What genetic test is used to diagnose myotonic dystrophy and how is it performed?

Genetic Testing for Myotonic Dystrophy Type 1

The genetic test for myotonic dystrophy type 1 (DM1) involves detecting CTG trinucleotide repeat expansions in the DMPK gene using a two-tiered approach: initial PCR-based screening followed by Southern blot analysis for confirmation and precise sizing of larger expansions. 1

Testing Methodology

Primary Testing Strategy

The diagnostic approach uses a combination of molecular techniques targeting the CTG repeat in the 3' untranslated region of the DMPK gene on chromosome 19q13.32:

Initial Screening:

• Conventional PCR or Triplet Repeat-Primed PCR (TP-PCR) serves as the first-line test 1
• PCR effectively detects normal alleles (5-34 CTG repeats) and variable alleles (35-49 repeats)
• Critical limitation: PCR alone cannot reliably detect or size larger expansions (≥50 repeats) due to preferential amplification of smaller alleles 1

Confirmatory Testing:

• Southern blot analysis is essential for:
  • Detecting fully-variable alleles (≥50 repeats) that confirm DM1 diagnosis 1
  • Accurate sizing of large expansions (can range from 50 to 6000+ repeats) 1
  • Identifying somatic mosaicism, which can be detected down to ~10% 1
  • Cases where only a single PCR band is detected, particularly in the variable allele range 1

When Southern Blot is Mandatory

You must proceed to Southern blot when:

• A homozygous peak is detected within normal range BUT the patient has mild clinical symptoms (mosaicism suspected) 1
• A single peak is detected in the variable allele range (35-49 repeats) to rule out size mosaicism 1
• PCR shows only one band, as this may represent allele dropout of a large expansion 1

Clinical Interpretation by Repeat Size

The number of CTG repeats directly correlates with clinical significance 1:

• 5-34 repeats: Normal - no disease association
• 35-49 repeats: Variable allele - no symptoms but high risk of expansion in offspring
• 50-100 repeats: Fully-variable allele - mild DM1 (cataracts, mild myotonia)
• ~100-1000 repeats: Classic DM1 (progressive weakness, myotonia, cardiac arrhythmias, respiratory insufficiency)
• 1000-6000 repeats: Congenital DM1 (severe hypotonia, respiratory distress, developmental delay, high neonatal mortality)

Critical Testing Pitfalls

NGS is NOT appropriate for DM1 diagnosis:

• Short-read next-generation sequencing cannot reliably detect expanded DMPK alleles and is explicitly not recommended 1
• Long-read sequencing can theoretically detect expansions but is not routinely used in clinical practice 1
• This is a major pitfall: Standard NGS panels for fetal akinesia or neuromuscular disorders will miss DM1 2

Allele dropout is a real concern:

• Large expansions may fail to amplify by PCR, creating false appearance of homozygosity 1
• This is why Southern blot confirmation is non-negotiable when clinical suspicion exists despite normal PCR results

Quality Control Requirements

Every clinical run must include 1:

• No DNA control
• Sensitivity control (normal sample diluted with fully-variable allele sample)
• Positive controls spanning normal, variable, and fully-variable ranges

Special Clinical Scenarios

Congenital DM1 considerations:

• Severe polyhydramnios and fetal akinesia sequence should prompt specific DM1 testing 2
• Standard chromosomal microarray and NGS panels will be normal - you must specifically order CTG repeat analysis 2
• Almost exclusively transmitted maternally with massive expansions (>1000 repeats) 1

Genetic counseling implications:

• Variable alleles (35-49 repeats) require counseling about expansion risk in offspring 1
• Fully-variable alleles have 50% transmission risk with anticipation (earlier onset and increased severity in subsequent generations) 1
• Prenatal diagnosis should be offered for at-risk pregnancies 1

Reporting Standards

Results should specify the exact number of CTG repeats for both alleles using HGVS nomenclature format: NM_004409.5:c.[*224_*283CTG[copy_number]] 1

The report must clearly state whether results are consistent with normal range, variable allele, or fully-variable allele, along with appropriate genetic counseling recommendations 1