This is another type of Dystrophy. Also known as Dystropia Myotonica.
Myotonic dystrophy is caused by a repeat expansion mutation in DNA. The defect is not in the muscle itself, but in gene regulation, which disrupts how multiple proteins are made.
There are two main types, each with a distinct genetic origin:
Type 1 (DM1 / Steinert disease)
This is caused by an abnormal expansion of CTG trinucleotide repeats in the DMPK gene on chromosome 19.
Normal people have about ~5–35 CTG repeats while affected individuals have 50–3000+ CTG repeats, the longer the repeat, the earlier and more severe the disease.
Type 2 (DM2)
Caused by CCTG tetranucleotide repeat expansion in the CNBP (Cellular Nucleic Acid–Binding Protein) gene on chromosome 3.
What the mutation actually does
The problem is not the protein itself. The toxic effect comes from mutant RNA.
Step-by-step mechanism:
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The expanded repeats are transcribed into RNA
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This abnormal RNA accumulates inside the nucleus
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The RNA traps important splicing proteins, mainly MBNL (Muscleblind-like) proteins
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Loss of MBNL function causes **mis-splicing of many other genes
A DNA repeat expands → produces toxic RNA → traps splicing regulators → wrong proteins are made → muscle and multi-organ dysfunction develops.
There is a correlation between disease severity, age at onset and approximate size of triplet repeat mutations. There is progressive distal muscle weakness, with ptosis, weakness and thinning of the face and sternomastoids. Myotonia is typically present. Muscle disease is part of a syndrome comprising:
- Cataracts
- Frontal baldness
- Cognitive impairment (mild)
- Oesophageal dysfunction (and aspiration)
- Cardiomyopathy and conduction defects (sudden death can occur in type 1)
- Small pituitary fossa and hypogonadism
- Glucose intolerance
- Low serum IgG
This gradually progressive condition usually becomes evident between 20 and 50 years.