Molecular dosimetry of endogenous and exogenous O(6)-methyl-dG and N7-methyl-G adducts following low dose [D3]-methylnitrosourea exposures in cultured human cells.

For DNA-reactive chemicals, a low dose linear assessment of cancer risk is the science policy default. In the present study, we quantitated the endogenous and exogenous N7-methyl-G and O(6)-methyl-dG adducts in human lymphoblastoid cells exposed to low dose [D3]-methylnitrosourea. Endogenous amounts of both adducts remained nearly constant, while the exogenous adducts showed linear dose-responses. The data show that O(6)-methyl-dG adducts ≥1.8/10(8) dG correlated with published studies that demonstrated significant increases of mutations under these conditions. The combined results do not support linear extrapolations to zero when data are available for science-based regulations.