Medications for neurodegenerative diseases

Huntington disease, or HD, is a rare neurodegenerative disease that involves a repeated sequence of DNA that causes an abnormal protein to form, leading to abnormal movements and cognitive problems.

In most people, a gene called huntingtin or HTT on chromosome 4, contains a triplet repeat, where the nucleotides C, A, and G are repeated 10-35 times in a row. In people with Huntington disease, this repeat goes on for 36 or more times in a row.

CAG codes for the amino acid glutamine, so people with Huntington disease will have 36 or more glutamines in a row in the huntingtin protein.

So, in addition to being a triplet repeat disorder, HD is, more specifically, a “polyglutamine” disease.

The specific way in which extra glutamines causes HD symptoms isn’t fully worked out, but some clues are that the mutated protein aggregates within the neuronal cells of the caudate and putamen of the basal ganglia causing neuronal cell death.

Cell death might be related to excitotoxicity, which is excessive signaling of these neurons, which leads to high intracellular calcium.

Now the symptoms of HD involve progressive CNS disturbances including movement, cognitive, and mood symptoms and they start appearing around the age of 40.

Remember, the age of onset depends on the number of CAG repeats, so more repeats means earlier onset.

Over time, if enough of the neurons die in the caudate and putamen, which together form the dorsal striatum, then it can cause actual loss of brain tissue volume in that area and expansion of the lateral ventricles.

The death of neurons also cause neurotransmitter imbalance in these regions and there’s a decrease in inhibitory neurotransmitters like GABA, and an increase in stimulatory neurons like dopamine.

This also decreases acetylcholine, which is released by interneurons that help other neurons communicate.

Now, the affected areas play an important role in movement, particularly inhibiting it, so cell death in the basal ganglia causes movement problems like chorea, which are purposeless, dance-like jerking movements, and athetosis which are slower, writhing, “snake-like” movements mainly affecting the hands.