Genetic Overlap in Five Mental Illnesses Traced to Common Inherited Variations

Certain major mental disorders may have more in common than we once thought. Researchers have conducted the largest genome-wide study of its kind and have discovered that five mental illnesses are traceable to the same common inherited genetic variations. The findings could have major implications for treating these diseases in the future.

Earlier research revealed the first evidence of overlap between five disorders, including schizophrenia, bipolar disorder, ADHD, depression and autism. In fact, it found that people with the disorders were more likely to have suspect variation at the same four chromosomal sites. But the extent of this overlap remained unclear. In order to examine these relationships a bit further, the researchers used the same genome-wide information and the largest data sets currently available in order 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. More specifically, the scientists looked for similarities in genetic variation among several thousand people with each illness and then compared them to controls.

So what did they find? It turns out that the researchers found overlap was highest between schizophrenia and biopolar disorder. It was moderate between biopolar disorder and depression and for ADHD and depression. And it was low between schizophrenia and autism. Overall, common genetic variation accounted for 17 to 28 percent of risk for the illnesses.

"Such evidence quantifying shared genetic risk factors among traditional psychiatric diagnoses will help us move toward classification that will be more faithful to nature," said Bruce Cuthbert, one of the researchers, in a news release.

The molecular genetic evidence linking schizophrenia and depression, in particular, has widespread implications. If replicated, it could be useful for diagnostics and research. Yet there were also some surprise findings. The scientists expected to see more overlap between ADHD and autism, but the modest schizophrenia-autism connection is consistent with other emerging evidence.

"It is encouraging that the estimates of genetic contributions to mental disorders trace those from more traditional family and twin studies," said Thomas Lehner, chief of the NIMH Genomics Research Branch, which funds the project, in a news release. "The study points to a future of active gene discovery for mental disorders."

The findings are published in the journal Nature Genetics.