PHEX, FGF23, DMP1 and beyond

Purinergic signaling through the P2Y2 receptor regulates osteocytes' mechanosensitivity

Osteocytes' response to dynamic loading plays a crucial role in regulating the bone mass but quickly becomes saturated such that downstream induction of bone formation plateaus. The underlying mechanisms that downregulate osteocytes' sensitivity and overall response to loading remain unknown. In other cell types, purinergic signaling through the P2Y2 receptor has the potential to downregulate the sensitivity to loading by modifying cell stiffness through actin polymerization and cytoskeleton organization. Herein, we examined the role of P2Y2 activation in regulating osteocytes' mechanotransduction using a P2Y2 knockout cell line alongside conditional knockout mice. Our findings demonstrate that the absence of P2Y2 expression in MLO-Y4 cells prevents actin polymerization while increasing the sensitivity to fluid flow-induced shear stress. Deleting osteocytes' P2Y2 expression in conditional-knockout mice enabled bone formation to increase when increasing the duration of exercise. Overall, P2Y2 activation under loading produces a negative feedback loop, limiting osteocytes' response to continuous loading by shifting the sensitivity to mechanical strain through actin stress fiber formation.

Acute adaptation of renal phosphate transporters in the murine kidney to oral phosphate intake requires multiple signals

AIMS

Dietary inorganic phosphate (Pi) modulates renal Pi reabsorption by regulating the expression of the NaPi-IIa and NaPi-IIc Pi transporters. Here, we aimed to clarify the role of several Pi-regulatory mechanisms including parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23) and inositol hexakisphosphate kinases (IP6-kinases) in the acute regulation of NaPi-IIa and NaPi-IIc.

METHODS

Wildtype (WT) and PTH-deficient mice (PTH-KO) with/without inhibition of FGF23 signalling were gavaged with Pi/saline and examined at 1, 4 and 12 h.

RESULTS

Pi-gavage elevated plasma Pi and decreased plasma Ca²⁺ in both genotypes after 1 h Within 1 h, Pi-gavage decreased NaPi-IIa abundance in WT and PTH-KO mice. NaPi-IIc was downregulated 1 h post-administration in WT and after 4 h in PTH-KO. PTH increased after 1 h in WT animals. After 4 h Pi-gavage, FGF23 increased in both genotypes being higher in the KO group. PTHrp and dopamine were not altered by Pi-gavage. Blocking FGF23 signalling blunted PTH upregulation in WT mice and reduced NaPi-IIa downregulation in PTH-KO mice 4 h after Pi-gavage. Inhibition of IP6-kinases had no effect.

CONCLUSIONS

(1) Acute downregulation of renal Pi transporters in response to Pi intake occurs also in the absence of PTH and FGF23 signalling, (2) when FGF23 signalling is blocked, a partial contribution of PTH is revealed, (3) IP6 kinases, intracellular Pi-sensors in yeast and bacteria, are not involved, and (4) Acute Pi does not alter PTHrp and dopamine. Thus, signals other than PTH, PTHrp, FGF23 and dopamine contribute to renal adaption.

Fibroblast Growth Factor 23 and Cardiovascular Risk in Diabetes Patients-Cardiologists Be Aware

Numerous clinical studies have indicated that elevated FGF23 (fibroblast growth factor 23) levels may be associated with cardiovascular (CV) mortality, especially in patients with chronic kidney disease. The purpose of this study was to examine the hypothesis that FGF23 may be a potent CV risk factor among patients with long-standing type 2 diabetes mellitus (T2DM). Research was performed utilizing patients with T2DM and regular outpatient follow-up care. Baseline characteristics determined by laboratory tests were recorded. Serum FGF23 levels were detected using a sandwich enzyme-linked immunosorbent assay. All patients underwent echocardiograms and 12-lead electrocardiograms. Data records of 102 patients (males: 57%) with a median age of 69 years (interquartile range (IQR) 66.0-74.0) were analyzed. Baseline characteristics indicated that one-third (33%) of patients suffered from ischemic heart disease (IHD), and the median time elapsed since diagnosis with T2DM was 19 years (IQR 14.0-25.0). The hemoglobin A1c, estimated glomerular filtration rate, and FGF23 values were, respectively, as follows: 6.85% (IQR 6.5-7.7), 80 mL/min/1.73 m2 (IQR 70.0-94.0), and 253.0 pg/mL (IQR 218.0-531.0). The study revealed that FGF23 was elevated in all patients, regardless of IHD status. Thus, the role of FGF23 as a CV risk factor should not be overestimated among patients with T2DM and good glycemic control.

FGF23: A Review of Its Role in Mineral Metabolism and Renal and Cardiovascular Disease

FGF23 is a hormone secreted mainly by osteocytes and osteoblasts in bone. Its pivotal role concerns the maintenance of mineral ion homeostasis. It has been confirmed that phosphate and vitamin D metabolisms are related to the effect of FGF23 and its excess or deficiency leads to various hereditary diseases. Multiple studies have shown that FGF23 level increases in the very early stages of chronic kidney disease (CKD), and its concentration may also be highly associated with cardiac complications. The present review is limited to some of the most important aspects of calcium and phosphate metabolism. It discusses the role of FGF23, which is considered an early and sensitive marker for CKD-related bone disease but also as a novel and potent cardiovascular risk factor. Furthermore, this review gives particular attention to the reliability of FGF23 measurement and various confounding factors that may impact on the clinical utility of FGF23. Finally, this review elaborates on the clinical usefulness of FGF23 and evaluates whether FGF23 may be considered a therapeutic target.

Genetic renal disease classification by hormonal axes

The kidneys, which regulate many homeostatic pathways, are also a major endocrinological target organ. Many genetic renal diseases can be classified according to the affected protein along such endocrinological pathways. In this review, we examine the hypothesis that a more severe phenotype is expected as the affected protein is located more distally along such pathways. Thus, the location of a defect along its endocrinological pathway should be taken into consideration, in addition to the mutation type, when assessing genetic renal disease severity.

Idiopathic infantile hypercalcemia: mutations in SLC34A1 and CYP24A1 in two siblings and fathers

Objectives Both CYP24A1 and SLC34A1 gene mutations are responsible for idiopathic infantile hypercalcemia, whereas loss-of-function mutations in CYP24A1 (25-OH-vitamin D-24-hydroxylase) lead to a defect in the inactivation of active 1.25(OH)2D; mutations in SLC34A1 encoding renal sodium phosphate cotransporter NaPi-IIa lead to primary renal phosphate wasting combined with an inappropriate activation of vitamin D. The presence of mutations in both genes has not been reported in the same patient until today. Case presentation Hypercalcemia was incidentally detected when a 13-month-old boy was being examined for urinary tract infection. After 21 months, hypercalcemia was detected in his six-month-old sister. High dose of vitamin D was not given to both siblings. Both of them also had hypophosphatemia and decreased tubular phosphate reabsorption. Intensive hydration, furosemide and oral phosphorus treatment were given. Bilateral medullary nephrocalcinosis was detected in both siblings and their father. Serum Ca and P levels were within normal limits at follow-up in both siblings. Siblings and their parents all carry a homozygous stop codon mutation (p.R466*) in CYP24A1. Interestingly, both siblings and the father also have a heterozygous splice-site mutation (IVS6(+1)G>A) in SLC34A1. The father has nephrocalcinosis. Conclusions A biallelic loss-of-function mutation in the CYP24A1 gene was identified as responsible for hypercalcemia, hypercalciuria and nephrocalcinosis. In addition, a heterozygous mutation in the SLC34A1 gene, although not being the main pathogenic factor, might contribute to the severe phenotype of both patients.

Interaction between serum FGF-23 and PTH in renal phosphate excretion, a case-control study in hypoparathyroid patients

BACKGROUND

phosphate homeostasis is mediated through complex counter regulatory feed-back balance between parathyroid hormone, FGF-23 and 1,25(OH)2D. Both parathyroid hormone and FGF-23 regulate proximal tubular phosphate excretion through signaling on sodium- phosphate cotransporters II_a and II_c. However, the interaction between these hormones on phosphate excretion is not clearly understood. We performed the present study to evaluate whether the existence of sufficient parathyroid hormone is necessary for full phosphaturic function of FGF-23 or not.

METHODS

In this case-control study, 19 patients with hypoparathyroidism and their age- and gender-matched normal population were enrolled. Serum calcium, phosphate, alkaline phosphatase,parathyroid hormone, FGF-23, 25(OH)D, 1,25(OH)2D and Fractional excretion of phosphorous were assessed and compared between the two groups, using SPSS software.

RESULTS

The mean serum calcium and parathyroid hormone level was significantly lower in hypoparathyroid patients in comparison with the control group (P < 0.001 and P < 0.001, respectively). We found high serum level of phosphate and FGF-23 in hypoparathyroid patients compared to the control group (P < 0.001 and P < 0.001, respectively). However, there was no significant difference in Fractional excretion of phosphorous or 1,25OH2D level between the two groups. There was a positive correlation between serum FGF-23 and Fractional excretion of phosphorous just in the normal individuals (P < 0.001, r = 0.79).

CONCLUSIONS

Although the FGF-23 is a main regulator of urinary phosphate excretion but the existence of sufficient parathyroid hormone is necessary for the full phosphaturic effect of FGF-23.

Identification of a novel variant in the PHEX gene using targeted gene panel sequencing in a 24-month-old boy with hypophosphatemic rickets

Familial hypophosphatemic rickets (FHR) is a disorder characterized by phosphate wasting and hypophosphatemia due to defects in renal phosphate transport regulation. There are 4 known inherited forms of FHR that differ in their molecular causes. Very few studies have been conducted that focused on the molecular analysis of FHR in Koreans. Eighteen mutations of the PHEX gene have been identified to this date in Korea. Herein, we report the clinical case of a 24-month-old boy presenting with bowed legs and short stature. The biochemical profile showed hypophosphatemia with decreased tubular reabsorption of phosphate. Several family members were identified with short stature and genu varum. Therefore, he was diagnosed with FHR. To identify the molecular causes of FHR, we performed targeted gene panel sequencing and found a novel hemizygous missense variant, c.1949T&gt;C (p.Leu650Pro), in the PHEX gene. This variant was also detected in the boy's mother who exhibited genu varum and short stature.

Functional analysis of a de novo mutation c.1692 del A of the PHEX gene in a Chinese family with X-linked hypophosphataemic rickets

Objectives

X-linked hypophosphataemic rickets (XLHR) is a disease of impaired bone mineralization characterized by hypophosphataemia caused by renal phosphate wasting. The main clinical manifestations of the disorder are O-shaped legs, X-shaped legs, delayed growth, and bone pain. XLHR is the most common inheritable form of rickets, with an incidence of 1/20 000 in humans. It accounts for approximately 80% of familial cases of hypophosphataemia and serves as the prototype of defective tubular phosphate (PO4³⁺) transport, due to extra renal defects resulting in unregulated FGF23 activity. XLHR is caused by loss-of-function mutations in the PHEX gene. The aim of this research was to identify the genetic defect responsible for familial hypophosphataemic rickets in a four-generation Chinese Han pedigree and to analyze the function of this mutation.

Methods

The genome DNA samples of all members in the pedigree were extracted from whole blood. We sequenced all exons of the PHEX and FGF23 genes, as well as the adjacent splice site sequence with Sanger sequencing. Next, we analyzed the de novo mutation c.1692 del A of the PHEX gene with an online digital service and investigated the mutant PHEX with SWISS-MODEL, immunofluorescence, and protein stability detection.

Results

Through Sanger sequencing, we found a de novo mutation, c.1692 del A, in exon 16 of the PHEX gene in this pedigree. This mutation can make the PHEX protein become unstable and decay rapidly, which results in familial XLHR.

Conclusion

We have found a de novo loss-of-function mutation, c.1692 del A, in exon 16 of the PHEX gene that can cause XLHR.

Cite this article: J. Huang, X. Bao, W. Xia, L. Zhu, J. Zhang, J. Ma, N. Jiang, J. Yang, Q. Chen, T. Jing, J. Liu, D. Ma, G. Xu. Functional analysis of a de novo mutation c.1692 del A of the PHEX gene in a Chinese family with X-linked hypophosphataemic rickets. Bone Joint Res 2019;8:405–413. DOI: 10.1302/2046-3758.88.BJR-2018-0276.R1.

Impaired function of fibroblast growth factor 23 / Klotho protein axis in prediabetes and diabetes mellitus: Promising predictor of cardiovascular risk

The discovery of clear molecular mechanisms of early cardiac and vascular complications in patients with prediabetes and known diabetes mellitus are core element of stratification at risk with predictive model creation further. Previous clinical studies have shown a pivotal role of impaired signaling axis of fibroblast growth factor 23 (FGF23), FGF23 receptor isoforms and its co-factor Klotho protein in cardiovascular (CV) complications in prediabetes and diabetes. Although there were data received in clinical studies, which confirmed a causative role of altered function of FGF-23/Klotho protein axis in manifestation of CV disease in prediabetes and type 2 diabetes mellitus (T2DM), the target therapy of these diseases directing on improvement of metabolic profiles, systemic and adipokine-relating inflammation by beneficial restoring of dysregulation in FGF-23/Klotho protein axis remain to be not fully clear. The aim of the review was to summarize findings regarding the role of impaired FGF-23/Klotho protein axis in developing CV complications in patients with prediabetes and type 2 diabetes mellitus. It has been elucidated that elevated levels of FGF-23 and deficiency of Klotho protein in peripheral blood are predictors of CV disease and CV outcomes in patients with (pre) diabetes, while predictive values of dynamic changes of the concentrations of these biomarkers require to be elucidated in detail in the future.

FGF23: Clinical usefulness and analytical evolution

Fibroblast Growth Factor 23 (FGF23) is a key hormone for the regulation of phosphate homeostasis. Over the past decades, FGF23 was the subject of intense research in the fields of nephrology and the cardiology. It presents a remarkable correlation with well-established biomarkers of cardiovascular disorders in both chronic kidney disease (CKD) and heart failure (HF) patients. The interest of FGF23 lies in its early-onset in the primary course of CKD as well as in the incremental prognosis information it conveys in both CKD and HF. Different types of assays of FGF-23 testing exist, those targeting the intact form (iFGF23), the other one detecting terminal fragments (cFGF23). The issue is still pending which assay suits best for clinical use. Recently, the implementation of this biomarker on multianalyzer platforms, on which other markers of phospho-calcic balance are set up, allows a rapid turn-around-time and a potential financial gain. However, despite the good analytical performances of the automated methods, there is a poor harmonization between assays. The introduction of an international certified reference material should standardize the measurement and improve the harmonization of results from different laboratories. A deeper understanding of physio-pathological mechanisms and processing of FGF-23 should reinforce its clinical indications and might also identify new therapeutic targets for the treatment of CKD and HF.

Hereditary hypophosphatemic rickets with hypercalciuria: pathophysiology, clinical presentation, diagnosis and therapy

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH; OMIM: 241530) is a rare autosomal recessive disorder with an estimated prevalence of 1:250,000 that was originally described by Tieder et al. Individuals with HHRH carry compound-heterozygous or homozygous (comp/hom) loss-of-function mutations in the sodium-phosphate co-transporter NPT2c. These mutations result in the development of urinary phosphate (Pi) wasting and hypophosphatemic rickets, bowing, and short stature, as well as appropriately elevated 1,25(OH)₂D levels, which sets this fibroblast growth factor 23 (FGF23)-independent disorder apart from the more common X-linked hypophosphatemia. The elevated 1,25(OH)₂D levels in turn result in hypercalciuria due to enhanced intestinal calcium absorption and reduced parathyroid hormone (PTH)-dependent calcium-reabsorption in the distal renal tubules, leading to the development of kidney stones and/or nephrocalcinosis in approximately half of the individuals with HHRH. Even heterozygous NPT2c mutations are frequently associated with isolated hypercalciuria (IH), which increases the risk of kidney stones or nephrocalcinosis threefold in affected individuals compared with the general population. Bone disease is generally absent in individuals with IH, in contrast to those with HHRH. Treatment of HHRH and IH consists of monotherapy with oral Pi supplements, while active vitamin D analogs are contraindicated, mainly because the endogenous 1,25(OH)₂D levels are already elevated but also to prevent further worsening of the hypercalciuria. Long-term studies to determine whether oral Pi supplementation alone is sufficient to prevent renal calcifications and bone loss, however, are lacking. It is also unknown how therapy should be monitored, whether secondary hyperparathyroidism can develop, and whether Pi requirements decrease with age, as observed in some FGF23-dependent hypophosphatemic disorders, or whether this can lead to osteoporosis.

Clinical and genetic characteristics of 15 families with hereditary hypophosphatemia: Novel Mutations in PHEX and SLC34A3

BACKGROUND

Hereditary hypophosphatemia is a group of rare renal phosphate wasting disorders. The diagnosis is based on clinical, radiological, and biochemical features, and may require genetic testing to be confirmed.

METHODOLOGY

Clinical features and mutation spectrum were investigated in patients with hereditary hypophosphatemia. Genomic DNA of 23 patients from 15 unrelated families were screened sequentially by PCR-sequencing analysis for mutations in the following genes: PHEX, FGF23, DMP1, ENPP1, CLCN5, SLC34A3 and SLC34A1. CytoScan HD Array was used to identify large deletions.

RESULTS

Genetic evaluation resulted in the identification of an additional asymptomatic but intermittent hypophosphatemic subject. Mutations were detected in 21 patients and an asymptomatic sibling from 13 families (86.6%, 13/15). PHEX mutations were identified in 20 patients from 12 families. Six of them were novel mutations present in 9 patients: c.983_987dupCTACC, c.1586+2T>G, c.1206delA, c.436+1G>T, c.1217G>T, and g.22,215,887-22,395,767del (179880 bp deletion including exon 16-22 and ZNF645). Six previously reported mutations were found in 11 patients. Among 12 different PHEX mutations, 6 were de novo mutations. Patients with de novo PHEX mutations often had delayed diagnosis and significantly shorter in height than those who had inherited PHEX mutations. Novel compound heterozygous mutations in SLC34A3 were found in one patient and his asymptomatic sister: c.1335+2T>A and c.1639_1652del14. No mutation was detected in two families.

CONCLUSIONS

This is the largest familial study on Turkish patients with hereditary hypophosphatemia. PHEX mutations, including various novel and de novo variants, are the most common genetic defect. More attention should be paid to hypophosphatemia by clinicians since some cases remain undiagnosed both during childhood and adulthood.

Physiologic and pathophysiologic roles of extra renal CYP27b1: Case report and review

Although the kidney was initially thought to be the sole organ responsible for the production of 1,25(OH)₂D via the enzyme CYP27b1, it is now appreciated that the expression of CYP27b1 in tissues other than the kidney is wide spread. However, the kidney is the major source for circulating 1,25(OH)₂D. Only in certain granulomatous diseases such as sarcoidosis does the extra renal tissue produce sufficient 1,25(OH)₂D to contribute to the circulating levels, generally associated with hypercalcemia, as illustrated by the case report preceding the review. Therefore the expression of CYP27b1 outside the kidney under normal circumstances begs the question why, and in particular whether the extra renal production of 1,25(OH)₂D has physiologic importance. In this chapter this question will be discussed. First we discuss the sites for extra renal 1,25(OH)₂D production. This is followed by a discussion of the regulation of CYP27b1 expression and activity in extra renal tissues, pointing out that such regulation is tissue specific and different from that of CYP27b1 in the kidney. Finally the physiologic significance of extra renal 1,25(OH)₂D₃ production is examined, with special focus on the role of CYP27b1 in regulation of cellular proliferation and differentiation, hormone secretion, and immune function. At this point the data do not clearly demonstrate an essential role for CYP27b1 expression in any tissue outside the kidney, but several examples pointing in this direction are provided. With the availability of the mouse enabling tissue specific deletion of CYP27b1, the role of extra renal CYP27b1 expression in normal and pathologic states can now be addressed definitively.

Mutational analysis of PHEX, FGF23 and CLCN5 in patients with hypophosphataemic rickets

CONTEXT

Hypophosphataemic rickets (HR) is a group of rare hereditary renal phosphate wasting disorders caused by mutations in PHEX, FGF23, DMP1, ENPP1, CLCN5 or SLC34A3.

OBJECTIVE

To investigate underlying genetic defects in patients with hypophosphataemic rickets.

METHODS

We analysed genomic DNA from nine unrelated families for mutations in the entire coding region of PHEX, FGF23, DMP1, ENPP1, CLCN5 or SLC34A3 by PCR sequencing and copy number analysis.

RESULTS

A total of 14 patients were studied. PHEX mutations were identified in 12 patients from seven families. Five of them were novel mutations present in eight patients: c.154G>T (p.E52*), c.401_402insGCCAAA (p.Q134_K135insPK), c.1600C>T (p.P534S), g.22016715_22056805del (40-kb deletion including promoter and exons 1-3) and c.2242_2243delCT (p.L748 fs*48). Four patients had previously reported mutations: c.1768+1G>A and c.1807G>A (p.W602*). Novel CLCN5 (c.1205G>A, p.W402*) and FGF23 (c.526C>G, p.R176G) mutations were found in two patients from the remaining two families. Many of the mutations were de novo: c.154G>T and c.2242_2243delCT in PHEX and c.526C>G in FGF23. Furthermore, we characterized the breakpoint of the novel PHEX g.22016715_22056805del and the c.2242_2243delCT, which is 6 bp from the stop codon, resulting in a frameshift and extension of the reading frame by 42 amino acids.

CONCLUSIONS

Novel and de novo mutations are frequent and PHEX mutations are still the most common genetic defects in the Turkish population. Gene copy number analysis should be considered in patients with negative results by conventional PCR-based sequencing analysis. The current study further expands the mutation spectrum underlying HR.

FGF23 from bench to bedside

There is a strong association between elevated circulating fibroblast growth factor-23 (FGF23) levels and adverse outcomes in patients with chronic kidney disease (CKD) of all stages. Initially discovered as a regulator of phosphate and vitamin D homeostasis, FGF23 has now been implicated in several pathophysiological mechanisms that may negatively impact the cardiovascular and renal systems. FGF23 is purported to have direct (off-target) effects in the myocardium, as well as canonical effects on FGF receptor/α-klotho receptor complexes in the kidney to activate the renin-angiotensin-aldosterone system, modulate soluble α-klotho levels, and increase sodium retention, to cause left ventricular hypertrophy (LVH). Conversely, FGF23 could be an innocent bystander produced in response to chronic inflammation or other processes associated with CKD that cause LVH and adverse cardiovascular outcomes. Further exploration of these complex mechanisms is needed before modulation of FGF23 can become a legitimate clinical target in CKD.

FGF23-Associated Tumor-Induced Osteomalacia in a Patient With Small Cell Carcinoma: A Case Report and Regulatory Mechanism Study

Tumor-induced osteomalacia (TIO) is typically caused by phosphaturic mesenchymal tumor (PMT) that secretes the phosphaturic hormone, fibroblast growth factor-23 (FGF23), resulting in decreased phosphate reabsorption in kidneys, hypophosphatemia, and finally osteomalacia. Rare cases of malignant tumor manifesting with TIO other than PMT had been reported, although in most of these reports, except one, circulating FGF23 levels were not evaluated and tissue expressing of FGF23 was not confirmed. In this article, we report a case of TIO in a patient with pulmonary small cell carcinoma with liver metastasis. The patient manifested with hypophosphatemia. His circulating level of FGF23 was markedly increased. The expression of FGF23 in tumor cells was confirmed. Furthermore, the regulatory mechanism of FGF23 in this patient was also investigated.

Intestinal Depletion of NaPi-IIb/Slc34a2 in Mice: Renal and Hormonal Adaptation

The Na(+) -dependent phosphate-cotransporter NaPi-IIb (SLC34A2) is widely expressed, with intestine, lung, and testis among the organs with highest levels of mRNA abundance. In mice, the intestinal expression of NaPi-IIb is restricted to the ileum, where the cotransporter localizes specifically at the brush border membrane (BBM) and mediates the active transport of inorganic phosphate (Pi). Constitutive full ablation of NaPi-IIb is embryonically lethal whereas the global but inducible removal of the transporter in young mice leads to intestinal loss of Pi and lung calcifications. Here we report the generation of a constitutive but intestinal-specific NaPi-IIb/Slc34a2-deficient mouse model. Constitutive intestinal ablation of NaPi-IIb results in viable pups with normal growth. Homozygous mice are characterized by fecal wasting of Pi and complete absence of Na/Pi cotransport activity in BBM vesicles (BBMVs) isolated from ileum. In contrast, the urinary excretion of Pi is reduced in these animals. The plasma levels of Pi are similar in wild-type and NaPi-IIb-deficient mice. In females, the reduced phosphaturia associates with higher expression of NaPi-IIa and higher Na/Pi cotransport activity in renal BBMVs, as well as with reduced plasma levels of intact FGF-23. A similar trend is found in males. Thus, NaPi-IIb is the only luminal Na(+) -dependent Pi transporter in the murine ileum and its absence is fully compensated for in adult females by a mechanism involving the bone-kidney axis. The contribution of this mechanism to the adaptive response is less apparent in adult males.

Retinol deprivation partially rescues the skeletal mineralization defects of Phex-deficient Hyp mice

X-linked hypophosphatemic rickets (XLH) is a genetic disorder caused by mutational inactivation of the PHEX gene, encoding a transmembrane endopeptidase expressed in osteoblasts. Since several experiments involving Phex-deficient Hyp mice have demonstrated that an increased expression of Fgf23 in osteoblasts is causative for the renal phosphate loss characteristic of XLH, we performed genome-wide expression analysis to compare differentiated osteoblasts from wildtype and Hyp mice. Here we did not only observe the expected increase of Fgf23 expression in the latter ones, but also a differential expression of genes that are either induced by or involved in retinoic acid signaling, which led us to analyze whether dietary retinol deprivation would influence the phenotype of Hyp mice. Unexpectedly, feeding a retinol-free diet resulted in a partial rescue of the growth plate and bone mineralization defects in 6 weeks old Hyp mice. When we fed the same diet for 24 weeks the amount of non-mineralized bone matrix (osteoid) was reduced by more than 70%, although phosphate homeostasis was unaffected. In contrast, a dietary normalization of serum phosphate levels in Hyp mice reduced the osteoid amount by less than 30%, thereby demonstrating a previously unknown impact of retinol on the cell-autonomous mineralization defect of Phex-deficient osteoblasts.

Fibroblast growth factor 23 and Klotho are present in the growth plate

INTRODUCTION

Regulation of phosphate homeostasis is essential for mineralization and enchondral ossification. Fibroblast growth factor 23 (FGF23) and its obligatory co-receptor Klotho (KL) play a key role in this process by influencing both renal phosphate reabsorption and vitamin D metabolism. In disease, excessive action of FGF23 leads to hypophosphatemic rickets, while its deficiency causes tumoral calcinosis. Although osteocytes and osteoblasts are widely seen as the primary source of FGF23 under physiological conditions, the origin of systemic FGF23 remains controversial. In this study, we investigated the expression of FGF23 and KL in porcine growth plate cartilage, adjacent tissues, and parenchymal tissues.

MATERIALS AND METHODS

Tissue samples were obtained from 4- to 6-week-old piglets. mRNA expression was quantified by real-time PCR and normalized to 18S rRNA. Immunohistochemical staining was performed for FGF23, KL, collagen type X, and FGF receptor 1. Growth plate chondrocyte subpopulations were acquired by collagenase digestion of growth plate explants and subsequent density gradient centrifugation.

RESULTS

We could detect both FGF23 and KL mRNA and protein in growth plate chondrocytes. FGF23 expression was mainly found in hypertrophic and resting chondrocytes. Furthermore, significant expression of both genes was observed in bone, liver, and spleen.

CONCLUSION

These data challenge previous expression analyses, in particular theories of bone as the exclusive source of FGF23. Moreover, significant expression of FGF23 and KL within the growth plate and adjacent tissues imply a potential local role of FGF23 in chondrocyte differentiation and tissue mineralization.

A novel Phex mutation in a new mouse model of hypophosphatemic rickets

X-linked hypophosphatemic rickets (XLH) is a dominantly inherited disease characterized by renal phosphate wasting, aberrant vitamin D metabolism, and defective bone mineralization. It is known that XLH in humans and in certain mouse models is caused by inactivating mutations in PHEX/Phex (phosphate-regulating gene with homologies to endopeptidases on the X chromosome). By a genome-wide N-ethyl-N-nitrosourea (ENU)-induced mutagenesis screen in mice, we identified a dominant mouse mutation that exhibits the classic clinical manifestations of XLH, including growth retardation, skeletal abnormalities (rickets/osteomalacia), hypophosphatemia, and increased serum alkaline phosphatase (ALP) levels. Mapping and sequencing revealed that these mice carry a point mutation in exon 14 of the Phex gene that introduces a stop codon at amino acid 496 of the coding sequence (Phex(Jrt) also published as Phex(K496X) [Ichikawa et al., 2012]). Fgf23 mRNA expression as well as that of osteocalcin, bone sialoprotein, and matrix extracellular phosphoglycoprotein was upregulated in male mutant long bone, but that of sclerostin was unaffected. Although Phex mRNA is expressed in bone from mutant hemizygous male mice (Phex(Jrt)/Y mice), no Phex protein was detected in immunoblots of femoral bone protein. Stromal cultures from mutant bone marrow were indistinguishable from those of wild-type mice with respect to differentiation and mineralization. The ability of Phex(Jrt)/Y osteoblasts to mineralize and the altered expression levels of matrix proteins compared with the well-studied Hyp mice makes it a unique model with which to further explore the clinical manifestations of XLH and its link to FGF23 as well as to evaluate potential new therapeutic strategies.

Biological variability of plasma intact and C-terminal FGF23 measurements

CONTEXT

FGF23 measurement may have a role in the management of patients with chronic kidney disease (CKD).

OBJECTIVE

Our objective was to study the biological variability of plasma intact FGF23 (iFGF23) and C-terminal FGF23 (cFGF23) concentrations.

DESIGN

Plasma samples were taken from 12 healthy adults at multiple time points on 2 consecutive days to assess diurnal variability of FGF23 concentrations. Early-morning fasting and nonfasting samples were also taken over a 6-wk period to estimate components of biological variation. Samples from 170 volunteers were used to define reference intervals. FGF23 concentrations were measured by commercial ELISA. Western blotting was used to analyze FGF23 species from the plasma of healthy adults and patients with predialysis CKD and those undergoing dialysis.

PARTICIPANTS AND SETTING

A total of 180 healthy adults and 18 adults with stage 3-5D CKD participated in this study at a hospital research unit.

MAIN OUTCOME MEASURE

Estimates were made of the biological variability of plasma FGF23 concentrations.

RESULTS

iFGF23, but not cFGF23, showed significant diurnal variation. cFGF23 had a significantly lower intra-individual variation than iFGF23 (8.3 vs. 18.3%) but higher inter-individual variation than iFGF23 (28.9 vs. 19.2%). Fourteen samples would be needed to estimate an individual's homeostatic set point (within 10%) for iFGF23 compared with only three samples for cFGF23. Using Western blotting, C-terminal FGF23 fragments were detected in the plasma of individuals with normal renal function and at all stages of renal disease. The percent cFGF23 was significantly higher in those without renal impairment (P < 0.001) and was positively correlated with plasma phosphate concentration in those with normal renal function.

CONCLUSIONS

The high intra-individual biological variability of iFGF23 may limit its clinical use as a diagnostic or management tool. Risk-related thresholds may be more appropriate for clinical decision making based on cFGF23 measurements than conventional reference intervals. FGF23 cleavage pathways may be an important natural regulatory mechanism for phosphate control.

Amelogenesis imperfecta and other biomineralization defects in Fam20a and Fam20c null mice

The FAM20 family of secreted proteins consists of three members (FAM20A, FAM20B, and FAM20C) recently linked to developmental disorders suggesting roles for FAM20 proteins in modulating biomineralization processes. The authors report here findings in knockout mice having null mutations affecting each of the three FAM20 proteins. Both Fam20a and Fam20c null mice survived to adulthood and showed biomineralization defects. Fam20b (-/-) embryos showed severe stunting and increased mortality at E13.5, although early lethality precluded detailed investigations. Physiologic calcification or biomineralization of extracellular matrices is a normal process in the development and functioning of various tissues (eg, bones and teeth). The lesions that developed in teeth, bones, or blood vessels after functional deletion of either Fam20a or Fam20c support a significant role for their encoded proteins in modulating biomineralization processes. Severe amelogenesis imperfecta (AI) was present in both Fam20a and Fam20c null mice. In addition, Fam20a (-/-) mice developed disseminated calcifications of muscular arteries and intrapulmonary calcifications, similar to those of fetuin-A deficient mice, although they were normocalcemic and normophosphatemic, with normal dentin and bone. Fam20a gene expression was detected in ameloblasts, odontoblasts, and the parathyroid gland, with local and systemic effects suggesting both local and/or systemic effects for FAM20A. In contrast, Fam20c (-/-) mice lacked ectopic calcifications but were severely hypophosphatemic and developed notable lesions in both dentin and bone to accompany the AI. The bone and dentin lesions, plus the marked hypophosphatemia and elevated serum alkaline phosphatase and FGF23 levels, are indicative of autosomal recessive hypophosphatemic rickets/osteomalacia in Fam20c (-/-) mice.

New mouse models for metabolic bone diseases generated by genome-wide ENU mutagenesis

Metabolic bone disorders arise as primary diseases or may be secondary due to a multitude of organ malfunctions. Animal models are required to understand the molecular mechanisms responsible for the imbalances of bone metabolism in disturbed bone mineralization diseases. Here we present the isolation of mutant mouse models for metabolic bone diseases by phenotyping blood parameters that target bone turnover within the large-scale genome-wide Munich ENU Mutagenesis Project. A screening panel of three clinical parameters, also commonly used as biochemical markers in patients with metabolic bone diseases, was chosen. Total alkaline phosphatase activity and total calcium and inorganic phosphate levels in plasma samples of F1 offspring produced from ENU-mutagenized C3HeB/FeJ male mice were measured. Screening of 9,540 mice led to the identification of 257 phenodeviants of which 190 were tested by genetic confirmation crosses. Seventy-one new dominant mutant lines showing alterations of at least one of the biochemical parameters of interest were confirmed. Fifteen mutations among three genes (Phex, Casr, and Alpl) have been identified by positional-candidate gene approaches and one mutation of the Asgr1 gene, which was identified by next-generation sequencing. All new mutant mouse lines are offered as a resource for the scientific community.

FGF23 and syndromes of abnormal renal phosphate handling

Fibroblast growth factor 23 (FGF23) is part of a previously unrecognized hormonal bone-parathyroid-kidney axis, which is modulated by 1,25(OH)(2)-vitamin D (1,25(OH)(2)D), dietary and circulating phosphate and possibly PTH. FGF23 was discovered as the humoral factor in tumors that causes hypophosphatemia and osteomalacia and through the identification of a mutant form of FGF23 that leads to autosomal dominant hypophosphatemic rickets (ADHR), a rare genetic disorder. FGF23 appears to be mainly secreted by osteocytes where its expression is up-regulated by 1,25(OH)(2)D and probably by increased serum phosphate levels. Its synthesis and secretion is reduced through yet unknown mechanisms that involve the phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX), dentin matrix protein 1 (DMP1) and ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1). Consequently, loss-of-function mutations in these genes underlie hypophosphatemic disorders that are either X-linked or autosomal recessive. Impaired O-glycosylation of FGF23 due to the lack of UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyl-transferase 3 (GALNT3) or due to certain homozygous FGF23 mutations results in reduced secretion of intact FGF23 and leads to familial hyperphosphatemic tumoral calcinosis. FGF23 acts through FGF-receptors and the coreceptor Klotho to reduce 1,25(OH)(2)D synthesis in the kidney and probably the synthesis of parathyroid hormone (PTH) by the parathyroid glands. It furthermore synergizes with PTH to increase renal phosphate excretion by reducing expression of the sodium-phosphate cotransporters NaPi-IIa and NaPi-IIc in the proximal tubules. Loss-of-function mutations in these two transporters lead to autosomal recessive Fanconi syndrome or to hereditary hypophosphatemic rickets with hypercalciuria, respectively.

Biomineralization of bone: a fresh view of the roles of non-collagenous proteins

The impact of genetics has dramatically affected our understanding of the functions of non-collagenous proteins. Specifically, mutations and knockouts have defined their cellular spectrum of actions. However, the biochemical mechanisms mediated by non-collagenous proteins in biomineralization remain elusive. It is likely that this understanding will require more focused functional testing at the protein, cell, and tissue level. Although initially viewed as rather redundant and static acidic calcium binding proteins, it is now clear that non-collagenous proteins in mineralizing tissues represent diverse entities capable of forming multiple protein-protein interactions which act in positive and negative ways to regulate the process of bone mineralization. Several new examples from the author's laboratory are provided which illustrate this theme including an apparent activating effect of hydroxyapatite crystals on metalloproteinases. This review emphasizes the view that secreted non-collagenous proteins in mineralizing bone actively participate in the mineralization process and ultimately control where and how much mineral crystal is deposited, as well as determining the quality and biomechanical properties of the mineralized matrix produced.

Three novel mutations in the PHEX gene in Chinese subjects with hypophosphatemic rickets extends genotypic variability

Mutations in the phosphate-regulating endopeptidase homolog, X-linked, gene (PHEX), which encodes a zinc-dependent endopeptidase that is involved in bone mineralization and renal phosphate reabsorption, cause the most common form of hypophosphatemic rickets, X-linked hypophosphatemic rickets (XLH). The distribution of PHEX mutations is extensive, but few mutations have been identified in Chinese with XLH. We extracted genomic DNA and total RNA from leukocytes obtained from nine unrelated Chinese subjects (three males and six females, age range 11-36 years) who were living in Taiwan. The PHEX gene was amplified from DNA by PCR, and the amplicons were directly sequenced. Expression studies were performed by reverse-transcription PCR of leukocyte RNA. Serum levels of FGF23 were significantly greater in the patients than in normal subjects (mean 69.4 ± 18.8 vs. 27.2 ± 8.4 pg/mL, P < 0.005), and eight of the nine patients had elevated levels of FGF23. Germline mutations in the PHEX gene were identified in five of 9 patients, including novel c.1843 delA, donor splice site mutations c.663+2delT and c.1899+2T>A, and two previously reported missense mutations, p.C733Y and p.G579R. These data extend the spectrum of mutations in the PHEX gene in Han Chinese and confirm variability for XLH in Taiwan.

Phosphate sensing

Human phosphate homeostasis is regulated at the level of intestinal absorption of phosphate from the diet, release of phosphate through bone resorption, and renal phosphate excretion, and involves the actions of parathyroid hormone, 1,25-dihydroxy-vitamin D, and fibroblast growth factor 23 to maintain circulating phosphate levels within a narrow normal range, which is essential for numerous cellular functions, for the growth of tissues and for bone mineralization. Prokaryotic and single cellular eukaryotic organisms such as bacteria and yeast "sense" ambient phosphate with a multi-protein complex located in their plasma membrane, which modulates the expression of genes important for phosphate uptake and metabolism (pho pathway). Database searches based on amino acid sequence conservation alone have been unable to identify metazoan orthologs of the bacterial and yeast phosphate sensors. Thus, little is known about how human and other metazoan cells sense inorganic phosphate to regulate the effects of phosphate on cell metabolism ("metabolic" sensing) or to regulate the levels of extracellular phosphate through feedback system(s) ("endocrine" sensing). Whether the "metabolic" and the "endocrine" sensor use the same or different signal transduction cascades is unknown. This article will review the bacterial and yeast phosphate sensors, and then discuss what is currently known about the metabolic and endocrine effects of phosphate in multicellular organisms and human beings.

Age-related stature and linear body segments in children with X-linked hypophosphatemic rickets

Children with X-linked hypophosphatemic rickets (XLH) are prone to severe stunting. A multicenter mixed-longitudinal study was conducted to assess age-related stature, sitting height, arm and leg length in XLH patients on continuous treatment with phosphate and calcitriol. Mean standard deviation scores (SDS) for all body dimensions were markedly reduced and differed significantly among each other at the initial and subsequent evaluations (baseline: stature -2.48 SDS; sitting height -0.99 SDS; arm length -1.81 SDS; leg length -2.90 SDS; each p<0.001). A strong association between stature and leg length (r (2)=0.87, p<0.001) was noted. Leg length SDS decreased progressively during childhood (2-9 years) and adolescence (12-15 years; each p<0.001). Sitting height SDS increased significantly during late childhood, indicating uncoupled growth of the legs and trunk and resulting in an ever increasing sitting height index (i.e. ratio of sitting height to stature; age 2 years 2.0 SDS; age 10 years 3.3 SDS; p<0.001) that was associated with the degree of stunting (r (2)=0.314, p<0.001). Mean serum phosphate levels were positively associated with stature and leg length, but negatively with sitting height index. Based on these results, we can conclude that growth of the legs and trunk is uncoupled in XLH and related to serum phosphate levels.

The emergence of phosphate as a specific signaling molecule in bone and other cell types in mammals

Although considerable advances in our understanding of the mechanisms of phosphate homeostasis and skeleton mineralization have recently been made, little is known about the initial events involving the detection of changes in the phosphate serum concentrations and the subsequent downstream regulation cascade. Recent data has strengthened a long-established hypothesis that a phosphate-sensing mechanism may be present in various organs. Such a phosphate sensor would detect changes in serum or local phosphate concentration and would inform the body, the local environment, or the individual cell. This suggests that phosphate in itself could represent a signal regulating multiple factors necessary for diverse biological processes such as bone or vascular calcification. This review summarizes findings supporting the possibility that phosphate represents a signaling molecule, particularly in bone and cartilage, but also in other tissues. The involvement of various signaling pathways (ERK1/2), transcription factors (Fra-1, Runx2) and phosphate transporters (PiT1, PiT2) is discussed.

Induction of an osteocyte-like phenotype by fibroblast growth factor-2

The purpose of this study was to characterize the molecular phenotype that occurs during the profound morphological shift of cultured osteogenic cells upon treatment with fibroblast growth factor-2 (FGF2). A time course of treatment with FGF2 was performed on an osteoblast cell line, primary bone marrow stromal cells and an osteocyte-like cell line. Morphologic changes were recorded, and gene profiling was carried out by real time PCR. By 8h of FGF2 treatment, there is a striking morphological shift of osteoblast and stromal cells to an elongated dendritic-like morphology that is remindful of osteocytes. In osteoblasts treated with FGF2, this morphologic shift is preceded by an induction of several osteocyte markers, including dentin matrix protein 1 (>20-fold) and E11 (>5-fold). There is a transient increase in the gene expression of sclerostin (3.5-fold) and PHEX (2.5-fold). Sclerostin regulation by FGF2 is complex, as gene expression becomes markedly inhibited by FGF2 at times points after 8h of treatment before rebounding at day 12. Analogous modulation of osteocyte markers is seen in bone marrow stromal cells and MLO-Y4 osteocyte-like cells. In conclusion, this study shows that FGF2 can regulate the transition of osteogenic cells towards the osteocyte lineage, as well as, regulate the expression of critical genes in osteocytes.

Ablation of systemic phosphate-regulating gene fibroblast growth factor 23 (Fgf23) compromises the dentoalveolar complex

Fibroblast growth factor-23 (FGF23) is a hormone that modulates circulating phosphate (P(i)) levels by controlling P(i) reabsorption from the kidneys. When FGF23 levels are deficient, as in tumoral calcinosis patients, hyperphosphatemia ensues. We show here in a murine model that Fgf23 ablation disrupted morphology and protein expression within the dentoalveolar complex. Ectopic matrix formation in pulp chambers, odontoblast layer disruption, narrowing of periodontal ligament space, and alteration of cementum structure were observed in histological and electron microscopy sections. Because serum P(i) levels are dramatically elevated in Fgf23(-/-), we assayed for apoptosis and expression of members from the small integrin-binding ligand, N-linked glycoprotein (SIBLING) family, both of which are sensitive to elevated P(i) in vitro. Unlike X-linked hypophosphatemic (Hyp) and wild-type (WT) specimens, numerous apoptotic osteocytes and osteoblasts were detected in Fgf23(-/-) specimens. Further, in comparison to Hyp and WT samples, decreased bone sialoprotein and elevated dentin matrix protein-1 protein levels were observed in cementum of Fgf23(-/-) mice. Additional dentin-associated proteins, such as dentin sialoprotein and dentin phosphoprotein, exhibited altered localization in both Fgf23(-/-) and Hyp samples. Based on these results, we propose that FGF23 and (P(i)) homeostasis play a significant role in maintenance of the dentoalveolar complex.

Hereditary disorders of renal phosphate wasting

Inherited diseases of renal phosphate handling lead to urinary phosphate wasting and depletion of total body phosphorus stores. Clinical sequelae of inherited disorders that are associated with increased urinary phosphate excretion are deleterious and can lead to abnormal skeletal growth and deformities. This Review describes hereditary disorders of renal phosphate wasting taking into account developments in our understanding of renal phosphate handling from the last decade. The cloning of genes involved in these disorders and further studies on their pathophysiological mechanisms have given important insights in to how phosphatonins, such as FGF-23, regulate renal phosphate reabsorption in health and disease. X-linked dominant hypophosphatemic rickets results from mutation of a metalloprotease (PHEX) that has an unidentified role in FGF-23 degradation. Mutation of an RXXR proteolytic cleavage site in FGF-23 prevents degradation and increases circulating levels of FGF-23 in autosomal dominant hypophosphatemic rickets. FGF-23 acts to remove sodium phosphate co-transporters from the luminal membrane of proximal tubular cells with resultant renal phosphate wasting. Loss of function mutations in genes encoding the transporters NaPi-IIc and NaPi-IIa also result in renal phosphate wasting and rickets.

Long-term clinical outcome and carrier phenotype in autosomal recessive hypophosphatemia caused by a novel DMP1 mutation

Homozygous inactivating mutations in DMP1 (dentin matrix protein 1), the gene encoding a noncollagenous bone matrix protein expressed in osteoblasts and osteocytes, cause autosomal recessive hypophosphatemia (ARHP). Herein we describe a family with ARHP owing to a novel homozygous DMP1 mutation and provide a detailed description of the associated skeletal dysplasia and carrier phenotype. The two adult patients with ARHP, a 78-year-old man and his 66-year-old sister, have suffered from bone pain and lower extremity varus deformities since early childhood. With increasing age, both patients developed severe joint pain, contractures, and complete immobilization of the spine. Radiographs showed short and deformed long bones, significant cranial hyperostosis, enthesopathies, and calcifications of the paraspinal ligaments. Biochemistries were consistent with hypophosphatemia owing to renal phosphate wasting; markers of bone turnover and serum fibroblast growth factor 23 (FGF-23) levels were increased significantly. Nucleotide sequence analysis of DMP1 revealed a novel homozygous mutation at the splice acceptor junction of exon 6 (IVS5-1G > A). Two heterozygous carriers of the mutation also showed mild hypophosphatemia, and bone biopsy in one of these individuals showed focal areas of osteomalacia. In bone, DMP1 expression was absent in the homozygote but normal in the heterozygote, whereas FGF-23 expression was increased in both subjects but higher in the ARHP patient. The clinical and laboratory observations in this family confirm that DMP1 has an important role in normal skeletal development and mineral homeostasis. The skeletal phenotype in ARHP may be significantly more severe than in other forms of hypophosphatemic rickets.

FGF-23: More than a regulator of renal phosphate handling?

Fibroblast growth factor 23 (FGF-23) is likely to be the most important regulator of phosphate homeostasis, which mediates its functions through FGF receptors and the coreceptor Klotho. Besides reducing expression of the sodium-phosphate cotransporters NPT2a and NPT2c in the proximal tubules, FGF-23 inhibits the renal 1α-hydroxylase and stimulates the 24-hydroxylase, and it appears to reduce parathyroid hormone (PTH) secretion in short-term studies. FGF-23 synthesis and secretion by osteocytes and osteoblasts is upregulated through 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] and through an increased dietary phosphate intake. FGF-23 levels are elevated or inappropriately normal in patients with tumor-induced osteomalacia and several inherited hypophosphatemic disorders, but the most significant increases are found in patients with chronic kidney disease (CKD). During the early stages of CKD, increased FGF-23 production enhances urinary phosphate excretion and thus prevents the development of hyperphosphatemia, reduces the circulating levels of 1,25(OH)(2)D(3), and therefore contributes to the development of secondary hyperparathyroidism. In patients with end-stage renal disease (ESRD), FGF-23 levels can be extremely high and were shown to be predictors of bone mineralization, left ventricular hypertrophy, vascular calcification, and mortality. It remains to be determined, however, whether FGF-23 represents simply a sensitive biomarker of an abnormal phosphate homeostasis or has, independent of serum phosphate levels, potentially negative "off-target" effects. Nonetheless, reducing the production and/or the biologic activity of FGF-23 may be an important therapeutic goal for this patient population.

Tumor-induced hypophosphatemic rickets in an adolescent boy--clinical presentation, diagnosis, and histological findings in growth plate and muscle tissue

CONTEXT

The mechanism behind disabling muscle weakness in tumor-induced hypophosphatemic rickets is obscure. Histological investigation of growth plate tissue of patients with tumor-induced osteomalacia has so far not been reported.

PATIENT

A mesenchymal tumor was detected in the left distal fibula by (68)Ga-DOTATOC in a 17-yr-old boy with adolescent onset of severe hypophosphatemic rickets. Disabling muscle weakness improved within days after surgery, and normal mobility was restored within months.

METHODS AND RESULTS

The resected tissue included part of the growth plate allowing immunohistochemical investigation. Positive staining of FGF23 was found in the tumor cells and in hypertrophic chondrocytes, osteoblasts, and osteoclasts of the adjacent growth plate. This distribution matched that found in growth plate tissue of a healthy control. We found positive staining for the somatostatin receptor not only in the tumor but also within the growth plate and adjacent bony tissue in the patient and the healthy control. Muscle tissue provided evidence for a partial defect in respiratory chain complexes I-IV. Biochemical markers were nearly or completely restored to normal 12 months after surgery.

CONCLUSIONS

Hypertrophic growth plate chondrocytes are a target or source of FGF23 in tumor-induced osteomalacia. Low serum phosphate, FGF23, or other factors produced by the tumor may interfere with mitochondrial function.

The bone and the kidney

Renal tubular diseases may present with osteopenia, osteoporosis or osteomalacia, as a result of significant derangements in body electrolytes. In case of insufficient synthesis of calcitriol, as in renal failure, the more complex picture of renal osteodystrophy may develop. Hypothetically, also disturbed renal production of BMP-7 and Klotho could cause bone disease. However, the acknowledgment that osteocytes are capable of producing FGF23, a phosphaturic hormone at the same time modulating renal synthesis of calcitriol, indicates that it is also bone that can influence renal function. Importantly, a feed-back mechanism exists between FGF23 and calcitriol synthesis, while Klotho, produced by the kidney, determines activity and selectivity of FGF23. Identification of human diseases linked to disturbed production of FGF23 and Klotho underlines the importance of this new bone-kidney axis. Kidney and bone communicate reciprocally to regulate the sophisticated machinery responsible for divalent ions homeostasis and for osseous or extraosseous mineralisation processes.

Renal phosphate handling in human--what can we learn from hereditary hypophosphataemias?

BACKGROUND

Renal reabsorption of inorganic phosphate is critical for the maintenance of phosphate homeostasis. The sodium dependent phosphate cotransporters NaPi-IIa and NaPi-IIc have been identified to fulfill this task at the brush border membrane of proximal tubule cells. Various factors including dietary phosphate intake, parathyroid hormone, or the so called phosphatonins such as FGF23 have been shown to regulate activity of these transporters.

DESIGN

This review seeks to give an update on our current knowledge about regulatory mechanisms involved in human renal phosphate reabsorption.

RESULTS

Recently, an increasing number of genes have been identified that are directly associated with inherited phosphate wasting disorders (Klotho, PHEX, DMP1 and NHERF1). Several of these genes are predominantly expressed by osteocytes and osteoclasts in the bone suggesting indispensable signalling pathways between kidneys and the skeleton.

CONCLUSION

In this review, the affected gene products in these inherited hypophosphataemias and their contribution to phosphate homeostasis are discussed.

Forging forward with 10 burning questions on FGF23 in kidney disease

The discovery of fibroblast growth factor 23 (FGF23) as the causal factor in the pathogenesis of rare forms of hypophosphatemic rickets is rapidly reshaping our understanding of disordered mineral metabolism in chronic kidney disease (CKD). Excessive production of FGF23 by osteocytes is an appropriate compensation to help maintain normal phosphorus metabolism in these patients. Beginning in early CKD, progressive increases in levels of FGF23 enhance phosphaturia on a per-nephron basis and inhibit calcitriol production, thereby contributing centrally to the predominant phosphorus phenotype of predialysis kidney disease: normal serum phosphate, increased fractional excretion of phosphate, and calcitriol deficiency. A proliferation of studies linking phosphorus and now FGF23 excess to adverse renal and cardiovascular outcomes in patients with CKD is setting the stage for novel clinical trials that could ultimately bring FGF23 testing into the clinic. Ten burning questions must be addressed to galvanize FGF23 research further in CKD.

Regulation of phosphate homeostasis by PTH, vitamin D, and FGF23

In contrast to the regulation of calcium homeostasis, which has been extensively studied over the past several decades, relatively little is known about the regulation of phosphate homeostasis. Fibroblast growth factor 23 (FGF23) is part of a previously unrecognized hormonal bone-parathyroid-kidney axis, which is modulated by PTH, 1,25(OH)(2)-vitamin D (1,25(OH)(2)D), dietary and serum phosphorus levels. Synthesis and secretion of FGF23 by osteocytes are positively regulated by 1,25(OH)(2)D and serum phosphorus and negatively regulated, through yet unknown mechanisms, by the phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) and by dentin matrix protein 1 (DMP1). In turn, FGF23 inhibits the synthesis of 1,25(OH)(2)D, and it may negatively regulate the secretion of parathyroid hormone (PTH) from the parathyroid glands. However, FGF23 synergizes with PTH to increase renal phosphate excretion by reducing expression of the renal sodium-phosphate cotransporters NaPi-IIa and NaPi-IIc in the proximal tubules. Most insights gained into the regulation of phosphate homeostasis by these factors are derived from human genetic disorders and genetically engineered mice, which are reviewed in this paper.

Growth-plate cartilage in chronic renal failure

Bone growth occurs in the growth-plate cartilage located at the ends of long bones. Changes in the architecture, abnormalities in matrix organization, reduction in protein staining and RNA expression of factors involved in cell signaling have been described in the growth-plate cartilage of nephrectomized animals. These changes can lead to a smaller growth plate associated with decrease in chondrocyte proliferation, delayed hypertrophy, and prolonged initiation of mineralization and vascular invasion. As a result, chronic renal failure can result in stunted body growth and skeletal deformities. Multiple etiologic factors can contribute to impaired bone growth in renal failure, including suboptimal nutrition, metabolic acidosis, and secondary hyperparathyroidism. Recent findings have also shown the tight connection between chondro/osteogenesis, hematopoiesis, and immunogenesis.

Osteocytes and WNT: the Mechanical Control of Bone Formation

Mechanical loading is of pivotal importance in the maintenance of skeletal homeostasis, but the players involved in the transduction of mechanical stimuli to promote bone maintenance have long remained elusive. Osteocytes, the most abundant cells in bone, possess mechanosensing appendices stretching through a system of bone canaliculi. Mechanical stimulation plays an important role in osteocyte survival and hence in the preservation of bone mechanical properties, through the maintenance of bone hydratation. Osteocytes can also control the osteoblastic differentiation of mesenchymal precursors in response to mechanical loading by modulating WNT signaling pathways, essential regulators of cell fate and commitment, through the protein sclerostin. Mutations of Sost, the sclerostin-encoding gene, have dramatic effects on the skeleton, indicating that osteocytes may act as master regulators of bone formation and localized bone remodeling. Moreover, the development of sclerostin inhibitors is opening new possibilities for bone regeneration in orthopedics and the dental field.

Familial tumoral calcinosis: from characterization of a rare phenotype to the pathogenesis of ectopic calcification

Familial tumoral calcinosis (FTC) refers to a heterogeneous group of inherited disorders characterized by the occurrence of cutaneous and subcutaneous calcified masses. Two major forms of the disease are now recognized. Hyperphosphatemic FTC has been shown to result from mutations in three genes: fibroblast growth factor-23 (FGF23), coding for a potent phosphaturic protein, KL encoding Klotho, which serves as a co-receptor for FGF23, and GALNT3, which encodes a glycosyltransferase responsible for FGF23 O-glycosylation; defective function of any one of these three proteins results in hyperphosphatemia and ectopic calcification. The second form of the disease is characterized by absence of metabolic abnormalities, and is, therefore, termed normophosphatemic FTC. This variant was found to be associated with absence of functional SAMD9, a putative tumor suppressor and anti-inflammatory protein. The data gathered through the study of these rare disorders have recently led to the discovery of novel aspects of the pathogenesis of common disorders in humans, underscoring the potential concealed within the study of rare diseases.

Loss-of-function ENPP1 mutations cause both generalized arterial calcification of infancy and autosomal-recessive hypophosphatemic rickets

The analysis of rare genetic disorders affecting phosphate homeostasis led to the identification of several proteins that are essential for the renal regulation of phosphate homeostasis; for example, fibroblast growth factor 23 (FGF23), which inhibits renal phosphate reabsorption and 1,25-dihydroxyvitamin D synthesis. Here, we report presumable loss-of-function mutations in the ENPP1 gene (ectonucleotide pyrophosphatase/phosphodiesterase) in members of four families affected with hypophosphatemic rickets. We provide evidence for the conclusion that ENPP1 is the fourth gene-in addition to PHEX, FGF23, and DMP1-that, if mutated, causes hypophosphatemic rickets resulting from elevated FGF23 levels. Surprisingly, ENPP1 loss-of-function mutations have previously been described in generalized arterial calcification of infancy, suggesting an as yet elusive mechanism that balances arterial calcification with bone mineralization.

Increased bone volume and correction of HYP mouse hypophosphatemia in the Klotho/HYP mouse

Inactivating mutations of PHEX cause X-linked hypophosphatemia and result in increased circulating fibroblast growth factor 23 (FGF23). FGF23 action is dependent upon Klotho, which converts FGF receptor 1 into an FGF23-specific receptor. Disruption of Klotho results in a complex bone phenotype and hyperphosphatemia, the converse phenotype of X-linked hypophosphatemia. We examined effects of disrupting both Klotho and PHEX by creating a double-knockout (Klotho/HYP) mouse. The combined disruption corrected the hypophosphatemia in HYP mice, indicating that Klotho is epistatic to PHEX. FGF23 levels remained elevated in all groups except wild-type, indicating that Klotho is necessary for FGF23-dependent phosphaturic activity. 1,25-Dihydroxyvitamin D levels, reduced in HYP mice, were comparably elevated in Klotho and Klotho/HYP mice, demonstrating that Klotho is necessary for FGF23's effect on vitamin D metabolism. Serum PTH levels were reduced in both Klotho and Klotho/HYP mice. Moreover, the Klotho null phenotype persisted in Klotho/HYP, maintaining the runty phenotype and decreased life span of Klotho null mice. Notably, microcomputed tomography analysis demonstrated greater trabecular bone volume fraction in Klotho/HYP mice than that in all other groups (Klotho/HYP, 56.2 +/- 6.3%; Klotho, 32.5 +/- 10.3%; HYP, 8.6 +/- 7.7%; and wild type, 21.4 +/- 3.4%; P < 0.004). Histomorphometric analysis confirmed the markedly increased trabecular bone density in Klotho/HYP mice and the well-established increase in osteoid volume in HYP mice. These observations suggest that with addition of Klotho loss of function, the overabundant osteoid typically produced in HYP mice (but fails to mineralize) is produced and mineralized in the double knockout, resulting in markedly enhanced trabecular bone density.