Huntington disease (Huntington's chorea) is an incurable, neurodegenerative, autosomal dominant inherited disorder caused by an elongated CAG repeat on the short arm of chromosome 4p16.3 in the Huntingtine gene. The signs and symptoms of the disease consist of motor, cognitive and psychiatric disturbances.
The salient clinical features of Huntington’s disease are listed below:
Diagnosis of Huntington’s disease can usually be made on clinical grounds; especially in the patients who present with the typical features of the disease, or those who have a genetically proven family history.
In other cases, certain investigations may be needed to establish the diagnosis with certainty.
There is no definitive treatment of Huntington’s disease. Palliative and symptomatic management is provided to the patient.
Once the disease develops, it is slowly progressive. The severity of the clinical features depends on the trinucleotide repeat length. Patients with 60 or more repeats often present early by the age of 20 years. In contrast, those with fewer than 40 repeats do not develop clinically apparent features .
On average, the patient lives for around 20 years after the onset of symptoms. Complications develop due to motor and cognitive abnormalities. Death most commonly results from pneumonia. Up to one third of the patients of Huntington’s disease die of this complication. Heart disease and suicide are the second and third most common causes of death in these patients .
Huntington’s disease almost always has positive family history since it is an autosomal dominant disease. It is caused by a mutation in the Huntingtin gene (HTT) on the short arm of chromosome 4 that codes the protein Huntingtin (Htt). A trinucleotide repeat sequence that is normally present in this gene is expanded in case of this disease. The abnormal protein thus produced is called mutated Huntingtin protein (mHtt) and is responsible for the underlying pathological mechanism of the disease.
The prevalence of Huntington’s disease varies greatly in different populations around the world. The prevalence is maximum in Western Europe (7 cases per 100,000 people) and much lower in Asia and Africa (one case per million people). A recent epidemiological study has calculated the average prevalence of Huntington’s disease in the UK during the past 20 years (1990 to 2010) to be 12.3 cases per 100,000 people . Worldwide, the prevalence of this disease is around 5 to 10 cases per 100,000 people.
Huntington’s disease is equally prevalent in both sexes. The age of onset is variable; however, onset in the first decade and after the seventh decade is extremely rare. The average age of onset ranges from 35 to 44 years.
Huntingtin protein (Htt) is present in high concentrations in the brain. The exact function of this protein is not clear but it is believed to have an anti-apoptotic and neuroprotective role. Mutated Huntingtin protein (mHtt) is not able to perform these neuroprotective functions and accumulates in the neurons in the form of aggregates to form inclusion bodies within them . Mutated Huntingtin protein causes cell death by effect on chaperone proteins, interaction with caspases, impairment of energy production within the cells and effects on gene expression.
Cell death resulting in atrophy is most prominent in the portion of basal ganglia referred to as the neostriatum (which consists of the caudate nucleus and putamen). Cell death also occurs to a lesser extent in substantia nigra, certain layers of the cerebral cortex (2, 5 & 6) and portions of the cerebellum, thalamus and hypothalamus. Basal ganglia play a role in the inhibition of a many neuronal circuits that are responsible for initiation of movements. If basal ganglia are damaged, this inhibitory effect is lost and as a result, involuntary movements such as those seen in Huntington’s disease result.
Since Huntington’s disease results from genetic abnormalities, there are no effective preventive measures against it.
Huntington’s disease is an autosomal dominant disorder caused by a mutation in the Huntingtin gene (HTT) on the short arm of chromosome 4. It is characterized by chorea, altered behavior, dementia and motor abnormalities. The disease is progressive and the patients survive for an average of 20 years after the onset of symptoms. There is no definite cure and the disease is treated symptomatically.
Huntington’s disease is a genetic disorder which is characterized by involuntary movements of the body, altered behavior, loss of memory and abnormalities of gait. There is no definite cure and the disease is treated symptomatically. The life expectancy after the onset of symptoms is around 20 years.