Scientists discover potential biomarkers for male infertility: the composition of microRNAs and their isoforms

One of the most unexpected findings of the Human Genome Project was that over 98% of the human genome does not encode proteins. For a while, this fraction of the genome was considered to have no function and was referred to as “junk” DNA. However, it is now known that non-coding DNA serves important biological functions.

For instance, certain DNA segments give rise to a type of non-coding yet functional RNA called microRNA or miRNA, which acts as a key regulator of biological processes and pathways. Close to 2000 loci in the human genome produce miRNAs cataloged and annotated in miRBase, a public database. These molecules are found not only in cells but are also secreted into biological fluids such as blood, urine, and semen, among others, where they can be characterized and quantified, reflecting important information about cellular health. Recently, isoforms of miRNAs or isomiRs (variants with heterogeneity in length and/or sequence) that are highly abundant have been identified.

Semen contains a unique concentration of miRNAs, most of which are enclosed in structures called extracellular vesicles that can be transferred to sperm, contributing to fertilization. Researchers from the Human Molecular Genetics group at the Bellvitge Biomedical Research Institute (DIBELL), specifically from the Molecular Genetics of Male Infertility and Urogenital Diseases research line led by Dr. Sara Larriba, in collaboration with Dr. Lluís Bassas from the Andrology Service (Fundació Puigvert), and Dr. Lauro Sumoy from the High-Content and Bioinformatics Unit (IGTP), using large-scale sequencing technologies (small RNAseq), demonstrated the presence of isomiRs in these seminal structures. “Given that some of these vesicles in semen derive from the testicle, alterations in the concentration of miRNA/isomiRs they contain may reflect changes in sperm production,” explains Dr. Larriba. Their study showed alterations in the profile of miRNAs as well as their isoforms in extracellular vesicles of semen among individuals with azoospermia of different origins, suggesting they could be used as potential biomarkers for male infertility. “The small RNAseq technique can characterize the different isoforms of miRNAs that differ by a single base,” comments Dr. Sumoy.

miRNAs modulate protein translation by binding to target mRNA. The interest in isomiRs lies in the fact that changes in their sequence can diversify the repertoire of target genes regulated by miRNAs, since isomiR variants may have different targets than their respective canonical miRNAs. Additionally, isomiRs need to be considered in the development of diagnostic assays based on the detection and quantification of miRNA biomarkers because they are relatively abundant and, due to their sequence similarity with canonical miRNAs, may technically interfere with the specificity of these assays. In fact, the results of their latest study, published in the International Journal of Molecular Sciences show that the RT-qPCR technique, used to validate the results of small RNAseq, is unable to discriminate between isomiRs. “This suggests that the presence of isomiRs compromises the quantification of miRNAs, impacting the interpretation of RT-qPCR results (designed for detecting canonical miRNAs),” explains Adriana Ferre, the first author of the article. Based on this, the research group suggests prioritizing those miRNAs with homogeneous expression between canonical miRNAs and their respective isomiRs as the ideal strategy for developing clinically useful tests.

Image: Adriana Ferre and Sara Larriba, first signer and corresponding author of the study, respectively.

The Bellvitge Biomedical Research Institute (IDIBELL) is a biomedical research center created in 2004. It is participated by the Bellvitge University Hospital and the Viladecans Hospital of the Catalan Institute of Health, the Catalan Institute of Oncology, the University of Barcelona and the City Council of L’Hospitalet de Llobregat.

IDIBELL is a member of the Campus of International Excellence of the University of Barcelona HUBc and is part of the CERCA institution of the Generalitat de Catalunya. In 2009 it became one of the first five Spanish research centers accredited as a health research institute by the Carlos III Health Institute. In addition, it is part of the “HR Excellence in Research” program of the European Union and is a member of EATRIS and REGIC. Since 2018, IDIBELL has been an Accredited Center of the AECC Scientific Foundation (FCAECC).