Therapy of Genetic Disorders-Novel Therapies for Duchenne Muscular Dystrophy

Abstract

Duchenne muscular dystrophy (DMD) is an inherited, progressive muscle wasting disorder caused by mutations in the dystrophin gene. An increasing variety of approaches are moving towards clinical testing that all aim to restore dystrophin production and to enhance or preserve muscle mass. Gene therapy methods are being developed to replace the defective dystrophin gene or induce dystrophin production from mutant genes. Stem cell approaches are being developed to replace lost muscle cells while also bringing in new dystrophin genes. This review summarizes recent progress in the field with an emphasis on clinical applications.

Keywords: Duchenne muscular dystrophy; adeno-associated viral vectors; antisense oligonucleotide; cxmd dog; dystrophin; gene therapy; inducible pluripotent stem cell; mdx mouse; mesoangioblast; morpholino; myoblast; pericyte; premature termination codon; satellite cell; stem cells; utrophin.

Figure

Functional dystrophin can be restored through rAAV-mediated gene replacement, exon skipping, mutation suppression (not shown), or cell therapies (not shown). For gene replacement, minimally essential regions of dystrophin are removed to fit the limited carrying capacity of rAAV vectors and generate short dystrophin constructs (microdystrophins). Despite the truncation, microdystrophins restored dystrophin protein production and ~90% of strength in dystrophic mice. Larger dystrophin constructs (minidystrophins) that incorporate additional regions necessary for recruiting important dystrophin binding partners (e.g. nNOS) result in even greater physiological improvements. Minidystrophins are delivered in pieces using multiple rAAV vectors and reassembled in muscle cells by various methods such as homologous recombination. In exon-skipping, synthetic antisense oligonucleotides (AON) are specifically designed to anneal to precursor mRNA (pre-mRNA) and alter RNA splicing to restore normal protein open reading frames, resulting in the production of a smaller, but functional dystrophin. These techniques can be used in combination with cell therapies to correct genetic mutations prior to transplantation back into the patient in ex vivo therapies.