Osteocyte regulation of phosphate homeostasis and bone mineralization underlies the pathophysiology of the heritable disorders of rickets and osteomalacia

Abstract

Although recent studies have established that osteocytes function as secretory cells that regulate phosphate metabolism, the biomolecular mechanism(s) underlying these effects remain incompletely defined. However, investigations focusing on the pathogenesis of X-linked hypophosphatemia (XLH), autosomal dominant hypophosphatemic rickets (ADHR), and autosomal recessive hypophosphatemic rickets (ARHR), heritable disorders characterized by abnormal renal phosphate wasting and bone mineralization, have clearly implicated FGF23 as a central factor in osteocytes underlying renal phosphate wasting, documented new molecular pathways regulating FGF23 production, and revealed complementary abnormalities in osteocytes that regulate bone mineralization. The seminal observations leading to these discoveries were the following: 1) mutations in FGF23 cause ADHR by limiting cleavage of the bioactive intact molecule, at a subtilisin-like protein convertase (SPC) site, resulting in increased circulating FGF23 levels and hypophosphatemia; 2) mutations in DMP1 cause ARHR, not only by increasing serum FGF23, albeit by enhanced production and not limited cleavage, but also by limiting production of the active DMP1 component, the C-terminal fragment, resulting in dysregulated production of DKK1 and β-catenin, which contributes to impaired bone mineralization; and 3) mutations in PHEX cause XLH both by altering FGF23 proteolysis and production and causing dysregulated production of DKK1 and β-catenin, similar to abnormalities in ADHR and ARHR, but secondary to different central pathophysiological events. These discoveries indicate that ADHR, XLH, and ARHR represent three related heritable hypophosphatemic diseases that arise from mutations in, or dysregulation of, a single common gene product, FGF23 and, in ARHR and XLH, complimentary DMP1 and PHEX directed events that contribute to abnormal bone mineralization.

Fig. 1

FGF23 protein domains and the ADHR mutations. FGF23 has a 24-amino acid signal peptide, followed by amino acids 25–179 that comprise the N-terminal FGF-like domain. The ADHR mutations at R₁₇₆ and R₁₇₉ (R176Q/W; R179Q/W) occur within the FGF23 subtilisin-like proprotein convertase (SPC) site, separating the conserved FGF-like domain from the more variable C-terminal tail (amino acids 180–251). FGF23 is glycosylated within two larger regions (denoted as ‘{GLY}’). T178 is a key glycosylated residue that may protect the mature, intact hormone from SPC degradation.

Fig. 2

Schematic structure of the human DMP1 gene and its mutations. The DMP1 gene is located on the long (q) arm of chromosome 4 at position 21 (4q21). All seven different mutations showed loss-of-function mutations in the DMP1 gene.

Fig. 3

The inter-related biomolecular abnormalities underlying the abnormal phosphate homeostasis and bone mineralization in XLH, ARHR, and ADHR. A. In XLH, the PHEX mutation results in suppression of Sgne1 (7B2) mRNA, by an as yet unknown mechanism. The resultant decreased 7B2 protein leads to decreased 7B2·PC2 enzyme activity, which directly limits FGF23 degradation, and through effects on BMP1 decreases DMP1 degradation. The restricted production of the C-terminal DMP1 peptide increases Fgf23 mRNA and decreases DKK1 mRNA. Together the limited FGF23 proteolysis and enhanced Fgf23 mRNA increase intact bioactive FGF23, which in turn results in renal phosphate wasting and hypophosphatemia. The decreased DKK1 mRNA and DKK1 protein enhance Wnt/β-catenin protein production, which, along with FGF23 mediated hypophosphatemia, impairs bone mineralization. B. In ARHR, the abnormal renal phosphate wasting and bone mineralization occur as a consequence of a DMP1 mutation that limits the production of the DMP1 protein. These phenotypic characteristics result from the same abnormal biomolecular events, as those that occur in response to the deficiency of the C-terminal DMP1 peptide in XLH. C. In ADHR, the abnormal renal phosphate wasting and bone mineralization occur as a consequence of a FGF23 mutation that limits FGF23 degradation at the subtilisin-like protein convertase enzyme site. In XLH, similar limited FGF23 proteolysis occurs due to decreased 7B2·SPC2 enzyme activity. Iron deficiency in patients with ADHR may increase Fgf23 mRNA, contributing to the increased serum FGF23 levels in this disease.