Autosomal dominant hypophosphatemic rickets/osteomalacia: clinical characterization of a novel renal phosphate-wasting disorder

Metabolic Bone Disease

Metabolic bone diseases encompass a group of disorders characterized by abnormalities in bone metabolism, structure, or mineralization. These disorders negatively impact overall health and quality of life and place individuals at high risk for fracture, which may increase morbidity and mortality. Clinicians should understand who is at risk for these disorders, select individuals who warrant further workup, determine appropriate laboratory and imaging evaluation, interpret results in a clinical context, and choose an optimal management strategy based on the individual patient.

Hypophosphatemic rickets and short stature

An 18-month-old male presented with gross motor delay and poor growth (weight z-score -2.21, length z-score -4.26). Radiographs showed metaphyseal irregularities suggesting metaphyseal dysplasia and sagittal craniosynostosis. Biochemical evaluation supported hypophosphatemic rickets [serum phosphorus 2.3 mg/dL (reference range (RR) 4.3-6.8), alkaline phosphatase 754 unit/L (RR 156-369)] due to renal phosphate wasting (TmP/GFR 4.3 mg/dL, normal for age 4.3-6.8), with C-terminal fibroblast growth factor 23 (FGF23) 125 RU/mL (>90 during hypophosphatemia suggests FGF23-mediated hypophosphatemia). Treatment was initiated with calcitriol and phosphate. Genetic analysis showed a pathogenic variant of FGF23: c.527G > A (p.Arg176Gln) indicative of autosomal dominant hypophosphatemic rickets (ADHR). Consistent with reports linking iron deficiency with the ADHR phenotype, low ferritin was detected. Following normalization of ferritin level (41 ng/mL) with oral ferrous sulfate replacement, biochemical improvement was demonstrated (FGF23 69 RU/mL, phosphorus 5.0 mg/dL and alkaline phosphatase 228 unit/L). Calcitriol and phosphate were discontinued. Three years later, the patient demonstrated improved developmental milestones, linear growth (length Z-score -2.01), radiographic normalization of metaphyses, and stabilization of craniosynostosis. While the most common cause of hypophosphatemic rickets is X-linked hypophosphatemia, other etiologies should be considered as treatment differs. In ADHR, normalization of iron leads to biochemical and clinical improvement.

Tumor-induced osteomalacia

Tumor-induced osteomalacia is one of paraneoplastic syndromes characterized by hypophosphatemia caused by excessive actions of fibroblast growth factor 23 (FGF23). Since the cloning of FGF23 about 20 years ago, more widespread awareness of this disease has been achieved. However, there still remain several difficulties in the management of patients with this disease. In this review, these clinical problems are discussed together with the physiological and pathophysiological functions of FGF23. Personal proposals in the management of patients with suspected patients with tumor-induced osteomalacia are also presented.

Global guidance for the recognition, diagnosis, and management of tumor-induced osteomalacia

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by mesenchymal tumors that secrete fibroblast growth factor 23 (FGF23). Patients present with progressive bone pain, muscle weakness, and fragility fractures. TIO is characterized by hypophosphatemia, excess renal phosphate excretion, and low/inappropriately normal 1,25-dihydroxyvitamin D (1,25(OH)₂ D) levels. Rarity and enigmatic clinical presentation of TIO contribute to limited awareness among the medical community. Accordingly, appropriate diagnostic tests may not be requested, leading to delayed diagnosis and poorer patient outcomes. We have developed a global guidance document to improve the knowledge of TIO in the medical community, enabling the recognition of patients with TIO and appropriate referral. We provide recommendations aiding diagnosis, referral, and treatment, helping promote a global standard of patient management. We reviewed the literature and conducted a three-round Delphi survey of TIO experts. Statements were drafted based on published evidence and expert opinions (≥70% consensus required for final recommendations). Serum phosphate should be measured in patients presenting with chronic muscle pain or weakness, fragility fractures, or bone pain. Physical examination should establish features of myopathy and identify masses that could be causative tumors. Priority laboratory evaluations should include urine/serum phosphate and creatinine to assess renal tubular reabsorption of phosphate and TmP/GFR, alkaline phosphatase, parathyroid hormone, 25-hydroxyvitamin D, 1,25(OH)₂ D, and FGF23. Patients with the clinical/biochemical suspicion of TIO should be referred to a specialist for diagnosis confirmation, and functional imaging should be used to localize causative tumor(s). Recommended treatment is tumor resection or, with unresectable/unidentifiable tumors, phosphate salts plus active vitamin D, or burosumab.

Bone characteristics of autosomal dominant hypophosphatemic rickets patients

OBJECTIVE

Autosomal dominant hypophosphatemic rickets (ADHR) is a rare disease caused by activating mutations in fibroblast growth factor 23 (FGF23) gene. With FGF23 activation, ADHR is a good model to explore the effects of FGF23 on skeletal development and mineralization. However, the bone microarchitecture of ADHR patients is poorly investigated. This study aims to illustrate the bone properties of ADHR patients and clarify the effect of FGF23 on load bearing and non-load bearing bone.

METHODS

Bone microarchitectures of 11 ADHR subjects and sex- and age-matched healthy controls were analyzed by HR-pQCT. The effect of FGF23 mutations on load bearing and non-load bearing bone was explored by comparison of bone microarchitecture in distal radius and distal tibia. The BMD, bone microarchitecture and bone strength were compared between 7 ADHR patients and 7 age- and sex-matched XLH patients.

RESULTS

Among 11 subjects with FGF23 mutations, 10 patients presented with obvious symptoms, five of which had received 1-3 years of iron supplement, neutral phosphate, and calcitriol treatments. The symptomatic patients presented with low bone density and fractures in X rays, with decreased Z score of aBMD (L1-L4: -1.3 ± 1.4, femoral neck: -2.1 ± 1.8, total hip: -1.85 ± 1.6). Compared with controls, HR-pQCT analysis of 5 untreated ADHR patients showed increased total area (+61.6 %, p = 0.03) and cortical perimeter (+17.2 %, p = 0.03) in distal radius. No significant differences were found in other parameters in distal radius. In distal tibia, the patients presented obvious defects in cancellous bone, with decreased trabecular vBMD (-62.9 %, p = 0.003), trabecular BV/TV (-48.7 %, p = 0.003) and trabecular number (-42.2 %, p = 0.001). The trabecular separation (+113.3 %, p = 0.007) and trabecular network inhomogeneity (+226.7 %, p = 0.001) were accordingly increased. In addition to another 5 treated patients, the bone microarchitecture changes revealed similar pattern, but the increase of total area and cortical perimeter in distal radius was no longer statistically significant. The non-symptomatic ADHR patient demonstrated slightly decreased total vBMD, trabecular vBMD and trabecular BV/TV in distal tibia. The changing pattern of bone geometry and microarchitecture of ADHR patients were similar to XLH patients but showed less deficit and stronger bone strength.

CONCLUSION

ADHR patients presented increased total area and cortical perimeter in distal radius, and obvious defect in cancellous bone in distal tibia. FGF23 have impairment effect on trabecular bone especially in weight bearing site.

Osteocyte Egln1/Phd2 links oxygen sensing and biomineralization via FGF23

Osteocytes act within a hypoxic environment to control key steps in bone formation. FGF23, a critical phosphate-regulating hormone, is stimulated by low oxygen/iron in acute and chronic diseases, however the molecular mechanisms directing this process remain unclear. Our goal was to identify the osteocyte factors responsible for FGF23 production driven by changes in oxygen/iron utilization. Hypoxia-inducible factor-prolyl hydroxylase inhibitors (HIF-PHI) which stabilize HIF transcription factors, increased Fgf23 in normal mice, as well as in osteocyte-like cells; in mice with conditional osteocyte Fgf23 deletion, circulating iFGF23 was suppressed. An inducible MSC cell line ('MPC2') underwent FG-4592 treatment and ATACseq/RNAseq, and demonstrated that differentiated osteocytes significantly increased HIF genomic accessibility versus progenitor cells. Integrative genomics also revealed increased prolyl hydroxylase Egln1 (Phd2) chromatin accessibility and expression, which was positively associated with osteocyte differentiation. In mice with chronic kidney disease (CKD), Phd1-3 enzymes were suppressed, consistent with FGF23 upregulation in this model. Conditional loss of Phd2 from osteocytes in vivo resulted in upregulated Fgf23, in line with our findings that the MPC2 cell line lacking Phd2 (CRISPR Phd2-KO cells) constitutively activated Fgf23 that was abolished by HIF1α blockade. In vitro, Phd2-KO cells lost iron-mediated suppression of Fgf23 and this activity was not compensated for by Phd1 or -3. In sum, osteocytes become adapted to oxygen/iron sensing during differentiation and are directly sensitive to bioavailable iron. Further, Phd2 is a critical mediator of osteocyte FGF23 production, thus our collective studies may provide new therapeutic targets for skeletal diseases involving disturbed oxygen/iron sensing.

Approach to Hypophosphatemic Rickets

Hypophosphatemic rickets typically presents in infancy or early childhood with skeletal deformities and growth plate abnormalities. The most common causes are genetic (such as X-linked hypophosphatemia), and these typically will result in lifelong hypophosphatemia and osteomalacia. Knowledge of phosphate metabolism, including the effects of fibroblast growth factor 23 (FGF23) (an osteocyte produced hormone that downregulates renal phosphate reabsorption and 1,25-dihydroxyvitamin-D (1,25(OH)2D) production), is critical to determining the underlying genetic or acquired causes of hypophosphatemia and to facilitate appropriate treatment. Serum phosphorus should be measured in any child or adult with musculoskeletal complaints suggesting rickets or osteomalacia. Clinical evaluation incudes thorough history, physical examination, laboratory investigations, genetic analysis (especially in the absence of a guiding family history), and imaging to establish etiology and to monitor severity and treatment course. The treatment depends on the underlying cause, but often includes active forms of vitamin D combined with phosphate salts, or anti-FGF23 antibody treatment (burosumab) for X-linked hypophosphatemia. The purpose of this article is to explore the approach to evaluating hypophosphatemic rickets and its treatment options.

A Unique Mechanism of a Novel Synonymous PHEX Variant Causing X-Linked Hypophosphatemia

CONTEXT

Synonymous mutations are usually nonpathogenic.

OBJECTIVE

We report here a family with X-linked hypophosphatemia (XLH) due to a novel synonymous PHEX variant with a unique mechanism.

METHODS

We studied a 4-member family (a mother, a son, and 2 daughters), all affected with XLH. Genomic DNA was extracted from peripheral leucocytes. Whole exome sequencing (WES) was used to identify the underlying genetic variant in the proband (the son). Sanger sequencing was used to confirm this variant in the proband and his family members. RT-PCR and sequencing of the cDNA revealed the effect of this variant on the PHEX structure and function.

RESULTS

A synonymous variant in the PHEX gene (c.1701A>C) was identified in all affected members. This variant changes the first nucleotide of exon 17 from adenine to cytosine. Using RT-PCR, this variant was shown to interfere with splicing of exons 16 with 17 resulting in a single shorter PHEX transcript in the proband compared to normal control. Sanger sequencing of the cDNA revealed a complete skipping of exon 17 and direct splicing of exons 16 and 18. This led to a frameshift and an introduction of a new stop codon in the next codon (codon 568), which ultimately led to truncation and loss of the final 183 amino acids of PHEX.

CONCLUSION

This novel variant shows how a synonymous exonic mutation may induce a complex series of changes in the transcription and translation of the gene and causes a disease, a mechanism that is not commonly recognized.

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.

PHEXL222P Mutation Increases Phex Expression in a New ENU Mouse Model for XLH Disease

PhexL222P mouse is a new ENU mouse model for XLH disease due to Leu to Pro amino acid modification at position 222. PhexL222P mouse is characterized by growth retardation, hypophosphatemia, hypocalcemia, reduced body bone length, and increased epiphyseal growth plate thickness and femur diameter despite the increase in PHEXL222P expression. Actually, PhexL222P mice show an increase in Fgf23, Dmp1, and Mepe and Slc34a1 (Na-Pi IIa cotransporter) mRNA expression similar to those observed in Hyp mice. Femoral osteocalcin and sclerostin and Slc34a1 do not show any significant variation in PhexL222P mice. Molecular dynamics simulations support the experimental data. P222 might locally break the E217-Q224 β-sheet, which in turn might disrupt inter-β-sheet interactions. We can thus expect local protein misfolding, which might be responsible for the experimentally observed PHEXL222P loss of function. This model could be a valuable addition to the existing XLH model for further comprehension of the disease occurrence and testing of new therapies.

Shedding Light on the Complex Regulation of FGF23

Early research has suggested a rather straightforward relation between phosphate exposure, increased serum FGF23 (Fibroblast Growth Factor 23) concentrations and clinical endpoints. Unsurprisingly, however, subsequent studies have revealed a much more complex interplay between autocrine and paracrine factors locally in bone like PHEX and DMP1, concentrations of minerals in particular calcium and phosphate, calciprotein particles, and endocrine systems like parathyroid hormone PTH and the vitamin D system. In addition to these physiological regulators, an expanding list of disease states are shown to influence FGF23 levels, usually increasing it, and as such increase the burden of disease. While some of these physiological or pathological factors, like inflammatory cytokines, may partially confound the association of FGF23 and clinical endpoints, others are in the same causal path, are targetable and hence hold the promise of future treatment options to alleviate FGF23-driven toxicity, for instance in chronic kidney disease, the FGF23-associated disease with the highest prevalence by far. These factors will be reviewed here and their relative importance described, thereby possibly opening potential means for future therapeutic strategies.

Fibroblast growth factor-23 rs7955866 polymorphism and risk of chronic kidney disease

Background

A missense gain-of-function fibroblast growth factor-23 (FGF23) gene single nucleotide polymorphism (SNP) (rs7955866) has been associated with FGF23 hypersecretion, phosphaturia, and bone disease. Excess circulating FGF23 was linked with atherosclerosis, hypertension, initiation, and progression of chronic kidney disease (CKD).

Methods

The study included 72 CKD stage 2/3 Egyptian patients (27–71 years old, 37 females) and 26 healthy controls matching in age and sex. Repeated measures of blood pressure were used to quantify hypertension on a semiquantitative scale (grades 0 to 5). Fasting serum urea, creatinine, uric acid, total proteins, albumin, calcium, phosphorus, vitamin D3, intact parathyroid hormone (iPTH), and intact FGF23 (iFGF23) were measured. DNA extracted from peripheral blood leucocytes was used for genotyping of FGF23 rs7955866 SNP using the TaqMan SNP genotyping allelic discrimination method.

Results

Major causes of CKD were hypertension, diabetic kidney disease, and CKD of unknown etiology. There was no significant difference in minor allele (A) frequency between the studied groups (0.333 in GI and 0.308 in GII). Median (IQR) serum iFGF23 was significantly higher in GI [729.2 (531.9–972.3)] than in GII [126.1 (88.5–152.4)] pg/mL, P < 0.001. Within GI, the minor allele (A) frequency load, coded for codominant inheritance, had a significant positive correlation with both hypertension grade (r = 0.385, P = 0.001) and serum iFGF23 (r = 0.259, P = 0.028). Hypertension grade had a significant positive correlation with serum phosphorus and iFGF23.

Conclusions

For the first time in an Egyptian cohort, we report a relatively high frequency of the rs7955866 SNP. It may remain dormant or become upregulated in response to some environmental triggers, notably dietary phosphorus excess, leading to increased circulating iFGF23 with ensuing hypertension and/or renal impairment. Subjects with this SNP, particularly in the homozygous form, are at increased risk for CKD of presumably “unknown” etiology, with a tendency for early onset hypertension and increased circulating iFGF23 out of proportion with the degree of renal impairment. Large-scale population studies are needed to confirm these findings and explore the role of blockers of the renin–angiotensin–aldosterone system and sodium chloride cotransporters in mitigating hypertension associated with FGF23 excess.

Rickets guidance: part I-diagnostic workup

Rickets is a disease of the growing child arising from alterations in calcium and phosphate homeostasis resulting in impaired apoptosis of hypertrophic chondrocytes in the growth plate. Its symptoms depend on the patients' age, duration of disease, and underlying disorder. Common features include thickened wrists and ankles due to widened metaphyses, growth failure, bone pain, muscle weakness, waddling gait, and leg bowing. Affected infants often show delayed closure of the fontanelles, frontal bossing, and craniotabes. The diagnosis of rickets is based on the presence of these typical clinical symptoms and radiological findings on X-rays of the wrist or knee, showing metaphyseal fraying and widening of growth plates, in conjunction with elevated serum levels of alkaline phosphatase. Nutritional rickets due to vitamin D deficiency and/or dietary calcium deficiency is the most common cause of rickets. Currently, more than 20 acquired or hereditary causes of rickets are known. The latter are due to mutations in genes involved in vitamin D metabolism or action, renal phosphate reabsorption, or synthesis, or degradation of the phosphaturic hormone fibroblast growth factor 23 (FGF23). There is a substantial overlap in the clinical features between the various entities, requiring a thorough workup using biochemical analyses and, if necessary, genetic tests. Part I of this review focuses on the etiology, pathophysiology and clinical findings of rickets followed by the presentation of a diagnostic approach for correct diagnosis. Part II focuses on the management of rickets, including new therapeutic approaches based on recent clinical practice guidelines.

FGF-23 transmitted tumor - induced hypophosphatemic osteomalacia: a rare case of a young woman with recurrent fractures and review of the literature

•

Tumor-transmitted osteomalacia is a rare FGF 23 transmitted phosphaturic disease.

•

Delay of diagnosis is common due to small and clinically silent underlying tumors.

•

The most sensitive imaging modality is ⁶⁸Ga-DOTA-TOC-PET/CT-scan.

•

Complete resection of the tumor mass cures this paraneoplastic disease.

We present a case of tumor-induced osteomalacia (TIO) in a young woman of 22 years. The fibroblast growth factor 23 transmitting tumor in her left foot remained undetected for several years. She suffered several fractures including insufficiency fractures of both femoral necks requiring bilateral proximal femoral nailing. After phosphaturia was diagnosed any known genetic etiology was excluded. Even advanced imaging modalities were unable to detect the clinically silent tumor until an ⁶⁸Ga-DOTA-TOC-PET/CT-scan revealed a mass with paraneoplastic activity in the left foot. Complete resection of the tumor proved to cure her condition after 9 years of uncertainty and suffering. Serum phosphate levels returned to normal within days.

After presentation of the case report, the current literature on published cases of TIO between 1956 and 2021 is summarized to emphasize the importance of an accurate and early diagnosis. Our case report aims to illustrate that a long latency period of diagnosis may be avoided utilizing the latest imaging techniques to spare affected patients from long treatment of symptoms instead of finding the underlying cause.

Autosomal recessive hypophosphatemic rickets type 2 (ARHR2) due to ENPP1-deficiency

Awareness for hypophosphatemic rickets has increased in the last years, based on the availability of specific medical treatments. Autosomal recessive hypophosphatemic rickets type 2 (ARHR2) is a rare form of hypophosphatemic rickets, which is known to develop in survivors of generalized arterial calcification of infancy (GACI). Both disorders are based on a deficiency of ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) and present with a high clinical variability and a lack of a phenotype-genotype association. ARHR2 is characterized by phosphate wasting due to elevated fibroblast growth factor 23 (FGF23) levels and might represent a response of the organism to minimize ectopic calcification in individuals with ENPP1-deficiency. This report reviews the recent clinical and preclinical data on this ultra-rare disease in childhood.

Autosomal Dominant Hypophosphatemic Rickets: A Case Report and Review of the Literature

Autosomal dominant hypophosphatemic rickets (ADHR) is an extremely rare form of genetic rickets caused by mutations in the fibroblast growth factor 23 gene. ADHR is characterized by hypophosphatemia secondary to isolated renal phosphate wasting. Only a few cases of ADHR have been reported in the literature to date. We describe the case of a 17-month-old girl who presented with severe failure to thrive (length: −4.08 standard deviation (SD), weight: −2.2 SD) and hypotonia. Hypophosphatemia, decreased tubular phosphate reabsorption (69%), and rachitic lesions were found. Genetic analysis showed the heterozygous variant c.536G>A (NM_020638.3:c.536G>A) in exon 3 of the FGF23 gene, leading to the diagnosis of ADHR. She was treated with phosphate salts and oral alfacalcidol. After 4 years of treatment, at 5 years of age, the patient’s ADHR resolved spontaneously. Considering the lack of knowledge regarding ADHR, we reviewed the literature to describe the features of this rare and poorly understood disease. Eleven ADHR pediatric cases have been described thus far, with cases tending to be more common in females than males. Similar to the general population, two groups of patients with ADHR can be described depending on the mutations present: patients with an R179 and R176 mutation have early-onset of disease and higher frequency of rickets, and a milder and late-onset of disease, respectively. Symptoms and disease severity may fluctuate. Spontaneous remission may occur during the pediatric age.

Phosphate restriction impairs mTORC1 signaling leading to increased bone marrow adipose tissue and decreased bone in growing mice

Bone marrow stromal cells (BMSCs) are multipotent cells that differentiate into cells of the osteogenic and adipogenic lineage. A striking inverse relationship between bone marrow adipose tissue (BMAT) and bone volume is seen in several conditions, suggesting that differentiation of BMSCs into bone marrow adipocytes diverts cells from the osteogenic lineage, thereby compromising the structural and mechanical properties of bone. Phosphate restriction of growing mice acutely decreases bone formation, blocks osteoblast differentiation and increases BMAT. Studies performed to evaluate the cellular and molecular basis for the effects of acute phosphate restriction demonstrate that it acutely increases 5' adenosine monophosphate-activated protein kinase (AMPK) phosphorylation and inhibits mammalian target of rapamycin complex 1 (mTORC1) signaling in osteoblasts. This is accompanied by decreased expression of Wnt10b in BMSCs. Phosphate restriction also promotes expression of the key adipogenic transcription factors, peroxisome proliferator-activated receptor γ (PPARγ) and CCAAT-enhancer binding protein α (CEBPα), in CXCL12 abundant reticular (CAR) cells, which represent undifferentiated BMSCs and are the main source of BMAT and osteoblasts in the adult murine skeleton. Consistent with this, lineage tracing studies reveal that the BMAT observed in phosphate-restricted mice is of CAR cell origin. To determine whether circumventing the decrease in mTORC1 signaling in maturing osteoblasts attenuates the osteoblast and BMAT phenotype, phosphate-restricted mice with OSX-Cre^ERT2 -mediated haploinsufficiency of the mTORC1 inhibitor, TSC2, were generated. TSC2 haploinsufficiency in preosteoblasts/osteoblasts normalized bone volume and osteoblast number in phosphate-restricted mice and attenuated the increase in BMAT observed. Thus, acute phosphate restriction leads to decreased bone and increases BMAT by impairing mTORC1 signaling in osterix-expressing cells. © 2021 American Society for Bone and Mineral Research (ASBMR).

FAM20C Overview: Classic and Novel Targets, Pathogenic Variants and Raine Syndrome Phenotypes

FAM20C is a gene coding for a protein kinase that targets S-X-E/pS motifs on different phosphoproteins belonging to diverse tissues. Pathogenic variants of FAM20C are responsible for Raine syndrome (RS), initially described as a lethal and congenital osteosclerotic dysplasia characterized by generalized atherosclerosis with periosteal bone formation, characteristic facial dysmorphisms and intracerebral calcifications. The aim of this review is to give an overview of targets and variants of FAM20C as well as RS aspects. We performed a wide phenotypic review focusing on clinical aspects and differences between all lethal (LRS) and non-lethal (NLRS) reported cases, besides the FAM20C pathogenic variant description for each. As new targets of FAM20C kinase have been identified, we reviewed FAM20C targets and their functions in bone and other tissues, with emphasis on novel targets not previously considered. We found the classic lethal and milder non-lethal phenotypes. The milder phenotype is defined by a large spectrum ranging from osteonecrosis to osteosclerosis with additional congenital defects or intellectual disability in some cases. We discuss our current understanding of FAM20C deficiency, its mechanism in RS through classic FAM20C targets in bone tissue and its potential biological relevance through novel targets in non-bone tissues.

Rickets in Children: An Update

Rickets refers to a deficient mineralization of the growth plate cartilage, predominantly affecting longer bones. Despite the fact that preventive measures are available, it is still a common disease worldwide; nutritional rickets, due to vitamin D deficiency or dietary calcium inadequate intake, remains the most common form. Medical history, physical examination, radiologic features and biochemical tests are essential for diagnosis. Although recent studies suggest hypophosphatemia as the leading alteration, rickets is classically divided into two categories: calcipenic rickets and phosphopenic rickets. Knowledge of this categorization and of respective clinical and laboratory features is essential for rapid diagnosis and correct management. The aim of this review is to analyze the epidemiological, pathogenetic, clinical, and therapeutic aspects of the different forms of rickets, describing the novelties on this “long-lived” disease.

Iron deficiency plays essential roles in the trigger, treatment, and prognosis of autosomal dominant hypophosphatemic rickets

By analyzing iron status of 14 ADHR patients, we found that iron deficiency was an important trigger of ADHR. Correcting iron deficiency significantly improved patients' symptoms. Meanwhile, patients' serum phosphate showed positive correlations with iron metabolism parameters and hemoglobin-related parameters, suggesting the necessity of monitoring and correcting the iron status in ADHR.

INTRODUCTION

Autosomal dominant hypophosphatemic rickets (ADHR) is unique for its incomplete penetrance, variety of disease onsets, and waxing and waning phenotypes. Iron deficiency is a trigger of ADHR. This study aimed to clarify the role of iron deficiency in ADHR.

METHODS

Data of clinical manifestations and laboratory examinations were collected from patients among eight kindreds with ADHR. Multiple regression and Pearson's correlation tests were performed to test the relationships of serum phosphate levels and other laboratory variables during the patients' follow-ups.

RESULTS

Among 23 ADHR patients with fibroblast growth factor 23 (FGF23) mutations, 14 patients presented with obvious symptoms. Ten patients had iron deficiency at the onset of ADHR, coinciding with menarche, menorrhagia, pregnancy, and chronic gastrointestinal bleeding. Two patients who did not have their iron status tested presented with symptoms after abortion and pregnancy in one patient each, which suggested that they also had iron deficiency at onset. Patients were treated with ferrous succinate tablets, vitamin C, and neutral phosphate and calcitriol. With correction of the iron status, the patients' symptoms showed notable improvement, with increased serum phosphate levels. Two patients' FGF23 levels also declined to the normal range. There were strong correlations between serum phosphate and serum iron levels (r = 0.7689, p < 0.0001), serum ferritin levels (r = 0.5312, p = 0.002), iron saturation (r = 0.7907, p < 0.0001), and transferrin saturation (r = 0.7875, p < 0.001). We also examined the relationships between serum phosphate levels and hemoglobin-related indices, which were significant (hemoglobin: r = 0.71, p < 0.0001; MCV: r = 0.7589, p < 0.0001; MCH: r = 0.8218, p < 0.0001; and MCHC: r = 0.7751, p < 0.0001). Longitudinal data of six patients' follow-up also showed synchronous changes in serum phosphate with serum iron levels.

CONCLUSIONS

Iron deficiency plays an important role in triggering ADHR. Monitoring and correcting the iron status are helpful for diagnosing and treating ADHR. Iron metabolism parameters and hemoglobin-related parameters are positively correlated with serum phosphate levels in patients with ADHR and iron deficiency, and these might serve as good indicators of prognosis of ADHR.

A case report of mesenchymal scapular FGF secreting tumor: Importance of follow up in tumor induced osteomalacia

A 46-year-old Asian male with history of atraumatic fracture of femur (requiring the use of a walker), muscle cramps and loosening teeth presents to Endocrine clinic. He had elevated parathyroid hormone, severely low phosphorus, elevated bone-specific ALP, with normal serum and urine calcium. He was found to have elevated FGF 23 levels, but initial functional and anatomic imaging was negative for any localizing tumor. With persistent follow-up and serial imaging, after 3 years, a 2.2 cm right scapular mass was found on MRI. Since it was also visualized on PET/CT, this was suspected to be the cause of his severe hypophosphatemia. He underwent surgical excision and pathology revealed a phosphaturic mesenchymal tumor after excision. Tumor induced osteomalacia is a rare, acquired paraneoplastic syndrome in which a tumor that secretes FGF23 leads to decreased renal phosphate reabsorption, resulting in hypophosphatemia, and bone demineralization. Diagnosis is challenging as common presenting symptoms are nonspecific, but when followed up closely with proper diagnostic modalities, identification & removal of the culprit lesion is usually curative.

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.

Congenital Conditions of Hypophosphatemia in Children

Great strides over the past few decades have increased our understanding of the pathophysiology of hypophosphatemic disorders. Phosphate is critically important to a variety of physiologic processes, including skeletal growth, development and mineralization, as well as DNA, RNA, phospholipids, and signaling pathways. Consequently, hypophosphatemic disorders have effects on multiple systems, and may cause a variety of nonspecific signs and symptoms. The acute effects of hypophosphatemia include neuromuscular symptoms and compromise. However, the dominant effects of chronic hypophosphatemia are the effects on musculoskeletal function including rickets, osteomalacia and impaired growth during childhood. While the most common causes of chronic hypophosphatemia in children are congenital, some acquired conditions also result in hypophosphatemia during childhood through a variety of mechanisms. Improved understanding of the pathophysiology of these congenital conditions has led to novel therapeutic approaches. This article will review the pathophysiologic causes of congenital hypophosphatemia, their clinical consequences and medical therapy.

New Therapies for Hypophosphatemia-Related to FGF23 Excess

FGF23 is a hormone produced by osteocytes in response to an elevation in the concentration of extracellular phosphate. Excess production of FGF23 by bone cells, or rarely by tumors, is the hormonal basis for several musculoskeletal syndromes characterized by hypophosphatemia due to renal phosphate wasting. FGF23-dependent chronic hypophosphatemia causes rickets and osteomalacia, as well as other skeletal complications. Genetic disorders of FGF23-mediated hypophosphatemia include X-linked hypophosphatemia (XLH), autosomal dominant hypophosphatemic rickets (ADHR), autosomal recessive hypophosphatemic rickets (ARHR), fibrous dysplasia of bone, McCune-Albright syndrome, and epidermal nevus syndrome (ENS), also known as cutaneous skeletal hypophosphatemia syndrome (CSHS). The principle acquired form of FGF23-mediated hypophosphatemia is tumor-induced osteomalacia (TIO). This review summarizes current knowledge about the pathophysiology and clinical presentation of the most common FGF23-mediated conditions, with a focus on new treatment modalities. For many decades, calcitriol and phosphate supplements were the mainstay of therapy. Recently, burosumab, a monoclonal blocking antibody to FGF23, has been approved for treatment of XLH in children and adults, and an active comparator trial in children has shown good efficacy and safety for this drug. The remainder of FGF23-mediated hypophosphatemic disorders continue to be treated with phosphate and calcitriol, although ongoing trials with burosumab for treatment of tumor-induced osteomalacia show early promise. Burosumab may be an effective treatment for the remainder of FGF23-mediated disorders, but clinical trials to support that possibility are at present not available.

Targeting fibroblast growth factor 23-responsive pathways uncovers controlling genes in kidney mineral metabolism

Fibroblast Growth Factor 23 (FGF23) is a bone-derived hormone that reduces kidney phosphate reabsorption and 1,25(OH)2 vitamin D synthesis via its required co-receptor alpha-Klotho. To identify novel genes that could serve as targets to control FGF23-mediated mineral metabolism, gene array and single-cell RNA sequencing were performed in wild type mouse kidneys. Gene array demonstrated that heparin-binding EGF-like growth factor (HBEGF) was significantly up-regulated following one-hour FGF23 treatment of wild type mice. Mice injected with HBEGF had phenotypes consistent with partial FGF23-mimetic activity including robust induction of Egr1, and increased Cyp24a1 mRNAs. Single cell RNA sequencing showed overlapping HBEGF and EGF-receptor expression mostly in the proximal tubule, and alpha-Klotho expression in proximal and distal tubule segments. In alpha-Klotho-null mice devoid of canonical FGF23 signaling, HBEGF injections significantly increased Egr1 and Cyp24a1 with correction of basally elevated Cyp27b1. Additionally, mice placed on a phosphate deficient diet to suppress FGF23 had endogenously increased Cyp27b1 mRNA, which was rescued in mice receiving HBEGF. In HEK293 cells with stable alpha-Klotho expression, FGF23 and HBEGF increased CYP24A1 mRNA expression. HBEGF, but not FGF23 bioactivity was blocked with EGF-receptor inhibition. Thus, our findings support that the paracrine/autocrine factor HBEGF could play novel roles in controlling genes downstream of FGF23 via targeting common signaling pathways.

Validation of a next-generation sequencing (NGS) panel to improve the diagnosis of X-linked hypophosphataemia (XLH) and other genetic disorders of renal phosphate wasting

BACKGROUND

Hypophosphataemic rickets (HR) comprise a clinically and genetically heterogeneous group of conditions, defined by renal-tubular phosphate wasting and consecutive loss of bone mineralisation. X-linked hypophosphataemia (XLH) is the most common form, caused by inactivating dominant mutations in PHEX, a gene encompassing 22 exons located at Xp22.1. XLH is treatable by anti-Fibroblast Growth Factor 23 antibody, while for other forms of HR such as therapy may not be indicated. Therefore, a genetic differentiation of HR is recommended.

OBJECTIVE

To develop and validate a next-generation sequencing panel for HR with special focus on PHEX.

DESIGN AND METHODS

We designed an AmpliSeq gene panel for the IonTorrent PGM next-generation platform for PHEX and ten other HR-related genes. For validation of PHEX sequencing 50 DNA-samples from XLH-patients, in whom 42 different mutations in PHEX and 1 structural variation have been proven before, were blinded, anonymised and investigated with the NGS panel. In addition, we analyzed one known homozygous DMP1 mutation and two samples of HR-patients, where no pathogenic PHEX mutation had been detected by conventional sequencing.

RESULTS

The panel detected all 42 pathogenic missense/nonsense/splice-site/indel PHEX-mutations and in one the known homozygous DMP1 mutation. In the remaining two patients, we revealed a somatic mosaicism of a PHEX mutation in one; as well as two variations in DMP1 and a very rare compound heterozygous variation in ENPP1 in the second patient.

CONCLUSIONS

This developed NGS panel is a reliable tool with high sensitivity and specificity for the diagnosis of XLH and related forms of HR.

Fibroblast Growth Factor-23 and Matrix Extracellular Phosphoglycoprotein Levels in Healthy Children and, Pregnant and Puerperal Women

INTRODUCTION AND OBJECTIVE

Fibroblast growth factor (FGF-23) and matrix extracellular phosphoglycoprotein (MEPE) are bone-related factors and their role in physiologic conditions and in different life stages are unknown. We aimed to evaluate age- and pregnancy-related changes in MEPE and FGF-23 levels and their correlations with calcium (Ca)-phosphate (PO4) metabolism.

METHODS

The study population included 96 healthy children (50 females) and 31 women (11 healthy, 10 pregnant, and 10 lactating). Intact FGF-23 (iFGF-23), MEPE, ferritin, parathyroid hormone (PTH), 25-OH vitamin D, alkaline phosphatase (ALP), IGF-I, IGFBP-3 and, Ca, PO4 and creatine (Cre) in serum (S) and urine (U) samples were determined. The renal phosphate threshold (TmPO4/GFR) and z-scores for the parameters that show age-related changes were calculated.

RESULTS

Serum iFGF-23 concentrations showed nonsignificant changes with age; however, MEPE decreased with age, reaching the lowest levels after 7 years. Additionally, higher serum MEPE concentrations were observed during pregnancy. Other than ALP, all other examined parameters demonstrated age-related changes. ALP, BUN, S-Cre, and U-Ca/Cre showed puerperal and pregnancy related changes together with MEPE. iFGF-23 was positively correlated with S-PO4 and TmPO4/GFR. MEPE was positively correlated with S-Ca, S-PO4 and TmPO4/GFR and negatively correlated with PTH, IGF-1, and IGFBP-3.

CONCLUSION

Not iFGF-23 but MEPE showed age-dependent changes and was affected by pregnancy. Although, MEPE and iFGF-23 did not correlate with each other, they seem to affect serum and urinary phosphate in the same direction. Additionally, we found evidence that ferritin and growth factors might have a role in serum calcium and phosphate regulation.

Oral Iron Replacement Normalizes Fibroblast Growth Factor 23 in Iron-Deficient Patients With Autosomal Dominant Hypophosphatemic Rickets

Autosomal dominant hypophosphatemic rickets (ADHR) is caused by mutations impairing cleavage of fibroblast growth factor 23 (FGF23). FGF23 gene expression increases during iron deficiency. In humans and mice with the ADHR mutation, iron deficiency results in increased intact FGF23 concentrations and hypophosphatemia. We conducted a prospective open label pilot clinical trial of oral iron replacement over 12 months in ADHR patients to test the hypothesis that oral iron administration would normalize FGF23 concentrations. Eligibility criteria included: FGF23 mutation; and either serum iron <50 μg/dL; or serum iron 50 to 100 μg/dL combined with hypophosphatemia and intact FGF23 >30 pg/mL at screening. Key exclusion criteria were kidney disease and pregnancy. Oral iron supplementation started at 65 mg daily and was titrated based on fasting serum iron concentration. The primary outcome was decrease in fasting intact FGF23 by ≥20% from baseline. Six adults (three male, three female) having the FGF23-R176Q mutation were enrolled; five completed the 12-month protocol. At baseline three of five subjects had severely symptomatic hypophosphatemia (phosphorus <2.5 mg/dL) and received calcitriol with or without phosphate concurrent with oral iron during the trial. The primary outcome was met by 4 of 5 (80%) subjects all by month 4, and 5 of 5 had normal intact FGF23 at month 12. Median (minimum, maximum) intact FGF23 concentration decreased from 172 (20, 192) pg/mL at baseline to 47 (17, 78) pg/mL at month 4 and 42 (19, 63) pg/mL at month 12. Median ferritin increased from 18.6 (7.7, 82.5) ng/mL at baseline to 78.0 (49.6, 261.0) ng/mL at month 12. During iron treatment, all three subjects with baseline hypophosphatemia normalized serum phosphorus, had markedly improved symptoms, and were able to discontinue calcitriol and phosphate. Oral iron repletion normalized FGF23 and phosphorus in symptomatic, iron-deficient ADHR subjects. Thus, the standard approach to ADHR should include recognition, treatment, and prevention of iron deficiency. © 2019 American Society for Bone and Mineral Research.

Iron replacement ameliorates hypophosphatemia in autosomal dominant hypophosphatemic rickets: A review of the role of iron

Autosomal dominant hypophosphatemic rickets (ADHR) is remarkable among the hypophosphatemic rickets syndromes for its variable age of presentation and periods of quiescence during which serum phosphate and fibroblast growth factor 23 (FGF 23) levels are normal without therapy. In contrast, hypophosphatemia in X-linked hypophosphatemic rickets (XLH) manifests soon after birth and requires lifelong therapy. This suggests that there are environmental factors which can alter FGF 23 activity in ADHR but not in XLH. We present an adult with ADHR in whom resolution of hypophosphatemia was achieved by correcting iron deficiency without the need for phosphate supplementation. Serial iron and FGF 23 levels revealed an inverse relationship (r=-0.79, p<0.04). All patients with ADHR who present with hypophosphatemia and worsening symptoms should be screened for iron deficiency. If iron deficiency is detected, therapy with a combination of calcitriol and iron supplementation should be considered without phosphate supplementation.

Earlier Onset in Autosomal Dominant Hypophosphatemic Rickets of R179 than R176 Mutations in Fibroblast Growth Factor 23: Report of 20 Chinese Cases and Review of the Literature

Autosomal dominant hypophosphatemic rickets (ADHR) is a rare hereditary disorder characterized by variant onset ages and diverse phenotypes. Our aim is to explore the genotype-phenotype correlations between ADHR patients with R176 and R179 mutations in FGF23 gene. Clinical manifestations, laboratory examinations, and genetic analyses were collected from 20 patients in six Chinese ADHR kindreds in our hospital. Previously published ADHR literatures were reviewed. Among 20 Chinese ADHR mutation carriers, 11 patients revealed overt symptoms. 10/11 (90.9%) of which were females. Patients with R179 mutations presented with earlier onset than those with R176 mutation [1.3 (1.0, 37.0) years vs. 28.5 (19.0, 44.0) years]. More patients with R179 mutations had a history of rickets with lower extremity deformity [3/4 (75%) vs. 1/7 (14.3%), p < 0.05]. The serum phosphate, i-FGF23 and c-FGF23 levels of patients with R179 and R176 mutations were 0.47 ± 0.14 mmol/L versus 0.57 ± 0.17 mmol/L, 79.6 ± 87.0 pg/mL versus 79.9 ± 107.4 pg/mL, and 33.4 ± 3.0 RU/mL versus 121.3 ± 177.6 RU/mL, respectively. 7/11 of patients had iron deficiency at onset of disease. When combined with previously reported seven ADHR families, difference was observed in the age of onset among symptomatic patients with R179 and R176 mutations [1.0 (0.9, 37.0) years vs. 24.5 (1.2, 57.0) years, p < 0.05]. Patients with R179 mutation were more likely to have rickets than R176 mutation (11/13, 84.6% vs. 5/20, 25.0%, p < 0.01) and lower extremity deformity (10/13, 76.9% vs. 6/19, 31.6%, p < 0.01). ADHR patients with R179 mutations had earlier onset age and more rickets compared to those with mutations in R176, which partially explained the clinical heterogeneity of ADHR.

Phosphate disorders and clinical management of hypophosphatemia and hyperphosphatemia

Serum phosphorus levels range from 2.5 and 4.5mg/dL (0.81-1.45 mmol/L) in adults, with higher levels in childhood, adolescence, and pregnancy. Intracellular phosphate is involved in intermediary metabolism and other essential cell functions, while extracellular phosphate is essential for bone matrix mineralization. Plasma phosphorus levels are maintained within a narrow range by regulation of intestinal absorption, redistribution, and renal tubular absorption of the mineral. Hypophosphatemia and hyperphosphatemia are common clinical situations, although changes are most often mild and oligosymptomatic. However, acute and severe conditions that require specific treatment may occur. In this document, members of the Mineral and Bone Metabolism Working Group of the Spanish Society of Endocrinology and Nutrition review phosphate disorders and provide algorithms for adequate clinical management of hypophosphatemia and hyperphosphatemia.

FGF23 at the crossroads of phosphate, iron economy and erythropoiesis

Fibroblast growth factor 23 (FGF23) was initially characterized as an important regulator of phosphate and calcium homeostasis. New research advances demonstrate that FGF23 is also linked to iron economy, inflammation and erythropoiesis. These advances have been fuelled, in part, by the serendipitous development of two distinct FGF23 assays that can substitute for invasive bone biopsies to infer the activity of the three main steps of FGF23 regulation in bone: transcription, post-translational modification and peptide cleavage. This 'liquid bone biopsy for FGF23 dynamics' enables large-scale longitudinal studies of FGF23 regulation that would otherwise be impossible in humans. The balance between FGF23 production, post-translational modification and cleavage is maintained or perturbed in different hereditary monogenic conditions and in acquired conditions that mimic these genetic disorders, including iron deficiency, inflammation, treatment with ferric carboxymaltose and chronic kidney disease. Looking ahead, a deeper understanding of the relationships between FGF23 regulation, iron homeostasis and erythropoiesis can be leveraged to devise novel therapeutic targets for treatment of anaemia and states of FGF23 excess, including chronic kidney disease.

FGF23 and Associated Disorders of Phosphate Wasting

Fibroblast growth factor 23 (FGF23), one of the endocrine fibroblast growth factors, is a principal regulator in the maintenance of serum phosphorus concentration. Binding to its cofactor αKlotho and a fibroblast growth factor receptor is essential for its activity. Its regulation and interaction with other factors in the bone-parathyroid-kidney axis is complex. FGF23 reduces serum phosphorus concentration through decreased reabsorption of phosphorus in the kidney and by decreasing 1,25 dihydroxyvitamin D (1,25(OH)2D) concentrations. Various FGF23-mediated disorders of renal phosphate wasting share similar clinical and biochemical features. The most common of these is X-linked hypophosphatemia (XLH). Additional disorders of FGF23 excess include autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets, fibrous dysplasia, and tumor-induced osteomalacia. Treatment is challenging, requiring careful monitoring and titration of dosages to optimize effectiveness and to balance side effects. Conventional therapy for XLH and other disorders of FGF23-mediated hypophosphatemia involves multiple daily doses of oral phosphate salts and active vitamin D analogs, such as calcitriol or alfacalcidol. Additional treatments may be used to help address side effects of conventional therapy such as thiazides to address hypercalciuria or nephrocalcinosis, and calcimimetics to manage hyperparathyroidism. The recent development and approval of an anti-FGF23 antibody, burosumab, for use in XLH provides a novel treatment option.

Sustained Klotho delivery reduces serum phosphate in a model of diabetic nephropathy

Diabetic nephropathy (DN) is a primary cause of end-stage renal disease and is becoming more prevalent because of the global rise in type 2 diabetes. A model of DN, the db/db uninephrectomized ( db/db-uni) mouse, is characterized by obesity, as well as compromised renal function. This model also manifests defects in mineral metabolism common in DN, including hyperphosphatemia, which leads to severe endocrine disease. The FGF23 coreceptor, α-Klotho, circulates as a soluble, cleaved form (cKL) and may directly influence phosphate handling. Our study sought to test the effects of cKL on mineral metabolism in db/db-uni mice. Mice were placed into either mild or moderate disease groups on the basis of the albumin-to-creatinine ratio (ACR). Body weights of db/db-uni mice were significantly greater across the study compared with lean controls regardless of disease severity. Adeno-associated cKL administration was associated with increased serum Klotho, intact, bioactive FGF23 (iFGF23), and COOH-terminal fragments of FGF23 ( P < 0.05). Blood urea nitrogen was improved after cKL administration, and cKL corrected hyperphosphatemia in the high- and low-ACR db/db-uni groups. Interestingly, 2 wk after cKL delivery, blood glucose levels were significantly reduced in db/db-uni mice with high ACR ( P < 0.05). Interestingly, several genes associated with stabilizing active iFGF23 were also increased in the osteoblastic UMR-106 cell line with cKL treatment. In summary, delivery of cKL to a model of DN normalized blood phosphate levels regardless of disease severity, supporting the concept that targeting cKL-affected pathways could provide future therapeutic avenues in DN. NEW & NOTEWORTHY In this work, systemic and continuous delivery of the "soluble" or "cleaved" form of the FGF23 coreceptor α-Klotho (cKL) via adeno-associated virus to a rodent model of diabetic nephropathy (DN), the db/db uninephrectomized mouse, normalized blood phosphate levels regardless of disease severity. This work supports the concept that targeting cKL-affected pathways could provide future therapeutic avenues for the severe mineral metabolism defects associated with DN.

Autosomal Dominant Hypophosphatemic Rickets Presenting in a Phenotypically Normal Adult Female

We describe a presentation of Autosomal Dominant Hypophosphatemic Rickets (ADHR) in a 22-year-old female with normal pubertal growth and development and a negative family history in first-degree relatives. The patient presented with a 2-year history of upper and lower extremity proximal muscle pain and weakness and bilateral femoral neck and pubic bone insufficiency fractures. She had a normal serum calcium but a low phosphate as well as 25-hydroxyvitamin D (25(OH)D) levels leading initially to a diagnosis of osteomalacia. Urine phosphate reabsorption was low confirming a phosphate wasting disorder. She had an elevated Fibroblast Growth Factor 23 (FGF23) level. After Tumor-Induced Osteomalacia was ruled out by extensive imaging, she was sent for genetic testing for hereditary rickets which showed a previously reported missense variant in FGF23. Subsequently, she found out that her father's maternal aunt and grandfather had ‘bone disorder' and were wheelchair-bound in adulthood. After replenishment of vitamin D, treatment with calcitriol and phosphate leads to complete resolution of patient's symptoms and laboratory abnormalities.

Hypophosphatemic Rickets

Hypophosphatemic rickets, mostly of the X-linked dominant form caused by pathogenic variants of the PHEX gene, poses therapeutic challenges with consequences for growth and bone development and portends a high risk of fractions and poor bone healing, dental problems and nephrolithiasis/nephrocalcinosis. Conventional treatment consists of PO4 supplements and calcitriol requiring monitoring for treatment-emergent adverse effects. FGF23 measurement, where available, has implications for the differential diagnosis of hypophosphatemia syndromes and, potentially, treatment monitoring. Newer therapeutic modalities include calcium sensing receptor modulation (cinacalcet) and biological molecules targeting FGF23 or its receptors. Their long-term effects must be compared with those of conventional treatments.

Bone function, dysfunction and its role in diseases including critical illness

The skeleton is one of the largest organs in the human body. In addition to its conventional functions such as support, movement and protection, the skeleton also contributes to whole body homeostasis and maintenance of multiple important non-bone organs/systems (extraskeletal functions). Both conventional and extraskeletal functions of the skeleton are defined as bone function. Bone-derived factors (BDFs) are key players regulating bone function. In some pathophysiological situations, including diseases affecting bone and/or other organs/systems, the disorders of bone itself and the subsequently impaired functions of extraskeletal organs/systems caused by abnormal bone (impaired extraskeletal functions of bone) are defined as bone dysfunction. In critical illness, which is a health status characterized by the dysfunction or severe damage of one or multiple important organs or systems, the skeleton shows rapid bone loss resulting from bone hyper-resorption and impaired osteoblast function. In addition, the dysfunctions of the skeleton itself are also closely related to the severity and prognosis of critical illness. Therefore, we propose that there is bone dysfunction in critical illness. Some methods to inhibit osteoclast activity or promote osteoblast function by the treatment of bisphosphonates or PTH1-34 benefit the outcome of critical illness, which indicates that enhancing bone function may be a potential novel strategy to improve prognosis of diseases including critical illness.

Regulation of Fibroblast Growth Factor 23 by Iron, EPO, and HIF

Purpose of review

Fibroblast growth factor-23 (FGF23) is the key hormone produced in bone critical for phosphate homeostasis. Elevated serum phosphorus and 1,25dihydroxyvitaminD stimulates FGF23 production to promote renal phosphate excretion and decrease 1,25dihydroxyvitaminD synthesis. Thus completing the feedback loop and suppressing FGF23. Unexpectedly, studies of common and rare heritable disorders of phosphate handling identified links between iron and FGF23 demonstrating novel regulation outside the phosphate pathway.

Recent Findings

Iron deficiency combined with an FGF23 cleavage mutation was found to induce the autosomal dominant hypophosphatemic rickets phenotype. Physiological responses to iron deficiency, such as erythropoietin production as well as hypoxia inducible factor activation, have been indicated in regulating FGF23. Additionally, specific iron formulations, used to treat iron deficiency, alter post-translational processing thereby shifting FGF23 protein secretion.

Summary

Molecular and clinical studies revealed that iron deficiency, through several mechanisms, alters FGF23 at the transcriptional and post-translational level. This review will focus upon the novel discoveries elucidated between iron, its regulators, and their influence on FGF23 bioactivity.

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.

Pharmacological management of X-linked hypophosphataemia

The most common heritable disorder of renal phosphate wasting, X-linked hypophosphataemia (XLH), was discovered to be caused by inactivating mutations in the phosphate regulating gene with homology to endopeptidases on the X-chromosome (PHEX) gene in 1995. Although the exact molecular mechanisms by which PHEX mutations cause disturbed phosphate handling in XLH remain unknown, focus for novel therapies has more recently been based upon the finding that the bone-produced phosphaturic hormone fibroblast growth factor-23 is elevated in XLH patient plasma. Previous treatment strategies for XLH were based upon phosphate repletion plus active vitamin D analogues, which are difficult to manage, fail to address the primary pathogenesis of the disease, and can have deleterious side effects. A novel therapy for XLH directly targeting fibroblast growth factor-23 via a humanized monoclonal antibody (burosumab-twza/CRYSVITA, henceforth referred to just as burosumab) has emerged as an effective, and recently approved, pharmacological treatment for both children and adults. This review will provide an overview of the clinical manifestations of XLH, the molecular pathophysiology, and summarize its current treatment.

Phosphate homeostasis disorders

Our understanding of the regulation of phosphate balance has benefited tremendously from the molecular identification and characterization of genetic defects leading to a number of rare inherited or acquired disorders affecting phosphate homeostasis. The identification of the key phosphate-regulating hormone, fibroblast growth factor 23 (FGF23), as well as other molecules that control its production, such as the glycosyltransferase GALNT3, the endopeptidase PHEX, and the matrix protein DMP1, and molecules that function as downstream effectors of FGF23 such as the longevity factor Klotho and the phosphate transporters NPT2a and NPT2c, has permitted us to understand the complex interplay that exists between the kidneys, bone, parathyroid, and gut. Such insights from genetic disorders have allowed not only the design of potent targeted treatment of FGF23-dependent hypophosphatemic conditions, but also provide clinically relevant observations related to the dysregulation of mineral ion homeostasis in health and disease.

Phosphaturic mesenchymal tumors: what an endocrinologist should know

Tumor-induced osteomalacia (TIO), also known as "oncogenic osteomalacia", is a rare cause of osteomalacia. TIO often has an insidious onset characterized clinically by progressive muscle weakness and bone pain with fractures. The hallmark biochemical finding is a persistent low serum phosphorus concentration due to renal phosphate wasting. The vast majority of cases of TIO result from production of the phosphaturic hormone fibroblast growth factor 23 (FGF23) by a histologically distinctive mesenchymal tumor, termed "phosphaturic mesenchymal tumor" (PMT). Circulating FGF23 induces internalization of renal sodium/phosphate co-transporters resulting in reduced proximal tubular phosphate reabsorption. FGF23 also inhibits production of 1α,25-dihydroxyvitamin D which is inappropriately low or normal in the context of hypophosphatemia. Diagnosis is often delayed owing to the rarity of the condition and an underappreciation for the role of phosphorus as a cause for the constellation of symptoms. Primary treatment for TIO is identification of the offending tumor and surgical removal. However, these tumors are notoriously difficult to find, precluding the opportunity for a curative surgery in many. In such cases, phosphate and calcitriol therapy is used to improve symptoms and heal the osteomalacia. Recently, molecular genetic studies have shown recurrent genetic events in PMT, including the novel fusions FN1-FGFR1 and less commonly FN1-FGF1. These fusion events are hypothesized to result in autocrine/paracrine signaling loops within the tumor, spurring tumorigenesis. This review will cover the clinical features, imaging characteristics, pathologic features, molecular genetic aspects, and therapy of PMT, with a brief discussion of other neoplasms that may cause TIO.

Genetic Causes of Rickets

Rickets is a metabolic bone disease that develops as a result of inadequate mineralization of growing bone due to disruption of calcium, phosphorus and/or vitamin D metabolism. Nutritional rickets remains a significant child health problem in developing countries. In addition, several rare genetic causes of rickets have also been described, which can be divided into two groups. The first group consists of genetic disorders of vitamin D biosynthesis and action, such as vitamin D-dependent rickets type 1A (VDDR1A), vitamin D-dependent rickets type 1B (VDDR1B), vitamin D-dependent rickets type 2A (VDDR2A), and vitamin D-dependent rickets type 2B (VDDR2B). The second group involves genetic disorders of excessive renal phosphate loss (hereditary hypophosphatemic rickets) due to impairment in renal tubular phosphate reabsorption as a result of FGF23-related or FGF23-independent causes. In this review, we focus on clinical, laboratory and genetic characteristics of various types of hereditary rickets as well as differential diagnosis and treatment approaches.

Heritable and acquired disorders of phosphate metabolism: Etiologies involving FGF23 and current therapeutics

Phosphate is critical for many cellular processes and structural functions, including as a key molecule for nucleic acid synthesis and energy metabolism, as well as hydroxyapatite formation in bone. Therefore it is critical to maintain tight regulation of systemic phosphate levels. Based upon its broad biological importance, disruption of normal phosphate homeostasis has detrimental effects on skeletal integrity and overall health. Investigating heritable diseases of altered phosphate metabolism has led to key discoveries underlying the regulation and systemic actions of the phosphaturic hormone Fibroblast growth factor-23 (FGF23). Both molecular and clinical studies have revealed novel targets for the development and optimization of therapies for disorders of phosphate handling. This review will focus upon the bridge between genetic discoveries involving disorders of altered FGF23 bioactivity, as well as describe how these findings have translated into pharmacologic application.

Late-onset hereditary hypophosphatemic rickets with hypercalciuria (HHRH) due to mutation of SLC34A3/NPT2c

OBJECTIVE

To identify a genetic basis for markedly reduced bone density and multiple fractures in an adult patient with hypophosphatemia and hypercalciuria.

SUBJECTS

A 54-year-old Vietnamese man, his unaffected two daughters and wife.

METHODS

We performed biochemical studies and sequenced the SLC34A3 gene using genomic DNA from peripheral blood mononuclear cells.

RESULTS

Biochemical evaluation of the proband revealed hypophosphatemia with increased renal phosphate wasting, hypercalciuria, low serum parathyroid hormone (PTH) and an elevated serum 1,25(OH)2D level. Mutation analysis of SLC34A3 gene revealed that the patient was a compound heterozygote for two nonsynonymous nucleotide substitutions: a novel c.571G>A (p.G191S) damaging mutation and the previously reported c.200G>A (p.R67H) polymorphism, consistent with the clinical diagnosis of late-onset hereditary hypophosphatemic rickets with hypercalciuria (HHRH). His wife and older daughter both carried the p.R67H polymorphism, while his younger daughter was compound heterozygous for p.R67H and p.G191S.

CONCLUSIONS

HHRH is an uncommon autosomal recessive disease that generally manifests in childhood as rickets or nephrolithiasis, but an adult onset phenotype may occur in heterozygous carriers of SLC34A3 mutations. The severe presentation of this proband in adulthood with marked nephrolithiasis, multiple fractures and low bone density emphasizes the importance of measuring the serum phosphorus level in patients with suspected but unexplained osteoporosis and/or recurrent renal stones. The recognition of late-onset HHRH facilitates timely institution of appropriate therapy.