Huntington’s disease is a very complex disorder involving wasting of nerve cells. It has been affecting thousands of families down the ages. The disease is named after George Huntington, an American physicist who identified it in 1872.
Though he was the first to actually note down the disorder in detail in terms of modern medicine, the matter was under observation from the time of the Greeks who called this disorder as Chorea. It was the name given to describe the uncontrollable dance like motions that an affected person goes through.
It was observed that the disorder was passed from the parents to the child and the name Hereditary Chorea was given. These days the doctors call it simply the Huntington’s Disease.
The disease is genetic, that is, it is passed from generation to generation. In America only, there are about 15000 known cases and almost ten times of that have 50 percent chance of acquiring it. In the gradual progression of the disease, the persons lose their ability to think, feel and move. However, there have been several breakthroughs in understanding of the disease and that is a good sign.
But What Is The Genetics That Lie Behind The Disease?
The human body consists of cells which in turn contains nucleus (except the red blood cells). The nucleus contains 46 chromosomes arranged in 23 pairs. Each of these chromosomes contains the tightly packed DNA strands. Genes are the building blocks of the DNA.
The 23 pairs of chromosomes contain one chromosome each from each parent. They are numbered 1 to 23 for reference. The first 22 pairs are called autosomal chromosomes while the 23rd pair is the sex chromosome that defines the sex of the person.
Huntington’s Disease gene is called an Autosomal Dominant. This means that the gene responsible for the disease is located in the first 22 pairs. Recently it has been identified to be in the 4th Chromosome. One of the genes of this 4th chromosome is the HD gene, the gene that causes all the problems. And this gene dominates the other gene which is working correctly thereby suppressing normal function.
As the gene does not belong to any sex chromosomes, there is equal chance of the disease passing on to the child from both mother and father. If even one parent is affected, with each pregnancy there is 50 percent chance of passing the gene on to the child.
The reason behind the HD gene causing the disease is a protein that this defective gene makes. It is called Huntingtin. Though the function of the protein is still unknown, it is known that it plays an important role on the nerve cells. It disrupts the normal activity and kills the nerve cells prematurely. The continuous loss of nerve cells leads to the abnormal condition and shows the symptoms that are associated with Huntington’s disease.
The abnormality of the gene lies in the abnormal repetition of a DNA sequence called CAG repeat. This extra repetition is responsible for the formation of Huntingtin. Normally the genetic test done to identify the gene is done by counting the number of repetitions of this sequence. In normal persons the repetition is fewer than 28 times. But the affected people have more than 40 such repetitions.
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