H5N1 Receptor Binding Domain Roulette
Recombinomics Commentary 18:35
January 7, 2008
"We now know what to look for," said Sasisekharan, whose study appears in the journal Nature Biotechnology.
The above quote from the lab authoring on the paper, "Glycan topology determines human adaptation of avian H5N1 virus hemagglutinin," cites the main result from the study, which identified assay targets that better approximate receptors used by influenza to infect a cell. Previous studies had shown a preference of seasonal flu H1 and H3 serotypes for alpha 2,6 gal linkages found in the upper respiratory tract, but the earlier assays require high concentrations of virus, which led to anomalies. The new assay, which is based on both the type of linkage as well as the shape of the receptor allows for a wide range of discriminating concentrations, which will permit identification of changes that produce intermediary levels of binding.
The differences in binding between human and avian influenza provides a straight forward reason for the inefficient transmission of avian influenza by humans. H5N1 is an avian virus that has already adapted to growth in humans, but it does not transmit easily. Screenings of H5N1 isolates in the past identify two H5N1’s that had enhanced binding to human alpha 2,6 gal receptors, and reduced affinity for avian alpha 2,3 gal. These two isolates, A/Hong Kong/212/2003 and A/Hong Kong/213/2003, had come from family members who were infected in Fujian province while on vacation from Hong Kong. A daughter had died in China before they returned, and the cluster was infected by an H5N1 that had the above altered affinities and had a change in its receptor binding domain (RBD) which changes the serine at position 227 to an asparagine (S227N).
This change was expected to increase the efficiency of human to human transmission, and the change could be appended onto a different H5N1 genetic background via recombination. The presence of donor sequences in H9N2 isolates in the Middle East lead to the prediction that such a change would happen in later 2005 in the Middle east when the Qinghai strain migrated into the region. At the time of the prediction, there had been no confirmed examples of Qinghai H5N1 infecting a person.
A cluster of four siblings developed H5N1 symptoms in Turkey at the end of 2005 and were confirmed H5N1 positive at the beginning of 2006. The H5N1 isolates from the index case had S227N. Although H5N1 from the sister of the cluster did not have S227N, a second isolate from another infected patient in Turkey did. Only four human isolates were obtained from patients in Turkey, but two of the for had S227N, and almost all isolates from Turkey were from clusters.
Subsequently, additional RBD changes were found in other Qinghai outbreaks in the Middle East and in some cases two RBD changes were found. These changes were associated with clusters in Turkey, Iraq (N186S and Q186R), Azerbaijan (N186K), and Egypt. S227N was found in two patients in Egypt, while other combinations were found in other clusters, including V223I and M230I in Egypt. These two changes were in the Gharbiya cluster, which was the largest cluster in Egypt. However, the same two changes were found in chicken isolates from Gharbiya and Beni Suef, creating opportunities for additional combinations of changes through exchanges via recombination.
The new assay will allow for quantification of the changes, either individually or in combination. The presence of the individual components in the same region will increase the likelihood of the acquisition of these changes by the same isolate.
Monitoring of receptor binding changes and combinations of changes is important, because efficient transmission is the only characteristic keeping H5N1 from evolving in to catastrophic pandemic. Small changes of one or two positions in or near the receptor binding domain can have effects on transmission efficiencies, which will be measure more precisely with the new assay.
These small changes are unlikely to lower the case fatality rate, and may in fact increase the rate, as was seen with M230I. All patients in Egypt infected with H5N1 containing M230I died last season. The involvement of M230I in the recent fatalities in Egypt is not publicly known, because the sequences from the recent outbreak have not been released.
However, based on prior sequences, RBD changes are flying around the Middle East in the Qinghai strain, and the new assay should be able to more quantitatively determine enhanced binding linked to the changes, either individually or in combination.
The rules of RDB roulette are somewhat governed by the various changes that are co-circulating, and the new assay will be able to better predict the consequences or various combinations.
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