Fragile X syndrome is caused by an inherited change on the X chromosome that may interfere with normal brain development and body function.

Historically, Fragile X was diagnosed in a family because a boy or several male relatives with mental retardation were tested for this common genetic syndrome. More recently, researchers have discovered an association between Fragile X syndrome and women with premature ovarian dysfunction (often called POF).

What Causes Fragile X?
Symptoms of Fragile X

What Causes Fragile X?

Fragile X syndrome is caused by changes to a particular region of the FMR1 gene (short for "Familial Mental Retardation 1"). This region is made up of a series of three DNA nucleotides, C-G-G, that are repeated a number of times in what is called a "triplet repeat."

Most people carry between 6 and 40 copies of the triplet repeat, C-G-G, in their FMR1 gene. And studies have shown that carrying up to 59 copies of the triplet repeat has no effect on the ability of the FMR1 gene to work properly. But difficulties begin to occur when the triplet repeat is greater than 59.

When the triplet repeat is between 60 and 200 copies, geneticists refer to this as a premutation for Fragile X syndrome. When the triplet repeat copies are more than 200, this is called a full mutation for Fragile X syndrome.

It is estimated that approximately 1 in 246 to 1 in 468 women carry a premutation for Fragile X. One in 3,600 boys and 1 in 6,000 girls are likely affected by the full mutation of Fragile X.

How premutations and full mutations affect the body depends on whether the person carrying these changes is a male or female.

Each of us carry a pair of sex chromosomes: women have two X chromosomes (XX), while men have one X and one Y chromosome (XY). The genes located on the X chromosome are not present on the Y chromosome, and vice versa.

When a gene on the X chromosome is altered or mutated, women may experience fewer effects because they carry a second X chromosome. Men, who only have the one X chromosome, usually experience more serious consequences.

Symptoms of Fragile X

Women who carry premutations for Fragile X syndrome are at increased risk for serious reproductive complications, but do not have other physical and mental features of Fragile X syndrome. The are also at risk for having a child who inherits a full mutation, because the premutation can expand during reproduction.

Although women who inherit a full mutation in the Fragile X gene are less severely affected than men, although approximately one-third do have mental retardation or learning and/or behavior issues. Women with full mutations do not have the same reproductive difficulties that women with premutations typically experience.

Men who inherit a full mutation in the Fragile X gene are almost always affected with mental retardation, which may range from mild to profound, and may also experience autistic-like behaviors in addition to some facial and physical changes.

Fragile X syndrome is a complex condition, and more detailed information regarding the condition man be found at the Fragile X Foundation (www.FragileX.org) and the Fragile X Research Foundation (www.fraxa.org).

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