Circulating cell-free miRNAs have emerged as promising minimally-invasive biomarkers for early detection, prognosis and monitoring of cancer. They can exist in the bloodstream incorporated into extracellular vesicles (EVs) and ribonucleoprotein complexes. However, it is still debated if EVs contain biologically meaningful amounts of miRNAs and may provide a better source of miRNA biomarkers than whole plasma. Researchers from the Latvian Biomedical Research and Study Centre set out to systematically compare the diagnostic potential of prostate cancer-associated miRNAs in whole plasma and in plasma EVs.
RNA was isolated from whole plasma and plasma EV samples from a well characterised cohort of 50 patient with prostate cancer (PC) and 22 patients with benign prostatic hyperplasia (BPH). Nine miRNAs known to have a diagnostic potential for PC in cell-free blood were quantified by RT-qPCR and the relative quantities were compared between patients with PC and BPH and between PC patients with Gleason score ≥ 8 and ≤6.
Only a small fraction of the total cell-free miRNA was recovered from the plasma EVs, however the EV-incorporated and whole plasma cell-free miRNA profiles were clearly different. Four of the miRNAs analysed showed a diagnostic potential in our patient cohort. MiR-375 could differentiate between PC and BPH patients when analysed in the whole plasma, while miR-200c-3p and miR-21-5p performed better when analysed in plasma EVs. EV-incorporated but not whole plasma Let-7a-5p level could distinguish PC patients with Gleason score ≥ 8 vs ≤6.
Workflow of the study and characterisation of plasma EVs
a Workflow of the study. b Representative transmission electron microscopy image of plasma EVs. c Quantification of EVs isolated from plasma of PC patients and healthy controls (HC) by nanoparticle tracking analysis. d Average size distribution of EVs isolated from plasma of PC patients and healthy controls. e Western blot analysis of EV markers (TSG101, CD9), endoplasmic reticulum protein Calnexin and β-actin in plasma EVs isolated from two healthy individuals and PC-3 cells (as a positive control)
This study demonstrates that for some miRNA biomarkers EVs provide a more consistent source of RNA than whole plasma, while other miRNAs show better diagnostic performance when tested in the whole plasma.