FET fusion oncoproteins disrupt physiologic DNA repair and create a targetable opportunity for ATR inhibitor therapy.

In cancers with genetic loss of specific DNA damage response (DDR) genes (i.e., BRCA1/2 tumor suppressor mutations), synthetic lethal targeting of compensatory DDR pathways has translated into clinical benefit for patients. Whether and how growth-promoting oncogenes might also create tumor-specific vulnerabilities within DDR networks is not well understood. Here we focus on Ewing sarcoma, a FET fusion oncoprotein (EWSR1-FLI1) driven pediatric bone tumor, as a model for the class of FET rearranged cancers. Native FET family members are among the earliest factors recruited to DNA double-strand breaks (DSBs), though the function of both native FET proteins and FET fusion oncoproteins in DNA repair remains to be defined. We discover that EWSR1-FLI1 and other FET fusion oncoproteins are recruited to DNA DSBs and impair the activation and downstream signaling of the DNA damage sensor ATM. In multiple FET rearranged cancers, we establish the compensatory ATR signaling axis as a collateral dependency and therapeutic target using patient-derived xenograft models. In summary, we describe how oncogenes can disrupt physiologic DNA repair and provide the preclinical rationale for specifically testing ATR inhibitors in FET rearranged cancers as part of ongoing early phase clinical trials.