Huntington's Disease Genetic Testing Basics
What is Huntington's disease?
Huntington's disease is an inherited disease that affects nerve cells in parts of the brain. A progressive disease (meaning it continues to get worse), Huntington's leads to mental deterioration and the loss of control over the body's muscle movements.
Symptoms usually start in the late 30's or early 40's. Some people have symptoms earlier or later. Once symptoms begin, they continue to get worse. Ultimately, the disease leads to premature death, typically 15 to 20 years after the first symptoms appear.
Different symptoms appear at different stages of the disease:
Early Stages — mental difficulties, poor coordination, possibly some involuntary movements.
Later Stages — a movement disorder called chorea (a "dancing" type of movement that involves uncontrollable jerking of the arms and legs). Muscles, such as those that control speech and swallowing, may become impaired.
Long Term — mental abilities (memory, comprehension, concentration, etc.) gradually worsen. Individuals may have outbursts of aggressive behavior or other psychiatric symptoms, including depression and paranoia. Worsening symptoms render Huntington's patients less able to perform jobs and regular daily activities.
How common is Huntington's disease?
Huntington's affects between three and seven Caucasians out of 100,000. In the United States, that equals about 9,000 to 21,000 people. The disease appears to be less common in other ethnic groups, such as Chinese, Japanese and African blacks.
Compared to some other diseases, Huntington's is rare. Multiple sclerosis, for example, affects between five and 10 times more people in the United States than Huntington's. Schizophrenia affects 1,000 people per 100,000 (or about 3 million people in the U.S.).
Who is at risk of Huntington's disease?
Huntington's disease is not caused by an infection or by exposure to a toxin in the environment. The disease is caused by a genetic change (an alteration). You only need to inherit one copy of the altered gene to be affected. You get one copy of a gene from your mother and one from your father. If either parent has Huntington's, you have a 50 percent chance of getting the altered gene.
If neither of your parents have Huntington's disease (and they are at the age when people normally start showing symptoms), you probably have nothing to worry about. But keep a few things in mind. Some people with the altered gene do not develop symptoms until several years after the usual age when symptoms begin. Also, a parent who might have eventually developed Huntington's disease might have died from another cause before showing any symptoms of the disease. Because of these exceptions, you may not be able to easily tell whether or not you are at risk just from your personal and family history.
If you have concerns about yourself or family members being affected, tell your doctor. Your doctor can direct you to a genetic counselor.
Is there a cure?
No. Huntington's disease cannot be cured, but there are some treatments for the symptoms.
Before you consider getting a genetic test to learn if you have inherited an altered gene for Huntington's disease, imagine how you might feel if you discovered that you would develop a debilitating disease with no cure. If you decide to get tested, talk to a medical professional (such as a genetic counselor) to help you deal with the implications of the test results.
The Gene For Huntington's Disease
What does it mean to have an altered gene for Huntington's?
A gene is like a recipe. In the case of Huntington's disease, part of the gene is "expanded," causing it to be defective. Imagine if the phrase "add 1 teaspoon of salt" appeared fifty times in a recipe. That much salt would ruin whatever you were making.
In general, a higher number of repeats in a gene has a worse effect on the "recipe" and is associated with the severity of the disease. With more repeats, the disease starts at an earlier age. This relationship is not perfect, so people with the Huntington's gene won't know exactly when they will start to show symptoms.
Should You Be Tested?
What are my chances of getting Huntington's?
This depends on your family history. With few exceptions, in order to qualify for testing, you must either have symptoms of the disease or have a close relative (a parent or sibling) who has been diagnosed with the disease. If you have no family history of the disease and you don't have any symptoms, you are not likely to have the altered gene.
Note: You may not have all the facts about your family history available. For example, if some members of your family died at an early age, you might not know the cause of death. And some families don't like to talk about their disease history. Or, you might not know both parents very well, or you might be adopted and not know your family history at all.
If you are concerned about Huntington's disease, talk to a knowledgeable medical professional, such as a genetic counselor, to find out whether testing is right for you.
Understanding Test Results and Options
How Do You Make Sense Of The Results?
If I test positive for Huntington's, what does that mean for my family and me?
If you don't have symptoms now, but you inherited the gene for Huntington's disease, you might start having symptoms at about age 40. If you have already experienced some of the symptoms, you know how challenging this disease can be.
Because the disease starts later in life, many people who decide to get a genetic test for Huntington's disease already have children. If you test positive, it means your brothers, sisters, and each of your children have a 50 percent chance of having inherited the gene. Your brothers and sisters may want to be tested as well.
According to generally accepted principles of medical ethics, children under 18 should not be tested. A positive test result creates a lot of worry for someone so young. Because there is no treatment for Huntington's disease, the test does not need to be done right away. It's better to wait until a child turns 18 so he or she can make an independent decision about whether to learn if they have the gene for Huntington's.
Does anyone ever get a positive test result, but not have the disease (a "false" positive)?
No. The test measures the number of abnormal DNA repeats in the gene. If there are too many repeats, the test result is positive. The test is extremely accurate in counting repeats, so it is unlikely that a positive result would be incorrect.
Does anyone ever get a negative test result, but actually have the disease (a "false" negative?)
If you receive a negative test result, you can be confident that you will not develop the disease. If a change is present, the test will find the change in the gene 98 percent of the time.
How will I deal with it if the test shows I'm going to develop Huntington's disease?
Learning that you are affected by a serious illness is obviously difficult. Research on the psychological effects of genetic testing for Huntington's disease has shown that people receiving the test results tend to accept the news after a period of adjustment.
Only about 20 percent of eligible people take the test. Perhaps only people who are ready to deal with the information want to get the test. Also, because only a qualified center can order the test, people being tested receive extra support to help them deal with the results.
Finally, it's not only the people who get positive test results who have trouble adjusting. People who receive a negative test result have reported feeling guilty that they did not get the disease, especially when one of their brothers or sisters was found to carry the gene. Everyone will deal with the information in his or her own way.
Ultimately, you should make sure you have the support of friends, family and professional counselors prepared to help you deal with these issues.
If I have the Huntington's disease gene, can I have children who don't have it?
Yes. If you have the Huntington's disease gene, you have a 50 percent chance of passing it on to each of your children, but also a 50 percent chance of not passing it along. If you find out you have the gene prior to becoming pregnant, or early in the pregnancy, you can have prenatal testing to find out if the fetus is affected. If you want prenatal testing, your obstetrician can tell you about how those tests are done and how the results might affect a pregnancy.
If I don't have the Huntington's disease gene, my children won't have it, right?
This is a tricky question. Think of the gene like a recipe. If you have too many repeats of one "ingredient," you will get the disease. But if you have only slightly more repeats than normal, you will not develop Huntington's. However, the number of DNA repeats in the gene can increase when being passed from a parent to a child at the time of conception. With enough of an increase, your child will develop Huntington's disease. This is more of an issue for someone who gets a test result that's in between the normal number of repeats and the number associated with the disease. If nobody in your family has Huntington's, you shouldn't start worrying about your children getting the disease.
Is there any harm in finding out if I have the gene?
Although it is stressful to receive positive test results, most people respond well once they are given time to adjust to the news. The rate of suicide is not dramatically higher among people taking the test as compared to the general population.
The period of adjustment will be, of course, different for different people. For this reason, testing is done through a medical professional who can provide the necessary support to a person struggling to accept this difficult news.
How does the test work?
If you decide to be tested for Huntington's, you need to enroll with a medical center that has a formal protocol for predictive genetic testing. If you're thinking about being tested, you probably have a family member with Huntington's. The neurologist caring for your family member often can refer you to a testing center. Testing centers have neurologists, medical geneticists and genetic counselors with expertise to help you face the complicated issues related to predictive testing.
Before you receive the test, you will have to sign a consent form indicating that you understand and agree to the test. The test usually requires taking about 1 teaspoon of blood from your arm and sending it to a lab that does testing. A lab obtains DNA from the blood sample and uses special techniques to determine the number of repeats in the gene.
If there are a low number of repeats, the result is reported as "negative" or "normal."
If there are a large number of repeats, the result is reported as "positive" or "abnormal."
An in-between number of repeats is more difficult to interpret because these people could be unaffected, but are still able to have children with an abnormal number of repeats.
If the test result is positive for the large number of repeats, however, that means the lab is very sure that there are enough repeats to cause the disease.
How much does the test cost?
The test costs approximately $300. Costs vary slightly depending on what lab does the testing.
Does insurance pay for the test?
Most health-insurance companies pay for "diagnostic testing," which means the test is being done to confirm a diagnosis in a person who already has symptoms. However, there are many different plans, and you should check whether diagnostic genetic testing is a covered service before having the test performed.
Many insurance companies will not pay for a predictive test (a test for a person who does not have symptoms), especially if there is nothing that can be done to prevent the condition (no change in patient management). If you are considering this test, call your insurance company and ask about its coverage.
How long does it take to get results?
Once you have blood taken for the test, you will receive your results in two to three weeks. Test results will not be reported directly from the laboratory to you. Instead, the laboratory provides the results directly to the medical center that ordered the test. You would then return to the center for another appointment to discuss the results. This way, family and friends can be there to support you when you hear the news.
Can a health-insurance company raise my rates or drop me from coverage if I test positive?
In 2008, the U.S. government passed a law called GINA (Genetic Information Nondiscrimination Act). This law prohibits discrimination by health insurers and employers on the basis of genetic information. Learn more here.
Also, this may depend on whether you have group insurance or are self-employed. Both federal and state laws usually cover people with group insurance, while state laws only cover people who are self-employed. Also, the Federal Health Insurance Portability and Accountability Act (HIPAA) of 1996 prohibits health insurance discrimination based on any "health status-related factor," (including genetic information) by group health plans. Unfortunately, this act does not apply to the self-employed.
Some states have enacted legislation to cover the gaps. More than half the states prohibit health-insurance companies from using genetic information to deny coverage. Other states require specific justification for the use of genetic information in denying a claim. Texas bans the use of genetic information by any group health plans, and Alabama prohibits discrimination based upon predisposition to cancer.
Life insurance, long-term care and disability insurance are generally not covered by these laws. People with life and disability coverage provided by their employers are unlikely to have this insurance affected by a genetic test result.
For information about laws in individual states, click on the following links:
Learn more about life insurance, disability insurance, and long-term care insurance laws.
Learn more about health insurance laws.