Huntington's disease is an inherited disorder characterized by the destruction of nerve cells in specific areas of the brain. Symptoms present in adulthood with the onset most frequently occurring during the third or fourth decade of life. Early symptoms include irritability, mood swings, memory problems and impaired judgment. Uncontrolled movements and difficulties speaking, swallowing and walking occur as the illness progresses. The National Human Genome Research Institute notes that approximately 30,000 Americans have Huntington's disease.
Chromosomal Location of the Huntington's Disease Gene
Human genetic material is contained in the chromosomes. There are 23 pairs of chromosomes for a total of 46 chromosomes in the human genome. Each parent contributes one copy of each chromosome in the pair. Chromosomes are sequentially numbered for identification purposes. Within each chromosome are many genes, each controlling some aspect of structure or function of the human body. The University of Kansas Medical Center Division of Endocrinology, Metabolism and Genetics reports the gene responsible for Huntington's disease is located on chromosome 4. The symbol for the gene in which the Huntington's disease defect occurs is HTT.
Inheritance Pattern of Huntington's Disease
The National Institute of Neurological Disorders and Stroke states Huntington's disease has an autosomal dominant pattern of inheritance. The term "autosomal" refers to the fact that the Huntington's disease gene is not located on a sex chromosome. Because it is located on an autosomal chromosome instead of a sex chromosome, Huntington's disease affects males and females in equal numbers. The term "dominant" means if one of the two copies of the HTT gene in the fourth chromosome pair is defective, the disease will occur. This is in contrast to recessive genetic disorders in which both copies of the gene must be defective for the disease to occur. The University of Kansas Medical Center Division of Endocrinology, Metabolism and Genetics explains that because of the autosomal dominant pattern of inheritance, the children of a parent with Huntington's disease have a 50 percent chance of inheriting the disorder.
Gene Abnormality in Huntington's Disease
The abnormal HTT gene that causes Huntington's disease has a defect known as CAG repeat expansion. Genes have codes, which occur in sets of three. CAG is one such code. The defect of the Huntington's disease gene is an abnormal repetition of the CAG code. The National Human Genome Research Institute explains that a normal HTT gene has 11 to 29 CAG code repeats. The abnormal HTT gene that causes Huntington's disease typically has 40 to 80 CAG code repeats. The Merck Manual for Healthcare Professionals points out that the more CAG repeats within the defective gene, the earlier the onset of the disease. The number of CAG repeats also correlates to the severity of Huntington's disease.
References
- National Human Genome Research Institute: Learn about Huntington's Disease
- National Institute of Neurological Disorders and Stroke: Huntington's Disease
- Merck Manual for Healthcare Professionals: Huntington's Disease
- Genetics Home Reference: Huntington Disease
- University of Kansas Medical Center Division of Endocrinology, Metabolism and Genetics: Genetics of Huntington Disease
