HD (huntingtin (Huntington disease)) is a human gene that codes for a protein called huntingtin. The exact function of this protein is not known, but it appears to play an important role in nerve cells. Within cells, huntingtin may be involved in signaling, transporting materials, binding proteins and other structures, and protecting against programmed cell death (apoptosis). The huntingtin protein is required for normal development before birth. It is expressed in many tissues in the body, with the highest levels of expression seen in the brain.

One region of the HD gene has a particular sequence of 3 DNA bases, CAG, that is repeated multiple times. (Bases are the building blocks of DNA.) This region is called a trinucleotide repeat, and normally it is repeated 10 to 35 times. The repeat leads to the production of a huntingtin protein that contains a stretch of glutamine, an amino acid building block used in many proteins.

The HD gene is located on the short (p) arm of chromosome 4 at position 16.3, from base pair 3,113,411 to base pair 3,282,655.

Related conditions

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Huntington disease is caused by mutations in the HD gene. The inherited mutation in the HD gene that causes Huntington disease is a sequence of 3 DNA bases, CAG, which is abnormally repeated from 36 to more than 120 times. This unstable region is called an expanded trinucleotide repeat. The CAG repeat expansion leads to the production of a huntingtin protein that contains an abnormally long stretch of the amino acid glutamine. Enzymes in the cell often cut this elongated protein into fragments that have "sticky" ends. The protein fragments form abnormal clumps inside nerve cells and may attract other, normal proteins into the clumps. These nerve cells do not function properly and ultimately die. This process is particularly likely to occur in the striatum (a part of the brain that coordinates movement) and the cortex (a part of the brain that controls thinking and emotions).

People with 36 to 40 CAG repeats may or may not develop the signs and symptoms of Huntington disease during their lifetime, while people with more than 40 repeats will almost definitely develop the disorder during a normal lifetime.

As the altered HD gene is passed from one generation to the next, the size of the CAG repeat expansion can change; it often increases in size. People with 27 to 35 CAG repeats have not been reported to develop Huntington disease, but their children are at risk of having the disease if the repeat expansion increases.

References

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External links

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• GeneCard

Genes