Huntington’s disease

Huntington's disease is an inherited, neurodegenerative disorder that results in the progressive breakdown of nerve cells in the brain. It is caused by a mutation in a single gene, the Huntingtin (HTT) gene, located on chromosome 4. This mutation involves an excessive number of repeats of a specific DNA sequence known as a CAG (cytosine-adenine-guanine) trinucleotide. In the unaffected gene, this section repeats 10 to 35 times, but in individuals with Huntington's disease, it repeats 36 times or more. The resulting faulty huntingtin protein is toxic to brain cells, particularly in the basal ganglia and cerebral cortex, regions responsible for movement, thought, and emotion.

The disease is characterized by a triad of symptoms affecting movement, cognition, and psychiatric health. The most distinctive motor symptom is chorea—involuntary, jerky, or writhing movements. As the disease progresses, motor impairments worsen to include dystonia (muscle rigidity), difficulty with speech and swallowing, and poor balance. Cognitive decline manifests as difficulty with planning and organizing, memory lapses, and impaired judgment, eventually leading to dementia. Psychiatric disturbances are also common and can precede motor symptoms; these include depression, anxiety, obsessive-compulsive disorder, irritability, and psychosis.

Huntington's disease belongs to a class of conditions known as trinucleotide repeat disorders and is one of the most common hereditary neurodegenerative diseases, alongside conditions like Alzheimer's and Parkinson's disease. However, it is unique in its inheritance pattern. It is an autosomal dominant disorder, meaning that an individual needs only one copy of the defective gene from an affected parent to develop the disease. Consequently, a child of a parent with Huntington's has a 50% chance of inheriting the condition. The age of onset is typically between 30 and 50, but it can appear earlier (juvenile Huntington's) or later. A phenomenon known as anticipation can occur, where the number of CAG repeats increases in successive generations, often leading to an earlier onset and more severe symptoms.

For patients and families, Huntington's disease presents profound challenges. There is currently no cure, and treatments focus on managing symptoms to improve quality of life, such as medications to control chorea or treat psychiatric conditions. The definitive genetic cause allows for predictive testing, which poses a significant ethical and emotional dilemma for individuals at risk. Knowing one's genetic status can impact major life decisions, including career choices and family planning, and carries a heavy psychological burden. However, this genetic clarity also makes Huntington's a key target for advanced therapeutic research. Gene-silencing therapies, such as those using antisense oligonucleotides (ASOs) to lower the production of the toxic huntingtin protein, represent a promising frontier in the effort to slow or halt the progression of this devastating disease.