Ectopic expression of cancer/testis antigen SSX2 induces DNA damage and promotes genomic instability

Abstract

SSX cancer/testis antigens are frequently expressed in melanoma tumors and represent attractive targets for immunotherapy, but their role in melanoma tumorigenesis has remained elusive. Here, we investigated the cellular effects of SSX2 expression. In A375 melanoma cells, SSX2 expression resulted in an increased DNA content and enlargement of cell nuclei, suggestive of replication aberrations. The cells further displayed signs of DNA damage and genomic instability, associated with p53-mediated G1 cell cycle arrest and a late apoptotic response. These results suggest a model wherein SSX2-mediated replication stress translates into mitotic defects and genomic instability. Arrest of cell growth and induction of DNA double-strand breaks was also observed in MCF7 breast cancer cells in response to SSX2 expression. Additionally, MCF7 cells with ectopic SSX2 expression demonstrated typical signs of senescence (i.e. an irregular and enlarged cell shape, enhanced β-galactosidase activity and DNA double-strand breaks). Since replication defects, DNA damage and senescence are interconnected and well-documented effects of oncogene expression, we tested the oncogenic potential of SSX2. Importantly, knockdown of SSX2 expression in melanoma cell lines demonstrated that SSX2 supports the growth of melanoma cells. Our results reveal two important phenotypes of ectopic SSX2 expression that may drive/support tumorigenesis: First, immediate induction of genomic instability, and second, long-term support of tumor cell growth.

The ‘melanoma-enriched’ microRNA miR-4731-5p acts as a tumour suppressor

“…The authors also showed that knockdown of SSX2 expression had a similar effect to the overexpressed constructs, with cell growth being diminished [27]. Therefore, based on these data, the authors concluded that melanoma appears dependent on an optimal level of SSX2 expression [27]. With these data in mind, we selected SSX4 as a representative of the SSX family for further investigation.…”

“…Strikingly, cell proliferation and migration were unaffected by the lack of SSX4 or SSX4B expression [26]. Furthermore, in A375 cells, induction of SSX2 expression using a lentiviral construct (above endogenous tumour levels) led to a reduction of cell viability and colony formation, and induced cell cycle G1 checkpoint arrest [27]. SSX2 expression was further found to induce DNA damage response which promoted genomic instability [27].…”

“…Furthermore, in A375 cells, induction of SSX2 expression using a lentiviral construct (above endogenous tumour levels) led to a reduction of cell viability and colony formation, and induced cell cycle G1 checkpoint arrest [27]. SSX2 expression was further found to induce DNA damage response which promoted genomic instability [27]. The authors also showed that knockdown of SSX2 expression had a similar effect to the overexpressed constructs, with cell growth being diminished [27].…”

“…SSX2 expression was further found to induce DNA damage response which promoted genomic instability [27]. The authors also showed that knockdown of SSX2 expression had a similar effect to the overexpressed constructs, with cell growth being diminished [27]. Therefore, based on these data, the authors concluded that melanoma appears dependent on an optimal level of SSX2 expression [27].…”

Meiosis-like Functions in Oncogenesis: A New View of Cancer

“…[19][20][21][22][23][24][25][26][27][28][29][30], including the fostering of genome instability, a driver of cancer evolution (24). However, given that the normal function of many CT genes in spermatogenesis is unknown, it remained unclear whether proteins that normally specifically orchestrate meiotic chromosome segregation events (such as interhomolog association/recombination and sister centromere monopolarity) contribute to maintenance and/or development/progression of cancers.…”