Specific miRNA Disease Biomarkers in Blood, Serum and Plasma: Challenges and Prospects

Significant effort has been devoted to discovering microRNA (miRNA) disease biomarkers. In particular, miRNAs in whole blood or specific blood components are candidates for improving the diagnosis of diseases, including life-threatening pathologies. This review covers the challenges crucial for the translation of miRNAs in body fluids (circulating miRNAs) from a research setting into a clinical care scenario. First, we discuss the specificity of miRNA biomarkers for the diagnosis of a disease. While single miRNAs such as miR-20a, miR-21, miR-155, and miR-126 are frequently not disease specific, miRNA signatures that consist of a plurality of different miRNAs may help to improve differentiation between pathologies. Second, we discuss the degree of reproducibility and highlight selected validation studies. While single miRNA markers are often confirmed by independent studies, miRNA signatures are less frequently verified. Third, we address challenges to the profiling of miRNAs in high-throughput settings and we discuss the appropriateness of various analytical platforms and bioinformatics towards a clinical application of miRNAs. Finally, we shed light on the suitability of enriched miRNA sources, e.g. fractionation of body fluids for extracellular vesicles such as exosomes or blood cells, to develop miRNA signatures. With an increasing number of verified miRNA signatures and with the advance of matured medium-throughput approaches in clinical settings, specific miRNA markers are increasingly likely to contribute to human healthcare.