A combination of EWSR1-FLI1 with loss of one EWSR1 allele promotes damage in the mitotic spindle and sensitization to microtubule-destabilizing agent

Microtubule-destabilizing agents (MDAs) and microtubule-stabilizing agents (MSAs) are commonly used chemotherapeutic agents due to its activity to induce cell death by compromising the dynamics of spindles during mitosis. Ewing sarcoma, the second most common pediatric bone cancer, is known to selectively respond to MDAs as a first-line treatment, but not to MSAs. Ewing sarcoma cells carry an aberrant EWSR1-FLI1 fusion gene and only one wildtype EWSR1 allele. To investigate the origin of this MDA sensitivity, we used an (AID-EWSR1/wt: EWSR1-FLI1-mCherry/wt) cell line that enables conditional induction of EWSR1-FLI1 expression (Tet-On system) and EWSR1 knockdown derived from one EWSR1 allele (auxin-degron system). A combination of EWSR1-FLI1 expression and EWSR1 knockdown induces apoptosis upon nocodazole treatment, a type of MDA. Our study revealed that the mitotic spindles of Ewing sarcoma cells (A673, RD-ES and SK-ES1) contain elevated levels of tubulin damage, visualized with GTP-tubulin, compared to mesenchymal stem cells (MSC). Consistently, the combination of EWSR1-FLI1 expression and EWSR1 knockdown in (AID-EWSR1/wt: EWSR1-FLI1-mCherry/wt) cell line also leads to an increased incidence of damage in mitotic spindles. Together, we propose that the sensitivity of Ewing sarcoma cells is derived from the increased levels of damage to mitotic spindles caused by EWSR1-FLI1 expression and EWSR1 knockdown.