Fragile X syndrome is the most common inherited cause of intellectual disability and autism.

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About Fragile X Syndrome

Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability and autism. Symptoms cover a wide range, from mild to very severe. About one-third of all people with FXS also have autism.1 Individuals with the disorder may also have behavioral issues such as, hyperactivity, social anxiety and aggression. Though FXS occurs in both sexes, males are more frequently affected than females, and generally with greater severity.

Approximately 1 in every 3,600 males and 1 in every 6,000 females is born with FXS.

Fragile X carriers are unaffected by the disorder, but they are at risk for certain related genetic conditions that can affect individuals in a variety of ways. Fragile X-associated tremor/ataxia syndrome is a condition that causes balance, tremor and memory problems in some older male (and less commonly, female) carriers. Fragile X-associated primary ovarian insufficiency is characterized by decreased ovarian function, which can lead to infertility and early menopause in some female carriers.

Now available with reflex to Xpansion Interpreter® for precise information about reproductive risk for patients with 55 to 90 CGG repeats.

Inheritance of Fragile X

In normal individuals, the FMR1 gene, located on the X chromosome, is characterized by a repetitive trinucleotide sequence of less than 45 CGG repeats. Fragile X syndrome is typically caused by a full mutation of the FMR1 gene (>200 CGG repeats).

A carrier of a fragile X premutation allele has between 55 and 200 CGG repeats, which may expand further when passed from mother to child, resulting in fragile X syndrome. Fragile X is an X-linked dominant disorder, so only the mother needs to be a carrier for the child to be at risk. If the mother is a carrier, there is up to a 1 in 2 (50%) chance the child will be affected. The risk can be calculated based upon the number of CGG repeats the mother carries. About 1 in 259 women in the US, across all ethnic groups, are carriers of fragile X.

Interpreting Carrier Screening Results

The results for fragile X testing fall into four categories:

Normal less than 45 CGG repeats Patient is not at risk to have a child with FXS.
Intermediate (gray zone) 45–54 CGG repeats There is no reported risk for patient to have an affected child. Prenatal diagnosis is available for reassurance.
Future generations may be carriers, because the number of repeats may increase as the gene is passed on.
Patient may be at risk of developing fragile X-associated disorders
Refer to genetic counselor.
Premutation (carrier) 55–200 CGG repeats Females are at risk to have a child with FXS.
Prenatal diagnosis is available.
Patient is at risk for fragile X-associated disorders.
Refer to genetic counselor.
Full mutation more than 200 CGG repeats The individual may be affected by FXS.
Females are at risk to have a child with FXS.
Prenatal diagnosis is available.
Refer to genetic counselor.
References
1. Autism spectrum phenotype in males and females with fragile X full mutation and premutation. J Autism Dev Disord. 2007 Apr;37(4):738-47.