[Genetic bases in Huntington disease]

Huntington disease is a neurodegenerative syndrome of late appearance in 80% of the cases. Its clinical course shows motor and cognitive disorders which lead to total incapacity of the individual 10 to 15 years after the appearance of the first syndromes. From a genetic point of view, this disease is of autosomic dominant inheritance with complete penetration. The gene (IT15) is localized in the short arm of chromosome 4 in the telemaric region 4p16.3 and it is known that the mutation is produced by an increase in the number of trinucleotides CAG (glutamine) localized in the 5' end of the gene, in the first exon. In the general population these are repeated in a number of less than 30 repetitions and in the disease population in a number greater than 36 and, in some cases, even greater than 100 repetitions. These sequences are unstable from one generation to another and this fact may explain the phenomenon of genetic anticipation shown in this disease since there is an inversely proportional correlation between the number of repetitions and the age of appearance of the first symptoms. The gene codifies for a protein known as "huntingtina" which is expressed not only in brain but also in different tissues. Although the function remains unknown, it is known that it is bound to other proteins related to the transmission of neuron signals and to the regulation of neuron death (apoptosis) among others. The defect is produced by a gain in function closely related to the number of repetitions. At present, presymptomatic and prenatal diagnosis of the disease may be achieved since the reliability of the studies is practically 100%. Nonetheless, the demand is lower than expected and this may be due to psychologic, social and legal problems, together with the lack of adequate infrastructure for completely guaranteeing the performance of these studies.