Discovered in 1872 by George Huntington, an American doctor from New York, Huntington disease is a genetic brain disorder. Huntington disease, also known as Huntington chorea and Huntington chronic progressive hereditary chorea, which is an illness that causes the deterioration of nerve cells in the brain cumulatively, over time. This transpires in the basal ganglia, which contains nerve cells that are responsible for movement and the cerebral cortex which is accountable for congenital development. Cerebral cortex also regulates movement. George Huntington examined cases of chorea and dementia of middle-aged patients as well as past reports from two generations of familial physicians. He then wrote an essay called “On Chorea” which presented his observations and studies of neuropsychology. It described the disease and its symptoms which corresponds to what we know today as Huntington disease, named after the man that discovered it himself. As we go further in depth of understanding the disease, why and how it occurs, and what are the symptoms. We will know what genetic testing is offered for the disease and major discoveries in how to cure or delay this condition.
Looking through the lens of a human genetics scientist, what causes Huntington’s disease? Well, it wasn’t until 1993 that Macdonald et al, a doctor, found the gene that causes this disorder. Mutations in chromosome four on the short arm, located in the HTT gene, causes Huntington disease. The HTT gene regulates the production of a protein in the nerve cells across the brain. The protein is called huntingtin and is unknown today of its role in the brain, but scientists are certain it is an essential part in functioning nerve cells or neurons in the brain. The HTT gene holds an irregular chain of reoccurring nucleotides of cytosine, adenine and guanine or CAG. This series of CAG, arises multiple times in the DNA molecule. This is called trinucleotide repeat. The increased number of CAG’s in the HTT gene causes defective nerve cells. On a normal scale, the CAG sequence is repeated ten to thirty-five instances in the HTT gene. People with Huntington disease, however, accumulate thirty-six to hundred- twenty or more CAG’s in the gene. “The dysfunction and eventual death of neurons in certain areas of the brain underlie the signs and symptoms of Huntington disease.” (National Institute of health, Genetics Home Reference 2017)
Parents who are carriers of HD, have a 50% chance have transmitting Huntington disease to their children regardless of the sex of the child. Huntington disease is inherited in an autosomal dominant form which implements that carriers pass down the HTT gene from one generation to the next one. A single copy of the mutated gene in each cell has the power to cause the disorder. This means a person with Huntington disease inherits the altered gene from one of their affected parents. It is extremely rare to have this disease if a parent doesn’t possess the disease themselves. What follows? As the gene is passed down from one generation to the next, the CAG trinucleotide repeat usually grows in size. The outcome of having a large number of repeats result in developing symptoms of the disorder at a young age. People who have this disease have forty to fifty repeats of CAG. If a person accumulates more than sixty, they are likely to have the juvenile form. Another circumstance of passes the gene on, is if a person had twenty-seven to thirty-five repeats. They don’t necessarily have the disease, but can have children who may become carriers. As the HTT gene is passed down, the size of the trinucleotide repeat may expand, resulting in the parent’s child to accumulate thirty-six or more repeats, exhibiting the disease. This is called genetic anticipation. It is uncertain if people with thirty-six to thirty-nine will have HD, but people who have forty or more have an advantage of obtaining the disease.
(The photo above shows that HD is inherited in an autosomal dominate pattern. It shows that parents have a 50% chance of passing on to their children)
Huntington disease is diagnosed through computerized tomography, magnetic resonance imaging, or electroencephalography. Clinical assessments and patient history are essential, informative and are taken into account before testing. Computerized tomography uses x-rays to take images of the brain. Magnetic resonance imaging takes images of the brain through magnetic force and radio waves. Electroencephalography tracks electrical motion in the brain. People who have Huntington disease experience symptoms of develop uncontrollable and abnormal muscle movements of the arms, legs, feet, and hands gradually. Mental disabilities like depression, trouble in retaining information, cognitive performance declines, and personality disorder develops. Most also experience memory lost over time. Pneumonia, falls, heart failure, or infections are life threatening occurrences that can lead to death in Huntington disease patients. “Many describe the symptoms of HD as having ALS, Parkinson’s, and Alzheimer’s- simultaneously.” (Huntington’s Disease Society of America,2017)
Symptoms are classified through three stages. The first stage of HD, people undergo mood swings and lack in coordination. Sudden movements of the body begin, making daily task hard to do. Mediation may be prescribed for depression. The second stage includes cognitive disabilities. It becomes hard to think problems through and use reasoning to make decisions. Speech is diminished and may develop complications with swallowing. Everything they experienced in the first stage worsen in the second stage. A speech language pathologist and physical therapist may be needed to help a person with Huntington disease function. The final stage involves twenty-four-hour care of the person who is diagnosed with HD. They are unable to feed themselves or do any other activities on their own. They are unable to walk or talk, but can understand when they are spoken to. Choking is a huge concern as some die from it. Weight lost within all stages relates to results from worsen symptoms. A stable diet is mandatory in sustaining their appetite. They live over a ten to twenty-five-year period after symptoms occur and symptoms are progressive within that time. Huntington disease “affects both sexes and all races and ethic groups around the world.” (Huntington’s Disease Society of America,2017) It approximately 30,000 people in and less than 200,000 people are at risk of having the disease in the United States.
10% of people who have Huntington disease are affected at a young age due to the expanded chain of CAG’s in the HTT, passed down generations. This is called Juvenile Huntington disease (JHD). Children and teenagers experience almost the same symptoms as adult onset Huntington disease, but one expectation. “JHD typically progresses more rapidly than adult onset HD.” (Huntington’s Disease Society of America,2017) They experience difficulty in walking and have a hard time speaking. The child skills that they once had deteriorates, and they find it difficult to remember information. They go through the three stages of Huntington disease, but at a faster rate.
There is no cure as of today, but there are treatments to help cope or handle the symptoms. “In August 2008, the Food and Drug Administration (FDA) approved tetrabenazine (Xenazine) for the treatment of the repetitive, involuntary movements (chorea associated with Hunting’s disease. This is the first and only FDA-approved treatments specifically for any symptom of HD” (National Organization for Rare Disorders,2007) Neuroleptic medication can help stop uncontrollable movements or ticks’ moments at a time. Other medication are given to patients with HD to help them deal with depression and emotional instability. To relieve them from severe cases of each symptom, a distinct high calorie diet is prepared for them.
In 1986, genetic testing for people with Huntington Disease was established. From then to today, it was advanced in its techniques to give the opportunity for people to know their risk for genetic disease and for giving them information about transfers these disease to their children. In 1994, gene test was offered. Because there is no cure for Huntington Disease, the reasoning behind genetic testing is to identify people who are at risk and to educate at risk patients on decisions to make for their future health wise. One common testing that is done for patients who are at risk of HD is presymptomatic testing. It consists of genetic counseling, neurological evaluation, and the DNA analysis. Genetic counseling informs the person who is seeking information the information they need to make a crucial decision wanting to know if they will have HD. Two sessions must be completed, this give them time to decide if they would like to move forward and if they don’t, they also have the chance in not proceeding with the following steps. Blood is drawn during the second visit if they choose to continue. Counseling is taken very seriously as the rate of suicide from results can be expected. Neurological evaluations are a pretest given to patients after genetic counseling for patients who think they are experiencing symptoms of the Huntington Disease. Patients who is given negative results in having symptoms of the disease, commonly don’t continue with gene tests. Psychological counseling is required for those who test positive to HD. Only 5% to 7% of patients who are at risk tend to go on to predictive testing. For children who are at risk, it is not recommended for them to take genetic test until they are the legal as of eighteen. This is to ensure that they done go through trauma as they may in fully uncomprehend the details of testing and its consequences of knowing. Nevertheless, children younger than eighteen can take the test if they are at risk of inhibiting the disease through family history.
Another test that is done is prenatal testing. In fact, “About one-half of 1% of all HD tests involve prenatal test…The majority of those who test gene-positive opt not to have children rather than undergo prenatal testing.” (Richard H. Myers,2004) They go through prenatal counselling session that is very similar of that of presymptomatic testing. Couples who are expecting a child and don’t want to abort the child tend to perform Preimplantation genetic test diagnosis (PDG). This option allows them to go through with carrying the child in greater hope that their child or children won’t have HD. “Eggs are harvested, fertilized in vitro, tested and those testing gene-negative are implanted.” (Richard H. Myers, 2004) This is usually done on the between ten to eighteen weeks of pregnancy. “The overall pregnancy rate was 24% per cycle started, 29% per egg collection, 38% per transfer, and 40% per couple treated.” (Richard H. Myers,2004) This means that the success rate of PDG is low and does not guaranteed success in having a child with HD. Couple who decide to go through PDG, typically never had a presymptomatic testing done beforehand.
On a positive note, there is hope for the future! In 2016, medical doctors Steve Finkbeiner and Ian Kratter from Gladstone Institutes are trying to come up with a therapeutic target for Huntington Disease. Their main focus of their research study was to reduce the toxicity that formed in the brain due to accumulated HTT gene through the process of phosphorylation. Phosphorylation is a protein which plays a key role in chemical signaling and helps normalize mechanisms with a cell. It does this by changing the structure of other proteins and changing its actions. One of their main concerns before conducting the experiment, was finding good testing subjects. Using mice, which does not develop HD, they had to manipulated and genetically modified their genes so that they could have HD. When that was complete, they induced endogenous S421 where the site of transcription starts in the gene. The S421 is a specific point of the huntingtin protein. “S421 mutates to mimic phosphorylation and mitigates neurodegeneration by increasing proteasome-dependent turnover of mHTT which reduces toxicity of mHTT.” (Ian H. Kratter, Hengameh Zahed, Aaron C.Daub, Kurt F. Weilberth, Steven Finkbeiner, 2016). The end results? Phosphorylation allows neurons to remove more of the damaging protein, so it won’t store up in the cell and harm it. It worked! Unfortunately, Doctor Finkbeiner and Kratter don’t know how long this affect will last in the mice and this hasn’t been tested on humans yet, we are one step closer to finding a new treatment or cure for Huntington Disease!
Huntington Disease is an illness that causes the deterioration of nerve cells in the brain through the overproduction of the HTT gene trinucleotide repeats. It’s a progressive disorder that shows symptoms of Abnormal and uncontrollable muscle movements, cognitive impairment, and change in personality progressively overtime. Researching HD and learning new, cut throat information, we be optimistic in discovering new finding of treatment and cure for these individual through new technologies. Hopefully in the near future patients of Huntington disease will no longer live with painful, emotional, and mental stress of dealing with the harsh realities of the disease itself and the decisions. And that soon they could live normal lives with family and friends.