Tourette’s syndrome and other tic disorders appear to be among the most heritable neuropsychiatric disorders, a new study suggests.
The study, published online June 17 in JAMA Psychiatry, showed that tic disorders had a heritability of approximately 77%.
The authors, led by David Mataix-Cols, PhD, from the Karolinska Institutet, Stockholm, Sweden, explain that while tic disorders are thought to be strongly familial, precise estimates of heritability are lacking.
For the current study, they estimated family clustering and heritability of tic disorders at the population level by using data from two Swedish population-based registers. They identified 4826 individuals diagnosed with Tourette’s syndrome or chronic tic disorders from 1969 through 2009.
Results showed that first-degree relatives of individuals with tic disorders had an 18-fold increased risk for Tourette’s syndrome or chronic tic disorders compared with controls. Second-degree relatives had a 4.5-fold increased risk and third-degree relatives had a 3-fold increase.
What Is Tourette Syndrome?
Tourette syndrome (TS) is a neurological disorder where a person has both motor and vocal tics. Doctors and scientists don’t know the exact cause of TS, but some research suggests that it occurs when there’s a problem with how nerves communicate in the brain. A disturbance in the balance in neurotransmitters - chemicals in the brain that carry nerve signals from cell to cell - may play a role in TS.
Tourette syndrome is not contagious. You can’t catch it from someone who has it. Studies suggest that TS is a genetic disorder, which means it’s the result of a change in genes that’s either inherited (passed on from parent to child) or happens during development in the womb.
As with other genetic disorders, someone may have a tendency to develop TS. But that doesn’t mean the person will definitely get the condition. Doctors and researchers are continually learning new information about TS and what might lead a person to develop it.
People with Tourette syndrome usually first notice symptoms while they’re kids or teens. TS affects people of all races and backgrounds, although more guys than girls have the condition.
Full siblings, parents, and children of individuals with Tourette’s syndrome or chronic tic disorder had similar risks. The results also indicate that risks in full siblings were higher than those in maternal half-siblings despite similar shared environmental exposures. First cousins had a 3-fold higher risk for Tourette’s syndrome or chronic tic disorders compared with control patients.
The researchers point out that although tic disorders are clearly more prevalent in males, their results suggest that the familial risk for tic disorders is similar in male and female probands regardless of the sex of the relative.
They suggest that when specific genes associated with tic disorders are identified, they will be associated with tic disorders in both sexes and will have similar effect sizes in males and females, but female sex may help protect against tic disorders in embryonic and fetal development.
The authors note that these results may not be generalizeable to non- European populations, who have a lower prevalence of tic disorders.
JAMA Psychiatry. Published online June 17, 2015.