Friday, April 13, 2012

More on Kawaoka's H5

Kawaoka presented his H5N1 study to the CEIRS network of researchers today via webinar.  Most of this has been covered in other write-ups describing his presentation at the Royal Society earlier in the month, but I will expound on some things that I feel have not been covered in what I have read.  I have a more detailed description of the experiments and results as well as some background on flu here

The experiment:
The researchers created an H5 HA that had greatly increased human cell specificity due to three mutations that were randomly added: N158D, N224K and Q226L. They put this HA with the other genes form the 2009 pandemic H1N1 and this virus was able to infect ferrets and showed very weak aerosol transmission (2 of 6 ferrets from 5 to 7 days after infection).  They took a sample of the virus from the animal that was infected via aerosol transmission and re-infected another ferret.  This time the virus transmitted more efficiently (4 of 6 ferrets, 3 to 7 day after infection).   When they sequenced this virus they found one additional mutation, T318I.
They also tested various combinations of the four mutations to dissect how they affect the behavior of the virus. The first three mutations are at positions that are known to affect the species specificity of a virus. The mutations at 224 and 226 affect the shape of the part of the protein that binds to cells. One set of amino acids cause the virus to bind bird cells, the other causes it to bind human cells, kind of like different keys fit in different locks. The mutation at 158 is known to eliminate a glycosylation site. Of all the mutations outlined in the study, this is the only one that has been found in nature in H5 viruses. Many if not most of H5 viruses isolated from birds since 2009 have lost this glycosylation site and every single human H5 lacks it.  According to Dr. Kawaoka’s work, this mutation also serves to stabilize the HA protein. The final mutation also serves this same function.
To me, the importance of this work is not that this is likely going to arise in nature and we should fear it.  The potential of this virus to occur naturally is very low since there would have to be reassortment followed by passage in the right host to build these or similar mutations. The real take away from this paper is how the mutations interact to create a transmissible virus. The two mutations in the receptor binding domain of HA (224, 226) play a major role in switching the host range of the virus, but it also causes the HA to become unstable. This instability prevents this virus from being very infectious and blocks transmission. The mutation at 158 further pushes the virus towards human cell preference but also increases the stability of the protein.  This allows the virus to be more infectious in ferrets and also gives it the ability to transmit through the air (albeit very poorly).  The final mutation at 318 greatly increases the stability of virus and allows the virus to be more infectious and to transmit more efficiently, though still far less infectious and efficient than the 2009 pandemic virus.

With more research on how various mutations affect the performance of a virus, we can start to build an index of mutations and functions.  When surveillance teams isolate new H5 viruses they can identify how new viruses differ from older viruses and can quickly identify how these mutations affect the virus. If officials can identify virus population before they become airborne then vaccine and drug stockpiles can be used more efficiently.  Rather than fully mobilizing our response efforts every time the virus pops up, we can take a more “don’t fire til use see the whites of their eyes” approach and save our ammunition to utilize in the most effective manner. This will ease strains on what experts believe are inadequate stockpiles of drugs and vaccines.

No comments:

Post a Comment