Fragile X Syndrome: FMR1 Trinucleotide Repeat Analysis

Clinical Information

Fragile X Syndrome is the most common form of inherited intellectual disability. Approximately 1/1250 males and 1/2500 females are affected by the condition. Some population studies have shown the carrier frequency to be as high as 1/250 individuals. The American College of Medical Genetics policy statement on Fragile X testing recommends consideration of testing for males or females with intellectual disability, developmental delay or autism, those with a family history of Fragile X syndrome or unexplained intellectual disability. Additionally, prenatal testing should be offered to known carrier females. Trinucleotide repeat analysis is the standard for Fragile X diagnosis. Patients with the above characteristics who had a previously normal cytogenetic Fragile X results should also be considered for trinucleotide repeat analysis.

Specimen Requirements

• The preferred sample type is 3-4 ml of peripheral blood collected in an EDTA (purple top) tube. Extracted DNA and saliva are also accepted for this test. Blood and saliva kits are available by request.
• Send approximately 5µg of extracted DNA at a requested concentration of 90-130 ng/µl.
• Saliva samples must be submitted in an approved saliva kit.

Transport Instructions

• The blood specimen should be kept at room temperature and delivered via overnight shipping. If shipment is delayed by one or two days, the specimen should be refrigerated and shipped at room temperature. Do not freeze the specimen. Samples collected on Friday can be safely designated for Monday delivery.
• Extracted DNA should be sent at room temperature via overnight delivery.
• Saliva is stable at room temperature and can be delivered via overnight or ground shipping.

Prenatal Testing Information

Prenatal diagnosis is available if a parent has a premutation or full allele expansion in FMR1. Prenatal diagnosis is not available if a parent has an intermediate (gray zone) allele. CVS and amniotic fluid samples are accepted; however, methylation analysis is only performed on amniotic fluid samples collected at a minimum of 16 weeks gestational age. Additional fees for cell culture and maternal cell contamination may apply. Maternal cell contamination studies are required for all prenatal molecular tests. Contact the laboratory prior to sending a prenatal specimen.

Prenatal Specimen Requirements

• Amniotic Fluid: Direct amniotic fluid will be accepted for analysis given there is sufficient volume for back-up culture to be established. A back-up culture at a reference lab OR Greenwood lab is required. Additional charges may apply if cell culture is required after receipt of the sample. If sending direct fluid for molecular analysis only, 10-20 ml of amniotic fluid is requested. Chromosome studies will require an additional 10-15 ml of fluid.
• Chorionic Villi Sample (CVS): 10-50 mg of chorionic villi is requested in a sterile tube with CVS media. Direct testing on CVS is not available; an additional 1-3 weeks may be needed for cultures to grow.
• Cultured amniocytes/CVS: 2x T25 confluent flasks
• A maternal sample is required, and accepted sample types include blood, saliva, and extracted DNA. Please refer to the specimen requirements above for more information.

Prenatal Transport Instructions

• Amniotic fluid should be kept at room temperature; do not freeze or refrigerate. Specimen should be sent by courier or overnight mail to arrive at the laboratory the next day. FedEx is preferred.
• CVS and cultured amniocytes should be kept at room temperature; do not freeze or refrigerate. Specimen should be sent by courier or overnight mail to arrive at the laboratory the next day. FedEx is preferred.
• Please refer to the transport instructions above for sending the maternal sample.