Science Daily 12/15/03: SARS Coronavirus Part Bird, Part Mammal: Study

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Science Daily December 15, 2003

 

SARS Coronavirus Part Bird, Part Mammal: Study

Source: University Of Toronto

 

Severe Acute Respiratory Syndrome (SARS) is formed by a combination of

mammalian and avian viruses, says a new study from the University of

Toronto.

 

The study, published in the January issue of the Journal of Virology,

sheds light on the SARS coronavirus, a deadly form of pneumonia caused

by the same viral family as the common cold. By tracing its

evolutionary history, researchers theorize that SARS is likely the

result of a rare recombination of viruses from both mammalian and

avian hosts. This, they believe, forms an entirely new virus

unrecognizable to human immune systems.

 

Understanding the evolution of SARS is a crucial step towards managing

future viral outbreaks, according to the study's co-author David

Guttman, a professor of evolutionary genomics in the Department of

Botany. Identifying the specific evolutionary changes that enables

this virus to spread into the human population should dramatically

improve our understanding of why this particular virus is so virulent.

" This will allow us to design more effective treatments and respond

more effectively to future outbreaks, " says Guttman.

 

In their study, Guttman and PhD student John Stavrinides deconstructed

and compared the SARS virus genome to related coronaviruses using

phylogenetic computational tools. They found that the protein encoded

on the genome's left side was of mammalian origin (such as cats, cows

and mice); while the proteins on the right were of avian origin (such

as chickens and ducks). The middle gene - the S gene - encodes a

protein that is a mix of mammalian and avian-like viruses.

 

In all coronaviruses, the S gene encodes a protein called spike

glycoprotein, which protrudes from the head of the virus. With most

coronaviruses, the immune system would recognize this protein as a

foreign molecule. However, the merging of mammalian and avian viruses

very likely altered the structure of this protein and allowed it to

sneak past immune surveillance.

 

Guttman says this type of genetic change can have far more dramatic

consequences than simple genetic mutations, in which only small

features in genes are changed at any one time. " These recombination

events have the potential to create an entirely new structure

essentially instantaneously, " he says. " Since our immune systems have

never seen this new viral form, it is more difficult for them to

respond to it in a timely and effective manner. " Similar genetic

exchange events are believed responsible for some of the most

devastating viral epidemics and pandemics, such as the 1918 Spanish

Influenza pandemic that killed over 20 million people worldwide.

 

In 2002, SARS spread to over 30 countries within six months and killed

over 700 people. It is believed the virus was transmitted to humans by

masked palm civets (an animal related to ferrets and cats) in the food

markets of southern China. What is not known, however, is exactly what

event led to the evolution of this new virus.

 

" It's possible that a civet picked up the virus from a bird, " says

Guttman. " This could have created the opportunity for a very rare

recombination event that produced a virus with a new host range.

Basically, the recombinant virus is infectious to humans, while the

two parent viruses are not. This new virus likely then spread to

humans due to poor hygiene and close quarters in the food markets of

Southern China. "

 

Although there have been promising developments in SARS vaccine

research, a truly effective vaccine is probably years away. " We hope

that this work will contribute to the design of specific and effective

vaccines, " says Guttman, " but perhaps it will be most useful in the

development of tests for the diagnosis of new SARS outbreaks. We will

be in a much better position to recognize new and potentially deadly

viral outbreaks if we can identify the specific evolutionary changes

that made SARS so deadly. " Guttman states that this is just a first

step in this process, and that real progress will require a more

thorough understanding of viruses existing in animal populations, and

how these viruses are transmitted to humans.

 

The study received funding from the Natural Sciences and Engineering

Research Council of Canada and the Canada Foundation for Innovation.

 

This story has been adapted from a news release issued by University

Of Toronto.

 

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