How Do Genes Contribute to Obsessive-Compulsive Disorder?

What you need to know

• Obsessive-compulsive disorder (OCD) is influenced by many genes, each having a small effect
• OCD symptoms exist on a spectrum in the general population, from no symptoms to severe
• There are genetic links between OCD and related disorders like anxiety and tic disorders

Understanding obsessive-compulsive disorder

Obsessive-compulsive disorder (OCD) is a mental health condition that affects 1-3% of people worldwide. People with OCD experience unwanted, intrusive thoughts (obsessions) and feel compelled to perform repetitive behaviors (compulsions) to relieve anxiety.

While OCD can severely impact daily functioning, milder obsessive-compulsive traits are actually quite common. Many people occasionally double-check that they’ve locked the door or worry about germs without it significantly affecting their lives. This suggests OCD may exist on a spectrum in the general population.

Recent genetic research supports this “dimensional” view of OCD. Rather than being a distinct condition that someone either has or doesn’t have, OCD-like traits may occur to varying degrees in everyone. People diagnosed with OCD likely fall on the extreme end of this spectrum.

The genetics of OCD

Scientists have long recognized that OCD tends to run in families, suggesting a genetic component. Twin studies estimate that genetic factors account for 40-50% of the risk for OCD. But pinpointing the exact genes involved has proven challenging.

Unlike some medical conditions caused by a single gene, OCD appears to be influenced by hundreds or even thousands of genes. Each gene likely has only a tiny effect on OCD risk. It’s the combined impact of many genetic variants that shapes a person’s likelihood of developing OCD.

This complex genetic architecture is known as polygenic inheritance. It helps explain why OCD doesn’t follow simple inheritance patterns in families. A child may inherit some OCD risk genes from their parents but not others. Environmental factors also play an important role in determining whether someone develops OCD.

Obsessive-compulsive traits in the general population

An important recent insight is that the same genes influencing diagnosed OCD also seem to impact milder obsessive-compulsive traits in the general population.

Researchers have conducted large genetic studies looking at obsessive-compulsive symptoms reported by people without an OCD diagnosis. They found significant genetic overlap between these everyday OC traits and clinical OCD. This supports the idea that OCD exists on a spectrum, with diagnosed cases representing the extreme end.

For example, one study examined genetic risk scores for OCD in the general population. People with higher genetic risk scores tended to report more obsessive-compulsive symptoms, even if they didn’t meet criteria for an OCD diagnosis. This suggests the same genes influence both clinical OCD and milder OC traits.

OCD symptom dimensions

OCD symptoms can manifest in different ways. Common subtypes include:

• Contamination fears and cleaning rituals
• Concerns about harm and checking behaviors
• Need for symmetry or exactness
• Unacceptable/taboo thoughts

Researchers have investigated whether these symptom dimensions have distinct genetic influences. Results have been mixed. Some studies find evidence for dimension-specific genetic factors, while others suggest a common genetic vulnerability underlying all OCD symptoms.

It’s likely that both shared and unique genetic influences are at play. There may be some genes that increase risk for OCD in general, as well as genes that shape how symptoms manifest. More research is needed to clarify the genetic basis of different OCD presentations.

Genetic links with other disorders

OCD often occurs alongside other mental health conditions. Common co-occurring disorders include anxiety disorders, depression, and tic disorders like Tourette syndrome.

Genetic studies have found significant genetic correlations between OCD and several of these conditions. This means many of the same genetic variants that increase risk for OCD also influence risk for related disorders.

Some of the strongest genetic links are between:

• OCD and anorexia nervosa (genetic correlation ~0.53)
• OCD and Tourette syndrome (genetic correlation ~0.4)

There are also genetic correlations between OCD and conditions like depression, autism spectrum disorders, and schizophrenia. This suggests partially overlapping genetic influences across a range of psychiatric disorders.

Interestingly, OCD shows a small negative genetic correlation with attention-deficit/hyperactivity disorder (ADHD). This means some genetic variants may increase risk for OCD while decreasing risk for ADHD, or vice versa.

These findings highlight the complexity of psychiatric genetics. Rather than being completely distinct conditions, many mental health disorders appear to have overlapping genetic risk factors.

Implications for understanding OCD

This genetic research has several important implications for how we conceptualize OCD:

1. OCD likely exists on a spectrum in the population, rather than being a discrete condition. The same genes influence both clinical OCD and milder obsessive-compulsive traits.

2. There is no single “OCD gene.” Rather, hundreds or thousands of genes each contribute a small amount to OCD risk.

3. Genetic risk for OCD overlaps with risk for several other psychiatric conditions. This may help explain why OCD often co-occurs with disorders like depression and anxiety.

4. Both common genetic variations in the population and rare genetic mutations may play a role in OCD. Common variants likely account for most of the genetic risk.

5. Environmental factors interact with genetic predisposition to determine whether someone develops OCD. Genes are not destiny.

Looking ahead

While we’ve learned a lot about the genetics of OCD, many questions remain. Larger studies are needed to identify more of the specific genes involved. Researchers are also working to better understand how genetic risk translates into OCD symptoms at a biological level.

In the future, genetic information may help guide OCD treatment. For example, genetic testing might predict which patients are likely to respond best to different medications or therapies. However, given the complexity of OCD genetics, such applications are still a long way off.

For now, this research reinforces that OCD has complex biological underpinnings. People with OCD shouldn’t blame themselves for their symptoms. At the same time, genes aren’t everything - psychological and social factors also play key roles in OCD. Effective treatments like cognitive-behavioral therapy can help people manage OCD regardless of their genetic risk.

Conclusions

• OCD is influenced by many genes, each having a small effect
• The same genes impact both clinical OCD and milder OC traits in the general population
• There are shared genetic risk factors across OCD and several related psychiatric disorders
• Both genes and environment play important roles in determining who develops OCD