Myotonic Dystrophy: DMPK Trinucleotide Repeat Analysis

Clinical Information

Myotonic dystrophy is the most common form of adult onset muscular dystrophy and has an incidence of 1/8000 individuals. This autosomal dominant disorder is characterized by myotonia, muscle wasting, frontal balding, hypogonadism, and ocular and ECG abnormalities. Genetic anticipation is commonly seen in families with myotonic dystrophy. In these families, extreme amplification can occur during mother to child transmission of the abnormal allele leading to a congenital form of the disease. Congenital myotonic dystrophy can be associated with a very severe disease state including generalized hypotonia and intellectual disability.

Technical Information

• This test is performed by trinucleotide repeat analysis.
• Genomic DNA is analyzed for the common trinucleotide expansion mutation within the myotonic dystrophy type 1 gene (DMPK). DNA is subjected to polymerase chain reaction (PCR) using gene-specific primers for the 3′ UTR region of the DMPK gene containing the CTG repeats. The resulting products are separated by agarose gel electrophoresis (AGE) and compared to the control DNA samples with known trinucleotide repeat lengths.
• Triplet-primed PCR-amplification of the region of the DMPK gene containing the expansion mutation is also performed.

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 the fetus is at risk of inheriting a DMPK repeat expansion. Testing can be performed on cultured CVS, direct amniotic fluid (if DNA requirements are met), or cultured amniocytes. 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.