Synovial sarcoma, X breakpoint (SSX) (SSX)

The Synovial sarcoma, X breakpoint (SSX) family consists of highly homologous proteins that primarily function as transcriptional repressors and are classified as cancer-testis antigens (CTAs) [8, 11]. In normal physiology, their expression is restricted to the testis, but they are aberrantly expressed in various malignancies, including melanoma and multiple myeloma [9, 15]. The family is most notably defined by its involvement in the t(X;18)(p11;q11) chromosomal translocation, which fuses the SS18 gene to SSX1, SSX2, or SSX4, creating the oncogenic SS18-SSX fusion protein characteristic of synovial sarcoma [1, 5]. This fusion protein disrupts chromatin remodeling by hijacking the BAF (SWI/SNF) complex, leading to the displacement of tumor suppressors and the activation of pro-proliferative genes like SOX2 [12, 13]. Therapeutically, SSX proteins are targeted via immunotherapies, such as TCR-engineered T cells and cancer vaccines, due to their tumor-specific expression [14, 17]. Additionally, the SS18-SSX fusion is targeted indirectly through HDAC inhibitors, which promote its degradation, and ATR inhibitors, which exploit the DNA damage vulnerabilities induced by the oncoprotein [1, 3]. Recent research also highlights the potential of SUMOylation inhibitors to disrupt the stability and function of the SSX-containing fusion protein [7].

Other names

SSX familyCancer-testis antigen family 5CT5Protein SSXSynovial sarcoma, X breakpoint proteinSSX1SSX2SSX4

Histone deacetylase inhibition leading to oncoprotein degradation, SUMOylation inhibition, ATR inhibition, and T-cell receptor (TCR) or CAR-T mediated cytotoxicity against SSX-derived peptides.