The Surprising Link Between Depression, ADHD, Bipolar, Schizophrenia and Autism

An important new study—the first of its kind—finds a genetic connection that may yield better treatments and less stigma for big mental illnesses.

Scientists had suspected that some of the disorders shared genetic traits, but were surprised to find commonalities across all five. (Getty Images/Tim Teebken)

Shari Roan is an award-winning health writer based in Southern California.

On the face of it, you'd never think autism, major depression, schizophrenia, bipolar disorder, and attention-deficit hyperactivity disorder (ADHD) would have much in common, except for the fact that they're all mental illnesses. Autism, for example, typically involves social withdrawal and trouble with language, while someone struggling with major depression has huge emotional lows.

But a new study suggests that five of the most serious types of mental health conditions share some genetic risk factors. The study, just published in the medical journal The Lancet, is the first to show that certain gene mutations increase the risk of these five mental disorders, which are the most common and, often, the most difficult to treat. The information should eventually help researchers better understand what causes these disorders and make it easier to make more effective treatments.

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So what links these five very different mental illnesses? The study found that variations in two genes involved in the balance of calcium in brain cells are implicated in several. The pathway that regulates this chemical balance has emerged as a key target for the development of new medications.

Researchers suspected that some of the disorders shared some genetic traits, but were surprised to find commonalities across all five, says Dr. Jordan W. Smoller, associate vice chair of the department of psychiatry at Massachusetts General Hospital, in Boston, and the lead author of the study. "We did have some suspicion because studies [done with twins] have suggested that there might be some shared genetic contribution in a couple of these disorders," he says. "But the notion that there would be shared biological pathways that would cut across all of these disorders was a surprise, and it points to some shared biology."

The fact that all five of the disorders shared biological pathways came as a surprise to the researchers.

Researchers have long debated how various mental illnesses are alike or different. Studies over the past two decades hinted that genetic risks are important in developing these illnesses. But it was the development of large databases of genetic information for several major psychiatric disorders that allowed scientists to examine the possible overlap of genetic traits. The database used in the Lancet study used genomes from over 33,000 people with a mental disorder and nearly 28,000 without one.

Scientists identified four places in genes that indicate an increased risk and that have significant and overlapping links with all five diseases—regions on chromosomes that help regulate the flow of calcium in brain cells. More analysis backed up the idea that calcium channel activity could play an important role in the development of all five disorders.

Which is not to say that these gene variants always cause mental illness, however, Smoller says. "Our findings represent a small portion of the genetic picture," he says. "For each of these disorders there are hundreds, if not thousands, of genes that could contribute."

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That's also why this information can't yet be used for diagnosis or to help an individual determine his or her risk of developing a disorder. "The individual variances...confer a very small increased risk," Smoller says. "They are not diagnostic. They can't be used as a test. Even if you had all of these variants you still might not develop a disorder. And there may be people with none of these variants who develop a disorder. They are risk factors."

Moreover, genes alone are not likely to be the sole cause of most mental illnesses. Environmental factors and life experiences are also important. "We've known from twin studies that these five disorders clearly have a big genetic component," Smoller says. "But it's not 100 percent. It's not all genetic."

'These five disorders have a big genetic component,' says the study's lead author. 'But it's not 100 percent genetic.'

Still, the information could eventually lead to an entirely different way of viewing these conditions that's based on biological or genetic features of the disorder, not a description of a person's symptoms. And that could help reduce the stigma around these mental illnesses, encouraging them to be seen more as biological problems like cancer or diabetes. "One hope is that we will move in that direction," Smoller says. "The current system for classifying these disorders is almost purely based on symptoms, not on causes."

Do you think genes play a big part in developing mental illnesses? Could this new study help destigmatize these five disorders?

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