Fibroblast Growth Factor 23 Gene Research Articles

Fibroblast growth factor 23 (FGF23) gene polymorphisms are associated with essential hypertension risk and blood pressure levels in Chinese Han population

ABSTRACTIn this case-control study, 246 EH patients and 157 healthy controls were selected from Chinese Han population to explore the associations between the fibroblast growth factor 23 (FGF23) gene polymorphisms and essential hypertension (EH).The SequenomMassarray system was used for the genotyping of three FGF23 gene Tag single-nucleotide polymorphisms, namely rs7955866, rs13312756, and rs3812822. The primers were designed by Assay Designer 3.1 software, and then the samples were added to a 384-well plate for the polymerase chain reaction amplification, shrimp alkaline phosphatase reaction, and desalting after extension. The distributions of the alleles, genotypes, and haplotypes were compared between the two groups. Confounding factors (sex, age, BMI, smoking, and drinking) were adjusted in the non-logistic regression, and the results showed that rs7955866 and rs3812822 polymorphisms were independently associated with the risk of developing EH (P < 0.05). The statistical analysis of the haplotype of rs7955866-rs13312756-rs3812822 showed that haplotype ACC could increase the risk of developing EH (P = 0.046; OR = 1.513, 95%CI: 1.005–2.278). The analysis of the control group showed that carrying rs7955866 A allele (P = 0.031) and rs3812822 C allele (P = 0.025) was associated with the increase of systolic blood pressure (SBP). The insulin (INS) level in the peripheral blood was significantly different between the case and control groups (P = 0.014). After confounding factors were excluded, the results showed that the serum INS level was also an independent risk factor of developing EH (P = 0.044; OR = 1.604, 95%CI: 1.014–2.539). In summary, our results suggest that FGF23 gene polymorphisms are associated with the risk of developing EH in Chinese Han population.

Circulating adiponectin modifies the FGF23 response to vitamin D receptor activation: a post hoc analysis of a double-blind, randomized clinical trial.

The fibroblast growth factor 23 (FGF23) response to phosphate load is suppressed in adiponectin gene null mice and substantially amplified in mice overexpressing the same gene and vitamin D receptor (VDR) activation markedly enhances FGF23 gene expression. We performed an analysis of the static (baseline adiponectin levels) and dynamic (fluctuations in adiponectin levels) interactions of serum adiponectin with the FGF23 response to paricalcitol and placebo in the setting of a double-blind, randomized clinical trial in chronic kidney disease (CKD) patients (NCT01680198). As compared with placebo, VDR activation by paricalcitol markedly increased serum FGF23 levels (P < 0.001), and such an increase was amplified in patients in the 4th adiponectin quartile as compared with other quartiles (P = 0.009) while no such an effect was noted in the placebo group (P = 0.49). Both baseline adiponectin (P for interaction = 0.009) and fluctuations in adiponectin levels following paricalcitol and placebo (P for interaction = 0.003) strongly modified the difference in the FGF23 response to these treatments. These interactions were specific because no similar effect modification by other factors with the FGF23 response to VDR activation was found. Furthermore, in a global correlation analysis, adiponectin and FGF23 were interrelated independent of the estimated glomerular filtration rate and other potential confounders (β = 0.22, P = 0.003). Adiponectin is a strong modifier of the FGF23 response to VDR activation in CKD patients. The adiponectin-FGF23 link discovered in genetically engineered mice is of mechanistic relevance in the FGF23 response to VDR activation in CKD patients.

Kidney fibroblast growth factor 23 does not contribute to elevation of its circulating levels in uremia

Fibroblast growth factor 23 (FGF23) secreted by osteocytesis a circulating factor essential for phosphate homeostasis. High plasma FGF23 levels are associated with cardiovascular complications and mortality. Increases of plasma FGF23 in uremia antedate high levels of phosphate, suggesting a disrupted feedback regulatory loop or an extra-skeletal source of this phosphatonin. Since induction of FGF23 expression in injured organs has been reported we decided to examine the regulation of FGF23 gene and protein expressions in the kidney and whether kidney-derived FGF23 contributes to the high plasma levels of FGF23 in uremia. FGF23 mRNA was not detected in normal kidneys, but was clearly demonstrated in injured kidneys, already after four hours in obstructive nephropathy and at 8 weeks in the remnant kidney of 5/6 nephrectomized rats. No renal extraction was found in uremic rats in contrast to normal rats. Removal of the remnant kidney had no effect on plasma FGF23 levels. Well-known regulators of FGF23 expression in bone, such as parathyroid hormone, calcitriol, and inhibition of the FGF receptor by PD173074, had no impact on kidney expression of FGF23. Thus, the only direct contribution of the injured kidney to circulating FGF23 levels in uremia appears to be reduced renal extraction ofbone-derived FGF23. Kidney-derived FGF23 does not generate high plasma FGF23 levels in uremia and is regulated differently than the corresponding regulation ofFGF23 gene expression in bone.

Serum fibroblast growth factor 23, serum iron and bone mineral density in premenopausal women

Fibroblast growth factor 23 (FGF23) circulates as active protein and inactive fragments. Low iron status increases FGF23 gene expression, and iron deficiency is common. We hypothesized that in healthy premenopausal women, serum iron influences C-terminal and intact FGF23 concentrations, and that iron and FGF23 associate with bone mineral density (BMD). Serum iron, iron binding capacity, percent iron saturation, phosphorus, and other biochemistries were measured in stored fasting samples from healthy premenopausal white (n=1898) and black women (n=994), age 20-55years. Serum C-terminal and intact FGF23 were measured in a subset (1631 white and 296 black women). BMD was measured at the lumbar spine and femur neck. Serum phosphorus, calcium, alkaline phosphatase and creatinine were lower in white women than black women (p<0.001). Serum iron (p<0.0001) and intact FGF23 (p<0.01) were higher in white women. C-terminal FGF23 did not differ between races. Phosphorus correlated with intact FGF23 (white women, r=0.120, p<0.0001; black women r=0.163, p<0.01). However, phosphorus correlated with C-terminal FGF23 only in black women (r=0.157, p<0.01). Intact FGF23 did not correlate with iron. C-terminal FGF23 correlated inversely with iron (white women r=-0.134, p<0.0001; black women r=-0.188, p<0.01), having a steeper slope at iron <50mcg/dl than ≥50mcg/dl. Longitudinal changes in iron predicted changes in C-terminal FGF23. Spine BMD correlated with iron negatively (r=-0.076, p<0.01) in white women; femur neck BMD correlated with iron negatively (r=-0.119, p<0.0001) in black women. Both relationships were eliminated in weight-adjusted models. BMD did not correlate with FGF23. Serum iron did not relate to intact FGF23, but was inversely related to C-terminal FGF23. Intact FGF23 correlated with serum phosphorus. In weight-adjusted models, BMD was not related to intact FGF23, C-terminal FGF23 or iron. The influence of iron on FGF23 gene expression is not important in determining bone density in healthy premenopausal women.

Upregulation of Oxidative Stress Related Genes in a Chronic Kidney Disease Attributed to Specific Geographical Locations of Sri Lanka.

Objective. To infer the influence of internal and external oxidative stress in chronic kidney disease patients of unknown etiology (CKDu) in Sri Lanka, by analyzing expression of genes related directly or indirectly to oxidative stress: glutamate-cysteine ligase catalytic subunit (GCLC), glutathione S-transferase mu 1 (GSTM1), glucose-6-phosphate dehydrogenase (G6PD), fibroblast growth factor-23 (FGF23), and NLR family pyrin domain containing 3 (NLRP3). Methods. Reverse transcription quantitative polymerase chain reaction (RT-qPCR) was carried out for the selected populations: CKDu patients (n = 43), chronic kidney disease patients (CKD; n = 14), healthy individuals from a CKDu endemic area (GHI; n = 9), and nonendemic area (KHI; n = 16). Fold changes were quantified relative to KHI. Results. GCLC had greater than threefold upregulation in all three study groups, with a maximum of 7.27-fold upregulation in GHI (p = 0.000). GSTM1 was not expressed in 25.6% of CKDu and 42.9% of CKD patients, but CKDu patients expressing GSTM1 showed upregulation of 2.60-fold (p < 0.05). Upregulation of FGF23 and NLRP3 genes in CKD and CKDu was observed (p < 0.01), with greater fold changes in CKD. Conclusion. Results suggest higher influence of external sources of oxidative stress in CKDu, possibly owing to environmental conditions.

Key role of the kidney in the regulation of fibroblast growth factor 23

High circulating levels of fibroblast growth factor 23 (FGF23) have been demonstrated in kidney failure, but mechanisms of this are not well understood. Here we examined the impact of the kidney on the early regulation of intact FGF23 in acute uremia as induced by bilateral or unilateral nephrectomy (BNX and UNX, respectively) in the rat. BNX induced a significant increase in plasma intact FGF23 levels from 112 to 267 pg/ml within 15 min, which remained stable thereafter. UNX generated intact FGF23 levels between that seen in BNX and sham-operated rats. The intact to C-terminal FGF23 ratio was significantly increased in BNX rats. The rapid rise in FGF23 after BNX was independent of parathyroid hormone or FGF receptor signaling. No evidence of early stimulation of FGF23 gene expression in the bone was found. Furthermore, acute severe hyperphosphatemia or hypercalcemia had no impact on intact FGF23 levels in normal and BNX rats. The half-life of exogenous recombinant human FGF23 was significantly prolonged from 4.4 to 11.8 min in BNX rats. Measurements of plasma FGF23 in the renal artery and renal vein demonstrated a significant renal extraction. Thus the kidney is important in FGF23 homeostasis by regulation of its plasma level and metabolism.

Where is the link between mineral bone markers and cardiovascular disease in CKD?

Where is the link between mineral bone markers and cardiovascular disease in CKD?

FGF23 gene regulation by 1,25-dihydroxyvitamin D: opposing effects in adipocytes and osteocytes.

In a closed endocrine loop, 1,25-dihydroxyvitamin D3 (1,25D) induces the expression of fibroblast growth factor 23 (FGF23) in bone, with the phosphaturic peptide in turn acting at kidney to feedback repress CYP27B1 and induce CYP24A1 to limit the levels of 1,25D. In 3T3-L1 differentiated adipocytes, 1,25D represses FGF23 and leptin expression and induces C/EBPβ, but does not affect leptin receptor transcription. Conversely, in UMR-106 osteoblast-like cells, FGF23 mRNA concentrations are upregulated by 1,25D, an effect that is blunted by lysophosphatidic acid, a cell-surface acting ligand. Progressive truncation of the mouse FGF23 proximal promoter linked in luciferase reporter constructs reveals a 1,25D-responsive region between -400 and -200 bp. A 0.6 kb fragment of the mouse FGF23 promoter, linked in a reporter construct, responds to 1,25D with a fourfold enhancement of transcription in transfected K562 cells. Mutation of either an ETS1 site at -346 bp, or an adjacent candidate vitamin D receptor (VDR)/Nurr1-element, in the 0.6 kb reporter construct reduces the transcriptional activity elicited by 1,25D to a level that is not significantly different from a minimal promoter. This composite ETS1-VDR/Nurr1 cis-element may function as a switch between induction (osteocytes) and repression (adipocytes) of FGF23, depending on the cellular setting of transcription factors. Moreover, experiments demonstrate that a 1 kb mouse FGF23 promoter-reporter construct, transfected into MC3T3-E1 osteoblast-like cells, responds to a high calcium challenge with a statistically significant 1.7- to 2.0-fold enhancement of transcription. Thus, the FGF23 proximal promoter harbors cis elements that drive responsiveness to 1,25D and calcium, agents that induce FGF23 to curtail the pathologic consequences of their excess.

The couple fibroblast growth factor 23 (FGF23)/Klotho

The fibrobalst growth factor 23 (FGF23) is a protein secreted in the plasma by bone cells. FGF23 controls phosphate and calcitriol plasma concentration. Its main physiological targets are the kidney and the parathyroid gland. Its production is stimulated by phosphate intake and plasma calcitriol levels. FGF23 binds a receptor at the cell surface that is composed by the association of a FGF receptor with the protein Klotho. Klotho expression is restricted to few organs including the kidney and the parathyroid gland. Klotho, which is expressed at the cell surface, can also be released in the plasma after an enzymatic cleavage. The role of the circulating form of Klotho is unknown. In addition to its role as a co-receptor Klotho may control the stability of the renal sodium phosphate transporter NPT2a and the calcium channel TRPV5 via enzymatic properties. FGF23 or Klotho gene disruption in mice leads to similar phenotypes except for the levels of FGF23, undetectable or increased respectively. FGF23 plasma concentration rises from the early stage of renal insufficiency to prevent plasma phosphate concentration from increasing. In parallel Klotho production seems to diminish. These modifications trigger the secondary hyperparathyroidism observed in renal insufficiency. The increase in FGF23 concentration and the decrease in Klotho levels are associated in animals and in human as well to an augmentation of mortality especially from cardiovascular causes and to heart modification: hypertrophy or dysfunction.

FGF23 gene variation and its association with phosphate homeostasis and bone mineral density in Finnish children and adolescents

Fibroblast growth factor 23 (FGF23), a bone-derived hormone, participates in the hormonal bone–parathyroid–kidney axis, which is modulated by PTH, 1,25-dihydroxyvitamin D, plasma phosphate (Pi), and diet. Inappropriately high serum FGF23, seen in certain genetic and acquired disorders, results in urinary phosphate wasting and impaired bone mineralization. This study investigated the impact of FGF23 gene variation on phosphate homeostasis and bone health. The study included 183 children and adolescents (110 girls) aged 7–19 years (median 13.2years). Urine and blood parameters of calcium and phosphate homeostasis were analyzed. Bone characteristics were quantified by DXA and peripheral quantitative computed tomography (pQCT). Genetic FGF23 variation was assessed by direct sequencing of coding exons and flanking intronic regions. Nine FGF23 polymorphisms were detected; three of them were common: rs3832879 (c.212-37insC), rs7955866 (c.716C>T, p.T239M) and rs11063112 (c.2185A>T). Four different haplotypes and six different diplotypes were observed among these three polymorphisms. The variations in FGF23 significantly associated with plasma PTH and urinary Pi excretion, even after adjusting for relevant covariates. FGF23 variations independently associated with total hip BMD Z-score, but not with other bone outcomes. In instrument analysis, genetic variance in FGF23 was considered a weak instrument as it only induced small variations in circulating FGF23, PTH and Pi concentrations (F statistic less than 10). The observed associations between FGF23 variations and circulating PTH, and Pi excretion and total hip BMD Z-scores suggest that FGF23 polymorphisms may play a role in mineral homeostasis and bone metabolism.

Abstract 3446: Fibroblast growth factor 23 promotes prostate cancer cell proliferation and invasiveness in vitro

Abstract Prostate cancer is the most common malignancy among American men and remains the second leading cause of male cancer mortality. Since age is thought to be one of the crucial factors in prostate cancer development, fibroblast growth factor 23 (FGF23), which forms a complex with Klotho, the age-related gene product, could play a key role in prostate development and progression. FGF23 is a peptide hormone member of the endocrine FGF subfamily (FGF19, FGF21, and FGF23), which signals through FGF receptors (FGFRs) in the presence of the coreceptor α-Klotho. FGF-23 has a critical role in phosphate homeostasis and vitamin D metabolism. The regulation of 1,25-Dihydroxyvitamin D by FGF-23 could also have implications for cancers, given that vitamin D metabolites have been shown to have anticarcinogenic effects. It has been reported that higher concentrations of FGF-23 were found in patients with advanced ovarian cancer compared with those with early-stage cancer or control subjects; circulating FGF-23 is associated with an increased risk of colorectal adenoma; however, to date, no investigations of the role of FGF-23 in prostate cancer have been reported. In the present study we investigated the function of FGF23 on human prostate cancer progression. FGF23 gene expression was determined in the commonly used prostate cancer cell lines: PC3, LNCaP, DU145, VCaP, and an immortalized normal prostate cell line PNTIA. RT-PCR showed that both FGF23 and αKlotho (KLA) genes are expressed in all these cell lines. Quantitative real-time RT-PCR indicated that FGF23 gene expression is elevated in prostate cancer compared to normal prostate tissue; FGF23 expression frequency is significantly higher in prostate cancer in compare with normal tissue (prostate cancer: total case number 35, positive 23; normal prostate tissue: total number 33, positive 15; p=0.04841). Biological function assays demonstrated that exogenous FGF23 stimulates prostate cancer cell proliferation (LNCaP by 30%, PC3 20%), anchorage-independent growth (by 60% for both LNCaP and PC3 cells), and invasion (LNCaP by 50% and PC3 55%). Suppression of FGF23 expression by FGF23 short hairpin RNA (shRNA) significantly inhibits PC3 and LNCaP cell proliferation, soft agar colony formation, and invasiveness in vitro. These data suggest that FGF23 promotes human prostate cancer progression and targeting FGF23 signaling pathway could decrease prostate cancer growth and metastasis. Citation Format: Shu Feng, Michael Ittmann. Fibroblast growth factor 23 promotes prostate cancer cell proliferation and invasiveness in vitro. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3446. doi:10.1158/1538-7445.AM2014-3446

A new missense mutation in FGF23 gene in a male with hyperostosis–hyperphosphatemia syndrome (HHS)

Hyperostosis-hyperphosphataemia syndrome (HHS) is a rare autosomal recessive metabolic disorder, characterized by recurrent painful swelling of long bones, periosteal new bone formation and cortical hyperostosis or intramedullary sclerosis, hyperphosphatemia and low intact fibroblast growth factor 23 (FGF23) protein levels. It is caused by mutations in 2 genes, N-acetylgalactosaminyltransferase 3 (GalNAc-transferase; GALNT3) and FGF23. We have performed mutation analysis of the GALNT3 and FGF23 genes in a patient with HHS and detected a homozygous mutation in exon 3 of FGF23 gene (NM_020638.2: c.471C>A) which results in amino acid change from phenylalanine 157 to leucin (p.F157L) in receptor interaction site.

Postzygotic HRAS Mutation Causing Both Keratinocytic Epidermal Nevus and Thymoma and Associated With Bone Dysplasia and Hypophosphatemia Due to Elevated FGF23

Epidermal nevus syndrome is a rare group of disorders characterized by the combination of congenital epidermal nevi and extracutaneous features, including skeletal, neurological, ocular, and other systemic findings. We report a case of keratinocytic epidermal nevus syndrome that includes a thymoma, bone dysplasia, and hypophosphatemia with elevated fibroblast growth factor 23 (FGF23) levels associated with postzygotic HRAS mutation. A 14-year-old boy was admitted due to recent limping. The physical examination revealed multiple right-sided linear epidermal nevi along Blaschko's lines. Magnetic resonance imaging showed cystic lesions in cervical bones and thymoma, and x-ray examination showed cystic lesions in the hands. Biochemical studies demonstrated severe hypophosphatemia, normocalcemia, high normal PTH, low 25-hydroxyvitamin D and low 1,25-dihydroxyvitamin D levels. The serum FGF23 C-terminal level was normal, but the intact FGF23 level was found to be elevated. Genetic evaluation revealed a heterozygote mutation in the HRAS gene in both the keratinocytic epidermal nevus and thymoma but not in DNA extracted from blood lymphocytes, thus establishing the mutation as postzygotic. Postzygotic mutations in HRAS lead to elevation of FGF23 levels, as found in mutated PHEX, FGF23, DMP1, and ENPP1 genes, which lead to hypophosphatemia. An identical postzygotic HRAS mutation was shown to be present in both keratinocytic epidermal nevus and thymoma and to be associated with bone lesions and hypophosphatemia due to elevated FGF23 levels. These may all be related to the HRAS mutation.

Fibroblast growth factor 23 (FGF23) gene polymorphism in children with Kawasaki syndrome (KS) and susceptibility to cardiac abnormalities

BackgroundFibroblast Growth Factor (FGF) 23 influences endothelial integrity and few reports have studied the association between FGF23 and Kawasaki syndrome (KS), a childhood vasculitis displaying a high risk of subsequent cardiac abnormalities (CaA).AimTo investigate the genetic variation in the FGF23 gene in a cohort of KS children and its association with serum FGF23 levels and eventual development of CaA, including both coronary artery dilatations and aneurysms.Patients and methods84 Italian KS children were recruited; 24/84 (28.6%) developed CaA. Each patient underwent evaluation of serum FGF23 levels and FGF23 genotype: the frequency of the c.212-37insC (rs3832879) polymorphism in intron 1 was examined and compared with sex, age at disease onset, fever duration, laboratory data, and occurrence of CaA. Univariate statistical analysis of categorical parameters was performed by the Pearson’s Chi-square test or Fisher’s exact test as appropriate. Parametric variables were assessed by Student’s t-test for unpaired data. Independent predictors of disease were studied by a logistic regression model.Results28/84 patients carried the FGF23 polymorphism (33.3%) and had higher serum FGF23 levels (p < 0.01). FGF23 polymorphism was significantly associated with CaA compared to wild type FGF23 children (respectively, p = 0.03 and p = 0.05). The comparison with demographical, clinical or laboratory data was not significant.ConclusionsThe prevalent segregation of the c.212-37insC polymorphism in children with CaA advocates a possible functional FGF23 role in the predisposition to higher serum levels of FGF23 and potential occurrence of any coronary artery abnormalities in KS.

1,25-Dihydroxyvitamin D3 Regulation of Fibroblast Growth Factor-23 Expression in Bone Cells: Evidence for Primary and Secondary Mechanisms Modulated by Leptin and Interleukin-6

Fibroblast growth factor-23 (FGF23) is a circulating hormone that acts to correct hyperphosphatemic states by inhibiting renal phosphate reabsorption and to prevent hypervitaminosis D by feedback repressing 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) biosynthesis. FGF23 gene expression in the osteoblast/osteocyte is induced by the nuclear vitamin D receptor (VDR) bound to 1,25(OH)2D3, but cycloheximide sensitivity of this induction suggests that it may occur largely via secondary mechanisms requiring cooperating transcription factors. We therefore sought to identify 1,25(OH)2D3-regulated transcription factors that might impact FGF23 expression. Although neither leptin nor interleukin-6 (IL-6) alone affects FGF23 expression, leptin treatment was found to potentiate 1,25(OH)2D3 upregulation of FGF23 in UMR-106 cells, whereas IL-6 treatment blunted this upregulation. Genomic analyses revealed conserved binding sites for STATs (signal transduction mediators of leptin and IL-6 action) along with transcription factor ETS1 in human and other mammalian FGF23 genes. Further, STAT3, STAT1, ETS1, and VDR mRNAs were induced in a dose-dependent manner by 1,25(OH)2D3 in UMR-106 cells. Bioinformatic analysis identified nine potential VDREs in a genomic interval containing human FGF23. Six of the putative VDREs were capable of mediating direct transcriptional activation of a heterologous reporter gene when bound by a 1,25(OH)2D3-liganded VDR complex. A model is proposed wherein 1,25(OH)2D3 upregulates FGF23 production directly via multiple VDREs and indirectly via induction of STAT3, ETS1, and VDR transcription factors that are then activated via cell surface and intracellular signaling to cooperate in the induction of FGF23 through DNA looping and generation of euchromatin architecture.

A Functional Allelic Variant of theFGF23Gene Is Associated with Renal Phosphate Leak in Calcium Nephrolithiasis

A significant percentage of patients with calcium nephrolithiasis and normal parathyroid function have renal phosphate leak. This disorder is characterized by idiopathic hypophosphatemia and reduced renal phosphate threshold normalized for the glomerular filtration rate (TmPi/GFR). The majority of these patients harbor high or inappropriately normal circulating levels of fibroblast growth factor 23 (FGF23), a hormone regulating phosphate homeostasis. The aim of this study was to define the role of FGF23 allelic variants in the pathogenesis of hypophosphatemic nephrolithiasis. We sequenced the regulative and coding regions of the FGF23 gene in 106 stone formers, 17 of which had renal phosphate leak, and in 87 healthy controls. We subsequently performed in vitro studies. A C716T nonsynonymous change (T239M, rs7955866) in the FGF23 gene was detected in seven of the 17 stone formers with renal phosphate leak. The prevalence of the T allele and of the CT genotype in stone formers with renal phosphate leak was significantly higher compared to that observed in stone formers without renal phosphate leak and in controls (P < 0.03 in all cases). In the whole study population, FGF23(716T) subjects showed levels of serum phosphate and TmPi/GFR significantly lower compared to FGF23(716C) subjects. In vitro studies showed that the T239M change increases FGF23 secretion and that the FGF23(239M) variant induces a higher activation of the FGF receptor/ERK pathway compared to FGF23(239T). Our results highlight a novel significant association between the C716T missense variation in the FGF23 gene and calcium nephrolithiasis with renal phosphate leak.

FGF23 Regulation of Phosphorus Homeostasis Is Dependent on PTH

Fibroblast growth factor-23 (FGF23) is a key regulator of phosphorus homeostasis (1). FGF23 is a member of the FGF19 subfamily of fibroblast growth factors. The FGF23 gene is located on human chromosome 12p13 and mouse chromosome 6 and is composed of three coding exons with an open reading frame of 125 residues. FGF23 is most highly expressed in bone but also found in thymus, brain, heart, and other tissues. In bone, FGF23 mRNA is found in osteoblasts, osteocytes, bone-lining cells, and osteoprogenitor cells. FGF23 is secreted primarily by osteoblasts and osteocytes in response to increased serum phosphorus or 1,25-dihydroxyvitamin D. Circulating FGF23 binds to the canonical FGF receptor 1c coupled to the single transmembrane protein Klotho, a coreceptor required for FGF23 signaling, to form the specific receptor for FGF23 function (2). FGF23 targets the parathyroid gland to decrease PTH secretion and the kidney to increase urinary phosphorus excretion and to suppress synthesis of 1,25-dihydroxyvitamin D by decreasing renal tubular 1 -hydroxylase expression. These changes result in increased urinary loss of phosphorus as well as decreased proximal renal tubular production of 1,25dihydroxyvitamin D, both of which lead to a return to normal serum phosphorus levels (Fig. 1). In the kidney, circulating FGF23 reduces the expression of proximal renal tubular brush border membrane sodium-phosphate cotransporters NaPi2a and NaPi2c, which reduces the ability of the kidney to reabsorb phosphorus from proximal renal tubular glomerular filtrate, thereby causing increased urinary loss of phosphorus (3). FGF23 also reduces the activity of proximal renal tubular 1 -hydroxylase, the enzyme required for production of 1,25-dihydroxyvitamin D and increases the activity of 24-hydroxylase, the proximal renal tubular enzyme that metabolizes 1,25-dihydroxyvitamin D. Decreased serum 1,25-dihydroxyvitamin D leads to decreased intestinal calcium and phosphorus absorption, and low serum calcium and low serum 1,25-dihydroxyvitamin D both lead to increased production of PTH. The main physiological short-term role of circulating FGF23 appears to be to stimulate renal tubular phosphorus excretion when excessive phosphorus intake leads to hyperphosphatemia, whereas its main long-term role appears to be to limit intestinal absorption of phosphorus. Progressive increases in serum FGF23 play a major role in mineral metabolism abnormalities that develop in disorders characterized by hyperphosphatemia. Serum FGF23 levels increase in early-stage chronic kidney disease (CKD) and are associated with progressive hyperphosphatemia and hypocalcemia in late-stage CKD, secondary hyperparathyroidism, and low 1,25-dihydroxyvitamin D levels. Serum FGF23 levels increase beginning as early as stage 2 CKD because of decreased renal tubular clearance of phosphorus and result in increased fractional phosphorus excretion, reduced serum phosphorus levels, and reduced renal 1 -hydroxylase activity, which lead to normalization of serum phosphorus and reduced serum 1,25-dihydroxyvitamin D levels. Increased serum phosphorus levels in later-stage CKD, despite increased serum FGF23 levels and low serum 1,25dihydroxyvitamin D levels, lead to persistent stimulation of PTH secretion. As CKD progresses, serum FGF23 continues to increase, whereas responsiveness to FGF23 decreases as the number of intact nephrons declines. Decreased intact nephrons are associated with reduced expression of the FGF23 coreceptor Klotho. In end-stage

Longitudinal evaluation of FGF23 changes and mineral metabolism abnormalities in a mouse model of chronic kidney disease

Fibroblast growth factor 23 (FGF23) is a phosphaturic and vitamin D-regulatory hormone of putative bone origin that is elevated in patients with chronic kidney disease (CKD). The mechanisms responsible for elevations of FGF23 and its role in the pathogenesis of chronic kidney disease-mineral bone disorder (CKD-MBD) remain uncertain. We investigated the association between FGF23 serum levels and kidney disease progression, as well as the phenotypic features of CKD-MBD in a Col4a3 null mouse model of human autosomal-recessive Alport syndrome. These mice exhibited progressive renal failure, declining 1,25(OH)(2)D levels, increments in parathyroid hormone (PTH) and FGF23, late-onset hypocalcemia and hyperphosphatemia, high-turnover bone disease, and increased mortality. Serum levels of FGF23 increased in the earliest stages of renal damage, before elevations in blood urea nitrogen (BUN) and creatinine. FGF23 gene transcription in bone, however, did not increase until late-stage kidney disease, when serum FGF23 levels were exponentially elevated. Further evaluation of bone revealed trabecular osteocytes to be the primary cell source for FGF23 production in late-stage disease. Changes in FGF23 mirrored the rise in serum PTH and the decline in circulating 1,25(OH)(2)D. The rise in PTH and FGF23 in Col4a3 null mice coincided with an increase in the urinary fractional excretion of phosphorus and a progressive decline in sodium-phosphate cotransporter gene expression in the kidney. Our findings suggest elevations of FGF23 in CKD to be an early marker of renal injury that increases before BUN and serum creatinine. An increased production of FGF23 by bone may not be responsible for early increments in FGF23 in CKD but does appear to contribute to FGF23 levels in late-stage disease. Elevations in FGF23 and PTH coincide with an increase in urinary phosphate excretion that likely prevents the early onset of hyperphosphatemia in the face of increased bone turnover and a progressive decline in functional renal mass.

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 D(3) 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 26years 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, 26years old, presented with fatigue at 24years 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.

PTH increases FGF23 gene expression and mediates the high-FGF23 levels of experimental kidney failure: a bone parathyroid feedback loop

Parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) target the kidney to cause a phosphaturia. FGF23 also acts on the parathyroid to decrease PTH expression, but in chronic kidney disease (CKD) there are high-serum PTH and FGF23 levels and resistance of the parathyroid to FGF23. We now report that PTH acts on bone to increase FGF23 expression and characterize the signal transduction pathway whereby PTH increases FGF23 expression. Remarkably, we show that PTH is necessary for the high-FGF23 levels of early kidney failure due to an adenine high-phosphorus diet. Parathyroidectomy before the diet totally prevented the fivefold increase in FGF23 levels in kidney failure rats. Moreover, parathyroidectomy of early kidney failure rats corrected their high-FGF23 levels. Therefore, in early kidney failure, the high-FGF23 levels are dependent on the high-PTH levels. PTH infusion for 3 days to mice with normal renal function increased serum FGF23 and calvaria FGF23 mRNA levels. To demonstrate a direct effect of PTH on FGF23, we added PTH to rat osteoblast-like UMR106 cells. PTH increased FGF23 mRNA levels (4-fold) and this effect was mimicked by a PKA activator, forskolin. PTH also decreased SOST mRNA levels (3-fold). SOST codes for sclerostin, a Wnt pathway inhibitor, which is a PTH receptor (PTH1R) target. The effect of PTH was prevented by added sclerostin. Therefore, PTH increases FGF23 expression which involves the PKA and Wnt pathways. The effect of PTH on FGF23 completes a bone-parathyroid endocrine feedback loop. Importantly, secondary hyperparathyroidism is essential for the high-FGF23 levels in early CKD.