What causes Fragile X syndrome ?
What causes fragile X syndrome? Mutations (changes) in the FMR1 gene cause fragile X syndrome (FXS). This gene carries instructions to make a protein called the fragile X mental retardation 1 protein. The FMR1 gene contains a section of DNA called a CGG triplet repeat, which normally repeats from 5 to around 40 times. In most cases of FXS, this section of DNA is repeated more than 200 times, which "turns off" the FMR1 gene and disrupts the function of the nervous system. In a small portion of cases, other types of changes in the FMR1 gene cause FXS. These changes may involve a deletion of all or part of the gene, or a change in the building blocks (amino acids) used to make the gene's protein. People with 55 to 200 repeats of the CGG segment are said to have an FMR1 premutation. Most people with a premutation are intellectually normal. In some cases, people with a premutation have lower levels of the gene's protein and may have some mild symptoms of FXS. About 20% of women with a premutation have premature ovarian failure, and some people with a premutation have an increased risk of developing fragile X-associated tremor/ataxia syndrome (FXTAS).