumors was evaluated in the C3H mouse, a strain that has a low incidence of spontaneous lung tumors. Lung tumors were induced in C3H mice by treatment with 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK), 50 mg/kg, or nitrosodimethylamine (NDMA), 3 mg/kg for 7 weeks (3 times/week, i.p.). Eleven tumors from each treatment group were evaluated for activated ras genes by direct sequencing and oligonucleotide hybridization to slot blots of amplified DNA from these tumors. An activated K-ras gene was detected in 100% of NDMA- and NNK-induced lung tumors, and the activating mutation detected in all samples was a GC to AT transition (GGT to GAT) in codon 12. In contrast, only 40% of the seven spontaneous lung tumors analyzed contained an activated K-ras gene and the mutations identified were not localized to either a specific base or codon. Both NNK and NDMA can be activated via alpha-hydroxylation to methylating agents. The GC to AT mutation observed in codon 12 in the nitrosamine-induced tumors is consistent with the formation of an O6-methylguanine (O6MG) adduct. Similar concentrations (13-15 pmoles/mumol deoxyguanosine) of this promutagenic adduct were detected in lungs during treatment with either NNK or NDMA. Thus, both these nitrosamines appear to activate the K-ras gene in lung through a direct genotoxic mechanism involving the formation of the O6MG adduct. The frequency of K-ras activation was similar in chemically induced lung tumors from the sensitive A/J strain and the C3H mouse, indicating that susceptibility for neoplasia in these stains is not related to the ability to activate this gene. Although tumors were induced in lung from 100% of C3H mice following chronic carcinogen exposure, both the size and the multiplicity was significantly less, while latency was longer than that observed in the A/J mouse. These differences could not be attributed to an altered propensity for DNA damage, but rather suggest that genetic loci which regulate clonal expansion and growth of initiated cells play a major role in the susceptibility of pulmonary neoplasia.