Reprogramming of mesenchymal stem cells by the synovial sarcoma-associated oncogene SYT-SSX2

Prevailing importance of the hedgehog signaling pathway and the potential for treatment advancement in sarcoma

The hedgehog signaling pathway is important in embryogenesis and post natal development. Constitutive activation of the pathway due to mutation of pathway components occurs in ~25% of medulloblastomas and also in basal cell carcinomas. In many other malignancies the therapeutic role for hedgehog inhibition though intriguing, based on preclinical data, is far from assured. Hedgehog inhibition is not an established part of the treatment paradigm of sarcoma but the scientific rationale for a possible benefit is compelling. In chondrosarcoma there is evidence of hedgehog pathway activation and an ontologic comparison between growth plate chondrocyte differentiation and different chondrosarcoma subtypes. Immunostaining epiphyseal growth plate for Indian hedgehog is particularly positive in the zone of pre-hypertrophic chondrocytes which correlates ontologically with conventional chondrosarcoma. In Ewing sarcoma/PNET tumors the Gli1 transcription factor is a direct target of the EWS-FLI1 oncoprotein present in 85% of cases. In many cases of rhabdomyosarcomas there is increased expression of Gli1 (Ragazzini et al., 2004). Additionally, a third of embryonal rhabdomyosarcomas have loss of Chr.9q22 that encompasses the patched locus (Bridge et al., 2000). The potential to treat osteosarcoma by inhibition of Gli2 and the role of the pathway in ovarian fibromas and other connective tissue tumors is also discussed (Nagao et al., 2011; Hirotsu et al., 2010). Emergence of acquired secondary resistance to targeted therapeutics is an important issue that is also relevant to hedgehog inhibition. In this context secondary resistance of medulloblastomas to treatment with a smoothened antagonist in two tumor mouse models is examined.

Genome-wide recruitment to Polycomb-modified chromatin and activity regulation of the synovial sarcoma oncogene SYT-SSX2

Background

SYT-SSX is the oncogene associated with synovial sarcoma (SS), a stem cell disease. SYT-SSX is thought to be responsible for sarcoma initiation and development. It interacts with components of Polycomb and SWI/SNF complexes, the two epigenetic controllers that maintain the heritable status of differentiation-specific genes in the stem/progenitor cell. Through these associations SYT-SSX is thought to alter gene expression programs by epigenetic mechanisms. Recently, we reported that SYT-SSX2 reprograms mesenchymal stem cells and myoblasts by dictating their commitment to the neural lineage while disrupting their normal differentiation. This reprogramming was due to the direct occupancy of proneural genes by the SYT-SSX2 nuclear complex. To gain a clear understanding of SYT-SSX2 control of gene expression networks, we conducted a thorough genome-wide analysis to determine the mechanism of its recruitment and identify signature sets of epigenetic markers that would predict its targeting and transcriptional activity.

Results

SYT-SSX2 was recruited to distinct loci across all chromosomes, and an overwhelming number of Polycomb-modified sites enriched with the trimethylated histone H3 on lysine 27 (H3K27me3) formed the main recruiting module for SYT-SSX2. Not all SYT-SSX2/H3K27me3-occupied genes had altered expression, denoting the requirement for additional signals upon oncogene binding. Differential binding and epigenetic patterns distinguished upregulated and downregulated genes. Most activated genes had SYT-SSX2 sites enriched with H3K27me3 within their body or near their transcription start site (TSS) whereas a majority of downregulated genes were characterized by SYT-SSX2/H3K27me3-rich regions at long-range, or by modifications associated with transcription activation within the gene body or near the TSS. Hierarchical and functional clustering identified H3K27me3 as the dominant epigenetic marker associated with SYT-SSX2 binding and gene expression. Notably, this analysis revealed a cluster of upregulated neuronal genes densely covered by H3K27me3, consistent with programming toward the neural lineage by SYT-SSX2 observed previously.

Conclusions

The data analysis revealed that Polycomb complexes or their modified chromatin and their stably silenced differentiation programs seem to be the main target for SYT-SSX2, suggesting that their perturbation is at the center of tumorigenesis driven by the oncogene. Further research into this mechanism is crucial to the full understanding of SS biology.

Downstream and intermediate interactions of synovial sarcoma-associated fusion oncoproteins and their implication for targeted therapy

Synovial sarcoma (SS), an aggressive type of soft tissue tumor, occurs mostly in adolescents and young adults. The origin and molecular mechanism of the development of SS remain only partially known. Over 90% of SS cases are characterized by the t(X;18)(p11.2;q11.2) translocation, which results mainly in the formation of SS18-SSX1 or SS18-SSX2 fusion genes. In recent years, several reports describing direct and indirect interactions of SS18-SSX1/SSX2 oncoproteins have been published. These reports suggest that the fusion proteins particularly affect the cell growth, cell proliferation, TP53 pathway, and chromatin remodeling mechanisms, contributing to SS oncogenesis. Additional research efforts are required to fully explore the protein-protein interactions of SS18-SSX oncoproteins and the pathways that are regulated by these partnerships for the development of effective targeted therapy.