Huntington’s disease is an incurable neurodegenerative disease that affects muscle coordination and cognitive function. Symptoms of the disease usually start to develop around middle age. Huntington’s disease used to be called Huntington’s chorea because it is the most commonly inherited condition to lead to the involuntary movements referred to as chorea.
This disease affects significantly more Western Europeans than Asians or Africans and affects males and females equally. Huntington’s disease is inherited in an autosomal dominant fashion, which means the disease will develop if a person inherits the mutation on either copy of their huntingtin gene (HTT). Any person born to a parent affected by the condition therefore has a 50% chance of inheriting the disease. In the extremely unlikely scenario that both parents carry an HTT mutation, this risk is increased to 75% and if either parent carries two mutated HTT copies, the risk of inheritance is 100%. Since the disease often begins during middle age, people can go on to have children before they are aware they have the disease or that their offspring would be at risk.
The HTT gene is located on chromosome 4 and codes for the huntingtin protein (Htt). The mutation occurs as an expansion mutation of the cytosine-adenine-guanine (CAG) triplet found on the HTT gene. The mutated gene codes gives rise to a mutated Htt (mHtt) protein that gradually damages brain cells.
Although the symptoms of Huntington’s disease can develop at any time in life, they usually begin when a person is aged between 35 and 44 years. As expanded copies of HTT are passed on through generations, the number of repeats can increase in successive generations causing a more severe disease to occur at an earlier age of onset than in previous generations. This is referred to as genetic anticipation. In around 6% of cases, Huntington’s disease develops in people aged under 21 years. This rapidly progressing form of the condition is referred to as as juvenile, akinetic-rigid or Westphal variant HD.
Symptoms vary between individuals but some of the most common earliest symptoms are changes in mood and cognitive function, lack of coordination and an unsteady gate. As the disease progress, movement may become jerky and uncontrolled and a decline in mental ability becomes more obvious, with dementia eventually developing.
There is no cure for Huntington’s disease and patients require full-time palliative care in the later stages. Various medications are available to help relieve symptoms associated with the condition.
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