Abstract A20: Tumorigenicity of Ewing sarcoma is critically dependent on the trithorax proteins MLL and menin

Ewing sarcoma is driven by the oncogenic fusion protein, EWS-FLI1, and arises via de-regulation of developmental transcriptional programs. Control of normal developmental transcription programs is governed by epigenetic regulation which, in the case of HOX programs, is dependent on coordinated and reciprocal actions of polycomb (PcG) and trithorax (TrxG) proteins. We recently reported that in Ewing sarcoma posterior HOX genes, in particular HOXD13, are abnormally over-expressed and that the promoters of these genes are marked with the TrxG-dependent activating histone modification, H3K4me3. H3K4me3 is deposited by the MLL methyltransferase, a key enzyme that is frequently mutated in leukemia, largely as a result of chromosomal translocations that induce the creation of oncogenic MLL fusion-proteins. In leukemia, MLL fusion-proteins cooperate with wild-type MLL and the scaffolding protein menin to induce malignant transformation via deregulation of HOXA genes. The purpose of this study was to test the hypothesis that MLL and menin contribute to tumorigenesis and to posterior HOX gene deregulation in Ewing sarcoma. Gene and protein expression were determined by microarray, qRT-PCR, western blot and immunohistochemistry. Loss of function was achieved by lentiviral shRNAs and by exposing cells to MI-503, a small molecule inhibitor of menin-MLL protein-protein interactions. Changes in gene and protein expression after MI-503 treatment were assessed by qRT-PCR and western blot. Chromatin immunoprecipitation (ChIP) was used to assess binding of MLL and menin at gene promoters. Our results confirm that MLL, menin and HOXD13 are all highly expressed by Ewing sarcoma tumors and cell lines relative to non-malignant tissues and stem cells, and loss of function studies implicate each as a tumor promoting oncogene. Specifically, knockdown of MLL, menin or HOXD13 resulted in reduced cell proliferation, increased death and/or reduced tumorigenic capacity, as determined by anchorage-independent growth in soft agar and subcutaneous tumor formation in vivo. At a molecular level, knockdown of MLL and menin led to reduced expression of HOXD13, implicating these proteins as key mediators of HOXD13 over-expression. Exposure of Ewing sarcoma cells to MI-503 inhibited proliferation and colony formation in soft agar, and resulted in reduced expression of HOXD13 as well as other posterior HOX genes. In contrast, MI-NC, a control compound with similar structure that lacks affinity for the menin-MLL interaction had no effect on cell growth, viability or HOX gene expression. Significantly, MI-503-treated cells also showed a marked reduction in EWS-FLI1 and wild-type EWSR1 expression, both of which are regulated by the EWSR1 promoter, and of EZH2, a well-established oncogene in Ewing sarcoma. ChIP studies of the EWSR1 and EZH2 promoters demonstrated reduced binding of MLL and menin in MI-503-treated cells. These data demonstrate a key role for MLL and menin in Ewing sarcoma pathogenesis and directly implicate the TrxG complex in regulation of EWS-FLI1, thus highlighting a novel opportunity for therapeutic intervention. Citation Format: Laurie K. Svoboda, Natashay Bailey, Melanie Krook, Raelene Van Noord, Ashley Harris, Rajiv M. Patel, Dafydd Thomas, Tomek Cierpicki, Jolanta Grembecka, Elizabeth R. Lawlor. Tumorigenicity of Ewing sarcoma is critically dependent on the trithorax proteins MLL and menin. [abstract]. In: Proceedings of the AACR Special Conference: Developmental Biology and Cancer; Nov 30-Dec 3, 2015; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(4_Suppl):Abstract nr A20.