Secreted frizzled-related protein 4 is a potent tumor-derived phosphaturic agent

The Intricacies of Renal Phosphate Reabsorption-An Overview

To maintain an optimal body content of phosphorus throughout postnatal life, variable phosphate absorption from food must be finely matched with urinary excretion. This amazing feat is accomplished through synchronised phosphate transport by myriads of ciliated cells lining the renal proximal tubules. These respond in real time to changes in phosphate and composition of the renal filtrate and to hormonal instructions. How they do this has stimulated decades of research. New analytical techniques, coupled with incredible advances in computer technology, have opened new avenues for investigation at a sub-cellular level. There has been a surge of research into different aspects of the process. These have verified long-held beliefs and are also dramatically extending our vision of the intense, integrated, intracellular activity which mediates phosphate absorption. Already, some have indicated new approaches for pharmacological intervention to regulate phosphate in common conditions, including chronic renal failure and osteoporosis, as well as rare inherited biochemical disorders. It is a rapidly evolving field. The aim here is to provide an overview of our current knowledge, to show where it is leading, and where there are uncertainties. Hopefully, this will raise questions and stimulate new ideas for further research.

A 50-Year-Old Man Presenting with Multiple Bone Lesions and a Diagnosis of Phosphaturic Mesenchymal Tumor of the Femur

BACKGROUND Phosphaturic mesenchymal tumor (PMT) is an extremely rare mesenchymal neoplasm that is commonly seen in bone and soft tissue. It is associated with a paraneoplastic syndrome, oncogenic osteomalacia, due to tumor-induced urinary phosphate wasting. It is demonstrated to be predominantly mediated by fibroblast growth factor 23 (FGF23)/fibroblast growth factor receptor 1 (FGFR1) axis. Clinically, PMT usually presents as a solitary lesion in the bone. The diagnosis of PMT is challenging due to its non-specific clinical manifestation, radiologic findings, and morphological features. CASE REPORT We report the case of a 50-year-old man presenting with multiple lytic bone lesions and associated pathologic fracture of the right femur, clinically suspicious for multiple myeloma or other metastatic malignant process. Resection from the right femur showed a hypercellular lesion composed of oval-to-spindled cells infiltrating the native trabecular bone with admixed multinucleated giant cells. Immunohistochemical (IHC) staining and in situ hybridization (ISH) demonstrated the tumor cells were positive for SATB2, ERG, FGFR1, and FGF23 ISH. DNA and RNA next-generation sequencing showed marked increases in mRNA levels of FGF23 and FGFR1. The constellation of clinicoradiologic, histomorphologic, IHC, and molecular findings supported a diagnosis of primary benign PMT. CONCLUSIONS This case report discusses a patient with PMT presenting with multifocal lesions due to tumor-induced osteomalacia at initial presentation. We hope that this report will increase the awareness of clinician and pathologists of PMT as a differential diagnosis in patients presenting with multifocal lytic bone lesions. In turn, this will prevent misdiagnosis and overtreatment of a typically benign process.

The contribution of a novel PHEX gene mutation to X-linked hypophosphatemic rickets: a case report and an analysis of the gene mutation dosage effect in a rat model

A Chinese family was identified to have two patients with rickets, an adult female and a male child (proband), both exhibiting signs related to X-linked hypophosphatemic rickets (XLH). Gene sequencing analysis revealed a deletion of adenine at position 1985 (c.1985delA) in the PHEX-encoding gene. To investigate the relationship between this mutation and the pathogenicity of XLH, as well as analyze the effects of different dosages of PHEX gene mutations on clinical phenotypes, we developed a rat model carrying the PHEX deletion mutation. The CRISPR/Cas9 gene editing technology was employed to construct the rat model with the PHEX gene mutation (c.1985delA). Through reproductive procedures, five genotypes of rats were obtained: female wild type (X/X), female heterozygous (-/X), female homozygous wild type (-/-), male wild type (X/Y), and male hemizygous (-/Y). The rats with different genotypes underwent analysis of growth, serum biochemical parameters, and bone microstructure. The results demonstrated the successful generation of a stable rat model inheriting the PHEX gene mutation. Compared to the wild-type rats, the mutant rats displayed delayed growth, shorter femurs, and significantly reduced bone mass. Among the female rats, the homozygous individuals exhibited the smallest body size, decreased bone mass, shortest femur length, and severe deformities. Moreover, the mutant rats showed significantly lower blood phosphorus concentration, elevated levels of FGF23 and alkaline phosphatase, and increased expression of phosphorus regulators. In conclusion, the XLH rat model with the PHEX gene mutation dosage demonstrated its impact on growth and development, serum biochemical parameters, and femoral morphology.

Phosphaturic Mesenchymal Tumors with or without Phosphate Metabolism Derangements

Phosphaturic mesenchymal tumors (PMT) are rare neoplasms, which can give rise to a multifaceted syndrome, otherwise called tumor-induced osteomalacia (TIO). Localizing these tumors is crucial to obtain a cure for the phosphate metabolism derangement, which is often the main cause leading the patient to seek medical help, because of invalidating physical and neuromuscular symptoms. A proportion of these tumors is completely silent and may grow unnoticed, unless they become large enough to produce pain or discomfort. FGF-23 can be produced by several benign or malignant PMTs. The phosphate metabolism, radiology and histology of these rare tumors must be collectively assessed by a multidisciplinary team aimed at curing the disease locally and improving patients' quality of life. This narrative review, authored by multiple specialists of a tertiary care hospital center, will describe endocrine, radiological and histological features of these tumors, as well as present surgical and interventional strategies to manage PMTs.

Phosphatonins: From Discovery to Therapeutics

OBJECTIVE

Phosphate is crucial for cell signaling, energy metabolism, nucleotide synthesis, and bone mineralization. The gut-bone-parathyroid-kidney axis is influenced by parathyroid hormone, 1,25-dihydroxyvitamin D, and phosphatonins, especially fibroblast growth factor 23 (FGF23). These hormones facilitate maintenance of phosphate homeostasis. This review summarizes current knowledge regarding the phosphate homeostasis, phosphatonin pathophysiology, and clinical implications of FGF23-related hypophosphatemic disorders, with specific focus on burosumab treatment.

METHOD

A focused literature search of PubMed was conducted.

RESULTS

Phosphatonins including FGF23, secreted frizzled-related protein 4, matrix extracellular phosphoglycoprotein, and fibroblast growth factor 7 play a pathogenic role in several hypophosphatemic disorders. Excess FGF23 inhibits sodium-dependent phosphate cotransporters (NaPi-2a and NaPi-2c), resulting in hyperphosphaturia and hypophosphatemia. Additionally, FGF23 suppresses 1,25-dihydroxyvitamin D synthesis in the proximal renal tubule, and thus, it indirectly inhibits intestinal phosphate absorption. Disorders of FGF23-related hypophosphatemia include X-linked hypophosphatemia (XLH), autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets, fibrous dysplasia/McCune-Albright syndrome, and tumor-induced osteomalacia (TIO). Complications of conventional therapy with oral phosphate and vitamin D analogs comprise gastrointestinal distress, hypercalcemia, nephrocalcinosis, and secondary/tertiary hyperparathyroidism. In both children and adults with XLH and TIO, the anti-FGF23 antibody burosumab exhibits a favorable safety profile and is associated with healing of rickets in affected children and improvement of osteomalacia in both children and adults.

CONCLUSION

The treatment paradigm for XLH and TIO is changing based on data from recent clinical trials. Research suggest that burosumab is effective and safe for pediatric and adult patients with XLH or TIO.

Bioinformatics Analysis of Common Differential Genes of Viral Myocarditis and Dilated Cardiomyopathy: Screening for Potential Pharmacological Compounds

(1) Background: The mechanism of viral myocarditis (VMC) progression to dilated cardiomyopathy (DCM) remains unclear. The aim of this study was to identify key genes in the progression of VMC to DCM, so as to find potential therapeutic drugs and provide insights for future research. (2) Methods: Differential expression analysis of GSE4172 and GSE17800 from the Gene Expression Omnibus (GEO) database was performed using GEO2R, which contained genome-wide analysis of myocardial biopsies from VMC and DCM, respectively. We used the Venn diagram analysis to screen the common differentially expressed genes (DEGs). GO functional enrichment analysis and KEGG pathway analysis were also performed. Then we conducted protein-protein interaction (PPI) networks using STRING and identified hub genes using Cytoscape. Finally, we used cMAP to screen out candidate compounds targeting these hub genes; (3) Results: In total, 2143 DEGs for VMC and 1365 DEGs for DCM were found. Then a total of 191 common DEGs were identified. Biological processes and pathway involved in these genes mainly include GABA-gated chloride ion channel activity and Rap1 signaling pathway. A total of 14 hub genes were identified. PPI network showed these hubs mainly enriched in regulation of WNT signaling pathway and GABA-gated chloride ion channel activity. Subgroup analysis of Severe VMC cohort revealed 10 hub genes which mainly clustered in GABA channel activity, extracellular matrix remodeling and sarcomere dysfunction. Using cMAP, we obtained top 10 potential medications, but only amlodipine is currently viable; (4) Conclusions: Our study finds the hub genes and reveals the important role of GABA-gated chloride ion channel, Rap1 signaling pathway, WNT signaling pathway, extracellular matrix remodeling and sarcomere dysfunction in the progression from VMC to DCM. Amlodipine is a potential viable drug in preventing the progression of VMC to DCM.

Identification of Ascorbic Acid and Gallic Acid as Novel Inhibitors of Secreted Frizzled-Related Protein for the Treatment of Obesity-Induced Type 2 Diabetes

Type 2 diabetes mellitus (T2D) has been reported as major public health issue rising at an alarming rate worldwide, and obesity is the leading risk factor for the development of T2D. Secreted frizzled-related protein 4 (SFRP4) released with inflammatory mediators from adipose tissues constrains the exocytosis of insulin containing granules from the pancreatic islets that leads towards the development to T2D. The significant overexpression of SFRP4 in diabetic patients and its involvement in islet dysfunction suggest its critical role in the development of diabetes. Thus, this study was designed to explore the potential of ascorbic acid (AA) and gallic acid (GA) against SFRP4 for the treatment of diabetes. Molecular docking approach was used for the prediction of binding interactions of AA and GA at the active pocket of SFRP4. Docking analysis indicated strong binding interactions of AA and GA to the amino acid residues at the active site of SFRP4. A significant reduction in the level of SFRP4 was observed in transfected cells treated with AA and GA. For the evaluation of the cytotoxicity of AA and GA against HepG2 cells, MTT assay was performed. The results of MTT assay demonstrated that AA and GA are non-cytotoxic towards HepG2 cells at concentration of 15 μM. The oral administration of AA and GA to diet-induced obese mice caused significant reduction in body weight, blood glucose level, and SFRP4 expression. The results of this study suggest that AA and GA have potential for the treatment of obesity-induced T2D.

[Tumor localization and treatment of tumor-induced osteomalacia]

Tumor-induced osteomalacia (TIO) or oncogenic osteomalacia (OOM) is a rare paraneoplastic renal phosphate wasting syndrome. The disease is mostly triggered by small, benign mesenchymal tumors that express somatostatin receptors (SSTR) and produce excessive levels of fibroblast growth factor 23 (FGF 23) or other phosphatonins. These reduce the phosphate back resorption in the proximal tubules of the kidneys, thereby causing hypophosphatemia and lead to an absolute or relatively low calcitriol serum concentration. The main symptoms include muscle weakness, bone pain and recurrent insufficiency fractures secondary to sometimes pronounced osteomalacia. The suspected diagnosis can only be confirmed by determination of the phosphate level. It can often take years before the tumor is successfully localized. The necessary tumor localization is often the most difficult step in the treatment before the OOM can be curatively treated by open surgical resection of the tumor. In recent years new approaches for faster tumor localization and treatment of the tumor have been developed. Positron emission tomography (PET) in co-registration with computed tomography (⁶⁸Ga-DOTA-TATE PET/CT) is currently the most sensitive imaging methodology for tumor detection. The application of the monoclonal FGF 23 antibody burosumab represents a promising new option in the treatment of inoperable adult OOM.

Hypophosphatemia in acute liver failure of a broad range of etiologies is associated with phosphaturia without kidney damage or phosphatonin elevation

Hypophosphatemia is a common and dangerous complication of acute liver failure (ALF) of various etiologies. While various mechanisms for ALF-associated hypophosphatemia have been proposed including high phosphate uptake into regenerating hepatocytes, acetaminophen (APAP)-associated hypophosphatemia was linked to renal phosphate wasting, and APAP-induced renal tubular injury was proposed as underlying mechanism. We studied 30 normophosphatemic and 46 hypophosphatemic (serum phosphate < 2.5 mg/dL) patients from the Acute Liver Failure Study Group registry with APAP- or non-APAP-induced ALF. Since kidney injury affects phosphate excretion, patients with elevated serum creatinine (>1.2 mg/dL) were excluded. Maximal amount of renal tubular phosphate reabsorption per filtered volume (TmP/GFR) was calculated from simultaneous serum and urine phosphate and creatinine levels to assess renal phosphate handling. Instead of enhanced renal phosphate reabsorption as would be expected during hypophosphatemia of non-renal causes, serum phosphate was positively correlated with TmP/GFR in both APAP- and non-APAP-induced ALF patients (R2 = 0.66 and 0.46, respectively; both P < 0.0001), indicating renal phosphate wasting. Surprisingly, there was no evidence of kidney damage based on urinary markers including neutrophil gelatinase-associated lipocalin and cystatin C even in the APAP group. Additionally, there was no evidence that the known serum phosphatonins parathyroid hormone, fibroblast growth factor 23, and α-Klotho contribute to the observed hypophosphatemia. We conclude that the observed hypophosphatemia with renal phosphate wasting in both APAP- and non-APAP-mediated ALF is likely the result of renal tubular phosphate leak from yet-to-be identified factor(s) with no evidence for proximal tubular damage or contribution of known phosphatonins.

A supergene underlies linked variation in color and morphology in a Holarctic songbird

The genetic architecture of a phenotype can have considerable effects on the evolution of a trait or species. Characterizing genetic architecture provides insight into the complexity of a given phenotype and, potentially, the role of the phenotype in evolutionary processes like speciation. We use genome sequences to investigate the genetic basis of phenotypic variation in redpoll finches (Acanthis spp.). We demonstrate that variation in redpoll phenotype is broadly controlled by a ~55-Mb chromosomal inversion. Within this inversion, we find multiple candidate genes related to melanogenesis, carotenoid coloration, and bill shape, suggesting the inversion acts as a supergene controlling multiple linked traits. A latitudinal gradient in ecotype distribution suggests supergene driven variation in color and bill morphology are likely under environmental selection, maintaining supergene haplotypes as a balanced polymorphism. Our results provide a mechanism for the maintenance of ecotype variation in redpolls despite a genome largely homogenized by gene flow.

Phosphate Concentrations and Modifying Factors in Healthy Children From 12 to 24 Months of Age

CONTEXT

Phosphate homeostasis and its modifiers in early childhood are inadequately characterized.

OBJECTIVE

To determine physiological plasma phosphate concentration and modifying factors in healthy infants at 12 to 24 months of age.

DESIGN

This study included 525 healthy infants (53% girls), who participated in a randomized vitamin D intervention trial and received daily vitamin D3 supplementation of either 10 or 30 μg from age 2 weeks to 24 months. Biochemical parameters were measured at 12 and 24 months. Dietary phosphate intake was determined at 12 months.

MAIN OUTCOME MEASURES

Plasma phosphate concentrations at 12 and 24 months of age.

RESULTS

Mean (SD) phosphate concentration decreased from 12 months (1.9 ± 0.15 mmol/L) to 24 months (1.6 ± 0.17 mmol/L) of age (P < 0.001 for repeated measurements). When adjusted by covariates, such as body size, creatinine, serum 25-hydroxyvitamin D, intact and C-terminal fibroblast growth factor 23, mean plasma phosphate was higher in boys than girls during follow-up (P = 0.019). Phosphate concentrations were similar in the vitamin D intervention groups (P > 0.472 for all). Plasma iron was associated positively with plasma phosphate at both time points (B, 0.006 and 0.005; 95% CI, 0.004-0.009 and 0.002-0.008; P < 0.001 at both time points, respectively). At 24 months of age, the main modifier of phosphate concentration was plasma creatinine (B, 0.007; 95% CI 0.003-0.011, P < 0.001).

CONCLUSION

Plasma phosphate concentration decreased from age 12 to 24 months. In infants and toddlers, the strongest plasma phosphate modifiers were sex, iron, and creatinine, whereas vitamin D supplementation did not modify phosphate concentrations.

The Causes of Hypo- and Hyperphosphatemia in Humans

Phosphate homeostasis involves several major organs that are the skeleton, the intestine, the kidney, and parathyroid glands. Major regulators of phosphate homeostasis are parathormone, fibroblast growth factor 23, 1,25-dihydroxyvitamin D, which respond to variations of serum phosphate levels and act to increase or decrease intestinal absorption and renal tubular reabsorption, through the modulation of expression of transcellular transporters at the intestinal and/or renal tubular level. Any acquired or genetic dysfunction in these major organs or regulators may induce hypo- or hyperphosphatemia. The causes of hypo- and hyperphosphatemia are numerous. This review develops the main causes of acquired and genetic hypo- and hyperphosphatemia.

FGF23 and Bone and Mineral Metabolism

FGF23 is a phosphotropic hormone produced by the bone. FGF23 works by binding to the FGF receptor-Klotho complex. Klotho is expressed in several limited tissues including the kidney and parathyroid glands. This tissue-restricted expression of Klotho is believed to determine the target organs of FGF23. FGF23 reduces serum phosphate by suppressing the expression of type 2a and 2c sodium-phosphate cotransporters in renal proximal tubules. FGF23 also decreases 1,25-dihydroxyvitamin D levels by regulating the expression of vitamin D-metabolizing enzymes, which results in reduced intestinal phosphate absorption. Excessive actions of FGF23 cause several types of hypophosphatemic rickets/osteomalacia characterized by impaired mineralization of bone matrix. In contrast, deficient actions of FGF23 result in hyperphosphatemic tumoral calcinosis with high 1,25-dihydroxyvitamin D levels. These results indicate that FGF23 is a physiological regulator of phosphate and vitamin D metabolism and indispensable for the maintenance of serum phosphate levels.

Clinical Characteristics and Surgical Outcomes of Sinonasal Lesions Associated With Tumor-Induced Osteomalacia

OBJECTIVE

To investigate the clinical characteristics and surgical outcomes of sinonasal tumors associated with tumor-induced osteomalacia (TIO).

STUDY DESIGN

Retrospective case series.

SETTING

Single tertiary center.

METHODS

We studied the clinical characteristics and surgical outcomes of 43 patients (22 male, 21 female) who had lesions in the nasal cavity and paranasal sinus associated with TIO and underwent surgery between August 2006 and November 2019.

RESULTS

The mean ± SD duration between the onset of symptoms and surgery was 3.9 ± 2.6 years. The most common tumor site was the ethmoid sinus (76.7%), and the skull base was involved in 12 cases. Phosphaturic mesenchymal tumors were diagnosed in 41 patients, among whom there was 1 multifocal case. Another 2 cases involved odontogenic fibroma and hemangiofibroma, respectively. Serum phosphorus normalized in 39 cases within 4.4 ± 2.3 days, and serum fibroblastic growth factor 23 normalized within 1 day; clinical symptoms, however, gradually improved within several months after the first operation. There was no significant difference in the recovery rate between endoscopic and open surgery (P = 0.639). Two patients with recurrent cases and 2 with nonremission cases recovered after a sinonasal reoperation. The patient with a multifocal case recovered after the resection of the tumors in the ethmoid sinus and mandible. The overall recovery rate was 97.7%.

CONCLUSION

Most sinonasal tumors associated with TIO are located in the ethmoid sinus, and the skull base is involved in some cases. Complete excision of the tumor leads to recovery, and endoscopic surgery could achieve recovery rates similar to those of open surgery.

Reduction of Secreted Frizzled-Related Protein 5 Drives Vascular Calcification through Wnt3a-Mediated Rho/ROCK/JNK Signaling in Chronic Kidney Disease

Vascular calcification (VC) is commonly associated with bone loss in patients with chronic kidney disease (CKD). The Wingless-related integration site (Wnt) regulates osteoblast activation through canonical signaling pathways, but the common pathophysiology of these pathways during VC and bone loss has not been identified. A rat model of adenine-induced CKD with VC was used in this study. The rats were fed 0.75% adenine (2.5% protein, 0.92% phosphate) with or without intraperitoneal injection of calcitriol (0.08 µg/kg/day) for 4 weeks. Angiotensin II (3 µM)-induced VC was achieved in high phosphate medium (3 mM) through its effect on vascular smooth muscle cells (VSMCs). In an mRNA profiler polymerase chain reaction assay of the Wnt signaling pathway, secreted frizzled-related protein 5 (sFRP5) levels were significantly decreased in the CKD rat model compared with the control group. The repression of sFRP5 on VSMC trans-differentiation was mediated through Rho/Rho-associated coiled coil containing protein kinase (ROCK) and c-Jun N-terminal kinase (JNK) pathways activated by Wnt3a. In a proof of concept study conducted with patients with CKD, serum sFRP5 concentrations were significantly lower in subjects with VC than in those without VC. Our findings suggest that repression of sFRP5 is associated with VC in the CKD environment via activation of the noncanonical Wnt pathway, and thus that sFRP5 might be a novel therapeutic target for VC in CKD.

Frequent overexpression of klotho in fusion-negative phosphaturic mesenchymal tumors with tumorigenic implications

Phosphaturic mesenchymal tumors (PMT) are tumors that cause hypophosphatemia/osteomalacia chiefly by secreting FGF23. We have identified FN1-FGFR1/FGF1 fusion genes in nearly half of PMT, suggesting a central role of FGFR1 pathways in the pathogenesis of PMT. Tumorigenic drivers are unknown for tumors where previous study detected neither fusion, including many in bone, where FISH failed because of tissue decalcification. To identify alternative fusions in PMT without known fusions, as well as to validate the positive FISH results and characterize the fusion junctions, 34 PMT were studied, including 12 with known FN1-FGFR1 fusion by FISH (Group A), 2 with FN1-FGF1 (B), 12 with neither fusion (C), and 8 with previous acid-based decalcification and hence unknown fusion status (D). In total, 23 archival samples were subjected to anchored multiplex PCR-based RNA-sequencing (AMP-seq) with primers targeting FN1, genes encoding the FGF/FGFR families, and KL (α-Klotho); five Group C cases were also studied with whole-transcriptomic and exome-captured RNA sequencing, respectively. The AMP-seq results were consistent with previous FISH and/or transcriptomic sequencing data, except in one old Group A sample. One case had a novel FGFR1 exon 9 breakpoint, confirmed by genomic DNA sequencing. One Group D bone tumor was found to harbor FN1-FGF1. All 3 RNA-sequencing platforms failed to identify convincing fusion genes in Group C (N = 10), which instead expressed significantly higher levels of either KL or KLB. This result was further confirmed with KL and KLB RNA CISH semi-quantification (RNAscope). Our results demonstrated the utility of AMP-seq, which was compromised by decalcification and prolonged archiving. Of potential importance, fusion-negative PMT frequently overexpressed α-Klotho (or instead β-Klotho less commonly), whose role as an obligatory co-receptor for FGF23-FGFR1 binding suggests its aberrant expression in osteocytes/osteoblasts might result in an FGF23-FGFR1 autocrine loop that in turn drives the overexpression of FGF23 and tumorigenesis through activated FGFR pathways.

Hyperphosphatemia with low FGF7 and normal FGF23 and sFRP4 levels in the circulation characterizes pediatric hypophosphatasia

Hypophosphatasia (HPP) is the inborn-error-of-metabolism caused by loss-of-function mutation(s) of the ALPL gene that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). TNSALP in healthy individuals is on cell surfaces richly in bone, liver, and kidney. Thus, TNSALP natural substrates accumulate extracellularly in HPP, including inorganic pyrophosphate (PPi), a potent inhibitor of hydroxyapatite crystal formation and growth. Superabundance of extracellular PPi (ePPi) in HPP impairs mineralization of bones and teeth, often leading to rickets during childhood and osteomalacia in adult life and to tooth loss at any age. HPP's remarkably broad-ranging severity is largely explained by nearly four hundred typically missense mutations throughout the ALPL gene that are transmitted as an autosomal dominant or autosomal recessive trait. In the clinical laboratory, the biochemical hallmark of HPP is low serum ALP activity (hypophosphatasemia). However, our experience indicates that hyperphosphatemia from increased renal reclamation of filtered inorganic phosphate (Pi) is also common. Herein, from our prospective single-center study, we document throughout the clinical spectrum of non-lethal pediatric HPP that hyperphosphatemia reflects increased renal tubular threshold maximum for phosphorus adjusted for the glomerular filtration rate (TmP/GFR). To explore its pathogenesis, we studied mineral metabolism and quantitated circulating levels of three phosphatonins [fibroblast growth factor 23 (FGF23), secreted frizzled-related protein 4 (sFRP4), and fibroblast growth factor 7 (FGF7)] in 41 pediatric patients with HPP, 73 with X-linked hypophosphatemia (XLH), and 15 healthy pediatric control (CTR) subjects. The HPP and XLH cohorts had normal serum total and ionized calcium and parathyroid hormone levels (Ps > 0.10) and uncompromised glomerular filtration. In XLH, serum FGF23 was characteristically elevated (P < 0.0001) and despite hypophosphatemia sFRP4 was normal (P > 0.4) while FGF7 was low (P < 0.0001). In HPP, despite hyperphosphatemia serum FGF23 and sFRP4 were normal (Ps > 0.8) while FGF7 was low (P < 0.0001). Subsequently, in rats, we confirmed that FGF7 is phosphaturic. Thus, hyperphosphatemia in non-lethal pediatric HPP is associated with phosphatonin insufficiency together with, as we discuss, ePPi excess and diminished renal TNSALP activity.

Mechanisms of phosphate transport

Over the past 25 years, successive cloning of SLC34A1, SLC34A2 and SLC34A3, which encode the sodium-dependent inorganic phosphate (P_i) cotransport proteins 2a-2c, has facilitated the identification of molecular mechanisms that underlie the regulation of renal and intestinal P_i transport. P_i and various hormones, including parathyroid hormone and phosphatonins, such as fibroblast growth factor 23, regulate the activity of these P_i transporters through transcriptional, translational and post-translational mechanisms involving interactions with PDZ domain-containing proteins, lipid microdomains and acute trafficking of the transporters via endocytosis and exocytosis. In humans and rodents, mutations in any of the three transporters lead to dysregulation of epithelial P_i transport with effects on serum P_i levels and can cause cardiovascular and musculoskeletal damage, illustrating the importance of these transporters in the maintenance of local and systemic P_i homeostasis. Functional and structural studies have provided insights into the mechanism by which these proteins transport P_i, whereas in vivo and ex vivo cell culture studies have identified several small molecules that can modify their transport function. These small molecules represent potential new drugs to help maintain P_i homeostasis in patients with chronic kidney disease - a condition that is associated with hyperphosphataemia and severe cardiovascular and skeletal consequences.

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.

Tumor-induced osteomalacia: experience from three tertiary care centers in India

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome characterized by recalcitrant hypophosphatemia. Reports from the Indian subcontinent are scarce, with most being single center experiences involving few patients. Herein, we conducted a retrospective analysis of 30 patients of TIO diagnosed at three tertiary care hospitals in India. Patients with persistent hypophosphatemia (despite correction of hypovitaminosis D), normocalcemia, elevated alkaline phosphatase, low TmP/GFR and elevated or 'inappropriately normal' FGF23 levels were labeled as having TIO. They were sequentially subjected to functional followed by anatomical imaging. Patients with a well-localized tumor underwent excision; others were put on phosphorous and calcitriol supplementation. The mean age at presentation was 39.6 years with female:male ratio of 3:2. Bone pain (83.3%) and proximal myopathy (70%) were the chief complaints; 40% of cases had fractures. The mean delay in diagnosis was 3.8 years. Tumors were clinically detectable in four patients (13.3%). The mean serum phosphate was 0.50 mmol/L with a median serum FGF23 level of 518 RU/mL. Somatostatin receptor-based scintigraphy was found to be superior to FDG-PET in tumor localization. Lower extremities were the most common site of the tumor (72%). Tumor size was positively correlated with serum FGF23 levels. Twenty-two patients underwent tumor resection and 16 of them had phosphaturic mesenchymal tumors. Surgical excision led to cure in 72.7% of patients whereas disease persistence and disease recurrence were seen in 18.2% and 9.1% of cases, respectively. At the last follow-up, serum phosphate in the surgically treated group was significantly higher than in the medically managed group.

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.

Genetics of Refractory Rickets: Identification of Novel PHEX Mutations in Indian Patients and a Literature Update

Refractory rickets is a genetic disorder that cannot be treated by vitamin D supplementation and adequate dietary calcium and phosphorus. Hereditary hypophosphatemic rickets is one of the major forms of refractory rickets in Indian children and caused due to mutations in the PHEX , FGF23 , DMP1 , ENPP1 , and SLC34A3 genes. This is the first study in India on a large number of patients reporting on mutational screening of the PHEX gene. Direct sequencing in 37 patients with refractory rickets revealed eight mutations in 13 patients of which 1 was nonsense, 2 were deletions, 1 was a deletion-insertion, and 4 were missense mutations. Of these mutations, four (c.566_567 delAG, c.651_654delACAT, c.1337delinsAATAA, and c.2048T > A) were novel mutations. This article discusses the mutations in Indian patients, collates information on the genetic causes of refractory rickets, and emphasizes the significance of genetic testing for precise diagnosis, timely treatment, and management of the condition, especially in developing countries.

Secreted frizzled related protein 4 (sFRP4) update: A brief review

Secreted frizzled-related proteins control a multitude of biological phenomena throughout development and adult life in humans. In parallel, aberrant gene expression and abnormal secreted protein levels accompany a wide range of pathologies in humans. In this review, we provide a brief introduction to sFRP4, an update of the pathways it's involved, its various physiological actions that are reported to contribute to diseases, outlining the importance of its wider research and specific modulation by pharmacologic interventions. First recognized as a novel molecule that co-purified with a disparate protein, its identity was based on its sequence homology to the frizzled receptors. Once multiple members of the family were cloned, their genetic loci, tissue and subcellular distributions were located. Nucleotide and amino acid sequences were characterized and homology to different organisms was found to be present that helped elucidate their actions. Following subsequent experimental studies, they were found to be secreted proteins with an affinity to bind to the Wnt ligands, participating in different developmental and adult homeostatic pathways by the virtue of their regulatory function to the Wnt signal transduction system. Secreted frizzled related protein 4 has garnered considerable attention in the recent years following breakthrough discoveries implicating them in the pathogenesis of various diseases. Studies investigating them can provide information not only regarding their association with a disease but can also help use them as potential biomarkers and therapeutic targets.

Role of the Wnt/β-Catenin Pathway in Renal Osteodystrophy

Vascular calcification and bone fragility are common and interrelated health problems that affect chronic kidney disease (CKD) patients. Bone fragility, which leads to higher risk of fracture and mortality, arises from the abnormal bone remodeling and mineralization that are seen in chronic kidney disease. Recently, sclerostin and Dickkopf-related protein 1 were suggested to play a significant role in CKD-related bone disease as they are known inhibitors of the Wnt pathway, thus preventing bone formation. This review focuses on new knowledge about the Wnt pathway in bone, how its function is affected by chronic kidney disease and how this affects bone structure. Expression of components and inhibitors of the Wnt pathway has been shown to be affected by the loss of kidney function, and a better understanding of the bone effects of Wnt pathway inhibitors could allow the development of new therapies to prevent bone fragility in this population.

The Foot That Broke Both Hips: A Case Report and Literature Review of Tumor-Induced Osteomalacia

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome characterized by hypophosphatemia and clinical symptoms of osteomalacia. Only discussed as case reports, there is still limited knowledge of this condition as a potentially curable cause of osteomalacia among clinicians and pathologists. In this article, we present a case of tumor-induced osteomalacia in a 59-year-old gentleman followed by an up-to-date review of the existing literature on TIO.

Tumor-induced Osteomalacia: A Sherlock Holmes Approach to Diagnosis and Management

Tumor-induced osteomalacia (TIO) is a subtype of paraneoplastic syndrome associated with hypophosphatemia due to renal phosphate wasting in adults. The humoral factor responsible for clinical picture known as fibroblast growth factor 23 (FGF23) is most often secreted by benign yet elusive mesenchymal tumors, difficult to localize, access, and excise completely; rarely, they are multiple and malignant. Paradoxical inappropriately normal or low levels of 1,25-dihydroxyvitamin D in the setting of hypophosphatemia is due to suppressive effect of FGF23. The following case report describes a 31-year-old male with symptoms of multiple fractures and severe muscle weakness, hypophosphatemia with elevated tubular maximum reabsorption of phosphate/glomerular filtration rate with low active Vitamin D, prompted assay for C-terminal FGF23, which was elevated multifold. The tumor was localized with whole body 68-Gadolinium DOTANOC positron emission tomography-computed tomography fusion scan in the left nasal cavity with ipsilateral maxillary antrum. It was excised through transnasal approach and found to be mesenchymal tumor on histopathology. At 1 week of follow-up, serum phosphate became normalized without supplementation. The patient is in follow-up for further measurement of FGF23 level and signs of recurrence. Because the occurrence of such a condition is rare and most often misdiagnosed or mismanaged for years, it is important to recognize this condition in differential diagnosis as potential curative surgical option is a reality.

Tumour-induced osteomalacia

Tumour-induced osteomalacia (TIO), also known as oncogenic osteomalacia, is a rare paraneoplastic disorder caused by tumours that secrete fibroblast growth factor 23 (FGF23). Owing to the role of FGF23 in renal phosphate handling and vitamin D synthesis, TIO is characterized by decreased renal tubular reabsorption of phosphate, by hypophosphataemia and by low levels of active vitamin D. Chronic hypophosphataemia ultimately results in osteomalacia (that is, inadequate bone mineralization). The diagnosis of TIO is usually suspected when serum phosphate levels are chronically low in the setting of bone pain, fragility fractures and muscle weakness. Locating the offending tumour can be very difficult, as the tumour is often very small and can be anywhere in the body. Surgical removal of the tumour is the only definitive treatment. When the tumour cannot be located or when complete resection is not possible, medical treatment with phosphate salts or active vitamin D is necessary. One of the most promising emerging treatments for unresectable tumours that cause TIO is the anti-FGF23 monoclonal antibody KRN23. The recent identification of a fusion of fibronectin and fibroblast growth factor receptor 1 (FGFR1) as a molecular driver in some tumours not only sheds light on the pathophysiology of TIO but also opens the door to a better understanding of the transcription, translocation, post-translational modification and secretion of FGF23, as well as suggesting approaches to targeted therapy. Further study will reveal if the FGFR1 pathway is also involved in tumours that do not harbour the translocation.

Clinical, cellular, microscopic, and ultrastructural studies of a case of fibrogenesis imperfecta ossium

Fibrogenesis imperfecta ossium is a rare disorder of bone usually characterized by marked osteopenia and associated with variable osteoporosis and osteosclerosis, changing over time. Histological examination shows that newly formed collagen is abnormal, lacking birefringence when examined by polarized light. The case presented demonstrates these features and, in addition, a previously undocumented finding of a persistent marked reduction of the serum C3 and C4. Osteoblasts established in culture from a bone biopsy showed abnormal morphology on electron microscopy and increased proliferation when cultured with benzoylbenzoyl-ATP and 1,25-dihydroxyvitamin D, contrasting with findings in normal osteoblasts in culture. A gene microarray study showed marked upregulation of the messenger RNA (mRNA) for G-protein-coupled receptor 128 (GPR 128), an orphan receptor of unknown function and also of osteoprotegerin in the patient's osteoblasts in culture. When normal osteoblasts were cultured with the patient's serum, there was marked upregulation of the mRNA for aquaporin 1. A single pathogenetic factor to account for the features of this disorder has not been defined, but the unique findings described here may facilitate more definitive investigation of the abnormal bone cell function.

sFRP4 signalling of apoptosis and angiostasis uses nitric oxide-cGMP-permeability axis of endothelium

Nitric oxide (NO) plays a critical role in endothelial functions such as cellular migration, vascular permeability and angiogenesis. Angiogenesis, the formation of new blood vessels from "pre-existing" ones is a carefully regulated process and essential during reproduction, development and wound healing. Previously our lab group reported that Secreted Frizzled-Related Protein 4 (sFRP4) could inhibit angiogenesis in both in vitro and in vivo conditions. sFRP4 belongs to a family of secreted glycoproteins that function as antagonists of the canonical Wnt signalling pathway. Although the pro-apoptotic role of sFRP4 is well discussed in literature, little is known in regards to its anti-angiogenic property. The objective of this study was to elucidate sFRP4 implications in NO biology of the endothelium. Results demonstrate that sFRP4 causes endothelial dysfunction by suppressing NO-cGMP signaling and elevating corresponding ROS levels. The imbalance between NO and ROS levels results in apoptosis and subsequent leakiness of endothelium as confirmed in vivo (Texas red/Annxin - CAM assay) and in vitro (Monolayer permeability assay) conditions. Furthermore utilizing peptides synthesized from the CRD domain of sFRP4, our results showed that while these peptides were able to cause endothelial dysfunctions, they did not cause apoptosis of the endothelial cells. Thereby confirming that sFRP4 can mediate its anti-angiogenic effect independent of its pro-apoptotic property. In conclusion, the current study reports that sFRP4-mediated anti-angiogenesis occurs as a result of impaired NO-cGMP signaling which in turn allow for elevation of redox levels and promotion of apoptosis of endothelial cells.

Characterization of FN1-FGFR1 and novel FN1-FGF1 fusion genes in a large series of phosphaturic mesenchymal tumors

Phosphaturic mesenchymal tumors typically cause paraneoplastic osteomalacia, chiefly as a result of FGF23 secretion. In a prior study, we identified FN1-FGFR1 fusion in 9 of 15 phosphaturic mesenchymal tumors. In this study, a total of 66 phosphaturic mesenchymal tumors and 7 tumors resembling phosphaturic mesenchymal tumor but without known phosphaturia were studied. A novel FN1-FGF1 fusion gene was identified in two cases without FN1-FGFR1 fusion by RNA sequencing and cross-validated with direct sequencing and western blot. Fluorescence in situ hybridization analyses revealed FN1-FGFR1 fusion in 16 of 39 (41%) phosphaturic mesenchymal tumors and identified an additional case with FN1-FGF1 fusion. The two fusion genes were mutually exclusive. Combined with previous data, the overall prevalence of FN1-FGFR1 and FN1-FGF1 fusions was 42% (21/50) and 6% (3/50), respectively. FGFR1 immunohistochemistry was positive in 82% (45/55) of phosphaturic mesenchymal tumors regardless of fusion status. By contrast, 121 cases of potential morphologic mimics (belonging to 13 tumor types) rarely expressed FGFR1, the main exceptions being solitary fibrous tumors (positive in 40%), chondroblastomas (40%), and giant cell tumors of bone (38%), suggesting a possible role for FGFR1 immunohistochemistry in the diagnosis of phosphaturic mesenchymal tumor. With the exception of one case reported in our prior study, none of the remaining tumors resembling phosphaturic mesenchymal tumor had either fusion type or expressed significant FGFR1. Our findings provide insight into possible mechanisms underlying the pathogenesis of phosphaturic mesenchymal tumor and imply a central role of the FGF1-FGFR1 signaling pathway. The novel FN1-FGF1 protein is expected to be secreted and serves as a ligand that binds and activates FGFR1 to achieve an autocrine loop. Further study is required to determine the functions of these fusion proteins.

Immunohistochemical and molecular detection of the expression of FGF23 in phosphaturic mesenchymal tumors including the non-phosphaturic variant

Background

Phosphaturic mesenchymal tumors (PMTs) are rare neoplasms that are often associated with tumor-induced osteomalacia (TIO) due to excessive serum levels of fibroblast growth factor 23 (FGF23). PMTs share overlapping histologic features with other types of tumors; thus, accurate pathological diagnosis may be challenging. We performed an immunohistochemical examination of FGF23 expression in PMTs and other types of tumors, together with pertinent molecular analyses.

Methods

Seven PMTs (5 with TIO and 2 without TIO) and 46 other types of bone and soft tissue tumors were retrieved, and immunohistochemistry was performed using a commercially available anti-FGF23 antibody. In addition, FGF23 mRNA expression was detected by reverse transcription-polymerase chain reaction (RT-PCR), using RNA extracted from formalin-fixed, paraffin-embedded tissues.

Results

Immunohistochemical analysis of FGF23 expression showed distinct, punctate staining in the cytoplasm in 5 PMTs with TIO, whereas FGF23 expression was negative in the 2 PMTs without TIO and the other 46 tumors. FGF23 mRNA expression was detected in all 4 PMTs examined, as well as in 1 chondromyxoid fibroma and 1 myxoid liposarcoma. The real-time RT-PCR data showed that the relative expression levels of the FGF23 mRNA tended to be higher in PMTs with TIO than in PMTs without TIO, or in the chondromyxoid fibroma specimen.

Conclusions

Our data suggested that the feasibility of immunohistochemical detection of FGF23 may depend on the level of secreted FGF23 from tumor cells. Thus, immunohistochemistry for FGF23 is an useful diagnostic adjunct for PMT, although its utility appears to be limited in cases without TIO.

Recombinant sFRP4 bound chitosan–alginate composite nanoparticles embedded with silver nanoclusters for Wnt/β-catenin targeting in cancer theranostics

Targeting a specific pathway aberrantly upregulated in cancer cells has shown immense potential in cancer therapy.

Surgical treatment of tumor-induced osteomalacia: a retrospective review of 40 cases with extremity tumors

Background

Tumor-induced osteomalacia (TIO) is a rare syndrome typically caused by mesenchymal tumors. It has been shown that complete tumor resection may be curative. However, to our knowledge, there has been no report of a large cohort to exam different surgical approaches. This study was aimed to assess outcomes of different surgical options of patients with tumor-induced osteomalacia at a single institution.

Methods

Patients with extremity tumors treated in our hospital from January, 2004 to July, 2012 were identified. The minimum follow-up period was 12 months. Patient’s demography, tumor location, preoperative preparation, type of surgeries were summarized, and clinical outcomes were recorded. Successful treatment was defined as significant symptom improvement, normal serum phosphorus and significant improvement or normalization of bone mineral density at the last follow-up. Differences between patients with soft tissue tumors and bone tumors were compared.

Results

There were 40 (24 male and 16 female) patients identified, with an average age of 44 years. The tumors were isolated in either soft tissue (25 patients) or bone (12 patients) and combined soft tissue and bone invasion was observed in 3 patients. For the primary surgery, tumor resection and tumor curettage were performed. After initial surgical treatment, six patients then received a second surgery. Four patients were found to have malignant tumors base on histopathology. With a minimum follow-up period of 12 months, 80% of patients (32/40) were treated successfully, including 50% of patients (2/4) with malignant tumors. Compared to patients with bone tumor, surgical results were better in patient with soft tissue tumor.

Conclusions

Surgical treatment was an effective way for TIO. Other than tumor curettage surgery, tumor resection is the preferred options for these tumors.

A novel chromogenic in situ hybridization assay for FGF23 mRNA in phosphaturic mesenchymal tumors

Phosphaturic mesenchymal tumors of the mixed connective tissue type (PMT) are very rare tumors of bone and soft tissues. Most patients with PMT have long-standing osteomalacia secondary to production of fibroblast growth factor 23 (FGF23), a hormone that inhibits phosphate reuptake within the renal proximal tubule. Previously, we have reported the detection of FGF23 mRNA in PMT by reverse transcription polymerase chain reaction (PCR); however, the low specificity and risk for nontumoral tissue contamination inherent in PCR-based methodology limit its clinical utility. We evaluated RNAscope as a semiquantitative method of in situ FGF23 mRNA detection in the diagnosis of PMT. Twenty-five PMTs (median 52 y, range 5 to 73 y) occurred in patients with tumor-induced osteomalacia (TIO), manifesting as masses (mean 3.9 cm, range 1.4 to 12 cm) in various bones and soft tissues. FGF23 mRNA was positive in 96% (22/23) informative cases of PMT: 16 cases scored 3+; 5 scored as 2+; 1 scored as 1+. Among these cases, FGF23 mRNA was detected in 3 malignant PMTs along with their metastases. Forty control cases included aneurysmal bone cyst (N=4), chondromyxoid fibroma (N=8), high-grade osteosarcomas (N=8), and (nonfamilial) tumoral calcinosis, as well as miscellaneous cartilage-forming tumors or osteoid-forming tumors and soft tissue tumors. All control cases were negative for FGF23 mRNA in the lesional cells. One aneurysmal bone cyst had rare FGF23 mRNA-expressing osteocytes clustered around remodeled bone. One ovarian serous carcinoma in a patient with disseminated disease, elevated serum FGF23, and TIO was negative for FGF23 mRNA in the neoplastic cells. We conclude that RNAscope is a highly sensitive and specific, semiquantitative in situ hybridization method of FGF23 mRNA detection applicable to formalin-fixed, paraffin-embedded tissues. Detection of FGF23 expression is a valuable diagnostic adjunct, especially in patients with occult TIO. Compared with reverse transcription PCR, this method preserves tissue morphology and reduces "false positives" related to detection of endogenous FGF23 mRNA expression by osteocytes.

Association of secreted frizzled-related protein 4 (SFRP4) with type 2 diabetes in patients with stable coronary artery disease

Background

Secreted frizzled-related proteins (SFRP) are regulators of Wnt-signalling. SFRP4 has been shown to regulate insulin exocytosis and is overexpressed in type 2 diabetes mellitus.

Here we characterized the relation of SFRP4 to glucose and triglyceride metabolism and outcomes in patients with stable coronary artery disease on statin treatment in the prospective Homburg Cream & Sugar Study (NCT00628524).

Methods

Fasting SFRP4 concentrations were measured by ELISA in 504 consecutive patients with stable CAD confirmed by angiography.

Results

The median age was 68 years and 83% of patients were male. Oral glucose tolerance tests were performed in all patients without known diabetes for metabolic characterization. 24.4% of patients showed normal glucose tolerance, 29.4% impaired glucose tolerance and 46.2% diabetes mellitus. SFRP4 concentrations correlated with insulin (R = 0.153, p = 0.001), HbA1c (R = 0.166, p < 0.0001), fasting triglycerides (R = 0.113, p = 0.011) and higher triglycerides after lipid challenge (postprandial triglycerides R = 0.124, p = 0.005; AUC R = 0.134, p = 0.003). Higher SFRP4 concentrations were associated with type 2 diabetes, metabolic syndrome, and severity of diabetes.

The primary outcome was the composite of cardiovascular death and cardiovascular hospitalization within 48 months follow-up. Comparison of event-free survival between SFRP4 tertiles showed that SFRP4 concentrations were not predictive for cardiovascular outcome.

Conclusions

SFRP4 concentrations are associated with impaired glucose and triglyceride metabolism but do not predict cardiovascular outcome in patients with stable coronary artery disease on treatment.

Computer aided screening of secreted frizzled-related protein 4 (SFRP4): a potential control for diabetes mellitus

Diabetes mellitus is a life threatening disease and scientists are doing their best to find a cost effective and permanent treatment of this malady. The recent trend is to control the disease by target base inhibiting of enzymes or proteins. Secreted frizzled-related protein 4 (SFRP4) is found to cause five times more risk of diabetes when expressed above average levels. This study was therefore designed to analyze the SFRP4 and to find its potential inhibitors. SFRP4 was analyzed by bio-informatics tools of sequence tool and structure tool. A total of three potential inhibitors of SFRP4 were found, namely cyclothiazide, clopamide and perindopril. These inhibitors showed significant interactions with SFRP4 as compared to other inhibitors as well as control (acetohexamide). The findings suggest the possible treatment of diabetes mellitus type 2 by inhibiting the SFRP4 using the inhibitors cyclothiazide, clopamide and perindopril.

Diagnostic Modalities for FGF23-Producing Tumors in Patients with Tumor-Induced Osteomalacia

Fibroblast growth factor 23 (FGF23) is a hormone that is produced by osteocytes and regulates phosphate and vitamin D metabolism through binding to the Klotho-FGF receptor complex. Excessive actions of FGF23 cause several kinds of hypophosphatemic rickets/osteomalacia. Tumor-induced rickets/osteomalacia (TIO) is a paraneoplastic syndrome caused by overproduction of FGF23 from the responsible tumors. Because TIO is cured by complete resection of the causative tumors, it is of great clinical importance to locate these tumors. Several imaging methods including skeletal survey by magnetic resonance imaging and octreotide scintigraphy have been used to identify the tumors that cause TIO. However, none of these imaging studies indicate that the detected tumors are actually producing FGF23. Recently, systemic venous sampling was conducted for locating FGF23-producing tumor in suspected patients with TIO and demonstrated that this test might be beneficial to a subset of patient. Further studies with more patients are necessary to establish the clinical utility of venous sampling in patients with TIO.

Multilineage somatic activating mutations in HRAS and NRAS cause mosaic cutaneous and skeletal lesions, elevated FGF23 and hypophosphatemia

Pathologically elevated serum levels of fibroblast growth factor-23 (FGF23), a bone-derived hormone that regulates phosphorus homeostasis, result in renal phosphate wasting and lead to rickets or osteomalacia. Rarely, elevated serum FGF23 levels are found in association with mosaic cutaneous disorders that affect large proportions of the skin and appear in patterns corresponding to the migration of ectodermal progenitors. The cause and source of elevated serum FGF23 is unknown. In those conditions, such as epidermal and large congenital melanocytic nevi, skin lesions are variably associated with other abnormalities in the eye, brain and vasculature. The wide distribution of involved tissues and the appearance of multiple segmental skin and bone lesions suggest that these conditions result from early embryonic somatic mutations. We report five such cases with elevated serum FGF23 and bone lesions, four with large epidermal nevi and one with a giant congenital melanocytic nevus. Exome sequencing of blood and affected skin tissue identified somatic activating mutations of HRAS or NRAS in each case without recurrent secondary mutation, and we further found that the same mutation is present in dysplastic bone. Our finding of somatic activating RAS mutation in bone, the endogenous source of FGF23, provides the first evidence that elevated serum FGF23 levels, hypophosphatemia and osteomalacia are associated with pathologic Ras activation and may provide insight in the heretofore limited understanding of the regulation of FGF23.

Renal phosphate handling: Physiology

Phosphorus is a common anion. It plays an important role in energy generation. Renal phosphate handling is regulated by three organs parathyroid, kidney and bone through feedback loops. These counter regulatory loops also regulate intestinal absorption and thus maintain serum phosphorus concentration in physiologic range. The parathyroid hormone, vitamin D, Fibrogenic growth factor 23 (FGF23) and klotho coreceptor are the key regulators of phosphorus balance in body.

WNT signaling in bone homeostasis and disease: from human mutations to treatments

Low bone mass and strength lead to fragility fractures, for example, in elderly individuals affected by osteoporosis or children with osteogenesis imperfecta. A decade ago, rare human mutations affecting bone negatively (osteoporosis-pseudoglioma syndrome) or positively (high-bone mass phenotype, sclerosteosis and Van Buchem disease) have been identified and found to all reside in components of the canonical WNT signaling machinery. Mouse genetics confirmed the importance of canonical Wnt signaling in the regulation of bone homeostasis, with activation of the pathway leading to increased, and inhibition leading to decreased, bone mass and strength. The importance of WNT signaling for bone has also been highlighted since then in the general population in numerous genome-wide association studies. The pathway is now the target for therapeutic intervention to restore bone strength in millions of patients at risk for fracture. This paper reviews our current understanding of the mechanisms by which WNT signalng regulates bone homeostasis.

Secreted frizzled-related protein 4 reduces insulin secretion and is overexpressed in type 2 diabetes

A plethora of candidate genes have been identified for complex polygenic disorders, but the underlying disease mechanisms remain largely unknown. We explored the pathophysiology of type 2 diabetes (T2D) by analyzing global gene expression in human pancreatic islets. A group of coexpressed genes (module), enriched for interleukin-1-related genes, was associated with T2D and reduced insulin secretion. One of the module genes that was highly overexpressed in islets from T2D patients is SFRP4, which encodes secreted frizzled-related protein 4. SFRP4 expression correlated with inflammatory markers, and its release from islets was stimulated by interleukin-1β. Elevated systemic SFRP4 caused reduced glucose tolerance through decreased islet expression of Ca(2+) channels and suppressed insulin exocytosis. SFRP4 thus provides a link between islet inflammation and impaired insulin secretion. Moreover, the protein was increased in serum from T2D patients several years before the diagnosis, suggesting that SFRP4 could be a potential biomarker for islet dysfunction in T2D.

Vitamin D and the kidney

The kidney is essential for the maintenance of normal calcium and phosphorus homeostasis. Calcium and inorganic phosphorus are filtered at the glomerulus, and are reabsorbed from tubular segments by transporters and channels which are regulated by 1α,25-dihydroxyvitamin (1α,25(OH)(2)D) and parathyroid hormone (PTH). The kidney is the major site of the synthesis of 1α,25(OH)(2)D under physiologic conditions, and is one of the sites of 24,25-dihydroxyvitamin D (24,25(OH)(2)D) synthesis. The activity of the 25(OH)D-1α-hydroxylase, the mixed function oxidase responsible for the synthesis of 1α,25(OH)(2)D, is regulated by PTH, 1α,25(OH)(2)D, fibroblast growth factor 23 (FGF23), inorganic phosphorus and other growth factors. Additionally, the vitamin D receptor which binds to, and mediates the activity of 1α,25(OH)(2)D, is widely distributed in the kidney. Thus, the kidney, by regulating multiple transport and synthetic processes is indispensible in the maintenance of mineral homeostasis in physiological states.

FGF23 and syndromes of abnormal renal phosphate handling

Fibroblast growth factor 23 (FGF23) is part of a previously unrecognized hormonal bone-parathyroid-kidney axis, which is modulated by 1,25(OH)(2)-vitamin D (1,25(OH)(2)D), dietary and circulating phosphate and possibly PTH. FGF23 was discovered as the humoral factor in tumors that causes hypophosphatemia and osteomalacia and through the identification of a mutant form of FGF23 that leads to autosomal dominant hypophosphatemic rickets (ADHR), a rare genetic disorder. FGF23 appears to be mainly secreted by osteocytes where its expression is up-regulated by 1,25(OH)(2)D and probably by increased serum phosphate levels. Its synthesis and secretion is reduced through yet unknown mechanisms that involve the phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX), dentin matrix protein 1 (DMP1) and ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1). Consequently, loss-of-function mutations in these genes underlie hypophosphatemic disorders that are either X-linked or autosomal recessive. Impaired O-glycosylation of FGF23 due to the lack of UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyl-transferase 3 (GALNT3) or due to certain homozygous FGF23 mutations results in reduced secretion of intact FGF23 and leads to familial hyperphosphatemic tumoral calcinosis. FGF23 acts through FGF-receptors and the coreceptor Klotho to reduce 1,25(OH)(2)D synthesis in the kidney and probably the synthesis of parathyroid hormone (PTH) by the parathyroid glands. It furthermore synergizes with PTH to increase renal phosphate excretion by reducing expression of the sodium-phosphate cotransporters NaPi-IIa and NaPi-IIc in the proximal tubules. Loss-of-function mutations in these two transporters lead to autosomal recessive Fanconi syndrome or to hereditary hypophosphatemic rickets with hypercalciuria, respectively.

Matrix extracellular phosphoglycoprotein is expressed in causative tumors of oncogenic osteomalacia

Oncogenic osteomalacia (OOM), or tumor-induced osteomalacia, is a rare disease characterized by renal phosphate wasting and osteomalacia. It arises due to the secretion of fibroblast growth factor 23 (FGF-23) from causative tumors. Matrix extracellular phosphoglycoprotein (MEPE) is predominantly expressed in odontoblasts, osteoblasts, and osteocytes. Although the presence of MEPE mRNA has been reported in some OOM tumors, little is known about the prevalence of MEPE expression in OOM tumors. In this study, the expression of MEPE and FGF-23 in OOM tumors was investigated at the transcriptional and translational levels. Eleven causative OOM tumors were analyzed by quantitative real-time reverse transcription-polymerase chain reaction and immunohistochemistry for MEPE and FGF-23 expression. Hemangiopericytomas and giant cell tumors, pathological diagnoses that are common in cases of OOM, were obtained from non-osteomalacic patients and analyzed as controls. The gene expression level of FGF23 and MEPE in OOM tumors was 10(4)- and 10(5)-times higher, respectively, than in non-OOM tumors. Immunohistochemical staining revealed that FGF-23 protein was expressed in all OOM tumors, and MEPE was expressed in 10 out of 11 OOM tumors. Thus, MEPE expression was common in OOM tumors, similar to FGF-23. These results indicate that, in addition to the hypophosphatemic effects of FGF-23, MEPE or the MEPE-derived acidic serine aspartate-rich MEPE-associated motif peptide may contribute to decreased bone mineralization in OOM patients.

Evidence for FGF23 involvement in a bone-kidney axis regulating bone mineralization and systemic phosphate and vitamin D homeostasis

Bone is involved in the maintenance of phosphate and vitamin D homeostasis via its production and secretion of FGF23 and serves as a reservoir for the storage and release of calcium and phosphate into the circulation. Alterations in mineralization of extracellular matrix and the remodeling activities of the skeleton are coupled to the kidney conservation of phosphate and production of 1,25(OH)2D via the regulation of FGF23 production by osteocytes through yet-to-be defined locally derived factors. In addition, FGF23 production is regulated by 1,25(OH)2D in a feedback loop where FGF23 stimulate Cyp24 mediated degradation of 1,25(OH)2D that serves to protect the organism from the toxic effects of vitamin D excess. In this chapter, we will review the regulation and function of FGF23.