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

Abstract

N-Nitrosodimethylamine (NDMA) is classified as an animal and probable human carcinogen. Murine liver DNA adducts, mutations, 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. O6-Methylguanine (m6G) remained near background for NDMA doses up to [~]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 weeks and mutations at 10 weeks. While the level of CYP2E1 was unchanged, MGMT gene transcription was induced in females at 10 weeks. Homologous recombination-mediated chromosomal rearrangements did not increase over background. Point mutations, however, were elevated substantially in both sexes. 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). Taken together, the data implicate m6G as the dominant mutagenic adduct under chronic dosing with NDMA. Furthermore, the genomic m6G level was identified at which its dedicated repair protein, MGMT, became saturated. The coordinated application of DNA adduct, mutational and biochemical analyses provides a new approach for early detection and cancer management. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=103 SRC="FIGDIR/small/687536v1_ufig1.gif" ALT="Figure 1"> View larger version (34K): org.highwire.dtl.DTLVardef@1e6ae72org.highwire.dtl.DTLVardef@1c6fad6org.highwire.dtl.DTLVardef@7e3716org.highwire.dtl.DTLVardef@52a87b_HPS_FORMAT_FIGEXP M_FIG C_FIG