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Huntington’s Disease – Can genetic testing help?
Reproductive Genetics
8 min read
Huntington’s disease is a disease that causes the breakdown and death of nerve cells in the brain. It’s progressive, which means it steadily worsens over time. Most people inherit the disorder from a parent, but in some cases, a person can have Huntington’s without a family history. It causes an individual to have jerky and involuntary movements, known as chorea, which is why this disease is sometimes referred to as Huntington’s chorea. There is no discrimination in Huntington’s disease. It affects both men and women in all races and ethnicities. According to the World Health Organization, Huntington’s disease affects about five to seven out of 100,000 people in Western countries. Symptoms typically start in a person’s 30s and 40s, and death usually comes 15-20 years from the time the symptoms started.
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Huntington’s disease is considered an autosomal dominant mutation. This means a person only needs one copy of the mutation to have the disease. It occurs when the Huntingtin gene, also known as the HTT gene, has a mutation. This gene itself isn’t completely understood by researchers, and it’s so named because Dr. George Huntington, who was one of the first to study the disease, named it after himself. The HTT gene is responsible for coding for the Huntingtin protein. When it mutates, it changes the structure of the Huntingtin protein. This change in the HTT gene alters how the protein works in the body, damaging brain cells by clumping up and killing them. The brain cells most affected are those that are responsible for voluntary movement, thought, and memory. If a parent has a mutated HTT gene, each of their children will have a 50% chance of having the disorder themselves. What actually causes the mutation in the HTT gene is an expansion of the cytosine-adenine-guanine, or CAG, repeat sequence. Chromosomes are made up of long strands of human DNA. DNA is made up of bases: A, C, G, or T that pair up. People who have Huntington’s have an abnormally long stretch of repeated sequences of CAGs. People with Huntington’s have an average of 40-50 of these r epeats. To put it in perspective, people who don’t have Huntington’s have about 17 of these repeat CAGs, and anything more than 36 is suspicious of Huntington’s. Unfortunately, the size of the CAG repeats tend to increase in size with every generation it is passed down to, and these long repeats are associated with an earlier onset of symptoms than the generations before them.
Signs and symptoms A person who has the gene mutation will start noticing signs that something is wrong around 30-40 years old. They can be subtle, with slight changes in someone’s mood or their ability to recall memories. They could start noticing they’re slightly unsteady on their feet, or feel like they’re body isn’t quite balanced. As the disease progresses, the involuntary movements worsen. Talking becomes more difficult because it’s hard for a person to now form words with their mouth. PDFmyURL easily turns web pages and even entire websites into PDF!
Rigid muscles and contractures occur, which make walking and standing upright harder. They could also have problems moving the muscles for their eyes. Many people with the disease underestimate how bad their symptoms really are. Much of this is due to the steady decline of cognitive function. Judgment and emotional state can be impaired. Anxiety, irrational anger, and becoming easily frustrated occur in many people. Depression is the most common psychiatric problem and this is actually due to changes in the brain as a result of the disease, rather than being upset with the diagnosis of Huntington’s disease. Weight loss is a very common issue in people diagnosed with Huntington’s. Much of this is due to the constant movements and the sheer amount of calories that are burned because of always moving. In the late stages of the disease, the damage to the nerve cells takes over almost every aspect of a person’s body. They may have worsening chorea — jerky movements — but in some cases these movements will stop. The person becomes unable to leave their bed, and they can no longer talk. However, they can remain aware of their surroundings. Dementia is a devastating result of long term Huntington’s.
Diagnosis Diagnosing Huntington’s disease involves getting a full medical and family history. Doctors may do magnetic resonance imaging (MRI) to scan the brain looking for areas of damage. This may not be an accurate form of diagnosis in the early stages of the disease, but it can help rule out other causes of a person’s symptoms. Other tests include a physical and neurological exam. Neurological testing looks for memory or language concerns, and a possible psychiatric evaluation looks for signs of anxiety, depression, or other mood disorders. If a person isn’t having any symptoms, a diagnosis of Huntington’s disease can be discovered by genetic testing, to find out if a person has the HTT gene mutation. As stated above, there may be a family history of Huntington’s, but there might not be. A lot of consideration needs to go into testing for Huntington’s. Since PDFmyURL easily turns web pages and even entire websites into PDF!
there is no way of preventing or stopping the disease, there is an ethical dilemma on whether or not to get the test. Some people may not want to know that they are going to get the disease at some point in their life. They can’t do anything about it, so they don’t want to spend their time focusing on it. But for some, if there is a family history, they want to know so that they can plan for the future, and the future of their family. Once a person knows they have the disease, there are several options to consider if they want to have children. They could choose to do in vitro fertilization, or IVF. In this process, eggs are taken from a woman’s ovaries and fertilized with her partner’s sperm in a laboratory setting. The resulting embryos that develops can then be tested, a process called preimplantation genetic diagnosis. This will tell which embryos have the disease or not. The embryos that don’t contain the HTT gene mutation can then be transferred back unto the woman’s uterus, ensuring she will not have a baby with Huntington’s. If the mother or father doesn’t want to use their own eggs or sperm, they can go through an egg or sperm donor instead, bypassing their genetics altogether. Another family-building option is to test the fetus once a woman is pregnant to find out if it has the mutation. This is done by taking a sample from the fetus and testing its DNA, through chorionic villus sampling (CVS) or amniocentesis. From there, if the results show the baby has the gene mutation for Huntington’s disease, the parents can decide if they want to continue with the pregnancy or plan for the future.
Treatments Tetrabenazine is a relatively new medication approved by the FDA to treat the symptoms of uncontrolled muscle movements. Antipsychotic medications can be given to help with the more difficult emotions, such as hallucinations and outbursts of anger. For depression and anxiety, doctors can prescribe medications to help ease symptoms. However, many of these medications cause side effects such as extreme tiredness, sedation and troubles focusing, so doctors and patients need to weigh the risks versus the benefits. Anxiety can be PDFmyURL easily turns web pages and even entire websites into PDF!
debilitating, and it can help to have support persons to provide encouragement, especially because antianxiety and antidepressant medications can take a few weeks to feel the effects. Since the jerky movements caused by muscle nerve damage can impinge on a person’s daily life, physical and occupational therapist are frequently involved to help a person learn to control their movements as much as possible and to assist with coping with their new lifestyle—finding new ways to perform the things they used to do without thinking. Speech Language Pathologists can help teach a person how to swallow without choking and learn to continue to speak after the muscles in their face and jaw have been affected. Since the jerky movements caused by muscle nerve damage can impinge on a person’s daily life, physical and occupational therapist are frequently involved to help a person learn to control their movements as much as possible and to assist with coping with their new lifestyle—finding new ways to perform the things they used to do without thinking. Speech Language Pathologists can help teach a person how to swallow without choking and learn to continue to speak after the muscles in their face and jaw have been affected.
Complications Usually, it’s not the disease itself that kills people, it’s the complications that arise from it. There is an increased risk of suicide which is why it’s so important to regularly get evaluated and treated for the emotional symptoms that Huntington’s disease may cause. Complications can come about as a result of a person falling due to the uncoordinated movements. They can become injured due to falls, which can lead to more time being bedridden. In end-stage Huntington’s, the disease causes people to need around-the-clock care. As losing weight is an issue for people with Huntington’s disease, food may have to be pureed, or fed from a feeding tube in later stages of the disease as the risk of choking is very high. Swallowing becomes near impossible, and aspiration PDFmyURL easily turns web pages and even entire websites into PDF!
pneumonia (the inhaling of food or liquids into the lungs causing infection) is a main cause of death. GenomeSmart can help you understand the role of genetic testing as you plan your family.
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