Muscular Dystrophy & Myopathy — Genetic Workup

Hereditary muscle disease covers the muscular dystrophies (DMD/BMD, LGMD, FSHD, CMD, EDMD), the myotonic dystrophies (DM1, DM2), the congenital myopathies, and treatable metabolic myopathies like Pompe disease. The first-line test depends heavily on the suspected subtype — DMD/BMD demands deletion/duplication analysis first; FSHD has its own bespoke methylation-sensitive Southern blot or molecular combing assay; DM1 and DM2 require repeat-primed PCR; LGMD goes straight to a broad NGS panel.

Duchenne and Becker Muscular Dystrophy (DMD / BMD)

Step 1: DMD Deletion/Duplication (MLPA)

• Test: DMD deletion/duplication analysis by MLPA.
• Yield: ~65–70% of DMD/BMD patients (deletions ~60–65%, duplications ~5–10%).
• Why first: largest gene in the human genome (79 exons); deletion/duplication is the commonest mechanism + sequencing alone misses these.

Step 2: DMD Sequencing

• If MLPA negative: DMD NGS sequencing of all 79 exons.
• Yield (combined MLPA + sequencing): ~95%.
• Findings: point mutations, small deletions/insertions, splice-site variants.

Reading Frame Rule (DMD vs. BMD)

• Out-of-frame deletion → DMD (severe; no functional dystrophin).
• In-frame deletion → BMD (milder; truncated but partially functional dystrophin).
• Exceptions exist; reading frame predicts but does not absolutely determine phenotype.

Carrier Testing (Females)

• X-linked recessive — female carriers may have elevated CK, cardiomyopathy, or mild proximal weakness (“manifesting carriers”).
• Carrier testing in mother + sisters once proband mutation identified.
• Cascade family testing including prenatal options if requested.

Treatment Relevance

• Exon-skipping therapies (DMD only): eteplirsen (exon 51 skip), golodirsen (exon 53), viltolarsen (exon 53), casimersen (exon 45). The patient’s specific deletion determines eligibility.
• Gene therapy: delandistrogene moxeparvovec (Elevidys) — AAV-mediated micro-dystrophin transfer.
• Glucocorticoids: prednisone or deflazacort — standard of care; vamorolone is the newest FDA-approved option.
• Cardiomyopathy surveillance: serial echo / cardiac MRI; ACE inhibitors / β-blockers.

Facioscapulohumeral Muscular Dystrophy (FSHD)

FSHD1 (~95%)

• Test: D4Z4 repeat contraction on chromosome 4q35 — measured by methylation-sensitive Southern blot or molecular combing.
• Normal: 11–100 D4Z4 repeats.
• FSHD1: 1–10 repeats + permissive 4qA haplotype (necessary for DUX4 expression).
• Caveat: NGS does NOT detect this — must order the dedicated D4Z4 assay.

FSHD2 (~5%)

• Test: SMCHD1 sequencing (chromosome 18); also requires permissive 4qA haplotype with hypomethylated D4Z4.
• Less common than FSHD1; same phenotype.

Clinical Clues

• Asymmetric facial / scapular / humeral weakness (Beevor sign, scapular winging).
• Progressive descending pattern.
• Family history (autosomal dominant).

Myotonic Dystrophy Type 1 (DM1, Steinert)

• Test: DMPK 3′ UTR CTG repeat expansion by repeat-primed PCR.
• Normal: 5–34 repeats.
• Premutation: 35–49.
• Mild DM1: 50–150.
• Classical DM1: 100–1000.
• Congenital DM1: >1000 repeats (typically maternal inheritance — anticipation).
• Anticipation: repeats expand from generation to generation, earlier and more severe disease.
• Clinical clues: grip myotonia, frontal balding, ptosis, cataracts, cardiac conduction abnormalities, insulin resistance, daytime sleepiness.

Myotonic Dystrophy Type 2 (DM2 / PROMM)

• Test: CNBP (ZNF9) intron 1 CCTG repeat expansion by repeat-primed PCR.
• Normal: <30 repeats.
• DM2: 75 – >11,000 repeats.
• No anticipation.
• Phenotype differences vs DM1: more proximal-than-distal weakness; less prominent facial features; less severe cardiac involvement; no congenital form.

Limb-Girdle Muscular Dystrophy (LGMD)

• First-line test: LGMD NGS panel covering >30 known LGMD genes.
• Major subtypes:
  • LGMD R1 (calpainopathy, CAPN3): commonest recessive.
  • LGMD R2 (dysferlinopathy, DYSF): Miyoshi pattern (distal posterior compartment).
  • LGMD R3-R6 (sarcoglycanopathies, SGCA/B/G/D): severe childhood-onset; mimic DMD.
  • LGMD D1 (DNAJB6), LGMD D2 (TNPO3), LGMD D3 (HNRNPDL): autosomal dominant.
  • LGMD R9 (FKRP): common dystroglycanopathy.
  • LGMD R12 (anoctaminopathy, ANO5).
• Yield: ~50–70% (varies by phenotype + ethnic background).
• If panel negative: trio WES (broader differential — congenital myopathy, metabolic myopathy, mitochondrial).

Emery-Dreifuss Muscular Dystrophy (EDMD)

• First-line test: LMNA sequencing (commonest cause; AD form, EDMD2).
• Reflex / panel: EMD sequencing (X-linked, EDMD1), SYNE1, SYNE2, FHL1, TMEM43.
• Clinical clues: early contractures (Achilles, elbows, posterior neck) + humeroperoneal weakness + cardiac conduction defects → permanent pacemaker required.
• Critical: ALL LMNA-related disease (including DCM and LGMD1B) needs cardiac surveillance — sudden death from arrhythmia.

Congenital Muscular Dystrophies (CMD)

• First-line test: CMD NGS panel.
• Major subtypes:
  • Merosin-deficient CMD (LAMA2): white matter abnormalities on MRI.
  • Ullrich CMD / Bethlem myopathy (COL6A1/2/3): distal hyperlaxity + proximal contractures.
  • Dystroglycanopathies (POMT1, POMT2, POMGNT1, FKTN, FKRP, LARGE): Walker-Warburg, MEB, Fukuyama, brain malformations.
  • SEPN1-related (SELENON): rigid spine syndrome.

Congenital Myopathies (Structural)

• First-line test: congenital myopathy NGS panel.
• Major subtypes:
  • Central core disease (RYR1): also predisposes to malignant hyperthermia.
  • Nemaline myopathy (NEB, ACTA1, TPM2, TPM3, TNNT1): nemaline rods on biopsy.
  • Myotubular / centronuclear myopathy (MTM1 X-linked; DNM2, BIN1 AD/AR): central nuclei on biopsy.
  • Multi-minicore disease (SELENON, RYR1).
• Treatment relevance: AAV-mediated MTM1 gene therapy (resamirigene bilparvovec) in trial; RYR1 identification avoids MH-triggering anesthesia.

Pompe Disease (GSD II)

• First-line test: GAA enzyme activity on dried blood spot (DBS).
• Confirm: GAA sequencing.
• Phenotypes:
  • Infantile-onset: severe hypotonia, hypertrophic cardiomyopathy, death < 1 year if untreated.
  • Late-onset (LOPD): childhood to adult; proximal myopathy + respiratory failure (diaphragm involvement out of proportion to limb weakness).
• Treatment — TREATABLE:
  • Alglucosidase alfa (Myozyme/Lumizyme) — enzyme replacement, IV every 2 weeks.
  • Avalglucosidase alfa (Nexviazyme) — improved muscle targeting.
  • Cipaglucosidase alfa + miglustat (Pombiliti + Opfolda) — combination ERT.
• When to suspect LOPD: any unexplained LGMD with respiratory weakness; routinely screen with DBS GAA in LGMD workup.

Metabolic Myopathies (Other)

• McArdle disease (GSD V, PYGM): second-wind phenomenon, exercise intolerance, myoglobinuria. Sequence PYGM.
• CPT II deficiency (CPT2): prolonged-exertion myoglobinuria; sequence CPT2.
• VLCAD deficiency (ACADVL): cold/fasting-induced rhabdomyolysis.
• LPIN1 deficiency: recurrent rhabdomyolysis in children.
• Recurrent rhabdo NGS panel: includes CPT2, ACADVL, LPIN1, RYR1, ANO5, others.

Disease → Test Quick Reference Table

Pitfalls and Pearls

• DMD step 1 = MLPA (catches deletions and duplications); step 2 = sequencing.
• Reading frame rule: in-frame = BMD (milder), out-of-frame = DMD (severe).
• FSHD ≠ NGS: order the dedicated D4Z4 + 4qA haplotype test. NGS will miss it.
• DM1 / DM2 need repeat-primed PCR; standard sequencing panels do NOT detect these expansions.
• LGMD NGS panel is the workhorse — yield ~50–70%; add DBS GAA in parallel to catch late-onset Pompe.
• EDMD / LMNA: cardiac conduction disease + early contractures → permanent pacemaker.
• Pompe is TREATABLE — never miss it. DBS GAA is cheap, fast, sensitive.
• Congenital myopathy with RYR1: malignant hyperthermia risk — alert anesthesia.
• Anticipation in DM1: explain to families that future generations may be affected earlier and more severely (especially maternal congenital DM1).
• Mother of DMD proband: carrier testing + cardiac surveillance (manifesting carrier risk for DCM).

References

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2. Bushby K, Finkel R, Birnkrant DJ, et al. Diagnosis and management of Duchenne muscular dystrophy. Lancet Neurol. 2010;9(1):77-93.
3. Tawil R, Kissel JT, Heatwole C, et al. Evidence-based guideline summary: Evaluation, diagnosis, and management of facioscapulohumeral muscular dystrophy. Neurology. 2015;85(4):357-364.
4. Thornton CA. Myotonic dystrophy. Neurol Clin. 2014;32(3):705-719.
5. Straub V, Murphy A, Udd B. 229th ENMC international workshop: Limb girdle muscular dystrophies — nomenclature and reformed classification. Neuromuscul Disord. 2018;28(8):702-710.
6. Kishnani PS, Steiner RD, Bali D, et al. Pompe disease diagnosis and management guideline. Genet Med. 2006;8(5):267-288.
7. Mendell JR, Sahenk Z, Lehman K, et al. Assessment of systemic delivery of rAAVrh74.MHCK7.micro-dystrophin in children with Duchenne muscular dystrophy. JAMA Neurol. 2020;77(9):1122-1131.