EXO1 Facilitates MiDAS and Prevents Genome Instability and Cell Death in Ewing Sarcoma

Ewing sarcoma (EwS) is an aggressive malignancy driven by EWSR1::ETS fusions, predominantly EWSR1::FLI1. Previous efforts using both direct and indirect approaches to target these chimeric oncoproteins have yielded limited clinical benefit. Although EWSR1::FLI1 is a well-known source of replication stress and genome instability, targeting DNA damage response (DDR) factors that mitigate these effects remain poorly understood. Here, we identified a marked dependency of EwS cells on exonuclease 1 (EXO1). We demonstrate that EXO1 is essential for EwS cell survival and tumor growth, highlighting its potential as a novel therapeutic target. Intriguingly, we unveil that EXO1 loss impairs mitotic DNA synthesis (MiDAS), promoting EWSR1::FLI1-associated genome instability and cell death. Collectively, our results support the idea that targeting DDR factors, which counteract replication stress and/or DNA damage induced by fusion oncoproteins, represents a promising therapeutic option for EwS.