Fragile X Syndrome
(Martin-Bell Syndrome; FXS)
by Rick Alan
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.
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:
The doctor will ask about symptoms and medical history. A physical exam will be done. FXS may be suspected if a child has:
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:
Medicines used to treat symptoms of FXS include:
Scientists are currently trying to understand how FMRP works in the body. This will help develop treatments specific to FXS.
Specific educational strategies depend on the degree of developmental delays and/or intellectual disability. Educational strategies include setting a stable educational environment for the patient that includes:
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.
FRAXA Research Foundation
The National Fragile X Foundation
National Institute of Child Health and Human Development
Fragile X Research Foundation of Canada
Bardoni B, Davidovic L, Bensaid M, Khandjian EW. The fragile X syndrome: exploring its molecular basis and seeking a treatment. Expert Rev Mol Med . 2006;8(8):1-16.
Committee on Genetics health supervision for children with fragile X syndrome. Pediatrics. 2011;127;994.
Fragile x syndrome. EBSCO DynaMed website. Available at: http://www.ebscohost.com/dynamed/what.php . Updated January 11, 2011. Accessed September 14, 2012.
FRAXA Research Foundation website. Available at: http://www.fraxa.org/fragilex.aspx . Accessed September 20, 2012.
National Institute of Child Health and Human Development website. Available at: http://www.nichd.nih.gov/health/topics/fragile_x_syndrome.cfm . Accessed September 20, 2012.
Willemsen R, Anar B, De Diego Otero Y, et al. Noninvasive test for fragile X syndrome, using hair root analysis. Am J Hum Genet . 1999;65(1):98-103.
Last reviewed October 2012 by Rimas Lukas, MD