In 2011, in an attempt to evaluate whether H5N1, also known as the bird flu, has the potential to become highly transmissible among humans, scientists in both the Netherlands and the United States made a new H5N1 flu by genetically altering current strains of the virus (1). Yoshihiro Kawaoka and his research team from the University of Wisconsin accomplished this by creating four mutations in hemagglutinin (HA) coding region, from an H5N1 virus and combining it with segments from the 2009 H1N1 swine flu virus (2). HA is a protein responsible for the virus binding to a cell. Researchers found that this mutant virus was capable of spreading in ferrets via droplet transmission, although it “was not highly pathogenic and did not cause mortality” (2).
Recently, researchers from the Medical Research Council’s National Institute for Medical Research in London studied how changes in the transmissible mutant HA created by Kawaoka’s group led to increased binding in ferrets and humans (3). This group found that one amino acid change from glutamine, a polar amino acid, to leucine, a nonpolar amino acid, was also present in flu types that caused the 1957 and 1968 pandemics (3). By determining the mutant virus’s crystal structure, researchers found that this mutation causes a change in orientation of the HA receptor (3). This change results in preferential binding to sialic acids, which vary from species to species and are necessary for infection. Such acids are found in human respiratory cells rather than in avian intestinal cells (3).
In addition to this mutation, researchers believe that an amino acid change that results in the loss of a large carbohydrate also plays a role in changing its binding preferences (3). This carbohydrate may have prevented receptors from binding to sialic acids, preventing it from infecting certain species (3). In addition, researchers speculate that the substitution of lysine, an amino acid with a positively charged side chain, for asparagine, an amino acid with an uncharged side chain, improved the ability of HA to bind to receptors on cell surfaces due to its charge (3).
The fourth mutation in the ferret-transmissible HA region is responsible for the fusion of virus membranes with target membranes (2). This mutation acts to increase the stability of the virus by offsetting destabilization caused by the mutations in and around the binding site (2). It achieves this by lowering the threshold pH needed for the membranes to fuse to the pH range found in the human respiratory tract (2).
Researchers concluded that this mutant H5N1 virus has a higher affinity for human receptors when compared to a wild type H5N1 strain and a much lower affinity for avian receptors compared to this wild type (3). However, when contrasted with the 1968 pandemic virus, they found that the mutant has much weaker binding with respect to both human and avian receptors (3). The ferret-transmissible strain has a 200 fold binding preference for human receptors over avian receptors, but this preference is mainly due to a huge loss in avian receptor binding capabilities rather than a large increase in human receptor binding capabilities (3).
Members of this project hope that their research will provide a foundation for further investigation into the evolution of wild type H5N1 viruses into highly transmissible strains with the potential to cause pandemics similar to those in the past.
1. Y. Kawaoka, Nature 482, 155 (2012)
2. M. Imai et al., Nature 486, 420-428 (2012)
3. X. Xiong et al., Nature 497, 392-396 (2013)