Qualitative or quantitative defects of dystrophin

Qualitative or quantitative defects of dystrophin encompass a group of neuromuscular disorders caused by mutations in the DMD gene located on the X chromosome, which encodes the protein dystrophin. Dystrophin is a critical structural protein that connects the cytoskeleton of muscle fibers to the surrounding extracellular matrix, providing mechanical stability during muscle contraction. When dystrophin is absent (quantitative defect) or structurally abnormal and partially functional (qualitative defect), the result is progressive muscle degeneration and weakness. This grouping includes the well-known dystrophinopathies: Duchenne muscular dystrophy (DMD), where dystrophin is virtually absent, and Becker muscular dystrophy (BMD), where dystrophin is reduced in quantity or altered in quality but partially functional. X-linked dilated cardiomyopathy associated with DMD gene mutations also falls within this spectrum. The primary body systems affected are the skeletal muscular system and the cardiovascular system. In Duchenne muscular dystrophy, symptoms typically begin in early childhood (ages 2-5) with progressive proximal muscle weakness, difficulty walking, Gowers' sign (using hands to rise from the floor), calf pseudohypertrophy, and eventual loss of ambulation usually by the early teenage years. Cardiomyopathy and respiratory insufficiency develop over time and are major causes of morbidity and mortality. Becker muscular dystrophy presents with a milder and more variable phenotype, with later onset and slower progression. Female carriers may also manifest symptoms ranging from mild muscle weakness to cardiomyopathy. Current treatments include corticosteroids (such as deflazacort and prednisone) to slow disease progression, cardiac management with ACE inhibitors or beta-blockers, respiratory support, and physical therapy. Gene-targeted therapies have emerged, including exon-skipping antisense oligonucleotides (eteplirsen, golodirsen, viltolarsen, casimersen) approved for specific mutations, and micro-dystrophin gene therapy (delandistrogene moxeparvovec) has received accelerated approval for ambulatory DMD patients. Ataluren has been conditionally approved in some regions for patients with nonsense mutations. Multidisciplinary care remains essential for optimizing quality of life and survival.

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