Abstract Background: ETS gene fusions, which are found in approximately half of all prostate cancers, result in overexpression of ETS transcription factors, of which ERG is the prototype. We previously discovered that ERG interacts with the DNA damage response protein poly (ADP-ribose) polymerase 1 (PARP1). Therefore, we hypothesized that the ERG-PARP1 interaction may confer radiation resistance by increasing DNA repair efficiency, and this radioresistance can be reversed with PARP1 inhibition. Methods: We used lentiviral approaches to establish isogenic models of ERG overexpression in PC3 and DU145 prostate cancer cell lines. We then assessed the role of ERG overexpression on 1) PARP1 activity (via immunoblots for poly ADP-ribose (PAR) formation), 2) DNA repair (via COMET assays and immunofluorescent microscopy for gamma-H2aX foci), and 3) clonogenic survival, before and following radiation +/− the PARP1 inhibitor olaparib. We also created ERG mutants with a point mutation (Y373) which disrupted the ERG-PARP interaction, and assessed the role of this interaction on radiation response. Finally, we established xenografts from ERG-overexpressing and control PC3 cells, and treated these xenografts with radiation +/− PARP1 inhibition. Results: In both PC3 and DU145 cells, ERG overexpression resulted in increased PARP1 activity and quicker repair of DNA damage, both of which were reversed with olaparib. ERG overexpression resulted in a 1.3 fold increase in clonogenic survival following radiation, consistent with radioresistance. PARP1 inhibition with olaparib preferentially radiosensitized ERG-positive cells, by a factor of 1.5 relative to ERG-negative cells (p<0.05). Additionally, disruption of ERG-PARP interaction via point mutation reversed the ERG-mediated increase in clonogenic survival following radiation. Within PC3 xenografts, ERG overexpression also conferred radioresistance which was reversed with PARP1 inhibition. Conclusions: In these prostate cancer cell models, ERG overexpression confers radiation resistance via increased efficiency of DNA repair following radiation. This radioresistance is dependent on the ERG-PARP1 interaction, and can be reversed via inhibition of PARP1 both in vitro and in vivo. These novel findings motivate the use of PARP1 inhibitors as radiosensitizers in patients with localized ETS fusion-positive prostate cancers. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1463. doi:1538-7445.AM2012-1463