Abstract 4969: Exosomal RNA based liquid biopsy detection of androgen receptor variant 7 in plasma from prostate cancer patients
Authors: Hurley James , O’Neill Vincent , Brock Graham , Siddiqui Javed , Malik Rohit , Chinnaiyan Arul , Skog Johan 
Published: July 2016

Abstract
Introduction
Androgenic signaling is a critical part in the growth and survival of prostate cancer (PCa), especially in advanced, metastatic or recurrent disease. Consequently, the androgen receptor (AR) is a target for hormone therapies. Full-length androgen receptor mRNA is derived from 8 exons and encodes an N-terminal transactivation domain (exons 1-2), a DNA-binding domain (exons 2-3) and C-terminal ligand binding domain (exons 4-8). However, AR variants have been identified in which the ligand binding domain is removed by splicing between exons 1-3 and intronic, cryptic exons. One of these variants, ARv7, is highly expressed in patients and cultured PCa cells, which demonstrate resistance to hormone therapy treatment. To avoid ineffective procedures, the development of a liquid biopsy test for ARv7 expression would be a useful and non-invasive means of determining treatment strategies for PCa patients.
Methods
We have developed a diagnostic test to detect the presence of full-length androgen receptor and ARv7 transcripts in plasma samples taken from patients with advanced prostate cancer. The ExoDx™ ARv7 assay comprises a column-based isolation step yielding total exosomal RNA from 1-4 mls of patient plasma, followed by detection of both full-length androgen and the v7 receptor variants, using qPCR. Assay quality is monitored by inclusion of internal and external controls. Following validation, using both synthetic spikes and human samples, we monitored AR expression in plasma samples from treatment-resistant PCa patients. The data was analyzed and aligned with patient's response data.
Results
Here, we present proof of concept data for the ExoDX ARv7 assay. Using this novel diagnostic test, we were able to detect ARv7 variant mRNA in exosomal RNA isolated from patient plasma. We determined the profile of ARv7/FL transcripts in a cohort of prostate cancer patients with both high sensitivity and specificity, and observed concordance of the qPCR-based plasma results.
Conclusions
Liquid biopsies represent a low-risk, non-invasive and viable approach to testing for biomarkers in prostate cancer patients. Here, we demonstrate the capability of our validated diagnostic test to determine the presence of AR alternate transcripts in plasma. Since this analyte is the result of alternative splicing, it would not be detected in cell-free DNA alone. Monitoring plasma levels of ARv7 transcripts could enable effective personalized treatment for patients with advanced prostate cancer and has clear clinical application.
Citation Format: James Hurley, Vincent O’Neill, Graham Brock, Javed Siddiqui, Rohit Malik, Arul Chinnaiyan, Johan Skog. Exosomal RNA based liquid biopsy detection of androgen receptor variant 7 in plasma from prostate cancer patients. [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 4969.