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 mental retardation in males and a significant cause of mental retardation in females.
Martin-Bell syndrome; Marker X syndrome
Causes, incidence, and risk factors
Fragile X syndrome is caused by a change in the FMR1 gene. A small section of the gene code (three letters only -- CGG) is repeated on a fragile area of the X chromosome. The more repeats, the more likely there is to be a problem.
Normally, the FMR1 gene makes a protein needed for your brain to grow properly. A defect in this 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.
Fragile X syndrome can be a cause of autism or related disorders, although not all children with fragile X syndrome have these conditions.
SymptomsBehavior problems associated with fragile X syndrome include:
- Delay in crawling, walking, or twisting
- Hand clapping or hand biting
- Hyperactive or impulsive behavior
- Mental retardation
- Speech and language delay
- Tendency to avoid eye contact
Physical signs may include:
- Flat feet
- Flexible joins 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 mental retardation, 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
The person will have a family history of Fragile X syndrome (especially a male relative).
There are very few outward signs of Fragile X syndrome in babies. Some signs may include:
- Large head circumference in babies
- Mental retardation
- Oversized testes (macro-orchidism) in males who have reached puberty
- Subtle differences in facial characteristics
In females, excess shyness may be the only sign of the problem.
A genetic test called polymerase chain reaction (PCR) is used to diagnose this disease. This test looks for a mutation (called a triplet repeat) in the FMR1 gene.
In the past, a specific type of chromosome analysis was done. Such testing may still be available.
There is no specific treatment for Fragile X syndrome. Instead, training and education help affected children function at as high a level as possible. Because the condition is not rare, educational approaches have been developed and tested.
National Fragile X Foundation -- www.nfxf.org
The outcome depends on the extent of intellectual impairment that is present as well as emotional and social skills.
Complications vary depending on the type and severity of symptoms.
- Recurrent infections in children
- Seizure disorder
Calling your health care provider
Call your health care provider if you suspect Fragile X syndrome in a child with mental retardation. Genetic counseling is recommended to help families understand the condition, and to understand the complex nature of DNA testing in Fragile X.
Genetic counseling may help both existing and prospective parents with a family history of Fragile X syndrome, or a family history of other symptoms such as tremor. Genetic testing can help determine the level of risk in these families.
Accurate diagnosis is important, because other family members can inherit Fragile X syndrome or other problems related to an increased number of repeats in FMR1.
ReferencesWise PH. Developmental disabilities and chronic illness. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF, eds. Nelson Textbook of Pediatrics. 18th ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 38.
Reviewed By: A.D.A.M. Editorial: David Zieve, MD, MPH, David Eltz. Previously reviewed by Diana Chambers, MS, EdD, Certified Genetics Counselor (ABMG), Charter Member of the ABGC, University of Tennessee, Memphis. Review provided by VeriMed HealthcareNetwork (8/11/2009).