Last Updated: 2003-07-18 9:20:33 -0400 (Reuters Health)

WASHINGTON (Reuters) - Having a short version of a certain gene may make people more susceptible to depression after stressful events, such as losing a job or a loved one, researchers reported on Thursday.

People only have to inherit one copy of the gene, from either parent, to become more vulnerable to depression, the researchers in Britain, New Zealand and the United States found.

The study, published in Friday's issue of the journal Science, could be the first to confirm stress as an environmental factor in causing disease, they said.

Stress events, such as divorce or illness, are well-known triggers for depression--one of the five leading causes of disability in the world. "However, not all people who encounter a stressful life experience succumb to (depression)," the researchers wrote.

They evaluated the 5-HTT, or serotonin transporter gene, because the protein it controls is affected by well-known anti-depression drugs called selective serotonin reuptake inhibitors. Serotonin is a message-carrying chemical linked with mood. The 5-HTT gene, also known as the serotonin transporter gene, controls the recycling of the chemical messenger.

This candidate gene was chosen based on animal studies and human brain imaging studies that showed an association with this gene and serotonergic responses to stressful stimuli, Dr. Terrie E. Moffitt, of King's College London, told Reuters Health.

The fact that rhesus monkeys have the same corresponding version of the gene suggested that it was involved somehow in the perception of stress and the capacity of organisms to adapt to stress, she explained.

Dr. Moffitt's group followed 847 Caucasian New Zealanders, born in the early l970s, from birth into their 20s.

About 17 percent of the subjects had two copies of the stress-sensitive short version of 5-HTT, 31 percent had two copies of the longer version of the gene and 51 percent had one of each.

The research team also kept tabs on potentially stressful life events involving employment, money, housing, health and relationship woes among the volunteers from ages 21 to 26.

At 26 years old, 17 percent of the participants had a diagnosis of major depression in the past year and three percent had either tried or thought about committing suicide.

Of those who had more than one major stressful event, 43 percent with at least one short version of the 5-HTT gene developed depression, the researchers found. Just 17 percent who had two long copies developed depression.

The researchers also found that volunteers who were abused as children were more likely to become depressed. They reported last year that a gene for the enzyme monoamine oxidase A, also a target of antidepressants and associated with 5-HTT, affected a child's response to abuse.

Dr. Moffitt said her group is now attempting to reproduce their findings in other patient groups. They are also looking for other genes involved in mental disorders as a result of environmental causes, she added.

This discovery of a group of individuals who are protected against life stress could "open the door to treatments that prevent depression," she concluded.

Earlier this month a team at the University of Pittsburgh said they had identified 19 different genetic areas linked with depression.

The National Institute of Mental Health estimates that 16 percent of Americans--more than 30 million people--will develop major depression at some point in their lives, costing employers more than $30 billion in lost productivity.