Box 1. RNA interference: a primer

RNA interference silences a target gene through the specific destruction of that gene's messenger RNA, the intermediary molecule between DNA and protein. Double-stranded RNA (dsRNA) is central to the technique: when dsRNA with identical sequences to a specific mRNA is introduced into cells, the mRNA is recognized and degraded by a multiprotein body called the RNA-induced silencing complex. Destruction of the target mRNA leads to a drop in the levels of its encoded protein, and thus to inhibition of the target gene.

In worms and flies, dsRNAs of hundreds of nucleotides can be used to target a gene. However, in mammalian cells long dsRNAs induce a potent anti-viral response, shutting down the synthesis of all proteins. So more sophisticated strategies are required, and small interfering RNAs (siRNAs) are used instead. These siRNAs are about 21 nucleotides long, and are efficiently used by the RNA-induced silencing complex but are too short to activate a full-blown anti-viral dsRNA response.

siRNAs can either be made in vitro and subsequently introduced into cells, or they can be made directly in cells through the expression of short hairpin RNAs (shRNAs). shRNAs fold back on themselves, creating a region of dsRNA and a loop. This hairpin is processed enzymatically to remove the loop and generate a mature siRNA. Expression of shRNAs can be used to induce RNAi in transgenic mice as well as in cell lines, so the technique can be applied to investigate gene function in whole animals.

A.F.