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S227N Change in H5N1 Receptor Binding Domains in Turkey
June 23, 2006
Recently the sequences of all 8 gene segments of four human isolates from Turkey were released by WHO Influenza Centre National Institute for Medical Research, National Institute for Medical Research, The Ridgeway Mill Hill, London. Two of the isolates, A/Turkey/12/2006(H5N1) and A/Turkey/15/2006(H5N1) were from the index case and his sister, respectively. It seems likely that the other two isolates, A/Turkey/651242/06(H5N1) and A/Turkey/65596/06(H5N1), were from the other two fatalities, a third sibling and a cousin, respectively.
Media reports had indicated that the H5N1 from the index case had a change in the receptor binding domain of the HA gene. This change, S227N, had been shown to increase the affinity for alpha 2.6 receptors, which are found in the upper respiratory tract of humans. The acquisition was predicted to take place in the Middle East via recombination with H9N2, which was another bird flu virus that was endemic to the region. However, the WHO had issued an update on mutations indicating that the S227N was transient because the H5N1 from sister of the index case did not have this change, suggesting that the change arose in the index case via random mutation.
Analysis of the newly released sequences indicated that one of the later isolates, Turkey/65596, which probably came from the cousin, also had S227N. This polymorphism is rare in H5N1. It was first seen in isolates from family members from Hong Kong who had been infected in Fujian Province in 2003. These isolates, A/Hong Kong/212/2003(H5N1) and A/Hong Kong/213/2003(H5N1) were from a father and son and both had S227N. There were also reports of S227N in a Vietnam isolate in 2005.
However, there were no reports of S227N in Qinghai isolates, so the likelihood that S227N would arise independently in the two Turkish cases via random mutation is remote. Moreover, the failure to find S227N in the other two isolates may be related to isolation procedures. In Hong Kong, both isolation procedures used mammalian MDCK dog kidney cells. Isolation in chicken eggs may select against S227N. Moreover, the timing or location of the tissue or fluid used to isolate the H5N1 may dictate the level of S227N, which would be less likely to grow in lung. Lung was the source of the positive PCR data on the fatal cases.
The finding of S227N in a second isolate suggests the acquisition was via recombination, and the change may have been more stable that initially thought. The cluster in Turkey was the largest reported and it involved several related families. There were also outbreaks nearby in Iraq and Azerbaijan. The HA of the Turkey isolates is closely related to HA from bird, cat, and human isolates from Iraq. The nearby Azerbaijan outbreak was also large and extended. Thus, S227N may have contributed to the size of these clusters.
The release of the four sequences add important information on the receptor binding domain change, S227N. The WHO is monitoring adjacent positions, 226 and 228 for additional receptor binding changes. Acquisition of human polymorphism at these positions may greatly increase the affinity of H5N1 for upper respiratory receptors in humans, leading to more efficient transmission, which is the missing condition required for a catastrophic pandemic.