Organ-specific effects of DNA methylation by alkylating agents in the inbred Swiss mouse.

Young adult inbred Swiss mice given single or repeated equitoxic doses of N-methyl-N-nitrosourea (MNUA) or methyl methanesulphonate (MMS) develop thymomas and pulmonary adenomas only following MNUA in spite of nearly identical overall alkylation of DNA of tumour target tissues by both agents due mainly to the biologically ineffective product 7-methylguanine. The main difference in DNA alkylation was the production of O6-methylguinine, a known pre-mutagenic product, by MNUA in amounts 10 or more times as large as following MMS. This supports the possibility that somatic mutations are a part of the process of carcinogenesis.