Ava Tran
Ava Tran
2/26/18
Huntington’s Disease Informative Essay
Huntington’s Disease is an autosomal dominant genetic disorder, which means that in order to inherit the disease, only one copy of the defective gene is needed. A person receives 1 copy of genes from each parent, so besides sex chromosomes, each person has 2 copies of each gene. A parent with the defective huntingtin gene has a 50% chance of passing on the defective copy rather than a healthy copy, which means that every child who has a parent with the defective gene has a 50% chance of getting the defective copy. Someone who has the defective gene will ultimately end up going through the process of Huntington’s Disease at some point.[1] The average age for someone to start experiencing symptoms of the disease 30-40. Juvenile Huntington’s disease is when the symptoms of the disease start to appear in those who are younger than 20. Huntington’s Disease is already a fairly rare disease, considering that it’s been found that only around 30,000 people in the United States currently have the disease. Juvenile Huntington’s Disease is a very rare version of Huntington’s Disease and appears to have much more rapid effects than those who start to experience symptoms as adults. The life expectancy for people with Huntington’s Disease is about 10-30 years after symptoms start appearing, while the 5-10% that experience the disease before the age of 20 have a life expectancy of approximately 15 years.[9] Ultimately the disease is a type of Alzheimer's disease, in which the process causes nerve cells in the brain to gradually die, leading to fatality.
The symptoms of Huntington’s Disease gradually worsens over time and is explained by many to be a combination of Alzheimer’s, ALS, and Parkinson's Disease. the overall explanation for the disease is that it causes one’s functional abilities to gradually worsen over time. In the beginning stage of the disease, mild chorea starts to take effect. Chorea, which is derived from the Greek word dance, and causes a person to experience involuntary body movements. Depression as well other similar emotional problems are also very common in people with the disease. One of the earliest symptoms is often close to unnoticeable, as it is very common for people to develop a difficulty towards learning new things and thinking things through. In the first stage of the disease, medication is sometimes recommended, mostly to help with mental instabilities such as depression. In the second stage of the disease, the chorea starts to worsens, and it becomes recommended by professionals to take medication to help with movement instabilities as well as mental instabilities. Physical therapists and speech therapists can also help someone to adjust to the new way their body functions. Speech may become affected and swallowing food may become more of a struggle as the ability of self-control gradually worsens. In the final stage of the disease, the person becomes completely dependent on others to take care of them, as they become unable to walk and struggle to talk at all. Choking becomes very common, as it becomes a difficulty to control themselves enough to be able to swallow food. The most common cause of death is often found to be pneumonia.[2]
Currently, there is much research to be done on the disease, as the exact explanation of what causes the specific symptoms are still only assumptions. In 1993, the gene that causes the disease was finally found by scientists on chromosome 4. This gene codes for the production of the huntingtin protein, which has a function that is still unknown to this day. What is known is that Huntington’s Disease is a neurodegenerative disorder, a sequence of DNA to repeat, that cause an abnormal protein to form, this then results in abnormal cognitive, and movement issues. As explained before, the fact that only one copy of the defective gene is needed to develop the disease makes the disease an autosomal dominant disorder. A normal huntingtin gene contains a triplet repeat disorder, which means that it has an excessive amount of the nucleotide sequence “CAG”. In the Huntington gene, the sequence is repeated 10-35 times, while it is found that in people with Huntington's disease the sequence is repeated 36 or more times. The sequence CAG is code for glutamine, which is an amino acid, so in other words, people with Huntington's disease are found to have 36 or more glutamines in the huntingtin protein, which also makes the disease a polyglutamine disease.[4] The unknown part of the disease is specifically how the excessive amount of polyglutamine directly correlates to causing the symptoms of the disease. What is assumed by most, from the information that is currently known, is that the mutated protein causes aggregation within neuronal cells, belonging to the putamen and the caudate (these two together are called the dorsal striatum) within the basalganglien. This part of the brain is essential to movement capabilities. This process causes neuronal cell death. The cell death might be caused by the excessive signaling of neurons, which is also called excitotoxicity, which causes high intracellular calcium (Ca2+).[5] Not only is the huntingtin protein affected by this, but DNA replication, in general, is also found to be affected. The process of neuronal cell death is currently the best explanation as to what cause movement problems such as chorea. Huntington’s disease is also one of the known (human affected) 14 trinucleotide repeat disorders, which are diseases that are caused by an excessive amount of nucleotide triplet in DNA.[4]
Sadly there is no prevention or cure for those who have the defective gene, only ways to help slow, manage, and make the process easier for those affected by the disease. Since a parent with the defective gene has a 50% of passing the gene onto their children, they have the option of preimplantation genetic diagnosis, as well as vitro fertilization. [1] Through this process eggs from the ovaries are removed and taken to a lab to be fertilized with the father’s sperm. The embryos are then tested for the Huntington gene, and the ones that test negative are put in the mother’s uterus.
Sources
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Huntington's Disease. (n.d.). Retrieved from Huntington's Disease.” Mayo Clinic, Mayo Clinic, www.mayoclinic.org/diseases-conditions/huntingtons-disease/symptoms-causes/syc-20356117.
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What is Huntington's Disease. (n.d.). Retrieved from http://hdsa.org/what-is-hd/
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HO, S., MD, & Leong, H. G. (m.d.). Chorea & Huntington's Disease. Retrieved February 28, 2018, from https://www.movementdisorders.org/MDS/About/Movement-Disorder-Overviews/Chorea--Huntingtons-Disease.htm
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Trinucleotide Repeat Disorders. (2014, October 28). Retrieved February 28, 2018, from http://web.stanford.edu/group/hopes/cgi-bin/hopes_test/trinucleotide-repeat-disorders/
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Learning About Huntington's Disease. (n.d.). Retrieved March 01, 2018, from https://www.genome.gov/10001215/learning-about-huntingtons-disease/
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Juvenile Huntington disease. (n.d.). Retrieved March 13, 2018, from https://rarediseases.info.nih.gov/diseases/10510/juvenile-huntington-disease