Cerebellum (in blue) of the human brain
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Cerebellar ataxia

Spinocerebellar ataxia (SCA) is a genetic disease with multiple types, each of which could be considered a disease in its own right. As with other forms of ataxia, SCA results in unsteady and clumsy motion of the body due to a failure of the fine coordination of muscle movements, along with other symptoms. more...

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It can be easily misdiagnosed as another neurological condition, such as multiple sclerosis (MS). There is no known cure for this degenerative condition, which lasts for the remainder of the sufferer's life. Treatments are generally limited to softening symptoms, not the disease itself. The condition is irreversible. A person with this disease will usually end up needing to use a wheelchair, and eventually they will need assistance to perform daily tasks. The symptoms of the condition vary with the specific type (there are several), and with the individual patient. Generally, a sufferer retains full mental capacity while they progressively lose physical control over their body until their death.

One means of identifying the disease is with an MRI to view the brain. Once the disease has progressed sufficiently, the cerebellum (a part of the brain) can be seen to have visibly shrunk. The most precise means of identifying SCA, including the specific type, is through DNA analysis. Some, but far from all, types of SCA may be inherited, so a DNA test may be done on the children of a sufferer, to see if they are at risk of developing the condition.

SCA is related to olivopontocerebellar atrophy (OPCA); SCA types 1, 2, and 7 are also types of OPCA. However, not all types of OPCA are types of SCA, and vice versa. This overlapping classification system is both confusing and controversial to some in this field.

Types

The following is a list of some, not all, types of Spinocerebellar ataxia. The first ataxia gene was identified in 1993 for a dominantly inherited type. It was called “Spinocerebellar ataxia type 1" (SCA1). Subsequently, as additional dominant genes were found they were called SCA2, SCA3, etc. Usually, the "type" number of "SCA" refers to the order in which the gene was found. At this time, there are at least 22 different gene mutations which have been found (not all listed).

Identifying the different types of SCA now requires knowledge of the normal genetic code, and faults in this code, which are contained in a person's DNA (Deoxyribonucleic acid). The "CAG" mentioned below is one of many three-letter sequences that makes up the genetic code, this specific one coding the aminoacid glutamine. Thus, those ataxias with poly CAG expansions, along with several other neurodegenerative diseases resulting from a poly CAG expansion, are referred to as polyglutamine diseases.

Notes

Both onset of initial symptoms and duration of disease can be subject to variation. If the disease is caused by a polyglutamine trinucleotide repeat CAG expansion, a longer expansion will lead to an earlier onset and a more radical progression of clinical symptoms, resulting in earlier death.

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Gene Defects Precede Degeneration - scientists research spinocerebellar ataxia type 1 - Brief Article
From Applied Genetics News, 2/1/00

Researchers studying mice that develop a neurodegenerative disorder similar to spinocerebellar ataxia type 1 (SCA1) have pinpointed abnormalities in gene expression that occur long before signs of the disease appear. Howard Hughes investigator Huda Zoghbi at the Baylor College of Medicine and colleagues reported their findings in the February 2000 issue of Nature Neuroscience.

In 1993, Zoghbi and her collaborator Harry Orr of the University of Minnesota found that SCA1 is caused by a "genetic stutter" of the three-nucleotide DNA sequence CAG. Genes that contain this stutter have an unusually high number of CAG repeats, which in turn causes them to produce proteins with an abnormally long glutamine tract. A normal SCA1 gene has about 30 CAG repeats, which are interrupted by another triplet repeat sequence. People who have SCA1, however, carry a gene with 40 to a 100 uninterrupted CAG repeats.

SCA1 is one of eight neurodegenerative diseases, including Huntington's disease that is caused by this type of mutation. Somehow, the "polyglutamine" proteins cause severe degeneration in specific groups of neurons, which vary from one disease to the next. Patients with SCA1 suffer the worst damage in their cerebellar Purkinje cells, and consequently lose their balance and motor coordination. Loss of muscle control worsens steadily until patients are no longer able to eat or to breathe. In 1998, Zoghbi and her colleagues showed that the abnormal SCA1 protein, ataxin- 1, accumulates in the cell nucleus where it presumably impairs normal functions. Although ataxin-1 is present in all brain cells, only the Purkinje cells appear to be seriously affected.

Using a technique known as "PCR-based cDNA subtraction," which allowed them to compare gene expression in normal and SCA1 mice, Zoghbi and colleagues were able to pinpoint other genes whose expression pattern was altered by the abnormal SCA1 gene. "We found six neuronal genes, all highly abundant in Purkinje cells, that were downregulated at a surprisingly early stage in the disease," says Zoghbi. "In fact, the earliest gene, called PCCMT, was downregulated only one day after the SCA1 gene turns on. Others were downregulated a few days or weeks afterward." Several of the downregulated genes produced proteins that helped regulate calcium levels in neurons. Calcium is critical to the ability of nerves to transmit signals.

A mouse homologue of the human gene called _ACT-1 was upregulated in their transgenic mice. The expression of _ACT-1 is also disturbed in people with Alzheimer's or Huntington's disease. The genes that were altered in their expression in the SCA-1 mice appear to be similarly altered in SCA-1 human patients. "In humans, these neurodegenerative diseases do not show up until later in life, so the changes in cells must be very subtle," says Zoghbi.

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