August 11 (HealthDay News) -- A gene variation may explain why some people are more prone to anxiety disorders, such as post-traumatic stress, a new study says.

People carrying two copies of the Met158 variation of the catechol-O-methyltransferase (COMT) gene showed greater anxiety on a standard personality test and also were significantly more startled than others when shown a series of unpleasant pictures, according to findings published in the August issue of Behavioral Neuroscience.

Depending on ethnicity, about a quarter of the population carries two copies of Met158. The COMT gene produces an enzyme that breaks down the brain chemical dopamine, weakening its signal.

The study's German and American authors said their finding offers a biochemical explanation for why some people find it harder to regulate emotional arousal. Their sensitivity, combined with other genetic and environmental factors, may make them more likely to have anxiety disorders.

This finding also confirms suspicions that variations in the gene that regulates dopamine signaling may be linked to negative emotionality. The Met158 variation, the authors speculated, may increase dopamine levels in the brain's memory and emotional support systems, resulting in an "inflexible attentional focus" on unpleasant stimuli. Essentially, people with the Met158 carriers can't tear themselves away from something that's arousing -- even if it's bad.

A single gene variation, though, can explain only some anxious behavior, said one of the researchers.

"This single gene variation is potentially only one of many factors influencing such a complex trait as anxiety," study co-author Christian Montag said in a news release issued by the publication. "Still, to identify the first candidates for genes associated with an anxiety-prone personality is a step in the right direction."

Montag said that if further research continues to back this finding, one day "it might be possible to prescribe the right dose of the right drug, relative to genetic makeup, to treat anxiety disorders."

SOURCE: American Psychological Association, news release, Aug. 10, 2008