Mutational spectrum of dimethylnitrosamine in the liver of 3- and 6-week-old lacI transgenic mice.

We determined the spectrum of mutations in the lacI gene in the liver of Big Blue(R) transgenic mice after exposure to five daily doses of 2 mg/kg dimethylnitrosamine (DMN) at 3 and 6 weeks of age. This dose has been reported to increase the mutant frequency 9-fold when the animals are 3 weeks old. The lacI mutations recovered when treated at 3 weeks consist of mainly G:C --> A:T transitions, predominantly at non-CpG sites, and thus are consistent with mutagenesis by DMN. No increase in mutant frequency was reported when the mice were treated at 6 weeks of age. As we have previously shown that changes in mutational spectrum can be detected even when no statistically significant increase in mutant frequency is seen, we also examined the spectrum after treatment at 6 weeks. No changes from the spontaneous spectrum were detected. The comparison of the outcome of DMN treatment at 3 and 6 weeks confirms a change in metabolic activation, adduct removal, or mutation fixation between 3 and 6 weeks of age.