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, 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 for schizophrenia and autism. Overall, common genetic variation accounted for 17-28 percent of risk for the illnesses. "Since our study only looked at common gene variants, the total genetic overlap between the disorders is likely higher," explained Naomi Wray, Ph.D., University of Queensland, Brisbane, Australia, who co-led the multi-site study by the Cross Disorders Group of the Psychiatric Genomics Consortium (PGC), which is supported by the NIH's National Institute of Mental Health (NIMH). "Shared variants with smaller effects, rare variants, mutations, duplications, deletions, and gene-environment interactions also contribute to these illnesses." Dr. Wray, Kenneth Kendler, M.D., of Virginia Commonwealth University, Richmond, Jordan Smoller, M.D., of Massachusetts General Hospital, Boston, and other members of the PGC group report on their findings online on August 11, 2013 in the journal Nature Genetics. "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, Ph.D., director of the NIMH Division of Adult Translational Research and Treatment Development and coordinator of the Institute's Research Domain Criteria (RDoC) project, which is developing a mental disorders classification system for research based more on underlying causes.Login Or Register To Read Full Story