Ewing sarcoma is a highly malignant bone-associated cancer of children and young adults that harbors the EWS/FLI fusion protein. EWS/FLI is the key driver mutation in Ewing sarcoma because it is expressed in nearly all cases of the disease, it functions as an oncoprotein in model systems, and inhibition of EWS/FLI expression or function in Ewing sarcoma cell lines causes a loss of the oncogenic phenotype. EWS/FLI functions as an aberrant transcription factor to both activate and repress critical target genes that modulate the cancerous phenotype. Because of its absolute reliance on a single mutant transcription factor, Ewing sarcoma provides an unparalleled model to understand the relationship between a "driver" mutation and "driver" versus "passenger" epigenetic events in oncogenic transformation. Our published data demonstrate that EWS/FLI functions both as a transcriptional activator, and as a transcriptional repressor, at important target genes. We found that EWS/FLI functions via interaction with the nucleosome remodeling and deacetylase (NuRD) co- repressor complex to mediate transcriptional repression, and that NuRD-associated histone deacetylase (HDAC) and lysine-specific demethylase 1 (LSD1) activities are required for this function. In contrast, the mechanisms by which EWS/FLI mediates transcriptional activation are less well understood, but our preliminary data indicate that LSD1 function is also required for this activity. We therefore hypothesize that LSD1 serves as a key epigenetic regulator in Ewing sarcoma, and that its specific genomic localization is specified via its interaction with EWS/FLI to direct "driver" epigenetic changes at target genes critical for the Ewing sarcoma cancerous phenotype.
Our current ability to discriminate between driver and passenger epigenetic events is rudimentary at best, primarily because the mechanisms by which epigenetic regulators get targeted to critical genomic loci is poorly understood. Our groups have focused on the EWS/FLI fusion transcription factor driver oncoprotein in Ewing sarcoma as a model to understand the link between the epigenetic machinery and oncogenic transformation. Here we propose to test our hypothesis that EWS/FLI mediates oncogenic transformation by actively relocalizing the histone demethylase LSD1 (and associated epigenetic complexes) to target genes critical for the development of Ewing sarcoma and in doing so to act instructively to induce driver epigenetic modifications.