April 13, 2009

Understanding a Common Cold Virus

Illustration of the human rhinovirus Human rhinovirus.Wellcome Images.

Rhinoviruses are a major cause of the common cold and may contribute to about half of asthma flare-ups. Researchers have now completed sequencing the genomes of all the known rhinovirus types, setting the stage for the development of medications and vaccines to combat the viruses.

The cold is the most common illness known, bringing the sneezing, scratchy throat and runny nose that we're all familiar with. People in the United States have an estimated 1 billion colds each year.

More than 200 different viruses are known to cause the symptoms of the common cold. An estimated 30-35% of all adult colds are caused by rhinoviruses. In people with asthma, particularly children, rhinovirus infections are also frequently associated with flare-ups. Scientists had previously identified 99 distinct rhinovirus types. Recently, however, a number of unknown types were detected in patients with severe flu-like illnesses.

A research team led by Dr. Stephen B. Liggett at the University of Maryland School of Medicine reasoned that strategies for combating rhinoviruses will depend on a better understanding of rhinovirus diversity and evolution. The team set out, using internal funds, to complete the genetic sequences of all known rhinovirus types. Dr. Ann Palmenberg at the University of Wisconsin, Madison, who is supported by NIH's National Institute of Allergy and Infectious Diseases (NIAID), collaborated on the analysis. The results appeared in the journal Science on April 3, 2009.

The scientists sequenced the complete genomes of 70 known human rhinoviruses and 10 others from nasal-wash samples of patients with rhinovirus upper respiratory tract infections. The final collection, including the previously published sequences, consisted of 138 full-length human rhinoviruses genomes.

The researchers compared all the sequences to determine how they are related. Based on these relationships, they discovered that there may be up to 4 different species of rhinovirus.

Rhinoviruses contain all their genetic information on a single strand of RNA (a molecule related to DNA). The researchers found that all the virus RNA strands feature a cloverleaf-like shape at one end. Nearly every virus had a unique sequence in a section of this region. Analogous regions in related viruses have been shown to affect how pathogenic the viruses are. The researchers believe this stretch of sequence might play a similar role in rhinoviruses.

The scientists also found evidence for distantly related strains swapping sections of RNA. Exactly where and how the viruses exchange genetic material in the body is uncertain, but multiple rhinoviruses are known to infect people simultaneously.

These study results provide a framework for analyzing human rhinoviruses that may strike in the future. The information can be used in studies to track the movement and evolution of new viruses. It may also prove valuable for developing antiviral medications and vaccines.

—by Harrison Wein, Ph.D.

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