Duchenne/Becker Muscular Dystrophy

Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are X-linked degenerative muscle disorders caused by pathogenic variants in the DMD gene. Testing for DMD variants can be used to confirm a diagnosis of DMD/BMD in symptomatic individuals or to determine carrier status for females with a family history of DMD/BMD or dilated cardiomyopathy (DCM). Prenatal testing for familial DMD variants is also available.

Disease Overview

Symptoms

• DMD
  • Delayed childhood milestones (e.g., sitting, standing, walking, climbing) due to progressive symmetrical muscular weakness
  • Cardiomyopathy onset: approximately 14 years
    • 95% have cardiovascular involvement
  • Wheelchair dependence: typically by 12 years
  • Laboratory findings
    • No observable dystrophin expression
    • Serum CK levels: significantly increased
• BMD
  • Later-onset muscle weakness
  • Cardiomyopathy onset: approximately 15 years
  • Wheelchair dependence: 20s-30s
  • Laboratory findings
    • Dystrophin expression: 20-100%
    • Serum CK levels: increased
• DMD-associated dilated cardiomyopathy (DCM)
  • Rapidly progressive disease course in the absence of skeletal myopathy
  • Male age of onset: teens and 20s
  • Female age of onset: 30s and 40s

Incidence

• DMD: 1/3,500 male births worldwide
• BMD: 1/19,000 male births worldwide

Genetics

Gene: DMD

Inheritance: X-linked

Penetrance

• Males: 100%
• Females: varies with X-chromosome inactivation

De novo variants: approximately one-third of cases

Typical Diagnostic Testing Strategy

• Initial testing for DMD/BMD
  • Serum creatine kinase (CK) concentration
  • Muscle biopsy with dystrophin studies
• Molecular testing
  • Deletion/duplication analysis
  • Sequencing analysis

Typical Carrier Testing Strategy

• For a known familial DMD variant, targeted testing is recommended.
• If there is a family history of DMD/BMD but the causative familial variant is unknown, test an affected relative then perform targeted testing for the identified variant in at-risk relatives.
• If an affected relative cannot be tested, at-risk relatives should be tested by deletion/duplication analysis first because most DMD variants are large deletions and duplications.
  • If negative, consider DMD sequencing.

Recommended Follow-Up Testing

Cardiac evaluation for affected individuals and carriers

Test Description

Clinical Sensitivity

• DMD
  • Deletion/duplication: 55-75%
  • Sequencing: 20-35%
• BMD
  • Deletion/duplication: 75-90%
  • Sequencing: 10-20%

Results

• Positive
  • One pathogenic variant detected in DMD gene
    • Causative for DMD/BMD in males
    • Female carriers are variably affected
• Negative
  • No pathogenic variants identified
    • Risk for being affected with, or a carrier of, DMD/BMD, is reduced but not excluded.
• Inconclusive
  • Variants of uncertain clinical significance detected
  • Whether variants are benign or pathogenic is unknown

Limitations

• A negative result does not exclude a heritable form of muscular dystrophy.
• Diagnostic errors can occur due to rare sequence variations.
• Interpretation of this test result may be impacted if the individual has had an allogeneic stem cell transplantation.
• The following will not be evaluated:
  • Variants outside the coding regions and intron-exon boundaries of the targeted gene(s)
  • Regulatory region variants and deep intronic variants
  • Breakpoints of large deletions/duplications
  • Noncoding transcripts
• The following may not be detected:
  • Deletions/duplications/insertions of any size by massively parallel sequencing
  • Single exon deletions/duplications based on the breakpoints of the rearrangement
  • Some variants due to technical limitations in the presence of pseudogenes, repetitive, or homologous regions
  • Low-level somatic variants

Analytic Sensitivity

• For MLPA: greater than 99%
• For massively parallel sequencing:

• Variant Class	Analytic Sensitivity (PPA) Estimatea (%)	Analytic Sensitivity (PPA) 95% Credibility Regiona (%)
  SNVs	99.2	96.9-99.4
  Deletions 1-10 bp	93.8	84.3-98.2
  Deletions 11-44 bp	100	87.8-100
  Insertions 1-10 bp	94.8	86.8-98.5
  Insertions 11-23 bp	100	62.1-100
  DMD gene is a subset of a larger methods-based validation from which the PPA values are derived. bp, base pairs; PPA, positive percent agreement; SNVs, single nucleotide variants