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Study shows extent of genetic overlap in mental illnesses

Schizophrenia and bipolar share highest overlap

Jo Carlowe

Monday, 12 August 2013

Scientists have quantified the genetic overlap between several major mental health conditions.

The findings from the largest genome-wide study of its kind, appear in the latest edition of Nature Genetics.

Researchers, funded in part by the National Institutes of Health, 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 17-28% of risk for the illnesses.

"Since our study only looked at common gene variants, the total genetic overlap between the disorders is likely higher," says Naomi Wray, of the University of Queensland, Australia, who co-led the multi-site study by the Cross Disorders Group of the Psychiatric Genomics Consortium (PGC).

"Shared variants with smaller effects, rare variants, mutations, duplications, deletions, and gene-environment interactions also contribute to these illnesses.”

Bruce Cuthbert, director of the NIMH Division of Adult Translational Research and Treatment Development and coordinator of the Institute's Research Domain Criteria (RDoC) project, added: "Such evidence quantifying shared genetic risk factors among traditional psychiatric diagnoses will help us move toward classification that will be more faithful to nature."

In February of this year, PGC researchers – more than 300 scientists at 80 research centers in 20 countries – reported the first evidence of overlap between all five disorders. People with the disorders were more likely to have suspect variation at the same four chromosomal sites. But the extent of the overlap remained unclear.

In the new study, they used the same genome-wide information and the largest data sets currently available to estimate the risk for the illnesses attributable to any of hundreds of thousands of sites of common variability in the genetic code across chromosomes. They looked for similarities in such genetic variation among several thousand people with each illness and compared them to controls – calculating the extent to which pairs of disorders are linked to the same genetic variants.

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

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