Huntington disease (HD) is caused by an abnormality
in the hungintin gene,
which is located on chromosome
4. Each of us has two copies of this gene. One copy is
inherited from our mother and the other copy is inherited
from our father. HD is caused by a change in one of these
two copies. A disease that is caused when only one of
the two copies of the gene is abnormal is called "autosomal
dominant."
As a result, most individuals with HD also have a parent
who had the abnormal gene and was diagnosed with the
disease. The children of an individual with HD are at
50% risk of inheriting the abnormal huntingtin gene
from their parent with the disease.
Most families with HD report a long history with this
disease, with each generation having some individuals
diagnosed with the disease.
What is the change in the huntingtin gene that causes
HD?
Within the huntingtin gene, there is a sequence of DNA
that has a variable number of CAG
repeats. This sequence of 3 nucleotides codes for an
amino
acid called glutamine. In a person with Huntington
disease, the change in the huntingtin gene that causes
the clinical symptoms of the disease is an increased
number of CAG nucleotides. The chart below explains
the interpretation of various numbers of CAG repeats
in the huntingtin gene.
Number of CAG Repeats Result
| 26 or below |
|
Normal |
| 27-35 |
|
Person normal but child may develop Huntington
disease |
| 36-39 |
|
Person more likely to develop Huntington disease |
| 40 and above |
|
Person will develop Huntington disease –
100% diagnosis |
In general, persons who have 40 or more CAG repeats
in their gene will develop Huntington disease unless
they die of other causes before disease signs and symptoms
appear. Persons having 36-39 CAG repeats may or may
not develop HD during their lifetime, but their children
are at greater risk for having the disease. Individuals
with 27-35 CAG repeats are not at risk for Huntington
disease, but their children might be at risk for developing
symptoms.
Individuals may be interested in storing some of their
genetic material (DNA) for future HD gene testing. Learn
More >
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