Evaluation and characterization of anti-RalA autoantibody as a potential serum biomarker in human prostate cancer

// Jitian Li 1, * , Liping Dai 1, 2, * , Ningjing Lei 1 , Mengtao Xing 1 , Pei Li 1 , Chenglin Luo 1 , Carlos A. Casiano 3, # , Jian-Ying Zhang 1, 2, # 1 Department of Biological Sciences, The University of Texas at El Paso, El Paso, TX 79968, USA 2 Henan Key Laboratory of Tumor Epidemiology and Henan Academy of Medical and Pharmaceutical Sciences, Zhengzhou University, Zhengzhou, Henan 450052, China 3 Center for Health Disparities and Molecular Medicine, Department of Basic Sciences, Loma Linda University School of Medicine, Loma Linda, CA 92354, USA * These authors contributed equally to this work # These authors share senior authorship Correspondence to: Jian-Ying Zhang, email: jzhang@utep.edu Keywords: autoantibody, tumor-associated antigens (TAAs), cancer immunodiagnosis, prostate cancer Received: March 28, 2016      Accepted: May 05, 2016      Published: June 07, 2016 ABSTRACT Autoantibodies against intracellular tumor-associated antigens (TAAs) are commonly found in human cancers. In this study, we characterized the serum autoantibody response to the RalA, Ras-like GTPase, in patients with prostate cancer (PCa). The autoantibodies were detected by immunofluorescence assay in PCa cell lines, ELISA, and immunoblotting in 339 serum samples from patients with PCa and benign prostatic hyperplasia (BPH), and in normal human sera (NHS). The expression of RalA in prostate tumor tissues was evaluated by immunohistochemistry (IHC) in tumor microarrays. The autoantibody level to RalA (median) in NHS was significantly lower than in PCa (0.053 vs 0.138; P < 0.001) and BPH (0.053 vs 0.132; P < 0.005) groups. The circulating anti-RalA autoantibody could distinguish PCa patients from normal individuals with the area under the receiver operating characteristic (ROC) curve (AUC) performing at 0.861, with sensitivity of 52.9% and specificity of 91.0%. Elevation in serum immunoreactivity was observed in PCa patients after radical prostatectomy. The combined use of both anti-RalA autoantibody and PSA showed a significantly higher discriminatory ability compared with either of those markers alone. RalA protein expression was detected by IHC in 85.3% of tumor tissues from PCa patients, but without significant difference compared to BPH or normal control tissues. Together, our study shows the additional benefits of anti-RalA autoantibody as a potential serological biomarker for PCa, particularly in patients with normal PSA, and further demonstrate the utility of biomarker combinations in the immunodiagnosis of PCa.