G TO A TRANSITION AND G TO T TRANSVERSIONS IN CODON-12 OF THE KI-RAS ONCOGENE ISOLATED FROM MOUSE LUNG-TUMORS INDUCED BY 4-(METHYLNITROSAMINO)-1-(3-PYRIDYL)-1-BUTANONE (NNK) AND RELATED DNA METHYLATING AND PYRIDYLOXOBUTYLATING AGENTS

Lung tumors were induced in A/J mice by the tobacco-specific nitrosami ne 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and the relate d compounds acetoxymethylmethylnitrosamine (AMMN) and acetoxymethylnit rosamino)-1-(3-pyridyl)-1-butanone (NNKOAc). NNK both methylates and p yridyloxobutylates DNA while AMMN and NNKOAc only methylate or pyridyl oxobutylate DNA, respectively. The lung tumors were analyzed for mutat ions in the Ki-ras oncogene by PCR amplification followed by either re striction fragment length polymorphism, hybridization, or sequencing p rocedures. NNK induced GGT to GAT mutations in codon 12 (26 of 28 samp les analyzed). AMMN induced GGT to GAT mutations in 18 of 18 samples. In contrast, NNKOAc induced a variety of changes including GGT to GAT (8/21), GGT to TGT (5/21) and GGT to GTT (4/21) mutations. These resul ts demonstrate that DNA methylation causes mainly G to A transitions i n the Ki-ras gene of A/J mouse lung tumors, consistent with previous r esults and a role for O6-methylguanine, while DNA pyridyloxobutylation induces G to A transitions as well as G to T transversions, perhaps d ue to the steric bulk of the adducts which are formed. The results are discussed with respect to mutations observed in rodent and human lung tumors.