The androgen receptor (AR) remains a key driver of disease progression even with the advent of new therapeutic agents such as abiraterone (A) and enzalutamide (E). Relapse after treatment with A or E has been associated with the emergence of tumour cells expressing AR splice variants, and at least one mutation in AR has been associated with resistance to E. We have investigated the expression of AR splice variants in late stage prostate cancers using Nanostring (nCounter®) to quantify the levels of AR transcripts and by IHC using antibodies targeting the N- and C-terminal AR domains. We also examined the expression of putative AR regulated genes found to be expressed in prostate cancer reported in publically available expression data.

Formalin fixed paraffin embedded tissue from 38 commercially sourced (TriStar Technology Group; Asterand; Avaden Biosciences) late stage prostate cancers including 10 castrate resistant prostate cancers (CRPC), were confirmed by pathology review. Total mRNA was isolated from a single 5μm section and gene expression from nCounter® analysis was expressed as normalised Log2 values. Immunohistochemical analysis of AR expression was determined by a pathologist using a H score.

Our analysis of AR splice variant expression, relative to a Nanostring detection threshold based on housekeeping genes, has shown the presence of more than one constitutively active AR splice variant in individual tumours. ARV7 was detected in 4/10 CRPC and always found together with ARV1, AR45, and the constitutively active ARV12. All of the CRPC that were negative for ARV7 expressed AR45 and ARV12. Unsupervised clustering of 507 putative AR regulated genes revealed three molecular subgroups among the 38 prostate tumours. One of these subgroups was characterised by the presence of 12/14 AR splice variants including ARV7. Of the other two subgroups, one contained AR45, the other ARV12. Pathway analysis of genes that were expressed to a higher level (>1.5 Log2 fold) in the ARV7 positive compared to ARV7 negative tumours were predominantly associated with cell cycle and proliferation functions.

Our data suggests complexity in the molecular landscape of prostate cancer associated with AR splice variants, and that multiple AR splice variants may be of importance to identify patients likely to relapse on treatment with emerging therapy.

Citation Format: Zara Ghazoui, Emma Jones, Margaret Veldman-Jones, Vivien N. Jacobs, Neil R. Smith, Michele Johnstone, J. Carl Barrett, Elizabeth A. Harrington, Paul Elvin. Expression of multiple androgen receptor splice variants in late stage prostate cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-110. doi:10.1158/1538-7445.AM2015-LB-110