Review on The Hungtington's Disease

A genetic disorder is a relatively rare genetic problem caused by one or more defects in an individual’s DNA. The size and severity of these abnormalities can vary, ranging from a point mutation changing a single base in one gene to entire chromosomes being affected. Some of these defects may be caused by new mutations in the DNA, for example in Down Syndrome. This is caused by a condition called Trisomy 21, whereby the individual has a third copy of chromosome 21. However, the parents of the affected individual are usually unaffected themselves and the extra chromosome occurs by chance (1). Other genetic disorders may be hereditary, whereby the faulty genes are inherited from the individual’s parents. There are many well-known examples of inherited genetic diseases, such as sickle cell anemia, cystic fibrosis, and muscular dystrophy. This essay will focus on an inherited genetic disorder known as Huntington’s disease, detailing the biology of this condition and its pattern of inheritance along with how the disease is diagnosed and treated.

Huntington’s disease is an autosomal dominant neurodegenerative disorder which affects approximately 5-10 per 100,000 in the European population, with a much lower prevalence in Asian and African communities (2). The disease is characterized by unpredictable, involuntary body movements, known as chorea, along with cognitive decline and behavioral differences (3). Typically, symptoms begin to show between the age of 30-50, however, a small portion of individuals may develop the disease before the age of 20 years old in a condition known as Juvenile Huntington’s Disease (4). Huntington’s Disease is typically fatal within 15–20 years from the onset, however complications arising such as pneumonia, cardiac disease or physical injuries can further reduce life expectancy (5). Additionally, the psychiatric and behavioral changes can result in depression and between 8–9 % of deaths in patients with Huntington’s are attributed to suicide (6).

The gene implicated in Huntington’s disease is known as the Huntingtin gene (HTT), which has been mapped to the short arm of chromosome 4p16.3 (7). The genetic mutation which occurs in this gene is known as a Trinucleotide Repeat, specifically of the CAG codon. The CAG repeat occurs in the coding region of the gene and encodes a glutamine amino acid which is included within the protein. In a normal Huntingtin gene, there are approximately 10-35 CAG repeats, which leads to the production of a normal protein. The inclusion of furthermore glutamines due to more CAG repeats causes a malformed protein and for this reason, Huntington’s is sometimes referred to as a ‘polyglutamine’ disease (7). Between 27-35 repeats is known as the ‘intermediate allele’, meaning that the individuals themselves are not affected but there is potential for affected offspring through further expansion of the CAG repeats. 36-39 repeats result in a reduced penetrance allele, whereby symptoms usually appear in later life. Therefore, individuals with full penetrance Huntington’s Disease typically have 40 or more CAG repeats (8). The presence of the CAG repeats also leads to a phenomenon known as ‘anticipation’, whereby the greater the number of CAG repeats the earlier the onset of the disease (3). Therefore, in individuals with Juvenile Huntington’s disease, the number of CAG repeats often exceeds 55 (9). It is not fully clear why the mutated Huntingtin protein is toxic to the cells of the brain, however, numerous theories have been proposed and it is thought that it is a combination of these mechanisms which contributes to the pathogenesis of Huntington’s disease (5).

As previously stated, Huntington’s disease is inherited in an autosomal dominant manner (7). This means that an affected individual only has to inherit one copy of the mutated gene from an affected parent. As is the case with most autosomal dominant diseases, there is a 50% chance that the children of an affected parent will inherit the disease. This is shown in Fig 1. below. The disease also tends to be present in every generation of the family and is not gender related (10). If a situation were to arise where both parents have a copy of the mutated HTT gene and are thus both affected, the risk that their children will inherit the disease increases to 75%. Furthermore, if one parent were to have two copies of the mutated gene, the risk would be 100% meaning that all of their children will be affected by the disease, however, this is a very rare occurrence (11).

When symptoms begin to develop, for example, chorea, then taking a history is the initial and most important step of diagnosis. If the individual has a parent with Huntington’s disease and is displaying the appropriate symptoms, genetic testing is often not required and diagnosis can be relatively straightforward. However, this can be more complicated when family history is not available. In this situation, the individual can undergo genetic testing to confirm a diagnosis by measuring the number of CAG repeats in the Huntingtin gene. Predictive testing is also available for those with a family history of Huntington’s disease (2). However, as there is currently no cure for the disease many at-risk individuals opt against this testing to the avoid the anxiety that a positive test may bring, whereas others choose to be tested to aid in major career and family decisions such as pregnancy. Prenatal testing is also available to examine for evidence of the mutated HTT gene (7).

As previously mentioned, there is currently no cure for Huntington’s disease. However, the severity of symptoms may be reduced through both medication and non-medicative therapy. Therapy may include physical therapy and rehabilitation to prevent loss of motor function, as well as perhaps occupational and speech therapy (12). Attempts to treat chorea may be made through the use of medications such as dopamine receptor blockers and psychiatric symptoms such as depression may be treated through medication such as serotonin reuptake inhibitors (5).

Huntington’s disease is a lifelong genetic disorder which causes severe physical and psychological problems for the affected individual. Due to the autosomal dominant inheritance pattern of the disease, it can be a great worry to children of affected parents whether they will inherit this debilitating disease. As stated, there is currently no cure and current treatments only deal with treating the symptoms. Therefore, research into Huntington’s disease represents an exciting field of research with new breakthroughs being made regularly, with the hope of developing more effective therapeutic treatments for this and many other neurodegenerative disorders.