Molecular genetic and biochemical analyses of FGF23 mutations in familial tumoral calcinosis

Epithelial-like transport of mineral distinguishes bone formation from other connective tissues

We review unique properties of bone formation including current understanding of mechanisms of bone mineral transport. We focus on formation only; mechanism of bone degradation is a separate topic not considered. Bone matrix is compared to other connective tissues composed mainly of the same proteins, but without the specialized mechanism for continuous transport and deposition of mineral. Indeed other connective tissues add mechanisms to prevent mineral formation. We start with the epithelial-like surfaces that mediate transport of phosphate to be incorporated into hydroxyapatite in bone, or in its ancestral tissue, the tooth. These include several phosphate producing or phosphate transport-related proteins with special expression in large quantities in bone, particularly in the bone-surface osteoblasts. In all connective tissues including bone, the proteins that constitute the protein matrix are mainly type I collagen and γ-carboxylate-containing small proteins in similar molar quantities to collagen. Specialized proteins that regulate connective tissue structure and formation are surprisingly similar in mineralized and non-mineralized tissues. While serum calcium and phosphate are adequate to precipitate mineral, specialized mechanisms normally prevent mineral formation except in bone, where continuous transport and deposition of mineral occurs.

The Successful Treatment of Deep Soft-tissue Calcifications with Topical Sodium Thiosulphate and Acetazolamide in a Boy with Hyperphosphatemic Familial Tumoral Calcinosis due to a Novel Mutation in FGF23

Hyperphosphatemic familial tumoral calcinosis (HFTC) is a rare autosomal recessive disorder. Topical sodium thiosulfate (STS) and acetazolamide can be a safe and effective treatment for patients who do not respond to conventional therapy for ectopic calcifications. We report the successful treatment of deep soft-tissue calcifications with topical STS and acetazolamide in a boy diagnosed with HFTC due to a novel homozygous mutation of FGF23.

A novel FGF23 mutation in hyperphosphatemic familial tumoral calcinosis and its deleterious effect on protein O-glycosylation

BACKGROUND

Hyperphosphatemic familial tumoral calcinosis (HFTC) is a rare disease characterized by hyperphosphatemia and ectopic calcification, predominantly at periarticular locations. This study was performed to characterize the clinical profile of tumoral calcinosis and to identify gene mutations associated with HFTC and elucidated its pathogenic role.

METHODS

The three subjects (two male and one female) were aged 30, 25 and 15 years, respectively. The clinical features, histopathological findings, and outcomes of three subjects with HFTC were retrospectively reviewed. The three subjects were analyzed for FGF23, GALNT3 and KL mutations. Function of mutant gene was analyzed by western blotting and wheat germ agglutinin affinity chromatography.

RESULTS

All subjects had hyperphosphatemia and elevated calcium-phosphorus product. Calcinosis positions included the left shoulder, left index finger, and right hip. Bone and joint damage were present in two cases and multiple foci influenced body growth in one case. The histopathological features were firm, rubbery masses comprising multiple nodules of calcified material bordered by the proliferation of mononuclear or multinuclear macrophages, osteoclastic-like giant cells, fibroblasts, and chronic inflammatory cells. The novel mutation c.484A>G (p.N162D) in exon 3 of FGF23 was identified in one subject and his family members. Measurement of circulating FGF23 in the subject confirmed low intact FGF23 and increased C-terminal fragment. In vitro experiments showed that the mutant FGF23 proteins had defective O-glycosylation and impaired protein proteolysis protection.

CONCLUSION

We identified a novel FGF23 missense mutation, and confirmed its damaging role in FGF23 protein O-glycosylation. Our findings expand the current spectrum of FGF23 variations that influence phosphorus metabolism.

Identification of a second Klotho interaction site in the C terminus of FGF23

FGF23 interacts with a FGFR/KL-receptor complex to propagate cellular signaling, where its C-terminal C26 peptide is critical for engaging the co-receptor KL. We identify a distinct peptide sequence C28 residing in the FGF23 C terminus that regulates its interaction with KL. C28 can independently function as an FGF23 antagonist, and we report an optimized peptide antagonist of much enhanced potency. FGF23 can use either of the two C-terminal sites to exert biological effects, as shown by in vitro and in vivo studies. The loss of both KL-interaction sites inactivates the protein. We conclude that the C terminus of FGF23 is a bidentate ligand possessing two independent KL-interaction sites. The identification of this second KL-association site provides an additional perspective in the molecular basis of FGF23-receptor signaling and raises questions pertaining to its structural mechanism of action and the potential for biased biological signaling.

Genetic renal disease classification by hormonal axes

The kidneys, which regulate many homeostatic pathways, are also a major endocrinological target organ. Many genetic renal diseases can be classified according to the affected protein along such endocrinological pathways. In this review, we examine the hypothesis that a more severe phenotype is expected as the affected protein is located more distally along such pathways. Thus, the location of a defect along its endocrinological pathway should be taken into consideration, in addition to the mutation type, when assessing genetic renal disease severity.

Defective O-glycosylation of novel FGF23 mutations in a Chinese family with hyperphosphatemic familial tumoral calcinosis

OBJECTIVES

Hyperphosphatemic familial tumoral calcinosis/hyperostosis-hyperphosphatemia syndrome (HFTC/HHS) is a rare disorder caused by deficiency or resistance of fibroblast growth factor 23 (FGF23). Here we reported a Chinese family with HFTC/HHS, aiming at clarifying the clinical features, bone microarchitectures and molecular mechanisms of the disease.

METHODS

Clinical manifestations, laboratory examinations and genetic analyses were collected from two HFTC patients. Bone microarchitectures were detected by HR-pQCT. In vitro expression and glycosylation of mutant and wild-type FGF23 proteins were analyzed by western blotting and wheat germ agglutinin affinity chromatography. Subcellular localizations of FGF23 proteins were detected by immunocytochemistry.

RESULTS

The two brothers carried previously unreported c.413T > G, p.Leu138Arg and c.491T > A, p.Ile164Asn compound heterozygous variants in the FGF23 gene, which was "likely pathogenic" according to American College of Medical Genetics (ACMG) Standards and Guidelines. Both patients had severe hyperphosphatemia and significantly elevated C-terminal FGF23. With HHS, patient 1 presented with lower extremity pain and widespread cardiovascular calcification. HR-pQCT of his distal radius and tibia revealed decreased volume BMD and cortical thickness, which were inconsistent with hyperostosis manifestations in X-ray. He received etidronate treatment, which improved his BMD and the ectopic calcification. His brother exhibited less bone involvement but had experienced recurrent painful calcified mass from a young age and undergone several resections. In vitro experiments showed that the mutant FGF23 proteins had defective O-glycosylation and impaired secretion. However, no difference in subcellular localization was found between the wild-type and mutant FGF23 proteins.

CONCLUSION

We have presented a Chinese HFTC/HHS family with novel FGF23 c.413T > G, p.Leu138Arg and c.491T > A, p.Ile164Asn variants. We clarified the bone microarchitectures of HFTC/HHS patients by HR-pQCT, and expanded the genotype-phenotype spectrum of the disease. In vivo studies suggested that O-glycosylation of FGF23 plays an important role in the pathogenesis of HFTC/HHS, providing further understanding of the disease mechanism.

Regulation of fibroblast growth factor 23 (FGF23) in health and disease

Fibroblast growth factor 23 (FGF23) is mainly produced in the bone and, upon secretion, forms a complex with a FGF receptor and coreceptor αKlotho. FGF23 can exert several endocrine functions, such as inhibiting renal phosphate reabsorption and 1,25-dihydroxyvitamin D3 production. Moreover, it has paracrine activities on several cell types, including neutrophils and hepatocytes. Klotho and Fgf23 deficiencies result in pathologies otherwise encountered in age-associated diseases, mainly as a result of hyperphosphataemia-dependent calcification. FGF23 levels are also perturbed in the plasma of patients with several disorders, including kidney or cardiovascular diseases. Here, we review mechanisms controlling FGF23 production and discuss how FGF23 regulation is perturbed in disease.

A Novel Heterozygous Deletion Variant in KLOTHO Gene Leading to Haploinsufficiency and Impairment of Fibroblast Growth Factor 23 Signaling Pathway

Hyperphosphatemia is commonly present in end-stage renal disease. Klotho (KL) is implicated in phosphate homeostasis since it acts as obligate co-receptor for the fibroblast growth factor 23 (FGF23), a major phosphaturic hormone. We hypothesized that genetic variation in the KL gene might be associated with alterations in phosphate homeostasis resulting in hyperphosphatemia. We performed sequencing for determining KL gene variants in a group of resistant hyperphosphatemic dialysis patients. In a 67-year-old female, blood DNA sequencing revealed a heterozygous deletion of a T at position 1041 (c.1041delT) in exon 2. This variation caused a frameshift with substitution of isoleucine for phenylalanine and introduction of a premature termination codon (p.Ile348Phefs*28). cDNA sequencing showed absence of deletion-carrier transcripts in peripheral blood mononuclear cells suggesting degradation of these through a nonsense-mediated RNA decay pathway. Experiments in vitro showed that p.Ile348Phefs*28 variant impaired FGF23 signaling pathway, indicating a functional inactivation of the gene. In the patient, serum levels of KL were 2.9-fold lower than the mean level of a group of matched dialysis subjects, suggesting a compromise in the circulating protein concentration due to haploinsufficiency. These findings provide a new loss-of-function variant in the human KL gene, suggesting that genetic determinants might be associated to clinical resistant hyperphosphatemia.

Hyperphosphatemic familial tumoral calcinosis secondary to fibroblast growth factor 23 (FGF23) mutation: a report of two affected families and review of the literature

Hyperphosphatemic familial tumoral calcinosis (HFTC), secondary to fibroblast growth factor 23 (FGF23) gene mutation, is a rare genetic disorder characterized by recurrent calcified masses. We describe young Lebanese cousins presenting with HFTC, based on a retrospective chart review and a prospective case study. In addition, we present a comprehensive review on the topic, based on a literature search conducted in PubMed and Google Scholar, in 2014 and updated in December 2017. While the patients had the same previously reported FGF23 gene mutation (homozygous c.G367T variant in exon 3 leading to a missense mutation), they presented with variable severity and age of disease onset (at 4 years in patient 1 and at 23 years in patient 2). A review of the literature revealed several potential patho-physiologic pathways of HFTC clinical manifestations, some of which may be independent of hyperphosphatemia. Most available treatment options aim at reducing serum phosphate level, by stimulating renal excretion or by inhibiting intestinal absorption. HFTC is a challenging disease. While the available medical treatment has a limited and inconsistent effect on disease symptomatology, surgical resection of calcified masses remains the last resort. Research is needed to determine the safety and efficacy of FGF23 replacement or molecular therapy, targeting the specific genetic aberration. Hyperphosphatemic familial tumoral calcinosis is a rare genetic disorder characterized by recurrent calcified masses, in addition to other visceral, skeletal, and vascular manifestations. It remains a very challenging disease.

Development and Validation of a Simple Diagnostic Method to Detect Gain and Loss of Function Defects in Fibroblast Growth Factor-23

BACKGROUND

Fibroblast growth factor-23 (FGF23) is a bone-derived hormone that regulates the homeostasis of phosphate and vitamin D. Three substitutions in the hormone are reported to cause autosomal dominant hypophosphatemic rickets and seven substitutions to cause autosomal recessive hyperphosphatemic familial tumoral calcinosis (HFTC). Both disorders are rare in the general population and occur most often in the Eastern Mediterranean region and Africa. None of the mutations could be identified using standard restriction fragment length polymorphism. The only technique currently available to confirm the clinical diagnosis is DNA sequencing.

METHODS

Using a tri-primer ARMS-PCR, in vitro site-directed mutagenesis and DNA sequencing, we developed, verified and validated a rapid and reliable diagnostic test for the ten mutations in FGF23.

RESULTS

We generated a test for all ten mutations and confirmed each test by DNA sequencing. We increased the specificity of the test by introducing a mismatch at position -2 in the 3'-terminus of the reverse primer of the normal and the mutant sequences. Finally, using DNA sequencing, we validated the technique for FGF23/S129F substitution by testing samples from 80 individuals from two unrelated Arab families harboring HFTC.

CONCLUSIONS

This inexpensive and specific method could be adopted where DNA sequencing is not available or affordable.

FGF23-S129F mutant bypasses ER/Golgi to the circulation of hyperphosphatemic familial tumoral calcinosis patients

FGF23 is essential for the homeostasis of phosphate, and vitamin D. Loss-of-function mutations in this hormone cause hyperphosphatemic familial tumoral calcinosis (HFTC). Earlier reports suggested that intact FGF23 from loss of function mutants such as FGF23/S129F (iFGF23/S129F) is retained intracellularly while the carboxy-terminal fragment is secreted. We sought to investigate the fate of iFGF23/S129F mutant hormone in vivo and in vitro. Five patients clinically diagnosed with HFTC and confirmed by DNA sequencing to carry the c.386 C>T; p.S129F mutation in the homozygous state were studied. Healthy and heterozygous individuals were used as controls in the study. Using ELISA assays, we showed that iFGF23/S129F was 2-5 folds higher in patients' plasma, compared to heterozygous or healthy controls. Importantly, the mutant hormone could not be detected in the patients' sera. However, using proteinase inhibition profiling, we found that a serum metalloproteinase degraded the iFGF23/S129F explaining our failure to detect it in sera. The serum metalloproteinase degrades the WT and the mutant at different rates. Also, confocal microscopy imaging using wild-type (WT) FGF23 or FGF23/S129F mutant in transiently transfected HEK293 and HeLa cells showed weak staining of the Golgi complex with some vesicular staining resembling the ER. Additionally, FGF23 variants (FGF23/WT, FGF23/S129F, FGF23/S71G, and FGF23/R176Q) from stably transfected HEK293 cells secreted high levels into a serum-free medium that can be detected by ELISA and Western blot. Our results suggest that iFGF23/S129F mutant bypasses the ER/Golgi quality control system to the circulation of HFTC patients by an unknown pathway. Finally, we hypothesize that either the mutant hormone is unable to bind α-Klotho-FGFR1c, or it binds the dyad receptor with low affinity and, therefore, incapable of initiating maximal intracellular signaling. Our findings raise the potential use of the WT hormone in therapies of some HFTC patients.

Conditional Deletion of Murine Fgf23: Interruption of the Normal Skeletal Responses to Phosphate Challenge and Rescue of Genetic Hypophosphatemia

The transgenic and knockout (KO) animals involving Fgf23 have been highly informative in defining novel aspects of mineral metabolism, but are limited by shortened lifespan, inability of spatial/temporal FGF23 control, and infertility of the global KO. To more finely test the role of systemic and genetic influences in FGF23 production, a mouse was developed that carried a floxed ("f")-Fgf23 allele (exon 2 floxed) which demonstrated in vivo recombination when bred to global-Cre transgenic mice (eIIa-cre). Mice homozygous for the recombined allele ("Δ") had undetectable serum intact FGF23, elevated serum phosphate (p < 0.05), and increased kidney Cyp27b1 mRNA (p < 0.05), similar to global Fgf23-KO mice. To isolate cellular FGF23 responses during phosphate challenge, Fgf23(Δ/f) mice were mated with early osteoblast type Iα1 collagen 2.3-kb promoter-cre mice (Col2.3-cre) and the late osteoblast/early osteocyte Dentin matrix protein-1-cre (Dmp1-cre). Fgf23(Δ/f) /Col2.3-cre(+) and Fgf23(Δ/f) /Dmp1-cre(+) exhibited reduced baseline serum intact FGF23 versus controls. After challenge with high-phosphate diet Cre(-) mice had 2.1-fold to 2.5-fold increased serum FGF23 (p < 0.01), but Col2.3-cre(+) mice had no significant increase, and Dmp1-cre(+) mice had only a 37% increase (p < 0.01) despite prevailing hyperphosphatemia in both models. The Fgf23(Δ/f) /Col2.3-cre was bred onto the Hyp (murine X-linked hypophosphatemia [XLH] model) genetic background to test the contribution of osteoblasts and osteocytes to elevated FGF23 and Hyp disease phenotypes. Whereas Hyp mice maintained inappropriately elevated FGF23 considering their marked hypophosphatemia, Hyp/Fgf23(Δ/f) /Col2.3-cre(+) mice had serum FGF23 <4% of Hyp (p < 0.01), and this targeted restriction normalized serum phosphorus and ricketic bone disease. In summary, deleting FGF23 within early osteoblasts and osteocytes demonstrated that both cell types contribute to baseline circulating FGF23 concentrations, and that targeting osteoblasts/osteocytes for FGF23 production can modify systemic responses to changes in serum phosphate concentrations and rescue the Hyp genetic syndrome. © 2016 American Society for Bone and Mineral Research.

Combined sequence and sequence-structure based methods for analyzing FGF23, CYP24A1 and VDR genes

FGF23, CYP24A1 and VDR altogether play a significant role in genetic susceptibility to chronic kidney disease (CKD). Identification of possible causative mutations may serve as therapeutic targets and diagnostic markers for CKD. Thus, we adopted both sequence and sequence-structure based SNP analysis algorithm in order to overcome the limitations of both methods. We explore the functional significance towards the prediction of risky SNPs associated with CKD. We assessed the performance of four widely used pathogenicity prediction methods. We compared the performances of the programs using Mathews correlation Coefficient ranged from poor (MCC = 0.39) to reasonably good (MCC = 0.42). However, we got the best results for the combined sequence and structure based analysis method (MCC = 0.45). 4 SNPs from FGF23 gene, 8 SNPs from VDR gene and 13 SNPs from CYP24A1 gene were predicted to be the causative agents for human diseases. This study will be helpful in selecting potential SNPs for experimental study from the SNP pool and also will reduce the cost for identification of potential SNPs as a genetic marker.

The Fibroblast Growth Factor signaling pathway

The signaling component of the mammalian Fibroblast Growth Factor (FGF) family is comprised of eighteen secreted proteins that interact with four signaling tyrosine kinase FGF receptors (FGFRs). Interaction of FGF ligands with their signaling receptors is regulated by protein or proteoglycan cofactors and by extracellular binding proteins. Activated FGFRs phosphorylate specific tyrosine residues that mediate interaction with cytosolic adaptor proteins and the RAS-MAPK, PI3K-AKT, PLCγ, and STAT intracellular signaling pathways. Four structurally related intracellular non-signaling FGFs interact with and regulate the family of voltage gated sodium channels. Members of the FGF family function in the earliest stages of embryonic development and during organogenesis to maintain progenitor cells and mediate their growth, differentiation, survival, and patterning. FGFs also have roles in adult tissues where they mediate metabolic functions, tissue repair, and regeneration, often by reactivating developmental signaling pathways. Consistent with the presence of FGFs in almost all tissues and organs, aberrant activity of the pathway is associated with developmental defects that disrupt organogenesis, impair the response to injury, and result in metabolic disorders, and cancer. For further resources related to this article, please visit the WIREs website.

FGF23 is endogenously phosphorylated in bone cells

Levels of serum phosphate are controlled by the peptide hormone FGF23, secreted from bone osteocytes. Elevated levels of circulating FGF23 are a key factor in several hypophosphatemic disorders and play a role in chronic kidney disease. Posttranslational processing of FGF23 includes multi-site O-glycosylation, which reduces intracellular cleavage by proprotein convertases. The FGF23 protein also contains four serine phosphorylation consensus sequences (S-X-D/E); in this work, we asked whether FGF23 is a substrate for secretory phosphorylation. Both HEK cells as well as IDG-SW3 cells, an osteocyte model, incorporated radiolabeled orthophosphate into intact FGF23, as well as into the 14-kDa carboxy-terminal-but not the 17-kDa N-terminal-fragment. Sequential serine-to-alanine site-directed mutagenesis of four kinase consensus sites showed that labeling occurred on three serines within the carboxy-terminal fragment, Ser180 (adjacent to the cleavage site), Ser207, and Ser212. Liquid chromatography-coupled mass spectroscopy indicated the presence of phosphate at Ser212 in recombinant R&D mouse FGF23(R179Q) , confirming labeling results. A phosphopeptide-specific antibody was raised against phospho-Ser212 and exhibited immunoreactivity in osteocytes present in mouse long bone, providing further evidence that FGF23 is naturally phosphorylated in bone. Bone SIBLING proteins are serine-phosphorylated by the ubiquitous Golgi secretory kinase FAM20C. Cotransfection of HEK and MC3T3 cells with FGF23 and active, but not inactive, FAM20C kinase increased the storage and release of FGF23 in radiolabeling experiments, indicating potential effects of phosphorylation on FGF23 stability. Collectively, these data point to an important role for phosphorylation of FGF23 in bone.

Endocrine FGFs: Evolution, Physiology, Pathophysiology, and Pharmacotherapy

The human fibroblast growth factor (FGF) family comprises 22 structurally related polypeptides that play crucial roles in neuronal functions, development, and metabolism. FGFs are classified as intracrine, paracrine, and endocrine FGFs based on their action mechanisms. Paracrine and endocrine FGFs are secreted signaling molecules by acting via cell-surface FGF receptors (FGFRs). Paracrine FGFs require heparan sulfate as a cofactor for FGFRs. In contrast, endocrine FGFs, comprising FGF19, FGF21, and FGF23, require α-Klotho or β-Klotho as a cofactor for FGFRs. Endocrine FGFs, which are specific to vertebrates, lost heparan sulfate-binding affinity and acquired a systemic signaling system with α-Klotho or β-Klotho during early vertebrate evolution. The phenotypes of endocrine FGF knockout mice indicate that they play roles in metabolism including bile acid, energy, and phosphate/active vitamin D metabolism. Accumulated evidence for the involvement of endocrine FGFs in human genetic and metabolic diseases also indicates their pathophysiological roles in metabolic diseases, potential risk factors for metabolic diseases, and useful biomarkers for metabolic diseases. The therapeutic utility of endocrine FGFs is currently being developed. These findings provide new insights into the physiological and pathophysiological roles of endocrine FGFs and potential diagnostic and therapeutic strategies for metabolic diseases.

Long-term clinical outcome and phenotypic variability in hyperphosphatemic familial tumoral calcinosis and hyperphosphatemic hyperostosis syndrome caused by a novel GALNT3 mutation; case report and review of the literature

BACKGROUND

Hyperphosphatemic Familial Tumoral Calcinosis (HFTC) and Hyperphosphatemic Hyperostosis Syndrome (HHS) are associated with autosomal recessive mutations in three different genes, FGF23, GALNT3 and KL, leading to reduced levels of fibroblast growth factor 23 (FGF23) and subsequent clinical effects.

RESULTS

We describe a consanguineous family with two affected siblings with HFTC and HHS caused by a novel homozygous G-to T substitution in exon 3 of GALNT3 (c.767 G > T; p.Gly256Val), demonstrating great phenotypic variation and long asymptomatic intervals. Calcific tumors appeared at 14 years of age in the male, and the female displayed episodic diaphysitis from age 9 years. Symptoms of eye involvement were present in both from childhood, and progressed into band keratopathy in the female. Abnormal dental roots and tooth loss, as well as myalgia were present in both from their mid-twenties, while the female also had calcifications in the placenta, the iliac vessels and thyroid cartilage. New calcific tumors appeared more than 20 years after the initial episodes, delaying diagnosis and treatment until the ages of 37 and 50 years, respectively. Both siblings had elevated serum phosphate levels, inappropriately elevated tubular maximum phosphate reabsorption per unit glomerular filtration rate (TmP/GFR), reduced levels of intact FGF23 and increased levels of c-terminal FGF23. Review of all 54 previously published cases of GALNT3, FGF23, and KL associated HFTC and HHS demonstrated that more subjects than previously recognized have a combined phenotype.

CONCLUSION

We have described HFTC and HHS in a consanguineous Caucasian family with a novel GALNT3 mutation, demonstrating new phenotypic features and significant variability in the natural course of the disease. A review of the literature, show that more subjects than previously recognized have a combined phenotype of HFTC and HHS. HHS and HFTC are two distinct phenotypes in a spectrum of GALNT3 mutation related calcification disorders, where the additional factors determining the phenotypic expression, are yet to be clarified.

A new missense mutation in FGF23 gene in a male with hyperostosis-hyperphosphatemia syndrome (HHS)

Hyperostosis-hyperphosphataemia syndrome (HHS) is a rare autosomal recessive metabolic disorder, characterized by recurrent painful swelling of long bones, periosteal new bone formation and cortical hyperostosis or intramedullary sclerosis, hyperphosphatemia and low intact fibroblast growth factor 23 (FGF23) protein levels. It is caused by mutations in 2 genes, N-acetylgalactosaminyltransferase 3 (GalNAc-transferase; GALNT3) and FGF23. We have performed mutation analysis of the GALNT3 and FGF23 genes in a patient with HHS and detected a homozygous mutation in exon 3 of FGF23 gene (NM_020638.2: c.471C>A) which results in amino acid change from phenylalanine 157 to leucin (p.F157L) in receptor interaction site.

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.

Hexa-D-arginine treatment increases 7B2•PC2 activity in hyp-mouse osteoblasts and rescues the HYP phenotype

Inactivating mutations of the "phosphate regulating gene with homologies to endopeptidases on the X chromosome" (PHEX/Phex) underlie disease in patients with X-linked hypophosphatemia (XLH) and the hyp-mouse, a murine homologue of the human disorder. Although increased serum fibroblast growth factor 23 (FGF-23) underlies the HYP phenotype, the mechanism(s) by which PHEX mutations inhibit FGF-23 degradation and/or enhance production remains unknown. Here we show that treatment of wild-type mice with the proprotein convertase (PC) inhibitor, decanoyl-Arg-Val-Lys-Arg-chloromethyl ketone (Dec), increases serum FGF-23 and produces the HYP phenotype. Because PC2 is uniquely colocalized with PHEX in osteoblasts/bone, we examined if PC2 regulates PHEX-dependent FGF-23 cleavage and production. Transfection of murine osteoblasts with PC2 and its chaperone protein 7B2 cleaved FGF-23, whereas Signe1 (7B2) RNA interference (RNAi) transfection, which limited 7B2 protein production, decreased FGF-23 degradation and increased Fgf-23 mRNA and protein. The mechanism by which decreased 7B2•PC2 activity influences Fgf-23 mRNA was linked to reduced conversion of the precursor to bone morphogenetic protein 1 (proBMP1) to active BMP1, which resulted in limited cleavage of dentin matrix acidic phosphoprotein 1 (DMP1), and consequent increased Fgf-23 mRNA. The significance of decreased 7B2•PC2 activity in XLH was confirmed by studies of hyp-mouse bone, which revealed significantly decreased Sgne1 (7B2) mRNA and 7B2 protein, and limited cleavage of proPC2 to active PC2. The expected downstream effects of these changes included decreased FGF-23 cleavage and increased FGF-23 synthesis, secondary to decreased BMP1-mediated degradation of DMP1. Subsequent Hexa-D-Arginine treatment of hyp-mice enhanced bone 7B2•PC2 activity, normalized FGF-23 degradation and production, and rescued the HYP phenotype. These data suggest that decreased PHEX-dependent 7B2•PC2 activity is central to the pathogenesis of XLH.

Rationale-Based Engineering of a Potent Long-Acting FGF21 Analog for the Treatment of Type 2 Diabetes

Fibroblast growth factor 21 (FGF21) is a promising drug candidate for the treatment of type 2 diabetes. However, the use of wild type native FGF21 is challenging due to several limitations. Among these are its short half-life, its susceptibility to in vivo proteolytic degradation and its propensity to in vitro aggregation. We here describe a rationale-based protein engineering approach to generate a potent long-acting FGF21 analog with improved resistance to proteolysis and aggregation. A recombinant Fc-FGF21 fusion protein was constructed by fusing the Fc domain of human IgG1 to the N-terminus of human mature FGF21 via a linker peptide. The Fc positioned at the N-terminus was determined to be superior to the C-terminus as the N-terminal Fc fusion retained the βKlotho binding affinity and the in vitro and in vivo potency similar to native FGF21. Two specific point mutations were introduced into FGF21. The leucine to arginine substitution at position 98 (L98R) suppressed FGF21 aggregation at high concentrations and elevated temperatures. The proline to glycine replacement at position 171 (P171G) eliminated a site-specific proteolytic cleavage of FGF21 identified in mice and cynomolgus monkeys. The derived Fc-FGF21(RG) molecule demonstrated a significantly improved circulating half-life while maintaining the in vitro activity similar to that of wild type protein. The half-life of Fc-FGF21(RG) was 11 h in mice and 30 h in monkeys as compared to 1-2 h for native FGF21 or Fc-FGF21 wild type. A single administration of Fc-FGF21(RG) in diabetic mice resulted in a sustained reduction in blood glucose levels and body weight gains up to 5-7 days, whereas the efficacy of FGF21 or Fc-FGF21 lasted only for 1 day. In summary, we engineered a potent and efficacious long-acting FGF21 analog with a favorable pharmaceutical property for potential clinical development.

Vitamin D: health panacea or false prophet?

Vitamin D deficiency, diagnosed when the serum 25-hydroxyvitamin D (25-OHD(3)) concentration is less than 20 ng/mL, has joined vitamin A deficiency as two of the most common nutrition-responsive medical conditions worldwide. There have been more scientific articles published about vitamin D in the 21st century than about any other vitamin, reflecting the massive expansion of the field of vitamin D research. Adequate vitamin D status has been linked to decreased risks of developing specific cancers, including cancers of the esophagus, stomach, colon, rectum, gallbladder, pancreas, lung, breast, uterus, ovary, prostate, urinary bladder, kidney, skin, thyroid, and hematopoietic system (e.g., Hodgkin's lymphoma, non-Hodgkin's lymphoma, multiple myeloma); bacterial infections; rheumatoid arthritis; Crohn's disease; periodontal disease; multiple sclerosis; asthma; type 2 diabetes; cardiovascular disease; stroke; peripheral artery disease; hypertension; chronic kidney disease; muscle weakness; cognitive impairment; Alzheimer's disease; clinical depression; and premature death. On the other hand, inadequate vitamin D status during human pregnancy may be associated with increased risk for the development of type 1 diabetes in the offspring. However, this point of view may be excessively optimistic. There also is evidence that despite the current heavy reliance on serum 25-OHD(3) concentration for the diagnosis of an individual's vitamin D status, local tissue vitamin D intoxication may be present in individuals with much lower serum 25-OHD(3) concentrations than are currently appreciated. Only rarely are the symptoms of local tissue vitamin D intoxication associated with vitamin D status or intake. An individual's serum 25-OHD(3) concentration may appear to be "low" for reasons totally independent of sunlight exposure or vitamin D intake. Serum 25-OHD(3) concentration is only poorly responsive to increases in vitamin D intake, and the prolonged routine consumption of thousands of international units of vitamin D may interfere with the regulation of phosphate homeostasis by fibroblast growth factor-23 (FGF23) and the Klotho gene product, with consequences that are detrimental to human health. In light of these counterbalancing observations, curbing excessive enthusiasm for universally increasing vitamin D intake recommendations may be in order.

Extraskeletal cartilaginous, osseous, and chordoid tumors in children and adolescents

Extraskeletal cartilaginous, osseous, and chordoid tumors are extraordinarily rare in children and adolescents. These lesions are diagnostically challenging due to their rarity and their overlap with metastatic osteosarcoma, reactive or metabolic calcifying and bone-forming masses, various pseudosarcomatous proliferations such as myositis ossificans, and other rare genetic or metabolic disorders. This article reviews the clinicopathologic features and differential diagnosis of extraskeletal cartilaginous, osseous, and chordoid neoplasms in the first two decades of life and highlights the use of diagnostic adjuncts.

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.

Hyperphosphatemic tumoral calcinosis: a 10-year follow-up

OBJECTIVE

To report a case of primary hyperphosphatemic tumoral calcinosis (TC) and its long-term 10-year follow-up.

PATIENT

The patient was an 18-year-old male, who had been diagnosed with TC at the age of 8 years. In spite of nine surgeries for tumoral resection and medical treatments (i.e., aluminum hydroxide, non-steroidal anti-inflammatory agents) the lesions continued to progress. Physical examination showed calcified masses on shoulders, hip, elbows and right foot. PTH, calcitonin, 25(OH) vitamin D, 1,25(OH)2 vitamin D, renal and liver function, electrolytes, alkaline phosphatase, calcium and magnesium were normal. Serum phosphorus was elevated. FGF-23 (C-terminal): 1960 RU/mL (<180) Radiological and histological studies were compatible with TC.

CONCLUSION

This long-term follow-up illustrates the morbidity and difficulty in treating these patients due to the progressive and recurrent nature of the calcified masses, for which there is no effective treatment as yet. The discovery of FGF-23 as the factor responsible for the hyperphosphatemic type of TC paves the way for forthcoming therapies.

Regulation of phosphate homeostasis by PTH, vitamin D, and FGF23

In contrast to the regulation of calcium homeostasis, which has been extensively studied over the past several decades, relatively little is known about the regulation of phosphate homeostasis. Fibroblast growth factor 23 (FGF23) is part of a previously unrecognized hormonal bone-parathyroid-kidney axis, which is modulated by PTH, 1,25(OH)(2)-vitamin D (1,25(OH)(2)D), dietary and serum phosphorus levels. Synthesis and secretion of FGF23 by osteocytes are positively regulated by 1,25(OH)(2)D and serum phosphorus and negatively regulated, through yet unknown mechanisms, by the phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) and by dentin matrix protein 1 (DMP1). In turn, FGF23 inhibits the synthesis of 1,25(OH)(2)D, and it may negatively regulate the secretion of parathyroid hormone (PTH) from the parathyroid glands. However, FGF23 synergizes with PTH to increase renal phosphate excretion by reducing expression of the renal sodium-phosphate cotransporters NaPi-IIa and NaPi-IIc in the proximal tubules. Most insights gained into the regulation of phosphate homeostasis by these factors are derived from human genetic disorders and genetically engineered mice, which are reviewed in this paper.

Clinical variability of familial tumoral calcinosis caused by novel GALNT3 mutations

The GALNT3 gene encodes GalNAc-T3, which prevents degradation of the phosphaturic hormone, fibroblast growth factor 23 (FGF23). Biallelic mutations in either GALNT3 or FGF23 result in hyperphosphatemic familial tumoral calcinosis or its variant, hyperostosis-hyperphosphatemia syndrome. Tumoral calcinosis is characterized by the presence of ectopic calcifications around major joints, whereas hyperostosis-hyperphosphatemia syndrome is characterized by recurrent long bone lesions with hyperostosis. Here we investigated four patients with hyperphosphatemia and clinical manifestations including tumoral calcinosis and/or hyperostosis-hyperphosphatemia syndrome to determine underlying genetic cause and delineate phenotypic heterogeneity of these disorders. Mutational analysis of FGF23 and GALNT3 in these patients revealed novel homozygous mutations in GALNT3. Although the presence of massive calcifications, cortical hyperostosis, or dental anomalies was not shared by all patients, all had persistent hyperphosphatemia. Three of the patients also had inappropriately normal 1,25-dihyroxyvitamin D [1,25(OH)(2)D] and confirmed low circulating intact FGF23 concentrations. The four novel GALNT3 mutations invariably resulted in hyperphosphatemia as a result of low intact FGF23, but other clinical manifestations were variable. Therefore, tumoral calcinosis and hyperostosis-hyperphosphatemia syndrome represent a continuous spectrum of the same disease caused by increased phosphate levels, rather than two distinct disorders.

Familial tumoral calcinosis: from characterization of a rare phenotype to the pathogenesis of ectopic calcification

Familial tumoral calcinosis (FTC) refers to a heterogeneous group of inherited disorders characterized by the occurrence of cutaneous and subcutaneous calcified masses. Two major forms of the disease are now recognized. Hyperphosphatemic FTC has been shown to result from mutations in three genes: fibroblast growth factor-23 (FGF23), coding for a potent phosphaturic protein, KL encoding Klotho, which serves as a co-receptor for FGF23, and GALNT3, which encodes a glycosyltransferase responsible for FGF23 O-glycosylation; defective function of any one of these three proteins results in hyperphosphatemia and ectopic calcification. The second form of the disease is characterized by absence of metabolic abnormalities, and is, therefore, termed normophosphatemic FTC. This variant was found to be associated with absence of functional SAMD9, a putative tumor suppressor and anti-inflammatory protein. The data gathered through the study of these rare disorders have recently led to the discovery of novel aspects of the pathogenesis of common disorders in humans, underscoring the potential concealed within the study of rare diseases.

Isolated C-terminal tail of FGF23 alleviates hypophosphatemia by inhibiting FGF23-FGFR-Klotho complex formation

Fibroblast growth factor (FGF) 23 inhibits renal phosphate reabsorption by activating FGF receptor (FGFR) 1c in a Klotho-dependent fashion. The phosphaturic activity of FGF23 is abrogated by proteolytic cleavage at the RXXR motif that lies at the boundary between the FGF core homology domain and the 72-residue-long C-terminal tail of FGF23. Here, we show that the soluble ectodomains of FGFR1c and Klotho are sufficient to form a ternary complex with FGF23 in vitro. The C-terminal tail of FGF23 mediates binding of FGF23 to a de novo site generated at the composite FGFR1c-Klotho interface. Consistent with this finding, the isolated 72-residue-long C-terminal tail of FGF23 impairs FGF23 signaling by competing with full-length ligand for binding to the binary FGFR-Klotho complex. Injection of the FGF23 C-terminal tail peptide into healthy rats inhibits renal phosphate excretion and induces hyperphosphatemia. In a mouse model of renal phosphate wasting attributable to high FGF23, the FGF23 C-terminal peptide reduces phosphate excretion, leading to an increase in serum phosphate concentration. Our data indicate that proteolytic cleavage at the RXXR motif abrogates FGF23 activity by a dual mechanism: by removing the binding site for the binary FGFR-Klotho complex that resides in the C-terminal region of FGF23, and by generating an endogenous inhibitor of FGF23. We propose that peptides derived from the C-terminal tail of FGF23 or peptidomimetics and small-molecule organomimetics of the C-terminal tail can be used as therapeutics to treat renal phosphate wasting.

Familial tumoral calcinosis and hyperostosis-hyperphosphataemia syndrome are different manifestations of the same disease: novel missense mutations in GALNT3

OBJECTIVE

To report on the biochemistry and clinical and genetic findings of two siblings, the younger sister presenting with recurrent bone pain of the radius and ulna, and medullary sclerosis, and the older brother with soft tissue calcific deposits (tumoral calcinosis) but who later developed bone pain. Both were found to be hyperphosphaturic.

MATERIALS AND METHODS

The index family comprised four individuals (father, mother, brother, sister). The affected siblings were the offspring of a non-consanguineous Indian family of Tamil origin. Bidirectional sequencing was performed on the DNA from the index family and on 160 alleles from a population of 80 unrelated unaffected control individuals of Tamil extraction and 72 alleles from individuals of non-Tamil origin.

RESULTS

Two symptomatic siblings were found to harbour previously unreported compound heterozygous missense UDP-N-acetyl-D-galactosamine: polypeptide N-acetylgalactosaminyltransferase 3 (GalNAc-transferase; GALNT3) mutations in exon 4 c.842A>G and exon 5 c.1097T>G. This sequence variation was not detected in the control DNA. This is the first report of siblings exhibiting stigmata of familial tumoral calcinosis and hyperostosis-hyperphosphataemia syndrome with documented evidence of autosomal recessive missense GALNT3 mutations.

CONCLUSION

The findings from this family add further evidence to the literature that familial tumoral calcinosis and hyperostosis-hyperphosphataemia syndrome are manifestations of the same disease and highlight the importance of appropriate metabolic and genetic investigations.

Molecular pathology of the fibroblast growth factor family

The human fibroblast growth factor (FGF) family contains 22 proteins that regulate a plethora of physiological processes in both developing and adult organism. The mutations in the FGF genes were not known to play role in human disease until the year 2000, when mutations in FGF23 were found to cause hypophosphatemic rickets. Nine years later, seven FGFs have been associated with human disorders. These include FGF3 in Michel aplasia; FGF8 in cleft lip/palate and in hypogonadotropic hypogonadism; FGF9 in carcinoma; FGF10 in the lacrimal/salivary glands aplasia, and lacrimo-auriculo-dento-digital syndrome; FGF14 in spinocerebellar ataxia; FGF20 in Parkinson disease; and FGF23 in tumoral calcinosis and hypophosphatemic rickets. The heterogeneity in the functional consequences of FGF mutations, the modes of inheritance, pattern of involved tissues/organs, and effects in different developmental stages provide fascinating insights into the physiology of the FGF signaling system. We review the current knowledge about the molecular pathology of the FGF family.

Defective O-glycosylation due to a novel homozygous S129P mutation is associated with lack of fibroblast growth factor 23 secretion and tumoral calcinosis

BACKGROUND

Homozygous mutations in fibroblast growth factor (FGF23) have recently been described as the genetic cause of one form of hyperphosphatemic tumoral calcinosis (HFTC). However, it remained unclear to date how these mutations lead to loss of biologically active FGF23 in the circulation.

METHODS

We here report a novel homozygous mutation, c.385T>C in FGF23 exon 2, which changes codon 129 from serine to proline (S129P) in a previously described individual affected by HFTC. The S129P mutation as well as two known FGF23 mutations, S71G and S129F, were introduced into an expression vector encoding wild-type (wt) human (h) FGF23 to yield [P129]hFGF23, [F129]hFGF23, and [G71]hFGF23; whole lysates, glycoprotein fractions, and conditioned media from HEK293 and COS-7 cells expressing these constructs were subjected to Western blot analysis using affinity-purified goat anti-hFGF23(51-69) and anti-hFGF23(206-222) antibodies.

RESULTS

We detected 25- and 32-kDa protein species in total lysates of HEK293 cells expressing wt-hFGF23. The 32-kDa band, representing O-glycosylated hFGF23, was not detectable in the glycoprotein fraction of lysates from HEK293 cells expressing [P129]hFGF23, and in comparison with wt-FGF23 only small amounts of [P129]hFGF23 were secreted into the medium. Similar results were obtained for cells expressing [G71]hFGF23 and [F129]hFGF23.

CONCLUSION

Our data for the first time directly show that FGF23 mutations associated with HFTC impair O-glycosylation in vitro resulting in poor secretion of the mutant hormone thereby explaining the characteristic hyperphosphatemic phenotype of homozygous carriers in vivo.

[Reduced bone density and bone pain :osteomalacia with hypophospatemia and hypophosphaturia]

Two patients aged 24 and 64 years presented at our hospital with similar symptoms including bone pain and muscle weakness. Basic laboratory tests and urinary diagnostics, bone densitometry and bone histology revealed severe osteomalacia with renal phosphate wasting. After the exclusion of other causes an extensive tumor search was performed due to suspected tumor-induced osteomalacia. In one patient a mesenchymal tumor was found in the thigh and completely resected. After surgery the patient showed a rapid recovery from osteomalacia. Because the search was unsuccessful in the other patient phosphorus supplementation in combination with calcitriol was started. Despite continuing renal phosphate wasting a significant increase in bone mineral density was observed.

Familial tumoral calcinosis and the role of O-glycosylation in the maintenance of phosphate homeostasis

Familial tumoral calcinosis refers to a group of disorders inherited in an autosomal recessive fashion. Hyperphosphatemic tumoral calcinosis is characterized by increased re-absorption of phosphate through the renal proximal tubule, resulting in elevated phosphate concentration and deposition of calcified deposits in cutaneous and subcutaneous tissues, as well as, occasionally, in visceral organs. The disease was found to result from mutations in at least 3 genes: GALNT3, encoding a glycosyltransferase termed ppGalNacT3, FGF23 encoding a potent phosphaturic protein, and KL encoding Klotho. Recent data showed that ppGalNacT3 mediates O-glycosylation of FGF23, thereby allowing for its secretion and possibly protecting it from proteolysis-mediated inactivation. Klotho was found to serve as a co-receptor for FGF23, thereby integrating the genetic data into a single physiological system. The elucidation of the molecular basis of HFTC shed new light upon the mechanisms regulating phosphate homeostasis, suggesting innovative therapeutic strategies for the management of hyperphosphatemia in common acquired conditions such as chronic renal failure.

Hypophosphatemia: the common denominator of all rickets

Rickets is a disease of the hypertrophic chondrocytes in the growth plate and is caused by hypophosphatemia-a derived defect in terminal chondrocyte apoptosis. This highlights the critical role of phosphorous in cartilage and bone metabolism. This review shows the role of phosphorous metabolism, transport and function in maintaining phosphorous supply to the growth plate, bone osteoblast and the kidney. Given that phosphorous is the common denominator of all rickets, this review proposes a new classification for the differential diagnosis of rickets, which is based on the mechanisms leading to hypophosphatemia-high PTH activity, high FGF23 activity or renal phosphaturia.