Seminal fluid: a useful source of prostate cancer biomarkers? (2015)

Biomarkers in MedicineVol. 9, No. 2 EditorialFree AccessSeminal fluid: a useful source of prostate cancer biomarkers?Matthew J Roberts, Renee S Richards, Robert A Gardiner & Luke A SelthMatthew J RobertsCentre for Clinical Research, The University of Queensland, Brisbane, QLD, AustraliaDepartment of Urology, Royal Brisbane & Women's Hospital, Brisbane, QLD, AustraliaSchool of Medicine, The University of Queensland, Brisbane, QLD, AustraliaCentre for Advanced Imaging, The University of Queensland, Brisbane, QLD, Australia, Renee S RichardsCentre for Clinical Research, The University of Queensland, Brisbane, QLD, Australia, Robert A GardinerCentre for Clinical Research, The University of Queensland, Brisbane, QLD, AustraliaDepartment of Urology, Royal Brisbane & Women's Hospital, Brisbane, QLD, Australia & Luke A SelthAuthor for correspondence: E-mail Address: luke.selth@adelaide.edu.auDame Roma Mitchell Cancer Research Laboratories & Adelaide Prostate Cancer Research Centre, School of Medicine, The University of Adelaide, SA, AustraliaFreemasons Foundation Centre for Men's Health, School of Medicine, The University of Adelaide, SA, AustraliaPublished Online:18 Feb 2015https://doi.org/10.2217/bmm.14.110AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareShare onFacebookTwitterLinkedInRedditEmail Keywords: biomarkerdiagnosishepsinmicroRNAPCA3prostate cancerseminal fluidProstate cancer (PCa) is the most common internal cancer in men, and is normally diagnosed at a localized stage through a combination of serum prostate-specific antigen (PSA) testing and digital rectal examination. However, PSA is not specific to PCa, with more than half the patients who have an elevated PSA test result having a negative nontargeted biopsy [1]. Moreover, because PCa is highly heterogeneous and often multifocal in nature, current diagnostic pathways result in extensive over-detection of indolent tumors not requiring treatment and, ironically, under-detection of aggressive low-volume tumors. These issues have contributed to conflicting recommendations for PSA-based detection from government and professional bodies. Multiparametric MRI is being increasingly used to localize and stratify prostatic lesions according to likelihood of significant PCa, which enables selective biopsying of patients. However, multiparametric MRI is expensive and fails to detect approximately 10% of significant tumors [2]. Thus, the need for a noninvasive and inexpensive test that accurately detects PCa and, more importantly, differentiates indolent from life-threatening PCa is urgently required.Seminal fluid (SF) is composed of secretions from glands in the male urogenital tract: approximately 40% of SF is prostatic material, released following global smooth muscle contraction and expulsion into the urethra, with the remainder contributed by the seminal vesicles and testes. In this editorial, we will present a case for the potential of SF as a clinically relevant 'liquid biopsy' of the prostate that could assist in diagnosis of PCa.Why use SF for diagnosis of PCa?SF has a number of advantages over blood and urine in terms of its potential as a source of PCa-specific biomarkers. First, prostatic constituents are highly enriched in SF compared with other bodily fluids. Indeed, PSA was originally discovered in SF, where it exists at a concentration approximately 5–6 orders of magnitude higher than in blood serum [3]. Second, unlike malignant prostatic epithelial cells and their products that only enter the circulation following transgression of blood–tissue barriers, cells and their secretions are released naturally into SF by both normal and malignant glands. Both of these factors suggest that biomarkers would be detected earlier in SF than in blood, highlighting the potential of this fluid for early detection including premalignant changes. Third, SF not only contains cell-free material from the prostate but also cancer cells that are detectable prior to biopsy-based PCa diagnosis [4], with the proliferation rate of these cells potentially valuable in monitoring patients with low-grade disease in active surveillance regimens [5]. Finally, unlike SF, biomarkers in blood and urine can be subject to high variability due to physical and/or metabolic processes, such as an increased body mass index [6].The major drawback of SF as a source of biomarkers is difficulty in physically receiving specimens from elderly men with erectile dysfunction. Although this is clearly a critical point, we have found that an ability to provide a SF specimen on request indicated a life expectancy for PCa patients comparable to cancer-free age-matched peers [Gardiner RA, Unpublished data]. Thus, men with adequate erectile function who are able to provide SF would be more likely to benefit from treatment with curative intent. Another major issue is that patients may not be inclined to provide SF due to personal, social, religious or ethical issues [7]. However, the experience in our research program is that the vast majority of men at risk of PCa have been willing to donate SF for research purposes. A further argument against SF as a diagnostic fluid has focused on the requirement for infrastructure and/or processing capacities for its collection [8], but routine practice in fertility clinics contradicts such an assertion. Moreover, for some molecules, such as RNA and protein, we have found that men can provide samples in appropriate collection vessels from their homes provided that they are returned to the laboratory within 2 h for processing [9].There are other body fluids enriched for prostatic material, and any discussion of the utility of SF as a source of PCa-specific biomarkers must involve a comparative evaluation. Expressed prostatic secretion (EPS) is obtained in the first void following vigorous digital rectal examination or prostatic massage. Although the value of EPS for PCa diagnosis remains uncertain, our preliminary findings suggest that disease-specific biomarkers are more readily detectable in SF [10]. Moreover, 'milking' of the prostate is largely confined to the posterior portion of the gland with this approach, resulting in risk of under-sampling anteriorly, where up to 30% of tumors are located [11]. Postejaculatory urine (usually collected together with ejaculate itself) represents another attractive biofluid, given that it contains both prostate-derived and urine-specific biomarkers [12]. However, key biomarkers would be present at lower concentrations in this fluid compared with SF, requiring the use of more sensitive detection methods.Identification of PCa-specific biomarkers in SFSF is a diverse and abundant molecular milieu, comprised of nucleic acids, proteins, lipids, sugars, small metabolites and ions. While earlier studies assayed for specific candidate biomarkers [12], recent advances in molecular technologies have enabled global profiling of various marker types in SF, promising to revolutionize this field.Attempts to identify PCa-specific biomarkers have been greatly facilitated by '-omic' methodologies. Proteomic profiling of SF using 2-DE and MALDI-TOF/MS identified proteins implicated in PCa and signatures that correspond with those derived from EPS [8,13]. Capillary electrophoresis mass spectrometry was recently used to profile SF samples from men with a range of prostatic conditions, with a panel of 11 proteins reported to accurately distinguish between localized and advanced PCa [14]. Another study showed that Dkk-3, an abundant protein in seminal plasma, is found at significantly higher levels in patients with biopsy-confirmed PCa: indeed, the combination of Dkk-3 with serum and SF PSA demonstrated improved diagnostic accuracy compared with serum PSA alone [15].Nucleic acids, including microRNA (miRNA), other coding and noncoding RNA species and DNA, are also present in SF. Our group recently used RNA sequencing to profile small RNAs in the nonsperm epithelial cell fraction of SF and identified a panel of putative miRNA markers of PCa. Many of these miRNAs are known to be differentially expressed in tumors, supporting their prostatic origin. In a validation cohort of 48 patients, miR-200b in combination with serum PSA was shown to exhibit a significantly higher diagnostic value for PCa than serum PSA alone [16]. Preliminary findings also indicate that the cell-free fraction of SF, including prostate-derived extracellular vesicles or 'prostasomes', is a rich source of highly stable miRNA and other molecular biomarkers of PCa that is shielded from enzymatic degradation [Richards RS, Unpublished data].Metabolites in SF may also be useful markers of PCa. Metabolic changes in SF from men with PCa were first identified in the 1960s [17]. Zinc and citrate are normally secreted from the prostate to assist with homeostasis of ions (e.g., calcium) and to facilitate sperm motility and fertilization [18]. In PCa, metabolic homeostasis is perturbed, leading to reduced intracellular and seminal citrate and zinc, a phenomenon that is thought to occur prior to histological changes. Changes in other metabolites in SF, such as spermine and myo-inositol, have been demonstrated using NMR spectroscopy [18]. These small molecule disturbances have also been demonstrated in prostatic tissue in vitro using high-resolution magic angle spinning magnetic resonance spectroscopy, while also being measured in vivo using magnetic resonance spectroscopic imaging.As for other biofluids, it is likely that no single SF-derived analyte will provide the desired diagnostic or prognostic accuracy for PCa in the clinic, and with this in mind we are working toward a multivariate index assay. For example, in a cohort of SF samples from 66 men, we assessed the diagnostic accuracy of PCA3, a PCa-associated long noncoding RNA, and Hepsin, a transmembrane protease previously associated with PCa [9,19]. A combination of Hepsin, PCA3 and serum PSA predicted PCa status and clinical risk more accurately than serum PSA alone. Moreover, integration of miRNA markers with PCA3 and Hepsin further improved diagnostic specificity and risk prediction [9].Conclusion & future perspectiveIn the next decade, PCa diagnosis will further incorporate new imaging (primarily MRI-based) techniques. However, we envision that during this period new biomarkers based on findings from '-omic' profiling studies will be used to better personalize diagnosis and prognosis. SF is a natural, prostatic-specific biofluid that represents global prostatic pathophysiology, and utilizing SF as a 'liquid biopsy' to achieve this vision is a genuine possibility – indeed, recent studies in this field indicate that SF is enriched for PCa-specific biomarkers. Establishing SF-based diagnostic methods could assist in the early diagnosis and prognosis of PCa, while also facilitating monitoring of disease progression or response to specific noninvasive, nonprocedurally-based management approaches. However, a SF-based biomarker test for PCa will only be used in the clinic if it can reliably outperform or complement the current diagnostic pathways, and achieving this will not be straightforward. Unlike blood, collection of SF is as yet uncommon and remains to be widely accepted as an approach – but this issue can be overcome. In order to prove the utility of SF, cohorts will need to be large enough and selected in such a way to robustly address specific clinical questions. Finally, validation of findings in independent sample sets will be critically important. Once these issues are addressed, we are confident that SF-derived biomarkers can contribute to the diagnostic and prognostic armamentarium for PCa in the future.Financial & competing interests disclosureMJ Roberts is supported by a Doctor in Training Research Scholarship from Avant Mutual Group Ltd., Cancer Council Queensland PhD Scholarship and Professor William Burnett Research Fellowship from the Discipline of Surgery, School of Medicine, The University of Queensland. RA Gardiner is supported by grants from the RBWH Foundation and PCFA/Cancer Council Queensland. LA Selth is supported by Young Investigator Awards from the Prostate Cancer Foundation (the Foundation 14 award) and the Prostate Cancer Foundation of Australia (PCFA; YI 0810) and a grant from PCFA/Cancer Australia (grant ID 1012337). The Adelaide Prostate Cancer Research Centre is supported by an establishment grant from the PCFA (ID 2011/0452). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.No writing assistance was utilized in the production of this manuscript.References1 Schröder FH, Hugosson J, Roobol MJ et al. Screening and prostate-cancer mortality in a randomized European study. N. Engl. J. Med. 360(13), 1320–1328 (2009).Crossref, Medline, Google Scholar2 Hamoen EHJ, De Rooij M, Witjes JA, Barentsz JO, Rovers MM. Use of the Prostate Imaging Reporting and Data System (PI-RADS) for prostate cancer detection with multiparametric magnetic resonance imaging: a diagnostic meta-analysis. Eur. Urol. doi:10.1016/j.eururo.2014.10.033 (2014) (Epub ahead of print).Medline, Google Scholar3 Lilja H. A kallikrein-like serine protease in prostatic fluid cleaves the predominant seminal vesicle protein. J. Clin. 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A randomized controlled trial protocolContemporary Clinical Trials, Vol. 50Prostate-based biofluids for the detection of prostate cancer: A comparative study of the diagnostic performance of cell-sourced RNA biomarkersProstate International, Vol. 4, No. 3Biomarker Discovery in Human Prostate Cancer: an Update in Metabolomics StudiesTranslational Oncology, Vol. 9, No. 4 Vol. 9, No. 2 STAY CONNECTED Metrics History Published online 18 February 2015 Published in print February 2015 Information© Future Medicine LtdKeywordsbiomarkerdiagnosishepsinmicroRNAPCA3prostate cancerseminal fluidFinancial & competing interests disclosureMJ Roberts is supported by a Doctor in Training Research Scholarship from Avant Mutual Group Ltd., Cancer Council Queensland PhD Scholarship and Professor William Burnett Research Fellowship from the Discipline of Surgery, School of Medicine, The University of Queensland. RA Gardiner is supported by grants from the RBWH Foundation and PCFA/Cancer Council Queensland. LA Selth is supported by Young Investigator Awards from the Prostate Cancer Foundation (the Foundation 14 award) and the Prostate Cancer Foundation of Australia (PCFA; YI 0810) and a grant from PCFA/Cancer Australia (grant ID 1012337). The Adelaide Prostate Cancer Research Centre is supported by an establishment grant from the PCFA (ID 2011/0452). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.No writing assistance was utilized in the production of this manuscript.PDF download