Bone Abstracts

Short stature and osteoporosis are common in DMD. Disease progression can be slowed by glucocorticoids but these are associated with further growth retardation and skeletal fragility. The defect in growth and skeletal development in children with DMD is probably multifactorial and not solely dependent on glucocorticoid exposure. The muscular dystrophy x-linked (mdx) mouse is the most commonly used animal model of DMD. However, its growth phenotype has not been studied in detail and the phenotype is relatively mild. Few medications that have shown therapeutic benefit in the mdx have also shown efficacy in DMD clinical trials. The utrophin heterozygous mdx mice might represent a more appropriate model but their growth and bone phenotype have not been investigated. We tested the hypothesis that: Mouse models of DMD (mdx and mdx:utr) have an intrinsic abnormality of linear growth and skeletal development. A cross-sectional study of 49 male mice sacrificed at 3, 5 and 7 weeks was performed. Mdx and mdx:utr mice were obtained from the Jackson laboratory, alongside C57BL/10 controls (WT). Animal growth was assessed twice weekly using digital weighing scales and ruler. Forelimb grip strength testing was performed according to the TREAT-NMD SOP. Creatine Kinase was measured using an Abnova assay kit, on blood taken at sacrifice. Histopathology was assessed using H+E sections of tibialis anterior muscle. Left tibiae were scanned using SkyScan microtomography to assess cortical and trabecular bone structure. 3-point bending determined biomechanical properties.

Muscle: WT mice had the greatest normalised grip strength at all ages. Mdx:utr had higher mean grip strength at 7 weeks than mdx mice. CK assay results indicated significantly higher serum values from mdx (P<0.02) and mdx:utr (P<0.002) mice, compared to WT. Muscle histology was consistent with these observations.

Growth: There was no significant difference in bodyweight gain between groups at any age and no difference in tail length by 7 weeks. Gain in body length was 0.3 mm less/day when comparing the mdx and mdx:utr to WT mice culled at 7 weeks, but Micro-CT of tibial length revealed no genotype difference.

Bone: There were no significant differences in trabecular bone parameters between groups at any age, except for structural model index (greater in 3-week WT mice, P<0.04). Cortical bone parameters and bone mechanical properties were similar at all ages. There are very limited strategies available to treat short stature and osteoporosis in DMD and the impaired osteoblast function described in DMD suggests that an anabolic treatment would be optimal. We have demonstrated that young mdx and mdx:utr mice exhibit muscle weakness, but do not show a bone or growth phenotype and therefore have clear limitations. Finding a more suitable pre-clinical mouse model is therefore essential.

Disclosure: The authors declared no competing interests.