Single Gene Swap Helps Bird Flu Virus Switch Hosts

Borrowing a single gene from a human influenza strain can make a dangerous strain of bird flu easily transmissible between guinea pigs, researchers report in a paper published online today in Science. The scientists conclude that there is a substantial risk that the strain, H5N1, which so far has not infected many people, could touch off a pandemic.

The paper is another example of so-called gain-of-function studies, a controversial field in which researchers deliberately manipulate viruses in ways that can make them more dangerous. Flu scientists argue that such studies are needed to better gauge pandemic risks—in this case, from H5N1, which has caused massive outbreaks in poultry and wild birds in Asia, Africa, and Europe since 2003. More than 600 people have also become infected—and more than half of them have died. But in almost all cases, their infection was due to contact with sick birds. H5N1 does not transmit efficiently from person to person. If it develops a way to do so, scientists say that a pandemic would be virtually inevitable.

Last year, controversial papers published by influenza scientists Yoshihiro Kawaoka of the University of Wisconsin, Madison, in Nature, and Ron Fouchier of Erasmus MC in Rotterdam, the Netherlands, in Science suggested that a few mutations in H5N1 sufficed to make the virus transmissible through coughing and sneezing between ferrets, the most widely used animal model for flu. The researchers interpreted the results as a sign that the same mutations might cause the strain to go pandemic. Most of the changes were in hemagglutinin, a protein on the viral surface, and they made the protein a better fit for human receptors, their entry point into cells. Publication of both papers was delayed because a U.S. biosecurity panel recommended against publishing them in full; it later reversed its opinion.

Chen Hualan of China's Harbin Veterinary Research Institute took a different approach. Instead of focusing on mutations in the hemagglutinin gene, she wondered whether H5N1 might become pandemic by picking up entire genes from H1N1, the human influenza strain that first swept the globe in 2009. The flu genome consists of eight gene segments that strains can readily swap; this exchange can happen, for instance, when a person or an animal is infected with both strains.

For the study, Chen's team created 127 hybrids, or reassortants, in which H5N1 had anywhere between one and seven gene segments from H1N1. They then took the most pathogenic hybrids—based on tests in mice—and gauged whether these composite viruses would transmit between guinea pigs that didn't have direct contact but were housed in adjoining cages, a common setup to test whether a virus can be spread by respiratory droplets through the air. To their surprise, introducing either the H1N1 gene for the polymerase enzyme or the gene for the so-called nonstructural protein was sufficient to cause airborne transmission. Several other H1N1 genes didn't do the trick by themselves but added to the effect if they came along.

In March, Fouchier's group published the results of similar reassortment experiments, but the tests used ferrets. In that species, mash-ups between H5N1 and H1N1 did not create airborne transmission—a somewhat more reassuring finding. "The question now is how does the guinea pig model weigh up against the ferret model, and what does either model tell us about the situation in humans," says Wendy Barclay, an influenza researcher at Imperial College London, who wasn't part of either research team. "I don't think anyone knows what the best model is," Kawaoka says.

Fouchier argues that a virus will probably not become pandemic if it doesn't also undergo the changes in receptor binding that he and Kawaoka identified. Chen, too, believes those changes are probably needed, but that the other genes may provide a big help. "Efficient transmission of influenza virus results from synergism among multiple viral gene segments," she says.

Chen says that she would have liked to test the reassortant viruses in ferret studies as well, but this was impossible because she joined a group of researchers who announced a global moratorium on H5N1 transmissibility studies at the height of the controversy over the Fouchier and Kawaoka papers in January 2012. The experiments in the current paper had just been completed when the moratorium started, Chen says. The moratorium ended in January of this year. Barclay says she expects more gain-of-function studies will help clarify just what ingredients H5N1 needs to become a pandemic virus.

But Richard Ebright, a microbiologist at Rutgers University in Piscataway, New Jersey, says that expanding the line of research is risky. Supporters of the Fouchier and Kawaoka studies said that they were important because they showed for the first time that H5N1 could potentially spread between mammalian hosts, he notes. "This argument—even if one accepts it, which I do not -- does not provide a rationale for the third, fourth, fifth, and nth research projects confirming the same point," Ebright writes in an e-mail.

Simon Wain-Hobson, a virologist at the Pasteur Institute in Paris, says that the new study—although of high quality—is "very dangerous work." He adds that, "One wonders why it is published in Science."