Duchenne muscular dystrophy (DMD) is an X-linked disease caused by mutations in the gene encoding dystrophin, a crucial protein for skeletal muscle integrity and function. Bone fragility is an additional complication resulting from the severe muscle pathology. The ability of microdystrophin over-expression to rescue the dystrophic muscle pathology in animal models has led to clinical trials assessing the safety and efficacy of microdystrophin gene replacement therapy for DMD. However, whether this approach confers protection against bone fragility is unknown. We hypothesised that transgenic over-expression of microdystrophin would rescue the bone phenotype in dystrophin-deficient mdx mice, a model of DMD.
Transgenic mdx (mdxTg) mice were generated using the ACTA1 promoter to target over-expression of the DR4-23/DC-microdystrophin variant to skeletal muscles in mdx mice. Male control C57BL/10 (BL10), mdx and mdxTg mice were studied. Contractile function of the tibialis anterior muscle was determined in 8-week-old mice by an in situ muscle function apparatus. Muscle histology and bone micro-computed tomography were conducted in femora and L6 vertebrae from mice euthanised at different stages of skeletal maturity (9, 12, or 26-weeks).
As expected, transgenic over-expression of microdystrophin resulted in normal weights, histology and improved function in muscles of mdx mice. Microdystrophin also restored the significantly lower vertebral and femoral trabecular bone volume, and number, and vertebral trabecular thickness of mdx mice to normal levels at all time points assessed, indicating a positive effect on trabecular bone mass. Additionally, mdx mice at all three time points had greater cross-sectional area in the proximal femur; this was also corrected in mdxTg mice.
In conclusion, transgenic expression of microdystrophin in skeletal muscles rescued both the low trabecular bone mass and widened proximal metaphyses associated with the dystrophic phenotype in mdx mice. This suggests a potential benefit on the skeleton of microdystrophin gene therapy in DMD.