Innovative Therapeutic Approaches for Duchenne Muscular Dystrophy

Abstract

Duchenne muscular dystrophy (DMD) is the most common childhood muscular dystrophy affecting ~1:5000 live male births. Following the identification of pathogenic variations in the dystrophin gene in 1986, the underlining genotype/phenotype correlations emerged and the role of the dystrophin protein was elucidated in skeletal, smooth, and cardiac muscles, as well as in the brain. When the dystrophin protein is absent or quantitatively or qualitatively modified, the muscle cannot sustain the stress of repeated contractions. Dystrophin acts as a bridging and anchoring protein between the sarcomere and the sarcolemma, and its absence or reduction leads to severe muscle damage that eventually cannot be repaired, with its ultimate substitution by connective tissue and fat. The advances of an understanding of the molecular pathways affected in DMD have led to the development of many therapeutic strategies that tackle different aspects of disease etiopathogenesis, which have recently led to the first successful approved orphan drugs for this condition. The therapeutic advances in this field have progressed exponentially, with second-generation drugs now entering in clinical trials as gene therapy, potentially providing a further effective approach to the condition.

Keywords: Duchenne muscular dystrophy; antisense oligonucleotide chemistry; dystrophin restoration; exon-skipping; gene therapy; innovative clinical trials; stop codon reversion.

Figure 1

Dystrophin isoforms and effect of DMD mutations. (A) Schematic representation of dystrophin isoforms. The picture shows the localization of each promoter along the DMD gene (arrows) and the tissue specificity. (B) The two graphs report the frequency of deletions, duplications, and small mutations as described both in many reports (left graph; [9,10,11] and in a large, Italian DMD gene mutation study recently published by the authors (right graph, [3])). As summarized in the bottom of the panel, the DMD mutations lead to the absence of dystrophin protein, inducing the activation of several biological processes that cause the progressive muscle weakening and loss of ambulation, together with respiratory and cardiac complications.

Figure 2

Schematic representation of exon skipping in the DMD transcript. (A) In DMD patients, the deletion of exon 50 (light gray block) generates an out-of-frame transcript that contains a premature stop codon, leading to the absence of the dystrophin protein. (B) The skipping of exon 51 using antisense oligonucleotides (AONs) targeting exon 51, such as eteplirsen (Exondys 51; NCT03218995, NCT03985878; NCT03992430; NCT04179409), restores the open reading frame, resulting in the synthesis of a short but functional dystrophin protein.