Reassortments may cause flu pandemics

Profound genetic shifts called reassortments in influenza viruses may be more likely to cause pandemics than point mutations, according to research recently published in PLoS Genetics.

Georgii Bazykin, the corresponding author on the paper, said the influenza virus genome consists of eight individual segments that are similar to the chromosomes in the human genome. When different strains co-infect a single cell, their genomes can exchange the segments in a process called reassortment, resulting in a novel genome with genetic material from both viruses.

"Most major flu epidemics that we know were caused by such reassortments," Bazykin said. "When you analyze the strains that have caused these outbreaks, you find that they had combinations of viral genome segments that were never seen together before. This was the case for the 1957 and the 1968 pandemics, as well as for the swine flu in 2009. The deadliest Spanish flu pandemic of 1918 had probably the same nature, although it is hard to be certain for the events so distant in time."

The strong genetic shift of the virus makes it unfamiliar to the immune system of most humans and allows it to spread efficiently. When the significantly different virus spreads, it is referred to as antigenic shift.

Another path for flu viruses to take is antigenic drift. This process occurs when a flu virus obtains smaller mutations gradually. Antigenic drift is responsible for seasonal flu outbreaks.

Researchers from the Lomonosov Moscow State University examined the relationship between antigenic shift and antigenic drift in the study. They found that the larger changes of antigenic shift resulted in multiple smaller changes through antigenic drift.

"We believe that this effect is connected to the fact that reassorted genes have to operate in a new genetic environment," Bazykin said. "Since genes are connected to each other, if gene A has changed, a new version of gene B is also likely to be preferable. As a result, every reassortment event is followed by a trail of additional point mutations."

As reassortments result in potential pandemic-causing strains, the findings of the study may be relevant to predicting future viral outbreaks.