Abstract
SSX was initially identified as a melanoma associated tumour antigen MEL2 and in the SS18/SSX fusion gene of synovial sarcoma. It consists of a family of nine, highly homologous X chromosome genes, being SSX1, SSX2 and SSX4 the most commonly expressed in tumours. In normal tissue, SSX expression is restricted to germ cells, trophoblasts, and mesenchymal stem cells. In malignant cells, SSX expression is over-represented in sarcomas. SSX expression is epigenetically regulated by methylation and histone deacetylation.
Aim
To investigate the oncogenic properties of SSX. Validate it as tumour target and identify lead compounds.
Methods
RNAi system for conditional silencing of SSX expression, Protein analysis and Mass Spec, PCR arrays, tumour xenographs and drug library screening.
Results
SSX expression peaks at the G1/S phase of the cell cycle where it interacts with histone H3, H4 and β-catenin. This interaction promotes the phosphorylation of β-catenin at tyrosine residues and the transactivation of β-catenin genes MMP2 and cyclin D1. The association of SSX and MMP2 expression resulted in an increased invasion capacity of tumour cells.
In vivo, silencing of SSX in tumour xenographs significantly affected the tumour growth. These tumours were characterized by large necrotic areas, impaired vascularization and cytoplasmic β-catenin. In contrast, tumour xenographs expressing SSX were highly proliferative, vascularized and exibited nuclear β-catenin. We further screened a chemical library and have identified lead compounds that target SSX.
Conclusion
Our results demonstrate that SSX is a tumour target for drug development. We hypothesise that drugs targeting of SSX could affect functions related with the mesenchymal cell phenotype having potential cytostatic effect in sarcomas and/or epithelial tumour cells that undergo a epithelial-to-mesenchymal transition prior to metastasis.
