Dystrophin is a protein located between the sarcolemma and the outermost layer of myofilaments in the muscle fiber. It is a cohesive protein, linking actin filaments to other support proteins that reside on the inside surface of each muscle fiber's plasma membrane. These support proteins on the inside surface of the sarcolemma in turn links to two other consecutive proteins for a total of three linking proteins. The final linking protein is attached to the fibrous endomysium of the entire muscle fiber. Dystrophin supports muscle fiber strength, and the absence of dystrophin reduces muscle stiffness, increases sarcolemmal deformability, and compromises the mechanical stability of costameres and their connections to nearby myofibrils. This has been shown in recent studies where biomechanical properties of the sarcolemma and its links through costameres to the contractile apparatus were measured, and helps to prevent muscle fiber injury. Movement of thin filaments creates a pulling force on the extracellular connective tissue that eventually becomes the tendon of the muscle. The dystrophin associated protein complex also helps scaffold various signalling and channel proteins, implicating the DAPC in regulation of signalling processes.
Pathology
Dystrophin deficiency has been definitively established as one of the root causes of the general class of myopathies collectively referred to as muscular dystrophy. The deletions of one or several exons of the dystrophin DMD gene cause Duchenne and Becker muscular dystrophies. The large cytosolic protein was first identified in 1987 by Louis M. Kunkel, after concurrent works by Kunkel and Robert G. Worton to characterize the mutated gene that causes Duchenne muscular dystrophy. Normal skeletal muscle tissue contains only small amounts of dystrophin, but its absence leads to the development of a severe and currently incurable constellation of symptoms most readily characterized by several aberrant intracellular signaling pathways that ultimately yield pronounced myofibernecrosis as well as progressive muscle weakness and fatigability. Most DMD patients become wheelchair-dependent early in life, and the gradual development of cardiac hypertrophy—a result of severe myocardial fibrosis—typically results in premature death in the first two or three decades of life. Variants in the DMD gene that lead to the production of too little or a defective, internally shortened but partially functional dystrophin protein, result in a display of a much milder dystrophic phenotype in affected patients, resulting in the disease known as Becker's muscular dystrophy. In some cases, the patient's phenotype is such that experts may decide differently on whether a patient should be diagnosed with DMD or BMD. The theory currently most commonly used to predict whether a variant will result in a DMD or BMD phenotype, is the reading frame rule. Though its role in airway smooth muscle is not well established, recent research indicates that dystrophin along with other subunits of dystrophin glycoprotein complex is associated with phenotype maturation.
