DNA adduct and mutational profiles reveal a threshold of cellular defenses against N-nitrosodimethylamine administered to mice in drinking water.

N-Nitrosodimethylamine (NDMA) is classified as an animal and probable human carcinogen. Murine liver DNA adducts, mutations, O (6)-methylguanine DNA methyltransferase (MGMT), and CYP2E1 were evaluated following chronic administration of NDMA in drinking water. In a dose-escalation study, 7-methylguanine (m7G) increased linearly with NDMA dose. O (6)-Methylguanine (m6G) remained near background for NDMA doses up to approximately 1 ppm, beyond which its level, and corresponding mutations, rose steeply. An extended study was done with 5 ppm NDMA, in which adducts were measured at 3 and 10 weeks and mutations at 10 weeks. We found that both sexes experienced elevated levels of point mutations closely tracking with the levels of m6G, which emerged as the dominant mutagenic adduct under chronic dosing with NDMA. Homologous recombination-mediated chromosomal rearrangements, however, did not increase over background. Mutational analysis over 96 trinucleotide contexts revealed predominantly GC-->AT mutations in 5'-purine-G-3' contexts in a pattern matching human COSMIC cancer mutational signature SBS11, with secondary features resembling SBS119 (AT-->GC). Moreover, we identified the m6G level ( approximately 2000 adducts/diploid genome) above which its dedicated repair protein, MGMT, became saturated. The coordinated application of DNA adduct, mutational, and biochemical analyses provides a new approach for studying mechanisms of carcinogenesis, with relevance to early cancer detection and cancer management.