Not all good things come in big packages

Introduction

The essence of molecular diagnostics typically begins with the extraction of nucleic acids from a patient sample such as blood, solid tissue, or other biological specimens. In past years, small fragments of nucleic acids observed during quality control checks were considered as degraded “junk” DNA or RNA, and therefore discarded. It was not until 1993, when Victor Ambros and his group at Dartmouth College identified the first microRNA (miRNA) in Caenorhabditis elegans, that the interest in these small molecules as regulators of key biological functions became evident [1]. As many research laboratories scrambled to identify these miRNAs and their biological functions, credit with the first link to clinical disease is given to Carlo Croce and his group at the Ohio State University for their seminal work in chronic lymphocytic leukemia [2].

Contrary to gene expression studies of messenger RNAs (mRNAs), miRNA quickly conquered the stage as promising new biomarkers of human diseases. Technically, these molecules were more stable than mRNA, were easily detectable using a variety of molecular biology techniques and could be quantified. Clinically, miRNAs were shown to be specific for a particular tissue or cell type, able to differentiate diseased vs. nondiseased tissues, associated with progression of disease, and showed potential for therapeutic intervention.

More than 20 years after the first discovery of miRNAs, this special issue of Clinical Chemistry and Laboratory Medicine reviews the impact that miRNA analysis has had on the biology of human disease, its diagnosis and treatment.

Preanalytical and analytical issues of miRNA measurement

Similarly to all the biomarkers used in our diagnostic laboratories, miRNA measurement in biological samples such as serum, plasma and tissues are prone to preanalytical variations [3]. It is quite clear in the field that discrepancies among studies are often because of a lack of standardization in preanalytical and analytical methodologies [4]. In addition, many studies reported biological variation of miRNAs based on age, sex, exercise and more [5], [6], [7]. A careful awareness of these factors is essential for a proper understanding of the potential and limitations of these small RNAs. In an effort to address these important aspects, we have put together relevant articles addressing the challenges in miRNA detection. Khan et al. [8] covers the best practice to limit preanalytical variation of miRNAs in body fluids and tissues. Mayr et al. [9] discuss the impact of hematocrit on miRNA measurement. Otsu et al. [10] present original data on intra-individual variation of miRNAs. Finally, Kappel et al. [11] review the analytical aspects of miRNA detection, covering the pros and cons of the strategies used in the field.

Clinical applications of the different circulating forms of miRNAs

miRNAs have been shown to be very stable in body fluids compared to their mRNA counterparts [12], [13]. It is now clear that these small RNAs can escape RNAse activity in body fluids because of protective shields while they are circulating. miRNAs in body fluids can be found in small vesicles, bound to protein, in lipoproteins and in blood cells as well. Interestingly, all these circulating fractions are detectable and can be useful in many different clinical applications. Takahashi et al. [14] review the principle and involvement of exosomal miRNAs in cancer. Focusing on the intracellular fraction of blood miRNAs, Provost [15] covers the clinical significance of platelets miRNAs. Desgagné et al. [16] review the particularities and functions of lipoprotein-associated miRNAs. Finally, Schulte et al. [17] review the clinical and diagnostic applications of miRNAs in cardiovascular diseases.

miRNAs in cancer

As previously mentioned, the first evidence of the involvement of miRNAs in human disease was shown in cancer, 15 years ago by Calin et al. [2]. Since then, the vast majority of publications on the clinical applications of miRNAs have been published in that field. It is now clear that miRNAs can act as oncogenes or tumor suppressors depending on their targets [18]. Furthermore, 50% of miRNAs are found in genomic fragile sites, amplified or deleted in different types of cancer [19]. In this special edition, Ling et al. [20] review the role of noncoding RNAs in cancer. Filella et al. [21] cover the implications of these small biomarkers in the management of prostate cancer. Original articles will follow with Diamantopoulos et al. [22] investigating the prognostic value of miR-16 in colorectal cancer, whereas Shen et al. [23] show the diagnostic potential of blood miR-4449 in multiple myeloma. Finally, giving us hope in the implementation of miRNAs as routine biomarkers in our diagnostic laboratories, de Abreu and colleagues [24] will share the Darthmouth experience in pancreatic cancer.

miRNAs and genomic

The genomic and molecular biology fields have evolved very fast in the last decades. The new high-throughput sequencing technologies allow massive and affordable sequencing of the human genome. This has raised an interest in the connection between single nucleotide polymorphisms (SNPs) and human disease [12]. SNPs not only elicit an impact on protein functions but are also conducive to impaired interactions between miRNAs and their targets [25]. SanGiovanni et al. [26] investigate the impact of SNPs on miRNA regulation in the context of age-related macular degeneration.

Although miRNA analysis continues to have enormous potential for diagnosis, prognosis and treatment strategies, adoption of routine analysis in the clinical setting remains challenging. We are hopeful that discoveries of novel biological impact will lead to a better understanding of human diseases and bring the promises of miRNA as a robust biomarker to fruition. We wish to thank all of the authors of this special issue for the time they took to write these manuscripts and for the never-ending efforts of their research in this important field.