In this section, you’ll find information about HD genetic cause.
If you want to better understand what Huntington Disease is, what means being affected by it or living with a person who is affected, have a look also to this video:
Is is a 45 minutes video titled “HD: the most curable among incurable diseases”, by Telecolor – private broadcasting of Cremona.
You will hear about HD by:
- Prof. Ferdinando Squitieri, HD doctor and researcher:
- Fabio, HD affected
- Gianna, mother and wife's; Gianfranco, husband's; Alessandra, dother's.
- LIRH, that is the non profit association supporting research on HD.
A GENETIC DISEASE
Huntington’s Disease (also known as “chorea”, dance, due to involuntary movements caused by the disease) is a rare neurodegenerative disease of nervous system, it is hereditary and it has a genetic cause, that was discovered in 1993. The gene that causes the disease is located on chromosome 4 and it encodes a protein called huntingtin (Htt).
In the first segment of gene, there is a sequence of three nucleotides, cytosine – adenine – guanine (CAG) that is repeated many times (es. CAG – CAG – CAG - ..). This is called “trinucleotide repetitions”. Depending on genetic code, the CAG codifies for the amino acid glutamine. So, a repetition of CAG originates a chain of glutamines or “poliglutaminics”. A trinucleotide repetitions until 35 unit of CAG is considered normal. When the gene HD presents more than 40 repetitions of CAG, the altered form of huntingtin protein that is produced causes the disease during life.
For this reason, the disease is caused by a development of CAG repetition and it is one of the several diseases due to poliglutamine.
A gene is formed by a DNA sequence (Desoxyribonucleic Acid) that codifies specific proteins. DNA is a polymer (a long chain) made by nucleotides with a double-helical structure. A nucleotide is a chemical compound formed from nitrogenous bases (adenine, guanine, cytosine and thymine) tied to a molecule of sugar (desoxyribose) and to a phosphate group. DNA contains genetic informations translated in a sequence of amino acid specific for every protein, based on “genetic code”. Humans are diploid. This means that have two pairs (also called alleles) of every gene, inherited by each parents. A human cell contains 23 pair of chromosomes. Each pair is formed by a father’s chromosome and a mother’s chromosome.
The mutated gene in Huntington Disease is present in all the cells of the organism, since the conception. This means that the disease can be inherited from a generation to another.
HD is a autosomal genetic disease. This means that both men and women can be damaged by the disease both men and women in the same way, because the pathological gene is located on a chromosome that is the same in both genders (autosomal chromosomes, not sexual). Most subjects affected by Huntington’s Chorea are heterozygote. This means that they have 2 different copies of the gene: a normal copy inherited by the parent not affected and a copy inherited by the parent affected. As an exception, when both parents are affected, the product of conception can inherit 2 copies pathological of the gene (one from each parents). In this case, the son is homozygote (2 copies of the gene identicals).
HD is also a dominant genetic disease. With this, we mean that is enough to have one copy of the pathological gene, which came from one parent, to inherit the disease. In other words, the mutation in HD gene has a dominant effect on normal gene, inherited by not affected parent.