Cancer/Testis Antigens: "Smart" Biomarkers for Diagnosis and Prognosis of Prostate and Other Cancers

Abstract

A clinical dilemma in the management of prostate cancer (PCa) is to distinguish men with aggressive disease who need definitive treatment from men who may not require immediate intervention. Accurate prediction of disease behavior is critical because radical treatment is associated with high morbidity. Here, we highlight the cancer/testis antigens (CTAs) as potential PCa biomarkers. The CTAs are a group of proteins that are typically restricted to the testis in the normal adult but are aberrantly expressed in several types of cancers. Interestingly, >90% of CTAs are predicted to belong to the realm of intrinsically disordered proteins (IDPs), which do not have unique structures and exist as highly dynamic conformational ensembles, but are known to play important roles in several biological processes. Using prostate-associated gene 4 (PAGE4) as an example of a disordered CTA, we highlight how IDP conformational dynamics may regulate phenotypic heterogeneity in PCa cells, and how it may be exploited both as a potential biomarker as well as a promising therapeutic target in PCa. We also discuss how in addition to intrinsic disorder and post-translational modifications, structural and functional variability induced in the CTAs by alternate splicing represents an important feature that might have different roles in different cancers. Although it is clear that significant additional work needs to be done in the outlined direction, this novel concept emphasizing (multi)functionality as an important trait in selecting a biomarker underscoring the theranostic potential of CTAs that is latent in their structure (or, more appropriately, the lack thereof), and casts them as next generation or "smart" biomarker candidates.

Keywords: biomarkers; cancer/testis antigens; centrosomal protein of 55 kDa (CEP55); intrinsically disordered protein; nucleolar protein 4 (NOL4); prostate cancer; prostate-associated gene 4 (PAGE4).

Figure 1

Cancer/Testis Antigen (CTA) expression in recurrent and non-recurrent prostate cancer. CTA expression in clinically localized prostate cancer with recurrence (Rec (+)) (n = 43) and without recurrence (Rec (−)) (n = 29). (A) centrosomal protein of 55 kDa (CEP55); (B) NDC80 kinetohore complex component NUF2; (C) prostate-associated gene 4 (PAGE4); (D) lymphokine-activated killer T-cell-originated protein kinase (PBK); (E) the dual specificity protein kinase TTK. Reproduced with permission from ref. [49].

Figure 2

Kaplan-Meier analyses. Kaplan-Meier curves showing biochemical recurrence-free survival against time after radical prostatectomy stratified by the mRNA expression of (A) CEP55; (B) NUF2; (C) PAGE4; (D) PBK; and (E) TTK (high versus low groups dichotomized by median value). Reproduced with permission from ref. [49].

Figure 3

Variability of predicted intrinsic disorder levels and peculiarities of intrinsic disorder distributions within amino acid sequences of several CATs, PAGE4 (A, UniProt ID: O60829), nucleolar protein 4—NOL4 (B, UniProt ID: O94818), CEP55 (C, UniProt ID: Q53EZ4), TTK (D, UniProt ID: P33981), NUF2 (E, UniProt ID: Q9BZD4) and PBK (F, UniProt ID: Q96KB5). Intrinsic disorder profiles for query proteins generated by PONDR^® VLXT [113], PONDR^® VL3 [114], PONDR^® VSL2 [114,115], PONDR^® FIT [116], IUPred_short and IUPred_long [117] are shown by black, red, green, pink, yellow and blue lines, respectively. Cyan dash-dot-dotted lines show the mean disorder propensity calculated by averaging disorder profiles of individual predictors. Light pink shadow around the PONDR^® FIT curve shows error distribution. In these analyses, the predicted intrinsic disorder scores above 0.5 are considered to correspond to the disordered residues/regions, whereas regions with the disorder scores between 0.2 and 0.5 are considered flexible.

Figure 4

PAGE4 levels correlate with survival of patients with hormonenaive PCa. Overall survival of patients with hormone-naive PCa after transurethral resection of the prostate (TURP) for local advanced obstructive PCa stratified for high versus negative/low (neg/low) epithelial PAGE4 levels on the advanced PCa tissue microarray (TMA) (third quartile of mean epithelial PAGE4 intensity was set as the cut-off level). Reproduced with permission from ref. [107].

Figure 5

Intrinsic disorder propensity and some important disorder-related functional information generated for human NOL4 (A) and CEP55 (B) by the D²P² database [127]. The D²P² is a database of predicted disorder for a large library of proteins from completely sequenced genomes [127]. D²P² database uses outputs of IUPred [117], PONDR^® VLXT [113], PrDOS [138], PONDR^® VSL2B [114,115], PV2 [127] and ESpritz [139] and is further supplemented by data concerning location of various curated posttranslational modifications and predicted disorder-based protein binding sites. Here, the green-and-white bar in the middle of the plot shows the predicted disorder agreement between nine predictors, with green parts corresponding to disordered regions by consensus. Yellow bar shows the location of the predicted disorder-based binding sites (molecular recognition features, MoRFs which are predicted by ANCHOR algorithm [140,141]), whereas colored circles at the bottom of the plot show location of various posttranslational modifications (PTMs).