The detection and quantitation of carcinogen-DNA adducts in human cells are the key parameters in the molecular dosimetry of human exposure to environmental carcinogens. For investigating the possible relevance of alkylating N-nitroso compounds as causative agents in human carcinogenesis, we have quantitated O4-ethyl-2′-deoxythymidine (O4-EtdThd) in human liver DNA obtained from 33 autopsy specimens, i.e., 13 cases with primary liver cancer (LC), 8 with cancers other than liver cancer (OC), and 12 with noncancerous diseases (NC). None of the cases analyzed had a history of known occupational exposure to ethylating agents. The detection limit for O4-EtdThd was 3 × 10-8 as a O4-EtdThd/dThd molar ratio in DNA, which was attained by the combination of prefractionation of DNA hydrolysates (= 20 mg of DNA/sample) by high performance liquid chromatography and competitive radioimmunoassay using anti-(O4-EtdThd) monoclonal antibody ER-01. Except for one case in each group, O4-EtdThd [or, alternatively, (an) unidentified structural modification(s) of DNA recognized by monoclonal antibody ER-01] was detected at mean (± SD) O4-EtdThd/dThd molar ratios of 39.9 ± 40.2 × 10-8, 53.5 ± 74.0 × 10-8, and 11.7 ± 6.5 × 10-8, respectively, in LC, OC, and NC. The difference of the O4-EtdThd content in DNA between LC and NC, or between LC + OC and NC, was statistically significant at P < 0.05. These results suggest that humans are exposed to ethylating agents in vivo and that a premutagenic DNA lesion (O4-EtdThd) eventually accumulates in DNA, possibly to a biologically significant extent.
This work was supported in part by grants for Cancer Research and for Special Project Research, Cancer-Bioscience from the Ministry of Education, Science and Culture of Japan and by the Deutsche Forschungsgemeinschaft (SFB 102/A7).