Fibroblast growth factor-23 mutants causing familial tumoral calcinosis are differentially processed

Familial tumoral calcinosis (TC, OMIM 211900) is a heritable disorder characterized by hyperphosphatemia, normal or elevated serum 1,25-dihydroxyvitamin D, and often severe ectopic calcifications. Two recessive mutations in fibroblast growth factor-23 (FGF23), serine 71/glycine (S71G) and serine 129/phenylalanine (S129F), were identified as causing TC. Herein, we undertook comprehensive biochemical analyses of an extended TC family carrying the S71G FGF23 mutation, which revealed that heterozygous (serine/glycine, S/G) individuals had elevated serum FGF23 C-terminal fragments compared with wild-type (serine/serine, S/S) family members (P < 0.025). To understand the differential processing of FGF23 in TC patients, we transiently expressed S71G as well as S129F FGF23. FGF23 ELISA in tandem with Western analyses revealed increased proteolytic cleavage of mutant FGF23 and a limited secretion of intact protein. Furthermore, S71G and S129F FGF23 carrying mutations that disrupt the furin-like protease RXXR motif in FGF23 rescued the secretion of the intact protein, and both TC mutant proteins harboring the R176Q mutation revealed no altered sensitivity to trypsin compared with the native (R176Q)FGF23. Finally, S71G, but not S129F mutant FGF23, is rescued by temperature. In summary, FGF23 mutations causing TC lead to increased intracellular proteolysis of FGF23, most likely by furin-like proteases, due to conformational changes of the mutant protein. The destabilizing nature of these mutations provides new insight into the pathophysiology of TC and exemplifies the physiological importance of FGF23 in phosphate and vitamin D metabolism.     

The role of mutant UDP-N-acetyl-alpha-D-galactosamine-polypeptide N-acetylgalactosaminyltransferase 3 in regulating serum intact fibroblast growth factor 23 and matrix extracellular phosphoglycoprotein in heritable tumoral calcinosis

CONTEXT: Familial tumoral calcinosis (TC) results from disruptions in phosphate metabolism and is characterized by high serum phosphate with normal or elevated 1,25 dihydroxyvitamin vitamin D concentrations and ectopic and vascular calcifications. Recessive loss-of-function mutations in UDP-N-acetyl-alpha-D-galactosamine-polypeptide N-acetylgalactosaminyltransferase 3 (GALNT3) and fibroblast growth factor-23 (FGF23) result in TC. OBJECTIVE: The objective of the study was to determine the relationship between GALNT3 and FGF23 in familial TC. DESIGN, SETTING, AND PATIENTS: We assessed the major biochemical defects and potential genes involved in patients with TC. Intervention: Combination therapy consisted of the phosphate binder Sevelamer and the carbonic anhydrase inhibitor acetazolamide. RESULTS: We report a patient homozygous for a GALNT3 exon 1 deletion, which is predicted to truncate the encoded protein. This patient had high serum FGF23 concentrations when assessed with a C-terminal FGF23 ELISA but low-normal FGF23 levels when tested with an ELISA for intact FGF23 concentrations. Matrix extracellular phosphoglycoprotein has been identified as a possible regulator of phosphate homeostasis. Serum matrix extracellular phosphoglycoprotein levels, however, were normal in the family with GALNT3-TC and a kindred with TC carrying the FGF23 S71G mutation. The tumoral masses of the patient with GALNT3-TC completely resolved after combination therapy. CONCLUSIONS: Our findings demonstrate that GALNT3 inactivation in patients with TC leads to inadequate production of biologically active FGF23 as the most likely cause of the hyperphosphatemic phenotype. Furthermore, combination therapy may be effective for reducing the tumoral burden associated with familial TC.     

Novel GALNT3 mutations causing hyperostosis-hyperphosphatemia syndrome result in low intact fibroblast growth factor 23 concentrations

CONTEXT: Hyperostosis-hyperphosphatemia syndrome (HHS) is a rare metabolic disorder characterized by hyperphosphatemia and localized hyperostosis. HHS is caused by mutations in GALNT3, which encodes UDP-N-acetyl-alpha-D-galactosamine:polypeptide N- acetylgalactosaminyltransferase 3. Familial tumoral calcinosis (TC), characterized by ectopic calcifications and hyperphosphatemia, is caused by mutations in the GALNT3 or fibroblast growth factor 23 (FGF23) genes. OBJECTIVE: Our objective was to identify mutations in FGF23 or GALNT3 and determine serum FGF23 levels in an HHS patient. DESIGN: Mutation detection in FGF23 and GALNT3 was performed by DNA sequencing, and serum FGF23 concentrations were measured by ELISA. PATIENTS OR OTHER PARTICIPANTS: A 5-year-old French boy with HHS and his family members participated. RESULTS: The patient presented with painful cortical lesions in his leg. Radiographs of the affected bone showed diaphyseal hyperostosis. The lesional tissue comprised trabeculae of immature, woven bone surrounded by fibrous tissue. Biochemistry revealed elevated phosphate, tubular maximum rate for phosphate reabsorption per deciliter of glomerular filtrate, and 1,25-dihydroxyvitamin D levels. The patient was a compound heterozygote for two novel GALNT3 mutations. His parents and brother were heterozygous for one of the mutations and had no biochemical abnormalities. Intact FGF23 level in the patient was low normal, whereas C-terminal FGF23 was elevated, a pattern similar to TC. CONCLUSION: The presence of GALNT3 mutations and elevated C-terminal, but low intact serum FGF23, levels in HHS resemble those seen in TC, suggesting that HHS and TC are different manifestations of the same disorder. The absence of biochemical abnormalities in the heterozygous individuals suggests that one normal allele is sufficient for secretion of intact FGF23.     

Hypophosphatemia with elevations in serum fibroblast growth factor 23 in a child with Jansen's metaphyseal chondrodysplasia

CONTEXT: Previous studies have suggested a regulatory relationship between serum phosphorus, vitamin D, and fibroblast growth factor 23 (FGF23), a hormone that promotes renal excretion of phosphate. Despite these associations, the identity of the primary regulator of serum FGF23 is unresolved. Jansen's metaphyseal chondrodysplasia is a rare autosomal dominant disorder associated with short-limbed dwarfism and other characteristic skeletal abnormalities. This condition is caused by mutations in the PTH/PTHrP receptor that result in ligand-independent cAMP accumulation, thus rendering the receptor constitutively active. These patients typically exhibit asymptomatic hypercalcemia and hypophosphatemia despite low or undetectable serum levels of PTH and PTHrP. EVIDENCE ACQUISITION: A literature search revealed that serum FGF23 levels had not been studied in patients with Jansen's syndrome, a disorder in which the biochemical features present a unique opportunity to study the possible relationship between FGF23 and calcium-phosphorus-vitamin D metabolism. A case of Jansen's syndrome is presented in which serum FGF23 concentrations, along with serum phosphorus and 1,25(OH)(2) vitamin D levels, were measured and compared with those of age-matched controls. Evidence Synthesis: Serum FGF23 concentrations in the patient with Jansen's syndrome were found to be markedly and persistently elevated, compared with values in healthy, age-matched controls, despite hypophosphatemia and normal 1,25(OH)(2) vitamin D levels. CONCLUSION: Together, our findings indicate that serum FGF23 could be governed by factor(s) other than serum phosphorus, potentially by activation of the PTH/PTHrP receptor in bone.     

Tumoral calcinosis presenting with eyelid calcifications due to novel missense mutations in the glycosyl transferase domain of the GALNT3 gene

CONTEXT: Familial tumoral calcinosis (TC) is a rare autosomal recessive disorder characterized by metastatic calcifications, often periarticular. Biochemical findings include hyperphosphatemia, high 1,25-dihydroxyvitamin D levels, and elevated tubular maximum for phosphate reabsorption per deciliter of glomerular filtrate (TmP/GFR). TC is caused by biallelic mutations of the genes encoding either fibroblast growth factor 23 (FGF23) or uridine diphosphate-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase 3 (GalNAc transferase 3 or GALNT3). OBJECTIVE: The objective was to identify mutations in FGF23 or GALNT3 responsible for a mild TC phenotype by DNA sequencing and to determine serum FGF23 levels by ELISA. PATIENTS OR OTHER PARTICIPANTS: The subject was a 25-yr-old Caucasian woman with eyelid calcifications and biochemical features of TC. RESULTS: Eyelid biopsy revealed superficial dermis calcifications. There was no history of metastatic calcifications, mineral homeostasis abnormalities, or renal dysfunction. Biochemistry revealed normal levels of calcium, creatinine, PTH, and 25-hydroxyvitamin D, with elevated phosphorous, TmP/GFR, and high normal 1,25-dihydroxyvitamin D levels. Intact FGF23 was undetectable (< 3 pg/ml), whereas C-terminal FGF23 was elevated (698.2 RU/ml). Mutation detection revealed compound heterozygosity for two novel mutations in the glycosyl transferase domain of the GALNT3 gene. CONCLUSION: Previously reported GALNT3 mutations in TC have been null mutations. This study shows that missense mutations affecting the glycosyl transferase domain of GalNAc transferase 3 also cause TC. Elevated C-terminal FGF23 fragments with undetectable intact FGF23 suggest that the mutant enzyme lacks the ability to glycosylate FGF23 and that glycosylation by GalNAc transferase 3 is necessary for secretion of functional full-length FGF23.     

Fibroblast growth factor 23 and its receptors

Fibroblast growth factor 23 (FGF23) is a circulating factor that plays critical roles in phosphate and vitamin D metabolism, as evidenced by the fact that FGF23 missense mutations cause autosomal dominant hypophosphatemic rickets (ADHR). Autosomal dominant hypophosphatemic rickets is characterized by hypophosphatemia with inappropriately normal 1,25-dihydroxyvitamin D concentrations, as well as bone pain, fracture and rickets. This phenotype parallels that of patients with tumor induced osteomalacia (TIO), X-linked hypophosphatemic rickets (XLH), and fibrous dysplasia (FD), in whom elevated serum FGF23 levels are often observed. The fibroblast growth factor receptors (FGFR1-4) play key roles in skeletal development, as well as in normal metabolic processes. Several FGFR isoforms that potentially mediate the activity of FGF23 have been implicated. In the short term, these findings will lead to further understanding of FGF23 function, and potentially in the long term, to targeted therapies in disorders of hypo- and hyperphosphatemia that involve FGF23.     

Mutation analysis and serum FGF23 level in a patient with pulmonary alveolar microlithiasis

Pulmonary alveolar microlithiasis (PAM) is a rare, hereditary disorder characterized by ectopic formation of calcium-phosphate microliths in the alveolar space. PAM has been reported to arise from inactivating mutations in SLC34A2, encoding a sodium-dependent phosphate co-transporter essential for phosphate transport in the lungs and small intestine. Serum levels of the phosphaturic hormone fibroblast growth factor-23 (FGF23) in PAM have not been determined. Our objectives were to investigate the genetic etiology and circulating level of FGF23 in a 50-year-old male with clinical characteristics of PAM and extra-pulmonary calcifications. The SLC34A2 and FGF23 genes were sequenced for mutations and serum FGF23 analyzed by ELISA. We found no disease-causing mutations or single nucleotide polymorphisms in the genes investigated. Importantly, repeated measurements revealed undetectable or markedly low serum FGF23 (<3-11 RU/ml). Surprisingly, in the face of low serum FGF23, 1,25-dihydroxy vitamin D? level was low-normal and parathyroid hormone mildly elevated. Total 24-h urinary excretion of phosphate and calcium were low, as was fractional urinary excretion of calcium. In contrast, fractional excretion of phosphate was above normal, likely due to elevated PTH. Collectively, PAM may be a polygenic disorder that arises from mutations other than in SLC34A2. The low FGF23 level in our PAM patient supports an intestinal-bone axis, leading to decreased FGF23 expression when intestinal phosphate absorption is compromised.     

Hyperphosphatemic tumoral calcinosis: effects of phosphate depletion on vitamin D metabolism, and of acute hypocalcemia on parathyroid hormone secretion and action

In hyperphosphatemic tumoral calcinosis, plasma 1,25-dihydroxyvitamin D [1,25(OH)2D] levels are inappropriately elevated, suggesting an abnormality in vitamin D metabolism. To define this abnormality further, we measured vitamin D metabolites in two patients and four controls before and after phosphate depletion. The patients showed elevated plasma levels of 1,25(OH)2D in the basal state. Phosphate depletion reduced serum phosphate in patients from a mean of 6.1 to 2.6 mg/dl; this was accompanied by a rise in plasma 25-hydroxyvitamin D from 33.6 to 41.9 ng/dl, and in 1,25(OH)2D from 67.7 to 93.2 pg/ml. The absolute rise in 1,25(OH)2D was similar to that of controls. EDTA infusion produced a normal increase of serum immunoreactive PTH levels and urinary cAMP excretion. In this form of tumoral calcinosis, 1,25(OH)2D levels are elevated despite hyperphosphatemia, normal immunoreactive PTH, and normal serum calcium concentrations, suggesting an abnormality in the regulation of 1,25(OH)2D synthesis or metabolism, or alternatively, another undefined stimulus for 1,25(OH)2D synthesis. These patients appear to have concurrent abnormalities of renal tubular phosphate transport and vitamin D metabolism.     

Skeletal resistance to 1,25-dihydroxyvitamin D3 in osteopetrotic rats

The osteopetrotic (op/op) rat mutation is a lethal mutation in which decreased osteoclast function (bone resorption) coexists with markedly elevated serum levels of 1 ,25-dihydroxyvitamin D3[1,25(OH)2D3]. Increased circulating levels of 1,25(OH)2D3 have been reported in other osteopetrotic animal mutations and in some osteopetrotic children. This study examined the effects of 1,25(OH)2D3 infusions on serum and skeletal parameters in normal and mutant rats of op stock. We also examined vitamin D receptor expression and binding in bone cells from op normal and mutant animals. Four-week-old normal and mutant rats were infused either with propylene glycol (used as controls) or with 12.5-125 ng of 1,25(OH)2D3/d using osmotic minipumps implanted subcutaneously for 1 wk. Sera were analyzed for calcium, phosphorus, and 1,25(OH)2D3 levels. Histomorphometric analyses of proximal tibiae from treated normal (50 ng/d) and op mutant (125 ng/d) rats and their vehicle-infused controls were performed. Normal animals infused with 1,25(OH)2D3 exhibited a dose-dependent increase in serum calcium levels. Histomorphometric analyses of metaphyseal bone within the primary spongiosae region showed that 1,25(OH)2D3 increased osteoclast number with a reduction in osteoblast surface associated with a decrease in growth plate cartilage thickness. However, similar analyses on secondary spongiosae showed a decrease in osteoclast number and surface associated with an anabolic response. Op mutants infused with 1,25(OH)2D3 did not exhibit any change in serum calcium levels or histomorphometric parameters related to growth plate cartilage and metaphyseal bone compared with mutant controls. Vitamin D mRNA and protein levels were increased twoto threefold in op mutants compared to age-matched normal rats. However, binding affinity of 1,25(OH)2D3 to its receptor was similar between op mutant and normal animals. High dose calcitriol therapy, under the conditions and period of treatment used in this study, failed to stimulate bone turnover in op rats, suggesting that they are resistant to the skeletal effects of 1,25(OH)2D3. The failure of osteoclast activation in response to 1,25(OH)2D3 treatment may be associated with osteoblast incompetence in this mutation.     

Hypercalcemia associated with infection by Cryptococcus neoformans and Coccidioides immitis

BACKGROUND: Of the 13 reported cases of hypercalcemia associated with fungal infection, 1 was caused by Cryptococcus neoformans and probably mediated by increased levels of 1,25-dihydroxyvitamin D [1,25(OH)2D]. Eight others were associated with Coccidioides immitis, of which only 2 had measured 1,25(OH)2D levels; in both, they were diminished. We report a patient with human immunodeficiency virus infection and simultaneous C. immitis and C. neoformans pneumonia and C. immitis fungemia associated with hypercalcemia. METHODS: Consecutive measurements of serum total and ionized calcium, phosphorous, blood urea nitrogen, creatinine, 25(OH)D, 1,25(OH)2D, parathyroid hormone (PTH), parathyroid hormone-related protein (PTHrp) and albumin were performed over a period of 46 months. RESULTS: While the patient was hypercalcemic, intact serum PTH and PTHrp were undetectable, serum 25(OH)D levels were normal, and serum 1,25(OH)2D levels were in the high normal range. Successful treatment of the C. immitis and C. neoformans infections resulted in resolution of the hypercalcemia and increase of PTH and PTHrp to the normal range. CONCLUSION: In some patients with HIV infection, coincident hypercalcemia, and severe fungal infection, the responsible factor may be 1,25(OH)2D. Although total serum levels of this compound may not be frankly elevated, they are inappropriately high for the circumstances. Free 1,25(OH)2D levels should be determined in this situation.     

PTH ablation ameliorates the anomalies of Fgf23-deficient mice by suppressing the elevated vitamin D and calcium levels

Fibroblast growth factor 23 (FGF23) is a key regulator of mineral ion homeostasis. Genetic ablation of Fgf23 in mice leads to severe biochemical disorders including elevated serum 1,25-dihydroxyvitamin D [1,25(OH)2D], hypercalcemia, hyperphosphatemia, and marked decreased PTH levels. Because PTH stimulates 1,25(OH)2D production and increases serum calcium levels, we hypothesized that ablation of PTH from the Fgf23 knockout (Fgf23-/-) mice could suppress these affects, thus ameliorating the soft tissue and skeletal anomalies in these animals. In this study, we generated a genetic mouse model with dual ablation of the Fgf23/PTH genes. The data show that deletion of PTH does suppress the markedly higher serum 1,25(OH)2D and calcium levels observed in Fgf23-/- mice and results in much larger, heavier, and more active double-knockout mice with improved soft tissue and skeletal phenotypes. On the contrary, when we infused PTH (1-34) peptide into Fgf23-/- mice using osmotic minipumps, serum 1,25(OH)2D and calcium levels were increased even further, leading to marked reduction in trabecular bone. These results indicate that PTH is able to modulate the anomalies of Fgf23-/- mice by controlling serum 1,25(OH)2D and calcium levels.     

Oncogenic hypophosphataemic osteomalacia: biomarker roles of fibroblast growth factor 23, 1,25-dihydroxyvitamin D3 and lymphatic vessel endothelial hyaluronan receptor 1

Oncogenic osteomalacia (OOM) is characterised by tumour production of fibroblast growth factor 23 (FGF23) that results in hypophosphataemia and renal phosphate wasting, reduced 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) synthesis and osteomalacia. Here, we demonstrate the roles of serum FGF23 and 1,25(OH)2D3, together with the lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1), as biomarkers for OOM. A previously well 52-year-old man presented with a 2-year history of generalised musculoskeletal pain and proximal myopathy. He had hypophosphataemia, elevated serum alkaline phosphatase activity, low serum 1,25(OH)2D3 and a reduced tubular maximum of phosphate/glomerular filtration rate. These findings indicated a diagnosis of OOM, but magnetic resonance imaging (MRI) and octreotide scintigraphy did not identify any tumours. Treatment with oral phosphate and calcitriol resolved the symptoms and biochemical abnormalities within 6 months. Four years later, he relapsed whilst on treatment with oral phosphate and calcitriol. Serum FGF23 concentration was elevated and MRI identified a 2 cm tumour within Hoffa's fat pad of the left knee. Removal of the tumour resulted in a complete resolution of symptoms and normalisation of the serum biochemical abnormalities including serum FGF23. Histology demonstrated a phosphaturic mesenchymal tumour, mixed connective tissue variant (PMTMCT), which revealed immunostaining with anti-LYVE-1 antibody and hence the presence of lymphatic vessels. Serum FGF23 and 1,25(OH)2D3 were found to be reliable biomarkers for OOM. In addition, the demonstration of lymphatics in the PMTMCT helps to distinguish this tumour from most typical benign haemangiomas.     

Fibroblast growth factor (FGF)-23 in patients with primary hyperparathyroidism

OBJECTIVE: We aimed to determine the serum level of fibroblast growth factor-23 (FGF-23) in patients with primary hyperparathyroidism (pHPT) to understand its physiological role in the disorder. PATIENTS AND METHODS: Ninety-eight patients with pHPT who underwent parathyroidectomy formed the study group. We also measured serum FGF-23 in 11 of these patients on postoperative day 6. RESULTS: Serum FGF-23 levels was significantly higher in pHPT patients than in healthy controls (35.6+/-17.8 ng/l vs 28.9+/-11.2 ng/l (mean+/-s.d.); P<0.001 (Pearson's correlation coefficient)), but there was no significant difference in the serum FGF-23 level between pHPT patients with normal renal function (creatinine clearance (Ccr) of >or=70 ml/min) and healthy controls. Serum FGF-23 correlated positively with serum calcium (P<0.0001) and intact parathyroid hormone (PTH) (P<0.01), and negatively with Ccr (P<0.001), serum phosphate (P<0.05), and serum 1,25-dihydroxyvitamin D (1,25(OH)(2)D) (P<0.05). Multiple linear regression analysis of factors potentially determining serum FGF-23 levels in pHPT patients showed serum calcium (P<0.01) and Ccr (P<0.001) to be significant predictors. The serum levels of FGF-23 did not change after parathyroidectomy despite the normalization of serum calcium values. Multiple linear regression analysis revealed that serum FGF-23 was not a significant predictor of serum phosphate or 1,25(OH)(2)D in pHPT patients. CONCLUSIONS: FGF-23 may not play a significant role in regulating phosphate or 1,25(OH)(2)D in pHPT patients, especially in those with normal renal function. Further studies are warranted to determine the role of FGF-23 in renal insufficiency or failure.     

COMPARISON OF INTESTINAL CALCIUM ABSORPTION AND CIRCULATING 1,25-DIHYDROXYVITAMIN D LEVELS IN MALIGNANCY-ASSOCIATED HYPERCALCAEMIA AND PRIMARY HYPERPARATHYROIDISM

The relation between circulating 1,25-dihydroxyvitamin D (1,25(OH)2D) levels and intestinal calcium absorption--as determined by an oral calcium load test--was studied in 16 patients with hypercalcaemia of malignancy (HM) and 16 with hypercalcaemic primary parathyroidism (HPT). In the HPT group serum calcium rose significantly after the oral calcium load and the increment correlated significantly with 1,25(OH)2D levels. While 1,25(OH)2D levels were raised to within the hyperparathyroid range in a number of HM patients, there was no correlation between change in serum calcium and 1,25(OH)2D level in the HM group and serum calcium did not rise significantly after the oral calcium load. HM patients with detectable or raised 1,25(OH)2D levels typically had few, or no, bone metastases in association with squamous lung cancers. A high proportion of these patients exhibited other aspects of hyperparathyroid-like activity such as increased renal tubular calcium reabsorption, depressed renal tubular phosphate reabsorption and elevated urinary cyclic AMP excretion. Conversely, HM patients with undetectable 1,25(OH)2D levels typically had extensive metastatic bone disease in association with breast carcinoma and were less likely to exhibit other hyperparathyroid-like features. It is postulated that in the former, the 'inappropriately' detectable or raised 1,25(OH)2D levels may have been due to enhanced renal 1 alpha-hydroxylase activity stimulated by the parathyroid hormone (PTH)-like effect of a non-PTH ectopic humoral mediator. In the latter the suppressed 1,25(OH)2D levels would be the predicted result of a non-humorally mediated hypercalcaemia. It is currently unclear why intestinal calcium absorption was depressed in all HM patients when 1,25(OH)2D levels were normal or raised in some cases. It is possible, however, that in HM there is 'end organ' resistance to the effects of 1,25(OH)2D due to a generalized malabsorptive process.     

Regulatory mechanisms of circulating fibroblast growth factor 23 in parathyroid diseases

Fibroblast growth factor-23 (FGF-23) is a circulating factor regulating phosphate and vitamin D homeostasis. Phosphate intake and an administration of 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) increase circulating FGF-23 levels, and elevated FGF-23 prevents hyperphosphatemia and vitamin D toxicity by hyperphosphaturia and suppression of circulating 1,25(OH)(2)D level, comprising a feedback loop to maintain phosphate and vitamin D homeostasis. Excess secretion of parathyroid hormone (PTH), another phosphaturic factor, elevates the serum calcium level in primary hyperparathyroidism. PTH also elevates circulating FGF-23 level, which cooperatively enhances the phosphaturia, resulting in hypophosphatemia. The circulating FGF-23 level increases as renal function declines in chronic kidney disease (CKD), and it exhibits significant and positive correlations with serum phosphate, calcium, and PTH in CKD patients on maintenance hemodialysis, suggesting that these parameters regulate circulating FGF-23 level. Tight associations between circulating FGF-23 and calcium levels were observed both in patients with primary hyperparathyroidism and in CKD patients on maintenance hemodialysis, suggesting the role of serum calcium on FGF-23 regulatory mechanisms. FGF-23 may have a physiological role in preventing tissue damage such as ectopic calcifications, partly via suppressing the serum calcium x phosphate product by accelerating urinary phosphate excretion and suppressing vitamin D activation.     

Fibroblast growth factor-23 in patients with Graves' disease before and after antithyroid therapy: its important role in serum phosphate regulation

OBJECTIVE: Hyperthyroidism is a well-described cause of hyperphosphatemia. We aimed to clarify the physiological role of fibroblast growth factor (FGF)-23 in serum phosphate homeostasis in patients with Graves' disease during the course of treatment for hyperthyroidism. CONTEXT: The study group comprised 56 patients (45 for a cross-sectional study and 11 for a longitudinal study) with Graves' disease. For the cross-sectional study, patients were assigned, on the basis of their serum phosphate level, to a hypophosphatemia group (n = 14), a normophosphatemia group (n = 16), or a hyperphosphatemia group (n = 15). Serum FGF-23, calcium, phosphate, PTH, and 1,25-dihydroxyvitamin D [1,25(OH)(2)D] levels were compared between the three groups. For the longitudinal study, we assessed changes in these biochemical indices before and after antithyroid treatment. RESULTS: In the cross-sectional study, the serum FGF-23 level was significantly higher (P < 0.05) in the hyperphosphatemia group than in the other groups (61 +/- 36 ng/liter vs. 31 +/- 22 ng/liter and 30 +/- 9 ng/liter). In the longitudinal study, serum levels of FGF-23 decreased significantly (P < 0.05) from a high of 54 +/- 12 ng/liter before treatment to 29 +/- 14 ng/liter after treatment. In contrast, the serum 1,25(OH)(2)D level increased significantly (P < 0.005) from 55 +/- 22 pmol/liter before treatment to 185 +/- 76 pmol/liter 3 months after treatment. Serum FGF-23 levels were positively correlated with serum phosphate levels (P < 0.0001) and negatively correlated with serum 1,25(OH)(2)D levels (P < 0.0001). CONCLUSIONS: The significant positive correlation between serum levels of phosphate and FGF-23 indicates that FGF-23 may play an important role in serum phosphate homeostasis by its up-regulation in the hyperphosphatemic condition.     

Low serum concentrations of 1,25-dihydroxyvitamin D in human magnesium deficiency

The effect of magnesium deficiency on vitamin D metabolism was assessed in 23 hypocalcemic magnesium-deficient patients by measuring the serum concentrations of 25-hydroxyvitamin D (25OHD) and 1,25-dihydroxyvitamin D [1,25-(OH)2D] before, during, and after 5-13 days of parenteral magnesium therapy. Magnesium therapy raised mean basal serum magnesium [1.0 +/- 0.1 (mean +/- SEM) mg/dl] and calcium levels (7.2 +/- 0.2 mg/dl) into the normal range (2.2 +/- 0.1 and 9.3 +/- 0.1 mg/dl, respectively; P less than 0.001). The mean serum 25OHD concentration was in the low normal range (13.2 +/- 1.5 ng/ml) before magnesium administration and did not significantly change after this therapy (14.8 +/- 1.5 ng/ml). Sixteen of the 23 patients had low serum 1,25-(OH)2D levels (less than 30 pg/ml). After magnesium therapy, only 5 of the patients had a rise in the serum 1,25-(OH)2D concentration into or above the normal range despite elevated levels of serum immunoreactive PTH. An additional normocalcemic hypomagnesemic patient had low 1,25-(OH)2D levels which did not rise after 5 days of magnesium therapy. The serum vitamin D-binding protein concentration, assessed in 11 patients, was low (273 +/- 86 micrograms/ml) before magnesium therapy, but normalized (346 +/- 86 micrograms/ml) after magnesium repletion. No correlation with serum 1,25-(OH)2D levels was found. The functional capacity of vitamin D-binding protein to bind hormone, assessed by the internalization of [3H]1,25-(OH)2D3 by intestinal epithelial cells in the presence of serum was not significantly different from normal (11.42 +/- 1.45 vs. 10.27 +/- 1.27 fmol/2 X 10(6) cells, respectively). These data show that serum 1,25-(OH)2D concentrations are frequently low in patients with magnesium deficiency and may remain low even after 5-13 days of parenteral magnesium administration. The data also suggest that a normal 1,25-(OH)2D level is not required for the PTH-mediated calcemic response to magnesium administration. We conclude that magnesium depletion may impair vitamin D metabolism.     

Inhibition of intestinal sodium-dependent inorganic phosphate transport by fibroblast growth factor 23

The mechanisms by which fibroblast growth factor 23 (FGF23) alters inorganic phosphate (Pi) homeostasis is not entirely clear. In the present study, we examined the effect of FGF23 on intestinal sodium-dependent Pi transport in mice. Injection of FGF23(R179Q) markedly reduced serum Pi and 1,25(OH)2D3 levels in normal mice. Those animals show the reduction of intestinal sodium-dependent Pi transport activity and the amount of type IIb sodium-dependent Pi cotransporter (type IIb NaPi) protein in the brush border membrane vesicles. In vitamin D receptor null mice (VDR-/-), FGF23(R179Q) had no effect on intestinal sodium-dependent Pi transport activity and type IIb NaPi protein levels. The present study suggests that FGF23(R179Q) reduces intestinal sodium-dependent Pi transport activity and type IIb NaPi protein levels by a mechanism that is dependent on VDR.     

Correlations of serum concentrations of 1,25-dihydroxyvitamin D, phosphorus, and parathyroid hormone in tumoral calcinosis

The inherited metabolic disorder tumoral calcinosis is characterized by elevated serum phosphorus and 1,25-dihydroxyvitamin D [1,25-(OH)2D] levels and paraarticular calcific tumors. The pathogenesis of this disease is obscure, but an elevated renal phosphate reabsorption threshold and increased production of 1,25-(OH)2D are postulated as defects. We studied nine affected patients and found that both serum phosphorus and renal phosphate reabsorption threshold (TmP/GFR) were positively correlated with serum 1,25-(OH)2D levels. Since tumoral calcinosis is a disorder with abnormal renal phosphate transport, we compared the TmP/GFR and serum 1,25-(OH)2D levels to values obtained in patients with two other diseases with renal phosphate transport defects: oncogenic osteomalacia and X-linked hypophosphatemic rickets. We found a significant correlation between TmP/GFR and 1,25-(OH)2D levels in all three diseases, suggesting that in these diseases 1,25-(OH)2D production is regulated in some manner by phosphate transport. Furthermore, previous work indicated that in tumoral calcinosis broad variation exists in serum phosphorus levels. In our patients a negative correlation was found between the serum PTH concentrations and both serum phosphorus levels and TmP/GFR values, respectively. We postulate that although the basic defect in tumoral calcinosis most likely resides in the proximal renal tubular cell, the variation in serum phosphorus levels and possibly disease expression is modulated in part by PTH.