Genes - What Causes Depression: Depression

Content provided by the Faculty of the Harvard Medical School

Genes

Every part of your body, including your brain, is controlled by genes. Genes make proteins that are involved in biological processes. Throughout life, different genes turn on and off, so that — in the best case — they make the right proteins at the right time. But if the genes get it wrong, they can alter your biology in a way that results in your mood becoming unstable. In a vulnerable person, any stress (a fight with your spouse, a missed deadline at work, or a medical illness, for example) can then push this system off balance.

To be sure, chemicals are involved in this process, but it is not a simple matter of one chemical being too low and another too high. Rather, many chemicals are involved, working both inside and outside nerve cells. There are millions, even billions, of chemical reactions that make up the dynamic system that is responsible for your mood, perceptions, and how you experience life. With this level of complexity, you can see how two people might have similar symptoms of depression, but the problem on the inside, and therefore what treatments will work best, may be entirely different.

The fact that depression and bipolar disorder run in families has long been clear, but experts now have a developing picture of how much of that tendency comes from nature and how much reflects nurture. Studies of twins and adopted children, plus a wealth of research from the Human Genome Project and the Human Genetics Initiative at the National Institute of Mental Health, have begun to answer some important questions.

The clearest genetic link is to bipolar disorder. Most experts believe it affects 1% of the general population, although some preliminary evidence suggests it could be even more common. Half of those with bipolar disorder have a relative with a similar pattern of mood fluctuations. Studies of identical twins, who share a genetic blueprint, show that if one twin has bipolar disorder, the other has a 60%–80% chance of developing it, too. These numbers don't apply to fraternal twins who, like other biological siblings, share only about half of their genes. If one fraternal twin has bipolar disorder, the other has a 20% chance of developing it.

The genetic components of other mood disorders are far harder to pin down. A person who has a first-degree relative who suffered major depression has a 1.5%–3% higher-than-normal risk of experiencing the condition as well. But researchers have found it quite difficult to sort out the actual influence of genes versus environmental factors.

Thus far, experts say genes alone are not responsible for causing mood disorders. Rather, these illnesses probably result when genes make a person vulnerable; then the illness is triggered by environmental factors like early losses or long-term stress.

Research indicates that a person's genes also affect how well he or she responds to different treatments. Although we do not yet have genetic tests to help us choose the best treatment, such tests may not be too far off (see "On the horizon").

Last updated:	January 23, 2007