Duchenne/Becker Muscular Dystrophy Deletion/Duplication with Reflex to Sequencing

Duchenne/Becker Muscular Dystrophy (DMD) Deletion/Duplication with Reflex to Sequencing 2011241
Method: Multiplex Ligation-dependent Probe Amplification/Massively Parallel Sequencing
  • Most comprehensive DMD gene test for DMD or BMD
  • Deletion/duplication analysis is performed first
    • If no large deletions or duplications are detected and/or results do not explain the clinical scenario, sequencing of the DMD gene is performed
  • Deletion/duplication and sequencing components are also orderable separately, see below
Duchenne/Becker Muscular Dystrophy (DMD) Deletion/Duplication 2011235
Method: Multiplex Ligation-dependent Probe Amplification
  • Appropriate first-tier genetic test for diagnostic testing or carrier screening for DMD or BMD; does not detect sequence variants
  • Recommended test for a known familial DMD large deletion or duplication previously identified in a family member
  • A copy of the family member’s test result documenting the known familial variant is required
  • Appropriate second-tier test for diagnostic or carrier screening for DMD or BMD after result of deletion/duplication analysis is negative
Familial Mutation, Targeted Sequencing 2001961
Method: Polymerase Chain Reaction/Sequencing
  • Recommended test for a known familial DMD sequence variant previously identified in a family member.
  • A copy of the family member's test result documenting the known familial variant is required.
Duchenne/Becker Muscular Dystrophy (DMD) Deletion/Duplication, Fetal 2011231
Method: Multiplex Ligation-dependent Probe Amplification
  • This test is performed on prenatal samples at risk for a known familial DMD deletion or duplication.
  • A copy of the family member's test result documenting the known familial variant is required.
Familial Mutation, Targeted Sequencing, Fetal 2001980
Method: Polymerase Chain Reaction/Sequencing
  • This test is performed on prenatal samples at risk for a known familial sequence variant.
  • A copy of the family member's test result documenting the known familial variant is required.

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 (eg, sitting, standing, walking, climbing) due to progressive symmetrical muscular weakness
    • Cardiomyopathy onset – ~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 – ~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 – ~1/3 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
    • Some variants due to technical limitations in the presence of pseudogenes, repetitive, or homologous regions
    • Low-level somatic variants

Analytical Sensitivity

  • For MLPA – greater than 99%
  • For massively parallel sequencing:
  • Variant Class Analytical Sensitivity (PPA) Estimatea (%)

    Analytical Sensitivity (PPA) 95% Credibility Region(%)

    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

References 

Darras B, Urion D, Ghosh P. Dystrophinopathies. In: Pagon RA, Adam MP, Ardinger HH, et al., editors. GeneReviews, University of Washington, 1993-2017. Seattle, WA [Last Update: Apr 2018; Accessed: Nov 2018]

Last Update: January 2019