Abstract
Purpose
Up to 70% of the differences in human bone mass have been attributed to genetic background. These differences are associated with alterations in the biomechanical properties, micro-architecture and remodeling of bone as well as its susceptibility to fracture and its capacity for repair. In previous work it was shown that C57Bl6 mice carrying one copy of the parathyroid hormone related protein (PTHrP+/−) gene developed osteopenia by four months of age. The current study was designed to determine if the haploinsufficient phenotype was maintained on a C3H background.
Method
PTHrP+/+ and PTHrP+/− mice on C57Bl6 and C3H backgrounds were euthanised between 6 and 18 months of age. The femurs were harvested, fixed in 4% paraformaldehyde overnight and scanned on a Skyscan 1172 equipped with a 10kV X-ray source and a 10 megapixel camera at a resolution 5μm. The amount and quality of cortical and trabecular bone was quantified from 2D images and 3D reconstructions using CTAn, CTvol and CTVox software. The undecalcified specimens were embedded at low temperature in MMA, sectioned at 5 μm and stained with Von Kossa and Toluidine Blue to distinguish mineralized from unmineralized tissue.
Results
A novel application of CTAn was developed to automatically and consistently separate cortical from trabecular bone for high throughput, independent quantification. At all ages, PTHrP+/− mice on the C57Bl6 background had less trabecular bone, which was of poorer quality, than their wild type counterparts. In contrast, no difference was seen between PTHrP+/− and PTHrP+/+ mice on the C3H background at any age. No difference in cortical thickness was seen between PTHrP+/− and PTHrP+/+ mice on either background at any age, although the femoral cortices of the C3H mice were consistently thicker than those of the C57Bl6 mice.
Conclusion
The osteopenic phenotype of young adult PTHrP+/− mice on a C57Bl6 background is lost when the mutation is bred onto a C3H background. This suggests that some other osteogenic agent can compensate for the lack of PTHrP during bone development in the C3H mice.
