The National Institutes of Health has revealed data showing that the five most common mental disorders — bipolar disorder, schizophrenia, depression, attention deficit hyperactivity disorder (ADHD), and autism — all have overlap in their genetic risk factors. Precisely, one genetic factor accounted for a 17-28 percent risk of having any one of the disorders.

“Since our study only looked at common gene variants, the total genetic overlap between the disorders is likely higher,” explained Naomi Wray, Ph.D., of 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.”

The first sign of a common link among these five diseases came in a report earlier this year that involved more than 300 scientists at 80 research centers in 20 countries. That study only pinpointed regions on chromosomes and did not peg down exact genes that the disorders shared in common.

In the current study, genetic overlap was seen on an individual gene level, with research looking at single nucleotide polymorphisms (SNPs), which are single-letter genetic variations within genes that occur between people. All humans share the same genes, but there are different variants or mutations that can happen spontaneously.

There was a 15 percent overlap between schizophrenia and bipolar disorder, a 10 percent overlap between bipolar disorder and depression, a nine percent overlap between schizophrenia and depression, and a three percent overlap between autism and schizophrenia. If the schizophrenia and depression correlation is true and reproducible, then this finding will have a large impact on the treatment of the diseases and drug development in the future.

Furthermore, the study has allowed researchers to determine the heritability versus spontaneous chances of having a mutation that would predispose a person to these diseases. For instance, a common genetic mutation — SNP — was seen in 23 percent of schizophrenia patients, but from analyzing twins in studies, researchers saw that up to 81 percent of the cases were from generic inheritance.

The same disparity between heritability and spontaneous genetic mutations was seen in other diseases — for instance, 25 percent spontaneous vs. 75 percent heritable 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.

“It is encouraging that the estimates of genetic contributions to mental disorders trace those from more traditional family and twin studies. The study points to a future of active gene discovery for mental disorders,” said Thomas Lehner, Ph.D., chief of the National Institute of Mental Health, in press statement.

Source: Cross-Disorder Group of the Psychiatric Genomics Consortium. Genetic relationship between five psychiatric disorders estimated from genome-wide SNPs. Nature Genetics. 2013.