Huntington’s Disease; Historical and Medical Overview
Huntington’s disease is a genetic neurological disorder involving abnormal body movements and a lack of coordination. Additionally it had mental and behavioural affects. This disease was one of the first inherited genetic disorders on which an accurate test can be performed as the gene for HD was discovered in 1993 (Thomson, A., 1993). Originally individuals with Huntington’s were diagnosed with alcoholism or manic depression, most deaths in the early 20th century were due to starvation or dehydration as individuals were misdiagnosed. Huntington’s may have been around before the Middle Ages, due to its abnormal motion it may have been referred to as St. Vitas’ dance. One of the first medical descriptions of this disease was made in Norway in 1860 when it was noted that a remote and secluded area had a high prevalence of dementia with a high pattern of abnormal movements that ran in families. In 1872 George Huntington, third generation medical practitioner, whose family shared HD symptoms realized that the conditions were linked and he provided an accurate description of the disease. (Bates, G., et al. 2002) Over a hundred years later in the late 1970s the U.S.-Venezuela Huntington’s Disease Collaborative Research Project began which provided the basis for this HD gene’s discovery (Avila Giron, R., 1973). The discovery of the HD gene occurred in 1993 and was an important beginning step for the human genome project. Since then much research has been done to further understand Huntington’s disease.
Of the many medical conditions to investigate, Huntington’s is most interesting as there is a lot of information available on how the disease works genetically and links to physiology. I have not known anyone directly with Huntington’s disease but due to the fact that the gene for this disease is actually dominant, whereas most genetically linked diseases are recessive, Huntington’s disease directly influenced the development and evolution of the genetics field.
There are many symptoms involved with Huntington’s and it is found that the symptoms severity ingresses progressively. However symptoms are not always recognized as being caused by Huntington’s until they are considered severe. The physical symptoms, jerky, random and uncontrolled movement, called chorea, are the most common. In the beginning stages of the disease these movements are generally thought to be due to a lack of coordination. As the disease progresses any function that utilizes muscle control will be affected, included stability, facial movements, eating movements, walking, and rapid eye movement sleep (Arnulf, I., et al. 2008). Slow movements and stiffness can develop at the later stages of Huntington’s and are more characteristic of juvenile HD, which can progress to the stage of seizures (Kremer, B., 2002).
Besides physical symptoms there are cognitive and psychopathological problems that arise due to Huntington’s. Cognitive abilities become impaired as the disease progresses affecting planning, abstract thinking, initiation actions, perceptual and spatial skills, methods of remembering, short-term memory, and the ability to learn new information or actions. Cognitive symptoms will differ depending on the individual. Psychopathological symptoms will vary more than the cognitive or physical symptoms based on the individual. These include anxiety, fear or sadness of others, depression, aggression, compulsive behaviour, inability to recognize negative emotions. These can lead to an increase in negative addictive behaviour if it is already present. (Johnson, S.A., et al. 2007, van Duijin, E., et al 2007)
The diagnosis of Huntington’s begins with a physical and/or psychological examination if initial symptoms or evident. This is mainly due to uncontrolled movements but for some individuals the disease may commence with cognitive or emotional symptoms that may be difficult to identify. The easiest way to determine if an individual has Huntington’s is by a blood test, which counts the number of repetitions in the gene. A Negative blood test means that the individual does not have Huntington’s and cannot pass it on to their children. A positive blood test means that the individual has an expanded repeat in the gene and will develop e the disease, have a 50% chance of passing it on to their children, but this is not considered a diagnosis as that person may not develop any symptoms of years. As this disease is not currently curable and considered life-changing test cannot be done for children under 18 and counselling is required. It is currently possible to pre-screen embryo’s for Huntington’s to ensure a child will not have or carrier the HD gene. (HDSA, 1996)
The gene involved with Huntington’s is called the HD gene. It is also known as Huntingtin (HTT) or IT15. It is located on the short arm of chromosome 4. The initial part of the gene on the %’ end has a repeated sequence of cytosine, adenine and guanine, or CAGCAGCAGCAG….. This trinucleotide repeat codes for glutamine and forms a chain called polyglutamine or polyQ. The Huntington disease results when the polyQ chain has more than 36 glutamines and produces a mutant form of Htt. This mutant Htt causes increased moratality of neurons in specific areas of the brain. The number of glutamines in the polyQ chain is related to the rate of acquiring functions, the more repeats the faster the onset of symptoms. (Kieburtz, K., et al 1994) Huntington’s has never been diagnosed with less than 36 glutamines in a polyQ chain. Huntington’s is dominant so only one affected gene is needed to cause the disease. It is possible for a parent who doesn’t have Huntington’s but has a count of glutamines close to 36 to give their child Huntington’s due to DNA replication dynamics. Huntington’s is not generally associated with any new mutations, so a parent that has 15 glutamines in its polyQ chain will not pass the disease on to their children. (Cong, S.S., et al. 1997) The most dominant symptoms of Huntington’s are when an individual receives the mutated gene from both parents. However whether you have two copies or one of the mutated genes the outcome of the disease is the same. (Wexler, N.S. et al. 1987)
There is no treatment to “cure” Huntington’s disease. There are treatments, medication and therapy available to treat the symptoms of Huntington’s disease. Emotional symptoms can be treated with antidepressants and other drugs affecting the psychosis. Psychological visits are recommended when the disease is first diagnoses, even if there are no symptoms yet as it is a life changing disease and the individual will eventually develop symptoms, it is all a matter of time. Counselling may also be necessary for family members as the disease progresses and the disease affects the individual’s manner and attitude. (van Duijin, E., et al 2007) Speech therapy is very common and if started early can be very effective as the ability to learn is reduced as the disease progresses (Zinzi, P., et al. 2007).
Possibly the most important treatment for Huntington’s is maintaining the correct nutrient balance. In order to maintain their correct body weight in the late stages of Huntington’s an individual will need to have a greater nutrient intake. This can be difficult as the ability to swallow is affected by the disease. Thickening agents can be added to drinks. These agents increase the nutrient content and are also safer and easier to swallow than a “watery” liquid. Consuming healthier foods at the onset of the disease appears to slow the progression of symptoms while healthy and/or junk foods consumed before onset or at the early stages speed up the progression of symptoms. (Gaba, A., 2008) The FDA has done a clinical study to determine the affects of an omega-3 fatty acid, EPA. Preliminary results show that there is an improvement in motor skills and it is thought that EPA may slow or possible reverse the progression of EPA. However this study has not been completed yet though the preliminary results are positive (Puri, B.K., et al. 2005).
At the late stages of Huntington’s there are significant safety risks to the individual and constant attention and care is needed. As the individual looses motor functions, restraints, like seat belts in wheel chairs and padding when appropriate should be used. Constant monitoring of the diet, especially when swallowing becomes difficult is important as choking or suffocating can occur. If the individual has significant psychological affects then their judgement may become impaired. During the late stages of Huntington’s the individual will need have constant care or be in surroundings, like a nursing home or home care that monitors them constantly. There is no one treatment that can be used singularly for Huntington’s but with careful surveillance and monitoring a variety of treatments can be used to help the individual.
Research has progressed since the discovery of HTT. Drug companies have reached Phase III trials, which are conducted on humans. Intrabody therapy, genetically engineered antibodies called intrabodies, bind to the end of the mutant HTT gene and reduces the mutant protein accumulation and formation mainly due to protein misfolding. This treatment has been used on drosophila flies and greatly increased their lifespan (Miller, T.W. and Messer, A. 2005) Since Huntington’s is linked to a single gene than gene silencing is a potential tool to control or reverse the disease. When using gene knockout on mutant HTT in mice, the disease was found to have a 60% reduction in the protein expression and stalled the progression of the disease (Harper, S.Q., et al. 2005). Additional research in gene silencing with mice in the late stages of Huntington’s recovered their motor skills once the mutant HTT gene was stopped (Diaz-Hernandez, M., et al 2005). An ongoing treatment that does not stop Huntington’s but stops the progression of Huntington’s symptoms is by using stem cell implants (Vince, G., 2005). Stem cells are able to form any type of cell so by implanting them in the affected areas they will form into the needed cell. This treatment has shown positive results but do to the controversy with stem cell treatment it may be difficult to have ongoing constant treatment needed to alleviate symptoms as the neurological damage still progresses because the mutant gene is still active. There is much research being conducted not only in genetics, but dealing with nutrition, exercise and psychological factors as well.
Huntington’s disease and the eventual cure of the disease will be a large step for the scientific field. The discovery of the HTT gene was instrumental to the genetics field. However the physical and emotional affects of the disease are dramatic and sad considering the fact that if you have got the mutated gene you cannot actually stop the disease, you can prevent and elongate the onset of the disease but ultimately you cannot stop the disease. It affects both the mind and the body. Scientifically Huntington’s is a bit exciting but at the emotional and human level it breaks the heart. It will be a great day when this mutation can be corrected particularly for those who have not developed symptoms but one day will.
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