Abstract C089: Characterization of the breast tumor immune microenvironment in women of African descent using single-nucleus RNA- and ATAC-sequencing

Abstract Women of African descent are at an increased risk of developing and dying from aggressive subtypes of breast cancer. A connection between aggressive disease and Western Sub-Saharan African ancestry has been postulated, but it remains largely unknown to what extent breast cancer in Africa is reminiscent of breast cancer in U.S. African American (AA) women who experience disproportionately high mortality rates. We performed ATAC- and RNA-sequencing on 9 human triple-negative breast cancer cell lines of U.S. origin and discovered that African ancestry influences the chromatin landscape, leading to disparate transcription factor (TF) activity and downstream gene expression patterns indicative of an aggressive tumor biology. Here, we describe an ambitious study that employs single-nucleus (sn) ATAC- and RNA-sequencing (snMultiome) of frozen breast tumors to characterize chromatin accessibility and gene expression patterns with single-cell resolution in AA (n=33), Kenyan (n=25), and European American (EA, n=24) women in relation to genetic ancestry, risk factor exposures, clinical characteristics, and 5-year survival. To achieve this, we successfully isolated intact, high-quality single nuclei from archival frozen breast tumor tissue through an optimized combination of enzymatic digestion and automated tissue homogenization. We performed snMultiome sequencing of 82 tumors using the 10x Genomics platform. Following filtering, normalization (SCT for snRNA; LSI for snATAC), peak calling (MACS2), and integration (Harmony), our dataset includes a total of 296,557 nuclei. Cancerous (163,419 nuclei) and non-cancerous (133,138 nuclei) cells were distinguished based on DNA copy number (CopyKat). Within the microenvironment, 11 major immune, epithelial, and stromal cell types were successfully annotated, exhibiting distinct patterns by population group (e.g. AA tumors showed markedly increased abundance of myeloid and T-cells, while Kenyan tumors showed increased abundance of pericytes and fibroblasts, relative to EA). A large number of enriched TFs within each cell type varied significantly by population group, suggesting distinct chromatin accessibility patterns related to genetic ancestry. CD45+, EPCAM- cells were further extracted, clustered, and manually annotated for subpopulations based on known marker genes. We detected 26 distinct immune subpopulations across our samples, including 2 inflammatory macrophage, 6 immunosuppressive macrophage, 1 fibronectin+/thrombospondin+ monocyte, 6 T-cell, 1 natural killer cell, 2 B-cell, 1 mast cell, and 4 dendritic cell subpopulations. Differential gene expression analyses of each immune cell subpopulation, adjusted for age, BMI, and tumor subtype, revealed ancestry-specific gene expression patterns, which could provide novel insights into functional differences that may impact tumor immune response. Current efforts focus on in-depth molecular characterization of ancestry-related differences in the tumor immune environment and distinct signatures present in lethal disease. Citation Format: Alexandra R. Harris, Huaitian Liu, Brittany D. Jenkins, Tiffany H. Dorsey, Francis Makokha, Shahin Sayed, Gretchen Gierach, Stefan Ambs. Characterization of the breast tumor immune microenvironment in women of African descent using single-nucleus RNA- and ATAC-sequencing abstract. In: Proceedings of the 17th AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2024 Sep 21-24; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2024;33(9 Suppl):Abstract nr C089.