Overexpression of Phex in osteoblasts fails to rescue the Hyp mouse phenotype.
Inactivating mutations of Phex, a phosphate-regulating endopeptidase, cause hypophosphatemia and impaired mineralization in X-linked hypophosphatemia (XLH) and its mouse homologue, Hyp. Because Phex is predominantly expressed in bone and cultured osteoblasts from Hyp mice display an apparent intrinsic mineralization defect, it is thought that reduced expression of Phex in mature osteoblasts is the primary cause of XLH. To test this hypothesis, we studied both targeted expression of Phex to osteoblasts in vivo under the control of the mouse osteocalcin (OG2) promoter and retroviral mediated overexpression of Phex in Hyp-derived osteoblasts (TMOb-Hyp) in vitro. Targeted overexpression of Phex to osteoblasts of OG2 Phex transgenic Hyp mice normalized Phex endopeptidase activity in bone but failed to correct the hypophosphatemia, rickets, or osteomalacia. OG2 Phex transgenic Hyp mice did exhibit a small, but significant, increase in bone mineral density and dry ashed weight, suggesting a partial mineralization effect from restoration of Phex function in mature osteoblasts. Similarly, retroviral mediated overexpression of Phex in TMOb-Hyp osteoblasts restored Phex mRNA levels, protein expression, and endopeptidase activity but failed to correct their intrinsic mineralization defect. In addition, we failed to detect the Phex substrate FGF-23 in osteoblasts. Taken together, these in vivo and in vitro data indicate that expression of Phex in osteoblasts is not sufficient to rescue the Hyp phenotype and that other sites of Phex expression and/or additional factors are likely to be important in the pathogenesis of XLH.
Liu
Shiguang
S
Department of Medicine, Duke University Medical Center, Durham, North Carolina 27710, USA.
Guo
Rong
R
Tu
Qisheng
Q
Quarles
L Darryl
LD
D000469:Alkaline Phosphatase / Q000378:metabolism; D000818:Animals; D001842:Bone and Bones / Q000201:enzymology; D002113:Calcification, Physiologic; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D000089703:Fibroblast Growth Factor-23; D005822:Genetic Vectors; D017674:Hypophosphatemia / Q000235:genetics*; D051379:Mice; D018345:Mice, Knockout; D008822:Mice, Transgenic; D010006:Osteoblasts / Q000378:metabolism*; D015675:Osteocalcin / Q000235:genetics; D010018:Osteomalacia / Q000235:genetics; D053402:PHEX Phosphate Regulating Neutral Endopeptidase; D010641:Phenotype; D011401:Promoter Regions, Genetic; D011506:Proteins / Q000235:genetics* / Q000378:metabolism; D012279:Rickets / Q000235:genetics; D018411:Spodoptera; D014162:Transfection
C000717428:Fgf23 protein, mouse; D011506:Proteins; D015675:Osteocalcin; D000089703:Fibroblast Growth Factor-23; D000469:Alkaline Phosphatase; D053402:PHEX Phosphate Regulating Neutral Endopeptidase; C506232:Phex protein, mouse