Methods for quantification and characterization of microRNAs in cell-free plasma/serum, normal exosomes and tumor-derived exosomes

Exosomes are actively released by all cell types under both normal and pathological conditions. The circulation of these membrane nanovesicles in various body fluids enables them to horizontally transfer their genetic information from cell to cell. An excessively high exosome secretion has been observed in cancer patients. Packaging of biomolecules, e.g., nucleic acids and proteins, into exosomes is assumed to be a selective process during tumor progression. Exosomes even contain microRNAs (miRNAs) associated with the RISC-Loading Complex, and thus, display cell-independent capacity to process precursor into mature miRNAs. These small non-coding RNA molecules are frequently deregulated in cancer and modulate the expression of numerous tumor-associated genes and cellular processes. Considering the characteristics of miRNAs and exosomes that reflect cancer development, tumor load, malignant progression towards metastatic relapse and drug resistance, they may be of potential clinical uses. To reach the high level of evidence required for their entry into clinical practice, it is crucial to develop standardized detection assays. The present review article focuses on most popular analytical techniques for isolating, quantifying and characterizing exosomes and miRNAs from plasma/serum of cancer patients, to end with an example for the fractionation of cancer-derived exosomal, normal exosomal and cell-free miRNAs.