African American Triple Negative Breast Cancer Research Articles

Abstract LB312: Subtype- and race-specific differences in immune landscape of breast cancer

Abstract Tumor mass comprises of not just the tumor cells but a variety of stromal cells. Immune cells residing within the tumor microenvironment modulate the growth and behavior of the tumor cells through a bi-directional crosstalk and affect the outcomes of cancer therapies. Breast cancer (BC) is a highly heterogeneous disease having four major molecular subtypes. Triple-negative breast cancer (TNBC) is the most aggressive subtype and has limited available therapeutic options. Further, it disproportionately affects African American (AA) women, with nearly two times higher incidence as compared to Caucasian American (CA) women. It is associated with an earlier onset and overall higher mortality of BC. Here, we investigated the differences in the immune landscape between TNBC and non-TNBC subtypes and examined if there existed any race-specific patterns. A total of 81 breast cancer samples with a distribution of 55 non-TNBC (28 CA and 27 AA) and 26 TNBC (12 CA and 14 AA) were selected after histopathologic analysis. Tissues were analysis using the NanoString nCounter® pancancer immune panel to determine tissue-specific immune cell populations. We observed a significantly higher presence of tumor-infiltrating lymphocytes in TNBC compared to non-TNBC, with a higher immune score for CD45+ (total lymphocytes), T-cells, B-cells and tumor-associated macrophages. Specifically, among the T-cell population, Treg cells, Th1 cells, and CD8+ cells were higher in TNBC samples. The pathway prediction indicated 'primary immunodeficiency' and 'cancer immunotherapy by PD-1 blockade' pathways being significantly up-regulated in TNBC samples. Overexpression of CTLA4, IDO1, PDL1 and PDL2 as well as Treg marker genes, such as FOXP3 and LAG3, which are involved in immunosuppressive pathways, was observed in TNBC. In race-wise comparisons of non-TNBC and TNBC sample between CA and AA patients, we found that AA non-TNBC tumors had higher Treg cell scores. On the other hand, CA TNBC samples had higher Th1 and CD8+ T-cell scores compared to AA TNBC. The pathway prediction from differentially expressed genes between CA and AA non-TNBC samples showed complement activation pathway to be down-regulated in AA patients, which could be associated with an increased presence of Treg cells. Further, when comparing CA vs AA TNBC samples, we found significant up-regulation of oncogene BMI1 and down-regulation of genes involved in T-cell activation such as CD47, CD44, FOS, and S100A8 in AA TNBC, supporting their more aggressive nature. In conclusion, our data show that TNBC patients have a higher infiltration of immune cells, which remain suppressed due to the presence of Treg cells and PD1 signaling pathway. The presence of higher Treg cells in AA non-TNBC and lower Th1 and CD8+ T-cells in AA TNBC samples suggests an overall immune suppressed microenvironment in AA patients compared to CA patients. Overall, our data suggest that TNBC could be good candidate for immune therapy. Citation Format: Amod Sharma, Kunwar Somesh Vikramdeo, Sarabjeet Kour Sudan, Mohammad Aslam Khan, Mohammad Tahir, James Elliot Carter, Cindy Nelson, Ajay P Singh, Seema Singh. Subtype- and race-specific differences in immune landscape of breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB312.

Next-Generation Sequencing and Triple-Negative Breast Cancer: Insights and Applications.

The poor survival of triple-negative breast cancer (TNBC) is due to its aggressive behavior, large heterogeneity, and high risk of recurrence. A comprehensive molecular investigation of this type of breast cancer using high-throughput next-generation sequencing (NGS) methods may help to elucidate its potential progression and discover biomarkers related to patient survival. In this review, the NGS applications in TNBC research are described. Many NGS studies point to TP53 mutations, immunocheckpoint response genes, and aberrations in the PIK3CA and DNA repair pathways as recurrent pathogenic alterations in TNBC. Beyond their diagnostic and predictive/prognostic value, these findings suggest potential personalized treatments in PD -L1-positive TNBC or in TNBC with a homologous recombination deficit. Moreover, the comprehensive sequencing of large genomes with NGS has enabled the identification of novel markers with clinical value in TNBC, such as AURKA, MYC, and JARID2 mutations. In addition, NGS investigations to explore ethnicity-specific alterations have pointed to EZH2 overexpression, BRCA1 alterations, and a BRCA2-delaAAGA mutation as possible molecular signatures of African and African American TNBC. Finally, the development of long-read sequencing methods and their combination with optimized short-read techniques promise to improve the efficiency of NGS approaches for future massive clinical use.

Abstract 441: Targeting MKK3-MYC complex to reduce racial survival disparity in breast cancer

Abstract Breast cancer is the most common cancer and the leading cause of cancer-related death in women. The triple-negative breast cancer (TNBC) is the most aggressive type of breast cancer. African American women experience up to 2-fold higher incidence of TNBC as compared to White women. Moreover, African Americans die from breast cancer at up to 40% higher rate than White and Hispanic women. However, there is no therapeutic options are currently available to reduce survival disparity in breast cancer. To address this unmet medical need, we develop novel computational methods combined with experimental chemical biology and high-throughput screening (HTS) technologies. Using our specially designed bioinformatics workflow, we discovered that high expression of mitogen-activated protein kinase kinase 3 (MKK3) strongly correlates with worsened clinical outcomes in African American TNBC patients. The analysis of TNBC patient genomic profiles revealed that MKK3 promotes TNBC in part by activating the major tumor driver transcription factor MYC. MYC is a highly appealing therapeutic target in TNBC, however, no approved MYC inhibitors are available. We found that MKK3 binds and activates MYC through protein-protein interaction (PPI) in TNBC cells. Both genetic and pharmacological inhibition of the MKK3/MYC complex suppresses MYC activity and the viability of TNBC cells. Thus, MKK3/MYC interaction represents a novel vulnerability for therapeutic intervention in TNBC. By combining computational modeling with experimental HTS, we have discovered the first small molecule MKK3/MYC PPI inhibitors. For example, a quinoline derivative SGI-1027 disrupts the MKK3/MYC complex in cells and in vitro, inhibits MYC transcriptional activity, and suppresses the viability of TNBC cells. Together, our studies revealed a novel mechanism of TNBC tumorigenesis through MKK3-induced MYC activation, establish a new HTS to enable the discovery of potent and selective inhibitors to regulate MKK3/MYC complex, and may open new clinical strategies to reduce racial disparity in TNBC mortality. Acknowledgments: This work was supported in part by Mary Kay Ash Foundation Grant for Cancer Research (A.A.I.), Emory initiative on Biological Discovery through Chemical Innovation (A.A.I.), NCI’s Informatics Technology for Cancer Research (ITCR) Program (R21CA274620, A.A.I.), NCI Cancer Target Discovery and Development (CTD^2) Network (U01CA217875, H.F.), NCI Emory Lung Cancer SPORE (P50CA217691, H.F.), Career Enhancement Program (A.A.I., P50CA217691), Winship Cancer Institute (NIH 5P30CA138292).

Abstract C027: miRNA-214-5P regulates RAD51, a biomarker for aggressive disease and racial disparities in triple-negative breast cancer

Abstract Breast cancer (BC) is one of the deadliest cancers in women. Among various subtypes, triple-negative breast cancer (TNBC) is the most aggressive and hard to treat subtype of breast cancer because it is highly metastatic and lacks targeted therapies. The death rate from BC is higher among African American (AA) women than among women of other races and ethnicities. The higher incidences of TNBCs and their aggressive growth in young AA women contributing to higher death rates indicate a biological basis for this difference. Thus, it is imperative to understand the molecular mechanisms that contribute to aggressive tumor growth in AA women, identify biomarkers to select patients who will respond to existing therapies, and develop effective therapeutics to reduce this disparity. Our previous findings showed that the DNA repair protein, RAD51, is overexpressed in AA TNBC patients and correlates with a poor prognosis relative to European American (EA) TNBC patients. However, the exact mechanism behind the regulation of RAD51 has not been identified. Our miRNA seq analysis shows a list of downregulated miRNAs in AA TNBC cell lines compared to EA TNBC cell lines. Interestingly, the miRDB-MicroRNA Target Prediction Database predicted that miR-214-5P has the seed sequence to bind and degrade RAD51 mRNA. Analysis of the TCGA database by UALCAN portal also shows a decreased expression of miR-214-5P in AA TNBC patients compared to EA TNBC patients. Treating the AA TNBC cell lines with miR-214-5P mimic downregulates RAD51 expression in a cell cycle-independent manner and also induces HR-deficiency as measured by Dr-GFP assay. Based on these results, we designed a synergistic lethality-based combination of miR-214-5P and Olaparib in TNBC cells. Data from our preclinical evaluations show miR-214-5P and Olaparib cause increased DNA strand breaks, and synergistic TNBC cell lethality compared to individual treatments. Together, our data indicate that miR-214-5P regulates RAD51 and either of these genes could be biomarkers for aggressive TNBC and racial disparity in BC therapeutic outcomes. Citation Format: Ganesh Acharya, Chinnadurai Mani, Upender Manne, Komaraiah Palle. miRNA-214-5P regulates RAD51, a biomarker for aggressive disease and racial disparities in triple-negative breast cancer [abstract]. In: Proceedings of the 15th AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2022 Sep 16-19; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2022;31(1 Suppl):Abstract nr C027.

Abstract A011: Whole genome sequencing of the Alabama Hereditary Cancer Cohort to identify African American breast cancer genetic risk variants

Abstract Regardless of having higher rates of triple-negative breast cancer (TNBC) and breast cancer-related deaths, African Americans are drastically understudied in hereditary breast cancer research. Our group has established the Alabama Hereditary Cancer Cohort to carry out next-generation sequencing (NGS) to address this gap. These efforts aim to identify inherited breast cancer risk variants in African Americans. Interestingly, African American TNBC tumors have been investigated through NGS, identifying somatic mutations unique to African American breast cancer. Now, African American-specific variants associated with inherited risk need to be identified, including those in genes harboring somatic mutations in African American TNBC. This study carried out whole genome sequencing (WGS) to investigate inherited African American breast cancer risk. All coding and non-coding regions of the nuclear genome and the entirety of the mitochondrial genome were explored, all of which can play a role in hereditary breast cancer risk. Individuals provided a blood or saliva sample for DNA extraction upon study consent. Subsequently, paired-end Illumina sequencing was carried out. Fastq files were trimmed using Trimmomatic and further processed according to the Genome Analysis ToolKit Best Practices, resulting in variant call format (VCF) files. Coding, non-coding, and mitochondrial regions were individually extracted using VCFtools and UCSC Table Browser tracks. Coding and mitochondrial variants were annotated using Annovar and MSeqDR, respectively. Coding variants were further filtered for minor allele frequency (MAF) classes (i.e., rare and low-frequency) and mutation type (i.e., protein-truncating variants; PTVs) using R (v. 4.2). Preliminary WGS data of 60 African American hereditary breast cancer cases have identified statistically significant rare and low-frequency PTVs in genes reported to harbor somatic mutations exclusively in African American TNBC tumors. Furthermore, we have identified variants in prostate cancer susceptibility genes that are statistically associated with African American breast cancer risk. We have also identified multiple variants in the mitochondrial genome that are statistically linked to breast cancer risk, some African American-specific. Our data shows that WGS is an invaluable tool for identifying breast cancer susceptibility variants. Further sequencing and functional analyses will validate our preliminary findings. Citation Format: Troy LoBue, Isaac Mcneely, Nancy Merner. Whole genome sequencing of the Alabama Hereditary Cancer Cohort to identify African American breast cancer genetic risk variants [abstract]. In: Proceedings of the 15th AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2022 Sep 16-19; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2022;31(1 Suppl):Abstract nr A011.

Abstract C051: Unraveling the biology of TNBC in African American women through the WHITNEY study

Abstract Cancer disparities arise from structural racism in the United States, which affects overall standard of living due to unequal access to crucial resources. Barriers in access to clinical care among under-served populations in combination with a vast underrepresentation at a scientific research level further perpetuate inequalities in cancer treatment. Specifically, Black women in the US are approximately twice as likely as Caucasian women to die of Triple-Negative Breast Cancer (TNBC), a particularly aggressive breast cancer subset, lacking expression of estrogen receptors, progesterone receptors, and human epidermal growth factor receptor-2. Our previous work and that of others has shown racial differences in TNBC tumor biology, with preference of basal-like subtypes in African American (AA) women. Herein, we present the Women’s Health Initiative for Triple-Negative Breast Cancer Equity (WHITNEY) study, a first of its kind, coordinated effort between Massachusetts General Hospital, and Boston Medical Center, the largest safety-net hospital in New England. The WHITNEY study seeks to understand factors contributing to biological differences and outcome of TNBC diagnoses in AA women. Response to neoadjuvant chemotherapy (NACT) response is a strong predictor of TNBC outcomes and failure to achieve a pathological complete response (pCR) in the breast following NACT is associated with a high probability of metastatic relapse. As Black women with TNBC have lower pCR rates, WHITNEY has collected pre-treatment biopsies from AA and other TNBC patients undergoing NACT to understand actionable insights into the biological differences in TNBC. Our prior published work has suggested that intratumoral heterogeneity is greater in TNBCs among AA patients than others, and that epigenetic silencing of key tumor suppressors is selectively increased in these TNBCs. Core needle baseline biopsies were obtained from consented patients diagnosed with TNBC at either MGH or BMC. We utilized single-cell RNA sequencing, spatial transcriptomics, genomic and epigenetic profiling to characterize minor cell subpopulations that drive poor outcomes among African American TNBC patients. Here, we outline our preliminary tissue collection, processing and analysis methods used in the WHITNEY study to better understand the biology of TNBC in African American women. Citation Format: Malalage N. Peiris, Emma Kelly, Christina Ennis, Kiana Mahdaviani, Aylin Dedeoglu, Adrian Ilinski, Esther Rheinbay, Ruben Dries, Naomi Ko, Leif Ellisen. Unraveling the biology of TNBC in African American women through the WHITNEY study [abstract]. In: Proceedings of the 15th AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2022 Sep 16-19; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2022;31(1 Suppl):Abstract nr C051.

Abstract C064: IGF-II regulates a critical mitochondrial protein, Magmas, with differential expression between African American and Non-Hispanic White women with TNBC

Abstract Breast cancer is the most common malignancy in women. Young African American (AA) women are affected by a higher incidence and mortality rate of triple-negative breast cancer (TNBC) compared to Non-Hispanic White (NHW) women. Currently, available treatment options are suboptimal, with a 5-year survival of less than 12% for metastatic TNBC. Our laboratory demonstrated that IGF-2 plays a pivotal role in the tumor aggressiveness of AA women. Moreover, tissues from AA women express high levels of IGF-2 and exhibit high activation of IGF-2 signaling pathways associated with tumorgenesis and chemoresistance. An emerging pathway linked to IGF-2 mediated chemoresistance and cell survival in TNBC is the regulation of mitochondrial and survival proteins. Mitochondrial Magmas is a mitochondrial protein found in all eukaryotic cells that is required for cell viability. It functions to transport nuclear proteins from the inner mitochondrial membrane to the mitochondrial matrix. Magmas can buffer increased reactive oxidative species (ROS) and promote survival of cancer cells. Overexpression of Magmas reduced ROS and enhanced cellular tolerance to oxidative stress; however, its downregulation raised cellular ROS and made cells more vulnerable to ROS-mediated death. Given the role of IGF-2 in regulating mitochondrial proteins, we investigated if IGF-2 regulated Magmas. We hypothesize that IGF-2 increases Magmas, and since AA women express higher levels of IGF-2, they will express higher Magmas levels. We used the CRL-2335 AA TNBC cell line and an IGF-2 knockdown of those same cells. Western blotting was used to detect the specific isoforms of IGF-2 and Magmas from cell lysates. Immunohistochemistry (IHC) characterized Magmas expression in paired, normal-tumor breast cancer tissues. Real time PCR was used to evaluate the expression of IGF2 and MAGMAS. The preliminary results showed that IGF-2 knockdown in CRL-2335 AA TNBC cells significantly decreased Magmas protein expression and its corresponding mRNA. In addition, tissue analysis demonstrated high levels of Magmas staining in TNBC tissue while little or no staining was detected in the normal tissues. Furthermore, tumor tissues of AA women showed significantly higher expression of Magmas as compared to tissues from NHW women. Targeting Magmas represents a novel and much needed tool to treat TNBC. Our team is currently validating Magmas as a target for Theranostics treatment, a promising emerging radiotherapy for TNBC patients. Citation Format: Jennifer D. Tran, Alfonso D. Duran, Qian-Wei Tan, Frankis G. Almaguel, Daisy De León. IGF-II regulates a critical mitochondrial protein, Magmas, with differential expression between African American and Non-Hispanic White women with TNBC [abstract]. In: Proceedings of the 15th AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2022 Sep 16-19; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2022;31(1 Suppl):Abstract nr C064.

Tumor immune microenvironment of self-identified African American and non-African American triple negative breast cancer

Differences in the tumor immune microenvironment may result in differences in prognosis and response to treatment in cancer patients. We hypothesized that differences in the tumor immune microenvironment may exist between African American (AA) and NonAA patients, due to ancestry-related or socioeconomic factors, that may partially explain differences in clinical outcomes. We analyzed clinically matched triple-negative breast cancer (TNBC) tissues from self-identified AA and NonAA patients and found that stromal TILs, PD-L1 IHC-positivity, mRNA expression of immune-related pathways, and immunotherapy response predictive signatures were significantly higher in AA samples (p < 0.05; Fisher’s Exact Test, Mann–Whitney Test, Permutation Test). Cancer biology and metabolism pathways, TAM-M2, and Immune Exclusion were significantly higher in NonAA samples (p < 0.05; Permutation Test, Mann–Whitney Test). There were no differences in somatic tumor mutation burden. Overall, there is greater immune infiltration and inflammation in AA TNBC and these differences may impact response to immune checkpoint inhibitors and other therapeutic agents that modulate the immune microenvironment.

Abstract PO-131: RAD51 is a biomarker for aggressive disease and racial disparities in triple-negative breast cancer

Abstract Breast cancer (BC) is the second most diagnosed malignant disease in women, and one of the leading causes of cancer-related deaths. Triple-negative breast cancer (TNBC) is the most aggressive and difficult to treat subtype of BC because it is highly metastatic and lacks targeted therapies. African American (AA) women have a higher death rate from BC than women of other races and ethnicities. The higher incidences of TNBCs and their aggressive growth in young AA women contributing to higher death rates indicate a biological basis for this difference. Thus, it is imperative to understand the molecular mechanisms that contribute to aggressive tumor growth in AA women, identify biomarkers to select patients who will respond to existing therapies, and develop effective therapeutics to reduce this disparity. Our studies identified that multiple TNBC cells derived from AA women are inherently chemoresistant and exhibit aggressive growth behavior compared to TNBC cells derived from European American (EA) patients. Our preliminary screenings showed that the DNA repair protein, RAD51, is overexpressed in AA TNBC patients and correlates a poor prognosis relative to EA TNBC patients. Analysis of AA and EA TNBC tumor specimens indicated the epigenetic regulation of RAD51 by promoter methylations and microRNAs. Furthermore, AA women diagnosed with TNBC, have a considerably lower incidence of germline BRCA1 mutations than women of other racial or ethnic groups. This indicates most TNBC tumors in AA patients are DNA repair proficient and have intact cell cycle checkpoint mechanisms that protect them from chemotherapy-induced DNA damage and promote therapeutic resistance. Our drug screenings identified CHK1 inhibitor, Prexasertib caused DNA repair deficiency in BRCA wild-type TNBC cells by promoting proteasome-mediated degradation of BRCA1 and RAD51 proteins. Therefore, we designed a synthetic lethality-based drug combination of Prexasertib with PARP inhibitors (PARPi) in DNA repair proficient TNBC cells. Data from our preclinical evaluations show Prexasertib and Olaparib cause increased DNA strand breaks, mitotic catastrophe, and synergistic TNBC cell lethality compared to individual drug treatments. Additionally, computational analysis of TCGA data revealed a RAD51 upregulation in TNBC tumors compared to normal breast tissues and other subtypes of BC which renders as a poor prognostic marker for these patients. Remarkably, there was an interesting discrepancy in RAD51 expression levels between different racial groupings, with AA and Asian BC patients having higher levels of RAD51 expression than Caucasian BC patients. Consistent with these observations, AA and Asian TNBC patients showed decreased survival probability. Together, our data indicate that RAD51 and its epigenetic regulators could be biomarkers for aggressive TNBC and racial disparity in BC therapeutic outcomes and suggests a novel combination therapy involving Prexasertib and Olaparib may improve prognosis and reduce racial disparity in TNBC. Citation Format: Ganesh N. Acharya, Chinnadurai Mani, Upender Manne, Komaraiah Palle. RAD51 is a biomarker for aggressive disease and racial disparities in triple-negative breast cancer [abstract]. In: Proceedings of the AACR Virtual Conference: 14th AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2021 Oct 6-8. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2022;31(1 Suppl):Abstract nr PO-131.

Differential expression of microRNA between triple negative breast cancer patients of African American and European American descent

ABSTRACT There are racial disparities in the outcome of triple negative breast cancer (TNBC) patients between women of African ancestry and women of European ancestry, even after accounting for lifestyle, socioeconomic and clinical factors. MicroRNA (miRNA) are non-coding molecules whose level of expression is associated with cancer suppression, proliferation and drug resistance; therefore, these have potential for biomarker applications in cancers including TNBC. Historically, miRNAs up-regulated in African American (AA) patients have received less attention than for patients of European ancestry. Using laser capture microdissection (LCM) to acquire ultrapure tumor cell samples, miRNA expression was evaluated in 15 AA and 15 European American (EA) TNBC patients. Tumor sections were evaluated using RNA extraction followed by miRNA analysis and profiling. Results were compared based on ethnicity and method of tissue fixation. miRNAs that showed high differential expression in AA TNBC patients compared to EA included: miR-19a, miR-192, miR-302a, miR-302b, miR-302c, miR-335, miR-520b, miR-520f and miR-645. LCM is a useful technique for isolation of tumor cells. We found a greater abundance of RNA in frozen samples compared to formalin fixed, paraffin embedded samples. miRNA appears to be a useful biomarker for TNBC to improve diagnosis and treatment.

YB1 Is a Major Contributor to Health Disparities in Triple Negative Breast Cancer.

Simple SummaryTriple negative breast cancer (TNBC) is a devastating disease that affects many women, due to the lack of FDA-approved targeted therapy. In the absence of cell surface receptors ER, PR, and Her2 that can be targeted with hormonal and antibody treatments, cytotoxic chemotherapy remains the major course of treatment, with a dismal response and rapid recurrence due to the acquisition of resistance. TNBC is also twice as more prevalent in African American (AA) when compared to Caucasian American (CA) women. This study investigated the role of the YB1 gene in the disparities in TNBC between AA and CA women. We found that YB1 is highly expressed in TNBC tumors of AA origin when compared to CAs. Increased expression levels and activity of YB1 correlates with poor disease outcomes, resistance to chemotherapy, and the activation of the cancer stem cell (CSC) phenotype, with higher levels in AA than in CA TNBC tumors. More importantly, we found that the targeted inhibition of the expression and activity of YB1 significantly inhibited the oncogenic behavior of AA tumors through sensitization to chemotherapy and inhibition of CSCs. Our study is the first to show that YB1 activity may be a major biological contributor to the health disparities in TNBC, and that development of therapies that specifically target YB1 could reduce these disparities.Triple negative breast cancer (TNBC) is the most aggressive amongst all breast cancer (BC) subtypes. While TNBC tumors represent less than 20% of all BC subtypes, they are responsible for the most BC-related deaths. More significantly, when considering TNBC incidence across all racial/ethnic groups, TNBC accounts for less than 20% of all BCs. However, in non-Hispanic black women, the incidence rate of TNBC is more than 40%, which may be a contributing factor to the higher BC-related death rate in this population. These disparities remain strong even after accounting for differences in socioeconomic status, healthcare access, and lifestyle factors. Increased evidence now points to biological mechanisms that are intrinsic to the tumor that contribute to disparate TNBC disease burdens. Here, we show that YB1, a multifunction gene, plays a major role in the TNBC disparities between African American (AA) and Caucasian American (CA) women. We show in three independent TNBC tumors cohorts, that YB1 is significantly highly expressed in AA TNBC tumors when compared to CAs, and that increased levels of YB1 correlate with poor survival of AA patients with TNBC. We used a combination of genetic manipulation of YB1 and chemotherapy treatment, both in vitro and in animal models of TNBC to show that YB1 oncogenic activity is more enhanced in TNBC cell lines of AA origin, by increasing their tumorigenic and aggressive behaviors, trough the activation of cancer stem cell phenotype and resistance to chemotherapeutic treatments.

A panel of miRNAs as prognostic markers for African-American patients with triple negative breast cancer

BackgroundTo investigate the global expression profile of miRNAs, their impact on cellular signaling pathways, and their association with poor prognostic parameters in African-American (AA) patients with triple negative breast cancer (TNBC).MethodsTwenty-five samples of AA TNBC patients were profiled for global miRNA expression and stratified considering three clinical-pathological parameters: tumor size, lymph node (LN), and recurrence (REC) status. Differential miRNA expression analysis was performed for each parameter, and their discriminatory power was determined by Receiver Operating Characteristic (ROC) curve analysis. KMplotter was assessed to determine the association of the miRNAs with survival, and functional enrichment analysis to determine the main affected pathways and miRNA/mRNA target interactions.ResultsA panel of eight, 23 and 27 miRNAs were associated with tumor size, LN, and REC status, respectively. Combined ROC analysis of two (miR-2117, and miR-378c), seven (let-7f-5p, miR-1255b-5p, miR-1268b, miR-200c-3p, miR-520d, miR-527, and miR-518a-5p), and three (miR-1200, miR-1249-3p, and miR-1271-3p) miRNAs showed a robust discriminatory power based on tumor size (AUC = 0.917), LN (AUC = 0.945) and REC (AUC = 0.981) status, respectively. Enrichment pathway analysis revealed their involvement in proteoglycans and glycan and cancer-associated pathways. Eight miRNAs with deregulated expressions in patients with large tumor size, positive LN metastasis, and recurrence were significantly associated with lower survival rates. Finally, the construction of miRNA/mRNA networks based in experimentally validated mRNA targets, revealed nodes of critical cancer genes, such as AKT1, BCL2, CDKN1A, EZR and PTEN.ConclusionsAltogether, our data indicate that miRNA deregulated expression is a relevant biological factor that can be associated with the poor prognosis in TNBC of AA patients, by conferring to their TNBC cells aggressive phenotypes that are reflected in the clinical characteristics evaluated in this study.

Abstract PS7-86: Molecular determinants of racial disparity guide triple-therapy for triple negative breast cancer

Abstract Background and Aim: Triple-negative breast cancer (TNBC) is a very aggressive subtype of breast cancer characterized by the absence of estrogen receptor (ER), progesterone receptor (PR) and HER2 receptor expression. Mortality from TNBC is significantly higher in African American (AA) women in comparison to White American (WA) women (5-year relative survival of only 14% for AA in comparison to 36% for WA) even though the incidence rates are lower in AA women. Irrespective of stage at diagnosis, AA-TNBC is more aggressive with higher metastasis and a poorer survival than WA-TNBC; therefore, it is imperative to understand the molecular determinants that drive aggressive progression of AA-TNBC. Overall aim of this study is to decipher the alterations in the molecular circuitry underlying racial disparity in TNBC progression in AA women compared to WA women. Results: AA-TNBC cells (HCC1806 and HCC1569) exhibited increased growth and higher migration potential; higher expression of stemness factors, increased number of mammospheres and higher CD44+/CD49f+ population in comparison to WA-TNBC (Hs578t, BT549, HCC1937 and HCC1187) cells. We analyzed RNA sequencing data of multiple AA and WA TNBC cell lines to query the status self-renewal pathways (Wnt/βCatenin, GLI1/Shh and YAP-TAZ) and observed significantly higher levels of GLI1 in AA-TNBC cell lines while no significant alterations were observed in other pathway components. Further analysis of TCGA dataset revealed a positive correlation between GLI1 and Notch1 in AA-TNBC with a negative correlation in WA-TNBC. Increased expression of components of GLI1 and Notch1 pathway (SHH, Jagged, cleaved-Notch (NICD), Hes1 and FOXM1) were noted in AA-TNBC compared to WA-TNBC cells. AA-TNBC cells showed increased nuclear localization of GLI1 and NICD as compared to WA-TNBC cells. We observed that GLI1 and NICD co-localize and co-immunoprecipitate in AA-TNBC indicating a direct interaction between these two transcription factors. High expression of GLI1 and Notch1 correlated with poor recurrence free survival in TNBC patients. Analyses of clinical samples revealed higher levels of nuclear NICD and GLI1 in AA-TNBC in comparison to WA-TNBC. Concomitant inhibition of GLI1 and Notch1 using respective small molecule inhibitors, GANT61 and DAPT, along with standard chemotherapeutic agents (Doxorubicin and carboplatin) effectively inhibited AA TNBC tumor growth in mice. Also, ex vivo analyses of tumor cells showed reduced migration and invasion potential as well as downregulation of CD44+/CD49f+ and ADLH1A1+ population in a synergistic manner. Combined treatment with GANT61+ DAPT+ Carboplatin effectively reduced stem cell frequency of AA-TNBC tumors in in vivo limiting dilution assay. Conclusions: In conclusion, these results show that AA-TNBC cells are inherently aggressive with increased growth, migration and stemness potential. We found aberrant activation of GLI1 and Notch pathway and a crosstalk between GLI1 and NICD whose inhibition effectively inhibits AA-TNBC and sensitizes AA-TNBC to standard chemotherapy. Our studies propose a ‘triple drug regimen’ for AA-TNBC tumors based on its molecular circuitry. Citation Format: Sumit Siddharth, Sheetal Parida, Nethaji Muniraj, Shawn M Hercules, Arumugam Nagalingam, Balázs Győrffy, Juliet Daniel, Dipali Sharma. Molecular determinants of racial disparity guide triple-therapy for triple negative breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS7-86.

Multiethnic PDX models predict a possible immune signature associated with TNBC of African ancestry.

Triple-negative breast cancer (TNBC) is an aggressive subtype most prevalent among women of Western Sub-Saharan African ancestry. It accounts for 15-25% of African American (AA) breast cancers (BC) and up to 80% of Ghanaian breast cancers, thus contributing to outcome disparities in BC for black women. The aggressive biology of TNBC has been shown to be regulated partially by breast cancer stem cells (BCSC) which mediate tumor recurrence and metastasis and are more abundant in African breast tumors. We studied the biological differences between TNBC in women with African ancestry and those of Caucasian women by comparing the gene expression of the BCSC. From low-passage patient derived xenografts (PDX) from Ghanaian (GH), AA, and Caucasian American (CA) TNBCs, we sorted for and sequenced the stem cell populations and analyzed for differential gene enrichment. In our cohort of TNBC tumors, we observed that the ALDH expressing stem cells display distinct ethnic specific gene expression patterns, with the largest difference existing between the GH and AA ALDH+ cells. Furthermore, the tumors from the women of African ancestry [GH/AA] had ALDH stem cell (SC) enrichment for expression of immune related genes and processes. Among the significantly upregulated genes were CD274 (PD-L1), CXCR9, CXCR10 and IFI27, which could serve as potential drug targets. Further exploration of the role of immune regulated genes and biological processes in BCSC may offer insight into developing novel approaches to treating TNBC to help ameliorate survival disparities in women with African ancestry.

Abstract PO-129: High expression of MKK3 correlates with poor clinical outcomes in African American breast cancer patients

Abstract African American women experience a 2-fold higher incidence of triple-negative breast cancer (TNBC) and are 40% more likely to die from breast cancer than women of other ethnicities. Previous studies have revealed a higher frequency of inactivating mutations in multiple tumor suppressor genes in African American breast cancer patients as compared to White patients. However, the molecular bases for the survival disparity in breast cancer remain unclear, and no race-specific therapeutically actionable targets have been proposed. To address this clinical need, we took the computational systems biology approach to uncover new potentially druggable genes that contribute in poor clinical outcomes of African American breast cancer patients. Through a comprehensive analysis of gene expression and survival data determined for The Cancer Genome Atlas (TCGA) breast cancer patient cohort, we found that more than 30% of all protein-coding genes are differentially expressed in White and African American breast cancer patients. The cluster analysis of co- regulated genes combined with the pathway enrichment analysis revealed a strong association of genes upregulated in African American patients with major oncogenic processes including cell cycle regulation, immunodeficiency, and oxidative phosphorylation. We have further identified more than 30 genes whose overexpression correlates with worsened clinical outcomes of African American breast cancer patients but not White patients. Among those genes, the overexpression of mitogen-activated protein kinase kinase 3 (MKK3) has one of the most dramatic and race-specific impacts on the survival of African American TNBC patients. We found that MKK3 can promote the TNBC tumorigenesis in African American patients in part by activating the master regulator MYC through protein- protein interaction. Together, this study uncovered new genetic features and molecular mechanisms of breast cancer tumorigenesis. As one example, we discovered new functional connectivity between poor clinical outcomes of African American TNBC patients and activation of the MYC transcriptional program through MKK3 signaling. These findings may open new opportunities for clinical investigations to reduce the survival disparity in breast cancer. Citation Format: Xuan Yang, Mohamed Amgad, Lee A.D. Cooper, Yuhong Du, Haian Fu, Andrey A. Ivanov. High expression of MKK3 correlates with poor clinical outcomes in African American breast cancer patients [abstract]. In: Proceedings of the AACR Virtual Conference: Thirteenth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2020 Oct 2-4. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(12 Suppl):Abstract nr PO-129.

Abstract LB-222: Aberrant activation of GLI1 and Notch1 contributes to racial disparity in triple-negative breast cancer progression

Abstract Background and Aim: Triple-negative breast cancer (TNBC) is a very aggressive subtype of breast cancer characterized by the absence of estrogen receptor (ER), progesterone receptor (PR) and HER2 receptor expression. Mortality from TNBC is significantly higher in African American (AA) women in comparison to European American (EA) women (5-year relative survival of only 14% for AA in comparison to 36% for EA) even though the incidence rates are lower in AA women. Irrespective of stage at diagnosis, AA-TNBC is more aggressive with higher metastasis and a poorer survival than EA-TNBC; therefore, it is imperative to understand the molecular determinants that drive aggressive progression of AA-TNBC. Overall aim of this study is to decipher the alterations in the molecular circuitry underlying racial disparity in TNBC progression in AA women compared to EA women. Results: AA-TNBC cells (HCC1806 and HCC1569) exhibited increased growth and higher migration potential; higher expression of stemness factors, increased number of mammospheres and higher CD44+/CD49f+ population in comparison to EA-TNBC (Hs578t, BT549, HCC1937 and HCC1187) cells. We analyzed RNA sequencing data of multiple AA and EA TNBC cell lines for self-renewal pathways (Wnt/βCatenin, GLI1/Shh and YAP-TAZ) and observed significantly higher levels of GLI1 in AA-TNBC cell lines while no significant alterations were observed in other pathway components. Further analysis of TCGA dataset revealed a positive correlation between GLI1 and Notch1 in AA-TNBC with a negative correlation in EA-TNBC. Increased expression of components of GLI1 and Notch1 pathway (SHH, Jagged, cleaved-Notch (NICD), Hes1 and FOXM1) were noted in AA-TNBC compared to EA-TNBC cells. AA-TNBC cells showed increased nuclear localization of GLI1 and NICD as compared to EA-TNBC cells. We observed that GLI and NICD co-localize in AA-TNBC. High expression of GLI1 and Notch1 correlated with poor overall survival in TNBC patients. AA TNBC Tumor samples show higher expression of GLI1 and NICD in comparison to TNBC tumors from EA patients. Concomitant inhibition of GLI1 and Notch1 using respective small molecule inhibitors, GANT61 and DAPT, along with standard chemotherapeutic agents (Doxorubicin and carboplatin) effectively inhibited AA TNBC growth and progression in mice; growth, proliferation, migration and invasion of ex vivo tumor cells and downregulated CD44+/CD49f+ and ADLH1A1+ population in a synergistic manner. Combined treatment with GANT61+ DAPT+ Carboplatin effectively reduced stem cell frequency of AA-TNBC tumors in in vivo limiting dilution assay. Conclusions: In conclusion, these results show that AA-TNBC cells are inherently aggressive with increased growth, migration and stemness potential. We found aberrant activation of GLI and Notch pathway and a crosstalk between GLI1 and NICD whose inhibition effectively inhibits AA-TNBC and sensitizes AA-TNBC to standard chemotherapy. Citation Format: Sumit Siddharth, Anirudh Saxena, Nethaji Muniraj, Sheetal Parida, Arumugam Nagalingam, Dipali Sharma. Aberrant activation of GLI1 and Notch1 contributes to racial disparity in triple-negative breast cancer progression [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr LB-222.

Abstract 4973: Comparison of tumor immune microenvironment in triple negative breast cancer (TNBC) of African American versus that of white patients

Abstract Introduction: Although there are many possible causes, it has been observed that Triple Negative Breast Cancer (TNBC) has a worse overall outcome in African American (AA) than white patients. Recently, there is also evidence that the patients response to the tumor, or the biological response expressed as the tumor immune microenvironment, impacts outcome. The goal of this study was to compare the Tumor Microenvironment (TME) of TNBC in the two mentioned groups in order to seek molecular parameters that are different in TNBC in AA patients as compared to whites. Methods: We selected N=50 AA and N=50 white TNBC samples from the Yale Pathology archives that were matched by diagnosis date. We measured CD45, CD3, CD8, CD20, CD14, CD68, PD-L1, THY1, aSMA and Fibroblast Activation Protein (FAP) expression in both the tumor and stromal compartments in whole slides from formalin fixed paraffin embedded (FFPE) tissues using multiplexed quantitative immunofluorescence (QIF). We also assessed the activation status of CD3-positive cells using Ki67 and Granzyme B markers. Median of each marker expression was calculated for each case by using quantitation from all Fields of View (FOV) that contained at least 10% tumor area. Results: AA tumors contained a significantly higher CD45-positive cells in overall, within tumor and stromal compartments (p = 0.0102, 0.0007 and 0.0280 respectively). Being more specific about CD45 marker, AA also showed to have a significantly higher level of overall lymphocyte population, measured by quantitating CD45+ CD14- cells (p =0.0081). AA tumors also had a significantly higher level of CD8 expression (p &amp;lt;0.0001). Even though we could not find any differences in CD3 expressions between the two groups, AA tumors contained a significantly higher level of activated T-cells (p =0.0432), where activation is defined as CD3 cells with high Ki67 or Granzyme B. AA tumors contained a significantly higher level of CD68 positive cells (p &amp;lt; 0.0001) and PD-L1 expression in CD68+ cells (p =0.0101). All other markers showed no difference between the two groups. Conclusion: Significant differences are seen in the TME of AA vs. white TNBCs. Further characterization of macrophages, fibroblasts, regulatory T-cells and Extracellular Matrix are underway to more specifically define TME and determine the relationship between TME features and outcome. Citation Format: Vesal Yaghoobi, Vasiliki Pelekanou, Tess O'Meara, Andrea Silber, Lajos Pusztai, David Rimm, Kim Blenman. Comparison of tumor immune microenvironment in triple negative breast cancer (TNBC) of African American versus that of white patients [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4973.

Abstract PR18: Prognostic role of androgen receptor in triple-negative breast cancer: A global multi-institutional experience

Abstract Background: African American (AA) triple-negative breast cancer (TNBC) patients experience worse clinical outcomes and exhibit 40% higher mortality rate than their European Americans (EAs) counterparts. There are currently no distinctions in inherent tumor biology between the ethnically distinct TNBC patients that serve as risk-predictive markers allowing new tailored treatments. Recently, androgen receptor (AR) has emerged as a new target for treating TNBC. However, the prognostic value of AR in TNBC remains controversial. To reconcile conflicting reports about the impact of AR loss on prognosis of TNBC and uncover the molecular pathways that may underlie the racial disparity in outcomes among AR-negative TNBCs, we determined the prognostic value of AR in diverse TNBC cohorts. Since loss of AR is associated with worse clinical outcome and African and AA women are more prone to aggressive disease course, we hypothesized that AR loss may underlie the global disparate burden in TNBC. Methods: We evaluated AR expression in well-annotated formalin-fixed, paraffin-embedded TNBC resection samples (n=1351) obtained from multiple hospitals from US, UK, Norway, Ireland, Nigeria and India. Samples with ≥1% nuclei staining positive for AR were deemed to be AR-positive. Associations between AR status, clinicopathologic variables and overall survival (OS) were evaluated. We performed gene set enrichment analysis for AR low and high group in AA and EA TNBCs. In vitro experiments were performed to examine whether AR loss increased the metastatic potential of TNBC cells. Results: We observed a significant difference in AR expression among EA and AA TNBCs (p=0.02) with AR loss associated with women of African ancestry (&amp;gt;90%, p&amp;lt;0.05). AR loss was significantly associated with poor OS in TNBC patients from US cohort (p=0.0324; n=316 for AR-negative, n=104 for AR-positive) and Nigerian cohort (p=0.0251; AR-negative=164, AR-positive=16). AR-negative was associated with poor OS in adjuvant-treated high Ki67 (&amp;gt;14%) (HR=1.72; p=0.095) AA TNBC (n=98) when compared to EA TNBCs (n=80). Furthermore, AR status retained its significant prognostic value (HR=1.549, p=0.036) after controlling for age, grade, Ki67, race and chemotherapy status. Gene set enrichment analysis revealed that Wnt/β-catenin signaling was the top-enriched gene ontology in the AR-low TNBC subgroup. Moreover, β-catenin protein levels are higher in AA AR-low TNBCs compared with AA AR-high TNBCs (p&amp;lt;0.05), suggesting Wnt signaling upregulation in AA women with AR-negative TNBC. In TNBC cell lines, loss of AR was significantly associated with higher cell proliferation, migration and invasion (p&amp;lt;0.05). Conclusion: Our study suggests a striking association of AR loss in TNBC with women of African ancestry. Our data offer compelling evidence that oncogenic Wnt/β-catenin signaling may link AR loss to more aggressive disease course and represent actionable biology in AA AR-negative TNBCs for whom no targeted treatments are currently on the horizon. This abstract is also being presented as Poster C102. Citation Format: Shristi Bhattarai, Sergey Klimov, Karuna Mittal, Uma Krishnamurthi, Xiaoxian Bill Li, Deepika Wali, Ceyda Sonmez Wetherilt, Ansa Riaz, Mohammad A. Aleskandarany, Andrew R. Green, Ian O. Ellis, Meenakshi Gupta, Lauren E. McCullough, Upender Manne, Johnson Agboola, Brett Baskovich, Emiel A. Janssen, Grace Callagy, Anurag Mehta, Tanuja Shet, Rakha A. Emad, Padmashree C.G. Rida, Ritu Aneja. Prognostic role of androgen receptor in triple-negative breast cancer: A global multi-institutional experience [abstract]. In: Proceedings of the Eleventh AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2018 Nov 2-5; New Orleans, LA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(6 Suppl):Abstract nr PR18.

Abstract A070: Myosin heavy chain 9 (MYH9) is a novel interacting partner of kinesin family member C1 (KIFC1) in African American TNBC: Implications for racial disparity

Abstract Women of African descent in the United States are twice as likely to develop triple-negative breast cancer (TNBC), compared to women of European descent. African American (AA) women with TNBC present a much more aggressive disease course relative to their European American (EA) counterparts. Thus, there is an urgent need to evaluate race-specific biomarkers and their molecular regulation to improve survival outcome in African American (AA)patients. Kinesin family member C1 (KIFC1), a minus-end-directed microtubule motor protein is involved in centrosomal clustering and chromosomal instability. KIFC1 is highly overexpressed (fold change=1.50, p=0.000392) in TNBC relative to non-TNBC. We previously showed that ablation of KIFC1 impaired proliferation and migration of AA TNBC cells to a greater extent than European American (EA) equivalent. Our published data also showed that the weighted index (WI) of nuclear KIFC1 (nKIFC1) is higher in AA TNBC (WI: 154.66 vs. 133.74, AA and EA, respectively, p = 0.036) and nKIFC1 is an independent poor prognosis biomarker exclusively for AA TNBC patients. While the centrosome clustering role of KIFC1 during mitosis is well-established, its predominant nuclear localization in interphase may underlie its previously unrecognized race-specific association. We and others have shown that nKIFC1 immunoprecipitates with various histones, helicases, and actinins. Immunoprecipitation (IP) of KIFC1 from nuclear fractions of TNBC cell lines followed by mass-spectrometry analysis revealed myosin heavy chain 9 (MYH9), a tumor suppressor protein that stabilizes p53 and promotes its nuclear retention, as the top nKIFC1 binding partner in AA TNBC cell lines (MDA-MB-468 and HCC1806). However, we observed a rather low affinity (rank 5th) or no binding of MYH9 with nKIFC1 in EA TNBC cell lines (MDA-MB-436, HCC1937). Our IP-MS data suggest that a) the relative abundance of MYH9 in nKIFC1 immunoprecipitates is higher in the AA TNBC cell lines and b) MYH9 expression is higher in AA TNBC compared to EA TNBC cells. We hypothesize that nKIFC1 impairs the tumor suppressor function of MYH9 more effectively in AA compared to EA TNBC cells. Intriguingly, KIFC1 immunoprecipitates also showed the presence of beta, alpha-, and gamma actin along with several dead box helicases such as DDX5, DDX17, exclusively in AA TNBC cell lines, suggesting that a combination therapy regimen of KIFC1 inhibitor (SR31527, CW069, and AZ82) and taxane/anthracycline/platinating agent might be more effective in AA than in EA TNBC cells. RNA seq analyses to identify other KIFC1partners using KIFC1 knock out using CRISPR-CAS9 approach in AA and EA TNBC cell lines are currently underway. Citation Format: Chakravarthy Garlapati, Shriya Joshi, Luciane Cavalli, Uma Krishnamurti, Shobhna Kapoor, Ritu Aneja. Myosin heavy chain 9 (MYH9) is a novel interacting partner of kinesin family member C1 (KIFC1) in African American TNBC: Implications for racial disparity [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 A070.

Abstract B042: Loss of nuclear alpha catenin contributes to chemoresistance and aggressive disease in African American triple-negative breast cancer patients

Abstract Triple-negative breast cancer (TNBC) is a particularly aggressive, 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 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 validate our findings of alpha catenin loss in TNBC, we analyzed over 600 breast cancer patient samples by immunohistochemistry. We found that alpha catenin protein loss was inversely correlated with survival in TNBC. Moreover, loss was observed more frequently in tumors from AA patients. Interestingly, both nuclear and cytosolic alpha catenin adhered to this pattern. While its cytosolic 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 developed several isogenic cell line pairs. Using CRISPR/Cas9-mediated gene editing, we generated 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 found that alpha catenin KO cells demonstrated greater capacity for anchorage-independent growth. Loss of alpha catenin also resulted in decreased sensitivity to DNA-damaging chemotherapies including platinum salts (cisplatin, carboplatin), topoisomerase poisons (doxorubicin, etoposide), and the PARP inhibitor olaparib. This is clinically relevant as doxorubicin, carboplatin, and olaparib are approved for treatment of TNBC patients. To identify binding partners, we performed co-immunoprecipitation of alpha catenin from nuclear lysates, followed by mass spectrometry. We found that nuclear alpha catenin can 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, which ensures that only cells with undamaged DNA may enter mitosis. We further found that 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 study, 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 in a subset of patients. Citation Format: Rania Bassiouni, Victoria Dirgo, Krystine Mansfield, Ritin Sharma, Lee D. Gibbs, Patrick Pirrotte, Nasreen Vohra, Kevin Gardner, John D. Carpten. Loss of nuclear alpha catenin contributes to chemoresistance and aggressive disease in African American triple-negative breast cancer patients [abstract]. In: Proceedings of the Eleventh AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2018 Nov 2-5; New Orleans, LA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(6 Suppl):Abstract nr B042.