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New Data Reveal Extent of Genetic Overlap Among Major Mental Disorders

Overlap highest between schizophrenia and bipolar disorder (August 12)

The largest genome-wide study of its kind has determined how much five major mental illnesses are traceable to the same common inherited genetic variations.

Researchers partly funded by the National Institutes of Health (NIH) found that the overlap was highest between schizophrenia and bipolar disorder; moderate for bipolar disorder and depression and for ADHD and depression; and low between schizophrenia and autism. Overall, common genetic variation accounted for 17 to 28 percent of the risk for the illnesses.

The overlap in heritability attributable to common genetic variation was about 15 percent between schizophrenia and bipolar disorder, about 10 percent between bipolar disorder and depression, about 9 percent between schizophrenia and depression, and about 3 percent between schizophrenia and autism.

The newfound molecular genetic evidence linking schizophrenia and depression, if replicated, could have important implications for diagnostics and research, say the researchers. They expected to see more overlap between ADHD and autism, but the modest schizophrenia–autism connection is consistent with other emerging evidence.

The study results also attach numbers to molecular evidence documenting the importance of heritability traceable to common genetic variation in causing these five major mental illnesses. Yet this still leaves much of the likely inherited genetic contribution to the disorders unexplained — not to mention non-inherited genetic factors.

For example, common genetic variation accounted for 23 percent of schizophrenia, but evidence from twin and family studies estimate total heritability of the diseaseat 81 percent. Similarly, the gaps are 25 percent vs. 75 percent for bipolar disorder, 28 percent vs. 75 percent for ADHD, 14 percent vs. 80 percent for autism, and 21 percent vs. 37 percent for depression.

Source: NIH; August 12, 2013.

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