Myogenin is a member of the basic helix-loop-helix (bHLH) gene family and converts multipotential mesodermal cells to myoblasts. The four members of the myoD family show unique spatio-temporal expression patterns and therefore may have different functions during myogenesis. Here we inactivate the myogenin gene in order to understand its role in myogenesis. Homozygous mutations are lethal perinatally owing to the resulting major defects in skeletal muscle. The extent of disorganization of muscle tissue differs in three regions. In the latero-ventral body wall, most cells, including myogenic cells, disappear and there is rapid accretion of fluid. In the limbs, cells of the myogenic lineage exist, but they are severely disrupted, and some of them are mono-nucleate with properties of myoblasts. In contrast, there are many axial, intercostal and back muscle fibres to be seen, although fibres are mainly disorganized and Z-lines are not present in most myofibrils. These findings are evidence that myogenin is crucial for muscle development in utero and demonstrate that other members of the myogenic gene family cannot compensate for the defect.

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Registered at the UMIN (University Hospital Medical Information Network) Clinical Trials Registry with study number UMIN000008343.

The establishment of human induced pluripotent stem cells (hiPSCs) has enabled the production of in vitro, patient-specific cell models of human disease. In vitro recreation of disease pathology from patient-derived hiPSCs depends on efficient differentiation protocols producing relevant adult cell types. However, myogenic differentiation of hiPSCs has faced obstacles, namely, low efficiency and/or poor reproducibility. Here, we report the rapid, efficient, and reproducible differentiation of hiPSCs into mature myocytes. We demonstrated that inducible expression of myogenic differentiation1 (MYOD1) in immature hiPSCs for at least 5 days drives cells along the myogenic lineage, with efficiencies reaching 70–90%. Myogenic differentiation driven by MYOD1 occurred even in immature, almost completely undifferentiated hiPSCs, without mesodermal transition. Myocytes induced in this manner reach maturity within 2 weeks of differentiation as assessed by marker gene expression and functional properties, including in vitro and in vivo cell fusion and twitching in response to electrical stimulation. Miyoshi Myopathy (MM) is a congenital distal myopathy caused by defective muscle membrane repair due to mutations in DYSFERLIN. Using our induced differentiation technique, we successfully recreated the pathological condition of MM in vitro, demonstrating defective membrane repair in hiPSC-derived myotubes from an MM patient and phenotypic rescue by expression of full-length DYSFERLIN (DYSF). These findings not only facilitate the pathological investigation of MM, but could potentially be applied in modeling of other human muscular diseases by using patient-derived hiPSCs.

Using the gene targeting technique, we have generated a new mouse model of congenital muscular dystrophy (CMD), a null mutant for the laminin oc2 chain. These homozygous mice, designated dy 3K ldy 3K , are characterized by growth retardation and severe muscular dystrophic symptoms and die by 5 weeks of age. Light microscopy revealed that muscle fiber degeneration in these mice begins no later than postnatal day 9. In degenerating muscles, considerable amounts of TUNEL positive nuclei were detected as well as DNA laddering, suggesting increased apoptotic cell death was involved in the process of muscle fiber degeneration.