Huntington’s disease (HD) is a rare genetic disorder that affects your brain. It is estimated that 30,000 people in the United States have a diagnosis of HD. HD is passed down (inherited) from parent to child in an autosomal dominant fashion meaning each child of an affected parent has a 50% chance of testing positive for HD. HD is caused by a mutation in the ‘huntingtin’ gene (HTT). This mutation affects nucleotides which are the basic building blocks of your DNA. In unaffected individuals, the huntingtin gene contains a repeating sequence of nucleotides CAG. Individuals affected by HD have too many repeats of this CAG sequence. When this sequence is repeated too many times, it causes the body to produce mutant huntingtin protein (mHTT), an abnormal protein that is toxic to cells in the brain (neurons). Buildup of this abnormal protein leads to cell death (neuronal loss).
Patients with HD gradually develop abnormal muscle movements that they cannot control. These movements, known as chorea, are thought to be caused by death of brain cells (neuronal loss) and changes in a chemical in the brain called dopamine which is involved in controlling movement. In early HD, too much dopamine leads to unwanted movements. Most patients with HD start to experience chorea symptoms in their forties or fifties. However, an estimated 10% of patients experience symptoms before the age of 20 (juvenile onset) and another 10% develop motor symptoms after the age of 60. In addition to chorea, patients with HD often experience difficulty with memory and attention (cognitive decline) and changes in mood and behavior.