Activation of c-Ha-ras-1 proto-oncogene by in vitro modification with a chemical carcinogen, benzo(a)pyrene diol-epoxide.

Chemical carcinogenesis generally proceeds via the formation of strongly electrophilic reactants, termed ultimate carcinogens. The observation that many ultimate carcinogens are potent mutagens and the results of studies on the covalent binding of carcinogens to cellular macromolecules suggest that tumour initiation results from mutations arising from the binding of ultimate carcinogens to DNA. Recently, gene transfer experiments have shown that some tumours contain activated oncogenes which are members of the ras gene family (reviewed in refs 6. 7) and which differ by single base pair substitutions from their non-transforming counterparts, the proto-oncogenes (see, for example, refs 8-11). Here we have used clones of the c-Ha-ras-1 proto-oncogene to show that reaction in vitro with an ultimate carcinogen generates a transforming oncogene when the modified DNA is introduced into NIH 3T3 cells. As DNA is the only cellular macromolecule present in the reactions, our experiments also show that reaction of an ultimate carcinogen with DNA alone can lead to the induction of mutations in mammalian cells.