CSIG-17. ADHESION GPCR BAI1/ADGRB1 CAN BLOCK IGF1R-MEDIATED GROWTH SIGNALLING, INCREASE RADIOSENSITIVITY AND AUGMENT SURVIVAL IN MEDULLOBLASTOMA

Abstract Medulloblastoma (MB) is an aggressive pediatric brain tumors and increased knowledge about the disease mechanisms is needed to develop new therapeutic approaches. Brain-specific Angiogenesis Inhibitor 1 (BAI1/ADGRB1) is an adhesion GPCR receptor and tumor suppressor epigenetically silenced in MB. Our prior studies showed that BAI1 can bind the MDM2 E3 ubiquitin ligase and relocate it to the cell membrane, thereby preventing it from degrading p53 in the nucleus (Zhu et al, Cancer Cell, 2018). Whether other MDM2 targets are affected by this relocation that may be functionally important in MB development is unknown. We hypothesized that BAI1-mediated relocation of MDM2 might destabilize cell surface oncogenic receptors and found that BAI1 stimulates IGF1R poly-ubiquitination and degradation. Mechanistically, BAI1 fosters MDM2-IGF1R interaction through beta-arrestins, adaptor proteins connecting MDM2 to IGF1R. Epigenetic reactivation of BAI1 expression suppressed Stat3 and Akt signaling, and radio-sensitized MB cells, leading to significantly improved survival in orthotopic MB xenograft models in mice regardless of tumor p53 mutation status. These findings are important as they demonstrate that BAI1 has dual anti-tumor effects in MB through MDM2 membrane re-localization, stabilizing p53 and blocking IGF1R signaling. They further suggest that epigenetic targeting of BAI1 is a promising therapeutic approach for clinical translation.