Fragile X syndrome


Are there earlier onset, later onset, or variant forms of fragile X syndrome?

Fragile X syndrome affects development from birth. While there is variable expression of fragile X syndrome, meaning some individuals are not as severely affected as others, there is not an early onset or late onset version of the disorder. Individuals who have the premutation in the FMR1 gene of 55 to 200 CGG repeats can have other disorders that are associated with this abnormal repeat number, but these individuals are not said to have fragile X syndrome.

Fragile X syndrome is part of a group known as the FMR1-related disorders. In addition to fragile X syndrome, this group includes fragile X-associated tremor/ataxia syndrome (FXTAS) and FMR1-related primary ovarian insufficiency (FXPOI). All the FMR1-related disorders can affect both men and women, with the exception of FXPOI (women only). All the FMR1-related disorders are passed from one generation to the next through mutation of the FMR1 gene, which is on the X chromosome. The most common cause of these FMR1-related disorders is having too many CGG repeats in the FMR1 gene. Every gene is made up of a string of nucleotide bases in a specific order; the nucleotide bases are A, T, G, and C. The length of each gene and the specific code are different, and sometimes specific parts of the code are repeated. For the FMR1 gene, having up to 44 CGG trinucleotide ("tri" meaning "three", "nucleotide" meaning "nucleotide base") repeats is typical. A person who has between 45 and 54 CGG repeats is considered "gray zone" or "borderline", meaning the potential exists for that repeat to become larger and then be considered a premutation. These individuals do not show any signs of any of the FMR1-related disorders. When a person has between 55 and 200 CGG repeats, they have the premutation and can have either FXTAS and/or FXPOI. If a person has over 200 CGG repeats in their FMR1 gene and it becomes methylated (meaning "turned off"), they have fragile X syndrome, meaning they have the full mutation and will have signs of the disorder. The number of CGG repeats in the FMR1 gene can be determined through molecular testing of the FMR1 gene.

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