Small Fractions of Muscular Dystrophy Embryonic Stem Cells Yield Severe Cardiac and Skeletal Muscle Defects in Adult Mouse Chimeras

Duchenne muscular dystrophy (DMD) is characterized by the loss of the protein dystrophin, leading to muscle fragility, progressive weakening, and susceptibility to mechanical stress. Although dystrophin‐negative mdx mouse models have classically been used to study DMD, phenotypes ap‐ pear mild compared to patients. As a result, characterization of muscle pathology, especially in the heart, has proven difficult. We report that in‐ jection of mdx embryonic stem cells (ESCs) into Wild Type (WT) blastocysts produces adult mouse chimeras with severe DMD phenotypes in the heart and skeletal muscle. Inflammation, regeneration and fibrosis are observed at the whole organ level, both in dystrophin‐negative and dystrophin‐ positive portions of the chimeric tissues. Skeletal and cardac muscle func‐ tion are also decreased to mdx levels. In contrast to mdx heterozygous carriers, which show no significant phenotypes, these effects are even ob‐ served in chimeras with low levels of mdx ESC incorporation (10‐30%). Chimeric mice lack typical compensatory utrophin upregulation, and show pathological remodeling of Connexin‐43. In addition, dystrophin‐negative and dystrophin–positive isolated cardiomyocytes show augmented calcium response to mechanical stress, similar to mdx cells. These global effects highlight a novel role of mdx ESCs in triggering muscular dystrophy even when only low amounts are present.