Inherited Fanconi renotubular syndromes: unveiling the intricacies of hypophosphatemic rickets/osteomalacia.

INTRODUCTION: Fanconi renotubular syndromes (FRTS) are a rare group of inherited phosphaturic disorders with limited Indian as well as global data on this condition. Here, we describe the experience of a single Endocrinology center from Western India on FRTS. MATERIALS AND METHODS: Comprehensive clinical, biochemical, radiological, management, and genetic details of FRTS patients managed between 2010 and 2023 were collected and analyzed. RESULTS: FRTS probands had mutations (eight novel) in six genes [CLCN5 (n = 4), SLC2A2 (n = 2), GATM, EHHADH, HNF4A, and OCRL (1 each)]. Among 15 FRTS patients (11 families), rickets/osteomalacia was the most common (n = 14) presentation with wide inter- and intra-familial phenotypic variability. Delayed diagnosis (median: 8.8 years), initial misdiagnosis (8/11 probands), and syndrome-specific discriminatory features (8/11 probands) were commonly seen. Hypophosphatemia, elevated alkaline phosphatase, normal parathyroid hormone (median: 36 pg/ml), high-normal/elevated 1,25(OH)2D (median: 152 pg/ml), hypercalciuria (median spot urinary calcium to creatinine ratio: 0.32), and variable proximal tubular dysfunction(s) were observed. Elevated C-terminal fibroblast growth factor 23 in two probands was misleading, till the genetic diagnosis was reached. Novel observations in our FRTS cohort were preserved renal function (till sixth decade) and enthesopathy in FRTS1 and FRTS3 families, respectively. CONCLUSION: Our findings underscore frequent under- and misdiagnosis of FRTS; hence, a high index of suspicion for FRTS in phosphopenic rickets/osteomalacia, with early consideration of genetic testing is essential to ensure timely diagnosis of FRTS. The novel variants and phenotypic manifestations described here expand the disease spectrum of FRTS.

The differential effect of modern intravenous iron on fibroblast growth factor 23 and phosphate in non-dialysis dependent CKD - the exploratory randomized controlled double-blind ExplorIRON-CKD study.

BACKGROUND: Intravenous iron is commonly used in patients with non-dialysis-dependent chronic kidney disease (CKD). Modern intravenous iron compounds (e.g. ferric derisomaltose (FDI), ferric carboxymaltose (FCM)) are increasingly utilized with similar efficacy. A differential effect in terms of hypophosphatemia has been noted following administration of FCM, which may be related to fibroblast growth factor 23 (FGF23). This study was designed to examine the comparative effects of FDI and FCM on FGF23, phosphate and other markers of bone turnover. METHODS: The single-center double-blind randomized controlled trial "Iron and Phosphaturia - ExplorIRON-CKD" primarily assessed the effects of FCM and FDI on intact FGF23 and phosphate, whilst also studying the impact on vitamin D, parathyroid hormone and phosphaturia. Bone markers including alkaline phosphatase, bone-specific alkaline phosphatase, procollagen type 1 N-terminal propeptide and carboxy-terminal collagen cross-linked telopeptide were monitored. Non-dialysis-dependent CKD patients (stage 3a-5) with iron deficiency with/without anemia (serum ferritin < 200 µg/L or transferrin saturation = 20% and serum ferritin 200-299 µg/L) were randomized to receive FDI or FCM in a 1:1 ratio. At baseline 1000 mg of intravenous iron was administered followed by 500-1000 mg at 1 month to achieve replenishment. Measurements were performed at baseline, 1-2 days following iron administration, 2 weeks, 1 month (second iron administration), 1-2 days following second administration, 2 months and 3 months following initial infusion. RESULTS: Twenty-six patients participated in the trial; 14 randomized to FDI and 12 to FCM. Intact FGF23 increased following administration of iron, and the increase was significantly higher with FCM compared to FDI (Baseline to 1-2 days following 1st administration: FDI: 3.0 (IQR: - 15.1 - 13.8) % vs. FCM: 146.1 (IQR: 108.1-203.1) %; p < 0.001 and Baseline to 1-2 days following 2nd administration: FDI: 3.2 (IQR: - 3.5 - 25.4) % vs. FCM: 235.1 (138.5-434.6) %; p = 0.001). Phosphate levels decreased in the FCM group, causing a significant difference versus FDI 2 weeks following administration of the first dose. A significantly greater decrease in 1,25 (OH)2 Vitamin D was noted with FCM. Several markers of bone turnover significantly changed following administration of FCM but not FDI. CONCLUSIONS: The study suggests a differential effect on FGF23 following administration of FCM compared to FDI in non-dialysis-dependent CKD patients, similar to other patient groups. This may lead to changes consistent with hypovitaminosis D and alterations in bone turnover with potential clinical consequences. Further definitive studies are required to understand these differences of intravenous iron compounds. TRIAL REGISTRATION: European Union Drug Regulating Authorities Clinical Trials Database (EudraCT) number: 2019-004370-26 ( https://www.clinicaltrialsregister.eu/ctr-search/trial/2019-004370-26/GB ) (First date of trial registration: 03/12/2019).

Acquired disorders of phosphaturia: Beyond tumor-induced osteomalacia.

Phosphate is an integral part of human cellular structure and function. Though most recognised disorders of phosphaturia are genetic in origin, phosphate loss due to acquired conditions is commonly encountered in clinical practice. Acquired hypophosphatemia is most commonly due to renal phosphate wasting and can produce significant morbidity. It also heralds future kidney damage, and continued exposure can lead to progressive kidney injury and potentially renal failure. These conditions are a diverse group of disorders with common shared mechanisms causing loss of phosphate in the urine. Renal phosphate loss can occur as an isolated entity or as a part of generalised proximal tubular dysfunction, i.e., Fanconi's syndrome. An insight into the pathophysiological mechanisms of acquired phosphaturia can help clinicians monitor their patients better and avoid potential harms.

Rapamycin downregulates α-klotho in the kidneys of female rats with normal and reduced renal function.

Both mTOR and α-klotho play a role in the pathophysiology of renal disease, influence mineral metabolism and participate in the aging process. The influence of mTOR inhibition by rapamycin on renal α-klotho expression is unknown. Rats with normal (controls) and reduced (Nx) renal function were treated with rapamycin, 1.3 mg/kg/day, for 22 days. The experiments were conducted with rats fed 0.6% P diet (NP) and 0.2% P diet (LP). Treatment with rapamycin promoted phosphaturia in control and Nx rats fed NP and LP. A decrease in FGF23 was identified in controls after treatment with rapamycin. In rats fed NP, rapamycin decreased mRNA α-klotho/GADPH ratio both in controls, 0.6±0.1 vs 1.1±0.1, p = 0.001, and Nx, 0.3±0.1 vs 0.7±0.1, p = 0.01. At the protein level, a significant reduction in α-klotho was evidenced after treatment with rapamycin both by Western Blot: 0.6±0.1 vs 1.0±0.1, p = 0.01, in controls, 0.7±0.1 vs 1.1±0.1, p = 0.02, in Nx; and by immunohistochemistry staining. Renal α-klotho was inversely correlated with urinary P excretion (r = -0.525, p = 0.0002). The decrease in α-klotho after treatment with rapamycin was also observed in rats fed LP. In conclusion, rapamycin increases phosphaturia and down-regulates α-klotho expression in rats with normal and decreased renal function. These effects can be observed in animals ingesting normal and low P diet.

Hepatic phosphate uptake and subsequent nerve-mediated phosphaturia are crucial for phosphate homeostasis following portal vein passage of phosphate in rats.

Fibroblast growth factor 23, parathyroid hormone, and 1,25-dihydroxyvitamin D are critical in phosphate homeostasis. Despite these factors' importance, regulators of phosphaturia in the acute postprandial phase remain largely unknown. This study investigated the mechanism of acute phosphate regulation in the postprandial phase in rats. Duodenal administration of radiolabeled phosphate (32P) showed that 32P levels in the inferior vena cava (IVC) blood were lower than those in the portal vein (PV) blood. Serum phosphate concentration transiently increased 5 min after phosphate solution administration through IVC, while it was maintained after the administration through PV. Phosphate administration through both IVC and PV resulted in increased fractional excretion of phosphate (FEPi) at 10 min without elevation of the known circulating factors, but urinary phosphate excretion during the period was 8% of the dose. Experiments using 32P or partial hepatectomy showed that the liver was one of the phosphate reservoirs. The elevation of FEPi and suppression of sodium-phosphate cotransporter 2a in the kidney at 10 min was attenuated in rats with SCH23390, hepatic denervation, or renal denervation, thus indicating that the liver communicated with the kidney via the nervous system to promote phosphaturia. These results revealed previously unknown mechanisms for serum phosphate maintenance.

Use of Teriparatide in Hyperphosphatemic Familial Tumor Calcinosis: Evaluating the Interaction Between FGF23 and PTH on the Phosphaturic Effect.

Hyperphosphatemic familial tumor calcinosis (HFTC) is a rare disease characterized by hyperphosphatemia and calcium and phosphorus crystal deposition. It occurs due to the loss of function of FGF23. Herein, we report a case of a 50-year-old woman diagnosed with HFTC (homozygous variant in the GALNT3 gene, c.803_804 C insertion) with a history of ectopic calcifications in the past 30 years. Laboratory tests on admission were as follows: phosphate (P) 7.1 mg/dL (Normal range (NR) 2.5-4.5 mg/dL), FGF23 c-terminal 2050 RU/mL (NR < 150 RU/mL), and intact FGF23 (iFGF23) 18.93 pg/mL (NR 12.0-69.0 pg/mL). Treatment with acetazolamide, sevelamer, and a phosphorus-restricted diet was started, but phosphatemia remained high and calcifications continued to progress. In an attempt to further decrease P, a 36-day cycle of teriparatide (TPTD) 20 mcg twice daily was added, decreasing P from 6.2 to 5.2 mg/dL and increasing the 1.25(OH)2 vitamin D by 34.2%. As urinalysis was not feasible at the end of the 36-day cycle, a second cycle was performed for another 28 days, producing a similar decrease in P (from 6.4 to 5.5 mg/mL) and an evident decrease in the rate of tubular reabsorption of P (from 97.2 to 85.3%), however, accompanied by a worrying increase in calciuria. The use of TPTD 20 mcg twice daily in a patient with genetic resistance to FGF23 (HFTC) was associated with consistent increase in phosphaturia and reduction in phosphatemia, in addition to an increase in calcitriol. The resulting hypercalciuria precludes the therapeutic use of TPTD in HFTC and suggests an important role of FGF23, not only in phosphate homeostasis but also in avoiding any excess of calcitriol.

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.

Burosumabe para o tratamento de hipofosfatemia ligada ao cromossomo X em adultos e crianças / Burosumabe for or treatment of hypophosphatemia linked to or chromosome X in adults and infants

CONTEXTO: A hipofosfatemia ligada ao cromossomo X (HLX) é considerada uma doença ultrarrara, cronicamente debilitante e deformante. É uma doença causada por mutações no gene regulador do fosfato com homologia às endopeptidases do cromossomo X (PHEX), que leva a perda de função do mesmo, gerando erros na detecção de fosfato e aumento dos níveis de fator de crescimento de fibroblastos 23 (FGF23). Os maiores achados clínicos na XLH são hipofosfatemia, retardo no crescimento, raquitismo e/ou osteomalácia. Anormalidades esqueléticassão sinais precoces de HLX. Fraturas, perdas auditivas, problemas dentários e osteomalácia podem aparecer além da perda de qualidade de vida associada. O tratamento convencional é realizado com administração de fosfato e vitamina D. Esta não é uma terapia com alvo no mecanismo fisiopatológico da doença, resumindo-se à tentativa de minorar a hipofosfatemia e o excesso de hormônio da paratireoide. A reposição oral de fosfato e vitamina D são insuficientes para atender aos objetivos do tratamento, não logrando alterar a densidade mineral óssea da coluna e quadril em adultos e estão associados a efeitos adversos importantes. Considera-se a possibilidade de inibição da atividade do FGF23 como uma medida terapêutica única para doenças hipofosfatêmicas causadas pelo excesso de FGF23. O burosumabe é um anticorpo monoclonal, sendo o primeiro medicamento desenvolvido para inibir a FGF23 e com isso aumentar a reabsorção de fosfato do rim, para que pela produção de vitamina D, melhore a absorção intestinal de cálcio e fosfato reduzindo os danos causados pela HLX. Agências internacionais aprovaram seu uso inicialmente nas populações pediátricas, em que o benefício clínico na melhora da HLX está mais estabelecido. Alguns países ampliaram o uso para a população adulta. PERGUNTA: O uso de burosumabe para o tratamento da hipofosfatemia ligada ao cromossomo X é eficaz e seguro quando comparado ao tratamento com fosfato e vitamina D ou placebo? TECNOLOGIA: burosumabe (CRYSVITA®). EVIDÊNCIAS CIENTÍFICAS: Em revisão sistemática da literatura, o demandante selecionou seis estudos clínicos que avaliaram o uso do burosumabe no tratamento da HLX. Três destes foram realizados em pacientes pediátricos e três em pacientes adultos. Há algumas limitações inerentes aos estudos de fase 2 e 3 selecionados. Os resultados apresentados demostram benefícios clínicos especialmente na melhora do raquitismo em crianças, da mobilidade e crescimento com tratamento com burosumabe em relação ao tratamento com fosfato e vitamina D. O estudo de Imel, et al., 2019 mostrou melhora do Escore Radiographic Global Impression of Change (RGI-C) ­ global ­ diferença média dos mínimos quadrados em 40 semanas: 1,1 (IC95% 0,8 ­ 1,5; p 0,0001) favorecendo o tratamento com burosumabe. O estudo de Carpenter et al., 2018, avaliou o mesmo escore em 40 semanas e obteve os seguintes resultados de acordo com a posologia: burosumabe, duas vezes por mês (+1,66 ± 0,09); burosumabe, uma vez por mês (+1,47 ± 0,14); burosumabe, todos os pacientes (+1,56 ± 0,08). Ao avaliar a gravidade do raquitismo pelo escore Thatcher, a diferença média dos mínimos quadrados em 40 semanas de acordo com a posologia foi: burosumabe, duas vezes por mês: -1,06 ± 0,11; p< 0,0001; burosumabe, uma vez por mês: -0,73 ± 0,10; p<0,0001; burosumabe, todos os pacientes: -0,89 ± 0,0,07. Em adultos também houve benefícios com o uso do medicamento, porém menos consistentes que na população pediátrica. Insogna et al., 2018, avaliaram a cura completa de fraturas ativas ou pseudofraturas em 24 semanas. Os seguintes resultados foram observados: burosumabe (43,1%) versus placebo (7,7%); Odds ratio OR 16,8 (IC95% não reportado); p <0,0001. Os estudos em adultos foram comparados com placebo, logo os resultados favoráveis ao burosumabe foram esperados. Em relação à segurança, tanto na população pediátrica quanto adulta, o burosumabe não acarretou nenhum evento adverso grave nos estudos realizados. AVALIAÇÃO ECONÔMICA: O demandante realizou uma análise de custo-efetividade. O modelo foi construído em uma planilha MS Excel com base em uma estrutura de Markov com horizonte temporal de toda a vida. Dois modelos de custo-efetividade foram apresentados, um considerando a população pediátrica (idades de 1 a 17 anos) e outro a população adulta (idade ≥18 anos). A razão de custo-efetividade incremental (RCEI) do burosumabe em relação ao uso de fosfato e vitamina D em pediatria foi de R$ 2.401.312,64/QALY e para a população adulta de R$ 2.534.873,52 /QALY. Nas análises de sensibilidade univariada para ambas as populações o modelo se mostrou sensível ao custo do burosumabe e idade de início do tratamento. O modelo possui limitações, entre elas a ausência de dados de incidência da HLX na população brasileira e os dados disponíveis no DATASUS relacionados a procedimentos e tratamentos provavelmente estão subestimados em relação a doença. Por tratar-se de doença rara, a falta de registro de dados e suas implicações podem impactar no modelo e suas análises de sensibilidade. AVALIAÇÃO DE IMPACTO ORÇAMENTÁRIO: A análise de impacto orçamentário (AIO) realizada pelo demandante previu população de pacientes elegíveis para o tratamento com burosumabe baseada em referências internacionais pela ausência de dados registrados no Brasil. Foi adotado um horizonte de 5 anos. O demandante apresentou um cenário base com incorporação de burosumabe para 100% dos pacientes. No cenário ­base os impactos orçamentários em cinco anos foram: R$ 197.249.738,34 para a população pediátrica e R$ 224.968.284,47 para a população adulta. Um outro cenário considerando que 50% seriam diagnosticados e tratados com burosumabe ou fosfato e vitamina D em 5 anos foi apresentado: R$ 98.624.869,17 para a população pediátrica e R$ 67.490.485,34 para a população adulta. O cenário 3 considerou a introdução gradual do burosumabe em 20%, 40%, 50% e 75%, chegando a 100% no quinto ano, assim o impacto em 5 anos foi de: R$ 65.209.976,69 para a população pediátrica R$ 48.026.685,80 para a população adulta. Apesar desse cenário ser o mais próximo do real, a disseminação pode ser maior já que há ausência de tratamento eficaz para a doença. Além disso, a AIO têm provavelmente subestimativas da população por usar uma média internacional e não considerar a incidência da doença ao longo do horizonte temporal. MONITORAMENTO DO HORIZONTE TECNOLÓGICO: De acordo com os critérios adotados nessa análise, não foram localizados medicamentos em desenvolvimento clínico para a doença. DISCUSSÃO: A análise da evidência clínica apresentada sugere que a tecnologia proposta apresenta efetividade superior à única alternativa disponível atualmente para tratamento da HLX no SUS, com confiabilidade moderada, corroborando atuais recomendações para seu uso em diretrizes internacionais. Por ser uma doença rara, com limitações de dados brasileiros disponibilizados, as análises econômicas submetidas pelo demandante indicam limitações metodológicas, reduzindo a confiabilidade das conclusões a respeito de custo-efetividade e impacto orçamentário no cenário brasileiro podendo ter os valores subestimados. RECOMENDAÇÃO PRELIMINAR DA CONITEC: Os membros do plenário não consideraram como robustas as evidências científicas de eficácia e segurança do borusomabe. Além disso, foi considerado elevado o valor de custo-efetividade incremental, assim como o impacto orçamentário. Sendo assim, no dia 7 de outubro de 2020, em sua 91ª reunião de plenário, os membros da Conitec foram desfavoráveis à recomendação de incorporação no SUS do "Burosumabe para o tratamento de hipofosfatemia ligada ao cromossomo X em adultos e crianças". CONSULTA PÚBLICA: a Consulta Pública nº 56 foi realizada entre os dias 04/11/2020 e 30/11/2020. Foram recebidas 619 contribuições, sendo 103 pelo formulário para contribuições técnico-científicas e 516 pelo formulário para contribuições sobre experiência ou opinião. Após apreciação das contribuições recebidas, o Plenário da Conitec entendeu que houve argumentação suficiente para mudança de entendimento acerca de sua recomendação preliminar. A maioria das contribuições foram contra recomendação e se concentraram nos seguintes benefícios do medicamento: eficácia do tratamento; melhora dos sintomas; melhora dos níveis de fosfato e qualidade de vida. Desse modo, a Comissão mudou a recomendação inicial. RECOMENDAÇÃO FINAL DA CONITEC: Os membros da Conitec presentes na 94ª reunião ordinária, no dia 04 de fevereiro de 2021, consideraram que os benefícios clínicos do tratamento foram mais acentuados na população pediátrica apresentando desfechos consistentes. Diante do exposto, os membros presentes deliberaram, por unanimidade, a recomendação do burosumabe para o tratamento de hipofosfatemia ligada ao cromossomo X em crianças conforme protocolo Clínico e Diretrizes Terapêuticas (PCDT) e recomendar a não incorporação do burosumabe para o tratamento de hipofosfatemia ligada ao cromossomo X em adultos. Assim, foi assinado o Registro de Deliberação nº 589/2021. DECISÃO: incorporar o burosumabe para o tratamento da hipofosfatemia ligada ao cromossomo X em crianças conforme Protocolo Clínico e Diretrizes Terapêuticas (PCDT) e não incorporar o burosumabe para o tratamento da hipofosfatemia ligada ao cromossomo X em adultos, do Sistema Único de Saúde - SUS, conforme Portaria nº 01, publicada no Diário Oficial da União nº 34, seção 1, página 93, em 22 de fevereiro de 2021.

Mutational landscape and genetic signatures of cell-free DNA in tumour-induced osteomalacia.

Tumour-induced osteomalacia (TIO) is a very rare paraneoplastic syndrome with bone pain, fractures and muscle weakness, which is mostly caused by phosphaturic mesenchymal tumours (PMTs). Cell-free DNA (cfDNA) has been regarded as a non-invasive liquid biopsy for many malignant tumours. However, it has not been studied in benign tumours, which prompted us to adopt the targeted next-generation sequencing approach to compare cfDNAs of 4 TIO patients, four patients with bone metastasis (BM) and 10 healthy controls. The mutational landscapes of cfDNA in TIO and BM groups were similar in the spectrum of allele frequencies and mutation types. Markedly, deleterious missense mutations in FGFR1 and loss-of-function mutations in MED12 were found in 3/4 TIO patients but none of BM patients. The gene ontology analysis strongly supported that these mutated genes found in TIOs would play a potential role in PMTs' process. The genetic signatures and corresponding change in expression of FGFR1 and FGF23 were further validated in PMT tissues from a test cohort of another three TIO patients. In summary, we reported the first study of the mutational landscape and genetic signatures of cfDNA in TIO/PMTs.

A paternally inherited non-sense variant c.424G>T (p.G142*) in the first exon of XLαs in an adult patient with hypophosphatemia and osteopetrosis.

XLαs, the extra-large isoform of alpha-subunit of the stimulatory guanine nucleotide-binding protein (Gsα), is paternally expressed. The significance of XLαs in humans remains largely unknown. Here, we report a patient who presented with increased bone mass, hypophosphatemia, and elevated parathyroid hormone (PTH) levels. His serum calcium was in the lower limit of the normal range. Whole exome sequencing of this subject found a novel non-sense variant c.424G>T (p. G142*) in the first exon of XLαs, which was inherited from his father and transmitted to his daughter. This variant was predicted to exclusively influence the expression of XLαs, while possibly having no significant effects on other gene products of this locus. Ellsworth-Howard test revealed normal renal response to PTH in proband. Human SaOS2 cells transfected with mutant XLαs failed to generate cyclic adenosine monophosphate under PTH stimulation, indicating skeletal resistance to this hormone. This subject showed higher circulating sclerostin, dickkopf1, and osteoprotegerin (OPG) levels, while lower receptor activator of nuclear factor kappa-B ligand/OPG ratio, leading to reduced bone resorption. Our findings indicate that XLαs plays a critical role in bone metabolism and GNAS locus should be considered as a candidate gene for high bone mass.

All-trans retinoic acid reduces the transcriptional regulation of intestinal sodium-dependent phosphate co-transporter gene (Npt2b).

Inorganic phosphate (Pi) homeostasis is regulated by intestinal absorption via type II sodium-dependent co-transporter (Npt2b) and by renal reabsorption via Npt2a and Npt2c. Although we previously reported that vitamin A-deficient (VAD) rats had increased urine Pi excretion through the decreased renal expression of Npt2a and Npt2c, the effect of vitamin A on the intestinal Npt2b expression remains unclear. In this study, we investigated the effects of treatment with all-trans retinoic acid (ATRA), a metabolite of vitamin A, on the Pi absorption and the Npt2b expression in the intestine of VAD rats, as well as and the underlying molecular mechanisms. In VAD rats, the intestinal Pi uptake activity and the expression of Npt2b were increased, but were reduced by the administration of ATRA. The transcriptional activity of reporter plasmid containing the promoter region of the rat Npt2b gene was reduced by ATRA in NIH3T3 cells overexpressing retinoic acid receptor (RAR) and retinoid X receptor (RXR). On the other hand, CCAAT/enhancer-binding proteins (C/EBP) induced transcriptional activity of the Npt2b gene. Knockdown of the C/EBP gene and a mutation analysis of the C/EBP responsible element in the Npt2b gene promoter indicated that C/EBP plays a pivotal role in the regulation of Npt2b gene transcriptional activity by ATRA. EMSA revealed that the RAR/RXR complex inhibits binding of C/EBP to Npt2b gene promoter. Together, these results suggest that ATRA may reduce the intestinal Pi uptake by preventing C/EBP activation of the intestinal Npt2b gene.

FGF23 and inflammation-a vicious coalition in CKD.

High serum concentrations of the phosphaturic hormone, fibroblast growth factor 23 (FGF23), contribute to various tissue injuries. In chronic kidney disease, the sources of FGF23 and the stimuli that control FGF23 production differ from those in the physiologic scenario. Mediators of inflammation are intensively studied as potential factors that contribute to FGF23 elevations and thereby might function as drug targets to lower FGF23 levels. The present study focuses on tumor necrosis factor.

Pharmacological Npt2a Inhibition Causes Phosphaturia and Reduces Plasma Phosphate in Mice with Normal and Reduced Kidney Function.

BACKGROUND: The kidneys play an important role in phosphate homeostasis. Patients with CKD develop hyperphosphatemia in the later stages of the disease. Currently, treatment options are limited to dietary phosphate restriction and oral phosphate binders. The sodium-phosphate cotransporter Npt2a, which mediates a large proportion of phosphate reabsorption in the kidney, might be a good therapeutic target for new medications for hyperphosphatemia. METHODS: The authors assessed the effects of the first orally bioavailable Npt2a inhibitor (Npt2a-I) PF-06869206 in normal mice and mice that had undergone subtotal nephrectomy (5/6 Nx), a mouse model of CKD. Dose-response relationships of sodium, chloride, potassium, phosphate, and calcium excretion were assessed in response to the Npt2a inhibitor in both groups of mice. Expression and localization of Npt2a/c and levels of plasma phosphate, calcium, parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF-23) were studied up to 24-hours after Npt2a-I treatment. RESULTS: In normal mice, Npt2a inhibition caused a dose-dependent increase in urinary phosphate (ED50 approximately 21 mg/kg), calcium, sodium and chloride excretion. In contrast, urinary potassium excretion, flow rate and urinary pH were not affected dose dependently. Plasma phosphate and PTH significantly decreased after 3 hours, with both returning to near baseline levels after 24 hours. Similar effects were observed in the mouse model of CKD but were reduced in magnitude. CONCLUSIONS: Npt2a inhibition causes a dose-dependent increase in phosphate, sodium and chloride excretion associated with reductions in plasma phosphate and PTH levels in normal mice and in a CKD mouse model.

1,25-Dihydroxyvitamin D Maintains Brush Border Membrane NaPi2a and Attenuates Phosphaturia in Hyp Mice.

Phosphate homeostasis is critical for many cellular processes and is tightly regulated. The sodium-dependent phosphate cotransporter, NaPi2a, is the major regulator of urinary phosphate reabsorption in the renal proximal tubule. Its activity is dependent upon its brush border localization that is regulated by fibroblast growth factor 23 (FGF23) and PTH. High levels of FGF23, as are seen in the Hyp mouse model of human X-linked hypophosphatemia, lead to renal phosphate wasting. Long-term treatment of Hyp mice with 1,25-dihydroxyvitamin D (1,25D) or 1,25D analogues has been shown to improve renal phosphate wasting in the setting of increased FGF23 mRNA expression. Studies were undertaken to define the cellular and molecular basis for this apparent FGF23 resistance. 1,25D increased FGF23 protein levels in the cortical bone and circulation of Hyp mice but did not impair FGF23 cleavage. 1,25D attenuated urinary phosphate wasting as early as one hour postadministration, without suppressing FGF23 receptor/coreceptor expression. Although 1,25D treatment induced expression of early growth response 1, an early FGF23 responsive gene required for its phosphaturic effects, it paradoxically enhanced renal phosphate reabsorption and NaPi2a protein expression in renal brush border membranes (BBMs) within one hour. The Na-H+ exchange regulatory factor 1 (NHERF1) is a scaffolding protein thought to anchor NaPi2a to the BBM. Although 1,25D did not alter NHERF1 protein levels acutely, it enhanced NHERF1-NaPi2a interactions in Hyp mice. 1,25D also prevented the decrease in NHERF1/NaPi2a interactions in PTH-treated wild-type mice. Thus, these investigations identify a novel role for 1,25D in the hormonal regulation of renal phosphate handling.

Phosphaturia in kidney stone formers: Still an enigma.

Calcium kidney stones are common worldwide. Most are idiopathic and composed of calcium oxalate. Calcium phosphate is present in around 80% and may initiate stone formation. Stone production is multifactorial with a polygenic genetic contribution. Phosphaturia is found frequently among stone formers but until recently received scant attention. This review examines possible mechanisms for the phosphaturia and its relevance to stone formation from a wide angle. There is a striking lack of clinical data. Phosphaturia is associated, but not correlated, with hypercalciuria, increased 1,25 dihydroxy-vitamin D [1,25 (OH)2D], and sometimes evidence of disturbances in proximal renal tubular function. Phosphate reabsorption in the proximal renal tubules requires tightly regulated interaction of many proteins. Paracellular flow through intercellular tight junctions is the major route of phosphate absorption from the intestine and can be reduced therapeutically in hyperphosphatemic patients. In monogenic defects stones develop when phosphaturia is associated with hypercalciuria, generally explained by increased 1,25 (OH)2D production in response to hypophosphatemia. Calcification does not occur in disorders with increased FGF23 when phosphaturia occurs in isolation and 1,25 (OH)2D is suppressed. Candidate gene studies have identified mutations in the phosphate transporters, but in few individuals. One genome-wide study identified a polymorphism of the phosphate transporter gene SLC34A4 associated with stones. Others did not find mutations obviously linked to phosphate reabsorption. Future genetic studies should have a wide trawl and should focus initially on groups of patients with clearly defined phenotypes. The global data should be pooled.

X-linked hypophosphatemia: Management and treatment prospects.

X-linked hypophosphatemia (XLH), due to a PHEX gene mutation, is the most common genetic form of rickets and osteomalacia. Manifestations in children consist of rickets, lower-limb bone deformities, bone pain, failure to thrive, dental abscesses, and/or craniostenosis. Adults may present with persistent bone pain, early osteoarthritis, hairline fractures and Looser zones, enthesopathy, and/or periodontitis. Regardless of whether the patient is an infant, child, adolescent or adult, an early diagnosis followed by optimal treatment is crucial to control the clinical manifestations, prevent complications, and improve quality of life. Treatment options include active vitamin D analogs and phosphate supplementation to correct the 1.25(OH)2 vitamin D deficiency and to compensate for the renal phosphate wasting, respectively. The recently introduced FGF23 antagonist burosumab is designed to restore renal phosphate reabsorption by the proximal tubule and to stimulate endogenous calcitriol production. In Europe, burosumab is licensed for use in pediatric patients older than 1 year who have XLH. This review discusses the diagnosis and treatment of XLH and describes the indications of the various available treatments.