Molecular genetic and immunological analysis of dystrophin of a young patient with X-linked muscular dystrophy

Systemic AAV Micro-dystrophin Gene Therapy for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a lethal muscle disease caused by dystrophin gene mutation. Conceptually, replacing the mutated gene with a normal one would cure the disease. However, this task has encountered significant challenges due to the enormous size of the gene and the distribution of muscle throughout the body. The former creates a hurdle for viral vector packaging and the latter begs for whole-body therapy. To address these obstacles, investigators have invented the highly abbreviated micro-dystrophin gene and developed body-wide systemic gene transfer with adeno-associated virus (AAV). Numerous microgene configurations and various AAV serotypes have been explored in animal models in many laboratories. Preclinical data suggests that intravascular AAV micro-dystrophin delivery can significantly ameliorate muscle pathology, enhance muscle force, and attenuate dystrophic cardiomyopathy in animals. Against this backdrop, several clinical trials have been initiated to test the safety and tolerability of this promising therapy in DMD patients. While these trials are not powered to reach a conclusion on clinical efficacy, findings will inform the field on the prospects of body-wide DMD therapy with a synthetic micro-dystrophin AAV vector. This review discusses the history, current status, and future directions of systemic AAV micro-dystrophin therapy.

Duchenne phenotype with in-frame deletion removing major portion of dystrophin rod: threshold effect for deletion size?

In a 9-year-old boy with Duchenne muscular dystrophy we found a large in-frame deletion, spanning exons 10 to 53 of the dystrophin gene. The deletion removed almost all of the central rod domain of dystrophin. Using carboxyterminal dystrophin antibodies the immunohistochemical reaction was normal in all muscle fibers. In immunoblot studies we found dystrophin of abnormal size (160 kDa) and normal amount (about 100%). The immunochemical features and the reading frame deduced from DNA analysis are usually associated with Becker muscular dystrophy, but the clinical characteristics were those of the severe Duchenne phenotype. All the cases of in-frame dystrophin deletions reported so far, which involved more than 36 exons, invariably resulted in a severe phenotype. Therefore, a threshold effect for dystrophin length may be reasonably suspected. Very short dystrophin molecules might induce a severe disarray of the dystrophin network.

Immunohistochemical analysis of dystrophin-associated proteins in Becker/Duchenne muscular dystrophy with huge in-frame deletions in the NH2-terminal and rod domains of dystrophin

The absence of dystrophin causes the drastic reduction of the dystrophin-associated proteins (DAPs) in the sarcolemma and the loss of the linkage between the subsarcolemmal cytoskeleton and the extracellular matrix in Duchenne muscular dystrophy (DMD) skeletal muscle. Here, we report a mild reduction of the DAPs in the unique Becker muscular dystrophy patients with huge deletions in the rod domain of dystrophin and a moderate reduction of the DAPs in patients with huge deletions that involve both the NH2-terminal and rod domains of dystrophin. The phenotype of the latter patients was more severe than that of the former. In both cases, however, the reduction in the DAPs was milder than in typical DMD patients or DMD patients lacking the COOH-terminal domains of dystrophin. Our results suggest that (a) the NH2-terminal and rod domains of dystrophin may not be essential for the interaction with the sarcolemmal glycoprotein complex; and (b) defects in the actin binding activity of dystrophin may cause disruption of the anchorage of the dystrophin-glycoprotein complex to the subsarcolemmal cytoskeleton, which may render muscle fibers susceptible to degeneration.