The role of miRNAs in male human reproduction: a systematic review

Background The presence of miRNAs in human reproductive tissue is intriguing and suggests the possibility that these important regulatory molecules play a role in reproductive function. However, the regulatory role of miRNAs in reproductive tissue remains poorly understood with a significant amount of controversial and contradicting data. Objectives To systematically review the high-quality studies published to date investigating miRNAs associated with male human reproduction in order to describe their roles and relations with infertility and update the knowledge in the field. Materials and methods A comprehensive systematic review of the published literature in MEDLINE-PubMed and EMBASE databases from the earliest available online indexing year until June 2018 (complimentary search until July 2019) was performed, in accordance with the PRISMA guidelines. We have included descriptive, case-control, cross-sectional, and observational prospective and retrospective studies in which fertile/infertile men were well-defined. The primary outcome was the miRNA expression in testis, epididymis, sperm cells, seminal plasma, and extracellular vesicles (i.e., exosomes and microvesicles). Results We identified 25,204 articles, of which 42 were selected for qualitative analysis. Of the 42 articles included, 15 evaluated the miRNAs in testis, five in epididymis, 13 in spermatozoa, and 11 in seminal plasma and/or extracellular vesicles. Two studies tackled more than one sub-group. As far as miRNA presence and content, the results of this systematic review indicated that every tissue/cell contains a well-defined and stable population of miRNAs that could be potentially related to spermatogenesis and embryogenesis. Discussion and conclusion Our systematic review of descriptive and observational studies shows a consistent relationship between aberrant miRNA expression and infertility. Therefore, it seems reasonable that measuring the expression of particular miRNAs might be useful not only as infertility biomarkers, but also for developing therapeutic strategies.