The following paragraphs are referenced from the April/May 2006 magazine, “Scientific American Mind”, article by Andrich Juergen & Joerg T. Epplen.
Huntington’s Disease was diagnosed as an inherited disorder more than 11 years ago, yet the mutation that causes it was not discovered until 1993. A DNA test on a blood sample was quickly devised to reveal whether a person carried the abnormal form of the gene, which leads to progressive destruction of the brain, crippling muscles and mental function.
Currently there is no effective treatment or cure for the 900 New Zealanders, or the many thousands worldwide who have been diagnosed with the disease.
The Huntington’s test is so certain because the disease is caused by a single gene – the Huntingtin Gene on chromosome 4 (the name of the gene is spelled differently than that of the illness). Typically the gene contains several occurrences of a set of DNA building blocks: cytosine, adenine and guanine, abbreviated as CAG. This set drives the production of the huntingtin protein. The more often the CAG sequence comes up in the gene, the more glutamine – an amino acid – in the Huntingtin protein. In healthy genes the CAG sequence may appear up to 28 times. But if it occurs more than 35 to 40 times, the glutamine chain in the Huntingtin protein becomes too long and causes trouble. The larger the number of CAG sets, the longer the chain, and the earlier and more severe the disease.
The genetic mutation that overproduces the CAG sequence is inherited from a single parent. A child, therefore, has a 50 percent chance of getting the disease-causing form of the gene if either parent has it. Disease symptoms typically begin when carriers are 35 to 45 years old, but even among close relatives the onset and course can differ significantly.
Huntington’s disease had been known for centuries before it was given its definitive name. In the Middle Ages, victims of what was then called “the dance” made pilgrimages to Ulm, Germany, to pray in the chapel of Saint Vitus, leading to the ailment’s name, “Saint Vitus’ dance”. The first to recognize the condition as an inherited disease was the young American Neurologist George Huntington in 1872; together with his father, he had tracked cases in a family on Long Island, outside New York City, and was able to differentiate them from chorea minor, caused by a streptococcal infection that has similar symptoms. Today about one in 10,000 people in the US suffers from Huntington’s. There are about 900 people suffering from Huntington’s in New Zealand.
The symptoms that gave the disease its original name are the “dancing” movements. The exaggerated motions of the limbs that are its most frequent and striking effects. In the beginning, patients try to disguise these jerks and twitches as shrugs or try to translate them into deliberate motions such as stretches.
But little by little their muscles go out of control. They are beset by sudden grimaces, and speaking and swallowing become increasingly difficult. In later stages, movements are slower; increased muscle contraction leads to painful contortions of the limbs that can last minutes or hours.
Characteristic mental symptoms often appear decades before the physical problems. The disease can cause repeated outbreaks of moodiness, yet patients who receive a positive test result can also fall victim to emotional swings driven by their knowledge of impending destruction. Relatives often notice personality changes – patients may become paranoid, tyrannise those around them with unfounded jealousy or react extraordinarily aggressively to trivial disagreements.
As the disease advances, they may obsess about minor issues for days or weeks, burdening their families and destroying their social connections. Patients’ cognitive abilities also wane; their memories deteriorate, and they find it increasingly difficult to concentrate. The problems progress to severe dementia and complete helplessness. Even early in the disease, the mounting mental breakdown can have catastrophic effects on a person’s personal and professional lives, and suicide attempts are not unusual.