With myotonic dystrophy, “myo” means muscle, “tonic” means spasm, “dys” means bad, and “troph” means nourish; so myotonic dystrophy refers to the muscle appearing poorly nourished and weak, and being contracted, without being able to relax.
Myotonic dystrophy is actually a group of disorders, all of which are caused by autosomal dominant genetic mutations.
This means that one affected copy of a gene is enough to cause disease.
Affected people are typically present in each generation, because an affected person (male or female) has a 50% chance of passing on the affected gene to a child, which causes that child to have the disease.
There are two major types of myotonic dystrophy: type 1, or DM1 for short, also known as Steinert’s disease and type 2, or DM2 for short.
In myotonic dystrophy type 1, the affected gene is on the long arm of chromosome 19 and is called DMPK.
The DMPK gene has a trinucleotide repeat, which means that a group of three DNA nucleotides is repeated multiple times in a row.
In DMPK, it’s the nucleotides cytosine, thymine, and guanine, or CTG.
These CTGs are found in the 3’ untranslated region of DMPK.
The 3’ untranslated region is at the end of the DMPK gene that’s made into mRNA but not protein, and it helps modulate gene expression.
Expressed DMPK mRNA gets translated into a protein called myotonic dystrophy protein kinase, and it helps in the communication between muscle cells, but also heart and brain cells.
In the muscle, this kinase shuts off a muscle protein called myosin phosphatase, which is involved in muscle tensing or contraction and relaxation.
In myotonic dystrophy type 2, the affected gene is located on the long arm of chromosome 3 and is called CNBP.
Instead of a trinucleotide repeat, the CNBP gene contains a tetranucleotide repeat where the nucleotides cytosine, cytosine, thymine, and guanine, or CCTG are repeated multiple times in a row.
These CCTGs are found in the first intron of CMBP, which is a part of the gene that’s made into mRNA but not protein, and helps modulate gene expression.
Expressed CNBP mRNA gets translated into a protein called Cellular nucleic acid- binding protein, which controls the function of various genes in the muscle and heart.
In both types of myotonic dystrophy, there is a repeat expansion, meaning there’s an increased number of CTG and CCTG repeats in the affected genes, respectively.