20 March 2006
Big Problems From Small Avian Flu Mutations
by Kate Melville
As the H5N1 time-bomb continues to tick away, the World Health Organization (WHO), its resources stretched to the limit, is trying to plan containment measures and identify a possible vaccine to stem a possible pandemic. While the 100 deaths and 177 documented cases of the avian influenza virus may seem modest, the WHO says that with each new case comes the possibility of the virus mutating into a form that is transmissible from human-to-human. Once this happens, say experts, there will be few, if any, places on Earth where anyone can escape the deadly virus. But researchers are hopeful that a newly developed technology, the glycan microarray, will at least be able to quickly identify new strains of the virus that have the potential to make that critical mutation.
"With continued outbreaks of the H5N1 virus in poultry and wild birds, further human cases are likely," said Ian Wilson, a Scripps Research professor and head of the laboratory that conducted the work on the glycan microarray. "The potential for the emergence of a human-adapted H5 virus, either by re-assortment or mutation, is a clear threat to public health worldwide."
Wilson's lab has been conducting experiments to observe the kind of mutation needed for the H5N1 virus to become transmissible from human-to-human. So far the researchers have examined one strain called A/Vietnam/1203/2004 (Viet04), which was sourced from a 10-year-old Vietnamese boy who died from the infection in 2004. Researchers say that the Viet04 virus is one of the most pathogenic H5N1 viruses studied to date. When they observed the virus, researchers found that the hemagglutinin (the agent responsible for binding the virus to the cell being infected) structure from Viet04 was closely linked to the 1918 virus HA, which caused 50 million deaths worldwide.
The new findings, reported in Science, were made possible using a newly developed microarray technology comprised of hundreds of microscopic assay sites on a single small surface. Using the microarray technology, researchers concluded that only minor mutations are needed for the binding site preference of the avian virus to switch from receptors in the intestinal tract of birds to the respiratory tract of humans. The researchers wrote that these mutations are already: "known in [some human influenza] viruses to increase binding for these receptors."
Receptor specificity for the influenza virus is determined by the glycoprotein hemagglutinin (HA) on the surface of the virus. These viral HAs bind to host cell receptors containing complex glycans-carbohydrates that in turn contain terminal sialic acids. Avian viruses mostly bind to 2-3 linked sialic acids on receptors of intestinal epithelial cells, while human viruses are usually specific for the 2-6 linkage on the epithelial cells of the lungs and upper respiratory tract. These exchanges allow the virus and host membranes to fuse, so that viral genetic material can be transferred to the cell.
The lack of a jump from a 2-3 to a 2-6 receptor is the major reason why human-to-human transmission following avian-to-human infection has not occurred. But the researchers also made careful note that their study is presuming only a single route for the virus to adapt, and that there are likely many as yet "unidentified mutations" that could surface. Alternate routes may allow the virus to switch receptor specificity and make the necessary jump from human-to-human, since "once a foothold in a new host species is made, the virus HA can optimize its specificity to the new host."
"Our recombinant approach to the structural analysis of the Viet04 virus showed that when we inserted HA mutations that had already been shown to shift receptor preference in H3 HAs to the human respiratory tract, the mutations increased receptor preference of the Viet04 HA towards specific human glycans that could serve as receptors on lung epithelial cells," Wilson explained. "The effect of these mutations on the Viet04 HA increases the likelihood of binding to and infection of susceptible epithelial cells."
Although the study seems only to reinforce previous gloomy avian flu predictions, the use of the glycan microarray technology is actually a positive step toward identifying new active virus strains in the field. "This technology allows researchers to assay hundreds of varieties in a single experiment," said Jeremy M. Berg, the director of the National Institute of General Medical Sciences. "The glycan microarray offers a detailed picture of viral receptor specificity that can be used to map the evolution of new human pathogenic strains, such as the H5N1 avian influenza, and could prove invaluable in the early identification of emerging viruses that could cause new epidemics."
Source: Scripps Research Institute