Gene Therapy for Duchenne Muscular Dystrophy

Abstract

Duchenne muscular dystrophy (DMD) is an X-linked, muscle wasting disease that affects 1 in 5000 males. Affected individuals become wheelchair bound by the age of twelve and eventually die in their third decade due to respiratory and cardiac complications. The disease is caused by mutations in the DMD gene that codes for dystrophin. Dystrophin is a structural protein that maintains the integrity of muscle fibres and protects them from contraction-induced damage. The absence of dystrophin compromises the stability and function of the muscle fibres, eventually leading to muscle degeneration. So far, there is no effective treatment for deteriorating muscle function in DMD patients. A promising approach for treating this life-threatening disease is gene transfer to restore dystrophin expression using a safe, non-pathogenic viral vector called adeno-associated viral (AAV) vector. Whilst microdystrophin gene transfer using AAV vectors shows extremely impressive therapeutic success so far in large animal models of DMD, translating this advanced therapy medicinal product from bench to bedside still offers scope for many optimization steps. In this paper, the authors review the current progress of AAV-microdystrophin gene therapy for DMD and other treatment strategies that may apply to a subset of DMD patients depending on the mutations they carry.

Keywords: Gene therapy; adeno-associated virus; antisense; duchenne; dystrophin; exon skipping; microdystrophin; muscular dystrophy.

Fig. 1

Full length dystrophin and dystrophin-associated protein complex (DAPC). The dystrophin protein contains an actin binding domain (which is also the N-terminal domain), a rod domain made of 24 spectrin repeats (labelled R), four hinges (labelled H1-H4), a cysteine-rich domain and a C-terminal domain. Dystrophin is attached to the DAPC via the cysteine-rich domain (which bind dystroglycan) and C terminus (which bind syntrophins and dystrobrevin), linking the internal cytoskeleton and extracellular matrix (12) (13). R16 and R17 contains nNOS binding site that is important for localisation of nNOS at the sarcolemma (19).

Fig. 2

Clinically relevant miniaturised dystrophin constructs. (Top) The promoters utilised in the therapeutic constructs are shown by yellow arrows. These promoters are shown to be muscle and heart specific and will preferentially express the transgene in those tissues. The transgene of all the constructs contain the coding sequence for protein domains that are clinically relevant and functional. The AAV serotype of choice is also based on tissue tropism, where they are muscle and heart tropic. Clinical constructs for Sarepta Therapeutics (in partnership with Roche), Pfizer, Solid Biosciences and Genethon adapted from (48) (49) (50) and (9) respectively. ABD: Actin-binding domain; H: hinge; R: rod; CRD: cysteine-rich domain; CTD: C-terminal domain. (a) The localisation of microdystrophin-1 (MD1) protein (utilised by Sarepta Therapeutics and Genethon) to the DAPC. (b) The localisation of mini-dystrophin protein utilised by Pfizer to the DAPC. There remains a possibility of the recruitment of nNOS to the sarcolemma by the mini-dystrophin via an unknown mechanism (114) (115). (c) The localisation of microdystrophin protein utilised by Solid Biosciences to the DAPC.