Trinucleotide Repeat Disorders
Part 8: Polyglutamine Diseases

Descriptions of other diseases that involve codon repeat expansions.

SCA7 (Spinocerebellar Ataxia Type 7)

SCA7 (Spinocerebellar Ataxia Type 7) is the last of the SCAs to fall under the category of polyglutamine diseases. Like the other SCAs, the most common symptom of SCA7 is loss of coordination. In addition to this, people with SCA7 often have difficulties with vision.

The Gene:

The gene involved in SCA7 lies on Chromosome 3 and is also named SCA7. Typically, in asymptomatic individuals there are between 4 and 19 copies of CAG in the SCA7 allele. In a person with the disease, however, the allele has anywhere between 37 and 306 copies. At present, not enough data exist to fully understand the effect that alleles with between 19 and 29 copies of CAG will have on individuals. Individuals with 30-36 copies of CAG are considered to be in the intermediate zone; they may or may not develop the symptoms of SCA7. If they do develop symptoms, the symptoms are likely to be milder and to appear later in life than they would for people with 37 or more copies of CAG.

The Protein:

The protein product of the SCA7 gene is called ataxin-7. Currently, the normal function of this protein is unknown. Scientists suspect that when ataxin-7 proteins are altered, they tend to clump together in the nucleus, producing what are called neuronal inclusions, or NIs (NIs have also been found in certain nerve cells of people with SBMA, HD, and some other SCAs). These inclusions have been associated with degeneration of the nerve cell, but whether or not they are in fact the direct cause of degeneration is yet to be determined. (See Figure F-5.)

How the Symptoms Come About:

The loss of coordination that people with SCA7 experience results from damage to the cerebellum. This mechanism is more-or-less the same as that of SCA1. (For a more detailed explanation of this mechanism, click here.)

The effect that SCA7 has on one’s vision is a little more complicated because vision is a process that involves many players. Contrary to popular belief, humans do not literally “see” with their eyes. Instead, the eyes are simply the first stop on a pathway for visual information that will eventually lead to the processing of this information in the brain. After light from an image comes into the eye, the information it contains is encoded into nerve impulses by the retina. (For a discussion of nerve impulses, click here.) These impulses are then sent down the optic tract to a part of the brain called the lateral geniculate body. Here the information undergoes something like a preliminary inspection, which involves a categorization of the data. The newly categorized info is then sent on to the visual cortex, which is part of the cerebral cortex of the brain. It is in the cerebral cortex where the brain assembles a processed image and we actually “see” what is in front of us. To see an image clearly and accurately, then, all pieces in this visual puzzle must be in good working order. In SCA7, however, there is noticeable damage to all parts of the visual pathway. While this by no means implies that people with SCA7 go blind, some problems with vision are likely to occur. (See Figure F-15.)

Last Modified: 9-18-02

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