ERG inhibition selectively sensitizes ERG-positive prostate cancer to radiation through DNA repair vulnerabilities and enhances response to androgen receptor inhibitors

Background ERG overexpression, driven by TMPRSS2-ERG gene fusion, occurs in ~ 50% of prostate cancers (PCa) and defines a molecular subtype with distinct therapeutic vulnerabilities. Radiotherapy (RT) is a cornerstone for localized and locally advanced PCa, yet substantial number of patients experience treatment failure. This study investigates whether direct ERG inhibition selectively impairs the resolution of DNA damage signaling and radiosensitizes ERG-positive PCa, providing a precision medicine strategy. Methods ERG-positive VCaP cells, ERG-negative DU145/BPH1 cells, ex vivo patient PCa tissue slice cultures (TSCs, n = 14 from 10 patients), and patient-derived organoids (PDOs, n = 2) were treated with ERG inhibitors (ERGi-USU, VPC-18005) or ERG siRNA ± irradiation (2–6 Gy). DNA double-strand break (DSB) repair was assessed via γ H2AX/53BP1 immunofluorescence and pEJ-GFP reporter assay. Clonogenic survival was evaluated in 2D/3D assays. Label-free quantitative LC–MS/MS proteomics identified differentially regulated pathways, validated by immunoblotting. Promoter activity was measured using GLuc-ON™ Promoter Reporter plasmids. Results Here, we show that direct ERG inhibition selectively delays the resolution of radiation-induced DSB-associated damage signals and reduces end-joining activity in ERG-positive models, leading to enhanced radiosensitivity. In ERG-positive VCaP cells, ex vivo patient PCa tissue slices, and patient-derived PCa organoids, treatment with ERG inhibitors or ERG-targeted siRNA significantly increased residual γ H2AX/53BP1 foci and reduced clonogenic survival following irradiation, whereas ERG-negative tumors remained unaffected. Quantitative LC–MS/MS proteomics, confirmed by immunoblotting and promoter activity assay, revealed that ERG inhibition downregulates key PARP1-mediated end-joining pathway components, including PARP1, LIG3, XRCC1, and the end-resection factor MRE11. Functional assays showed that ERG and PARP inhibition converge on a common mechanism to block alternative end joining, whereas combined ERG and androgen receptor (AR) pathway inhibition further suppressed growth in ERG-positive PCa spheroids and PDOs, probably through metabolic reprogramming. Conclusions These data position ERG as a master regulator of DNA repair vulnerabilities in PCa and provide a strong mechanistic rationale for clinical evaluation of ERG-directed radiosensitization and combination therapy strategies in molecularly defined ERG-positive disease.