MicroRNAs in prostate cancer: from biomarkers to molecularly ... - Nature

Prostate Cancer and Prostatic Diseases (2012) 15, 314 - 319 & 2012 Macmillan Publishers Limited All rights reserved 1365-7852/12 www.nature.com/pcan

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MicroRNAs in prostate cancer: from biomarkers to molecularly-based therapeutics A Gordanpour1, RK Nam2, L Sugar3 and A Seth1,3,4 MicroRNAs (miRNAs) are effective regulators of gene expression that have a significant role in the pathogenesis of prostate and various other cancers. The high prevalence of aberrant miRNA expression in prostate cancer, and miRNAs’ distinctive properties, give much hope that they can be used as biomarkers and next generation of molecular anticancer therapeutics. Herein, we review the literature on miRNA involvement in prostate cancer pathogenesis and the current understanding of their role as oncogenes, tumor suppressors and metastasis-regulators. We also review the latest research on miRNAs in prostate cancer preclinical studies and clinical trials, and highlight the advantages and challenges of possible miRNA-based therapies. The emerging information regarding the biology of miRNAs in prostate cancer is promising, and may lead to a role(s) for these molecules as diagnostic/prognostic markers and effective therapeutic tools for better molecularly targeted treatment of prostate cancer. Prostate Cancer and Prostatic Diseases (2012) 15, 314--319; doi:10.1038/pcan.2012.3; published online 14 February 2012 Keywords: microRNA; biomarker; therapeutics

INTRODUCTION MicroRNAs (miRNAs) constitute a large family of endogenous, evolutionarily conserved, single-stranded RNA molecules that regulate gene expression. They are 18 -- 25 nt long, and are not translated to proteins. The canonical mechanism of miRNAmediated gene regulation is by its binding to the 30 untranslated region of a target messenger RNA (mRNA) and inhibiting protein production by translation inhibition or mRNA destabilization.1 Although the majority of published data has focused on miRNAs that act via this canonical pathway, there are no mechanistic requirements that restrict miRNA action to only the 30 untranslated region. In fact, miRNAs have also been shown to use 50 untranslated region and open reading frame binding sites to regulate mRNA expression.2,3 In addition, there have been reports of miRNAs that influence gene expression by directly binding to DNA,4 - 6 and some miRNAs can even activate, rather than inhibit gene expression.7 Taken together, these findings highlight the complexity of gene regulation by the miRNAs. The field of cancer genetics has rapidly expanded from identifying the first cancer-related miRNAs,8 to exploring their potential as therapeutic options in less than 10 years. Based on release 18 of miRBase database,9 there are currently more than 1700 miRNAs in the human genome, and an estimated 60% of mRNAs are predicted to be regulated by miRNAs.10 - 13 miRNAs have been associated with almost every cellular process, such as development,14 differentiation,15 apoptosis16 and cell cycle regulation.17 Consequently, misregulation at any of these processes owing to abnormal miRNA expression or mutations can potentially lead to cancer, as the aberrant expression of many miRNAs has been characterized in numerous cancer subtypes, including prostate cancer. In recent years, miRNAs have emerged as possible diagnostic and prognostic biomarkers that may stratify prostate tumors based on specific genetic profiles and thereby improve aspects of

patient management such as staging and treatment. Although many cancer drugs poorly distinguish between cancer and normal cells, development of more precise and specific ways to recognize tumor cells would be of vital importance. The use of such genetic markers can create a unique new niche for the development of molecularly-based drugs that can move prostate cancer research closer to personalized medicine and complement the current standard of care. This review will summarize the current understanding of miRNAs as potential prostate cancer biomarkers, as well as denote how they may be harnessed as agents of molecularly-based therapeutics. MIRNAS AS BIOMARKERS IN PROSTATE CANCER PATHOGENESIS The initial studies of miRNA deregulation in prostate cancer were performed by miRNA microarray profilings, and since then several studies have analyzed prostate cancer-specific miRNA profiles by using genome-wide screenings and validation by quantitative PCR technology.18 - 23 Examination of prostate tumor miRNA expression profiles has revealed widespread dysregulation of miRNAs in primary prostate cancer compared with normal prostate tissue. In addition, there are miRNAs that are abnormally expressed in different stages of disease progression and metastasis, and are hence implicated as prognostic markers of clinical aggressiveness and recurrence. There are some miRNAs that have been found to influence tumor microenvironment.24 This extensive body of knowledge points to miRNAs as potentially being novel cancer biomarkers, and as such they have several advantageous features. The small size and distinctive biochemical structure of miRNAs makes them resistant to endogenous RNase activity.25 miRNAs are extremely stable in formalin-fixed tissues, which improves their detection.26 They are also detectable in plasma and serum fluids,

1 Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Ontario, Canada; 2Division of Urology, Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada; 3Department of Pathology, Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada and 4Biological Sciences, Sunnybrook Research Institute, Toronto, Ontario, Canada. Correspondence: Dr A Seth, Department of Laboratory Medicine and Pathobiology, University of Toronto, Sunnybrook Research Institute, 2075 Bayview Avenue, S224, Toronto, Ontario M4N 3M5, Canada. E mail: [email protected] Received 7 November 2011; revised 19 December 2011; accepted 5 January 2012; published online 14 February 2012

miRNAs as biomarkers and molecularly-based therapeutics A Gordanpour et al

as Mitchell et al.25 were able to distinguish patients with prostate cancer from healthy controls by measuring miR-141 levels in serum. This remarkable stability allows miRNAs to be readily detectable and makes them potentially invaluable biomarkers. miRNAs as oncogenes or tumor suppressors miRNA expression is deregulated in cancer cells compared with the corresponding normal tissue, and depending on the specific cell type and stage of development, miRNAs can function as either tumor-suppressors or oncogenes. Together, they have been termed as oncomirs. Table 1 shows numerous miRNAs that have been reported to be abnormally expressed in primary prostate cancer in comparison with normal tissue. Some miRNAs, specifically marked in Table 1, have shown conflicting results---having been reported to be upregulated in some studies and downregulated in others. Different methods of sample collection, varied study designs and diverse specificity of the platforms used, could explain some of these inconsistencies. Aberrant expressions of some miRNAs such as miR-25, miR-34a, miR-145 and miR-205 have been reported in several miRNA studies. Oncogenic miRNAs that are upregulated in cancer may promote tumorigenesis by negatively regulating tumor suppressor genes inhibiting proliferation, or by repressing genes associated with apoptosis and differentiation. In this case, inhibition of miRNA activity can be achieved by antisense synthetic oligonucleotides (antagomirs) or by drugs that can inhibit the oncogenic activity of the miRNA. Conversely, downregulation of a miRNA that functions as a tumor-suppressor can promote neoplastic development by enhancing proliferation. Restoration of these miRNAs with synthetic mimics or by using viral vectors encoding the miRNAs could have a therapeutic benefit by halting or even reversing tumor growth. It is important to note that there is a vast number of miRNAs that have been linked to prostate cancer in genome-wide screening studies, but there is still no consensus

Table 1.

on exactly which miRNAs are involved in prostate cancer formation and progression. Hence, further experimental validation is critical in better elucidating miRNAs’ biological function and role in the pathogenesis of prostate cancer. Metastasis-regulatory miRNAs Cancer metastasis, rather than the primary tumor itself, is the cause of death in prostate cancer patients. In addition to their role as potential oncogenes and tumor-suppressors, miRNAs can also be involved in the regulation of metastasis.37 These ‘metastamirs’ could be regulating various steps of the metastatic cascade such as epithelial-to-mesenchymal transition, migration, invasion, angiogenesis, adhesion and colonization of distant organs. So far, only a few studies have investigated miRNAs associated with metastatic prostate cancer. One recent study by Watahiki et al.38 has used next generation sequencing to compare the miRNA profiles of a transplantable metastatic versus a non-metastatic prostate cancer xenograft, and has identified many miRNAs that are differentially expressed in prostate cancer metastasis. Table 2 lists miRNAs that have been linked to metastasis in prostate cancer as either pro-metastatic or anti-metastatic. Some miRNAs such as miR-21, miR-331 -- 3p, miR-205 and miR-203 have been associated with prostate cancer metastasis. For instance, miR-21 has been shown to be upregulated in the majority of human cancers, including prostate, and has been associated with increased invasiveness of prostate LNCaP cells.48 miR-205 has been reported by Gandellini et al.35 to inhibit epithelial-tomesenchymal transition and reduce cell migration and invasion through downregulation of protein kinase Cepsilon. miRNAs in preclinical studies There is a small but increasing numbers of preclinical studies focusing on the role of miRNAs in prostate cancer, and have been

miRNAs associated with primary prostate cancer

Upregulated in prostate tumors let-7aa22 let-7da23 let-7i19,23 miR-16a23 miR-17-5p23 miR-20a22,23 miR-2123 miR-2519,22,23 miR-26aa23 miR-26a-119 miR-26a-219 miR-27aa23 miR-29ba23 miR-30ca23 miR-31a19 miR-3219,23 miR-34aa23 miR-34b19 miR-92a23 miR-92-119 miR-92-219 miR-9319,23 miR-9523 miR-9621,33 miR-99b19 miR-101a23 miR-106a23

miR-106b19 miR-122a22 miR-124a23 miR-125aa19 miR-12922 miR-130b21 miR-13523 miR-141a22,25 miR-14623 miR-14823 miR-181a-119 miR-181a-219 miR-181ba23 miR-18219,33 miR-182*21 miR-18321,33 miR-18420,23 miR-18723 miR-18819 miR-19123 miR-194-119 miR-194-219 miR-195a23 miR-19623 miR-196a-119 miR-196a-219 miR-19723

Downregulated in prostate tumors miR-19820,23 miR-19923 miR-200c19 miR-20220 miR-203a23 miR-20623 miR-21020 miR-21423 miR-221a31 miR-222a31 miR-22323 miR-29620 miR-30222 miR-302c20 miR-32020 miR-345a20 miR-37019,20 miR-373a20 miR-37519,21 miR-42519 miR-44919 miR-49120 miR-49820 miR-50320 miR-51320 miR-524*21 miR-63421

let-7aa20,23 let-7b19,20 let-7c20 let-7da20 let-7f20 let-7g20 miR-1-219 miR-7-119 miR-7-219 miR-15a32 miR-16a20,21 miR-16-132 miR-19b20 miR-2220 miR-23a20 miR-23b20,22 miR-2422,23 miR-26aa20 miR-26b20 miR-27aa20 miR-27b20 miR-29a20 miR-29ba20 miR-30a-5p20 miR-30b20 miR-30ca20 miR-31a21

miR-34aa19 miR-92a20 miR-9920 miR-10020,22 miR-101a28 miR-10320 miR-125aa20 miR-125b20,22 miR-12619 miR-128a19,23 miR-133a-119 miR-141a20 miR-14320,22 miR-14519,20,22 miR-148a20 miR-14921,23 miR-181ba21 miR-195a20 miR-199a20 miR-203a34 miR-20519 - 22,35 miR-21823 miR-218-219 miR22019 miR-221a19 - 22 miR-222a20 - 22 miR-33036

miR-32919 miR-331-3p27 miR-34019 miR-345a19 miR-36821 miR-373a29 miR-41019 miR-449a30 miR-48719 miR-49019 miR-49419 miR-49720 miR-49919 miR-520ca29 miR-520h19

Abbreviation: miR, microRNA. a Opposite expression of miRNAs in different studies.

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Prostate Cancer and Prostatic Diseases (2012), 314 - 319

315


316 Table 2.

miRNAs associated with metastatic prostate cancer

Pro-metastatic miRNAs 38

let-7d let-7g38 let-7g*38 let-7i38 miR-738 miR-938 miR-9*38 miR-1738 miR-18a38 miR-18b38 miR-20b*38 miR-2148,44 miR-27a38 miR-27b38 miR-30a38 miR-30a*38 miR-3138 miR-34c-5p38 miR-99a38 miR-106a38 miR-125b38 miR-125b-2*38 miR-12638

Anti-metastatic miRNAs 38

miR-128 miR-13638 miR-13838 miR-140-5p38 miR-142-5p38 miR-14438 miR-144*38 miR-148b*38 miR-151-3p38 miR-15238 miR-181a-2*38 miR-19144 miR-200a38 miR-21038 miR-21838 miR-223*38 miR-301*38 miR-340*38 miR-37345 miR-374a38 miR-37938 miR-449a38 miR-450a38

38

miR-451 miR-486-3p38 miR-486-5p38 miR-520c45 miR-542-3p38 miR-744*38 miR-124638

let-7c39 miR-142 miR-7-1*38 miR-15a32 miR-15b38 miR-1638,46 miR-16-132 miR-17-3p47 miR-2438 miR-24-2*38 miR-26b38 miR-28-5p38 miR-29a38 miR-29b50 miR-29c38 miR-33b38 miR-34a38,53 miR-9538 miR-10039 miR-101*38 MiR-106b38 MiR-126*38,56 miR-133a42

miR-14340,41 MiR-14538,41 MiR-146a43,44 miR-146b-5p38 miR-18538 miR-18638 miR-188-5p38 miR-19138 miR-193a-3p38 miR-19538 miR-196a38 miR-200b49 MiR-200b*38 miR-200c*38 MiR-20338,51,34 MiR-20538,35,52 miR-21839 MiR-22154,55 miR-324-5p38 miR-331-3p38,27 miR-33538 miR-339-5p38 miR-342-3p38

miR-361-5p38 miR-36338 miR-42438 miR-42538 miR-45438 miR-49738 miR-50338 miR-542-5p38 miR-556-5p38 miR-582-5p38 miR-590-5p38 miR-62738 miR-65138 miR-65238 miR-66038 miR-66438 miR-70838 miR-118038 miR-126938 miR-128738 miR-2115*38 miR-3065-5p38

Abbreviation: miR, microRNA.

Table 3.

miRNAs in prostate cancer preclinical studies

miRNA

Target

References

miR-15a, miR16-1 miR16 miR-17-3p miR-203

BCL2, CCND1, WNT3A CDK1, CDK2 Vimentin Runx2, Dlx5, ZEB2, Bmi, survivin CD44 P27

Bonci et al.32 Takeshita et al.46 Zhang et al.47 Saini et al.51

miR-34a miR-221, miR-222

Liu et al.57 Mercatelli et al.31

Abbreviation: miR, microRNA.

reported in Table 3. Bonci et al.32 have shown that miR-15a and mir-16-1 are downregulated in cancer cells of advanced prostate cancer, resulting in BCL2, CCND1 and WNT3A upregulation, which leads to increased cell survival and invasion. They showed that the loss of function of miR-15a and miR-16 causes the nontumorigenic prostate cell line RWPE-1 to form tumors in NODSCID mice. The authors propose that miR-15a and miR-16 may have significant therapeutic potential, as single agents or in combination with chemotherapy, as delivery of miR-15a and miR-16 to prostate cancer xenografts caused tumor regression. Interestingly, Takeshita et al.46 reported that systemic delivery of miR-16 by atelocollagen inhibits the growth of metastatic prostate cancer in bone of nude mice, and may potentially be used for the treatment of metastatic prostate cancer. Zhang et al.47 observed that miR-17-3p regulates the expression of vimentin and altered expression of miR-17-3p decreases the tumorigenic behavior and reduces tumor size in nude mice. Saini et al.51 also noted that relative miR-203 expression is lower in prostate cancer cell lines derived from bone metastasis, and over-expression of miR-203 has a negative effect on the development of metastases in nude mice. Significantly, miR-203 has been shown to repress bone-specific transcriptional regulators, such as Runx2 and Dlx5, in addition to regulating pro-metastatic genes ZEB2, Bmi and Survivin. A recent preclinical study by Liu et al.57 showed convincing preclinical therapeutic evidence, implicating miR-34a in prostate cancer Prostate Cancer and Prostatic Diseases (2012), 314 - 319

metastasis. Intratumoral injection of miR-34a into subcutaneous tumors inhibited tumor growth, and systemic delivery of miR-34a into the tail vein could reduce tumor burden by half. In orthotopic LAPC9 tumors, they showed that miR-34a can reduce lung metastasis without changing tumor growth, and can increase survival of mice. These results make a strong case for the effectiveness of miR-34a as a therapeutic agent. Putative targets of miRNAs The major functional role of miRNAs is marked by the proteincoding mRNAs that they target. Because of partial complementarity between a miRNA molecule and its binding sequence, every miRNA can potentially bind to numerous mRNA targets, and each mRNA could be regulated by many different miRNAs. This makes elucidating the biological function of a miRNA challenging, as identification of a single target may not adequately reflect the entire function of a miRNA. Most studies thus far have used bioinformatic tools to predict targets of different miRNAs, and algorithms on the most frequently used bioinformatic programs often give hundreds of putative targets for each miRNA. Different computational target prediction software, such as TargetScan (Cambridge, MA, USA), PicTar (New York, NY, USA), miRanda (New York, NY, USA) and miRBase (Manchester, UK), can use varying methods combining thermodynamics-based models and sequence alignment of miRNA -- mRNA molecules and their conservation in different species to name putative targets of miRNAs.58 Although these algorithms are a good starting point for the prediction of miRNA targets, ultimately functional experiments must be used to validate a gene target.

MIRNAS AS THERAPEUTIC AGENTS---BREAKTHROUGHS AND CHALLENGES The association between the differential expression of miRNAs and cancer is now so convincing that there has been considerable attention focused on applying these gene regulators as therapeutic agents. Although the utilization of miRNA-targeted therapy has not yet been implemented in clinical practice, & 2012 Macmillan Publishers Limited


miRNA-based therapeutics can potentially be applied as effective therapeutics by utilizing them as either drugs or drug targets: miRNA molecules that act as tumor suppressors or antagomirs of oncogenic miRNAs may serve as cancer treatments. This approach offers a new paradigm for treating human diseases, one that can utilize the entire human genome for therapeutic purposes. After establishment of a miRNA’s role in cancer pathogenesis and identification of its targets, expression of the specific transcripts bearing the complementary sequence of the miRNA can be manipulated by small nucleic acids that mimic or antagonize miRNAs, using endogenous miRNA machinery. miRNAs are naturally occurring molecules, and so they benefit from million years of evolutionary ‘fine tuning’ of their function, and there are likely distinct advantages in applying miRNAs as therapeutic agents over the current conventional drugs. Unlike other nucleic acid therapeutics such as siRNAs, specificity for a single target is not the purpose for miRNA drugs, as one miRNA can target multiple downstream effectors.59 Consequently, miRNA-based drugs have the advantage that they can target multiple genes in a pathway concurrently by either inhibiting or mimicking a single miRNA. Also, multiple tumor-suppressive miRNAs can be used simultaneously on one or a group of target genes to augment the effect of the therapy. For example, if there is a mutation in the binding sequence of an oncogenic target, one miRNA may not be able to bind to it, but combination of several miRNAs that target the same gene would reduce the probability of mutation-induced resistance.60 Another advantage of using miRNAs as drugs is that they are small in size, and are therefore less antigenic than protein-coding gene replacement therapies.60 Despite the great potential of miRNAs as anticancer drugs, there are some challenges that need to be addressed in order to bring miRNAs into the clinic. The development of miRNA-based therapies requires that miRNAs pertinent to cancer formation and progression be first identified and validated. As previously mentioned, oligonucleotides or virus-based constructs can be used to directly introduce miRNAs to the system, or drugs can be used to regulate miRNA expression and processing.61 Perhaps the main hurdle in using miRNA-based therapy is delivering the drug to cancer cells, whether locally in the prostate or to metastatic cells disseminated throughout the body. Several delivering strategies are considered plausible, such as incorporating the miRNAs within liposomes, or conjugating them to peptides that can penetrate the plasma membrane.62 For example, a lipid-based system was used to deliver chemically synthesized miR-34a, resulting in inhibition of tumor growth in mouse models of nonsmall-cell lung cancer.63 Also, tandem arrays of miRNA mimics delivered by lentiviral vectors proved effective against Bcr -- Abl lymphoid leukemia.64 Although the multispecific nature of miRNAs makes them very effective in regulating cellular processes associated with normal cell function and neoplastic development, Table 4.

this property can also be a weakness of miRNA-based therapy, as perturbing miRNA levels may affect the expression of unintended mRNA target. Another major concern would be that the introduction of exogenous miRNAs may overwhelm the RNAinduced silencing complex and inhibit the processing of other miRNAs that are involved in normal cellular function.65 An additional challenge for developing miRNA-based therapies is the issue of toxicity and safety. There is concern that miRNAs would affect off-target genes, delivery with liposomes might be toxic,66 and high concentration of small RNAs may cause liver damage.67 Ongoing and future clinical trials will be crucial in assessing the safety of miRNAs as therapeutics. miRNAs in clinical trials There are currently a number of major pharmaceutical companies that have miRNA therapeutics programs. The most advanced miRNA therapy program is being done in liver, benefiting from the fact that oligonucleotides administered systematically will largely localize to the liver. In 2010, Santaris Pharma A/S, a biopharmaceutical company that focuses on the development of RNA-targeted therapies, announced that their drug miravisen (SPC3649), a miR-122 inhibitor, has been advanced into Phase II studies. This drug is being used to treat Hepatitis C virus infections by sequestering miR-122, and thereby inhibiting the replication of Hepatitis C virus. Miravirsen was the first miRNA-targeted drug to enter clinical trials. This drug was developed using Santaris Pharma A/S Locked Nucleic Acid Drug Platform. Data from the drug’s phase I trial on healthy volunteers showed that this miRNA-targeted therapy is well tolerated. Recently, the FDA gave the miRagen Therapeutics’ compound MGN-4893 that targets miR-451 orphan drug status, a designation intended for drugs used for treatment of rare conditions, and clinical trials for this compound are scheduled to begin in 2012. This drug is being used to treat polycythemia vera, a myeloproliferative disease, which is characterized by the abundance of blood cells and platelets in the body. Prostate cancer-associated miRNAs currently in clinical trials Recently, Mirna Therapeutics and researchers at the University of Texas MD Anderson Cancer Center have had success in inhibiting prostate cancer tumor growth, decreasing lung metastasis and extending survival in mice by using miRNAs. This represents a very exciting step towards the clinical use of miRNA-based drugs. As previously mentioned, Liu et al.57 showed that miR-34a is underexpressed in prostate cancer stem cells, and systemic delivery of miR-34a using a liposome-based delivery agent inhibited prostate cancer stem cells from replicating by suppressing the adhesion molecule CD44. This group hopes to advance miR-34a as a treatment option for prostate cancer patients. Although impeding cancer in mice might be a lot easier than in human patients, these

miRNAs in prostate cancer clinical trials

Trial title

Study type

Institution

Trial identifier

MicroRNA expression profiles in high risk prostate cancer

Observational

NCT01220427

Molecular correlates of sensitivity and resistance to therapy in prostate cancer Trial of vaccine therapy in curative resected prostate cancer patients using autologous dendritic cells loaded with mRNA from primary prostate cancer tissue, hTERT and Survivin Phase II randomized study of combined androgen deprivation comprising Bicalutamide and Goserelin or Leuprolide Acetate with versus without Cixutumumab in patients with newly diagnosed hormone-sensitive metastatic prostate cancer

Observational

Wu¨rzburg University Hospital, Germany University of Washington

NCT01050504

Treatment

Rikshospitalet University Hospital, Norway

NCT01197625

Biomarker/laboratory analysis, treatment

Saint Anthony0 s Hospital at Saint Anthony0 s Health Center, Illinois

NCT01120236

Abbreviations: mRNA, messenger RNA; miRNA, microRNA.



317


318 Biomarkers

MicroRNAs

• ~1700 human miRNAs • Small and very stable • Can be measured non-invasively • Readily detectable in tissue, blood, urine, and saliva • Easily and accurately quantitated by qRT-PCR

Therapeutics

• May serve as drug or drug target • Can utilize the entire transcriptome for therapeutic purposes • Suitable for molecularly based personalized medicine • Naturally occurring molecules

Figure 1. miRNAs as potential biomarkers and therapeutic agents. miRNAs, microRNAs; qRT-PCR, quantitative reverse transcriptase-PCR.

results are indeed promising as miRNA-targeted therapeutic tools that have clinical value. Currently there are several observational clinical trials that aim to study miRNAs in prostate cancer (Table 4) (www.clinicaltrials.gov). ‘Micro-RNA expression profiles in high risk prostate cancer’, NCT01220427, is being conducted at Wu¨rzburg University Hospital. Its main goal is to examine whether specific miRNA expression profiles are correlated with prostate cancer outcome. Another trial ‘Molecular correlates of sensitivity and resistance to therapy in prostate cancer’, NCT01050504, is based at the University of Washington. It aims to study differences in gene expression patterns of miRNAs, in addition to other genes, in order to discover new biomarkers and drug targets. Both of these trials are currently recruiting participants. There are also two clinical trials that briefly study miRNA expression as a secondary objective of their study. Although there are no clinical trials that use miRNAs as a treatment option for prostate cancer, the emerging field of miRNA-targeted therapeutics is undoubtedly progressing and the vast body of knowledge from basic science and pre-clinical work is reassuring. CONCLUSION We have sought here to illustrate and expand the notion of miRNAs as biomarkers and future therapeutics for prostate cancer (Figure 1). Despite the great deal of attention given to miRNAs, our current understanding of their role in the development and progression of prostate cancer is very limited. Nonetheless, miRNAs are promising to be efficient and specific biomarkers and therapeutic tools. As the field of miRNA continues to grow, a deeper understanding of miRNA expression and function in normal and cancer cells will surely influence the development of miRNA-based therapeutics. We envision that in the coming years there will be profound advances in the field of molecularly-based therapeutics, and it remains to be seen whether these will result in effective miRNA-based drugs. CONFLICT OF INTEREST The authors declare no conflict of interest.

ACKNOWLEDGEMENTS This work was supported by Canadian Cancer Society Research Institute (grant no. 019038).

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