How The Huntington's Disease Gene Affects How It's Inherited
By: Rachel Lustbader
Updated February 05, 2020
Medically Reviewed By: Kelly Coker, M.B.A., Ph.D., LPC, NCC
Genetics play an important factor when it comes to many diseases. Over 6,000 diseases are inherited through genetics. Sometimes, certain genetics don't give you the disease directly but may make you more prone to inheriting it. If you have a family history of cancer, you may be at risk for cancer, and you should check up with your doctor and live a healthy lifestyle to prevent it. Other times, genetics can mean that you're going to get the disease no matter what. Case in point, Huntington's disease, or HD.
What Is Huntington's Disease?
HD causes your nerve cells to decay over a period, leading to a decrease in all functions. It's genetic, and it typically develops in a person's 30s or 40s, but it may develop even earlier than that. Because it causes your nerve cells to break down, this leads to a decline in physical and cognitive abilities and may lead to different mental diseases.
The Gene That Causes Huntington's Disease
A mutation in the HTT gene causes HD. This gene is responsible for telling your brain to create huntingtin, an essential protein. Huntingtin's exact purpose is not fully understood, but it's found in the brain as well as body tissue. What we do know about huntingtin is that it probably plays a key role in nerve cell health. It keeps them strong and prevents them from self-destructing. If something were to change the HTT gene, then you can see how it can affect your nerve cells.
Because most cases of HD happen in the 30s and 40s, most people who want to have kids have already had them, and had them before the symptoms of the disease began to take effect.
If one of your parents has Huntington's disease, you may wonder what the chance of you inheriting the mutated gene is. Regardless of the sex of the parent affected, the odds of you inheriting it is around 50 percent. For many, this is quite anxiety-inducing. A genetic flip of the coin determines your fate.
HD occurs in about 3 in every 100,000 people, so the odds of both parents having the disease aren't too common. However, if they do have the gene, the odds, as you may expect, go up. Sometimes, it will go up to 75 percent. In some cases, all children may get it at a 100 percent rate.
If your parents have HD, but you don't, then there is still a risk in passing it down to your children.
This means that the best way to cure Huntington's disease is to breed it out. As mentioned before, its late-stage occurrence means that most parents would have already had kids by the time symptoms begin to show, but it can occur at a younger age, such as your 30s, when you're about to have your first child.
If that's the case, then many parents may consider adoption or a surrogate parent as a solution.
The Preimplantation Controversy
Some parents who want kids of their genetic makeup can take certain measures to make sure none of their children have HD through preimplantation genetic diagnoses. This is where a lab uses the parents' sperm and eggs to create multiple embryos, and the embryos are tested for any signs of the HTT mutation. If one does not have it, it is implanted into the mother.
It's not the cheapest way to do it, nor is it the most uncontroversial. Some may have a problem with the idea of this kind of breeding, but if you don't, then it may worth being looked into.
Huntington's Disease And National Origins
What is quite interesting is that your risk of HD may depend on your national origin. Those with European origins have the most likely chance of getting it. It's still rare at 3 in 100,000 cases, but it's the highest. Asian countries tend to have the lowest cases of HD. This may be due to the origins of the mutations.
Symptoms of Huntington's Disease
As the disease involves the gradual breakdown of your nerve cells, the symptoms tend to be cognitive or affect your movement. Mental disorders may develop due to HD as well.
When HD affects your movement, it may be subtle changes at first, but eventually, it will affect you enough where you'll need care. It isn't uncommon for those who are suffering from HD to be injured or die due to falling or losing their balance. Other symptoms include:
- Sudden twitching or jerking movements. These will happen quite often.
- Trouble swallowing. Many patients with HD may need assistance swallowing their foods, as choking is quite common.
- Talking may become difficult with HD because mouth movements are hard.
Then there is cognition. This involves processing and thinking, which will grow worse over time. These include:
- The inability to focus. Everyone spaces out on occasion, but people with HD will have it happen to them all the time.
- Your thoughts may freeze. As you're trying to speak, you may stumble or repeat a thought.
- There may be worse impulse control than usual. Those with HD should not gamble or go shopping without assistance. Also, not thinking before you speak is another problem.
- People with HD may have trouble with their self-awareness. They are not living in the present, but instead on autopilot.
- You may have memory problems. Besides not being able to recall past events, you may have difficulty retaining things you've learned.
HD can also lead to mental disorders, including:
- OCD. Obsessive-compulsive disorder is when you repeat certain tasks over and over again because you're not satisfied.
- Bipolar disorder. This is when your mood suddenly changes. Sometimes, you'll feel happy and superior to everyone else. On other days, you may feel apathetic and depressed.
- Depression, in general, is another symptom of HD. It's not only situational, where the patient is depressed because they have a disease that will cause their brain power to break down over time, but changes in your brain also cause it.
Juvenile HD is when children or adolescents develop HD. It's much rarer, but much deadlier, as the symptoms may come across as children being children. A child with JHD may have trouble in school, have behavior problems, or forget things. Sometimes, this can be mistaken as normal teenage behavior, with the only puzzling symptoms being seizures. People with JHD usually have a lower life expectancy too.
At first, people who have HD may be able to live normal lives, but over time, their symptoms will worsen, and they will probably need assistance in living. Sometimes, the disease progresses slowly, and the person who gets it may have 30 years to live. Other times, particularly in JHD cases, it can kill you in just ten years.
Because the cells in your brain are breaking down, the weight of your brain will go down. The average adult brain is about three pounds. At the late stages of HD, the brain may drop a pound.
Sometimes, it's not the disease that kills you, but the symptoms. Choking, falling, and seizures are just a few reasons why one would die. Suicide is another big problem, occurring in almost 10 percent of patients. Suicide tends to come early. It can be due to depression, or because the patient wants to end their life before they lose all their cognition.
Treatment of Huntington's Disease
Prevention through genetic counseling is the only way to eradicate the disease. If you have it, there is no cure or a way to stop it, but there are ways to slow it down. This usually happens by treating the symptoms.
If you exercise and have a healthy diet, this can make your physical and mental health much better, helping you in the long run. Physical therapy can help your movements and reduce the number of falls and swallowing issues you may have. Mental stimulation can help your cognitive abilities. Medication can not only help your physical and mental symptoms but may be able to treat your depression or bipolar disorder.
HD can be a heavy burden for the patient. They may become apathetic and let themselves succumb to the disease, rather than slow it down and live the longest life possible. They may have suicidal thoughts, too. A counselor can help the patient cope with the disease, and make a plan to live the best life possible under these circumstances.
The caretaker may feel depressed or tired over taking care of the patient, especially if the caretaker is a family member. Counseling can help keep the caretaker's spirit up and teach them techniques to take care of the patient.
Huntington's disease is quite horrifying, but if you know your genetic history and take steps to prevent it from passing it on to your children, you can take one step in helping to end it.
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