The potential of modulating small RNA activity in vivo

Small RNAs have shown to be ubiquitous, useful, post-transcriptional gene silencers in a diverse array of living organisms. As a result of homologous sequence interactions, these small RNAs repress gene expression. Through a process called RNA interference (RNAi), double strand RNA molecules are processed by an enzyme called Dicer, which cleaves RNA duplexes into 21-23 base pair oligomers. Depending on their end-point functions, these oligomers are named differently, the two most common being small interfering RNAs (siRNAs) and microRNAs (miRNAs). These small RNAs are the effector molecules for inducing RNAi, leading to post-transcriptional gene silencing by guiding the RNAi-induced silencing complex (RISC) to the target mRNA. By exploiting these small RNAs, it is possible to regulate the expression of genes related to human disease. The knockdown of such target genes can be achieved by transfecting cells with synthetically engineered small RNAs or small RNA expressing vectors. Within recent years, studies have also shown the important role of miRNAs in different diseases. By using several chemically engineered anti-miRNA oligonucleotides, disease related miRNAs can be specifically and effectively silenced. Since RNAi has developed into an everyday method for in vitro knockdown of any target gene of interest, the next step is to further explore its potential in vivo and the unique opportunities it holds for the development of novel therapeutic strategies. This review explores the various applications of small RNA technology in in vivo studies, and its potential for silencing genes associated with various human diseases. We describe the latest development in small RNA technology for both gene knockdown, and the inhibition of translational silencing in animal studies. A variety of small RNA formulations and modifications will be reviewed for their improvement on stability and half-life, their safety and off-target effects, and their efficiency and specificity of gene silencing.