HD affects the whole brain, but certain areas are more vulnerable than others. Pictured above are the basal ganglia – a group of nerves cell clusters, called nuclei. These nuclei play a key role in movement and behavior control and are the parts of the brain most prominently affected in early HD.
What Is Huntington’s Disease?
Huntington’s disease (HD) is a fatal genetic disorder that causes the progressive breakdown of nerve cells in the brain. It deteriorates a person’s physical and mental abilities usually during their prime working years and has no cure. HD is known as the quintessential family disease because every child of a parent with HD has a 50/50 chance of inheriting the faulty gene. Today, there are approximately 41,000 symptomatic Americans and more than 200,000 at-risk of inheriting the disease.
The symptoms of HD are described as having ALS, Parkinson’s and Alzheimer’s – simultaneously.
Symptoms usually appear between the ages of 30 to 50, and worsen over a 10 to 25-year period. Ultimately, the weakened individual succumbs to pneumonia, heart failure or other complications. Everyone has the gene that causes HD, but only those that inherit the expansion of the gene will develop HD and perhaps pass it on to each of their children. Every person who inherits the expanded HD gene will eventually develop the disease. Over time, HD affects the individual’s ability to reason, walk and speak.
- Personality changes, mood swings & depression
- Forgetfulness & impaired judgment
- Unsteady gait & involuntary movements (chorea)
- Slurred speech, difficulty in swallowing & significant weight loss
The Huntingtin Gene
Huntington’s disease is a hereditary neurodegenerative disorder caused by an expansion of a repeating CAG triplet series in the huntingtin gene on chromosome 4, which results in a protein with an abnormally long polyglutamine sequence. HD is one of a larger family of polyglutamine repeat disorders, all of which areneurodegenerative diseases. It is inherited in an autosomal dominant fashion, so that each child of an affected parent has a 50% chance of developing the disease. There is currently no cure or treatment which can halt, slow or reverse the progression of the disease.
The HD gene was identified in 1993. It contains a repeating sequence of three base-pairs, called a “triplet repeat” or “trinucleotide repeat.” An excess number of CAG repeats in the gene results in a protein containing an excess number of glutamine units.
The normal function of huntingtin is not known, but the expanded polyglutamine sequence in the huntingtin protein is in some way toxic to brain cells. Just as in other polyglutamine expansion disorders, certain neurons appear to be more vulnerable to damage in HD. Atrophy is most marked in the corpus striatum of the basal ganglia, including the caudate and putamen. In later phases of the disease, other regions of the brain are also affected.
The Progression of HD
Huntington’s disease manifests as a triad of motor, cognitive, and psychiatric symptoms which begin insidiously and progress over many years, until the death of the individual. The average length of survival after clinical diagnosis is typically 10-20 years, but some people have lived thirty or forty years. Late stage HD may last up to a decade or more.