ATRT-15. ATRTS HIJACK EPIGENETIC MECHANISMS TO PROMOTE IMMUNOSUPPRESSION AND ESCAPE FROM NK CELL THERAPY

Atypical Teratoid/Rhabdoid Tumors (ATRTs) are malignant tumors of the central nervous system in children. They are almost universally characterized by genetic loss of SWI/SNF Related, Matrix Associated, Actin Dependent Regulator of Chromatin Subfamily member B1 (SMARCB1) - a core subunit of the ATP-dependent SWI/SNF chromatin remodeling complex, and dysregulation of lineage restricted enhancers. Here, we show that tumor subgroup-specific upregulation of a distant member of the transforming growth factor-β (TGF-β), bone morphogenic factor (BMF) family of proteins drives insensitivity to NK-mediated cytolysis in vitro and in vivo. Mechanistically, subgroup-specific loss of chromatin occupancy by the RE1 Silencing Transcription Factor (REST) to its consensus RE1 motif, allowed deposition of enhancer-specific histone H3 lysine4 (K4) mono-methylation and histone H3K27 acetylation marks to activate an enhancer-promoter interaction. Restoration of REST binding or BET-domain inhibition resulted in a significant reduction in the enhancer marks and enhancer-promoter interaction and involved histone H3K27me3 deposition at the REST binding site, to facilitate gene silencing and sensitization of ATRTs to NK-mediated cytolysis. Thus, our study has identified a novel function for REST as a modulator of tumor-intrinsic enhancer activity and tumor-extrinsic immune microenvironment. Of equal significance, we highlight a role of REST in recruiting the polycomb repressive complex-2 (PRC2) in gene silencing in ATRTs.