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What is RNAi? A Brief Primer

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What is RNA interference? This technology, known as RNAi for short, triggers a natural gene silencing process that occurs when a virus invades a cell.

As many viruses have double-stranded RNA, the cell has a built-in mechanism to attack this RNA: When double-stranded RNA enters the cell, an enzyme called Dicer chops it up into short 21-25 base-pair segments (called siRNA, for short interfering RNA), which then combine with another group of enzymes to form a complex called RISC, or RNA-induced silencing complex. This complex unwinds the short double-stranded siRNAs into single strands, then uses them in its hunt for RNA which is complementary to these strands (potential viral RNA).

The RISC complex can copy bait siRNAs numerous times, using RNA polymerases. When these single strands hybridize to a complementary target mRNA, one of RISC's enzymes chops this sequence up into meaningless bits.

In 1998, Andy Fire of the Carnegie Institution and Craig Mello of the University of Massachusetts discovered that they could trigger this silencing process for a select strand of target mRNA by introducing a double-stranded RNA of a complementary sequence into the cell of a model organism. They patented their invention. In organisms such as C. elegans and Drosophila, introducing RNAi completely or partially shuts down the expression of the target gene, and the resulting phenotypic changes reveal the function of that gene. But this process does not work for mammals without inducing a fatal interferon response.

In 2001, Tom Tuschl from the Max Planck Institute for Biophysical Chemistry and colleagues solved this mammalian RNAi problem when they demonstrated that introducing siRNAs of 21 base pairs, with two-nucleotide overhangs, to human cells could silence genes in the cell. (Philip Zamore of the University of Massachusetts, Philip Sharp and David Bartel of the Whitehead Institute also share credit for experiments that led to this discovery. See US Patent Application 20020086356, and WIPO application WO 0244321.)

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