[A remarkable advancement in structural biology aimed at elucidating the ‍mechanism of synovial sarcoma development]

Synovial sarcoma is a type of soft tissue sarcoma that predominantly occurs near the joints of the extremities in young adults. Its hallmark is a recurrent and pathogenic chromosomal translocation, t(X;18)(p11.2;q11.2), which results in the fusion of the SSX1 or SSX2 gene with SS18. The expressed SS18-SSX fusion protein induces abnormalities in the SWItch/Sucrose Non-Fermentable (SWI/SNF) complex, a chromatin remodeling complex. In this paper, we refer specifically to the human SWI/SNF complex as mSWI/SNF. Since 2020, significant progress has been made in elucidating the molecular mechanisms underlying the initial event in synovial sarcomagenesis, particularly in structural biology, thereby opening new possibilities for structure-based drug design (SBDD). SS18-SSX1 replaces the wild-type SS18, an essential subunit of mSWI/SNF, and in turn ejects SMARCB1, another core subunit of the complex. This aberrant mSWI/SNF complex (ssSWI/SNF) is then relocated to nucleosomes containing H2A K119Ub. H2A is one of the core histone proteins, and its 119th lysine residue is ubiquitinated to form H2A K119Ub. Chromatin domains harboring nucleosomes with this modification typically exhibit suppressed gene expression patterns. Furthermore, this region is occupied by polycomb complexes, but ssSWI/SNF competes with them, leading to gene activation, which constitutes the initial event in synovial sarcomagenesis. Given that SSX1 is normally expressed primarily in the testes, it is plausible that its ectopic expression leads to aberrant function within the chromatin remodeling complex. Ultimately, the C-terminal region of SSX1 was found to bind to the acidic patch within the nucleosome, and its structural details have been elucidated through cryo-electron microscopy.