SSX and the synovial-sarcoma-specific chimaeric protein SYT-SSX co-localize with the human Polycomb group complex

Abstract

Chromosome translocation t(X;18)(p11.2;q11.2) is unique to synovial sarcomas and results in an`in frame' fusion of the SYT gene with the SSX1 or closely-related SSX2 gene. Wild-type SYT and SSX proteins, and the SYT-SSX chimaeric proteins, can modulate transcription in gene reporter assays. To help elucidate the role of these proteins in cell function and neoplasia we have performed immunolabelling experiments to determine their subcellular localization in three cell types. Transient expression of epitope-tagged proteins produced distinctive nuclear staining patterns. The punctate staining of SYT and SYT-SSX proteins showed some similarities. We immunolabelled a series of endogenous nuclear antigens and excluded the SYT and SYT-SSX focal staining from association with these domains (e.g. sites of active transcription, snRNPs). In further experiments we immunolabelled the Polycomb group (PcG) proteins RING1 or BMI-1 and showed that SSX and SYT-SSX proteins, but not SYT, co-localized with these markers. Consistent with this we show that SSX and SYT-SSX associate with chromatin, and also associate with condensed chromatin at metaphase. Noteably, SSX produced a dense signal over the surface of metaphase chromosomes whereas SYT-SSX produced discrete focal staining. Our data indicate that SSX and SYT-SSX proteins are recruited to nuclear domains occupied by PcG complexes, and this provides us with a new insight into the possible function of wild-type SSX and the mechanism by which the aberrant SYT-SSX protein might disrupt fundamental mechanisms controlling cell division and cell fate.

Activation of the non-canonical Dvl–Rac1–JNK pathway by Frizzled homologue 10 in human synovial sarcoma

“…Hence, efforts have been made to understand the molecular basis of synovial sarcoma, but the origin tissue of synovial sarcoma is still not clarified. A SYT-SSX fusion gene, a product of chromosomal translocation t(X;18)(p11;q11), is a wellknown feature of synovial sarcoma and it is considered to play a critical role in the oncogenesis of synovial sarcoma (Turc-Carel et al, 1986;Clark et al, 1994;Brett et al, 1997;Lim et al, 1998;Soulez et al, 1999). Furthermore, several groups reported the gene expression profile analysis using cDNA microarray and revealed that synovial sarcoma showed a distinct gene expression pattern compared with other spindle cell sarcomas (Nagayama et al, 2002;Nielsen et al, 2002;Segal et al, 2003).…”

Concise Review: Cancer/Testis Antigens, Stem Cells, and Cancer

“…Importantly, several findings afford a role of the CTA SSX in stem cell biology. SSX is recruited to the nuclear domains occupied by the PcG protein Bmi-1 [44], a regulator of stem cell self-renewal [45]. SSX is involved in mesenchymal-to-epithelial transition (MET) by promoting matrix metalloproteinase 2 expression and changing E-cadherin levels, consequently affecting cell migration [28].…”

Immunostaining for SYT protein discriminates synovial sarcoma from other soft tissue tumors: analysis of 146 cases

“…10,11,15 The N-terminus of SYT determines its nuclear localization and the 34 amino acids of C-terminus (also called SSXRD domain) mediates SSX nuclear localization. The SSXRD domain also plays a pivotal role in determining the subnuclear localization of the SYT-SSX chimeric oncoprotein.…”