The role of the PHEX gene (PEX) in families with X-linked hypophosphataemic rickets

Active sites of human MEPE-ASARM regulating bone matrix mineralization

The proteolytic fragment ASARM (acidic serine- and aspartate-rich motif) of MEPE (matrix extracellular phosphoglycoprotein) (MEPE-ASARM) may act as an endogenous anti-mineralization factor involved in X-linked hypophosphatemic rickets/osteomalacia (XLH). We synthesized MEPE-ASARM peptides and relevant peptide fragments with or without phosphorylated Ser residues (pSer) to determine the active site(s) of MEPE-ASARM in a rat calvaria cell culture model. None of the synthetic peptides elicited changes in cell death, proliferation or differentiation, but the peptide (pASARM) with three pSer residues inhibited mineralization without causing changes in gene expression of osteoblast markers tested. The anti-mineralization effect was maintained in peptides in which any one of three pSer residues was deleted. Polyclonal antibodies recognizing pASARM but not ASARM abolished the pASARM effect. Deletion of six N-terminal residues but leaving the recognition sites for PHEX (phosphate regulating endopeptidase homolog, X-linked), a membrane endopeptidase responsible for XLH, intact and two C-terminal amino acid residues did not alter the anti-mineralization activity of pASARM. Our results strengthen understanding of the active sites of MEPE-pASARM and allowed us to identify a shorter more stable sequence with fewer pSer residues still exhibiting hypomineralization activity, reducing peptide synthesis cost and increasing reliability for exploring biological and potential therapeutic effects.

A novel c.2179T>C mutation blocked the intracellular transport of PHEX protein and caused X-linked hypophosphatemic rickets in a Chinese family

BACKGROUND

X-linked hypophosphatemic rickets (XLH) is a heterogeneous genetic phosphate wasting disorder that occupies the majority of inheritable hypophosphatemic rickets (HR). XLH is caused by loss-of-function mutations in the phosphate-regulating endopeptidase gene (PHEX) located on the X chromosome.

METHOD

In this study, we performed whole-exome sequencing (WES) on the proband to identify the causative gene. The mutations were analyzed by predictive online software, such as PolyPhen-2. Plasmids containing the wild-type (WT) and mutant cDNA of the candidate gene were transfected into HEK293, then, the expression, cellular localization, and glycosylation state of the candidate proteins were detected by western blot, immunostaining, and endoglycosidase H digestion. The expression and concentration of related factor were measured by RT-PCR and ELISA.

RESULTS

We identified a novel missense mutation c.2179T>C in the PHEX that results in the substitution of p.Phe727Leu (F727L). This mutation was predicted to be disease-causing by all four predictive online software. In vitro studies demonstrated that the F727L substitution hindered the intracellular trafficking of the mutant PHEX, with ~59% of mutant PHEX protein retained in the endoplasmic reticulum (ER) and only ~16% of the mutant protein localized on the cell surface. Endoglycosidase H digestion assay showed that the mutant F727L PHEX protein was not fully glycosylated. The concentration of intact FGF23 in hFOB1.19 cell culture medium collected from the mutant PHEX group was the highest (62.9 pg/ml) compared to the WT group (32.1 pg/ml) and control group (23.5 pg/ml).

CONCLUSION

Our results confirmed that the mutant PHEX protein was lowly glycosylated and retarded within the ER, the intact FGF23 level in cell culture media caused by the mutant PHEX protein was significantly elevated compared to that of the WT group, which may explain why the single base mutation in the PHEX led to XLH syndrome in this family.

A Novel PHEX Mutation in Japanese Patients with X-Linked Hypophosphatemic Rickets

X-linked hypophosphatemic rickets (XLH) is a dominant inherited disorder characterized by renal phosphate wasting, aberrant vitamin D metabolism, and abnormal bone mineralization. Inactivating mutations in the gene encoding phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) have been found to be associated with XLH. Here, we report a 16-year-old female patient affected by hypophosphatemic rickets. We evaluated her serum fibroblast growth factor 23 (FGF23) levels and conducted sequence analysis of the disease-associated genes of FGF23-related hypophosphatemic rickets: PHEX, FGF23, dentin matrix protein 1, and ectonucleotide pyrophosphatase/phosphodiesterase 1. She was diagnosed with XLH based on her clinical features and family history. Additionally, we observed elevated FGF23 levels and a novel PHEX exon 9 mutation (c.947G>T; p.Gly316Val) inherited from her father. Although bioinformatics showed that the mutation was neutral, Gly316 is perfectly conserved among humans, mice, and rats, and there were no mutations in other FGF23-related rickets genes, suggesting that in silico analysis is limited in determining mutation pathogenicity. In summary, we present a female patient and her father with XLH harboring a novel PHEX mutation that appears to be causative of disease. Measurement of FGF23 for hypophosphatemic patients is therefore useful for the diagnosis of FGF23-dependent hypophosphatemia.

Identification of two novel mutations in the PHEX gene in Chinese patients with hypophosphatemic rickets/osteomalacia

Objective

X-linked dominant hypophosphatemia (XLH) is the most prevalent form of inherited rickets/osteomalacia in humans. The aim of this study was to identify PHEX gene mutations and describe the clinical features observed in 6 unrelated Chinese families and 3 sporadic patients with hypophosphatemic rickets/osteomalacia.

Methods

For this study, 45 individuals from 9 unrelated families of Chinese Han ethnicity (including 16 patients and 29 normal phenotype subjects), and 250 healthy donors were recruited. All 22 exons and exon-intron boundaries of the PHEX gene were amplified by polymerase chain reaction (PCR) and directly sequenced.

Results

The PHEX mutations were detected in 6 familial and 3 sporadic hypophosphatemic rickets/osteomalacia. Altogether, 2 novel mutations were detected: 1 missense mutation c.1183G>C in exon 11, resulting in p.Gly395Arg and 1 missense mutation c.1751A>C in exon 17, resulting in p.His584Pro. No mutations were found in the 250 healthy controls.

Conclusions

Our study increases knowledge of the PHEX gene mutation types and clinical phenotypes found in Chinese patients with XLH, which is important for understanding the genetic basis of XLH. The molecular diagnosis of a PHEX genetic mutation is of great importance for confirming the clinical diagnosis of XLH, conducting genetic counseling, and facilitating prenatal intervention, especially in the case of sporadic patients.

The chicken or the egg: PHEX, FGF23 and SIBLINGs unscrambled

The eggshell is an ancient innovation that helped the vertebrates' transition from the oceans and gain dominion over the land. Coincident with this conquest, several new eggshell and noncollagenous bone-matrix proteins (NCPs) emerged. The protein ovocleidin-116 is one of these proteins with an ancestry stretching back to the Triassic. Ovocleidin-116 is an avian homolog of Matrix Extracellular Phosphoglycoprotein (MEPE) and belongs to a group of proteins called Small Integrin-Binding Ligand Interacting Glycoproteins (SIBLINGs). The genes for these NCPs are all clustered on chromosome 5q in mice and chromosome 4q in humans. A unifying feature of the SIBLING proteins is an Acidic Serine Aspartate-Rich MEPE (ASARM)-associated motif. The ASARM motif and the released ASARM peptide play roles in mineralization, bone turnover, mechanotransduction, phosphate regulation and energy metabolism. ASARM peptides and motifs are physiological substrates for phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX), a Zn metalloendopeptidase. Defects in PHEX are responsible for X-linked hypophosphatemic rickets. PHEX interacts with another ASARM motif containing SIBLING protein, Dentin Matrix Protein-1 (DMP1). DMP1 mutations cause bone-renal defects that are identical with the defects caused by loss of PHEX function. This results in autosomal recessive hypophosphatemic rickets (ARHR). In both X-linked hypophosphatemic rickets and ARHR, increased fibroblast growth factor 23 (FGF23) expression occurs, and activating mutations in FGF23 cause autosomal dominant hypophosphatemic rickets (ADHR). ASARM peptide administration in vitro and in vivo also induces increased FGF23 expression. This review will discuss the evidence for a new integrative pathway involved in bone formation, bone-renal mineralization, renal phosphate homeostasis and energy metabolism in disease and health.

Regulation of bone-renal mineral and energy metabolism: the PHEX, FGF23, DMP1, MEPE ASARM pathway

More than 300 million years ago, vertebrates emerged from the vast oceans to conquer gravity and the dry land. With this transition, new adaptations occurred that included ingenious changes in reproduction, waste secretion, and bone physiology. One new innovation, the egg shell, contained an ancestral protein (ovocleidin-116) that likely first appeared with the dinosaurs and was preserved through the theropod lineage in modern birds and reptiles. Ovocleidin-116 is an avian homolog of matrix extracellular phosphoglycoprotein (MEPE) and belongs to a group of proteins called short integrin-binding ligand-interacting glycoproteins (SIBLINGs). These proteins are all localized to a defined region on chromosome 5q in mice and chromosome 4q in humans. A unifying feature of SIBLING proteins is an acidic serine aspartate-rich MEPE-associated motif (ASARM). Recent research has shown that the ASARM motif and the released ASARM peptide have regulatory roles in mineralization (bone and teeth), phosphate regulation, vascularization, soft-tissue calcification, osteoclastogenesis, mechanotransduction, and fat energy metabolism. The MEPE ASARM motif and peptide are physiological substrates for PHEX, a zinc metalloendopeptidase. Defects in PHEX are responsible for X-linked hypophosphatemic rickets (HYP). There is evidence that PHEX interacts with another ASARM motif containing SIBLING protein, dentin matrix protein-1 (DMP1). DMP1 mutations cause bone and renal defects that are identical with the defects caused by a loss of PHEX function. This results in autosomal recessive hypophosphatemic rickets (ARHR). In both HYP and ARHR, increased FGF23 expression plays a major role in the disease and in autosomal dominant hypophosphatemic rickets (ADHR), FGF23 half-life is increased by activating mutations. ASARM peptide administration in vitro and in vivo also induces increased FGF23 expression. FGF23 is a member of the fibroblast growth factor (FGF) family of cytokines, which surfaced 500 million years ago with the boney fish (i.e., teleosts) that do not contain SIBLING proteins. In terrestrial vertebrates, FGF23, like SIBLING proteins, is expressed in the osteocyte. The boney fish, however, are an-osteocytic, so a physiological bone-renal link with FGF23 and the SIBLINGs was cemented when life ventured from the oceans to the land during the Triassic period, approximately 300 million years ago. This link has been revealed by recent research that indicates a competitive displacement of a PHEX-DMP1 interaction by an ASARM peptide that leads to increased FGF23 expression. This review discusses the new discoveries that reveal a novel PHEX, DMP1, MEPE, ASARM peptide, and FGF23 bone-renal pathway. This pathway impacts not only bone formation, bone-renal mineralization, and renal phosphate homeostasis but also energy metabolism. The study of this new pathway is relevant for developing therapies for several diseases: bone-teeth mineral loss disorders, renal osteodystrophy, chronic kidney disease and bone mineralization disorders (CKD-MBD), end-stage renal diseases, ectopic arterial-calcification, cardiovascular disease renal calcification, diabetes, and obesity.

PHEX neutralizing agent inhibits dentin formation in mouse tooth germ

The mutation of phosphate-regulating gene with homologies to endopeptidases on the X-chromosome (PHEX) can lead to human X-linked hypophosphatemic rickets which displays hypo-mineralization in bone and dentin. To study its possible roles in teeth, PHEX antibody was injected into pregnant mice on E15 to explore its roles on the formation of enamel and dentin. Mallory trichrome staining results showed that arrangements of ameloblasts and odontoblasts were irregular after PHEX antibody treatment. Differentiation of odontoblasts and the formation of dentin were inhibited. Spatiotemporal distribution of PHEX protein was observed in various stages of tooth germ. Immunohistochemical results showed positive PHEX signals appeared in the inner enamel epithelium on E16 and became stronger on E18. Ameloblasts and odontoblasts showed much higher PHEX expression on P1 and P3. Expression of PHEX in odontoblasts decreased accordingly. However, enamel formation was only slightly affected. The findings proved that a decrease in PHEX expression could suppress dentin formation.

Abnormal presence of the matrix extracellular phosphoglycoprotein-derived acidic serine- and aspartate-rich motif peptide in human hypophosphatemic dentin

Severe dental troubles are associated with X-linked hypophosphatemic rickets and are mainly related to impaired dentin mineralization. In dentin matrix, matrix extracellular phosphoglycoprotein (MEPE) may be protected from proteolysis by a specific interaction with PHEX (phosphate regulating gene with homologies to endopeptidases on the X chromosome). The objective of our work was to determine whether PHEX impairment induces MEPE cleavage in dentin and the subsequent release of the C-terminal acidic serine- and aspartate-rich motif (ASARM) peptide, which is known to inhibit mineralization. By Western blot analysis, we explored dentin extracts from seven hypophosphatemic patients with mutations of the PHEX gene. A proteomic approach combining immunoprecipitation, surface-enhanced laser desorption/ionization-time of flight-mass spectrometry and matrix-assisted laser desorption ionization-time of flight analysis of the samples completed this exploration. This study shows a 4.1-kDa peptide containing the MEPE-derived ASARM peptide in hypophosphatemic samples. The presence of ASARM was less marked in patients treated with 1-hydroxylated vitamin D and phosphate during growth. Moreover, recombinant ASARM implanted in a rat pulp injury model disturbed the formation of the reparative dentin bridge. These results suggest that abnormal MEPE cleavage occurs when PHEX activity is deficient in humans, the ASARM peptide may be involved in the mineralization defects and the PHEX-MEPE interaction may be indirect, as ensuring a better phosphate and vitamin D environment to the mineralizing dentin prevents MEPE cleavage.

Mutational analysis of the PHEX gene: novel point mutations and detection of large deletions by MLPA in patients with X-linked hypophosphatemic rickets

X-Linked hypophosphatemic rickets (HYP, XLH) is a disorder of phosphate homeostasis, characterized by renal phosphate wasting and hypophosphatemia, with normal to low 1,25-dihydroxy vitamin D3 serum levels. The purpose of our study was the detection of inactivating mutations in the PHEX gene, the key enzyme in the pathogenesis of XLH. The 16 patients, representing eight families, presented with suspected XLH from biochemical and clinical evidence. All 16 were referred for mutational analysis of the PHEX gene. We detected three novel disease-causing mutations, C59S, Q394X, and W602, for which a loss of function can be predicted. A G28S variation, found in two healthy probands, may be a rare polymorphism. Another mutation, A363 V, is localized on the same allele as the C59S mutation, thus its functional consequences cannot be proven. Furthermore, we detected a deletion of three nucleotides in exon 15 which resulted in the loss of amino acid threonine 535. Heterozygosity of this mutation in a male patient without any chromosomal aberrations suggests its presence as a mosaic. Novel large deletions were detected using multiplex ligation-dependent probe amplification (MLPA) analysis. Two of these deletions, loss of exon 22 alone or exons 21 and 22 together, may result in the translation of a C-terminal truncated protein. Two large deletions comprise exons 1-9 and exons 4-20, respectively, and presumably result in a nonfunctional protein. We conclude that molecular genetic analysis confirms the clinical diagnosis of XLH and should include sequence analysis as well as the search for large deletions, which is facilitated by MLPA.

Oncogenic hypophosphatemic osteomalacia associated with a nasal hemangiopericytoma

We report a patient with a nasal hemangiopericytoma associated with an oncogenic hypophosphatemic osteomalacia (OHO). This syndrome results from tumor products that decrease renal tubular phosphate resorption, leading to the osteomalacia. This patient presented with classic bone manifestations of osteomalacia and a nasal tumor. Laboratory studies performed before the first resection of the tumor included normal serum calcium, hypophosphatemia due to decreased tubular reabsorption of phosphate, and an undetectable serum 1,25 dihydroxy vitamin D level. Serum parathormone level was normal. Anterior iliac crest bone biopsy showed characteristic signs of osteomalacia that included increased osteoid and delayed mineralization. A partial resection of the nasal tumor was performed. After the first surgery the patient showed detectable serum level of 1,25 dihydroxy vitamin D, and transient normalization of the tubular reabsorption of phosphate. The patient was also treated with phosphate supplements and vitamin D with transient control of her clinical manifestations and improvement of the radiographic signs of osteomalacia. Three months after surgery, the serum level of 1,25 dihydroxy vitamin D level again became undetectable. After selective embolization of the tumor, followed by an apparent complete tumor resection and postoperative radiation therapy, her hypophosphatemia and decreased phosphate tubular reabsorption persisted. Therefore, biochemical changes associated with hemangiopericytoma induced OHO may persist even after apparent total tumor resection. Clinicians should be aware of the oncogenic basis for some osteomalacia, as seen in this patient.

Proteomics in renal research

Proteomic technologies are used with increasing frequency in the renal community. In this review, we highlight the use in renal research of a number of available techniques including two-dimensional gel electrophoresis, liquid chromatography/mass spectrometry, surface-enhanced laser desorption/ionization, capillary electrophoresis/mass spectrometry, and antibody and tissue arrays. These techniques have been used to identify proteins or changes in proteins specific to regions of the kidney or associated with renal diseases or toxicity. They have also been used to examine protein expression changes and posttranslational modifications of proteins during signaling. A number of studies have used proteomic methodologies to look for diagnostic biomarkers in body fluids. The rapid rate of development of the technologies along with the combination of classic physiological and biochemical techniques with proteomics will enable new discoveries.

Serum MEPE-ASARM-peptides are elevated in X-linked rickets (HYP): implications for phosphaturia and rickets

MEPE (Matrix Extracellular PhosphoglycoprotEin) expression is markedly elevated in X-linked-hypophosphatemic-rickets (HYP) and tumor-induced osteomalacia (TIO). In normal individuals, circulating serum-levels of MEPE are tightly correlated with serum-phosphorus, parathyroid hormone (PTH) and bone mineral density (BMD). Also, MEPE derived, C-terminal ASARM-peptides are candidate minhibins and/or phosphatonins. Our aims were to determine: 1. whether MEPE-ASARM-peptide(s) are abnormally elevated in HYP/hyp serum, and, 2. whether the ASARM-peptide(s) accumulate in hyp mice kidney renal-tubules. Using a specific competitive ELISA we measured a five fold increase (P=0.007) of serum ASARM-peptide(s) in human HYP patients (normal subjects 3.25 microM n=9; S.E.M.=0.51 and HYP-patients 15.74 microM, n=9; S.E.M.=3.32). A 6.23 fold increase (P=0.008) was measured in hyp male mice compared with their normal male siblings (normal-siblings, 3.73 muM, S.E.M.=0.57, n=3; and hyp-mice 23.4 microM, n=3, S.E.M.=4.01). Renal immuno-histological screening also revealed a dramatic increase of ASARM-peptides in regions anatomically consistent with the proximal convoluted tubules. This study demonstrates for the first time that markedly elevated serum levels of protease-resistant ASARM-peptide(s) occur in HYP/hyp and they accumulate in murine hyp kidneys. These peptides are thus likely responsible for the phosphaturia and defective mineralization in HYP/hyp and TIO.

The wrickkened pathways of FGF23, MEPE and PHEX

The last 350 years since the publication of the first medical monograph on rickets (old English term wrickken) (Glisson et al., 1651) have seen spectacular advances in our understanding of mineral-homeostasis. Seminal and exciting discoveries have revealed the roles of PTH, vitamin D, and calcitonin in regulating calcium and phosphate, and maintaining healthy teeth and skeleton. However, it is clear that the PTH/Vitamin D axis does not account for the entire picture, and a new bone-renal metabolic milieu has emerged, implicating a novel set of matrix proteins, hormones, and Zn-metallopeptidases. The primary defects in X-linked hypophosphatemic rickets (HYP) and autosomal-dominant hypophosphatemic rickets (ADHR) are now identified as inactivating mutations in a Zn-metalloendopeptidase (PHEX) and activating mutations in fibroblast-growth-factor-23 (FGF23), respectively. In oncogenic hypophosphatemic osteomalacia (OHO), several tumor-expressed proteins (MEPE, FGF23, and FRP-4) have emerged as candidate mediators of the bone-renal pathophysiology. This has stimulated the proposal of a global model that takes into account the remarkable similarities between the inherited diseases (HYP and ADHR) and the tumor-acquired disease OHO. In HYP, loss of PHEX function is proposed to result in an increase in uncleaved full-length FGF23 and/or inappropriate processing of MEPE. In ADHR, a mutation in FGF23 results in resistance to proteolysis by PHEX or other proteases and an increase in half-life of full-length phosphaturic FGF23. In OHO, over-expression of FGF23 and/or MEPE is proposed to result in abnormal renal-phosphate handling and mineralization. Although this model is attractive, many questions remain unanswered, suggesting a more complex picture. The following review will present a global hypothesis that attempts to explain the experimental and clinical observations in HYP, ADHR, and OHO, plus diverse mouse models that include the MEPE null mutant, HYP-PHEX transgenic mouse, and MEPE-PHEX double-null-mutant.

Musculoskeletal manifestations of osteomalacia and rickets

Osteomalacia (OM) is still an important metabolic bone disease with increased prevalence in certain regions of the world as well as in the urban population of elderly confined. The disease presents with a wide variety of clinical, biochemical and radiographic manifestations mimicking other musculoskeletal disorders, including 'osteoporosis'. In this chapter, we provide the basis for its clinical diagnosis and management. There have been significant recent advances in the understanding of vitamin D deficiency and hypophosphataemic osteomalacia, which can now assist clinicians in the precise diagnosis and treatment of this disease. In this chapter we also review the various underlying aetiologies. The successful management of OM depends on the underlying aetiology.

Reduced fertility in male mice deficient in the zinc metallopeptidase NL1

Members of the M13 family of zinc metalloendopeptidases have been shown to play critical roles in the metabolism of various neuropeptides and peptide hormones, and they have been identified as important therapeutic targets. Recently, a mouse NL1 protein, a novel member of the family, was identified and shown to be expressed mainly in the testis as a secreted protein. To define its physiological role(s), we used a gene targeting strategy to disrupt the endogenous murine Nl1 gene by homologous recombination and generate Nl1 mutant mice. The Nl1(-/-) mice were viable and developed normally, suggesting that zygotic expression of Nl1 is not required for development. However, Nl1(-/-) males produced smaller litters than their wild-type siblings, indicating specific male fertility problems. Reduced fertility may be explained by two impaired processes, decreased egg fertilization and perturbed early development of fertilized eggs. These two phenotypes did not result from gross anatomical modifications of the testis or from impaired spermatogenesis. Basic sperm parameters were also normal. Thus, our findings suggest that one of the roles of NL1 in mice is related to sperm function and that NL1 modulates the processes of fertilization and early embryonic development in vivo.

Mepe, the gene encoding a tumor-secreted protein in oncogenic hypophosphatemic osteomalacia, is expressed in bone

The MEPE (matrix extracellular phosphoglycoprotein) gene is a strong candidate for the tumor-derived phosphaturic factor in oncogenic hypophosphatemic osteomalacia (OHO). 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. Here we report the isolation of the murine homologue of MEPE, from a bone cDNA library, that encodes a protein of 433 amino acids, 92 amino acids shorter than human MEPE. Mepe, like Phex, is expressed by fully differentiated osteoblasts and down-regulated by 1,25-(OH)2D3. In contrast to Phex, Mepe expression is markedly increased during osteoblast-mediated matrix mineralization. Greater than normal Mepe mRNA levels were observed in bone and osteoblasts derived from Hyp mice, the murine homologue of human XLH. Our data provide the first evidence that MEPE/Mepe is expressed by osteoblasts in association with mineralization.

Phosphate wasting in oncogenic osteomalacia: PHEX is normal and the tumor-derived factor has unique properties

Oncogenic osteomalacia (OOM) is characterized by renal phosphate wasting and abnormal metabolism of vitamin D, somewhat similar to the phenotype of X-linked hypophosphatemic rickets (HYP). DNA from OOM tumor cells was analyzed for mutations in the PHEX gene, which is mutated in HYP. Screening for mutations by single-strand conformation polymorphism analysis and subsequent sequencing of all the exons revealed no mutations. Conditioned media from long-term cultures of OOM tumor cells were used to further characterize the physical properties of the phosphate-regulating factor and its mechanism of action. Inhibition of OK 3B2 cell renal phosphate transport by conditioned media was dose-dependent and maximal after 20 h. This time course differed from that of parathyroid hormone (PTH). The bioactivity was stable to mild acid and alkali treatment and freeze drying and was retained in the aqueous phase following organic solvent extraction. The activity was not suppressed by heat or by treatment with trypsin but was suppressed by the protease papain and had an apparent molecular weight of < 5000. No change was detected in the expression of type II sodium/phosphate cotransporter (NaPi) mRNA in OK 3B2 cells in response to conditioned media, unlike the reduction seen in Hyp mice. In the presence of colchicine or cytochalasin D, the inhibitory response to conditioned media was reduced, similar to the effect of these agents on the response to PTH. Cycloheximide also suppressed the inhibitory response of conditioned media, but not the response to PTH. 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.

Three novel PHEX gene mutations in Japanese patients with X-linked hypophosphatemic rickets

X-linked hypophosphatemic rickets (XLH) is an X-linked dominant disorder characterized by renal phosphate wasting, abnormal vitamin D metabolism, and defects of bone mineralization. The phosphate-regulating gene on the X-chromosome (PHEX) that is defective in XLH has been cloned, and its location identified at Xp22.1. It has been recognized to be homologous to certain endopeptidases. So far, a variety of PHEX mutations have been identified mainly in European and North American patients with XLH. To analyze the molecular basis of four unrelated Japanese families with XLH, we determined the nucleotide sequence of the PHEX gene of affected members. We detected a new nonsense mutation (R198X) in exon 5, a new 3 nucleotides insertion mutation in exon 12 and a new missense mutation (L160R) in exon 5 as well as a previously reported nonsense mutation in exon 8 (R291X). These results suggest that: 1) PHEX gene mutations are responsible for XLH in Japanese patients, and 2) PHEX gene mutations are heterogeneous in the Japanese population similarly to other ethnic populations.

MEPE, a new gene expressed in bone marrow and tumors causing osteomalacia

Oncogenic hypophosphatemic osteomalacia (OHO) is characterized by a renal phosphate leak, hypophosphatemia, low-serum calcitriol (1,25-vitamin-D3), and abnormalities in skeletal mineralization. Resection of OHO tumors results in remission of the symptoms, and there is evidence that a circulating phosphaturic factor plays a role in the bone disease. This paper describes the characterization and cloning of a gene that is a candidate for the tumor-secreted phosphaturic factor. This new gene has been named MEPE (matrix extracellular phosphoglycoprotein) and has major similarities to a group of bone-tooth mineral matrix phospho-glycoproteins (osteopontin (OPN; HGMW-approved symbol SPP1), dentin sialo phosphoprotein (DSPP), dentin matrix protein 1 (DMP1), bone sialoprotein II (IBSP), and bone morphogenetic proteins (BMP). All the proteins including MEPE contain RGD sequence motifs that are proposed to be essential for integrin-receptor interactions. Of further interest is the finding that MEPE, OPN, DSPP, DMP1, IBSP, and BMP3 all map to a defined region in chromosome 4q. Refined mapping localizes MEPE to 4q21.1 between ESTs D4S2785 (WI-6336) and D4S2844 (WI-3770). MEPE is 525 residues in length with a short N-terminal signal peptide. High-level expression of MEPE mRNA occurred in all four OHO tumors screened. Three of 11 non-OHO tumors screened contained trace levels of MEPE expression (detected only after RT-PCR and Southern 32P analysis). Normal tissue expression was found in bone marrow and brain with very-low-level expression found in lung, kidney, and human placenta. Evidence is also presented for the tumor secretion of clusterin (HGMW-approved symbol CLU) and its possible role as a cytotoxic factor in one of the OHO patients described.