The Prospect of Silencing Disease Using RNA Interference

Gene silencing by RNA interference can be initiated by introducing synthetic small interfering RNAs (siRNAs) or their precursors into cells where they are taken up by the endogenous microRNA processing machinery. The siRNA precursors are cleaved by the Dicer enzyme into siRNAs, which are 19- to 21-nucleotide double-stranded RNAs with a 2- to 3-nucleotide overhang and characteristic 5′-phosphate and 3′-hydroxyl groups at each end. As Dicer, an RNAse III–like enzyme, hands off the siRNA to the RISC (the cellular complex ultimately responsible for “slicing” messenger RNA [mRNA]), the double-stranded RNA unwinds, the RISC becomes activated, and 1 strand (the guide strand) remains within the activated complex. If the guide strand is homologous to an mRNA sequence, Argonaute 2, an enzyme within the RISC complex, cleaves the mRNA in the center of the region of homology. The cleaved mRNA is rapidly degraded and the protein for which it encodes is not produced. The guide strand small RNA is protected from degradation by the RISC and can direct the cleavage of many mRNAs. Alternately, plasmids or viral vectors encoding small stem-loop RNAs (called short hairpin RNAs [shRNAs]) can be introduced into cells where they are incorporated into the cell’s genome, resulting in the expression of shRNAs that are processed in the nucleus by the same mechanism as endogenous microRNA precursors and exported to the cytoplasm where they are processed similarly to the process described above.