Genetics and Manic-Depression
To learn more about the genetics of manic-depression, we turned to John Nurnberger, Jr., M.D., Ph.D., director of the Institute of Psychiatric Research at Indiana University and professor of psychiatry and medical neurobiology at the Indiana University School of Medicine.
Post: Could you explain the evidence that manic-depressive illness is hereditary?
Dr. Nurnberger: Well, we know there are inheritable or genetic factors involved with these disorders. For instance, the disorder is very much more likely to happen in one twin if the other twin has the disorder. You also see the same kind of preponderance in families of people who have the condition, and you see it in adoption studies. If a person is adopted from a family and develops the disorder, and you trace back to the biological family, it's more likely you,ll see the illness occurring in the biological family of a person with depression than you will in a biological family of somebody else.
The genetic determination of these disorders is about two thirds. In other words, about two thirds of the variance in whether somebody is likely to get the disorder or not is probably related to genetic factors. This means another one third of the variance is probably environmental. We don't understand very well yet what the specific genetic factors are, nor do we understand what the specific environmental factors are that may cause this in some people.
Post: What studies are you working on now in this area?
Dr. Nurnberger: The studies at the Institute relate to primarily genetic factors that may be involved in predisposing people to develop bipolar or unipolar disorder. The current evidence in the field suggests there may be a mixture of genes causing these disorders. There is evidence for genes on chromosome 18, X, 11, and 21 as well as some evidence for other areas, but those areas are the ones most likely implicated at present. It seems to be a complex pattern of inheritance. The other thing we are looking at is the effect of stress in causing depression. We have been studying animal models to look at the effects of stress.
Post: Are some populations more prone to bipolar disease than others?
Dr. Nurnberger: That's a difficult question to answer, because it's not clear that the same methods of diagnosing these disorders have been used in different populations. There is some evidence that some populations have relatively higher rates of mania. The Amish have been studied quite intensively, and it does appear, in the Amish in Pennsylvania, that the rate of bipolar disorder is about equal to the rate of unipolar disorder, which would mean that mania may be relatively more common in that population. Also in the Mediterranean populations that have been studied, it appears as though the relative incidence of mania is higher. In Scandinavian populations, it appears that the relative incidence of depression is higher. But again, it's somewhat hard to compare different populations unless you're really clear that you're using exactly the same methods of diagnosing.
Post: What is the prevalence of depression?
Dr. Nurnberger: It depends on how you define depression as to how many people are affected. In its milder forms, you may be talking about 40 percent of the population who, at one time or another in their lives, have symptoms of depression. In its severe, incapacitating forms, it's more like seven percent of the population. In manic-depressive illness, which has highs as well as lows, it's more like one percent.
Post: Can you describe these severe, incapacitating cases?
Dr. Nurnberger: An incapacitating depression is a state that lasts for weeks or months and involves sadness, inability to enjoy anything, and changes in sleep and appetite. There may be increased sleep an appetite or decreased sleep and appetite. There may also be increased speed of thought and movement together with agitation, or a person may be slowed down and not able to move or think very quickly. …