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Antidepressant drugs occasionally make depressed patients think about suicide. The problem is the only to know if someone will have this side effect is to give them the drug. Now, researchers have found two genetic markers that may predict who will have problems with the drugs before they take them.

NPR's Joanne Silberner reports.

JOANNE SILBERNER: To many people antidepressant drugs known as SSRIs work well. But in about two percent to four percent of people, the drugs lead the thoughts suicide, not suicide itself, but still nobody wants to give depressed people drugs that make them think about killing themselves.

Francis McMahon of the National Institute of Mental Health and his colleagues thought maybe genes could identify people likely to have problems. The whole human genome is a big place to look but McMahon is a pretty optimistic guy.

Dr. FRANCIS McMAHON (Unit Chief, National Institute of Mental Health): Well, we sat down with a number of smart people and we came up with a list of genes we thought were likely to be affected by antidepressants, genes that encode neurotransmitters, the chemical messengers in the brain, the neurotransmitter receptors as well as a variety of genes that had been reported in the past in the scientific literature to be involved in an antidepressant outcome.

SILBERNER: They had DNA samples from nearly 2,000 people who had taken the SSRI citalopram, also known as Celexa. They looked at 68 genes and they published their report in The American Journal of Psychiatry.

Dr. McMAHON: Well, what we found was that there were two genes that were both receptors for the brain chemical glutamate that carried versions that were more common in people who developed new suicidal thinking while receiving citalopram.

SILBERNER: A study published last June by a different group showed that a different gene could be somewhat predictive but only in men. McMahon and his colleagues got strong results for both men and women depending on the particular genetic make-up, people's incidence as suicidal thinking at the teleprompt increased anyway from two-fold to eight-fold.

In one small group with a particularly unfortunate genetic make-up, the risk increased 15-fold. Tom Insel is head of the National Institute of Mental Health, which funded the study. He said the research begins to address one of the biggest challenges in psychiatry, how to decide between drugs, talk therapy and electroshock treatments for depression?

Dr. TOM INSEL (Director, National Institute of Mental Health): The problem is we know that all of these treatments work but we don't know who's going to respond best to which treatment. So what we need is to be able to personalize or individualize the way we approach any given person with depression.

SILBERNER: Using genes will take some more work.

Dr. INSEL: We don't think anybody should go out there having their glutamate receptor sequenced because this is not ready for primetime. It may be in a matter of a couple of years even. We'll have a chance to look at how strong the prediction is in a prospective study.

SILBERNER: At the moment though, he says, it's not ready to go commercially. The National Institutes of Mental Health has applied for patent on using the genetic information to develop a blood test for sensitivity to SSRIs. Insel says that's to keep a private company for marketing a test before the research shows it's ready to go or for from applying for its own patent and blocking others from working on it.

Joanne Silberner, NPR News.

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