Abstract 5159: Identification of ARv567es expression profile in the prostate cancer clinical samples with a newly developed antibody

Abstract

Abstract Background: Androgen receptor splice variants (AR-Vs) have emerged as an important diagnostic biomarker and potential therapeutic oncotarget in castration-resistant prostate cancer. Extensive RNA sequencing of a large cohort of metastases from men treated with androgen deprivation therapy (ADT) including abiraterone and enzalutamide, revealed a marked increase in the amount and variety of AR-Vs when compared to primary tumor samples. These and other data suggest that the AR-Vs should have a selective growth advantage over the full-length AR (AR-FL). However, this growth advantage only occurs after ADT even though AR-FL and AR-V RNA expression are both present in the tumor. Methods: In order to determine how this progression to AR-V predominance occurs, we used the LuCaP 86.2 xenograft, which has a genomic rearrangement of the AR gene so that tumor cells either express AR-FL or ARv567es. Xenografts were implanted sc into the flanks of SCID mice. When tumors reached a volume of 200 mm3 cohorts of mice were castrated and followed then euthanized at 0, 1 day, 1 week, or 3 weeks post-castration. Intact control mice bearing LuCaP 86.2 xenografts were euthanized at similar time points. Tumors were analyzed for AR-FL and ARv567es mRNA and protein. In addition canonical and AR-V regulated gene expression was analyzed by quantitative PCR. We then used an ARv567es antibody to examine the presence of ARv567es in 155 metastases. Results: The ARv567es antibody is a rabbit monoclonal antibody against the unique exon-9 amino acid sequence expressed by ARv567es (CERAASVHF). This antibody does not react with AR-FL. As previously described, there is no difference in growth rates for castration-resistant LuCaP 86.2 xenografts grown in intact and castrate mice. Following castration there was a marked increase in ARv567es nuclear staining (p < 0.01), with no significant change in ARv567es mRNA. AR-amino terminal staining was unchanged following castration; however, consistent with the loss of expression of AR-FL protein and increase in ARv567es protein, there was no detectable carboxy-terminal AR protein staining. PSA gene expression did not change following castration but a marked increase in TMPRSS2, UGT2B17, and UBE2C gene expression was seen, consistent with a shift from ligand-mediated AR-FL activity to AR-V activity. Finally, immunohistochemistry on a tissue microarray (TMA) of 155 metastases from 55 patients in the University of Washington rapid autopsy program demonstrated a significant increase in AR-V staining compared to a primary tumor TMA. Summary: This study demonstrates that expression of ARv567es in development of a castrate resistant xenograft may be regulated at the level of protein translation and that the presence of an AR gene rearrangement for ARv567es prior to castration may be a marker that the tumor can become resistant to castration by an AR-V mechanism. Citation Format: Gang Liu, Aihua Li, Shihua Sun, Cynthia Sprenger, Eva Corey, Colm Morrisey, Scott Dehm, Stephen Plymate. Identification of ARv567es expression profile in the prostate cancer clinical samples with a newly developed antibody. [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 5159. doi:10.1158/1538-7445.AM2015-5159