Wednesday, 27 June 2012

Much a-flu about nothing revisited (post 1 of 2)

Last November I posted a blog regarding the biology of influenza virus and the controversial work of Ron Fouchier at the Erasmus Medical Centre in the Netherlands (here’s a link). A lot has happened since those blog posts. I’m sure many people have heard a little about the controversies surrounding the Fouchier paper and a similar paper by Yoshihiro Kawaoka, but I’ll give a brief summary for any that may have missed it. Two papers, studying the transmissibility of bird flu, were produced and submitted to the journals Science and Nature towards the end of last year. A huge debate then emerged over the fact that the studies contained potential dual-use. The argument was made that somebody may be able to use the information in the papers to weaponize bird flu. Since the virus has a reported case fatality rate of close to 60% (something that is in itself debatable) there is a large fear that the virus could be used for bio-terror attacks. As a result, the United States based National Science Advisory Board for Biosecurity (NSABB) voted in favour of blocking publication of the two papers. However, following certain revisions to the papers and clarification of some of the issues, the NSABB reversed its decision and voted in favour of publication in March this year. Both papers have now been published in their respective journals after an 8 month wait. Being that I posted about this issue back when it started, I thought it would be nice to do a follow up post looking at the two papers that have caused such a furore.

 Both of the papers are looking at the ability of avian influenza to transmit between mammalian hosts. Since it is generally frowned upon to use humans for such an experiment the researchers used ferrets which act as a good model to study how the virus could potentially spread in humans. Ferrets aren’t humans (in case there was any confusion), so even though they act as a good model for study, what holds true in ferrets does not necessarily hold true for humans. I don’t usually explicitly encourage people to read my other blog posts (I like to think that reading one may inspire you to look at more), however I am making an exception here since it may help with understanding of this post for anyone who is new to influenza. So here are the links to part 1 and part 2

Now that everyone is up to speed I’ll get into it. The first paper was published by Nature in May this year and came from the lab of Yoshihiro Kawaoka of the University of Wisconsin-Madison. As I described in my previous posts the key difference between avian influenza and a mammalian counterpart is in the receptor specificity. Avian influenza binds to receptors carrying an α2,3 linked sialic acid whereas mammalian viruses target α2,6 receptors. In order for H5N1 to transmit between humans the haemagglutinin (HA) protein must develop the ability to bind to α2,6 receptors. This was the starting point for Kawaoka’s team who added random mutations to a collection of H5N1 viruses isolated from a patient in Vietnam. The team then took all of their mutant viruses and selected those that were able to bind to α2,6 receptors. The H5 with the highest affinity to α2,6 receptors was found to have three mutations; N158D, N224K and Q226L. For the non-scientists reading, the letters refer to amino acids (the building blocks of proteins) and the numbers refer to the position of the amino acid in the HA protein. N158D means that the N (asparagine) amino acid at position 158 has mutated to a D (aspartic acid). The alterations at 224 and 226 are in the region of the protein which binds the receptor. The mutation at 158 prevents the addition of a sugar to the protein.

Having produced an H5N1 virus with specificity to α2,6 receptors the team moved on to produce a hybrid virus. The hybrid virus was made of the mutant H5 protein on an H1N1 virus backbone. In my previous blog I described the influenza genome as similar to a jigsaw with 8 pieces. Kawaoka’s team essentially took away a piece of the H1N1 jigsaw (the H1) and replaced it with a piece from the H5N1 jigsaw (the H5). This hybrid virus is therefore a full H1N1 virus, except for the replacement of H1 with H5. The H1N1 backbone is from the virus type responsible for the Swine Flu outbreak in 2009 and is still responsible for many flu cases now, meaning it is known to transmit in mammals. The only hindrance to the spread of this hybrid between mammals is therefore the HA protein taken from the mutant H5N1. The hybrid H5N1 was found to transmit through the air to two out of six ferrets. So while airborne transmission is occurring it is fairly ineffective.
Studying airborne transmission. The ferrets are separated by wire mesh so not to contact each other but can have spread of air between the two halves of the cage (and therefore hopefully spread of the virus).

The team then isolated virus from one of the infected ferrets, which brought to light an additional mutation. This mutation is T318I and was found to alter the stability of the HA protein. When the three initial mutations occur the protein becomes less stable which may explain the limited transmission. The T318I mutation compensates for the loss of stability and improves transmissibility. The virus containing all 4 mutations (N158D/N224K/Q226L/T318I) was much more effective at aerosol spread between the ferrets and at replicating within them than the triple mutant virus. However, even though the virus could spread easily between the ferrets, it was not seen to be lethal to them, indicating the virus loses some of its virulence in order to be transmissible.

The HA protein and locations of the mutations.
Image taken from the published paper (linked above)
So what does all that tell us? Kawaoka’s team have shown that four mutations to the H5 protein can allow it to effectively transmit between ferrets. With that knowledge it will be possible to survey H5N1 viruses in the wild and look for the presence of these mutations, thus allowing us to prepare for any potential pandemic. As I’ve already mentioned ferrets are not humans, so while these specific mutations have allowed spread in ferrets they may not necessarily allow spread in humans. Any surveillance should not be blinkered to these exact mutations, a fact which I will come back to when discussing the Fouchier paper. The results also hint at a loss of virulence when the virus can transmit by an aerosol route. This may occur if H5N1 naturally acquires the ability for aerosol spread in mammals, meaning the potential pandemic may not be as bad as feared (though this is mere speculation on my behalf and hard to test).

I don’t like to take up too much of people’s time with these blogs so look to keep them as short as possible. As such, I’m going to leave this post here. I’ll post the second part discussing the Fouchier paper tomorrow to round it all off. So you’ll have to come back to find out about that one.

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