An autoregulatory feedback loop converging on H2AK119ub1 drives synovial sarcoma [CUT&RUN]. An autoregulatory feedback loop converging on H2AK119ub1 drives synovial sarcoma [CUT&RUN]

The SS18-SSX fusion drives oncogenic transformation in synovial sarcoma by bridging SS18, a member of mSWI/SNF complex, to polycomb repressive complex 1 (PRC1) target genes. Here we show that the SSX C-terminus, via its SSXRD domain, directs SS18-SSX chromatin binding independently of SS18. SSXRD specific targeting is mediated by interaction with H2AK119ub1 and histone MacroH2A with which the fusion overlaps genome wide. Variant Polycomb Repressive Complex 1.1 (PRC1.1) acts as the main depositor of H2AK119ub1 and consequently is required for SS18-SSX occupancy. Importantly, SSX C-terminus not only depends on H2AK119ub1 for localization but also further promotes it and consequently, high H2AK119ub1 levels are acquired during synovial sarcoma development. These results reveal an SS18-SSX/PRC1 autoregulatory feedback-loop that enforces fusion binding and could play a role in a wide-range of cancers and physiological settings where SSX proteins are overexpressed. Overall design: Chromatin profiles of endogenous SS18-SSX1/2, H2AK119ub1 in human synovial sarcoma cells and HA occupancy in osteosarcoma or synovial sarcoma cells expressing MSCV-HA-eGFP-SS18-PGK-Puro, MSCV-HA-eGFP-SS18-SSX1-PGK-Puro and MSCV-HA-eGFP- SSX-C-PGK-Puro

SS18-SSX融合蛋白通过将哺乳动物SWI/SNF（mSWI/SNF）复合物成员SS18桥接至多梳抑制复合体1（polycomb repressive complex 1, PRC1）的靶基因，驱动滑膜肉瘤的致癌转化。本研究证实，SSX蛋白C端通过其SSXRD结构域，可独立于SS18介导SS18-SSX的染色质结合。SSXRD的特异性靶向依赖于与组蛋白H2AK119泛素化（H2AK119ub1）以及融合蛋白在全基因组范围内共定位的组蛋白MacroH2A的相互作用。变异型多梳抑制复合体1.1（variant Polycomb Repressive Complex 1.1, PRC1.1）是H2AK119ub1的主要沉积酶，因此是SS18-SSX占据染色质所必需的。值得注意的是，SSX蛋白C端不仅依赖H2AK119ub1实现定位，还可进一步促进该修饰的形成，进而在滑膜肉瘤发生过程中获得高水平的H2AK119ub1。本研究结果揭示了SS18-SSX/PRC1自调控反馈环路，该环路可强化融合蛋白的染色质结合，且可能在SSX蛋白过表达的多种癌症及生理场景中发挥作用。
实验设计概述：对人滑膜肉瘤细胞中内源性SS18-SSX1/2、H2AK119ub1的染色质谱，以及表达MSCV-HA-eGFP-SS18-PGK-Puro、MSCV-HA-eGFP-SS18-SSX1-PGK-Puro与MSCV-HA-eGFP-SSX-C-PGK-Puro的骨肉瘤或滑膜肉瘤细胞中HA的结合谱进行检测。