Topping and tailing vegetables gets rid of their unpalatable ends, and now it seems that an analogous way of delivering RNAi provides a more digestible means of knocking down mammalian genes in a tissue-specific fashion.

Toshie Shinagawa and Shunsuke Ishii have come up with a new method of delivering long double-stranded RNA (dsRNA) to mammalian cells that avoids the interferon response, which is a viral defence mechanism that causes non-specific RNA degradation and cell death. Until now, the main problem with using an RNA polymerase II (Pol II) promoter in a vector for delivering RNAi was that Pol II transcripts are transferred from the nucleus to the cytosol, where they provoke the interferon response.

Shinagawa and Ishii circumvented this problem by constructing a vector — pDECAP — that expresses mRNAs that lack the 5' 7-methylguanosine (m7G) cap (the 'top') and the 3' poly(A) tail that are required for transport to the cytosol. The ingenious addition of a ribozyme cassette ensures the m7G caps are cut-off, whereas the omission of a poly(A) addition sequence effectively 'tails' the dsRNA transcripts.

So, topping and tailing the dsRNA blocks its export to the cytosol. It is only after the long dsRNA is processed into siRNA in the nucleus that it moves to the cytoplasm, where it degrades the target mRNA.

Using this strategy, the authors knocked down expression of the Ski oncogene in mice. They were able to effectively mimic the phenotype of Ski-knockout mice without the time-consuming process that the generation of such knockouts entails.

The big pay off from using this new strategy is that we are no longer limited to using promoters for RNAi vectors, such as Pol III, which are equally active in all cell types. By contrast, the Pol II promoter that transcribes all protein-coding genes in mammals can be made tissue-specific or inducible depending on interactions with various transcriptional regulators. So, this new approach allows easy and efficient generation of tissue-specific or inducible knockdown mice.

Doubtless RNAi transgenic systems that use small hairpin-type RNAs will retain their popularity in the near future, however, this new dsRNA strategy certainly represents a significant challenge to their dominance.