Huntington's Disease: The Genetics

Imagine a Perfect Life

Imagine the perfect life. The perfect life, in some minds, would include a well-paying job, a sizable home, marriage, and a family. Now, on the other end of the spectrum, imagine second guessing every decision that you could possibly make; deciding whether to buy a new car, deciding whether or not to go to college, or contemplating having children. Which scenario sounds more appealing? It is an accepted (somewhat common) thought that human nature is to act on whim and do things impulsively. Humans do things without thinking of the consequences. But, if there was some force, some unwanted and unrelenting force, issuing a person to rethink every decision in their life, then that force would be a terrible thing. Unfortunately, there is such a terrible thing and it may never simply go away. Huntington’s disease is this unwanted and unrelenting force.

Personal Note

On a personal note to you dear reader, is, this research article, paper, reflection, or what have you, is a sensitive topic for me. I know someone who has been diagnosed with HD (Huntington's disease) and I am very close with their family. I write this not only for a grade, but more to further my understanding of the disease on the genetic level. Through my experience, not only is the individual who has been diagnosed with HD affected, but so is the immediate family. For some members of the family, the emotional stress that arises from the mere implication that they may be a carrier of HD weighs heavy on their mind.

Heredity

In this Punnett Square, the capital H means the disease is present. Since the HD allele is dominant, only one parent needs to carry it in order for the disease to occur at least 50% of the time.
In this Punnett Square, the capital H means the disease is present. Since the HD allele is dominant, only one parent needs to carry it in order for the disease to occur at least 50% of the time.

How Does it Happen?

To understand why people are affected so deeply by Huntington’s disease, one must first take a look into the genetics of Huntington's disease. "Huntington’s disease (HD) is a dominantly transmitted neurodegenerative disorder with wide variation in onset age but with an average age at onset of 40 years" (Myers 2004). The HD allele (DNA coding that occupies a given position on a chromosome) would be the allele that is dominant. Huntington's disease would then, because it is the dominant allele, reside inside someone and make them a carrier of HD. The second part concerning the average onset age of 40 is typically when carriers start showing symptoms that they possess HD. Because of the late age of onset, people often times do not know that they are carriers of HD. This is cause for concern because these carriers may have already had children, who in turn, may also be carriers. "Children of HD gene carriers have a 50% chance of inheriting the disease" (Myers).

Location of Mutation

Scientists and researchers have discovered the specific allele that causes Huntington's disease, what happens to the allele when it is synthesized in the body, and the reason why certain characteristics are portrayed. Huntington’s disease has been widely researched and much is known about the disease genetically. "The gene is located on chromosome 4p16.32 and the genetic alteration which causes the disease is an increase of the number of repetitions of three nucleic acids (C, A, and G) in the coding region of the first exon of the HD gene" (Myers). These repetitions of the nucleic acids (C for Cytosine, A for Adenine, and G for Guanine) overstimulate protein production in the body. This overproduction of protein starts blocking nerve endings which results in many of Huntington’s disease's symptoms. Notable HD symptoms include "involuntary chorieform movements, cognitive impairment, mood disorders, and behavioral changes that are chronic and progressive over the course of the illness" (Myers).

HTT Gene

The gene responsible for Huntington’s disease is called “huntingtin” and is otherwise known as the HTT gene. The HTT gene creates a protein called huntingtin and is thought to be essential in nerve cell (neurons) development in the brain, but is not yet conclusive (Genetics Home Reference 2008). As stated previously, the Huntington’s disease allele can be found on chromosome 4p16.32. This means it is found on chromosome 4 between base pairs 3,076,407 to 3,245,686 (Genetics Home Reference). For a more distinct look at chromosome 4 the image shown below displays where the HTT gene can be found.

Chromosome 4

Source

In the DNA replication process, the HTT gene contains a segment of trinucleotide CAG that is normally duplicated anywhere between 10 and 35 times. However, in the case of Huntington’s disease, this trinucleotide CAG replicates itself 36 to over 120 times.“The expanded CAG segment leads to the production of an abnormally long version of the huntingtin protein. The elongated protein is cut into smaller, toxic fragments that bind together and accumulate in neurons, disrupting the normal functions of these cells” (Genetics Home Reference).

Trinucleotide CAG Repititions

Individuals who have the trinucleotide CAG replicated 27 to 35 times will not develop Huntington’s disease themselves, but they are at risk for carrying the disease and giving it to their children. As the number of generations grow in a family with a history of Huntington’s disease, the number of repetitions of the trinucleotide CAG grows (Genetics Home Reference). This means a carrier parent that has 35 repetitions of the trinucleotide CAG and shows no signs of Huntington’s disease could have a child that has 37 repetitions, which results in the child developing Huntington’s disease. These repeats of the CAG strand can vary widely depending on whether the gene is inherited via the mother or the father. If the offspring received the HTT gene mutation from the father, it has a higher chance of repeating larger amount of times. Maternal transmission of the gene are relatively smaller, typically ranging from 1 to 3 added repeats. Paternal repetitions are far larger and more typical, which explains how many Huntington’s disease carriers/developers often receive their disorder from the father (Myers).

Age ranges of Huntington's disease onset
Age ranges of Huntington's disease onset

Average Age of Onset

Huntington’s disease is not apparent, often, in the early stages of life. In fact, in most
cases Huntington’s disease does not develop until one reaches their thirties. “The
age at onset distribution in Huntington’s disease is very broad and may vary from
as young as 3 or 4 years of age to as old as 85” (Myers). The average age of
developing Huntington’s disease is forty years old. As seen in the graph to the
right, developing Huntington’s disease follows a bell-shaped curve. The highest
frequency occurs in the age 40 – 44 range and slightly drops as the age both
increase and decreases.

Notable Symptoms

More often than not, the symptoms that carry themselves along with Huntington’s disease are quite apparent. Behavior changes and unusual movements may in sue due to the huntingtin protein overproducing and attacking nerve endings. Wild jerking of appendages is a very visible sign, as is seen in most Huntington’s disease patients (National Institute of Health 2011). Dementia is another effect of developing Huntington’s disease. Dementia will move in much sooner than in the average healthy human (NIH).

Complications

Family members and others will be left to deal with some of the complications that go a long with a person having Huntington’s disease. A notable complication that one may find is the lack of self care. As the length of time the disorder has been apparent increases, the complications and symptoms worsen (NIH). Self care can range from setting up doctor’s visits to bathing one’s self properly. This is cause for concern because if self care is deteriorating, then the person will not feel adequate enough to keep living and may give up.

Treatment

Unfortunately, at the current time there is no cure for Huntington’s disease. Prenatal testing is available to the parents if there is concern the baby has Huntington’s disease. In fact, prenatal testing for Huntington’s disease has been a major forerunner in prenatal testing and has provided a lot of insight for other late-onset genetic disorders. The only treatments that are out on the market only aim to slow the symptoms of the disease (NIH). There are many clinical trials being done that target Huntington’s disease. One clinical trial study will “look at how safe, tolerable and effective SD-809 ER is in HD subjects who switch from tetrabenazine to SD-809ER” (HDSA 2013). These clinical trials are hopeful to the Huntington’s Disease Sociey of America.

A new research program, started by Steven Finkbeiner, MD, PhD, and Paul Muchowski, PhD, is underway. Finkbeiner and Muchowski have anticipated a treatment or cure by the year 2020. Although funded through various agencies, not a lot of exposure has been given to Huntington’s disease and could affect the progress of treatment research. The duo intends on using new medical technology as a means of furthering scientific research on the Huntington’s disease gene, more specifically, the best way to approach the replication process of the trinucleotide CAG in order to disrupt the process (Medical News Today). There has been other previous research looking into the trinucleotide CAG repeats for a specific sequence that could possibly explain the mutation, specifically a G8 (Harper 2004).

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Outlook hopeful

Many are hopeful in a breakthrough for this destructive disease. However, unless exposure to this disease is spread to the public, it will remain silent and not receive proper funding to research any possible cures or treatments. Much about Huntington’s disease is known already and more updated scientific research techniques can only further the rate at which scientists can discover more.

References

For more information, visit the sites below.

Differences in duration of Huntington's disease based on age at onset; Foroud, Tatiana; Gray, Jacqueline; Ivashina, Julia; Conneally, P Michael .Journal of Neurology, Neurosurgery and Psychiatry66.1 (Jan 1999): 52-6.

http://ghr.nlm.nih.gov/condition/huntington-disease

http://ghr.nlm.nih.gov/gene/HTT

http://www.hdsa.org/

Landmarks in Medical Genetics: Classic Papers with Commentaries; Peter S. Harper

http://books.google.com/books?hl=en&lr=&id=A1hChCwPefsC&oi=fnd&pg=PA153&dq=Huntington%27s+disease&ots=_ww0jBgVqf&sig=qYp2yu2v0Eyh7w-dHmTwtpgc3yc#v=onepage&q=Huntington's%20disease&f=false

http://www.medicalnewstoday.com/releases/143861.php

Myers, Richard H. “Huntington's disease genetics.” NeuroRx 1.2 (2004): 255-262.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC534940/

http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001775/

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