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Background: Epithelial-mesenchymal transition (EMT) in breast cancer drives tumor invasion, metastasis and drug resistance. BRCA1 mutation carriers have a high risk for developing aggressive basal-like triple-negative breast cancers with EMT characteristics. It has been described that normal mammary epithelium of BRCA1-mutation carriers is comprised of aberrant luminal progenitor cell population resembling basal-like breast cancer cells. Yet, the role of BRCA1 in regulating epithelial cell plasticity in normal mammary gland remains largely obscure. Aim: Here, we used patient-derived normal and cancer organoid cultures from BRCA1-mutation carriers and noncarriers, to examine the effect of the BRCA1 mutation background on epithelial cell plasticity and the susceptibility to EMT. Methods and results: Mammary organoids were established from normal or cancer mammary tissues obtained from consenting patients undergoing lumpectomy or mastectomy. Isolated cells were plated in adherent basement membrane extract (BME) drops and overlaid with optimized organoid culture medium. EMT regulation in breast cancer is usually studied using cell lines and murine models. To determine the possibility to study EMT on patient-derived organoids, organoid culture media was optimized and established organoids were exposed to TGFβ to induce EMT. Morphological and phenotypic alterations were characterized using immunolabeling and visualization with confocal microscopy. Breast cancer organoids induced with TGFβ demonstrated EMT-like changes including the downregulation of E-Cadherin and upregulation of N-Cadherin. Moreover, breast cancer organoids showed typical cytoskeleton rearrangements. Here, the transformation from cortical actin into stress fibers formed in dedifferentiated mesenchymal cells, was visualized with actin staining. However, normal mammary organoids behaved differently. The cytoskeleton of BRCA-wild type (noncarriers) normal mammary organoids was not affected by the treatment. Curiously, normal mammary organoids derived from BRCA1-mutation carriers demonstrated EMT like changes upon exposure to TGFβ. To further determine mechanisms facilitating cell plasticity in BRCA1-mutation carriers, single cell RNA sequencing analysis on BRACA1-mutation carriers and noncarriers derived organoids is ongoing. Conclusion: The results suggest that BRCA1 germline mutation predisposes normal mammary epithelium dedifferentiation due to increased susceptibility to EMT. Citation Format: Rakefet Ruth Ben-Yishay, Naama Herman, Vered Noy, Eyal Mor, Aiham Mansur, Dana Ishay-Ronen. Normal mammary epithelium of BRCA1 mutation carriers demonstrates increased susceptibility to cell plasticity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5847.

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