Major depressive disorder (MDD) affects approximately seven percent of U.S. adults. The cause of this disorder is multi-factorial and the treatments are diverse. With the emergence of newer technology to investigate the human genome, genetics now plays a role in depression. Various genes have been identified which may predispose an individual to a depressive episode. Additionally, genetic testing is available to guide treatment. This is known as pharmacogenetics. There are genes which are known to predict metabolism of certain drugs, as well as genes which code for various receptors implicated in depression. Testing patients for variants of these genes is a relatively new but promising practice. Results from genetic testing can inform decisions regarding which antidepressant a patient may respond best to and at what dose.
As with many diseases, particularly in the field of psychiatry, depression is polygenic. This means that there is no single gene which can predict the disorder. However, progress is being made in identifying genetic variants associated with depression. Several scientific consortiums along with direct-to-consumer genetic testing companies have been able to identify some of these genetic variants. With continued research and increasing sample size, a better understanding of the genetic underpinning of the disease will emerge. As the specific genes are characterized and analyzed, they represent potential targets for future treatments.
Though this research is promising, to date there are no classes of medication for depression which are aimed at specific genetic targets. However, with the medications which currently do exist, there are genetic variants that can aid in treatment decisions. For example, the most widely used class of medications for depression are selective serotonin reuptake inhibitors (SSRIs). As the name implies, these drugs act by blocking the reuptake of serotonin (an important neurotransmitter implicated in mood) which allows for more active serotonin in the brain. The gene which codes for the serotonin transporter has been identified. Individuals have different variants of this gene, which allow for variations in expression of this transporter and possible differences in response to SSRIs.
While these novel approaches to treatment of MDD are exciting, it is important to keep in mind that environmental factors, age, sex, diet, alcohol use, hormonal status, and general health all play a role in the risk of disease and response to treatment. Ultimately, treatment decisions are made by analyzing each of these elements for an individual patient. Though genetic testing is only a small piece of the puzzle, it can add important information which was not available decades ago.