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Fragile X Syndrome, DNA Analysis, Prenatal With Southern Blot Analysis
Testing performed on fetal sample (amniotic fluid or CVS) for a fetus at risk for fragile X syndrome.
Polymerase chain reaction (PCR) followed by capillary electrophoresis, and, if required, Southern blot hybridization
Fragile X syndrome (OMIM 309550) is the most common known form of inherited mental retardation, affecting 16 to 25 of 100,000 males. Prevalence among females is approximately half what is reported for males. In almost every case the disorder arises from an expansion of a CGG repeat polymorphism in the 5′ untranslated region of the FMR1 gene. The expansion commonly occurs in the maternal line of transmission. The normal repeat range is defined as 54 repeats and fewer. Patients with 55 to 200 CGG repeats are regarded as premutation carriers. Premutations are unstable and prone to further expansion in the next generation, such that females with premutations are at high risk of having a child with a full mutation (>200 repeats). The high end of the normal range (45-54 repeats) is considered a gray zone. Gray zone carriers are unlikely to have children with full mutations, but subsequent generations may be at risk. The full mutation is associated with methylation of the FMR1 promoter region that abrogates gene expression and elicits the fragile X phenotype of mental retardation/developmental delay, commonly with characteristic facial and gonadal features. Premutation carriers will not exhibit the features. Females may experience primary ovarian insufficiency (POI), while men (and, less commonly, women) older than 50 may exhibit an ataxia and tremor phenotype (FXTAS) with similarities to parkinsonism
Amniotic fluid, CVS, or cultured amniocytes. Maternal blood should be submitted for Maternal Cell Contamination , as it is required for fetal testing. If submitted for confirmation, paternal blood can also be tested.
10 mL amniotic fluid, 10 mg CVS, two T25 flasks of cultured amniocytes, or two T25 flasks of cultured villi; 7 mL whole blood for parental samples
Causes for Rejection
Frozen specimen; hemolysis; quantity not sufficient for analysis; improper container
If culture is needed, an additional 7 to 12 days may be required. A maternal blood sample is required for Maternal Cell Contamination  and confirmation purposes. A paternal blood sample is also highly recommended for confirmation purposes. Parental blood samples should be submitted on a separate test request form. Please contact CMBP genetic counselors at 800-345-4363 prior to submitting prenatal samples for testing.