Growth Inhibition and Induction of Innate Immune Signaling of Chondrosarcomas with Epigenetic Inhibitors.

Chondrosarcomas are inherently resistant to chemotherapy and radiotherapy, pointing to an unmet need for new treatment options. Immune checkpoint inhibitors, which have shown remarkable promise in multiple solid cancer types, have limited efficacy in chondrosarcomas. Mutations in IDH1/2 genes, which result in progressive increases in DNA and histone methylation, are observed in 50% of conventional chondrosarcomas, suggesting that epigenetic dysregulation represents a potential barrier for tumor progression and target for therapeutic intervention. Here, we demonstrated that combined treatment of FDA-approved inhibitors of DNA methyltransferases (DNMTs) 5-aza-2'-deoxycytidine (5-aza), and histone deacetylases (HDACs) suberanilohydroxamic acid (SAHA) impaired the proliferation of chondrosarcoma cell lines in vitro and in xenograft studies. Transcriptomic analysis reveals that chondrosarcoma cells treated with 5-aza and SAHA markedly elevated the expression of IFN-stimulated genes including PD-L1, indicating that these epigenetic drugs induced a potent innate immune response. We demonstrated that 5-aza and SAHA resulted in both genomic and epigenomic instability, as shown by elevated DNA damage response and derepression of retrotransposons, respectively, which in turn activated pattern recognition receptors (PRRs) and the downstream IFN signaling pathways. Importantly, the cytotoxic effects of 5-aza and SAHA can be rescued by depletion of PRRs such as cGAS and MAVS, and potentiated by depletion of the RNA-editing enzyme ADAR1. Together, our results demonstrate preclinical activity of combined DNMT and HDAC inhibition against chondrosarcomas and suggest that targeted epigenetic therapies could represent a new therapeutic approach in the treatment of chondrosarcomas, and this is being tested in an ongoing clinical trial (NCT04340843). ©2021 American Association for Cancer Research.