Respiratory Pathology in a Humanized Mouse Model of Duchenne Muscular Dystrophy

Abstract

Duchenne muscular dystrophy (DMD) is the most common X‐linked disease affecting 1 in 3500 male births. DMD is characterized by mutations in the DMD gene, which encodes the protein dystrophin that provides elasticity in skeletal muscle. Mutations in the DMD gene result in a lack of dystrophin which causes muscle fibers to degenerate and leads to inflammation, fibrosis, and muscle atrophy. Boys with DMD have progressive muscle weakness within the diaphragm that leads to respiratory failure in late adolescence/early adulthood. The most common DMD mouse model – the mdx mouse – does not have the same genetic defect as those in humans which makes gene editing impossible in this model. Therefore, a novel mouse model carrying the human gene was created which has the human exon 52 deletion in the dystrophin gene (hDMD/Δ52;mdx). Since respiratory failure is the major cause of morbidity in DMD and needs to be a target for future therapies, we sought to characterize the respiratory pathology in this novel DMD model. Whole body plethysmography (WBP) was used to assess respiration in normoxic air (FiO2: 0.21; nitrogen balance; baseline) and during a challenge with hypercapnic and hypoxic conditions (FiCO2: 0.07, FiO2: 0.10; nitrogen balance). Post mortem studies included immunohistochemistry of the diaphragm and tongue and diaphragm neuromuscular junction analysis. At baseline hDMD/Δ52;mdx mice are indistinguishable from either mdx or wildtype (WT) mice at 2, 6, and 12 months for most measures. However, during the challenge hDMD/Δ52;mdx mice have reduced frequency and minute ventilation by 6 months, which continues to decline at 12 months compared to WT mice. Starting at 6 months of age diaphragm neuromuscular junctions in the hDMD/Δ52;mdx and the mdx mice show similar pathology of decreased colocalization of pre‐ and post‐synaptic endplates. Myofiber atrophy, fibrosis, and regeneration are significant in the diaphragms, but only mild in the tongues of both models. In conclusion, the hDMD/Δ52;mdx exhibits moderate respiratory pathology, and serves as a relevant animal model to study the impact of novel gene editing therapies on respiratory function.