Unveiling the role of the Mediator complex in Ewing sarcoma tumorigenesis

[eng] Ewing sarcoma is an aggressive tumor arising in bones and soft tissues that affects children and young adults. This disease is determined by an aberrant chromosomal translocation that forms an oncogenic fusion protein, being EWSR1-FLI1 the most common. The oncogene is responsible for tumorigenesis through epigenetic deregulation in a particular cell context. It generates de novo super-enhancers that regulate crucial genes for cellular maintenance and tumorigenesis, and which are largely enriched in cofactors associated to RNA-pol II such as the Mediator complex. In order to characterize the specific functions of super-enhancer-regulated genes that enable tumorigenesis, we have identified in silico the Mediator complex subunit MED13L and RERE as genes regulated by EWSR1-FLI1- bound super-enhancers. We found that EWSR1-FLI1 regulates the expression of MED13L and RERE, which are differentially expressed in Ewing sarcoma. Both proteins bind to overlapping regions with EWSR1- FLI1 on CpG islands corresponding to genes involved in protein translation and alternative splicing. In addition, MED13L and RERE regulate the expression of spliceosome components by altering the splicing pattern of important genes for Ewing sarcoma tumorigenesis. We observed that Ewing sarcoma cell lines are highly sensitive to the splicing inhibitor Pladienolide B, and that cells respond differently depending on MED13L and RERE levels. Therefore, we propose that the synergistic effect due to the combination of Pladienolide B and the CDK8 Mediator kinase inhibitor BRD6989 might be further studied for consideration as a new therapeutic approach in Ewing sarcoma.