Endocrine Abstracts

A key mechanism of persistent cell survival under testosterone suppression in advanced prostate cancer (PC) is continued Androgen Receptor (AR) activation. This results from AR mutation, overexpression, hyper-activation, and/or expression of constitutively-active AR transcript variants (AR-Vs). AR has an unusually long 3’ untranslated region (3’UTR), which performs vital regulatory roles but is remarkably understudied. Its contribution to continued AR activation under androgen-depletion, and to disease progression, has not been investigated. Indeed, its associations with RNA-binding proteins and non-coding RNAs (e.g. microRNAs – AR is the most commonly microRNA-targeted oncogene in PC) have dramatic effects on transcript stability and translational efficiency. Further, shortening of oncogene 3’UTRs, via alternative polyadenylation (APA), avoids repression by microRNAs and is a widespread feature of cancer associated with poor outcome. Opposingly, 3’UTR circularisation is linked to mRNA stabilisation and increased translation. We identified mechanistically-undescribed APA sites within AR in prostate tissue. We used in silico approaches to examine 3’UTR length of AR and other cancer-implicated transcripts in a cohort of >5000 PC patients (Decipher Biosciences), assessing their association with disease progression and patient outcome. AR transcript-length was significantly increased in high vs low Gleason grade tumours (in contrast to the majority of oncogenes showing transcript shortening), and is the most significantly length-altered transcript. This is consistent with reduced AR 3’UTR splicing observed in cancerous vs non-cancerous prostate, and may increase AR activity through retention of binding sites for miRs shown to stabilise AR transcript. We present novel findings from long-read RNA-sequencing from 22RV1 cells and patient-derived xenografts treated ±anti-androgen, Enzalutamide, revealing novel cancer-associated transcript isoforms and length alterations that may contribute to maintenance of AR activity in advanced disease. These hitherto under-investigated AR-regulatory mechanisms could be exploited in the design of new therapies, particularly for scenarios driving resistance to current therapeutics