Muscular dystrophies are disorders of skeletal muscle where the specific disease process ultimately leads to a loss of muscle mass (atrophy), a loss of strength, and associated disabilities ranging from mild to severe. Several of these disorders are, unfortunately, fatal.
Duchenne Muscular Dystrophy
When muscular dystrophy is discussed, many people routinely think of Duchenne Muscular Dystrophy (DMD), a progressive, fatal muscle disease, most often affecting young boys. This disease is caused by mutations in the gene that produces the protein which has been named dystrophin. This is one of the largest genes ever identified, and as such it is often a target of genetic mistakes such as deletions of important information. The dystrophin protein plays a critical role at the cell membrane of the muscle. Without it, muscle gets progressively damaged and ultimately lost, usually leading to the death of the affected children, often from respiratory failure, before they reach age 20.
Becker Muscular Dystrophy
Less frequently discussed is Becker Muscular Dystrophy, which is not as severe as DMD, but to the surprise of many is also caused by mutations in the gene for dystrophin. In the case of Becker Muscular Dystrophy it is not uncommon for symptoms such as weakness to not appear until late adolescence or early adulthood. The muscle of Becker patients can have diminished levels of dystrophin protein, or only a partially functional version of the protein.
The Duchenne and Becker forms are the two most common forms of muscular dystrophies.
Limb-Girdle Muscular Dystrophy
There are a number of different forms of limb-girdle muscular dystrophy (LGMD). The causes of the different forms relate to mutations in different genes including the sarcoglycans, dysferlin, TRIM32, Fukutin-related protein, calpain, and telethonin. Many of these disorders appear in childhood, but can appear later into adulthood. It is important to distinguish them from Duchenne and Becker dystorphies by testing for the presence and function of dystrophin.
Emery-Dreifuss Muscular Dystrophy (EDMD)
In EDMD, the mutations that are causative are associated with proteins found along the membrane that encircles the nucleus of the muscle cell. The types of EDMD are defined by whether the mutation is in the gene encoding the protein emerin or in those encoding the protein lamin A/C. As compared to the other dystrophies, EDMD patients can routinely show problems with heart function, such as electrical conduction disturbances and heart rhythm alterations.
Facioscapulohumeral Muscular Dystrophy (FSHD)
In FSHD, as implied by the name, weakness develops in the muscles of the face, the scapula (or shoulder blade) and the humerus (the upper arm bone), but can also involve muscles in the lower leg. This disorder does not typically shorten lifespan and it has not yet been linked to mutations in a specific gene, but rather to a series of repeated DNA sequences found on chromosome 4.
Myotonic Dystrophy
Myotonic dystrophy can be classified into two types. For type I, there are found specific trinucleotide repeat expansions in the gene encoding myotonin protein kinase, while in type 2 no specific gene mutation has been identified as causative for myotonic dystrophy. Rather, a specific repeated sequence of DNA can be found to be reproduced many times. Patients with Myotonic dystrophy can also have an association with altered glucose metabolism, cataracts of the optic lens, and an inability to relax muscles after they have contracted.
The muscular dystrophies can display a wide range of genetic alterations with a commonality of muscle weakness and loss of function, ranging from mild to very severe. To learn more about the muscular dystrophies, visit the website of the Muscular Dystrophy Association.
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