Flu-free by design? (Image: Norrie Russell, courtesy of Valerie White and The Roslin Institute)

Flu is hard to handle in chickens. Simply vaccinating them doesn’t work very well: the vaccine must be precisely matched to the type of flu that is circulating, and even then vaccinated birds still transmit some virus. That virus then evolves, which may be what produced the H5N1 bird flu that has killed at least 306 people – as well as countless birds. So it would be great if we could genetically engineer chickens to resist all flu.

Flu carries its genes in eight chunks of RNA, each of which must bind to an RNA-replicating enzyme produced by the virus to reproduce itself – and the virus – during an infection. In an attempt to thwart this process, Laurence Tiley at the University of Cambridge and colleagues equipped chickens with DNA that produces a short hairpin-shaped molecule of RNA. This matches the short sequence on each of the eight chunks of flu RNA where the replicating enzyme binds. In tests in cultured chicken cells, the hairpin RNA bound to the replicating enzyme and prevented it from reproducing flu RNA.

It wasn’t so simple in whole chickens, however. When birds engineered to make the hairpin RNA were infected with H5N1, the virus replicated in them nevertheless – to lethal effect. The birds still shed the virus too. But somehow none of the birds housed with them, engineered or not, caught the flu – although they caught it readily from non-engineered birds infected with H5N1.


Obscure mechanism

“The mechanism underlying this effect is not known,” the team admits. The reason why non-infected chickens stayed flu-free wasn’t that the virus mutated to become less dangerous, so that the target birds caught it but just didn’t get sick – they didn’t even develop antibodies to H5N1, showing they didn’t catch the virus in the first place. It could be that the hairpin RNA interferes with recently discovered small RNA molecules made by the flu virus that help regulate infection.

“We consider these birds to be a first step towards producing robustly resistant chickens, and clearly it is important to understand the mechanism in as much detail as possible,” says Tiley. The team plans to pass flu through the modified chickens repeatedly to see how it works – and assess any effects on the virus’s evolution.

Whatever the mechanism, the hairpin RNA should work against all kinds of flu, unlike current vaccines. And the researchers say flu is unlikely to evolve to evade the hairpin’s effect on its replicating enzyme – that would require the enzyme and its binding sites on all eight of flu’s genome segments to mutate at once, which seems unlikely.

Ways around the block

Other ways to evade the block on transmission may well evolve, however, as maximising transmission is the main selection pressure on disease organisms. The question, says Andrew Read of Pennsylvania State University in University Park, is whether this makes the virus more or less dangerous.

“You can picture this leading to a more aggressive strain that would overcome whatever is stopping transmission,” he says. “But it could also select for less virulent flu, that wouldn’t kill the bird so fast, giving it more time to transmit to another host.”

The way to test this would be to compare how viruses with low virulence and high virulence evolve in resistant hosts, says Read. Any animal with engineered resistance should be tested for its effect on pathogen evolution, he says, as similar experiments are likely to be tried in pigs and other animals that get flu, and with other animal diseases.

Even if such tests demonstrate that the GM animals are an effective way to combat flu, people may not want to eat the flu-proof animals. “Further development will undoubtedly stimulate debate about the application of this technology in food production,” the team drily concludes.

Journal reference: Science, DOI: 10.1126/science.1198020