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Background X-linked hypophosphatemia (XLH) is an inherited disease of phosphate metabolism in which inactivating mutations of the Phosphate Regulating Endopeptidase Homolog, X-Linked ( PHEX ) gene lead to local and systemic effects including impaired growth, rickets, osteomalacia, bone abnormalities, bone pain, spontaneous dental abscesses, hearing difficulties, enthesopathy, osteoarthritis, and muscular dysfunction. Patients with XLH present with elevated levels of fibroblast growth factor 23 (FGF23), which is thought to mediate many of the aforementioned manifestations of the disease. Elevated FGF23 has also been observed in many other diseases of hypophosphatemia, and a range of animal models have been developed to study these diseases, yet the role of FGF23 in the pathophysiology of XLH is incompletely understood. Methods The role of FGF23 in the pathophysiology of XLH is here reviewed by describing what is known about phenotypes associated with various PHEX mutations, animal models of XLH, and non-nutritional diseases of hypophosphatemia, and by presenting molecular pathways that have been proposed to contribute to manifestations of XLH. Results The pathophysiology of XLH is complex, involving a range of molecular pathways that variously contribute to different manifestations of the disease. Hypophosphatemia due to elevated FGF23 is the most obvious contributor, however localised fluctuations in tissue non-specific alkaline phosphatase (TNAP), pyrophosphate, calcitriol and direct effects of FGF23 have been observed to be associated with certain manifestations. Conclusions By describing what is known about these pathways, this review highlights key areas for future research that would contribute to the understanding and clinical treatment of non-nutritional diseases of hypophosphatemia, particularly XLH.

Fibroblast growth factor‐23 (FGF23) is a bone‐derived hormone regulating renal phosphate reabsorption and vitamin D synthesis in renal proximal tubules. Here, we show that FGF23 directly regulates the membrane abundance of the Na + :Cl − co‐transporter NCC in distal renal tubules by a signaling mechanism involving the FGF receptor/αKlotho complex, extracellular signal‐regulated kinase 1/2 (ERK1/2), serum/glucocorticoid‐regulated kinase 1 (SGK1), and with‐no lysine kinase‐4 (WNK4). Renal sodium (Na + ) reabsorption and distal tubular membrane expression of NCC are reduced in mouse models of Fgf23 and α Klotho deficiency. Conversely, gain of FGF23 function by injection of wild‐type mice with recombinant FGF23 or by elevated circulating levels of endogenous Fgf23 in Hyp mice increases distal tubular Na + uptake and membrane abundance of NCC, leading to volume expansion, hypertension, and heart hypertrophy in a αKlotho and dietary Na + ‐dependent fashion. The NCC inhibitor chlorothiazide abrogates FGF23‐induced volume expansion and heart hypertrophy. Our findings suggest that FGF23 is a key regulator of renal Na + reabsorption and plasma volume, and may explain the association of FGF23 with cardiovascular risk in chronic kidney disease patients. Synopsis FGF23 serum levels are elevated in chronic kidney disease patients. FGF23 is here shown to be a regulator of the sodium‐chloride channel NCC in distal renal tubules, thus affecting renal sodium retention, plasma expansion, hypertension, and heart hypertrophy. FGF23 regulates membrane abundance and activity of the renal sodium‐chloride channel NCC through the ERK1/2‐SGK1‐WNK4 signaling pathway. FGF23 is a sodium‐conserving hormone. Elevated circulating FGF23 leads to volume expansion, hypertension, and cardiac hypertrophy in a Klotho‐dependent fashion. The NCC inhibitor chlorothiazide blunts FGF23‐induced hypertension. A low sodium diet aggravates the hypertensive action of FGF23 through crosstalk with aldosterone signaling at the level of SGK1. Graphical Abstract FGF23 serum levels are elevated in chronic kidney disease patients. FGF23 is here shown to be a regulator of the sodium‐chloride channel NCC in distal renal tubules, thus affecting renal sodium retention, plasma expansion, hypertension, and heart hypertrophy.

Fibroblast growth factor 23 (FGF23) plays a key role in the complex network between the bones and other organs. Initially, it was thought that FGF23 exclusively regulated phosphate and vitamin D metabolism; however, recent research has demonstrated that an excess of FGF23 has other effects that may be detrimental in some cases. The understanding of the signaling pathways through which FGF23 acts in different organs is crucial to develop strategies aiming to prevent the negative effects associated with high FGF23 levels. FGF23 has been described to have effects on the heart, promoting left ventricular hypertrophy (LVH); the liver, leading to production of inflammatory cytokines; the bones, inhibiting mineralization; and the bone marrow, by reducing the production of erythropoietin (EPO). The identification of FGF23 receptors will play a remarkable role in future research since its selective blockade might reduce the adverse effects of FGF23. Patients with chronic kidney disease (CKD) have very high levels of FGF23 and may be the population suffering from the most adverse FGF23-related effects. The general population, as well as kidney transplant recipients, may also be affected by high FGF23. Whether the association between FGF23 and clinical events is causal or casual remains controversial. The hypothesis that FGF23 could be considered a therapeutic target is gaining relevance and may become a promising field of investigation in the future.

Background X-linked hypophosphatemia (XLH) is a rare, hereditary, progressive, renal phosphate-wasting disorder characterized by a pathological increase in FGF23 concentration and activity. Due to its rarity, diagnosis may be delayed, which can adversely affect outcomes. As a chronic disease resulting in progressive accumulation of musculoskeletal manifestations, it is important to understand the natural history of XLH over the patient’s lifetime and the impact of drug treatments and other interventions. This multicentre, international patient registry (International XLH Registry) was established to address the paucity of these data. Here we present the findings of the first interim analysis of the registry. Results The International XLH Registry was initiated in August 2017 and includes participants of all ages diagnosed with XLH, regardless of their treatment and management. At the database lock for this first interim analysis (29 March 2021), 579 participants had entered the registry before 30 November 2020 and are included in the analysis (360 children [62.2%], 217 adults [37.5%] and 2 whose ages were not recorded [0.3%]; 64.2% were female). Family history data were available for 319/345 (92.5%) children and 145/187 (77.5%) adults; 62.1% had biological parents affected by XLH. Genetic testing data were available for 341 (94.7%) children and 203 (93.5%) adults; 370/546 (67.8%) had genetic test results; 331/370 (89.5%) had a confirmed PHEX mutation. A notably longer time to diagnosis was observed in adults ≥ 50 years of age (mean [median] duration 9.4 [2.0] years) versus all adults (3.7 [0.1] years) and children (1.0 [0.2] years). Participants presented with normal weight, shorter length or height and elevated body mass index (approximately − 2 and + 2 Z-scores, respectively) versus the general population. Clinical histories were collected for 349 participants (239 children and 110 adults). General data trends for prevalence of bone, dental, renal and joint conditions in all participants were aligned with expectations for a typical population of people with XLH. Conclusion The data collected within the International XLH Registry, the largest XLH registry to date, provide substantial information to address the paucity of natural history data, starting with demographic, family history, genetic testing, diagnosis, auxology and baseline data on clinical presentation.

The primary objective of this study was to determine the role of fibroblast growth factor 23 (FGF-23) in the pathogenesis of diffuse idiopathic skeletal hyperostosis (DISH). A total of 61 patients with DISH and 61 age- and sex-matched control patients without DISH were included in this study. The serum FGF-23, creatinine, inorganic phosphate, calcium, albumin, albumin-adjusted calcium and alkaline phosphatase, and C-reactive protein were assessed in both groups. Based on the extent of ossification, DISH group was further divided into T-DISH and L-DISH subgroups. Data were comparatively analyzed between DISH and Non-DISH groups and among T-DISH, L-DISH, and Non-DISH groups, respectively. Besides, the number of ossification segments of all DISH patients was quantified and the correlation between the number of ossification segments and the serum concentration of FGF-23 was analyzed. The results revealed that serum FGF-23 was significantly higher in DISH group than in Non-DISH group, regardless of gender. Interestingly, serum Pi was significantly lower in DISH group than in Non-DISH group. Moreover, a significant difference in serum FGF-23 among T-DISH, L-DISH, and Non-DISH groups was also observed. In contrast to Non-DISH group, both T-DISH and L-DISH subgroups displayed significantly higher serum FGF-23 level. Although the mean value was relatively higher in L-DISH subgroup, no statistically significant difference was found between T-DISH and L-DISH subgroups. In addition, a moderately positive correlation was identified between the number of ossification segments and the serum level of FGF-23. It can be concluded that serum FGF-23 could serve as a positive biomarker for DISH and may play a significant role in ectopic ossification in DISH.

X-linked hypophosphatemia (XLH) is a rare metabolic disorder characterized by elevated FGF23 and chronic hypophosphatemia, leading to impaired skeletal mineralization and enthesopathies that are associated with pain, stiffness, and diminished quality of life. The natural history of enthesopathies in XLH remains poorly defined, partly due to absence of a sensitive quantitative tool for assessment and monitoring. This study investigates the utility of 18F-NaF PET/CT scans in characterizing enthesopathies in XLH subjects. In 19 adult XLH subjects, enthesopathy burden was assessed by quantifying calcified sites on CT and 18F-NaF PET uptake at 16 common tendon/ligament insertion locations. Parameters obtained were (1) number of enthesopathy sites, (2) characterization of each site as CT-positive (CT +) and/or PET-positive (PET +), (3) a semiquantitative score based on severity of affected enthesopathies (CT-score global and PET-score global ). Biochemical and self-reported questionnaires results were correlated with 18F-NaF PET/CT parameters. 18F-NaF PET/CT detected at least one enthesopathy in all subjects, with 18F-NaF PET positivity often detected before CT (19.4% of all enthesopathies). Age negatively correlated with the number of PET + /CT− enthesopathies and positively with PET−/CT + enthesopathies. PET-score global was positively associated with ALP. While PET-score global showed no correlation with any applied survey, CT-score global was associated with worse functionality and pain. These associations suggest a progression from an actively mineralizing lesion to a more established, inactive lesion. Overall, although 18F-NaF PET/CT is not yet indicated for routine clinical use, it is a promising research tool for evaluating enthesopathy burden in XLH, offering valuable insights into the disease’s progression and potentially enabling early therapeutic assessment.

Uterine sarcoma (US) is a highly malignant cancer with poor prognosis and high mortality in women. In this study, we evaluated the expression of human fibroblast growth factor 23 (FGF23) in different US subtypes and the relationship between survival and clinicopathological characteristics. We conducted a comparative analysis of FGF23 gene expression in different pathological types of US. Utilizing a cohort from The Cancer Genome Atlas of 57 patients, a 50-patient microarray dataset (GSE119043) from the Gene Expression Omnibus and a Suining cohort of 44 patients, we analyzed gene expression profiles and corresponding clinicopathological information. Immunohistochemistry was used to examine the expression level of FGF23 in four US subtypes. Survival analysis was used to assess the relationship between FGF23 expression and prognosis in US patients. Compared with uterine normal smooth muscle and uterine leiomyoma, FGF23 expression was significantly upregulated in US and was differentially expressed in four US subtypes. Uterine carcinosarcoma exhibited the highest expression of FGF23 among the subtypes. Survival analysis revealed no correlation between FGF23 expression and either overall survival or progression-free survival in US (  > 0.05). Similar results were obtained from the validation cohorts. Univariate and multivariate analyses showed no significant correlation between FGF23 expression and the US prognosis. Tumor stage, CA125, and tumor recurrence were independent prognostic factors for survival of US patients. FGF23 was highly expressed in US and was promising as a novel potential biomarker for the diagnosis and prognosis of US.

Apart from its phosphaturic action, the bone‐derived hormone fibroblast growth factor‐23 (FGF23) is also an essential regulator of vitamin D metabolism. The main target organ of FGF23 is the kidney, where FGF23 suppresses transcription of the key enzyme in vitamin D hormone (1,25(OH)2D) activation, 1α‐hydroxylase, and activates transcription of the key enzyme responsible for vitamin D degradation, 24‐hydroxylase, in proximal renal tubules. The circulating concentration of 1,25(OH)2D is a positive regulator of FGF23 secretion in bone, forming a feedback loop between kidney and bone. The importance of FGF23 as regulator of vitamin D metabolism is underscored by the fact that in the absence of FGF23 signaling, the tight control of renal 1α‐hydroxylase fails, resulting in overproduction of 1,25(OH)2D in mice and men. During recent years, big strides have been made toward a more complete understanding of the mechanisms underlying the FGF23‐mediated regulation of vitamin D metabolism, especially at the genomic level. However, there are still major gaps in our knowledge that need to be filled by future research. Importantly, the intracellular signaling cascades downstream of FGF receptors regulating transcription of 1α‐hydroxylase and 24‐hydroxylase in proximal renal tubules still remain unresolved. The purpose of this review is to highlight our current understanding of the molecular mechanisms underlying the regulation of vitamin D metabolism by FGF23, and to discuss the role of these mechanisms in physiology and pathophysiology. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Background This meta-analysis aims to investigate the impact of abnormalities in mineral metabolic markers, including serum phosphate and calcium, intact parathyroid hormone (iPTH), and fibroblast growth factor 23 (FGF23) on the risk of fractures in patients with chronic kidney disease (CKD). Methods A systematic search was conducted across MEDLINE, Web of Science, EMBASE, ClinicalTrials.gov, and the Cochrane Central Register for Controlled Trials. The outcomes were association of mineral metabolic markers with the risk of fractures in patients with chronic kidney disease. Pooled risk estimates and 95% confidence intervals (CIs) were calculated using fixed-effects or random-effects models. Results Thirty-two studies were included in the meta-analysis. High and low levels of serum phosphate in hemodialysis (HD) patients were both associated with an increased risk of fractures (RR = 1.08, 95% CI 1.02–1.15, P  = 0.013; RR = 1.13, 95% CI 1.02–1.25, P  = 0.022, respectively). Similarly, abnormal levels of iPTH in CKD patients, both high and low, were associated with increased fracture risk (RR = 1.25, 95% CI 1.20–1.31, P  < 0.001; RR = 1.41, 95% CI 1.10–1.82, P  = 0.007, respectively). Elevated FGF23 levels were also linked to an increased risk of fractures (RR = 1.32, 95% CI 1.06–1.66, P  = 0.015). While a higher level of calcium exhibited a trend towards reduced fracture incidence without statistical significance (RR = 0.90, 95% CI 0.77–1.05, P  = 0.181), lower calcium levels tended to increase fracture risk without statistical significance (RR = 1.11, 95% CI 0.99–1.24, P  = 0.087). Notably, subjects treated with calcium and phosphorus modulating drugs demonstrated a statistically significant reduction in fractures among CKD patients undergoing dialysis (phosphate binders, RR = 0.79, 95% CI 0.70–0.89; cinacalcet, RR = 0.74, 95% CI 0.59–0.93; vitamin D analogues, RR = 0.82, 95% CI 0.74–0.92, respectively). Conclusion This meta-analysis underscores the association between abnormal mineral metabolic markers, including high serum phosphate, iPTH, and FGF23, and an increased risk of fractures in CKD patients. Notably, both elevated and decreased levels of phosphate and iPTH contribute to fracture risk. The efficacy of active vitamin D, phosphorus binders, and cinacalcet in preventing fractures was observed in HD patients but not in the non-dialysis CKD population. Trial registration PROSPERO CRD42023493951.

Fibroblast growth factor (FGF) 23, a circulating 26-kDa peptide produced by osteogenic cells, is a novel phosphaturic factor. In our previous study, binge-eating/purging type anorexia nervosa (AN-BP) patients had elevated plasma intact FGF23 (iFGF23) levels, while restricting type (AN-R) patients had plasma iFGF23 levels similar to healthy controls. Although bulimia nervosa (BN) patients as well as some patients with AN-BP regularly engage in binge eating, there have been no studies regarding plasma iFGF23 levels in BN patients. Therefore, this study was performed to determine plasma iFGF23 concentrations in BN patients and healthy controls. The study population consisted of 13 female BN patients and 11 healthy female controls. Blood samples were collected from all subjects after overnight fasting. Plasma iFGF23 was measured using an ELISA kit in a cross-sectional manner. The two-tailed Mann-Whitney U-test was used to assess differences between BN patients and healthy controls. In addition, BN patients were divided into two groups based on questionnaire-reported binge eating frequency immediately prior to participation in this study: high frequency of binge eating (once a week or more; HF group; n = 8) and low frequency of binge eating (less than once a week; LF group; n = 5). Two-tailed Mann-Whitney U-test with Bonferroni's correction was performed after the Kruskal-Wallis test to assess differences between HF group, LF group, and healthy controls. Median (quartiles) plasma iFGF23 levels were greater in BN patients (35.5 [14.8-65.0] pg/ml) than in controls (3.8 [not detected-5.3] pg/ml; p = 0.002). In addition, median (quartiles) plasma iFGF23 levels were greater in the HF group (62.3 [44.4-73.4] pg/ml) than in controls (p < 0.001) and in the LF group (12.9 [not detected-30.3] pg/ml; p = 0.011), while there were no differences between the LF group and controls (p = 0.441). This is the first study to show that BN patients have elevated plasma iFGF23 levels. Moreover, this study showed that BN patients with a high frequency of binge eating have elevated plasma iFGF23 levels, while iFGF23 levels are similar to healthy controls in those with a low frequency of binge eating. Plasma iFGF23 level may be a suitable indicator of binge eating in BN patients.