OCD Genetics: Where we are and where we’re going

Obsessive Compulsive Disorder and Related disorders (OCDR) are viewed as genetically complex, multifactorial disorders. What we mean by ‘complex’ is that it is unlikely that a single gene predisposes or directly causes the disorder. Rather, multiple genes may contribute to disease risk. In addition, OCDR is ‘multifactorial’ meaning that genes alone do not cause disease, but rather a combination of genes and environmental factors most likely underlie causation.

Since shared environment and random factors can both contribute to disease risk, studies have been  designed to specifically tease out those risks that are genetically-based. With our growing ability to examine DNA sequencing, we are able to look at genetic material that is shared in families of affected vs non-affected relatives. Historically, twin and family studies have been used to look at patterns of disease occurrence in related individuals to determine if there are shared genetic risk factors.
Twin studies examine concordance of illness (i.e., both twins with diagnosis or both twins without diagnosis)  and discordance of illness (one twin with diagnosis and the other without). Monozygotic (MZ) twins share almost all their genes and dizygotic (DZ) twins share only half their genes on average, similar to any pair of siblings. As a result, rates of co-occurrence of disease in MZ twins compared to DZ twins can be used to estimate genetic heritability. Almost all studies to date have reported MZ concordance rates of OCD around .50 and DZ rates of OCD around .20 (Browne et al., 2014), suggesting that OCD is partly heritable.
Family studies go beyond twins studies. They estimate the likelihood that a disease or trait present in one family member will occur again in other family members. This estimate is often referred to as the ‘recurrence risk’ (RR). In the case of OCD, first degree relatives (i.e., biological parents, full siblings, and offspring) share on average 50% of their genetic material and the estimated recurrence risk is between 10% – 20%. This means that there is a slight likelihood that an OCD trait or disorder present in one family member will occur again in other family members in the same or subsequent generations.
Future Directions: The need for larger studies
Because of the complexity of OCDR and its relationship to a number of other disorders such as Tourette Syndrome, it may be particularly important to gather research information about a number of different variables (e.g., specific symptoms, symptom severity, onset, co-morbid disorders, and demographic variables like sex and race/ethnicity).  Looking at these factors will help us better understand this complex disorder. With this in mind, even bigger samples of OCDR research participants are needed. Thus we are happy to announce that we, at the Keck School of Medicine at USC, have received NIMH funding to collect information from 5,000 individuals with OCD, Hoarding Disorder, Body Dysmorphic Disorder, or other OCDR. The goal is to better understand how genes contribute to this group of disorders. Participants can join the research via the internet, and then follow up face to face with research clinicians. Using this innovative research design, we hope to reach as many individuals as we can, even those who do not typically come to academic research centers for care.
A website for clinicians and patients has been established for the USC OCD research study (keck.usc.edu/gpc-ocd).
References and Helpful Resources
Browne, H.A., Gair, S.L., Scharf, J.M., & Grice, D.E. (2014). Genetics of obsessive-compulsive disorder and related disorders. Psychiatric Clinics of North America, 37, 319-335.
Sobell, J., Pato, M.T., Pato, C.N., Knowles, J.A. (in press). OCD Genetics: Current and Future Directions. In FOCUS – OCD and Related disorders: Diagnosis to Treatment.
International OCD Foundation website: www.iocdf.org