Huntington Disease

Cause: Huntington disease (HD), a fatal neurodegenerative disorder characterized by progressive deterioration of cognitive and motor functions, is caused by an expansion of a trinucleotide (CAG) repeat encoding glutamine in the protein huntingtin. Neurodegeneration, the loss of structure and function of brain cells, occurs initially and most severely in the medium-spiny neurons of the striatum, and later in the deep layers of the cortex.

Diagnosis: HD is normally diagnosed in the clinic on the basis of motor performance (onset of chorea). Several other tests may also be performed and measured using the unified Huntington's disease rating scale in order to obtain an overall assessment of disease progression. As HD is an adult-onset disorder, individuals are generally between 35 and 50 years of age when onset of the motor symptoms occur. However, while rare, juvenile cases have been described. Subsequent to the clinical motor diagnosis, DNA testing is done to conclusively determine that it is HD. The DNA test measures the number of CAG repeats present in the HD gene and determines if it falls in the normal or expanded (HD) range. As there is an inverse correlation between CAG size and age of onset, for those individuals that have HD in the family but do not have symptoms, they can elect to have the HD gene test to determine if they will develop HD in the future.

Current Research:

At the present time there is no cure for HD. However, numerous labs across the world are working hard to find potential treatments for HD and substantial progress is being made.

The major focus now is to investigate in more detail key molecular events that influence the pathogenesis of HD, and to evaluate potential compounds in HD models (and some are now in clinical trials) for their ability to provide protection against the toxicity of mutant huntingtin in order to bridge the gap between HD and a cure. Some of these efforts are listed below.

Gene Silencing: One of the main goals in HD research at the present time is on gene silencing. In HD, an incorrect version of the HD gene is present. This in turn makes an incorrect version of the huntingtin protein (what is called mutant huntingtin). If we can lower the amount, or get rid altogether, of the mutant huntingtin protein, this would decrease or remove the damage caused by the mutant protein and protect against its toxic effects. A number of techniques are currently being investigated which lower the amount of the mutant huntingtin protein. These are very important experiments and compounds are currently being testing in HD mouse models.

Excitotoxicity: This is a pathological process that involves dysfunction of glutamate receptors, in particular N-Methyl-D-aspartic acid (NMDA) receptors. Excitotoxins bind to these receptors and can cause excitotoxic stress due to high levels of calcium ions entering the cell, protease activation and eventual cell death. Excitotoxic neuropathological changes are observed in HD and are considered to be an early event in the pathogenesis of this disease. Efforts are ongoing to screen drug libraries in order to identify inhibitors of excitotoxic stress. Once the compounds have been identified they are then tested in HD mouse models in order to determine if protection and amelioration of the symptoms is observed and to identify compounds that may prove beneficial in human clinical trials.

Apoptosis: Programmed cell death or apoptosis is a program found within in all cells that once triggered leads to activation of caspases, cleavage of specific proteins and eventual death of the injured cell. This is a fundamental biological process essential for development and normal tissue homeostasis. In neurodegenerative diseases, alterations in components of the cell death machinery occur and enhance apoptosis. There is strong evidence to suggest that apoptotic events play a role in the neurodegeneration observed in HD. Work is ongoing to identify inhibitors and test them in cell and animal models of HD.

There are a number of other important HD research efforts, including stem cell research, also ongoing. Information on these, and current clinical trials, can be found in the websites below.

Websites for information on Huntington disease :

European HD Network

http://www.euro-hd.net/html/network

CHDI

http://www.highqfoundation.org/

CMMT

http://www.cmmt.ubc.ca/outreach/hd-clinic

HDbuzz

http://en.hdbuzz.net/

Huntington Society of Canada

http://www.huntingtonsociety.ca/

Sites internet pour de l'information sur la maladie d'Huntington: