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Schizophrenia, Bipolar Disorder Share Common Genetic Cause

January 21, 2009 — The largest family study of schizophrenia and bipolar disorder ever conducted shows that these 2 disorders share a common genetic cause, a finding that challenges the current view that they are separate and distinct conditions.

The analysis, which included more than 9 million individuals from more than 2 million families over a 30-year period, showed that first-degree relatives of individuals with either schizophrenia (35,985 individuals) or bipolar disorder (40,487) had a significantly increased risk for these disorders.

Investigators at the Karolinksa Institutet, in Stockholm, Sweden, found that full siblings were 9 times more likely than the general population to have schizophrenia and 8 times more likely to have bipolar disorder. Maternal half-siblings were 3.6 times more likely to have schizophrenia and 4.5 times more likely to have bipolar disorder than the general population.

This risk was lower for paternal half-siblings, who were 2.7 times more likely to have schizophrenia and 2.4 times more likely to have bipolar disorder.

"Our results were fairly clear-cut and demonstrated that there was a significant increased risk of both schizophrenia and bipolar disorder among first-degree relatives with either of these diseases," study investigator Christina Hultman, PhD, told Medscape Psychiatry. "There was also evidence from the half-siblings and adoptive relatives that this risk is substantially due to genetic factors vs environmental factors, which allows us to conclude that there is a common genetic variation between these 2 psychotic disorders," Dr. Hultman added.

The study is published in the January 17 issue of the Lancet.