What is the diagnostic approach for Myotonic Dystrophy Type 1 (DM1)?

Diagnostic Testing for Myotonic Dystrophy Type 1 (DM1)

The definitive diagnostic test for DM1 is molecular genetic testing that detects and quantifies CTG trinucleotide repeat expansions in the DMPK gene, with ≥50 CTG repeats confirming the diagnosis. 1

Molecular Genetic Testing Approach

Primary Testing Method

• Direct DNA testing using a combination of PCR and Southern blot analysis is the gold standard for DM1 diagnosis, as this dual approach can detect all DM1 alleles across the full range of repeat expansions 1
• PCR is essential for detecting smaller expansions (<100 repeats) and provides rapid, cost-effective results for most clinically significant cases 1
• Southern blot analysis is necessary for detecting and characterizing larger expansions (>100 repeats), which often appear as diffuse smears due to somatic mosaicism 1
• Triplet repeat-primed PCR is now included as an updated methodology that can improve detection sensitivity 1

Interpretation of CTG Repeat Numbers

The diagnosis is based on specific CTG repeat thresholds that correlate with disease severity 1:

• Normal alleles: 5-34 repeats - No disease association, stable transmission 1
• Variable alleles: 35-49 repeats - No current disease but increased risk of expansion in offspring; these individuals are asymptomatic carriers 1
• Fully-variable alleles (diagnostic): ≥50 repeats - Confirms DM1 diagnosis 1
  • 50-100 repeats: Mild disease (cataracts, mild myotonia) 1
  • 100-1000 repeats: Classic disease (progressive muscle weakness, myotonia, cardiac arrhythmias, respiratory insufficiency) 1
  • 1000-6000 repeats: Congenital disease (developmental delay, respiratory distress, hypotonia, high neonatal mortality) 1

Clinical Indications for Testing

Testing should be ordered in the following scenarios 1:

• Symptomatic confirmatory testing in patients presenting with characteristic features: progressive distal muscle weakness, myotonia, cataracts, cardiac conduction defects, or multisystem involvement 1
• Equivocal presentations such as isolated cataracts in younger adults or unexplained cardiac conduction abnormalities 1
• Predictive testing of asymptomatic at-risk family members after identification of a fully-variable allele in an affected relative 1
• Prenatal diagnosis in at-risk pregnancies, particularly when fetal ultrasound shows hypotonia, positional abnormalities, or hydramnios 1

Important Testing Considerations

Technical Limitations

• Next-generation sequencing (NGS) has technological limitations for detecting DMPK CTG expansions and should not be used as a standalone method 1
• Recent research suggests short-read whole genome sequencing can identify expansion-range alleles and characterize sequence interruptions, but cannot accurately size expanded alleles 2
• Maternal cell contamination studies must be performed when testing prenatal samples (amniotic fluid or chorionic villus) to confirm fetal origin 1

Critical Pitfalls to Avoid

• Do not rely solely on PCR for diagnosis, as larger expansions (>100 repeats) may not amplify reliably and will be missed 1
• Be aware that approximately 3-8% of DM1 patients have CCG or other interruptions within their CTG expansions, which can result in milder phenotypes and later onset despite similar repeat lengths 1
• Clinical sensitivity and specificity approach 100% for expansions >100 repeats, but variable alleles (35-49 repeats) may be identified in asymptomatic individuals, requiring careful genetic counseling 1
• The American College of Medical Genetics and Genomics emphasizes that DM1 is the only gene associated with this condition - variants outside the CTG expansion have not been described 1

Quality Assurance

• Laboratories must participate in biannual DM1 proficiency testing and use validated reference materials available from the CDC Genetic Testing Reference Materials Coordination Program 1
• Testing must comply with Clinical Laboratory Improvement Amendments (CLIA) and College of American Pathologists (CAP) requirements 1

Complementary Diagnostic Studies

While molecular testing is definitive, these studies support the clinical diagnosis 3:

• Electromyography shows characteristic myopathic changes with polyphasic motor unit action potentials of short duration and low amplitude 3
• Cardiac screening with ECG and echocardiography is essential given that cardiac manifestations occur in approximately 80% of DM1 patients and are a major cause of mortality 3
• Muscle biopsy is no longer routinely necessary since direct DNA testing became available, but may help distinguish inflammatory from non-inflammatory myopathies in unclear cases 3