Gregory Hannon has established RNA interference in mammalian cells. Credit: MARLENA EMMONS

RNA interference (RNAi) is a selective method of silencing protein expression at the post-transcriptional level. Double-stranded RNA specific to the gene to be silenced is introduced into the cell, where it is processed into short single-stranded RNA fragments. Antisense strands, complementary to the fragments, bind to the target RNA and prompt enzymes to disable it. The effect is to destroy all the target messenger RNA, effectively halting production of the protein.

Genetica, a start-up company in Cambridge, Massachusetts, co-founded by Gregory Hannon at Cold Spring Harbor Laboratory, New York, is developing RNAi technology for high-throughput target validation in mammalian cells. Hannon and his colleagues in New York have done much of the recent work in the stable RNAi suppression of gene expression in mammalian cells.

Scientists at Bristol-Myers Squibb, based in Princeton, New Jersey, are trying to create RNAi reagents for the entire proteome, to allow analysis of all expressed genes. But there are still problems with the method, says Pam Carroll, senior research investigator in applied genomics at the company. “Most researchers use synthetic RNA double-stranded oligos that are expensive and there is still variability in response. Nonetheless, it has been an amazing year for RNA interference as a mammalian target validation technology,” she says. For example, only recently has RNAi been successfully used in mammalian model systems.

Another exciting development in RNAi is the use of small interfering RNA (siRNA) compounds. These are “efficient, specific and relatively non-toxic”, says Dmitry Samarsky, manager of technology development at functional-genomics firm Sequitur in Natick, Massachusetts. But he notes that getting these compounds into cells is still a challenge. For researchers interested in a convenient approach to RNAi, companies such as OligoEngine in Seattle, Washington, and Ambion in Austin, Texas, offer siRNAi kits and expression vectors, as well as custom siRNA synthesis. Custom siRNAs are also now supplied by many RNA companies such as Dharmacon in Lafayette, Colorado; Proligo in Hamburg, Germany; and QIAGEN in Venlo, the Netherlands.

The biotech firm Benitec, in St Lucia, Australia, has circumvented the difficulties of introducing double-stranded RNA into cells by developing a DNA template that produces double-stranded hairpin-loop molecules within the cell, which trigger RNAi. Last September, Benitec announced that it had made the first transgenic mice in which targeted endogenous genes were suppressed using RNAi.