It would take just a small change in DNA to create a mutation of the 2009 H1N1 “swine flu” virus that could spread more easily, concluded US scientists in a recent PLoS ONE paper where they also showed that their method could be used to monitor the evolution of H1N1, particularly that part of its DNA that determines how well the virus binds to the respiratory tract and potentially result in a more widespread pandemic with more deaths.

The fear among scientists and health experts is that the 2009 H1N1 virus will follow the same pattern that we have seen in previous pandemics: one year a mild version spreads around the world, and then it evolves into a more virulent mutation that causes a second wave of pandemic of much deadlier proportions.

This is what happened in 1917-18, where the first wave was like a typical flu epidemic, and then the second wave killed tens of millions of people worldwide.

H1N1 is a mix of human, swine and avian flu genes, which is why, when it broke out in Mexico in March 2009, there were serious concerns that it would be more widespread and cause many more deaths than seasonal flu.

But the death toll was lower than feared, mostly because the virus was not very good at spreading from person to person.

While the H1N1 virus continues to circulate and evolve, the key question is, which of the new strains will have greater ability to infect humans. And by anticipating what they might be, the greater the chance of staying ahead of the game and developing effective vaccines and other defences.

Now, senior author Dr Ram Sasisekharan, the Edward Hood Taplin Professor and director of the Harvard-MIT Division of Health Sciences and Technology in Cambridge, Massachusetts, and colleagues, hope they have given the World Health Organization (WHO) scientists who track the evolution of influenza viruses, something to look out for.

WHO labs all over the world collect samples of human and avian flu strains for scientists to analyze. One such analysis involves sequencing the DNA of the viruses to see how they are mutating. If you imagine the DNA of the virus to be like the text of a book, the WHO scientists look for changes in the way some words or phrases are spelled. But they don’t just look for any change: they know which chapters and sections to monitor more closely, because these parts of the DNA influence key proteins where a change in their behaviour could have important consequences for how the virus behaves in human hosts.

For example, one such “chapter” in the DNA book of H1N1 is that which codes for hemagglutinin (HA), a protein that helps the virus get a foothold in the human host. A slight change in this protein could make it more efficient at binding to the HA receptors on the surfaces of the cells in the respiratory tract, increasing the chance of infection.

What Sasisekharan and colleagues have established is that it takes only a simple two-letter change in that part of the H1N1’s DNA code to create such a mutation. In other words, as the first part of the title of their paper states: “A Single Base-Pair Change in 2009 H1N1 Hemagglutinin Increases Human Receptor Affinity”.

They created a virus with a single mutation in the HA region, where they replaced the amino acid isoleucine with another amino acid, lysine. That switch was enough to greatly increase the binding strength of the HA protein.

They then tested this lab-created mutation on ferrets and showed that it was indeed an H1N1 strain that could be transmitted easily via the “airborne” route. Ferrets are used in such tests because their respiratory system is similar enough to that of humans to use as a lab model.

The location on the HA protein that this finding points to is known to be prone to frequent mutation because it is near what is called an “antigenic site”, a location that interacts frequently with human antibodies. This is why flu vaccines have to follow a new formula every year.

An expert in investigating mechanisms of influenza virus infections, Dr Qinghua Wang, assistant professor of biochemistry at Baylor College of Medicine, told MIT News that this discovery is an important step because it is usually very difficult to determine which HA protein mutation of the many possible ones will have an effect on human health.

“These are exactly the types of mutations that we need to watch out for in order to safeguard human from future disastrous flu pandemics,” he said.

Sasisekharan and colleagues wrote that their study also provides a systematic approach to monitoring the evolution of currently circulating 2009 H1N1 strains, that might result in mutations better able to bind to HA receptors and spread more efficiently among humans.

“A Single Base-Pair Change in 2009 H1N1 Hemagglutinin Increases Human Receptor Affinity and Leads to Efficient Airborne Viral Transmission in Ferrets.”
Akila Jayaraman, Claudia Pappas, Rahul Raman, Jessica A Belser, Karthik Viswanathan, Zachary Shriver, Terrence M Tumpey, Ram Sasisekharan Ram Sasisekharan.
PLoS ONE, published online 02 Mar 2011

Additional source: MIT News (9 Mar 2011).

Written by: Catharine Paddock, PhD