Caroline Miller

Background: Ewing Sarcoma (ES) is a highly malignant primary bone tumor mainly affecting children and young adults. There are approximately 200 new cases annually in the US, with macro-metastases in 20-25% of patients at the time of diagnosis. The EWS-FLI1 fusion protein is the driving oncoprotein of ES, present in >85% of cases, resulting from a balanced translocation of the ES breakpoint gene (EWSR1) from chromosomes 22 to 11 near the friend of leukemia virus integration site 1 gene (FLI1)^1,2. EWS-FLI1 is a transcription factor, binding to GGAA microsatellites at accessible sites in chromatin. Hundreds of downstream proteins are dysregulated, promoting tumor growth. Although EWS-FLI1 was first identified in 1992, there are no existing oncological therapies that specifically target it³.

Methods: A PubMed search was performed, yielding results from 2017 to 2024, using key terms “EWS-FLI1”, “EWSR-FLI1”, “Ewing Sarcoma”, and “targeting therapy”.

Results: The EWS-FLI1 protein is difficult to target directly due to its intrinsically disorganized structure⁴. Possible targets to indirectly achieve EWS-FLI1 inhibition include its regulators, site of action, and downstream transcribed proteins. The FACT complex is an epigenetic modulator of EWS-FLI1 and an enhancing agent of EWS-FLI1 oncogenic activity. Inhibition of FACT resulted in significantly decreased EWS-FLI1 mRNA and depressed growth of tumor xenografts in mice⁵. Similar effects were seen in a separate study examining the inhibition of TRIM8, a ubiquitin ligase that degrades EWS-FLI1 in tumor cells to maintain pathophysiologic concentrations. Inhibition of TRIM8 caused apoptosis of tumor cells and significantly impaired tumor xenograft growth⁶. Looking to target the site of EWS-FLI1 action, the antineoplastic antibiotic, mithramycin, was tested in a 2017 clinical trial after results of tumor suppression in ES mouse models were observed in previous studies, however, mithramycin proved to be hepatotoxic to humans at subtherapeutic doses⁷. Inhibiting the downstream protein EYA3, an angiogenic tyrosine kinase transcribed due to EWS-FLI1 activity, proved to significantly decrease tumor vascularity and angiogenesis. However, this approach failed to address the hundreds of other tumorigenic proteins transcribed by EWS-FLI1⁸.

Conclusions: Targeting regulators of EWS-FLI1 has proven to be more effective in suppressing the proliferation of ES tumors than targeting EWS-FLI1 directly, its site of action, or downstream proteins. It is important to note that no single approach examined here is effective in actively shrinking tumor models. A novel, hybrid approach is proposed, first investigating the upregulation of TRIM8, then combining this action with FACT inhibition to degrade existing EWS-FLI1 proteins and inhibit additional production.

Works Cited

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