Genetics and Mental Illness: What’s the Connection?
It’s long been known that certain diseases tend to run in families. In fact, most of our understanding of mental illness comes from family, adoption, and twin studies. Now, researchers have discovered a gene that can double the risk of depression, another that can increase the risk of schizophrenia , and others that may play a role in the risk of bipolar disorder .
The Genetics of Depression
Each year an estimated 9.5% of American adults suffer from mood disorders. People suffering from depression experience symptoms of sadness, emptiness, or hopelessness that interfere with their lives. One of the questions scientists have been trying to answer is why some people become depressed in reaction to life stresses while others do not.
One study sheds some light on this question.
Researchers followed 847 New Zealanders over five years and charted their reaction to different life stressors. Stressors included things like loss of job, death of a loved one, broken relationships, or prolonged illness. What they found was that participants with a short version of a particular gene were more likely to become depressed than those who had the long version of the same gene.
Specifically, 43% of people with the short form of the gene developed depression, in contrast to 17% of those with the long form. The findings suggest that some people—because of their genes—are more vulnerable to depression when certain stressors activate the gene. Other researchers have found similar findings.
The Genetics of Schizophrenia
The family link to schizophrenia is well-established, but paring down the risk to one specific gene has proven difficult. For example, we each inherit two copies of the COMT (catecho-O-methyltransferase) gene, one from each parent.
There are two COMT types possible: the val type (short for the amino acid valine) and the met type (short for the amino acid methionine). Research suggests that the val type may increase the susceptibility to schizophrenia by reducing dopamine activity.
One study tested the working memory of 181 schizophrenic patients, 219 of their well siblings, and 75 controls. The researchers found that individuals who had two copies of the val type gene performed worse than those with only one copy, and those with the met type gene performed best of all. What this suggests is that the val type gene may increase an individual’s susceptibility to schizophrenia.
The Genetics of Bipolar Disorder
Bipolar disorder is a disease that causes extreme shifts of mood from depression to mania to a mixed state. While family and twin studies have shown that bipolar disease can be inherited, they have not yet pinpointed a particular gene. In fact, evidence suggests that bipolar disorder is affected by the presence and interaction of many genes.
In another study, researchers found that patients with bipolar disorder had a significantly higher frequency of the COMT and short 5-HTT genes. Again, these findings suggest that genes may affect people's susceptibility to this condition.
Despite the growing number of genetic tests available, there are no genetic tests for depression, schizophrenia, or bipolar disorder. Genetic testing also causes concerns among many people, particularly in regards to privacy. Therefore, as new tests are developed, people who choose to be tested must weigh the benefits against the risks of finding out what their genes contain.
As exciting as these discoveries may be, researchers are quick to note that mental illness, like other diseases, is an equation of sorts, with genetics being only one of the variables. For example, while your genes may increase your risk for a disease, other factors also play a role. These factors can be exposures to toxins, bacteria, and viruses during fetal development, as well as nutritional status, stress, emotional trauma, childhood development, environment, and a history of other medical conditions.
National Human Genome Research Institute
National Institute of Mental Health
Canadian Mental Health Association
Mental Health Canada
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Last reviewed May 2008 by Ryan Estévez, MD, PhD, MPH
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