Huntington disease (HD) is a brain disorder caused by a defective gene.
It affects one in 10,000 people in Canada and across the world, among both men and women. HD gradually destroys certain structures in the brain (the caudate nucleus and the putamen) before eventually attacking the cerebral cortex.
The gene that causes HD was identified in 1993. What we know about it is that it produces a protein called “huntingtin” and that, in persons with the disease, this protein splits into two pieces – one short and one a bit longer. The short segments form little protein clusters. Researchers are now trying to determine exactly what causes the destruction of the brain cells – the protein itself, the fact that it splits into uneven pieces, the formation of clusters, or an as-yet unknown mechanism? To date, these questions remain unanswered.
Since HD is genetic, people who carry the defective gene can pass it on to their children, who then have a 50% chance of inheriting the gene. If they do inherit the gene, they will also develop the disease sooner or later in their lifetime.
The first signs and symptoms of HD usually appear between the ages of 30 and 50. The disease sometimes manifests before the age of 20, in which case it is referred to as juvenile HD.
HD progresses in five stages, from Stage I where affected individuals are still fully autonomous, to Stage V where they require full-time specialized nursing care. Each stage can last several years. Life expectancy after diagnosis varies from one person to another. HD doesn’t directly cause an affected person’s death, but it leads to complications that can lead to death.
Individuals can be affected in different ways by the wide range of signs and symptoms of HD, which fall under three main categories: motor, cognitive, and emotional disturbances.
Motor impairment usually affects the person’s dexterity, coordination and balance. The disease typically begins with the appearance of slight involuntary movements (e.g. spasms) that tend to worsen as the disease progresses. These movements have a growing impact on the person’s autonomy, particularly in terms of mobility and the ability to speak and swallow. In advanced stages, these uncontrollable movements tend to abate, as progressive rigidity of the limbs sets in. At this stage, affected individuals usually require assistance in every aspect of their life.
Cognitive disturbances usually manifest as problems concentrating, planning or making decisions, and even a lack of judgment. The person may also have problems recalling past events and memories. Just like the motor impairment, these cognitive disturbances tend to worsen as the disease progresses.
Emotional disturbances vary greatly from one person to another, but depression, changes in personality, irritability and mood swings are common. Less commonly, persons with HD may experience hallucinations and paranoia.
The onset and severity of symptoms also varies greatly from one person to another, and from one stage of the disease to the next. As a result, it is impossible to predict how the disease will progress, even among members of the same family.
While studies in animals have helped scientists identify some promising treatment avenues, research isn’t advanced enough yet to have resulted in a treatment that could be used in humans. Currently, doctors can only help their HD patients deal with the specific symptoms of the disease as they arise (e.g. medication to help the patient overcome depression).
There is a screening test designed to identify whether high-risk individuals (usually children of people with HD) carry the defective gene. It can be difficult to decide whether or not to take the test, since there is a 50 percent chance that the result will be positive. When considering taking the test, it is therefore recommended that individuals consult a genetic counsellor, who can provide guidance in making an informed decision.
For more information on Huntington disease, visit the websites of the Huntington Society of Canada (huntingtonsociety.ca – bilingual) or of the Société Huntington du Québec (huntingtonqc.org – French only).