Abstract 6708: RUNX3 restoration in breast tumors leads to profound growth regression via c-Myc inhibition

Abstract Genetic alterations in the RUNX family genes and their co-factor CBFB are frequently observed in cancer. In breast cancer, RUNX1 and CBFB are identified as significantly mutated genes, while RUNX3 expression is often silenced through hypermethylation. These observations highlight the importance of RUNX genes in mammary homeostasis. Previous studies on luminal-type breast cancer demonstrated that RUNX3 restoration inhibits tumor progression by destabilizing the estrogen receptor α (ERα) protein. However, despite the therapeutic promise of reactivating RUNX3 in breast cancer, its role in hormone receptor-negative subtypes remains inadequately understood. Furthermore, RUNX3 has been reported to exhibit dualistic roles and depending on the cancer type and stage, may function as either tumor suppressor or oncogene. Yet, the molecular mechanisms underlying this duality remain unclear. To investigate the role of RUNX3 in breast pathogenesis, we developed the MMTV-PyMT; Krt8-CreERT2; Rosa26-lsl-RUNX3 mouse model and analysed the effect of conditional induction of RUNX3 expression in Krt8+ mammary epithelial cells and tumor cells within the MMTV-PyMT-driven breast cancer. Without RUNX3 induction, MMTV-PyMT mice developed an average of eight tumors per mouse, and transcriptomic analyses revealed robust activation of the Myc-signaling pathway. Conversely, the induction of RUNX3 significantly suppressed tumor growth, concomitant with a diminished c-Myc expression. This observation underscores the tight relationship between RUNX3 and c-Myc and identifies RUNX3-mediated suppression of Myc pathway as a major aspect of RUNX3 tumor suppressive role. Remarkably, histological analyses of RUNX3-induced breast tumors showed two distinct clusters of cancer cells: proliferating RUNX3-positive cells and non-proliferating RUNX3-positive cells. Spatial transcriptomic profiling revealed elevated c-Myc expression and suppressed RUNX1 expression in the proliferating RUNX3-positive cells compared to non-proliferating RUNX3-positive cells. This observation raises the intriguing possibility that interplay between RUNX1 and RUNX3 determines cellular proliferative potential and implicates RUNX1 as a regulator of RUNX3’s paradoxical traits. Consistently, overexpression of RUNX3 in human breast cancer cells induced significant cell death and upregulated RUNX1 expression. Our findings suggest the convergence of RUNX1 and RUNX3 pathways in driving cell states within the tumor microenvironment and establish a causal link between RUNX3, c-Myc activity, and tumor growth dynamics. This study provides critical insights into dual roles of RUNX3 in breast cancer pathogenesis. Citation Format: Nawaphat Jangphattananont, Junichi Matsuo, Wei Peng Yong, Linda SH Chuang, Yoshiaki Ito. RUNX3 restoration in breast tumors leads to profound growth regression via c-Myc inhibition [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 6708.