Abstract 5141: ADAR1 maintains cancer stem cell proliferation in a mouse model of TNBC-derived breast cancer brain metastases

Abstract Background: Essential for preventing autoinflammation, adenosine deaminase acting on double-stranded RNA (ADAR1) is also a recognized driver of epi-transcriptomic remodeling and tumor immune silencing. High levels of ADAR1 have been associated with a worse prognosis and reduced survival in patients with high-grade breast cancer (BC), especially in the HER2+ and triple-negative BC (TNBC) subgroup, which are the most likely to develop brain metastasis (BrM), arising in up to 30% of TNBC and HER2+ cases. Nevertheless, the role of ADAR1 in the metastatic context is only partially understood. We have hypothesized that ADAR1 activation and upregulation of the p150 isoform play a pivotal role in promoting BC-BrM invasion by maintaining BC stem cell metastatic niches. We focused on the brain, one of the most aggressive and less studied metastatic sites. Methods: We developed a BC-BrM model based on intracerebroventricular implants of MDA-MD-231 TNBC and primary BC-BrM patient cells in Rag2-/-γc-/- mice. Before implantation, tumor cells were transduced with a nanoluc-GFP reporter for tracing ADAR1 enzymatic activity in the brain niche, or shRNA vectors for ADAR1 down regulation studies. Results: Upon establishment of BrM, scRNA sequencing and qRT-PCR revealed that ADAR1 p150 was upregulated, alongside WNT/β-Catenin pathway activation, a switch in the CD47 isoforms, and extra cellular matrix remodeling. Preliminary results from FFPE imaging of patient samples further suggested a correlation between ADAR1 and WNT/β-Catenin activation, showing that high levels of ADAR1 are associated with nuclear translocation of β-Catenin. Additionally, sc-RNA sequencing of ADAR1 knocked-down TNBC-derived BrM compared to scrambled controls, identified a cluster with increased protein stress and arrest of growth-related processes, predominantly represented by the ADAR1 KD cells. These findings suggest that ADAR1 is essential for maintaining cancer cell proliferation in the brain. Future Directions: We are now further validating the role of ADAR1 and its molecular interactors in maintaining the metastatic brain niche through scRNA-sequencing of BC-BrM patient samples collected from UCSD hospitals. Overall, this study provides new insights into the molecular role of ADAR1 deaminase activity in establishing and maintaining metastatic niches. Citation Format: Teresa Sposito, Emma Klacking, Jessica Pham, Roman Sasik, Inge van der Werf, Najla Kfoury-Beaumont, Alexander Khalessi, Thomas Whisenant, Catriona Jamieson. ADAR1 maintains cancer stem cell proliferation in a mouse model of TNBC-derived breast cancer brain metastases [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 5141.