Permissive Secondary Mutations Enable the Evolution of Influenza Oseltamivir Resistance

Influenza Escape Tricks Tamiflu, or oseltamivir, has been extensively stockpiled by several governments in anticipation of a dangerous influenza pandemic. So far, its large-scale use has not been required, but, despite this, resistance has emerged in seasonal strains mediated by a single point mutation of histidine to tyrosine in the 274 residue (H274Y) of neuraminidase. When the resistant virus was first discovered in 1998, it grew poorly, but by 2008 the virus was reinvigorated and the mutation had spread worldwide in seasonal influenza. So what happened that improved viral fitness so radically? Bloom et al. (p. 1272; see the Perspective by Holmes) show that the H274Y mutation hinders the folding of the neuraminidase enzyme. In the more vigorous recent oseltamivir-resistant isolates, other mutations compensate for the deleterious effect of H274Y and restore fitness to the virus. Compensatory mutations offset the structural costs of acquiring a drug-resistance mutation in a virus. The His274→Tyr274 (H274Y) mutation confers oseltamivir resistance on N1 influenza neuraminidase but had long been thought to compromise viral fitness. However, beginning in 2007–2008, viruses containing H274Y rapidly became predominant among human seasonal H1N1 isolates. We show that H274Y decreases the amount of neuraminidase that reaches the cell surface and that this defect can be counteracted by secondary mutations that also restore viral fitness. Two such mutations occurred in seasonal H1N1 shortly before the widespread appearance of H274Y. The evolution of oseltamivir resistance was therefore enabled by “permissive” mutations that allowed the virus to tolerate subsequent occurrences of H274Y. An understanding of this process may provide a basis for predicting the evolution of oseltamivir resistance in other influenza strains.