The accumulation and persistence of somatic mutations in vivo

Mutant frequencies at the native Dlb-1 and the lacI transgene reach a plateau within 1 week of mutagenesis in the small intestine of mice. This indicates that these mutations are selectively neutral and should accumulate additively. In a test with three mutagens, one potent (ethylnitrosourea) and two weak (1,2-dimethylhydrazine and methyl methanesulphonate), ten weekly treatments induced ten times the frequency of Dlb-1 mutations induced by a single treatment in each case, a result that is consistent with perfect additivity. These observations indicate that the mutation frequency at neutral loci will be an integrated measure of the effects of aging, endogenous mutagens, environmental mutagens and antimutagens. They suggest a simple protocol for transgenic mutation assays which would increase sensitivity, require fewer animals, and conform to standard toxicological practice.