Huntington’s disease is a neurodegenerative disorder marked by a progressive loss of motor control and thinking ability, as well as emotional and behavioral changes, and psychiatric problems.
Huntington’s is caused by an inherited mutation in the huntingtin (HTT) gene, which provides the instructions to make the HTT protein. While HTT’s exact role in the body is still uncertain, the protein it is known to be active in the brain.
Genes are made of a specific sequence of four types of so-called DNA bases (A, T, G, and C) that are “read” in groups of three to indicate each part of the protein. The HTT gene has a section called a CAG repeat, where multiple copies of the three bases “CAG” are present.
The number of CAG segment repeats varies naturally from one individual to the other, and repeats of 10 to 35 times are normal and do no affect the protein. But if a mutation increases the number of repeats to more than 36, the resulting abnormally large protein may lead to Huntington’s disease.
A diagnosis of Huntington’s disease is generally confirmed through a genetic test, to check the presence of the abnormally expanded HTT gene. However, a specialist may first review the patient’s family’s medical history, and evaluate the symptoms to rule out other causes.
Huntington’s disease is inherited in an autosomal dominant manner, meaning that if one parent has the condition, there is a 50 percent chance that he or she will pass it onto their children. One copy of the mutated gene is sufficient to cause this disorder.
Therefore, a diagnosis of Huntington’s is more likely to be quickly reached if there is a family history of the disease.
A specialist can help identify the characteristic symptoms of Huntington’s disease as part of a diagnosis. This includes exams to measure any changes in motor function, neuropsychological assessments that look for cognitive changes, and psychiatric evaluations to assess any behavioral changes.
A brain imaging scan, such as magnetic resonance imaging (MRI), may also be carried out. This may be used to identify changes that Huntington’s disease causes in the brain, but also to rule out other conditions that may affect the brain.
For a definitive diagnosis, a genetic test is required. This will normally involve a blood sample being taken and sent off to a specialized center for examination. The test can determine the size of the HTT gene, or how many CAG repeats are present.
Huntington’s disease will almost always be present in people with 40 or more CAG repeats. The CAG segment can be repeated more than 120 times and, generally, the more expanded the gene is, the earlier disease symptoms will start.
It is possible to be tested for Huntington’s disease before any symptoms appear if someone already has the disease in the same family. However, a diagnosis can be difficult if there are between 36 to 40 CAG repeats, as the disease may or may not develop in the person. For this reason, a decision to undergo such testing is recommended only after careful discussion with a genetic counselor, in order to better understand the impact results can have on the person and his or her family.
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