Gene editing could help protect chickens from bird flu

Gene editing can be used to create chickens that are partially resistant to infection by avian influenza, according to a proof-of-concept study by UK researchers. Their findings, published in the journal Nature Communications, present a potential strategy to help mitigate the spread of avian influenza — commonly known as bird flu — into farmed poultry from wild bird sources.

Bird flu is a major global threat, with a devastating impact in both farmed and wild bird populations; and in rare instances, mutations in the bird flu virus allow it to infect people and cause serious illness. Efforts to control the spread of the disease are urgently needed — but poultry vaccination against avian influenza has not yet been reliable due to the rapid antigenic drift of field viruses, and is controversial owing to political and economic implications.

In chickens, avian influenza viruses hijack the host protein ANP32A to help replicate themselves. Scientists from The University of Edinburgh, Imperial College London and The Pirbright Institute have now altered the section of DNA responsible for producing the ANP32A protein, by editing the ANP32A gene in chicken germ cells (precursors of reproductive cells).

When the ANP32A gene-edited chickens were exposed to a normal dose of the H9N2-UDL strain of avian influenza virus, nine out of 10 birds remained uninfected and there was no spread to other chickens. The birds showed no adverse health or egg-laying productivity effects when monitored for over two years.

The research team then exposed the gene-edited birds to an artificially high dose of avian influenza virus — 1000 times higher, to be exact — to further test their resilience. While five out of 10 birds became infected, the amount of virus in the infected gene-edited chickens was much lower than the level typically seen during infection in non-gene-edited chickens. The gene edit also helped to limit onward spread of the virus to just one of four non-gene-edited chickens placed in the same incubator. There was no transmission to gene-edited birds.

The scientists found that in the ANP32A gene-edited birds, the virus had adapted to enlist the support of two related proteins — ANP32B and ANP32E — to replicate. Following lab tests, they found that some of the mutations enabled the virus to utilise the human version of ANP32, though its replication remained low in cell cultures from the human airway. But while experts have said that additional genetic changes would be needed for the virus to infect and spread effectively in humans, the research team concluded that the single ANP32A gene edit is not robust enough for application in the production of chickens.

To prevent the emergence of escape viruses — viruses that adapt to evade the gene edit and cause infection — the research team targeted additional sections of DNA responsible for producing all three proteins (ANP32A, ANP32B and ANP32E) inside lab-grown chicken cells. In cell cultures in the lab, growth of the virus was successfully blocked in cells with the three gene edits. The next step will be to try to develop chickens with edits to all three genes.

“Gene editing offers a promising route towards permanent disease resistance, which could be passed down through generations, protecting poultry and reducing the risks to humans and wild birds,” said principal investigator Professor Mike McGrew, from The University of Edinburgh’s Roslin Institute. “Our work shows that stopping the spread of avian influenza in chickens will need several simultaneous genetic changes.”

Professor Wendy Barclay, from Imperial College London, added, “This work is an exciting collaboration that fuses our expertise in virology with the world-leading genetic capability at the Roslin Institute. Although we haven’t yet got the perfect combination of gene edits to take this approach into the field, the results have told us a lot about how influenza virus functions inside the infected cell and how to slow its replication.”

Members of the scientific community have had a cautiously optimistic reaction to the news. Professor Raina MacIntyre, Head of the Biosecurity Program at UNSW’s Kirby Institute and an expert in influenza and emerging infectious diseases, noted that avian influenza spreads globally not just through poultry trading, but also through wild waterfowl such as ducks and geese as they migrate between countries and continents. “So engineering farmed chickens alone is not enough,” she said.

“The other main concern is influenza A viruses are highly mutable and subject to continual antigenic drift; this means the virus itself will likely evolve to overcome engineered traits in the birds,” MacIntyre added. She did say the technology could potentially be used for influenza vaccine manufacturing.

“Many vaccines are manufactured using embryonated hens’ eggs,” she said, “… [so] having engineered eggs that are resistant to highly pathogenic avian influenza can be of great benefit for vaccine manufacturing. Having said that, new vaccine technologies for influenza vaccines are becoming more common now, and reliance on eggs for vaccine manufacturing may not be as common in a few years.”

Associate Professor Dimitri Perrin, lead of the Biomedical Data Science group from Queensland University of Technology (QUT), said it is crucial for any gene edit to ensure that there are no unintended modifications or unintended consequences elsewhere in the genome. “One edit could produce the desired effect on one specific function but also a detrimental effect for another one,” he said.

While Perrin said it was encouraging that no differences were found, a single edit was not sufficient to achieve perfect results. “Targeting more genes increases the desired effect, but also the risk of other detrimental outcomes,” he said. “More research is needed to strike the right balance.”

Professor James Wood, Head of Department of Veterinary Medicine at the University of Cambridge, concluded that the study provides an important proof of principle for the genetic control of avian influenza. He said it is essential that the scientists demonstrate the same benefits, and the same health checks, in live birds as they did in the chicken cells that had three genetic mutations introduced.

“It would further be highly valuable to determine if these changes were advantageous in turkeys and ducks, other important food species — and it will be important to demonstrate protection in animals against the more virulent viruses circulating as well,” he said.

Image caption: A non-gene-edited chicken (left) pictured next to an ANP32A gene-edited chicken (right). Image credit: Norrie Russell.