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Genetic Testing for Huntington's Disease Disclaimer Statements and opinions in this guide are not necessarily those of the
Huntington's Disease Society of America, nor does HDSA promote,
endorse, or recommend any treatment or therapy mentioned herein.
The reader should consult a physician or other appropriate health care
professional concerning any advice, treatment or therapy set forth in
this book
Contents I--HUNTINGTON'S DISEASE AND THE HD GENE What is Huntington's Disease (HD)?
II--GENETIC TESTING
Huntington's Disease and the HD Gene Huntington's Disease (HD) is a hereditary degenerative brain disorder
which leads to both physical and mental disabilities. Symptoms usually
appear between the ages of 30 and 50, but the disease may strike
those as young as two or as old as 80. Initial symptoms may include
difficulty in concentration, depression or twitching, but each person
who has HD is affected differently and early symptoms vary from
person to person.
Testing is available to determine whether a person has the gene for HD. The decision about whether or not to be tested for HD is one which
requires careful consideration. This booklet will answer the most common
questions regarding testing.
How do I know if I am at risk for Huntington's Disease? Huntington's Disease is inherited in an autosomal dominant pattern.
This means that every child born to a person who has HD, regardless of
gender, has a 50 percent, or one-in-two, chance of inheriting the gene
that causes HD. Men and women are equally affected, so that either
parent may pass on the HD gene to any offspring.
person with HD will develop the disease and 50 percent will not?
for HD regardless of whether or not any brothers and sisters have it.
No, you cannot "catch" HD from another person. You must be born with
the gene for HD in order to one day develop this disorder.
though there is no known family history of the disorder. There are
many reasons why this may be. Perhaps the parent who had the
gene for HD died at an early age, before the symptoms appeared, or
maybe other family members had HD but were misdiagnosed as
having Parkinson's disease or another disorder with HD-like characteristics.
In cases like these, doctors may recommend genetic testing to confirm
or rule out a diagnosis of HD.
We inherit our genes from our parents and therefore we are born with
them. Some genes are "turned on" only at later stages of our lives.
A good example is the gene for baldness. Many people are born with
this gene but it is only in adulthood that it is activated and that people
begin to lose their hair. Also, people who have the same gene for
baldness may begin losing their hair at different ages.
Yes, the gene for HD was found in 1993.
Genes lie on these chromosomes and are made up of deoxyribonucleic acid,
or DNA. It is estimated that humans have about 100,000 genes which
influence development, growth and functioning of the body, and are passed
down from parents to children.
segment of DNA which contains a strip of repeated "trinucleotides" at one
end. Nucleotides are the building blocks of DNA. The trinucleotide repeat
involved in HD is cytosine-adenine-guanine, or CAG.
determined by the number of CAG (or trinucleotide) repeats contained within
their copies of the HD genes. People who have inherited the gene which
causes HD have a higher number of repeats, usually above 40. No one with
35 or fewer CAG repeats has been known to develop HD symptoms.
Individuals who have an HD gene with 36-39 repeats may or may not
develop the symptoms of HD within a normal life-span.
to child - so that the child may have slightly more or fewer repeats than the
parent. The tendency for an increased number of repeats is more common
when the gene is passed from father to child than when a mother transmits
the HD gene.
only one copy if the gene with a high number of repeats in order to develop
the disease.
determining whether a person will or will not develop HD. The number of
repeats is not known to be correlated with the course of the disease, and
a particular repeat number does not predict a specific age of onset. The
exception to this is that a very high number of repeats is indicative of the
juvenile form of HD with age of onset prior to age 20.
How can I be tested?
whether a person has the HD gene. This test cannot, however, predict
when symptoms will begin or the course of the disease.
who wish to undergo presymptomatic testing do so at an HD testing center.
The testing centers involve teams of professionals who are knowledgeable
about HD. A list of these testing centers is available from HDSA.
includes one session devoted to each of the following: genetic counseling; a
neurological exam; a psychological interview; discussion of the results; and
follow-up. The genetic test itself is a blood test.
testing understands the implications of this knowledge and is prepared to
receive the results. The neurologic exam will determine if any early symptoms
of HD are present. If the person is found to be symptomatic, he/she will be
offered the option of discontinuing the test procedure.
more and require repeat visits to a testing center. Exceptions are sometimes
made if a person lives far away from the center or in other circumstances.
Decisions concerning scheduling of appointments can only be made by those
who will perform the testing.
to all testing sessions. The partner should be a spouse, companion or a close
friend. It is not advisable to bring a sibling or another person who is at risk for HD.
awaiting test results, upon receiving the results, and thereafter.
the testing centers do encourage you to have available either a blood sample from
a family member who has HD or the results of his or her genetic test. This is to
ensure that the illness in the family is indeed HD. Some people are misdiagnosed
as having HD when, in fact, they have another neurological disorder. Therefore it
is important to establish that the affected family member has HD. This type of
testing is usually reimbursable by insurance.
and will perform the test free of charge for those involved in their studies, some
have sliding fee scales, and others charge anywhere from $600 to $1,500.
testing. However, before doing so, you should weigh the risks and benefits of
submitting a claim. Unfortunately, genetic discrimination does exist. Though
unusual, it is possible for an insurance company to deny health coverage or to
cancel an existing policy should a person be found to carry the HD gene. A positive
test result may also be considered a "pre-existing condition" which could make it
difficult to obtain future health insurance coverage. In a number of states, laws
do exist which prohibit genetic discrimination by insurers, and a similar federal
law is being considered.
seeking reimbursement from their insurance company. They then do so only if
they do not have the gene.
another family member is known to have the gene for HD. Test results cannot
determine if a person will develop HD if the number of CAG repeats falls into the
intermediate range. Also, positive test results cannot predict when the symptoms
of HD will begin.
with your written permission. On an initial call to a testing center, you should ask
how confidentiality is ensured.
I am beginning to show symptoms?
if you have symptoms of HD.
be taken lightly. Testing should never be mandated or forced upon an at-risk
individual. There are no "right" or "wrong" answers. Of course, everyone will
have his/her own circumstances to take into consideration. The following are
some of the issues to think about and/or discuss with those who are closest
to you:
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