Gene 'silencing' with small interfering RNA (siRNA) is attracting considerable interest at present as a strategy for target validation that also has potential therapeutic applications. Originally discovered as part of a natural mechanism for gene regulation in nematodes, fruitflies and plants, interference of gene expression by siRNA — in which siRNA guides sequence-specific messenger RNA degradation and thus prevents it being translated into protein — has recently been shown to be applicable using designed siRNA in mammalian cells. To achieve in vivo gene silencing in mammalian tissues would require expression from intracellular transcription, a challenge that has been tackled by Davidson and colleagues, who now describe a virus-mediated delivery mechanism for siRNA that can be used to diminish expression of exogenous and endogenous genes markedly in vitro and in vivo.

To show proof-of-principle in vitro, human cancer cells that express the enzyme β-glucuronidase were infected with recombinant adenovirus-based vectors that contained the coding sequence for siRNA against β-glucuronidase, which led to a 60% decrease in enzyme activity. The authors then assessed the ability of virally expressed siRNA to diminish target-gene expression in adult mouse tissues in vivo. A significant reduction in the expression of green-fluorescent protein in the brain of transgenic mice engineered to express this marker protein, and in the endogenous expression of β-glucuronidase in the liver, was achieved, providing the first demonstration of the use of siRNA to diminish target-gene expression in brain and liver tissue.

One potential therapeutic application of siRNA would be to reduce the expression of toxic gene products associated with disorders such as Huntington's disease, in which the production of a form of a protein that contains an expanded sequence of glutamine residues leads to protein aggregation. The authors showed that expression of a protein containing an expanded polyglutamine sequence in neural cells could be reduced by 90% using virally expressed siRNA, and this reduction in expression resulted in a significant reduction in the formation of polyglutamine aggregates. Using viral vectors based on adeno-associated viruses and lentiviruses, which have the ability to infect cells in the central nervous system more efficiently than adenovirus vectors, should further extend the application of siRNA to viral-based therapies and target validation.