Bone fracture healing is regulated by a series of complex physicochemical and biochemical processes. One of these processes is bone mineralisation, which is vital for normal bone development, its biomechanical competence and fracture healing. Phosphatase, orphan 1 (PHOSPHO1), a bone-specific phosphatase, has been shown to be involved in the mineralisation of the extracellular matrix in bone. It can hydrolyse phosphoethanolamine and phosphocholine to generate inorganic phosphate, which is crucial for bone mineralisation. Phospho1−/− mice show hypomineralised bone and spontaneous fractures. All these data led to the hypothesis that PHOSPHO1 is essential for bone mineralisation and its structural integrity. However, no study to our knowledge has shown the effects of PHOSPHO1 on bone fracture healing. In this study, we examined how PHOSPHO1-deficiency might affect the healing and quality of the fractured bones in Phospho1−/− mice. We performed rodded immobilised fracture surgery on the right tibia of control wild type (WT) and Phospho1−/− mice (n=16 for each group) at eight weeks of age. Bone was left to heal for four weeks and then the mice were euthanised and their tibias were analysed using Faxitron X-ray analyses, microCT, histology and histomorphometry and three-point bending test. Our microCT and X-ray analyses revealed that the appearance of the callus and several static parameters of bone remodeling at the fracture sites were markedly different in WT and Phospho1−/− mice. We observed a significant increase of BS/BV, BS/TV and trabecular number and decrease in trabecular thickness and separation in Phospho1−/− callus in comparison to the WT callus. These observations were further confirmed by histomorphometry. The increased bone mass at the fracture sites of Phospho1−/− mice appears to be caused by increased bone formation as there is a significant increase of osteoblast number, while osteoclast numbers remained unchanged. There was a marked increase of osteoid volume over bone volume (OV/BV) in the Phospho−/− callus. Interestingly, the amount of osteoid was markedly higher at the fracture sites than that of normal trabecular bones. The three-point bending test showed that Phospho 1 −/− fractured bone had more of an elastic characteristics than the WT bone as they underwent more of a plastic deformity before the breakage point compare to the WT. Our work suggests that PHOSPHO1 plays an integral role during bone fracture repair. PHOSPHO1 can be an interesting target to improve the fracture healing process.
