The psychiatric illnesses seem very different — schizophrenia, bipolar disorder, autism, major depression and attention deficit hyperactivity disorder.
Yet they share several genetic glitches that can nudge the brain along a path to mental illness, researchers report. Which disease, if any, develops is thought to depend on other genetic or environmental factors.
Their study, published online Wednesday in The Lancet, was based on an examination of genetic data from more than 60,000 people worldwide.
Its authors say it is the largest genetic study yet of psychiatric disorders. The findings strengthen an emerging view of mental illness that aims to make diagnoses based on the genetic aberrations underlying diseases instead of on the disease symptoms.
Two of the aberrations discovered in the new study were in genes used in a major signaling system in the brain, giving clues to processes that might go awry and suggestions of how to treat the diseases.
“What we identified here is probably just the tip of an iceberg," said Dr. Jordan Smoller, lead author of the paper and a professor of psychiatry at Harvard Medical School and Massachusetts General Hospital. “As these studies grow, we expect to find additional genes that might overlap."
The new study does not mean that the genetics of psychiatric disorders are simple. Researchers say there seem to be hundreds of genes involved and the gene variations discovered in the new study only confer a small risk of psychiatric disease.
Steven McCarroll, director of genetics for the Stanley Center for Psychiatric Research at the Broad Institute of Harvard and MIT, said it was significant that the researchers had found common genetic factors that pointed to a specific signaling system.
“It is very important that these were not just random hits on the dartboard of the genome," said McCarroll, who was not involved in the new study.
The work started in 2007 when a large group of researchers began investigating genetic data generated by studies in 19 countries and including 33,332 people with psychiatric illnesses and 27,888 people free of the illnesses for comparison. The researchers studied scans of people’s DNA, looking for variations in any of several million places along the long stretch of genetic material containing 3 billion DNA letters. The question: Did people with psychiatric illnesses tend to have a distinctive DNA pattern in any of those locations?
Researchers had already seen some clues of overlapping genetic effects in identical twins. One twin might have schizophrenia while the other had bipolar disorder. About six years ago, around the time the new study began, researchers had examined the genes of a few rare families in which psychiatric disorders seemed especially prevalent. They found a few unusual disruptions of chromosomes that were linked to psychiatric illnesses. But what surprised them was that, while one person with the aberration might get one disorder, a relative with the same mutation got a different one.
Jonathan Sebat, chief of the Beyster Center for Molecular Genomics of Neuropsychiatric Diseases at the University of California, San Diego, and one of the discoverers of this effect, said that work on these rare genetic aberrations had opened his eyes. “Two different diagnoses can have the same genetic risk factor," he said.
In fact, the new paper reports, distinguishing psychiatric diseases by their symptoms has long been difficult. Autism, for example, was once called childhood schizophrenia. It was not until the 1970s that autism was distinguished as a separate disorder.
But Sebat, who did not work on the new study, said that until now it was not clear whether the rare families he and others had studied were an exception or whether they were pointing to a rule about multiple disorders arising from a single genetic glitch.
“No one had systematically looked at the common variations" in DNA, he said. “We didn’t know if this was particularly true for rare mutations or if it would be true for all genetic risk."