Health Library

Fragile X syndrome (FXS) is a problem of the X chromosome. It is a condition that is inherited from the parents. FXS is the most common cause of inherited intellectual disability. It is also the most common known cause of autism. FXS affects 1 in 4000 males and 1 in 6000 females.

FXS is caused by problems with the FMR1 gene. These problems prevent a certain protein from developing. The protein called fragile X mental retardation protein (FMRP) helps to make connections in the brain. Without the protein, certain connections in the brain cannot be made. This causes developmental problems and prevents children from fully developing some higher cognitive functions.

Cognitive Mapping of the Brain

Copyright © Nucleus Medical Media, Inc.

The main risk factor for FXS is having a parent with an FMR1 mutation. These mutations vary in degree. Most people who inherit a minor mutation, which is sometimes called a premutation, do not develop the symptoms and signs of FXS, though a few premutated children may show signs resembling autism, and others (primarily males) may develop a set of neurological symptoms called “fragile X tremor ataxia syndrome” in later adult life.

However, children of mothers with FMR1 premutations are at risk of inheriting a fully mutated FMR1 gene severe enough to cause symptomatic FXS. If a woman does not have symptoms and is a carrier of an FMRI mutation or premutation, each child of hers has a 50% chance of inheriting that gene. Of the children that inherit that gene, boys are much more likely to develop symptoms than girls. The severity of the disorder may vary quite a bit between different individuals.

In general, each generation tends to have worse mutations and a higher risk of FXS than the previous one.

The number and severity of symptoms varies widely. Symptoms tend to be less frequent and milder in females. Symptoms can include:

• Mental impairment ranging from learning disabilities to intellectual disability
• Behavioral difficulties, including:
  • Attention deficit hyperactivity disorder (ADHD)
  • Anxiety
  • Unstable moods and emotional outbursts
  • Hypersensitivity to loud noises or bright lights
  • Aggressive behavior
  • Extreme shyness (particularly in girls)
• Autistic behaviors
• Physical problems and abnormalities, including:
  • Long face with protruding jaw
  • Large, protruding ears
  • Flat feet
  • Hyperextended joints
  • High-pitched voice and enlarged testes in males after puberty
  • Mitral valve prolapse
  • Seizures
  • Low muscle tone

The doctor will ask about symptoms and medical history. A physical exam will be done. FXS may be suspected if a child has:

• Physical, cognitive, or emotional symptoms of FXS
• Unexplained developmental delays or intellectual disability

The diagnosis can be confirmed by genetic testing ( DNA blood test).

There is no cure for FXS. Treatment is aimed at controlling symptoms. Treatment may include:

There are no known ways to prevent FXS once a person is born. If you have FXS or risk factors for FXS, you can talk to a genetic counselor when deciding to have children.