Inhibition of the oncogenic fusion protein EWS-FLI1 causes G2/M cell cycle arrest and enhanced vincristine sensitivity in Ewing sarcoma

A chimeric fusion between the RNA binding protein EWS and the ETS family transcription factor FLI1 (EWS-FLI1), created from a chromosomal translocation, is implicated in driving the majority of Ewing sarcomas (ES) by modulation of transcription and alternative splicing. The small molecule YK-4-279 inhibits EWS-FLI1 function and induces apoptosis. We tested 69 anti-cancer drugs in combination with YK-4-279 and found that vinca alkaloids exhibited synergy with YK-4-279 in five ES cell lines. The combination of YK-4-279 and vincristine reduced tumor burden and increased survival in mice bearing ES xenografts. We determined that independent drug-induced events converged to cause this synergistic therapeutic effect. YK-4-279 rapidly induced G2/M arrest, increased the abundance of cyclin B1, and decreased EWS-FLI1–mediated expression of microtubule-associated proteins, which rendered cells more susceptible to microtubule depolymerization by vincristine. YK-4-279 reduced the expression of the EWS-FLI1 target gene encoding ubiquitin ligase UBE2C, and this in part contributed to the increase in cyclin B1. Biochemical assays revealed that YK-4-279 also increased the abundance of proapoptotic isoforms of MCL1 and BCL2, presumably through inhibition of alternative splicing by EWS-FLI1, thus promoting cell death in response to vincristine. Thus a combination of vincristine and YK-4-279 might be therapeutically effective in ES patients. Examination of mRNA profiles of TC32 on knockdown of EWS-FLI1 or treatment with YK-4-279: 3 samples Total: 1 TC32 WT Control, 1 TC32 shEF, 1 TC32 YK