Genetic and environmental factors can interact and increase an individual's chances of developing mental illness, as researchers demonstrated in a 2003 study published in the journal Science. In the study, researchers found that individuals with one or two copies of the S allele of the serotonin transporter gene had a greater chance of becoming depressed or suicidal following stressful events than those with two copies of the L allele. Since then, researchers have continued to investigate the connections between specific genes and stress reactions. Two recent studies, both published in the journal Biological Psychiatry, have found new information about the ways in which genetic factors can make an individual more likely to develop depression after experiencing stress.
A team of researchers, led by Dr. Ian Gotlib of Stanford University, studied the connection between the serotonin transporter gene and depression in 67 girls, none of whom had a previous history of mental illness. Researchers genotyped the girls, recording whether they possessed two S alleles, two L alleles, or an S and an L, and then exposed them to a stress test. The researchers also measured the level of cortisol, a stress hormone, in the girls' blood before, during, and after the test. While girls with one or two L alleles produced, at most, slightly higher levels of cortisol immediately following the stress test, girls with two S alleles produced significantly higher levels of cortisol. Additionally, the cortisol levels of these girls continued to rise up until 30 minutes after the stress test before declining. The researchers suggest that this association between the S allele and elevated cortisol production may play an important role in why those with the S allele display a heightened vulnerability to depression following environmental stressors.
A second study investigated the role of a second gene, serotonin 2A receptor, which has also been linked to depression. Dr. Vibe Frokjaer, of the Copenhagen University in Denmark, and colleagues looked for an association between increased binding of this receptor and the inability of their subjects to cope with stress. Each of the study's 83 participants took a standardized personality test. Then, using PET imaging, the researchers measured the amount of serotonin 2A receptor-binding in the subjects' brains. Those who generally had difficulty dealing with stress, according to the results of their personality test, also had more binding in the frontal cortex. The researchers believe these results may indicate a connection between serotonin 2A receptor and personality traits which can contribute to depression.
Both studies found connections between genes and the development of depression following a stressor, but there is no current evidence that the two genes work together to create depression. More research is necessary to determine if there is a link between these genes as well as to investigate the roles of other genes. These studies have further illuminated some of the mechanisms by which genes can increase an individual's risk for developing depression, and future studies should allow us to better understand how genetic factors and environmental triggers both contribute to mental illness.
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