What is the genetic basis of Huntington's disease?

Huntington's disease is caused by a mutation in the HTT gene, which leads to the production of an abnormal protein.

Huntington's disease is a genetic disorder that is inherited in an autosomal dominant manner. This means that an individual only needs to inherit one copy of the faulty gene from either parent to develop the disease. The gene in question is the HTT gene, which provides instructions for making a protein called huntingtin. Although the function of this protein is not fully understood, it is known to be important for nerve cells (neurons) in the brain.

The mutation that causes Huntington's disease involves a DNA segment known as a CAG trinucleotide repeat. This segment is made up of a series of three DNA building blocks (cytosine, adenine, and guanine) that appear multiple times in a row. In most people, the CAG segment is repeated 10 to 35 times within the gene. However, in people with Huntington's disease, the CAG segment is repeated 36 to more than 120 times. The length of the CAG repeat seems to correlate with the age at which symptoms appear; the longer the repeat, the earlier the onset of symptoms.

The abnormal huntingtin protein, produced as a result of the mutation, is cut into smaller, toxic fragments that accumulate in neurons, disrupting their function and eventually leading to cell death. This process primarily affects neurons in the part of the brain called the striatum, which is involved in controlling movement. Over time, the loss of these neurons results in the characteristic signs and symptoms of Huntington's disease, including uncontrolled movements, emotional problems, and loss of thinking ability.

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