Nine researchers from U of T’s Faculty of Medicine contributed to a major international study, published in the journal Science, showing the underlying genetic similarities between mental illnesses like schizophrenia and bipolar disorder.
The team, led by Harvard University’s Broad Institute, determined that psychiatric disorders share many genetic variants, while neurological disorders such as Parkinson’s or Alzheimer’s appear more distinct.
For the current study, international consortia pooled their data to examine the genetic patterns across 25 psychiatric and neurological diseases. Because each genetic variant only contributes a tiny percentage of the risk for developing a given disorder, the analyses required huge sample sizes to separate reliable signals from noise.
The researchers measured the amount of genetic overlap across the disorders of 265,218 patients and 784,643 controls. They also examined the relationships between brain disorders and 17 physical or cognitive measures, such as years of education, from 1,191,588 individuals.
The final results indicated widespread genetic overlap across different types of psychiatric disorders, particularly between attention-deficit/hyperactivity disorder (ADHD), bipolar disorder, major depressive disorder, and schizophrenia. The data also indicated strong overlap between anorexia nervosa and obsessive-compulsive disorder (OCD), as well as between OCD and Tourette syndrome.
In contrast, neurological disorders such as Parkinson’s and multiple sclerosis appeared more distinct from one another and from the psychiatric disorders — except for migraine, which was genetically correlated to ADHD, major depressive disorder, and Tourette syndrome.
According to the researchers, the high degree of genetic correlation among the psychiatric disorders suggests that current clinical categories do not accurately reflect the underlying biology.
“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,” says Neale.
As a hypothetical example, a single mechanism regulating concentration could drive both inattentive behavior in ADHD and diminished executive function in schizophrenia.
Further exploration of these genetic connections could help define new clinical phenotypes and inform treatment development and selection for patients.
Additionally, within the cognitive measures, the researchers were surprised to note that genetic factors predisposing individuals to certain psychiatric disorders — namely anorexia, autism, bipolar disorder, and OCD — were significantly correlated with factors associated with higher childhood cognitive measures, including more years of education and college attainment.