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Huntington’s disease

May 6, 2007

In the April/May 2006 (Yes, it’s an oldie, but I’m just catching up) edition of Scientific American Mind, there’s an interesting article written by Juergen Andrich and Joerg T. Epplen about Huntington’s disease.

It starts with simple incidents, such as forgetting a familiar address, or dropping a cup. But they are not incidents. Not clumsiness, forgetfulness or overreaction either. At least, when you have Huntington’s disease, an inherited disease of which the mutating gene was discovered in 1993. It leads to “progressive destruction of the brain, crippling muscles and mental function“. This mutation wreaks havoc inside the brain.

A single gene on chromosome 4 (the huntingtin gene (no misspelling)) is the cause. DNA consists of 4 bases: Cytosine, Adenine, Guanine and Thymine. If the CAG sequence on this gene occurs more than 35 to 40 times (instead of the regular 28 times), this chain becomes too long and causes trouble. The longer the chain of CAG-sequences, the earlier the disease starts showing, and the more severe it gets.

The symptoms usually show up when at age 35-45, but this also depends on the length of the chain.

Saint Vitus dance, an “involuntary movement disorder“, is characterized by “brief, irregular contractions that are not repetitive or rhythmic, but appear to flow from one muscle to the next. These ‘dance-like’ movements of chorea (from the same root word as “choreography”) often occur with athetosis, which adds twisting and writhing movements.” This is also seen in Huntington’s disease. But mental symptoms often occur before the physical problems, which also leads to social problems (relatives, friends, etc.), and even suicide. Before 1993 (when the responsible gene was discovered) people were often misdiagnosed as “mentally ill or alcoholic”.

What follows is something I don’t fully understand and therefore may not be very clear, but I’ve included it nonetheless:
When the elongated protein starts binding with other proteins, the function of those proteins is in danger.
Glutamate is a neurotransmitter, a chemical which helps a neuron “talk” with another cell. Synapses allow the neurons to form a network and communicate, and function as a system.
Via some complex process, some neurotransmitters won’t be removed “such as glutamate from the synapses“, resulting in “adjacent neurons continually excited” which will damage the cell.
Because of some other difficult process, it’s inpossible for the huntingtin protein to bind to the HIP-1 protein: “the neurons are driven to kill themselves.”

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