Myotonic Dystrophy Type 2

Histopathology of DM2. Muscle biopsy showing mild myopathic changes and grouping of atrophic fast fibres (type 2, highlighted).

Immunohistochemical staining for type-1 (“slow”) myosin. DM2 is caused by a defect of the ZNF9 gene on chromosome 3. The specific defect is a repeat of the cytosine-cytosine-thymine-guanosine (CCTG) tetranucleotide in the ZNF9 gene. As it involves the repeat of four nucleotides, it is not a trinucleotide repeat disorder, but rather a tetranucleotide repeat disorder.

The repeat expansion for DM2 is much larger than for DM1, ranging from 75 to over 11,000 repeats. Unlike in DM1, the size of the repeated DNA expansion in DM2 does not appear to make a difference in the age of onset or disease severity. Anticipation appears to be less significant in DM2 and most current reviews only report mild anticipation as a feature of DM2.