U of T researchers have discovered that the SARS virus was formed by a combination of viruses from bird and mammalian hosts. Graduate student John Stavrinides, who made Severe Acute Respiratory Syndrome his Ph.D. project under the supervision of Dr. David Guttman, had his findings published in the January issue of the Journal of Virology.

Although Stavrinides normally works on plant pathogens, he was curious about SARS. “When the SARS coronavirus [DNA sequence was published]…I was genuinely interested in the [virus’s history]. So from pure curiosity, I had the tools, and I knew how to extract information from the DNA sequence.” An organism’s evolutionary history can be determined from its DNA sequence, which is unique to each organism. Stavrinides said, “you can trace the evolutionary history of an organism by using the DNA sequence, but not just one DNA sequence. You need lot of different DNA sequences within the same family.” By comparing the DNA of closely related organisms, you can determined how closely each one is related to the others. He downloaded all the related coronavirus sequences from publicly accessible databases. He then generated phylogenetic, or evolutionary, trees that showed the history of the coronavirus. He found that the trees for different parts of the coronavirus genome did not look the same, and therefore had different evolutionary histories. This told the scientists that SARS had more than one origin.

Somehow a combination of DNA information from two different viruses had occurred. “We believe that recombination occurred in a gene that encodes a protein called spike glycoprotein. This protein is located on the surface of the virus particle-it’s what’s exposed to the environment,” said Dr. Guttman, a professor of evolutionary genomics in the Department of Botany.

The result of this combination was a new protein that was unrecognizable to the human immune system. We cannot fight SARS because of this inability to recognize the virus.

So how did the two contributing viruses come into contact? The predominant hypothesis says that civet cats, from which a virus very similar to SARS has been isolated, acquired a different coronavirus from a bird. The two viruses then combined inside the cat host, and the new virus was transmitted to humans. It is believed that this occurred due to the close proximity in which animals are held in Chinese food markets.

What makes the virus so dangerous to humans is still uncertain. Stavrinides mentioned that other scientists have recently isolated certain genes in humans that might make us more susceptible to SARS.

Stavrinides’ term project on SARS is finished, but he continues to work on evolution. He is interested in the process in general. “Evolutionary inference you can use on pretty much any system and get some really good information out of it.”

The future of SARS is uncertain. Although new drugs and vaccines are being developed, another outbreak is still possible. Dr. Guttman said, “It’s certainly possible that the virus is still in whatever reservoir of animal hosts there is, and is just waiting for the right conditions to emerge again. Or it’s very possible that it was a flashing of hand, and now the virus has changed enough so it’s no longer infectious to humans.”

In 2003 SARS spread to more than 30 countries, infected more than 8,000 people, and killed more than 700 victims.