Huntington’s disease is a neurodegenerative disorder marked by a progressive loss of motor control and cognitive ability, as well as emotional and behavioral changes, and psychiatric problems.
The disease 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 is known to be active in the brain.
Genes are made of a specific sequence of four types of DNA bases (A, T, G, and C) that are “read” in groups of three, called codons, to indicate each part of the protein. The HTT gene has a section called a CAG trinucleotide repeat, where multiple copies of the three bases “CAG” are present.
The number of CAG segment repeats varies from one individual to another, and repeats of 10 to 35 times are normal and do not affect the protein. However, 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 for the presence of the abnormally expanded HTT gene. Before that happens, a specialist may first review the patient’s family medical history, and evaluate symptoms to rule out other causes.
Every person inherits two copies of the HTT gene, one from each biological parent. Huntington’s disease is inherited in an autosomal dominant manner, meaning that only one copy of the mutated gene is sufficient to cause the disorder. A person who carries one mutant copy of the HTT gene has a 50% chance of passing the mutated gene to their biological children.
Because the disease-causing gene can be passed from parents to their children, 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, may also be carried out. The scan can help identify changes in the brain caused by Huntington’s disease and also rule out other conditions that may affect the brain and cause similar symptoms.
For a definitive diagnosis, a genetic test is required. This will normally involve a blood sample being taken and sent to a specialized center for examination. The test can determine how many CAG repeats are present in the HTT gene.
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 there is a known family history of the disease. 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 to better understand how the results may affect the person and their family.
Last updated: June 9, 2021
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