Huntington's Disease: Genetics, Symptoms, and Research
This paper provides a comprehensive overview of Huntington's Disease (HD), a hereditary neurodegenerative disorder caused by a single defective gene. It examines the genetic mechanism underlying HD — specifically the CAG trinucleotide repeat expansion in the HTT gene — and explains how repeat length correlates with disease onset and severity. The paper traces the physical, cognitive, and psychiatric manifestations of the disease, describes its progressive and fatal course, and reviews current diagnostic approaches including neurological, psychiatric, and genetic testing. It also surveys emerging research directions, including antisense therapy and the so-called "Huntingtin Holiday" finding, while noting the disease's higher prevalence in populations of Anglo-Saxon and European ancestry.
- Introduction to Huntington's Disease: Overview of HD's genetic cause and general nature
- The CAG Trinucleotide Repeat Mechanism: How CAG repeats drive HD onset and severity
- Manifestations and Disease Progression: Physical, cognitive, and psychiatric symptoms over time
- Diagnosis and Testing: Neurological, psychiatric, and genetic diagnostic methods
- Research and Emerging Therapies: Antisense therapy and the Huntingtin Holiday finding
- Conclusion: Summary of HD's rarity, fatality, and open questions
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What makes this paper effective
- It integrates multiple peer-reviewed sources to build a coherent picture of a complex genetic disease, moving logically from cause to mechanism to symptoms to diagnosis to treatment.
- The paper accurately uses technical terminology — such as "CAG trinucleotide repeat," "polyglutamine expansion," and "monogenetic etiology" — while remaining accessible to a general academic audience.
- It maintains a consistent focus on the disease's fatal and incurable nature while still conveying genuine optimism about emerging research directions, providing nuanced balance.
Key academic technique demonstrated
The paper demonstrates effective synthesis of primary research literature. Rather than summarizing each source individually, the author weaves findings from Lu and Yang (2012), Margolis and Ross (2003), Landles and Bates (2004), and others into a single coherent narrative. This technique — integrating multiple citations to support a unified argument rather than presenting a source-by-source review — is a hallmark of competent undergraduate academic writing.
Structure breakdown
The paper opens with a definition and overview of HD, then narrows to the molecular mechanism (the CAG repeat). It broadens again to cover clinical manifestations, followed by a detailed section on diagnostic approaches. It then pivots to emerging research before closing with a summary conclusion that revisits key open questions. This funnel-and-widen structure is well-suited to a scientific overview paper at the undergraduate level.
Introduction to Huntington's Disease
Huntington's Disease (HD) is a heritable genetic disease that is neurodegenerative in nature. It impacts every aspect of the human condition — the physical (muscular), the mental (memory and cognition), and the behavioral (mood). The onset of the disease varies, with the majority of cases appearing in adults. Because it is a genetic disease caused by inherited genes, all persons who carry the defective gene may pass it on to offspring. A single copy of the defective Huntington gene is all that is needed for an individual to develop the disease. Once the gene is possessed, onset at some point is certain, as the "monogenetic etiology" consists of a single defective inherited gene that requires no other external factors or variants for causation (Lu & Yang, 2012, p. 964).
The disease process usually begins in middle age, with symptoms ranging from feelings of "clumsiness" to "balance trouble" (Margolis & Ross, 2003, p. 1726). These symptoms are typically accompanied by cognitive disruptions. Mood disorders are also commonly associated with the onset of HD. Once onset begins, the disease progresses and is fatal. The sole predisposing factor is possession of the defective Huntington gene. There is currently no prophylaxis or method of prevention, as the disease is incurable. However, studies are being conducted in which therapies are being tested for the alleviation of symptoms (Lu & Yang, 2012). These studies include antisense therapy and the "sustained benefit of transient mHtt lowering" (Lu & Yang, 2012). While it remains unclear exactly why or how this therapy works, the relief of symptoms has been observed as the result.
HD remains a relatively mysterious phenomenon. The pathogenesis of HD is not fully understood, with various researchers offering different theories about how the disease progresses (Landles & Bates, 2004). However, the causative effects have been identified by Lu and Yang (2012) as beginning with a "CAG expansion encoding an elongated polyglutamine (polyQ) repeat near the N terminus of the Huntingtin (Htt) protein," though it remains uncertain how these molecules interact and function (p. 965). The temporal nature of the onset can be latent for many years and depends on the number of CAG repetitions, but it has been "seen in individuals with a juvenile onset" (Kojovic, Cordivari, & Bhatia, 2011, p. 50). Once the disease reaches onset in middle age, however, progression is acute, with precipitating factors determined on a case-by-case basis. Normal onset occurs around age 40, in men and women alike and irrespective of race (Lu & Yang, 2012).
The CAG Trinucleotide Repeat Mechanism
The CAG trinucleotide repeat is the segment of the HTT gene that is defective. It consists of a DNA sequence that repeats a number of times. In a normal CAG segment, the repeat occurs anywhere from 10 to 35 times. Huntington's Disease is characterized by a repeat occurring anywhere from 36 to 120 times. Individuals with fewer repeats — in the 36 to 40 range — may not show or develop any signs or symptoms of HD, whereas individuals with over 40 repeats typically do show signs and symptoms of the disorder and are most heavily affected by it (Warby, Graham, & Hayden, 2014).
Manifestations and Disease Progression
The manifestations of Huntington's Disease include moodiness, irritability, inability to function normally, loss of short-term memory, fumbling of objects, and loss of coordination. Because it is a neurological disease that impacts the brain, the nervous system is affected and degeneration of the body's ability to behave normally is the primary manifestation. The condition worsens over time, and it is expected that the patient will eventually require full-time care — typically being fed, bathed, and overseen by a caretaker (Huntington's Disease, 2013).
According to Stoler and Meltzer (2013), HD "introduces a large and extraneous risk of early mortality and morbidity" (p. 132). Complications arise as the disease permeates the "motor, cognitive and psychiatric" processes of the body, thereby affecting virtually every aspect of the person, in both mind and body (Stoler & Meltzer, 2013, p. 133). It is thus a bioneurological disorder that cannot be reversed. The genetic probability of inheriting Huntington's Disease from a parent who carries the defective gene is 50%, as either a copy of the healthy gene or a copy of the defective gene will be passed on. As the disease progresses, disability worsens, and the patient is eventually reduced to a state in which he or she is unable to provide self-care and requires full assistance (Huntington's Disease, 2013).
Personality and mood disorders are associated with the onset of the disease, and a loss of the ability to think rationally is also typical. In children — though less common than adult onset — the disease manifests and progresses rapidly, and early mortality is common. Adult onset is more gradual, though it may also be considered acute, as it progresses steadily and there is no method of prevention. While scientists and researchers continue to work toward understanding the pathways of the disease and the way in which cells regulate and respond to the expansion of the proteins involved, more light is being shed on the nature of HD (Lu & Yang, 2012).
Conclusion
In summation, Huntington's Disease is a hereditary genetic disorder caused by a single defective gene. It is still unclear to researchers how and why onset begins, although it is established that CAG segment repetitions are related to onset and that the more frequently they occur, the more likely symptoms are to develop. Why some individuals carry only a few CAG repeats while others carry many remains unknown (Lu & Yang, 2012). There is currently no known cure for the disease, and it is invariably fatal. Nevertheless, evidence indicates that relief of symptoms is possible through lowering mHtt levels in the patient (Lu & Yang, 2012). Diagnosis depends on the identification of symptoms as well as assessment of family medical history. Huntington's Disease is rare, fatal, and most commonly found in Western as opposed to Eastern societies. For this reason, researchers continue to investigate whether the genetic makeup of these populations is a determining factor, or whether reporting and identification are simply inadequate in Eastern regions.
References
Huntington Disease. (2013). Genetics Home Reference. Retrieved from http://ghr.nlm.nih.gov/condition/huntington-disease
Kojovic, M., Cordivari, C., & Bhatia, K. (2011). Myoclonic disorders: A practical approach for diagnosis and treatment. Therapeutic Advances in Neurological Disorders, 4(1), 47–62. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3036960/
Landles, C., & Bates, G. (2004). Huntingtin and the molecular pathogenesis of Huntington's disease. EMBO Reports, 5(10), 958–963. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1299150/
Lu, X., & Yang, W. (2012). Huntingtin Holiday: Progress toward an antisense therapy for Huntington's Disease. Neuron, 74(6), 964–966. Retrieved from http://www.cell.com/neuron/fulltext/S0896-6273(12)00512-0
Margolis, R., & Ross, C. (2003). Diagnosis of Huntington Disease. Clinical Chemistry, 49(10), 1726–1732. Retrieved from
Stoler, A., & Meltzer, D. (2013). Mortality and morbidity risks and economic behavior. Health Economics, 22(2), 132–143. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3349765/
Warby, S., Graham, R., & Hayden, M. (2014). GeneReviews. NCBI. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK1305/
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