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Myotonic and other dystrophies; congenital myopathies

  • Lesion

Many other gene defects weaken muscle: myotonic dystrophy with a grip that will not let go, FSHD of face and shoulders, congenital myopathies of floppy babies.

Updated

What it is

Myotonic dystrophy is the commonest adult muscular dystrophy: type 1 from a CTG repeat expansion in DMPK, type 2 from a CCTG expansion in CNBP. The expanded RNA traps splicing proteins, causing myotonia, distal weakness, cataracts, cardiac conduction defects, insulin resistance, frontal balding and testicular atrophy, with anticipation in type 1. Facioscapulohumeral dystrophy comes from inappropriate expression of DUX4 when the D4Z4 repeat contracts or is hypomethylated. Limb-girdle dystrophies are dozens of disorders of sarcolemmal and other proteins (sarcoglycans, dysferlin, calpain-3). Emery–Dreifuss dystrophy affects nuclear envelope proteins (emerin, lamin A/C) with early contractures and heart block. Congenital myopathies present at birth with hypotonia and have distinctive structures on biopsy.

Structure

  • Myotonic dystrophy: many internal nuclei, chains of nuclei, ring fibres, sarcoplasmic masses, type I atrophy.
  • Limb-girdle: dystrophic pattern like Duchenne; absent sarcoglycan or dysferlin on immunostains.
  • Central core disease: central zones without oxidative enzyme activity in type I fibres.
  • Nemaline myopathy: red rods on Gomori trichrome from Z-disc material.
  • Centronuclear myopathy: small fibres with central nuclei like fetal myotubes.

Step by step

  1. In DM1, the expanded CUG repeat RNA stays in the nucleus as foci.
  2. The foci sequester MBNL splicing regulators and raise CELF1 activity.
  3. Many genes revert to fetal splicing patterns: the chloride channel CLCN1 (myotonia), insulin receptor (resistance), cardiac troponin T.
  4. Longer repeats in the next generation (anticipation) cause earlier and more severe disease.
  5. In FSHD, a contracted, hypomethylated D4Z4 array lets DUX4 be expressed in muscle and become toxic.

Clinical notes

  • Handshake that will not release, ptosis and cataracts in a young adult: myotonic dystrophy; screen the ECG for conduction block.
  • Congenital myotonic dystrophy causes severe hypotonia and breathing problems in a baby born to an affected mother.
  • Patients with RYR1 myopathies must avoid triggering anaesthetics.

Test yourself

Think of the answer, then tap the question to see it.

What mutation causes myotonic dystrophy type 1?

A CTG trinucleotide repeat expansion in the 3' untranslated region of DMPK.

How does the expanded repeat cause disease in myotonic dystrophy?

The repeat RNA sequesters MBNL splicing proteins, causing fetal splicing of genes such as CLCN1.

Which gene is wrongly expressed in FSHD?

DUX4.

Which congenital myopathy carries a high risk of malignant hyperthermia?

Central core disease (RYR1).

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Myotonic dystrophy: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2025); Facioscapulohumeral muscular dystrophy: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2025); Nemaline myopathy: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2025).

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Representative model, for educational purposes only.