Case Report: Unexplained Mild Hypophosphatemia and Very High Serum FGF23 Concentrations

Fibroblast growth factor (FGF)23 is one of the major regulators of phosphate homeostasis. Hypophosphatemia can lead to muscle weakness, fatigue, and osteomalacia. In the setting of hypophosphatemia, serum FGF23 can be measured to differentiate between FGF23-mediated and non-FGF23-mediated renal phosphate wasting. C-terminal FGF23 (cFGF23) assays detect both cFGF23 and intact FGF23 (iFGF23). Circulating FGF23 is regulated by 1.25-dihydroxy-vitamin D, parathyroid hormone (PTH), serum phosphate, and serum calcium but also by, for example, iron status, inflammation, erythropoietin, and hypoxia-inducible-factor-1-α. We present the case of a 48-year-old woman with unexplained mild hypophosphatemia, very high cFGF23, and normal iFGF23. The patient proved to have an iron deficiency. Iron deficiency alters the iFGF23-to-cFGF23 ratio. After initiation of iron treatment, cFGF23 strongly decreased. This case report illustrates the limitation of cFGF23 assays and urges clinicians to be aware that cFGF23 concentrations do not necessarily reflect iFGF23 concentrations and that alternative causes for its elevation should be considered (eg, iron deficiency). © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Rare and Common Variants in GALNT3 May Affect Bone Mass Independently of Phosphate Metabolism

Anabolic treatment options for osteoporosis remain limited. One approach to discovering novel anabolic drug targets is to identify genetic causes of extreme high bone mass (HBM). We investigated a pedigree with unexplained HBM within the UK HBM study, a national cohort of probands with HBM and their relatives. Whole exome sequencing (WES) in a family with HBM identified a rare heterozygous missense variant (NM_004482.4:c.1657C > T, p.Arg553Trp) in GALNT3, segregating appropriately. Interrogation of data from the UK HBM study and the Anglo-Australasian Osteoporosis Genetics Consortium (AOGC) revealed an unrelated individual with HBM with another rare heterozygous variant (NM_004482.4:c.831 T > A, p.Asp277Glu) within the same gene. In silico protein modeling predicted that p.Arg553Trp would disrupt salt-bridge interactions, causing instability of GALNT3, and that p.Asp277Glu would disrupt manganese binding and consequently GALNT3 catalytic function. Bi-allelic loss-of-function GALNT3 mutations alter FGF23 metabolism, resulting in hyperphosphatemia and causing familial tumoral calcinosis (FTC). However, bone mineral density (BMD) in FTC cases, when reported, has been either normal or low. Common variants in the GALNT3 locus show genome-wide significant associations with lumbar, femoral neck, and total body BMD. However, no significant associations with BMD are observed at loci coding for FGF23, its receptor FGFR1, or coreceptor klotho. Mendelian randomization analysis, using expression quantitative trait loci (eQTL) data from primary human osteoblasts and genome-wide association studies data from UK Biobank, suggested increased expression of GALNT3 reduces total body, lumbar spine, and femoral neck BMD but has no effect on phosphate concentrations. In conclusion, rare heterozygous loss-of-function variants in GALNT3 may cause HBM without altering phosphate concentration. These findings suggest that GALNT3 may affect BMD through pathways other than FGF23 regulation, the identification of which may yield novel anabolic drug targets for osteoporosis. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Renal Contributions to Age-Related Changes in Mineral Metabolism

Aging results in a general decline in function in most systems. This is particularly true with respect to the skeleton and renal systems, impacting mineral homeostasis. Calcium and phosphate regulation requires tight coordination among the intestine, bone, parathyroid gland, and kidney. The role of the intestine is to absorb calcium and phosphate from the diet. The bone stores or releases calcium and phosphate depending on the body's needs. In response to low plasma ionized calcium concentration, the parathyroid gland produces parathyroid hormone, which modulates bone turnover. The kidney reabsorbs or excretes the minerals and serves as the final regulator of plasma concentration. Many hormones are involved in this process in addition to parathyroid hormone, including fibroblast growth factor 23 produced by the bone and calcitriol synthesized by the kidney. Sclerostin, calcitonin, osteoprotegerin, and receptor activator of nuclear factor‐κB ligand also contribute to tissue‐specific regulation. Changes in the function of organs due to aging or disease can perturb this balance. During aging, the intestine cannot absorb calcium efficiently due to decreased expression of key proteins. In the bone, the balance between bone formation and bone resorption tends toward the latter in older individuals. The kidney may not filter blood as efficiently in the later decades of life, and the expression of certain proteins necessary for mineral homeostasis declines with age. These changes often lead to dysregulation of organismal mineral homeostasis. This review will focus on how mineral homeostasis is impacted by aging with a particular emphasis on the kidney's role in this process. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Long-Term Effects of Sglt2 Deletion on Bone and Mineral Metabolism in Mice

Sodium‐glucose cotransporter 2 (SGLT2) inhibitors improve kidney and cardiovascular outcomes in patients with type 2 diabetes mellitus (T2DM). However, bone fragility has emerged as a side effect in some but not in all human studies. Because use of SGLT2 inhibitors in humans affects mineral metabolism, we investigated the long‐term effects of genetic loss of Sglt2 function on bone and mineral metabolism in mice. Slc5a2 nonsense mutation in Sweet Pee (SP) mice results in total loss of Sglt2 function. We collected urine, serum, and bone samples from 15‐week‐old and 25‐week‐old wild‐type (WT) and SP mice fasted from food overnight. We measured parameters of renal function and mineral metabolism and we assessed bone growth, microarchitecture, and mineralization. As expected, 15‐week‐old and 25‐week‐old SP mice showed increased glucosuria, and normal kidney function compared to age‐matched WT mice. At 15 weeks, SP mice did not show alterations in mineral metabolism parameters. At 25 weeks, SP mice showed reduced fasting 24‐hour urinary calcium excretion and increased fractional excretion of phosphate, but normal serum calcium and phosphate, parathyroid hormone (PTH), vitamin D (1,25(OH)₂D), and fibroblast growth factor (FGF23) levels. At 25 weeks, but not at 15 weeks, SP mice showed reduced body weight compared to WT. This was associated with reduced femur length at 25 weeks, suggesting impaired skeletal growth. SP mice did not show trabecular or cortical bone microarchitectural modifications but showed reduced cortical bone mineral density compared to WT mice at 25 weeks. These results suggest that loss of Sglt2 function in mice in the absence of T2DM does not alter regulatory hormones FGF23, PTH, and 1,25(OH)₂D, but may contribute to bone fragility over the long term. Future studies are required to determine how loss of Sglt2 function impacts bone fragility in T2DM. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The CKD-MBD Syndrome: Hysteresis in PTH Involvement and PTH Administration for Its Management

Chronic kidney disease (CKD) disturbs mineral homeostasis, leading to mineral and bone disorders (MBD). CKD-MBD is a significant problem and currently available treatment options have important limitations. Phosphate retention is thought to be the initial cause of CKD-MBD but serum phosphate remains normal until the late stages of CKD, due to elevated levels of the phosphaturic hormone fibroblast growth factor-23 (FGF-23), and parathyroid hormone (PTH). Reduction of 1,25-dihydroxy-vitamin D (1,25[OH]₂ D) concentration is the next event in the adaptive response of the homeostatic system. We argue, and provide the rationale, that calcium retention which takes place concurrently with phosphate retention, could be the reason behind the hysteresis in the response of PTH. If indeed this is the case, intermittent administration of PTH in early CKD could prevent the hysteresis, which arguably leads to the development of secondary hyperparathyroidism, and provide the platform for an effective management of CKD-MBD. © 2020 American Society for Bone and Mineral Research (ASBMR).

X-Linked Hypophosphatemia: Uniquely Mild Disease Associated With PHEX 3'-UTR Mutation c.*231A>G (A Retrospective Case-Control Study)

X-linked hypophosphatemia (XLH), the most prevalent heritable renal phosphate (Pi) wasting disorder, is caused by deactivating mutations of PHEX. Consequently, circulating phosphatonin FGF23 becomes elevated and hypophosphatemia in affected children leads to rickets with skeletal deformity and reduced linear growth while affected adults suffer from osteomalacia and forms of ectopic mineralization. In 2015, we reported uniquely mild XLH in six children and four of their mothers carrying the non-coding PHEX 3'-UTR mutation c.*231A>G. Herein, we characterize this mild XLH variant by comparing its features in 30 individuals to 30 age- and sex-matched patients with XLH but without the 3'-UTR mutation. The "UTR" and "XLH" groups, both comprising 17 children (2 to 17 years, 3 girls) and 13 adults (23 to 63 years, 10 women), had mean ages of 23 years. Only 43% of the UTR group versus 90% of the XLH group had received medical treatment for their disorder, including 0% versus 85% of the females, respectively (ps < .0001). The UTR group was taller: mean ± SD height Z-score (HZ) -1.0 ± 1.0 versus -2.0 ± 1.4 (p = .0034), with significantly greater height for females (-0.9 ± 0.7 versus -2.3 ± 1.4; p = .0050) but not males (-1.2 ± 1.1 versus -1.9 ± 1.5; p = .1541), respectively. Mean ± SD "arm span Z-score" (AZ) did not differ between the UTR -0.8 ± 1.3 versus XLH -1.3 ± 1.8 groups (p = .2269). Consequently, the UTR group was more proportionate with a mean ∆Z (AZ - HZ) of 0.1 ± 0.6 versus 0.7 ± 1.0 (p = .0158), respectively. Compared to the XLH group, the UTR group had significantly higher fasting serum Pi and renal tubular threshold maximum for phosphorus per glomerular filtration rate (TmP/GFR) (ps ≤ .0060), serum FGF23 concentrations within the reference range (p = .0068), and similar serum alkaline phosphatase levels (p = .6513). UTR lumbar spine bone mineral density Z-score was higher (p = .0343). Thus, the 3'-UTR variant of XLH is distinctly mild, especially in girls and women, posing challenges for its recognition and management. © 2020 American Society for Bone and Mineral Research.

Treatment of Human Immunodeficiency Virus Infection With Tenofovir Disoproxil Fumarate-Containing Antiretrovirals Maintains Low Bone Formation Rate, But Increases Osteoid Volume on Bone Histomorphometry

Bone mineral density (BMD) loss is a known complication of human immunodeficiency virus (HIV) infection and its treatment, particularly with tenofovir disoproxil fumarate (TDF)-containing antiretroviral regimens. Although renal proximal tubular dysfunction and phosphaturia is common with TDF, it is unknown whether BMD loss results from inadequate mineralization. We evaluated change in BMD by dual-energy X-ray absorptiometry (DXA) and bone histomorphometry by tetracycline double-labeled transiliac crest biopsies in young men living with HIV before (n = 20) and 12 months after (n = 16) initiating TDF/lamivudine/efavirenz. We examined relationships between calciotropic hormones, urinary phosphate excretion, pro-inflammatory and pro-resorptive cytokines, and bone remodeling-related proteins with changes in BMD and histomorphometry. Mean age was 29.6 ± 5.5 years, with mean CD4 + T cell count of 473 ± 196 cells/mm³ . At baseline, decreased bone formation rate and increased mineralization lag time were identified in 16 (80%) and 12 (60%) patients, respectively. After 12 months, we detected a 2% to 3% decrease in lumbar spine and hip BMD by DXA. By histomorphometry, we observed no change in bone volume/total volume (BV/TV) and trabecular parameters, but rather, increases in cortical thickness, osteoid volume, and osteoblast and osteoclast surfaces. We did not observe significant worsening of renal phosphate excretion or mineralization parameters. Increases in PTH correlated with decreased BMD but not histomorphometric parameters. Overall, these data suggest abnormalities in bone formation and mineralization occur with HIV infection and are evident at early stages. With TDF-containing antiretroviral therapy (ART), there is an increase in bone remodeling, reflected by increased osteoblast and osteoclast surfaces, but a persistence in mineralization defect, resulting in increased osteoid volume. © 2019 American Society for Bone and Mineral Research.

Acute Tissue Mineral Deposition in Response to a Phosphate Pulse in Experimental CKD

Pathogenic accumulation of calcium (Ca) and phosphate (PO₄ ) in vasculature is a sentinel of advancing cardiovascular disease in chronic kidney disease (CKD). This study sought to characterize acute distribution patterns of radiolabeled ³³ PO₄ and ⁴⁵ Ca in cardiovascular tissues of rats with CKD (0.25% dietary adenine). The disposition of ³³ PO₄ and ⁴⁵ Ca was assessed in blood and 36 tissues after a 10-minute intravenous infusion of one of the following: (i) PO₄  pulse + tracer ³³ PO₄ ; (ii) PO₄  pulse + tracer ⁴⁵ Ca; or (iii) saline + tracer ⁴⁵ Ca in CKD and non-CKD animals. After the infusion, ³³ PO₄ in blood was elevated (2.3× at 10 minutes, 3.5× at 30 minutes, p < 0.05) in CKD compared with non-CKD. In contrast, there was no difference in clearance of ⁴⁵ Ca from the blood. Compared with controls, CKD rats had a markedly increased ³³ PO₄ incorporation in several tissues (skeletal muscle, 7.8×; heart, 5.5×), but accrual was most pronounced in the vasculature (24.8×). There was a significant, but smaller, increase in ⁴⁵ Ca accrual in the vasculature of CKD rats (1.25×), particularly in the calcified rat, in response to the acute phosphate load. Based on the pattern of tissue uptake of ³³ PO₄ and ⁴⁵ Ca, this study revealed that an increase in circulating PO₄ is an important stimulus for the accumulation of these minerals in vascular tissue in CKD. This response is further enhanced when vascular calcification is also present. The finding of enhanced vascular mineral deposition in response to an acute PO₄  pulse provides evidence of significant tissue-specific susceptibility to calcification. © 2018 American Society for Bone and Mineral Research.

Alkaline Phosphatase: Discovery and Naming of Our Favorite Enzyme

Alkaline phosphatase can be considered "our favorite enzyme" for reasons apparent to those who diagnose and treat metabolic bone diseases or who study skeletal biology. Few might know, however, that alkaline phosphatase likely represents the most frequently assayed enzyme in all of medicine. Elevated activity in the circulation is universally recognized as a marker for skeletal or hepatobiliary disease. Nevertheless, the assay conditions in many ways are nonphysiological. The term alkaline phosphatase emerged when it became necessary to distinguish "bone phosphatase" from the phosphatase in the prostate that features an acidic pH optimum. Beginning in 1948, studies of the inborn-error-of-metabolism hypophosphatasia would identify the natural substrates and establish the physiological role of alkaline phosphatase, including in biomineralization. Here, we recount the discovery in 1923 and then eventual naming of this enzyme that remains paramount in our field. © 2017 American Society for Bone and Mineral Research.

Therapeutic Effects of FGF23 c-tail Fc in a Murine Preclinical Model of X-Linked Hypophosphatemia Via the Selective Modulation of Phosphate Reabsorption

Fibroblast growth factor 23 (FGF23) is the causative factor of X-linked hypophosphatemia (XLH), a genetic disorder effecting 1:20,000 that is characterized by excessive phosphate excretion, elevated FGF23 levels and a rickets/osteomalacia phenotype. FGF23 inhibits phosphate reabsorption and suppresses 1α,25-dihydroxyvitamin D (1,25D) biosynthesis, analytes that differentially contribute to bone integrity and deleterious soft-tissue mineralization. As inhibition of ligand broadly modulates downstream targets, balancing efficacy and unwanted toxicity is difficult when targeting the FGF23 pathway. We demonstrate that a FGF23 c-tail-Fc fusion molecule selectively modulates the phosphate pathway in vivo by competitive antagonism of FGF23 binding to the FGFR/α klotho receptor complex. Repeated injection of FGF23 c-tail Fc in Hyp mice, a preclinical model of XLH, increases cell surface abundance of kidney NaPi transporters, normalizes phosphate excretion, and significantly improves bone architecture in the absence of soft-tissue mineralization. Repeated injection does not modulate either 1,25D or calcium in a physiologically relevant manner in either a wild-type or disease setting. These data suggest that bone integrity can be improved in models of XLH via the exclusive modulation of phosphate. We posit that the selective modulation of the phosphate pathway will increase the window between efficacy and safety risks, allowing increased efficacy to be achieved in the treatment of this chronic disease. © 2017 American Society for Bone and Mineral Research.

Conditional Deletion of Murine Fgf23: Interruption of the Normal Skeletal Responses to Phosphate Challenge and Rescue of Genetic Hypophosphatemia

The transgenic and knockout (KO) animals involving Fgf23 have been highly informative in defining novel aspects of mineral metabolism, but are limited by shortened lifespan, inability of spatial/temporal FGF23 control, and infertility of the global KO. To more finely test the role of systemic and genetic influences in FGF23 production, a mouse was developed that carried a floxed ("f")-Fgf23 allele (exon 2 floxed) which demonstrated in vivo recombination when bred to global-Cre transgenic mice (eIIa-cre). Mice homozygous for the recombined allele ("Δ") had undetectable serum intact FGF23, elevated serum phosphate (p < 0.05), and increased kidney Cyp27b1 mRNA (p < 0.05), similar to global Fgf23-KO mice. To isolate cellular FGF23 responses during phosphate challenge, Fgf23(Δ/f) mice were mated with early osteoblast type Iα1 collagen 2.3-kb promoter-cre mice (Col2.3-cre) and the late osteoblast/early osteocyte Dentin matrix protein-1-cre (Dmp1-cre). Fgf23(Δ/f) /Col2.3-cre(+) and Fgf23(Δ/f) /Dmp1-cre(+) exhibited reduced baseline serum intact FGF23 versus controls. After challenge with high-phosphate diet Cre(-) mice had 2.1-fold to 2.5-fold increased serum FGF23 (p < 0.01), but Col2.3-cre(+) mice had no significant increase, and Dmp1-cre(+) mice had only a 37% increase (p < 0.01) despite prevailing hyperphosphatemia in both models. The Fgf23(Δ/f) /Col2.3-cre was bred onto the Hyp (murine X-linked hypophosphatemia [XLH] model) genetic background to test the contribution of osteoblasts and osteocytes to elevated FGF23 and Hyp disease phenotypes. Whereas Hyp mice maintained inappropriately elevated FGF23 considering their marked hypophosphatemia, Hyp/Fgf23(Δ/f) /Col2.3-cre(+) mice had serum FGF23 <4% of Hyp (p < 0.01), and this targeted restriction normalized serum phosphorus and ricketic bone disease. In summary, deleting FGF23 within early osteoblasts and osteocytes demonstrated that both cell types contribute to baseline circulating FGF23 concentrations, and that targeting osteoblasts/osteocytes for FGF23 production can modify systemic responses to changes in serum phosphate concentrations and rescue the Hyp genetic syndrome. © 2016 American Society for Bone and Mineral Research.

FGFR Inhibitor Ameliorates Hypophosphatemia and Impaired Engrailed-1/Wnt Signaling in FGF2 High Molecular Weight Isoform Transgenic Mice

High molecular weight FGF2 transgenic (HMWTg) mouse phenocopies the Hyp mouse, homolog of human X-linked hypophosphatemic rickets with hypophosphatemis, and abnormal FGF23, FGFR, Klotho signaling in kidney. Since abnormal Wnt signaling was reported in Hyp mice we assessed whether Wnt signaling was impaired in HMWTg kidneys and the effect of blocking FGF receptor (FGFR) signaling. Bone mineral density and bone mineral content in female HMWTg mice were significantly reduced. HMWTg mice were gavaged with FGFR inhibitor NVP-BGJ398, or vehicle and were euthanized 24 h post treatment. Serum phosphate was significantly reduced and urine phosphate was significantly increased in HMWTg and was rescued by NVP-BGJ398. Analysis of kidneys revealed a significant reduction in Npt2a mRNA in HMWTg that was significantly increased by NVP-BGJ398. Increased FGFR1, KLOTHO, P-ERK1/2, and decreased NPT2a protein in HMWTg were rescued by NVP-BGJ398. Wnt inhibitor Engrailed-1 mRNA and protein was increased in HMWTg and was decreased by BGJ398. Akt mRNA and protein was decreased in HMWTg and was increased by NVP-BGJ398. The active form of glycogen synthase 3 beta (pGSK3-β) and phosphor-β-catenin were increased in HMWTg and were both decreased by NVP-BGJ398 while decreased active-β-catenin in HMWTg was increased by NVP-BGJ398. We conclude that FGFR blockade rescued hypophosphatemia by regulating FGF and WNT signaling in HMWTg kidneys. J. Cell. Biochem. 117: 1991-2000, 2016. © 2016 Wiley Periodicals, Inc.

Xeno-Klotho Inhibits Parathyroid Hormone Signaling

Although fibroblast growth factor (FGF) 23 was recently identified as a phosphatonin that influences vitamin D metabolism, the underlying signaling mechanisms remain unclear. FGF23 elevates the renal levels of membrane-associated klotho as well as soluble klotho. Klotho is expressed on distal tubules. Upon enzymatic cleavage, soluble klotho is released into the renal interstitial space and then into the systemic circulation. The expression of 25-hydroxyvitamin D3 1α-hydroxylase (1-OH) on proximal tubular cells is controlled by parathyroid hormone (PTH). Klotho binds to various membrane proteins to alter their function. Here, the interaction between the PTH receptor and klotho was studied using various approaches, including immunoprecipitation, in vitro cell culture, and in vivo animal experiments. Immunoprecipitation studies demonstrate, for the first time, that recombinant human klotho protein interacts with human PTH receptors to inhibit the binding of human PTH. Furthermore, when applied to human proximal tubular cells, recombinant human klotho suppresses PTH-stimulated generation of inositol trisphosphate in vitro. Moreover, PTH-induced increase of cyclic AMP secretion and 1α,25-dihydroxyvitamin D3 (1,25VD) was attenuated by recombinant human klotho in vivo. In addition, recombinant human klotho inhibits the expression of 1-OH by PTH both in vitro and in vivo. These results suggest that free klotho mediates the FGF23-induced inhibition of 1,25VD synthesis.

Osteogenic cell cultures cannot utilize exogenous sources of synthetic polyphosphate for mineralization

Phosphate is critical for mineralization and deficiencies in the regulation of free phosphate lead to disease. Inorganic polyphosphates (polyPs) may represent a physiological source of phosphate because they can be hydrolyzed by biological phosphatases. To investigate whether exogenous polyP could be utilized for mineral formation, mineralization was evaluated in two osteogenic cell lines, Saos-2 and MC3T3, expressing different levels of tissue non-specific alkaline phosphatase (tnALP). The role of tnALP was further explored by lentiviral-mediated overexpression in MC3T3 cells. When cells were cultured in the presence of three different phosphate sources, there was a strong mineralization response with β-glycerophosphate (βGP) and orthophosphate (Pi) but none of the cultures sustained mineralization in the presence of polyP (neither chain length 17-Pi nor 42-Pi). Even in the presence of mineralizing levels of phosphate, low concentrations of polyP (50 μM) were sufficient to inhibit mineral formation. Energy-dispersive X-ray spectroscopy confirmed the presence of apatite-like mineral deposits in MC3T3 cultures supplemented with βGP, but not in those with polyP. While von Kossa staining was consistent with the presence or absence of mineral, an unusual Alizarin staining was obtained in polyP-treated MC3T3 cultures. This staining pattern combined with low Ca:P ratios suggests the persistence of Ca-polyP complexes, even with high residual ALP activity. In conclusion, under standard culture conditions, exogenous polyP does not promote mineral deposition. This is not due to a lack of active ALP, and unless conditions that favor significant processing of polyP are achieved, its mineral inhibitory capacity predominates.

Increased osteopontin contributes to inhibition of bone mineralization in FGF23-deficient mice

Excessive FGF23 has been identified as a pivotal phosphaturic factor leading to renal phosphate-wasting and the subsequent development of rickets and osteomalacia. In contrast, loss of FGF23 in mice (Fgf23(-/-) ) leads to high serum phosphate, calcium, and 1,25-vitamin D levels, resulting in early lethality attributable to severe ectopic soft-tissue calcifications and organ failure. Paradoxically, Fgf23(-/-) mice exhibit a severe defect in skeletal mineralization despite high levels of systemic mineral ions and abundant ectopic mineralization, an abnormality that remains largely unexplained. Through use of in situ hybridization, immunohistochemistry, and immunogold labeling coupled with electron microscopy of bone samples, we discovered that expression and accumulation of osteopontin (Opn/OPN) was markedly increased in Fgf23(-/-) mice. These results were confirmed by qPCR analyses of Fgf23(-/-) bones and ELISA measurements of serum OPN. To investigate whether elevated OPN levels were contributing to the bone mineralization defect in Fgf23(-/-) mice, we generated Fgf23(-/-) /Opn(-/-) double-knockout mice (DKO). Biochemical analyses showed that the hypercalcemia and hyperphosphatemia observed in Fgf23(-/-) mice remained unchanged in DKO mice; however, micro-computed tomography (µCT) and histomorphometric analyses showed a significant improvement in total mineralized bone volume. The severe osteoidosis was markedly reduced and a normal mineral apposition rate was present in DKO mice, indicating that increased OPN levels in Fgf23(-/-) mice are at least in part responsible for the osteomalacia. Moreover, the increased OPN levels were significantly decreased upon lowering serum phosphate by feeding a low-phosphate diet or after deletion of NaPi2a, indicating that phosphate levels contribute in part to the high OPN levels in Fgf23(-/-) mice. In summary, our results suggest that increased OPN is an important pathogenic factor mediating the mineralization defect and the alterations in bone metabolism observed in Fgf23(-/-) bones.

Neonatal iron deficiency causes abnormal phosphate metabolism by elevating FGF23 in normal and ADHR mice

Fibroblast growth factor 23 (FGF23) gain of function mutations can lead to autosomal dominant hypophosphatemic rickets (ADHR) disease onset at birth, or delayed onset following puberty or pregnancy. We previously demonstrated that the combination of iron deficiency and a knock-in R176Q FGF23 mutation in mature mice induced FGF23 expression and hypophosphatemia that paralleled the late-onset ADHR phenotype. Because anemia in pregnancy and in premature infants is common, the goal of this study was to test whether iron deficiency alters phosphate handling in neonatal life. Wild-type (WT) and ADHR female breeder mice were provided control or iron-deficient diets during pregnancy and nursing. Iron-deficient breeders were also made iron replete. Iron-deficient WT and ADHR pups were hypophosphatemic, with ADHR pups having significantly lower serum phosphate (p < 0.01) and widened growth plates. Both genotypes increased bone FGF23 mRNA (>50 fold; p < 0.01). WT and ADHR pups receiving low iron had elevated intact serum FGF23; ADHR mice were affected to a greater degree (p < 0.01). Iron-deficient mice also showed increased Cyp24a1 and reduced Cyp27b1, and low serum 1,25-dihydroxyvitamin D (1,25D). Iron repletion normalized most abnormalities. Because iron deficiency can induce tissue hypoxia, oxygen deprivation was tested as a regulator of FGF23, and was shown to stimulate FGF23 mRNA in vitro and serum C-terminal FGF23 in normal rats in vivo. These studies demonstrate that FGF23 is modulated by iron status in young WT and ADHR mice and that hypoxia independently controls FGF23 expression in situations of normal iron. Therefore, disturbed iron and oxygen metabolism in neonatal life may have important effects on skeletal function and structure through FGF23 activity on phosphate regulation.

Effects of iron deficiency anemia and its treatment on fibroblast growth factor 23 and phosphate homeostasis in women

Fibroblast growth factor 23 (FGF23) is an osteocyte-derived hormone that regulates phosphate and vitamin D homeostasis. Through unknown mechanisms, certain intravenous iron preparations induce acute, reversible increases in circulating FGF23 levels that lower serum phosphate in association with inappropriately low levels of calcitriol, similar to genetic diseases of primary FGF23 excess. In contrast, studies in wild-type mice suggest that iron deficiency stimulates fgf23 transcription but does not result in hypophosphatemia because FGF23 is cleaved within osteocytes by an unknown catabolic system. We tested the association of iron deficiency anemia with C-terminal FGF23 (cFGF23) and intact FGF23 (iFGF23) levels in 55 women with a history of heavy uterine bleeding, and assessed the longitudinal biochemical response over 35 days to equivalent doses of randomly-assigned, intravenous elemental iron in the form of ferric carboxymaltose (FCM) or iron dextran. Iron deficiency was associated with markedly elevated cFGF23 (807.8 ± 123.9 relative units [RU]/mL) but normal iFGF23 (28.5 ± 1.1 pg/mL) levels at baseline. Within 24 hours of iron administration, cFGF23 levels fell by approximately 80% in both groups. In contrast, iFGF23 transiently increased in the FCM group alone, and was followed by a transient, asymptomatic reduction in serum phosphate <2.0 mg/dL in 10 women in the FCM group compared to none in the iron dextran group. Reduced serum phosphate was accompanied by increased urinary fractional excretion of phosphate, decreased calcitriol levels, and increased parathyroid hormone levels. These findings suggest that iron deficiency increases cFGF23 levels, and that certain iron preparations temporarily increase iFGF23 levels. We propose that intravenous iron lowers cFGF23 in humans by reducing fgf23 transcription as it does in mice, whereas carbohydrate moieties in certain iron preparations may simultaneously inhibit FGF23 degradation in osteocytes leading to transient increases in iFGF23 and reduced serum phosphate.