Fragile X syndrome
Diagnosis and Testing
Is there more than one test for fragile X syndrome?
The "gold standard" for fragile X syndrome testing has been polymerase chain reaction (PCR) testing to determine the number of CGG repeats and Southern blot analysis, which is another method used to detect the number of repeats that can also determine whether the gene is methylated (turned off) or not. Most (greater than 99%) mutations in the FMR1 gene are due to an increased number of CGG repeats. Less than 1% of mutations are due to deletions or duplications in/of the FMR1 gene (this means loss or gain of genetic material) or sequence variants (a small change in the DNA code of the FMR1 gene). The technology used to detect these types of genetic changes include a method called FISH (fluorescent in situ hybridization), deletion/duplication analysis, and sequence analysis.
Protein testing to look at the amount of FMRP (fragile X mental retardation protein) is available but is not performed routinely. This test may be useful to screen males who have intellectual disability (mental retardation) but no diagnosis yet, but this screening test would not provide a definitive diagnosis. Testing of the FMR1 gene would still be needed to confirm whether or not that person has fragile X syndrome.
All of this testing must be ordered by a physician or genetic counselor who can interpret the results for the patient and their family. A full list of the laboratories currently offering testing for fragile X syndrome can be found by visiting GeneTests and entering "fragile X syndrome" into the search box.