The Genetics of Tourette Syndrome
by Adam T. McGruther
In 1825, Georges Gilles de la Tourette, a French neurologist, diagnosed
a woman with the first case of what would come to be known as Tourette Syndrome
(TS).1 She exhibited symptoms such as facial tics, sudden outbursts of obscene
language, and the repetition of things that were said to her. Since then,
we have learned that TS is a genetically inherited disorder that may affect
as many as 1 in 100 people, making it one of the most common genetic disorders
known to man.1 In order to better explain TS, a case study will be utilized
to give the reader a clear example of the implications of TS within a family.
While much is known about the symptoms of TS, the mode of inheritance is
still very complicated model. TS is believed to be an autosomal dominant
disorder, meaning that the gene (or genes) that causes the disorder is located
on one of the chromosomes numbered 1-22, and not on one of the sex chromosomes
(X,Y).1 The reason that genes was placed in parenthesis is that recent studies
have indicated that the disorder may not be limited to one gene, but have
multifactoral implications.2 Since TS is a dominant disorder, those individuals
who are homozygous for the disorder (TT, this refers to the presence of
the gene(s) for TS on two paired chromosomes) are affected, as well as those
who are heterozygous (Tt). However, the expressiveness of the gene(s), and
hence the disorder is variable. Of the individuals who have the Tt genotype,
only 2.2% of males and 0.3% of females are affected with the disorder.2
These numbers, however, are subject to change based on the interpretation
of what qualifies as TS. Recent literature has expanded the scope of TS
to include the following symptoms:2
motor and/or vocal tics
ADD (attention deficit disorder)
Motor tics are generally the most common symptom associated with TS. In
individuals with TS, multiple motor and vocal tics are usually observed,
as well as some of the other behavioral symptoms mentioned. The tics can
be classified as:3
motor: eye blinking, shoulder shrugging, head jerking, facial grimacing
vocal: throat clearing, yelping, tongue clicking, sniffing
motor: jumping, twirling about, touching others, self-injurious behaviors
vocal: uttering words or phrases out of context (obscenities or racial slurs)
It is important to realize, however, while as many in 1 in 100 people may
have TS (TT or Tt), very few actually manifest the complex tics or multiple
As mentioned before, the inheritance model is very complicated, and is best
explained using a case study. Since the disorder is dominant, it tends to
affect entire familial lines, and can be seen across the generations. Consider
Mike and Carrie Riley; happy parents of six children, but confused by their
childrenís peculiar habits. Two of their boys, Greg and Ken, both
tend to repeat what is said to them, and both have been in trouble in school
for yelling out obscenities in class. Two of their other children, Alicia
and Kate, both have problems with constant fidgeting, which disrupts their
ability to sit still in school. For years Mike and Carrie had attributed
their boy's behavior to childhood mischievousness, and dismissed their girlís
actions since both Mike and his mother, Rita, exhibited the same behavior.
Mike and Carrieís other two children, Joe and Cindy, have shown no
signs of any behavior similar to their siblings.
Mike's sister, Suzanne, and her husband, Adam, also have a couple of children
with peculiar habits. Their son, Steve, is dyslexic, and is very poor at
math. One of their daughters, Edie, constantly takes showers or is washing
her hands. Their other child, Jenny, seems to exhibit no strange behavior
or learning disabilities.
While all of these cases seem to be unrelated, when one considers the broad
definition of TS, a common thread can be seen throughout the Riley family.
Suppose Rita is TT, meaning that she has full blown Tourette syndrome. If
she married a man who was Tt, a carrier with no symptoms, any children they
would have would be either TT or Tt. Mike, who has TS, is probably TT, and
his sister, Suzanne, is probably Tt, but is not affected. Supposing that
Adam is tt, he does not have TS and is not a carrier, Steve and Edie are
probably Tt and are affected with TS. It is probable that their third child,
Jenny, is also tt and therefore will not have TS. As for Mikeís children,
since Carrie is probably tt, all of their children are Tt. However, since
TS is expressed variably, not all of the children will necessarily develop
the disorder (Joe and Cindy), and those that do have varying degrees of
severity in their symptoms.
Obviously there is a great deal of uncertainty in trying to diagnose Tourette
syndrome. Many of it's symptoms are common to one or more other neurological
disorders. Unfortunately, however, until the gene(s) responsible for Tourette
syndrome is found, and a genetic test developed, much of the guesswork associated
with this disorder will continue.
1Merz, Beverly, Journal of the American Medical Association, vol. 260, no.
18, pg 2619.
2Walkup, J.T., LaBuda, M.C., Singer, H.S., Brown, J., Riddle, M.A., Hurko,
O., American Journal of Human Genetics, 59:684-693, 1996.
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