MicroRNAs or miRNAs are a class of small non-coding, conserved RNAs with a stretch of 19-24 nucleotides long and able to mediate a large number of biological activities. miRNA plays an important role in the regulation of gene expression at post-transcriptional level including stem cell differentiation, embryogenesis, hematopoiesis, metabolism and secretions, immune responses or infections 1. In 1993, the first miRNA, lin-4 (primary small miRNA) was discovered in Caenorhabditis elegans, through a hereditary screening in nematodes. Later around the same time, it was identified that lin-14 is regulated by lin-4, which exhibited the regulatory capacity of small RNAs 2. MiRNAs partial compatibility with various mRNAs, bringing about the downregulation of gene expression in an assortment of modes, including translational suppression, mRNA cleavage, and deadenylation. Different miRNA has different biological functions, like in animals, miRNA is also conserved in plants and plays a crucial role in the developmental process. Pri-miRNA processed to form a mature miRNA and this entire process involved the action of two RNase III enzymes Drosha and Dicer, in core nucleus and cytoplasm, respectively. The miRNA genes are transcribed by RNA polymerase II and generate a long pri-miRNA (primary miRNA) within the nucleus. The hairpin loop structure in pri-miRNA is recognized by microprocessor complex DGCR8 (DiGeorge syndrome critical region gene -8 or Pasha in flies) 2. DGCR8 along with Drosha cleaves the pri-miRNA and initiate the primary process by producing pre-miRNA. The pre-miRNA has a short stem of 2–3 nucleotide with 3? overhangs, which is perceived by exportin 5 (EXP5) that intervenes transport to the cytoplasm. In the cytoplasm, pre-miRNA is processed by Dicer to form mature miRNA duplex. Among these two strands of mature miRNA one strand is integrated into the RISC complex (RNA Inducing Silencing Complex) where it can regulate the expression of target mRNAs in the cytosol. In the multiprotein complex, RISC, one important protein is Argonaute (AGO) which is mainly responsible for the differentiation and developmental processes 3. Another strand of mature miRNA may get degraded, or conceivably transported out of the cell 1.
Mutation or alteration in miRNA may results in the abnormal miRNA processing due to which gene expression get affected and leads diseased condition. Circulating miRNAs (cell-free) in blood components i.e. plasma or serum, can be used as a key source or indicative biomarker to distinguish the differential gene expression levels of miRNA and this phenomenal expression level of circulating miRNAs have been seen in various diseases like cancer, infectious diseases, Cardiovascular diseases, Neurodegenerative disorders and many more. For this purpose, healthy individuals can be used as a control against diseased one. Due to the stable, quantifiable and tissue-specific expression of mature miRNA in serum, they serve as a potential biomarker for the evaluation of many diseases4. Here, in this article, we will discuss the role of circulating miRNAs as potential biomarker in infectious diseases
Methods for profiling of Circulating miRNAs
MicroRNAs are very stable in blood and due to this property, they recognize as a potential biomarker. Comprehensive analysis of circulating miRNAs in serum and plasma samples can be characterized by qRT-PCR, microarray, and high throughput deep sequencing approaches. QRT-PCR amplification is applied to measure the low levels of circulating miRNA and it can be used for small-scale experiments. Its high specificity and sensitivity make it a novel approach. The basic principle behind this approach is the conversion of miRNA to cDNA, which further analyzed by using standard qPCR approach. The second approach, microarray, is also used to detect the cell-free miRNAs
1. Etheridge, A., et al., Extracellular microRNA: a new source of biomarkers. Mutat Res, 2011. 717(1-2): p. 85-90.
2. Wahid, F., et al., MicroRNAs: synthesis, mechanism, function, and recent clinical trials. Biochim Biophys Acta, 2010. 1803(11): p. 1231-43.
3. Cenik, E.S. and P.D. Zamore, Argonaute proteins. Current Biology, 2011. 21(12): p. R446-R449.
4. Chen, X., et al., Characterization of microRNAs in serum: a novel class of biomarkers for diagnosis of cancer and other diseases. Cell research, 2008. 18(10): p. 997-1006.