LYCOS RETRIEVER 
Fragile X Syndrome
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Fragile X Syndrome is the most common form of inherited mental retardation. More than 99% of cases are due to the expansion of a polymorphic (CGG) repeat within the FMR1 gene. Approximately 1/1250 males and 1/2500 females are affected by the condition. Some population studies have shown the carrier frequency to be as high as 1/250 individuals. The American College of Medical Genetics policy statement on Fragile X testing recommends consideration of testing under the following circumstances:
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Fragile X Syndrome is an X-linked dominant disorder with variable penetrance characterized by developmental delay, mental retardation (severe in males and milder in females) and macroorchidism in post-pubertal males. The genetic defect is an expansion of a trinucleotide repeat (CGG) pattern that causes loss of function of the FMR1 gene localized in chromosome Xq27. Increases in clinical severity are frequently observed after parent-to-child transmission. Female carriers are usually mildly- or unaffected and there may be unaffected males that are carriers of a premutation.
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Fragile X Syndrome occurs when a certain gene (FMR-1) mutates or changes. The FMR-1 gene mutates by expanding or repeating certain chemical components of DNA (CGG). Everyone has the FMR-1 gene, but whereas the general population has between 5 and 40 repeats, carriers of FXS have between 55 and 200 repeats. This is called the premutation stage. Individuals with the full mutation have over 200 repeats. The full mutation causes the FMR-1 gene to turn off.
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A person with Fragile X Syndrome has a mutation in the FMR1 (Fragile X mental retardation 1) gene in the DNA that makes up the X chromosome. That mutation causes the cell to methylate a regulatory region of the FMR1 gene. The methylation turns off the FMR1 gene. Since the gene is turned off, the person doesn't make Fragile X mental retardation protein (FMRP). That lack of a specific protein triggers Fragile X Syndrome.
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Fragile X Syndrome is now recognized as the most common inherited cause of mental retardation. The clinical and behavioral changes are not always obvious and can lead to delay in diagnosis. The most common findings are developmental delays, including speech problems and behavioral features such as hyperactivity and hand biting or hand flapping. Mental retardation can range from mild to severe and females tend to be more mildly affected than males. Physical signs in males include long faces, large prominent ears and large testes which are more apparent after childhood. Hagerman et al have put together a screening check list which can be used by the clinician considering referral to genetics for further evaluation.
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Fragile X Syndrome is caused by a mutation in a specific gene carried on the X chromosome. This gene, called the Fragile X Mental Retardation 1 (FMR1) gene, is a sometimes unstable gene that mutates frequently in human beings. The particular type of mutation involved, known as a ”trinucleotide repeat disorder,” involves multiple errors in the genetic code that make up the affected gene. Because the FMR1 gene is so easily destabilized, it is more likely to show up suddenly and frequently in families with no previous history of the disease than most other genetic disorders. Carriers of the disease (those with “premutations” of the FMR1 gene) usually do not show any symptoms, but their children and grandchildren have a much greater chance of being affected.
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Fragile X Syndrome