Discovery and Evaluation of Extracellular MicroRNA Biomarkers in Plasma, Ascites, and Urine.

MicroRNAs (miRNAs) comprise a large group of small noncoding RNAs within a heterogeneous entity of noncoding RNAs, forming potent functional tools regulating the crucial biological processes within cells and the body. Cell-free miRNAs have become one of the novel promising diagnostic, predictive, and prognostic biomarkers for various diseases extensively investigated in recent years. This is due to their presence within extracellular fractions of various body fluids suggesting their potential as noninvasive "liquid biopsy" in case of their dysregulated expression.Among the body fluids, blood plasma and serum along with urine are the most commonly investigated sources of various types of cell-free miRNAs. Another body fluid, i.e., ascites (effusion, peritoneal/pleural fluid) may be the clinically important fluid particularly associated with carcinogenesis in ovarian carcinomas and hepatocellular carcinomas or in case of liver cirrhosis.Here, we provide a protocol for an expression profiling study based on qPCR analyses aimed at finding novel candidate miRNAs via small-scale or large-scale screening and evaluation experiments using liquid biopsies of blood plasma, ascites, and urine. Using this approach may be worth in cases where no (or limited) information is available on miRNA expression in particular diseases and geographic regions, for validation of previously published miRNAs with promising diagnostic potential, particularly in situations where follow-up study is aimed at validating miRNAs coming from (micro) array or NGS experiments, or where funding for large-scale experiments is not available. We demonstrate that assessment of plasma, ascites, and urine miRNAs expression may represent a feasible method to explore the potential for finding novel diagnostic, predictive, and prognostic biomarkers for various diseases.