Abstract 4090: ADAR1 acts as a metastatic switch by sustaining survival programs in breast cancer brain metastasis

Abstract Background: Essential for preventing autoinflammation, adenosine deaminase acting on double-stranded RNA (ADAR1) has been widely implicated in tumor progression and immunotherapy resistance. However, its role in the establishment and progression of metastasis remains unexplored. High-grade breast cancer (BC) patients harboring ADAR1 overexpression have been correlated with a worse prognosis and reduced survival, suggesting that ADAR1 activation may also play a role in BC stem cells transformation. This project focused on the brain niche, one of the most aggressive and least studied sites for BC metastasis. Methods: We investigated ADAR1 dependencies in BC brain metastasis (BC-BrM) using single-cell RNA sequencing (scRNA-seq) of orthotopically derived triple-negative breast cancer (TNBC) BrM. The model was generated by intra-cerebroventricular implantation of TNBC cells carrying a nanoluc-GFP reporter for tracing ADAR1 activity into Rag2γc-/-mice. Data from this mouse model were compared with scRNA-seq and imaging of metastatic patient samples. Results: When TNBC cells were implanted into the mouse brain to form BrM, we observed upregulation of the tumor extracellular matrix (ECM) and JAK/STAT-driven inflammatory response, together with the activation of the WNT/β-Catenin pathway and ADAR1, compared to non-implanted TNBC cells. Consistent with these findings, we observed acorrelation between nuclear ADAR1 and WNT/β-Catenin activation in human tumors. As a readout for increased β-Catenin stability, we detected a marked inhibition of its negative regulator, GSK3β, in samples with high nuclear ADAR1/β-Catenin levels. Sc-RNA seq analysis of patient samples identified a cluster with high ADAR1 level characterized by fast dividing cells and activation of Protein Arginine Methyltransferase 1 (PRMT1), a driver of de novo fatty acids synthesis in BC cells. This cluster positively correlated with cancer cell proliferation programs, lipid remodeling and suppression of inflammatory stimuli. ADAR1 downregulation in the mouse model confirmed a decrease in PRMT1 across several clusters and revealed a reduction in the metastatic drive of the tumors, by impairing cell-cycle progression, ECM deposition and lipid remodeling. Finally, we identified a highly stressed cluster, predominantly composed of ADAR1 KD cells, characterized by endoplasmic reticulum stress, a robust unfolded protein response, and an apoptotic signature. Conclusions: Our findings identify ADAR1 as a critical “metastatic switch” that enables BC cells to coordinate proliferation and stress adaptation programs required for survival in the brain microenvironment. ADAR1 loss destabilizes metastatic proteostasis ultimately leading to programmed cell death. This study uncovers a key dependency of BC-BrM on ADAR1, highlighting it as a promising therapeutic target for disrupting the metastatic niche. Citation Format: Teresa Sposito, Emma Klacking, Jessica Pham, Roman Sasik, Inge Van der Werf, Najla Kfoury-Beaumont, Thomas Beaumont, Vanessa Goodwill, Alexander A. Khalessi, Thomas Whisenant, Catriona Jamieson. ADAR1 acts as a metastatic switch by sustaining survival programs in breast cancer brain metastasis [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 4090.