RAD21 is a driver of chromosome 8 gain in Ewing sarcoma to mitigate replication stress

Aneuploidy, defined as whole chromosome gain or loss, causes cellular stress but, paradoxically, is a frequent occurrence in cancers. Here, we investigate why around 50% of Ewing sarcomas, driven by the EWS-FLI1 fusion oncogene, harbor chromosome 8 gains. Expression of the EWS-FLI1 fusion in primary cells causes replication stress that can result in cellular senescence. Using an evolution approach, we show that trisomy 8 mitigates EWS-FLI1 induced replication stress through gain of a copy of RAD21. Low-level ectopic expression of RAD21 is sufficient to dampen replication stress and improve proliferation in EWS-FLI1 expressing cells. Conversely, deleting one copy in trisomy 8 cells largely neutralizes the fitness benefit of chromosome 8 gain and reduces tumorgenicity of an Ewing sarcoma cancer cell line in soft agar assays. We propose that RAD21 promotes tumorigenesis through single gene copy gain. Such genes may explain some recurrent aneuploidies in cancer.