**2. Molecular mechanisms for triggering silencing through microRNAs**

In principle, miRNAs are generated in the nucleus via long primary miRNA transcript (primiRs), which are converted by the microprocessor complex into a 70-nucleotide stem-loop structure. This complex consists of Drosha (an RNase III enzyme) and a dsRNA binding protein critical region 8 (DGCR8) DNA sequence. After binding, the dsRNA component of premiRNA is cleaved. As partial cleavage occurs, the pre-miRNA is transferred from the nucleus to the cytoplasm through the exportin-5 pathway and further processed by Dicer; the dsRNAs now consists of an inactive passenger strand and an active mature strand. When the 'mature' miRNA is incorporated into the RISC complex it triggers a silencing effect on the target.-

With regard to gene silencing and its possible applications in the clinic, miRNA can target multiple sites and thus modulate the expression of many genes. Recognition of mRNA occurs when it binds to a short sequence of nucleotides rather than to the total 21 nucleotides that form an siRNA.

To initiate RNAi, an miR can be partially complementary, binding to multiple mRNAs to block their expression. It is noteworthy that the mechanism of action of an miR is distinct from that of an RNAi in that miRs inhibit the translation of the mRNA [6]. In contrast, a small siRNA is perhaps the most frequently employed RNAi mechanism to silence protein coding genes in the short term. SiRNA is a synthetic RNA duplex structure designed to target specific mRNA in order to degrade it. In the laboratory, gene knockdown is commonly achieved in many cell types using siRNA. Normally, a perfect match is required between the siRNA oligonucleotide and the target mRNA sequence. Finally, siRNA can also be used to knockdown non-protein coding genes e.g. long non-coding RNAs (lnRNA) [9].

siRNAs and miRNAs are highly potent compared to small therapeutic molecules. In addition, both can act on proteins which lack enzymatic functions as well as those which cannot be reached by conventional drug molecules. Currently, two main therapeutic strategies are based on miRNAs, namely: (1) replacement of miRNA or (2) miRNA inhibition. Inhibition of miRNA can be conceptualized as antisense therapy, since synthetic, single stranded RNAs, act as miR antagonists which inhibit the activity of endogenous miRNAs. In contrast, synthetic miRNAs can replicate, substitute, or enhance the function of endogenous miRNAs. Thus, exogenous miR therapy results in mRNA degradation or inhibition leading to gene silencing.

siRNA, for its part involves the introduction of a man-made siRNA into the target cells to trigger RNA interference (RNAi), leading to inhibition of the mRNA expression and thus to gene silencing. Both miRNAs and siRNAs have similar physicochemical characteristics, but their functions are quite separate. Although their mechanisms of action are the same- both are short RNA duplexes that target mRNAs to silence gene, miRNA employs translational repression, the degradation of the mRNA and occasionally, endonucleolytic cleavage of mRNA, while siRNA works exclusively via the endonucleolytic cleavage of target mRNA [2].
