A. Introduction. Diseases that have autosomal dominant inheritance affect individuals who receive only one defective copy of the gene from either parent. An example of an autosomal dominant inherited disease is Huntington's disease (HD). Other autosomal dominant inherited disorders are listed in the Appendix.
B. Huntington disease (HD)
1. The characteristic dysfunction is the cell death of cholinergic neurons and GABA-ergic neurons within the caudate nucleus (corpus striatum). This results clinically in choreic (dance-like) movements, mood disturbances, and progressive loss of mental activity. No treatment is available.
2. HD is caused by autosomal dominant mutation so that an individual need receive only one defective copy of the HD gene to have the disease.
3. The HD gene is located on the short arm (p arm) of chromosome 4 (4p).
4. The HD gene encodes for a protein that has yet to be identified.
5. The mechanism for neuronal cell death in Huntington's disease may involve a hyperactive N-methyl-D-aspartate (NMDA) receptor as indicated below:
a. Because glutamate is the main excitatory transmitter in the brain, almost all neurons have glutamate receptors, one of which is called the NMDA receptor. The NMDA receptor is so named because it is selectively activated by the glutamate agonist called NMDA.
b. In normal synaptic transmission, glutamate levels rise transiently within the synaptic cleft. However, excessive and diffuse release of glutamate results in neuronal cell death called glutamate toxicity.
C. Glutamate toxicity is the result of an excessive influx of calcium into the neurons because of the sustained action of glutamate on the NMDA receptor, d. It is thought that the mutation of the HD gene on 4p somehow results in a hyperactive NMDA receptor so that excessive influx of calcium into neurons of the caudate nucleus occurs with resultant cell death.
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