Causes and origins of Tourette syndrome
Causes and origins of Tourette syndrome have not been fully elucidated. Tourette syndrome (abbreviated as Tourette's or TS) is an inherited neurodevelopmental disorder that begins in childhood or adolescence, characterized by the presence of multiple motor tics and at least one phonic tic, which characteristically wax and wane. Tourette's syndrome occurs along a spectrum of tic disorders, which includes transient tics and chronic tics. The exact cause of Tourette's is unknown, but it is well established that both genetic and environmental factors are involved. The overwhelming majority of cases of Tourette's are inherited, although the exact mode of inheritance is not yet known, and no gene has been identified. Tics are believed to result from dysfunction in the thalamus, basal ganglia, and frontal cortex of the brain, involving abnormal activity of the brain chemical, or neurotransmitter, dopamine. In addition to dopamine, multiple neurotransmitters, like serotonin, GABA, glutamate, and histamine (H3-receptor), are involved. Non-genetic factors—while not causing Tourette's—can influence the severity of the disorder. Some forms of Tourette's may be genetically linked to obsessive-compulsive disorder (OCD), while the relationship between Tourette's and attention-deficit hyperactivity disorder (ADHD) is not yet fully understood. Genetic factors The exact cause of Tourette's is unknown, but it is well established that both genetic and environmental factors are involved. Genetic epidemiology studies have shown that Tourette's is highly heritable, and 10 to 100 times more likely to be found among close family members than in the general population. The exact mode of inheritance is not known; no single gene has been identified, and hundreds of genes are likely involved. Genome-wide association studies were published in 2013 and 2015 in which no finding reached a threshold for significance. Twin studies show that 50 to 77% of identical twins share a TS diagnosis, while only 10 to 23% of fraternal twins do. But not everyone who inherits the genetic vulnerability will show symptoms. A few rare highly penetrant genetic mutations have been found that explain only a small number of cases in single families (the SLITRK1, HDC, and CNTNAP2 genes). In some cases, tics may not be inherited; these cases are identified as "sporadic" Tourette syndrome (also known as tourettism) because a genetic link is missing. A person with Tourette syndrome has about a 50% chance of passing the gene(s) to one of their children. Gender appears to have a role in the expression of the genetic vulnerability, with males more likely to express tics than females. Tourette syndrome is a condition of incomplete penetrance, meaning not everyone who inherits the genetic vulnerability will show symptoms. Tourette's also shows variable expression—even family members with the same genetic makeup may show different levels of symptom severity. The gene(s) may express as Tourette syndrome, as a milder tic disorder (transient or chronic tics), or as obsessive compulsive symptoms with no tics at all. Only a minority of the children who inherit the gene(s) will have symptoms severe enough to require medical attention. There is currently no way to predict the symptoms