Objective: The BRCA1 C-Terminal (BRCT) domain of BRCA1 has been found to interact with three accessory proteins (PRR5, RICTOR, and SIN1) of mTOR complex 2 (mTORC2). BRCA1 BRCT domain dissociates the members of the mTORC2 complex to inhibit the activity of this kinase and block activation of Akt. Most breast cancers exhibit hyperactivation of the PI3K/Akt/mTOR pathway, and we hypothesize that genetic and epigenetic loss of functional BRCA1 contributes this signaling activation event. Therefore, the goal of this project is to study the potential cancer impacts of BRCA1-deficient breast cancers and their response to mTOR inhibition.

Methods: Yeast two-hybrid and orthogonal mammalian overexpression systems were used to confirm the interactions of PRR5, RICTOR, and SIN1 with the BRCT domain. MCF-10A breast cells were used to examine BRCA1-γH2AX foci formation under DNA damage conditions with cisplatin and mTOR inhibitors. Three-dimensional MCF-10A spheroids were used to study the effects of breast acini formation with shRNA-mediated repression BRCA1 and RICTOR. Breast cancer cell lines MCF-7, MDA-MB-231, and HCC1937 were used to study the effects of BRCA1 deficiency and response to pan-mTOR and pan-PI3K/mTOR inhibitors.

Results: Wild type BRCA1 BRCT domain was found to have stronger interactions with PRR5, RICTOR, and SIN1 than with the M1775R mutated BRCA1 BRCT domain and empty vector control. MCF-10A breast cells treated with rapamycin, which hyperactivates mTORC2, exhibited significantly higher amounts of BRCA1-γH2AX foci formation in comparison to control and pan-mTOR PP242-treated cells. This was comparable to the effect of cisplatin-treated cells. BRCA1-RICTOR double KD rescued the effect of distorted breast cell acini formation displayed by BRCA1 loss that could lead to tumorigenesis. This suggests that inhibition of the mTORC2 complex can be an effective target, especially for cells that lack functional BRCA1. MCF-7 and MDA-MB-231 cells with BRCA1 knockdown were significantly more sensitive to pan-mTOR inhibition than non-targeting scramble control. HCC1937 cells, which have a truncated, non-functional BRCA1 BRCT domain, were more sensitive to pan-PI3K/mTOR inhibition that HCC1937 cells with restored full-length BRCA1. Significance: Breast cancer is the most commonly diagnosed cancer among women in the U.S. BRCA1 is the most characterized gene associated with breast cancer. Genetic alterations in BRCA1 appear in about 5-10% of breast cancer cases, and epigenetic loss of BRCA1 appears in about 10-15% of breast cancer cases. Also, about 70% of breast tumors have hyperactive PI3K/Akt/mTOR pathways. This would suggest that breast cancer is heavily reliant on this oncogenic pathway, and therefore BRCA1-deficient patients may benefit from pan-mTOR cancer therapeutics.

Citation Format: Kimiko L. Krieger, Wen-Feng Hu, Tyler Ripperger, Nicholas T. Woods. Evaluating the functional impacts of the BRCA1-mTORC2 interaction in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1716.