Abstract 2497: ET-743 interferes with EWS-FLI1 driven WRN expression in Ewing sarcoma cells and sensitizes to topoisomerase I inhibitors

Abstract

Abstract The hallmark of Ewing sarcoma is the EWS-FLI1 transcription factor believed to be responsible for oncogenesis and tumor progression. ET-743 (Trabectedin, Yondelis™) is a natural product recently identified as a particularly active agent for Ewing sarcoma cells in vitro. We have recently demonstrated that ET-743 interferes with the activity of the oncogenic transcription factor EWS-FLI1. In this report, we examine the link between EWS-FLI1 suppression and the DNA damage response. We hypothesize that blocking the EWS-FLI1 associated DNA damage response may confer additional sensitivity to specific cytotoxic agents. DNA repair pathways and EWS-FLI1 targets were evaluated by microarray expression, western blot analysis and siRNA interference. Microarray expression and western blot analysis revealed that ET-743 suppresses WRN and XRCC4 expression, critical proteins in the Non-Homologous End Joining Pathway (NHEJ) repair pathway. Further siRNA data showed that the EWS-FLI1 transcription factor drives the expression of WRN. These results suggest that WRN suppression by ET-743 is occurring through an EWS-FLI1 dependent mechanism. Since the NHEJ pathway is associated with resistance to specific chemotherapeutic agents, several combination therapies were investigated in vitro. ESFT cells show heightened sensitivity to the combination of ET-743 and the topoisomerase I inhibitor, SN-38. This result was unique to topoisomerase I inhibitors and further restricted to ESFT cell lines. Consistent with these results, previous reports have established that WRN deficient cells are hypersensitive to topoisomerase I but not topoisomerase II inhibitors. Additionally, we found that irinotecan augments the ET-743 mediated transcriptional inhibition of EWS-FLI1. Taken together these mechanisms account for the heightened sensitivity of the combination of ET-743 and irinotecan observed both in vitro and in vivo in two different ESFT xenograft models. In summary, by employing a mechanism based approach, we have identified a novel combination therapy of ET-743 and irinotecan. By linking the EWS-FLI1 transcriptional program to the DNA damage response in ESFT cells, we expose an inherent weakness to topoisomerase I inhibitors thus creating a cell type specific sensitivity to the drug. This report provides a novel method of oncogenic transcription factor targeting that involves direct suppression of downstream targets and exploitation of the resulting changes in gene expression. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2497. doi:1538-7445.AM2012-2497