domingo, 9 de septiembre de 2012

FXTAS - Genetics Home Reference

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FXTAS - Genetics Home Reference

Genetics Home Reference: your guide to understanding genetic conditions 

Fragile X-associated tremor/ataxia syndrome

(often shortened to FXTAS)
Reviewed August 2012

What is FXTAS?

Fragile X-associated tremor/ataxia syndrome (FXTAS) is characterized by problems with movement and thinking ability (cognition). FXTAS is a late-onset disorder, usually occurring after age 50, and its signs and symptoms worsen with age. This condition affects males more frequently and severely than females. Affected individuals have areas of damage in the part of the brain that controls movement (the cerebellum) and in a type of brain tissue known as white matter, which can be seen with magnetic resonance imaging (MRI). This damage leads to the movement problems and other impairments associated with FXTAS.
The characteristic features of FXTAS are intention tremor, which is trembling or shaking of a limb when trying to perform a voluntary movement such as reaching for an object, and problems with coordination and balance (ataxia). Typically intention tremors will develop first, followed a few years later by ataxia, although not everyone with FXTAS has both features. Many affected individuals develop other movement problems, such as a pattern of movement abnormalities known as parkinsonism, which includes tremors when not moving (resting tremor), rigidity, and unusually slow movement (bradykinesia). In addition, affected individuals may have reduced sensation, numbness or tingling, pain, or muscle weakness in the lower limbs. Some people with FXTAS experience problems with the autonomic nervous system, which controls involuntary body functions, leading to the inability to control the bladder or bowel.
People with FXTAS commonly have cognitive disabilities. They may develop short-term memory loss and loss of executive function, which is the ability to plan and implement actions and develop problem-solving strategies. Loss of this function impairs skills such as impulse control, self-monitoring, focusing attention appropriately, and cognitive flexibility. Many people with FXTAS experience anxiety, depression, moodiness, or irritability.
Some women develop immune system disorders, such as hypothyroidism or fibromyalgia, before the signs and symptoms of FXTAS appear.

How common is FXTAS?

Studies show that approximately 1 in 450 males has the genetic change that leads to FXTAS, although the condition occurs in only about 40 percent of them. It is estimated that 1 in 3,000 men over age 50 is affected. Similarly, 1 in 200 females has the genetic change, but only an estimated 16 percent of them develop signs and symptoms of FXTAS.

What genes are related to FXTAS?

Mutations in the FMR1 gene increase the risk of developing FXTAS. The FMR1 gene provides instructions for making a protein called FMRP, which helps regulate the production of other proteins. FMRP plays a role in the development of synapses, which are specialized connections between nerve cells. Synapses are critical for relaying nerve impulses.
Individuals with FXTAS have a mutation in which a DNA segment, known as a CGG triplet repeat, is expanded within the FMR1 gene. Normally, this DNA segment is repeated from 5 to about 40 times. In people with FXTAS, however, the CGG segment is repeated 55 to 200 times. This mutation is known as an FMR1 gene premutation. An expansion of more than 200 repeats, a full mutation, causes a more serious condition called fragile X syndrome, which is characterized by intellectual disability, learning problems, and certain physical features.
For unknown reasons, the premutation leads to the overproduction of abnormal FMR1 mRNA that contains the expanded repeat region. The FMR1 mRNA is the genetic blueprint for the production of FMRP. Researchers believe that the high levels of mRNA cause the signs and symptoms of FXTAS. The mRNA has been found in clumps of proteins and mRNA (intranuclear inclusions) in brain and nerve cells in people with FXTAS. It is thought that attaching to FMR1 mRNA and forming clumps keeps the other proteins from performing their functions, although the effect of the intranuclear inclusions is unclear. In addition, the repeat expansion makes producing FMRP from the mRNA blueprint more difficult, and as a result, people with the FMR1 gene premutation can have less FMRP than normal. A reduction in the protein is not thought to be involved in FXTAS. However, it may cause mild versions of the features seen in fragile X syndrome, such as prominent ears, anxiety, and mood swings.
Read more about the FMR1 gene.
Read more about fragile X syndrome.

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