Abstract 4476: A novel nonsense mutation in androgen receptor confers resistance to CYP17 inhibitor treatment in prostate cancer
Authors: Han Dong , Owiredu Jude , Valencia Kevin , Cai Changmeng 
Published: July 2017

Background: The standard treatment for prostate cancer (PCa) is androgen deprivation therapy (ADT) that blocks AR transcriptional activity, which is responsible to the initiation and progression of PCa. However, ADT invariably leads to the castration-resistant PCa (CRPC) with restored activity of AR. CRPC can be further treated with more intensive ADTs, including CYP17-inhibitors to block intratumoral androgen synthesis and more potent AR antagonist. By analyzing the tumor mRNA from a CRPC patient biopsy who developed resistance to CYP17-inhibitor treatment, we identified a novel nonsense AR mutation on ligand binding domain (Q784*), which produces a C-terminal truncated form of AR protein containing only a fraction of ligand binding domain (LBD) and may mimic the function of some AR splice variants. We hypothesized that AR-Q784* may gain the androgen-independent activity, or may enhance the transcriptional activity of full-length AR (AR-FL) under low androgen environment through dimerization with AR-FL.

Method: Luciferase reporter assays were applied to assess the transcriptional activity of AR-Q784* in absence or presence of androgens, and with or without AR-FL. Immunoblotting and immunofluorescence assays were used to examine the protein stability and cellular localization of AR-Q784*. Chromatin immunoprecipitation assays were also used to assess the chromatin binding ability of AR-Q784*. Moreover, the stable cell line that expresses endogenous AR-FL and tetracycline-inducible AR-Q784* was generated to determine the transcription activity on AR target genes and assess the effects on cell proliferation, particularly under low dose androgen condition.

Result: Although it was constitutively expressed in nucleus, AR-Q784* does not elicit transcriptional activity despite the conditions of ligands. However, when it was co-expressed with AR-FL, AR-Q784* can enhance AR-mediated transcriptional activity and thus increased PCa cell proliferation particularly under low androgen conditions. Mechanistically, AR-Q784* can dimerize with AR-FL, enhance DNA binding ability of AR-FL, and strengthen AR recruitment of p300 coactivator.

Conclusion: We show that CYP17 inhibitor treatment in CRPC may select for a distinct class of AR mutations/variations, including nonsense (AR-Q784*) or point-deletion mutations within LBD and certain splice variants that only lose a fraction of LBD domain. This class of AR mutants/variants is transcriptionally inactive but can enhance the activity of AR-FL due to their abilities of constitutive DNA binding and dimerization with AR-FL. However, this enhancing activity of those AR mutants/variants can be prevented by antagonist treatments such as enzalutamide. Therefore, this study provides an additional rationale for treating PCa patients with combination therapy of abiraterone and enzalutamide, which may prevent the selection for such AR mutations/variations in CRPC.

Citation Format: Dong Han, Jude Owiredu, Kevin Valencia, Changmeng Cai. A novel nonsense mutation in androgen receptor confers resistance to CYP17 inhibitor treatment in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4476. doi:10.1158/1538-7445.AM2017-4476