How Huntington’s Disease is Diagnosed
Everyone is born with the huntingtin gene; however, some people have an abnormal copy of the gene that causes the huntingtin gene to expand and cause the symptoms of the disease. Someone can only have an abnormal copy of the gene if one of their parents has an abnormal copy. If a parent has the abnormal gene, each of their children has a 50% chance of having the abnormal gene. Every person who carries the abnormal gene will eventually develop symptoms if they live long enough.
In 1983 scientists discovered a gene marker linked to HD. In 1993 they isolated the gene on chromosome 4 and identified that people with HD have a mutant expanded form of the gene. A blood test was developed to identify if a person has abnormal gene. This test is highly accurate, but it can’t determine when the symptoms of the disease will start or the severity of the symptoms. People can test positive for the abnormal HD gene and not have any symptoms for many years.
If you use a blood test to figure out if you have the abnormal gene, it’s called predictive testing. Typically, only adults over the age of 18 are tested for HD. The rare exception is when medical professionals believe a youth has the symptoms of Juvenile Onset HD. If someone waits until they or someone they know believes they have symptoms of HD, they will see a doctor to confirm the diagnosis.
When blood samples are analyzed for Huntington’s disease, they are looking to see how many times the “letters” CAG repeat on the huntingtin gene. Here is breakdown of what they are looking for in the lab:
Less than 27 CAG repeats – Negative, will not develop HD and children are not at risk
27-35 CAG repeats – will not develop HD, but their children have a small risk
36-39 CAG repeats – Positive, may develop symptoms at an older age. Their children have a 50% chance of having HD.
40+ CAG repeats – Positive, person will develop symptoms. Their children have a 50% chance of having HD.
Should you get tested?
Only you can make the decision about whether you want to be tested. Family, partners, and others may pressure you one way or the other, but it remains your decision.
HD Reach offers an At-Risk Decision Path and Anonymous Genetic Testing Path program. These programs will help guide you through the decision-making process, build resiliency and develop coping skills. The Decision Path is designed to adjust to your needs, and you can always choose not to take the actual test.
The Genetic Testing Path offered by HD Reach is anonymous. It differs from most other programs because HD Reach is not part of a medical center and we do not accept insurance. All records are kept in a secure private database and our client’s control who sees the results. The results are provided in-person or via teletherapy by two trained counselors. If you do test positive for the abnormal gene, HD Reach will help you figure out the many steps you can take to prepare for going through the stages of the disease.
If you do decide to get tested, you need to think about things like life insurance. If you are going to purchase life insurance, it should be done before you begin a testing process.
There are options available to reduce or eliminate the risk of having a child with the abnormal gene. Options include:
- Egg/sperm/embryo donation
- Prenatal testing – this option is usually only done when the couple is certain they will terminate the pregnancy if the genetic test is positive
- Pre-implantation Genetic Diagnosis (PGD) – this procedure involves fertilizing the egg and the sperm in a laboratory and then testing the embryos to see if they carry the abnormal gene. Only the embryos with the normal gene are implanted in the woman’s womb.
All of these options can be discussed with a genetic counselor.