In my early 40s I began to have more difficulty concentrating, and I remember feeling unusually forgetful. For a while, I ignored these symptoms until I started to uncontrollably twitch and make fidgety motions. When the chorea started, it was clear to my doctor that I was dealing with something more than just forgetfulness. The fact that my mom also suffered from the disease confirmed the doctor’s predictions, and that is when I was diagnosed with Huntington’s Disease. From then on things only got worse, my symptoms of chorea began to occur more frequently, primarily in my hands, feet, and face, causing involuntary body movements, and making it very hard to go about daily life. Now, it is even difficult for me to remember specific conversations, TV shows or books. My disease affects my ability to plan, make decisions and process complex topics. I have trouble navigating, even to places I have been many times, although I can retain memories and continue to do things I have been for a long time. I get easily agitated, irritated and aggressive over little things that I can’t control. All these symptoms induce anxiety and depression and I often go into periods where I am constantly frustrated and down, but I don’t know why. I have recently noticed that my voice has become hoarse, slurred, and I either speak too quickly or too slowly. While all these symptoms make life much harder to enjoy, the worse part of having Huntington’s disease is knowing that it doesn’t just affect me, but potentially my children and our entire family for years to come. I am 55 and I have lived with Huntington’s Disease for just over 10 years, and I cherish every moment I have left with my family and friends. (Living with Huntington’s 2013)
Huntington’s disease was discovered in 1872 by George Huntington, a 22-year-old American doctor. He published a paper called On Chorea which lead the disease to be first recognized as an inherited disorder known as Huntington’s Chorea. In the 1980s and strong push to find the gene at fault began. Researchers built long and detailed pedigrees and using blood samples from large families with HD discovered its location, on chromosome 4. In 1993, the mutation was identified. This allowed HD families to be now tested to find out if they carried the mutation before the symptoms began to develop. (Kaplan 2019) Huntington’s Disease has affected many well-known individuals including; Woody Guthrie, Trey Gray and Charles Sabine. Probably the most famous person to suffer from Huntington’s Disease was Woody Guthrie, an icon in the folk music world, who was diagnosed with HD in 1952. Sadly, at the time little about the disease was known so his illness was essentially left untreated. Woody died at age 55, in 1967, only 15 years after he was diagnosed. His death helped increase public awareness for Huntington’s and lead to the development of the Huntington’s Disease Society of America (HDSA). For a long time, even after George Huntington discovered the disease, it was poorly understood. People believed the twitches characterized by HD were possessed by devils. Furthermore, people with the HD gene were dying before the symptoms could even develop, making it very hard for research to be done. So, when the gene was finally discovered in 1993, it was a breakthrough in the history of HD. (Zachary 2019)
Huntington’s Disease causes movement, cognitive and psychiatric disorders. Symptoms of Huntington’s vary greatly from person to person. Oftentimes symptoms increase in harshness or frequency as the disease progresses, and one disorder tends to be more dominant than the other. Specific symptoms that affect movement include; involuntary jerking (chorea), muscle rigidity or contracture (dystonia), slow eye movements, impaired posture, balance, and difficulty speaking or swallowing. (Mayo 2018) These symptoms cause both involuntary movements to be altered, and impairments to voluntary movements, making daily life very difficult. Huntington’s also causes cognitive issues like, difficulty organizing and prioritizing, tendency to get stuck on a thought, acting without thinking, lack of awareness, and difficultly obtaining new information. (Huntington’s Disease Genetics 2019) Finally, symptoms of psychiatric disorders also appear causing irritability, apathy, social withdrawal, insomnia, fatigue and suicidal thoughts. Symptoms of juvenile HD are mostly like regular Huntington’s but there are some differences including; stiffness of the legs, clumsiness, poor school performance, speech problems and behavioral disturbances. Seizures also occur in 25-30% of cases in adolescents, this is almost never seen in adults with HD. (Huntington’s Disease, 2019)
Huntington’s Disease affects one in every 10,000 people and around 30,000 people in the United States. While 150,000 or more people are at risk of developing the disorder due to inheritance from their parents. The condition is inevitably fatal, with an average life expectancy of 20 years after the first symptoms are identified. HD is found most commonly in Western Countries, affecting around 3 to 7 per 100,000 people with European descent. The disorder is less common in populations including people of Japanese, Chinese and African ancestry. (Huntington’s Disease Alzheimer’s Association, 2019) In certain populations and regions you are at a greater risk of developing the disease. For example, there is an unusually high concentration of people with HD in the Lake Maracaibo region of Venezuela, where the prevalence averages to around 700 affected individuals per 100,000 people. Similarly, in Australia over 1,800 people have HD and approximately 9,000 are at risk. Huntington’s Disease is inherited in an autosomal dominate fashion. So, the probability of each offspring inheriting the affected gene is 50%. Huntington’s is independent of gender, and the trait doesn’t skip generations due to its dominance. There is a wide variation of occurrence based on age, but the average onset is 40 years old. However, symptoms can manifest as early as age 2 or as late as 80. (What is Huntington’s Disease? 2015)
a) Genetic Cause
Huntington’s Disease is caused by a mutation in the HTT gene found on the short arm of chromosomes 4. The HTT gene gives instructions for making the protein huntingtin. The HTT mutation involves a DNA segment that all HTT genes contain, known as CAG trinucleotide repeat, which encodes the huntingtin protein. Huntingtin is important in the proper functioning of nerve cells in the brain. It is crucial for proper cell division and apoptosis to occur, and it interacts with many other proteins in the brain making it also involved in cell transport, signaling, and intra-cellular transportation. It is found in many of the body tissues, but the highest level of activity is carried out in the brain. The CAG trinucleotide repeat segment is made up of a series of three nucleotides, cytosine, adenine, and guanine. In normal humans, the segment is repeated between 10 to 35 times within the gene, whereas for people with Huntington’s, the gene is repeated 36 to more than 120 times. The codon CAG codes for the amino acid glutamine. During transcription, the CAG repeats create a polyglutamate section within the polypeptide. The non-mutated allele would then fold into its 3D shape and form the normal huntingtin protein that interacts with others to support healthy brain cells. The mutated allele however, folds in a shape that alters the tertiary structure of the huntingtin protein and causes neurons in the brain to malfunction and die. The extra-long polyglutamate chains break of very easily and because glutamine is a charged molecule the excess fragments that are broken off causes proteins to link and clump together altering their normal protein shape as well. These resulting masses are called protein aggregates, and this is why Huntington’s is so deadly as not only does it cause an alteration in the huntingtin protein but others around it too. The death and dysfunctions of [image: Image result for pedigree of huntington’s disease]these neurons are what cause the symptoms of Huntington’s. Huntington’s Disease is inherited in an autosomal dominant fashion. This means that a person only needs to inherit one copy of the mutated allele from either of their parents to get the disease. Most people with HD are heterozygotes because the disease is so rare and the probably of both of your parents having Huntington’s is very slim. In the pedigree, it is shown that the only way for offspring to not inherit the disease is having a homozygous recessive genotype and, in that case, you will not pass the disease to subsequent generations. If you are homozygous dominant or heterozygous you will eventually develop the disease. Furthermore, the HD gene is located on the short arm of chromosomes 4, meaning it is autosomal or not found on the sex chromosomes.
b) Biochemical Result
Huntingtin is the protein that is created by the CAG trinucleotide repeat, found on chromosome 4. All HTT genes have CAG repeats however, the longer the repeat the more likely you are to develop the disease and have an earlier onset. The more CAG codons there are, the more glutamine produced. Glutamine is a vital protein used for synthesizing proteins however too much glutamine makes the huntingtin polypeptide sticky. The proteins, such as huntingtin and others that are critical to brain cell health all get caught up in aggregates due to this stickiness, making them unable to carry out their functions. The protein aggregates also become toxic to the brain cells which results in the loss of brain tissue. Huntingtin is present in cells throughout the body, however, HD selectively kills nerve cells. This suggests that the huntingtin protein regularly interacts with proteins found only in the brain and that the altered form of huntingtin disrupts the interaction causing nerve cell death. The proteins that huntingtin is discovered to mostly interact with is the huntingtin’s interactor protein (HIP-1) and huntingtin’s associated protein (HAP-1). The number of CAG trinucleotide repeats determine how much or how little huntingtin interacts with these surrounding proteins. As the amount of CAG repeats increase, huntingtin binds to HIP-1 and HAP-1 less. Scientists have discovered that although these two proteins interact overall less, the mutated huntingtin protein and HAP-1 bind more tightly than the normal huntingtin protein does. The interaction between these two proteins, is believed to contribute to the degeneration of the nerve cells. These interactions cause the nerve cells of people with HD to become more sensitive to glutamate, an amino acid precursor. This sensitivity leads to the activation of proteins called caspases that chop huntingtin into small fragments. The fragments are now small enough to slip into the nucleus of the nerve cell and interfere with normal production of proteins. The interference causes cellular stress which could lead to more huntingtin polypeptides being broken into fragments. This cycle continues are eventually leads to nerve cell death. Cells in the basal ganglia, the part of the brain that is responsible for coordinating movement, as well as the cortex, which control memory and thoughts, are most affected by HD. This causes the symptoms of chorea, emotional disturbance and cognitive decline. However, when the patients are examined after death, no area of the brain is left completely unaffected. Currently, there are no treatment options that will prevent or stop Huntington’s from progressing but there are medications that can keep patients’ symptoms under control. First there are medications that suppress the involuntary movement component of HD such as Tetrabenazine or Antipsychotic drugs (Haloperidol, Chlorpromazine, Risperdal, Seroquel). There are also medications to treat psychiatric disorders including, Antidepressants and Mood-stabilizing drugs (Valproate, Carbamazepine, Lamictal). Lastly, most patients are involved into multiple kinds of therapies in addition to their medications like, psychotherapy, speech therapy, physical therapy and occupational therapy.
c) Resources for Patients and their Families
While scientists are still searching for a cure for Huntington’s Disease, it is very important patients and families know exactly what they are dealing with, so they can plan for now and the future. Below is a list of helpful resources to inform those affected that will hopefully answer some common questions as well.
About Huntington’s Disease https://www.hopkinsmedicine.org/psychiatry/specialty_areas/huntingtons_disease/patient_family_resources/education_whatis.html
Glossary of Frequently Used Terms https://www.hopkinsmedicine.org/psychiatry/specialty_areas/huntingtons_disease/patient_family_resources/HD_terms.html
Common Misconceptions and Myth About HD https://www.hopkinsmedicine.org/psychiatry/specialty_areas/huntingtons_disease/patient_family_resources/myths_about_HD.html
Publications, Links, and Support Groups https://www.hopkinsmedicine.org/psychiatry/specialty_areas/huntingtons_disease/patient_family_resources/resources.html
- Living with Huntington’s Disease. Huntington’s Disease News. [accessed 2019 Feb 27]. https://huntingtonsdiseasenews.com/living-with-huntingtons-disease/
- Kaplan Lewis. Huntington’s New South Wales. What Is The History Of Huntington’s Disease (HD)? | Huntington’s NSW. [accessed 2019 Feb 27]. https://www.huntingtonsnsw.org.au/information/hd-facts/history
- Zachary H. Celebrities With Huntingtons Disease: Woody Guthrie, Trey Gray. Health Care Zone. [accessed 2019 Feb 27]. https://healthcarenewsblog.com/celebrities-with-huntingtons-disease/
- Huntington’s disease. Mayo Clinic. 2018 May 16 [accessed 2019 Feb 27]. https://www.mayoclinic.org/diseases-conditions/huntingtons-disease/symptoms-causes/syc-20356117
- Huntington disease – Genetics Home Reference – NIH. U.S. National Library of Medicine. [accessed 2019 Feb 27]. https://ghr.nlm.nih.gov/condition/huntington-disease#statistics
- Huntington’s disease. Wikipedia. 2019 Feb 23 [accessed 2019 Feb 27]. https://en.m.wikipedia.org/wiki/Huntington’s_disease
- Huntington’s Disease. Alzheimer’s Association. [accessed 2019 Feb 27]. https://www.alz.org/alzheimers-dementia/what-is-dementia/types-of-dementia/huntington-s-disease
- What is Huntington’s disease? Stories. 2015 Jun 19 [accessed 2019 Feb 27]. https://www.yourgenome.org/facts/what-is-huntingtons-disease