Molecular dosimetry of DNA adducts in mice exposed to ethylene oxide

Ethylene oxide (EtO) is a highly reactive industrial chemical and known human carcinogen with a mutagenic mode of action (MOA). Its genotoxicity is primarily mediated through alkylation of DNA, forming the mutagenic adduct O6-(2-hydroxyethyl)-2'-deoxyguanosine (O6-HE-dG), albeit in small quantities, and the more abundant but less- or nonmutagenic N7-(2-hydroxyethyl)guanine (N7-HE-G) adduct. However, dose-response relationships of these DNA adducts, particularly at low inhalation exposure levels (< 3 ppm), remain unknown. These data are necessary to inform the biological plausibility of different statistical dose-response models that have been applied to human or animal data used for cancer risk assessment. In this study, B6C3F1 mice were exposed to EtO (0 to 200 ppm) for 6 h/d over 28 consecutive days. DNA adducts in lung, liver, bone marrow, and mammary gland were quantified using highly sensitive mass spectrometry platforms. N7-HE-G was detected in all tissues and exposure groups, showing linear dose-response relationships in the low-dose range (≤ 1 ppm) and increased sharply and exposure-disproportionately in the high-dose range (≥ 50 ppm). Despite high sensitivity, O6-HE-dG was undetectable in any tissue at exposure < 50 ppm, reflecting adduct levels that are below the current quantifiable limit. At higher exposures (≥ 50 ppm), O6-HE-dG exhibited a dose-response pattern of N7-HE-G. Notably, the mammary gland, despite being anatomically distant from the site of inhalation, exhibited the second-highest levels of both adducts at higher doses. This study provides the first reliable quantitative dose-response evidence of DNA adducts in tumor target and nontarget (liver) tissues across a wide range of EtO exposures. The two DNA adducts differ markedly in their abundance, repairability and mutagenic potential and together provide a molecular MOA dose-response framework to provide the biological foundation for informing quantitative cancer risk assessment and genotoxic hazard characterization.