Targeting of SUMOylation leads to cBAF complex stabilization and disruption of the SS18::SSX transcriptome in synovial sarcoma [HSJHD_ChIP-seq]

Synovial Sarcoma (SS) is driven by the SS18::SSX translocation event and is often diagnosed in patients under 30. In general, SS is refractory to chemotherapy and other therapeutic approaches. SS18::SSX does not directly bind to DNA. Instead, SS18::SSX alters ATP-dependent chromatin remodeling complex BAF (mammalian SWI/SNF complex) complexes away from a wild-type SS18-containing canonical (cBAF) complex towards a SS18::SSX-containing non-canonical (nc)BAF complex to drive a SS-specific transcription program with subsequent tumor formation. Small molecule targeted therapies are expanding beyond the kinome, providing additional tools to treat cancers that are not yet amenable to targeted therapies. Here, we demonstrate that SS18::SSX activates the SUMOylation program and SS are sensitive to the small molecule SAE1/2 inhibitor, TAK-981 (subasumstat). Mechanistically, we demonstrate TAK-981 de-SUMOylates the cBAF and Polybromo-associated BAF complex (PBAF) complex member, SMARCE1. The result of which is stabilization and restoration of cBAF complexes on chromatin, shifting away from the dominant SS18::SSX-ncBAF-complex driven transcriptome. This phenotypic shift is associated with DNA damage and cell death, resulting in tumor inhibition across both human and mouse SS tumor models. As such, TAK-981 combined with standard-of-care chemotherapy enhances induced synergistic activity through increased DNA damage, leading to tumor regressions. Targeting the SUMOylation pathway in SS restores cBAF complexes and blocks the SS18::SSX-ncBAF transcriptome, identifying a therapeutic vulnerability in SS, and positioning the in-clinic TAK-981 as a novel candidate to treat this refractory cancer. Overall design: Chromatin Immunoprecipitation DNA sequencing (ChIP seq) for SS18::SSX, SMARCA4, KDM2B and H3K27ac in SYO.1 and HS-SY-II synovial sarcoma cell lines