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. 2016 Oct;70(4):599-608.
doi: 10.1016/j.eururo.2016.03.049. Epub 2016 Apr 23.

Analytical Validation and Clinical Qualification of a New Immunohistochemical Assay for Androgen Receptor Splice Variant-7 Protein Expression in Metastatic Castration-resistant Prostate Cancer

Jonathan Welti  1 , Daniel Nava Rodrigues  1 , Adam Sharp  1 , Shihua Sun  2 , David Lorente  3 , Ruth Riisnaes  1 , Ines Figueiredo  1 , Zafeiris Zafeiriou  1 , Pasquale Rescigno  1 , Johann S de Bono  4 , Stephen R Plymate  5
Affiliations

Analytical Validation and Clinical Qualification of a New Immunohistochemical Assay for Androgen Receptor Splice Variant-7 Protein Expression in Metastatic Castration-resistant Prostate Cancer

Jonathan Welti et al. Eur Urol. 2016 Oct.

Abstract

Background: The androgen receptor splice variant-7 (AR-V7) has been implicated in the development of castration-resistant prostate cancer (CRPC) and resistance to abiraterone and enzalutamide.

Objective: To develop a validated assay for detection of AR-V7 protein in tumour tissue and determine its expression and clinical significance as patients progress from hormone-sensitive prostate cancer (HSPC) to CRPC.

Design, setting, and participants: Following monoclonal antibody generation and validation, we retrospectively identified patients who had HSPC and CRPC tissue available for AR-V7 immunohistochemical (IHC) analysis.

Outcome measurements and statistical analysis: Nuclear AR-V7 expression was determined using IHC H score (HS) data. The change in nuclear AR-V7 expression from HSPC to CRPC and the association between nuclear AR-V7 expression and overall survival (OS) was determined.

Results and limitations: Nuclear AR-V7 expression was significantly lower in HSPC (median HS 50, interquartile range [IQR] 17.5-90) compared to CRPC (HS 135, IQR 80-157.5; p<0.0001), and in biopsy tissue taken before (HS 80, IQR 30-136.3) compared to after (HS 140, IQR 105-167.5; p=0.007) abiraterone or enzalutamide treatment. Lower nuclear AR-V7 expression at CRPC biopsy was associated with longer OS (hazard ratio 1.012, 95% confidence interval 1.004-1.020; p=0.003). While this monoclonal antibody primarily binds to AR-V7 in PC biopsy tissue, it may also bind to other proteins.

Conclusions: We provide the first evidence that nuclear AR-V7 expression increases with emerging CRPC and is prognostic for OS, unlike antibody staining for the AR N-terminal domain. These data indicate that AR-V7 is important in CRPC disease biology; agents targeting AR splice variants are needed to test this hypothesis and further improve patient outcome from CRPC.

Patient summary: In this study we found that levels of the protein AR-V7 were higher in patients with advanced prostate cancer. A higher level of AR-V7 identifies a group of patients who respond less well to certain prostate cancer treatments and live for a shorter period of time.

Keywords: Androgen receptor; Androgen receptor variant-7; Castration-resistant prostate cancer; Metastatic biopsy; Predictor of outcome; Treatment resistance.

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Figures

Fig. 1
Fig. 1
EP343 identifies the AR-V7 splice variant. Immunoprecipitation (IP) of M12 cells stably transfected with PCDNA3 plasmid expressing 3×FLAG-tagged cDNAs encoding either AR-FL, ARv567es, or AR-V7. Following IP of cell lysates with a FLAG antibody, blots were probed for (A) AR-NTD or (B) AR-V7 to determine the specificity of the EP343 antibody. (C) Immunoblot staining of various prostate cancer cell lines, both positive and negative for AR-V7 and AR-FL, shows AR-V7 staining (top arrow) along with a number of nonspecific bands and AR-FL (bottom arrow). (D) LNCaP95s were transfected with siRNAs targeting either cryptic exon 3B (si V7), exon 7 (si ex7), exon 1 (si ex1), scrambled SiRNA (si SCR), or an input control. Note that siRNAs directed at components of AR-V7 (si V7 and si ex 1) reduced EP343 staining, but siRNA directed at exon 7 had no effect. (E) IHC on 22Rv1 (positive for AR-NTD and AR-V7), DU145 (negative for AR-NTD and AR-V7) and PC3s which are negative for AR-NTD and should be negative for AR-V7 but have obvious EP343 staining. Both PC3 and 22Rv1 were transfected with AR-V7 (cryptic exon 3B) siRNA and then pelleted for EP343 IHC staining. The targeted siRNA reduced AR-V7 in 22Rv1 IHC but not in PC3.
Fig. 2
Fig. 2
AR-V7 and AR-NTD protein and mRNA in prostate cancer cell lines and patient samples. (A) Protein and mRNA expression results for cell lines and patient castration resistant prostate cancer (CRPC) biopsy lysates. Patient protein samples show that EP343 binds to AR-V7 and some nonspecific band/s. Densitometry and qPCR results are normalised against 22Rv1 results. (B) AR-V7 and AR-NTD staining of patient hormone-sensitive prostate cancer (HSPC) and CRPC tissues, highlighting increases in both AR-V7 and AR-NTD.
Fig. 3
Fig. 3
Nuclear AR-V7 and AR-NTD expression increases from hormone-sensitive prostate cancer (HSPC) to castration-resistant prostate cancer (CRPC). (A) Nuclear AR-V7 immunohistochemical H score (HS) for matched HSPC and CRPC patient samples. Patients with decreased (grey line) and increased (black line) nuclear AR-V7 expression are shown, with p values for a Wilcoxon matched-pairs signed-rank test. (B) Nuclear AR-NTD HS for matched HSPC and CRPC patient samples. Patients with decreased (grey line) and increased (black line) nuclear AR-NTD expression are shown, with p values for a Wilcoxon matched-pairs signed-rank test. (C) Nuclear AR-V7 HS for HSPC biopsies (orange circles), CRPC biopsies before abiraterone acetate (AA) and enzalutamide (EZ) treatment (black squares), and CRPC biopsies after AA or EZ (grey triangles). Black crosshairs denote the median HS with interquartile range, and p values are for Mann-Whitney tests. (D) Nuclear AR-NTD HS for HSPC biopsies (orange circles), CRPC biopsies before AA or EZ (black squares), and CRPC biopsies after AA or EZ (grey triangles). Black crosshairs denote the median HS with interquartile range, and p values are for Mann-Whitney tests.
Fig. 4
Fig. 4
Nuclear AR-V7 and AR-V7/AR-NTD ratio but not nuclear AR-NTD expression are correlated with overall survival (OS) from castration-resistant prostate cancer biopsy. The patient cohort was divided by tertiles for (A) nuclear AR-V7, (B) nuclear AR-NTD, and (C) nuclear AR-V7/AR-NTD expression ratio, and p values for a linear trend test (orthogonal polynomial coefficients) are reported. HS = immunohistochemical H score; CI = confidence interval; HR = hazard ratio.
Fig. 5
Fig. 5
High nuclear AR-V7 and AR-V7/AR-NTD expression ratio are associated with shorter overall survival (OS) in patients with castration-resistant prostate cancer (CRPC). Kaplan-Meier curves show OS from time of CRPC biopsy for patients divided by tertiles for (A) nuclear AR-V7, (B) nuclear AR-NTD, and (C) nuclear AR-V7/AR-NTD expression ratio. Median OS and hazard ratio with 95% confidence intervals are shown, and p values are for a linear trend test (orthogonal polynomial coefficients).
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