What happens in Huntington’s disease, exactly?

What is it?

Huntington’s disease is a hereditary, neurodegenerative disorder that leads to severe problems with muscle coordination, mental ability, and behavior. Symptoms usually begin between ages 35 to 44 and get worse with time. The affected are typically expected to live for no more than twenty years after the development of symptoms, due to complications such as pneumonia, heart disease, and injury from falling.

HD is most common in Western European people, with around 7 cases in 100 000 people, and much less common in African and Asian people. HD rarely affects reproduction because of the late onset. This means that the disease is easily transmitted to future generations, and sometimes the parent doesn’t know they carry the disease gene before they have already had children.

What causes it?

HD is caused by a faulty gene. Each gene is the recipe for a protein, and in HD a sequence of a gene coding a protein essential to development has multiplied so many times that it cannot produce a functioning protein.

All humans have two different forms, alleles, of each gene, one from both of our parents. This means that someone whose parent has the gene is 50% likely to have it too. The gene that causes HD is dominant: having one diseased and one healthy gene is enough to become affected, because the diseased gene is more “loud” than the healthy one.

What happens during it?

The early symptoms of HD (loss of control over movement and problems with higher cognitive tasks and mood) are attributable to damage to a part of the brain called basal ganglia. It is known that the faulty protein is active everywhere in the body. The fact that it is particularly toxic to the brain indicates that the real issues arise, when the faulty protein is supposed to interact with other proteins found only in neurons, but isn’t able to because of its altered shape. This disruption in interaction is thought to lead to the death of neurons.

Mitochondria are like little organs of the cell whose job it is to turn energy from glucose into energy the cell can use. It is suggested that the faulty gene in HD is normally associated with mitochondria. One theory proposes that HD leads to cell deaths because it damages mitochondria and for that reason makes the cells less able to deal with a chemical transmitter called glutamate, which in excessive amounts or in weak cells becomes toxic. The increased glutamate sensitivity leads to the cutting up of the faulty protein and the fragments moving to the nucleus (the gene storage of the cell) and interfering with the production of other proteins, eventually causing the death of the cell.

The basal ganglia is involved with the motor cortex through two different pathways. In one of these pathways, damage to the basal ganglia leads to the understimulation of the motor cortex through a complex series of reactions , causing the slowness of movement characteristic to HD. In the other pathway the effect is overstimulation, which leads to the involuntary, jerky movements.

How can it be treated?

Unfortunately, a cure for HD hasn’t been found. Treatment is based on relieving the symptoms and providing care as the disease progresses.

Difficulties with eating are common, and during the disease it becomes increasingly important to make sure that the patient gets the nutrition they need. Food and liquids should be made easy to ingest, and if the problems grow too severe, a feeding tube can be used. Physical therapists can help the affected to maintain the ability to move for as long and as safely as possible, and help with breathing difficulties. Medication can be used to alleviate physical symptoms as well as depression and other psychiatric problems.

 

 

Sources: http://web.stanford.edu/group/hopes/cgi-bin/hopes_test/the-basic-neurobiology-of-huntingtons-disease-text-and-audio/, https://en.wikipedia.org/wiki/Huntingtin, https://en.wikipedia.org/wiki/Huntington%27s_disease#Mechanism

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