Oncogenic hijacking of a developmental transcription factor evokes therapeutic vulnerability for ROS-induction in Ewing sarcoma

Abstract

Ewing sarcoma (EwS) is an aggressive childhood cancer likely originating from mesenchymal stem cells or osteo-chondrogenic progenitors. It is characterized by fusion oncoproteins involving EWSR1 and variable members of the ETS-family of transcriptionfactors (in 85% FLI1). EWSR1-FLI1 can induce target genes by using GGAAmicrosatellites (mSats) as enhancers.Here, we show that EWSR1-FLI1 hijacks the developmental transcription factor SOX6 -a physiological driver of proliferation of osteo-chondrogenic progenitors -by binding to an intronic GGAA-mSat, which promotes EwS growth in vitro and in vivo. Through integration of transcriptome-profiling, published drug-screening data, and functional in vitro and in vivo experiments, we discovered that SOX6 interferes with the antioxidant system resulting in constitutively elevated reactive oxygen species (ROS) levels that create a therapeutic vulnerability toward the ROS-inducing drug Elesclomol. Collectively, our results exemplify how aberrant activation of a developmentaltranscription factor by a dominant oncogene can promote malignancy, but provide opportunities for targeted therapy.Ewing sarcoma (EwS) is the second most common bone or soft-tissue cancer in children and adolescents 1 . Even though the cell of origin of EwS is still debated, increasing evidence suggests that it may arise from mesenchymal stem cell (MSC)-derived early committed osteochondrogenic progenitors 2,3 . Indeed, EwS cells display a highly undifferentiated and embryonal phenotype. Clinically, EwS is a rapidly metastasizing cancer, and ~25% of cases are metastatic at initial diagnosis 1 . While great advances in treatment of localized disease have been achieved, established therapies still have limited success in advanced stages despite high toxicity 1 . Thus, more specific and in particular less toxic therapies are urgently required.As a genetic hallmark, EwS tumors express chimeric EWSR1-ETS (EwS breakpoint region 1 -E26 transformation specific) fusion oncoproteins generated through fusion of the EWSR1 gene and variable members of the ETS-family of transcription factors (TF), most commonly FLI1 (85% of all cases) 4,5 . Prior studies demonstrated that EWSR1-FLI1 acts as a pioneer transcription factor that massively rewires the tumor transcriptome ultimately promoting the malignant phenotype of EwS 6,7 . This is in part mediated through interference with and/or aberrant activation of developmental pathways 3,8 . Remarkably, EWSR1-FLI1 regulates approximately 40% of its target genes by binding to otherwise non-functional GGAA-

Prognostic profiling of the immune cell microenvironment in Ewing´s Sarcoma Family of Tumors

“…Expression of the EWS-FLI1 oncoprotein in ESFT highjacks the developmental transcription factor SOX6, leading to constitutively elevated ROS levels and therapeutic vulnerability (synthetic lethality) to Elesclomol. 54 Our findings are in line with previous hypotheses that HIF1α promotes tumor progression through suppressive myeloid and T cell populations and by creating a metabolically hostile environment for immune effector cells. 33,55 Reprogramming of the tumor metabolic program into glycolysis in part via HIF1α results in "metabolic competition" between cancer cells and T cells, which may explain the paucity of T cells in ESFT.…”