A common mutation in a single gene could make the difference between fighting back against life’s assaults and sinking into clinical depression, according to recent research.
The discovery could one day help to provide early warning of vulnerability to mental distress.
The World Health Organisation says that at any time 120-million people experience the lethargy, constant sadness and recurrent thoughts of death that are symptoms of depression.
It is now the world’s fourth leading cause of ”disease burden”, the time people spend with a disability or sickness.
”If current trends continue, by 2020 depression will be the first cause of disease burden worldwide and in the developed world will be second only to heart disease,” said Terrie Moffitt, a psychologist at King’s College London and the University of Wisconsin-Madison in the United States.
A lifelong study of 847 people born 30 years ago in Dunedin, New Zealand, has focused on a serotonin transporter gene called 5-HTT. Serotonin is one of the mood signals in the brain. The gene comes in two forms, known as short and long. Everybody inherits one copy from each parent, so a person will have two short copies, two long copies, or one of each.
Moffitt and her co-author Avshalom Caspi report in the journal Science that they talked to the volunteers over five years about stresses, including unemployment, money worries, physical illness, abuse and broken relationships. They also watched for signs of depression.
The subjects who had inherited the short version of the 5-HTT gene were more likely to experience depression when things went wrong than those with the long version.
A third of those with at least one copy of the short gene who had also faced a number of stressful events became depressed. Among those with two copies of the short variant and who also had multiple troubles in life, 43% became depressed. Of those who faced similar challenges with two copies of the protective version of the gene, only 17% became depressed.
”We are not reporting a gene that causes disease,” Moffitt said. ”Instead, we believe the gene helps influence whether people are resistant to the negative psychological effects of the unavoidable stresses of life.”
The 5-HTT gene is unlikely to be the only gene involved in depression. Another, yet unidentified, could explain why women seem more vulnerable than men.
The research is a reminder that biology is not destiny: some people with the weak form of the gene survived their troubles without being depressed and some people with the strong form succumbed. But it reinforces the idea that human outcomes depend on a cocktail of genetics, environment and personal history, the researchers say.
Working with their volunteers in New Zealand, the two scientists last year identified another genetic variation that seemed to make some men more resilient to abuse or unhappiness during childhood, when other abused boys with a different form of the gene became violent or abusive later in life.
Like the 5-HTT gene, both variations were common — and they could only be detected against a background of stresses. In effect, nurture could be used to identify nature. — Â