Scientists Crack SARS Genetic Sequence

Stefan Lovgren
for National Geographic News
April 15, 2003
Scientists have cracked the genetic sequence of the virus believed to cause severe acute respiratory syndrome (SARS). The sequencing reveals a virus that began its life in an animal, then mutated before picking up the power to infect people.

This past weekend, researchers from the Michael Smith Genome Sciences Centre in Vancouver, Canada, became the first to complete the DNA sequencing of the deadly virus. On Monday, the Centers for Disease Control and Prevention in Atlanta, Georgia, released its own sequencing results, confirming the Canadian findings that the mysterious disease probably originated in animals.

The epidemic has so far infected 3,169 people and killed at least 144. SARS originated in mainland China. While cases there may be declining, according to health officials, they seem to be on the rise in far-apart places like Hong Kong and Canada.

But the successful sequencing of the virus could turn the tide in the battle against the disease by helping scientists to develop testing for the virus. "This is a huge step forward in the fight to control the spread of SARS," said Caroline Astell, projects leader at the Genome Sciences Centre.

A Mutating Virus

The DNA results confirm that SARS is linked with a family of viruses known as coronaviruses, which can cause severe diseases in animals and are responsible for approximately 20 to 30 percent of common colds in humans.

"Now, the virus has become more virulent," said Michael Lai of University of Southern California in Los Angeles, a coronavirus expert. "It's causing more severe illness and is seemingly more contagious than before."

Although the SARS virus shares its structure with other coronaviruses, the proteins that make up its content are completely different, making it a brand new virus.

Scientists have offered several explanations for why SARS has turned deadly. An unknown human coronavirus could have undergone some mutations and changed its genetic makeup. Or an animal coronavirus could have mutated before jumping to humans. A third explanation suggests that an animal virus underwent "recombination" (an exchange of genetic material) with other coronaviruses, allowing it to infect humans and cause disease.

The new findings suggest that the SARS virus probably underwent natural mutations. "We don't see any recombination," said Rob Holt, head of sequencing at the Genome Sciences Centre. "This is most likely a coronavirus that circulated among animals and which mutated rapidly,"

While the DNA sequencing does not establish the source of the virus, Holt believes it's most likely to come from livestock. How it entered humans is not known.

Hard Work—and Some Luck

Bronchial tissue from a victim of SARS in Toronto yielded the virus sample that Canadian scientists sequenced. In a process known as "reverse transcription," the team made a DNA copy to be used in a biochemical reaction.

About 30 scientists at the Genome Sciences Centre set aside their regular cancer research to work on the sequencing. Aided by advanced molecular technology and some good luck with the volatile chemical reactions, they unraveled the genetic code in just a week, a remarkably fast achievement.

"It took three years to find the cause of AIDS and HIV," said Dick Thompson, spokesperson for the World Health Organization. "It took eight days to find the cause of this disease."

Scientists at the Centers for Disease Control in Atlanta, meanwhile, used a sample from a SARS victim in Hanoi, Vietnam, to complete their genetic sequencing. The two studies produced identical results.

The lab work has taken on crucial significance after WHO officials, in China to investigate the origins of the SARS outbreak, this weekend said they suspected field work may never reveal the source of the disease.

The outbreak began in November, but Chinese authorities didn't acknowledge it until several months later. Health experts have strongly criticized the Chinese government for trying to keep the epidemic a secret.

Testing Suspected Carriers

The scientists have posted the viral sequence on the Internet for researchers around the world to investigate further. They hope the genetic blueprint will lead to the development of new diagnostic tests for the disease. Such tests, which may require only a saliva sample, could determine if a potential patient has SARS and help in segregating those affected by the virus.

The sequencing may also help explain how the virus mutated into something so deadly. "The next step for the Genome Sciences Centre is to analyze the proteins that the virus produces, to find clues for why this is such a virulent pathogen," said Steven Jones of the Genome Sciences Centre.

Ultimately, the new information could lead to the discovery of a new vaccine against SARS, but this may take many years. Because the coronaviruses mutate so rapidly, it's very difficult to devise a vaccine for them. There is, for example, no vaccine for the cold. "We still have a long way to go," said Lai. "We don't have the upper hand yet, but we gained an important armament in the fight against SARS."

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