MicroRNAs as biomarkers in rheumatic diseases

Abstract

MicroRNAs (miRNAs) are endogenous, noncoding, single-stranded RNAs of 19-25 nucleotides in length. They regulate gene expression and are important in a wide range of physiological and pathological processes. MiRNAs are attractive as potential biomarkers because their expression pattern is reflective of underlying pathophysiologic processes and they are specific to various disease states. Moreover, miRNAs can be detected in a variety of sources, including tissue, blood and body fluids; they are reasonably stable and appear to be resistant to differences in sample handling, which increases their appeal as practical biomarkers. The clinical utility of miRNAs as diagnostic or prognostic biomarkers has been demonstrated in various malignancies and a few nonmalignant diseases. There is accumulating evidence that miRNAs have an important role in systemic rheumatic diseases and that various diseases or different stages of the same disease are associated with distinct miRNA expression profiles. Preliminary data suggest that miRNAs are promising as candidate biomarkers of diagnosis, prognosis, disease activity and severity in autoimmune diseases. MiRNAs identified as potential biomarkers in pilot studies should be validated in larger studies designed specifically for biomarker validation.

Figure 1. Biogenesis of mammalian microRNAs

The first step entails the generation of pri-miRNAs, usually transcribed by RNA polymerase II. Pri-miRNAs are cleaved in the nucleus to hairpin pre-miRNAs (typically 70–100 nucleotides) by Drosha, which is part of two multiprotein complexes including RNA helicases, heterogeneous nuclear ribonucleoproteins and a double-stranded RNA-binding protein . Pre-miRNAs are then actively transported to the cytoplasm by the nuclear pore exportin-5 for further processing by the cytoplasmic endonuclease Dicer, which, in collaboration with RNA binding proteins, cleaves both strands of the pre-miRNA hairpin duplex generating a double stranded RNA, 19–25 nucleotides long. One strand is then loaded into a protein complex called RISC, which is a ribonucleoprotein complex consisting of an RNA helicase A and proteins including Argonaute-2 and TRBP, which facilitate the binding of miRNAs to their mRNA targets. MiRNA binding can lead either to mRNA degradation or to repression of translation. Abbreviations: pri-miRNAs, primary miRNA transcripts; RISC, RNA-induced silencing complex; TRBP, trans-activation-response RNA-binding protein.

Figure 2. MicroRNA biomarkers in rheumatic diseases

Genetic alterations in the genes encoding miRNAs, their processing machinery or their binding sites could predispose to an autoimmune response or end-stage organ damage, or both. Similarly, various environmental exposures are associated with specific miRNA expression patterns that might contribute to the pathogenesis of autoimmune diseases. MiRNAs can be detected in a wide range of readily accessible biologic sources. Different microarray platforms or sequencing can be used to analyze global miRNA expression patterns, whereas real-time PCR and in situ hybridization allows for the targeted detection of selected miRNAs. The central role miRNAs have in the regulation of physiologic and pathologic processes, combined with their relative stability and the availability of technology for their detection, makes them promising biomarker candidates for diagnosis, prognosis and disease activity in autoimmune diseases. In addition, studying the biologic impact of differences in miRNA expression between controls and various disease states will improve our insight of the pathogenesis of these conditions. Abbreviation: FFPE, formalin fixed paraffin embedded