Loss of DMP1 causes rickets and osteomalacia and identifies a role for osteocytes in mineral metabolism

The osteocyte, a terminally differentiated cell comprising 90%-95% of all bone cells, may have multiple functions, including acting as a mechanosensor in bone (re)modeling. Dentin matrix protein 1 (encoded by DMP1) is highly expressed in osteocytes and, when deleted in mice, results in a hypomineralized bone phenotype. We investigated the potential for this gene not only to direct skeletal mineralization but also to regulate phosphate (P(i)) homeostasis. Both Dmp1-null mice and individuals with a newly identified disorder, autosomal recessive hypophosphatemic rickets, manifest rickets and osteomalacia with isolated renal phosphate-wasting associated with elevated fibroblast growth factor 23 (FGF23) levels and normocalciuria. Mutational analyses showed that autosomal recessive hypophosphatemic rickets family carried a mutation affecting the DMP1 start codon, and a second family carried a 7-bp deletion disrupting the highly conserved DMP1 C terminus. Mechanistic studies using Dmp1-null mice demonstrated that absence of DMP1 results in defective osteocyte maturation and increased FGF23 expression, leading to pathological changes in bone mineralization. Our findings suggest a bone-renal axis that is central to guiding proper mineral metabolism.

Tissue-nonspecific alkaline phosphatase and plasma cell membrane glycoprotein-1 are central antagonistic regulators of bone mineralization

Osteoblasts mineralize bone matrix by promoting hydroxyapatite crystal formation and growth in the interior of membrane-limited matrix vesicles (MVs) and by propagating the crystals onto the collagenous extracellular matrix. Two osteoblast proteins, tissue-nonspecific alkaline phosphatase (TNAP) and plasma cell membrane glycoprotein-1 (PC-1) are involved in this process. Mutations in the TNAP gene result in the inborn error of metabolism known as hypophosphatasia, characterized by poorly mineralized bones, spontaneous fractures, and elevated extracellular concentrations of inorganic pyrophosphate (PP(i)). PP(i) suppresses the formation and growth of hydroxyapatite crystals. PP(i) is produced by the nucleoside triphosphate pyrophosphohydrolase activity of a family of isozymes, with PC-1 being the only member present in MVs. Mice with spontaneous mutations in the PC-1 gene have hypermineralization abnormalities that include osteoarthritis and ossification of the posterior longitudinal ligament of the spine. Here, we show the respective correction of bone mineralization abnormalities in knockout mice null for both the TNAP (Akp2) and PC-1 (Enpp1) genes. Each allele of Akp2 and Enpp1 has a measurable influence on mineralization status in vivo. Ex vivo experiments using cultured double-knockout osteoblasts and their MVs demonstrate normalization of PP(i) content and mineral deposition. Our data provide evidence that TNAP and PC-1 are key regulators of the extracellular PP(i) concentrations required for controlled bone mineralization. Our results suggest that inhibiting PC-1 function may be a viable therapeutic strategy for hypophosphatasia. Conversely, interfering with TNAP activity may correct pathological hyperossification because of PP(i) insufficiency.

FGF-23 inhibits renal tubular phosphate transport and is a PHEX substrate

Oncogenic osteomalacia (OOM), X-linked hypophosphatemia (XLH), and autosomal dominant hypophosphatemic rickets (ADHR) are phenotypically similar disorders characterized by hypophosphatemia, decreased renal phosphate reabsorption, normal or low serum calcitriol concentrations, normal serum concentrations of calcium and parathyroid hormone, and defective skeletal mineralization. XLH results from mutations in the PHEX gene, encoding a membrane-bound endopeptidase, whereas ADHR is associated with mutations of the gene encoding FGF-23. Recent evidence that FGF-23 is expressed in mesenchymal tumors associated with OOM suggests that FGF-23 is responsible for the phosphaturic activity previously termed "phosphatonin." Here we show that both wild-type FGF-23 and the ADHR mutant, FGF-23(R179Q), inhibit phosphate uptake in renal epithelial cells. We further show that the endopeptidase, PHEX, degrades native FGF-23 but not the mutant form. Our results suggest that FGF-23 is involved in the pathogenesis of these three hypophosphatemic disorders and directly link PHEX and FGF-23 within the same biochemical pathway.

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.

FGF23, PHEX, and MEPE regulation of phosphate homeostasis and skeletal mineralization

There is evidence for a hormone/enzyme/extracellular matrix protein cascade involving fibroblastic growth factor 23 (FGF23), a phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX), and a matrix extracellular phosphoglycoprotein (MEPE) that regulates systemic phosphate homeostasis and mineralization. Genetic studies of autosomal dominant hypophosphatemic rickets (ADHR) and X-linked hypophosphatemia (XLH) identified the phosphaturic hormone FGF23 and the membrane metalloprotease PHEX, and investigations of tumor-induced osteomalacia (TIO) discovered the extracellular matrix protein MEPE. Similarities between ADHR, XLH, and TIO suggest a model to explain the common pathogenesis of renal phosphate wasting and defective mineralization in these disorders. In this model, increments in FGF23 and MEPE, respectively, cause renal phosphate wasting and intrinsic mineralization abnormalities. FGF23 elevations in ADHR are due to mutations of FGF23 that block its degradation, in XLH from indirect actions of inactivating mutations of PHEX to modify the expression and/or degradation of FGF23 and MEPE, and in TIO because of increased production of FGF23 and MEPE. Although this model is attractive, several aspects need to be validated. First, the enzymes responsible for metabolizing FGF23 and MEPE need to be established. Second, the physiologically relevant PHEX substrates and the mechanisms whereby PHEX controls FGF23 and MEPE metabolism need to be elucidated. Finally, additional studies are required to establish the molecular mechanisms of FGF23 and MEPE actions on kidney and bone, as well as to confirm the role of these and other potential "phosphatonins," such as frizzled related protein-4, in the pathogenesis of the renal and skeletal phenotypes in XLH and TIO. Unraveling the components of this hormone/enzyme/extracellular matrix pathway will not only lead to a better understanding of phosphate homeostasis and mineralization but may also improve the diagnosis and treatment of hypo- and hyperphosphatemic disorders.

Possible involvement of pregnane X receptor-enhanced CYP24 expression in drug-induced osteomalacia

Vitamin D controls calcium homeostasis and the development and maintenance of bones through vitamin D receptor activation. Prolonged therapy with rifampicin or phenobarbital has been shown to cause vitamin D deficiency or osteomalacia, particularly in patients with marginal vitamin D stores. However, the molecular mechanism of this process is unknown. Here we show that these drugs lead to the upregulation of 25-hydroxyvitamin D(3)-24-hydroxylase (CYP24) gene expression through the activation of the nuclear receptor pregnane X receptor (PXR; NR1I2). CYP24 is a mitochondrial enzyme responsible for inactivating vitamin D metabolites. CYP24 mRNA is upregulated in vivo in mice by pregnenolone 16alpha-carbonitrile and dexamethasone, 2 murine PXR agonists, and in vitro in human hepatocytes by rifampicin and hyperforin, 2 human PXR agonists. Moreover, rifampicin increased 24-hydroxylase activity in these cells, while, in vivo in mice, pregnenolone 16alpha-carbonitrile increased the plasma concentration of 24,25-dihydroxyvitamin D(3). Transfection of PXR in human embryonic kidney cells resulted in rifampicin-mediated induction of CYP24 mRNA. Analysis of the human CYP24 promoter showed that PXR transactivates the sequence between -326 and -142. We demonstrated that PXR binds to and transactivates the 2 proximal vitamin D-responsive elements of the human CYP24 promoter. These data suggest that xenobiotics and drugs can modulate CYP24 gene expression and alter vitamin D(3) hormonal activity and calcium homeostasis through the activation of PXR.

DMP1 depletion decreases bone mineralization in vivo: an FTIR imaging analysis

The role of DMP1 in mineralization was analyzed by comparing bone mineral and matrix properties in dmp1-null female mice to heterozygous and wildtype controls by FTIR imaging spectroscopy. The observed decreased mineral content in dmp1 null mice indicates a key role for dmp1 in bone mineralization. Indirect effects of DMP1 on other systems also determine the KO phenotype.

INTRODUCTION

Dentin matrix protein 1 (DMP1), an acidic phosphorylated extracellular matrix protein, is highly expressed in mineralized tissues. In vitro, DMP1 peptides can promote or inhibit mineralization depending on the extent of phosphorylation, the peptide size, and concentration. To clarify the biological function of DMP1 protein on in vivo mineralization, this study analyzed bone properties of dmp1 knockout (KO) mice compared with heterozygous (HET) and wildtype (WT) controls.

MATERIALS AND METHODS

Tibias from dmp1 KO and age-, sex-, and background-matched HET and WT mice at 4 and 16 weeks (N(total) = 60) were examined by Fourier transform infrared imaging (FTIRI), histology (n = 6 per genotype and age; N = 36), and geometry by muCT (n = 4 per genotype and age; N = 24). Serum ionic calcium and phosphate concentrations were also determined.

RESULTS

The mineral-to-matrix ratios (spectroscopic parameter of relative mineral content) were significantly lower in dmp1 KO mice tibias compared with WT and HET at 4 and 16 weeks. The mineral crystallinity (crystal size/perfection) was significantly increased in dmp1 KO and HET mice relative to WT. Collagen cross-link ratios (a spectroscopic parameter related to the relative amounts of nonreducible/reducible collagen cross-links) in dmp1 KO were not significantly different from WT and HET. Based on muCT, cortical bone cross-sectional areas at 16 but not 4 weeks were significantly reduced in the KO compared with controls. Maximum, minimum, and polar cross-sectional moments of inertia were significantly lower in dmp1 KO than in HET at 16 weeks but not at 4 weeks. Histological analysis and muCT 3-D images suggested that dmp1 KO mice had osteomalacia. Dmp1 KO mice had significantly lower ionic calcium and phosphate concentrations relative to WT, whereas in the HET, values for phosphate were equivalent, and calcium values were decreased relative to WT values.

CONCLUSIONS

The findings of decreased mineral-to-matrix ratio and increased crystal size in bones of dmp1 KO mice suggest that DMP1 has multiple roles (both direct and indirect) in the regulation of postnatal mineralization. We suggest that direct effects on mineral formation, crystal growth, and indirect effects on regulation of Ca x P concentrations and matrix turnover all contribute to the dominant phenotype in the dmp1 KO mouse.

Hypocalcemia and osteopathy in mice with kidney-specific megalin gene defect

Megalin is an endocytic receptor highly expressed in the proximal tubules of the kidney. Recently, we demonstrated that this receptor is essential for the renal uptake and conversion of 25-OH vitamin D3 to 1,25-(OH)2 vitamin D3, a central step in vitamin D and bone metabolism. Unfortunately, the perinatal lethality of the conventional megalin knockout mouse model precluded the detailed analysis of the significance of megalin for calcium homeostasis and bone turnover in vivo. Here, we have generated a new mouse model with conditional inactivation of the megalin gene in the kidney by using Cre recombinase. Animals with a renal-specific receptor gene defect were viable and fertile. However, lack of receptor expression in the kidney results in plasma vitamin D deficiency, in hypocalcemia and in severe bone disease, characterized by a decrease in bone mineral content, an increase in osteoid surfaces, and a lack of mineralizing activity. These features are consistent with osteomalacia (softening of the bones) as a consequence of hypovitaminosis D and demonstrate the crucial importance of the megalin pathway for systemic calcium homeostasis and bone metabolism.

Calcification of entheses associated with X-linked hypophosphatemic osteomalacia

We undertook a retrospective analysis of 26 patients with X-linked hypophosphatemic osteomalacia (or rickets), whose ages ranged from 1 to 62 years and who were from 11 different kindreds, to determine the prevalence and clinical characteristics of a unique disorder of the entheses (tendons, ligaments, and joint capsules). We found a generalized involvement of the entheses, with exuberant calcification of tendon and ligament insertions and of joint capsules, in 69 per cent of the subjects. The prevalence and extent of disease increased with age but were not correlated with sex. Commonly affected sites included the hand and sacroiliac joints. Histologic evaluation in a selected patient revealed intratendinous lamellar bone but no inflammatory cells. Our observations indicate that this disorder is an integral part of X-linked hypophosphatemic osteomalacia and exhibits clinical, radiographic, and histologic characteristics that differentiate it from degenerative disorders of these tissues and seronegative spondyloarthropathies.

Tumors associated with oncogenic osteomalacia express genes important in bone and mineral metabolism

Oncogenic osteomalacia (OOM) is associated with primitive mesenchymal tumors that secrete phosphaturic factors resulting in low serum concentrations of phosphate and calcitriol, phosphaturia, and defective bone mineralization. To identify overexpressed genes in these tumors, we compared gene expression profiles of tumors resected from patients with OOM and histologically similar control tumors using serial analysis of gene expression (SAGE). Three hundred and sixty-four genes were expressed at least twofold greater in OOM tumors compared with control tumors. A subset of 67 highly expressed genes underwent validation with an extended set of OOM and control tumors using array analysis or reverse-transcription polymerase chain reaction (RT-PCR). Ten of these validated genes were consistently overexpressed in all OOM tumors relative to control tumors. Strikingly, genes with roles in bone matrix formation, mineral ion transport, and bone mineralization were highly expressed in the OOM tumors.

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.

Pathogenic role of Fgf23 in Dmp1-null mice

Autosomal recessive hypophosphatemic rickets (ARHR), which is characterized by renal phosphate wasting, aberrant regulation of 1alpha-hydroxylase activity, and rickets/osteomalacia, is caused by inactivating mutations of dentin matrix protein 1 (DMP1). ARHR resembles autosomal dominant hypophosphatemic rickets (ADHR) and X-linked hypophosphatemia (XLH), hereditary disorders respectively caused by cleavage-resistant mutations of the phosphaturic factor FGF23 and inactivating mutations of PHEX that lead to increased production of FGF23 by osteocytes in bone. Circulating levels of FGF23 are increased in ARHR and its Dmp1-null mouse homologue. To determine the causal role of FGF23 in ARHR, we transferred Fgf23 deficient/enhanced green fluorescent protein (eGFP) reporter mice onto Dmp1-null mice to create mice lacking both Fgf23 and Dmp1. Dmp1(-/-) mice displayed decreased serum phosphate concentrations, inappropriately normal 1,25(OH)(2)D levels, severe rickets, and a diffuse form of osteomalacia in association with elevated Fgf23 serum levels and expression in osteocytes. In contrast, Fgf23(-/-) mice had undetectable serum Fgf23 and elevated serum phosphate and 1,25(OH)(2)D levels along with severe growth retardation and focal form of osteomalacia. In combined Dmp1(-/-)/Fgf23(-/-), circulating Fgf23 levels were also undetectable, and the serum levels of phosphate and 1,25(OH)(2)D levels were identical to Fgf23(-/-) mice. Rickets and diffuse osteomalacia in Dmp1-null mice were transformed to severe growth retardation and focal osteomalacia characteristic of Fgf23-null mice. These data suggest that the regulation of extracellular matrix mineralization by DMP1 is coupled to renal phosphate handling and vitamin D metabolism through a DMP1-dependent regulation of FGF23 production by osteocytes.

Hereditary forms of rickets and osteomalacia

1. We have described here various forms of rickets and osteomalacia that we have studied ourselves and have come to recognise as comprising definite syndromes. We have included only diseases in which hereditary factors have been proved or might be suspected to play a part.

2. There are more of these syndromes than previously defined.

3. All these hereditary diseases can be treated, usually very satisfactorily, using the same general principles and with only very few simple medicines such as vitamin D and sodium bicarbonate. Careful control and long follow-up, however, may be required to achieve best results and to avoid accidents.

4. A study of the genetics of the diseases is a help in diagnosis and treatment. Broadly speaking the group of diseases which mimic vitamin D deficiency more or less closely tend to be inherited in dominant fashion. The groups with more severe degree of renal-tubule abnormality are inherited as recessives.

5. When more than one member of a family is affected it has been found that in each family each syndrome runs true to type.

Actions and mode of actions of FGF19 subfamily members

Fibroblast growth factors (FGFs) are humoral factors with diverse biological functions. While most FGFs were shown to work as local factors regulating cell growth and differentiation, recent investigations indicated that FGF19 subfamily members, FGF15/19, FGF21 and FGF23 work as systemic factors. FGF15/19 produced by intestine inhibits bile acid synthesis and FGF21from liver is involved in carbohydrate and lipid metabolism. In addition, FGF23 was shown to be produced by bone and regulate phosphate and vitamin D metabolism. Furthermore, these FGFs require klotho or betaklotho for their actions in addition to canonical FGF receptors. It is possible that these FGFs together with their receptor systems might be targets for novel therapeutic measures in the future.

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

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.

Distinct roles for intrinsic osteocyte abnormalities and systemic factors in regulation of FGF23 and bone mineralization in Hyp mice

X-linked hypophosphatemia (XLH) is characterized by hypophosphatemia and impaired mineralization caused by mutations of the PHEX endopeptidase (phosphate-regulating gene with homologies to endopeptidases on the X chromosome), which leads to the overproduction of the phosphaturic fibroblast growth factor 23 (FGF23) in osteocytes. The mechanism whereby PHEX mutations increase FGF23 expression and impair mineralization is uncertain. Either an intrinsic osteocyte abnormality or unidentified PHEX substrates could stimulate FGF23 in XLH. Similarly, impaired mineralization in XLH could result solely from hypophosphatemia or from a concomitant PHEX-dependent intrinsic osteocyte abnormality. To distinguish between these possibilities, we assessed FGF23 expression and mineralization after reciprocal bone cross-transplantations between wild-type (WT) mice and the Hyp mouse model of XLH. We found that increased FGF23 expression in Hyp bone results from a local effect of PHEX deficiency, since FGF23 was increased in Hyp osteocytes before and after explantation into WT mice but was not increased in WT osteocytes after explantation into Hyp mice. WT bone explanted into Hyp mice developed rickets and osteomalacia, but Hyp bone explanted into WT mice displayed persistent osteomalacia and abnormalities in the primary spongiosa, indicating that both phosphate and PHEX independently regulate extracellular matrix mineralization. Unexpectedly, we observed a paradoxical suppression of FGF23 in juvenile Hyp bone explanted into adult Hyp mice, indicating the presence of an age-dependent systemic inhibitor of FGF23. Thus PHEX functions in bone to coordinate bone mineralization and systemic phosphate homeostasis by directly regulating the mineralization process and producing FGF23. In addition, systemic counterregulatory factors that attenuate the upregulation of FGF23 expression in Hyp mouse osteocytes are present in older mice.

Serum 1,25-dihydroxyvitamin D levels in subjects with X-linked hypophosphatemic rickets and osteomalacia

In order to determine whether a defect in vitamin D metabolism might play a role in the pathogenesis of X-linked hypophosphatemic rickets and osteomalacia (XLH), we compared the serum 1,25-dihydroxyvitamin D [1,25(OH)2D] level in 52 normal subjects and 37 patients with XLH. In untreated patients, adults were found to have values similar to age-matched controls, while youths had values similar to growth-rate-matched controls but significantly lower than the levels of age-matched controls who were growing at a normal rate. In contrast, treated XLH patients of all ages had serum levels significantly lower than both controls and untreated XLH patients. Further, the serum levels of 1,25(OH)2D in these treated patients had a significant inverse linear correlation with serum 25-(OH)D concentrations. We propose that subjects with XLH have serum 1,25(OH)2D levels within appropriate age- and growth-rate-matched normal ranges. However, in the presence of hypophosphatemia, we would have anticipated elevated levels of 1,25(OH)2D; viewed in this light the serum 1,25(OH)2D levels are inadequate, suggesting the presence of a relative deficiency of this active vitamin D metabolite.

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.

Rescue of the skeletal phenotype in CasR-deficient mice by transfer onto the Gcm2 null background

To understand the role of the calcium-sensing receptor (CasR) in the skeleton, we used a genetic approach to ablate parathyroid glands and remove the confounding effects of elevated parathyroid hormone (PTH) in CasR-deficient mice. CasR deficiency was transferred onto the glial cells missing 2-deficient (Gcm2-deficient) background by intercrossing CasR- and Gcm2-deficient mice. Superimposed Gcm2 deficiency rescued the perinatal lethality in CasR-deficient mice in association with ablation of the parathyroid glands and correction of the severe hyperparathyroidism. In addition, the double homozygous CasR- and Gcm2-deficient mice demonstrated healing of the abnormal mineralization of cartilage and bone associated with CasR deficiency, indicating that rickets and osteomalacia in CasR-deficient mice are not due to an independent function of CasR in bone and cartilage but to the effect of severe hyperparathyroidism in the neonate. Analysis of the skeleton of 6-week-old homozygous CasR- and Gcm2-deficient mice also failed to identify any essential, nonredundant role for CasR in regulating chondrogenesis or osteogenesis, but further studies are needed to establish the function of CasR in the skeleton. In contrast, concomitant Gcm2 and CasR deficiency failed to rescue the hypocalciuria in CasR-deficient mice, consistent with direct regulation of urinary calcium excretion by CasR in the kidney. Double Gcm2- and CasR-deficient mice provide an important model for evaluating the extraparathyroid functions of CasR.

Hypophosphatemic osteomalacia and bone sclerosis caused by a novel homozygous mutation of the FAM20C gene in an elderly man with a mild variant of Raine syndrome

BACKGROUND

Hypophosphatemia and increased serum fibroblast growth factor 23 (FGF23) levels have been reported in young brothers with compound heterozygous mutations for the FAM20C gene; however, rickets was not observed in these cases. We report an adult case of Raine syndrome accompanying hypophosphatemic osteomalacia with a homozygous FAM20C mutation (R408W) associated with increased periosteal bone formation in the long bones and an increase in bone mineral density in the femoral neck.

CASE

The patient, a 61-year-old man, was born from a cousin-to-cousin marriage. A short stature and severe dental demineralization were reported at an elementary school age. Hypophosphatemia was noted inadvertently at 27years old, at which time he started to take an active vitamin D metabolite (alphacalcidol) and phosphate. He also manifested ossification of the posterior longitudinal ligament. On bone biopsy performed at the age of 41years, we found severe osteomalacia surrounding osteocytes, which appeared to be an advanced form of periosteocytic hypomineralized lesions compared to those reported in patients with X-linked hypophosphatemic rickets. Laboratory data at 61years of age revealed markedly increased serum intact-FGF23 levels, which were likely to be the cause of hypophosphatemia and the decreased level of 1,25(OH)2D. We recently identified a homozygous FAM20C mutation, which was R408W, in this patient. When expressed in HEK293 cells, the R408W mutant protein exhibited impaired kinase activity and secretion.

DISCUSSION

Our findings suggest that certain homozygous FAM20C mutations can cause FGF23-related hypophosphatemic osteomalacia and indicate the multiple roles of FAM20C in bone.

Metallothionein-null mice are more susceptible than wild-type mice to chronic CdCl(2)-induced bone injury

Cadmium (Cd) is an environmental pollutant and is toxic to a number of organs. Chronic exposure to Cd causes loss of bone mass and increased incidence of bone fractures, as seen in Itai-itai patients and laboratory animals. Metallothionein (MT), a low-molecular weight, cysteine-rich, metal-binding protein, has been shown to play an important role in the detoxication of Cd. Thus, this study was designed to test the hypothesis that MT protects against Cd-induced bone injury. Wild-type and MT-I/II knockout (MT-null) mice were given repeated sc injections of CdCl(2) over a wide range of doses for 10 weeks, and Cd-induced bone injury was examined. Cd produced dose- and time-dependent increases in bone Cd content. However, the concentration of Cd in bone was much lower than that found in the liver and kidney (11 vs 400 and 120 microg/g, respectively) of the same mice. There was no difference in bone Cd content between wild-type and MT-null mice. Repeated Cd injections produced a dose-dependent loss of bone mass (up to 25%), as shown by analysis of the femur, tibia, and lumbar vertebrae. The loss of bone mass was more marked in MT-null mice than in wild-type mice, as shown by dry bone weight, defatted bone weight, bone ash weight, and total calcium content. X-ray photography showed decreasing bone density along the entire bone length with increasing dose and time of Cd exposure. The decrease in bone density was more marked in MT-null mice than in wild-type mice at the same dose and time points. Histopathology showed dilatation of haversian canals with increased osteoid seams, rounded osteocytes with expanded pericellular space, and expansion of hyperplastic bone marrow into metaphyseal cortical bone. Again, these lesions were more marked in MT-null mice. In conclusion, this study demonstrates that deficiency in MT renders animals more susceptible to Cd-induced bone mass loss and bone injury, and thus indicates that MT plays a protective role in Cd-induced toxicity in bone, as it does in other tissues.

Impaired hearing in X-linked hypophosphataemic (vitamin-D-resistant) osteomalacia

We assessed the hearing of 25 patients with X-linked hypophosphataemic osteomalacia. Twelve patients had subjective hearing loss and 2 had had episodic tinnitus, deafness, and vertigo similar to that in Ménière's disease. On pure-tone audiometry, 19 patients were shown to have sensorineural hearing loss, 3 of whom had evidence of an added conductive element. Tests were done to delineate sensory (cochlear) from neural deafness. Analysis of the stapedius reflex threshold, percentage speech discrimination score, tone decay, and loudness recruitment showed a disturbance of cochlear function. The possible mechanisms responsible for this dysfunction are discussed in the light of pathologic characteristics of familial hypophosphataemia and its extraosseous manifestations. Sensorineural hearing loss, a previously unrecognised complication of familial hypophosphataemic osteomalacia, occurs frequently.

MEPE's diverse effects on mineralization

Matrix extracellular phosphoglycoprotein (MEPE) is an inhibitor of mineralization in situ and in cell cultures where altered expression is associated with oncogenic osteomalacia and hypophosphatemic rickets. The purpose of this study was to determine whether the intact protein or the peptide(s) originating from this protein was responsible for the inhibition. The ability of the intact protein and the acidic, serine- and aspartate-rich MEPE-associated motif (ASARM) peptide to promote or inhibit de novo hydroxyapatite formation and growth of hydroxyapatite seed crystals, in both phosphorylated and dephosphorylated forms, was assessed at room temperature in a dynamic gel diffusion system at 3.5 and 5 days. The most effective nucleator concentration was also examined when associated with fibrillar type I collagen. The phosphorylated intact protein was an effective promoter of mineralization in the gelatin gel diffusion system, while the ASARM peptide was an effective inhibitor. When dephosphorylated both the intact protein and the ASARM peptide had no effect on mineralization. Associated with collagen fibrils, some of the effect of the intact protein was lost. This study demonstrates the importance of posttranslational modification for the site-specific activity of MEPE and its ASARM peptide.