Genetic Factors
Genetic studies, including twin studies, have proven that the overwhelming majority of cases of Tourette syndrome are inherited, although the exact mode of inheritance is not yet known. Tic disorders have long been thought to be inherited as an autosomal dominant gene, but recent research challenges the autosomal dominant hypothesis, and suggests an additive model involving multiple genes. According to Roger Freeman, MD, "Genes that cause tics have not yet been identified; it's very unlikely there's just one. Tics are very familial, but not in a Mendelian pattern (dominant, recessive, etc.). You can't 'inherit' the committee decision to define a problem in a specific way, so TS itself can't be inherited. It's a tic disorder that is inherited."
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 his/her 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 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. The finding of a chromosomal abnormality appears to apply to a very small minority of cases (1–2%). Studies to locate all of the genes implicated in Tourette's syndrome are ongoing.
Read more about this topic: Causes And Origins Of Tourette Syndrome
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