Bipolar mania and depression independently inherited

By Sara Freeman, medwireNews Reporter

Bipolar mania and depression may be independently inherited in families, findings of a US study suggest.

Although they often occur together, mania and depression may represent distinct pathologic pathways underlying bipolar disorder rather than increasingly severe manifestations of a common underlying genetic predisposition, say Kathleen Merikangas (National Institute of Mental Health, Bethesda, Maryland) and colleagues.

“This work has major implications for future genetic studies as well as relevance for classification, etiology and treatment of mood disorders,” the researchers write in Molecular Psychiatry.

Merikangas et al studied 447 mood disorder patients from the National Institute of Mental Health (NIMH) Family Study of Affective Disorders. Of these probands, 62 had bipolar I disorder, 66 had bipolar II disorder, 162 had major depressive disorder (MDD), and there were 157 individuals with none of these mental health problems who served as controls.

A total of 2082 first-degree, living or deceased, relatives of the probands were also studied, of whom 246 were relatives of the bipolar I disorder patients, 311 of the bipolar II disorder patients, 726 of the MDD patients, and 799 of the controls. Information on the relatives’ mental health history was obtained by direct semi-structured interview, if possible, or by questioning multiple family members about the individual’s history.

Results showed that probands with bipolar I disorder were more than eight times more likely to have relatives with bipolar I disorder (odds ratio [OR]=8.4) and those with MDD were more than twice as likely to have relatives with MDD (OR=2.26).

The lifetime prevalence rates of bipolar I disorder and MDD were 20.7% and 30.3%, respectively, for relatives of probands with the same condition.

“The familial aggregation of [bipolar I disorder] was primarily attributable to the familial specificity of manic episodes after adjusting for both proband and relative comorbid anxiety and substance use disorders,” Merikangas and team observe.

No such association was seen for bipolar II disorder, with relatives of probands with bipolar II disorder having a lifetime prevalence rate of 4.8% and a nonsignificant OR of 2.55. But they did have a 31.2% prevalence of MDD, with a significant OR of 2.24.

However, having mania or hypomania was not associated with an increased likelihood for having a relative with depression, or vice versa. This suggests “that the familial transmission of manic and major depressive episodes is independent despite the high magnitude of comorbidity between these mood states,” say the researchers.

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