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What causes fragile X syndrome?
In 1991, a March of Dimes-supported researcher discovered that fragile X syndrome is caused by a mutation (change) in a gene (called FMR-1) located on the X chromosome (6). Each person has 23 pairs of chromosomes, or 46 individual chromosomes. One pair, referred to as the sex chromosomes (called X and Y), determines whether a person is male or female. Normally, females have two X chromosomes, and males have one X chromosome and one Y chromosome.
A female who inherits one X chromosome with the abnormal FMR-1 gene has, in effect, a spare X with the normal gene. Therefore, females are affected by fragile X syndrome less frequently than males, and when affected, tend to have less severe symptoms. Males are generally more severely affected because they have one X chromosome containing only the abnormal gene. The mutation that causes fragile X syndrome is a genetic “stutter” in which a small section of genetic material within the gene is repeated too many times. Most individuals who do not have fragile X syndrome have between six and 45 repeats of this section of three “letters” (called a trinucleotide repeat) that help spell out the gene (7).
When an individual has more than about 200 repeats, the gene turns off and fails to make the protein it usually makes. It is not known how lack of this protein causes the symptoms of fragile X syndrome, but studies suggest that this protein may help regulate communication between nerve cells in the brain (5).
Fragile X syndrome gets its name from the appearance of the section of the X chromosome where the gene mutation occurs. In certain conditions, under a microscope, the section of the chromosome looks fragile, as if it is dangling by a thread.
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