Abstract PR13: Loss of alpha-catenin expression is associated with race, aggressive disease, and chemoresistance in triple-negative breast cancer
Authors: Bassiouni Rania , Singhal Sandeep , Gibbs Lee D. , Li Yunchi , Pirotte Patrick , Vohra Nasreen , Gardner Kevin , Carpten John D. 
Published: September 2020

Abstract
Triple-negative breast cancer (TNBC) is an aggressive and difficult-to-treat subtype of the disease. A well-documented 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 CTNNA1 gene, 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, particularly in AA patients. To explore the association of alpha-catenin loss with race, we first examined gene expression data available in the TCGA. We determined that race was significantly associated with alpha-catenin expression, with lower expression found in AA patients. We also found a significant over-representation of AA patients in the lowest subset of expressers. We validated this association in an independent, racially diverse cohort comprising 460 breast cancer patients. Interestingly, we observed loss of nuclear alpha-catenin specifically to be associated with race. We also found strong associations between low alpha-catenin expression and survival, tumor stage, and lymph node metastasis. While its junctional role has been well studied, little is known about nuclear alpha-catenin and its role in disease. To understand the nuclear function of alpha-catenin, we sought to identify its nuclear binding partners by performing co-immunoprecipitation followed by mass spectrometry. We found nuclear alpha-catenin to interact with ATR, a kinase critical to the DNA damage response and G2/M checkpoint. To determine whether loss of alpha-catenin affected ATR function, we developed several isogenic cell line pairs. Using CRISPR/Cas9-mediated gene editing, we generated CTNNA1 knockout BT-549, MB-MDA-436, and MDA-MB-231 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 found that loss of alpha-catenin promoted greater ATR activation in response to DNA damage. This was accompanied by increased phosphorylation of downstream effector proteins, including Chk1 and RPA2. Consistent with ATR hyperactivation, we found cells lacking alpha-catenin to be more sensitive to inhibitors of ATR and the G2/M checkpoint kinases Chk1 and Wee1. Additionally, alpha-catenin loss promoted DNA damage repair, thus decreasing sensitivity to DNA-damaging chemotherapies clinically utilized for TNBC treatment. In our studies, we have identified nuclear alpha-catenin as a tumor suppressor that effects TNBC’s susceptibility to chemotherapy by playing a role in ATR-directed DNA repair. Our data suggest that loss of alpha-catenin is more common in AA patients, and is associated with aggressive disease and poor prognosis. Therefore, CTNNA1 status may be important in determining appropriate therapeutic strategies for this subset of patients, which is disproportionally affected by the disease.
This abstract is also being presented as Poster B088.
Citation Format: Rania Bassiouni, Sandeep Singhal, Lee D. Gibbs, Yunchi Li, Patrick Pirotte, Nasreen Vohra, Kevin Gardner, John D. Carpten. Loss of alpha-catenin expression is associated with race, aggressive disease, and chemoresistance in triple-negative breast cancer [abstract]. In: Proceedings of the Twelfth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2019 Sep 20-23; San Francisco, CA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(6 Suppl_2):Abstract nr PR13.