Fragile X syndrome test

Specimen:

10-20 mL blood in EDTA tube.

Method:

PCR-based analysis to detect an expanded triplet repeat mutation in the regulatory region of the FMR-1 (fragile-site mental retardation) gene.

Application:

Investigation of familial or sporadic mental retardation without obvious cause, in conjunction with cytogenetics, to detect the fragile X mutation.

Prenatal diagnosis for the Fragile X syndrome is available - consult pathologist.

Interpretation:

In normal individuals the triplet repeat region is less than 50 triplets long (150 nucleotides).

The identification of the characteristic expansion of the triplet repeat is diagnostic for the presence of the fragile X mutation.

Individuals with expansions of 60-200 triplets are unaffected but may transmit the disorder (premutation); those with expansions of approximately 200 triplets may be affected and/or transmit the disorder; males with expansions of >200 triplets will be affected.

The mothers of affected children are obligate carriers. Molecular genetic studies are indicated in other family members.