Fragile X Syndrome (cont.)

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.

Medically Reviewed by a Doctor on 4/4/2014

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