Outwhitting influenza : permissive mutations may compensate for fitness defects of the influenza drug-resistance mutation via multiple mechanisms

Neuraminidase (NA), a surface glycoprotein enzyme on influenza virus, is the target of the most commonly used influenza antiviral, oseltamivir. The H274Y mutation in NA confers resistance to oseltamivir; however, this mutation also decreases viral fitness and NA expression on the cell surface. Permissive mutations buffer the deleteriousness of H274Y by rescuing NA cell surface expression, viral growth in tissue culture, and ferret transmission. These permissive mutations enabled the fixation of H274Y in circulating H1N1 influenza viruses prior to the replacement of this clade with oseltamivir-sensitive (H274) "swine flu” pandemic H1N1 in 2009. Interestingly, several of these have fixed in the oseltamivir-sensitive pandemic H1N1 lineage since 2009, which may make it possible for oseltamivir resistance to emerge in the future on a global scale. We hypothesized that the permissive mutations may buffer the effects of H274Y by increasing the melting temperature (Tm) of NA. To test this, we measured the Tm of wild type (WT) NA, H274Y, three permissive mutations, and the H274Y + permissive double mutants. We conclude that while some permissive mutations increase the Tm of NA, others may counteract the effects of H274Y through an alternative mechanism.