Fragile X Syndrome

What is fragile X syndrome?

Fragile X syndrome is a genetic condition that can cause a range of learning and developmental problems. These can include:

Some children with fragile X may also have mildly characteristic features such as prominent ears.

Fragile X is more common in boys than girls, and boys with fragile X usually have more severe symptoms.

What are the symptoms of fragile X syndrome?

Both boys and girls with fragile X can have certain behavioral and intellectual traits, though girls often have milder symptoms.

Kids with fragile X may have:

  • delayed speech
  • delayed motor skills
  • repetitive and unclear speech (especially in boys)
  • autistic-like behaviors, including poor eye contact, hand biting, hand flapping, and sensitivity to noise and crowds
  • learning difficulties
  • social anxiety
  • hyperactivity and short attention spans (especially in boys)
  • shyness (mostly seen in girls)

Some children with fragile X also have physical traits, which may become more noticeable after puberty:

  • large ears
  • a long and narrow face
  • flat feet
  • in boys: large, but functional, testicles
  • very flexible joints

Because the symptoms of fragile X are similar to, or may mimic, those of other conditions, such as autism and Prader-Willi syndrome, it’s important to see a doctor for an exact diagnosis.

What are the causes of fragile X syndrome?

Fragile X is caused by a mutation in the FMR1 gene that prevents the body from making an important protein, called FMRP. This protein helps create and maintain connections between brain cells and the nervous system. When FMRP is missing, signals from the brain may be misdirected. This causes the developmental and learning problems found in fragile X.

The FMR1 gene is found on the X chromosome, which means that females (who have two X chromosomes) have two copies of the FMR1 gene, and males (who have one X chromosome) have only one. If this one X chromosome is not working, there’s no backup. This is why boys often have more severe symptoms than girls.

Are there any medical concerns with fragile X?

Unlike many other genetic conditions, fragile X syndrome does not cause many medical complications. Health problems in children with fragile X may include:

As adults, people with fragile X may also develop:

  • mitral valve prolapse (leaky heart valve); this is seen in about half of all adults with fragile X
  • aortic enlargement in a small proportion
  • high blood pressure that can be treated with medication
  • scoliosis

Care for fragile X syndrome

At the Boston Children’s Hospital Fragile X Program, we provide specialty care for children and adults with fragile X. We’re also actively involved in doing research to learn more about the condition and to find better treatments.

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