THURSDAY, June 21, 2018 (HealthDay News) -- Mental illnesses ranging from depression to schizophrenia show a great degree of overlap in the genes that may contribute to them, a large, new study shows.
Researchers said the findings should deepen the understanding of how various psychiatric disorders arise. Eventually, they might even change the way the conditions are diagnosed and treated.
The "high degree" of genetic correlation among different psychiatric conditions suggests that the current thinking -- where the disorders are viewed as distinct -- may be off, the researchers added.
"The tradition of drawing these sharp lines when patients are diagnosed probably doesn't follow the reality, where mechanisms in the brain might cause overlapping symptoms," senior researcher Benjamin Neale said in a statement.
"If we can uncover the genetic influences and patterns of overlap between different disorders, then we might be able to better understand the root causes of these conditions -- and potentially identify specific mechanisms appropriate for tailored treatments," said Neale, a researcher at the Broad Institute of MIT and Harvard.
The findings come from a massive collaboration among over 600 research institutions worldwide. It involved close to 785,000 healthy people and more than 265,000 patients with psychiatric conditions or neurological disorders such as Parkinson's disease, multiple sclerosis or Alzheimer's.
Each person underwent a genome-wide association study -- where researchers rapidly scan an individual's entire set of DNA. When those studies are done on large groups of people with and without a given disease, researchers can identify gene variants that seem to be associated with the disease.
Overall, the current study found, psychiatric disorders shared many of the same underlying genetic factors. Some of the greatest genetic overlap was seen among major depression, bipolar disorder, schizophrenia and attention deficit hyperactivity disorder (ADHD). There was also a high degree of overlap between anorexia and obsessive-compulsive disorder (OCD), the researchers said.
In sharp contrast, neurological disorders appeared genetically distinct from each other, and from psychiatric conditions. The one exception was migraine headache -- which shared some gene variants with ADHD, depression and Tourette syndrome.
The findings were published June 21 in the journal Science.
It's not surprising that psychiatric disorders had so much in common, genetically speaking, said Verneri Anttila, a postdoctoral researcher at the Broad Institute who also worked on the study.
It's known that various mental health disorders share some symptoms, and research has already shown genetic correlations among different disorders. But, Anttila said, the scale of the current study is much larger than previous work.
Now the task is to identify specific genes that actually help lead to the disorders, according to Anttila. (Gene variants can be associated with a disease, without being a direct cause.)
Dr. Jeffrey Borenstein is president of the Brain and Behavior Research Foundation, a nonprofit based in New York City. He was not involved with the study.
On a basic level, Borenstein said, "these findings really highlight the fact that these psychiatric disorders are biologically based -- just like any other medical condition."
That's an important message, he said, because there is still stigma surrounding mental health disorders.
Eventually, Borenstein said, the results could improve researchers' understanding of the molecular mechanisms that cause psychiatric conditions. And that, he added, could yield new and more refined treatments.
Disorders like depression, ADHD and schizophrenia manifest in different ways, of course. But, Anttila explained, they can still share some "deeper mechanisms" at their origins.
The researchers gave this hypothetical example: A single brain mechanism regulating concentration might drive both inattention in kids with ADHD and some similar problems that people with schizophrenia can have.
In other findings, the researchers discovered some genetic overlap between certain psychiatric disorders and measures of early life mental abilities -- such as years of education and college attainment. Some of the genetic factors tied to diseases like anorexia, OCD and bipolar disorder were also associated with markers of better mental performance early in life.
On the other hand, genetic factors tied to early mental prowess were "negatively correlated" with gene variants associated with neurological disorders -- particularly Alzheimer's and stroke.
Past studies have linked higher education levels with a lower risk of Alzheimer's -- though the reasons are not yet clear. This study, Anttila said, adds the element of genetic data.
"What does it all mean?" Anttila said. "We don't have a good answer -- yet."
The National Alliance on Mental Illness has an overview of mental health disorders.