Activity and mechanism of action of histone deacetylase inhibitors in synovial sarcoma

Abstract

Synovial sarcoma is a rare but aggressive paediatric soft tissue sarcoma that is driven by the genomic rearrangement t(X;18)(p11.2;q11.2). The resulting fusion protein, SS18- SSX, exhibits oncogenic properties through protein-protein interactions that alter gene transcriptions and chromatin remodelling. Synovial sarcomas respond poorly to the conventional chemotherapies. Hence, it is necessary to develop novel therapeutic targets to improve the outcome of the treatment in synovial sarcoma. Considering that the expression of the fusion protein SS18-SSX is specific and essential for synovial sarcoma development and survival, targeting this chimeric oncoprotein or its functional protein network is a potential therapeutic strategy. To date there are no known compounds that directly target SS18-SSX, the driving fusion protein of synovial sarcoma.

SS18 is a transcriptional activator, member of the SWI/SNF complex that is ubiquitously expressed in normal cells. In the SS18-SSX fusion protein, the last nine amino acids of SS18 are replaced by 78 C-terminal amino acids of SSX. The resultant fusion protein therefore inherits oncogenic activities that are independent of the wild type SS18 and SSX. It recruits proteins that are involved in epigenetic gene regulation such as histone deacetylases and polycomb group proteins to form a fusion protein complex that regulates gene transcription and subsequently drives malignant transformation.

The work in this thesis explores the role of two class III histone deacetylases, SIRT1 and SIRT2, in the proliferation and survival of synovial sarcoma. We found that SIRT1 is overexpressed in primary synovial sarcomas. Both siRNA mediated and pharmacological knock down of SIRT1 and SIRT2 in synovial sarcomas and rhabdomyosarcomas impairs cell proliferation and autophagy flux. The overexpression of SIRT1 in synovial sarcomas was not associated with the clinical outcome of patients. However, in experimental in vitro cell assays, we observed that nutrient deprivation enhanced the sensitivity of synovial sarcoma and rhabdomyosarcoma cells to sirtuin inhibition. These results suggest a role of SIRT1 and SIRT2 in tumor cell survival under conditions of nutrient deficiency.

Mechanistically, we found that SIRT1 and SIRT2 form a complex with SS18-SSX and their histone substrate H4K16ac in a synovial sarcoma cell line and a patient-derived synovial sarcoma. This complex is disrupted shortly after exposure to the sirtuin inhibitors tenovin-6 and AGK2. These findings indicate that the interactions between SIRT1/SIRT2 and SS18-SSX may contribute to the malignant phenotype of synovial sarcoma by modifying acetylation of the SIRT1 and SIRT2 substrate, H4K16. The acetylation of H4K16 is associated with autophagy. Future studies should address whether autophagy genes are targets of the SS18-SSX/SIRT1/SIRT2 complex.

We set up a proximity ligation assay (PLA) as a screening method to search for small compounds that disrupt the SS18-SSX fusion protein complex. We first validated the method and showed that the association of SS18-SSX with TLE1 is specific for synovial sarcoma and can be visualised by PLA. We then screened a library of 16000 molecules and identified class I HDAC inhibitors and a novel compound, SXT1596, as agents that are able to dissociate the SS18-SSX/TLE1 complex and induce apoptosis in synovial sarcomas. We further showed that the disruption of the SS18-SSX/TLE1 complex by SXT1596 in synovial sarcoma released repression of EGR1 and rescued normal signaling.

The studies in this thesis provide direct evidences that SS18-SSX interacts with proteins that regulate transcription by epigenetic modification of targets, such as class I histone deacetylases and sirtuins. The disruption of the SS18-SSX protein complex with small molecular HDAC inhibitors induces rapid death of synovial sarcoma cells proving that targeting the driving complex of synovial sarcoma may give an opportunity to develop effective therapies for synovial sarcoma patients. Furthermore, PLA based drug screening is shown to be a reliable and valuable technique to identify lead compounds that disrupt protein-protein interactions and could be applied in other cancer types that are driven by fusion transcription factors.