Contributions of EWS/FLI target genes and cellular context in ewing sarcoma pathogenesis

Adult cancers often arise due to the accumulation of mutations over time, while pediatric malignancies occur early in life and may result from disruption of normal developmental processes. Ewing sarcoma is an undifferentiated neoplasm and the second most common bone associated tumor in children and young adults. This is an aggressive disease and effective treatment for those with metastatic disease is lacking. The EWS/FLI translocation encodes an oncogenic transcription factor and the principle driver of the disease. This fusion protein relies on a very specific cellular context in order to dysregulate transcription of several thousand genes including many developmental regulators. The presence of the translocation in a permissive cell type may be sufficient to drive the transformation process. The specific oncogenic contributions made by EWS/FLI target genes and identification of the cell of origin - which remains unknown - will lead to targeted treatment options based on the molecular underpinnings of the disease. Toward this goal, we identified BCL11B as an up-regulated EWS/FLI target necessary for maintenance of transformation. Genome wide expression profiling of BCL11B regulated genes revealed that BCL11B is responsible for a significant portion of the EWS/FLI repressed gene signature. The normal function of BCL11B as a transcriptional repressor in many developmental processes, suggests that inappropriate expression of BCL11B in the incipient Ewing sarcoma cell leads to aberrant expression of genes involved in several cell lineages, thus contributing to the poorly differentiated iv phenotype in Ewing sarcoma. Next we shifted our focus away from EWS/FLI target genes to identify genes present in the precursor cell that maintain Ewing sarcoma in an undifferentiated state and may foster a permissive cellular environment for EWS/FLI. ZEB2 expression is not dependent on EWS/FLI levels, but it is highly expressed in Ewing sarcoma tumors. ZEB2 represses epithelial gene expression in Ewing sarcoma cells. Reduction of ZEB2 allows for expression of epithelial genes and corresponding decreases in cell mobility and actin cytoskeleton rearrangements. Furthermore, the emergence of these epithelial phenotypes decreased the metastatic potential of Ewing sarcoma cells in a mouse metastasis model. This heretofore unrecognized epithelial plasticity may provide insight into the histogenesis of this disease.