Abstract 6318: Distinct molecular signatures in dense breast tissue as potential drivers of breast cancer development

Abstract Breast density is a significant independent risk factor for breast cancer; women with dense breasts have a 4-6-fold increased risk of the disease compared to women with non-dense breasts. It has been suggested that 30% of all breast cancer cases occur in women with > 50% dense areas. Breast density reflects variations in breast tissue composition. It is characterized by high proportions of stroma, containing collagen and other ECM proteins, fibroblasts, endothelial cells, and immune cells, suggesting a pro-tumor inflammatory microenvironment. To investigate molecular differences between dense and nondense breast tissue and their association with triple-negative breast cancer (TNBC), we used the PanCancer IO 360 panel to explore RNA expression and signaling pathway regulation across the samples. We observed differences in RNA expression among subjects, thereby identifying distinct molecular groups in dense breasts. The subjects were clustered into 3 subtypes with distinct molecular signatures and biological pathways. We identified a group (G1) with increased expression of genes involved in inflammatory processes, but still maintains a closer association with the phenotype of non-dense breasts. We also identified two other groups (G2 and G3) that are more distinct regarding the non-dense group and different from each other. These groups are related to reorganization and cellular architecture in the microenvironment (G2) and to proliferative regulation, cancer pathways, and increased gene expression in tumor development (G3). These groups do not seem to correlate with age or menopause stage in this first screen; instead, they resemble more subtypes of density or stages of density development. Additionally, analyzing RNA expression in TNBC samples, we identify an association with dense G3 samples that express more RNAs related to DNA repair, with upregulation of pathways in DNA damage, epigenetic regulation, apoptosis, and metabolic stress. At the same time, the pathways such as interferon signaling, immune cell adhesion and migration, and NF-kappa-beta are downregulated. These results suggest that specific density subtypes with distinct molecular signatures may promote tumor development more than others. To better understand this association and determine whether there are similarities in the resident cells of dense breast and TNBC samples, we are conducting spatial biology analyses to explore their microenvironments. The accurate classification of these subtypes has the potential to impact breast cancer prevention strategies and early diagnosis. Further investigations with a larger cohort are underway to understand better how molecular alterations in dense groups contribute to tumor development. Citation Format: Jay William Fox, Natalia Dworak, Patcharin Pramoonjago, Christopher A. Moskaluk, Ana Karina de Oliveira. Distinct molecular signatures in dense breast tissue as potential drivers of breast cancer development [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 6318.