Huntington's disease

What do we know about heredity and Huntington’s disease?

Huntington’s disease

Huntington’s disease (HD) is an inherited disorder that causes brain cells, called neurons, to die in various areas of the brain, including those that help to control voluntary (intentional) movement. Symptoms of the disease, which gets progressively worse, include uncontrolled movements (called chorea), abnormal body postures, and changes in behavior, emotion, judgment, and cognition. People with HD also develop impaired coordination, slurred speech, and difficulty feeding and swallowing. HD typically begins between ages 30 and 50. An earlier onset form called juvenile HD occurs under age 20.  Its symptoms differ somewhat from adult onset HD and include rigidity, slowness, difficulty at school, rapid involuntary muscle jerks called myoclonus, and seizures. More than 30,000 Americans have HD.

Huntington’s disease is caused by a mutation in the gene for a protein called huntingtin. The defect causes the cytosine, adenine, and guanine (CAG) building blocks of DNA to repeat many more times than is normal. Each child of a parent with HD has a 50-50 chance of inheriting the HD gene. A child who does not inherit the HD gene will not develop the disease and generally cannot pass it to subsequent generations. A person who inherits the HD gene will eventually develop the disease. HD is generally diagnosed based on a genetic test, medical history, brain imaging, and neurological and laboratory tests.

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Is there a test for Huntington’s disease?

The discovery of the HD gene led to a genetic test to make or confirm the diagnosis of Huntington’s disease. Using a blood sample, the genetic test analyzes DNA for the HD mutation by counting the number of CAG repeats in the huntingtin gene. Individuals who do not have HD usually have 28 or fewer repeats. Individuals with HD usually have 40 or more repeats.

Deciding to be tested for Huntington’s disease can be difficult. Individuals consider genetic testing to confirm a diagnosis when clear symptoms are present and there is a documented family history of HD. Others who have a parent with the disease elect to be tested to resolve uncertainty about their future. A negative test relieves anxiety and uncertainty. A positive test enables individuals to make decisions about careers, marriage and families.

Some who are at risk choose not to take the test. They choose to live with the uncertainty of being at risk and to forgo the emotional consequences of a positive result, as well as possible losses of insurance and employment. Genetic counselors can help individuals make the difficult decisions about testing.

Prospective parents consider prenatal testing when one parent has been diagnosed with Huntington’s disease or has been found to carry the gene. Prenatal testing can show whether the child will inherit the defective gene. To test the fetus, DNA is extracted from fetal cells via CVS (chorionic villi sampling) or amniocentesis. If the fetus tests positive, parents can make decisions about whether to terminate the pregnancy.

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Treatment

There is no treatment that can stop or reverse the course of HD. Tetrabenazine and deuterabenazine can treat chorea associated with HD.  Antipsychotic drugs may ease chorea and help to control hallucinations, delusions, and violent outbursts. Drugs may be prescribed to treat depression and anxiety. Side effects of drugs used to treat the symptoms of HD may include fatigue, sedation, decreased concentration, restlessness, or hyperexcitability, and should be only used when symptoms create problems for the individual.

Courtesy: https://www.genome.gov/Genetic-Disorders/Huntingtons-Disease

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