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Fragile X syndrome

Fragile X syndrome is a genetic condition involving changes in part of the X chromosome. It is the most common form of inherited intellectual disability in boys.

Alternative Names

Martin-Bell syndrome; Marker X syndrome

Causes, incidence, and risk factors

Fragile X syndrome is caused by a change in a gene called FMR1. A small part of the gene code is repeated on a fragile area of the X chromosome. The more repeats, the more likely there is to be a problem

The FMR1 gene makes a protein needed for your brain to grow properly. A defect in the 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.

A family history of fragile X syndrome, developmental problems, or intellectual disability may not be present.

Symptoms

Physical signs may include:

  • Flat feet
  • Flexible joints and low muscle tone
  • Large body size
  • Large forehead or ears with a prominent jaw
  • Long face
  • Soft skin

Some of these problems are present at birth, while others may not develop until after puberty.

Family members who have fewer repeats in the FMR1 gene may not have intellectual disability, 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.

Signs and tests

There are very few outward signs of Fragile X syndrome in babies. Some signs may include:

  • Large head circumference in babies
  • Intellectual disability
  • Large testicles after the start of puberty
  • Subtle differences in face features

In females, excess shyness may be the only sign of the disorder.

Genetic testing can diagnose this disease.

Treatment

There is no specific treatment for Fragile X syndrome. Instead, training and education have been developed to help affected children function at as high a level as possible.

Support Groups

National Fragile X Foundation -- www.fragilex.org

Expectations (prognosis)

How well the patient does depends on the amount of intellectual disability.

Complications

Complications vary depending on the type and severity of symptoms.

  • Recurrent infections in children
  • Seizure disorder

Fragile X syndrome can be a cause of autism or related disorders, although not all children with fragile X syndrome have these conditions.

Prevention

Genetic counseling may be helpful if you have a family history of this syndrome and are planning to become pregnant.

References

Shapiro BK, Batshaw ML. Intellectual disability. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF, eds. Nelson Textbook of Pediatrics. 19th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 33.

Updated: 9/6/2011

A.D.A.M. Editorial Team: David Zieve, MD, MHA, and David R. Eltz. Previously reviewed by Chad Haldeman-Englert, MD, Wake Forest School of Medicine, Department of Pediatrics, Section on Medical Genetics, Winston-Salem, NC. Review provided by VeriMed Healthcare Network (8/4/2011).


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