[Huntington disease. A review]

Huntington's disease (HD) is a hereditary autosomal dominant neurodegenerative disease characterized by motor, cognitive and psychiatric symptoms. It affects about 1 in 10,000 individuals. The onset of symptoms typically occurs in the third or fourth decade of life, though it may appear at any age. The molecular basis of the disease is the expansion of the trinucleotide CAG in the first exon of a gene on chromosome four (4p 16.3). This gene encodes the protein huntingtin of 3136 amino acids. The mutation of huntingtin produces an expanded stretch of glutamine (Gln) residues. This CAG/polyGln expansion has 6 to 39 units in normal individuals and 36 to 180 units in HD patients. The normal function of huntingtin and the pathogenic mechanisms caused by the expanded polyGln of mutant huntingtin remain incompletely characterized. Huntingtin appears to be associated with synaptic vesicles and/or microtubules and seems to have an important role in vesicular transport and/or the binding to the cytoskeleton. It is thought that this protein is important in embryogenesis and that its mutant form alters the function of the mitochondrial respiratory chain. The toxic gain of function caused by huntingtin could either be an overactivity of the normal function or the introduction of a novel function. Its interactions with other proteins could lead to an impairment of the cellular function or to its own polymerization to form insoluble aggregates. The intraneuronal aggregates could affect gene transcription, protein interactions, protein transport inside the nucleus and cytoplasm, and the vesicular transport. However, since a dissociation between the aggregation of huntingtin and the selective pattern of striatal neuronal loss has been demonstrated, it is believed that other properties of the mutant huntingtin, like proteolysis and the interactions with other proteins that affect vesicular trafficking and nuclear transport, could be responsible for the neurodegeneration. On gross examination, 80% of HD brains show atrophy of the frontal lobes. A bilateral, symmetric atrophy of the striatum is observed in 95% of the HD brains. The mean brain weight in HD patients is approximately 30% lower than in normal individuals. Striatal degeneration has an ordered and topographic distribution. The tail of the caudate nucleus shows more degeneration than the head. The caudate atrophy is associated to a gradual atrophy and neuronal loss in other brain regions as the disease progresses. The striatal and cerebral cortex projection neurons are much more susceptible to the disease than interneurons. In the neostriatum, the levels of GABA, dynorphin and substance P are decreased, but the concentrations of somatostatin and neuropeptide Y increase. An impairment of energy metabolism in HD and a sensitivity to oxidative stress and to the cytotoxic effects of glutamate seem to contribute to the neuronal death in HD. It is proposed that melatonin should be assayed in cell cultures and in transgenic animals due to its potent antioxidant and free radical scavenger properties.