What is  Fragile X syndrome?

Fragile X syndrome (also called Fragile X)  is the most common inherited form of mental retardation.* It results from a change, or mutation, in a single gene, which can be passed from one generation to  the next. Fragile X appears in families of every ethnic group and income level. 

Symptoms of Fragile X syndrome occur because  the mutated gene cannot produce enough of  a protein that is needed by  the body's cells, especially cells in the brain,  to develop and function normally. The amount and usability  of this protein, in part, determine how severe  the effects of Fragile X are. 

The most noticeable and consistent effect  of Fragile X is on intelligence. More than 80 percent of males  with Fragile X  have an IQ (intelligence quotient) of 75 or less.1 The effect of Fragile X on intelligence is more variable in females. Some females have  mental impairment, some have learning disabilities,  and some have a normal IQ. 

People with Fragile X syndrome also share certain medical problems as well as many common physical characteristics,2  such as large ears  and a long face. In addition, having Fragile X is often associated  with problems with sensation, emotion, and behavior. 

The National Institute of Child Health and Human Development (NICHD), part of the National Institutes of Health (NIH), is a major sponsor of research on Fragile X syndrome. Since 1991, when researchers funded by the NICHD discovered the gene that causes Fragile X, scientists have learned a great deal about that gene's structure and functions. The NICHD continues to support clinicians and scientists around the world who are working to find effective  behavioral or other therapies, medical treatments,  and prevention strategies  for Fragile X. 

What causes  Fragile X syndrome?

The underlying cause of Fragile  X is a change in a single gene,  the Fragile X Mental Retardation 1 (FMR1) gene, which is found on the X chromosome. (See the Human cells 101 section for more information about the X chromosome.) But how does this change  cause Fragile X? 

Causes  of Mental Retardation 

Mental retardation is associated with more than 500 conditions. Some of the most commonly  known genetic causes among these are: 

• Down syndrome, due  to an error in  the number of chromosomes 
• Fragile X, due to a repeat  in the genetic code that leads to a lack of production of a certain protein 
• PKU (phenylketonuria), due  to an error  in a single gene that makes a defective enzyme 

Other causes of mental retardation that can occur during pregnancy  include the mother's excessive use of alcohol, exposure to poisons in the environment, and diseases such  as rubella.

Genes contain the information used by  other parts of a cell to make proteins. Proteins  are the body's building blocks. Each protein performs a specific job. They make up the structure  of your organs and tissues  and are needed for all of your body's chemical functions. 

Each gene contains information for making at least one protein. If this information is changed, then the cell may not be able to make that protein, or it may not be able to make  a form of the protein  that the body can use. Fragile X occurs because the FMR1 gene is unable to make normal amounts of usable Fragile X Mental Retardation Protein, or FMRP. 

The amount of FMRP in the body is one factor that determines how severe the effects of having Fragile X are. A person  with nearly normal levels of FMRP usually has mild or no symptoms, while  a person with very little or no normal FMRP has more severe symptoms. 

Scientists are still studying the role  of FMRP in the body. One current research study revealed  that certain cell processes brain cells use  to communicate  with one another occur in excess in mice that have little  or no FMRP; that is, he brain cells may communicate too much or may communicate inappropriately. Researchers believe that FMRP may regulate the amount  of communication between cells and keep it under control. Scientists are hopeful that they can identify a similar function for FMRP  in humans. 

Notes: Dr. Nelson Crumfield

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EditText of this page (last edited June 10, 2010)