Abstract A022 Characterizing the cell of origin of Ewing sarcoma

Abstract Ewing sarcoma (ES) is a malignant bone and soft tissue tumor that affects children, adolescents, and young adults. ES is characterized by the presence of a driver fusion oncogene, most frequently EWSR1-FLI1. The cell of origin of Ewing sarcoma and the mechanisms of EWSR1-FLI1-driven cell transformation are the subjects of long-standing debates, largely due to the absence of a representative mouse model. Previous reports have suggested that bone-marrow mesenchyme or/and neural crest cells may serve as potential cells of origin for Ewing sarcoma. These hypotheses have never been tested in vivo, as the EWSR1-FLI1 fusion causes severe developmental toxicity in most cell types. To address these questions, we developed a stable model enabling controlled expression of the human EWSR1-FLI1 oncofusion specifically in neural crest cells. Using this model, we demonstrated that expression of human EWSR1-FLI1 oncofusion in neural crest cells can lead to their transformation and the development of tumors in vivo. Here we applied innovative single-cell transcriptomic, genetic, and high-resolution imaging approaches to characterize cells giving rise to EWSR1-FLI1 driven tumors in zebrafish model of Ewing sarcoma. EWSR1-FLI1 hindered the normal differentiation of neural crest cells into glial and neuronal lineages. Instead, EWSR1-FLI1 strongly reprogrammed neural crest cells by upregulating the expression of mesodermal regulators, including the key mesodermal regulator tbxta (Brachyury or T) transcription factor. tbxta/TBXT expression was maintained in subset of zebrafish and human Ewing sarcomas. Our model provides a mechanism by which a neural crest cell population can be transformed to Ewing sarcoma, a malignancy with predominant mesenchymal features. Taken together, these findings shed light on the cellular and molecular mechanisms underlying Ewing sarcoma development and provide valuable insights into its pathogenesis. Citation Format: Elena Vasileva, Claire Arata, Gage Crump, James Amatruda. Characterizing the cell of origin of Ewing sarcoma [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pediatric Cancer Research; 2024 Sep 5-8; Toronto, Ontario, Canada. Philadelphia (PA): AACR; Cancer Res 2024;84(17 Suppl):Abstract nr A022.