RNAi scoops medical Nobel
Gene silencers get something to shout about.
Two US geneticists who discovered one of the fundamental mechanisms by which gene expression is controlled have received a Nobel prize for their achievement. Andrew Fire and Craig Mello, who revealed the process of RNA interference (RNAi) in 1998, will share the US$1.4-million award.
RNAi, which occurs naturally in plants and animals, allows a gene to be specifically 'silenced'. This helps to regulate gene expression, and protects against viral infection and 'jumping genes' that can replicate and spread through the genome.
The process can also be induced experimentally by injecting tailor-made genetic sequences into cells, giving scientists a method for deliberately silencing a target gene. The method is now widely used as a basic genetic tool and is a promising candidate for future therapies.
"They have opened up a whole new area of biology, which was unsuspected before," says Nick Hastie, director of the Medical Research Council Human Genetics Unit in Edinburgh, UK. "It is also one of the quickest recognitions of a discovery; to find this in 1998 and get a Nobel Prize in 2006 is remarkable."
Fire, then working at the Carnegie Institution of Washington in Baltimore, Maryland, and Mello, who was at the University of Massachusetts Cancer Center in Worcester, made the discovery when studying the worm Caenorhabditis elegans, a much used workhorse for research geneticists.
They were investigating the process by which the information encoded in genes, made of DNA, forms a template for the manufacture of proteins the 'central dogma' of molecular biology. The first step of this process is the transcription of the DNA code, housed in the cell nucleus, into a related molecule, messenger RNA (mRNA), which exits the nucleus.
Fire, Mello and their team wanted to see whether they could influence the production of muscle protein in the worms by tinkering with the mRNA transcribed from the relevant gene. When they injected more of the naturally produced mRNA, it had no effect. Likewise, when they injected a tailor-made 'antisense' sequence to bind to the natural 'sense' sequence, nothing happened to the worms.
But when they injected double-stranded RNA made up of both sense and antisense sequences bound together, the worms displayed twitching behaviour similar to that of genetic mutants with no muscle gene at all. They had silenced the gene.
Subsequent investigation showed that injecting specific double-stranded RNA can silence any gene, and that you only need to inject a few molecules to do it. When Fire and Mello published their findings in Nature1 in 1998, a new world was opened to geneticists.
Fire and Mello also unravelled the mechanism by which the gene interference works, and the reason why simply injecting antisense RNA doesn't silence genes.
Double-stranded RNA is recognized by a protein called Dicer, which breaks it up into tiny double-stranded fragments. These fragments are then bound by a protein complex, RISC, which strips away one of the strands, leaving a complex bearing a tiny strip of RNA. (Although this process results in single strands, starting with a single strand of RNA does not have the same effect as it activates neither Dicer nor RISC).
The resulting complex binds to naturally produced mRNA, cutting it into strips and destroying it, silencing its parent gene.
On the defence
The mechanism serves as a natural defence against viruses, which attempt to co-opt a host's protein-production mechanism by inserting their own genes into the host DNA. The process also protects a cell from rampant expression of host gene fragments that replicate and insinuate themselves all over the genome.
New opportunities for using the technique are still emerging. Besides fighting viral infection, the method could be adapted to combat cancer, endocrine disorders and cardiovascular disease.
In animal trials, RNAi was recently successful in silencing a gene that causes high cholesterol levels. But other work has sounded an alarm about the potential dangers of RNAi one recent trial, for example, showed that it could prove fatal to mice (see ' RNA treatment kills mice') . Researchers are approaching clinical trials with caution.
Some have referred to the discovery as ushering in an RNA revolution, because it overrides the previous assumption that DNA is in control. In a 2004 review, Mello replied that "it is perhaps more apt to call it an RNA 'revelation'. RNA is not taking over the cell it has been in control all along. We just didn't realize it until now."
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- Fire A., et al. Nature, 391. 806 - 811 (1998).