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Huntington's disease

Huntington's disease (HD), also known as Huntington's chorea, is an inherited neurodegenerative disorder primarily affecting motor skills, mood, and cognitive abilities. It is a basal ganglia disease resulting in a hyperkinetic movement disorder called chorea. Symptoms typically manifest between the ages of 30 and 50, although juvenile cases exist.
Several neurons coloured yellow and having a large central core with up to two dozen tendrils branching out of them, the core of the neuron in the foreground contains an orange blob about a quarter of its diameter
An edited microscopic image of a medium spiny neuron (yellow) with an inclusion body (orange), which occurs as part of the disease process (image width 360 μm)

Signs and Symptoms

Initial symptoms often include subtle mood changes and cognitive decline. Physical symptoms typically follow, characterised by involuntary movements (chorea), lack of coordination, and an unsteady gait. As the disease progresses, symptoms worsen, leading to severe motor dysfunction, speech and swallowing difficulties, and dementia. Juvenile HD presents with symptoms resembling Parkinson's disease, such as slow movements and rigidity, often coupled with seizures.

Genetics

Huntington's disease is caused by a mutation in the huntingtin gene (HTT) on chromosome 4. The mutation involves an expansion of CAG trinucleotide repeats. Individuals with fewer than 36 repeats generally do not develop HD, while those with more than 40 repeats will be affected. The disease follows an autosomal dominant inheritance pattern, meaning a child has a 50% chance of inheriting the disorder if one parent is affected.

Graphic showing at top normal range of repeats, and disease-causing range of repeats.
Classification of trinucleotide repeats, and resulting disease status, depending on the number of CAG repeats

Mechanisms

The mutant huntingtin protein (mHtt) causes neurodegeneration, primarily affecting the basal ganglia and later the cerebral cortex. This results in the hallmark symptoms of HD. Other affected brain areas include the substantia nigra, hippocampus, and thalamus. The protein's toxicity manifests through multiple pathways, including mitochondrial dysfunction, impaired protein clearance, and transcriptional dysregulation.

Diagram of a side view of the brain and part of the spinal cord, the front of the brain is to the left, in the centre are red and blue masses, the red mass largely overlaps the blue and has an arm that starts at its leftmost region and forms a spiral a little way out tapering off and ending in a nodule directly below the main mass
The area of the brain most damaged in early Huntington's disease is the dorsal striatum made up of the caudate nucleus and the putamen.

Diagnosis

Diagnosis begins with the observation of physical symptoms, supported by genetic testing to confirm the presence of the expanded CAG repeats in the HTT gene. Predictive genetic testing is available for at-risk individuals. Imaging techniques like MRI and CT scans can show brain atrophy, particularly in the caudate nuclei.

Cross section of a brain showing undulating tissues with gaps between them, two large gaps are evenly spaced about the centre.
Coronal section from an MRI brain scan of a patient with HD, showing atrophy of the heads of the caudate nuclei, enlargement of the frontal horns of the lateral ventricles (hydrocephalus ex vacuo), and generalised cortical atrophy

Treatment

There is no cure for HD, but treatments can manage symptoms and improve quality of life. Tetrabenazine is approved for treating chorea, while antipsychotics and benzodiazepines can help reduce other motor symptoms. Antidepressants and antipsychotic medications are used to manage psychiatric symptoms. Physical, occupational, and speech therapies are essential for maintaining function and quality of life. Nutritional support is also very important due to difficulties in swallowing and weight loss.

Chemical structure of tetrabenazine, an approved compound for the management of chorea in HD
Chemical structure of tetrabenazine, an approved compound for the management of chorea in HD

Prognosis

Life expectancy for individuals with HD is generally 10 to 30 years following symptom onset. Common causes of death include pneumonia, heart disease, and complications from falls or choking. Suicide is a significant risk, particularly in the mid-stages of the disease.

Epidemiology

HD prevalence varies globally, with the highest rates in individuals of Western European descent. In the UK, the average prevalence is 12.3 per 100,000. Some isolated populations, such as the Lake Maracaibo region in Venezuela, show much higher prevalence due to founder effects.

History

The first detailed description of HD was provided by George Huntington in 1872. The genetic basis of the disease was identified in 1993 through international collaboration led by the Hereditary Disease Foundation. This discovery has paved the way for extensive research into the mechanisms and potential treatments for HD.

On the right is a young man, dressed in suit and tie, sporting a moustache and tuft of hair on the chin; on the left is the top half of a medical journal titled 'Medical and Surgical Reporter'
In 1872, George Huntington described the disorder in his first paper "On Chorea" at the age of 22.

Support Organisations

Numerous organisations support HD research and provide resources for affected individuals and their families. Key organisations include the Hereditary Disease Foundation and the Huntington's Disease Society of America.

A black-and-white photograph taken indoors of Woody Guthrie wearing pinstripe trousers, a tartan shirt with the top button undone, and a cap. He sits playing a six-string acoustic guitar, which is supported on one knee, and he appears to be singing. 'This Machine Kills Fascists' is written in all capital letters on a rectangular sticker, which is fixed onto the guitar.
The death of Woody Guthrie led to the foundation of the Committee to Combat Huntington's Disease.

Self-assessment MCQs (single best answer)

What is the primary cause of Huntington's Disease (HD)?



Which brain area is most affected in the early stages of Huntington's Disease?



What type of inheritance pattern does Huntington's Disease follow?



Which medication is approved for treating chorea in Huntington's Disease?



What is the typical age range for the onset of Huntington's Disease symptoms?



Which of the following is a common cause of death in individuals with Huntington's Disease?



What is the primary symptom of juvenile Huntington's Disease that resembles Parkinson's Disease?



What does an MRI scan of a patient with Huntington's Disease typically show?



In which population is the prevalence of Huntington's Disease particularly high due to founder effects?



Who provided the first detailed description of Huntington's Disease?



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