The biggest study yet into genetics and mental health has come up with a stunning result: The five most common mental illnesses -- autism, attention deficit disorder, bipolar disease, schizophrenia and major depression -- all have a common genetic root.
The finding, published in the journal Lancet on Wednesday, may eventually lead to a complete rewrite of the medical understanding of the causes of mental illness.
"We have been able to discover specific genetic variants that seem to overlap among disorders that we think of as very clinically different," Dr. Jordan Smoller of Massachusetts General Hospital in Boston, who led the study, said in a telephone interview.
The study does not explain every case of psychiatric disease, the researchers stress.
"We think this is one tiny fraction of the genetic component of these disorders. They involve hundreds and possibly thousands of genes," Smoller said.
And it didn't show every case was related. But it demonstrated on a genetic level that the five diseases are more like a continuum of dysfunction than five separate and discrete conditions.
Smoller's international team included dozens of researchers who looked at the genetics of more than 33,000 psychiatric patients and compared them to nearly 28,000 people without mental illness. They did what is called a genome-wide association study -- a scan of all the DNA.
"We aimed to identify specific variants underlying genetic effects shared between the five disorders in the Psychiatric Genomics Consortium: autism spectrum disorder, attention deficit-hyperactivity disorder, bipolar disorder, major depressive disorder, and schizophrenia," they wrote in their report.
They linked a considerable number to four places in the genome: a big stretch of chromosome 3; another part of chromsome 10, and two very specific genetic areas known to be involved in controlling cell function called calcium channels.
It wasn't a complete surprise, Smoller says. Doctors have noted some overlap of symptoms and knew that in families prone to one psychiatric disease, another could also occur. "Autism was once known as childhood schizophrenia and the two disorders were not clearly differentiated until the 1970s," the team wrote.
This finding could suggest that a genetic weakness upstream in the development of the brain could lead to a variety of psychiatric symptoms, perhaps influenced by other genes, or by the environment as well.
"We didn't know going in that we would be able to find commonality with such a broad array of disorders," Smoller said. "The fact that a particular pathway emerged as being relevant was also surprising. We didn't know about that one before."
Dr. Ken Duckworth, medical director of the National Alliance on Mental Illness, hopes the findings may help dispel some of the stigma that still surrounds psychiatric diseases.
"Ultimately this kind of research will give us a return in terms of social attitudes toward brain-based illness," Duckworth said in a telephone interview. "If you can understand an illness process, it doesn't seem so mysterious and terrifying."
Duckworth said every psychiatrist knows of patients whose symptoms don't clearly meet the definition of any one disease, and he noted that Sigmund Freud defined schizophrenia as a group of diseases. "This is a corner piece of the jigsaw puzzle," he said.
And it might lead to better treatments, said Dr. Bruce Cuthbert, who is director of the National Institute of Mental Health's division of Adult Translational Research. "We are finally starting to make inroads where we have actual physiological mechanisms that we can target," he said. "We can really start to understand the biology instead of having to guess at it."