Analysis of a large general population-based sample of Swedish twins revealed that genetic risk factors for psychiatric disorders — including autism, ADHD, tic disorders, intellectual disability, anxiety disorders and major depressive disorder — were associated with milder traits of these disorders.
“Psychiatric disorders are impairing and associated with genetic factors,” Mark J. Taylor, PhD, from the department of medical epidemiology and biostatistics at Karolinska Institute, Stockholm, and colleagues wrote. “Although clinical practice and most genetic studies follow a case-control conceptualization of these disorders, subclinical traits of these disorders are common among unaffected individuals and are as heritable as the disorders themselves based on twin methods.”
Combining a novel twin analytic approach and polygenic risk score analyses, researchers examined whether genetic risk factors linked to psychiatric disorders were also linked to continuous variation in milder population traits of the same disorders. Using questionnaires, they evaluated traits of ASD, ADHD, learning difficulties, tic disorders, OCD, anxiety, MDD, mania and psychotic experiences in a large Swedish twin sample.
The study included genetic data from 13,412 people and phenotype data from 13,923 twin pairs at age 9 years, 5,165 pairs at age 15 years and 4,273 pairs at age 18 years.
The disorders analyzed using novel twin models showed modest to strong genetic correlations with corresponding traits, with point estimates ranging from 0.31 to 0.69. Polygenic risk scores for psychiatric disorders were linked to related subclinical traits for ASD, ADHD, tic disorders, OCD, anxiety, MDD and schizophrenia, but not bipolar disorder, according to Taylor and colleagues.
After analyzing polygenic risk scores for quantitative traits tied to psychiatric diagnoses, the researchers found that the polygenic risk scores for depressive symptoms were linked to an increased risk for MDD diagnosis in the sample (OR = 1.16; 95% CI, 1.02-1.32).
Although most of the confidence intervals around the genetic connections overlapped, Taylor and colleagues observed greater genetic correlations for anxiety and MDD at age 18 years. Furthermore, the report revealed that shared genetic factors accounted for 60% to 100% of the phenotypic covariance between each trait and psychiatric diagnosis.
“These converging results using two contemporary methods revealed that many psychiatric disorders may share genetic risks with continuous symptom dimensions in the population,” the investigators wrote. “Nonetheless, the sample was young. Although many disorders develop during childhood and adolescence, disorders such as schizophrenia, [bipolar disorder], and severe depression become more common with age. Studies of older individuals are thus needed as a next step.” – by Savannah Demko
Disclosure: Taylor reports no relevant financial disclosures. Please see the study for all other authors’ relevant financial disclosures.