Abstract A080 PRC1/BMI1 activity alters chromatin accessibility to regulate differentiation in atypical teratoid rhabdoid tumors

Abstract Atypical Teratoid Rhabdoid Tumor (ATRT) is a highly aggressive pediatric brain tumor and characterized by SMARCB1 deletion. SMARCB1 is a member of the SWI/SNF chromatin remodeling complex that is responsible for determining cellular pluripotency and lineage commitment. We performed an RNAi-based functional genomic screen to identify key epigenetic regulators that co-operate with SMARCB1 loss and contribute to tumorigenesis in ATRT. 406 genes associated with epigenetic regulation in patient derived ATRT cell lines were targeted. This screen identified BMI1, a component of the Polycomb Repressive Complex 1 (PRC1), as one of the top targets essential for ATRT cell growth. Using RNA-seq and ChIP-Seq analysis we demonstrated that BMI1 co-operates with SMARCB1 deletion to suppress transcription of pro-differentiation pathways and promote self-renewal of tumor stem cells. In SMARCB1 deficient cells BMI1 forms a partial PRC1 complex devoid of DNA binding components. We found that genetic inactivation of BMI1 suppresses ATRT cell clonogenicity in vitro and decreases S-phase length. By Doxycycline-inducible SMARCB1 re-expression system we revealed activation of proapoptotic proteins p16 and p21. With immunoprecipitation for BMI1, followed by mass spectrometry analysis we demonstrated that re-expression of SMARCB1 activates two PRC1 chromatin localizing components CBX4 and CBX8. Moreover, inhibition of BMI1 significantly prolongs mice survival and decreases tumor size as evaluated by IVIS imaging system and T2-MRI. Thus, we demonstrated that SMARCB1 deletion results in reprograming of BMI1 chromatin occupancy away from lineage specification by altering the components of the PRC1 complex and that PRC1/BMI1 inhibition is a novel therapeutic approach in ATRT. Citation Format: Irina Irina Alimova, Gillian Murdock, Angela M. Pierce, Dong Wang, Nathan Dahl, Sujatha Venkataraman, Rajeev Vibhakar. PRC1/BMI1 activity alters chromatin accessibility to regulate differentiation in atypical teratoid rhabdoid tumors [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pediatric Cancer Research; 2024 Sep 5-8; Toronto, Ontario, Canada. Philadelphia (PA): AACR; Cancer Res 2024;84(17 Suppl):Abstract nr A080.