Genetic Counseling
Genetic testing for Huntington’s disease (HD) is a process that involves much more than just getting the results of a blood test. Many issues are at stake: financial, emotional, and social issues that involve not only the person seeking testing, but also his or her immediate family — and often the extended family as well. Before giving a blood sample, the person seeking a test deserves an opportunity to examine these issues thoroughly with people who are experienced with genetic testing for HD.
Genetic counseling is essential part of the HD testing process. Genetic counselors are non-directive and non-judgmental. Their job is to help people become better informed and to support independent decision-making around these very emotional and personal testing decisions. By discussing the issues involved with caring professionals (often together with other family members), people who are thinking about getting tested have the best opportunity to make informed decisions about whether testing is right for them at this time in their lives.
Genetic Testing
A genetic test, performed on a small amount of blood, can determine whether an individual is at risk to develop HD.
As a reminder, everyone has a specific piece of DNA that makes up the HD gene. This segment of DNA repeats in units of three, and is called a CAG trinucleotide repeat. Tri means three, and nucleotides are the building blocks of DNA. People who have inherited the gene change or mutation that causes HD have a CAG trinucleotide that repeats too many times. People who have CAG repeats in the normal range have not inherited HD.
There are four possible genetic test results
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Negative/Normal (26 or fewer CAG repeats)
- This individual will not develop HD.
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Intermediate (27-35 CAG repeats)
- This individual will not develop HD.
- There is a small possibility the CAG repeat may expand into the HD-causing range in the next generation.
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Reduced Penetrance (36-39 CAG repeats)
- This individual may or may not develop HD in their lifetime. If symptoms do develop, it is often later in life.
- About 1% of people who are tested fall into this area.
- Each child of a parent who has a CAG repeat in this range is at 50% risk for inheriting the HD mutation.
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Positive (40 or more CAG repeats)
- This individual will develop symptoms of HD over the course of a natural lifespan.
- The test can not tell if a person has symptoms of HD. There is a great deal of variability in the symptoms, their severity, and the age of onset, even within the same family.
- If an individual has 60 or more CAG repeats this would result in Juvenile onset HD.
The number of CAG repeats remains the same throughout your life. However, the number can be different from the number of CAG repeats in your affected parent. For example, your dad you could have 41 CAG repeats and you could have 44 CAG repeats.
If you inherit the HD-causing gene from your father, the number of CAG repeats may increase, expanding as the gene is passed from father to child. But if you inherit the gene from your mother, the number of CAG repeats usually remains stable. Regardless of the gender of your parent with HD, most often the number of CAG repeats is similar to that of your parent.
In general, the higher the number of CAG repeats, the earlier the onset of symptoms — although exceptions often occur. Other genetic and non-genetic variables also contribute to the complex issues that determine the age of onset and severity of symptoms. Even when two family members have the same number of CAG repeats – their age of onset and symptoms can be dramatically different.
More Information
To learn about the emotional implications of genetic testing our Impact of Test Results page. If you are considering pursuing presymptomatic testing, also known as predictive testing, visit our Presymptomatic Testing page.