ATRT-05. PRC1 IS AN ESSENTIAL DEPENDENCY AND THERAPEUTIC TARGET IN SMARCB1 DEFICIENT ATYPICAL TERATOID RHABDOID TUMORS Free

Atypical Teratoid Rhabdoid Tumors (ATRT) occur in young children with poor survival. ATRT are characterized by the absence of the chromatin remodeling protein SMARCB1 resulting in epigenetic dysregulation. To identify specific epigenetic factors essential for ATRT tumorigenesis, we performed a shRNA screen targeting 408 genes classified as epigenetic/chromatin-associated molecules in patient-derived ATRT cell lines. This unbiased screen identified BMI1, a component of the Polycomb Repressive Complex 1 (PRC1), as essential for ATRT cell viability. We hypothesized that BMI1 promotes ATRT tumorigenesis by co-operating with SMARCB1 loss to suppress scheduled transcription of pro-differentiation pathways and promote self-renewal of tumor stem cells. We found that BMI1 expression is elevated in ATRT patient samples and cell lines compared to the normal pediatric cerebellum. Genetic depletion of BMI-1 suppressed ATRT cell clonogenicity and decreased in vitro stem cell maintenance. Using RNA-seq-based gene set enrichment analysis we show that BMI1 depletion decreases c-MYC driven transcriptomic programs and induces p53 driven apoptotic programs. This suggests that BMI-1 drives ATRT tumorigenesis by promotingc-Myc driven tumor stem cell self-renewal and driving resistance to therapy by inactivating p53 mediated apoptotic pathways. Finally, we sought to evaluate whether BMI1 is a therapeutic vulnerability in ATRT. We found that PTC596, the clinical lead compound inhibitor of BMI-1, significantly inhibited clonogenic potential, decreased S-phase length and induced apoptosis in ATRT cells in vitro. Furthermore, using ChIP-seq we show that PTC596 alters chromatin occupancy of BMI1 away from differentiation associated gene promoters in ATRT cells. In vivo PTC 596 significantly decreased growth of intracranial orthotopic ATRT tumors compared to control animals as evaluated by MRI imaging. Moreover, treated mice survived significantly longer than control animals. These studies are the first pre-clinical validation of PRC1/BMI1 inhibition as a novel therapeutic approach in ATRT.