Diagnosis of Fragile X Syndrome

Determining the length of the repeat region in the FMR1 gene

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How is Fragile X Syndrome Diagnosed?

The FMR1 genetic test examines the repeat lengths in the FMR1 gene. The genetic test will determine the size of the tri-nucleotide repeat to see if an individual has a full mutation, a premutation, an intermediate allele or a normal allele.

People with a normal number of repeats (5-44) or an intermediate number (45-54) do not have fragile X syndrome. Individuals with the premutation (55-200 repeats) may exhibit mild symptoms. The presence of a premutation can cause fertility issues and tremors. When the repeat size exceeds 200, these patients are predicted to have fragile X syndrome.

The signs of fragile X syndrome are nonspecific in early childhood with developmental delay being the most universal manifestation of the disease. Fragile X testing is recommended for any child with a delay of speech, language, or motor development and intellectual disability, particularly if there is a family history of fragile X syndrome, any physical or behavioral characteristics of fragile X syndrome or a relative with an undiagnosed intellectual disability.

Fragile X syndrome genetic testing is also recommended for men and women who develop intention tremors after the age of 50 or if there is family history of FMR1-related disorders. Women with unexplained fertility issues should also consider testing for fragile X-syndrome.

Testing expectant parents will determine whether they are carriers of the premutation which can affect their risk of having children with fragile X syndrome. Since babies with fragile X syndrome will not show symptoms until developmental deficiencies manifest, early detection is extremely useful to reduce the developmental defects and symptoms associated with fragile X syndrome, through timely intervention.

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Recommended Links: Fragile X-associated Disorders. National Fragile X Foundation

Saul RA, Tarleton JC (1998, Updated 2012 Apr 26). FMR1-Related Disorders. In: Pagon RA, Adam MP, Ardinger HH, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2016.