Predicting Huntington’s Disease in European Descent

Rationale

“Congenital diseases are disorders that are present before or at birth.” (spine-health, 2019) “Huntington’s disease is a hereditary disease that is more common in European descent.” (ghr, 2019) It will usually affect hosts at the ages between 30-40, but there are cases of juvenile Huntington’s disease and late-onset. 'Huntington’s disease causes the breakdown of nerve cells in the brain' (mayoclinic, 2019)

“A Pedigree is a diagram that depicts the biological relationships between an organism and its ancestors.” (biologydictionary, 2019) A pedigree is important to locate hereditary diseases and traits. Pedigrees help deciphers whether the trait or disease is dominant or recessive. This means that you can find the probability of a disease or trait in offspring. To trace a dominant disease, you would see if it is authoritative compared to the other possibility, that would normally be seen in the first generation in a pedigree. Huntington's disease is a dominant gene and can usually be traced through a pedigree. There can be inconsistent information in a pedigree, a host could die before they have had any symptoms or the disease. This can make tracking the disease harder and that's why pedigrees are so helpful in the case of Huntington's disease. They can trace the generations to see the probability that their offspring could have the disease.

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Huntington's disease is not common in Asian countries compared to European. In European countries, 5-7 in 100,000 people will have HD, this is the same as countries of European descent like the USA. Asian countries are less likely to have HD with 1 in 100,000 people will have HD. This refines the disease to one heritage which helps with predicting Huntington's disease in a pedigree, therefore someone with no European descent is less likely to have HD, this means that a pedigree is more likely to predict whether someone has HD from looking at their ethnical background and their parents. Thus, a pedigree and European descent are essential in predicting Huntington’s disease. Therefore, this essay purposes the following question:

Can Huntington’s disease be predicted through pedigrees and European descent?

Background

Huntington’s disease is a rare progressive neurogenerative disease that will typically manifest at the age of 30-45. Huntington’s causes the loss of motor control leading to jerky movements, altered personality and psychiatric symptoms, and a decline in congenital function. Most HD patients have already had children before the symptoms have become noticeable. There is a 50% chance to pass on the gene to offspring, normally in large families, there will be HD in every generation. Researchers have realized that the juvenile form of HD relates to anticipation. This is when a phenotype becomes more severe from one generation to the next. Anticipation occurs when the gene is inherited from the father. (Chia, 2019)

In Juvenile HD the patient will be 30-50% more likely that a seizure could occur. With juvenile Huntington’s disease, the individual will usually die between 10 to 15 years after signs and symptoms first occur. The DNA segment is CAG trinucleotide repeat, this segment is a series of three DNA building blocks cytosine, adenine and guanine. The increase of the CAG segment leads to long versions of the huntingtin protein. This protein is then cut into smaller, toxic pieces that bind together and bunch in neurons, disrupting the normal function of these cells. The dysfunction and death of neurons in areas of the brain underlie the symptoms of Huntington’s disease. Individuals with 27 to 35 CAG repeats in the HTT gene don’t develop HD, but their children are at risk of developing the disorder. As the gene is passed down in generations the size of the CAG trinucleotide repeat should lengthen into the range associated with HD. (ghr, 2019)

With Huntington’s disease, there are four stages of the disease and its effects, the first is the early stage where the person can function fully both at work and at home. The second is early intermediate stage, the person can still work but at a reduced capacity. The third stage is the late intermediate stage, they can no longer manage work are household responsibilities, they would also be no longer able to handle financial affairs. Then it’s the early advanced stage, the person is no longer independent with daily activities. The last stage is the advanced stage, they will require complete support in daily activities. The direct cause of death from HD is never Huntington’s disease, instead, are effects of Huntington’s disease. The causes of death can range from pneumonia to heart failure, 7% of all patient deaths are suicide. The treatments found for HD still only alleviate the symptoms, through pharmaceutical and non-pharmaceutical treatment. Huntingtin the protein itself is found or “expressed everywhere throughout the body with the highest concentration in the brain. The protein should be a necessity to humans due to an experiment where it was lethal to mouse embryos when the gene was subtracted. The most affected area if the brain is striatum, this area controls planning and controlling movement. As HD progresses it starts to affect the cortex, this deteriorates the cognitive thinking. As Huntington’s disease progresses is slowly deteriorates the whole brain. (EHDN, 2019)

Evidence

Figure 1: Pedigree of an American Huntington's disease family.

Figure 1: Pedigree of an American Huntington's disease family. This Pedigree shows an American family whose mother/grandmother had HD, All the offspring of Generation I had HD, the second generation all together had 11 HD offspring out of 13. This just shows how deadly the disease is. This was an American family meaning they most likely had a European background; therefore knowing your heritage would be so important for a patient to know before having children

Figure 2: Length of the HTT CAG repeat

CAG repeat length Disease-causing? Consequences for offspring? Name

Below 27 No None Normal repeat length

27 – 35 No Repeats of 27 and more can be unstable and might increase when passed on to offspring Intermediate repeat length

36 – 39 Maybe Yes, offspring have a 50% probability of inheriting the expanded gene Reduced penetrance repeat length

40 and above Yes Yes, offspring have a 50% probability of inheriting the expanded gene Fully penetrant repeat length

Figure 2: Length of the HTT CAG repeat, this table shows how the CAG is linked to the severity of the disease. When the repeat length is below 27 repeats there should be no problem causing or offspring having the disease. Then once it is above 27 there is a possibility of offspring carrying the disease, but they are still not at risk. Once you get to the reduced penetrance repeat length is where you and your offspring will possibly have HD. Once the amount of CAG in your body is over 40 you will have the disease, but their offspring will still have a 50% chance of inheriting HD.

Figure 3: What determines the age at symptom onset?

Figure 3: What determines the age at symptom onset? This table shows how the amount of CAG directly relates to the age in which the onset will occur. As you can see when the amount of CAG repeat increases above 40, juvenile Huntington’s disease starts increasing. This shows reducing the CAG would directly help the quality and amount of life the patient would have. This means that a pedigree that would also add the amount of CAG repeats would help determine the quality of life for the offspring.

Evaluation

Nature.com is a reliable source according to https://mediabiasfactcheck.com/nature/, and it has a reliable author. The main problem is that it is 11 years old, I have not found sources that disagree with the cite. The information given is very useful since I now have a pedigree base on the first known case of Huntington’s disease. The author is Heidi Chial, PhD and she has written many articles for nature.com.

Edhn.com, the cite is a secondary source that is relaying background information. This site is very useful and is corroborated by (ghr, 2019). The author is unknown but the website itself is an organization for Huntington’s disease. It seems like a reliable source with useful information surrounding Huntington’s disease. This website is an organization that covers European Huntington’s disease, which means that it is focused on this field of work and should be one of the most reliable sources that I can use.

Ghr.nlm is a government site and is apart of the U.S national library of medicine, the source was reviewed in 2013 and then published in 2019. The source is corroborated by (EHDN, 2019). The author is unknown, but it is a gov website, so I believe that it is still valuable and reliable.

Conclusion

And in conclusion, the claim was proven, but it could be added to. Huntington’s disease can be predicted with the use of pedigrees and knowing if the individual has European descent. What could be extended to the statement that would help predict the severity of the disease that the offspring, would be knowing how many CAG repeats there are in the previous generations to predict the offspring’s quality of life. This would help contain the disease and prevent CAG repeats above 40 if controlled and studied adequately. Overall the claim was proven and justified due to the sources and evidence provided.

Bibliography

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10. Stanley J. Swierzewski, I. M., 2015. Huntington’s Disease Overview, Incidence and Prevalence of HD. [Online] Available at: http://www.healthcommunities.com/huntingtons-disease/overview-of-huntingtons.shtml [Accessed 17 September 2019].