Developmental expression and tissue distribution of Phex protein: Effect of the Hyp mutation and relationship to bone markers

Mutations in PHEX, a phosphate-regulating gene with homology to endopeptida ses on the X chromosome, are responsible for X-linked hypophosphatemia (XLH ). The murine Hyp homologue has the phenotypic features of XLH and harbors a large deletion in the 3' region of the Phex gene. We characterized the de velopmental expression and tissue distribution of Phex protein, using a mon oclonal antibody against human PHEX, examined the effect of the Hyp mutatio n on Phex expression, and compared neprilysin (NEP), osteocalcin, and parat hyroid hormone/parathyroid hormone-related protein (PTH/PTHrP) receptor gen e expression in bone of normal and Hyp mice. Phex encodes a 100- to 105-kDa glycoprotein, which is present in bones and teeth of normal mice but not H yp animals. These results were confirmed by in situ hybridization (ISH) and ribonuclease protection assay. Phex protein expression in femur and calvar ia decreases with age, suggesting a correlation between Phex expression and bone formation. Immunohistochemical studies detected Phex protein in osteo blasts, osteocytes, and odontoblasts, but not in osteoblast precursors. In contrast to Phex, the abundance of NEP messenger RNA (mRNA) and protein is not significantly altered in Hyp bone. Similarly, osteocalcin and PTH/PTHrP receptor gene expression are not compromised in bone of Hyp mice. Our resu lts are consistent with the hypothesis that loss of Phex function affects t he mineralizing activity of osteoblasts rather than their differentiation.