Myotonic dystrophy (DM) was one the first autosomal dominant disease discovered to be caused by a repeat expansion.

As it is dominant, if one parent is affected, there is a 50% chance that the future child will inherit the disease. However, the mutation may first appear in a generation, which is called a mutation “de novo”.

All this transcription of the repeat expansion accumulate and, as toxic RNAs, disrupt the normal function of up to twenty other genes, causing all the symptoms typic of Myotonic Dystrophy.

Although the two different types of myotonic dystrophy have common symptoms, they have basically two different origins. The two diseases (DM1 and DM2) are caused by distinct expansions located both in the regions of different genes.

Causes of Myotonic Dystrophy Type 1 (DM1)

The genetic anomaly that characterizes this form of Mytonic Dystrophy it’s none other than an expanded and unstable (CTG) trinucleotide repeat, situated on the region of the Dystrophia Myotonic-Protein Kinase (DMPK) gene on chromosome 19.

Once the number of triplet repeats, on the DMPK gene, exceeds the number of 37, the expanded sequence becomes more unstable and slippage become more frequent.

Myotonic Dystrophy Type 1’s severity is strictly related to the number of repeats.

Causes of Myotonic Dystrophy Type 2 (DM2)

This form of Myotonic Dystrophy is well known also as Proximal Myotonic Myopathy (PROMM), is caused by an expanded and unstable region repeat in the chromosome 3 (CCTG). The structure repeated occurring in DM2 it seems to be more complex than the triplet repeat seen in DM1.The normal repeat structure is approximately 10-20 repeats of a complex motif that consist in 104 to 176 nucleotides.

Individuals with 22-33 uninterrupted region repeats are said to carry a pre-mutation. These individuals don’t show any of the symptomps typical of the disease, however, they are at risk of having affected childrens.

The expanded region has been shown to display an even greater instability than the mutation toccurring in patients affected by DM1.

Even Myotonic Dystrophy type 2’s severity  depends on the the number of repeats as follow,