A GENE that may explain why some people are more likely to suffer depression has been discovered by scientists, paving the way for improved treatments for a mental illness that strikes one in five people.

Research in the United States has revealed that a variation in a single gene significantly affects the brain’s production of serotonin, a chemical messenger that plays a crucial role in depression.

Though the mutation has so far been identified only in mice, the findings have excited mental health researchers, who believe that similar genetic factors could leave people vulnerable to psychiatric disorders.

The discovery also promises to assist the development of new drugs for treating depression, anxiety, post-traumatic stress disorder and attention deficit hyperactivity disorder, in which serotonin is known to be important.

An entire class of modern antidepressant drugs — the selective serotonin reuptake inhibitors (SSRIs) that include Prozac (fluoxetine) and Seroxat (paroxetine) — work by increasing the amount of serotonin available to the brain.

Although they have transformed the treatment of depression, SSRIs can have side-effects in a minority of patients, including provoking suicidal thoughts. A more thorough understanding of the genetics of serotonin could allow the design of more precisely targeted drugs, or a genetic test that could predict a patient’s response to SSRIs.

Serotonin is a neurotransmitter, a signalling chemical that allows neurons to send messages to their neighbours.

Serotonin levels have a profound effect on brain function and behaviour, influencing mood, emotion, sleep and appetite. Low levels are strongly implicated in depression and other mental health problems.

In the study, a team led by Marc Caron, Professor of Cell Biology at Duke University in North Carolina, found that a tiny difference in a gene called Tph2 has a profound effect on serotonin production in mice.

Mice with the newly-discovered variant, in which only a single unit or “letter” of DNA is changed, make between 50 and 70 per cent less serotonin than normal animals. Details of the research are published today in the journal Science.

“For the first time, we’ve identified a naturally occurring genetic difference that controls the production of serotonin in the brain,” Professor Caron said.

Marjorie Wallace, chief executive of the mental health charity SANE, welcomed the findings, which underline the contribution that genes make to mental illness.

Although conditions such as depression, bipolar disorder and schizophrenia are all known to have a genetic component, it has not yet been possible to pin down the precise genes that are involved, she said.

“This offers further evidence that depression is a genetic condition, to which your genes make you vulnerable,” Ms Wallace said. “If people accept that there is a genetic vulnerability, then it helps to erase the stigma of mental illness. People are less likely to be blamed for being sufferers.”

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