Huntington's Disease Testing Process – Wayne's Story
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Wayne, from England, very honestly discusses the testing process and his experience of testing positive.
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Huntington’s disease, or HD, is an inherited neurodegenerative disorder in which brain cells are damaged and die over time, leading to progressive loss of mental and physical abilities.
The diseases is caused by an abnormal version of a protein named huntingtin, or HTT. A normal version of HTT has a stretch of 10-35 repeats of CAG nucleotide triplets, which encode for a stretch of the amino acid glutamine. In people with HD, the HTT gene has more than 35 CAG repeats, sometime more than 120 (juvenile onset). 26-35 = intermediate. The abnormally long stretch of polyglutamine changes the structure of the normal HTT protein, causing fragmentation and aggregations, which are toxic to nerve cells. The effect is specific in the basal ganglia of the brain, most notably in the striatum, which controls movement by inhibiting unwanted movements. This explains the most prominent symptoms of the diseases – uncontrollable movement, known as Huntington’s chorea – dance-like movements of the face and arms; and motor speech disorders, dysphasia aspiration – most common cause of death in HD patients.
A person has 2 copies of the HTT gene but one abnormal copy is enough to cause the disease. Children of an affected parent has a 50% chance of inheriting the abnormal copy of the gene, and hence the disease. This pattern of inheritance is known as autosomal dominant.
The onset and progression of the disease depends on the number of CAG repeats, or the size of expansion – the higher the number of repeats, the earlier the age of onset and the faster it progresses.
The high degree of repetition also increases the likelihood of inaccurate reading during DNA replication. The “confused” DNA polymerase may add more repeats as it loses track of repeat number. As the expanded HTT gene is transmitted to the next generation, the number of repeats often increases. This means a phenotypically-normal parent with 30-35 repeats may give his child a 40-repeats HTT gene who would develop the disease. As the size of polyglutamine increases from generation to generation, the onset of symptoms is getting earlier with each generation. This phenomenon is called anticipation.
An average person with a 40-50 CAG repeats in the HTT gene usually develop symptoms in their 40s. People with more than 60 repeats could show signs of the disease in their childhood (juvenile form). The first signs are subtle mental and cognitive disturbances that may go unnoticed. As the disease progress, chorea becomes most prominent, followed by motor speech disorders, rigidity, swallowing difficulty and dysphasia, personality changes, memory loss and other cognitive and psychiatric impairments.
Diagnosis is confirmed with genetic testing, genetics counseling is available for people with family history of HD.
Life expectancy in HD is generally around 10 to 20 years following the onset of visible symptoms. Most life-threatening complications result from muscle coordination with pulmonary aspiration, which also increases the risks of pneumonia, being the most common cause of death in HD patients.
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