My daughter's father's sister had a son with fragile X syndrome. We are concerned about whether the gene was carried by her father and may have been passed down to her. Should my daughter be concerned about this before she starts a family?
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 FMR1premutation. 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).
Last updated: 1/29/2015
How is fragile X syndrome inherited?
Fragile X syndrome (FXS) is inherited in an X-linked dominant manner. A condition is X-linked if the responsible gene is located on the X chromosome. The inheritance is dominant if having only one changed (mutated) copy of the responsible gene is enough to cause symptoms of the condition.
In women who carry an FMR1 gene premutation (approximately 55 to 200 CGG repeats), the repeats can expand to more than 200 repeats in their cells that develop into eggs. This means that women with a premutation (or a full mutation) have an increased risk to have a child with FXS. The size of the risk corresponds to the number of CGG repeats they have. By contrast, men with premutations are not at risk for the repeats expanding to over 200 when passing the gene to offspring. However, men with a premutation will pass the premutation on to all of their daughters and none of their sons. This is because boys receive only a Y chromosome from their fathers.
Last updated: 3/6/2016
Is genetic testing available for fragile X syndrome?
Yes, genetic testing is available for fragile X syndrome. Carrier testing for at-risk relatives and prenatal testing for pregnancies at increased risk are possible if the diagnosis of an FMR1-related disorder (including fragile X syndrome) has been confirmed in a family member.
The Genetic Testing Registry (GTR) provides information about the labs that offer genetic testing for fragile X syndrome. The intended audience for the GTR is health care providers and researchers. People with questions about genetic testing should speak with a health care provider or genetics professional.
Last updated: 1/29/2015
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