Huntington's Disease

Huntington’s disease

1. What is Huntington’s disease?

Huntington’s disease (HD) is an autosomal dominant hereditary disorder that causes degeneration in the brain and spinal cord. The degeneration leads to abnormalities of movement and of cognition, as well as psychiatric disturbances. This disorder is often also referred to as Huntington’s chorea. Chorea is the most characteristic movement disorder of classical HD and is characterized by brief, involuntary, abnormal movements, which appear unpredictably in all the parts of the body (Roze E, Cahill E, Martin E et al. 2011). Chorea is present in more than 90% of individuals, increasing during the first ten years (Warby SC, Graham RK, Hayden MR 1998). The deterioration of the brain and loss of neurons will lead to lack of focus, difficulties concentrating, forgetfulness, short-term memory lapses, etc. One who is battling Huntington’s is likely to endure dementia due to their cognitive decline and sometimes are initially diagnosed with Alzheimer’s due to their similar traits when the involuntary movements haven’t been recognized. Another effect of Huntington’s is mental disturbances. Individuals begin to have significant behavioral changes such as depression, agitation, anxiety, hallucinations, delusions, etc.

2. What causes this disorder?

The mutation responsible for HD is located at the 5’ terminal part of the HTT gene on chromosome 4p163. The mutation consists of an unstable expansion of CAG repeat sequence. (Roze E, Cahill E, Martin E et al. 2011). The mutation occurs when the protein for huntingtin CAG (glutamine) is repeated more than the normalcy of 15-20 repeats to greater than that of 36. The protein below shows the trinucleotides of glutamine (CAG) being repeated; once the repeats have exceeded 36 the mutated protein begins its dysfunction and eventually leads to death.

Figure 1 Mutated DNA chain of glutamines trinucleotides (Kelley 2002)

3. Who is affected by Huntington’s disease?

Approximately 1 person in 10,000 has Huntington’s disease and is most common in European descent. Although the disease is rare, its genetic nature means it affects a large number of family members (Quarrel O 2008). Since it is a hereditary disease, each possible offspring has a 50% chance of inheriting the mutated gene from the carrying parent. The disease is not gender specific; it really just depends on which copy of the DNA the offspring receives from each parent on whether they will be a boy or girl and/or affected by Huntington’s. Below is a great representation of the reproduction and the likelihood for the passing of the mutated gene from generation to generation.

Figure 2 Reproduction example of Huntington’s disease (What is HD? 2015)

4. What are the symptoms and how soon do they appear?

The average age of onset is 35 to 44 years old. Clinical features of HD can be divided into three groups: movement disorders, cognitive impairment, and psychiatric manifestations (Roze E, Cahill E, Martin E et al. 2011). Various movement disorders that may be displayed are tics, involuntary movements, jerks, swaying, clumsiness, etc. Some symptoms of cognitive impairment that will be displayed are: speech difficulties, slow reaction times, inability to speak, inability to care for oneself, difficulty in mental planning. Psychiatric symptoms that occur in individuals with HD but not limited to are: hostility, irritability, alcohol abuse, social withdrawal, obsessive-compulsive disorder, and manic episodes. Other general symptoms that one may display are sleep disorders, seizures, and weight loss. In early stages,