Abstract 2882: Mithramycin impedes EWS-FLI1 driven transcription by preventing target promoter clearance
Authors: Flores Guillermo , Kitchen-Goosen Susan , Oswald Brandon , Boguslawski Elissa , Adams Marie , Beddows Ian , Madaj Zachary , Grohar Patrick 
Published: August 2019

Abstract
Background: Ewing sarcoma (ES) is a primary bone tumor of adolescence with poor prognosis. ES is characterized by the presence of EWS-FLI1 which acts as an oncogenic transcription factor to promote and maintain a tumorigenic phenotype. All ES cells express EWS-FLI1, however, other actionable mutations are exceedingly rare. We previously identified mithramycin (MMA) as a potent and specific EWS-FLI1 inhibitor. MMA effectively decreases ES tumor cell viability and tumor volume in vivo by reversing the EWS-FLI1 transcriptional signature. Structural and biochemical data suggest that sub-micromolar MMA increases the stability of the DNA binding domain of EWS-FLI1 at GGAA repeats. These data suggest that MMA may prevent EWS-FLI1 from successfully promoting Pol II transcriptional elongation.
Methods: We used ChIP-qPCR to detect EWS-FLI1 binding at the promoter of well characterized targets. We used a Pol II processivity assay to measure the efficiency of transcriptional elongation at EWS-FLI1 targets. We use RT-qPCR, western blotting, and RNAseq to confirm that MMA sensitizes ES cells to elongation inhibitors. Finally, we use GROseq to identify the changes to Pol II dynamics at EWS-FLI1 targets following MMA treatment.
Results: Our ChIP-qPCR data demonstrate an increased fraction of EWS-FLI1 DNA binding at the promoters of target genes with GGAA microsatellite regions. MMA significantly inhibits the processivity of Pol II at EWS-FLI1 targets when added both into a run-on reaction or following ES cell pretreatment. Combining MMA with elongation inhibitors induces strong synergy as measured by Bliss-Independence, renders multiple ES cell lines unviable by inhibiting the effects of EWS-FLI1 at the mRNA and protein level, and is effective in vivo. MMA biases GROseq reads toward the 5’ end of genes, a result consistent with a promoter trapping mechanism. We are currently correlating GROseq data to global EWS-FLI1 DNA binding using ChIPseq.
Conclusions: We characterized a mechanism of MMA-induced EWS-FLI1 inhibition that involves increased stabilization of the fusion protein at target promoter regions. This mechanism is supported by changes to EWS-FLI1 DNA-binding, EWS-FLI1 target transcriptional processivity, and a reversal of the EWS-FLI1 transcriptional signature by combining MMA with downstream elongation inhibition. We hope that this work will inform future endeavors to develop a targeted therapy for ES that inhibits the main driver of disease, EWS-FLI1.
Citation Format: Guillermo Flores, Susan Kitchen-Goosen, Brandon Oswald, Elissa Boguslawski, Marie Adams, Ian Beddows, Zachary Madaj, Patrick Grohar. Mithramycin impedes EWS-FLI1 driven transcription by preventing target promoter clearance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2882.