An international team has found evidence of substantial overlap for genetic risk factors shared between bipolar disorder, major depressive disorder and schizophrenia and less overlap between those conditions and autism and attention deficit-hyperactivity disorder (ADHD).

The root causes of psychiatric illnesses are not known. Instead, for the past 125 years, clinicians have based diagnosis on a collection of symptoms observed in patients, something medical science has long left behind - and so the race has been on to find biological links.

Now, the authors say five psychiatric disorders share a common genetic link, which is the path to understanding the molecular underpinnings of psychiatric illness. The project is led by the Cross-Disorder Group of the Psychiatric Genomics Consortium and is the largest genetic study of psychiatric illness to date.  Nature has published similar papers with discoveries about psychiatric genetics, in 2008 and and one with 5 loci in 2011 (Nature Genetics 43, 969-976 doi:10.1038/ng.940).

They believe that insight into biological pathways that may predispose an individual to disease and could ultimately lead to the development of new therapeutic avenues to treat the five major psychiatric illnesses.

Evidence for genome-wide pleiotropy between psychiatric disorders. Proportion of variance in liability (SNP-based heritability) and proportion of covariance in liability between disorder (SNP-based coheritability) for five major psychiatric disorders. Credit and link: Nature Genetics doi:10.1038/ng.2711

"This is a very large scale study using a new, innovative statistical method," said study co-senior author Kenneth S. Kendler, M.D., professor of psychiatry, and human and molecular genetics in the Virginia Commonwealth University School of Medicine. "Prior to this model, we have not been able to address these questions. These results give us by far the clearest picture available to date of the degree of genetic similarity between these key psychiatric disorders. We hope that this will help us both in developing a more scientifically based diagnostic system and understanding the degree of sharing of the biological foundation these illnesses." 

A paper earlier this year in The Lancet reported that specific single nucleotide polymorphisms, or SNPs, are associated with a range of psychiatric disorders that can occur during childhood or adulthood. Next, the group will examine other disorders for which molecular genetic data is accumulating including eating disorders, obsessive compulsive disorder and drug use disorders.

Citation: Cross-Disorder Group of the Psychiatric Genomics Consortium, 'Genetic relationship between five psychiatric disorders estimated from genome-wide SNPs', Nature Genetics
11 August 2013 doi:10.1038/ng.2711