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UBC Theses and Dissertations

BRCA1 regulates the PLK1-mediated spindle positioning pathway that promotes luminal features and may suppress tumorigenesis in mammary epithelial cells He, Zhengcheng


Female carriers of mutations in Breast cancer, early onset 1 (BRCA1) show an elevated risk to develop breast cancers that resemble the primitive and proliferative cells of the mammary gland. BRCA1 is proposed to regulate the homeostasis of mammary progenitor cells, but its actions are not completely known. I hypothesize that BRCA1 regulates the positioning of the mitotic spindle, which ultimately controls the luminal features displayed in the progeny cells produced. I first used lentivirus transduction to silence BRCA1 expression in human mammary epithelial cells, including immortal MCF10A cells and primary cells isolated from reduction mammoplasty tissues. I found that the loss of BRCA1 function perturbed the cell division axis, which induced aneuploidy in progeny cells, reduced colony output, and perturbed the expression of luminal features. Mechanistically, a dynein-based pathway was disturbed by loss of BRCA1 function. I then studied the ex vivo growth of human mammary cells isolated directly from female carriers with pathogenic BRCA1 mutations. These progenitor-derived cells exhibited lower BRCA1 levels, higher radiosensitivity, and changes to the cell division axis. I used genome editing in MCF10A cells to model and study heterozygous BRCA1 mutations. These studies identified low BRCA1 expression and an inability to correctly orient the cell division axis in cells encoding pathogenic mutations. Subsequent proteomic analysis indicated PLK1 hyperactivity and treatment with a PLK1 inhibitor recovered the cell division axis and the acquisition of luminal features in primary mammary cells isolated from Brca1 mutant mice or female carriers with pathogenic BRCA1 mutations. Finally, I examined the tumorigenic processes that are altered through tissue-specific overexpression of a BRCA1 modifier, termed HMMR. From BLG-Cre;Brca1f/f;Trp53+/- mammary tissues, I isolated and studied epithelial cells that showed a loss of genome stability and activation of cGAS-STING and NF-κB signaling. HMMR overexpression increased the expression of immune-related genes that are known to recruit macrophages and promote a pro-tumorigenic microenvironment. Together, these research findings indicate BRCA1 controls mitotic spindle orientation. The disruption of this mechanism may underlie the changes observed in mammary epithelial cells and the increased risk to develop basal-like breast cancer that is observed in female carriers of BRCA1 mutations.

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