ABSTRACT
Duchenne muscular dystrophy (DMD) is a fatal, X-linked muscle wasting disease characterized by extensive cycles of muscle degeneration that arise in male patients due to mutations in the gene encoding dystrophin. The dystrophin protein, expressed from a 14 kb mRNA, serves to maintain myofiber integrity during muscle contraction by linking the actin cytoskeleton to the extracellular matrix through the dystrophin-associated protein complex (DPC). The dystrophin protein attaches to theDPC complex at its N-terminal region and anchors to actin in the myofiber cytoskeleton at its C-terminal region; the intervening central rod domain is composed of many spectrinlike repeats that act in a buffering capacity during excessive muscle force. Many of these repeat elements are actually dispensable, as the dystrophin protein in Becker muscular dystrophy (BMD) patients is truncated in this
region and is encoded for by shorter mini-dystrophin mRNAs. Many BMD patients survive far longer than those with DMD, having an almost normal life expectancy, and in a number of studies viral vectors expressing dystrophin mini-genes have been constructed and shown to be of potential therapeutic value for DMD patients. As one would expect, female carriers of DMD, who have one allele of the normal dystrophin gene, only express approximately 50% of normal dystrophin levels. However, a degree of dystrophin normalization occurs so that by later years dystrophin is expressed at more than 80% of myofibers. This has been proposed to be due to the combined effects of myofiber stabilization and muscle stem cells (satellite cells or myoblasts), a natural process which could be exploited in gene therapy of DMD if integrating vectors are used as the gene transfer vehicles. For example, in DMD patients, after myofiber degeneration and satellite stem cell activation, an integrating vector could be introduced to stably transduce rapidly dividing myoblasts to provide a permanent gene therapy for this debilitating disease. One such vector with medium insert capacity that has been used in a large number of gene transfer experiments and gene therapy protocols thus far is the retroviral vector. This vector has the potential to deliver a therapeutic mini-dystrophin gene and hence provide substantial benefits to DMD patients. In this chapter we will provide a brief introduction to the molecular biology of retroviruses and a more detailed analysis of the preclinical studies that have been carried out using this vector in mdx mice (the standard mouse model of muscular dystrophy). Finally, we will introduce some of the more modern concepts of retroviral-mediated gene therapy that may be of particular relevance in the successful treatment of DMD.
