Tracking Ewing sarcoma origin by developmental and trans-species genomics

A major obstacle to drug development in pediatric cancers is a lack of pre-clinical models recapitulating the human disease due to incomplete knowledge of tissue origin. This is specifically true for Ewing sarcoma (EwS) which is caused by EWSR1/ETS gene rearrangements. Mesenchymal stem cells (MSC) have been proposed as candidate cell types of origin, however, targeting of EWS-FLI1 to the bone mesenchymal lineage during mouse embryogenesis so far failed to result in tumorigenesis. Little is known about the precise developmental trajectories of different MSC cell types during normal and perturbed differentiation. In this project, we will follow three approaches to decipher the tissue and differentiation state of origin for EwS: i) Based on single-cell transcriptome analyses, we will establish the first time- and lineage-resolved single-cell reference atlas of human MSC development, naïve and along induced differentiation. We will then pinpoint alteration of normal differentiation trajectories after inducing ectopic EWS-FLI1 expression to define the cell type, differentiation stage, and chromatin architecture resembling EwS the closest and trace back the development of EwS tumor cells from single-cell and bulk analysis of EwS tumor samples by aligning them to the MSC differentiation reference atlas. ii) Based on the observation that the cancer epigenome retains memory of the tissue of origin in its enhancer usage, we will screen for convergence in trans-species activity of these enhancers on specific cell types and developmental stages during zebrafish development. Using these enhancers to irreversibly switch-on fluorescent reporter activity by Cre-mediated recombination, we will follow the developmental fate of these cells into adulthood. iii) Finally, we will perform lineage tracing experiments of rare EwS-like tumors developing in zebrafish with mosaic EWS-FLI1 expression to identify cell types permissive for EWS-FLI1 mediated transformation.