Facts you should know about the genetics of Tourette syndrome.
There are more than 4,000 inherited disorders known to geneticists, and among them is Tourette Syndrome (TS). As you read this brochure, please bear in mind that while it contains the most up-to-date information that we have about the inheritance of TS, it by no means represents the final answer.
Work now being carried out by TSA-funded genetic researchers holds real promise for providing us with a more complete picture of the inheritance and causes of TS.
The descriptions of TS symptoms and associated behaviors are given so that families will be as informed as possible; the discussions are not meant to suggest that carriers of the TS gene will be affected by any or all of them. While prediction of symptom severity cannot be made at this time, we emphasize that the majority of people with TS have mild cases.
The Tourette Syndrome Association hopes this brochure will be helpful to people with TS and their families as they strive to reach informed, personal choices about family planning.
What is Tourette Syndrome?
Tourette Syndrome is a neurological disorder characterized by involuntary movements (motor tics) and vocalizations (phonic or vocal tics). The disorder generally begins before the individual is 21 years old and is usually lifelong, with symptoms following a waxing and waning course. Both motor and vocal or phonic tics must be present for at least one year for the diagnosis of TS to be made. The severity of symptoms ranges from very mild and barely noticeable to severe and disabling. Fortunately, for most individuals TS symptoms are mild and do not require medical treatment. In addition, it occurs worldwide and affects all races.
How common is Tourette Syndrome?
Once thought to be rare, TS is now seen as relatively common among inherited disorders. In its complete form it may affect up to 1 person in every 2500, with perhaps 3 times that number showing partial expressions such as chronic tic disorder and OCD.
What other conditions may be related to Tourette Syndrome?
The manifestations of one of several of the following conditions may be very mild. However, for some individuals with TS, the related symptoms can be severe enough to interfere with everyday life.
1. Obsessions (recurrent or persistent thoughts; for example, preoccupations with dirt or disease) and compulsions (recurrent persistent actions or rituals; for example, excessive checking, counting, and cleaning) may be prominent, representing symptoms of obsessive-compulsive disorder (OCD). Recent evidence indicates that for some people, obsessive-compulsive behaviors may accompany Tourette Syndrome, and in some families with TS, those symptoms may be the sole manifestation of the disorder, particularly for females.
2. Attention deficit hyperactivity disorder -- ADHD (impaired attention, hyperactivity, and impulsivity) also may be associated with TS. Although the exact relationship between ADHD and TS remains to be determined, it appears that ADHD symptoms may co-occur with TS.
3. A variety of other behavioral problems may occur with TS, but their relationship with the ailment remains to be determined.
4. Chronic tic disorder (motor or phonic tics, but not both) and possibly transient tic disorder (tics present for less than one year) appear to be milder forms of Tourette Syndrome.
What is a genetic disorder?
It is a disease or condition that is caused by an alteration in the make-up of genes or chromosomes. Often there is a family history of a particular genetic disorder among close relatives. However, a genetic disorder also can appear in a family in which nobody else is known to have been affected. Occasionally, a careful examination of family members may reveal minor signs of a disorder that were not recognized before. For some genetic disorders, recent advances in research have changed the outlook from hopelessness to optimism. Much effort is currently being invested in the search for methods of early detection and treatment.
Is Tourette Syndrome a genetic disorder?
Based on the best available scientific information, it is now clear that TS is a genetic disorder. The vulnerability to TS is transmitted from one generation to another. When we speak of "vulnerability," we imply that the individual receives the genetic or constitutional basis for developing a tic disorder; the precise type of disorder and the severity of its expression may differ from one generation to another. Genetic vulnerability is transmitted from a gene-carrying mother or father, and can be passed on to either sons or daughters. When one parent has TS or is a TS gene carrier, there is about a 50% chance that each child that couple has may inherit the genetic vulnerability for TS. That mode of inheritance is called autosomal dominant.
What is Genetic Penetrance?
Not everyone who inherits genetic vulnerability for TS will develop symptoms of the disorder. A gene that does not always cause symptoms is called incomplete penetrance. Sometimes the degree of penetrance (expression of symptoms) is different in males and females. For example, in TS there is a 70% chance that female offspring who have inherited the gene will have at least one of the symptoms of TS. Male offspring who have inherited the gene are believed to have a 99% chance of showing some clinical expression of the gene.
Thus penetrance is sex-specific for TS, which means that the penetrance is higher in males than in females. Males are more likely to have some form of expression of the genetic vulnerability. There is a full 30% chance that female gene carriers will show no symptoms at all. For males, the figure is 1%.
What is Variable Expression?
The gene for TS causes different symptoms - variable expression - in different people. There is a range of forms the symptoms may take, including full-blown TS, chronic tic disorder, OCD, and possibly transient tic disorder or ADHD. For example, some individuals have TS (or chronic tics) and OCD together; others may have just one condition. There are also differences between the sexes in the expression of the TS gene. Males are more likely to have TS or chronic tics; females are more likely to have OCD. However, both male and female offspring may express any combination of symptoms or any degree of severity. In addition, it is not possible to predict the severity of symptoms in a child based on the severity of symptoms in the affected parent. The severity of the disorder is also highly variable. Most individuals who inherit the TS genetic vulnerability have very mild conditions for which they do not seek medical attention.
Do non-genetic factors influence Tourette Syndrome?
Although the vast majority of cases of TS occur on a hereditary basis, individual variations in the nature, course, and degree of severity of TS probably cannot be fully explained by genetic factors alone. It appears that non-genetic factors may be responsible for modifying the expression of the ailment.
Genetic Counseling for Tourette Syndrome
With the recognition that TS is a genetic disorder, families naturally have become interested in seeking genetic counseling. Genetic counseling should be provided by knowledgeable physicians or counselors who can impart accurate information about the mode of inheritance and work with families to help them deal with the complex findings which may be aroused.
Genetic testing and research in Tourette Syndrome
TSA-funded researchers are actively engaged in searching for the chromosomal location of the TS gene. For some genetic disorders (e.g., Huntingdon's disease and dystonia), scientists have already discovered a genetic marker on a chromosone very near the genes for those disorders. A genetic marker for TS will help to determine which people carry the TS gene. The next steps, would be to locate the abnormal gene itself, determine what went wrong, and work toward developing more specific medications and ultimately, a cure.
At present, there is no genetic or biochemical test to determine if a person with TS or an unaffected individual carries the gene. Nor is there a prenatal test to determine the potential range of symptom expression or severity of TS. Once the TS gene is located, scientists will work to develop such tests.
The Tourette Syndrome Association is a national voluntary health organization dedicated to identifying the cause, finding the cure, and controlling the effects of TS. Our programs of research, professional and public education, and individual and family services are made possible through the generosity of our donors.
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|Publication:||Pamphlet by: Tourette Syndrome Association|
|Date:||Sep 23, 1991|
|Next Article:||Questions and answers on Tourette syndrome.|