Genetic Code of Common Cold Cracked
THURSDAY Feb. 12, 2009 -- Today's medical breakthrough: a cure for the common cold.
Well, researchers are reporting what could at least be a significant step in that direction: the genetic codes of the 99 known viruses responsible for those pesky nasal infections.
"There has been brilliant work done trying to synthesize compounds against the common cold," said Dr. Stephen B. Liggett, director of the cardiopulmonary genomics program at the University of Maryland, and senior author of a report on the discoveries published in the Feb. 12 online issue of Science. "But we have not been working with a full knowledge of the genetics of rhinoviruses. Now that we have the full complement of known ones, we see there are subfamilies of rhinoviruses clustering together. The hope is that there could be a drug for each subfamily."
Liggett and the other researchers in the project used virus samples collected by nasal swabs in doctors' offices over two decades and sent to the American Type Culture Collection, a private nonprofit organization headquartered in Virginia. Some rhinovirus genomes had already been sequenced from those samples. The current study adds 80 new full genome sequences, showing their relationships.
"We made a family tree," Liggett said. "The major branches are going to be fixed. We have sequenced rhinoviruses from 10,000 individuals at different locations in the United States, and we see major branches in the trees, with lots of little twigs in the tree."
The viruses are still evolving, Liggett said. "We compared some samples taken in 2005 with some from the 1970s, and found a fair number of mutations," he said. "But the major evolutionary structure of the tree is not going to change."
Rhinoviruses can't change too much, Liggett explained. "If they mutate to become super virulent, they destroy the cells they live in. They might become less deadly and narrow down the window they operate in. We don't understand that window very well."
There is hope that a careful study of the viral genomes will reveal one central point of attack that could be exploited by drug makers. "What we would like is a single Achilles' heel for all the viruses that we have found so far, and we could attack in that direction," Liggett said.
But the viruses are found to have impressive powers of change. The study shows that some human rhinoviruses result from the exchange of genetic material from two separate strains infecting the same person. Such recombination had not been thought possible for rhinoviruses.
That recombination is one reason why a vaccine against the common cold appears to be impossible, said Ann C. Palmenberg, director of the Institute for Molecular Virology at the University of Wisconsin, and lead author of the sequencing effort. The viruses just keep changing too much.
A second reason is that the rhinoviruses do their dirty work on the mucosa, the outer lining of the inner nose, Palmenberg said. "We're not good at making vaccines that give mucosal protection," she said.
But there is hope for a single drug that would be effective against many rhinoviruses, Palmenberg and Liggett agreed. "Drugs have the probability of taking out multiple serotypes at one time," Palmenberg said.
Who would develop such a drug? "The most likely scenario is that a small biotechnology company would begin the work, and as they made progress, they would get the attention of a larger drug company," Liggett said. "Occasionally, a biotechnology can keep the ball rolling all the way through. It will be interesting to see how this unfolds."
Detailed information on the common cold and the viruses that cause it is available from the U.S. National Institute of Allergy and Infectious Diseases.
Posted: February 2009