Two studies have found differential expression of miRNAs during AR of kidney allografts. One study characterized the association between intrarenal miRNAs and clinicohistological

status of renal allografts.74 A subset of 17 miRNAs, out of 365, was found to be differentially expressed in AR biopsies compared with normal allograft biopsies. The altered expression of 6 of the 17 miRNAs identified was validated with quantitative analysis. Impressively, they reported that AR can be predicted accurately using intragraft levels of miR-142-5p (100% sensitivity and 95% specificity) or miR-155 (100% sensitivity and 95% specificity). In addition, miRNA levels were evaluated in isolated PBMC and human renal tubular epithelial cells. Some of the miRNAs found to be increased in AR were also expressed in PBMC. This indicates that cellular infiltration of immunological cells may explain the changes in miRNA expression. Using a similar PD98059 in vitro approach, Sui et al. reported 20 miRNAs that were differentially expressed, of which 12 were downregulated and 8 upregulated in AR, when compared with normal allograft biopsies.75 The next challenge in this research is to determine if changes in miRNA expression are due to AR alone, or due to other factors such as renal function, viral infection status and time since transplantation. A growing number of studies have found several human viruses such

as cytomegalovirus, Epstein-Barr Y 27632 virus (EBV) and BK virus that encode viral miRNAs and their specific expression can be associated with different phases of viral infection. Furthermore, there is differential expression of EBV-encoded miRNAs in peripheral blood cells of EBV Ceramide glucosyltransferase carriers (latent infection) and

patients with acute EBV infection.76 This might provide a diagnostic test to differentiate active viral infection from carriage that is important in the management of renal transplant patients.76–79 Further research is needed to examine the role and function of these miRNAs in the pathophysiology of the infection. Recent progress in miRNA research presents opportunities for understanding kidney diseases, including identification of new diagnostic biomarkers. The potential value of miRNAs as biomarkers for human cancer research has been demonstrated and may provide more accurate tumour classification than mRNA analysis.80 MiRNA profiles offer some important potential advantages over standard mRNA or other protein-based profiles. MiRNAs appear to be very stable in tissues and biological fluids, including serum and are protected from endogenous RNase by virtue of their small size and perhaps by packaging within exosomes.81 In addition, the tissue-specific nature of miRNA expression makes them ideal candidates for biomarkers.82 The total number of human miRNAs, estimated to be between 700 and 1000, is considerably smaller than the number of protein-coding mRNAs (about 22 000).