siRNA Versus miRNA as Therapeutics for Gene Silencing

Abstract

Discovered a little over two decades ago, small interfering RNAs (siRNAs) and microRNAs (miRNAs) are noncoding RNAs with important roles in gene regulation. They have recently been investigated as novel classes of therapeutic agents for the treatment of a wide range of disorders including cancers and infections. Clinical trials of siRNA- and miRNA-based drugs have already been initiated. siRNAs and miRNAs share many similarities, both are short duplex RNA molecules that exert gene silencing effects at the post-transcriptional level by targeting messenger RNA (mRNA), yet their mechanisms of action and clinical applications are distinct. The major difference between siRNAs and miRNAs is that the former are highly specific with only one mRNA target, whereas the latter have multiple targets. The therapeutic approaches of siRNAs and miRNAs are therefore very different. Hence, this review provides a comparison between therapeutic siRNAs and miRNAs in terms of their mechanisms of action, physicochemical properties, delivery, and clinical applications. Moreover, the challenges in developing both classes of RNA as therapeutics are also discussed.

Figure 1

Gene silencing mechanisms of siRNA and miRNA. siRNA: dsRNA (either transcribed or artificially introduced) is processed by Dicer into siRNA which is loaded into the RISC. AGO2, which is a component of RISC, cleaves the passenger strand of siRNA. The guide strand then guides the active RISC to the target mRNA. The full complementary binding between the guide strand of siRNA and the target mRNA leads to the cleavage of mRNA. miRNA: Transcription of miRNA gene is carried out by RNA polymerase II in the nucleus to give pri-miRNA, which is then cleaved by Drosha to form pre-miRNA. The pre-miRNA is transported by Exportin 5 to the cytoplasm where it is processed by Dicer into miRNA. The miRNA is loaded into the RISC where the passenger strand is discarded, and the miRISC is guided by the remaining guide strand to the target mRNA through partially complementary binding. The target mRNA is inhibited via translational repression, degradation or cleavage.

Figure 2

Target recognition by siRNA and miRNA. (a) siRNA is usually fully complementary to the coding region of its target mRNA; (b) miRNA is partially complementary to its target miRNA. Complementary binding usually occurs at the seed region (nucleotides (nt) 2–7 of the 5' end) of miRNA and the 3' UTR of the target mRNA.

Figure 3

Structures of chemically modified RNA. (i) In the ribose 2' –OH group modification, the 2' –OH group is modified with 2' –O-methyl (2' O-Me), 2' –fluoro (2' –F) or 2' –methoxyethyl (3' –O-MOE). (ii) In locked nucleic acid (LNA) modification, the ribose is locked in a C3' endo conformation by introducing a 2'-O and 4'-C methylene bridge; In unlocked nucleic acid (UNA), the ribose ring is cleaved between 2' -C and 3' -C. (iii) In backbone modification, the phosphodiester backbone linkage is being substituted. The nonbridging phosphate atom is replaced with a sulfur atom to give a phosphorothioate modification, or replaced with a borane (BH₃) moiety to give a boranophosphate modification.

Figure 4

Therapeutic indications of siRNA and miRNA therapeutics.