Patients with FGF23-related hypophosphatemic rickets/osteomalacia do not present with left ventricular hypertrophy

X-linked hypophosphataemia

X-linked hypophosphataemia is a genetic disease caused by defects in the phosphate regulating endopeptidase homolog X-linked (PHEX) gene and is characterised by X-linked dominant inheritance. The main consequence of PHEX deficiency is increased production of the phosphaturic hormone fibroblast growth factor 23 (FGF23) in osteoblasts and osteocytes. Chronic exposure to circulating FGF23 is responsible for renal phosphate wasting and decreased synthesis of calcitriol, which decreases intestinal phosphate absorption. These mechanisms result in lifelong hypophosphataemia, impaired growth plate and bone matrix mineralisation, and diverse manifestations in affected children and adults, including some debilitating morbidities and possibly increased mortality. Important progress has been made in disease knowledge and management over the past decade; in particular, targeting FGF23 is a therapeutic approach that has substantially improved outcomes. However, patients affected by this complex disease need lifelong care and innovative treatment strategies, such as gene repair of PHEX, are necessary to further limit the disease burden.

Roles of Parathyroid Hormone and Fibroblast Growth Factor 23 in Advanced Chronic Kidney Disease

Parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) each play a central role in the pathogenesis of chronic kidney disease (CKD)-mineral and bone disorder. Levels of both hormones increase progressively in advanced CKD and can lead to damage in multiple organs. Secondary hyperparathyroidism (SHPT), characterized by parathyroid hyperplasia with increased PTH secretion, is associated with fractures and mortality. Emerging evidence suggests that these associations may be partially explained by PTH-induced browning of adipose tissue and increased energy expenditure. Observational studies suggest a survival benefit of PTHlowering therapy, and a recent study comparing parathyroidectomy and calcimimetics further suggests the importance of intensive PTH control. The mechanisms underlying the regulation of FGF23 secretion by osteocytes in response to phosphate load have been unclear, but recent experimental studies have identified glycerol-3-phosphate, a byproduct of glycolysis released by the kidney, as a key regulator of FGF23 production. Elevated FGF23 levels have been shown to be associated with mortality, and experimental data suggest off-target adverse effects of FGF23. However, the causal role of FGF23 in adverse outcomes in CKD patients remains to be established. Further studies are needed to determine whether intensive SHPT control improves clinical outcomes and whether treatment targeting FGF23 can improve patient outcomes.

Cardiovascular health in pediatric patients with X-linked hypophosphatemia under two years of burosumab therapy

Introduction

X-linked hypophosphatemia (XLH) is caused by an inactivating mutation in the phosphate-regulating endopeptidase X-linked (PHEX) gene whose defective product fails to control phosphatonin fibroblast growth factor 23 (FGF23) serum levels. Although elevated FGF23 levels have been linked with detrimental cardiac effects, the cardiologic outcomes in XLH patients have been subject to debate. Our study aimed to evaluate the prevalence and severity of cardiovascular morbidity in pediatric XLH patients before, during, and after a 2-year treatment period with burosumab, a recombinant anti-FGF23 antibody.

Methods

This prospective observational study was conducted in a tertiary medical center, and included 13 individuals with XLH (age range 0.6-16.2 years) who received burosumab every 2 weeks. Clinical assessment at treatment initiation and after .5, 1, and 2 years of uninterrupted treatment included anthropometric measurements and cardiologic evaluations (blood pressure [BP], electrocardiogram, conventional echocardiography, and myocardial strain imaging).

Results

The linear growth of all patients improved significantly (mean height z-score: from -1.70 ± 0.80 to -0.96 ± 1.08, P=0.03). Other favorable effects were decline in overweight/obesity rates (from 46.2% to 23.1%) and decreased rates of elevated BP (systolic BP from 38.5% to 15.4%; diastolic BP from 38.5% to 23.1%). Electrocardiograms revealed no significant abnormality throughout the study period. Cardiac dimensions and myocardial strain parameters were within the normative range for age at baseline and remained unchanged during the study period.

Conclusion

Cardiologic evaluations provided reassurance that 2 years of burosumab therapy did not cause cardiac morbidity. The beneficial effect of this treatment was a reduction in cardiovascular risk factors, as evidenced by the lower prevalence of both overweight/obesity and elevated BP.

Non-Classical Effects of FGF23: Molecular and Clinical Features

This article reviews the role of fibroblast growth factor 23 (FGF23) protein in phosphate metabolism, highlighting its regulation of vitamin D, parathyroid hormone, and bone metabolism. Although it was traditionally thought that phosphate-calcium homeostasis was controlled exclusively by parathyroid hormone (PTH) and calcitriol, pathophysiological studies revealed the influence of FGF23. This protein, expressed mainly in bone, inhibits the renal reabsorption of phosphate and calcitriol formation, mediated by the α-klotho co-receptor. In addition to its role in phosphate metabolism, FGF23 exhibits pleiotropic effects in non-renal systems such as the cardiovascular, immune, and metabolic systems, including the regulation of gene expression and cardiac fibrosis. Although it has been proposed as a biomarker and therapeutic target, the inhibition of FGF23 poses challenges due to its potential side effects. However, the approval of drugs such as burosumab represents a milestone in the treatment of FGF23-related diseases.

Reassuring Data on the Cardiovascular Risk in Adults With X-linked Hypophosphatemia Receiving Conventional Therapy

CONTEXT

X-linked hypophosphatemia (XLH) is a rare genetic disorder that results in increased plasma levels of fibroblast growth factor 23 (FGF23). Several studies have demonstrated a direct association between FGF23 and cardiovascular mortality in cohorts of patients with chronic renal failure. However, in patients with XLH, studies on the cardiovascular impact of the disease are rare, with contradictory results.

OBJECTIVE

The aim was to assess whether the disease led to an increased cardiovascular risk.

METHODS

We conducted a single-center retrospective observational study on a local cohort of adult patients with XLH. The primary endpoint was a composite endpoint of the frequency of left ventricular hypertrophy (LVH) or presence of high blood pressure. Our secondary objectives were to assess echocardiographic, pulse wave velocity, and central blood pressure data as other markers of CV health. Independently of this cohort, tissue sodium content with magnetic resonance imaging was studied in 2 patients with XLH before and after burosumab.

RESULTS

Twenty-two patients were included. Median serum phosphate was 0.57 (0.47-0.72) mmol/L and FGF23 94 pg/L (58-2226). Median blood pressure was 124 (115-130)/68 (65-80) mm Hg, with only 9% of patients being hypertensive. A majority of patients (69%) had no LVH, only 1 had a left ventricular mass >100 g/m² and 25% of patients had left ventricular remodeling. Pulse wave velocity was normal in all patients. No differences in skin and muscle sodium content were observed before and after burosumab in the 2 patients who underwent sodium magnetic resonance imaging.

CONCLUSION

We found no elevated risk of developing hypertension or LVH in patients with XLH.

FGF-23, Left Ventricular Hypertrophy, and Mortality in Patients With CKD: A Revisit With Mediation Analysis

Background

In patients with chronic kidney disease (CKD), fibroblast growth factor (FGF)-23 is suspected to cause death or cardiovascular disease by inducing left ventricular hypertrophy (LVH).

Objectives

This study aims to quantify the mediational effect of LVH in the hypothetical causal pathway from FGF-23 to long-term adverse outcomes.

Methods

From 3,939 adults with CKD stages 2 to 4 enrolled in the CRIC (Chronic Renal Insufficiency Cohort) study, 2,368 participants with available data of FGF-23, left ventricular mass index at 1 year, and covariates were included. We employed linear and Cox proportional hazards regression models to investigate the association between FGF-23 and LVH, all-cause mortality, atrial fibrillation (AF), or congestive heart failure (CHF). Mediation analysis was used within a counterfactual framework to decompose the effect of FGF-23 into natural direct and indirect effects.

Results

Among 2,368 participants (mean age: 57.7 years, 1,252 males, median FGF-23 level: 138.8 RU/mL), left ventricular mass index was positively correlated with FGF-23. During a median of 12.0, 11.1, and 11.1 years, FGF-23 was associated with all-cause mortality (HR: 1.62, 95% CI: 1.24-2.12), AF (HR: 1.58, 95% CI: 1.12-2.24), and CHF (HR: 1.32, 95% CI: 0.95-1.84) when the highest quartile was compared to the lowest quartile. LVH mediated 7.4%, 11.2%, and 21.9% of the effect of FGF-23 on all-cause mortality, AF, and CHF, respectively.

Conclusions

In CKD patients, FGF-23 had a minor effect on the development of long-term adverse outcomes through LVH. Other potential mediators and the validity of negative effect of FGF-23 should be explored.

FGF23 and klotho at the intersection of kidney and cardiovascular disease

Cardiovascular disease is the leading cause of death in patients with chronic kidney disease (CKD). As CKD progresses, CKD-specific risk factors, such as disordered mineral homeostasis, amplify traditional cardiovascular risk factors. Fibroblast growth factor 23 (FGF23) regulates mineral homeostasis by activating complexes of FGF receptors and transmembrane klotho co-receptors. A soluble form of klotho also acts as a 'portable' FGF23 co-receptor in tissues that do not express klotho. In progressive CKD, rising circulating FGF23 levels in combination with decreasing kidney expression of klotho results in klotho-independent effects of FGF23 on the heart that promote left ventricular hypertrophy, heart failure, atrial fibrillation and death. Emerging data suggest that soluble klotho might mitigate some of these effects via several candidate mechanisms. More research is needed to investigate FGF23 excess and klotho deficiency in specific cardiovascular complications of CKD, but the pathophysiological primacy of FGF23 excess versus klotho deficiency might never be precisely resolved, given the entangled feedback loops that they share. Therefore, randomized trials should prioritize clinical practicality over scientific certainty by targeting disordered mineral homeostasis holistically in an effort to improve cardiovascular outcomes in patients with CKD.

Exploring the implications of blocking renin-angiotensin-aldosterone system and fibroblast growth factor 23 in early left ventricular hypertrophy without chronic kidney disease

Background

Whether fibroblast growth factor 23 (FGF23) directly induces left ventricular hypertrophy (LVH) remains controversial. Recent studies showed an association between FGF23 and the renin-angiotensin-aldosterone system (RAAS). The aim of this study was to investigate changes in FGF23 levels and RAAS parameters and their influences on LVH.

Methods

In the first experiment, male C57BL/6J mice were divided into sham and transverse aortic constriction (TAC) groups. The TAC group underwent TAC at 8 weeks of age. At 1, 2, 3, and 4 weeks after TAC, the mice were sacrificed, and blood and urine samples were obtained. Cardiac expressions of FGF23 and RAAS-related factors were evaluated, and cardiac histological analyses were performed. In the second experiment, the sham and TAC groups were treated with vehicle, angiotensin-converting enzyme (ACE) inhibitor, or FGF receptor 4 (FGFR4) inhibitor and then evaluated in the same way as in the first experiment.

Results

In the early stage of LVH without chronic kidney disease, serum FGF23 levels did not change but cardiac FGF23 expression significantly increased along with LVH progression. Moreover, serum aldosterone and cardiac ACE levels were significantly elevated, and cardiac ACE2 levels were significantly decreased. ACE inhibitor did not change serum FGF23 levels but significantly decreased cardiac FGF23 levels with improvements in LVH and RAAS-related factors, while FGFR4 inhibitor did not change the values.

Conclusions

Not serum FGF23 but cardiac FGF23 levels and RAAS parameters significantly changed in the early stage of LVH without chronic kidney disease. RAAS blockade might be more crucial than FGF23 blockade for preventing LVH progression in this condition.

Roles of PTH and FGF23 in kidney failure: a focus on nonclassical effects

Parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) each play a central role in the pathogenesis of chronic kidney disease-mineral and bone disorder (CKD-MBD). Both hormones increase as kidney function declines, presumably as a response to maintain normal phosphate balance, but when patients reach kidney failure, PTH and FGF23 fail to exert their phosphaturic effects, leading to hyperphosphatemia and further elevations in PTH and FGF23. In patients with kidney failure, the major target organ for PTH is the bone, but elevated PTH is also associated with mortality presumably through skeletal and nonskeletal mechanisms. Indeed, accumulated evidence suggests improved survival with PTH-lowering therapies, and a more recent study comparing parathyroidectomy and calcimimetic treatment further suggests a notion of "the lower, the better" for PTH control. Emerging data suggest that the link between SHPT and mortality could in part be explained by the action of PTH to induce adipose tissue browning and wasting. In the absence of a functioning kidney, the classical target organ for FGF23 is the parathyroid gland, but FGF23 loses its hormonal effect to suppress PTH secretion owing to the depressed expression of parathyroid Klotho. In this setting, experimental data suggest that FGF23 exerts adverse nontarget effects, but it remains to be confirmed whether FGF23 directly contributes to multiple organ injury in patients with kidney failure and whether targeting FGF23 can improve patient outcomes. Further efforts should be made to determine whether intensive control of SHPT improves clinical outcomes and whether nephrologists should aim at controlling FGF23 levels just as with PTH levels.

X-linked hypophosphatemia, obesity and arterial hypertension: data from the XLH21 study

BACKGROUND

The underlying mechanisms of obesity in X-linked hypophosphatemia (XLH) are not known. We aimed to evaluate whether FGF21, an endocrine FGF involved in the regulation of carbohydrate-lipid metabolism, could be involved.

METHODS

We performed a prospective multicenter cross-sectional study comparing FGF23, Klotho, and FGF21 levels in teenagers with XLH compared to healthy controls (VITADOS cohort) after matching for age, gender, and puberty. Non-parametric tests were performed (results presented as median (min-max)).

RESULTS

A total of 40 XLH teenagers (n = 20 Standard Of Care, SOC, n = 20 burosumab) were included. While patients receiving burosumab displayed increased BMI as compared to patients receiving SOC, systolic blood pressure expressed as percentile was progressively and significantly lower when comparing the three groups: 77 (4-99) in SOC, 47 (9-98) in burosumab, and 28 (1-94) in controls (p = 0.007). When compared to patients receiving SOC, patients receiving burosumab displayed significantly increased phosphate and 1,25(OH)2D levels. We found increased Klotho levels in patients receiving burosumab. No differences were found for either carbohydrate-lipid biomarkers or FGF21 between the three groups. A total of 21 XLH patients (53%) had insulin resistance (HOMA > 2.4, N = 10 SOC, N = 11 burosumab).

CONCLUSION

FGF21 does not explain obesity/overweight in XLH. Of note, this study was performed in France in 2018-2019, early after the approval authorizing burosumab only in case of severe XLH despite SOC. As such, the data on systolic blood pressure highlighting a possible impact of burosumab to decrease blood pressure as well as increase Klotho levels deserve further studies given their potential effect on long-term cardiovascular risk. A higher resolution version of the Graphical abstract is available as Supplementary information.

Physiological Mechanisms of Hypertension and Cardiovascular Disease in End-Stage Kidney Disease

PURPOSE OF REVIEW

In this article, we summarize recent advances in understanding hypertension and cardiovascular disease in patients with end-stage kidney disease.

RECENT FINDINGS

Factors such as anemia, valvular and vascular calcification, vasoconstrictors, uremic toxins, hypoglycemia, carbamylated proteins, oxidative stress, and inflammation have all been associated with the progression of cardiovascular disease in end-stage kidney disease but the causality of these mechanisms has not been proven. The high risk of cardiovascular mortality has not improved as in the general population despite many advancements in cardiovascular care over the last two decades. Mechanisms that increase hypertension risk in these patients are centered on the control of extracellular fluid volume; however, over-correction of volume with dialysis can increase risks of intradialytic hypotension and death in these patients. This review presents both recent and classic work that increases our understanding of hypertension and cardiovascular disease in end-stage kidney disease.

Strategies to lower fibroblast growth factor 23 bioactivity

Fibroblast growth factor 23 (FGF23) is a circulating hormone derived from the bone whose release is controlled by many factors and exerts a multitude of systemic actions. There are congenital and acquired disorders of increased and decreased FGF23 levels. In chronic kidney disease (CKD), elevations of FGF23 levels can be 1000-fold above the upper physiological limit. It is still debated whether this high FGF23 in CKD is a biomarker or causally related to morbidity and mortality. Data from human association studies support pathogenicity, while experimental data are less robust. Knowledge of the biology and pathobiology of FGF23 has generated a plethora of means to reduce FGF23 bioactivity at many levels that will be useful for therapeutic translations. This article summarizes these approaches and addresses several critical questions that still need to be answered.

Left Ventricular Hypertrophy in Patients with X-Linked Hypophosphataemia

X-linked hypophosphatemia (XLH) is a rare genetic disorder with X-linked dominant inheritance. Mutations in the PHEX gene increase fibroblast growth factor 23 (FGF23) concentrations, causing loss of phosphorus at the proximal tubule. Most pediatric patients debut in the first two years with short stature and bowed legs. Conventional treatment consists of oral supplements with phosphorus and calcitriol. Since 2018, burosumab has been approved as a novel therapeutic option for XLH, with promising results. The purpose of this study was to share our experience with two cases of XLH treated with burosumab. These patients presented with a broad phenotypical differences. One had the most severe radiological phenotype and developed left ventricular hypertrophy (LVH) and left ventricular dysfunction with preserved ejection fraction. Treatment with burosumab was well-tolerated and was followed by radiological stability and a striking improvement in both blood biochemistry and quality of life. The LVH was stable and left ventricular function normalized in the patient with cardiac involvement. In recent years many studies have been carried out to explain the role of FGF23 in cardiovascular damage, but the exact pathophysiological mechanisms are as yet unclear. The most intensively studied populations are patients with XLH or chronic kidney disease, as both are associated with high levels of FGF23. To date, cardiovascular involvement in XLH has been described in patients treated with conventional treatment, so it would be of interest to investigate if early use of burosumab at the time of diagnosis of XLH would prevent the occurrence of cardiovascular manifestations.

Activation of RAAS Signaling Contributes to Hypertension in Aged Hyp Mice

High circulating levels of fibroblast growth factor-23 (FGF23) are associated with left ventricular hypertrophy as well as increased morbidity and mortality in patients suffering from chronic kidney disease. However, the mechanisms underlying this association are controversial. Here, we aimed to further characterize the cardiovascular sequelae of long term endogenous FGF23 hypersecretion using 14-month-old male Hyp mice as a model of FGF23 excess. Hyp mice were characterized by a ~10-fold increase in circulating intact FGF23, hypophosphatemia, increased serum aldosterone, but normal kidney function, relative to wildtype (WT) controls. Cardiovascular phenotyping did not reveal any evidence of left ventricular hypertrophy or functional impairment in 14-month-old Hyp mice. Fractional shortening, ejection fraction, molecular markers of hypertrophy (Anp, Bnp), and intracardiac markers of contractility and diastolic function were all unchanged in these animals. However, intraarterial catheterization revealed an increase in systolic, diastolic, and mean arterial pressure of ~12 mm Hg in aged Hyp mice relative to WT controls. Hypertension in Hyp mice was associated with increased peripheral vascular resistance. To test the hypothesis that a stimulation of the renin–angiotensin–aldosterone system (RAAS) contributes to hypertension in aged Hyp mice, we administered the angiotensin receptor blocker losartan (30 mg/kg twice daily) or the mineralocorticoid receptor antagonist canrenone (30 mg/kg once daily) to aged Hyp and WT mice over 5 days. Both drugs had minor effects on blood pressure in WT mice, but reduced blood pressure and peripheral vascular resistance in Hyp mice, suggesting that a stimulation of the RAAS contributes to hypertension in aged Hyp mice.

In-center Nocturnal Hemodialysis Reduced the Circulating FGF23, Left Ventricular Hypertrophy, and All-Cause Mortality: A Retrospective Cohort Study

Fibroblast growth factor 23(FGF23) is the most important biomarker and pathogenic factor in Chronic Kidney Disease–Mineral and Bone Disorder (CKD–MBD). In the moderate and severe stages of chronic renal failure, abnormally elevated circulating FGF23 can lead to some complications, including myocardial hypertrophy, which is positively correlated with all-cause mortality. However, the circulating FGF23 level of different hemodialysis modalities, the underlying essential regulatory factors, and potential clinical benefits remain to be elucidated. In this retrospective cohort study, 90 in-center nocturnal hemodialysis (INHD) and 90 matched conventional hemodialysis (CHD) patients were enrolled. The complete blood count, intact FGF23(iFGF23), calcium, phosphorus, PTH, and other biochemical and echocardiographic parameters of INHD and CHD patients were collected and analyzed at 1-year follow-up. The all-cause mortality was recorded during the 7-year follow-up. Furthermore, the regulatory factors of iFGF23 and its association with echocardiographic parameters and mortality were investigated by multivariate regression. The levels of iFGF23 and serum phosphate in patients undergoing INHD were significantly lower than those in patients undergoing CHD. The left ventricular volume index (LVMI) in patients with INHD was significantly attenuated and positively correlated with the drop of serum iFGF23. The INHD group had reduced all-cause mortality compared to the CHD group. Multivariate analysis showed that iFGF23 was positively correlated with serum calcium, serum phosphorus, and calcium-phosphate product. The calcium-phosphate product is an independent determining factor of serum iFGF23. Compared with the CHD group, the INHD group presented with a significantly reduced circulating iFGF23 level, which was closely associated with attenuation of left ventricular hypertrophy, but INHD reduced all-cause mortality in an FGF23 independent manner.

Changes of FGF23 and the Renin-Angiotensin-System in Male Mouse Models of Chronic Kidney Disease and Cardiac Hypertrophy

Serum fibroblast growth factor 23 (FGF23) levels and the renin-angiotensin-aldosterone system (RAAS) are elevated in chronic kidney disease (CKD) patients, and their association with left ventricular hypertrophy (LVH) has been reported. However, whether the FGF23 elevation is the cause or result of LVH remains unclear. At 10 weeks, male C57BL/6J mice were divided into 4 groups: sham, CKD (5/6 nephrectomy), LVH (transaortic constriction), and CKD/LVH group. At 16 weeks, the mice were euthanized, and blood and urine, cardiac expressions of FGF23 and RAAS-related factors, and cardiac histological analyses were performed. Heart weight, serum FGF23 levels, and cardiac expression of FGF23 and RAAS-related factors, except for angiotensin-converting enzyme 2, were more increased in the CKD/LVH group compared to the other groups. A significant correlation between LVH and cardiac expressions of FGF23 and RAAS-related factors was observed. Furthermore, there was a significantly close correlation of the cardiac expression of FGF23 with LVH and RAAS-related factors. The coexisting CKD and LVH increased serum and cardiac FGF23 and RAAS-related factors, and there was a significant correlation between them. A close correlation of cardiac, but not serum FGF23, with LVH and RAAS suggests that local FGF23 levels may be associated with LVH and RAAS activation.

What are the benefits of the anti-FGF23 antibody burosumab on the manifestations of X-linked hypophosphatemia in adults in comparison with conventional therapy? A review

X-linked hypophosphatemia (XLH) is a genetic disease mostly related to PHEX gene mutations which increases FGF23 serum levels, leading to hypophosphatemia and osteomalacia in adults, while affected children, in addition, develop rickets. Most of adults with XLH suffer from reduced quality of life and physical disability due to chronic bone and joint pain related to limb deformities, early osteoarthritis, delayed-healing of insufficiency fractures, and enthesopathies. Dental infections, muscle dysfunction, and deafness are also frequent. The current treatment consists of 2-5 times daily oral administration of phosphate combined to active vitamin D, often badly tolerated with immediate digestive side effects, responsible for poor compliance. In the long term, it may induce nephrocalcinosis and hyperparathyroidism. Burosumab, an anti-FGF23 blocking antibody, was approved for treating children with XLH in many countries. A randomized 24-week-long placebo-controlled trial, followed by an open-label period of equal duration was conducted in 134 XLH adults treated with 1 mg/kg burosumab/4 weeks. During burosumab treatment, 94% of the patients normalized serum phosphate values versus 7% in the placebo group. Fracture healing was increased 16.7 times compared with placebo-treated patients. All pain and disability tests improved significantly in a time-dependent manner. Burosumab for 48 weeks improved histological lesions of osteomalacia in a single-arm longitudinal study analyzing paired bone biopsies. Another single-arm, open-label study investigated the long-term safety and efficacy of burosumab in 20 adult patients followed for 3.2 years. Burosumab was beneficial on pain and disability scores and on bone remodeling markers. No major side effects especially no hyperphosphatemic episodes were reported. Overall, the benefit/risk ratio of burosumab is positive in adult patients with clinical and/or biological complications of XLH. Burosumab corrects hypophosphatemia, promotes fracture healing, and induces a modest but significant effect on XLH-induced subjective pain and disability symptoms.

Plain language title and summary

Effects of conventional treatment and burosumab in adults with X-linked hypophosphatemia.X-linked hypophosphatemia (XLH) is a disease of genetic origin that affects mineralized tissues (skeleton and teeth) and impairs muscle function. It induces a decrease in blood phosphate levels. This leads to under mineralization of bones and insufficiency fractures that heal slowly, associated with poor dental health characterized by spontaneous dental abscesses. Adults with XLH suffer from chronic pain and limb deformities that alter their quality of life. They are currently treated with daily administration of vitamin D and several daily doses of phosphate. This treatment may induce parathyroid gland dysfunction and mineral deposits in the kidney. If not tightly monitored, these side effects may lead to tertiary hyperparathyroidism and the need for parathyroid gland surgery, or to nephrocalcinosis which may proceed to chronic kidney disease. Burosumab is an antibody that blocks the action of FGF23 the factor that circulates in excess in blood and is responsible for phosphate renal leak in XLH. Three studies demonstrated that burosumab, injected every 4 weeks, is efficient and safe for treating adults with XLH.

FGFR4 and Klotho Polymorphisms Are Not Associated with Cardiovascular Outcomes in Chronic Kidney Disease

INTRODUCTION

High plasma fibroblast growth factor 23 (FGF-23) predicts cardiovascular events in chronic kidney disease (CKD) patients. Experimental evidence suggests FGF receptor 4 (FGFR4) activation by FGF-23, and deficiency of the soluble form of its co-receptor Klotho promotes left-ventricular hypertrophy (LVH). To evaluate the clinical relevance of these findings, a Mendelian randomization study analyzed the association of genetic variants of FGFR4 and Klotho with echocardiographic parameters and cardiac events in CKD patients.

METHODS

The prospective Cardiovascular and Renal Outcome in CKD 2-4 Patients-The Fourth Homburg Evaluation study recruited CKD G2-G4 patients, of whom 519 consented to SNP genotyping (FGFR4: rs351855; Klotho: rs9536314). Echocardiographic examinations at baseline and 5 years later assessed prevalence of LVH by measurement of left-ventricular mass index (LVMI). Patients were followed for 5.1 ± 2.1 years for the primary endpoints of cardiac decompensation and atherosclerotic cardiovascular disease (ASCVD).

RESULTS

Carriers of the different alleles did neither differ in baseline LVMI (rs351855: p = 0.861; rs9536314: p = 0.379) nor in LVMI changes between baseline and follow-up (rs351855: p = 0.181; rs9536314: p = 0.995). Hundred and four patients suffered cardiac decompensation, and 144 patients had ASCVD. Time to cardiac decompensation (rs351855: p = 0.316; rs9536314: p = 0.765) and ASCVD (p = 0.508 and p = 0.800, respectively) did not differ between carriers of different alleles.

DISCUSSION/CONCLUSION

rs351855 and rs9536314 were not associated with LVMI or cardiac events. These findings do not provide evidence for a relevant clinical role of either FGFR4 stimulation or soluble form of Klotho deficiency in LVH development.

Intact fibroblast growth factor 23 levels and outcome prediction in patients with acute heart failure

Elevated fibroblast growth factor 23 (FGF23) levels are associated with adverse outcome in populations with cardiovascular disease and chronic kidney failure. It is unclear if FGF23 has significance in prognosis estimation in patients with acute heart failure (HF) when compared to traditional risk estimation tools. Serum levels of intact FGF23 were assessed in 139 patients admitted to the Intermediate Care Unit of a tertiary hospital for acute HF. Patients were followed-up for one year. After exclusion of patients who were lost to follow-up, data outliers, and patients with sampling errors, the final study cohort comprised 133 patients. The Seattle Heart Failure (SHF) Model was used to estimate one-year survival. FGF23 levels correlated with HF severity and were strongly associated with one-year mortality. Associations between one-year outcome and FGF23, assessed on day 1 after admission, were still evident after multivariable adjustment (OR 15.07; 95%CI 1.75-129.79; p = 0.014). FGF23 levels predicted the one-year outcome with similar accuracy as the SHF Model, both if assessed on day 1 and on day 2 after admission (FGF23d1: AUC 0.784; 95%CI 0.669-0.899; FGF23d2: AUC 0.766; 95%CI 0.631-0.901; SHF: AUC 0.771; 95%CI 0.651-0.891). The assessment of FGF23 in patients with acute HF might help identify high-risk patients that are more prone to complications, need a closer follow-up and more aggressive treatment.

Inflammation: a putative link between phosphate metabolism and cardiovascular disease

Dietary habits in the western world lead to increasing phosphate intake. Under physiological conditions, extraosseous precipitation of phosphate with calcium is prevented by a mineral buffering system composed of calcification inhibitors and tight control of serum phosphate levels. The coordinated hormonal regulation of serum phosphate involves fibroblast growth factor 23 (FGF23), αKlotho, parathyroid hormone (PTH) and calcitriol. A severe derangement of phosphate homeostasis is observed in patients with chronic kidney disease (CKD), a patient collective with extremely high risk of cardiovascular morbidity and mortality. Higher phosphate levels in serum have been associated with increased risk for cardiovascular disease (CVD) in CKD patients, but also in the general population. The causal connections between phosphate and CVD are currently incompletely understood. An assumed link between phosphate and cardiovascular risk is the development of medial vascular calcification, a process actively promoted and regulated by a complex mechanistic interplay involving activation of pro-inflammatory signalling. Emerging evidence indicates a link between disturbances in phosphate homeostasis and inflammation. The present review focuses on critical interactions of phosphate homeostasis, inflammation, vascular calcification and CVD. Especially, pro-inflammatory responses mediating hyperphosphatemia-related development of vascular calcification as well as FGF23 as a critical factor in the interplay between inflammation and cardiovascular alterations, beyond its phosphaturic effects, are addressed.

Genetically Predicted Fibroblast Growth Factor 23 and Major Cardiovascular Diseases, Their Risk Factors, Kidney Function, and Longevity: A Two-Sample Mendelian Randomization Study

Introduction

Fibroblast growth factor 23 (FGF23), a potential biomarker for kidney function, is related to cardiovascular disease (CVD) and diabetes, although it is unclear whether the relation is causal. This study evaluated the associations of genetically predicted FGF23 with major CVDs, their risk factors, kidney function, and longevity using Mendelian randomization (MR).

Methods

This is a two-sample MR study using summary statistics from large genome-wide association studies. Primary outcomes included coronary artery disease (CAD), myocardial infarction, heart failure, and atrial fibrillation. Secondary outcomes included cardiovascular risk factors, kidney function, and longevity. We used four single-nucleotide polymorphisms (SNPs) predicting FGF23, excluding rs2769071 in the ABO gene, which likely violates the MR exclusion-restriction assumption. We used inverse-variance weighted (IVW) as the primary statistical method to assess associations of FGF23 with the outcomes. Sensitivity analyses included weighted median (WM) and MR-Egger. We repeated the analyses including all five SNPs. Last, we validated the positive findings from the main analyses in a smaller study, i.e., FinnGen.

Results

Using IVW, genetically predicted higher FGF23 was inversely associated with risk of CAD [odds ratio (OR): 0.69 per logtransformed FGF23 (pg/ml) increase, 95% confidence interval (CI): 0.52–0.91] and type 2 diabetes mellitus (T2DM) (OR: 0.70, 95% CI: 0.52–0.96), but not with the other outcomes. The WM and MR-Egger estimates were directionally consistent.

Conclusion

This study suggests that genetically predicted higher FGF23 may be protective against CAD and T2DM. Future studies should explore the underlying mechanisms related to the potential protective effect of FGF23. FGF23 was unlikely a cause of poorer renal function.

Rare PHEX variant with insidious presentation leads to a delayed diagnosis of X-linked hypophosphatemia

A 7-year-old girl without a significant previous medical history was diagnosed with X-linked hypophosphatemic rickets (XLHR) due to a rare, most likely pathogenic, PHEX gene variant after a 4-year delayed diagnosis due to mild clinical presentation. At 2 years of age, her intoeing and femoral bowing were attributed to physiologic bowing and borderline vitamin D sufficiency, despite phosphorus not being measured. Hypophosphatemia was eventually detected after incomplete improvement of bowing and leg length discrepancy with suboptimal linear growth. This rare PHEX variant (c.1949T>C, p.Leu650Pro) further supported the clinical diagnosis of XLHR. Treatment with burosumab (an anti-FGF23 monoclonal antibody) normalised phosphorus and alkaline phosphatase levels and improved her bowing. The diverse phenotypic presentation of this variant can result in delayed diagnosis and highlights the importance of prompt assessment of phosphorus levels in patients with skeletal deformities to ensure timely recognition and treatment.

High FGF23 Levels Failed to Predict Cardiac Hypertrophy in Animal Models of Hyperphosphatemia and Chronic Renal Failure

Increased fibroblast growth factor 23 (FGF23) levels are an independent predictor for adverse cardiac events suggesting a role as a link that drives cardiomyopathic changes in cardiorenal syndrome. The search for the underlying mechanism driving this interaction has led to the hypothesis that FGF23 causes pathogenic changes in the heart. Increased serum FGF23 has been independently shown to cause increased cardiac morbidity, mortality, and hypertrophy by signalling through FGF receptor 4. This mechanistic concept was based on preclinical studies demonstrating inhibition of FGF23 signaling through FGF4, which led to suppression of left ventricular hypertrophy and fibrosis in a 2-week rat 5/6 nephrectomy study and a 12-week (2%) high-phosphate diet mouse model in which FGF23 levels were markedly elevated. In this report, renal dysfunction was observed in the 5/6 nephrectomy model, and FGF23 levels were significantly elevated, whereas no changes in left ventricular hypertrophy were observed at 2 or 4 weeks postnephrectomy. Mice placed on a high-phosphate diet that did not cause significant renal dysfunction resulted in significantly elevated FGF23 but no changes in left ventricular hypertrophy. The in vivo studies reported here, which were performed to recapitulate the observations of FGF23 as a driver of cardiac hypertrophy, did not lend support to the FGF23-driven cardiac remodelling hypothesis.

Investigating the causal effect of fibroblast growth factor 23 on osteoporosis and cardiometabolic disorders: A Mendelian randomization study

Pathological excess of fibroblast growth factor 23 (FGF23) causes mineral and bone disorders. However, the causality of FGF23 in the development of osteoporosis remains unknown. Whether FGF23 has systemic effects on cardiometabolic disorders beyond regulating mineral metabolism is also controversial. In this study, we investigated the causal effect of FGF23 on osteoporosis and cardiometabolic disorders using Mendelian randomization (MR) analysis. Summary statistics for single-nucleotide polymorphisms with traits of interest were obtained from the relevant genome-wide association studies. As a result, FGF23 was found to be inversely associated with femoral neck-BMD (odds ratio [OR] 0.682, 95% confidence interval [CI] 0.546-0.853, p = 8e-04) and heel estimated BMD (eBMD) (OR 0.898, 95%CI 0.820-0.985, p = 0.022) in the inverse-variance-weighted analysis, but not lumbar spine-BMD and fractures. The results were supported by the weighted-median analysis, and there was no evidence of pleiotropy in the MR-Egger analysis. FGF23 was associated with FN-BMD and eBMD after adjustment for estimated glomerular filtration rate, height, and body mass index in multivariable MR analysis. On the other hand, there was no association between FGF23 and cardiometabolic traits including cardio artery disease, brachial-ankle pulse wave velocity, intima-media thickness of carotid arteries, systolic and diastolic blood pressure, fasting glucose, high and low-density lipoprotein cholesterol, and triglycerides. Therefore, this MR study established that FGF23 was involved in bone loss and, in contrast, was not involved in cardiometabolic disorders. Our findings provide important insights into the role of FGF23 in the pathogenesis of osteoporosis and cardiometabolic disorders.

Fibroblast Growth Factor 23 and Left Ventricular Hypertrophy in Chronic Kidney Disease-A Pediatric Perspective

Cardiovascular diseases (CVD) are a hallmark in pediatric patients with chronic kidney disease (CKD) contributing to an enhanced risk of all-cause and CV morbidity and mortality in these patients. The bone-derived phosphaturic hormone fibroblast growth factor (FGF) 23 progressively rises with declining kidney function to maintain phosphate homeostasis, with up to 1,000-fold increase in patients with kidney failure requiring dialysis. FGF23 is associated with the development of left ventricular hypertrophy (LVH) and thereby accounts to be a CVD risk factor in CKD. Experimentally, FGF23 directly induces hypertrophic growth of cardiac myocytes in vitro and LVH in vivo. Further, clinical studies in adult CKD have observed cardiotoxicity associated with FGF23. Data regarding prevalence and determinants of FGF23 excess in children with CKD are limited. This review summarizes current data and discusses whether FGF23 may be a key driver of LVH in pediatric CKD.

A Land of Controversy: Fibroblast Growth Factor-23 and Uremic Cardiac Hypertrophy

Cardiac hypertrophy is a common feature in patients with CKD. Recent studies revealed that two phosphate regulators, fibroblast growth factor-23 and α-Klotho, are highly involved in the pathophysiologic process of CKD-induced cardiac hypertrophy. With decreasing renal function, elevated fibroblast growth factor-23 and decreased α-Klotho may contribute to cardiac hypertrophy by targeting the heart directly or by inducing systemic changes, such as vascular injury, hemodynamic disorders, and inflammation. However, several studies have demonstrated that disturbances in the fibroblast growth factor-23/α-Klotho axis do not lead to cardiac hypertrophy. In this review, we describe the cardiac effects of the fibroblast growth factor-23/α-Klotho axis and summarize recent progress in this field. In addition, we present not only the main controversies in this field but also provide possible directions to resolve these disputes.

Endocrine fibroblast growth factors as potential biomarkers for chronic kidney disease

INTRODUCTION

Among the family of fibroblast growth factors (FGFs), FGF19, FGF21, and FGF23 act as circulating hormones and are called endocrine FGFs. FGF19 and FGF21 regulate bile acid and energy homeostasis, respectively, whereas FGF23 regulates vitamin D and phosphate homeostasis. Accumulating evidence suggests that FGF23 plays a critical role in disturbed mineral metabolisms, left ventricular hypertrophy, immunosuppression, inflammation, among others in patients with chronic kidney disease (CKD), highlighting the potential both as a biomarker and a therapeutic target. Several studies have also examined the potential role of FGF19 and FGF21 in CKD patients.

AREAS COVERED

In this review, we present a brief overview of the biology of FGF19, FGF21, and FGF23, and summarize recent clinical and experimental studies on the pathophysiological roles of endocrine FGFs, mainly FGF23, in CKD patients.

EXPERT OPINION

Among the endocrine FGFs, FGF23 represents the most promising biomarker in CKD patients. If future studies confirm that FGF23 is directly toxic in CKD patients, FGF23 could be regarded as a therapeutic target and its measurement would be valuable if applied in clinical practice. Despite their potentially important roles, the clinical relevance of FGF19 and FGF21 in CKD patients is unclear, and much more studies are required.

FGF23 measurement in chronic kidney disease: What is it really reflecting?

Fibroblast growth factor can be measured in clinical practice using ELISA, with acceptable validity. Different from many metabolites and minerals, its value can differ by a thousand-fold between individuals, largely because of differences in kidney function and dietary habits. This wide range complicates the proper interpretation of the concentration of FGF23, both in terms of the appropriateness of a given value for a given estimated GFR, and in terms of estimating the magnitude of risk for clinical events, with which FGF23 is clearly associated. In this narrative review, the impact of kidney function, exposure to phosphate from diet, and novel emerging factors that influence FGF23 concentrations are discussed. These and yet to define determinants of FGF23 question the causality of the association of FGF23 with hard (cardiovascular) endpoints, as observed in several epidemiological studies.

Fibroblast growth factor 23: are we ready to use it in clinical practice?

Patients with chronic kidney disease (CKD) have a greatly enhanced risk of cardiovascular morbidity and mortality. Over the past decade it has come clear that a disturbed calcium-phosphate metabolism, with Fibroblast Growth Factor-23 as a key hormone, is partly accountable for this enhanced risk. Numerous studies have been performed unravelling FGF23s actions and its association with clinical conditions. As FGF23 is strongly associated with adverse outcome it may be a promising biomarker for risk prediction or, even more important, targeting FGF23 may be a strategy to improve patient outcome. This review elaborates on the clinical usefulness of FGF23 measurement. Firstly it discusses the reliability of the FGF23 measurement. Secondly, it evaluates whether FGF23 measurement may lead to improved patient risk classification. Finally, and possibly most importantly, this review evaluates if lowering of FGF23 should be a target for therapy. For this, the review discusses the current evidence indicating that FGF23 may be in the causal pathway to cardiovascular pathology, provides an overview of strategies to lower FGF23 levels and discusses the current evidence concerning the benefit of lowering FGF23.

Fibroblast growth factor 23 is associated with fractional excretion of sodium in patients with chronic kidney disease

BACKGROUND

Recent studies suggest that the phosphaturic hormone fibroblast growth factor 23 (FGF23) is involved in regulation of renal sodium excretion and blood pressure. There is evidence of both direct effects via regulation of the sodium-chloride symporter (NCC) in the distal tubule, and indirect effects through interactions with the renin-angiotensin-aldosterone system. However, clinical data on the association between FGF23 and renal sodium regulation is lacking. Herein, we investigated the associations of FGF23 with renal sodium handling and blood pressure in non-dialysis CKD patients.

METHODS

This was a cross-sectional study encompassing 180 CKD patients Stage 1-5, undergoing renal biopsy. Plasma intact FGF23, 24-h urinary sodium excretion, fractional excretion of sodium (FENa) and blood pressure were measured at baseline. The association between FGF23 and renal sodium handling was explored by multivariate regression analysis.

RESULTS

The median age was 52.8 years, 60.6% were men and the median estimated glomerular filtration rate (eGFR) was 50.6 mL/min/1.73 m2. In univariate analysis, FGF23 was positively associated with FENa (Spearman's rho = 0.47; P < 0.001) and systolic blood pressure (rho = 0.17, P < 0.05), but not with plasma sodium, 24-h urinary sodium excretion or mean arterial blood pressure. The association between FGF23 and FENa remained significant after adjustment for potential confounders (multivariable adjusted β coefficient 0.60, P < 0.001). This association was stronger among the 107 individuals with eGFR <60 mL/min/1.73 m2 (β = 0.47, P = 0.04) and in the 73 individuals on any diuretics (β = 0.88, P < 0.001). Adjustment for measured GFR instead of eGFR did not alter the relationship.

CONCLUSIONS

FGF23 is independently associated with increased FENa in non-dialysis CKD patients. These data do not support the notion that FGF23 causes clinically significant sodium retention. Further studies are warranted to explore the mechanism underlying this association.

Management of X-linked hypophosphatemia in adults

X-linked hypophosphatemia (XLH) is caused by mutations in the PHEX gene which result in Fibroblast Growth Factor-23 (FG-F23) excess and phosphate wasting. Clinically, XLH children present with rickets, bone deformities and short stature. In adulthood, patients may still be symptomatic with bone and joint pain, osteomalacia-related fractures or pseudofractures, precocious osteoarthrosis, enthesopathy, muscle weakness and severe dental anomalies. Besides these musculoskeletal and dental manifestations, adult XLH patients are also prone to secondary and tertiary hyperparathyroidism, cardiovascular and metabolic disorders. Pathophysiology of hyperparathyroidism is only partially understood but FGF23 excess and deficient production of calcitriol likely contributes to its development. Similarly, the pathophysiological mechanisms of potential cardiovascular and metabolic involvements are not clear, but FGF-23 excess may play an essential role. Treatment should be considered in symptomatic patients, patients undergoing orthopedic or dental surgery and women during pregnancy and lactation. Treatment with oral phosphate salts and active vitamin D analogs has incomplete efficacy and potential risks. Burosumab, a recombinant human monoclonal antibody against FGF-23, has proven its efficacy in phase 2 and phase 3 clinical trials in adult patients with XLH, but currently its position as first line or second line treatment differ among the countries.

Circulating FGF23 is not associated with cardiac dysfunction, atherosclerosis, infection or inflammation in hemodialysis patients

Fibroblast growth factor (FGF) 23 is a bone-derived hormone regulating serum inorganic phosphate (Pi) concentration. FGF23 is also involved in the development of chronic kidney disease (CKD)-mineral and bone disorder. Serum FGF23 concentration begins to increase early in the progression of CKD and can be remarkably high in hemodialysis patients with end-stage renal disease. It has been reported that high FGF23 concentration is a risk factor for cardiac dysfunction, atherosclerosis, infection or systemic inflammation in CKD patients. FGF23 was also shown to induce cardiac hypertrophy directly acting on cardiomyocytes. However, it is still controversial whether high FGF23 is causing cardiac dysfunction, atherosclerosis, infection or systemic inflammation in CKD patients. In the current study, we investigated whether FGF23 concentration is associated with cardiac dysfunction, atherosclerosis, infection or systemic inflammation in Japanese hemodialysis patients. We recruited 119 hemodialysis patients and examined the association between serum FGF23 concentration and several parameters concerning mineral metabolism, cardiac dysfunction, atherosclerosis, infection, and systemic inflammation. Serum FGF23 concentration was independently associated with serum calcium and Pi concentration (β = 0.276, p < 0.001; β = 0.689, p < 0.001). However, serum FGF23 concentration was not associated with parameters of cardiac dysfunction, atherosclerosis, infection, and systemic inflammation, either. Our results do not support the hypothesis that high FGF23 in dialysis patients is the cause of cardiac dysfunction, atherosclerosis, infection or systemic inflammation.

Renal and extrarenal effects of fibroblast growth factor 23

Fibroblast growth factor 23 (FGF23) is a hormone with a central role in the regulation of phosphate homeostasis. This regulation is accomplished by the coordinated modulation of renal phosphate handling, vitamin D metabolism and parathyroid hormone secretion. Patients with kidney disease have increased circulating levels of FGF23 and in other patient populations and in healthy individuals, FGF23 levels also rise following an increase in dietary phosphate intake. Maladaptive increases in FGF23 have a detrimental effect on several organs and tissues and, importantly, these pathological changes most likely contribute to increased morbidity and mortality. For example, in the context of heart disease, FGF23 is involved in the development of pathological hypertrophy that can lead to congestive heart failure. Increased FGF23 concentrations can also lead to microcirculatory changes, in particular reduced vasodilatory capacity, and collectively these cardiovascular changes can compromise tissue perfusion. In addition, FGF23 is associated with inflammation and an increased risk of infection; other potentially detrimental effects of FGF23 are likely to emerge in the future. Most importantly, recent insights demonstrate that FGF23 can be therapeutically targeted, which holds promise for the treatment of many patients in a variety of clinical settings.

αKlotho-FGF23 interactions and their role in kidney disease: a molecular insight

Following the serendipitous discovery of the ageing suppressor, αKlotho (αKl), several decades ago, a growing body of evidence has defined a pivotal role for its various forms in multiple aspects of vertebrate physiology and pathology. The transmembrane form of αKl serves as a co-receptor for the osteocyte-derived mineral regulator, fibroblast growth factor (FGF)23, principally in the renal tubules. However, compelling data also suggest that circulating soluble forms of αKl, derived from the same source, may have independent homeostatic functions either as a hormone, glycan-cleaving enzyme or lectin. Chronic kidney disease (CKD) is of particular interest as disruption of the FGF23-αKl axis is an early and common feature of disease manifesting in markedly deficient αKl expression, but FGF23 excess. Here we critically discuss recent findings in αKl biology that conflict with the view that soluble αKl has substantive functions independent of FGF23 signalling. Although the issue of whether soluble αKl can act without FGF23 has yet to be resolved, we explore the potential significance of these contrary findings in the context of CKD and highlight how this endocrine pathway represents a promising target for novel anti-ageing therapeutics.

FGF23 and Phosphate-Cardiovascular Toxins in CKD

Elevated levels of fibroblast growth factor 23 (FGF23) and phosphate are highly associated with increased cardiovascular disease and mortality in patients suffering from chronic kidney disease (CKD). As the kidney function declines, serum phosphate levels rise and subsequently induce the secretion of the phosphaturic hormone FGF23. In early stages of CKD, FGF23 prevents the increase of serum phosphate levels and thereby attenuates phosphate-induced vascular calcification, whereas in end-stage kidney disease, FGF23 fails to maintain phosphate homeostasis. Both hyperphosphatemia and elevated FGF23 levels promote the development of hypertension, vascular calcification, and left ventricular hypertrophy by distinct mechanisms. Therefore, FGF23 and phosphate are considered promising therapeutic targets to improve the cardiovascular outcome in CKD patients. Previous therapeutic strategies are based on dietary and pharmacological reduction of serum phosphate, and consequently FGF23 levels. However, clinical trials proving the effects on the cardiovascular outcome are lacking. Recent publications provide evidence for new promising therapeutic interventions, such as magnesium supplementation and direct targeting of phosphate and FGF receptors to prevent toxicity of FGF23 and hyperphosphatemia in CKD patients.

FGF23, Hypophosphatemia, and Emerging Treatments

FGF23 is an important hormonal regulator of phosphate homeostasis. Together with its co-receptor Klotho, it modulates phosphate reabsorption and both 1α-hydroxylation and 24-hydroxylation in the renal proximal tubules. The most common FGF23-mediated hypophosphatemia is X-linked hypophosphatemia (XLH), caused by mutations in the PHEX gene. FGF23-mediated forms of hypophosphatemia are characterized by phosphaturia and low or low-normal calcitriol concentrations, and unlike nutritional rickets, these cannot be cured with nutritional vitamin D supplementation. Autosomal dominant and autosomal recessive forms of FGF23-mediated hypophosphatemias show a similar pathophysiology, despite a variety of different underlying genetic causes. An excess of FGF23 activity has also been associated with a number of other conditions causing hypophosphatemia, including tumor-induced osteomalacia, fibrous dysplasia of the bone, and cutaneous skeletal hypophosphatemia syndrome. Historically phosphate supplementation and therapy using analogs of highly active vitamin D (eg, calcitriol, alfacalcidol, paricalcitol, eldecalcitol) have been used to manage conditions involving hypophosphatemia; however, recently a neutralizing antibody for FGF23 (burosumab) has emerged as a promising treatment agent for FGF23-mediated disorders. This review discusses the progression of clinical trials for burosumab for the treatment of XLH and its recent availability for clinical use. Burosumab may have potential for treating other conditions associated with FGF23 overactivity, but these are not yet supported by trial data. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia

X-linked hypophosphataemia (XLH) is the most common cause of inherited phosphate wasting and is associated with severe complications such as rickets, lower limb deformities, pain, poor mineralization of the teeth and disproportionate short stature in children as well as hyperparathyroidism, osteomalacia, enthesopathies, osteoarthritis and pseudofractures in adults. The characteristics and severity of XLH vary between patients. Because of its rarity, the diagnosis and specific treatment of XLH are frequently delayed, which has a detrimental effect on patient outcomes. In this Evidence-Based Guideline, we recommend that the diagnosis of XLH is based on signs of rickets and/or osteomalacia in association with hypophosphataemia and renal phosphate wasting in the absence of vitamin D or calcium deficiency. Whenever possible, the diagnosis should be confirmed by molecular genetic analysis or measurement of levels of fibroblast growth factor 23 (FGF23) before treatment. Owing to the multisystemic nature of the disease, patients should be seen regularly by multidisciplinary teams organized by a metabolic bone disease expert. In this article, we summarize the current evidence and provide recommendations on features of the disease, including new treatment modalities, to improve knowledge and provide guidance for diagnosis and multidisciplinary care.

Risk of cardiovascular involvement in pediatric patients with X-linked hypophosphatemia

OBJECTIVE

To find out if cardiovascular alterations are present in pediatric patients with X-linked hypophosphatemia (XLH).

STUDY DESIGN

Multicentre prospective clinical study on pediatric patients included in the RenalTube database ( www.renaltube.com ) with genetically confirmed diagnosis of XLH by mutations in the PHEX gene. The study's protocol consisted of biochemical work-up, 24-h ambulatory blood pressure monitoring (ABPM), carotid ultrasonography, and echocardiogram. All patients were on chronic treatment with phosphate supplements and 1-hydroxy vitamin D metabolites.

RESULTS

Twenty-four patients (17 females, from 1 to 17 years of age) were studied. Serum concentrations (X ± SD) of phosphate and intact parathyroid hormone were 2.66 ± 0.60 mg/dl and 58.3 ± 26.8 pg/ml, respectively. Serum fibroblast growth factor 23 (FGF23) concentration was 278.18 ± 294.45 pg/ml (normal < 60 pg/ml). Abnormally high carotid intima media thickness was found in one patient, who was obese and hypertensive as revealed by ABPM, which disclosed arterial hypertension in two other patients. Z scores for echocardiographic interventricular septum end diastole and left ventricular posterior wall end diastole were + 0.77 ± 0.77 and + 0.94 ± 0.86, respectively. Left ventricular mass index (LVMI) was 44.93 ± 19.18 g/m^2.7, and four patients, in addition to the obese one, had values greater than 51 g/m^2.7, indicative of left ventricular hypertrophy. There was no correlation between these echocardiographic parameters and serum FGF23 concentrations.

CONCLUSIONS

XLH pediatric patients receiving conventional treatment have echocardiographic measurements of ventricular mass within normal reference values, but above the mean, and 18% have LVMI suggestive of left ventricular hypertrophy without correlation with serum FGF23 concentrations. This might indicate an increased risk of cardiovascular involvement in XLH.

FGF23 effects on the heart-levels, time, source, and context matter

Fibroblast growth factor 23 (FGF23) has appeared as a hormone that is massively elevated in patients with chronic kidney disease. Whether FGF23 is a risk factor that associates with cardiac pathologies and cardiovascular mortality, as suggested by a variety of clinical studies, or additionally acts as a causative factor that induces cardiac injury, as more recently indicated by cell culture and animal studies, is under debate and the center of many ongoing experimental studies.

FGF23 beyond Phosphotropic Hormone

Fibroblast growth factor (FGF) 23 is a bone-derived phosphotropic hormone that regulates phosphate and vitamin D metabolism. FGF23 mainly affects kidney function via the FGF receptor (FGFR)/α-Klotho complex. The physiological roles of FGF23 and α-Klotho in the regulation of mineral homeostasis have been well established. In addition, recent studies have reported that FGF23 has various effects on many other tissues, sometimes in an α-Klotho-independent manner, especially under pathological conditions. However, how FGF23 works in these tissues without α-Klotho is not entirely clear. Here we review the recent reports concerning the actions of FGF23 on various tissues and discuss the remaining questions about FGF23.

Left Ventricular Structure in Patients With Mild-to-Moderate CKD-a Magnetic Resonance Imaging Study

Introduction

The high burden of left ventricular (LV) abnormalities in patients with advanced chronic kidney disease (CKD) is well established. However, less is known about the prevalence, patterns, and determinants of LV abnormalities in patients with early CKD.

Methods

We examined LV structure in 290 patients with a median estimated glomerular filtration rate (eGFR) of 51 ml/min per 1.73 m² by magnetic resonance imaging (MRI). We explored associations with clinical and hemodynamic parameters, hydration (bioimpedance), endothelial function, inflammation (including C-reactive protein and tumor necrosis factor−α and its soluble receptors) and mineral bone disease (MBD) markers (including vitamin D, parathyroid hormone, α-klotho and fibroblast growth factor−23).

Results

Normal geometry was found in 56% of patients, dilation in 4%, concentric remodeling in 10%, and LV hypertrophy in 29%. Linear regression analysis revealed that greater LV mass was independently associated with male sex, greater body mass index (BMI), and higher 24-hour systolic blood pressure (24-hour SBP). Concentric remodeling was independently associated with age, male sex, higher 24-hour SBP, and greater hemoglobin levels. Surprisingly, neither hydration status, nor endothelial function, nor any of the inflammatory or MBD parameters added significantly to these models.

Conclusion

Abnormal LV structure was found in almost one-half of the patients. Reducing BMI and 24-hour SBP and avoiding high hemoglobin concentrations appear to be the key factors to prevent abnormal LV remodeling in patients with mild-to-moderate CKD.

Cardiovascular Interactions between Fibroblast Growth Factor-23 and Angiotensin II

Both the activation of the renin angiotensin aldosterone system (RAAS) and elevations of circulating Fibroblast Growth Factor-23 (FGF-23) have been implicated in the pathogenesis of left ventricular hypertrophy (LVH) in chronic kidney disease. To investigate potential cross-talk between RAAS and FGF-23, we administered angiotensin II (Ang II) to wild-type rodents and the Hyp mouse model of excess FGF-23. Ang II administration for four weeks to wild-type rodents resulted in significant increases in systolic blood pressure and LVH. Unexpectedly, FGF-23 circulating levels were increased by 1.5-1.7 fold in Ang II treated animals. In addition, Ang II treatment increased expression of FGF-23 message levels in bone, the predominant tissue for FGF-23 production, and induced expression of FGF-23 and its co-receptor α-Klotho in the heart, which normally does not express FGF-23 or α-Klotho in physiologically relevant levels. Hyp mice with elevated FGF-23 exhibited increased blood pressure and LVH at baseline. Ang II administration to Hyp mice resulted further increments in blood pressure and left ventricular hypertrophy, consistent with additive cardiovascular effects. These findings suggest that FGF-23 may participate in unexpected systemic and paracrine networks regulating hemodynamic and myocardial responses.

X-Linked Hypophosphatemia and FGF23-Related Hypophosphatemic Diseases: Prospect for New Treatment

Phosphate plays essential roles in many biological processes, and the serum phosphate level is tightly controlled. Chronic hypophosphatemia causes impaired mineralization of the bone matrix and results in rickets and osteomalacia. Fibroblast growth factor 23 (FGF23) is a bone-derived hormone that regulates phosphate metabolism. FGF23 excess induces hypophosphatemia via impaired phosphate reabsorption in the renal proximal tubules and decreased phosphate absorption in the intestines. There are several types of genetic and acquired FGF23-related hypophosphatemic diseases. Among these diseases, X-linked hypophosphatemia (XLH), which is caused by inactivating mutations in the phosphate-regulating endopeptidase homolog, X-linked (PHEX) gene, is the most prevalent form of genetic FGF23-related hypophosphatemic rickets. Another clinically relevant form of FGF23-related hypophosphatemic disease is tumor-induced osteomalacia (TIO), a paraneoplastic syndrome associated with FGF23-producing tumors. A combination of active vitamin D and phosphate salts is the current medical therapy used to treat patients with XLH and inoperative TIO. However, this therapy has certain efficacy- and safety-associated limitations. Several measures to inhibit FGF23 activity have been considered as possible new treatments for FGF23-related hypophosphatemic diseases. In particular, a humanized monoclonal antibody for FGF23 (burosumab) is a promising treatment in patients with XLH and TIO. This review will focus on the phosphate metabolism and the pathogenesis and treatment of FGF23-related hypophosphatemic diseases.

Increased Circulating FGF23 Does Not Lead to Cardiac Hypertrophy in the Male Hyp Mouse Model of XLH

Serum levels of fibroblast growth factor 23 (FGF23) markedly increase with renal impairment, with FGF23 levels correlating with the presence of left ventricular hypertrophy (LVH) and mortality in patients with chronic kidney disease (CKD). FGF23 activates calcineurin/nuclear factor of activated T cell (NFAT) signaling and induces hypertrophy in murine cardiomyocytes. X-linked hypophosphatemia (XLH) is characterized by high circulating levels of FGF23 but, in contrast to CKD, is associated with hypophosphatemia. The cardiac effects of high circulating levels of FGF23 in XLH are not well defined. Thus, studies were undertaken to define the cardiac phenotype in the mouse model of XLH (Hyp mice). Echocardiographic and histological analyses demonstrated that Hyp left ventricles (LVs) are smaller than those of wild-type mice. Messenger RNA expression of cardiac hypertrophy markers was not altered in the LV or right ventricle of Hyp mice. However, the Hyp LVs had increased expression of the NFAT target genes NFATc1 and RCAN1. To determine whether phosphate alone can induce markers of hypertrophy, differentiated C2C12 myocytes were treated with phosphate. Phosphate treatment increased expression of cardiac hypertrophy markers, supporting a primary role for phosphate in inducing LVH. Although previous studies showed that increased circulating FGF23 and phosphate levels are associated with LVH, our results demonstrated that in XLH, high circulating levels of FGF23 in the setting of hypophosphatemia do not induce cardiac hypertrophy.

FGF23 Actions on Target Tissues-With and Without Klotho

Fibroblast growth factor (FGF) 23 is a phosphaturic hormone whose physiologic actions on target tissues are mediated by FGF receptors (FGFR) and klotho, which functions as a co-receptor that increases the binding affinity of FGF23 for FGFRs. By stimulating FGFR/klotho complexes in the kidney and parathyroid gland, FGF23 reduces renal phosphate uptake and secretion of parathyroid hormone, respectively, thereby acting as a key regulator of phosphate metabolism. Recently, it has been shown that FGF23 can also target cell types that lack klotho. This unconventional signaling event occurs in an FGFR-dependent manner, but involves other downstream signaling pathways than in "classic" klotho-expressing target organs. It appears that klotho-independent signaling mechanisms are only activated in the presence of high FGF23 concentrations and result in pathologic cellular changes. Therefore, it has been postulated that massive elevations in circulating levels of FGF23, as found in patients with chronic kidney disease, contribute to associated pathologies by targeting cells and tissues that lack klotho. This includes the induction of cardiac hypertrophy and fibrosis, the elevation of inflammatory cytokine expression in the liver, and the inhibition of neutrophil recruitment. Here, we describe the signaling and cellular events that are caused by FGF23 in tissues lacking klotho, and we discuss FGF23's potential role as a hormone with widespread pathologic actions. Since the soluble form of klotho can function as a circulating co-receptor for FGF23, we also discuss the potential inhibitory effects of soluble klotho on FGF23-mediated signaling which might-at least partially-underlie the pleiotropic tissue-protective functions of klotho.

Genetic Ablation of Fgf23 or Klotho Does not Modulate Experimental Heart Hypertrophy Induced by Pressure Overload

Left ventricular hypertrophy (LVH) ultimately leads to heart failure in conditions of increased cardiac pre- or afterload. The bone-derived phosphaturic and sodium-conserving hormone fibroblast growth factor-23 (FGF23) and its co-receptor Klotho have been implicated in the development of uremic LVH. Using transverse aortic constriction (TAC) in gene-targeted mouse models, we examine the role of Fgf23 and Klotho in cardiac hypertrophy and dysfunction induced by pressure overload. TAC profoundly increases serum intact Fgf23 due to increased cardiac and bony Fgf23 transcription and downregulation of Fgf23 cleavage. Aldosterone receptor blocker spironolactone normalizes serum intact Fgf23 levels after TAC by reducing bony Fgf23 transcription. Notably, genetic Fgf23 or Klotho deficiency does not influence TAC-induced hypertrophic remodelling, LV functional impairment, or LV fibrosis. Despite the profound, aldosterone-mediated increase in circulating intact Fgf23 after TAC, our data do not support an essential role of Fgf23 or Klotho in the pathophysiology of pressure overload-induced cardiac hypertrophy.

Is Fibroblast growth factor 23 the leading cause of increased mortality among chronic kidney disease patients? A narrative review

The death rate among chronic kidney disease patients is the highest compared to other chronic diseases. 60% of these fatalities are cardiovascular. Cardiovascular calcifications and chronic inflammation affect almost all chronic kidney disease patients and are associated with cardiovascular mortality. Fibroblast growth factor 23 is associated with vascular calcification. Systemic inflammation in chronic kidney disease patients is multifactorial. The role of systemic inflammation in the pathogenesis of vascular calcification was recently reappraised. Fibroblast growth factor 23 was accused as a direct stimulus of left ventricular hypertrophy, uremic inflammation, and impaired neutrophil function. This review will discuss the underlying mechanisms that underlie the link between Fibroblast growth factor 23 and increased mortality encountered among chronic kidney disease patients.