Duchenne muscular dystrophy

Abstract

Duchenne muscular dystrophy is a severe, progressive, muscle-wasting disease that leads to difficulties with movement and, eventually, to the need for assisted ventilation and premature death. The disease is caused by mutations in DMD (encoding dystrophin) that abolish the production of dystrophin in muscle. Muscles without dystrophin are more sensitive to damage, resulting in progressive loss of muscle tissue and function, in addition to cardiomyopathy. Recent studies have greatly deepened our understanding of the primary and secondary pathogenetic mechanisms. Guidelines for the multidisciplinary care for Duchenne muscular dystrophy that address obtaining a genetic diagnosis and managing the various aspects of the disease have been established. In addition, a number of therapies that aim to restore the missing dystrophin protein or address secondary pathology have received regulatory approval and many others are in clinical development.

Fig. 1 |. Schematic depiction of DMD and dystrophin protein.

a | The ~2.4 Mb full-length DMD gene contains eight promoters and 79 exons. Three upstream promoters (Dp427b, Dp427m and Dp427p) produce the ~11.4 kb full-length cDNA and the 427 kDa full-length dystrophin protein. Four internal promoters (Dp260, Dp140, Dp116 and Dp71) generate N-terminal truncated non-muscle isoforms of dystrophin. Alternative splicing at the 3’-end and alternative polyadenylation (the addition of a poly(A) tail to RNA) yield additional isoforms of dystrophin such as Dp40. The full-length protein generated from Dp427m is the primary muscle isoform. b | Full-length dystrophin can be divided into four major domains, including the N-terminal F-actin-binding domain (ABD; encoded by exons 1–8), rod (R; encoded by exons 8–64), cysteine-rich (CR; encoded by exons 64–70) and C-terminal (CT; encoded by exons 71–79) domains. The rod domain can be further divided into 24 spectrin-like repeats and four interspersed hinges. c | In patients with Duchenne muscular dystrophy (DMD), protein production is prematurely truncated and the resulting protein is not functional. This leads to the loss of connections between the cytoskeleton and the extracellular matrix. d | By contrast, in patients with Becker muscular dystrophy, a partially functional dystrophin is produced that contains the crucial domains required to connect to F-actin and the extracellular matrix.

Fig. 2 |. Healthy muscle and DMD muscle histology.

Cross-sectional staining of healthy muscle (panels a–d) and skeletal muscle from a patient with Duchenne muscular dystrophy (DMD; panels e–h). Haematoxylin and eosin (HE) staining shows centrally nucleated myofibers, inflammatory cell infiltration, variable myofiber size, and endomysium and perimysium connective tissue deposition (panels a and e). Masson trichrome (MT) staining shows increased fibrosis (blue staining) in a patient with DMD compared with healthy muscle (panels b and f). Immunofluorescence labelling of dystrophin and laminin shows a lack of dystrophin in a patient with DMD compared with healthy muscle (panels c and g) and variation in myofiber size in DMD muscle (panels d and h).

Fig. 3 |. Dystrophin and binding partners.

Dystrophin binding partners can be categorized as cytoskeletal proteins, transmembrane proteins, extracellular proteins, cytosolic signalling and scaffolding proteins, endocytic proteins, and cardiac-specific interacting proteins. ABD, actin-binding domain; CNS, central nervous system; CR, cysteine-rich domain; CT, C-terminal domain; DG, dystroglycan; NCX, sodium-calcium exchanger; nNOS, neuronal nitric oxide synthase; NOX2, NADPH oxidase 2; PMCA, plasma membrane calcium ATPase.

Fig. 4 |. Diagnostic decision tree to confirm the genetic diagnosis of dystrophinopathies.

Boys with typical symptoms of a dystrophinopathy and increased plasma creatine kinase (CK) levels should undergo genetic testing to confirm the diagnosis. Initially, multiplex ligation-dependent probe amplification (MLPA) or array comparative genome hybridization (CGH) should be used to identify causative DMD variants; however, if a causative variant is not identified (which occurs in ~25% of patients), small mutation analysis should be used. Boys without an identified causative variant after small mutation analysis should be referred for muscle biopsy and protein or mRNA analysis.

Fig. 5 |. Dystrophin-restoring approaches.

a | Micro-dystrophin gene therapy. To accommodate the limited capacity of adeno-associated viral vectors, cDNA constructs encoding only the most crucial parts of dystrophin have been generated to enable the expression of micro-dystrophins. b | Exon skipping. Antisense oligonucleotides (AONs) are used to target a specific exon during pre-mRNA splicing. The AON hides the targeted exon from the splicing machinery causing it to be skipped, thus restoring the reading frame, allowing the production of a Becker-like dystrophin. c | Genome editing. Guide RNAs are used to direct Cas9 to target regions in the DNA to delete an exon from the gene, thereby restoring the reading frame of mRNA produced from this gene, allowing the production of Becker-like dystrophin. d | Stop codon readthrough. For patients with nonsense mutations (indicated by an asterisk), compounds can suppress the premature stop codon use, facilitating instead the inclusion of an amino acid. Thus, a full-length dystrophin can be produced. ABD, actin-binding domain; CR, cysteine-rich domain; CT, C-terminal domain.