Loss of SS18-SSX1 Inhibits Viability and Induces Apoptosis in Synovial Sarcoma

Abstract

Background Most synovial sarcomas contain a chromosomal translocation t(X;18), which results in the formation of an oncoprotein SS18-SSX critical to the viability of synovial sarcoma. Questions/purposes We (1) established and characterized three novel synovial sarcoma cell lines and asked (2) whether inhibition of SS18-SSX1 decreases cell viability in these cell lines; and (3) whether reduction in viability after SS18-SSX1 knockdown is caused by apoptosis. After identifying a specific posttranscriptional splice variant in our cell lines, we asked (4) whether this provides a survival benefit in synovial sarcoma. Methods Cells lines were characterized. SS18-SSX1 knockdown was achieved using a shRNA system. Cell viability was assessed by WST-1 analysis and apoptosis examined by caspase-3 activity. Results We confirmed the SS18-SSX1 translocation in all cell lines and identified a consistent splicing variant. We achieved successful knockdown of SS18-SSX1 and with this saw a significant reduction in cell viability. Decreased viability was a result of increased apoptosis. Reintroduction of the exon 8 sequence into cells reduced cell viability in all cell lines. Conclusions We confirmed the presence of the SS18-SSX1 translocation in our cell lines and its importance in the survival of synovial sarcoma. We have also demonstrated that reduction in cell viability is related to an increase in apoptosis. In addition, we have identified a potential mediator of SS18-SSX function in exon 8. Clinical Relevance SS18-SSX represents a tumor-specific target in synovial sarcoma. Exploitation of SS18-SSX and its protein partners will allow us to develop potent tumor-specific therapeutic agents.

Preclinical Evidence of Anti-Tumor Activity Induced by EZH2 Inhibition in Human Models of Synovial Sarcoma

“…In synovial sarcoma a recurrent chromosomal translocation fuses the SS18 gene (a subunit of the SWI/SNF chromatin remodeling complex [ 10 ]) on chromosome 18 to one of three related genes on the X chromosome, SSX1 , SSX2 and rarely SSX4 , resulting in the expression of a SS18-SSX fusion protein [ 11 – 16 ]. This translocation is believed to be the sole driving event in synovial sarcomas [ 17 ], as expression of the translocation product in mouse models induces synovial sarcoma tumors at 100% penetrance [ 18 ], and expression of the translocation in synovial sarcoma cell lines is required for their survival [ 19 ]. Additionally, the SS18-SSX translocation is frequently the sole chromosomal aberration observed in synovial sarcoma, and there are very few additional mutations present in this indication [ 17 ].…”

Synovial Sarcoma: Recent Discoveries as a Roadmap to New Avenues for Therapy

“…Multiple lines of evidence implicate SS18-SSX as the central genetic “driver” in this cancer: i) its presence as the sole cytogenetic anomaly in up to a third of cases (8), ii) the low frequency of additional mutations (9), iii) its preservation in metastatic and advanced lesions (8), iv) the death of synovial sarcoma cells upon SS18-SSX knockdown (10) and v) its ability to induce tumors in conditional mouse models with identical histology, orthologous gene expression and immunophenotype, with 100% penetrance (11). …”

Localized synovial sarcoma: A single institutional study of 191 patients with a minimum follow‐up of 5 years for survivors

“…However, it may occur at all ages and affect various locations . The unique chromosomal translocation t(18; X) (p11; q11), which leads to the formation of a fusion protein, is considered to cause this disease . The most reliable treatment is wide surgical resection with or without radiotherapy .…”