A team of scientists from the US and Japan have identified a combination of three genes in the flu virus that was most likely responsible for making the 1918 flu strain so deadly that it caused the most devastating outbreak of infectious disease ever known to humankind, leaving tens of millions dead in its wake.
The study was the work of researchers at the University of Wisconsin-Madison in the US and two other research centres in Japan and was published online before print on 29 December in the Proceedings of the National Academy of Sciences, PNAS. It was led by Yoshihiro Kawaoka, an internationally renowned expert on influenza and a professor of pathobiological sciences in the University of Wisconsin-Madison School of Veterinary Medicine.
Autopsies of people killed by the 1918 flu epidemic show fluid-filled lungs badly damaged by massive hemorrhaging. Although it was clear that the virus had colonized lung tissue, exactly which genes gave the virus this deadly capacity and how was still somewhat of a mystery.
Using ferrets, because flu spreads in their bodies in a similar way as it does in humans, Kawaoka and his team were able to pinpoint a cluster of three flu virus genes that gave the 1918 flu virus the ability to reproduce in the victim’s lungs instead of being confined to the upper respiratory tract. Once the flu virus takes hold in the lungs then there is a greater chance of developing primary viral pneumonia, which together with secondary bacterial pneumonia killed between 20 and 50 million people in the 1918 outbreak.
For the study the researchers used a method called reverse genetics. They extracted genes from the 1918 flu virus (recovered from preserved lung tissue in the 1990s) and substituted them one by one into a variant of a modern day human flu virus (they used H1N1 A/Kawasaki/173/2001; K173), creating many, slightly different versions of it. They tested each version on the ferrets to see how the virus spread in the body.
In the majority of cases, the modified versions of the H1N1 virus made by substituting single genes (more correctly called reassortants rather than variants) all caused infections that stayed mainly in the upper respiratory tract. But there was one exception: when they tested a reassortant comprising three susbstituted genes, it colonized lung tissue and reproduced there. The three genes (PA, PB1, and PB2) alllowed the virus to make RNA polymerase in lung cells, which helps it make the proteins from which it can make copies of itself.
The pattern of virus infection with the three-gene version of the virus was very similar to what the researchers called the “wild type” of 1918 pandemic virus. They concluded that:
“Our findings strongly implicate the viral RNA polymerase complex as a major determinant of the pathogenicity of the 1918 pandemic virus. This new insight may aid in identifying virulence factors in future pandemic viruses that could be targeted with antiviral compounds.”
Kawaoka said that the three-gene complex was a potential target for new types of antiviral drugs which could be really useful because vaccines are unlikely to be ready in sufficient numbers to stem a new pandemic.
Four years ago, in 2004, Kawaoka and his team found that another gene from the 1918 virus makes flu more virulent in mice. The gene codes for a protein called hemagglutinin which is found on the surface of the virus and helps it get into host cells.
But this latest study is about another mechanism said Kawaoka. While hemagglutinin helps the virus get inside a host cell, the three-gene cluster then helps the virus make the RNA polymerase that it needs to replicate itself.
“Viral RNA polymerase complex promotes optimal growth of 1918 virus in the lower respiratory tract of ferrets.”
Tokiko Watanabe, Shinji Watanabe, Kyoko Shinya, Jin Hyun Kim, Masato Hatta, and Yoshihiro Kawaoka.
PNAS published online before print December 29, 2008.
Sources: Journal abstract, University of Wisconsin-Madison.
Written by: Catharine Paddock, PhD