FGF23 concentrations vary with disease status in autosomal dominant hypophosphatemic rickets.

We measured FGF23 concentrations in subjects with ADHR. FGF23 and serum phosphate correlated positively in controls and negatively in subjects with ADHR. Elevated FGF23 concentrations were associated with lower phosphate values. The variable phenotype in ADHR may be caused by fluctuations in FGF23. INTRODUCTION: Autosomal dominant hypophosphatemic rickets (ADHR) is a rare disorder of phosphaturia, hypophosphatemia, inappropriately low/normal calcitriol, and rickets/osteomalacia. ADHR is caused by mutations in a circulating peptide, fibroblast growth factor 23 (FGF23). We present the first report of FGF23 concentrations in subjects with ADHR. The aim was to test the hypotheses that subjects with ADHR have elevated FGF23 concentrations and that FGF23 concentrations are associated with disease severity. MATERIALS AND METHODS: This was an observational study at a tertiary referral center. Subjects were from three kindreds with FGF23 mutations causing ADHR (n = 42). Controls were participants from these families without mutations (n = 55). Fasting blood and urine samples were obtained. Biochemistries were determined, and FGF23 concentrations were measured using two ELISAs. RESULTS: Three cases are presented illustrating activity of disease and FGF23 concentrations. One case shows persistent hypophosphatemia and elevation of FGF23 concentrations, whereas another shows remission of hypophosphatemia corresponding to a decrease in previously elevated FGF23 concentrations. Overall cross-sectional group differences were nonsignificant for serum phosphate and FGF23 concentrations. C-terminal FGF23 concentration in controls was 61.0 +/- 28.6 (SD) RU/ml (median, 52.5 RU/ml), and in subjects with mutations was 148.8 +/- 374.5 RU/ml (median, 63.1 RU/ml). Mean intact FGF23 concentration in controls was 44.7 +/- 14.9 pg/ml (median, 40.4 pg/ml), and in subjects with mutations was 83.2 +/- 233.0 pg/ml (median, 39.0 pg/ml). C-terminal FGF23 concentrations were at least +2 SD in 10/42(24%), and intact FGF23 concentrations were at least +2 SD in 3/34(9%). Phosphate correlated positively with C-terminal and intact FGF23 in both controls and in subjects with mutations with phosphate >2.5 mg/dl but correlated significantly negatively with C-terminal and intact FGF23 in ADHR subjects with phosphate < or =2.5 mg/dl. CONCLUSIONS: Elevated FGF23 concentrations are associated with hypophosphatemia in ADHR, and remission of the phenotype is associated with lower FGF23 concentrations. FGF23 has an opposite relationship with phosphate in ADHR compared with controls. We conclude that ADHR symptoms and disease severity likely fluctuate with FGF23 concentrations.     

Serum FGF23 levels in normal and disordered phosphorus homeostasis.

We investigated if the circulating levels of the phosphaturic factor FGF23 are elevated in subjects with XLH. Although we failed to find a statistically significant increase, FGF23 levels were significantly correlated with the degree of hypophosphatemia in XLH. In contrast, FGF23 levels were markedly increased in subjects with ESRD and correlated inversely with the degree of hyperphosphatemia. INTRODUCTION: Inactivating mutations of PHEX cause renal phosphate wasting in X-linked hypophosphatemic rickets (XLH) because of the accumulation of a phosphaturic hormone called phosphatonin. The recent discovery that FGF23 is the circulating phosphaturic factor in autosomal dominant hypophosphatemia raises the possibility that FGF23 is phosphatonin. METHODS: Fasting serum FGF23 levels and serum biochemical parameters were measured using a human FGF23 (C-terminal) ELISA assay in 11 subjects with XLH and 42 age-matched controls, 5 subjects with hypophosphatemia of unknown cause, and 14 hyperphosphatemic subjects with end stage renal disease (ESRD). Associations between variables were examined using the Spearman's correlation coefficient and linear regression analysis. RESULTS AND CONCLUSIONS: FGF23 (RU/ml) concentrations were not different (p = 0.11) between control and hypophosphatemic XLH subjects, but were significantly increased in hyperphosphatemic subjects with ESRD (p < 0.001). Western blot analysis found the presence of both full-length and C-terminal FGF23 fragments in serum from ESRD subjects. There was a strong inverse correlation between FGF23 and serum phosphorus (r = -0.60) and calcium and phosphorus (Ca x P) product (r = -0.65) in XLH, and a strong positive relationship between FGF23 and Pi (r = 0.50) and Ca x P product (r = 0.62) in ESRD. FGF23 levels were variably elevated in subjects with hypophosphatemia of unknown cause, one of which had tumor-induced osteomalacia (TIO). Removal of the tumor resulted in rapid reduction in serum FGF23 levels. These findings suggest that FGF23 has a possible role in mediating hypophosphatemia in XLH and TIO, but the overlapping levels of FGF23 in hypophosphatemic disorders and normal subjects indicate that serum phosphorus and FGF23 can also be independently regulated.     

FGF23 analysis of a Chinese family with autosomal dominant hypophosphatemic rickets

Autosomal dominant hypophosphatemic rickets (ADHR; MIM 193100) is a hereditary disorder characterized by isolated renal phosphate wasting, hypophosphatemia, and inappropriately normal 1,25-dihydroxyvitamin D₃ levels. Recent studies have shown that the fibroblast growth factor 23 (FGF23) gene is responsible for this disease. FGF23 protein is a phosphaturic factor that is elevated in several diseases associated with hypophosphatemia and rickets but varies with disease status in ADHR. In the present study we observed a Chinese family of Han ethnic origin diagnosed with ADHR. The proband is a 30-year-old woman with no history of rickets but with multiple tooth abscesses as a young adult. She presented with progressive painful swelling of the left ankle after a blunt trauma at 26 years of age. She developed back pain, generalized weakness, and fatigue, and she could barely walk at age 27. She was found to have severe hypophosphatemia, low ratio of phosphorus tubule maximum (TmP) to glomerular filtration rate (GFR) (TmP/GFR), and elevated alkaline phosphatase at age 28. Her brother, 26 years old, presented with fatigue at 24 years of age and is normophosphatemic. The parents of this family had no history of rickets or hypophosphatemia. Direct sequence analysis of genomic DNA demonstrated a single heterozygous c.527G>A (p.R176Q) mutation in the FGF23 gene in three family members, including the proband, her brother, and their mother. Intact FGF23 assay of seven time points during the oral phosphate loading test showed no significant relationship between intact FGF23 and serum phosphorus levels of the subject with ADHR and a control. It is probably the first report of a Chinese family with ADHR.     

Fibroblast growth factor 23 concentrations in healthy term infants during the early postpartum period

Fibroblast growth factor 23 (FGF23) is a potent regulator of Pi and 1,25-(OH)₂D homeostasis. Early postpartum infants show intriguing changes in serum levels of Ca, Pi, PTH and 1,25-(OH)₂D. However, the role of FGF23 in the early neonatal mineral metabolism has not been clarified. In order to evaluate the significance of FGF23 during the early postpartum period, we examined the circulating FGF23 levels using an intact FGF23 ELISA and a C-terminal FGF23 ELISA either in 22 umbilical cord blood samples (the cord blood) or in 22 term infants at 5 days of life (the 5-day-old infant). We also compared these ranges with those of 11 healthy adults. Data were expressed as mean ± SD, and analyzed by two-way ANOVA, followed by the Tukey's test. C-terminal FGF23 in the cord blood, the 5-day-old infants and the healthy adults were 73.3 ± 22.4, 81.0 ± 28.2 and 39.0 ± 7.8 RU/ml, respectively. Intact FGF23 in the cord blood, the 5-day-old infants and the healthy adults were 3.9 ± 1.6, 21.8 ± 17.6, and 27.6 ± 7.3 pg/ml, respectively. Immunoprecipitation assays using anti-FGF23 antibodies demonstrated that the intact 32 kDa FGF23 was low and the fragmented FGF23 of 18 kDa was abundant in the cord blood compared with those in the healthy adults. In conclusion, our observations indicated that the intact FGF23/C-terminal FGF23 ratio was very low due to the fragmentation of FGF23 during the early postpartum period and might have a considerable contribution to the Pi homeostasis in the healthy term infants.     

Hypophosphatemia induced by intravenous administration of saccharated ferric oxide: Another form of FGF23-related hypophosphatemia

Fibroblast growth factor 23 (FGF23) is a humoral factor that is produced by osteocytes and regulates phosphate and vitamin D metabolism. Several hypophosphatemic diseases including X-linked, autosomal dominant and autosomal recessive hypophosphatemic rickets/osteomalacia and tumor-induced rickets/osteomalacia are caused by excess actions of FGF23. These diseases are characterized by hypophosphatemia associated with impaired proximal tubular phosphate reabsorption and inappropriately low serum 1,25-dihydroxyvitamin D [1,25(OH)₂D] levels for hypophosphatemia. Saccharated ferric oxide is widely used in Japan for iron-deficiency anemia. While it has been shown that saccharated ferric oxide induces hypophosphatemic osteomalacia, the mechanism of this hypophosphatemia remains to be clarified. We here describe three hypophosphatemic patients caused by intravenous administration of saccharated ferric oxide. Hypophosphatemia in these patients were associated with impaired renal tubular phosphate reabsorption, rather low serum 1,25(OH)₂D and high FGF23 levels. All these biochemical features improved by the cessation of saccharated ferric oxide. These results indicate that hypophosphatemia caused by saccharated ferric oxide is another form of FGF23-related hypophosphatemia.     

Clinical usefulness of measurement of fibroblast growth factor 23 (FGF23) in hypophosphatemic patients: proposal of diagnostic criteria using FGF23 measurement

Fibroblast growth factor 23 (FGF23) plays important roles in the development of hypophosphatemic diseases such as tumor-induced osteomalacia (TIO) and X-linked hypophosphatemic rickets/osteomalacia (XLH). However, clinical usefulness of measurement of FGF23 has not been established. The objective of this study is to examine the importance of FGF23 measurement in the diagnosis of hypophosphatemic diseases. Biochemical parameters concerning phosphate metabolism were analyzed in a cross-sectional study. 32 patients with TIO, 28 patients with XLH and 16 hypophosphatemic patients with other causes including vitamin D deficiency, Fanconi's syndrome and Cushing's syndrome were studied. In patients with TIO and XLH, FGF23 was above the upper limit of the reference range in most patients irrespective of medical treatment. The lowest FGF23 in these patients was 38.0 pg/ml. FGF23 in hypophosphatemic patients with other causes was undetectable (less than 3 pg/ml) in 12 patients and the highest FGF23 in this group was 23.9 pg/ml. Relationship between phosphate and FGF23 indicated that TIO and XLH are diseases with high FGF23 and hypophosphatemia judged by age-dependent reference ranges for serum phosphate. FGF23 measurement is useful for differential diagnosis of hypophosphatemic diseases caused by excess actions of FGF23 and other etiologies. High FGF23 with low phosphate judged by age-dependent reference ranges for phosphate establishes the diagnosis of diseases caused by excess actions of FGF23.     

Minimally invasive colon resection is associated with a transient increase in plasma sVEGFR1 levels and a decrease in sVEGFR2 levels during the early postoperative period

Introduction  Plasma vascular endothelial growth factor (VEGF) levels are elevated for 2–4 weeks after minimally invasive colorectal resection (MICR). VEGF induces wound and tumor angiogenesis by binding to endothelial cell (EC)-bound VEGF-receptor 1 (VEGFR1) and VEGFR2. Soluble receptors (sVEGFR1, sVEGFR2) sequester VEGF in the blood and decrease VEGF’s proangiogenic effect. The importance of the MICR-related VEGF changes depends on the effect of surgical procedures on sVEGFR1 and sVEGFR2; this study assessed levels of these proteins after MICR for benign indications. Methods  Blood samples were taken (n = 39) preoperatively (preop) and on postoperative days (POD) 1 and 3; in most cases a fourth sample was drawn between POD 7 and 30. sVEGFR1 and sVEGFR2 levels were measured via enzyme-linked immunosorbent assay (ELISA), which detects free and VEGF bound soluble receptor. Late samples were bundled into POD 7–13 and POD 14–30 time points. Results are reported as mean and standard deviation. The data was assessed with paired-samples t-test. Results  Preop, mean plasma sVEGFR2 level (9,203.7 ± 1,934.3 pg/ml) was significantly higher than the sVEGFR1 value (132.5 ± 126.2 pg/ml). sVEGFR2 levels were significantly lower on POD 1 (6,957.8 ± 1,947.7 pg/ml,) and POD 3 (7,085.6 ± 2,000.2 pg/ml), whereas sVEGFR1 levels were significantly higher on POD 1 (220.0 ± 132.8 pg/ml) and POD 3 (182.7 ± 102.1 pg/ml) versus preop results. No differences were found on POD 7–13 or 14–30. Conclusions  sVEGFR2 values decreased and sVEGFR1 levels increased early after MICR; due to its much higher baseline, the sVEGFR2 changes dominate. The net result is less VEGF bound to soluble receptor and more free plasma VEGF.     

Oral Iron Replacement Normalizes Fibroblast Growth Factor 23 in Iron-Deficient Patients With Autosomal Dominant Hypophosphatemic Rickets

Autosomal dominant hypophosphatemic rickets (ADHR) is caused by mutations impairing cleavage of fibroblast growth factor 23 (FGF23). FGF23 gene expression increases during iron deficiency. In humans and mice with the ADHR mutation, iron deficiency results in increased intact FGF23 concentrations and hypophosphatemia. We conducted a prospective open label pilot clinical trial of oral iron replacement over 12 months in ADHR patients to test the hypothesis that oral iron administration would normalize FGF23 concentrations. Eligibility criteria included: FGF23 mutation; and either serum iron <50 μg/dL; or serum iron 50 to 100 μg/dL combined with hypophosphatemia and intact FGF23 >30 pg/mL at screening. Key exclusion criteria were kidney disease and pregnancy. Oral iron supplementation started at 65 mg daily and was titrated based on fasting serum iron concentration. The primary outcome was decrease in fasting intact FGF23 by ≥20% from baseline. Six adults (three male, three female) having the FGF23-R176Q mutation were enrolled; five completed the 12-month protocol. At baseline three of five subjects had severely symptomatic hypophosphatemia (phosphorus <2.5 mg/dL) and received calcitriol with or without phosphate concurrent with oral iron during the trial. The primary outcome was met by 4 of 5 (80%) subjects all by month 4, and 5 of 5 had normal intact FGF23 at month 12. Median (minimum, maximum) intact FGF23 concentration decreased from 172 (20, 192) pg/mL at baseline to 47 (17, 78) pg/mL at month 4 and 42 (19, 63) pg/mL at month 12. Median ferritin increased from 18.6 (7.7, 82.5) ng/mL at baseline to 78.0 (49.6, 261.0) ng/mL at month 12. During iron treatment, all three subjects with baseline hypophosphatemia normalized serum phosphorus, had markedly improved symptoms, and were able to discontinue calcitriol and phosphate. Oral iron repletion normalized FGF23 and phosphorus in symptomatic, iron-deficient ADHR subjects. Thus, the standard approach to ADHR should include recognition, treatment, and prevention of iron deficiency. © 2019 American Society for Bone and Mineral Research.     

Genetic dissection of phosphate- and vitamin D-mediated regulation of circulating Fgf23 concentrations

Fibroblast growth factor-23 (FGF23) is a circulating factor that plays critical roles in phosphate and vitamin D metabolism. The goal of our studies was to dissect the pathways directing the vitamin D–phosphate–FGF23 homeostatic axis. To test the role of diet in the regulation of Fgf23, wild-type (WT) mice were fed either a standard (0.44% phosphorus) or a low-phosphate (0.02%) diet. WT mice on standard diet had a serum phosphate of 9.5 ± 0.3 mg/dl and an Fgf23 concentration of 99.0 ± 10.6 pg/ml; mice on the low-phosphate diet had a phosphate of 5.0 ± 0.2 mg/dl (P < 0.01) and an Fgf23 of 10.6 ± 3.7 pg/ml (P < 0.01). To genetically separate the effects of phosphate and vitamin D on Fgf23, we examined vitamin D receptor null (VDR^−/−) mice, which are hypocalcemic and hypophosphatemic secondary to hyperparathyroidism. On standard diets, WT and VDR^+/− mice had Fgf23 levels of 106.0 ± 30.7 and 90.6 ± 17.3 pg/ml, respectively, whereas Fgf23 was undetectable in the VDR^−/−. Animals were then placed on a diet that normalizes serum calcium and phosphorus. This ‘rescue’ increased Fgf23 in WT to 192.3 ± 32.5 pg/ml and in VDR^+/− to 388.2 ± 89.6pg/ml, and importantly, in VDR^−/− to 476.9 ± 60.1 pg/ml (P < 0.01 vs. WT). In addition, renal vitamin D 1-alpha hydroxylase (1-OHase) mRNA levels were corrected to WT levels in the VDR^−/− mice. In summary, Fgf23 is suppressed in diet-induced hypophosphatemia and in hypophosphatemia associated with secondary hyperparathyroidism. Normalization of serum phosphate by diet in VDR^−/− mice increases Fgf23. Thus, our results demonstrate that Fgf23 is independently regulated by phosphate and by vitamin D.     

An autosomal dominant hypophosphatemic rickets phenotype in a Tunisian family caused by a new FGF23 missense mutation

Autosomal dominant hypophosphatemic rickets (ADHR) is a rare disease, characterized by isolated renal phosphate wasting, hypophosphatemia, and inappropriately normal 1,25-dihydroxyvitamin D₃ (calcitriol) levels. This syndrome involves rickets with bone deformities in childhood and osteomalacia, osteoporosis, articular and para-articular pain, and fatigue in adulthood. It is caused by mutations in a consensus sequence for proteolytic cleavage of the FGF23 protein. Normally, this protein actively regulates phosphate homeostasis. Here we report a Tunisian family in which one parent and three children show clinical and biological features of ADHR. Mutation analysis of the FGF23 gene finds a heterozygous substitution of the C at position 526 by a T (526 C → T), leading to an amino acid replacement of the FGF23 protein (R176W) at position 176. This causative new mutation is located in the consensus sequence for the proteolytic cleavage domain. These results confirm the importance of this site in FGF23 function and its essential role in ADHR physiopathology.     

Intact fibroblast growth factor 23 and fragments in plasma from Gambian children

Summary

Fibroblast growth factor 23 (FGF23) is grossly elevated in Gambian children with rickets and, at a lower prevalence, in those without bone deformities. We used western blotting to mimic the detection capabilities of the C-terminal FGF23 enzyme-linked immunosorbent assay (ELISA). Only intact FGF23 hormone was present in Gambian plasma samples from children with and without rickets.

Introduction

Elevated circulating FGF23 concentrations have been detected in plasma samples from Gambian children using the C-terminal Immutopics ELISA. The Immutopics ELISA detects both the intact FGF23 hormone and the C-terminal fragment. The aim of this study was to determine whether the elevated FGF23 concentrations as detected by the ELISA were predominantly due to a high proportion of intact FGF23 hormone and/or C-terminal FGF23 fragments.

Methods

Stored, frozen plasma samples from previous studies of Gambian children with known concentrations of FGF23 as determined by C-terminal Immutopics ELISA assay, were selected for western blotting analysis: from children with rickets-like bone deformities (n?=?4) and local controls (n?=?4), with elevated >900 RU/ml (n?=?2) and normal <30 RU/ml (n?=?2; from each group). The anti-FGF23 polyclonal antibody that recognizes the C-terminal of FGF23 (as used in the Immutopics kit) was used as the primary antibody and the anti-IgG polyclonal antibody conjugated to horseradish peroxidase (HRP) was used as the secondary antibody.

Results

Firstly, C-terminal FGF23 fragments, although detectable in standards from the Immutopics ELISA kit, were not in the Gambian plasma samples. Secondly, there was no difference in the size of FGF23 molecules present in plasma from children with rickets-like bone deformities and children from the local community.

Conclusions

Western blotting has provided evidence that elevated FGF23 concentrations, as determined by the C-terminal Immutopics ELISA, measured in Gambian children with and without rickets-like bone deformities was not caused by an increased proportion of circulating inactive C-terminal fragments.     

Serum fibroblast growth factor-23 levels in chronic haemodialysis patients

Background  Fibroblast growth factor-23 (FGF23) is a circulating factor that regulates renal reabsorption of inorganic phosphate. Serum FGF23 level is increased in chronic kidney disease (CKD) patients as a compensatory mechanism to hyperphosphataemia. FGF23 directly signals in the parathyroid glands and can be used to predict future secondary hyperparathyroidism in dialysis patients. We examined the relationship between FGF23 and serum calcium, phosphate, 1,25(OH)₂D₃, and PTH levels in haemodialysis patients. Methods  FGF23 and the above mentioned characteristics were measured in 50 chronic haemodialysis patients. We analysed the correlation between FGF23 and the other characteristics by using the Pearson correlation coefficient and multiple regression analysis. Results  FGF23 was significantly increased in haemodialysis patients compared with healthy controls (1525 ± 373 vs. 37 ± 9 pg/ml, P < 0.0001). There was a significant negative correlation between log FGF23 and 1,25(OH)₂D₃ (R = −0.375, P = 0.009) and a significant positive correlation between log FGF23 and log PTH values (R = 0.287, P = 0.041). In multiple regression analysis log PTH and 1,25(OH)₂D₃ values were independent predictors of log FGF23 (P = 0.037 and 0.009, respectively). Conclusions  Our results revealed a marked increase in FGF23 levels in haemodialysis patients. PTH and vitamin D3 were independent predictors of FGF23 in the study group. Serum phosphate did not correlate with or predict FGF23 level despite the high prevalence of hyperphosphataemia in the study group.     

Relation between fibroblast growth factor-23, body weight and bone mineral density in elderly men

Summary  We evaluated the relation between serum FGF23 and bone mineral density (BMD) in a community-based cohort of elderly men. There was a weak correlation between FGF23 and BMD, which was primarily dependent on body weight. Introduction  FGF23 is a hormonal factor produced in bone and regulates serum levels of phosphate (Pi) and vitamin D. FGF23 over-expression is associated with skeletal abnormalities, including rickets/osteomalacia. The relation between FGF23 and Bone Mineral Density (BMD) in the community remains unexplored. Methods  We employed a large, population-based cohort of 3014 Swedish men aged 69–80 years, without known renal disease. BMD was measured with dual X-ray absorptiometry (DXA) in the hip and lumbar spine. Serum intact FGF23 was analyzed with a two-site monoclonal ELISA. Results  There was a weak but significant correlation between FGF23 and BMD in femoral neck (r = 0.04, p < 0.05), femoral trochanter (r = 0.05, p = 0.004), total hip (r = 0.06, p = 0.0015) and lumbar spine (r = 0.07, p = 0.0004). The correlations remained significant when adjusting for biochemical covariates (Pi, calcium, PTH, 25(OH)D and renal function). However, the association became insignificant in all regions when adjusting for established confounding variables including age, height, weight and smoking. Further analysis confirmed a significant correlation between FGF23 and body weight (r = 0.13, p < 0.0001). Conclusions  The weak correlation between FGF23 and BMD in elderly male subjects is mainly due to an association between FGF23 and body weight. Therefore, FGF23 may not play a significant role in the hormonal regulation of BMD. Richard Marsell and Majd A. I. Mirza contributed equally to this work. Funding source: this study was supported by the Swedish Research Council, the Novo Nordisk Foundation, the Swedish Kidney Foundation and the Swedish Society of Medicine.     

Effects of Tumor-Induced Osteomalacia on the Bone Mineralization Process

Fibroblast growth factor 23 (FGF23) overexpression has been identified as a causative factor for tumor-induced osteomalacia (TIO) characterized by hypophosphatemia due to increased renal phosphate wasting, low 1,25(OH)₂D₃ serum levels, and low bone density. The effects of long-lasting disturbed phosphate homeostasis on bone mineralization are still not well understood. We report on a patient with a 12-year history of TIO, treated with 1,25(OH)₂D₃ and phosphate, who finally developed hyperparathyroidism with gland hyperplasia before the tumor could be localized in the scapula and removed. During surgery a transiliac bone biopsy was obtained. FGF23 expression in the tumor cells was confirmed by in situ hybridization. Serum FGF23 levels as measured by ELISA were found to be extremely elevated before and decreased after removal of the tumor. Bone histology/histomorphometry and measurement of bone mineralization density distribution using quantitative backscattered electron imaging were performed on the bone biopsy. The data showed important surface osteoidosis and a slightly increased osteoblast but markedly decreased osteoclast number. The mineralized bone volume (−11%) and mineralized trabecular thickness (−18%) were low. The mean degree of mineralization of the bone matrix (−7%), the most frequent calcium concentration (−4.1%), and the amounts of fully mineralized bone (−40.3%) were distinctly decreased, while the heterogeneity of mineralization (+44.5%) and the areas of primary mineralization (+131.6%) were dramatically increased. We suggest that the elevated levels of FGF23 and/or low phosphate concentrations disturb the mineralization kinetics in vivo without affecting matrix mineralization of pre-existing bone packets. K. Nawrot-Wawrzyniak and F. Varga contributed equally to this work.     

A case of familial tumoral calcinosis/hyperostosis–hyperphosphatemia syndrome due to a compound heterozygous mutation in GALNT3 demonstrating new phenotypic features

Summary  A new case of familial tumoral calcinosis (FTC)/hyperostosis–hyperphosphatemia syndrome (HHS) due to a novel compound heterozygous mutation in N-acetylgalactosaminyltransferase 3 (GALNT3) and with new phenotypic findings is presented. The response in serum phosphate and fibroblast growth factor 23 (FGF23) to medical treatment is detailed. This case expands the genotype and phenotype of FTC/HHS and gives insight into its treatment and pathophysiology. Introduction  FTC and HHS are caused by mutations in FGF23, GALNT3, or KLOTHO. They are characterized by hyperphosphatemia, increased phosphate reabsorption, and elevated or inappropriately normal serum 1,25-dihydroxyvitamin D₃ (1,25-D₃); FTC is associated with calcific masses, and HHS with diaphyseal hyperostosis. Methods  A 36-year-old woman presented with abnormal dental X-rays at age 12 and was hyperphosphatemic at 22. She underwent radiographic, biochemical and genetic testing, and medical treatment. Results  Serum phosphorus was 7.3 mg/dL (2.5–4.8), TmP/GFR 6.99 mg/100 mL (2.97–4.45), 1,25-D₃ 35 pg/mL (22–67). Radiographs revealed tooth anomalies, thyroid cartilage calcification, calcific masses in vertebral spaces, calcification of the interstitial septa of the soft tissue in the lower extremities, and cortical thickening of the long bones. Her total hip Z score was 1.9. C-terminus serum FGF23 was 1,210 RU/mL (20–108), but intact FGF23 was 7.4 pg/mL (10–50). DNA sequencing determined she was a compound heterozygote for mutations in GALNT3. Treatment with niacinamide and acetazolamide decreased TmP/GFR and serum phosphate, which was paralleled by a decrease in serum C-terminus FGF23. Conclusions  This case broadens the spectrum of phenotypic and genotypic features of FTC/HHS and suggests treatments to decrease renal phosphate reabsorption in the setting of a low intact FGF23.     

Plasma levels of fibroblast growth factor-23 and mineral metabolism in diabetic and non-diabetic patients on chronic hemodialysis

Objective  Fibroblast growth factor (FGF) 23 is a circulating factor that regulates phosphate (P) metabolism. Since higher P levels are associated with vascular calcification, we examined the role of serum FGF-23 levels in P metabolism and vascular calcification in hemodialysis (HD) patients with and without diabetes mellitus (DM). Materials and methods  Chronic HD patients with DM (n = 39) and without DM (n = 50) were enrolled. Serum samples were obtained before the start of dialysis sessions, and the FGF-23 levels were determined by enzyme-linked immunosorbent assay. Abdominal computed tomography (CT) scan was performed, and the aortic calcification index (ACI) was determined by one examiner, blinded to the patient characteristics. Measurements of bone mineral density (BMD) were performed at the time of ACI estimation. Results  Log plasma FGF-23 levels were higher in non-DM (3.74 ± 0.71 pg/ml) than in DM (3.35 ± 0.74 pg/ml) patients. The log FGF-23 correlated positively with serum creatinine (r = 0.424, P < 0.0001), albumin (r = 0.225, P = 0.0337), Ca (r = 0.392, P = 0.0001), P (r = 0.735, P < 0.0001), and Ca × P product (r = 0.780, P < 0.0001). There were negative correlations between log FGF-23 and age (r = −0.208, P = 0.0497), glucose (r = −0.231, P = 0.0294), and CRP (r = −0.222, P = 0.0359). Multiple regression analyses were performed to explore the correlations between plasma FGF-23 and other factors associated with vascular calcification in all HD patients. Independent variables were selected based on the results of univariate analyses. The significant factors associated with FGF-23 in HD patients were age, serum levels of creatinine, albumin, glucose, Ca, P, and Ca × P product. Plasma FGF levels did not correlate significantly with either ACI or BMD in these patients. Conclusion  Our findings indicate that the plasma FGF-23 level is associated with calcium-phosphate metabolism disorders, but not with aortic calcification, in both non-DM and DM patients on chronic HD. In addition, plasma FGF-23 is associated with serum levels of creatinine and albumin. Therefore, the plasma FGF-23 level may provide a reliable marker for Ca and P imbalance and nutritional status in HD patients.     

The expanding family of hypophosphatemic syndromes

Investigation of X-linked hypophosphatemia (XLH) has led to the identification of a novel phosphate-regulating homeostatic system. Initially considered vitamin D-refractory rickets, renal phosphate wasting was identified as the cardinal biochemical feature of XLH and several related disorders. Current therapy employs calcitriol and phosphate, which usually improves, but does not completely heal deformities and short stature. Later complications of XLH include development of osteophytes, entheses, and osteoarthritis. The mutated gene in XLH, PHEX, is expressed in osteocytes, but its role in the pathogenesis of phosphate wasting is poorly understood. Many hypophosphatemic disorders are mediated by FGF23, a unique fibroblast growth factor with endocrine properties. Renal action of FGF23 leads to reduced expression of type II sodium-phosphate co-transporters, as well as reduced expression of CYP27B1, which encodes vitamin D 1α-hydroxylase. FGF23-mediated hypophosphatemia is characterized by inappropriately normal circulating 1,25-dihydroxyvitamin D together with renal phosphate wasting. The FGF23 system serves as a novel mechanism by which the mineralizing skeleton can communicate phosphate supply to the kidney and thereby mediate excretion or conservation of this important skeletal component. Other forms of FGF23-mediated hypophosphatemia represent various aberrations in this axis. Secretion of excess FGF23 (as in tumor-induced osteomalacia), and mutations preventing proteolytic cleavage of FGF23 result in similar clinical features. Other hypophosphatemic disorders are discussed.     

Decrease in serum FGF23 levels after intravenous infusion of pamidronate in patients with osteogenesis imperfecta

Fibroblast growth factor 23 (FGF23) plays a central role in phosphate (P) homeostasis. However, the precise mechanism of how FGF23 secretion is regulated remains to be elucidated. In the present study, we examined the effect of intravenous pamidronate administration on serum levels of FGF23. Thirteen patients with osteogenesis imperfecta were treated with two cycles of 3-day pamidronate infusion. Blood samples at pre- and post-drip pamidronate infusion were evaluated for serum calcium, P, intact PTH (iPTH), 1,25(OH)₂D, intact FGF23 (FGF23), type I collagen cross-linked N-telopeptides (NTx), bone-specific alkaline phosphatase (BAP), and TmP/GFR. During the two cycles, FGF23 levels decreased significantly preceding the decline in P levels. Although the change in P levels became less apparent during the second cycle, the reduction in FGF23 levels was similar during both cycles. Moreover, absence of correlation between FGF23 and P indicates that FGF23 attenuation is independent of the decrease in P levels during pamidronate infusion. Significant correlation between NTx suppression and the decrease in FGF23 levels during the 1st cycle (r = 0.665, P = 0.013) suggests that inhibition of osteoclast function may have some role in suppressing FGF23 levels. Because pamidronate dose was most associated with the decrease in FGF23 levels during the second cycle, pamidronate may directly attenuate osteocyte/osteoblast-mediated FGF23 production. This is the first evidence of a rapid fall in FGF23 levels following pamidronate infusion, raising the possibility that inhibition of bone resorption and/or direct effects of pamidronate may suppress secretion of FGF23.     

Elevated level of endothelin-1 in cerebrospinal fluid and lack of nitric oxide in basilar arterial plasma associated with cerebral vasospasm after subarachnoid haemorrhage in rabbits

Background  The role of endothelin-1 (ET-1) and nitric oxide (NO) as two important mediators in the development of cerebral vasospasm (CVS) after subarachnoid haemorrhage (SAH) is controversial. The objective of this study was to determine whether local levels of ET-1 and NO in cerebral arterial plasma and/or in cerebrospinal fluid (CSF) are associated with the occurrence of CVS after SAH. Methods  CVS was induced using the one-haemorrhage rabbit model and confirmed by digital subtraction angiography of the rabbits’ basilar artery on day 5. Prior to sacrifice, local CSF and basilar arterial plasma samples were obtained by a transclival approach to the basilar artery. Systemic arterial plasma samples were obtained. ET-1 levels were determined by immunometric technique (pg/ml ± SEM) and total nitrate/nitrite level spectrophotometrically (μmol/l ± SEM). Findings  Angiographic CVS was documented after SAH induction (n = 12, P < 0.05). The ET-1 level in CSF was significantly elevated by 27.3% to 0.84 ± 0.08 pg/ml in SAH animals (n = 7) in comparison to controls (0.66 ± 0.04 pg/ml, n = 7, P < 0.05). There was no significant difference in ET-1 levels in systemic and basilar arterial plasma samples of SAH animals compared to controls. A significant lack of local NO metabolites was documented in basilar arterial plasma after SAH (36.8 ± 3.1 μmol/l, n = 6) compared to controls (61.8 ± 6.2 μmol/l, n = 6, P < 0.01). Conclusion  This study demonstrates that an elevated ET-1 level in CSF and local lack of NO in the basilar arterial plasma samples are associated with CVS after experimental SAH.     

Urinary interleukin-6 is useful in distinguishing between upper and lower urinary tract infections

This study was designed to determine whether the measurement of interleukin (IL)-6 in urine is useful for distinguishing between acute pyelonephritis and lower urinary tract infection. This observational study was carried out at León Hospital (Spain) on 35 patients (ten boys) aged between 0 and 14 years with urinary tract infection. Urinary levels of IL-6 were determined with enzyme-linked immunosorbent assay (ELISA) at diagnosis and after recovery. Renal dimercaptosuccinate acid (DMSA) scan was performed on all patients to discard or confirm acute pyelonephritis. The mean urinary concentration [x ± standard deviation (SD)] of IL-6 at diagnosis was 20.3 ± 23.3 and 5.3 ± 9.7 pg/ml in patients with acute pyelonephritis and lower urinary infection, respectively [95% confidence interval (CI): 2.6–27.4; p < 0.01]. Specificity for a value of IL-6 >15 pg/ml, was 94.1% (95% CI: 91.1–97.1). Positive predictive value for IL-6 >15 pg/ml was 87.5% (95% CI: 81.1–93.8). IL-6 was undetectable in the urine of both groups of patients at the time of recovery. Urinary levels of IL-6 are useful in differentiating between upper and lower urinary tract infection in children. In this clinical setting, a value >15 pg/ml is a strong indicator of acute pyelonephritis.