While taxane-class drugs remains the only chemotherapy agents shown to improve survival of patients with metastatic castration-resistant prostate cancer (mCRPC), most patients with mCRPC ultimately become refractory due to the development of drug resistance. The molecular mechanisms involved in this resistance remain an unmet need. We previously showed that ERG, an oncogenic transcription factor, directly interacts with soluble tubulin dimers impairing the ability of taxanes to bind to microtubules in vitro and in circulating tumor cells (CTCs) from CRPC patients. We also demonstrated that expression of AR-v7 confers taxane resistance in vitro and in animal models of CRPC, owing to the lack of the microtubule-binding hinge domain which renders AR-v7 insensitive to microtubule stabilization by the taxanes. The purpose of the present study is to evaluate the impact of ERG and AR-v7 co-expression in prostate cells and to characterize their cooperative role in mediating taxane resistance. We generated multiple isogenic cell lines over-expressing ERG alone or in combination with AR-v7, and engineered VCaP cells (harboring endogenous ERG rearrangement and expressing AR-v7) to express a TetOn-shRNA targeting ERG mRNA. We also developed and optimized a Digital Droplet PCR (DD-PCR) assay to quantify the expression of ERG derived from the TMPRSS2-ERG gene fusion and of AR-v7. We tested the sensitivity and specificity of the assays using VCaP cells spiked into healthy donor blood samples. We detected ERG and AR-v7 co-expression in single VCaP cells. We also found that ERG and AR-v7 form a protein complex and we are testing their potential co-binding at common target genes using ChIP-seq. To determine the clinical relevance of our findings we queried available RNAseq data from benign prostate, locally advanced hormone naïve prostate cancer and a subset of the Stand-up-to-Cancer cohort of CRPC patients. We found high levels of AR-v7 expression in CRPC samples and in none of the benign or PCa samples and a co-overexpression of AR-v7 and ERG with 75% of ERG positive CRPC also expressing AR-v7. We are currently using our assays to characterize ERG and AR-v7 co-expression in CTCs of mCRPC patients samples collected at baseline, on treatment and relapse- as part of a fully-enrolled clinical trial (TAXYNERGY). We expect an enrichment overtime of CTCs that are positive for both ERG and AR-v7. In conclusion, determining ERG and AR-v7 status in mCRPC patients and their involvement in taxane resistance mechanism will aid refer ERG and AR-v7 positive mCRPC patients to an effective therapy.

Citation Format: Adeline Berger, Seaho Kim, Ada Gjyrezi, Giuseppe Galletti, Andrea Sboner, Mark A. Rubin, Scott Tagawa, Paraskevi Giannakakou, David S. rickman. ERG and AR-v7 involvement in taxane resistance of metastatic castration-resistant prostate cancers. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2125.