Both obsessive-compulsive disorder (OCD) and Tourette syndrome (TS) are psychiatric disorders that often get in the way of people’s daily lives — whether it’s repetitive, obsessive thoughts (OCD) or uncontrollable movement (TS) — and scientists have long found it challenging to identify which genes increase the risk of developing either.

Now, researchers at Massachusetts General Hospital and the University of Chicago are the first to directly confirm that both disorders are highly heritable, and that there are clear genetic differences between the two. “Both [Tourette syndrome] and OCD appear to have a genetic architecture of many different genes – perhaps hundreds in each person – acting in concert to cause disease,” Dr. Jeremiah Scharf, lead author of the study, said in a press release. They found that OCD heritability seems to be concentrated in certain chromosomes, such as chromosome 15 in particular. Tourette syndrome, on the other hand, is spread across many chromosomes.

OCD is the fourth most common psychiatric illness, and in severe cases can disrupt a person’s normal activities. It’s characterized by obsessive, uncontrollable thoughts that lead to compulsive actions such as washing hands constantly, counting to a certain number, or an extreme need to keep things in order. TS, meanwhile, is a chronic neurological disorder involving ‘tics,’ which are repetitive and involuntary movements or vocalizations — simple tics include eye blinking, facial grimacing, or shoulder shrugging; more complex tics are often a combination of all of these. Vocal tics include sniffing, throat-clearing, and grunting.

To the researchers tackling the project, “[t]rying to find a single causative gene for diseases with a complex genetic background is like looking for the proverbial needle in the haystack,” Lea Davis of the University of Chicago, an author of the study, said in the press release. In their approach, which is called genome-wide association studies (GWAS), they tested “the whole haystack” in order to figure out where the needles may be.

"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,” Nancy Cox of the University of Chicago said in the press release. “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."

The study, which was an international collaboration between the Tourette Syndrome Association International Consortium for Genetics and the International OCD Foundation Genetics Collaboration, representing 43 institutions from 12 different countries, is a step in furthering research about the affected genes, and how gene expression affects the development of OCD and TS. Farther down the road, scientists hope to discover the exact biological pathways that these disorders disrupt, which may have therapeutic benefits to OCD and TS patients.