Clinical practice. Fibroblast growth factor (FGF)23: a new hormone

Skeletal and extraskeletal disorders of biomineralization

The physiological process of biomineralization is complex and deviation from it leads to a variety of diseases. Progress in the past 10 years has enhanced understanding of the genetic, molecular and cellular pathophysiology underlying these disorders; sometimes, this knowledge has both facilitated restoration of health and clarified the very nature of biomineralization as it occurs in humans. In this Review, we consider the principal regulators of mineralization and crystallization, and how dysregulation of these processes can lead to human disease. The knowledge acquired to date and gaps still to be filled are highlighted. The disorders of mineralization discussed comprise a broad spectrum of conditions that encompass bone disorders associated with alterations of mineral quantity and quality, as well as disorders of extraskeletal mineralization (hyperphosphataemic familial tumoural calcinosis). Included are disorders of alkaline phosphatase (hypophosphatasia) and phosphate homeostasis (X-linked hypophosphataemic rickets, fluorosis, rickets and osteomalacia). Furthermore, crystallopathies are covered as well as arterial and renal calcification. This Review discusses the current knowledge of biomineralization derived from basic and clinical research and points to future studies that will lead to new therapeutic approaches for biomineralization disorders.

Congenital Conditions of Hypophosphatemia Expressed in Adults

The main congenital conditions of hypophosphatemia expressed in adulthood include several forms of hereditary hypophosphatemic rickets and a congenital disorder of vitamin D metabolism characterized by osteomalacia and hypophosphatemia in adult patients. Hypophosphatemia in adults is defined as serum phosphate concentration < 0.80 mmol/L. The principal regulators of phosphate homeostasis, as is well known, are parathyroid hormone (PTH), activated vitamin D, and Fibroblast Growth Factor 23 (FGF23). Differential diagnosis of hypophosphatemia is based on the evaluation of mechanisms leading to this alteration, such as high PTH activity, inadequate phosphate absorption from the gut, or renal phosphate wasting, either due to primary tubular defects or high FGF23 levels. The most common inherited form associated to hypophosphatemia is X-linked hypophosphatemic rickets (XLH), caused by PHEX gene mutations with enhanced secretion of the FGF23. Until now, the management of hypophosphatemia in adulthood has been poorly investigated. It is widely debated whether adult patients benefit from the conventional treatments normally used for pediatric patients. The new treatment for XLH with burosumab, a recombinant human IgG1 monoclonal antibody that binds to FGF23, blocking its activity, may change the pharmacological management of adult subjects with hypophosphatemia associated to FGF23-dependent mechanisms.

Investigation of the serum vitamin D level in infants followed up with the diagnosis of laryngomalacia: a case-control study

OBJECTIVE

The possible etiological relationship of the vitamin D with laryngomalacia is unclear. The aim of the study was to demonstrate the relationship between laryngomalacia and vitamin D levels.

METHODS

Twenty-three non-syndromic babies under the age of 1 year who were diagnosed with laryngomalacia were included in the study group. Forty healthy babies were included in the control group. The detailed anamnesis was obtained and a complete systemic physical examination, a flexible endoscopic laryngeal examination, and laboratory tests [calcium (Ca), phosphorus (P), alkaline phosphatase (ALP), parathormone (PTH), alanine transaminase (ALT), aspartate transaminase (AST), blood urea nitrogen (BUN), creatinine (Cre), and 25-hydroxy vitamin D (25-OH D)] were performed in all patients. All laboratory tests of the groups were compared.

RESULTS

It was observed that there was no significant difference in the parameters that could affect vitamin D levels, namely type of feeding, vitamin D supplement intake, and the season when the serum sample was taken (p > 0.05). The vitamin D level was significantly lower (p = 0.003, p < 0.05) and the P and ALP levels were significantly higher (p = 0.016 and p = 0.001, respectively; p < 0.05) in the laryngomalacia group. Although the correlation between vitamin D and PTH was not statistically significant according to the Pearson correlation analysis, it was lower in the laryngomalacia group compared to the control group (p = 0.381, p > 0.05).

CONCLUSION

In this study, it was observed that the vitamin D levels were lower in infants with laryngomalacia compared to the control group. We consider that vitamin D deficiency may be a factor in the etiology of laryngomalacia with a yet-to-be-clarified etiology.

Fibroblast growth factor 23 as a risk factor for cardiovascular events and mortality in patients in the EVOLVE trial

INTRODUCTION

High mortality rates in patients with chronic kidney disease-mineral and bone disorder (CKD-MBD) receiving maintenance hemodialysis are largely due to cardiovascular (CV) events.

METHODS

We evaluated associations between MBD parameters, fibroblast growth factor 23 (FGF23) concentrations, and clinically adjudicated CV events from the Evaluation of Cinacalcet Hydrochloride Therapy to Lower Cardiovascular Events (EVOLVE) trial. Patients enrolled in EVOLVE, who had not experienced any study endpoints between randomization and week 20 with evaluable baseline and week 20 values for key laboratory parameters (parathyroid hormone, calcium, phosphate, and FGF23), were assessed. We used adjusted Cox proportional hazards regression models to estimate relative risk of outcomes (primary composite, all-cause mortality, and CV events) based on FGF23 and MBD parameters. Laboratory values were modeled with linear terms and using natural cubic splines with two degrees of freedom.

FINDINGS

For the primary endpoint, patients assessed (N = 2309) were followed up over a mean duration of 3.1 years, during which 1037 CV events (497 deaths, 540 nonfatal events) occurred. Adjusted models showed an association between FGF23 and the risk of CV events. Hazard ratio per log unit of FGF23 at week 20 was 1.09 [95% CI: 1.03-1.16], and the hazard ratio per log unit change in FGF23 from week 0 to week 20 was 1.09 [95% CI: 1.00-1.17].

DISCUSSION

Our data highlight FGF23 as an independent CV risk factor and potential biomarker and therapeutic target for patients with CKD-MBD receiving maintenance hemodialysis.

Hypophosphataemic Rickets: Diagnosis Algorithm-How Not to Make a Mistake

Hypophosphataemic rickets is a heterogeneous group of entities characterized by rickets or osteomalacia due to a phosphate deficit caused mainly by decreased renal reabsorption. They are also characterized by defective intestinal absorption of calcium and rickets or osteomalacia unresponsive to cholecalciferol. These metabolic alterations lead to growth retardation, bone pain and deformities, and short stature. For a correct diagnosis and treatment of all forms of rickets, the basic aspects of pathophysiology of the calcium-phosphorus metabolism and the relevance of the bone-kidney axis modulated by the presence of phosphaturic agents need to be known. Diagnosis of these diseases includes clinical assessment, blood and urine analytical tests, and bone x-ray. The aim of this article is to briefly describe the pathophysiology, signs, symptoms, and clinical forms of hypophosphataemic rickets, proposing a diagnosis algorithm that can help in the clinical practice.

Disorders of phosphate metabolism

Phosphate in both inorganic and organic form is essential for several functions in the body. Plasma phosphate level is maintained by a complex interaction between intestinal absorption, renal tubular reabsorption, and the transcellular movement of phosphate between intracellular fluid and bone storage pools. This homeostasis is regulated by several hormones, principally the parathyroid hormone, 1,25-dihydroxyvitamin D and fibroblast growth factor 23. Abnormalities in phosphate regulation can lead to serious and fatal complications. In this review phosphate homeostasis and the aetiology, pathophysiology, clinical features, investigation and management of hypophosphataemia and hyperphosphataemia will be discussed.

Use of calcimimetics in children with normal kidney function

The calcium-sensing receptor (CaSR) plays an important role in the homeostasis of serum ionized calcium by regulating parathyroid hormone (PTH) secretion and tubular calcium handling. Calcimimetics, which act by allosteric modulation of the CaSR, mimic hypercalcemia resulting in suppression of PTH release and increase in calciuria. Mostly used in children to treat secondary hyperparathyroidism associated with advanced renal failure, we have shown that calcimimetics can also be successfully used in children with bone and mineral disorders in which elevated PTH plays a detrimental role in skeletal pathophysiology in the face of normal kidney function. The current review briefly discusses the role of the CaSR and calcimimetics in calcium homeostasis, and then addresses the potential applications of calcimimetics in children with normal kidney function with disorders in which suppression of PTH is beneficial.

Phosphate wasting disorders in adults

A cause of hypophosphatemia is phosphate wasting disorders. Knowledge concerning mechanisms involved in phosphate wasting disorders has greatly increased in the last decade by the identification of phosphatonins, among them FGF-23. FGF-23 is a primarily bone derived factor decreasing renal tubular reabsorption of phosphate and the synthesis of calcitriol. Currently, pharmacological treatment of these disorders offers limited efficacy and is potentially associated to gastrointestinal, renal, and parathyroid complications; therefore, efforts have been directed toward newer pharmacological strategies that target the FGF-23 pathway. This review focuses on phosphate metabolism, its main regulators, and phosphate wasting disorders in adults, highlighting the main issues related to diagnosis and current and new potential treatments.

Clinical significance of increased serum levels of FGF-23 in fibrous dysplasia

INTRODUCTION AND OBJECTIVE

Fibrous dysplasia (FD) can be associated with the development of hypophosphatemic osteomalacia, caused by the production of FGF-23 by dysplastic bone tissue. This study analysed FGF-23 levels in patients with FD, and their association with disease activity and serum phosphate values.

PATIENTS AND METHODS

Twelve adult patients with FD were included in the study. Clinical history, disease extension and activity and treatments received were reviewed, and the relationship of those values with FGF-23 and serum P levels was analysed.

RESULTS

FGF-23 was elevated in 6/12 patients (50%). Patients with high FGF-23 levels had similar age and disease activity and extension than those who did not. No differences were observed in serum phosphate values between both groups (increased FGF-23: 3.9±0.9 mg/dl vs. decreased FGF-23: 3.5±0.6 mg/dl). In fact, none of the patients with increased FGF-23 had low serum phosphate values.

CONCLUSION

Adult FD patients frequently present elevated FGF-23 values with no serum phosphate level repercussion, suggesting an alteration in the processing of this protein in the dysplastic bone tissue for this pathology.

Vitamin D3 increased intestinal Na/Pi-IIb and CYP27B1 mRNA level in rats fed low-phosphorus diets

The objective of the study was to determine the role of vitamin D₃ (VD₃) in regulating adaptation and mechanism of rats to low-phosphorus (P) diets. Rats were assigned to 4 diets containing 0.2%, 0.4%, 0.6%, or 0.8% P consisting of 5 replicate cages with 6 rats per replicate cage and fed for 7 days. Four rats from each replicate cage were treated with ethane-1-hydroxy-1,1-diphosphonicacid, tetrasodium salt (EHDP) and 2 rats remained untreated. Twelve hours prior to preparation on d 7, two of the EHDP-treated rats received an intraperitoneal injection of VD₃ [1,25-(OH)₂D₃] at 600 ng per kg body weight, while two rats did not receive the injection. Rats that did not receive VD₃ injection had decreased (P < 0.001) P absorption, but injection of VD₃ resulted in increased (P < 0.001) absorption. The effect of VD₃ injection was greater (P < 0.001) for rats fed 0.2% P diet than rats fed 0.8% P diet in ileum. Sodium dependent phosphate cotransporter type Ⅱb (Na/Pi-II b) and 25-hydroxyvitamin D 1-α hydroxylase (CYP27B1) mRNA level showed the same trend with P absorption. Serum concentration of VD₃ and 1α-hydroxylase activity in rats fed 0.2% P diet were lower than those fed 0.8% P diet. The injection of VD₃ increased (P < 0.001) serum concentration of VD₃ and 1α-hydroxylase activity. Thus, VD₃ increased Na/Pi–IIb and CYP27B1 mRNA level and improved serum concentration of VD₃ and 1α-hydroxylase activity in rats fed low-P diets.

Phosphate homeostasis and disorders

Recent studies of inherited disorders of phosphate metabolism have shed new light on the understanding of phosphate metabolism. Phosphate has important functions in the body and several mechanisms have evolved to regulate phosphate balance including vitamin D, parathyroid hormone and phosphatonins such as fibroblast growth factor-23 (FGF23). Disorders of phosphate homeostasis leading to hypo- and hyperphosphataemia are common and have clinical and biochemical consequences. Notably, recent studies have linked hyperphosphataemia with an increased risk of cardiovascular disease. This review outlines the recent advances in the understanding of phosphate homeostasis and describes the causes, investigation and management of hypo- and hyperphosphataemia.

The role of vitamin D in human fracture healing: a systematic review of the literature

INTRODUCTION

Vitamin D is essential for bone mineralization and for the subsequent maintenance of bone quality. Mineralization is part of hard callus formation and bone remodelling, processes, which are part of fracture healing. We provide a comprehensive review of the literature to summarize and clarify if possible, the cellular effects of vitamin D and its clinical involvement in the process of fracture healing in human.

MATERIAL AND METHODS

We conducted a literature search in PubMed, Embase (OVID version), and Web of Science.

RESULTS

A total of 75 in vitro and 30 in vivo studies were found with inconsistent results about the cellular effect of vitamin D on fracture involved inflammatory cells, cytokines, growth factors, osteoblasts, osteoclasts and on the process of mineralization. With only five in vitro studies performed on material derived from a fracture site and one in vivo study in fracture patients, the exact cellular role remains unclear. Seven studies investigated the circulating vitamin D metabolites in fracture healing. Although it appears that 25(OH)D and 24,25(OH)2D3 are not affected by the occurrence of a fracture, this might not be the case with serum concentrations of 1,25(OH)2D3. The potential clinical effect of vitamin D deficiency is only described in one case series and three case controlled studies, where the results tend to show no effect of a vitamin D deficiency. No clinical studies were found investigating solely vitamin D supplementation. Two clinical studies found a positive effect of vitamin D supplementation and calcium, of increased bone mineral density or respectively increased fracture callus area at the fracture site. One study found indirect evidence that vitamin D and calcium promoted fracture healing.

CONCLUSION

Despite these results, and the presumed beneficial effect of vitamin D supplementation in deficient patients, clinical studies that address the effects of vitamin D deficiency or supplementation on fracture healing are scarce and remain inconclusive. We conclude that vitamin D has a role in fracture healing, but the available data are too inconsistent to elucidate how and in what manner.

[Clinical usefulness of the determination of fibroblast growth factor 23 in the evaluation of patients with osteomalacia]

BACKGROUND AND OBJECTIVE

The aim of the present study was to analyze the usefulness of the determination of fibroblast growth factor 23 (FGF23), a regulatory hormone of phosphate metabolism, in the evaluation of patients with osteomalacia of different causes.

PATIENTS AND METHOD

Seventeen patients with osteomalacia were included: 12 hypophosphatemic osteomalacia (by several causes), 4 vitamin D-deficiency osteomalacia and one with hypophosphatasia. Plasma C-terminal FGF23 was determined in all patients.

RESULTS

FGF23 levels were increased in 6/12 (50%) of patients with hypophosphatemic osteomalacia (2 X-linked, one autosomal dominant, one related HIV therapy and 2 not elucidated). No patient with vitamin D-deficiency osteomalacia or hypophosphatasia presented increased FGF23 levels.

CONCLUSION

The determination of FGF23 could be useful in the evaluation of the different types of hypophosphatemic osteomalacia and also in the identification of their associated etiopathogenic mechanisms. Thus, depending on the cause, 50% of the patients with hypophosphatemic osteomalacia showed increased FGF23 values, whereas in vitamin D-deficiency osteomalacia and in hypophosphatasia FGF23 levels were normal.

Hypophosphatemic rickets due to perturbations in renal tubular function

The common denominator for all types of rickets is hypophosphatemia, leading to inadequate supply of the mineral to the growing bone. Hypophosphatemia can result from insufficient uptake of the mineral from the gut or its disproportionate losses in the kidney, the latter being caused by either tubular abnormalities per se or the effect on the tubule of circulating factors like fibroblast growth factor-23 and parathyroid hormone (PTH). High serum levels of the latter result in most cases from abnormalities in vitamin D metabolism which lead to decreased calcium absorption in the gut and hypocalcemia, triggering PTH secretion. Rickets is a disorder of the growth plate and hence pediatric by definition. However, it is important to recognize that the effect of hypophosphatemia on other parts of the skeleton results in osteomalacia in both children and adults. This review addresses the etiology, pathophysiologic mechanisms, clinical manifestations and treatment of entities associated with hypophosphatemic rickets due to perturbations in renal tubular function.

Lower fibroblast growth factor 23 levels in young adults with Crohn disease as a possible secondary compensatory effect on the disturbance of bone and mineral metabolism

Fibroblast growth factor 23 (FGF-23) is a bone-derived circulating phosphaturic factor that decreases serum concentration of phosphate and vitamin D, suggested to actively participate in a complex renal-gastrointestinal-skeletal axis. Serum FGF-23 concentrations, as well as various other laboratory parameters involved in bone homeostasis, were measured and analyzed with regard to various diseases and patients' characteristics in 44 patients with Crohn disease (CD) and 20 healthy controls (HCs) included in this cross-sectional study. Serum FGF-23 levels were significantly lower in patients with CD (900.42 ± 815.85pg/mL) compared with HC (1410.94 ± 1000.53pg/mL), p = 0.037. Further analyses suggested FGF-23 as a factor independent from various parameters including age (r = -0.218), body mass index (r = -0.115), 25-hydroxy vitamin D (r = 0.126), parathyroid hormone (r = 0.084), and bone mineral density (BMD) of hip and lumbar (r = 0.205 and r = 0.149, respectively). This observation remained even after multivariate analyses, exhibiting that BMD was not affected by FGF-23, although parameters such as age (p = 0.026), cumulative prednisolone dose (p < 0.0001), and smoking status (p = 0.024) were strong determinants of BMD regarding hip. Lower FGF-23 levels in patients with bowel inflammation are accompanied but not directly correlated with lower vitamin D levels, showing no impact on BMD determination of young adults with CD. The downregulation of serum FGF-23 levels in CD appears as a secondary compensatory effect on the bone and mineral metabolism induced by chronic intestinal inflammation.

Calcitriol, calcidiol, parathyroid hormone, and fibroblast growth factor-23 interactions in chronic kidney disease

Objective

To review the inter-relationships between calcium, phosphorus, parathyroid hormone (PTH), parent and activated vitamin D metabolites (vitamin D, 25(OH)-vitamin D, 1,25(OH)₂-vitamin D, 24,25(OH)₂-vitamin D), and fibroblast growth factor-23 (FGF-23) during chronic kidney disease (CKD) in dogs and cats.

Data Sources

Human and veterinary literature.

Human Data Synthesis

Beneficial effects of calcitriol treatment during CKD have traditionally been attributed to regulation of PTH but new perspectives emphasize direct renoprotective actions independent of PTH and calcium. It is now apparent that calcitriol exerts an important effect on renal tubular reclamation of filtered 25(OH)-vitamin D, which may be important in maintaining adequate circulating 25(OH)-vitamin D. This in turn may be vital for important pleiotropic actions in peripheral tissues through autocrine/paracrine mechanisms that impact the health of those local tissues.

Veterinary Data Synthesis

Limited information is available reporting the benefit of calcitriol treatment in dogs and cats with CKD.

Conclusions

A survival benefit has been shown for dogs with CKD treated with calcitriol compared to placebo. The concentrations of circulating 25(OH)-vitamin D have recently been shown to be low in people and dogs with CKD and are related to survival in people with CKD. Combination therapy for people with CKD using both parental and activated vitamin D compounds is common in human nephrology and there is a developing emphasis using combination treatment with activated vitamin D and renin-angiotensin-aldosterone-system (RAAS) inhibitors.

Fibroblast growth factor-23 levels are associated with vascular calcifications in peritoneal dialysis patients

BACKGROUND

The aim of the study was to assess the relationship between fibroblast growth factor-23 (FGF-23) and vascular calcifications (VC) in peritoneal dialysis (PD) patients.

METHODS

A cross-sectional study was performed in 55 PD patients who underwent pelvic X-ray to assess for VC. Patients with and without linear calcifications were recorded.

RESULTS

Fifteen patients (27.3%) had linear calcifications on pelvic X-ray. FGF-23 levels were higher in patients with VC (299.5 (30.4-2,410.0) vs. 74.4 (14.8-1,030) pg/ml, p < 0.01). Diabetic patients had lower FGF-23 values (43.2 (14.9-134.0) vs. 103.5 (14.8-2,410) pg/ml, p < 0.01). Patients with residual renal function (RRF) had lower FGF-23 levels (70.6 (14.8-513) vs. 179.5 (30.4-2,410) pg/ml, p = 0.06); however, this did not reach statistical significance. FGF-23 levels, age, creatinine, Ca, dialysis duration and HbA1c were positively correlated with VC, whereas RRF, Ca intake and ALP were negatively associated. Multivariate logistic analysis confirmed FGF-23 levels, age, dialysis duration and RRF to be associated with VC.

CONCLUSIONS

FGF-23 levels are associated with VC in PD patients. Further studies are needed to clarify whether it is simply a marker or a potential factor. It may prove to be an important therapeutic target for VC management.

FGF23 and Phosphate Wasting Disorders

A decade ago, only two hormones, parathyroid hormone and 1,25(OH)2D, were widely recognized to directly affect phosphate homeostasis. Since the discovery of fibroblast growth factor 23 (FGF23) in 2000 (1), our understanding of the mechanisms of phosphate homeostasis and of bone mineralization has grown exponentially. FGF23 is the link between intestine, bone, and kidney together in phosphate regulation. However, we still do not know the complex mechanism of phosphate homeostasis and bone mineralization. The physiological role of FGF23 is to regulate serum phosphate. Secreted mainly by osteocytes and osteoblasts in the skeleton (2,3), it modulates kidney handling of phosphate reabsorption and calcitriol production. Genetic and acquired abnormalities in FGF23 structure and metabolism cause conditions of either hyper-FGF23 or hypo-FGF23. Hyper-FGF23 is related to hypophosphatemia, while hypo-FGF23 is related to hyperphosphatemia. Both hyper-FGF23 and hypo-FGF23 are detrimental to humans. In this review, we will discuss the pathophysiology of FGF23 and hyper-FGF23 related renal phosphate wasting disorders (4).

FGF23 production by osteocytes

Fibroblast growth factor 23 (FGF23), a known regulator of phosphate homeostasis, is produced by cells residing in bone, namely, osteocytes, to target a distant organ, the kidney. Elevated FGF23 levels have recently been found systemically and in osteocytes in patients and animal models of chronic kidney disease. Associations between serum FGF23 level and vascular dysfunction, vascular calcification, and increased risk of cardiovascular disease have also been observed. In this review we discuss FGF23 expression in osteocytes and the potential means to regulate expression and function of this protein at the osteocyte level.

Pharmacological inhibition of fibroblast growth factor (FGF) receptor signaling ameliorates FGF23-mediated hypophosphatemic rickets

Fibroblast growth factor 23 (FGF23) is a circulating factor secreted by osteocytes that is essential for phosphate homeostasis. In kidney proximal tubular cells FGF23 inhibits phosphate reabsorption and leads to decreased synthesis and enhanced catabolism of 1,25-dihydroxyvitamin D3 (1,25[OH]2 D3 ). Excess levels of FGF23 cause renal phosphate wasting and suppression of circulating 1,25(OH)2 D3 levels and are associated with several hereditary hypophosphatemic disorders with skeletal abnormalities, including X-linked hypophosphatemic rickets (XLH) and autosomal recessive hypophosphatemic rickets (ARHR). Currently, therapeutic approaches to these diseases are limited to treatment with activated vitamin D analogues and phosphate supplementation, often merely resulting in partial correction of the skeletal aberrations. In this study, we evaluate the use of FGFR inhibitors for the treatment of FGF23-mediated hypophosphatemic disorders using NVP-BGJ398, a novel selective, pan-specific FGFR inhibitor currently in Phase I clinical trials for cancer therapy. In two different hypophosphatemic mouse models, Hyp and Dmp1-null mice, resembling the human diseases XLH and ARHR, we find that pharmacological inhibition of FGFRs efficiently abrogates aberrant FGF23 signaling and normalizes the hypophosphatemic and hypocalcemic conditions of these mice. Correspondingly, long-term FGFR inhibition in Hyp mice leads to enhanced bone growth, increased mineralization, and reorganization of the disturbed growth plate structure. We therefore propose NVP-BGJ398 treatment as a novel approach for the therapy of FGF23-mediated hypophosphatemic diseases.

Renal phosphate handling in Gitelman syndrome--the results of a case-control study

BACKGROUND

Patients with Gitelman syndrome, a hereditary salt-wasting tubulopathy, have loss-of-function mutations in the SLC12A3 gene coding for the thiazide-sensitive sodium chloride co-transporter in the distal convoluted tubule. Since the bulk of filtered phosphate is reabsorbed in the proximal tubule, renal phosphate wasting is considered exceptional in Gitelman syndrome.

METHODS

We investigated the renal handling of inorganic phosphate in 12 unselected Italian patients affected with Gitelman syndrome (5 females and 7 males, aged 6.0-18 years, median age 12 years) and in 12 healthy subjects matched for gender and age (controls). The diagnosis of Gitelman syndrome among the patients had been made clinically and confirmed by molecular biology studies.

RESULTS

The biochemical hallmarks of Gitelman syndrome, namely hypochloremia, hypokalemia, hypomagnesemia, increased urinary excretion of sodium, chloride, potassium and magnesium and reduced urinary excretion of calcium, were present in the 12 patients. In addition, both the plasma inorganic phosphate concentration (median and interquartile range: 1.28 [1.12-1.36] vs. 1.61 [1.51-1.66)] mmol/L) and the maximal tubular reabsorption of inorganic phosphate (1.08 [0.99-1.22] vs. 1.41 [1.38-1.47] mmol/L) were significantly lower (P < 0.001) in Gitelman patients than in control subjects. Circulating levels of 25-hydroxyvitamin D, intact parathyroid hormone and osteocalcin were similar in patients and controls.

CONCLUSIONS

The results of our case-control study disclose a hitherto unrecognized tendency towards renal phosphate wasting with mild to moderate hypophosphatemia in Gitelman syndrome.

Hypophosphatemic rickets

PURPOSE OF REVIEW

Description of the recent advances on the regulation of phosphate metabolism, gene mutations, and new approaches to treatment in patients with hypophosphatemic rickets.

RECENT FINDINGS

Fibroblast growth factor 23 (FGF23) overproduction may be a primary cause of hypophosphatemic rickets. Inactivating mutations of phosphate-regulating gene with homologies to endopeptidases on the X chromosome, dentin matrix acidic phosphoprotein 1, and ectonucleotide pyrophosphatase/phosphodiesterase 1 are associated with X-linked hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets 1, and autosomal recessive hypophosphatemic rickets 2, respectively. Activating mutations of FGF23 gene is the cause of autosomal dominant hypophosphatemic rickets. Iron deficiency may affect autosomal dominant hypophosphatemic rickets phenotype by regulating FGF23 production.Current treatment with activated vitamin D metabolites and oral inorganic phosphate salts may partially correct skeletal lesions and linear growth in patients with hypophosphatemic rickets. However, some patients have poor improvement by the current treatment.

SUMMARY

Identification of the causative mutation in patients with hypophosphatemic rickets may be useful to confirm the diagnosis and probably for prognosis. Inhibition of FGF23 overproduction by anti-FGF23 neutralizing antibodies could be a future approach for treatment of patients with FGF23-dependent hypophosphatemic rickets.

Phosphate homeostasis and its role in bone health

Phosphate is one of the most abundant minerals in the body, and its serum levels are regulated by a complex set of processes occurring in the intestine, skeleton, and kidneys. The currently known main regulators of phosphate homeostasis include parathyroid hormone (PTH), calcitriol, and a number of peptides collectively known as the "phosphatonins" of which fibroblast growth factor-23 (FGF-23) has been best defined. Maintenance of extracellular and intracellular phosphate levels within a narrow range is important for many biological processes, including energy metabolism, cell signaling, regulation of protein synthesis, skeletal development, and bone integrity. The presence of adequate amounts of phosphate is critical for the process of apoptosis of mature chondrocytes in the growth plate. Without the presence of this mineral in high enough quantities, chondrocytes will not go into apoptosis, and the normal physiological chain of events that includes invasion of blood vessels and the generation of new bone will be blocked, resulting in rickets and delayed growth. In the rest of the skeleton, hypophosphatemia will result in osteomalacia due to an insufficient formation of hydroxyapatite. This review will address phosphate metabolism and its role in bone health.

Genetic disorders of phosphate regulation

Regulation of phosphate homeostasis is critical for many biological processes, and both hypophosphatemia and hyperphosphatemia can have adverse clinical consequences. Only a very small percentage (1%) of total body phosphate is present in the extracellular fluid, which is measured by routine laboratory assays and does not reflect total body phosphate stores. Phosphate is absorbed from the gastrointestinal tract via the transcellular route [sodium phosphate cotransporter 2b (NaPi2b)] and across the paracellular pathway. Approximately 85% of the filtered phosphate is reabsorbed from the kidney, predominantly in the proximal tubule, by NaPi2a and NaPi2c, which are present on the brush border membrane. Renal phosphate transport is tightly regulated. Dietary phosphate intake, parathyroid hormone (PTH), 1,25 (OH)2 vitamin D3, and fibroblast growth factor 23 (FGF23) are the principal regulators of phosphate reabsorption from the kidney. Recent advances in genetic techniques and animal models have identified many genetic disorders of phosphate homeostasis. Mutations in NaPi2a and NaPi2c; and hormonal dysregulation of PTH, FGF23, and Klotho, are primarily responsible for most genetic disorders of phosphate transport. The main focus of this educational review article is to discuss the genetic and clinical features of phosphate regulation disorders and provide understanding and treatment options.

Bone disease in pediatric idiopathic hypercalciuria

Idiopathic hypercalciuria (IH) is the leading metabolic risk factor for urolithiasis and affects all age groups without gender or race predominance. IH has a high morbidity with or without lithiasis and reduced bone mineral density (BMD), as described previously in pediatric patients as well as in adults. The pathogenesis of IH is complex and not completely understood, given that urinary excretion of calcium is the end result of an interplay between three organs (gut, bone and kidney), which is further orchestrated by hormones, such as 1,25 dihydroxyvitamin D, parathyroid hormone, calcitonin and fosfatonins (i.e., fibroblast growth-factor-23). Usually, a primary defect in one organ induces compensatory mechanisms in the remaining two organs, such as increased absorption of calcium in the gut secondary to a primary renal loss. Thus, IH is a systemic abnormality of calcium homeostasis with changes in cellular transport of this ion in intestines, kidneys and bones. Reduced BMD has been demonstrated in pediatric patients diagnosed with IH. However, the precise mechanisms of bone loss or failure of adequate bone mass gain are still unknown. The largest accumulation of bone mass occurs during childhood and adolescence, peaking at the end of the second decade of life. This accumulation should occur without interference to achieve the peak of optimal bone mass. Any interference may be a risk factor for the reduction of bone mass with increased risk of fractures in adulthood. This review will address the pathogenesis of IH and its consequence in bone mass.

Are fibroblast growth factor 23 concentrations in renal transplant patients different from non-transplanted chronic kidney disease patients?

To compare the pattern of serum FGF23 levels in pediatric renal transplant recipients and GFR-matched controls. We performed a cross-sectional matched pair study in 19 stable pediatric renal transplant recipients and 19 GFR-matched controls with native CKD. After assessment for normal distribution, demographic and bone metabolism parameters were compared with Student's t-test, Wilcoxon's matched pairs (for non-normal distribution) test, and correlation analysis. The groups were comparable for anthropometric parameters, cystatin C eGFR (71.10 ± 37.28 vs. 76.11 ± 26.80 mL/min/1.73 m(2) ), cystatin C, urea, creatinine, intact PTH, pH, CRP, alkaline phosphatase, phosphate, calcium, ionized calcium, FGF-23 (63.44 [IQR 38.42, 76.29], 49.92 [IQR 42.48, 76.97]), albumin, and urinary calcium/creatinine ratio. The renal transplant patients had significantly lower 25-(OH) vitamin D levels (66.63 ± 17.54 vs. 91.42 ± 29.16 ng/mL), and higher 1,25-(OH)(2) vitamin D levels (95.78 ± 34.54 vs. 67.11 ± 35.90 pm). FGF-23 levels correlated negatively with cystatin C eGFR (r = -0.3571, p = 0.02770) and positively with PTH (r = 0.5063, p = 0.0026), but not with serum phosphate (r = 0.2651, p = 0.1077). We conclude that the increase in FGF23 levels with GFR decline in pediatric renal transplant patients remains similar to that in the patients with CKD. The relationship between FGF23 and serum vitamin D needs further evaluation.

Clinical practice: diagnostic approach to the rachitic child

Rickets remains a common problem among infants and children in many countries worldwide. Although the classical presentation associated with bone abnormalities is well known, paediatricians need to be aware of atypical presentations, especially in the first 6 months of life. Furthermore, although vitamin D deficiency rickets remains the commonest form of rickets in most countries, health care providers need to be aware of other possible causes and their typical clinical and biochemical presentations. This article discusses these and highlights the characteristic features of various forms of rickets and possible pitfalls clinicians should be aware of when confronted with a patient with suspected rickets. In conclusion, the recent advances made in understanding the underlying pathogeneses of the various forms of rickets has helped to delineate the diagnostic tests that assist in the diagnosis and management of the disease in children.