Causes and origins of Tourette syndrome

Causes of Tourette's 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,

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.

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, Genome wide association studies were published in 2013 and 2015 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. 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 a child may display, even if the gene(s) are inherited. Recent research suggests that a small number of Tourette syndrome cases may be caused by a defect on chromosome 13 of gene SLITRK1. Some cases of tourettism (tics due to reasons other than inherited Tourette's syndrome) can be caused by mutation. in the cortical and subcortical regions, the thalamus, basal ganglia, and frontal cortex of the brain. Research presents considerable evidence that abnormal activity of the brain chemical, or neurotransmitter, dopamine is involved. Dopamine excess or supersensitivity of the postsynaptic dopamine receptors may be an underlying mechanism of Tourette syndrome. Multiple neurotransmitters, like histamine (H3R), dopamine, serotonin, GABA and glutamate are involved in the etiology. The striatum is the main input nucleus of the basal ganglia circuit in the disorder, which is linked to the involvement of the histaminergic H3 receptor.

Psychosocial or other non genetic factors—while not causing Tourette's—can affect the severity of TS in vulnerable individuals and influence the expression of the inherited genes. PANDAS and the newer PANS (pediatric acute onset neuropsychiatric syndrome) hypotheses are the focus of clinical and laboratory research, but remain unproven. There is also a broader hypothesis that links immune system abnormalities and immune dysregulation with TS.

Some forms of OCD may be genetically linked to Tourette's, although the genetic factors in OCD with and without tics may differ. A genetic link between autism and Tourette's has not been established as of 2017.