Fragile X syndrome (FXS) is the most frequent inherited cause of intellectual disability globally. The condition is linked to a genetic change in the FMR1 gene on the X chromosome. This alteration involves an unstable trinucleotide repeat segment (a cytosine-guanine-guanine or CGG sequence). The number of these CGG repeats determines if an individual is unaffected, a carrier, or has the syndrome. Since FXS presentation varies significantly, specialized genetic testing is the only definitive method to confirm the FMR1 gene mutation.
When Diagnostic Testing Is Recommended
Healthcare providers recommend FMR1 gene testing when a person presents with specific developmental or medical concerns. Testing is used for individuals with unexplained developmental delay, intellectual disability, or autism spectrum disorder. Identifying FXS helps determine the underlying genetic cause of these conditions.
Physical characteristics associated with FXS, such as a long face, large ears, or flexible joints, also prompt testing. Genetic testing is considered when there is a known family history of FXS or other related conditions.
Related conditions include Fragile X-associated Tremor/Ataxia Syndrome (FXTAS) and Fragile X-associated Primary Ovarian Insufficiency (FXPOI). Individuals with a family history of unexplained intellectual disability, movement disorders, or early menopause are candidates for testing. Identifying the FMR1 status guides medical management and intervention strategies.
Molecular Methods Used for Diagnosis
Laboratory diagnosis of FXS measures the number of CGG trinucleotide repeats within the FMR1 gene. Testing often uses a combination of two molecular techniques to ensure comprehensive results.
Polymerase Chain Reaction (PCR) sizes smaller repeat lengths, generally within the normal, intermediate, and premutation ranges. PCR rapidly amplifies the DNA segment, allowing for a precise count of the CGG repeat number.
PCR becomes unreliable when the number of repeats is very large, such as in the full mutation range. Southern Blot analysis confirms full mutations, defined as over 200 CGG repeats. Southern Blotting also determines the gene’s methylation status, a chemical change that silences gene expression and characterizes the full mutation.
Specialized PCR methods, such as Triplet-Repeat PCR (TP-PCR), handle larger expansions, sometimes reducing the necessity for Southern Blotting. Laboratories use both PCR and Southern Blot, or advanced PCR techniques, to ensure accurate sizing and detect mosaicism.
Understanding the Test Results
Results from FMR1 gene testing are categorized based on the number of CGG repeats found. The Normal Range (5 to 44 CGG repeats) means individuals are not carriers and are not at risk for FXS or related disorders.
The Intermediate Range involves 45 to 54 repeats. Individuals in this range are not considered carriers and do not have an increased risk of having a child with a full mutation. However, there is a minor possibility of expansion to a premutation in future generations.
A Premutation Range is characterized by 55 to 200 CGG repeats. Individuals with a premutation are carriers who do not have FXS. They are at risk for developing FXTAS, a neurological disorder, or, in women, FXPOI (early menopause).
The Full Mutation is defined by having more than 200 CGG repeats. The large repeat number leads to the silencing of the FMR1 gene, resulting in a deficiency of the FMR protein necessary for proper brain development.
Carrier Screening and Prenatal Options
Testing for the FMR1 gene is an established component of reproductive planning. Carrier screening is offered to asymptomatic individuals who wish to determine their risk of passing a gene variant to their children.
Screening is recommended for women planning a pregnancy or in the early stages of one, especially with a family history of FXS or unexplained intellectual disability. Identifying a woman as a premutation carrier is important because the premutation is unstable and can expand to a full mutation when passed to offspring.
For parents identified as carriers, prenatal diagnostic testing determines the fetal FMR1 status during pregnancy. Procedures like Chorionic Villus Sampling (CVS, 10 to 13 weeks) or amniocentesis (16 to 20 weeks) provide the fetal DNA necessary for analysis.
An alternative for couples undergoing in vitro fertilization (IVF) is Preimplantation Genetic Diagnosis (PGD). This tests embryos for the full mutation before implantation, allowing selection of unaffected embryos. Genetic counseling discusses the risks of expansion and reproductive choices.