MBE Advance Access published online on May 23, 2007
Molecular Biology and Evolution, doi:10.1093/molbev/msm103
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Research Article |
The Genesis and Spread of Reassortant Human Influenza A/H3N2 Viruses Conferring Adamantane Resistance
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* National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20818, USA
Fogarty International Center, National Institutes of Health, Bethesda, MD 20892, USA
Center for Infectious Disease Dynamics, Department of Biology, Pennsylvania State University, University Park, PA 16802, USA
¶ Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544, USA
Canterbury Health Laboratories, Christchurch, New Zealand
** Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Department of Microbiology, Aichi Prefectural Institute of Public Health, Nagoya 462-8576, Japan
Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK
Address for Correspondence: Dr. Edward C. Holmes, Center for Infectious Disease Dynamics, Department of Biology, Pennsylvania State University, University Park, PA 16802, USA. Tel: 814 863 4689 Fax: 814 865 9131 Email: ech15{at}psu.edu
Received for publication March 29, 2007. Accepted for publication May 15, 2007.
A dramatic rise in the frequency of resistance to adamantane drugs by influenza A (H3N2) viruses has occurred in recent years – from 
2% to 90% in multiple countries worldwide – and associated with a single S31N amino acid replacement in the viral matrix M2 protein. To explore the emergence and spread of these adamantane resistant viruses we performed a phylogenetic analysis of recently sampled complete A/H3N2 genome sequences. Strikingly, all adamantane resistant viruses belonged to a single lineage (the ‘N-lineage’) characterized by 17 amino acid replacements across the viral genome. Further, our analysis revealed that the genesis of the N-lineage was due to a 4+4 segment reassortment event involving two distinct lineages of influenza A/H3N2 virus. A subsequent study of hemagglutinin HA1 sequences suggested that the N-lineage was circulating widely in Asia during 2005, and then dominated the Northern hemisphere 2005-2006 season in Japan and the USA. Given the infrequent use of adamantane drugs in many countries, as well as the decades of use in the US associated with little drug resistance, we propose that the globally increasing frequency of adamantane resistance is more likely attributable to its interaction with fitness-enhancing mutations at other genomic sites rather than to direct drug selection pressure. This implies that adamantanes may not be useful for treatment and prophylaxis against influenza viruses in the long term. More generally, these findings illustrate that drug selection pressure is not the sole factor determining the evolution and maintenance of drug resistance in human pathogens.
Key Words: influenza virus • adamantane resistance • reassortment • hitch-hiking • natural selection
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