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SS18-SSX co-opts P300 to sustain oncogenic transcription independent of SWI/SNF activity

Doherty, A.; Lagan, E.; Elliott, R. J. R.; Wynne, K.; Monger, C.; Bracken, A.; Shi, J.; Vakoc, C.; Armstrong, S.; Oliviero, G.; Ott, C.; Carragher, N.; Brien, G.

2026-01-27 cancer biology
10.64898/2026.01.26.701706 bioRxiv
Show abstract

Synovial sarcoma is driven by the SS18-SSX fusion oncoprotein, which has been assumed to promote tumorigenesis through its incorporation into the SWI/SNF chromatin remodeling complexes. Accordingly, therapeutic efforts have focused on targeting SS18-SSX containing SWI/SNF assemblies, yet these approaches have produced limited clinical benefit. Here, we demonstrate that SS18-SSX sustains oncogenic transcription independent of SWI/SNF activity. Despite efficient degradation and dismantling of SWI/SNF complexes, fusion occupancy at target loci and associated gene expression programs remain largely intact. Instead, we identify the acetyltransferase P300 as an essential co-factor supporting SS18-SSX chromatin binding and transcriptional activation. Targeting P300 displaces the fusion from chromatin, suppresses its transcriptional output, compromising synovial sarcoma viability. Notably, dual PROTAC mediated degradation of P300 and SWI/SNF produces strong synergistic effects, broadly disrupting SS18-SSX localization and function. These findings redefine the mechanistic basis of synovial sarcoma and reveal a mechanistically anchored therapeutic strategy for targeting its core oncogenic driver.

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