Huntington Disease is a rare genetic condition that most people have never even heard of unless a) they study it b) they personally know someone with the disease or c) they are a fan of House. Luckily for me only a) and c) apply in this situation. However, I believe Huntington’s like a variety of other diseases is something the public needs to be educated about because awareness really is the greatest way to inspire research into any field.
As autosomal dominant disorder this makes Huntington’s especially dangerous because as a dominant trait a person only needs one affected allele to develop the disorder. Were the trait recessive such as the trait for hemophaelia, for example, then the likelihood of having Huntington’s is significantly lowered. When a trait is autosomal this means that it is no carried by any of the sex chromosomes (X or Y), rather is carried by any of the other 22 chromosomes the human body has. In Huntington’s the gene affected is located on chromosome 4, specially on the p (upper, shorter) arm.
The symptoms of Huntington’s has already been discussed on a previous post on basal ganglia disorders; however, in summation it results in damage to the striatum and cerebral cortex causing changes in personality including mood swings, involuntary movements known as hypokinesia and eventually dementia. As is common with most genetic disorders, the symptoms do not appear until adulthood. In Huntington’s the symptoms usually arise around mid-age, but unfortunately it can arise earlier than our 30s or 40s if unlucky. Once symptoms start appearing the person usually has about another 5 to 15 years until death. The age at which symptoms appear directly correlates with the genetics behind the abnormal gene.
Huntington’s is part of numerous diseases including varies ataxias and fragile X syndrome that result due to trinucleotide repeat. Specifically, Huntington’s is due to a repeat of the CAG trinucleotide. Normal alleles carry about 10 to 35 copies, but those suffering from Huntington’s and various other neurodegenerative diseases have more than 40 repeats. People with around 60 repeats with develop Huntington’s around the age of 20. These repeats in CAG result in the production of a “mutant protein” that eventually fill the striatum and cerebral cortex causing degeneration and ultimately death of these brain cells. In healthy individuals the gene involved in Huntington’s encodes for a large protein known as huntingtin (Htt), which when normal enhances the production of a protein (BDNF) necessary for the survival of the cells in the striatum and cerebral cortex.
Stay tuned for a post later this week on current experimental treatment on Huntington’s! Thank you for reading
Cummings, Michael. “Genetics of Behavior.” Human Heredity: Principles and Issues. 9th ed. Belmont: Brooks/Cole, 2011. 405-06. Print.