Abstract 1423: SSX-2 expression and function in metastatic prostate cancer
Authors: Bloom Jordan E. , McNeel Douglas G. 
Published: August 2015

Background: The SSX family of proteins, whose function and aberrant expression in prostate cancer isn't fully understood, may lead to disease progression from localized to metastatic disease.

Cancer-testis antigens (CTAs) are a group of proteins whose expression is restricted to germline cells among normal somatic cells. CTAs have been found to be aberrantly expressed in many different cancers. Due to their restricted expression to tumor cells among MHC class I-expressing cells, there has been interest in CTAs as tumor-specific targets for CD8+ T cell-directed therapies. Amongst the CTA genes is the Synovial Sarcoma X chromosome breakpoint (SSX) gene family. The SSX family consists of ten highly homologous members, several of which have been detected in a wide variety of cancers. We have previously shown that SSX expression is confined to metastatic prostate tissue, and is not detected in primary prostate tumors. The function of the SSX family is largely unknown, and further insights into its function may reveal avenues for future directed therapies.

Methods: We determined the identity and prevalence of SSX family members in metastatic prostate cancer tissues and in peripheral blood samples of patients with prostate cancer. Specifically, cDNA from metastatic prostate tissues was examined by PCR with primers specific for each SSX family member; SSX family member expression was detected in peripheral blood samples using rt-PCR. To further understand SSX-2 function, prostate cell lines with SSX-2 expression knocked down or overexpressed were evaluated in vitro for morphological changes, tumorigenicity (soft agar colony formation), proliferation, and invasion (scratch test). Finally using qPCR, expression levels of markers of the epithelial to mesenchymal transition (EMT) were examined in the knocked down and overexpressed cell lines.

Results: SSX-2 was found to be the dominant family member expressed in prostate cancer, with 47% (7/15) of metastatic cDNA samples expressing SSX-2. SSX-1 was detected in one sample (1/15), and no additional family members were detected. SSX-2 mRNA was similarly detectable in the peripheral blood of 17 of 33 (51%) patients with recurrent prostate cancer and 0% (0 of 10) control blood donors without prostate cancer. In in vitro functional studies, SSX-2 knockdown of the 22Rv1 cell line led to cell morphology changes, increased cell proliferation, and a decrease in soft agar colony formation. Knockdown was also accompanied by altered expression of genes involved in EMT (Twist-1, ZEB1/2, Slug, vimentin). Complementary changes were observed in the RWPE-1 normal prostate epithelial cell line transfected to overexpress SSX-2.

Conclusions: Due to the preferential expression of SSX-2 in disseminated prostate cancer, as well as the effects of SSX knockdown and overexpression on EMT-related genes, we believe that SSX-2 is involved in the EMT process, and potentially in the progression of prostate cancer to metastatic disease.

Citation Format: Jordan E. Bloom, Douglas G. McNeel. SSX-2 expression and function in metastatic prostate cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1423. doi:10.1158/1538-7445.AM2015-1423