# 5061
During the 18 months since it first emerged, the novel A/H1N1/2009 virus has remained remarkably stable. Yet there remain ongoing concerns that the (former) pandemic virus might morph into a more virulent strain.
While we have seen sporadic oseltamivir resistance crop up, and a small percentage of samples have demonstrated small genetic changes (see Eurosurveillance: More On H1N1 Mutations), the evidence is inconclusive as to whether (or how much) any of these mutations actually affect the virulence or transmissibility of the virus.
This morning we’ve an open access article with an impressive pedigree (including Robert Webster/Richard Webby) from mBio - the online journal of the American Society for Microbiology – that looks at the potential for the pdmH1N1 virus to mutate or reassort with another virus, and produce a more virulent progeny.
I’ve reproduced the abstract below, the full text of the study can be accessed HERE.
Fair warning: Parts of this study are fairly technical, and may be tough sledding for those not well versed in virology.
Go ahead and read the (slightly reformatted) abstract below or proceed to the full text.
I’ll return with more after the break.
Does Pandemic A/H1N1 Virus Have the Potential To Become More Pathogenic?
- Natalia A. Ilyushina, Mariette F. Ducatez, Jerold E. Rehg, Bindumadhav M. Marathe, Henju Marjuki, Nicolai V. Bovin, Robert G. Webster, and Richard J. Webby
ABSTRACT
Epidemiologic observations that have been made in the context of the current pandemic influenza virus include a stable virulence phenotype and a lack of propensity to reassort with seasonal strains.
In an attempt to determine whether either of these observations could change in the future, we coinfected differentiated human airway cells with seasonal oseltamivir-resistant A/New Jersey/15/07 and pandemic A/Tennessee/1-560/09 (H1N1) viruses in three ratios (10:90, 50:50, and 90:10) and examined the resulting progeny viruses after 10 sequential passages.
When the pandemic virus was initially present at multiplicities of infection equal to or greater than those for the seasonal virus, only pandemic virus genotypes were detected. These adapted pandemic strains did, however, contain two nonsynonymous mutations (hemagglutinin K154Q and polymerase acidic protein L295P) that conferred a more virulent phenotype, both in cell cultures and in ferrets, than their parental strains.
The polymerase acidic protein mutation increased polymerase activity at 37°C, and the hemagglutinin change affected binding of the virus to α2,6-sialyl receptors. When the seasonal A/H1N1 virus was initially present in excess, the dominant progeny virus was a reassortant containing the hemagglutinin gene from the seasonal strain and the remaining genes from the pandemic virus.
Our study demonstrates that the emergence of an A/H1N1 pandemic strain of higher virulence is possible and that, despite their lack of detection thus far in humans, viable seasonal/pandemic virus reassortants can be generated.
IMPORTANCE This report supplies a key piece of information for investigating future evolution scenarios of pandemic A/H1N1 influenza in the human population.
We report that the emergence of an A/H1N1 pandemic strain of higher virulence is possible and that, despite their lack of detection thus far in humans, viable seasonal/pandemic virus reassortants can be generated.
Influenza viruses change, evolve, or mutate over time via two well established routes; Antigenic drift and Antigenic Shift.
Antigenic drift causes small, incremental changes in the virus over time. Drift is the standard evolutionary process of influenza viruses, and often come about due to replication errors that are common with single-strand RNA viruses.
Shift occurs when one virus swap out chunks of their genetic code with gene segments from another virus. This is known as reassortment. While far less common than drift, shift can produce abrupt, dramatic, and sometimes pandemic inducing changes to the virus.
For shift to happen, a host (human, swine, bird) must be infected by two influenza viruses at the same time. While that is relatively rare, it does happen.
Without getting too detailed (you can read the report for yourself), what the researchers here have done is to co-infect normal human bronchial epithelial (NHBE) cells in vitro with varying ratios of two influenza viruses; the 2009 H1N1 strain and an older (Tamiflu resistant) seasonal H1N1.
Although not a perfect model for the human model (NHBE lack the immune response that humans would mount), this is believed to be a reasonable test platform for viral reassortment and/or mutation.
In the end, while in most cases the novel H1N1 strain predominated, two nonsynonymous mutations (labeled G1 and G2) were identified and analyzed.
Ferret testing showed that both mutated strains produced higher viral titers, and longer virus shedding, than their parental counterparts.
Ferrets infected with the G1 strain demonstrated substantially more necrotizing bronchiolitis and alveolitis than test animals infected with G2, or either of the parental strains.
Leading the authors to state:
Taken together, our findings showed that two variants selected by coinfection of human cells acquired increased replicative fitness and virulence both in vitro and in vivo
From the discussion portion of the study, they authors write:
Our findings suggest that generation of viable intrasubtype reassortment between currently circulating oseltamivir-resistant seasonal and pandemic viruses is possible but requires initial dominance of the seasonal A/H1N1 strain.
Although we did not observe the emergence of a drug-resistant reassortant, the G2 variant was more fit than its parental strains for replication in ferrets. The lack of detection of such reassortants before now may be explained by a too-low ratio of seasonal to pandemic A/H1N1 strains.
With very little known co-circulating seasonal H1N1 at this time, opportunities for a reassortment of the novel H1N1 virus are admittedly slim. The authors state that the odds currently favor evolution via a less radical antigenic drift rather than an abrupt shift.
But that could change over time. There are other potential contributor viruses beyond seasonal H1N1, such as H5N1.
And changes via antigenic drift could enhance virulence as well.
So the authors suggest that future surveillance of the novel H1N1 virus watch for the adaptive changes identified in this study, as they may serve as a possible early warning sign of increased virulence in the virus.
