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Fragile X Syndrome

Fragile X syndrome facts*

*Fragile X syndrome facts medically reviewed by Charles Patrick Davis, MD, PhD on 1/15/2015.

  • Fragile X syndrome (also called Fragile X) is the most common inherited form of mental problems (mental retardation).
  • Fragile X syndrome is caused by changes in a single X chromosome (FMR1).
  • FMR1 does not produce enough protein (FMRP) that works cell communication.
  • Normal X genes that control FMR1 levels have promoter regions consisting of about 6 to 40 CGG; Fragile X genes have about 55 to 200 (termed premutation) or more than about 200 regions (termed full mutation).
  • About 100 trillion cells make up a human body and each should have 46 chromosomes, but males have only one X chromosome; so, if it contains a FMR1 chromosome, the effect of the lack of FMRP is more significant than in females.
  • Approximately one in 3,600 to 4,000 males is affected by the full mutation of the FMR1 gene, and about one in 468 males has the premutation form of the FMR1 gene; about one in 151 females in the general population has the premutation while about one in 4,000 to 6,000 females is affected by the full mutation (these are estimates only).
  • The Fragile X gene is only carried on an X chromosome; both males and females may transmit a Fragile X (FMR1) gene on their X chromosome so either a man or a woman may be a carrier.
  • The signs and symptoms of Fragile X syndrome have five general categories where individuals often show altered or arrested development.
    1. Intelligence and learning (low IQs)
    2. Physical (long face or jaw, short stature, enlarged testicles)
    3. Social and emotional (anxiety)
    4. Speech and language (stuttering and disorganized speech)
    5. Sensory (sensitive to certain stimuli like sounds, balance problems)
  • Each individual may exhibit variations in the above five categories.
    • Features of autism and fragile X overlap, but the core deficit in autism is social interaction while the core deficit in fragile X is intellectual function and hyper arousal/anxiety.
    • Angelman syndrome is a genetic disorder that also has features of fragile X with delayed development and severe speech impairment along with seizures, small head size, and hand-flapping movements; but is due to genetic defects or alterations in a gene termed UBE3A.
    • Myotonic dystrophy (muscular degeneration and tissue loss) related to fragile X syndrome as both have similar genetic alterations (repeat expansions of genetic coding)
  • Currently, there is no cure for Fragile X syndrome nor are there any definitive treatments other than helping the individual develop to their maximal potential through education; therapies such as speech, language, behavioral, and physical; and other therapies. Fragile X detects FMR1 DNA (PCR or Southern blot methods)
  • Medication therapy may only help certain individuals with certain severe problems and should be prescribed only by the patients' doctors; there is no medication that can cure Fragile X syndrome.
  • Adults with Fragile X syndrome may benefit from a team approach to help the patient adjust to independent living, if possible.
  • IIf a family member has been diagnosed with Fragile X syndrome, other family members may want to be tested to determine if they carry a Fragile X chromosome; there are several accurate DNA tests available for gene testing (see above) and for determining the amount of FMRP produced. Prenatal tests are also available.
  • Research is ongoing; gene therapy, gene activation, and FMRP replacement therapy are several areas that may lead to treatments, or possible cure or prevention, in the future.
  • Several national organizations may provide additional information on Fragile X syndrome (for example, American Association on Mental Retardation and the National Institute of Child Health and Human Development).


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