Diseases and Disorders

Huntington's Disease: An Overview

Jason Luo


Introduction

 It is no doubt that neurological disorders and conditions are extremely lethal as they can attack one of the most vital organs in the body: the brain. Our brains are responsible for our daily functions such as regulating our coordination, encoding information, and storing short/long term memory through our neurons and brain cells. Huntington's disease is a genetic and neurological disease that causes the death of nerve cells in the brain. This article will look at the effects and symptoms of Huntington's disease as well as how it is diagnosed and treated in infected patients.
 

What is Huntington’s Disease?

Huntington’s disease is an autosomal dominant brain disease that results from an expansion mutation in the HTT gene on chromosome 4 [1]. The wild-type HTT gene is responsible for synthesizing a protein called huntingtin that plays a role in chemical signaling, transporting materials, and the prevention of apoptosis. Huntingtin, while present all over the body, is most abundant in the brain and is related to the proper function of nerve and brain cells [1]. The mutation in the HTT gene causes a DNA segment of the gene called the CAG (cytosine, adenine, guanine) trinucleotide to appear more than normal. Normally, the HTT gene has 10-35 CAG trinucleotides but the resulting mutation results in the CAG trinucleotide appearing 36-120 times [1]. Typically Huntington's disease develops in patients between the ages 30-50 but could develop as early as 2 years old to as late as 80 years old[2].

 

Effects and Symptoms 

The mutant HTT gene causes the synthesis of an abnormally long huntingtin protein that contains increased numbers of polyglutamate (polyQ) tracts on exon 1[3]. The increased amounts of polyQ tracts have been shown to lead to excessive amounts of the neurotransmitter glutamate, which acts as an excitotoxin, causing excessive firing and activation of the neurons. It is supposedly responsible for the symptoms in Huntington’s, although the specifics of this are not fully understood yet [4]. Many studies performed on rats show that excessive amounts of glutamate have caused them to exhibit symptoms of Huntington’s [4].

Research done on mutant huntingtin has shown that it can cause various disruptions in gene expression that are necessary for a nerve cell's survival. For instance, mutant huntingtin has been shown to decrease the expression of GLT-1, a glutamate transporter which when expressed in normal amounts is responsible for uptaking and clearing the amount of glutamate in the synapse [5]. Without GLT-1, glutamate will accumulate in the synapse without any way to remove it. This accumulation can be damaging as extremely high concentrations of glutamate mean that the postsynaptic neuron will almost always have at least one glutamate ligand bound to its receptor. This can cause the neuron to become overexcited and fire absurdly, eventually leading to cell death [6]. The mutant huntingtin has also been shown to cause an imbalance in the amount of synaptic and extrasynaptic NMDA receptors (NMDAR), which allow the binding of glutamate as a ligand in order to trigger cellular responses. Synaptic NMDAR is a pro-survival receptor that activates survival genes in the neuron when glutamate binds to it, whereas extrasynaptic NMDAR activates genes that induce apoptosis when glutamate binds to it [7]. As a result, the increased amounts of glutamate and overexpression of extrasynaptic NMDAR results in accelerated nerve cell death.

The resulting loss of neurons can cause a wide range of symptoms. This can include developing movement disorders ranging from being as mild as having difficulty speaking, to more severe disorders such as chorea, the unpredictable and irregular movement of certain body parts like the arms and legs [8]. Death in the nerve cells has also led to many cognitive disorders such as lack of awareness, difficulty focusing on certain tasks/learning new information, and random outbursts of anger and control of emotion [8]. Patients with Huntington's have also shown signs of sadness, depression, and even suicide due to the sudden changes in the brain [8]. The area in the brain where the most nerve cell death occurs is in the basal ganglia, which is responsible for regulating both motor movements and emotions and explains why most of the symptoms that occur are kinesiological or cognitive [9].

 

Diagnosis

Huntington's shows a wide range of symptoms so there are many ways to diagnose a person who may be suspected of having the disease. A neurologist may interview the patient and ask them about any previous symptoms as well as their medical history. They can then perform various neurological and physical tests such as reflex, movement, hearing, and walking tests to evaluate certain motor and neurological functions and find any abnormalities in them  [9]. Genetic tests may also be used since Huntington's is a heritable disorder and has an equal chance of affecting men and women. Blood samples may be taken from the patient and then be examined to determine the number of CAG repeats in a person’s HTT gene [9]. Finally, brain imaging may be used to examine the shape and condition of a patient's brain. Computed tomography (CT) or Magnetic Resonance Imaging (MRI) are usually used as they give the most accurate picture of what a patient’s brain looks like  [9]. Those with Huntington’s can expect to see shrinkages in areas of the brain such as the striatum of the basal ganglia. However, these results do not fully conclude a patient has Huntington’s as these symptoms can be seen in other disorders as well [9]. 

 

Treatments 

As of now, there is no definitive cure for Huntington’s disease, and the treatment options currently available only lessen the existing symptoms. Such options include using drugs such as Xenazine and Klonopin to suppress the jerky and sudden movements caused by chorea  [10]. While these drugs sound promising, they do come with their fair share of side effects such as worsening the existing symptoms of Huntington's like depression [10]. Antipsychotic and antidepressants are also another form of treatment but they too can cause side effects such as dry mouth and can even worsen muscle rigidity and impair movement [10]. One a brighter note, speech therapy, psychotherapy, and physical therapy can help cope with the most common symptoms of Huntington’s such as trouble swallowing/speaking, thoughts of suicide, and trouble walking [10]. In addition, research being done now on Huntington’s revolves around blocking the activity of NMDAR with NMDAR antagonist drugs like memantine [11]. In the lab, scientists were able to use low doses of memantine in mice with Huntington-like symptoms and it has been shown to reduce downstream cell death and improve the motor movement of the mice [11]. Gene therapy is also being researched, particularly antisense oligonucleotide therapy. This type of treatment involves synthetic DNA and RNA strands called antisense oligonucleotides that bind to the mutant HTT gene, acting as a repressor similar to how microRNA regulates the expression of post-transcriptional mRNA [12]. The binding would limit the ability of the gene to synthesize the toxic protein and thus limit the symptoms of the disease.

 

Final Words

Huntington’s disease has affected thousands of people in the US, killing those who cannot receive treatment and permanently handicapping those who manage to survive. Hopefully, gene therapy and medication can be administered in the near future and create a more hopeful future for Huntington patients around the world.


References


  1. (09/06/2002). HTT Gene. Genetics Home Reference, Your Understanding to Genetic Conditions. https://ghr.nlm.nih.gov/gene/HTT#. Retrieved: 14/06/2020

  2. Huntington's Disease. Alzheimer's Association. https://www.alz.org/alzheimers-dementia/what-is-dementia/types-of-dementia/huntington-s-disease#:~:text=Symptoms%20of%20Huntington's%20disease%20usually,or%20as%20late%20as%2080. Retrieved: 14/06/2020

  3. Andre, Veronique., Cepeda, Carlos., & Levine, Michael. (08/04/2010 ). Dopamine and Glutamate in Huntington's Disease: A Balancing Act. US National Library of Medicine National Institutes of Health. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3118459/. Retrieved: 15/06/2020

  4. Bichell, Terry & Bowman, Aaron. (2015). Gene-Environment Interactions in Huntington’s Disease. ScienceDirect. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/polyglutamine-tract. Retrieved: 14/06/2020

  5. Anglada-Huguet, M., Vidal-Sancho, L., Cabezas-Llobet, N., Alberch, J., & Xifró, X. (2017, March 22). Pathogenesis of Huntington's Disease: How to Fight Excitotoxicity and Transcriptional Dysregulation. https://www.intechopen.com/books/huntington-s-disease-molecular-pathogenesis-and-current-models/pathogenesis-of-huntington-s-disease-how-to-fight-excitotoxicity-and-transcriptional-dysregulation. Retrieved: 15/06/2020

  6. Stephanie Liou. (26/06/2011). About Glutamate Toxicity. Huntington’s Outreach Project For Education at Stanford. https://hopes.stanford.edu/about-glutamate-toxicity/#:~:text=There%20are%20two%20general%20ways,of%20the%20receiving%20nerve%20cell. Retrieved: 15/06/2020

  7. Jimenez-Sanchez, Maria., Licitra, Floriana., Underwood Benjamin., & Rubinsztein David. (09/12/2016). Huntington’s Disease: Mechanisms of Pathogenesis and Therapeutic Strategies. Cold Spring Harbor Perspectives in Medicine. http://perspectivesinmedicine.cshlp.org/content/7/7/a024240.full#ref-27. Retrieved: 15/06/2020

  8. Mayo Clinic Staff. (14/04/2020). Huntington’s Disease. Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/huntingtons-disease/symptoms-causes/syc-20356117#:~:text=Huntington's%20disease%20is%20a%20rare,(cognitive)%20and%20psychiatric%20disorders. Retrieved: 14/06/2020

  9. (December 2017). Huntington’s Disease, Hope Through Research. National Institute of Neurological Disorders and Strokes. https://www.ninds.nih.gov/disorders/patient-caregiver-education/hope-through-research/huntingtons-disease-hope-through. Retrieved: 14/06/2020

  10. Approved Treatments for Huntington’s Disease. Huntington’s Disease News. https://huntingtonsdiseasenews.com/approved-treatments-for-huntingtons-disease/. Retrieved: 14/06/2020

  11. Giring, KD & Wang, YT. (23/09/2016). Neuroprotective strategies for NMDAR-mediated excitotoxicity in Huntington’s Disease. bioRxiv. https://www.biorxiv.org/content/10.1101/076885v1.full. Retrieved: 15/06/2020

  12. (2020). Huntington’s Disease. American Society of Gene + Cell Therapy. https://www.asgct.org/education/huntingtons-disease. Retrieved: 15/06/2020

Jason Luo

Jason Luo


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