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Tuesday, May 13, 2025 (Halifax) __ An international collaborative study led by Dalhousie University has significantly advanced the understanding of obsessive-compulsive disorder (OCD), providing genetic clues as to who might be at risk of developing the mental health condition that affects roughly one in 100 people and involves unwanted repetitive thoughts and behaviours.

While previous studies have shown that OCD can run in families, this is the first time researchers have made substantial progress in identifying genetic-risk loci, or specific locations on a chromosome where DNA variants are associated with higher susceptibility of a specific disease or trait.

Estimated to affect more than 400,000 Canadians, OCD causes intrusive thoughts (obsessions) and urges to repeat specific behaviours (compulsions) — symptoms that can be debilitating.

Researchers have now conducted the largest genetic study of OCD to date, comparing the DNA of more than 53,000 people with OCD to more than two million people without the disorder. This massive dataset allowed them to search for tiny differences in DNA -- known as genetic variants or SNPs -- that are more common in people with OCD.

"Our findings confirm our believe that OCD is caused by a complex combination of environmental and genetic causes," said Manuel Mattheisen, a Dalhousie University Research Chair and lead senior author on the paper. "As part of this complex combination, many genomic regions contribute to the etiology of OCD affecting not a single specific region, but multiple brain regions."

This study, published today in Nature Genetics, looked at the biological basis of OCD and showed that the risk is influenced by many genes, often the same ones involved in other psychiatric disorders. Identifying these specific genes and understanding their roles in particular brain cells brings us closer to figuring out the exact mechanisms behind OCD, which could help in the development of new medications and therapies.

The study identified 30 specific regions in our DNA that are associated with an increased risk of developing OCD. This marks a significant increase from previous, smaller studies. By looking closely at the identified DNA regions, scientists pinpointed 249 potential genes involved in OCD and then narrowed this down to 25 particularly strong "candidate" genes.

More than 200 investigators, from dozens of institutes around the world, were also able to estimate that around 11,500 different genetic variants likely contribute to OCD risk, with each having a very small effect, making OCD a highly "polygenic" condition.

"After decades of fumbling in the dark, we have finally cracked the piñata of OCD genetics. This study required unprecedented international collaboration, and I am proud of Sweden's contribution to this effort," said Mataix-Cols, professor at the Karolinska Institute in Stockholm, one of the international collaborators.

Dr. Mattheisen explains that the genetic risk factors identified by the team seem to be most active in specific types of brain cells, particularly in excitatory neurons, or nerve cells that stimulate activity, in brain areas that respond to the chemical messenger dopamine. This supports existing theories about which brain circuits might be involved in OCD.

Importantly, the study confirmed strong genetic links between OCD and other mental health conditions. People with genetic variants increasing OCD risk are also more likely to have variants linked to anxiety disorders, depression, anorexia nervosa, Tourette syndrome and PTSD.

The research also found that the genetic factors linked to higher OCD risk were associated with a lower genetic risk for inflammatory bowel diseases, higher educational attainment, higher body mass index and alcohol dependence.

"The genes and genetic relationships identified in this study are a huge step forward in the quest to understand the biology of OCD, and eventually to improve early detection and treatment efforts for this common and impairing psychiatric illness," said Carol Mathews, chair of the UF College of Medicine's Department of Psychiatry in Gainesville, Fla., and one of the senior authors on the work.

This work substantially advances the field of OCD genetics by identifying new OCD genetic risk loci and multiple credible candidate causal genes.

"These findings emphasize the need for a comprehensive exploration of the contribution of both common and rare genetic factors, as well as their interplay to OCD risk. Future studies will require ancestrally diverse samples to facilitate the discovery of additional OCD risk variants, understanding that these results may be useful for drug repurposing and yield new, more effective treatments," concludes Mattheisen, a faculty member at the faculties of Medicine and Computer Science at Dalhousie.

Media contact:

Alison Auld
Senior Research Reporter
Dalhousie University
Cell: 1-902-220-0491
Email: alison.auld@dal.ca