For several years scientists have been using RNA interference, commonly known as RNAi, to post-transcriptionaly silence the expression of selected proteins. This technique exploits the fact that short double-stranded RNA (dsRNA) molecules guide the destruction of homologous messenger RNAs by endogenous endonucleases. Thus, the introduction of dsRNA oligonucleotides 21-23 bases in length can specifically and dramatically reduce expression of targeted genes.
In addition to its use as a research tool, RNAi also has the potential to be used therapeutically. In yesterday's Nature online, researchers in the labs of Premlata Shanka at Harvard Medical School, and Philip Sharp at MIT, together with colleagues at the University of Pennsylvania, report that RNAi can be an effective means of stopping infection by the HIV virus.
The researchers used RNAi to silence expression of the CD4 receptor, the portal through which HIV enters cells, in modified HeLa cell lines. They found that RNAi reduced viral entry into these cells by almost 75 percent. They also used RNAi to target viral genes including gag, which codes for a polyprotein that is cleaved to yield the major structural components of the viral core. Destroying gag mRNA reduced the expression of viral proteins by up to 25-fold, and dramatically inhibited viral replication. The researchers then used RNAi to silence viral protein expression in human T-cell cultures. They obtained similar results, except that about two thirds of the cells weren't infected with the dsRNA and so went on to produce viral progeny.
RNAi also successfully silenced HIV genes after the viral genome had integrated into the host genome, thus the technique can inhibit not only viral uptake through silencing of host genes, but also virus production either pre or post integration. A drawback of the first approach is that CD4 is essential for a working immune response, and the authors suggest the HIV co-receptor CCR5 as an alternative. Individuals expressing homozygous mutants of this gene not only have an apparently normal immune response, but they are also protected from infection by the deadly virus. Silencing the wild-type CCR5 may, therefore, be a promising approach to take.—Tom Fagan
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