Are Most Genetic Conditions as Easy to Predict as Cystic Fibrosis or Huntingtons Disease

Huntington's disease is an autosomal dominant hereditary condition for which there is genetic testing. The gene abnormality, an increase in CAG repeats, is found on chromosome 4.

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What Is Huntington's Disease?

Huntington's disease is a hereditary condition that is marked by a severe movement disorder and progressive dementia. Huntington's disease begins around age 40 on the average, and, less commonly, begins during the teenage years. Those who are afflicted with Huntington's disease have a life expectancy of about 10-20 years after the onset of symptoms.

The condition causes substantial disability and increased dependence on caregivers as it progresses. According to the University of California San Francisco, Huntington's disease is a relatively uncommon condition, affecting approximately 1 out of every 10,000-20,000 people in the U.S., adding men and women are equally affected at the average age of onset between ages 30-55.

Unfortunately, Huntington's disease is currently incurable, and those who are diagnosed with the disease have no treatment options except for supportive care, which is focused on injury prevention and avoidance of complications, such as malnutrition and infections.

The gene responsible for Huntington's disease was discovered in 1993, and a blood test is available to identify whether you carry the gene.

The outcome of the disease and the lack of treatment are among the reasons why people who have known Huntington's disease in the family seek out genetic testing. Identifying the illness through genetic testing can help families prepare for impending difficulties, and can help in making decisions about family planning.

Heredity of Huntington's Disease

One of the reasons that the genetic test for Huntington's disease is so useful is that the condition is autosomal dominant. This means that if a person inherits only one defective gene for Huntington's disease, then that person is very likely to develop the disease.

The Huntington's gene is located on chromosome 4. All people inherit two copies of each gene; so all people have two copies of chromosome 4. The reason this disease is characterized as dominant is that having only one defective gene is enough to cause the condition, even if a person has another normal chromosome. If you have a parent who has the disease, your parent has one chromosome with the defect and one chromosome without the defect. You and each of your siblings have a 50 percent chance of inhering the disease from your affected parent.

Genetics of Huntington's Disease

Gene coding is made of a sequence of nucleic acids, which are molecules on our DNA that code for the proteins that our bodies need for normal function. The specific coding deficit in Huntington's disease is an increase of the number of repetitions of three nucleic acids, cytosine, adenine, and guanine, in the region of the first exon of the HD gene. This is described as a CAG repeat.

Normally, we should have about 20 CAG repeats in this particular location. If you have less than 26 repeats, you are not expected to develop Huntington's disease. If you have between 27 and 35 CAG repeats, you are unlikely to develop the condition, but you are at risk of passing on the disease to your offspring. If you have between 36 and 40 repeats, you may develop the condition yourself. People who have over 40 CAG repeats are expected to develop the disease.

Another observation with this genetic defect is that the number of repeats often increases with each generation, a phenomenon known as anticipation. So, for example, if you have a parent who has 27 CAG repeats in the region responsible for Huntington's disease, you could have a sibling with 31 repeats, and your sibling may have a child who has more repeats. The importance of anticipation in the genetics of Huntington's disease is that a person who has more CAG repeats is expected to develop symptoms of the disease earlier than a person who has fewer repeats.

Huntington's Disease Genetic Test Logistics

The way to get tested for Huntington's disease is through a diagnostic blood test. The accuracy of the test is very high. Normally, because Huntington's disease is such a serious condition, counseling is recommended before and after your genetic test results.

There are some strategies that your healthcare provider may use to better interpret your test results. For example, if you know that you have a parent with Huntington's disease, your healthcare provider may test your other parent as well to see how the number of CAG repeats on each of your chromosomes compares to the number of repeats of each of your parents' chromosomes. Testing of siblings can also help put the results in perspective.

How the HD Gene Causes Huntington's Disease

The genetic problem of Huntington's disease, the CAG repeats, causes an abnormality in the production of a protein called the huntingtin protein. It is not completely clear exactly what this protein does in people who do not have Huntington's disease. What is known, however, is that in Huntington's disease, the huntingtin protein is longer than usual and becomes prone to fragmentation (breaking into smaller sections). It is believed that this elongation or the resulting fragmentation may be toxic to nerve cells in the brain that affects mood and behavior.

The specific region of the brain impacted by Huntington's disease is the basal ganglia, a region deep in the brain that is also known to be responsible for Parkinson's disease. Like Parkinson's disease, Huntington's disease is characterized by movement problems, but Huntington's disease is more rapidly progressive, is fatal, and dementia is the most prominent symptom of the disease.

A Word From Verywell

Huntington's disease is a devastating condition that affects people in early middle age and leads to death within 20 years. Despite the scientific understanding of the disease, there is, unfortunately, no treatment that can slow disease progression and no cure for the disease.

Genetic testing for Huntington's disease is a huge advancement that can help with decision-making. The results of genetic testing can assist with family planning and preparation for disability and early death.

If you or someone you know has Huntington's disease or has been told that the disease will emerge in upcoming years, it is helpful to connect with support groups as a way to understand about how to cope and to lean on those who have similar experiences.

Additionally, while there is no treatment at this time, research is advancing, and you might consider participating in a research study as a way to learn about your options and to gain access to the latest developing and emerging therapeutic options.

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

1. McColgan P, Tabrizi SJ. Huntington's disease: a clinical review.European Journal of Neurology. 2018;25(1):24-34. doi. 10.1111/ene.13413.

2. Kerschbamer E, Biagioli M. Huntington's disease as neurodevelopmental disorder: altered chromatin regulation, coding, and non-coding rna transcription.Frontiers in Neuroscience. 2016;9. doi.  10.3389/fnins.2015.00509.

3. Colpo GD, Rocha NP, Furr Stimming E, Teixeira AL. Gene expression profiling in huntington's disease: does comorbidity with depressive symptoms matter?International Journal of Medical Sciences. 2020;21(22):8474. doi. 10.3390/ijms21228474.

Additional Reading

• Bates GP, Dorsey R, Gusella JF. Et al. Huntington disease. Nat Rev Dis Primers. 2015 Apr 23;1:15005. doi:10.1038/nrdp.2015.5

• Richard H. Myers. Huntington's Disease Genetics. NeuroRx. 2004 Apr; 1(2): 255–262. doi:10.1602/neurorx.1.2.255

By Heidi Moawad, MD
Heidi Moawad is a neurologist and expert in the field of brain health and neurological disorders. Dr. Moawad regularly writes and edits health and career content for medical books and publications.

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Source: https://www.verywellhealth.com/genetic-testing-for-huntingtons-disease-4161192

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