Myotonic Dystrophy: DMPK Trinucleotide Repeat Analysis

DMPK trinucleotide repeat analysis is a molecular test used to identify expanded CTG repeats in the gene associated with type 1 myotonic dystrophy.

Turnaround Time

3 weeks

CPT Code(s)

81234

Cost

$350

Genes

• Myotonic dystrophy Type 1 (DM1)

Myotonic dystrophy is the most common form of adult onset muscular dystrophy and has an incidence of 1/8000 individuals. The genetic defect in the disorder is the expansion of a (CTG) trinucleotide repeat in the DMPK gene. 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. Molecular diagnosis of myotonic dystrophy involves a combination of Southern blotting tests and direct PCR analysis to determine the (CTG) repeat number.

Molecular testing is useful to confirm the diagnosis and to identify the disease causing mutations within a family to allow for carrier testing and prenatal diagnosis.

Trinucleotide Repeat

PCR and Southern blot analysis are used in combination to determine allele repeat size. PCR will detect up to approximately 100 CTG repeats while Southern blot is used to detect the larger repeats.

The preferred sample type is 3-5 ml of peripheral blood collected in an EDTA (purple top) tube. Extracted DNA is also accepted for this test.

The 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.

Prenatal diagnosis is available if the familial mutations are known. 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.