Phosphate Reabsorption Research Articles

P49 An unusual cause of transient migratory bone oedema: fibroblast growth factor-23 secreting a mesenchymal tumour

Abstract Background Fibroblast growth factor-23 (FGF-23) is a phosphate regulator primarily expressed by osteocytes. Excess FGF-23 leads to decreased hydroxylation of 25-hydroxyvitamin D and poor renal phosphate reabsorption. This leads to hypophosphatemia and represents a rare cause of osteomalacia, resulting in bone oedema and stress fractures. Methods A 57-year-old man with known skin psoriasis presented with a two-year history of left foot and ankle pain. On examination, he had chronic dactylitis of the big toe and extra-articular features included skin psoriasis on the scalp, elbows, knees, and nail pitting. Serum inflammatory markers were normal, and autoimmune screens for RF, anti-CCP and HLA-B27 were negative. Bilateral foot and ankle X-rays showed no bony abnormality. This was diagnosed as likely psoriatic arthritis and MRI showed talar bone oedema, felt to be related to inflammatory arthritis. Symptoms settled well on non-steroidal anti-inflammatories. However, he presented with focal distal tibial swelling and pain, two months later. There was no history of trauma, Repeat MRI2 showed proximal migration of bone oedema with stress fractures of the left posterior talus and distal tibial metaphysis. Bloods tests showed low phosphate, elevated PTH, normal adjusted calcium, raised ALP and low 25-OH vitamin D. DEXA scan confirmed osteoporosis of the hip. The patient was commenced on bisphosphonate with 25-hydroxyvitamin D and phosphate supplementation. Despite this, the patient continued to have migratory joint pain affecting the ankle, hip and sacroiliac joints. Methotrexate was started for psoriasis and whilst his skin improved, his pain remained and further MRI showed left talar and right neck of femur insufficiency fractures. Results Although taking vitamin D3 supplementation, he remained hypophosphateamic 0.5mmol/L. Myeloma screen and PET FDG were normal. However, he was noted to have an increased fractional urinary phosphate excretion indicating poor renal phosphate reabsorption. One possible cause of this is elevated FGF-23, which was confirmed with FGF-23 assay (include levels and normal range). The patient underwent PET Ga-DOTATE imaging, utilising a tracer specific for somatostatin receptors found on neuroendocrine tumours. This showed a T9 pedicle lesion and a CT-guided biopsy confirmed a mesenchymal tumour as the cause of FGF-23 secretion, resulting in TBMO and insufficiency fractures. The patient has become asymptomatic on calcitriol and phosphate supplementation. He is now being considered for radiofrequency ablation therapy. Conclusion This case illustrates the need for a thorough investigation of symptomatic, treatment-refractory hypophosphataemia. Although mild hypophosphatemia could indicate adult-onset rickets, rarer causes such as FGF-23 secreting tumours should be considered. These tumours are notoriously difficult to locate; Ga-DOTATE PET is probably superior to other imaging modalities including FDG-PET in isolating mesenchymal tumours. Disclosures K. Fisher None. S. Melath None. S. Patel None.

P41 Searching high and low for a cause of transient migratory bone oedema: fibroblast growth factor-23 secreting mesenchymal tumour

Abstract Background Fibroblast Ggrowth factor-23 (FGF-23) is a phosphate regulator primarily expressed by osteocytes. Excess FGF-23 leads to decreased hydroxylation of 25-hydroxyvitamin D and poor renal phosphate reabsorption. This leads to hypophosphataemia and represents a rare cause of osteomalacia, resulting in bone oedema and stress fractures. Methods A 57-year-old man with known skin psoriasis presented with a two-year history of left foot and ankle pain. On examination, he had chronic dactylitis of the left, big toe associated with skin psoriasis and nail pitting. Serum inflammatory markers were normal, and autoimmune screens for RF, anti-CCP and HLA-B27 were negative. Bilateral foot and ankle X-rays showed no bony abnormality. This was diagnosed as likely psoriatic arthritis and MRI showed talar bone oedema, felt to be related to the inflammatory arthritis. Symptoms settled well on non-steroidal anti-inflammatories. However, he presented two months later with focal distal tibial pain and swelling. There was no history of trauma. Repeat MRI showed proximal migration of bone oedema with stress fractures of the left posterior talus and distal tibial metaphysis. Results Blood tests showed low phosphate, elevated PTH, normal adjusted calcium, raised ALP and low 25-hydroxyvitamin D. DEXA scan confirmed osteoporosis of the hip. The patient was commenced on intravenous three-monthly pamidronate and colecalciferol (vitamin D3) supplements. Despite this, the patient continued to have migratory joint pain affecting the ankle, hip and sacroiliac joints. Methotrexate was prescribed to improve his psoriasis, but whilst his skin improved his legs remained painful. A repeat MRI showed new insufficiency fractures to the left talus and right neck of femur. Although taking colecalciferol supplementation, his serum phosphate remained low (0.5mmol/L). On further investigation, Myeloma screen and FDG PET were normal but he was noted to have an increased fractional urinary phosphate excretion indicating poor renal phosphate reabsorption. Serum FGF-23 assay was elevated at 131 mIU/L (normal <100mIU/L). The patient underwent 68-Gallium DOTATATE PET imaging, utilising a tracer specific for somatostatin receptors found on neuroendocrine tumours, which showed a T9 pedicle lesion. CT-guided biopsy confirmed a mesenchymal tumour as the cause of FGF-23 secretion, resulting in transient bone marrow oedema and insufficiency fractures. The patient has become asymptomatic on calcitriol (1,25-dihydroxyvitamin D) and phosphate supplementation, and is being considered for radiofrequency ablation therapy. Conclusion This case illustrates the need for thorough investigation of symptomatic, treatment-refractory hypophosphataemia. Although mild hypophosphatemia could indicate adult onset rickets, rarer causes such as FGF-23 secreting tumours should be considered. These tumours are notoriously difficult to locate; 68-Gallium DOTATATE PET may offer superior specificity to other imaging modalities, including FDG-PET, in detecting these mesenchymal tumours. FGF-23 decreases hydroxylation of vitamin D3 and renal phosphate reabsorption, and calcitriol alongside phosphate supplementation is advisable for symptomatic management until definitive treatment. Disclosures K. Fisher None. S. Melath None. S. Patel None.

Expression of NaPi-IIb in rodent and human kidney and upregulation in a model of chronic kidney disease.

Na+-coupled phosphate cotransporters from the SLC34 and SLC20 families of solute carriers mediate transepithelial transport of inorganic phosphate (Pi). NaPi-IIa/Slc34a1, NaPi-IIc/Slc34a3, and Pit-2/Slc20a2 are all expressed at the apical membrane of renal proximal tubules and therefore contribute to renal Pi reabsorption. Unlike NaPi-IIa and NaPi-IIc, which are rather kidney-specific, NaPi-IIb/Slc34a2 is expressed in several epithelial tissues, including the intestine, lung, testis, and mammary glands. Recently, the expression of NaPi-IIb was also reported in kidneys from rats fed on high Pi. Here, we systematically quantified the mRNA expression of SLC34 and SLC20 cotransporters in kidneys from mice, rats, and humans. In all three species, NaPi-IIa mRNA was by far the most abundant renal transcript. Low and comparable mRNA levels of the other four transporters, including NaPi-IIb, were detected in kidneys from rodents and humans. In mice, the renal expression of NaPi-IIa transcripts was restricted to the cortex, whereas NaPi-IIb mRNA was observed in medullary segments. Consistently, NaPi-IIb protein colocalized with uromodulin at the luminal membrane of thick ascending limbs of the loop of Henle segments. The abundance of NaPi-IIb transcripts in kidneys from mice was neither affected by dietary Pi, the absence of renal NaPi-IIc, nor the depletion of intestinal NaPi-IIb. In contrast, it was highly upregulated in a model of oxalate-induced kidney disease where all other SLC34 phosphate transporters were downregulated. Thus, NaPi-IIb may contribute to renal phosphate reabsorption, and its upregulation in kidney disease might promote hyperphosphatemia.

FGF23 and Cardiovascular Risk

FGF23 is a protein secreted in the plasma by bone cells. In the kidney, FGF23 can activate an FGF receptor in the presence of its co-receptor αKlotho. FGF23 controls the renal phosphate reabsorption and calcitriol metabolism. When renal function declines, plasma FGF23 concentration raises and FGF23 can stimulate FGFRs in the absence of αKlotho. This induces cardiac hypertrophy, modifies cardiomyocyte contractility and increases the risk of arrhythmic events in cardiac cells. There is still no evidence that decreasing FGF23 concentration in patients on dialysis could improve their survival. In different cardiac disorders cardiomyocyte can produce FGF23, which can reveal a way of adaptation to the stress.

Phosphate-sensing and regulatory mechanism of FGF23 production.

Inorganic phosphate (Pi) is an essential mineral for human. Hypophosphatemia and hyperphosphatemia cause rickets/osteomalacia and ectopic calcification, respectively, indicating that serum Pi level needs to be regulated. Fibroblast growth factor (FGF) 23 is a principal hormone to regulate serum Pi level. FGF23 is produced by the bone, especially by the osteoblasts and osteocytes, and works by binding to FGF receptor (FGFR) 1c and α-Klotho complex in the kidney. FGF23 reduces serum Pi level by inhibiting both renal phosphate reabsorption and intestinal phosphate absorption via reduction of serum 1,25-dihydroxyvitamin D level. It has been unclear how the bone senses changes of serum Pi level and how the bone regulates the production of FGF23. Our recent results indicate that the post-translational modification of FGF23 protein through a gene product of GALNT3 is the main regulatory mechanism of enhanced FGF23 production by high dietary Pi. Furthermore, high extracellular Pi directly activates FGFR1 and its downstream intracellular signaling pathway regulates the expression level of GALNT3. We propose that FGFR1 works as a Pi-sensing receptor in the regulation of FGF23 production and serum Pi level. There is a negative feedback system, which is a basic mechanism of endocrine regulation, in the regulation of serum Pi involving FGFR1, and FGF23. These findings may lead to the development of new therapeutic methods to treat diseases caused by abnormal Pi level.

Positive Response to One-Year Treatment With Burosumab in Pediatric Patients With X-Linked Hypophosphatemia.

X-linked hypophosphatemia (XLH) causes significant burden in pediatric patients in spite of maintained treatment with phosphate supplements and vitamin D derivatives. Administration of burosumab has shown promising results in clinical trial but studies assessing its effect in the everyday practice are missing. With this aim, we analyzed the response to one-year treatment with burosumab, injected subcutaneously at 0.8 mg/kg every 2 weeks, in five children (three females) aged from 6 to 16 years, with genetically confirmed XLH. Patients were being treated with phosphate and vitamin D analogs until the beginning of burosumab treatment. In all children, burosumab administration led to normalization of serum phosphate in association with marked increase of tubular reabsorption of phosphate and reduction of elevated serum alkaline phosphatase levels. Baseline height of patients, from −3.56 to −0.46 SD, increased in the three prepubertal children (+0.84, +0.89, and +0.16 SD) during burosumab treatment. Growth improvement was associated with reduction in body mass index (−1.75, −1.47, and −0.17 SD, respectively), suggesting a salutary effect of burosumab on physical activity and body composition. Burosumab was well-tolerated, mild local pain at the injection site and transient and mild headache following the initial doses of burosumab being the only reported undesirable side effects. No patient exhibited hyperphosphatemia, progression of nephrocalcinosis, worsening of metabolic control or developed hyperparathyroidism. Mild elevation of serum PTH present at the beginning of treatment in one patient 4 was not modified by burosumab administration. These results indicate that in the clinical setting, beyond the strict conditions and follow-up of clinical trials, burosumab treatment for 1 year exerts positive effects in pediatric patients with XLH without major adverse events.

The Osteocyte: New Insights.

Osteocytes are an ancient cell, appearing in fossilized skeletal remains of early fish and dinosaurs. Despite its relative high abundance, even in the context of nonskeletal cells, the osteocyte is perhaps among the least studied cells in all of vertebrate biology. Osteocytes are cells embedded in bone, able to modify their surrounding extracellular matrix via specialized molecular remodeling mechanisms that are independent of the bone forming osteoblasts and bone-resorbing osteoclasts. Osteocytes communicate with osteoclasts and osteoblasts via distinct signaling molecules that include the RankL/OPG axis and the Sost/Dkk1/Wnt axis, among others. Osteocytes also extend their influence beyond the local bone environment by functioning as an endocrine cell that controls phosphate reabsorption in the kidney, insulin secretion in the pancreas, and skeletal muscle function. These cells are also finely tuned sensors of mechanical stimulation to coordinate with effector cells to adjust bone mass, size, and shape to conform to mechanical demands.

V-ATPase a3 isoform mutations identified in osteopetrosis patients abolish its expression and disrupt osteoclast function

The a3 isoform of vacuolar-type proton-pumping ATPase (V-ATPase) is essential for bone resorption by osteoclasts. Although more than 90 mutations of the human a3 gene have been identified in patients with infantile malignant osteopetrosis, it is unclear whether they lead to osteoclast dysfunction. We have established an in vitro assay to induce osteoclasts from spleen macrophages derived from a3-knockout mice. Here, we examined the effects of these mutations in a3-knockout osteoclasts. We were interested in four mutations, two short deletions and two missense mutations, previously identified in the a3 cytosolic domain. a3 harboring either of the two short deletions was hardly expressed in osteoclasts and calcium phosphate resorption was impaired. On the other hand, osteoclasts expressing a3 with either of the two missense mutations exhibited no defects. Specifically, expression levels of the mutant proteins, V-ATPase assembly, and calcium phosphate resorption activity were similar to those of the wild type. Moreover, these missense mutants interacted with Rab7, a small GTPase that regulates lysosomal trafficking. These results suggest that the short deletions impair a3 expression and thus disrupt V-ATPase subunit assembly essential for bone resorption, while the missense mutations do not cause osteoclast dysfunction without an additional mutation(s) or impair resorption of bone, but not of calcium phosphate.

Multidisciplinary patient care in X-linked hypophosphatemic rickets: one challenge, many perspectives.

SummaryX‑linked hypophosphatemic rickets (XLH, OMIM #307800) is a rare genetic metabolic disorder caused by dysregulation of fibroblast-like growth factor 23 (FGF23) leading to profound reduction in renal phosphate reabsorption. Impaired growth, severe rickets and complex skeletal deformities are direct consequences of hypophosphatemia representing major symptoms of XLH during childhood. In adults, secondary complications including early development of osteoarthritis substantially impair quality of life and cause significant clinical burden. With the global approval of the monoclonal FGF23 antibody burosumab, a targeted treatment with promising results in phase III studies is available for children with XLH. Nevertheless, complete phenotypic rescue is rarely achieved and remaining multisystemic symptoms demand multidisciplinary specialist care. Coordination of patient management within the major medical disciplines is a mainstay to optimize treatment and reduce disease burden. This review aims to depict different perspectives in XLH patient care in the setting of a multidisciplinary centre of expertise for rare bone diseases.

Anorexia Nervosa with Markedly High Bone Turnover and Hyperphosphatemia During Refeeding Rectified by Denosumab.

We describe a unique case of hyperphosphatemia associated with a very high bone turnover rate in a 51-year-old postmenopausal woman with undiagnosed anorexia nervosa (AN) who presented with a low-trauma hip fracture. In view of her severely malnourished state, she was not fit for surgery. She was treated according to a refeeding protocol that mandated bed rest. Contrary to expectation, she developed sustained hyperphosphatemia and borderline hypercalcemia. Bone remodelling markers, both resorption and formation, were markedly elevated. Parathyroid hormone (PTH) was low-normal at 1.7pmol/L, C-terminal fibroblast growth factor 23 (FGF23) was high at 293RU/ml, but tubular maximum reabsorption of phosphate (TmPO4/GFR) was elevated at 1.93mmol/L. Denosumab 60mg was administered that was followed by: rapid normalisation of serum phosphate; normalisation of resorption markers, transient hypocalcaemia with secondary hyperparathyroidism, and normalisation of both TmPO4/GFR and C-terminal FGF23. We speculate that prolonged immobilization as part of AN management led to a high remodelling state followed by hyperphosphatemia and high-normal calcium with appropriate suppression of PTH and that marked hyperphosphatemia and high TmP/GFR despite high FGF23 indicates the necessity of PTH adequacy for excess FGF23 to lower TmP/GFR.

Do Aqua Regia and EPA 3051A methods underestimate the pseudo-total P content in clay soil fraction?

ABSTRACT It is common the use of 3051A and aqua regia methods to assess the pseudo-total and total P contents in soil and in its fractions. However, previous study has shown the formation of large amounts of short-range order materials (SRO) in the residues of these extractions. The objective of this work was to evaluate whether phosphate resorb in the SRO formed in the residue of 3051A and aqua regia extractions in clay soil fraction. Residues obtained from the 3051A and aqua regia were recovered from filter papers. To characterize the SRO in these residues, sequential extractions were performed with 0.2 mol L-1 ammonium oxalate and 0.5 mol L-1 NaOH. The maximum pseudo-total P content extracted by aqua regia was 6800 mg kg-1. If SRO formed during this acid extraction was not extracted , 698 mg kg-1 P would have been not accounted (underestimation of 10.3%). This high level of P resorption in the 3051A and aqua regia extraction lead to misinterpretations of P reserve in clay fraction. Phosphate resorption was mainly in SRO extracted by ammonium oxalate for both 3051A and aqua regia residues. The chemical environment of the 3051A and aqua regia extractions facilitated the inner sphere adsorption of H2PO4 - in ferrol and aluminol groups of the SRO- ammonium oxalate. There was larger resorption of H2PO4 - in the 3051A-NaOH in relation to NaOH- aqua regia.

Renal phosphate loss in Iraqi chronic myeloid leukemia patients treated by imatinib mesylate

BACKGROUND: The use of BCR-ABL tyrosine kinase inhibitor imatinib mesylate improved outcomes for patients with chronic myeloid leukemia (CML). Hypophosphatemia is found to be associated with imatinib mesylate use; the exact mechanism of this is not clear yet but mostly related to a drug-induced proximal renal tubular defect.OBJECTIVE: The objective was to measure the renal phosphate loss in CML patients treated by imatinib mesylate.PATIENTS AND METHODS: A cross-sectional study included 40 patients (25 females) who were already diagnosed cases and treated with imatinib mesylate (400 mg/day). The mean age was 40.2 ± 7.8 years. The study was conducted at the medical city teaching hospital, Baghdad, hematology outpatient clinic from July 2016 to December 2016. Serum and random urine samples were measured phosphate and creatinine in serum and urine, respectively. Serum and urine phosphate were measured using the colorimetric method, whereas serum and urine creatinine were calculated by the kinetic method. Fractional excretion of phosphate and tubular maximum of phosphate reabsorption were calculated. After the completion of the study, we tested ten newly diagnosed patients at 0- and 3 months of treatment.RESULTS: Sixteen patients (40%) developed hypophosphatemia. The fractional excretion of phosphate increased (FEPO4 = 21.1%) with a comparable reduction in tubular reabsorption of phosphate to the glomerular filtration rate (Tmpi/GFR = 2.3 mg/dl). There was a significant direct correlation between SPO4 level and white blood cell count (R = 0.451; P = 0.001). The mean intact parathyroid hormone and Vitamin D levels were normal for the study group. All ten newly diagnosed cases developed hypophosphatemia at 3 months. This was statistically significant (P = 0.002). There increase in FEPO4 and decrease in Tmpi/GFR was statistically significant (P < 0.001 and 0.002), respectively.CONCLUSION: Hypophosphatemia while using imatinib mesylate is due to increased urinary phosphate excretion.

Developmental Changes in Phosphate Homeostasis.

Phosphate is a multivalent ion critical for a variety of physiological functions including bone formation, which occurs rapidly in the developing infant. In order to ensure maximal bone mineralization, young animals must maintain a positive phosphate balance. To accomplish this, intestinal absorption and renal phosphate reabsorption are greater in suckling and young animals relative to adults. This review discusses the known intestinal and renal adaptations that occur in young animals in order to achieve a positive phosphate balance. Additionally, we discuss the ontogenic changes in phosphotropic endocrine signalling as it pertains to intestinal and renal phosphate handling, including several endocrine factors not always considered in the traditional dogma of phosphotropic endocrine signalling, such as growth hormone, triiodothyronine, and glucocorticoids. Finally, a proposed model of how these factors may contribute to achieving a positive phosphate balance during development is proposed.

The Difficult Diagnosis of Hypophosphatemic Rickets-A Review of 8 Clinical Cases

Hypophosphatemic rickets is a rare, usually genetic disease associated with decreased phosphate reabsorption in the proximal renal tubule and vitamin D resistance. Several genetic mutations have been discovered, the most common being the X-linked PHEX mutation with high fibroblast growth factor 23 (FGF23) circulating levels. The hypophosphatemic type osteomalacia is usually hereditary or tumour-induced (TIO). In the past 20 years, we have discovered, treated and followed up overall 8 cases of the disease-3 women and 5 men, aged 18 to 52 years. In all patients the diagnostic process was long (a mean of 2-3 years) and involved multiple clinical consults, laboratory evaluations: Biochemical Standard Research, Hormonal Tests [PTH, 25 (OH) D, 1,25 (OH) 2D], Specialized Research (FGF23), Instrumental Research (Ultrasonography of whole body; Computed tomography; Magnetic resonance imaging; DXA examination with an assessment of T-score and Z-score of spine/hip). All of these studies aimed at ruling out different neurological, hematological, oncological, rheumatic, gastroenterological, urological, nephrological diseases and conditions. One of the patients had Fanconi syndrome, five had X-linked hypophosphatemia (XLH) and two had TIO. In the last two patients, we found a high level of FGF23 secreting a small lung neoplasm in the first case and a mesenchymal tumor in the median upper part of the right thigh in the second case. The surgical removal of the tumor mass lead to a fast decrease in FGF23 levels and correction of metabolic disturbances. We present clinical cases with hypophosphatemic rickets/osteomalacia and discuss the etiopathogenesis and treatment of this rare disease. Historically until now phosphate supplementation and therapy using analogs of highly active vitamin D (calcitriol, alfacalcidol, paricalcitol) have been used to manage conditions involving hypophosphatemia. In recent years there has been a progression of clinical trials for monoclonal anti-FGF23 antibodies for the treatment of XLH. These monoclonal anti-FGF23 antibodies may have potential for treating other conditions associated with FGF23 overactivity. However, clinical trials to support that possibility are not available at present.

Description of a novel SLC34A3.c.671delT mutation causing hereditary hypophosphatemic rickets with hypercalciuria in two adolescent boys and response to recombinant human growth hormone

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is an autosomal recessive disorder characterized by hypophosphatemia, rickets, hyperphosphaturia, elevated 1,25(OH)2D, and hypercalciuria. Mutations in SLC34A3, the gene encoding the sodium-dependent cotransporter NPT2c, have previously been described as a cause of HHRH. Here, we describe two male siblings with rickets and hypercalciuric nephrolithiasis born to unrelated parents, and their response to oral phosphate supplementation and growth hormone therapy. Whole exome sequencing of the oldest brother, and polymerase chain reaction and Sanger sequence analysis of the identified SLC34A3 mutations, was performed for confirmation and to evaluate his siblings and parents. Serum and urine biochemical parameters of mineral homeostasis before and after therapy were evaluated. Whole exome sequencing analysis identified a previously reported heterozygous deletion SLC34A3.g.2259-2359del101bp on the maternal allele, and a novel heterozygous single nucleotide deletion SLC34A3.c.671delT on the paternal allele of the two affected brothers. The parents and the unaffected brother are heterozygous carriers. Recombinant human growth hormone (rHGH) plus oral phosphate in one affected brother improved the renal phosphate leak and resulted in accelerated linear growth superior to that seen with oral phosphate supplementation alone in the other affected brother. Our case study is the first to demonstrate that rHGH can be considered in addition to oral supplementation with phosphorus to improve linear growth in patients with this disorder, and suggests that renal phosphate reabsorption in response to rHGH is NPT2c-independent.

Predicting the Impact of PHEX, FGF23 and DMP1 Gene Variants Found in Malaysian Malay Patients with Hypophosphataemic Rickets Through In Silico Analysis of Protein Function and mRNA Secondary Structure

Hypophosphataemic Rickets (HR) is a rare bone disorder characterised by chronic hypophosphataemia caused by defective phosphate reabsorption in the renal tubules. Variants in phosphate-regulating endopeptidase homolog, X-linked (PHEX), fibroblast growth factor-23 (FGF23) and dentin matrix protein-1 (DMP1) genes contribute to X-linked dominant, autosomal dominant and autosomal recessive forms of HR, respectively. In this study, four Malaysian patients’ DNA samples were subjected to polymerase chain reaction and Sanger sequencing to identify the types and locations of the variants. Then, in silico study was conducted based on the variants found to predict the effects of amino acid substitution on protein functions using SIFT and PolyPhen-2 software and RNAfold was used to construct the mRNA secondary structure. Mutational analyses had revealed two variants in PHEX; c.10G&gt;C (E4Q), c.1970A&gt;G (Y657C), one mutation in FGF23; c.716C&gt;T (T239M) and three variants on DMP1; c.309A&gt;T (S69C), c.1322C&gt;T (S406S), c.1334G&gt;A (E410E). The variants in these Malay patients were previously reported in different ethnic HR patients. Protein prediction programs suggested that the PHEX Y657C and DMP1 S69C variants may affect protein function. All variants were predicted to alter the secondary mRNA structure. These findings suggest that these missense and silent variants may lead to changes in protein function and mRNA secondary structure that are associated with the manifestation of HR phenotype.

Local application of alendronate controls bone formation and β‐tricalcium phosphate resorption induced by recombinant human bone morphogenetic protein‐2

This study examined the ability of local alendronate (ALN) administration to control β-tricalcium phosphate (β-TCP) resorption as well as the induction of bone formation by recombinant human bone morphogenetic protein-2 (rhBMP-2). A 15-mm critical-sized bone defect was created in the diaphysis of rabbit ulnae. Nine female rabbits (4 to 5 months-old) were divided into 3 groups. Group 1 (n = 6 ulnae) animals received implants consisting of β-TCP granules and 25 μg of rhBMP-2 in 6.5% collagen gel. Group 2 (6 ulnae) and Group 3 (6 ulnae) animals received the same implants, but with 10-6 M and 10-3 M ALN-treated TCP granules, respectively. Two weeks postsurgery, tartrate-resistant acid phosphatase-positive cell counts, new bone formation, and residual β-TCP were evaluated. This study showed that a high dose of ALN strongly reduced osteoclastic resorption of β-TCP induced by rhBMP-2, resulting in decreased bone formation. In contrast, a low dose of ALN slightly reduced the bone resorptive effect but increased bone formation. These results suggest that osteoclast-mediated resorption plays an important role in bone formation and a coupling-like phenomenon could occur in the β-TCP-implanted area, and that administration of a low dose of ALN may solve clinical bone resorptive problems induced by rhBMP-2.

1,25(OH)2D3 in Brain Function and Neuropsychiatric Disease.

1,25(OH)2D3 (1,25-dihydroxy-vitamin D3 = calcitriol) is a powerful regulator of mineral metabolism. The hormone increases calcium and phosphate plasma concentrations in part by stimulation of intestinal absorption and renal reabsorption of calcium and phosphate. It is primarily, but not exclusively, produced in the kidney. Renal 1,25(OH)2D3 formation is stimulated by calcium and phosphate deficiency and by parathyroid hormone which is up-regulated by hypocalcemia. 1,25(OH)2D3 formation is inhibited by fibroblast growth factor FGF23, which is up-regulated by phosphate excess and requires Klotho to become effective. Klotho- or FGF23-deficiency leads to excessive plasma 1,25(OH)2D3-, Ca2+- and phosphate-concentrations with severe soft tissue calcification and accelerated aging. Tissue calcification and premature aging are prevented by NH4Cl without affecting 1,25(OH)2D3-formation. 1,25(OH)2D3 has powerful effects apparently unrelated to mineral metabolism, including anti-inflammatory actions and modification of multiple brain functions. Excessive 1,25(OH)2D3 formation in klotho-deficient NH4Cl-treated mice leads to an amazing surge of exploratory behavior, lack of anxiety and decreased depression, effects dissipated by low vitamin D diet. Conversely, vitamin D deficient mice display reduced explorative behavior, enhanced anxiety, aberrant grooming, submissive social behavior, social neglect and maternal cannibalism. 1,25(OH)2D3 is generated in human brain, and acts on diverse structures including prefrontal cortex, hippocampus, cingulate gyrus, thalamus, hypothalamus, and substantia nigra. In neurons 1,25(OH)2D3 suppresses oxidative stress, inhibits inflammation, provides neuroprotection, down-regulates a variety of inflammatory mediators and up-regulates a wide variety of neurotrophins. Diseases postulated to be favorably modified by 1,25(OH)2D3 include multiple sclerosis, Parkinson´s disease, Alzheimer´s disease, depression, bipolar disorder and schizophrenia. Clearly, substantial additional experimentation is required to fully understand the neuro-psycho-pathophysiological role of 1,25(OH)2D3 and to exploit 1,25(OH)2D3 or related agonists in the treatment of neuro-psychiatric disorders.

Discerning the effects of phosphate status on the metabolism of hybrid poplar.

Accumulation of phosphate in leaves as external environmental phosphate concentrations increase has been observed across the plant kingdom. The excess storage of anions, such as phosphate, has various metabolic trade-offs, including a corresponding influx of counter-ions to maintain charge balance and/or the reduction in organic acid content to maintain internal pH. The leaves and roots of four hybrid poplar genotypes were tested for differences in metabolic response to increasing external phosphate and further effects on patterns of anion resorption among hybrid poplar and willow were explored. Organic acid concentrations increased or remained constant across treatments, suggesting that metabolic adjustments were made in response to greater influxes of inorganic cations rather than a response to increasing phosphate. During senescence, the hybrid poplar Tristis had higher sulfate and organic acid resorption, while hybrid willow, AAFC-5, had higher phosphate resorption proficiencies, suggesting differing anion remobilization mechanisms. Furthermore, phosphate accumulation was shown to continue well after bud-set in poplar hybrids, which may contribute to the low phosphorus resorption efficiency. This indicates that closely related species, with similar growth strategies, show preferential resorption toward different nutrients.

Sclerostin Antibody Treatment Increases Bone Mass and Normalizes Circulating Phosphate Levels in Growing Hyp Mice.

X-linked hypophosphatemia (XLH), caused by a loss-of-function mutation in the phosphate regulating gene with homology to endopeptidase located on the X chromosome (PHEX), is the most common form of vitamin D-resistant rickets. Loss of functional PHEX results in elevated fibroblast growth factor 23 (FGF23) levels, impaired phosphate reabsorption, and inhibited skeletal mineralization. Sclerostin, a protein produced primarily in osteocytes, suppresses bone formation by antagonizing Wnt signaling and is reported to be elevated in XLH patients. This study used the Hyp mouse model to investigate sclerostin's role in the pathophysiology of XLH by evaluating the use of a monoclonal antibody to sclerostin in a mouse model of XLH, the Hyp mouse. Male and female wild-type and Hyp littermates were injected with 25 mg/kg of vehicle or sclerostin antibody (Scl-Ab) twice weekly, beginning at 4 weeks of age and euthanized at 8 weeks of age. Scl-Ab treatment increased serum phosphate levels and suppressed circulating levels of intact FGF23 in treated wild-type and Hyp mice of both sexes. Cortical area, trabecular bone volume fraction (BV/TV), metaphyseal apparent density, and the peak load increased with Scl-Ab treatment in both sexes. This short-term treatment study suggests that Scl-Ab treatment can effectively improve some of the pathologies associated with XLH, including normalization of phosphate, and that sclerostin may play a role in regulating FGF23 and phosphate metabolism in XLH. © 2019 American Society for Bone and Mineral Research.