Disease relevance of PHEX

• Clinical and laboratory findings in this disorder are similar to those in oncogenic osteomalacia, in which tumors abundantly express FGF-23 messenger RNA, and to those in X-linked hypophosphatemia, which is caused by inactivating mutations in a phosphate-regulating endopeptidase called PHEX [1].
• Patient 2 had autonomous hyperparathyroidism associated with chronic renal insufficiency, and patient 3, with XLH, developed autonomous hyperparathyroidism after 8 years of therapy with Pi and calcitriol [2].
• INTRODUCTION: Inactivating mutations of PHEX cause renal phosphate wasting in X-linked hypophosphatemic rickets (XLH) because of the accumulation of a phosphaturic hormone called phosphatonin [3].
• PHEX gene and hypophosphatemia [4] [5].
• A case of neuroendocrine oncogenic osteomalacia associated with a PHEX and fibroblast growth factor-23 expressing sinusidal malignant schwannoma [6].

Psychiatry related information on PHEX

• As hypothesized, the HYP group reported more bipolar disorders and major depressive episodes than the control group [7].
• The HYP group also exceeded the control group on the severity of psychotic-like experiences, symptoms of borderline personality disorder, and rates of substance use disorders [7].
• St. John's Wort for depressive disorders: results of an outpatient study with the Hypericum preparation HYP 811 [8].
• In XLH, it is likely that the poor developmental outcome of spasticity, contractures and severe mental retardation results from a disturbance of neuronal connectivity, fasciculation, and synapse formation rather than aqueductal stenosis, increased intracranial pressure, or abnormal neuroblast migration [9].
• HYP group members with elevated scores on the Impulsive-Nonconformity Scale (Chapman et al., 1984) experienced greater rates of bipolar mood disorders, poorer overall adjustment, and higher rates of arrest than the remaining HYP or control participants [7].

High impact information on PHEX

• These studies have revealed important new insights regarding the mechanism of protein translocation across the peroxisomal membrane, the conservation of PEX genes through evolution, the role of peroxins in fatal human peroxisomal disorders, and the biogenesis of the organelle [10].
• Human peroxisome biogenesis disorders (PBDs) are a group of genetically heterogeneous autosomal-recessive disease caused by mutations in PEX genes that encode peroxins, proteins required for peroxisome biogenesis [11].
• The difference in bone disease between X-linked hypophosphatemia (severe) and hypophosphatemic bone disease (mild) at comparable low serum levels of phosphate implies that 1,25-(OH)2D and phosphate may have independent roles in the pathogenesis of defective bone mineralization [12].
• Today, thanks to genetically tractable model organisms and Chinese hamster ovary cells, 23 PEX genes have been cloned that encode the machinery ("peroxins") required to assemble the organelle [13].
• Orthophosphate transport in the erythrocyte of normal subjects and of patients with X-linked hypophosphatemia [14].
• The mineralization-inhibiting protein osteopontin [15] abundant in the extracellular matrix of bone, and in tooth dentin and cementum, is a substrate for PHEX  [16] [5]; PHEX essentially completely degrades inhibitory OPN [16].

Chemical compound and disease context of PHEX

• Inactivating mutations and/or deletions of PHEX ( Phosphate-regulating gene with Homologies to Endopeptidase on the X chromosome) are responsible for X-linked hypophosphatemic rickets in humans [17].
• INTRODUCTION: Matrix extracellular phosphoglycoprotein (MEPE) was recently identified as a candidate for the phosphaturic hormone phosphatonin, which has been implicated in disturbed phosphate metabolism, rickets, and osteomalacia associated with X-linked hypophosphatemic rickets (XLH) and oncogenic hypophosphatemic osteomalacia (OHO) [18].
• Patients with X-linked hypophosphatemic rickets (XLH) have normal or marginally low serum 1,25-dihydroxyvitamin D [1,25-(OH)2D] levels despite manifesting hypophosphatemia and phosphate depletion, which increase 1,25-(OH)2D production in many animal species [19].
• Of the bone resorption assays, HYP, TRAP, GHYL, and PYD/DPD, we recommend the urine PYD/DPD assay (adjusted for creatinine) because it is commercially available and because, along with the urine GHYL assay, it is the most sensitive bone resorption assay [20].
• Dipyridamole therapy is unlikely to improve the bone disease in children with XLH [21].

Biological context of PHEX

• Mutations in PHEX, a phosphate-regulating gene with homology to endopeptidases on the X chromosome, are responsible for X-linked hypophosphatemia (XLH) [22].
• The aim of this study was, therefore, to examine whether transgenic overexpression of PHEX under the human beta-actin promoter would rescue the Hyp phenotype [23].
• X-linked hypophosphatemia (XLH) is phenotypically similar to OHO and results from mutations in PHEX, a putative metallopeptidase believed to process a factor(s) regulating bone mineralization and renal phosphate reabsorption [24].
• FGF23, PHEX, and MEPE regulation of phosphate homeostasis and skeletal mineralization [25].
• Seven different PHEX mutations were detected in 8 patients: 2 missense mutations, 2 nonsense mutations, and 3 short deletions [26].

Anatomical context of PHEX

• We also present two other patients whose parathyroid glands were analyzed for PHEX mRNA expression following parathyroidectomy [2].
• The abundance of PHEX mRNA, relative to beta-actin mRNA, in parathyroid glands from patients 2 and 3 was several-fold greater than that in human fetal calvaria, as estimated by ribonuclease protection assay [2].
• Human recombinant endopeptidase PHEX has a strict S1' specificity for acidic residues and cleaves peptides derived from fibroblast growth factor-23 and matrix extracellular phosphoglycoprotein [27].
• These studies indicate that mutations in the PHEX gene are unlikely to be responsible for OOM and suggest that the tumor-derived factor that inhibits phosphate uptake is a small protein that does not downregulate type II NaPi mRNA, and requires an intact cytoskeleton and protein synthesis for activity [28].
• Altered cathepsin D metabolism in PHEX antisense human osteoblast cells [29].

Associations of PHEX with chemical compounds

• We found that affected individuals have a missense mutation in PHEX exon 16 that results in an amino acid change from leucine to proline in residue 555 [30].
• The peptide Abz-GFSDYK(Dnp)-OH, which contains the most favourable residues in the P(2) to P(2)' positions, was hydrolysed by PHEX at the N-terminus of aspartate with a k(cat)/ K(m) of 167 mM(-1) x s(-1) [27].
• In order to study PHEX substrate specificity, combinatorial fluorescent-quenched peptide libraries containing o -aminobenzoic acid (Abz) and 2,4-dinitrophenyl (Dnp) as the donor-acceptor pair were synthesized and tested as PHEX substrates [27].
• We hypothesized that the processing of DMP1 and DSPP is catalyzed by the PHEX enzyme, since this protein, an endopeptidase that is predominantly expressed in bone and tooth, has a strong preference for cleavage at the NH2-terminus of aspartyl residue [31].
• Of the three mutant PHEX proteins, the S711R was the least stable and the only one that could be rescued from the ER to the plasma membrane in cells grown at 26 degrees C. The chemical chaperone glycerol failed to correct defective targeting of all three mutant proteins [32].

Physical interactions of PHEX

• Our aims were to determine (1) whether PHEX binds specifically to MEPE, (2) whether the binding involves the ASARM motif region, and (3) whether free ASARM peptide affects mineralization in vivo in mice [33].

Enzymatic interactions of PHEX

• We found that both recombinant MEPE and synthetic phosphorylated ASARM peptide (ASARM-PO(4)) inhibit PHEX enzyme activities in an in vitro fluorescent-quenched PHEX enzyme activity assay [34].
• PHEX cleaves ASARM peptides from MEPE [35] and OPN [36], and full-length OPN [16].

Regulatory relationships of PHEX

• Recombinant MEPE also inhibits PHEX activity (K(i) = 2 nM and V(max-i) = 26%) [34].
• FGF-23 inhibits renal tubular phosphate transport and is a PHEX substrate [37].
• MATERIALS AND METHODS: We tested this hypothesis by generating two mouse lines expressing human PHEX under the control of a human beta-actin promoter (PHEX-tg) [23].
• It is unclear whether the mutant PHEX gene can induce hyperparathyroidism by abnormal regulation of peptidases [38].
• 24,25(OH)2 D3 improves skeletal lesions in a murine model of XLH and suppresses PTH secretion in animals [39].

Other interactions of PHEX

• Besides osteopontin, it was tempting to speculate that FGF23 is a substrate of PHEX, but studies have been inconclusive so far [40].
• PHEX expression in parathyroid gland and parathyroid hormone dysregulation in X-linked hypophosphatemia [2].
• We report a 5-year-old girl with XLH (patient 1) who had significant hyperparathyroidism at presentation, prior to initiation of therapy [2].
• Finally, our results clearly indicate that PHEX does not have neprilysin-like substrate specificity [27].
• X-linked hypophosphatemic rickets results from mutations in the PHEX gene, which codes for a protein that is a member of the neutral endopeptidase family [41].

Analytical, diagnostic and therapeutic context of PHEX

• Sequence analyses of the PHEX and FGF23 genes were normal [42].
• Protein interactions between MEPE and recombinant soluble PHEX (secPHEX) were measured using surface plasmon resonance (SPR) [33].
• PHEX mRNA expression increased 1.3-fold after treatment with 2 mM phosphate [43].
• Sequestration of the disease-causing mutant proteins in the endoplasmic reticulum (ER) and plasma membrane localization of wild-type and E581V PHEX proteins was demonstrated by immunofluorescence and cell surface biotinylation [32].
• METHODS: PCR and direct sequencing was performed for all exons and intron-exon boundaries of the PHEX gene in two XLH families [44].

References