Huntington's disease

Huntington's disease is a rare neurodegenerative disorder of the central nervous system with an autosomal dominant heredity (therefore there is a 50% chance of passing it to the offspring). It affects both genres equally.
==Genetics==
Huntington's disease is an autosomal dominantly inherited disease caused by an elongated CAG repeat on the short arm of chromosome 4p16.3 in the Huntingtin gene. This gene encodes a Huntingtin protein and on exon 1 contains the CAG tract. CAG is the codon for the amino acid glutamic. Whereas the wild-type contains a CAG repeat, coding for a polyglutamine stretch, of 6 to 26 trinucleotides, the Huntington's disease is associated with 36 repeats or more. An anticipation phenomenon was observed in Huntington families. The normal wild-type Huntingtin protein plays a role in synaptic function. It is necessary for the post-embryonic period and probably has an anti-apoptotic and protective function against the toxic mutant, huntingtin.
==History==
The Huntington disease was first observed and described by George Huntington in 1872. In 1983, a linkage on chromosome 4 was established and in 1993 the gene for HD was found. Finding the gene opened new research lines and new models how to treat this disease. 
==Symptoms==
As described above, Huntington's disease is associated with an expansion of trinucleotide repetitions CAG in an ORF of a gene called huntingtin. Based on the number of these CAG repeats the symptoms can vary, but usually, they appear after 35-44 years of age. In some cases, symptoms start before the age of 20 years with behavior disturbances and learning difficulties at school (Juvenile Huntington's disease; JHD). An increasing number of repetitions causes a more severe phenotype. Definite clinical manifestation will occur if the number of repeats exceeds 40.Whereas the range of 36-39 leads to an incomplete penetrance of the disease or to a very late onset, a person with 35 to 121 repeats will already develop a pathological phenotype. In cases of JHD, the repeat often exceeds 55.
Symptoms of earlier stages changes in personality, cognition and physical skills. The most characteristic symptoms of this disease include non-coordinated, random, jerky movements. These movements are called chorea. As the disorder progresses psychomotor and cognitive functions become more and more impaired and the patient becomes dependent on his surroundings. Although the disorder itself is not mortal, the affected individual usually dies within 15 years after the first symptoms appear due to the weakening of the immune system. There is no cure, only symptomatic treatments are available.
==References==
Roos R.A.C., Huntington's disease: a clinical review. Orphanet Journal of Rare Diseases (2010), 5:40 
Gudesblatt M. and Tarsy D., 'Huntington’s Disease: 'A Clinical Review. Neurology reviews (2011), S1