Myotonic dystrophy is a type of muscular dystrophy, the symptoms of which usually appear during adulthood, although they can show up at any age. It is an inherited condition characterized by progressive muscle wasting and loss of muscle strength.
Genetic causes of myotonic dystrophy
Myotonic dystrophy is diagnosed in two types. Type 1 (DM1) is caused by a mutation in the DMPK gene and primarily affects the muscles of the neck, face, lower legs, and hands. Type 2 (DM2) is caused by variations in the CNBP gene and is characterized by muscle weakness mainly in the shoulders, neck, hips, and elbows.
The specific function of these genes is unclear. The DMPK gene carries instructions to make dystrophia myotonica protein kinase, a protein that may have a role in cell-to-cell communication, especially in the brain, heart, and skeletal muscles. The CNBP gene encodes for CCHC-type zinc finger nucleic acid binding protein that is primarily found in the heart and skeletal muscles.
Myotonic dystrophy is inherited in an autosomal dominant pattern. Autosomal means that the faulty gene is carried in one of the autosomes or non-sex chromosomes. The DMPK and CNBP genes are both located on an autosome. The DMPK gene is located on chromosome 19, while the CNBP gene is found on chromosome 3. Dominant means that one mutated copy of the gene is sufficient to cause the disease. (We all have two copies of each gene, one inherited from each parent).
A person with an autosomal dominant disease has a 50% chance of passing the disease onto their children, irrespective of their gender.
Although myotonic dystrophy symptoms usually appear in adulthood, the congenital form of DM1 can be present at birth. For reasons not well-understood, this rare type of condition almost always occurs if the mutated DMPK gene is inherited from the mother.
DM1 also exhibits a genetic inheritance pattern called anticipation. This means that the symptoms of the condition worsen and may appear earlier as the mutation is passed on from generation to generation.
The type of mutation that causes myotonic dystrophy is called a nucleotide repeat expansion. In this type of genetic change, a set of three or four nucleotides (depending on the type of myotonic dystrophy) – the building blocks of DNA – are repeated (expanded) multiple times in a stretch of the gene and interferes with protein synthesis.
Genetic anticipation is the increase in the number of repeats every time a mutation is passed from parent to child. Successive generations accumulate increasingly larger stretches of the repeats, making the gene highly unstable and contributing to the increased severity of the symptoms.
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