Abstract 3488: Loss of nuclear alpha-catenin is associated with race, aggressive disease, and chemoresistance in triple negative breast cancer
Authors: Bassiouni Rania , Li Yunchi , David-Dirgo Victoria , Garcia-Mansfield Krystine , Sharma Ritin , Pirrotte Patrick , Vohra Nasreen , Singhal Sandeep , Gardner Kevin , Carpten John D. 
Published: August 2019

Abstract
Triple negative breast cancer (TNBC) is a particularly aggressive and difficult-to-treat subtype of the disease. A known health disparity exists within TNBC: African American (AA) women are more likely to be diagnosed with and die from the disease. Our group previously reported homozygous deletions in the CTNNA1gene, which encodes the protein alpha-catenin, in AA TNBC. We have undertaken a basic and translational research study to understand the mechanistic role and clinical impact of alpha-catenin loss in TNBC. To validate our findings of alpha-catenin loss in TNBC, we analyzed over 500 breast cancer patient samples by immunohistochemistry. We found loss of both nuclear and cytoplasmic alpha-catenin to be inversely correlated with survival in TNBC. However, loss of nuclear alpha-catenin was observed more frequently in tumors from AA patients, and corresponded to poorer survival in this group. While its cytosolic role has been well studied, little is known about nuclear alpha-catenin and its role in disease. To examine the nuclear function of alpha-catenin, we used CRISPR/Cas9-mediated gene editing to generate CTNNA1 knockout (KO) BT-549 and MB-MDA-436 cell lines. We also reintroduced CTNNA1 into MDA-MB-468 cells - a line derived from an AA woman with an endogenous deletion in CTNNA1. Using these models, we confirmed that all lines contained a pool of nuclear alpha-catenin. To identify binding partners, we performed co-immunoprecipitation of alpha-catenin from nuclear lysates, followed by mass spectrometry. Nuclear alpha-catenin was found to interact directly with ATR, a kinase critical to the DNA damage response (DDR). ATR mediates both the repair of DNA lesions and the G2/M cell cycle checkpoint to ensure that only cells with undamaged DNA may enter mitosis. We found high levels of activated ATR in KO cells, which corresponded with more efficient DNA repair after exposure to UV light. Loss of alpha-catenin also resulted in decreased sensitivity to the DNA-damaging chemotherapeutics cisplatin, carboplatin, doxorubicin, etoposide, and olaparib, several of which are approved for treatment of TNBC patients. Furthermore, alpha-catenin KO cells were more sensitive to inhibitors of ATR, as well as to inhibitors of the G2/M checkpoint proteins Chk1 and Wee1. Tellingly, the KO cells were less sensitive to inhibitors of ATM or DNA-PK, two regulators of alternate DDR pathways. This suggests that nuclear alpha-catenin likely plays a specific role in ATR-directed processes. In our studies, we have identified nuclear alpha-catenin as a tumor suppressor that affects TNBC’s susceptibility to chemotherapy by playing a role in the DDR and the G2/M checkpoint. Our data suggest that loss of alpha-catenin is more common in AA patients, and is associated with poor prognosis. Therefore, CTNNA1 status may be important in determining appropriate therapeutic strategies for this subset of patients.
Citation Format: Rania Bassiouni, Yunchi Li, Victoria David-Dirgo, Krystine Garcia-Mansfield, Ritin Sharma, Patrick Pirrotte, Nasreen Vohra, Sandeep Singhal, Kevin Gardner, John D. Carpten. Loss of nuclear alpha-catenin is associated with race, aggressive disease, and chemoresistance in triple negative 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 3488.