Analysis of the surface protein genes of human influenza virus (A/H1N1) isolated in northern Taiwan in 2001, 2006, and 2007 and biological characterization of viruses with large and small plaque formation

The influenza A virus is a member of the Orthomyxoviridae family. It is a single-stranded RNA virus with envelope. The genome consists of eight RNA fragments and can encode 10 viral proteins. There are 16 H (H1~H16) and 9 N (N1~N9) subtypes. Currently, the H1N1 and H3N2 subtypes are circulating among humans. According to the Center for Disease Control, Taiwan, the A/H1N1 strain reached peak prevalence in the years 2001/2002 and 2005/2006, and was the major strain isolated in the influenza seasons of these years. In order to understand the variation of genetic sequences and biological characteristics of these viruses, this study was focused on the A/H1N1 virus strains isolated in northern Taiwan in the years 2001, 2006, and 2007. The study consists of two parts. The first part includes molecular epidemiological studies, which focus on the surface glycoprotein hemagglutinin (HA) and neuraminidase (NA) genes. In second part, the biological function of A/H1N1 isolates with large or small viral plaques and the role of NA protein (N1) in the virulence of the influenza virus were examined. The structure modeling and the key positions of NA protein were discussed. The results revealed that the dominant strains isolated in 2001 was an A/New Caledonia/20/99-like strain, whereas the strains isolated in 2006 and 2007 was similar to the A/Solomon Islands/3/06 strain. Through phylogenetic analysis of the HA and NA of clinical isolates of the A/H1N1 virus, we found that these strains were clustered into two groups. The correlation between plaque size and neuraminidase activity was not existed. The inconsistent results were also found between the plaque size and the virus growth activity. However, by using plaque purification, there was one unique strain (6593) showed different biological characteristics between small plaques and large plaques. The results showed that the amino acid residue at position 443 of neuraminidase enzyme in the 6593S virus (small plaque) was asparagine ifferent from the serine residue of the large plaque derived from the same parental strain. This change resulted in a decrease in neuraminidase activity. Further, a hemagglutination elution test revealed that the 6593S virus was unable to release from hemagglutinated RBC. Through the protein structure modeling, it was suggested that the change in amino acid residue 443 of neuraminidase might alter the neuraminidase catalytic sites. The variation in neuraminidase structure may interfere the interactions among sialic acid, neuraminidase, and enzyme inhibitor. However, this finding needs to be elucidated by further studies.