Duchenne muscular dystrophy: recent insights in brain related comorbidities

Abstract

Duchenne muscular dystrophy (DMD), the most common childhood muscular dystrophy, arises from DMD gene mutations, affecting the production of muscle dystrophin protein. Brain dystrophin-gene products are also transcribed via internal promoters. Their deficiency contributes to comorbidities, including intellectual disability ( ~ 22% of patients), autism ( ~ 6%) and attention deficit disorders ( ~ 18%), representing a major unmet need for patients and families. Thus, improvement of their diagnosis and treatment is needed. Dystrophic mouse models exhibit similar phenotypes, where genetic therapies restoring brain dystrophins improve their behaviour. This suggests that future genetic therapies could address both muscle and brain dysfunction in DMD patients.

Fig. 1. DMD gene organization and associated protein products.

The full-length dystrophins (Dp427m,c,p) have a modular structure containing the N-terminus (NT) with F-actin binding sites, an extensive central rod domain that consists of β-spectrin-like repeats (oval shape symbols) with binding sites for F-actin, sarcolemma and multiple cellular proteins, and proline-rich hinge regions (H1-H4) predicted to form triple-helical coiled coils, followed by cysteine-rich (CR) and C-terminal (CT) domains. The shorter dystrophin isoforms (Dp260, Dp140, Dp116, Dp71) exhibit at least the cysteine-rich (CR) and C-terminal (CT) domains, except Dp40 that share the Dp71 first exon but misses the CT domain due to an alternative polyadenylation that generates a stop codon. The CR region contains a WW domain, two EF-hands, a β−dystroglycan binding site, and one ZZ domain. The C-terminal region contains two syntrophin binding sites and a coiled-coil domain interacting with dystrobrevins. The light blue rectangular boxes indicate the position of the promoter region of each isoform. The main tissues expressing the distinct dystrophin isoforms are indicated on the right. The figure was Created in BioRender.

Fig. 2. Schematic Representations of Dystrophin Interactions in the Nervous System.

A Dp427 and Postsynaptic Complexes in Central Inhibitory Synapses. This diagram illustrates the Dp427-associated complex, including α and β dystroglycans, and its role in postsynaptic scaffolding (the black U-shape form represents dystrobrevin). The interaction with key proteins such as Neuroligin-2 (NLGN2), neurexin, S-SCAM, and IQsec3, which are crucial for GABAAR clustering, is depicted. The GABAAR subunits are colour-coded to identify α subunits in light blue, β subunits in purple, and γ2 subunits in red. B Dp140 complexes in Synapses. This illustration shows the Dp140 complex associated with Ca_v2.1 and glutamate axis. C Dp71 and its Association with AQP4 and Kir4.1 Channels in Astrocyte Endfeet. This panel demonstrates the interaction of Dp71 with AQP4 and Kir4.1 channels within the perivascular astrocyte endfeet, indicating its potential role in neurovascular coupling. α-DG α-dystroglycan, β-DG β-dystroglycan, nNOS neuronal nitric oxide synthase, NLGN2 neuroligin 2, IQsec3 gephyrin-associated IQ motif and SEC7 domain-containing protein 3 (also called SynArfGEF), S-SCAM synaptic scaffolding molecule (also called MAGI-2 for membrane-associated guanylate kinase inverted 2); Ca_V2.1 calcium channels Kir: potassium channels, AQP-4 aquaporin-4, α-Dbv-1 α-dystrobrevin-1, DG dystroglycan, Syn syntrophin. Created in BioRender.

Fig. 3. Domains and areas of brain function affected in DMD.

Systematic overview of brain related comorbidities in DMD (Big ten of Duchenne) representing four domains and ten areas of (dys)functioning: A neurocognitive (executive functions, working memory and intellectual functioning); B Academics (reading and arithmetic); C emotional: (anxiety and depression; D neuropsychiatric (autism, Attention-Deficit Hyperactivity Disorder: ADHD, Obsessive Compulsive Disorder: OCD).

Fig. 4. Behavioural phenotyping of DMD mouse models.

A Expression pattern of brain dystrophins in dystrophin-deficient mouse lines. Mutation position and differential expression of the three brain dystrophins are shown for mouse lines carrying spontaneous mutations (mdx), chemically-induced mutations (mdx4cv, mdx3cv), transgenic deletion of a specific exon (mdx52) or whole dmd gene (dmd-null), and transgenic insertion (Dp71-null). E: exon; I: intron; del-: deletion; ( + ) protein expressed; (-) protein absent. B–E Illustrative images of the main behavioural tests that revealed key phenotypes in DMD mouse models. The tests tackled distinct brain-related functions including: B motor functions using the inverted grid (left) and rotarod (right) tests; C emtional and stress reactivity following scruff restraint (left drawing) or using elevated plus maze, light-dark choice and open-field anxiety tests (three last drawings, respectively); D social behaviour during social approach test (left) and socially-induced ultrasonic vocalisations (right); E cognitive functions using, respectively from top to bottom drawings, an object recognition test, fear conditioning, spatial working memory tasks in radial or T mazes, and spatial learning tasks in water maze. Created in BioRender.

Fig. 5. Impact of genetic therapies on mouse neurobehavioural phenotypes.

A Possible exon skipping strategies to restore the open reading frame in DMD mouse model carrying a deletion of exon 52. Exon 52 deleted dmd mRNA is out of frame and no functional Dp427 and Dp140 are thus produced. Skipping of exon 51 can restore Dp427 expression but not Dp140 expression because exon 51 contains Dp140 start codon. In contrast, exon 53 skipping offers the opportunity to restore both Dp427 and Dp140. B The brain delivery of genetic tools allowing dystrophin isoforms restoration improves key neurobehavioural phenotypes in DMD mouse models including emotional and stress reactivity (e.g. using fear response, elevated plus maze, light-dark choice tests), cognitive functions using a fear conditioning test and social behaviour during social approach test. This figure was created using Biorender.com, panel A and B (Created in BioRender).