(Corrects spelling of Goodyer and misattribution of quote in paras 15, 16)
* Depression affects 350 million people worldwide
* Biomarker means screening could find teenagers at risk
* Early intervention, treatment can limit mental illness
By Kate Kelland
LONDON, Feb 17 (Reuters) - British brain scientists have identified the first biomarker, or biological signpost, for clinical depression and say it could help find boys in particular who are at risk of developing the debilitating mental illness.
In a study in the Proceedings of the National Academies of Science (PNAS) journal, the team found that teenage boys who have a combination of depressive symptoms and raised levels of the stress hormone cortisol are up to 14 times more likely to develop major depression than those who show neither trait.
The findings suggest teenagers could in future be screened for such signals, and those at highest risk could be helped to develop the kind of coping strategies and "brain fitness" to help them avoid becoming depressive.
"We're very bad about looking after our mental health, and yet the problems of mental health are extremely common," said Barbara Sahakian, a Cambridge University professor of clinical neuropsychology who worked on the study.
"Depression is one of the greatest global burdens of disease - it's a much bigger problem than heart disease or cancer and it's much more expensive."
Depression affects around 350 million people worldwide and at its worst can blight patients' lives for decades, affecting their relationships, work and ability to function. It can also lead to suicide, which alone leads to a million deaths a year.
"Depression is a terrible illness," said Ian Goodyer, a child and adolescent psychiatrist who led the research team. "(And) we now have a very real way of identifying those teenage boys most likely to develop clinical depression."
He said armed with such knowledge, doctors and other carers could target prevention strategies at depression-vulnerable boys and "hopefully help reduce their risk of serious episodes of depression and their consequences in adult life".
According to the World Health Organisation, prevention programmes - including boosting cognitive, problem-solving and social skills in children - have been shown to reduce depression, and earlier intervention is more effective.
Different factors are thought to influence the development of depression, including genetics, brain chemistry, lifestyle and upbringing. Key triggers for the condition can include stressful life events, medical illness and alcohol abuse.
For their study, Goodyer's team measured levels of cortisol in saliva from two large separate groups of teenagers. The first group of 660 provided samples on four school mornings within a week and then again 12 months later. A second group of 1,198 teenagers gave samples over three school mornings.
Using self-reports, collected over 12 months, of any symptoms of depression - such as feeling sad or anxious - and combining them with the cortisol results, the researchers then divided the teenagers into four sub-groups ranging from those with normal levels of cortisol and low symptoms of depression in Group 1 through to those teenagers with more cortisol and high symptoms of depression in Group 4.
Tracking the teenagers for three years, the team found that those in Group 4 were on average seven times more likely than those in Group 1, and two to three times more likely than in the other two groups, to develop clinical depression.
Further analysis showed that boys in Group 4 were 14 times more likely to develop clinical depression than those in Group 1, and two to four times more likely to develop it than either of the other two groups.
Commenting on the findings, Oliver Howes at the Institute of Psychiatry and the Maudsley Hospital at King's College London noted that depression is "incredibly costly to society" and cited a 2011 European College of Neuropsychopharmacology report that said mood disorders cost Europe alone more than 110 billion euros ($150 bln) a year.
"We desperately need ways to identify people at high risk of depression early so we can potentially prevent its onset and treat it early to reduce its burden. In this context, (this) study... is a landmark in the field," he said. ($1 = 0.7307 euros) (Editing by Jon Boyle)
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