Abstract A005: Androgen receptor variants mRNA absolute quantification in prostate cancer cell models

Abstract

Abstract Background: Patients treated with Androgen Deprivation Therapy (ADT) have been shown to increase the expression of constitutively active Androgen Receptor (AR) splice variants (AR-Vs). These splice variants lack the ligand binding domain at the C-terminus of AR, required for binding to antiandrogen therapies. AR-V7 has also been correlated to the development and progression of castration resistant prostate cancer (CRPC). As not all circulating tumor cells (CTCs) contain AR-V7, we hypothesized that other AR-Vs are involved in the mechanism of resistance to ADT commonly seen in CRPC. To test this, we developed an mRNA quantitative assay targeting AR-Vs and then correlated our results with a targeted mass spectrometry (MS) based protein assay. This work will provide a feasible quantification of AR-V transcripts and that may provide a stratification for predicted responses to ADT treatment in patients that could be useful to clinicians. Methods: The absolute copy number of total AR, AR-V2, AR-V7, AR-V12, and AR-V23 were determined in eight different PCa cell lines and 48 patient derived xenografts (PDXs) using qRT-PCR assays. We performed siRNA targeting of these variants to evaluate primer specificity by evaluating knockdown efficiencies. These results will be correlated to targeted MS quantification of AR-Vs in each cell model. Results: We found that the most highly expressed variants in PCa cell lines were AR-V7 and AR-V12. In the LuCaP PDX model, AR-V12 was shown to have the highest expression of the AR-Vs tested. Specificity of AR-V knockdown was tested using AD-1 (which only expresses the full- length AR transcript) and R1-D567 (which only expresses AR-V567es, an AR-V12-like variant) cell lines. Conclusions: We were able to determine the landscape of AR-V mRNA expression in eight PCa cell lines and in 48 PDX tumor samples. This data will be correlated to protein expression currently being analyzed using a targeted MS method developed in our lab. Our work will be useful to help clinicians stratify PCa patients based on their AR-V expression profiles. This will establish a prognostic biomarker program that measures AR-V proteins in real time from clinical biopsy tissues, circulating tumor cells, or exosomes and informs the clinician on which course of treatment may be effective for each patient. Establishing a CRPC biomarker other than AR-V7 that can help explain the AR-V7 negative CRPC patients could eventually improve clinical application and predictive treatment outcomes. Citation Format: Gabrianne Larson, Zoi Sychev, Stephen Plymate, Eva Corey, Justin M. Drake. Androgen receptor variants mRNA absolute quantification in prostate cancer cell models [abstract]. In: Proceedings of the AACR Special Conference: Advances in Prostate Cancer Research; 2023 Mar 15-18; Denver, Colorado. Philadelphia (PA): AACR; Cancer Res 2023;83(11 Suppl):Abstract nr A005.