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| DiseaseID |
HGD8 |
| Genetic
Disorder |
Fragile X syndrome
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| Gene
Name |
FMR1 gene |
| Description |
Fragile X syndrome is the most common form of inherited mental retardation in males and is also a significant cause of mental retardation in females. It affects about 1 in 4,000 males and 1 in 8,000 females and occurs in all racial and ethnic groups.
Nearly all cases of fragile X syndrome are caused by an alteration (mutation) in the FMR1 gene where a DNA segment, known as the CGG triplet repeat, is expanded. Normally, this DNA segment is repeated from 5 to about 40 times. In people with fragile X syndrome, however, the CGG segment is repeated more than 200 times. The abnormally expanded CGG segment inactivates (silences) the FMR1 gene, which prevents the gene from producing a protein called fragile X mental retardation protein. Loss of this protein leads to the signs and symptoms of fragile X syndrome. Both boys and girls can be affected, but because boys have only one X chromosome, a single fragile X is likely to affect them more severely
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| Symptoms |
Mental retardation Large testicles (macro-orchidism) after the beginning of puberty Large body size Tendency to avoid eye contact Hyperactive behavior Large forehead or ears with a prominent jaw Family members who have fewer repeats in the FMR1 gene may not have mental retardation, but may have other problems. Women with less severe changes may have premature menopause or difficulty becoming pregnant. Both men and women may have problems with tremors and poor coordination.
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| Causes |
Fragile X syndrome is caused by a change in the FMR1 gene. The gene's code is repeated on a fragile area of the X chromosome. The more repeats, the more likely there is to be a problem.
Normally, the FMR1 gene makes a protein you need for for your brain to grow properly. A defect in this gene makes your body produce too little of the protein, or none at all.
Boys and girls can both be affected, but because boys have only one X chromosome, a single fragile X is likely to affect them more severely. You can have Fragile X syndrome even if your parents do not have it. |
| Diagnosis |
The person will have a family history of Fragile X syndrome (especially a male relative).
There are very few outward signs of Fragile X syndrome in babies. Babies may have a large head circumference. Measurement of oversized testes in males who have reached puberty may also suggest the diagnosis. An experienced geneticist may note subtle differences in facial characteristics. Mental retardation is the hallmark of this condition and, in females, this may be the only sign of the problem.
A specific genetic test called polymerase chain reaction (PCR) is used to diagnose this disease. This test looks for an expanded mutation (called a triplet repeat) in the FMR1 gene.
In the past, a specific type of chromosome analysis was done. Such testing may still be available.
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| Treatment |
There is no specific treatment available for fragile X syndrome. Supportive therapy for children who have fragile X syndrome includes:
• Special education and anticipatory management including avoidance of excessive stimulation to decrease behavioral problems.
• Medication to manage behavioral issues, although no specific medication has been shown to be beneficial.
• Early intervention, special education and vocational training.
• Vision, hearing, connective tissue problems, and heart problems when present are treated in the usual manner. |
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