Cancer is a disease that begins with mutation of critical genes: oncogenes and tumor suppressor genes. Our research on carcinogenic aromatic hydrocarbons indicates that depurinating hydrocarbon-DNA adducts generate oncogenic mutations found in mouse skin papillomas (Proc. Natl. Acad. Sci. USA 92:10422, 1995). These mutations arise by mis-replication of unrepaired apurinic sites derived from the loss of depurinating adducts. This relationship led us to postulate that oxidation of the carcinogenic 4-hydroxy catechol estrogens (CE) of estrone (E 1 ) and estradiol (E 2 ) to catechol estrogen-3,4-quinones (CE-3, 4-Q) results in electrophilic intermediates that covalently bind to DNA to form depurinating adducts. The resultant apurinic sites in critical genes can generate mutations that may initiate various human cancers. The noncarcinogenic 2-hydroxy CE are oxidized to CE-2,3-Q and form only stable DNA adducts. As reported here, the CE-3,4-Q were bound to DNA in vitro to form the depurinating adduct 4-OHE 1 (E 2 )-1(␣,␤)-N7Gua at 59-213 mol͞mol DNA-phosphate whereas the level of stable adducts was 0.1 mol͞mol DNA-phosphate. In female Sprague-Dawley rats treated by intramammillary injection of E 2 -3,4-Q (200 nmol) at four mammary glands, the mammary tissue contained 2.3 mol 4-OHE 2 -1(␣,␤)-N7Gua͞molDNA-phosphate. When 4-OHE 1 (E 2 ) were activated by horseradish peroxidase, lactoperoxidase, or cytochrome P450, 87-440 mol of 4-OHE 1 (E 2 )-1(␣, ␤)-N7Gua was formed. After treatment with 4-OHE 2 , rat mammary tissue contained 1.4 mol of adduct͞mol DNA-phosphate. In each case, the level of stable adducts was negligible. These results, complemented by other data, strongly support the hypothesis that CE-3,4-Q are endogenous tumor initiators.

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Catechol estrogens and catecholamines are metabolized to quinones, and the metabolite catechol (1,2-dihydroxybenzene) of the leukemogenic benzene can also be oxidized to its quinone. We report here that quinones obtained by enzymatic oxidation of catechol and dopamine with horseradish peroxidase, tyrosinase or phenobarbital-induced rat liver microsomes react with DNA by 1,4-Michael addition to form predominantly depurinating adducts at the N-7 of guanine and the N-3 of adenine. These adducts are analogous to the ones formed with DNA by enzymatically oxidized 4-catechol estrogens (Cavalieri,E.L., et al. (1997) PROC: Natl Acad. Sci., 94, 10937). The adducts were identified by comparison with standard adducts synthesized by reaction of catechol quinone or dopamine quinone with deoxyguanosine or adenine. We hypothesize that mutations induced by apurinic sites, generated by the depurinating adducts, may initiate cancer by benzene and estrogens, and some neurodegenerative diseases (e.g. Parkinson's disease) by dopamine. These data suggest that there is a unifying molecular mechanism, namely, formation of specific depurinating DNA adducts at the N-7 of guanine and N-3 of adenine, that could initiate many cancers and neurodegenerative diseases.

Exposure to estrogens has been associated with an increased risk of developing breast cancer. Breast biopsy tissues from 49 women without breast cancer (controls) and 28 with breast carcinoma (cases) were analyzed by HPLC with electrochemical detection for 31 estrogen metabolites and catechol estrogen quinone-glutathione conjugates. The levels of estrone and estradiol were higher in cases. More 2-catechol estrogen (CE) than 4-CE was observed in controls, but the 4-CE were three times higher than 2-CE in cases. In addition, the 4-CE were nearly four times higher in cases than in controls. Less O-methylation was observed for the CE in cases. The level of catechol estrogen quinone conjugates in cases was three times that in controls, suggesting in the cases a higher probability for the quinones to react with DNA and generate mutations that may initiate cancer. The levels of 4-CE and quinone conjugates were highly significant predictors of breast cancer. These results suggest that some catechol estrogen metabolites and conjugates could serve as biomarkers to predict risk of breast cancer.