The Genetics
of Huntington’s Disease
The genetic science that
explains Huntington’s disease may seem a bit complicated but it helps us to
understand why one person develops HD while someone else in the same family
does not. Although it is clear from family
histories that HD is an inherited disease, it wasn't until scientists
discovered the gene that causes it.
Everyone has the Huntington gene normal for the gene to have several
sequences of bases called CAG repeats (cytosine, adenine and guanine). A certain mutation can cause the gene to
expand with abnormal numbers of CAG repeats.
As the number of repeats increases so does the likelihood of HD. DNA
testing is used to determine the number of CAG repeats on the Huntington
gene. Generally, if the number of
repeats is less than 35 a person will not get HD. There is a borderline area around 36 to 41 repeats where a person
may or may not get HD; if the number of repeats is greater than 41, the person
will sooner or later exhibit symptoms of HD. Children with HD usually have more
than 75 repeats. The more repeats, the
earlier the symptoms appear. Scientists
have yet to discover how the increased number of CAG repeats actually causes
Huntington’s disease.
If one parent has a mutated
Huntington gene, then children of that parent have a 50% chance of inheriting
the disease. About 1 in 10,000 people
have the disease. But because of the
inherited nature of the disease and its profound effect on families, it is
thought to affect the lives of about 1 in 1,000 people.
Although genetic studies make
clear who will get Huntington’s disease, other unknown factors influence the
form the disease will take. An
Australian researcher, Dr. Edmond Chiu, and his colleagues decided that environment
makes a difference. They studied twins
with HD and found that one twin developed physical disabilities while the other
twin had more serious thinking and reasoning problems with greater overall
deterioration. The reasons for the
differences remain a mystery but continued research offers possibilities for
greater discovery and help for people with HD.
September 1999
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