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Causes of Muscular Dystrophy



All muscular dystrophies are inherited (genetic) conditions (even though some forms may form from an ex-novo mutation). Different muscular dystrophies follow various inheritance patterns:

Duchenne muscular dystrophy (DMD)

It is inherited in an X-linked recessive pattern, meaning that the mutated gene that causes the disorder is located on the X chromosome (one of the two sex chromosomes, the other being Y) and is thus considered sex-linked. In males (who have only one X chromosome) one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes) a mutation must generally be present in both copies of the gene to cause the disorder (relatively rare exceptions, manifesting carriers, do occur due to dosage compensation/X-inactivation). Males are therefore affected by X-linked recessive disorders much more often than females.

A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons. In about two thirds of DMD cases, an affected male inherits the mutation from a mother who carries one altered copy of the DMD gene. The other one third of cases probably result from new mutations in the gene. Females who carry one copy of a DMD mutation may have some signs and symptoms related to the condition (such as muscle weakness and cramping), but these are typically milder than the signs and symptoms seen in affected males.

Duchenne muscular dystrophy and Becker's muscular dystrophy are caused by mutations of the gene for the dystrophin protein and lead to an overabundance of the enzyme creatine kinase.

Myotonic Muscular Dystrophy
Autosomal dominant pattern of inheritance. Myotonic dystrophy results from the expansion of a short repeat in the DNA sequence (CTG in one gene or CCTG in another gene). In other words, the the gene defect is an abnormally long repetition of a three- or four-letter "word" in the genetic code. While the exact mechanism of action is not known, this molecular change may interfere with the production of important muscle proteins
Limb-Girdle Muscular Dystrophy
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. 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.
Congenital Muscular Dystrophy
Autosomal recessive or autosomal dominant; these diseases are sometimes inherited through both parents and sometimes inherited from one parent. They can also occur spontaneously because of a newly developed genetic mutation
Distal Muscular Dystrophy
May be autosomal dominant, (a faulty gene is inherited from one parent); or autosomal recessive (when a faulty gene is inherited from both parents).
Facioscapulohumeral Muscular Dystrophy
Lunt and Harper (1991) concluded that there is a dominantly inherited scapulohumeral or scapuloperoneal syndrome genetically distinct from FSHD that does not have facial weakness as a feature. Many cases (as much as 25%) may represent new, ex-novo mutations
Emery-Dreifuss Muscular Dystrophy
Can be X-linked recessive, primarily affecting males, who inherit the disease through their mothers. Another type is autosomal dominant, meaning it can be inherited through either parent; an autosomal recessive type occurs when a faulty gene is inherited from both parents.
Oculopharyngeal Muscular Dystrophy
May be autosomal dominant, meaning OPMD is inherited from one parent; or autosomal recessive, occurring when a faulty gene is inherited from each parent.

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