Condition
Duchenne and Becker Muscular Dystrophies
Editors: Alexander Rae-Grant MD, FRCPC, FAAN ; Esther Jolanda van Zuuren MD; Stacey Valentine MD, MPH
Background Information
Description
- X-linked genetic disorders that result in deficient activity of dystrophin, a protein that helps link cytoskeleton to extracellular matrix,,
- Duchenne/Becker muscular dystrophy characterized by onset in boys of progressive limb girdle weakness leading to wheelchair dependency, cardiomyopathy, and respiratory insufficiency,,
Also Called
- DMD (Duchenne muscular dystrophy)
- Duchennne's muscular dystrophy
- BMD (Becker muscular dystrophy)
- Becker's muscular dystrophy
- pseudohypertrophic muscular dystrophy (no longer used)
Definitions
- dystrophinopathy
- spectrum of muscle disease caused by pathogenic variants of DMD gene that encodes dystrophin protein
- mild forms include asymptomatic disease with elevated serum creatine phosphokinase or muscle cramps with myoglobinuria
- severe forms are progressive and classified as Duchenne muscular dystrophy or Becker muscular dystrophy
Types
- Duchenne muscular dystrophy (DMD),
- almost always associated with complete inability to produce functional dystrophin protein
- usually presents in boys aged 2-5 years
- in absence of treatment
- wheelchair dependence before age 13 years
- death occurs by about age 20 years
- Becker muscular dystrophy (BMD),
- associated with ability to produce variable amounts of partially functional dystrophin, resulting in widely variable phenotype
- often presents in boys by age 10 years and mean age of death is in mid-40s
- wheelchair dependence (if present) manifests at age ≥ 16 years
- neck flexor muscle strength is preserved, which differentiates BMD from DMD
- intermediate muscular dystrophy - has been used to describe manifestation in which patients able to walk after age 12 years but lose ambulation by age 15 years (refers to disease severity greater than typical BMD, but less severe than typical DMD)
- Duchenne muscular dystrophy-associated dilated cardiomyopathy
- characterized by dilated cardiomyopathy with heart failure
- in males, typically presents at age 20-40 years and progresses rapidly to death in several years
- in females, presents later with slower progression over ≥ 10 years
- usually no evidence of skeletal muscle disease
- may be classified as subclinical Becker muscular dystrophy
Epidemiology
Who Is Most Affected
Incidence/Prevalence
- STUDY SUMMARYestimated worldwide prevalence range 1.7-4.2 per 100,000 for Duchenne muscular dystrophy (DMD) and 0.4-3.6 per 100,000 for Becker muscular dystrophy (BMD)SYSTEMATIC REVIEW: Neuroepidemiology 2014;43(3-4):259
- STUDY SUMMARYin United States, estimated prevalence of Duchenne/Becker muscular dystrophy 1.51-2.05 per 10,000 boys aged 5-9 yearsCOHORT STUDY: Pediatrics 2015 Mar;135(3):513
Associated Conditions
- boys with Duchenne muscular dystrophy reportedly have increased risk for developing
- X-linked disorders that may occur with contiguous gene deletions include
- retinitis pigmentosa
- chronic granulomatous disease
- McLeod red cell phenotype (McLeod neuroacanthocytosis syndrome), primarily neurologic disorder with multisystem features including choreatic movement disorder, psychiatric manifestations, cognitive decline, myopathy, and axonal neuropathy (Orphanet J Rare Dis 2011 Oct 25;6:68)
- glycerol kinase deficiency, associated with congenital adrenal hypoplasia and developmental delay with or without congenital myopathy (Am J Hum Genet 1987 Mar;40(3):212PDF)
- adrenal hypoplasia, characterized by hypogonadotropic hypogonadism due to insufficiently developed adrenal cortex (J Med Genet 2008 Sep;45(9):e1)
Etiology and Pathogenesis
Causes
- dystrophin protein activity deficiency due to mutation of X-linked DMD gene (chromosomal locus Xp21.2-p21.1),,
- DMD gene is largest known gene, with 79 exons across 2.4 million nucleotides on X chromosome,
- > 5,000 pathogenetic variants identified in patients with Duchenne or Becker muscular dystrophy
- disease-causing alleles highly variable
- deletions of ≥ 1 exon account for 60%-70% of pathogenic mutations
- exon duplication mutations account for about 5%-10% of pathogenic mutations
Pathogenesis
- in normal physiologic setting,
- dystrophin is a membrane-associated protein present in muscle cells and some neurons, and is part of a protein complex that links cytoskeleton to extracellular matrix
- functions include
- linking cytoskeleton to extracellular matrix
- supporting muscle fiber integrity
- in setting of Duchenne and Becker muscular dystrophy, dystrophin deficiency leaves muscles susceptible to membrane damage,
- Duchenne associated with pathogenic variants that lead to complete absence of dystrophin expression and loss of muscle integrity that results in
- myofiber necrosis
- muscle fibrosis
- cycles of regeneration and degeneration
- impaired muscle regeneration capacity
- fatty replacement of muscle
- significant myopathic change over time
- Becker associated with variable amounts of partially functional dystrophin protein and reduced muscle integrity that results in
- features of Duchenne in some muscles
- less severe phenotype caused by
- variation in fiber size
- mild fibrosis
- variable degeneration or necrosis
- Duchenne associated with pathogenic variants that lead to complete absence of dystrophin expression and loss of muscle integrity that results in
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