Abstract 2367: Changes in breast hormone signaling and inflammation underlie radiologic features of cancer risk

Abstract Background: Image-based features of the normal breast such as mammographic density and background parenchymal enhancement (BPE) on contrast-enhanced breast magnetic resonance imaging (MRI) are independent breast cancer risk factors. Both are associated with estrogen signaling and respond to endocrine therapy. Yet, molecular and cellular mechanisms underlying these risk factors remain unclear. We aimed to investigate such mechanisms with the goal of identifying novel approaches for cancer risk stratification and personalized prevention. Methods: Single-nucleus RNA sequencing on a 10x Chromium platform was performed on normal human breast tissues from 28 women undergoing prophylactic mastectomies (n = 11), lumpectomies (n = 10), or mastectomies for breast cancer (n = 7) who consented to our biobanking protocol. Differences in cell composition and signaling pathways between individuals with different breast densities and BPE levels were determined. Additionally we assessed estrogen responsiveness in patient-derived organoid (PDOs) from women with different levels of BPE using qRT-PCR. Evaluation of immune infiltration as potential mediator of BPE is underway using immunofluorescence and spatial transcriptomics. Results: 12, 377 nuclei from 28 women were analyzed, including 16 with low (minimal or mild) and 12 with high (moderate or marked) BPE levels on MRI, and 5 cases with not dense and 23 with dense breasts on mammography. All main breast epithelial and stromal cell types were represented in our data. We observed great heterogeneity in cell composition between patients. Samples from BPE-high patients contained luminal secretory cells with reduced expression of differentiation markers and luminal hormone sensing cells with higher expression of estrogen response genes like PGR and GREB1. In agreement with this, we observed increased response to estrogen signaling in PDOs from high (n = 5) versus low BPE cases (n = 5). Moreover, luminal cells from BPE-high samples expressed significantly higher levels of CD74 which has been correlated with immune infiltration in cancer. Supporting this, interactions between luminal cells with macrophages via CD74 and APP/COPA were predicted in BPE-high but not in low samples. Comparing samples with distinct mammographic densities, we observed a higher proportion of luminal secretory cells, basal cells, and fibroblasts in patients with high mammographic densities suggesting that distinct mechanisms mediate BPE levels and breast density. Conclusions: Our findings indicate that BPE is influenced by two distinct mechanisms: estrogen-mediated changes in cellular composition and chronic inflammation, both potentially increasing breast cancer susceptibility. Understanding the basis of modifiable breast cancer risk factors will enable risk stratification and personalized treatment, ultimately reducing breast cancer incidence in high-risk populations. Citation Format: Nadine Goldhammer, Marin Bont, Michael Choi, Shruti Warhadpande, Heather Greenwood, Soumi Gottipati, Tam Binh Bui, Michael Bruck, Shoko Emily Abe, Michael Alvarado, Cheryl Ewing, Karen Goodwin, Rita Mukhtar, Jasmine Wong, Laura van't Veer, Nola Hylton, Laura J. Esserman, Jennifer M. Rosenbluth. Changes in breast hormone signaling and inflammation underlie radiologic features of cancer risk [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2367.