Emma Mactaggart 42920069
Huntington’s chorea is a devastating human genetic disease. It is a progressive disorder of motor, cognitive (attention, memory, producing and understanding language) and psychiatric disturbances. The average age onset is between 35 – 44 years of age with an average 15 – 18 years of survival time after onset.
It is an involuntary movement disease where those affected writhe, twist and jerk with non-repetitive, non-periodic jerks of the face, trunk or limbs. Symptoms for Huntington’s begin subtly with slight changes in coordination, minor involuntary movements and often a depressed or irritable mood. In late stages, behavioral problems are gradually lessened, motor disability becomes severe and the individual is totally dependent, mute and incontinent.
People who inherit this genetic disease have an abnormal dominant allele that causes degeneration of nerve cells in certain areas of the brain including cells in the basal ganglia and structures deep in the brain that coordinate movement. The brain’s outer surface is also affected which controls thought, perception and memory.
The Huntington’s allele is dominant so individuals with just one parent with Huntington’s have a 50-50 chance of developing the disease themselves. The disease is caused by an autosomal (chromosome that’s not an allosome (sex chromosome) ) dominant mutation in either of an individual's two copies of a gene called Huntingtin. In 1993, it was discovered that a stretch of DNA that is repeated over and over again, was responsible. This was CAGCAGCAGCAGCAG… and so on. People with more than 40 repeats of CAG develop the disease later in life but people with 60 or more will have juvenile onset around 21 years old. This mutation happens in majority of cases although some people who have no family history may still develop the disease as a result of new mutations.
To make things worse, Huntington’s chorea belongs to a class of genetic diseases that largely escape natural selection. It becomes “invisible” to natural selection as it does not generally affect people until after they have reproduced so therefore the alleles sneak into the next generation. Early-onset cases of Huntington’s are rare and are an exception. They are strongly selected against.
In general, Huntington’s is rare, 30 to 70 cases per million people in the world. Lake Maracaibo in Venezuela has the more cases than anywhere in the world, this has been found to be due to something called the founder effect. Around 200 years ago a single woman who carried the Huntington’s allele bore 10 children and today, many residents trace their ancestry back to this lineage.
Unfortunately, at this point in time, there is no cure for Huntington’s chorea. Symptoms can be treated including anti-depressants or anti-epileptic medicines for mental illness associated, anti-parksonian agents for rigidity and neuroleptics for chorea but there is no treatment for cognitive impairments. Genetic testing can identify people who carry a Huntington’s allele long before the onset of the disease and so can influence reproductive choices. The area of genetic testing raises much ethical debate. What age is considered appropriate for an individual to have a genetic test? Do parents have the right to get their children tested?
There are many organisations around the world that held and support people with Huntington’s chorea and research will always be continued.
University of California Museum of Paleontology, 2000, Huntington's Chorea: Evolution and Genetic Disease, viewed 4th September 2012, <http://evolution.berkeley.edu/evolibrary/article/medicine_05
Huntington Disease Society of America (HDSA), 2006, Huntington’s Chorea, viewed 4th September 2012, <http://www.meddean.luc.edu/lumen/MedEd/genetics/diseases/huntingtons.htm
Campbell, N. A., Reece, J. B. & Meyers, N. (2009) Biology, Frenchs Forest, NSW, Pearson Education Australia.