Fragile X syndrome and its associated conditions are caused by changes (mutations) in the FMR1 gene found on the X chromosome. This mutation affects how the body makes a protein called FMRP. The mutation causes the body to make only a little bit or none of the protein, which can cause the symptoms of Fragile X.
In a gene, the information for making a protein has two parts: the introduction, and the instructions for making the protein itself. Researchers call the introduction the promoter because of how it helps to start the process of building the protein.
The promoter part of the FMR1 gene includes many repeats—repeated instances of a specific DNA sequence called the CGG sequence. The FMR1 gene usually has between 6 and 40 repeats in the promoter; the average is 30 repeats.
People with between 55 and 200 repeats have a premutation of the gene. The premutation may cause the gene to not work properly, but it does not cause intellectual and developmental disability (IDD). The premutation is linked to the disorders FXPOI and FXTAS. However, not all people with the premutation show symptoms of FXPOI or FXTAS.
People with 200 or more repeats in the promoter part of the gene have a full mutation, meaning the gene might not work at all. People with a full mutation often have Fragile X syndrome.
The number of repeats, also called the “size of the mutation,” affects the type of symptoms and how serious the symptoms of Fragile X syndrome will be.
Inheriting Fragile X Syndrome
Fragile X syndrome is inherited, which means it is passed down from parents to children. Anyone with the FMR1 gene mutation can pass it to their children. However, a person who inherits the gene mutation may not develop Fragile X syndrome. Males will pass it down to all of their daughters and not their sons. Females have a 50/50 chance to pass it along to both their sons and daughters. In some cases, an FMR1 premutation can change to a full mutation when it is passed from parent to child. Read more about how FMR1 changes as it is passed from parent to child.