Disease relevance of Phex

• To determine which of these abnormalities is responsible for impaired chondrocyte apoptosis and subsequent rachitic changes, two additional models were examined: diet-induced hypophosphatemia/hypercalcemia and hypophosphatemia secondary to mutations in the Phex gene [1].
• These mice had a normal life span and fertility and a normal growth rate except for a reduction in body weight due to a loss of bone mass that was consistent with the observation that the derangement in phosphate metabolism is due to the loss of the Phex gene and was not restored [2].
• Fibroblast growth factor-23 (FGF-23) has been implicated in the renal phosphate wasting in X-linked hypophosphatemia, tumor-induced osteomalacia, and autosomal dominant hypophosphatemic rickets [3].
• X-linked hypophosphatemia is a metabolic bone disease occurring in both humans and mice [4].
• Rickets and osteomalacia are characteristic features of the Hyp mouse model of human X-linked hypophosphatemia [5].
• Hyperparathyroidism, therefore, is an integral component in the pathophysiology of Hyp, and likely X-linked hypophosphatemic rickets and serves to prevent severe hypocalcemia in mice and perhaps in patients afflicted with the disorder [6].

High impact information on Phex

• Disruption of angiogenesis by PEX, a noncatalytic metalloproteinase fragment with integrin binding activity [7].
• PEX blocks MMP-2 activity on the chick chorioallantoic membrane where it disrupts angiogenesis and tumor growth [7].
• Importantly, a naturally occurring form of PEX can be detected in vivo in conjunction with alphavbeta3 expression in tumors and during developmental retinal neovascularization [7].
• Levels of PEX in these vascularized tissues suggest that it interacts with endothelial cell alphavbeta3 where it serves as a natural inhibitor of MMP-2 activity, thereby regulating the invasive behavior of new blood vessels [7].
• Renal brush border membrane adaptation to phosphorus deprivation in the Hyp/Y mouse [8].
• The mineralization-inhibiting protein osteopontin [9] abundant in the extracellular matrix of bone, and in tooth dentin and cementum, is a substrate for PHEX [10] [11]; PHEX essentially completely degrades inhibitory OPN [10].

Chemical compound and disease context of Phex

• In summary, the Gy mutation is associated with a recessively expressed mutation of the spermine synthase gene, leading to spermine deficiency, and a dominantly expressed mutation of the Pex gene, leading to hypophosphatemia [12].
• Patients with X-linked hypophosphatemia have hypophosphatemia due to renal phosphate wasting and low or inappropriately normal levels of 1,25-dihydroxyvitamin D [13].
• The vitamin D metabolites, 25-hydroxyvitamin D (250HD) and 1,25-dihydroxyvitamin D [1,25-(OH)2D], were measured in samples of plasma from normal and X-linked hypophosphatemic (Hyp) mice, which are a model for this common form a human vitamin D-resistant rickets [14].
• Effect of the Hyp mutation and diet-induced hyperparathyroidism on renal parathyroid hormone- and forskolin-stimulated adenosine 3',5'-monophosphate production and brush border membrane phosphate transport [15].
• Besides rickets and osteomalacia and tooth dentin mineralization defects[11], the X-linked hypophosphatemic male mouse (Hyp/Y) presents with low serum calcium (Ca) and increased urinary hydroxyproline (OH-Pro) excretion, suggesting a parathyroid hormone (PTH)-stimulated bone resorption despite reduced magnesium (Mg) bone content [16].

Biological context of Phex

• The causative mutation, named Skeletal abnormality 1 (Ska1), was shown to be a single base pair mutation in a splice donor site immediately following exon 8 of the Phex (phosphate-regulating gene with homologies to endopeptidases located on the X-chromosome) gene [17].
• This experimentally induced phenotype mirrors the human condition X-linked hypophosphatemia; directly confirms the role of Phex in phosphate homeostasis, normal skeletal development, and rickets; and illustrates the power of mutagenesis in exploring animal models of human disease [17].
• This point mutation caused skipping of exon 8 from Phex mRNA, hypophosphatemia, and features of rickets [17].
• Phex protein expression in femur and calvaria decreases with age, suggesting a correlation between Phex expression and bone formation [18].
• Although recent observations indicate that Phex protein is expressed primarily in bone and may play an important role in osteoblast function and bone mineralization [10], the pattern of the Phex protein expression in the developing skeleton and its subcellular localization in osteoblasts remain unknown [19].

Anatomical context of Phex

• In addition, we failed to detect the Phex substrate FGF-23 in osteoblasts [20].
• Moderate expression of Phex was seen in odontoblasts and slight immunoreactivity was observed in ameloblasts [19].
• Calvaria from day 18 pc mice showed Phex epitopes in osteoblasts [19].
• In postnatal mice, Phex expression was observed in osteoblasts and osteocytes [19].
• No Phex immunoreactivity was detected in lung, heart, hepatocytes, kidney, intestine, skeletal muscle, or adipose tissue of mouse embryos [19].

Associations of Phex with chemical compounds

• This disorder results from mutations in the PHEX/Phex (Phosphate-regulating gene with homologies to endopeptidases on the X chromosome) gene, which is expressed in fully differentiated osteoblasts [21].
• Phex encodes a 100- to 105-kDa glycoprotein, which is present in bones and teeth of normal mice but not Hyp animals [18].
• Recently, we demonstrated that Hyp mice have greater urinary PGE2 levels compared with C57/B6 mice and that indomethacin administration in vivo and in vitro ameliorates the phosphate transport defect in Hyp mice [3].
• We find that PGE2 production was higher in Hyp mice than in C57/B6 mice [3].
• To determine further whether altered prostaglandin metabolism plays a role in the renal phosphate transport defect in Hyp mice, we incubated renal proximal tubules with arachidonic acid [3].

Regulatory relationships of Phex

• Additional factors, associated with either osteocyte differentiation and/or extracellular matrix, are necessary for Phex deficiency to stimulate Fgf23 gene transcription in bone [22].
• These findings suggest that Phex may control mineralization and removal of hypertrophic chondrocytes and cartilage matrix in growth plate by regulating the synthesis and deposition of certain bone matrix proteins and proteases such as MMP-9 [23].
• These data suggested that two alternative promoters control the renal expression of Npt2a gene and both Npt2a variant transcripts are down regulated in Hyp mice [24].
• However, the inability of PTH and hypophosphatemia to stimulate enzyme activity in a manner analogous to that in normal and phosphate-depleted mice indicates that a generalized defect of 1 alpha-hydroxylase regulation is manifest in Hyp-mice [25].
• The present study was undertaken to evaluate the response of Hyp mice to regulators known to inhibit renal 25-hydroxyvitamin D3-1-hydroxylase (1-hydroxylase) and stimulate renal 25-hydroxyvitamin D-24-hydroxylase (24-hydroxylase) [26].

Other interactions of Phex

• We demonstrate that the underlying mutations are nested deletions which lie in the Phex-Amelx chromosomal segment conserved between man and mouse [27].
• The authors found that Hyp mice had increased expression of the MEPE and another phosphaturic factor, Fgf23 [28].
• Transfer of Mepe deficiency onto the Phex-deficient Hyp mouse background failed to correct hypophosphatemia and aberrant serum 1,25(OH)(2)D(3) levels [28].
• We show that the most likely gene order in the distal portion of the mouse X chromosome is Pgk-1-DXSmh43-Hyp-Cbx-rs1-Amg, from proximal to distal [29].
• We have established a Mus spretus/Mus musculus domesticus interspecific backcross segregating for two X-linked mutant genes, Ta and Hyp, using in vitro fertilization [29].
• The mineralization-inhibiting protein osteopontin [9] abundant in the extracellular matrix of bone, and in tooth dentin and cementum, is a substrate for PHEX [10] [11]; PHEX essentially completely degrades inhibitory OPN [10].  PHEX also degrades the ASARM peptide from MEPE [30] and OPN [31].

Analytical, diagnostic and therapeutic context of Phex

• Phex protein levels were analyzed by immunoprecipitation of 35S-methionine-labeled osteoblast lysates or by immunoblotting of calvaria membrane extracts using a polyclonal rabbit antiserum raised against a mouse Phex carboxy-terminal peptide [21].
• By Western blot analysis, we demonstrated that Phex is a membrane-bound 100-kDa glycosylated monomer [32].
• Northern blot hybridization analysis of +/Y and Hyp/Y mice osteoblast mRNAs, using a cDNA probe for mouse OPN, revealed no difference in the steady state levels of osteopontin mRNA [33].
• In the current investigation, we employed a polymerase chain reaction (PCR) strategy to amplify a PEX cDNA from a human bone cell cDNA library [34].
• Using specific radioimmunoassays and Western ligand blot analysis, we evaluated serum levels of IGFs (IGF-1 and IGF-II) and IGFBPs, respectively, in normal and Hyp mice [35].

References