Data from The TMPRSS2–ERG Gene Fusion Blocks XRCC4-Mediated Nonhomologous End-Joining Repair and Radiosensitizes Prostate Cancer Cells to PARP Inhibition

Abstract

<div>Abstract<p>Exposure to genotoxic agents, such as ionizing radiation (IR), produces DNA damage, leading to DNA double-strand breaks (DSB); IR toxicity is augmented when the DNA repair is impaired. We reported that radiosensitization by a PARP inhibitor (PARPi) was highly prominent in prostate cancer cells expressing the TMPRSS2–ERG gene fusion protein. Here, we show that TMPRSS2–ERG blocks nonhomologous end-joining (NHEJ) DNA repair by inhibiting DNA-PKcs. VCaP cells, which harbor TMPRSS2–ERG and PC3 cells that stably express it, displayed γH2AX and 53BP1 foci constitutively, indicating persistent DNA damage that was absent if TMPRSS2–ERG was depleted by siRNA in VCaP cells. The extent of DNA damage was enhanced and associated with TMPRSS2–ERG's ability to inhibit DNA-PKcs function, as indicated by its own phosphorylation (Thr2609, Ser2056) and that of its substrate, Ser1778-53BP1. DNA-PKcs deficiency caused by TMPRSS2–ERG destabilized critical NHEJ components on chromatin. Thus, XRCC4 was not recruited to chromatin, with retention of other NHEJ core factors being reduced. DNA-PKcs autophosphorylation was restored to the level of parental cells when TMPRSS2–ERG was depleted by siRNA. Following IR, TMPRSS2–ERG-expressing PC3 cells had elevated Rad51 foci and homologous recombination (HR) activity, indicating that HR compensated for defective NHEJ in these cells, hence addressing why TMPRSS2–ERG alone did not lead to radiosensitization. However, the presence of TMPRSS2–ERG, by inhibiting NHEJ DNA repair, enhanced PARPi-mediated radiosensitization. IR in combination with PARPi resulted in enhanced DNA damage in TMPRSS2–ERG-expressing cells. Therefore, by inhibiting NHEJ, TMPRSS2–ERG provides a synthetic lethal interaction with PARPi in prostate cancer patients expressing TMPRSS2–ERG. <i>Mol Cancer Ther; 14(8); 1896–906. ©2015 AACR</i>.</p></div>