Abstract P1-07-10: Estrogen-stimulated DNA damage in mammary epithelium varies among strains of rodents and women
Authors: Dunphy Karen A. , Sharma Aman , Majhi Prabin , Schneider Sallie S , Crisi Giovanna M , Makari-Judson Grace , Jerry D Joseph 
Published: March 2022

Abstract
Background: Estrogens play an important role in breast development, but prolonged exposures and higher levels have been associated with increased risk of breast cancer. Recently, estrogens and environmental estrogenic chemicals have been shown to promote the formation of estrogen receptor-mediated co-transcriptional R-loops that lead to DNA double strand breaks in breast cancer cell lines. However, the DNA damage mediated by the estrogen receptors in the breast epithelium may vary among individuals. Experimental Design: Strains of mice and rats were used to investigate the genetic basis for variation in estrogen-induced DNA damage. The BALB/c and C57BL/6 strains of mice were selected as they differ in susceptibility to mammary tumors (Majhi et al., 2021). Similarly, the ACI and BN strains of rats differ in their susceptibility to mammary tumors induced by estrogen exposure. The mice and rats were treated with 17β-estradiol (E2) for up to 28 days, then tissues were analyzed for γH2AX foci to assess the presence of DNA double strand breaks. Normal breast tissues from average-risk and high-risk patients were maintained in culture (patient-derived explant cultures or PDEs) and exposed to either control media or supplemented with 10nM E2, then analyzed for γH2AX foci. Results: Proliferative responses were similar in both BALB/c and C57BL/6 mouse strains. However, DNA damage and R loops were significantly higher in the mammary epithelium of BALB/c mice whereas only background levels were detected in C57BL/6 mice. The highest level of γH2AX foci was found colocalized in nuclei of cells that were positive for estrogen receptor alpha (ERα+). The E2-stimulated γH2AX in BALB/c mice was not associated with differences in estrogen receptor (Esr1) level or transcriptional responses in target genes Pgr, Areg or expression of Apobec3. Similarly, DNA damage was increased significantly in the mammary epithelium of mammary tumor-susceptible ACI rats treated with E2, but not in the tumor-resistant BN strain. The effect of E2 was also examined in humans using breast explant cultures. Levels of E2-induced γH2AX were significantly higher in tissues from donors undergoing prophylactic mastectomy due to familial risk compared to tissues from average risk women undergoing reduction mammoplasty. Among average risk donors, significant increases in γH2AX by E2 treatment was observed in only 1 of five donors. Conclusions: These data demonstrate that E2-induced DNA damage varies among strains of rodents and women. The levels of E2-stimulated DNA damage can provide a valuable diagnostic tool to identify subsets of individuals for whom estrogen exposure may be pathogenic in breast epithelial cells due to impaired DNA double strand break repair due to inherited risk alleles or epigenetic alterations.
Citation Format: Karen A. Dunphy, Aman Sharma, Prabin Majhi, Sallie S Schneider, Giovanna M Crisi, Grace Makari-Judson, D Joseph Jerry. Estrogen-stimulated DNA damage in mammary epithelium varies among strains of rodents and women [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-07-10.