To be or not to be? The question of virulence as posed by polio

In a previous post we discussed the identity of polio.
If you haven’t yet read it and aren’t well-versed in who or what polio is, might I recommend a brief glance? If you’re already up to date on cVDPVs, let’s proceed, deep into the tangled web of RNA that is… poliovirus.

(Previous article: )

Step One – Find the Trouble Maker

Of the three types of attenuated poliovirus contained within the oral vaccine, it appears that Type II is by far the most likely to regain virulence. With this in mind, many researchers have focused on Type II, trying to pin down precisely what is occurring, at the genetic level.
Note that poliovirus is an RNA virus rather than a DNA virus, which means it is more prone to mutate, as during replication many errors are made and not corrected, unlike with DNA.

Some background on viruses (differences in DNA vs. RNA viruses), as well as an explanation of live vaccines, can be found here:

Step Two – What Makes a Trouble-Maker Troublesome

For the rest of this blog, most of the material discussed is referenced from an article titled: The Evolutionary Pathway to Virulence of an RNA Virus.

Combining genetic sequencing and experimental evolution (E.E.) – a handy method of studying evolutionary processes under experimental conditions – authors attempted to identify whether or not the mutations found in cVDPVs were liable to reoccur. By doing this they were able to help answer an important question – was there a parallelism between independent cVDPVs?

(In short, could there be a common element in the many cases of attenuated poliovirus reverting to virulence?)

Looking first at cVDPV sequences from Belarus, China, Egypt, Madagascar, and Nigeria, a series of nine common mutations were seen across the countries. This indicated that even without any interaction, avirulent vaccine strains were undergoing similar evolutionary changes to revert to virulence.
To delve deeper, researchers set up a model where human cells (a cell culture, not a human trial) were infected with the attenuated Type II. After a set time these cells were taken and the virus extracted. Some of the viral particles were then reintroduced into new cells, and some of them were sequenced. This cycle was repeated several times, with both a 33°C and 39.5°C model.

Retrieved from: Stern et al. “The Evolutionary Pathway to Virulence of an RNA Virus“, 2016.
Fig 1. A diagram showing the path of mutation from attenuated vaccine strain to virulence.

(These two apparently arbitrary temperatures might cause a raised eyebrow, but the rational is that vaccines are ordinarily produced at 33°C , and a human body under an immune response often reaches a 39.5°C febrile state.)

Whilst little synonymous mutation was seen at 33°C (there’s as always a base rate but no particular mutation was steadily seen significantly more than others), the 39.5°C model followed a similar trajectory to that seen in the cVDPVs analysed. Of the nine noted mutations seen in cVDPVs, four were observed to occur at heightened levels in cell culture, as shown below.

Retrieved from: Stern et al. “The Evolutionary Pathway to Virulence of an RNA Virus“, 2016.
Fig 2. The four mutations seen at heightened levels at 39.5°C (as indicated by the bold, coloured line). The baseline (in grey) shows the expected frequency, were the mutation not being actively selected. Dashes indicate levels at 33°C .

Three of these four were dubbed “gateway” mutations (seen as red lines in the above image) and were found to occur much more frequently than by random chance – indicating they were being selected.
The term “gateway” was used by the authors to clarify that in order of evolution, these mutations tended to precede further mutations. It appears that they are acting as an opening passage, where once they occur a series of further mutations can then occur, leading to reversion. To clarify whether or not these gatekeeper mutations were indeed likely to be “leading the charge”, samples were taken from vaccinated individuals, 14 days after vaccination. The sequences of these samples were compared to the initial attenuated Type II sequence used in vaccine production, and it was found that not only were these gatekeeper mutations tending to precede further mutations, A481G was usually seen prior to the other two gatekeeper mutations.

Taming the Shrew – Ahem, Trouble

All of this suggests a very delicate evolutionary pattern is taking place, and lends hope that with understanding, prevention can occur.
Now that particular key sites have been identified, one option would be to proactively remove an additional portion of the attenuated Type II strain, adding in another step it must take in order to regain virulence. A good target for this would be the gatekeeper mutation A481G, as it has been shown to be a key player through cVDPV analysis, cell culture E.E. and screening of vaccinated individuals.

In expanding this work to other vaccines, it is likely that similar patterns are detectable and could be prevented, before events progress to the point where polio currently is. This might include running a short E.E. experiment to see if there’s a sudden reversion when the vaccine of interest is taken from it’s 33°C production environment into a ~39.5°C environment. Any rapid reversion to a sequence resembling the initial virulent virus would be a red flag indicating a need to alter the vaccine.

It can be concerning to hear of a vaccine causing illness, but as is the case here, there is often much more to the picture. The number of cases of polio has dropped drastically world-wide since vaccinations began and were there a more effective vaccination system in place, cVDPVs wouldn’t have had the opportunity to develop. This, more than any other outcome, may be the most important finding, and something that needs to be amended for further eradication efforts.

Thank you for taking the time to read this (and the previous) post, I hope it was informative and left you wanting to do more of your own research in the future.

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7 Responses to To be or not to be? The question of virulence as posed by polio

  1. jcturnbullnz says:

    Thanks for posting. Why does type II appear to be the most likely to regain virulence?

  2. msbdavies says:

    Hi Jo

    This is an area that has been previously documented and is confirmed by the W.H.O.’s official site (see below).

    The reasoning behind this is that Type II has a shorter mean return to virulence time, sitting at 210 days v.s. 300 and 390 for Type I and Type III respectively. One of the reasons for this is that Type II is liable to transform with Type I, regaining an important virulence element. These specific data points are from an article found here:

    From there (and this is personal speculation), it’s possible that the key mutation sites in Type II could have a higher base rate of mutation than key sites in Type I or Type III.

    I hope that helps address your question, if you need any more information please feel free to let me know.

    Briar Davies

  3. Pingback: You’ve heard about measles, you’ve probably even heard about smallpox… but what do you know about polio? | This Microbial Life

  4. aearnshaw9 says:

    Hi Briar,

    A well-written follow up from your previous post. It is interesting to see common mutations occurring, causing the virus to regain virulence. I was wondering your opinion on if it is worth vaccinating against these strains, considering how likely they are to pop up again. Would this be a preventative measure or cause more strains to develop?
    Look forward to hearing your thoughts


    • msbdavies says:

      Hi Alyssa

      In my opinion, it is best to continue with vaccinations and upscale the efforts for full coverage in remote areas, because “herd immunity” is one of the main ways of preventing the spread of the virus, so by removing this (halting vaccinations), we would likely see a resurgence in polio cases and undo all the work thus far.

      Eventually, were an official declaration of eradication made, vaccinations could then be phased out, as with smallpox.

      In terms of creating new strains of cVDPV, poliovirus strain II is now considered eradicated from the wild so has already been phased out of the oral vaccine, and both strain I and strain III are less likely to mutate into cVDPV. This, coupled with increased coverage for immunisations, are good counter-measures for cVDPV development.

      Thanks for your questions,

  5. annabehlingnz says:

    I really enjoyed this post, Briar.
    I think it is more important than ever, given the recent measles outbreaks in New Zealand (and in general, preventable-disease outbreaks worldwide) to practice responsible scientific communication regarding these subjects, as opposed to the typical click-bait frenzy-inducing articles often favoured by the media. So very well done.

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