Prostate cancer (PCa) is the only one of the four solid tumour types, breast, lung and colorectal cancer, which has a clinical useful protein biomarker for diagnostics and follow-up after treatment. Prostate-specific antigen (PSA) has shown reasonable sensitivity for detection of incipient cancer and can also predict response to treatment (Freedland et al, 2005
). One of the drawbacks with PSA is its low specificity, such that benign hyperplasic conditions can also be associated with a PSA increase (Catalona et al, 1994
). Thus, additional PCa biomarkers are needed, especially the ones that give information about the severity of the disease and can predict high or low risk for future metastases.
In this article, we evaluated a novel approach to find predictive markers for PCa, and analysed RNA in urine exosomes from PCa patients. Two types of microvesicles are present in prostate secretions: (1) prostasomes (150–500
nm), produced by prostatic ductal epithelial cells that are a normal component of seminal fluid and play a role in male fertility (Burden et al, 2006
); and (2) exosomes, specialised nanovesicles (30–100
nm) with a cup-shaped morphology, actively secreted by a variety of normal and tumour cells. Elevated exosome secretion has been found in malignancy effusions, serum and urine from cancer patients (Mitchell et al, 2009
). In addition, it was shown that exosomes released from a mast cell line (Valadi et al, 2007
) and glioblastoma tumour cells (Skog et al, 2008
) contain intact mRNA that can be transferred to recipient cells and therein translated into functional proteins (Skog et al, 2008
). Certain RNA transcripts are enriched several 100-fold in the exosomes compared with the donor cells, supporting a specific packing mechanism (Skog et al, 2008
). The exosomes lack essentially all of the ribosomal RNA, which represent ~80% of the total RNA in cells, and thus contains mainly mRNAs and microRNAs (miRNAs). Therefore, exosomes are enriched in unique transcripts specific to tumour cells that may be below detection limit even in the tumour cells themselves (Skog et al, 2008
; Taylor and Gercel-Taylor, 2008
). In addition, individual PCa are often very heterogeneous in their phenotype, and a biopsy taken from one region of the tumour can have a distinct genotype from another region. Therefore, analysing the transcriptome in secreted PCa exosomes in urine has the advantages of being both noninvasive and informative as to the overall tumour malignancy status, including tumour-specific splice variants, mutations, and mRNA and miRNA levels known to be diagnostic for PCa.
Other groups have shown that tumour cells shed into the urine after mild prostate massage can be used as an mRNA source for the discovery of tumour biomarkers (Hessels et al, 2007
; Laxman et al, 2008
). In this study we have taken this concept a step further to analyse the tumour exosome fraction in urine. Using a nested PCR-based approach, we were able to show that tumour exosomes carry genetic information specific for PCa, and as a proof-of-principle we used tumour exosomes to detect two PCa mRNA biomarkers, PCA-3
(Hessels et al, 2007
; Laxman et al, 2008
). This study supports the use of RNA in exosomes isolated from urine as diagnostic markers for PCa, and offers an alternative, sensitive and unique new type of PCa biomarkers.