Duchenne Muscular Dystrophy (DMD) Testing | Dystrophin Gene Analysis | Cytogenomix® Malaysia
X-Linked Muscular Dystrophy

Duchenne Muscular Dystrophy Testing

Comprehensive genetic analysis for Duchenne (DMD) and Becker (BMD) muscular dystrophy. DMD gene deletion/duplication analysis, sequencing, and carrier testing.

1:5,000 MALE BIRTHS
79 EXONS
2.2Mb GENE SIZE
14-21 DAYS TAT

Test Specifications

Technical details for DMD genetic testing

Test Code DMD001 (Deletion/Duplication - MLPA), DMD002 (Sequencing), DMD003 (Comprehensive)
Methodology MLPA for deletion/duplication analysis (all 79 exons), NGS sequencing, Sanger confirmation
Gene Analyzed DMD (dystrophin) - Xp21.2, 79 exons, 2.2 Mb genomic locus
Mutations Detected Large deletions (60-65%), large duplications (5-10%), point mutations (25-30%)
Sample Type 3-5 mL whole blood (EDTA purple top) | Prenatal: Amniotic fluid/CVS
Turnaround Time 14-21 working days
Reporting Reading frame prediction (in-frame/out-of-frame), phenotype correlation (DMD vs BMD)
Genetic Counseling Pre- and post-test counseling included

Understanding Duchenne Muscular Dystrophy

Duchenne Muscular Dystrophy (DMD) is an X-linked recessive disorder caused by mutations in the DMD gene, which encodes dystrophin - a protein critical for muscle membrane stability. It is the most common inherited neuromuscular disease of childhood.

Key Facts:

  • Duchenne (DMD): Out-of-frame mutations, absent dystrophin, severe progressive weakness, loss of ambulation by age 12
  • Becker (BMD): In-frame mutations, reduced dystrophin, milder course, ambulation beyond age 16
  • Inheritance: X-linked recessive - affects primarily males
  • Gene: DMD - largest human gene (2.2 Mb, 79 exons)

The reading frame rule predicts phenotype: out-of-frame deletions cause DMD, in-frame deletions cause BMD. Exceptions occur in ~5-10% of cases.

DMD X-linked Inheritance

The Reading Frame Rule

Predicting Duchenne vs Becker phenotype from genetic findings

Out-of-Frame Deletion

Disrupts the triplet codon reading frame

Result: No dystrophin produced

Phenotype: Duchenne MD (severe)

In-Frame Deletion

Preserves the reading frame

Result: Partially functional dystrophin

Phenotype: Becker MD (milder)

Point Mutations

Nonsense, missense, splice-site

Result: Variable dystrophin production

Phenotype: Variable (usually DMD if nonsense)

Common Deletion Hotspots

Two major deletion-prone regions in the DMD gene

Hotspot 1: Exons 45-53 (proximal rod domain)

Accounts for ~50% of all deletions

45-50 45-47 45-53 46-50 48-50
Hotspot 2: Exons 2-20 (N-terminal/actin-binding domain)

Accounts for ~30% of all deletions

3-7 3-17 5-7 8-9 10-11

Clinical Progression of Duchenne MD

Natural history and disease stages

Presymptomatic
Early Ambulatory (2-5 yrs)
Late Ambulatory (6-12 yrs)
Non-ambulatory (12+ yrs)

Early Signs

  • Delayed walking (>18 months)
  • Gowers' sign
  • Calf pseudohypertrophy
  • Toe-walking

Ambulatory Stage

  • Progressive weakness
  • Frequent falls
  • Difficulty climbing stairs
  • Lordosis, contractures

Non-ambulatory

  • Wheelchair dependence
  • Scoliosis progression
  • Respiratory decline
  • Cardiomyopathy

Late Complications

  • Respiratory failure
  • Dilated cardiomyopathy
  • Sleep-disordered breathing
  • Contractures, pain

Carrier Testing in Females

Essential for family planning and risk assessment

Mothers of Affected Boys

~2/3 of mothers are carriers. If not a carrier, the mutation may have occurred de novo in the child.

Sisters of Affected Males

50% risk of being carriers if mother is carrier. Essential for reproductive planning.

Manifesting Carriers

8-10% of carriers have some muscle weakness due to skewed X-inactivation.

Cardiomyopathy Risk

Female carriers have increased risk of dilated cardiomyopathy (up to 15%).

X-Linked Recessive Inheritance

Understanding the risk in families

Carrier Mother × Normal Father

Xᴰ X
X Y
X X
Xᴰ X
Xᴰ Y
X Y

50% sons affected, 50% daughters carriers

Affected Father × Normal Mother

X Y
Xᴰ Y
Xᴰ X
X Y
Xᴰ X
X Y

100% daughters carriers, 0% sons affected

Treatment and Management

Multidisciplinary care and emerging therapies

Genetic Therapies

  • Exon skipping (eteplirsen, golodirsen - exon 51, 53)
  • Nonsense readthrough (ataluren)
  • Gene therapy (micro-dystrophin)

Cardiac Care

  • Annual cardiology review from diagnosis
  • ACE inhibitors/ARBs from early teens
  • Echocardiogram every 1-2 years

Respiratory Care

  • Annual FVC, cough peak flow
  • Nocturnal BiPAP when indicated
  • Cough assist device

Supportive Care

  • Corticosteroids (prednisone, deflazacort)
  • Physical therapy
  • Orthopedic management (scoliosis)

Frequently Asked Questions

Common questions about DMD testing

What is the difference between Duchenne and Becker MD?

Duchenne has absent dystrophin (out-of-frame mutations), leading to severe progression and loss of ambulation by age 12. Becker has reduced but present dystrophin (in-frame mutations), with milder progression and ambulation beyond age 16.

Can females be affected by DMD?

Rarely. Females can be manifesting carriers due to skewed X-inactivation or have Turner syndrome (45,X) with the mutated X. They may have mild weakness and are at increased risk for cardiomyopathy.

What is the risk of having another affected child?

If mother is a carrier: 50% of sons affected, 50% of daughters carriers. If de novo mutation in child: recurrence risk is very low (<1%), but germline mosaicism possible.

Can DMD be detected prenatally?

Yes, prenatal diagnosis is available through CVS (10-12 weeks) or amniocentesis (15-18 weeks) when the familial mutation is known. PGT for IVF cycles is also available.

What is exon skipping therapy?

Exon skipping uses antisense oligonucleotides to "skip" specific exons during splicing, restoring the reading frame and producing shortened but functional dystrophin. Only applicable for certain deletions (e.g., exon 51, 53).

Why is genetic testing important for treatment?

Genetic testing identifies which patients are eligible for specific therapies (exon skipping, readthrough). It also predicts progression and guides clinical trial enrollment.

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