C-terminal FGF23 Research Articles

Klotho and fibroblast growth factor 23 in cerebrospinal fluid in children.

The fibroblast growth factor (FGF) 23/Klotho axis is a principal regulator of phosphate hemostasis and vitamin D metabolism, but limited data is available on its role in the central nervous system. Here, we investigate soluble α-Klotho (sKlotho) and C-terminal as well as intact FGF23 in cerebrospinal fluid (CSF) and plasma and their relationship to mineral metabolism parameters in humans. In 39 children aged 0.3-16.8years undergoing lumbar puncture for the exclusion of inflammatory neurological disease, sKlotho and FGF23 were investigated by Western blot analysis, followed by ELISA quantification in CSF and plasma. The percentage of intrathecal synthesis of both proteins was calculated by measuring both the expected and observed CSF/plasma ratios of sKlotho and FGF23. The secreted (KL1) and cleaved (KL1+KL2) isoforms of sKlotho, and FGF23 were clearly detected in CSF in all subjects, although protein levels were lower compared to those of plasma samples (each p<0.01). The intrathecal percentage of CSF sKlotho and FGF23 synthesis amounted to 98 and 99%, respectively. CSF sKlotho levels were higher in boys than in girls (p<0.01), and correlated positively with plasma C-terminal FGF23 concentrations (p<0.05) and standardized height (p<0.01). Importantly, there were no significant correlations between plasma and CSF levels of sKlotho or FGF23. Plasma sKlotho as well as C-terminal and intact FGF23, respectively, were associated with parameters of mineral metabolism These results provide evidence that cleaved and secreted sKlotho and FGF23 are present in CSF, mainly derived from brain and affected by sex, height, and mineral metabolism parameters in children. Nevertheless, the absence of significant associations between plasma and CSF levels of Klotho and FGF23, respectively, suggest that the regulation of Klotho and FGF23 may be different between organs secreting these hormones into blood and CSF.

Calcific uraemic arteriolopathy (calciphylaxis): data from a large nationwide registry.

Calcific uraemic arteriolopathy (CUA, calciphylaxis) is a rare disease predominantly in dialysis patients and associated with high mortality. Painful skin ulcerations and calcification of cutaneous arterioles characterize calciphylaxis. We established an observational, Internet-based registry allowing online notification for all German CUA cases. The registry recorded data about patient characteristics, biochemistry and therapies. Blood samples were stored in a central biobank. Between 2006 and 2015, 253 CUA patients were recorded: median age 70 [interquartile range (IQR) 61-76] years, 60% females and 86% ( n = 207) dialysis patients, translating into an estimated annual incidence rate of 0.04% in German dialysis patients. Fifty-two per cent received vitamin K antagonists (VKAs) prior to CUA. Skin lesions were localized in 71% on the legs or gluteal region. In dialysis CUA patients median total serum calcium was 2.20 (IQR 2.06-2.37) mmol/L, phosphorus 1.67 (IQR 1.35-2.03) mmol/L, intact parathyroid hormone 147 (IQR 72-276) pg/mL and fetuin-A 0.21 (IQR 0.16-0.26) g/L (normal range 0.35-0.95). Median sclerostin, osteoprotegerin, TRAP5b, bone-specific alkaline phosphatase and c-terminal FGF23 levels were all elevated. The most frequently recorded therapeutic procedures in dialysis CUA patients were as follows: wound debridement (29% of cases), stopping VKA (25%), lowering calcium supply (24%), sodium thiosulphate (22%), application of vitamin K (18%), increase of dialysis duration/frequency (17%) and stoping active vitamin D (16%). Approximately 50% of CUA patients used VKA. Our data suggest that uncontrolled hyperparathyroidism is not the key determinant of calciphylaxis. Therapeutic strategies were heterogeneous. The experience of the German registry will help substantially to initiate a large-scale multinational CUA registry.

Up-regulation of FGF23 release by aldosterone

The fibroblast growth factor (FGF23) plasma level is high in cardiac and renal failure and is associated with poor clinical prognosis of these disorders. Both diseases are paralleled by hyperaldosteronism. Excessive FGF23 levels and hyperaldosteronism are further observed in Klotho-deficient mice. The present study explored a putative aldosterone sensitivity of Fgf23 transcription and secretion the putative involvement of the aldosterone sensitive serum & glucocorticoid inducible kinase SGK1, SGK1 sensitive transcription factor NFκB and store operated Ca2+ entry (SOCE). Serum FGF23 levels were determined by ELISA in mice following sham treatment or exposure to deoxycorticosterone acetate (DOCA) or salt depletion. In osteoblastic UMR106 cells transcript levels were quantified by qRT-PCR, cytosolic Ca2+ concentration utilizing Fura-2-fluorescence, and SOCE from Ca2+ entry following store depletion by thapsigargin. As a result, DOCA treatment and salt depletion of mice elevated the serum C-terminal FGF23 concentration. In UMR106 cells aldosterone enhanced and spironolactone decreased SOCE. Aldosterone further increased Fgf23 transcript levels in UMR106 cells, an effect reversed by mineralocorticoid receptor blockers spironolactone and eplerenone, SGK1 inhibitor EMD638683, NFκB-inhibitor withaferin A, and Ca2+ channel blocker YM58483. In conclusion, Fgf23 expression is up-regulated by aldosterone, an effect sensitive to SGK1, NFκB and store-operated Ca2+ entry.

Baseline FGF23 is Associated with Cardiovascular Outcome in Incident PD Patients.

♦ Fibroblast growth factor 23 (FGF23) is a phosphate regulating protein. Several studies demonstrated that elevated FGF23 is independently associated with mortality for early-stage chronic kidney disease and incident hemodialysis (HD) patients. However, little is known about the significance of elevated FGF23 in peritoneal dialysis (PD) patients. Here, we analyzed the association of FGF23 with cardiovascular (CV) events, all-cause mortality, residual renal function (RRF), and CV parameters in PD patients. ♦ The present study is a single-center, retrospective study. Patients who started PD at Seoul National University Hospital between January 2005 and July 2011 and whose baseline serum samples were available were enrolled. C-terminal FGF23 was measured. Subjects were divided into 2 groups; lower 2 tertiles (FGF23 ≤ 119.0 RU/mL) and top tertile (FGF23 > 119.0 RU/mL). The primary outcome was time to fatal or non-fatal CV events. In the subgroup analysis, the associations of FGF23 with aortic stiffness or with vascular calcification were analyzed. ♦ A total of 205 incident PD patients were analyzed. Mean duration of follow-up was 41.6 ± 20.0 months. The baseline median FGF23 level was 78.6 RU/mL (inter-quartile range [IQR], 34.1 - 155.0). At baseline, subjects in the higher FGF23 group were younger, and had a lower RRF, lower prevalence of diabetes mellitus (DM), and cerebrovascular disease. During follow-up, 22 of the 205 patients (10.7%) reached primary outcome. After adjustment for age, DM, pre-existing coronary artery disease, cerebrovascular disease, congestive heart failure, and left ventricular mass index, the higher FGF23 group exhibited significantly higher risk of primary outcome, compared with the lower group (hazard ratio [HR], 2.54; 95% confidence interval [CI], 1.05 - 6.12; p = 0.045). There were no significant differences in all-cause mortality and development of anuria between the 2 FGF23 groups. In the subgroup analysis, FGF23 groups were not associated with pulse wave velocity and abdominal aortic calcification score. ♦ Elevated FGF23 is associated with higher risk of adverse CV outcome for incident PD patients.

C-Terminal to Intact Fibroblast Growth Factor 23 Ratio in Relation to Estimated Glomerular Filtration Rate in Elderly Population

Background/Aims: An analytical equivalence between intact fibroblasts growth factor(iFGF23) and C-terminal(cFGF23) assays is logically expected, however, numerous studies demonstrate lack of a strong association between them. Previously, we have demonstrated the increase in cFGF23 slightly precedes the increase of iFGF23 with the impairment of kidney excretory function; without actually analyzing the ratio between both assays, which are postulated to be affected by declining kidney function. Therefore, the aim of this study was to analyze the ratio between C and iFGF23 in relation to the estimated glomerular filtration rate (eGFR) in an elderly population. Methods: We analysed the variability of c/iFGF23 ratio in the population of 3264 elderly PolSenior study participants (≥ 65years) in the relation to eGFR calculated according full Modification of Diet in Renal Disease, serum levels of C-reactive protein (hs-CRP), and iron. Results: The log₁₀(c/i FGF23 ratio) increased in the subsequent CKD stages. Serum iron and CRP levels reduced the log₁₀ and increased it with age in multivariate regression analysis. Conclusions: Our results suggest impairment in the cleavage of the C-terminal FGF23 fragments with the deterioration of kidney excretory function and age in the elderly population. Inflammation and low serum iron level seems to diminish degradation capacity of FGF23 fragments.

Impact of Annexin A 7 Deficiency on FGF23 Plasma Concentrations

Background/Aims: The release of fibroblast growth factor FGF23, a powerful regulator of 1,25(OH)₂D₃ formation and mineral metabolism, is stimulated by store-operated Ca²⁺ entry (SOCE), which is accomplished by the pore forming Ca²⁺ release activated channel protein Orai1. Regulators of Orai1 and thus FGF23 release include serum & glucocorticoid inducible kinase SGK1, a kinase up-regulated by glucocorticosteroids. Some effects of glucocorticoids require the presence of annexin A7, such as suppression of prostaglandin E2 in gastric glands. The present study thus explored whether annexin A7 impacts on FGF23 plasma levels. Methods: Comparisons were made between gene targeted mice lacking functional annexin A7 (Anx7^-/-) and their wild type littermates (Anx7^+/+). Serum C-terminal-FGF23, intact FGF23, 1,25(OH)₂D₃ and PTH concentrations were measured by ELISA or EIA. The serum and urinary phosphate concentrations were measured by colorimetry, the serum Ca²⁺ concentration and the urinary Ca²⁺ concentration by flame photometry. Results: Serum C-terminal FGF23 levels and corticosterone levels were significantly higher and serum 1,25(OH)₂ D₃ and PTH levels were significantly lower in Anx7^-/- than in Anx7^+/+ mice. Water intake was slightly but significantly higher in Anx7^-/- mice than in Anx7^+/+ mice. No significant difference was observed between Anx7^-/- and Anx7^+/+ mice in urinary fluid excretion, plasma Ca²⁺ concentration, plasma phosphate concentration and urinary Ca²⁺ output. The urinary phosphate output was significantly lower in Anx7^-/- mice than in Anx7^+/+ mice. Conclusion: Annexin A7 deficiency upregulates FGF23 plasma levels, an effect paralleled by increased corticosterone plasma levels, as well as decreased 1,25(OH)₂ D₃ and PTH plasma levels.

The effects of maternal iron deficiency on infant fibroblast growth factor-23 and mineral metabolism

Fibroblast growth factor-23 (FGF23), a phosphate(Phos)-regulating hormone, is abnormally elevated in hypophosphataemic syndromes and an elevated FGF23 is a predictor of mortality in kidney disease. Recent findings suggest iron deficiency as a potential mediator of FGF23 expression and murine studies have shown in utero effects of maternal iron deficiency on offspring FGF23 and phosphate metabolism.Our aim was to investigate the impact of maternal iron status on infant FGF23 and mineral metabolites over the first 2years of life. Infants born to mothers with normal (NIn=25,) and low (LIn=25) iron status during pregnancy, from a mother-infant trial (ISRCTN49285450) in rural Gambia, West Africa, had blood and plasma samples analysed at 12, 24, 52, 78 and 104weeks (wk) of age.Circulating intact-FGF23 (I-FGF23), Phos, total alkaline phosphatase (TALP) and haemoglobin (Hb) decreased and estimated glomerular filtration rate increased over time [all P≤0.0001)]. C-terminal-FGF23 (C-FGF23) and TALP were significantly higher in LI compared with NI, from 52wk for C-FGF23 [Beta coefficient (SE) 18.1 (0.04) %, P=0.04] and from 24wk for TALP [44.7 (29.6) U/L, P=0.04]. Infant Hb was the strongest negative predictor of C-FGF23 concentration [−21% (4%) RU/mL, P≤0.0001], Phos was the strongest positive predictor of I-FGF23 [32.0(3.9) pg/mL, P≤0.0001] and I-FGF23 did not predict C-FGF23 over time [−0.5% (0.5%), P=0.3].In conclusion, this study suggests that poor maternal iron status is associated with a higher infant C-FGF23 and TALP but similar I-FGF23 concentrations in infants and young children. These findings further highlight the likely public health importance of preventing iron deficiency during pregnancy. Whether or not children who are born to iron deficient mothers have persistently high concentrations of these metabolites and are more likely to be at risk of impaired bone development and pre-disposed to rickets requires further research.

Checkpoint kinase Chk2 controls renal Cyp27b1 expression, calcitriol formation, and calcium-phosphate metabolism.

Checkpoint kinase 2 (Chk2) is the main effector kinase of ataxia telangiectasia mutated (ATM) and responsible for cell cycle regulation. ATM signaling has been shown to upregulate interferon-regulating factor-1 (IRF-1), a transcription factor also expressed in the kidney. Calcitriol (1,25 (OH)2D3), a major regulator of mineral metabolism, is generated by 25-hydroxyvitamin D 1α-hydroxylase in the kidney. Since 25-hydroxyvitamin D 1α-hydroxylase expression is enhanced by IRF-1, the present study explored the role of Chk2 for calcitriol formation and mineral metabolism. Chk2-deficient mice (chk2 (-/-)) were compared to wild-type mice (chk2 (+/+)). Transcript levels of renal 25-hydroxyvitamin D 1α-hydroxylase, Chk2, and IRF-1 were determined by RT-PCR; Klotho expression by Western blotting; bone density by μCT analysis; serum or plasma 1,25 (OH)2D3, PTH, and C-terminal FGF23 concentrations by immunoassays; and serum, fecal, and urinary calcium and phosphate concentrations by photometry. The renal expression of IRF-1 and 25-hydroxyvitamin D 1α-hydroxylase as well as serum 1,25 (OH)2D3 and FGF23 levels were significantly lower in chk2 (-/-) mice compared to chk2 (+/+) mice. Plasma PTH was not different between the genotypes. Renal calcium and phosphate excretion were significantly higher in chk2 (-/-) mice than in chk2 (+/+) mice despite hypophosphatemia and normocalcemia. Bone density was not different between the genotypes. We conclude that Chk2 regulates renal 25-hydroxyvitamin D 1α-hydroxylase expression thereby impacting on calcium and phosphate metabolism.

Exome sequencing reveals a mutation in DMP1 in a family with familial sclerosing bone dysplasia

IntroductionHypophosphatemic rickets (HR) comprises a rare group of inherited diseases. Very recently, mutations in the dentin matrix protein 1 (DMP1) gene were identified in patients with an extremely rare autosomal recessive form of HR (ARHR). To date, very few cases of these mutations were reported. Materials and methodsA Lebanese consanguineous family with 2 affected sisters was studied. Patients aged 45 and 47years old presented with short stature, severe genu varum, cranial hyperostosis and a very high bone density that led to a diagnosis of a familial sclerosing bone dysplasia. Molecular analysis of known genes involved in osteopetrosis showed normal results. A combination of genotyping and exome sequencing was performed in order to elucidate the genetic basis of this pathology. ResultsBiochemical analysis was consistent with normal serum calcium and 1-25(OH)2D levels, low to normal serum phosphorus and elevated PTH values. Serum c-terminal FGF-23 was elevated in one of the two patients. A homozygous mutation disrupting the initiation codon of the DMP1 gene (OMIM 600980), NM_001079911.2: c.1A>G, p.Met1Val, was identified by exome sequencing and confirmed by Sanger sequencing. ConclusionWe report here a family of ARHR secondary to a DMP1 mutation located in the first coding exon of the gene. Our cases show that some ARHR cases may develop with age an unaccountable increase in bone density and bone overgrowth.

Changes in bone mineral metabolism parameters, including FGF23, after discontinuing cinacalcet at kidney transplantation.

Little is known about the effects of the administration of cinacalcet in dialytic patients who are scheduled for kidney transplantation, and in particular about the changes in FGF23 and other mineral metabolism parameters after surgery compared with recipients not on cinacalcet at kidney transplantation. We performed a prospective observational cohort study with recruitment of consecutive kidney transplant recipients at our institution. Patients were classified according to whether they were under treatment with cinacalcet before transplantation. Bone mineral metabolism parameters, including C-terminal FGF23, were measured at baseline, on day 15, and at 1, 3, and 6 months after transplantation. In previously cinacalcet-treated patients, cinacalcet therapy was discontinued on the day of surgery and was not restarted after transplantation. A total of 48 kidney transplant recipients, 20 on cinacalcet at surgery and 28 cinacalcet non-treated patients, completed the follow-up. Serum phosphate declined significantly in the first 15 days after transplantation with no differences between the two groups, whereas cinacalcet-treated patients showed higher FGF23 levels, although not significant. After transplantation, PTH and serum calcium were significantly higher in cinacalcet-treated patients. We conclude that patients receiving cinacalcet on dialysis presented similar serum phosphate levels but higher PTH and serum calcium levels during the initial six months after kidney transplantation than cinacalcet non-treated patients. The group previously treated with cinacalcet before transplantation showed higher FGF23 levels without significant differences, so further studies should investigate its relevance in the management of these patients.

Hyperphosphatemic familial tumoral calcinosis: Response to acetazolamide and postulated mechanisms

Hyperphosphatemic familial tumoral calcinosis (HFTC) is characterized by enhanced renal phosphate absorption, hyperphosphatemia, and tumor-like extraosseous calcifications due to inactivating mutations in FGF23 or associated proteins. Surgical excision is needed when low phosphate diet and phosphate binders are ineffective. Sporadic reports have supported acetazolamide use. We report on a 7-year-old African American boy who presented with severe HFTC requiring numerous surgical excisions. Tumors continued to appear and others reoccurred despite phosphate restriction and sevelamer carbonate. At the age of 9.5 years, acetazolamide (40 mg/kg/day) was added and resulted in mild metabolic acidosis (bicarbonate 25.3 mEq/L vs. 21.4 mEq/L, P < 0.001; serum pH 7.38 vs. 7.31, P = 0.013, pre- and post-acetazolamide, respectively) but no change in tubular reabsorption of phosphate (TRP) (96.9% vs. 95.9%, P = 0.34) or serum phosphate (6.6 mg/dl vs. 6.9 mg/dl, P = 0.52 pre- and post-acetazolamide, respectively). Following the initiation of acetazolamide therapy, the patient experienced significant improvement in disease course as indicated by resolution of localized bone pain, cessation of tumor formation, and no tumor recurrence. Despite mild metabolic acidosis, our patient had improved linear growth and did not develop any other side effects related to therapy. Intact FGF23 remained abnormally low throughout disease course, while C-terminal FGF23 increased with acetazolamide. We conclude that acetazolamide can control severe HFTC by inducing mild metabolic acidosis despite no change in serum phosphate or TRP. This effect may be exerted though improved calcium-phosphate complex solubility and increased FGF23 locally.

Effect of cholecalciferol recommended daily allowances on vitamin D status and fibroblast growth factor-23: An observational study in acute burn patients

ObjectiveBurn patients are at risk of hypovitaminosis D. Optimal vitamin D (VD) intakes are not defined in burn nutrition guidelines and studies mostly focused on ergocalciferol (VD2) supplementation in burn children. Aim of our study was to describe adult burns VD status, to measure effects of our cholecalciferol (VD3) supplementation on VD metabolism during acute burn care, and to assess correlation between FGF23 and C-reactive protein (CRP). DesignCohort study. MethodsFrom March 2012 to January 2013, patients >18 years, admitted within 24h after injury with burn surface area (BSA) ≥10% were included. Patients daily received VD3 from oral or enteral nutrition (400–600IU) and from oral or intravenous multivitamin complex (200–220IU). Serum levels of 25(OH)-D, 1-25(OH)2-D, 3rd generation PTH, C-terminal FGF23, total calcium, phosphate, albumin and CRP were measured at admission (D0) and every week during 4 weeks of follow-up. Data are expressed as percentage or median (min–max). Paired data were compared using Wilcoxon test. Correlation between CRP and FGF23 was assessed using nonparametric Spearman test. A p value <0.05 was considered to be statistically significant. ResultsWe initially included 24 patients. Median age and BSA were, respectively, 46 [19–86] years and 15 [10–85]%. At D0, 75% presented a VD insufficiency (25(OH)-D 21–29ng/ml) and 17% presented a deficiency (25(OH)-D ≤20ng/ml). We followed 12 patients until day 28: 25(OH)-D was unchanged while 1-25(OH)2-D and FGF23 decreased without reaching significance. We observed a significant positive correlation between FGF23 and CRP (r=0.59, 95% CI: 0.22–0.82, p=0.0032). ConclusionsMost of our adult burns presented hypovitaminosis D regardless of age. Nutrition supplemented with low dose of VD3 (intakes reaching recommended daily allowances) was insufficient to correct 25(OH)-D level. Moreover, an interesting correlation between CRP and FGF23 was found.

Hémodiafiltration online et hypoparathyroïdie chez l’enfant : une série de cas monocentrique

BackgroundDue to technical requirements and cost, hemodiafiltration (HDF) is not widely used in pediatrics. We have been using online HDF (oHDF) since 2009 and we observed low parathyroid hormone (PTH) levels despite the accurate management of CKD-MBD. MethodsWe reviewed the medical charts and parameters of mineral metabolism assessed on a before/after session basis in the 6 children undergoing chronic oHDF in our centre. ResultsWe observed low (<80pg/mL) PTH levels in all 6 patients and very low (<45pg/mL) PTH levels in 5, two of them presenting with pathological fractures. These low PTH levels were reversed after decreasing calcium concentration to 1.25 mmol/L in the dialysate, suggesting that high-efficiency oHDF may expose children to calcium during sessions in a too important amount when using 1.5 mmol/L dialysates. Last, C-terminal FGF23 levels before sessions were relatively low (<1600RU/mL), with a 32% clearance by oHDF. ConclusionPTH levels should be closely monitored in pediatric oHDF and solutions with a calcium concentration of 1.25 mmol/L should be used as first line in these patients.

Literature search and review related to specific preparatory work in the establishment of Dietary Reference Values for Phosphorus, Sodium and Chloride

Literature search and review related to specific preparatory work in the establishment of Dietary Reference Values for Phosphorus, Sodium and Chloride

Effect of ferric carboxymaltose on serum phosphate and C-terminal FGF23 levels in non-dialysis chronic kidney disease patients: post-hoc analysis of a prospective study

BackgroundSome parenteral iron therapies have been found to be associated with hypophosphatemia. The mechanism of the decrease in serum phosphate is unknown. The aim of this study is to examine the effect of IV ferric carboxymaltose(FCM) on phosphate metabolism and FGF23 levels in patients with chronic kidney disease(CKD).MethodsThis is a post-hoc analysis of a prospective study carried out in 47 non-dialysis CKD patients with iron-deficiency anaemia who received a single 1000 mg injection of FCM. Markers of mineral metabolism (calcium, phosphate, 1,25-dihydroxyvitamin D, PTH and FGF23[c-terminal]) were measured prior to FCM administration and at week 3 and week 12 after FCM administration. Based on the measured levels of serum phosphate at week 3, patiens were classified as hypophosphatemic or non-hypophosphatemic.ResultsSerum phosphate levels decreased significantly three weeks after FCM administration and remained at lower levels at week 12 (4.24 ± 0.84 vs 3.69 ± 1.10 vs 3.83 ± 0.68 mg/dL, respectively, p < 0.0001. Serum calcium, PTH and 1,25-dihydroxyvitamin D did not change over the course of the study. Serum FGF23 decreased significantly from 442(44.9-4079.2) at baseline to 340(68.5-2603.3) at week 3 and 191.6(51.3-2465.9) RU/mL at week 12, p < 0.0001. Twelve patients were non-hypophosphatemic and 35 hypophosphatemic. FGF23 levels decreased in both groups, whereas no changes were documented in any of the other mineral parameters.ConclusionsIn non-dialysis CKD patients, FCM induces reduction in serum phosphate levels that persists for three months. FCM causes a significant decrease in FGF23 levels without changes to other bone metabolism parameters.

C-terminal FGF23 fragments: present but not seen?

C-terminal FGF23 fragments: present but not seen?

Biological Variability of Plasma Intact and C-Terminal FGF23 Measurements

FGF23 measurement may have a role in the management of patients with chronic kidney disease (CKD). Our objective was to study the biological variability of plasma intact FGF23 (iFGF23) and C-terminal FGF23 (cFGF23) concentrations. Plasma samples were taken from 12 healthy adults at multiple time points on 2 consecutive days to assess diurnal variability of FGF23 concentrations. Early-morning fasting and nonfasting samples were also taken over a 6-wk period to estimate components of biological variation. Samples from 170 volunteers were used to define reference intervals. FGF23 concentrations were measured by commercial ELISA. Western blotting was used to analyze FGF23 species from the plasma of healthy adults and patients with predialysis CKD and those undergoing dialysis. A total of 180 healthy adults and 18 adults with stage 3-5D CKD participated in this study at a hospital research unit. Estimates were made of the biological variability of plasma FGF23 concentrations. iFGF23, but not cFGF23, showed significant diurnal variation. cFGF23 had a significantly lower intra-individual variation than iFGF23 (8.3 vs. 18.3%) but higher inter-individual variation than iFGF23 (28.9 vs. 19.2%). Fourteen samples would be needed to estimate an individual's homeostatic set point (within 10%) for iFGF23 compared with only three samples for cFGF23. Using Western blotting, C-terminal FGF23 fragments were detected in the plasma of individuals with normal renal function and at all stages of renal disease. The percent cFGF23 was significantly higher in those without renal impairment (P < 0.001) and was positively correlated with plasma phosphate concentration in those with normal renal function. The high intra-individual biological variability of iFGF23 may limit its clinical use as a diagnostic or management tool. Risk-related thresholds may be more appropriate for clinical decision making based on cFGF23 measurements than conventional reference intervals. FGF23 cleavage pathways may be an important natural regulatory mechanism for phosphate control.

Iron deficiency drives an autosomal dominant hypophosphatemic rickets (ADHR) phenotype in fibroblast growth factor-23 (Fgf23) knock-in mice

Autosomal dominant hypophosphatemic rickets (ADHR) is unique among the disorders involving Fibroblast growth factor 23 (FGF23) because individuals with R176Q/W and R179Q/W mutations in the FGF23 (176)RXXR(179)/S(180) proteolytic cleavage motif can cycle from unaffected status to delayed onset of disease. This onset may occur in physiological states associated with iron deficiency, including puberty and pregnancy. To test the role of iron status in development of the ADHR phenotype, WT and R176Q-Fgf23 knock-in (ADHR) mice were placed on control or low-iron diets. Both the WT and ADHR mice receiving low-iron diet had significantly elevated bone Fgf23 mRNA. WT mice on a low-iron diet maintained normal serum intact Fgf23 and phosphate metabolism, with elevated serum C-terminal Fgf23 fragments. In contrast, the ADHR mice on the low-iron diet had elevated intact and C-terminal Fgf23 with hypophosphatemic osteomalacia. We used in vitro iron chelation to isolate the effects of iron deficiency on Fgf23 expression. We found that iron chelation in vitro resulted in a significant increase in Fgf23 mRNA that was dependent upon Mapk. Thus, unlike other syndromes of elevated FGF23, our findings support the concept that late-onset ADHR is the product of gene-environment interactions whereby the combined presence of an Fgf23-stabilizing mutation and iron deficiency can lead to ADHR.

Early post-transplantation hypophosphatemia is associated with elevated FGF-23 levels

We examined the hypothesis that high FGF-23 levels early after transplantation contribute to the onset of hypophosphatemia, independently of parathyroid hormone (PTH) and other factors regulating phosphate metabolism. We measured serum phosphate levels (sPi), renal tubular reabsorption of Pi (TmPi/GFR), estimated GFR (eGFR), intact PTH (iPTH), calcitriol, intact (int) and C-terminal (Cter) FGF-23, dietary Pi intake and cumulative doses of glucocorticoids in 69 patients 12 days (95% confidence interval, 10-13) after renal transplantation. Hypophosphatemia was observed in 43 (62%) of the patients 12 days after transplantation. Compared with non-hypophosphatemic subjects, their post-transplantation levels of intact and CterFGF-23 were higher (195 (108-288) vs 48 (40-64) ng/l, P<0.002 for intFGF-23; 205 (116-384) vs 81 (55-124) U/ml, P<0.002, for CterFGF-23). In all subjects, Cter and intFGF-23 correlated inversely with sPi (r=-0.35, P<0.003; -0.35, P<0.003, respectively), and TmPi/GFR (r=-0.50, P<0.001; -0.54, P<0.001, respectively). In multivariate models, sPi and TmPi/GFR were independently associated with FGF-23, iPTH and eGFR. Pre-transplant iPTH levels were significantly higher in patients developing hypophosphatemia after renal transplantation. Pre-transplant levels of FGF-23 were not associated with sPi at the time of transplantation. In addition to PTH, elevated FGF-23 may contribute to hypophosphatemia during the early post-renal transplant period.

Ablation of the Galnt3 gene leads to low-circulating intact fibroblast growth factor 23 (Fgf23) concentrations and hyperphosphatemia despite increased Fgf23 expression.

Familial tumoral calcinosis is characterized by ectopic calcifications and hyperphosphatemia. The disease is caused by inactivating mutations in fibroblast growth factor 23 (FGF23), Klotho (KL), and uridine diphosphate-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase 3 (GALNT3). In vitro studies indicate that GALNT3 O-glycosylates a phosphaturic hormone, FGF23, and prevents its proteolytic processing, thereby allowing secretion of intact FGF23. In this study we generated mice lacking the Galnt3 gene, which developed hyperphosphatemia without apparent calcifications. In response to hyperphosphatemia, Galnt3-deficient mice had markedly increased Fgf23 expression in bone. However, compared with wild-type and heterozygous littermates, homozygous mice had only about half of circulating intact Fgf23 levels and higher levels of C-terminal Fgf23 fragments in bone. Galnt3-deficient mice also exhibited an inappropriately normal 1,25-dihydroxyvitamin D level and decreased alkaline phosphatase activity. Furthermore, renal expression of sodium-phosphate cotransporters and Kl were elevated in Galnt3-deficient mice. Interestingly, there were sex-specific phenotypes; only Galnt3-deficient males showed growth retardation, infertility, and significantly increased bone mineral density. In summary, ablation of Galnt3 impaired secretion of intact Fgf23, leading to decreased circulating Fgf23 and hyperphosphatemia, despite increased Fgf23 expression. Our findings indicate that Galnt3-deficient mice have a biochemical phenotype of tumoral calcinosis and provide in vivo evidence that Galnt3 plays an essential role in proper secretion of Fgf23 in mice.