Circulating microRNAs, valuable biomarkers in biological fluids

MicroRNAs (miRNAs) are severely protected sequences and single stranded structures approximately 18 to 25 nucleotides in length. The crucial role of miRNAs has been previously proved in the regulation of the gene expression in post transcriptional modification events of messenger RNA. The precise mechanism by which miRNAs modulate translational repression of mRNAs is not fully determined. However, two-third of human messenger RNAs might be miRNA targets nearly. MiRNAs involved in the regulation of gene expression and they play fundamental roles in routine cellular functions, such as cell growth and development, proliferation, differentiation and apoptosis. In addition, increasing evidence has suggested that miRNAs play an essential role in the generation, regulation and homeostasis of immune responses. Several studies in malignancies, infectious and autoimmune diseases, have shown changes in the expression level of miRNAs in cells and tissues and also their direct and indirect associations with diseases are shown. MiRNAs not only exist intracellularly, but also are found in body fluids in combination with lipoproteins or enclosed in the packaging of the bilayer membrane, which are called exosomes. Biological body fluids consist of different samples with specific features, including serum (plasma), saliva, urine, cerebrospinal fluid, tear, semen, Pleural fluid, Peritoneal Fluid, Broncho alveolar lavage (BAL), amniotic fluid, as well as milk (Colostrum). Despite the presence of abundant ribonuclease in body fluids such as serum and urine, circulating miRNAs are stable and easy to be quantitatively measured. The variation in the expression of circulating miRNAs in the biological fluids of patients has enhanced the possibility that miRNAs may serve as novel and accessible diagnostic and prognostic biomarkers. However, the biological function and secretory mechanisms, as well as the meaning of this variation in the expression of miRNAs, remain largely uncertain. Recently, study of circulating miRNAs has opened new window in research to find novel, safe and noninvasive biomarkers in medical science. The potential of circulating miRNAs as biomarkers of disease has mainly been demonstrated for various types of malignancies and autoimmune disease. Newly, however, attention has focused on the use of circulating miRNAs as biomarkers infectious diseases such as human tuberculosis viral hepatitis. The purpose of present review is to analyze and describe the expression of circulating miRNAs in biological fluids. Therefore, we represent the recent studies which are conducted in this field to identify and introduce effective biomarkers for the diagnosis and monitoring treatment in different diseases.