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Gene swap clue to mystery malady

New Delhi, April 6: Two viruses may have swapped genes and produced the mystery virus that has caused Severe Acute Respiratory Syndrome (SARS) in more than 2,200 people across 16 countries and sparked a global health alert.

The evidence, for the moment, indicates that the SARS virus may have emerged from the exchange of genes between a bird virus and a human virus, said a leading Dutch virologist, whose study provides a clue to the possible origin of the syndrome.

Investigators at the US Center for Disease Control had last week detected evidence that implicated a coronavirus — a group of virus that looks like the sun — as the cause of SARS that has killed over 90 people so far.

The study by Peter Rottier and his colleagues at Utrecht University in the Netherlands has shown that the change of a single gene can dramatically alter the properties of the coronavirus. The study had started long before the outbreak of SARS.

The scientists substituted a single gene in a feline coronavirus that only infects cats with a gene from a mouse hepatitis virus. The resulting “mutant” feline coronavirus acquired the capability to infect mouse cells and lost the ability to grow in cat cells.

“The induced mutation gave an exclusively cat virus the ability to infect mouse cells,” Rottier said. The Dutch scientists have published their findings in the April issue of the Journal of Virology.

“This suggests that it is possible for a coronavirus in a bird or an animal to exchange genes with a human coronavirus to give rise to a new coronavirus that is more virulent to humans,” Rottier has told The Telegraph.

The World Health Organisation has said SARS has affected 2,270 people in 16 countries with more than 90 deaths. China has the largest number of cases with 1,190 patients and 46 deaths.

Gene exchange, called recombination, is one explanation for the origin of SARS. Another possibility is that a harmless human coronavirus spontaneously acquired genetic mutations that gave it the ability to cause life-threatening pneumonia.

“In this case, it looks like recombination has occurred,” Rottier said. He said preliminary sequencing of the SARS virus suggests that it has genetic similarities with avian (bird) coronaviruses.

This raises the possibility that in the recent past, a human coronavirus and a bird coronavirus swapped genes, leading to the emergence of the SARS virus. Rottier cautioned that this hypothesis would require validation through detailed genetic study on the SARS virus.

Coronaviruses are ubiquitous and besides humans, they are found in cats, dogs, pigs, mice, chicken and other birds. All known human coronaviruses are relatively harmless and cause only minor colds. “But we cannot predict the properties a virus may acquire after gene exchange,” said Rottier.

International disease sleuths believe that SARS originated in China’s Guangdong province where health authorities had recorded over 792 cases and 31 deaths between November 2002 and February 2003.

Rottier said although recombination is a statistical event, it is more likely to occur in areas where humans and animals or birds — in this case, perhaps, chickens — are living in close proximity.

Attempts to sequence the SARS virus are underway in international research laboratories. There is no diagnostic test yet for SARS, but CDC and WHO are developing a test that detects antibodies to the new coronavirus. Both agencies have said they will share it with international laboratories.

A senior Indian health ministry official said they have requested WHO for the SARS test kit. The National Institute of Virology and the National Institute of Communicable Diseases, the leading agencies that deal with outbreaks in India, are expected to receive these kits.

Rottier and his colleagues had begun their experiments on the genetic manipulation of coronaviruses long before the outbreak of SARS. The studies are part of an effort to develop vaccines against certain feline coronaviruses that are lethal to cats.

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