An international research group has provided the first direct evidence that both obsessive-compulsive disorder (OCD) and Tourette syndrome (TS) are highly heritable.
Their report, published in the journal PLOS Genetics, also reveals major differences between the underlying genetic makeup of the disorders.
“Both TS and OCD appear to have a genetic architecture of many different genes — perhaps hundreds in each person — acting in concert to cause disease,” said senior author Jeremiah Scharf, M.D., Ph.D., of Massachusetts General Hospital.
“By directly comparing and contrasting both disorders, we found that OCD heritability appears to be concentrated in particular chromosomes — particularly chromosome 15 — while TS heritability is spread across many different chromosomes.”
The fourth most common mental health disorder, OCD is an anxiety condition characterized by obsessions and compulsions, while TS is a chronic disorder characterized by motor and vocal tics that usually begin in childhood. TS is often accompanied by conditions such as OCD or attention-deficit hyperactivity disorder.
Both disorders are considered heritable, but identifying specific genes linked to greater risk has been challenging.
“Trying to find a single causative gene for diseases with a complex genetic background is like looking for the proverbial needle in a haystack,” said co-author Lea Davis, Ph.D., of the University of Chicago.
“With this approach, we aren’t looking for individual genes. By examining the properties of all genes that could contribute to TS or OCD at once, we’re actually testing the whole haystack and asking where we’re more likely to find the needles.”
Using genome-wide complex trait analysis (GCTA), the researchers analyzed almost 1,500 individuals affected with OCD compared with more than 5,500 controls, and nearly 1,500 TS patients compared with more than 5,200 controls.
“Despite the fact that we confirm there is shared genetic liability between these two disorders, we also show there are notable differences in the types of genetic variants that contribute to risk. TS appears to derive about 20 percent of genetic susceptibility from rare variants, while OCD appears to derive all of its susceptibility from variants that are quite common, which is something that has not been seen before,” said co-senior author Nancy Cox, Ph.D., also at the University of Chicago.
Further investigation of these findings could lead to identification of the affected genes and how the expression changes contribute to the development of TS and OCD.
Research covering even larger patient populations—some of which are in the planning stages—could identify the biologic pathways disrupted in the disorder, possibly leading to new therapy options.
Source: Massachusetts General Hospital