This case–control study demonstrated that plasma estrogen-DNA adduct ratios were associated with breast cancer in Taiwan. The multiple logistic regression model indicated that a unit increase in the natural log of the estrogen-DNA adduct ratio was a significant predictor of breast cancer risk in premenopausal women, with an estimated hazard ratio of 1.718. Age, alcohol consumption, and the number of births were associated with estrogen-DNA adduct ratios with both a Student t test and ANOVA, but only age was statistically significant with the Mann–Whitney U test or Kruskal–Wallis test. Of the three estrogen metabolism-related genes, only the COMT SNP was associated with estrogen-DNA adduct ratios using the Kruskal–Wallis test. With regard to breast cancer, the ER/HER2 subtypes were not associated with estrogen-DNA adduct ratio.
In postmenopausal women, studies have demonstrated a positive association between breast cancer risk and circulating concentrations of estrogens. A reanalysis of nine prospective studies indicated that the relative risk of breast cancer for postmenopausal women with increasing quintiles of estradiol concentrations, relative to the lowest quintile, were 1.42, 1.21, 1.80, and 2.00 (Ptrend < 0.001) . For premenopausal women, evidence is limited because hormone measurements are complicated by the dynamic change of serum estrogen levels across menstrual cycles. However, a collaborative reanalysis of seven prospective studies demonstrated that breast cancer risk was associated with a doubling in concentrations of estradiol (OR 1.19, 95% CI: 1.06–1.35), but the association in premenopausal women was modest, and these seven studies were conducted in Western countries . The present study indicated a strong association between estrogen-DNA adduct ratios and breast cancer risk in premenopausal women. In contrast to serum estrogen levels, the timing of the blood sampling for estrogen-DNA adduct measurements was not limited by menstruation cycles
A pilot study published in 2008 demonstrated that urinary estrogen-DNA adduct ratios were higher in women at high risk of breast cancer (n = 12) and with breast cancer (n = 17) than in control group participants (n = 46). This finding was validated in a study with a larger number of participants (women at high risk of breast cancer: n = 40; women with breast cancer: n = 40; controls: n = 40) . Serum samples were measured in a third study, demonstrating that an estrogen-DNA adduct ratio ≥ 77 predicted breast cancer risk in logistic regression models (women at high risk of breast cancer: n = 63; women with breast cancer: n = 80; controls: n = 79) .
The present study differed from the previous three studies with respect to participants, sample type, and the subgroup analysis by menopausal status. With regard to participants, the previous three studies enrolled women from Italy and the United States whereas the present study enrolled women from Taiwan. The differences in estrogen profiles and metabolism between Asian, Asian American, and Caucasian women have been demonstrated in previous studies [32–35]; therefore, conducting a large-scale case–control study with Asian participants is crucial to verify whether estrogen-DNA adduct ratio can serve as a predictive biomarker for breast cancer risk in Asian women. In addition, a 5-year Gail Model score of ≥ 1.66% was used to define a high breast cancer risk in the previous three studies, but in the present study, histology was used to identify proliferative disease, which is a more stringent criteria for defining high risk groups. Regarding sample type, plasma samples were used in the present study whereas urine samples or serum samples were used in the previous three studies. Furthermore, we conducted a subgroup to stratify menopausal status because of the rapidly increasing incidence of premenopausal breast cancer with a unique tumor biology in East Asian patients. The present study demonstrated that estrogen-DNA adduct ratios were significantly associated with breast cancer risk in premenopausal women. The previous three studies did not identify significant interactions between estrogen-DNA adduct ratios and menopausal status in terms of risk prediction for breast cancer.
In contrast to the three earlier studies, we examined three gene polymorphisms related to estrogen metabolism. Of these, only COMT polymorphism exhibited a marginal association with estrogen-DNA adduct ratios. COMT catalyzes an inactivation pathway for catechol estrogen. The previous in vitro study indicated that a pretreatment of 2,3,7,8-tetrachlorodibenzo-p-dioxin followed by the inhibition of COMT activity increased the formation of depurinating 4-OHE1(E2)-1-N3Ade and 4-OHE1(E2)-1-N7Gua adducts in human breast epithelial cells . With regard to the SNP, the COMT gene Met/Met homozygotes yielded a 3-to-4-fold reduction in COMT activity relative to the Val/Val homozygotes, and Val/Met heterozygotes demonstrated intermediate activity . The frequency of the variant (low-activity) COMT allele was lower in Asian women than in Caucasian women (summarized in a previous article) . In the present study, the estrogen-DNA adduct ratios varied in the three COMT SNPs (heterogeneity of the SNP groups: P = 0.025 using the Kruskal–Wallis test), and the Val/Met group had the highest ratio (mean ratios: Val/Val, 41.28; Val/Met, 54.32; Met/Met, 48.61). However, each of the three statistical comparisons between the two COMT genotypes did not reach statistical significance (data not shown). A potential explanation is that interaction of environmental pollutant exposure or lifestyle factors with decreased COMT activity, but not COMT itself, contributes to production of estrogen-DNA adducts.
To evaluate whether the formation of estrogen-DNA adducts preferentially contributes to ER-positive breast cancer, we examined the association between estrogen-DNA adduct ratios and ER/HER2 status. However, we were unable to identify a significant association. Estrogen-DNA adducts might mediate the initial steps of breast carcinogenesis in various pluripotent cell types, and therefore, estrogen-DNA adduct ratios could differentiate between the control group participants and the high breast cancer risk participants regardless of ER/HER2 status.
In summary, our study suggests that the carcinogenesis of rapidly increasing incidence of breast cancer in young women in Taiwan or other East Asian countries is related to the formation of estrogen-DNA adducts, and estrogen-DNA adduct ratios can act as biomarkers for breast cancer risk.