Genes for depression

For many years researchers have suspected the existence of specific genes that make certain individuals more susceptible to depression than others.

The suspicion arose as it became obvious through observation that some people seem to be more sensitive to stress than others and some develop depression regardless of the events in their lives. There are also many studies indicating that depression runs in families and is more common in identical twins than non-identical twins.

A recent study completed at the University of Wisconsin in the US has provided evidence of what that gene might do. The study pinpointed one of the specific genes that appear to make a person more likely to become depressed.

The gene, called 5-HTT, is a serotonin transporter gene and was chosen for study because of the known link between serotonin response in the brain to stress levels and depression. This gene is the blueprint for a specific protein that is critical for transporting serotonin through the neuron cell membrane.

Serotonin is a chemical messenger in the brain that is known to be involved in depression and other psychiatric disorders. The most effective antidepressant medications today affect levels of this chemical in the brain and many work by blocking the re-uptake of serotonin through the cell membrane.

Everyone has the gene 5-HTT, but there are different versions of it. Some people have two short copies, others have two longer copies and still others have a combination, with one long and one short copy of the gene.

What this study discovered, is that those with one or two short copies of the gene are more likely to develop depression if they experience multiple stressful events than those with two copies of the protective longer version of the same gene.

In this study, more than 800 people were observed from birth into adulthood. During that time, the effects of multiple life stresses were evaluated.

Seventeen per cent of study participants carried two copies of the short, stress sensitive gene, while 31% carried two copies of the long protective version and 51% had one copy of each. These numbers are thought to be representative of the general Caucasian population.

During the study, stressful life events (such as situations with jobs, finances, health, housing or relationships) were charted when participants were between the ages of 21 and 26. By the age of 26, 17% had experienced major depression in the past year and three per cent had contemplated or attempted suicide.

Numbers were significantly higher in the group who had experienced multiple stressful events and also had at least one copy of the short version of the gene - 11% had attempted or contemplated suicide by the time they were 26 years old. Of those who had experienced multiple stresses, but had two copies of the long version of the gene, only four per cent had contemplated or attempted suicide.

Clearly, this shows a connection between suicide risk and the gene 5-HTT.

Only 10% of study participants were carriers of at least one copy of the short version of the gene and also experienced four or more stresses, but this group represented 25% of the total depression cases. In fact, 43% of this group developed depression by age 26. Only 17% of those with two copies of the longer version of the gene were depressed.

Although the study showed a connection between gene 5-HTT and risk for depression and suicide, it is clearly not the only factor involved.

The gene's effects may only be triggered in people who experience stress or trauma in their lives and perhaps not in all of those people. This supports the theory that our environment also plays an important role in determining the course of our mental health.

It is also important to note, that while those with the two copies of the longer gene were less likely than others to develop depression, this was not a completely protective gene. Given enough stressful experiences, some of these people still developed depression.

This study is the first to identify a specific gene for depression and may lead to treatments that are streamlined for people with a particular expression of this gene. It is even more significant that we already know what this gene does.

However, much more research is needed in order to further examine this link and to discover all the genetic and environmental factors involved in depression. It is unlikely that this is the sole susceptibility gene for depression.


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