Muscular dystrophy is a group of diseases
that cause progressive weakness and loss of muscle mass. In muscular
dystrophy, abnormal genes (mutations) interfere with the production of
proteins including dystrophin needed to form healthy muscle.
There are many different kinds of muscular dystrophy. Symptoms of the most common variety begin in childhood, primarily in boys. Other types don't surface until adulthood.
Some people who have muscular dystrophy will eventually lose the ability to walk. Some may have trouble breathing or swallowing.
There is no cure for muscular dystrophy at the present time. There are medications and therapy can help manage symptoms and slow the course of the disease.
Dystrophin is part of a complex structure involving several other protein components. The "dystrophin-glycoprotein complex" helps anchor the structural skeleton (cytoskeleton) within the muscle cells, through the outer membrane (sarcolemma) of each cell, to the tissue framework (extracellular matrix) that surrounds each cell. Due to defects in this assembly, contraction of the muscle leads to disruption of the outer membrane of the muscle cells and eventual weakening and wasting of the muscle.
muscular dystrophy (DMD) is the most common childhood form of muscular
dystrophy; it generally affects only boys (with extremely rare
exceptions), becoming clinically evident when a child begins walking. By
age 10, the child may need braces for walking and by age 12, most patients
are unable to walk. Life span ranges from 15 to 45 max. But there are a
few exceptions. In the early 1990s, researchers identified the gene for
the protein dystrophin which, when absent, causes DMD. The amount of
dystrophin correlates with the severity of the disease (i.e., the less
dystrophin present, the more severe the phenotype). Since the gene is on
the X chromosome, this disorder affects primarily males, and females who
are carriers have milder symptoms. Sporadic mutations in this gene occur
frequently, accounting for a third of cases. The remaining two-thirds of
cases are inherited in a recessive pattern.
Becker muscular dystrophy (BMD) is a less severe variant of Duchenne muscular dystrophy and is caused by the production of a truncated, but partially functional form of dystrophin. Survival is usually into old age. Affects only boys (with extremely rare exceptions)
Congenital muscular dystrophy includes several disorders with a range of symptoms. Muscle degeneration may be mild or severe. Problems may be restricted to skeletal muscle, or muscle degeneration may be paired with effects on the brain and other organ systems. A number of the forms of the congenital muscular dystrophies are caused by defects in proteins that are thought to have some relationship to the dystrophin-glycoprotein complex and to the connections between muscle cells and their surrounding cellular structure. Some forms of congenital muscular dystrophy show severe brain malformations, such as lissencephaly and hydrocephalus.
Distal muscular dystrophies' age at onset: 20 to 60 years; symptoms include weakness and wasting of muscles of the hands, forearms, and lower legs; progress is slow and not life-threatening. Miyoshi myopathy, one of the distal muscular dystrophies, causes initial weakness in the calf muscles, and is caused by defects in the same gene responsible for one form of LGMD (Limb Girdle Muscular Dystrophy).
Emery-Dreifuss Muscular Dystrophy patients normally present in childhood and the early teenage years with contractures. Clinical signs include muscle weakness and wasting, starting in the distal limb muscles and progressing to involve the limb-girdle muscles. Most patients also suffer from cardiac conduction defects and arrhythmias which, if left untreated, increase the risk of stroke and sudden death. There are three subtypes of Emery-Dreifuss Muscular Dystrophy, distinguishable by their pattern of inheritance: X-Linked, autosomal dominant and autosomal recessive. The X-linked form is the most common. Each type varies in prevalence and symptoms. The disease is caused by mutations in the LMNA gene, or more commonly, the EMD gene. Both genes encode for protein componenets of the nuclear envelope. However, how these mutations cause the pathogenesis is not well understood.
Facioscapulohumeral muscular dystrophy (FSHD) initially affects the muscles of the face, shoulders, and upper arms with progressive weakness. Symptoms usually develop in the teenage years. Some affected individuals become severely disabled. The pattern of inheritance is autosomal dominant, but there are a significant number of spontaneous mutations. Seminal research published in August 2010 documents that two defects are needed for FSHD, which for the first time provides a unifying theory for the underlying genetics of FSHD. The first is the deletion of D4Z4 repeats and the second is a "toxic gain of function" of the DUX4 gene. Facioscapulohumeral muscular dystrophy (FSHD) occurs both in males and females.
Limb-girdle muscular dystrophy is also called LGMD. Affects both boys and girls. LGMDs all show a similar distribution of muscle weakness, affecting both upper arms and legs. Many forms of LGMD have been identified, showing different patterns of inheritance (autosomal recessive vs. autosomal dominant). In an autosomal recessive pattern of inheritance, an individual receives two copies of the defective gene, one from each parent. The recessive LGMDs are more frequent than the dominant forms, and usually have childhood or teenage onset. The dominant LGMDs usually show adult onset. Some of the recessive forms have been associated with defects in proteins that make up the dystrophin-glycoprotein complex. Though a person normally leads a normal life with some assistance, in some extreme cases, death from LGMD occurs due to cardiopulmonary complications.
Myotonic muscular dystrophy is an autosomal dominant condition that presents with myotonia (delayed relaxation of muscles) as well as muscle wasting and weakness. Myotonic dystrophy varies in severity and manifestations and affects many body systems in addition to skeletal muscles, including the heart, endocrine organs, eyes, and gastrointestinal tract.
Myotonic muscular dystrophy type 1 (DM1), also known as Steinert disease, is the most common adult form of muscular dystrophy. It results from the expansion of a short (CTG) repeat in the DNA sequence of the DMPK (myotonic dystrophy protein kinase) gene. Myotonic muscular dystrophy type 2 (DM2) is much rarer and is a result of the expansion of the CCTG repeat in the ZNF9 (zinc finger protein 9) gene. While the exact mechanisms of action are not known, these molecular changes may interfere with the production of important muscle proteins.
Oculopharyngeal MD's age at onset: 40 to 70 years; symptoms affect muscles of eyelids, face, and throat followed by pelvic and shoulder muscle weakness, has been attributed to a short repeat expansion in the genome which regulates the translation of some genes into functional proteins.
* Information from various sources on the internet. Any suggested corrections are welcome.