(Sat, 23 Oct 2010 12:09:00 +0000)
# 5002
This morning we’ve a joint study from Imperial College London and the University of Marburg that may shed some light on why at least some cases of pandemic H1N1 proved severe (or fatal) while the great majority remained mild.
The `Norway’ or D222G (D225G in influenza H3 Numbering) mutation first announced by Norwegian Scientists last November has sparked repeated speculation that it might be associated with increased virulence.
Although we’ve covered this territory a number of times over the past year, a brief (and hopefully simple) review is in order. If you are up to speed on receptor binding, and the history of the D222G variant, feel free to skip the next section.
The D222G mutation had actually been detected months earlier, and in several other countries, but Norway was the first country to announce a possible link between that mutation and greater virulence.
This mutation involves a single amino acid change in the HA1 gene at position 222 from aspartic acid (D) to glycine (G).
The pdmH1N1 virus carrying this mutation appeared to bind more readily to receptor cells (α2-3) found deep in the lungs, whereas unmutated seasonal flu strains bind preferentially to the (α2-6) receptor cells found in the upper airway.
A virus’s ability to bind to specific cells is controlled by its RBD or Receptor Binding Domain; an area of its genetic code that allows it to attach to, and infect, specific types of host cells.
(A Very Simplified Illustration of RBDs)
Like a key into a padlock, the RBD must `fit’ in order to open the cell to infection.
For some deeper background you may wish to read Looking For the Sweet Spot, and a follow-up blog called Receptor Binding Domains:Take Two.
The World Health Organization’s take on this mutation has been that it is worth following, and studying, but there is no evidence (as yet) that it poses a substantial public health hazard.
In January, in a blog entitled WER Review: D222G Mutation In H1N1, I quoted the latest WHO report that stated:
`Based on currently available virological, epidemiological and clinical information, the D222G substitution does not appear to pose a major public health issue.’
This view is not universally held, however. There are some who have maintained that that the WHO is underestimating the impact of this mutation.
In March of this year, researchers from the Norwegian Institute of Public Health in Oslo reported that they found the mutation in 11 of 61 severe illness cases that they analyzed, but that it was not found in any of the 205 mild cases they looked at (see CIDRAP Report On The H1N1 Mutation Debate).
The WHO WER Review reported that the overall prevalence of D222G was <1.8% (52 detections among >2755 HA sequences) in contrast to a rate of 7.1% in fatal cases.
The WHO paper also reported on the occurrence of two other mutations at this amino acid position, D222E and D222N, although their significance is unclear.
While this all may sound like fairly damning evidence, it should be noted that mild cases have been detected with this D222G mutation in other studies, and many severe and fatal cases of pandemic H1N1 that have been examined did not have this mutation.
Some recent blogs on this mutation include:
Study: Receptor Binding Changes With H1N1 D222G Mutation
Eurosurveillance On Recently Isolated H1N1 Mutations
Referral: Virology Blog On D225G Mutation
Today’s study, which appears in the Journal of Virology, is called:
Altered receptor specificity and cell tropism of D222G haemagglutinin mutants from fatal cases of Pandemic A(H1N1) 2009 influenza
Yan Liu, Robert A. Childs, Tatyana Matrosovich, Stephen Wharton, Angelina S. Palma, Wengang Chai, Rodney Daniels, Victoria Gregory, Jennifer Uhlendorff, Makoto Kiso, Hans-Dieter Klenk, Alan Hay, Ten Feizi*, and Mikhail Matrosovich*
Admittedly a daunting title, but the abstract is a bit easier to follow. I’ve re-paragraphed, and added a couple of highlights to it for readability.
Abstract
Mutations in the receptor-binding site of the haemagglutinin of pandemic influenza A(H1N1) 2009 viruses have been detected sporadically. An Asp222Gly (D222G) substitution has been associated with severe or fatal disease.
Here we show that 222G variants infected a higher proportion of ciliated cells in cultures of human airway epithelium than viruses with 222D or 222E which targeted mainly non-ciliated cells.
Carbohydrate microarray analyses showed that 222G variants bind a broader range of {alpha}2-3-linked sialyl receptor sequences of a type expressed on ciliated bronchial epithelial cells and on epithelia within the lung.
These features of 222G mutants may contribute to exacerbation of disease.
The discovery that D222G enhances the binding to ciliated cells is important because cilia are motile hair-like protuberances that line the airway and help move mucus (and debris) up and out of the lungs.
SEM micrograph of the cilia projecting from respiratory epithelium in the lungs
If you infect (and impair) the lung’s cilia, you (theoretically, at least) increase the odds of that person developing pneumonia.
In this study, researchers tested 6 different variants of the pdmH1N1 virus, including 3 (Lvi, Nor, Ham-E) with the D222G mutation.
The `money quote’ from the study is:
The viruses with 222D (Mol and Ham) and 222E (Dak) showed a pattern of cell tropism typical of seasonal influenza A and B viruses infecting predominantly non-ciliated cells known to be rich in α2-6 Sia sequence: less than 5% of infected cells were ciliated.
By contrast, the three viruses with 222G, Lvi, Nor and Ham-e, infected both ciliated and non-ciliated cells, and 20% or more of infected cells were ciliated, known to express α2-3 Sia sequences.
This change in the cell tropism, with a 5-10 fold increase in infection of ciliated cells, thus correlated with the presence of the D222G substitution in the HA, and other amino acid differences, in particular D222E, had little or no effect.
Where then, does all of this leave us?
Well, the authors state that:
Whether the selection of the D222G mutation is a cause or a consequence of more severe lower respiratory tract infection has still to be resolved. It is evident, however, that its emergence is likely to exacerbate the severity of disease.
Luckily, this mutation has been slow to spread.
It has been detected in less than 2% of the samples tested, and that suggests that (right now, anyway) it may be less fit for transmission than other competing strains.
The fact that it tends to promote deep lung infections, and reduces the ability to expel mucus (and therefore cough productively), may help inhibit its spread.
A scenario not unlike what we’ve seen with the H5N1 (bird flu) virus, which as an avian virus, binds even more preferentially to α2-3 receptor cells.
What is true today, however, may not hold true tomorrow. Influenza viruses are capable of swift and sometimes dramatic mutations.
This research shows that even a seemingly mild strain of influenza can easily pick up virulence, and if it can retain transmissibility, could spark a serious public health hazard.
Which is why continued influenza research, the monitoring of this and other influenza strains, and the maintaining of pandemic readiness remain vital even after the pandemic of 2009 has passed.
