title: Influenza A virus NS1 gene mutations F103L and M106I increase replication and virulence.
abstract: BACKGROUND
  To understand the evolutionary steps required for a virus to become virulent in a new host, a human influenza A virus (IAV), A/Hong Kong/1/68(H3N2) (HK-wt), was adapted to increased virulence in the mouse. Among eleven mutations selected in the NS1 gene, two mutations F103L and M106I had been previously detected in the highly virulent human H5N1 isolate, A/HK/156/97, suggesting a role for these mutations in virulence in mice and humans.
  
  
  RESULTS
  To determine the selective advantage of these mutations, reverse genetics was used to rescue viruses containing each of the NS1 mouse adapted mutations into viruses possessing the HK-wt NS1 gene on the A/PR/8/34 genetic backbone. Both F103L and M106I NS1 mutations significantly enhanced growth in vitro (mouse and canine cells) and in vivo (BALB/c mouse lungs) as well as enhanced virulence in the mouse. Only the M106I NS1 mutation enhanced growth in human cells. Furthermore, these NS1 mutations enhanced early viral protein synthesis in MDCK cells and showed an increased ability to replicate in mouse interferon β (IFN-β) pre-treated mouse cells relative to rPR8-HK-NS-wt NS1. The double mutant, rPR8-HK-NS-F103L + M106I, demonstrated growth attenuation late in infection due to increased IFN-β induction in mouse cells. We then generated a rPR8 virus possessing the A/HK/156/97 NS gene that possesses 103L + 106I, and then rescued the L103F + I106M mutant. The 103L + 106I mutations increased virulence by >10 fold in BALB/c mice. We also inserted the avian A/Ck/Beijing/1/95 NS1 gene (the source lineage of the A/HK/156/97 NS1 gene) that possesses 103L + 106I, onto the A/WSN/33 backbone and then generated the L103F + I106M mutant. None of the H5N1 and H9N2 NS containing viruses resulted in increased IFN-β induction. The rWSN-A/Ck/Beijing/1/95-NS1 gene possessing 103L and 106I demonstrated 100 fold enhanced growth and >10 fold enhanced virulence that was associated with increased tropism for lung alveolar and bronchiolar tissues relative to the corresponding L103F and I106M mutant.
  
  
  CONCLUSIONS
  The F103L and M106I NS1 mutations were adaptive genetic determinants of growth and virulence in both human and avian NS1 genes in the mouse model.
authors.affiliation: Department of Biochemistry, Microbiology and Immunology, Faculty of Medicine, University of Ottawa 451 Smyth Rd, Ottawa, Ontario K1H8M5, Canada.
authors.forename: Samar K
authors.initials: SK
authors.lastname: Dankar
authors.forename: Shuai
authors.initials: S
authors.lastname: Wang
authors.forename: Jihui
authors.initials: J
authors.lastname: Ping
authors.forename: Nicole E
authors.initials: NE
authors.lastname: Forbes
authors.forename: Liya
authors.initials: L
authors.lastname: Keleta
authors.forename: Yishan
authors.initials: Y
authors.lastname: Li
authors.forename: Earl G
authors.initials: EG
authors.lastname: Brown
mesh_terms: D000220:Adaptation, Biological; D000818:Animals; D004195:Disease Models, Animal; D005260:Female; D006801:Humans; D053122:Influenza A Virus, H3N2 Subtype / Q000235:genetics / Q000302:isolation & purification / Q000472:pathogenicity*; D007251:Influenza, Human / Q000821:virology; D051379:Mice; D008807:Mice, Inbred BALB C; D050505:Mutant Proteins / Q000235:genetics / Q000378:metabolism; D020125:Mutation, Missense; D009976:Orthomyxoviridae Infections / Q000473:pathology / Q000821:virology; D016019:Survival Analysis; D019562:Viral Load; D017361:Viral Nonstructural Proteins / Q000235:genetics / Q000378:metabolism*; D010948:Viral Plaque Assay; D014774:Virulence; D037521:Virulence Factors / Q000235:genetics / Q000378:metabolism*; D014779:Virus Replication
chemical_list: C085820:INS1 protein, influenza virus; D050505:Mutant Proteins; D017361:Viral Nonstructural Proteins; D037521:Virulence Factors