ABSTRACT

The development of therapies for Duchenne muscular dystrophy (DMD) is a daunting task. There are over 640 muscles in the human body and the majority of them are affected in this disorder with the intriguing exception of the extraocular muscles (1). DMD is caused by the absence or dysfunction of dystrophin, a subsarcolemmal protein that links the cytoskeleton of the muscle fiber to the extracellular matrix via proteins associated with the muscle membrane (2-4). Dystrophin attaches to the actin cytoskeleton at its amino terminus and to a series of proteins at its carboxy terminus, including some embedded in the cell membrane, in particular b-dystroglycan. The membrane-associated b-dystroglycan is in turn linked to merosin in the extracellular matrix via a-dystroglycan. Between the amino and carboxy termini, the majority of the dystrophin molecule is arranged as a long rod-like structure. In the absence of dystrophin, the muscle fibers are prone to damage, leading to cycles of muscle fiber necrosis and repair. The muscle damage is associated with fibrosis and loss of muscle fibers as repair fails. Consequently, there is muscle wasting and the development of joint contractures that lead to the loss of independent ambulation between the ages of 7 and 13, and subsequently to premature death from respiratory or cardiac

SECTION IV: EXPERIMENTAL THERAPEUTICS

failure. Additionally, not only are the skeletal muscles affected by the lack of dystrophin, but the cardiac and smooth muscles are also involved, and in some patients there is clear evidence of developmental abnormalities in the brain that lead to nonprogressive cognitive defects (5). A milder allelic variant of DMD is Becker muscular dystrophy (BMD). BMD has a very variable presentation, ranging from a DMD-like clinical progression to a much milder condition with significant muscle weakness developing much later in life. Although both conditions are due to mutations in the same gene, the difference in most cases relates to the effect of the mutation on the reading frame of the mRNA. In general, DMD results from mutations that disrupt the reading frame, leading to a failure to generate dystrophin protein. In contrast, BMD mutations generally retain the reading frame leading to the production of an in-frame but internally truncated dystrophin (6). Patients with BMD have been useful in understanding critical regions of the protein structure of dystrophin, and several cases have shown that large regions of the central rod domain part of the protein can be deleted while still retaining significant function of the molecule and thereby causing a mild clinical condition (7,8). Such observations have been important in the development of several of the gene therapy strategies.