For reasons that only became clear after the gene responsible for HD was discovered in 1993, individuals affected with a very early onset of HD are far more likely to have an affected father than an affected mother. It
very unlikely for HD to appear in a child whose parent was not also affected
with HD. If this situation appears to be present, the physician should
consider diagnosis other than HD.
Occasionally an HD-affected child without an affected parent can be explained by the early death of a parent (before the parent's symptoms were evident), misdiagnosis or lack of diagnosis in a parent who was affected, non-paternity (a biological father who is not the same as the apparent father), onset of symptoms in the child before the parent's onset, or adoption.
Documenting the diagnosis of HD in other relatives can be helpful to the physician
as the child is evaluated for HD. A parent who suspects that
his or her child has juvenile HD can help the physician by assembling infor-mation
about the family history.
If the family history is missing because the child was adopted, it may still be
possible to obtain the missing information if it is important to the child's
diagnosis. Adoption agencies and county or regional social services
given an understanding of the serious nature of HD and its hereditary pattern,
may be able to contact the birth parents to obtain more information. Similarly,
a mother or family who becomes aware of the family history of HD should
be honest and open with the adoption agency, so the adopting parents are
aware of the child's genetic risks and are able to plan appropriately.
TESTING IN CHILDREN
The ability to detect changes in the HD gene itself has made confirmation of the
diagnosis of HD much simpler. The HD gene can be isolated from a
blood sample, and examined chemically; the abnormality in the gene that
causes HD is called a "CAG repeat expansion." The HD gene normally
has a variable number of "CAG repeats" - any number up to 35 repeats is normal.
There is a relationship between the repeat numbers and the age that HD symptoms
, so that higher repeat numbers are associated with younger ages of onset.
Most adults with HD have between 40-50 CAG repeats in their abnormal HD gene.
Usually, juvenile HD is associated with CAG repeat numbers of 50 or higher,
although it is not possible to define rigid boundaries. Very young age
of onset are associated with very high CAG repeat numbers; children with
HD onset at age 2-3 years and with over 100 CAG repeats have been
The gene test is close to 100% accurate. If the tests show two normal HD
genes, the child will never develop HD and is not at-risk for passing HD
on to his children.
If the test shows an abnormal HD gene, the child will someday develop HD. The gene test, however, cannot predict when a particular person's symptoms
Occasionally, individuals will have a CAG repeat length in the 36-39 range, which
may or may not be associated with the development of HD symptoms during
a normal life span. Results in this "intermediate range" are
not usually a factor when testing children for possible symptomatic HD.
And rarely, very high CAG repeat numbers (over 100 CAG repeats) are not detected
by the standard gene test. If HD is strongly suspected in a very young
child who has a normal gene test results, the physician may want to contact
the laboratory or a genetic counselor to discuss the possibility of a special
analysis to look for very large CAG repeat numbers.
DIAGNOSTIC GENETIC TESTING IN CHILDREN continued
The potential risks of testing a child for the HD gene inappropriately or prematurely cannot be emphasized enough. Individuals
who develop HD have
the CAG repeat expansion in one of their HD genes from the moment of conception - years or decades before their symptoms begin.
There are two ways in which a premature gene test can be misleading or damaging
to a child's care. First, an abnormal gene test result may incorrectly
be assumed to "explain" a child's symptoms, when in fact the symptoms are
not clearly related to HD. For example, an adult whose gene
test was felt to "explain" his blurry
vision and headaches was incorrectly diagnosed as having HD and, as
a result, the diagnosis of his pituitary tumor was delayed.
Secondly, it is possible that a gene test will show a small CAG repeat expansion,
one that is likely to be associated with adult-onset HD but not juvenile
onset HD. This is equivalent to a predictive gene test and
does not help to explain the child's current symptoms.
Due to the very sensitive nature of the gene test results, it is important for
counseling to occur before the results are given, so there are no misunderstandings
about their significance. If the physician is unable or does not
have the time to explain the gene test in details, a genetic counselor may be
asked to help with this part of the process.
TESTING CHILDREN WHO
DO NOT HAVE SYMPTOMS
When one child has been diagnosed with HD, parents may want to have their other
children tested as well. Testing of a person who does not have
symptoms of HD is called predictive testing, in order to distinguish it
from diagnostic testing for a person who has symptoms suggestive of HD.
Although, at first thought, it may seem re-assuring for parents to find out that
other children do not carry the HD gene, it is very important to
consider the complex potential effects of
the tests results on the entire family, as well as on the individual or
individuals being tested.
The risks of premature genetic diagnosis of HD have already been discussed and
genetic testing experts believe that predictive tests should be reserved
for individuals who are able to understand the potential risks and benefits
of the test and who are able to give informed consent.
Experience in the United States has shown that
most adults at-risk for HD do not choose to undergo predictive
genetic testing, so a parent who requests predictive
testing for a child is most likely doing something that the child
would not want if he or she were able to make the choice.
In addition, at this time, there is no medical advantage to knowing that someone
carries the HD gene - treatments that prevent or delay the disease have
not been developed yet.
For all these reasons, most genetic pro-fessionals in North American decline parental
requests for predictive tests on their asymptomatic children. Occasional
might be made for adolescents in adult situations, such as an "emancipated
minor" or a married teenager.
In the United States, potential adoption is not usually felt to be an appropriate
indication for HD predictive testing, because of the potential for social,
financial, educational, insurance and employment discrimination based on
the test results and the lack of medical treatment or care to balance the
potential social harms.
Other countries may have different practices.