Four genes in particular were linked to all five of these disorders. The four genes that were linked are all involved in the function of brain cells.
The researchers concluded that understanding the common genetic link among these disorders may lead to better treatments in the future.
"Tell a psychiatrist about all your symptoms."
The study was authored by the Cross-Disorder Group of the Psychiatric Genomics Consortium - a coalition of researchers and experts in various fields of mental health.
The researchers looked at the genes of 33,332 people with one or more of the following disorders: autism, bipolar disorder, major depression, schizophrenia and ADHD. The genes of 27,888 healthy people were used as a comparison. All the people in the study were of European descent.
The researchers wanted to see if any particular versions of genes were linked to mental health disorders.
They found that four gene versions were linked to all five disorders: ADHD, autism, bipolar disorder, depression and schizophrenia.
Analysis showed that the linked genes were all involved in brain cells' use of calcium.
Calcium flows into and out of nerve cells in the brain through channels. If those channels don’t function right, then brain cells may have trouble regulating the level of calcium.
The authors concluded that these results suggest that all five disorders may share some common underlying cause. The common genetics may be the reason people often have more than one of these disorders.
The authors also said that defining these disorders based on symptoms may not be best. Instead, these disorders may represent a spectrum of disorders caused by genetic weakness in calcium channels.
Currently, all five of these disorders are classified by symptoms. If people have a particular array of symptoms, then they are diagnosed with a disorder defined by those symptoms. There are currently no biological tests that can confirm or rule out these five disorders.
Therefore, the genetic findings of this study are a first step in understanding what the root cause of these disorders might be.
The genes that were linked to the disorders in this study are known to be involved in calcium regulation, but the study did not look at the function of calcium channels. So it is not known if the people with the risky genes actually had altered calcium channel function.
The study was published February 27 in The Lancet. The study was funded by the National Institutes of Health. The authors declared no conflicts of interest.