Tic disorders are among the most heritable neuropsychiatric conditions, concluded researchers who explored heritability of tic disorders, such as Tourette syndrome, in Swedish families.
While long thought to be of strong familial and heritable nature, research on such attributes of tic disorders has been lacking. Only recently are more efforts being made to look closely at genetics.
In this study, David Mataix-Cols, PhD, of the Karolinska Institutet in Stockholm, and colleagues estimated heritability based on two Swedish population-based registers, in which they identified 4,826 people diagnosed with Tourette syndrome or another chronic tic disorder from 1969 through 2009. Of those diagnosed with a tic disorder, 72.8% had at least one lifetime psychiatric comorbidity.
Using structural equation modeling, the researchers compared risk of Tourette syndrome or chronic tic disorders in biological relatives of probands to relatives of those unaffected. They found that risk for tic disorders increased proportionally to the degree of genetic relatedness, with risk for first-degree relatives significantly higher than second-degree relatives and third-degree relatives.
Full siblings, parents, and children of people with Tourette syndrome or chronic tic disorder, all with 50% genetic similarity, all had comparable risks (odds ratio [OR], 18.69; 95% CI, 14.53-24.05), compared to second- (OR, 4.58; 95% CI, 3.22-6.52) and third-degree relatives (OR, 3.07; 95% CI, 2.08-4.51). The risks for full siblings (50% genetic similarity; OR, 17.68; 95% CI, 12.90-24.23) were significantly higher than risks for maternal half siblings (25% genetic similarity; OR, 4.41; 95% CI, 2.24-8.67) despite similar environmental exposure. Overall, the risk of inheriting a tic disorder was approximately 77%, with no differences in risk observed between male and female individuals.