A loss-of-function mutation in NaPi-IIa and renal Fanconi's syndrome

Effects of SLC34A3 or SLC34A1 variants on calcium and phosphorus homeostasis

BACKGROUND

Variants in SLC34A1 and SLC34A2 genes, which encode co-transporters NaPi2a and NaPi2c, respectively, can lead to hypophosphatemia due to renal phosphate loss. This condition results in hypercalcitriolemia and hypercalciuria, leading to formation of kidney stones and nephrocalcinosis. Phenotype is highly variable. Management includes hyperhydration, dietary modifications, and/or phosphate supplementation. Thiazides and azoles may be used, but randomized studies are needed to confirm their clinical efficacy.

METHODS

We conducted a retrospective study in the pediatric nephrology unit at Grenoble University Hospital from January 2010 to December 2023. The study aimed to describe clinical and biological symptoms of patients with confirmed SLC34A1 and SLC34A3 gene variants and their outcomes.

RESULTS

A total of 11 patients (9 females) from 6 different families had variants in the SLC34A1 (5 patients) and SLC34A3 (6 patients) genes. Median age at diagnosis was 72 [1-108] months. Average follow-up duration was 8.1 ± 4.5 years. Presenting symptom was nephrocalcinosis (4 cases), followed by renal colic (3 cases). At diagnosis, 90% of patients had hypercalciuria and 45% had hypercalcitriolemia. Management included hyperhydration and dietary advice. All patients showed favorable outcomes with normal growth and school attendance. One patient with an SLC34A3 variant showed regression of nephrocalcinosis. Kidney function remained normal.

CONCLUSION

Clinical and biological manifestations of SLC34 gene variants are highly variable, even among siblings; therefore, management must be personalized. Hygienic and dietary measures (such as hyperhydration, a low sodium diet, and age-appropriate calcium intake) result in favorable outcomes in most cases. Use of azoles (e.g., fluconazole) appears to be a promising therapeutic option.

Phenotypic variability in phosphate transport disorders highlights need for individualized treatment strategies

Pathogenic variants in the SLC34A1 and SLC34A3 genes, encoding sodium-phosphate cotransporters 2a (NPT2a) and 2c (NPT2c), are linked to rare phosphate-wasting disorders. In this issue, Brunkhorst et al. explore the clinical presentations, biochemical profiles, and treatment outcomes associated with these genetic variants in 113 individuals. The study highlights distinct phenotypes, potential treatment challenges, and the need for further research to optimize therapeutic strategies and understand long-term outcomes for affected individuals.

CCDC158: A novel regulator in renal proximal tubular endocytosis unveiled through exome sequencing and interactome analysis

Renal proximal tubular reabsorption of proteins and polypeptides is tightly regulated by a concerted action of the multi-ligand receptors with subsequent processing from the clathrin-coated pits to early/recycling and late endosomes and towards lysosomes. We performed whole exome-sequencing in a male patient from a consanguineous family, who presented with low- and intermediate molecular weight proteinuria, nephrocalcinosis and oligospermia. We identified a new potential player in tubular endocytosis, coiled-coil domain containing 158 (CCDC158). The variant in CCDC158 segregated with the phenotype and was also detected in a female sibling with a similar clinical kidney phenotype. We demonstrated the expression of this protein in kidney tubules and modeled its structure in silico. We hypothesized that the protein played a role in the tubular endocytosis by interacting with other endocytosis regulators, and used mass spectrometry to identify potential interactors. The role of CCDC158 in receptor-mediated endocytosis was further confirmed by transferrin and GST-RAP trafficking analyses in patient-derived proximal tubular epithelial cells. Finally, as CCDC158 is known to be expressed in the testis, the presence of oligospermia in the male sibling further substantiated the pathogenic role of the detected missense variant in the observed phenotype. In this study, we provide data that demonstrate the potential role of CCDC158 in receptor-mediated endocytosis, most likely by interaction with other endocytosis-related proteins that strongly correlate with the proximal tubular dysfunction phenotype as observed in the patients. However, more studies are needed to fully unravel the molecular mechanism(s) in which CCDC158 is involved.

FAN1 Deletion Variant in Basenji Dogs with Fanconi Syndrome

Background: Fanconi syndrome is a disorder of renal proximal tubule transport characterized by metabolic acidosis, amino aciduria, glucosuria, and phosphaturia. There are acquired and hereditary forms of this disorder. A late-onset form of Fanconi syndrome in Basenjis was first described in 1976 and is now recognized as an inherited disease in these dogs. In part because of the late onset of disease signs, the disorder has not been eradicated from the breed by selective mating. A study was therefore undertaken to identify the molecular genetic basis of the disease so that dogs could be screened prior to breeding in order to avoid generating affected offspring. Methods: Linkage analysis within a large family of Basenjis that included both affected and unaffected individuals was performed to localize the causative variant within the genome. Significant linkage was identified between chromosome 3 (CFA3) makers and the disease phenotype. Fine mapping restricted the region to a 2.7 Mb section of CFA3. A whole genome sequence of a Basenji affected with Fanconi syndrome was generated, and the sequence data were examined for the presence of potentially deleterious homozygous variants within the mapped region. Results: A homozygous 317 bp deletion was identified in the last exon of FAN1 of the proband. 78 Basenjis of known disease status were genotyped for the deletion variant. Among these dogs, there was almost complete concordance between genotype and phenotype. The only exception was one dog that was homozygous for the deletion variant but did not exhibit signs of Fanconi syndrome. Conclusions: These data indicate that the disorder is very likely the result of FAN1 deficiency. The mechanism by which this deficiency causes the disease signs remains to be elucidated. FAN1 has endonuclease and exonuclease activity that catalyzes incisions in regions of double-stranded DNA containing interstrand crosslinks. FAN1 inactivation may cause Fanconi syndrome in Basenjis by sensitization of kidney proximal tubule cells to toxin-mediated DNA crosslinking, resulting in the accumulation of genomic and mitochondrial DNA damage in the kidney. Differential exposure to environmental toxins that promote DNA crosslink formation may explain the wide age-at-onset variability for the disorder in Basenjis.

Phosphate metabolism: its impact on disorders of mineral metabolism

Regulatory molecules typically work cooperatively to ensure the efficient functioning of hormonal systems. Examples include LH and FSH in reproductive biology, insulin and glucagon in glucose metabolism. Similarly, calcium and phosphorus are important regulators of skeletal homeostasis. In the circulation, these molecules are under the control of PTH, 1,25(OHD), and FGF23. In turn, these hormones depend upon a mutual and complex interplay among themselves. For example, alterations in calcium metabolism influence phosphorus homeostasis, as occurs in primary hyperparathyroidism (PHPT). Not as well recognized is the influence that abnormalities in phosphorus metabolism can have on calcium homeostasis. In this review, we call attention to the impact that abnormalities in phosphorus can have on calcium metabolism.

Severe hypophosphatemia induced by excessive production of FGF23 in acute hepatitis: from bedside to bench

Background

Although hepatic production of FGF23 has been suggested in chronic settings, there are no data indicating hypophosphatemia resulting from acute hepatic FGF23 production. Based on two clinical observations of profound hypophosphatemia in the setting of acute hepatitis, our study investigates the hypothesis of acute FGF23 liver expression.

Methods

Retrospective analyses were conducted to estimate FGF23 liver expression both qualitatively (in situ hybridization) and quantitatively (relative FGF23 gene expression and protein production) on histological specimens of human and murine acute hepatitis livers, compared with controls of hepatic fibrosis or healthy liver.

Results

The index clinical case involves acute alcoholic hepatitis complicated by profound hypophosphatemia due to phosphate diabetes, revealing a major production of both FGF23 C-terminal fraction (cFGF23) and bio-intact form (iFGF23, 39 751 RU/mL, N: 21-91; and 228.6 pg/mL, N: 22.7-93.1, respectively). A second case of acute hepatitis related to erythrocytic protoporphyria also exhibited comparable abnormalities. In both cases, no other cause of renal phosphate wasting was identified, and the hydroelectrolytic disorders disappeared in parallel with normalization of the liver balance and FGF23 levels. Histological data of acute hepatitis compared with cirrhosis and healthy liver confirmed our hypothesis of hepatic FGF23 overproduction. Furthermore, mouse models showed a significant increase in FGF23 mRNA relative liver expression in acute hepatitis and a moderate increase in cirrhosis, compared with healthy liver (respectively 60.55 ± 16.75 and 3.70 ± 0.87 vs 1.00 ± 0.65, both P < .05). These findings were also confirmed at the protein level.

Conclusion

This translational study raises the hypothesis of renal phosphate wasting induced by excessive hepatic production of FGF23 in case of acute hepatitis.

Complex phenotype in Fanconi renotubular syndrome type 1: Hypophosphatemic rickets as the predominant presentation

GATM-related Fanconi renotubular syndrome 1 (FRTS1) is a form of renal Fanconi syndrome (RFS), which is a disorder of solute and water reabsorption caused by defects in the function of the entire proximal tubule. Recent findings reveal the molecular basis of FRTS1: Intramitochondrial fiber aggregation triggered by mutant GATM provides a starting point for proximal tubule damage and drives disease progression. As a rare and newly recognized inherited kidney disease, the complex manifestations of FRTS1 are easily underdiagnosed or misdiagnosed. We discuss the complex phenotype of a 26-year-old woman with onset in infancy and a long history of hypophosphatemic rickets. We also identified a novel heterozygous missense variant in the GATM gene in this patient. The novel variant and phenotype we report expand the disease spectrum of FRTS1. We recommend screening for GATM in children with RFS, especially in patients with resistant rickets who have previously had negative genetic testing. In addition, we found pathological deposition of mutant GATM proteins within mitochondria in the patient's urinary sediment cells by a combination of electron microscopy and immunofluorescence. This unique urine cytology experiment has the potential to be a valuable tool for identifying patients with RRTS1.

Genetic causes of hypophosphatemia

Phosphate is a key component of mineralized tissues and is also part of many organic compounds. Phosphorus homeostasis depends especially upon intestinal absorption, and renal excretion, which are regulated by various hormones, such as PTH, 1,25-dihydroxyvitamin D, and fibroblast growth factor 23. In this review we provide an update of several genetic disorders that affect phosphate transporters through cell membranes or the phosphate-regulating hormones, and, consequently, result in hypophosphatemia.

Inherited non-FGF23-mediated phosphaturic disorders: A kidney-centric review

Phosphate is freely filtered by the glomerulus and reabsorbed exclusively in the proximal tubule by two key transporters, NaPiIIA and NaPiIIC, encoded by SLC34A1 and SLC34A3, respectively. Regulation of these transporters occurs primarily through the hormone FGF23 and, to a lesser degree, PTH. Consequently, inherited non-FGF23 mediated phosphaturic disorders are due to generalised proximal tubular dysfunction, loss-of-function variants in SLC34A1 or SLC34A3 or excess PTH signalling. The corresponding disorders are Renal Fanconi Syndrome, Infantile Hypercalcaemia type 2, Hereditary Hypophosphataemic Rickets with Hypercalciuria and Familial Hyperparathyroidism. Several inherited forms of Fanconi renotubular syndrome (FRTS) have also been described with the underlying genes encoding for GATM, EHHADH, HNF4A and NDUFAF6. Here, we will review their pathophysiology, clinical manifestations and the implications for treatment from a kidney-centric perspective, focussing on those disorders caused by dysfunction of renal phosphate transporters. Moreover, we will highlight specific genetic aspects, as the availability of large population genetic databases has raised doubts about some of the originally proposed gene-disease associations concerning phosphate transporters or their associated proteins.

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.

Ocular manifestations of the genetic renal tubulopathies

BACKGROUND

The genetic tubulopathies are rare and heterogenous disorders that are often difficult to identify. This study examined the tubulopathy-causing genes for ocular associations that suggested their genetic basis and, in some cases, the affected gene.

METHODS

Sixty-seven genes from the Genomics England renal tubulopathy panel were reviewed for ocular features, and for retinal expression in the Human Protein Atlas and an ocular phenotype in mouse models in the Mouse Genome Informatics database. The genes resulted in disease affecting the proximal tubules (n = 24); the thick ascending limb of the loop of Henle (n = 10); the distal convoluted tubule (n = 15); or the collecting duct (n = 18).

RESULTS

Twenty-five of the tubulopathy-associated genes (37%) had ocular features reported in human disease, 49 (73%) were expressed in the retina, although often at low levels, and 16 (24%) of the corresponding mouse models had an ocular phenotype. Ocular abnormalities were more common in genes affected in the proximal tubulopathies (17/24, 71%) than elsewhere (7/43, 16%). They included structural features (coloboma, microphthalmia); refractive errors (myopia, astigmatism); crystal deposition (in oxalosis, cystinosis) and sclerochoroidal calcification (in Bartter, Gitelman syndromes). Retinal atrophy was common in the mitochondrial-associated tubulopathies. Structural abnormalities and crystal deposition were present from childhood, but sclerochoroidal calcification typically occurred after middle age.

CONCLUSIONS

Ocular abnormalities are uncommon in the genetic tubulopathies but may be helpful in recognizing the underlying genetic disease. The retinal expression and mouse phenotype data suggest that further ocular associations may become apparent with additional reports. Early identification may be necessary to monitor and treat visual complications.

Genetic Diagnosis and Treatment of Inherited Renal Tubular Acidosis

Background

Renal tubular acidosis (RTA) is caused by various disruptions to the secretion of H+ by distal renal tubules and/or dysfunctional reabsorption of HCO3- by proximal renal tubules, which causes renal acidification dysfunction, ultimately leading to a clinical syndrome characterized by hyperchloremic metabolic acidosis with a normal anion gap. With the development of molecular genetics and gene sequencing technology, inherited RTA has also attracted attention, and an increasing number of RTA-related pathogenic genes have been discovered and reported.

Summary

This paper focuses on the latest progress in the research of inherited RTA and systematically reviews the pathogenic genes, protein functions, clinical manifestations, internal relationship between genotypes and clinical phenotypes, diagnostic clues, differential diagnosis, and treatment strategies associated with inherited RTA. This paper aims to deepen the understanding of inherited RTA and reduce the missed diagnosis and misdiagnosis of RTA.

Key Messages

This review systematically summarizes the pathogenic genes, pathophysiological mechanisms, differential diagnosis, and treatment of different types of inherited RTA, which has good clinical value for guiding the diagnosis and treatment of inherited RTA.

Transmembrane Transport of Inorganic Phosphate by a Strapped Calix[4]pyrrole

Synthetic anion receptors are increasingly being explored for the transport of anions across lipid membranes because of their potential therapeutic applications. A considerable amount of research focuses on the transport of chloride, whereas the transmembrane transport of inorganic phosphate has not been reported to date, despite the biological relevance of this anion. Here we present a calix[4]pyrrole with a bisurea strap that functions as a receptor and transporter for H₂PO₄^-, relying on the formation of eight hydrogen bonds and efficient encapsulation of the anion. Using a phosphate-sensitive lanthanide probe and ³¹P NMR spectroscopy, we demonstrate that this receptor can transport phosphate into vesicles by H₂PO₄^-/Cl^- antiport, H₂PO₄^- uniport, and Cs⁺/H₂PO₄^- symport mechanisms. This first example of inorganic phosphate transport by a neutral receptor opens perspectives for the future development of transporters for various biological phosphates.

Inherited Fanconi syndrome

BACKGROUND

Fanconi-Debré-de Toni syndrome (also known as Fanconi renotubular syndrome, or FRST) profoundly increased the understanding of the functions of the proximal convoluted tubule (PCT) and provided important insights into the pathophysiology of several kidney diseases and drug toxicities.

DATA SOURCES

We searched Pubmed and Scopus databases to find relevant articles about FRST. This review article focuses on the physiology of the PCT, as well as on the physiopathology of FRST in children, its diagnosis, and treatment.

RESULTS

FRST encompasses a wide variety of inherited and acquired PCT alterations that lead to impairment of PCT reabsorption. In children, FRST often presents as a secondary feature of systemic disorders that impair energy supply, such as Lowe's syndrome, Dent's disease, cystinosis, hereditary fructose intolerance, galactosemia, tyrosinemia, Alport syndrome, and Wilson's disease. Although rare, congenital causes of FRST greatly impact the morbidity and mortality of patients and impose diagnostic challenges. Furthermore, its treatment is diverse and considers the ability of the clinician to identify the correct etiology of the disease.

CONCLUSION

The early diagnosis and treatment of pediatric patients with FRST improve the prognosis and the quality of life.

Exposure to SARS-CoV-2 and Infantile Diseases

Background and Aim  Immune response against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in newborns and children after prophylactic immunization is currently a relevant research topic. The present study analyzes the issue by examining the possibility that the anti-SARS-CoV-2 immune responses are not uniquely directed against the virus but can-via molecular mimicry and the consequent cross-reactivity-also hit human proteins involved in infantile diseases. Methods  Human proteins that-if altered-associate with infantile disorders were searched for minimal immune pentapeptide determinants shared with SARS-CoV-2 spike glycoprotein (gp). Then, the shared pentapeptides were analyzed for immunologic potential and immunologic imprinting phenomena. Results  Comparative sequence analysis shows that: (1) numerous pentapeptides (namely, 54) are common to SARS-CoV-2 spike gp and human proteins that, when altered, are linked to infantile diseases; (2) all the shared peptides have an immunologic potential since they are present in experimentally validated SARS-CoV-2 spike gp-derived epitopes; and (3) many of the shared peptides are also hosted in infectious pathogens to which children can have already been exposed, thus making immunologic imprint phenomena feasible. Conclusion  Molecular mimicry and the consequent cross-reactivity can represent the mechanism that connects exposure to SARS-CoV-2 and various pediatric diseases, with a fundamental role of the immunologic memory and the history of the child's infections in determining and specifying the immune response and the pathologic autoimmune sequela.

Kidney glycolysis serves as a mammalian phosphate sensor that maintains phosphate homeostasis

How phosphate levels are detected in mammals is unknown. The bone-derived hormone fibroblast growth factor 23 (FGF23) lowers blood phosphate levels by reducing kidney phosphate reabsorption and 1,25(OH)2D production, but phosphate does not directly stimulate bone FGF23 expression. Using PET scanning and LC-MS, we found that phosphate increases kidney-specific glycolysis and synthesis of glycerol-3-phosphate (G-3-P), which then circulates to bone to trigger FGF23 production. Further, we found that G-3-P dehydrogenase 1 (Gpd1), a cytosolic enzyme that synthesizes G-3-P and oxidizes NADH to NAD+, is required for phosphate-stimulated G-3-P and FGF23 production and prevention of hyperphosphatemia. In proximal tubule cells, we found that phosphate availability is substrate-limiting for glycolysis and G-3-P production and that increased glycolysis and Gpd1 activity are coupled through cytosolic NAD+ recycling. Finally, we show that the type II sodium-dependent phosphate cotransporter Npt2a, which is primarily expressed in the proximal tubule, conferred kidney specificity to phosphate-stimulated G-3-P production. Importantly, exogenous G-3-P stimulated FGF23 production when Npt2a or Gpd1 were absent, confirming that it was the key circulating factor downstream of glycolytic phosphate sensing in the kidney. Together, these findings place glycolysis at the nexus of mineral and energy metabolism and identify a kidney-bone feedback loop that controls phosphate homeostasis.

Mineral Metabolism in Children: Interrelation between Vitamin D and FGF23

Fibroblast growth factor 23 (FGF23) was identified at the turn of the century as the long-sought circulating phosphatonin in human pathology. Since then, several clinical and experimental studies have investigated the metabolism of FGF23 and revealed its relevant pathogenic role in various diseases. Most of these studies have been performed in adult individuals. However, the mineral metabolism of the child is, to a large extent, different from that of the adult because, in addition to bone remodeling, the child undergoes a specific process of endochondral ossification responsible for adequate mineralization of long bones’ metaphysis and growth in height. Vitamin D metabolism is known to be deeply involved in these processes. FGF23 might have an influence on bones’ growth as well as on the high and age-dependent serum phosphate concentrations found in infancy and childhood. However, the interaction between FGF23 and vitamin D in children is largely unknown. Thus, this review focuses on the following aspects of FGF23 metabolism in the pediatric age: circulating concentrations’ reference values, as well as those of other major variables involved in mineral homeostasis, and the relationship with vitamin D metabolism in the neonatal period, in vitamin D deficiency, in chronic kidney disease (CKD) and in hypophosphatemic disorders.

A novel variant in GATM causes idiopathic renal Fanconi syndrome and predicts progression to end-stage kidney disease

Renal Fanconi syndrome (RFS) is a generalised disorder of the proximal convoluted tubule. Many genes have been associated with RFS including those that cause systemic disorders such as cystinosis, as well as isolated RFS. We discuss the case of a 10-year-old female who presented with leg pain and raised creatinine on a screening blood test. Her mother has RFS and required a kidney transplant in her thirties. Further investigations confirmed RFS in the daughter. Exome sequencing was performed on the affected mother, child, and unaffected father. We identified a novel variant in GATM; c.965G>C p.(Arg322Pro) segregating dominantly in the mother and daughter. We validated our finding with molecular dynamics simulations and demonstrated a dynamic signature that differentiates our variant and two previously identified pathogenic variants in GATM from wildtype. Genetic testing has uncovered a novel pathogenic variant that predicts progression to end stage kidney failure and has important implications for family planning and cascade testing. We recommend that GATM is screened for in children presenting with RFS, in addition to adults, particularly with kidney failure, who may have had previous negative gene testing.

De novo 11q13.3q13.4 deletion in a patient with Fanconi renotubular syndrome and intellectual disability: Case report and review of literature

Objective

To explore the genetic etiology of a child with facial dysmorphia, developmental delay, intellectual disability, Fanconi renotubular syndrome, and Chiari malformations.

Materials and methods

Whole exome sequencing (WES), Copy number variation sequencing (CNV-seq), and mitochondrial gene detection (Long-PCR + NGS) were applied to detect possible pathogenic mutations and chromosomal copy number variations (CNVs), together with databases and literature reviews to clarify the pathological significance of the candidate mutations.

Results

The WES revealed a 2.10 Mb interstitial deletion from 11q13.3 to 11q13.4, which was later confirmed by CNV-seq involving 11 OMIM genes, among which SHANK2, DHCR7, NADSYN1, FADD, NUMA1, IL18BP, ANO1, and FGF3 are disease-causing. The mitochondrial gene shows no variations.

Conclusion

The child has carried a de novo 11q13.3q13.4 microdeletion, in which SHANK2 genes may be the key gene responsible for the phenotype of intellectual disability. The renal manifestation of the child, which can be diagnosed as Fanconi renotubular syndrome, has an unknown cause but may result from the effect of the ANO1 gene. This case adds a new phenotype to the deletion of this region.

Approach to Hypophosphatemic Rickets

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

Altered Serum Uric Acid Levels in Kidney Disorders

Serum uric acid levels are altered by kidney disorders because the kidneys play a dominant role in uric acid excretion. Here, major kidney disorders which accompany hyperuricemia or hypouricemia, including their pathophysiology, are discussed. Chronic kidney disease (CKD) and hyperuricemia are frequently associated, but recent clinical trials have not supported the pathogenic roles of hyperuricemia in CKD incidence and progression. Diabetes mellitus (DM) is often associated with hyperuricemia, and hyperuricemia may be associated with an increased risk of diabetic kidney disease in patients with type 2 DM. Sodium-glucose cotransporter 2 inhibitors have a uricosuric effect and can relieve hyperuricemia in DM. Autosomal dominant tubulointerstitial kidney disease (ADTKD) is an important hereditary kidney disease, mainly caused by mutations of uromodulin (UMOD) or mucin-1 (MUC-1). Hyperuricemia and gout are the major clinical manifestations of ADTKD-UMOD and ADTKD-MUC1. Renal hypouricemia is caused by URAT1 or GLUT9 loss-of-function mutations and renders patients susceptible to exercise-induced acute kidney injury, probably because of excessive urinary uric acid excretion. Hypouricemia derived from renal uric acid wasting is a component of Fanconi syndrome, which can be hereditary or acquired. During treatment for human immunodeficiency virus, hepatitis B or cytomegalovirus, tenofovir, adefovir, and cidofovir may cause drug-induced renal Fanconi syndrome. In coronavirus disease 2019, hypouricemia due to proximal tubular injury is related to disease severity, including respiratory failure. Finally, serum uric acid and the fractional excretion of uric acid are indicative of plasma volume status; hyperuricemia caused by the enhanced uric acid reabsorption can be induced by volume depletion, and hypouricemia caused by an increased fractional excretion of uric acid is the characteristic finding in syndromes of inappropriate anti-diuresis, cerebral/renal salt wasting, and thiazide-induced hyponatremia. Molecular mechanisms by which uric acid transport is dysregulated in volume or water balance disorders need to be investigated.

Valproic-induced Fanconi syndrome: Clinical features, risk factors, diagnosis and management

Objective

Although Fanconi syndrome (FS) induced by valproate (VPA) has occasionally been reported, the detailed clinical features of the disease remain unclear. The aim of this study was to elucidate the clinical features of patients with VPA-induced FS.

Methods

We searched Chinese and English databases for all original studies, clinical reports, and case reports on VPA-induced FS published before March 2022.

Results

A total of 29 articles including 54 patients (28 males and 24 females) were included. The patients had a median age of 7 years (range 2–34 years), had severely disabled (87.0%), tube feeding (64.8%), and received an average of 1.8 medications other than VPA. The median duration of VPA treatment was 4 years (range 0.7–15.5). Pathological fractures (25.9%), unexplained fever (11.1%), muscle weakness (9.3%), and edema (9.3%) were the most common symptoms, while 18 patients were diagnosed in incidental laboratory tests. Blood tests revealed hypokalemia (69.2%), hypophosphatemia (98.0%), and hypouricemia (93.3%). Urinalysis revealed glucosuria (96.1%), proteinuria (100.0%), generalized hyperaminoaciduria (100.0 %), β2 macroglobulin (100.0%). Decreased percent total reabsorption of phosphate (%TRP) found in 94.1% of patients, and increased fractional excretion of uric acid (FEUA) were found in 100% of patients. The median time to resolution of FS after discontinuation of drug therapy was 3 months (range 0.25–18).

Conclusions

The possibility of FS needs to be considered with long-term VPA administration, especially in young, tube-fed, severely disabled patients who are co-administered with anticonvulsants. Patients receiving VPA should have regular blood and urine tests. Abnormal laboratory values returned to normal levels after VPA discontinuation.

Hypouricemia and Urate Transporters

Hypouricemia is recognized as a rare disorder, defined as a serum uric acid level of 2.0 mg/dL or less. Hypouricemia is divided into an overexcretion type and an underproduction type. The former typical disease is xanthinuria, and the latter is renal hypouricemia (RHUC). The frequency of nephrogenic hypouricemia due to a deficiency of URAT1 is high in Japan, accounting for most asymptomatic and persistent cases of hypouricemia. RHUC results in a high risk of exercise-induced acute kidney injury and urolithiasis. It is vital to promote research on RHUC, as this will lead not only to the elucidation of its pathophysiology but also to the development of new treatments for gout and hyperuricemia.

Genetic causes of neonatal and infantile hypercalcaemia

The causes of hypercalcaemia in the neonate and infant are varied, and often distinct from those in older children and adults. Hypercalcaemia presents clinically with a range of symptoms including failure to thrive, poor feeding, constipation, polyuria, irritability, lethargy, seizures and hypotonia. When hypercalcaemia is suspected, an accurate diagnosis will require an evaluation of potential causes (e.g. family history) and assessment for physical features (such as dysmorphology, or subcutaneous fat deposits), as well as biochemical measurements, including total and ionised serum calcium, serum phosphate, creatinine and albumin, intact parathyroid hormone (PTH), vitamin D metabolites and urinary calcium, phosphate and creatinine. The causes of neonatal hypercalcaemia can be classified into high or low PTH disorders. Disorders associated with high serum PTH include neonatal severe hyperparathyroidism, familial hypocalciuric hypercalcaemia and Jansen's metaphyseal chondrodysplasia. Conditions associated with low serum PTH include idiopathic infantile hypercalcaemia, Williams-Beuren syndrome and inborn errors of metabolism, including hypophosphatasia. Maternal hypocalcaemia and dietary factors and several rare endocrine disorders can also influence neonatal serum calcium levels. This review will focus on the common causes of hypercalcaemia in neonates and young infants, considering maternal, dietary, and genetic causes of calcium dysregulation. The clinical presentation and treatment of patients with these disorders will be discussed.

Vitamin D-dependent Hypercalcemia

Vitamin D metabolism represents a well-integrated, hormonally regulated endocrine unit interlinking calcium and phosphate metabolism. Pathophysiologic processes disturbing vitamin D metabolism comprise classic defects of vitamin D activation and action presenting as different forms of vitamin D-dependent rickets as well as disorders with increased vitamin D activity. The latter may result in hypercalcemia, hypercalciuria, and renal calcifications. Acquired and hereditary disorders causing hypervitaminosis D are discussed, including vitamin D intoxication, granulomatous disease, and idiopathic infantile hypercalcemia that may be caused by either a defective vitamin D degradation or by a primary defect in phosphate conservation.

Comprehensive Genetic Analysis Reveals Complexity of Monogenic Urinary Stone Disease

Introduction

Because of phenotypic overlap between monogenic urinary stone diseases (USD), gene-specific analyses can result in missed diagnoses. We used targeted next generation sequencing (tNGS), including known and candidate monogenic USD genes, to analyze suspected primary hyperoxaluria (PH) or Dent disease (DD) patients genetically unresolved (negative; N) after Sanger analysis of the known genes. Cohorts consisted of 285 PH (PHN) and 59 DD (DDN) families.

Methods

Variants were assessed using disease-specific and population databases plus variant assessment tools and categorized using the American College of Medical Genetics (ACMG) guidelines. Prior Sanger analysis identified 47 novel PH or DD gene pathogenic variants.

Results

Screening by tNGS revealed pathogenic variants in 14 known monogenic USD genes, accounting for 45 families (13.1%), 27 biallelic and 18 monoallelic, including 1 family with a copy number variant (CNV). Recurrent genes included the following: SLC34A3 (n = 13), CLDN16 (n = 8), CYP24A1 (n = 4), SLC34A1 (n = 3), SLC4A1 (n = 3), APRT (n = 2), CLDN19 (n = 2), HNF4A1 (n = 2), and KCNJ1 (n = 2), whereas ATP6V1B1, CASR, and SLC12A1 and missed CNVs in the PH genes AGXT and GRHPR accounted for 1 pedigree each. Of the 48 defined pathogenic variants, 27.1% were truncating and 39.6% were novel. Most patients were diagnosed before 18 years of age (76.1%), and 70.3% of biallelic patients were homozygous, mainly from consanguineous families.

Conclusion

Overall, in patients suspected of DD or PH, 23.9% and 7.3% of cases, respectively, were caused by pathogenic variants in other genes. This study shows the value of a tNGS screening approach to increase the diagnosis of monogenic USD, which can optimize therapies and facilitate enrollment in clinical trials.

BCS1L mutations produce Fanconi syndrome with developmental disability

Fanconi syndrome is a functional disorder of the proximal tubule, characterized by pan-aminoaciduria, glucosuria, hypophosphatemia, and metabolic acidosis. With the advancements in gene analysis technologies, several causative genes are identified for Fanconi syndrome. Several mitochondrial diseases cause Fanconi syndrome and various systemic symptoms; however, it is rare that the main clinical symptoms in such disorders are Fanconi syndrome without systematic active diseases like encephalomyopathy or cardiomyopathy. In this study, we analyzed two families exhibiting Fanconi syndrome, developmental disability and mildly elevated liver enzyme levels. Whole-exome sequencing (WES) detected compound heterozygous known and novel BCS1L mutations, which affect the assembly of mitochondrial respiratory chain complex III, in both cases. The pathogenicity of these mutations has been established in several mitochondria-related functional analyses in this study. Mitochondrial diseases with isolated renal symptoms are uncommon; however, this study indicates that mitochondrial respiratory chain complex III deficiency due to BCS1L mutations cause Fanconi syndrome with developmental disability as the primary indications.

Unusual presentation of a five-month-old boy with NaPi2a homozygous mutation without hyperphosphaturia: Case report and review of the literature

Deletions of the NaPi2a gene and mutations in the SLC34A gene should be considered in patients with atypical presentation, without phosphaturia, with mild hypo to normal phosphatemia, and nephrocalcinosis.

Inflammation-like changes in the urothelium of Lifr-deficient mice and LIFR-haploinsufficient humans with urinary tract anomalies

Congenital anomalies of the kidney and urinary tract (CAKUT) are the most common cause of end-stage kidney disease in children. While the genetic aberrations underlying CAKUT pathogenesis are increasingly being elucidated, their consequences on a cellular and molecular level commonly remain unclear. Recently, we reported rare heterozygous deleterious LIFR variants in 3.3% of CAKUT patients, including a novel de novo frameshift variant, identified by whole-exome sequencing, in a patient with severe bilateral CAKUT. We also demonstrated CAKUT phenotypes in Lifr-/- and Lifr+/- mice, including a narrowed ureteric lumen due to muscular hypertrophy and a thickened urothelium. Here, we show that both in the ureter and bladder of Lifr-/- and Lifr+/- embryos, differentiation of the three urothelial cell types (basal, intermediate and superficial cells) occurs normally but that the turnover of superficial cells is elevated due to increased proliferation, enhanced differentiation from their progenitor cells (intermediate cells) and, importantly, shedding into the ureteric lumen. Microarray-based analysis of genome-wide transcriptional changes in Lifr-/- versus Lifr+/+ ureters identified gene networks associated with an antimicrobial inflammatory response. Finally, in a reverse phenotyping effort, significantly more superficial cells were detected in the urine of CAKUT patients with versus without LIFR variants indicating conserved LIFR-dependent urinary tract changes in the murine and human context. Our data suggest that LIFR signaling is required in the epithelium of the urinary tract to suppress an antimicrobial response under homeostatic conditions and that genetically induced inflammation-like changes underlie CAKUT pathogenesis in Lifr deficiency and LIFR haploinsufficiency.

The Complexities of Organ Crosstalk in Phosphate Homeostasis: Time to Put Phosphate Sensing Back in the Limelight

Phosphate homeostasis is essential for health and is achieved via interaction between the bone, kidney, small intestine, and parathyroid glands and via intricate processes involving phosphate transporters, phosphate sensors, and circulating hormones. Numerous genetic and acquired disorders are associated with disruption in these processes and can lead to significant morbidity and mortality. The role of the kidney in phosphate homeostasis is well known, although it is recognized that the cellular mechanisms in murine models and humans are different. Intestinal phosphate transport also appears to differ in humans and rodents, with recent studies demonstrating a dominant role for the paracellular pathway. The existence of phosphate sensing has been acknowledged for decades; however, the underlying molecular mechanisms are poorly understood. At least three phosphate sensors have emerged. PiT2 and FGFR1c both act as phosphate sensors controlling Fibroblast Growth Factor 23 secretion in bone, whereas the calcium-sensing receptor controls parathyroid hormone secretion in response to extracellular phosphate. All three of the proposed sensors are expressed in the kidney and intestine but their exact function in these organs is unknown. Understanding organ interactions and the mechanisms involved in phosphate sensing requires significant research to develop novel approaches for the treatment of phosphate homeostasis disorders.

The genetic polymorphisms of XPR1 and SCL34A3 are associated with Fanconi syndrome in Chinese patients of tumor-induced osteomalacia

PURPOSE

Tumor-induced osteomalacia (TIO) is an acquired form of hypophosphatemia caused by tumors with excess production of fibroblast growth factor 23 (FGF23). Some reports showed that TIO patients had renal Fanconi syndrome (FS) with unidentified mechanism. In this study, we investigated the association between genetic polymorphisms of phosphate transporters in renal proximal tubules and TIO with FS.

METHODS

We recruited 30 TIO patients with FS (TIO-FS) as well as 30 TIO patients (TIO-nonFS) without any urine abnormalities matched by age and gender. We collected clinical manifestations and conducted targeted sequencing of SLC34A1, SLC34A3 and XPR1 genes and the association analysis between variants in TIO with FS and phenotypes.

RESULTS

TIO-FS group had lower levels of serum phosphate (0.44 ± 0.12 vs. 0.51 ± 0.07 mmol/L, p < 0.05) than TIO-nonFS group. Among the 16 SNPs in SLC34A1, SLC34A3 and XPR1 genes, GG/GC genotypes of rs148196667 in XPR1 and AA/TA genotypes of rs35535797 in SLC34A3 were associated with a reduced susceptibility to have FS. The G allele of rs148196667 in XPR1 decreased the risk of FS. The GGAA haplotype in SLC34A3 and GCT haplotype in XPR1 were associated with a decreased risk for FS.

CONCLUSIONS

The polymorphisms of XPR1 and SCL34A3 are associated with TIO patients with Fanconi syndrome. It provides novel insight to the relationship of phosphate transportation and general functions of renal proximal tubules.

Membrane Transport Proteins in Osteoclasts: The Ins and Outs

During bone resorption, the osteoclast must sustain an extraordinarily low pH environment, withstand immense ionic pressures, and coordinate nutrient and waste exchange across its membrane to sustain its unique structural and functional polarity. To achieve this, osteoclasts are equipped with an elaborate set of membrane transport proteins (pumps, transporters and channels) that serve as molecular ‘gatekeepers’ to regulate the bilateral exchange of ions, amino acids, metabolites and macromolecules across the ruffled border and basolateral domains. Whereas the importance of the vacuolar-ATPase proton pump and chloride voltage-gated channel 7 in osteoclasts has long been established, comparatively little is known about the contributions of other membrane transport proteins, including those categorized as secondary active transporters. In this Special Issue review, we provide a contemporary update on the ‘ins and outs’ of membrane transport proteins implicated in osteoclast differentiation, function and bone homeostasis and discuss their therapeutic potential for the treatment of metabolic bone diseases.

Chromatin architecture reveals cell type-specific target genes for kidney disease risk variants

BACKGROUND

Cell type-specific transcriptional programming results from the combinatorial interplay between the repertoire of active regulatory elements. Disease-associated variants disrupt such programming, leading to altered expression of downstream regulated genes and the onset of pathological states. However, due to the non-linear regulatory properties of non-coding elements such as enhancers, which can activate transcription at long distances and in a non-directional way, the identification of causal variants and their target genes remains challenging. Here, we provide a multi-omics analysis to identify regulatory elements associated with functional kidney disease variants, and downstream regulated genes.

RESULTS

In order to understand the genetic risk of kidney diseases, we generated a comprehensive dataset of the chromatin landscape of human kidney tubule cells, including transcription-centered 3D chromatin organization, histone modifications distribution and transcriptome with HiChIP, ChIP-seq and RNA-seq. We identified genome-wide functional elements and thousands of interactions between the distal elements and target genes. The results revealed that risk variants for renal tumor and chronic kidney disease were enriched in kidney tubule cells. We further pinpointed the target genes for the variants and validated two target genes by CRISPR/Cas9 genome editing techniques in zebrafish, demonstrating that SLC34A1 and MTX1 were indispensable genes to maintain kidney function.

CONCLUSIONS

Our results provide a valuable multi-omics resource on the chromatin landscape of human kidney tubule cells and establish a bioinformatic pipeline in dissecting functions of kidney disease-associated variants based on cell type-specific epigenome.

The Causes of Hypo- and Hyperphosphatemia in Humans

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

Congenital Conditions of Hypophosphatemia in Children

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

Novel Fanconi renotubular syndromes provide insights in proximal tubule pathophysiology

The various forms of Fanconi renotubular syndromes (FRTS) offer significant challenges for clinicians and present unique opportunities for scientists who study proximal tubule physiology. This review will describe the clinical characteristics, genetic underpinnings, and underlying pathophysiology of the major forms of FRST. Although the classic forms of FRTS will be presented (e.g., Dent disease or Lowe syndrome), particular attention will be paid to five of the most recently discovered FRTS subtypes caused by mutations in the genes encoding for L-arginine:glycine amidinotransferase (GATM), solute carrier family 34 (type Ii sodium/phosphate cotransporter), member 1 (SLC34A1), enoyl-CoAhydratase/3-hydroxyacyl CoA dehydrogenase (EHHADH), hepatocyte nuclear factor 4A (HNF4A), or NADH dehydrogenase complex I, assembly factor 6 (NDUFAF6). We will explore how mutations in these genes revealed unexpected mechanisms that led to compromised proximal tubule functions. We will also describe the inherent challenges associated with gene discovery studies based on findings derived from small, single-family studies by focusing the story of FRTS type 2 (SLC34A1). Finally, we will explain how extensive alternative splicing of HNF4A has resulted in confusion with mutation nomenclature for FRTS type 4.

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.

Hnf4a Is Required for the Development of Cdh6-Expressing Progenitors into Proximal Tubules in the Mouse Kidney

BACKGROUND

Hepatocyte NF 4α (Hnf4a) is a major regulator of renal proximal tubule (PT) development. In humans, a mutation in HNF4A impairs PT functions and is associated with Fanconi renotubular syndrome (FRTS). In mice, mosaic deletion of Hnf4a in the developing kidney reduces the population of PT cells, leading to FRTS-like symptoms. The molecular mechanisms underlying the role of Hnf4a in PT development remain unclear.

METHODS

The gene deletion tool Osr2Cre removed Hnf4a in developing nephrons in mice, generating a novel model for FRTS. Immunofluorescence analysis characterized the mutant phenotype, and lineage analysis tested whether Cadherin-6 (Cdh6)-expressing cells are PT progenitors. Genome-wide mapping of Hnf4a binding sites and differential gene analysis of Hnf4a mutant kidneys identified direct target genes of Hnf4a.

RESULTS

Deletion of Hnf4a with Osr2Cre led to the complete loss of mature PT cells, lethal to the Hnf4a mutant mice. Cdh6^high, lotus tetragonolobus lectin-low (LTL^low) cells serve as PT progenitors and demonstrate higher proliferation than Cdh6^low, LTL^high differentiated PT cells. Additionally, Hnf4a is required for PT progenitors to differentiate into mature PT cells. Genomic analyses revealed that Hnf4a directly regulates the expression of genes involved in transmembrane transport and metabolism.

CONCLUSIONS

Hnf4a promotes the differentiation of PT progenitors into mature PT cells by regulating the expression of genes associated with reabsorption, the major function of PT cells.

Phosphate Transport in Epithelial and Nonepithelial Tissue

Phosphate is an essential nutrient for life and is a critical component of bone formation, a major signaling molecule, and structural component of cell walls. Phosphate is also a component of high-energy compounds (i.e., AMP, ADP, and ATP) and essential for nucleic acid helical structure (i.e., RNA and DNA). Phosphate plays a central role in the process of mineralization, normal serum levels being associated with appropriate bone mineralization, while high and low serum levels are associated with soft tissue calcification. The serum concentration of phosphate and the total body content of phosphate are highly regulated, a process that is accomplished by the coordinated effort of two families of sodium-dependent transporter proteins. The three isoforms of the SLC34 family (SLC34A1-A3) show very restricted tissue expression and regulate intestinal absorption and renal excretion of phosphate. SLC34A2 also regulates the phosphate concentration in multiple lumen fluids including milk, saliva, pancreatic fluid, and surfactant. Both isoforms of the SLC20 family exhibit ubiquitous expression (with some variation as to which one or both are expressed), are regulated by ambient phosphate, and likely serve the phosphate needs of the individual cell. These proteins exhibit similarities to phosphate transporters in nonmammalian organisms. The proteins are nonredundant as mutations in each yield unique clinical presentations. Further research is essential to understand the function, regulation, and coordination of the various phosphate transporters, both the ones described in this review and the phosphate transporters involved in intracellular transport.

Inherited Renal Tubulopathies-Challenges and Controversies

Electrolyte homeostasis is maintained by the kidney through a complex transport function mostly performed by specialized proteins distributed along the renal tubules. Pathogenic variants in the genes encoding these proteins impair this function and have consequences on the whole organism. Establishing a genetic diagnosis in patients with renal tubular dysfunction is a challenging task given the genetic and phenotypic heterogeneity, functional characteristics of the genes involved and the number of yet unknown causes. Part of these difficulties can be overcome by gathering large patient cohorts and applying high-throughput sequencing techniques combined with experimental work to prove functional impact. This approach has led to the identification of a number of genes but also generated controversies about proper interpretation of variants. In this article, we will highlight these challenges and controversies.

Mechanisms of phosphate transport

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

Proximal renal tubular acidosis with and without Fanconi syndrome

Proximal renal tubular acidosis (RTA) is caused by a defect in bicarbonate (HCO₃⁻) reabsorption in the kidney proximal convoluted tubule. It usually manifests as normal anion-gap metabolic acidosis due to HCO₃⁻ wastage. In a normal kidney, the thick ascending limb of Henle’s loop and more distal nephron segments reclaim all of the HCO₃⁻ not absorbed by the proximal tubule. Bicarbonate wastage seen in type II RTA indicates that the proximal tubular defect is severe enough to overwhelm the capacity for HCO₃⁻ reabsorption beyond the proximal tubule. Proximal RTA can occur as an isolated syndrome or with other impairments in proximal tubular functions under the spectrum of Fanconi syndrome. Fanconi syndrome, which is characterized by a defect in proximal tubular reabsorption of glucose, amino acids, uric acid, phosphate, and HCO₃⁻, can occur due to inherited or acquired causes. Primary inherited Fanconi syndrome is caused by a mutation in the sodium-phosphate cotransporter (NaP_i-II) in the proximal tubule. Recent studies have identified new causes of Fanconi syndrome due to mutations in the EHHADH and the HNF4A genes. Fanconi syndrome can also be one of many manifestations of various inherited systemic diseases, such as cystinosis. Many of the acquired causes of Fanconi syndrome with or without proximal RTA are drug-induced, with the list of causative agents increasing as newer drugs are introduced for clinical use, mainly in the oncology field.

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.

Endocrine regulation of MFS2 by branchless controls phosphate excretion and stone formation in Drosophila renal tubules

How inorganic phosphate (Pi) homeostasis is regulated in Drosophila is currently unknown. We here identify MFS2 as a key Pi transporter in fly renal (Malpighian) tubules. Consistent with its role in Pi excretion, we found that dietary Pi induces MFS2 expression. This results in the formation of Malpighian calcium-Pi stones, while RNAi-mediated knockdown of MFS2 increases blood (hemolymph) Pi and decreases formation of Malpighian tubule stones in flies cultured on high Pi medium. Conversely, microinjection of adults with the phosphaturic human hormone fibroblast growth factor 23 (FGF23) induces tubule expression of MFS2 and decreases blood Pi. This action of FGF23 is blocked by genetic ablation of MFS2. Furthermore, genetic overexpression of the fly FGF branchless (bnl) in the tubules induces expression of MFS2 and increases Malpighian tubule stones suggesting that bnl is the endogenous phosphaturic hormone in adult flies. Finally, genetic ablation of MFS2 increased fly life span, suggesting that Malpighian tubule stones are a key element whereby high Pi diet reduces fly longevity previously reported by us. In conclusion, MFS2 mediates excretion of Pi in Drosophila, which is as in higher species under the hormonal control of FGF-signaling.

A novel heterozygous mutation c.680A>G (p. N227S) in SLC34A1 gene leading to autosomal dominant hypophosphatemia: A case report

RATIONALE

Currently, the relationship between heterozygous mutations in SLC34A1 and hypophosphatemia is controversial. Here we report an autosomal dominant hypophosphatemia pedigree carrying a novel heterozygous mutation in SLC34A1.

PATIENT CONCERNS

The proband is a 32-year old young man, presented with progressive pain and weakness in his lower extremities for more than 5 years. The proband showed persistent hypophosphatemia and low TmPO4/GFR values, indicating renal phosphate leak. His grandfather, father, and one of his uncles showed the similar symptoms.

DIAGNOSES

Autosomal dominant hypophosphatemia.

INTERVENTIONS AND OUTCOMES

Phosphorus supplement was prescribed to the proband and his affected uncle. Both their serum phosphorus levels recovered to normal and their symptoms such as back pain and lower extremity weakness were completely relieved. Whole exome sequencing was performed to identify disease-causing mutations in proband.

LESSONS

A novel heterozygous missense mutation c.680A>G (p. N227S) in exon 7 of SLC34A1 was found in proband by whole exome sequencing, which was also found in other 4 family members of this pedigree. Our report of an autosomal dominant hypophosphatemia pedigree with 5 mutant carriers enriches the clinical phenotype caused by the SLC34A1 mutations and further affirms the heterozygous mutations are causative for hypophosphatemia.

Renal phosphate handling and inherited disorders of phosphate reabsorption: an update

Renal phosphate handling critically determines plasma phosphate and whole body phosphate levels. Filtered phosphate is mostly reabsorbed by Na⁺-dependent phosphate transporters located in the brush border membrane of the proximal tubule: NaPi-IIa (SLC34A1), NaPi-IIc (SLC34A3), and Pit-2 (SLC20A2). Here we review new evidence for the role and relevance of these transporters in inherited disorders of renal phosphate handling. The importance of NaPi-IIa and NaPi-IIc for renal phosphate reabsorption and mineral homeostasis has been highlighted by the identification of mutations in these transporters in a subset of patients with infantile idiopathic hypercalcemia and patients with hereditary hypophosphatemic rickets with hypercalciuria. Both diseases are characterized by disturbed calcium homeostasis secondary to elevated 1,25-(OH)₂ vitamin D₃ as a consequence of hypophosphatemia. In vitro analysis of mutated NaPi-IIa or NaPi-IIc transporters suggests defective trafficking underlying disease in most cases. Monoallelic pathogenic mutations in both SLC34A1 and SLC34A3 appear to be very frequent in the general population and have been associated with kidney stones. Consistent with these findings, results from genome-wide association studies indicate that variants in SLC34A1 are associated with a higher risk to develop kidney stones and chronic kidney disease, but underlying mechanisms have not been addressed to date.

Hypophosphatemic Rickets

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

Inherited proximal tubular disorders and nephrolithiasis

The proximal tubule is responsible for reclaiming water, phosphates and amino acids from the tubular filtrate. There are genetic defects in both phosphate and amino acid transporters leading to nephrolithiasis. This review also explores genetic defects in regulators of phosphate and calcium transport in this nephron segment that lead to stone formation.

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.

Physiological regulation of phosphate by vitamin D, parathyroid hormone (PTH) and phosphate (Pi)

Inorganic phosphate (Pi) is an abundant element in the body and is essential for a wide variety of key biological processes. It plays an essential role in cellular energy metabolism and cell signalling, e.g. adenosine and guanosine triphosphates (ATP, GTP), and in the composition of phospholipid membranes and bone, and is an integral part of DNA and RNA. It is an important buffer in blood and urine and contributes to normal acid-base balance. Given its widespread role in almost every molecular and cellular function, changes in serum Pi levels and balance can have important and untoward effects. Pi homoeostasis is maintained by a counterbalance between dietary Pi absorption by the gut, mobilisation from bone and renal excretion. Approximately 85% of total body Pi is present in bone and only 1% is present as free Pi in extracellular fluids. In humans, extracellular concentrations of inorganic Pi vary between 0.8 and 1.2 mM, and in plasma or serum Pi exists in both its monovalent and divalent forms (H2PO4- and HPO42-). In the intestine, approximately 30% of Pi absorption is vitamin D regulated and dependent. To help maintain Pi balance, reabsorption of filtered Pi along the renal proximal tubule (PT) is via the NaPi-IIa and NaPi-IIc Na+-coupled Pi cotransporters, with a smaller contribution from the PiT-2 transporters. Endocrine factors, including, vitamin D and parathyroid hormone (PTH), as well as newer factors such as fibroblast growth factor (FGF)-23 and its coreceptor α-klotho, are intimately involved in the control of Pi homeostasis. A tight regulation of Pi is critical, since hyperphosphataemia is associated with increased cardiovascular morbidity in chronic kidney disease (CKD) and hypophosphataemia with rickets and growth retardation. This short review considers the control of Pi balance by vitamin D, PTH and Pi itself, with an emphasis on the insights gained from human genetic disorders and genetically modified mouse models.