DNA adducts,mutant frequencies,and mutation spectra in various organs of λlacZ mice exposed to ethylating agents

To investigate tissue-specific relations between DNA adducts and mutagenesis in vivo, λlacZ transgenic mice were treated i.p. with N-ethyl-N-nitrosourea (ENU), diethylnitrosamine (DEN), and ethyl methanesulphonate (EMS). In liver, bone marrow, and brain DNA from mice sacrificed at several time points after treatment O⁶-ethylguanine (O⁶-EtG) and N7-ethylguanine (N7-EtG) levels were determined as well as the mutant frequency (MF) in lacZ. In liver DNA of ENU- and DEN-treated mice, the bulk of O⁶-EtG was removed at 3 days after treatment, while the MF continued to increase thereafter. This suggests that O⁶-EtG is not the major premutagenic lesion in the liver. Indeed, sequence analysis of mutants showed only 24% GC → AT transitions, consistent with the O⁶-EtG lesion, and 28% TA → AT transversions, expected from O²-ethylthymine. In bone marrow after ENU treatment, a maximum mutation induction occurred at 3 days post-treatment, of which 43% were GC → AT mutations and 22% were TA → AT mutations. This suggests that in bone marrow O⁶-EtG may be a major premutagenic lesion at the 3-day time point. In liver and bone marrow, EMS treatment gave rise to a high level of N7-EtG and a low level of O⁶-EtG but no increase in MF. No adducts or mutation induction were observed in bone marrow of DEN-treated mice. No MF increase was observed in the brain of either ENU- or EMS-treated mice, although O⁶- and N7-adducts were present. Environ. Mol. Mutagen. 31:18–31, 1998. © 1998 Wiley-Liss, Inc.