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.

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.

Congenital hypophosphataemia in adults: determinants of bone turnover markers and amelioration of renal phosphate wasting following total parathyroidectomy

Congenital hypophosphataemia (CH) is a collection of disorders that cause defective bone mineralisation manifesting with rickets in childhood and osteomalacia in adulthood. Bone turnover markers (BTMs) are surrogate measures of metabolic bone disease severity. We explored the utility of BTMs in 27 adults with CH: 23 had X-linked hypophosphataemia (XLH), of whom 2 were hypoparathyroid post-total parathyroidectomy (PTx); 2 had autosomal dominant hypophosphataemic rickets (ADHR), and 2 had none of the known mutations. We measured the renal tubular maximum reabsorption rate of phosphate (TmP/GFR), C-terminal fibroblast growth factor 23 (FGF23), parathyroid hormone (PTH), ionised calcium, 1,25-dihydroxyvitamin D [1,25(OH)₂D], and a panel of BTMs: serum bone-specific alkaline phosphatase (bone ALP), osteocalcin (Oc), total procollagen type I amino-terminal propeptide (PINP), and carboxy-terminal telopeptide of type I collagen (CTX); and urine amino-terminal telopeptides of type I collagen (uNTX). After excluding 2 patients with XLH and PTx, the frequency of abnormal elevation in BTMs was: bone ALP (96%); CTX (72%); PINP (52%); uNTX (48%); Oc (28%). The strongest association with bone ALP was TmP/GFR. Those patients receiving phosphate supplements and alfacalcidol had significant elevation in CTX. The 2 patients with XLH and PTx had normalisation of TmP/GFR and near normalisation of BTMs post-operatively, despite marked elevation in both C-terminal and intact FGF23. In conclusion, BTMs in our CH patients indicated that most have abnormalities consistent with osteomalacia and many have mild secondary hyperparathyroidism; and the normalisation of TmP/GFR after total PTx in 2 cases of XLH remains unexplained, but possible causes are speculated.

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.

Hyper-expression of Osteocalcin mRNA in Odontoblasts of Hyp Mice

The Hyp mouse is a murine homologue of human X-linked hypophosphatemia that displays hypo-mineralization in bone and dentin. In this study, we tested the hypothesis that the defect in Hyp mice leads to alterations in the expression of dentin matrix proteins that may be associated with the hypo-mineralization changes in the tissues. Quantitative RT-PCR analyses showed that expression of the osteocalcin gene in Hyp mice tooth germ samples was significantly higher than in wild-type mice, whereas the gene expressions of osteonectin, osteopontn, dentin matrix protein 1, and type I collagen in both types of mice were similar. Further, cultured Hyp mice tooth germ samples exhibited a higher expression of the osteocalcin gene than did those from wild-type mice, which was in accord with the results of our in vivo analysis. These findings suggest that osteocalcin mRNA is highly expressed in Hyp mice odontoblasts and may be associated with dentin hypo-mineralization.

Prostaglandin-E2 Mediated Increase in Calcium and Phosphate Excretion in a Mouse Model of Distal Nephron Salt Wasting

Contribution of salt wasting and volume depletion to the pathogenesis of hypercalciuria and hyperphosphaturia is poorly understood. Pendrin/NCC double KO (pendrin/NCC-dKO) mice display severe salt wasting under basal conditions and develop profound volume depletion, prerenal renal failure, and metabolic alkalosis and are growth retarded. Microscopic examination of the kidneys of pendrin/NCC-dKO mice revealed the presence of calcium phosphate deposits in the medullary collecting ducts, along with increased urinary calcium and phosphate excretion. Confirmatory studies revealed decreases in the expression levels of sodium phosphate transporter-2 isoforms a and c, increases in the expression of cytochrome p450 family 4a isotypes 12 a and b, as well as prostaglandin E synthase 1, and cyclooxygenases 1 and 2. Pendrin/NCC-dKO animals also had a significant increase in urinary prostaglandin E2 (PGE-2) and renal content of 20-hydroxyeicosatetraenoic acid (20-HETE) levels. Pendrin/NCC-dKO animals exhibit reduced expression levels of the sodium/potassium/2chloride co-transporter 2 (NKCC2) in their medullary thick ascending limb. Further assessment of the renal expression of NKCC2 isoforms by quantitative real time PCR (qRT-PCR) reveled that compared to WT mice, the expression of NKCC2 isotype F was significantly reduced in pendrin/NCC-dKO mice. Provision of a high salt diet to rectify volume depletion or inhibition of PGE-2 synthesis by indomethacin, but not inhibition of 20-HETE generation by HET0016, significantly improved hypercalciuria and salt wasting in pendrin/NCC dKO mice. Both high salt diet and indomethacin treatment also corrected the alterations in NKCC2 isotype expression in pendrin/NCC-dKO mice. We propose that severe salt wasting and volume depletion, irrespective of the primary originating nephron segment, can secondarily impair the reabsorption of salt and calcium in the thick ascending limb of Henle and/or proximal tubule, and reabsorption of sodium and phosphate in the proximal tubule via processes that are mediated by PGE-2.

Vitamin D metabolism in hypophosphatemic vitamin D-resistant rickets

Plasma levels of 1,25-(OH)2-D were low in children with hypophosphatemic vitamin D-resistant rickets (HVDRR), but increased after very large doses of 1 alpha-OH-D3. These results suggest that the metabolism of 1,25-(OH)2-D is accelerated in HVDRR. In addition, the lower level of plasma 1,25-(OH)2-D in untreated HVDRR was correlated with the lower level of serum phosphate and renal threshold phosphate concentration (TmP/GFR). The administration of 1 alpha-OH-D3 to the patients with HVDRR could enhance the renal threshold phosphate concentration.

Association of circulating fibroblast growth factor-23 with renal phosphate excretion among hemodialysis patients with residual renal function

BACKGROUND AND OBJECTIVES

High serum levels of fibroblast growth factor-23 (FGF-23) are associated with mortality in patients with ESRD, but whether it still acts as a phosphaturic factor is unknown. This study aimed to explore the role of circulating FGF-23 on urinary phosphate excretion and phosphate balance in maintenance hemodialysis (MHD) patients with residual renal function (RRF).

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

There were 134 MHD patients enrolled in this cross-sectional study from June to July 2010. Demographics, laboratory data, and excretion capacity of phosphate were recorded. Multivariable linear regression was used to analyze the relationship of serum phosphate and the tubular reabsorption rate of phosphate with other factors.

RESULTS

The median age of the patients was 61.0 years and 47.8% were male. Thirty percent of the patients had high urinary output (>200 ml/d) accompanied by lower serum levels of phosphate, calcium, intact parathyroid hormone, and FGF-23 compared with those with low urine output (≤200 ml/d). The independent predictors of serum phosphate were normalized protein nitrogen appearance, intact parathyroid hormone, and FGF-23 in the low urine output group and female sex and GFR in the high urine output group. The tubular reabsorption rate of phosphate decreased to 50% of the normal level in patients with RRF. Elevated circulating FGF-23 was significantly associated with lower tubular phosphate reabsorption after adjusting for GFR.

CONCLUSIONS

RRF is associated with significant capacity to excrete phosphate in MHD patients and high levels of serum FGF-23 may promote phosphate excretion by remnant nephrons.

Fanconi-Bickel syndrome and autosomal recessive proximal tubulopathy with hypercalciuria (ARPTH) are allelic variants caused by GLUT2 mutations

CONTEXT

Many inherited disorders of calcium and phosphate homeostasis are unexplained at the molecular level.

OBJECTIVE

The objective of the study was to identify the molecular basis of phosphate and calcium abnormalities in two unrelated, consanguineous families.

PATIENTS

The affected members in family 1 presented with rickets due to profound urinary phosphate-wasting and hypophosphatemic rickets. In the previously reported family 2, patients presented with proximal renal tubulopathy and hypercalciuria yet normal or only mildly increased urinary phosphate excretion.

METHODS

Genome-wide linkage scans and direct nucleotide sequence analyses of candidate genes were performed. Transport of glucose and phosphate by glucose transporter 2 (GLUT2) was assessed using Xenopus oocytes. Renal sodium-phosphate cotransporter 2a and 2c (Npt2a and Npt2c) expressions were evaluated in transgenically rescued Glut2-null mice (tgGlut2-/-).

RESULTS

In both families, genetic mapping and sequence analysis of candidate genes led to the identification of two novel homozygous mutations (IVS4-2A>G and R124S, respectively) in GLUT2, the gene mutated in Fanconi-Bickel syndrome, a rare disease usually characterized by renal tubulopathy, impaired glucose homeostasis, and hepatomegaly. Xenopus oocytes expressing the [R124S]GLUT2 mutant showed a significant reduction in glucose transport, but neither wild-type nor mutant GLUT2 facilitated phosphate import or export; tgGlut2-/- mice demonstrated a profound reduction of Npt2c expression in the proximal renal tubules.

CONCLUSIONS

Homozygous mutations in the facilitative glucose transporter GLUT2, which cause Fanconi-Bickel syndrome, can lead to very different clinical and biochemical findings that are not limited to mild proximal renal tubulopathy but can include significant hypercalciuria and highly variable degrees of urinary phosphate-wasting and hypophosphatemia, possibly because of the impaired proximal tubular expression of Npt2c.

Familial hypophosphataemic rickets affecting a father and his two daughters: a case report

BACKGROUND

Hypophosphataemic rickets (HR) is a rare cause of short stature associated with limb deformities.

OBJECTIVE

To report the clinical and laboratory features of HR in two siblings and their father.

METHODS

Following the diagnosis of HR in a 4-year-old girl, her siblings and parents were screened using clinical, laboratory, and radiological parameters.

RESULTS

Short stature, lower limb deformities, frontal bossing and hypophosphataemia were present in all three patients. Serum alkaline phosphatase (ALP) was markedly elevated in both siblings who were aged two and 11 years but only minimally raised in their 43-year-old father. While spontaneous mutation is the presumed aetiology in the father, X linked dominant inheritance is the likely cause in both daughters.

CONCLUSIONS

Hypophosphataemic rickets should be considered in the differential diagnosis of short stature associated with limb deformities regardless of a family history of HR. Serum ALP may not be remarkably elevated when the diagnosis is made in adulthood.

A familial disorder with low bone density and renal phosphate wasting

Hereditary forms of renal phosphate wasting have been studied thoroughly in the past years. X-linked Hypophosphatemic rickets (XLH), autosomal dominant hypophosphatemic rickets/osteomalacia (ADHR) and autosomal recessive hypophosphatemic rickets (ARHR) are known genetic disorders in which a disturbance of phosphatonins is a causative factor in the pathogenesis. We describe a comparable but yet undescribed disorder in a family in which a 53 year old man presented with a spontaneous fracture after suffering for years with severe fatigue and musculoskeletal pains. A low serum phosphate was discovered. The two subsequent generations of this family developed the same symptoms but at an earlier age. Almost all family members have been investigated and the majority appears to have low bone density and/or renal phosphate wasting and/or low serum phosphate. Remarkably no rickets was found. No elevation of FGF23 or mutations in the gene encoding FGF23 were found. We believe this is a new familial disorder of bone metabolism and phosphate homeostasis in which a disturbance of bone modulators may play a central role.

Hyperaminoaciduria in mild phosphate diabetes in adults

Quantitative urinary amino acid excretion has been studied in 23 adult patients with mild phosphate diabetes (MPD), in 22 adult control patients with various renal disorders and in 15 children, 7--19 years old, with atopic disorders (normal controls). Statistically significant increases in urinary excretion of glutamine (p < 0.01), glycine (p < 0.01) and cystine (p < 0.05) were found in the MPD patients compared to the normal controls. The urinary excretion of glutamine was significantly increased while the increases in cystine and glycine excretions were non-significant when MPD patients were compared to the control patients. In addition to clinical signs and analyses of plasma and urinary phosphate, a quantitative evaluation of urinary amino acids might be a tool in the diagnosis of MPD. The significance of the increased urinary amino acid excretion in MPD is discussed.

Normal FGF23 levels in adult idiopathic phosphate diabetes

Fibroblast growth factor 23 (FGF23), a recently discovered phosphaturic substance playing a key role in genetic and oncogenic phosphate diabetes, is involved in the physiological regulation of phosphate metabolism. Moderate idiopathic phosphate diabetes (IPD) leading to male osteoporosis and diffuse pain resembling fibromyalgia has been described. The aim of our study was to define the role of FGF23 in the mechanism of IPD. The study concerned 29 patients with IPD, mean age 53 +/- 11 years, of whom 72% were men. Fifteen subjects without bone disease and with normal serum phosphate and calcium levels were used as controls. Phosphate diabetes was confirmed by phosphate reabsorption level <85% and phosphate reabsorption threshold (TmPO4/GFR) <0.83. Known causes of phosphate diabetes were excluded. Fasting level of FGF23, serum phosphate, 1-25(OH)2D3, and parathyroid hormone were measured in patients and compared with FGF23 and serum phosphate in healthy controls. Spinal and hip bone mineral density (BMD) were measured by osteodensitometry. Sixteen of 29 patients had diffuse pain, 10 had osteoporosis according to the World Health Organization criteria, and 11 had osteopenia. Serum phosphate was significantly lower in patients than in controls, but FGF23 levels did not differ. Compared to patients with normal bone status, patients with osteopenia and osteoporosis had significantly decreased FGF23 levels, whereas serum phosphate was identical in the two groups. In all patients, serum phosphate and FGF23 were positively correlated and FGF23 and 1-25(OH)2D3 were negatively correlated. FGF23 seems not be a cause of IPD, and the FGF23/phosphate/1-25(OH)2D3 axis appeared to be functional.

Pathogenic role of Fgf23 in Hyp mice

Inactivating mutations of the PHEX (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) endopeptidase, the disease-causing gene in X-linked hypophosphatemia (XLH), results in increased circulating levels of fibroblastic growth factor-23 (FGF23), a bone-derived phosphaturic factor. To determine the causal role of FGF23 in XLH, we generated a combined Fgf23-deficient enhanced green fluorescent protein (eGFP) reporter and Phex-deficient Hyp mouse model (Fgf23(+/-)/Hyp). eGFP expression was expressed in osteocytes embedded in bone that exhibited marked upregulation of eGFP in response to Phex deficiency and in CD31-positive cells in bone marrow venules that expressed low eGFP levels independently of Phex. In bone marrow stromal cells (BMSCs) derived from Fgf23(-/-)/Hyp mice, eGFP expression was also selectively increased in osteocyte-like cells within mineralization nodules and detected in low levels in CD31-positive cells. Surprisingly, eGFP expression was not increased in cell surface osteoblasts, indicating that Phex deficiency is necessary but not sufficient for increased Fgf23 expression in the osteoblast lineage. Additional factors, associated with either osteocyte differentiation and/or extracellular matrix, are necessary for Phex deficiency to stimulate Fgf23 gene transcription in bone. Regardless, the deletion of Fgf23 from Hyp mice reversed the hypophosphatemia, abnormal 1,25(OH)(2)D(3) levels, rickets, and osteomalacia associated with Phex deficiency. These results suggest that Fgf23 acts downstream of Phex to cause both the renal and bone phenotypes in Hyp mice.

[From gene to disease; hypophosphataemic rickets and the PHEX gene]

X-linked hypophosphataemic rickets is associated with mutations in the PHEX gene on the short arm of the X chromosome, encoding a membrane-bound endoprotease which is predominantly expressed in osteoblasts. Defective PHEX function leaves phosphaturic peptides such as FGF23 uncleaved, enabling these peptides, known as phosphatonins, to fully exert their phosphaturic potential in the proximal tubule of the kidney. An autosomally inherited form of hypophosphataemic rickets is caused by mutations in the proteolytic processing site of FGF23 itself, while in tumour-induced osteomalacia overproduction of FGF23 and possibly other phosphatonins causes the processing capacity to be exceeded, resulting in phosphaturic hypophosphataemia and osteomalacia.

Renal phosphate-wasting disorders

The renal regulation of phosphate is a complex process. Clinical disorders of phosphate handling have led to the identification of several genes and proteins involved in the maintenance of phosphate homeostasis. Further work is required to elucidate the precise pathways that regulate renal phosphate transport.

Dentinal defects in Hyp mice not caused by hypophosphatemia alone

The Hyp mouse is a murine homolog of human X-linked hypophosphatemic rickets and displays hypo-mineralization in bone and dentin due to a defect of the phosphate-regulating gene with homology to endopeptidase on the X chromosome (Phex) gene. It has long been considered that the bone and dentin defects in Hyp mice are caused by hypophosphatemia alone, however, several recent studies have indicated the possibility that intrinsic defects are present in Hyp mice osteoblasts. Further, we previously found a hyper-expression of osteocalcin (OC) mRNA in Hyp mouse odontoblasts and suggested the possibility of the presence of intrinsic defects. In the present study, we evaluated morphological features and OC mRNA expression levels in tooth germs of Nor mice with a normal phex gene and a low concentration of serum phosphate, and compared them to those in Hyp and wild-type mice. Nor mice exhibited low serum phosphate levels, however, did not show the characteristic features of dentin defects seen in Hyp mice, such as widened predentin and hyper-expression of OC mRNA. These results suggest that the hypo-mineralization of dentin in Hyp mice is not dependent on serum phosphate level, but rather is affected by intrinsic defects in odontoblasts.

Fibroblast growth factor-23 increases mouse PGE2 production in vivo and in vitro

Fibroblast growth factor-23 (FGF-23) has been implicated in the renal phosphate wasting in X-linked hypophosphatemia, tumor-induced osteomalacia, and autosomal dominant hypophosphatemic rickets. Recently, we demonstrated that Hyp mice have greater urinary PGE2 levels compared with C57/B6 mice and that indomethacin administration in vivo and in vitro ameliorates the phosphate transport defect in Hyp mice. To determine further whether altered prostaglandin metabolism plays a role in the renal phosphate transport defect in Hyp mice, we incubated renal proximal tubules with arachidonic acid. We find that PGE2 production was higher in Hyp mice than in C57/B6 mice. Incubation of C57/B6 mouse renal proximal tubules with FGF-23R176Q, an active mutant form of FGR23, increased tubular PGE2 production, an effect that was inhibited by 50 microM PD-98059 and 10 microM SB-203580, inhibitors of the MAP kinase pathway. C57/B6 mice injected with FGF-23R176Q had a approximately 10-fold increase in PGE2 excretion 24 h after intraperitoneal injection of FGF-23R176Q compared with vehicle-treated controls. Finally, we show that PGE2 inhibited both phosphate and volume absorption in mouse proximal convoluted tubules perfused in vitro and reduced brush-border membrane vesicle NaPi-2a protein abundance from renal cortex incubated in vitro with PGE2. In conclusion, FGF-23 increases urinary and renal tubular PGE2 production via the MAP kinase pathway and PGE2 inhibits proximal tubule phosphate transport.

Inorganic phosphate homeostasis and the role of dietary phosphorus

Inorganic phosphate (Pi) is required for cellular function and skeletal mineralization. Serum Pi level is maintained within a narrow range through a complex interplay between intestinal absorption, exchange with intracellular and bone storage pools, and renal tubular reabsorption. The crucial regulated step in Pi homeostasis is the transport of Pi across the renal proximal tubule. Type II sodium-dependent phosphate (Na/Pi) cotransporter (NPT2) is the major molecule in the renal proximal tubule and is regulated by Pi, parathyroid hormone and by 1,25-dihydroxyvitamin D. Recent studies of inherited and acquired hypophosphatemia [X-linked hypophosphatemic rickets/osteomalacia (XLH), autosomal dominant hypophosphatemic rickets/osteomalacia (ADHR) and tumor-induced rickets/osteomalacia (TIO)], which exhibit similar biochemical and clinical features, have led to the identification of novel genes, PHEX and FGF23, that play a role in the regulation of Pi homeostasis. The PHEX gene, which is mutated in XLH, encodes an endopeptidase, predominantly expressed in bone and teeth, but not in kidney. FGF-23 may be a substrate of this endopeptidase and may therefore accumulate in patients with XLH. In the case of ADHR mutations in the furin cleavage site, which prevent the processing of FGF-23 into fragments, lead to the accumulation of a "stable" circulating form of the peptide which also inhibits renal Pi reabsorption. In the case of TIO, ectopic overproduction of FGF-23 overwhelms its processing and degradation by PHEX, leading to the accumulation of FGF-23 in the circulation and inhibition of renal Pi reabsorption. Mice homozygous for severely hypomorphic alleles of the Klotho gene exhibit a syndrome resembling human aging, including atherosclerosis, osteoporosis, emphysema, and infertility. The KLOTHO locus is associated with human survival, defined as postnatal life expectancy, and longevity, defined as life expectancy after 75. In considering the relationship of klotho expression to the dietary Pi level, the klotho protein seemed to be negatively controlled by dietary Pi.

Comparison of two assays for fibroblast growth factor (FGF)-23

FGF-23 was recently shown to be involved in the development of several hypophosphatemic diseases, including X-linked hypophosphatemic rickets/osteomalacia (XLH) and tumor-induced rickets/osteomalacia (TIO). FGF-23 is processed between Arg179 and Ser180, and only full-length FGF-23 was shown to cause hypophosphatemia. Two assays for FGF-23 have been reported. One assay detects only full-length FGF-23. In contrast, the C-terminal assay recognizes both full-length and processed C-terminal fragment of FGF-23. However, discrepant results concerning circulatory levels of FGF-23 in patients with TIO and XLH have been reported using these two assays. We simultaneously measured FGF-23 levels in 13 patients with adult-onset hypophosphatemic osteomalacia and 29 patients with XLH by these two assays. The full-length assay indicated that FGF-23 was above the upper limit of the reference range in all patients with osteomalacia and in 24 of 29 patients with XLH. However, the C-terminal assay in dicated that FGF-23 was within the reference range in 3 of 13 patients with osteomalacia and 16 of 29 patients with XLH. In addition, there was no correlation between FGF-23 levels measured by these assays in patients with XLH whose FGF-23 was within the reference range by C-terminal assay. These results indicate that FGF-23 within the reference range by C-terminal assay does not rule out an increase in full-length FGF-23. In addition, because FGF-23 was high in most of these hypophosphatemic patients, these results support the notion that FGF-23 plays a major role in the development of hypophosphatemia in patients with TIO and XLH.

The role of fibroblast growth factor 23 for hypophosphatemia and abnormal regulation of vitamin D metabolism in patients with McCune-Albright syndrome

McCune-Albright syndrome (MAS) is sometimes complicated by hypophosphatemia and abnormally low levels of 1,25(OH)(2)D in the presence of hypophosphatemia. Recently, fibroblast growth factor 23 (FGF-23) was reported as a phosphaturic and a causal factor of abnormal vitamin D metabolism. This abnormal phosphate and vitamin D metabolism is well known to be found in oncogenic and X-linked hypophosphatemia. We furthermore reported increased circulating plasma FGF-23 levels in patients with oncogenic and X-linked hypophosphatemia. To determine whether FGF-23 may be involved in the pathogenesis of MAS, we measured plasma FGF-23 levels in six MAS patients. As a control for hypophosphatemia, we also investigated the plasma FGF-23 levels in two patients with hereditary hypophosphatemic rickets with hypercalciuria (HHRH). We also investigated the correlation of plasma FGF-23 levels with serum phosphate and 1,25(OH)(2)D levels after short-term pamidronate therapy in three MAS patients. Plasma FGF-23 levels were significantly increased in patients with MAS compared to normal controls, whereas they were not increased in HHRH patients. Serum phosphate levels of the MAS patients were significantly lower than those observed in normal controls. Plasma FGF-23 levels showed significant negative correlation with serum phosphate concentrations. In three MAS patients, pamidronate therapy decreased plasma FGF-23 levels, which showed significant negative correlation with serum 1,25(OH)(2)D concentrations. These data suggested that FGF-23 is a possible causal factor for hypophosphatemia and abnormal vitamin D metabolism in MAS.

Epidermal nevus syndrome associated with adnexal tumors, spitz nevus, and hypophosphatemic vitamin D-resistant rickets

The epidermal nevus syndrome is the association of epidermal nevi with abnormalities in other organ systems, most commonly the central nervous system, the skeletal system, and the eyes. We present a patient with epidermal nevus syndrome associated with hypophosphatemic vitamin D-resistant rickets and multiple adnexal and spindle cell tumors.

The wrickkened pathways of FGF23, MEPE and PHEX

The last 350 years since the publication of the first medical monograph on rickets (old English term wrickken) (Glisson et al., 1651) have seen spectacular advances in our understanding of mineral-homeostasis. Seminal and exciting discoveries have revealed the roles of PTH, vitamin D, and calcitonin in regulating calcium and phosphate, and maintaining healthy teeth and skeleton. However, it is clear that the PTH/Vitamin D axis does not account for the entire picture, and a new bone-renal metabolic milieu has emerged, implicating a novel set of matrix proteins, hormones, and Zn-metallopeptidases. The primary defects in X-linked hypophosphatemic rickets (HYP) and autosomal-dominant hypophosphatemic rickets (ADHR) are now identified as inactivating mutations in a Zn-metalloendopeptidase (PHEX) and activating mutations in fibroblast-growth-factor-23 (FGF23), respectively. In oncogenic hypophosphatemic osteomalacia (OHO), several tumor-expressed proteins (MEPE, FGF23, and FRP-4) have emerged as candidate mediators of the bone-renal pathophysiology. This has stimulated the proposal of a global model that takes into account the remarkable similarities between the inherited diseases (HYP and ADHR) and the tumor-acquired disease OHO. In HYP, loss of PHEX function is proposed to result in an increase in uncleaved full-length FGF23 and/or inappropriate processing of MEPE. In ADHR, a mutation in FGF23 results in resistance to proteolysis by PHEX or other proteases and an increase in half-life of full-length phosphaturic FGF23. In OHO, over-expression of FGF23 and/or MEPE is proposed to result in abnormal renal-phosphate handling and mineralization. Although this model is attractive, many questions remain unanswered, suggesting a more complex picture. The following review will present a global hypothesis that attempts to explain the experimental and clinical observations in HYP, ADHR, and OHO, plus diverse mouse models that include the MEPE null mutant, HYP-PHEX transgenic mouse, and MEPE-PHEX double-null-mutant.

What have we learnt about the regulation of phosphate metabolism?

PURPOSE OF REVIEW

The search for hormones which specifically regulate phosphate metabolism has fuelled recent tantalizing studies. These studies have been motivated by diseases involving renal phosphate wasting, including tumor-induced osteomalacia, X-linked hypophosphatemic rickets, and autosomal dominant hypophosphatemia. This review focuses on likely candidate 'phosphatonins' and their possible physiological significance.

RECENT FINDINGS

Candidate phosphatonins include fibroblast growth factor 23, matrix extracellular phosphoglycoprotein, stanniocalcin, and Frizzled-related protein 4. Fibroblast growth factor 23 has emerged as the prime candidate explaining pathophysiology of these diseases. FGF-23 is expressed in most tumors in tumor-induced osteomalacia. Serum fibroblast growth factor 23 is increased in most patients with X-linked hypophosphatemic rickets and tumor-induced osteomalacia. Injection of recombinant fibroblast growth factor 23 induces phosphaturia, hypophosphatemia, and suppression of 1,25-dihydroxyvitamin D in animals. Many unanswered questions remain, including the relationship between PHEX (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) mutations and elevated fibroblast growth factor 23. It is also not clear whether these candidate phosphatonins play a role in phosphate or vitamin D metabolism in healthy humans, or that this role is endocrine. The most compelling evidence derives from the fibroblast growth factor 23-knockout mouse which shows hyperphosphatemia and increased serum 1,25-dihydroxyvitamin D. A physiologically relevant phosphatonin should explain renal adaptation to variable dietary phosphate intake. The tissue source and determinants of serum fibroblast growth factor 23 are unknown.

SUMMARY

Pathophysiological and animal studies serve as a logical foundation on which to base further questions of human physiology. The definition of what is or is not a phosphatonin may need to be refined. There is a need to return to 'old-fashioned' human physiology studies to place recent findings in perspective.

Genetic advances, biochemical and clinical features and critical approach to treatment of patients with X-linked hypophosphatemic rickets

X-linked hypophosphatemic rickets (XLH) is an hereditary form of rickets due to isolated renal tubular phosphate wasting and impaired production of 1,25-dihydroxyvitamin D [1,25(OH)2D]. XLH is caused by mutations in the PHEX (phosphate regulating gene with homology to endopeptidases) gene, which is located on Xp22.1. The pathogenetic mechanisms by which mutations in the PHEX gene cause XLH are not completely known. Hypophosphatemia associated with disproportionate short stature and bone deformities of the lower limbs are the main findings in XLH patients. Some studies have shown that conventional treatment with vitamin D metabolites, such as 1,25(OH)2D3 or 1 alpha-hydroxyvitamin D3, combined with inorganic phosphate salts is able to improve serum phosphate concentrations and linear growth, as well as healing rickets. However, some patients may have poor beneficial effects by this therapy. On the other hand, some important treatment complications, such as hypervitaminosis D, nephrocalcinosis and secondary/tertiary hyperparathyroidism may occur during the current therapy. Despite conventional treatment, some patients may require surgical correction of bone deformities. In the light of the recent genetic advances the mechanisms that could be involved in the pathogenesis of XLH are discussed. Furthermore, the article reviews the effects of the medical treatment providing current recommendations for the management of XLH patients.

Cartilage abnormalities are associated with abnormal Phex expression and with altered matrix protein and MMP-9 localization in Hyp mice

X-linked hypophosphatemic rickets (HYP) in humans is caused by mutations in the PHEX gene. This gene mutation is also found in Hyp mice, the murine homologue of the human disease. At present, it is unknown why loss of Phex function leads to cartilage abnormalities in Hyp mice. In the present study, we compared in wild-type and Hyp mice Phex protein localization in cartilage of developing long bone as well as localization of skeletal matrix proteins and matrix metalloproteinase-9 (MMP-9). Also compared were chondrocyte apoptosis in the growth plate, mineralization and cartilage remnant retention in the metaphysis, and chondroclast/osteoclast characteristics in the primary spongiosa. Phex protein was detected in proliferating and hypertrophic chondrocytes in growth plate cartilage of wild-type mice, but not in Hyp mice. Hyp mice exhibited a widened and irregular hypertrophic zone in growth plate cartilage showing hypomineralization, increased cartilage remnants from the growth plate in both metaphyseal trabecular and cortical bone, and fewer and smaller chondroclasts/osteoclasts in the primary spongiosa. Increased link protein and C-propeptide of type II procollagen of Hyp mice reflected the increase in chondrocytes and matrix in the cartilaginous growth plate and in bone. In addition, growth plate osteocalcin and bone sialoprotein levels were decreased, while osteonectin was increased, in hypertrophic chondrocytes and cartilage matrix in Hyp mice. MMP-9 in hypertrophic chondrocytes was also reduced in Hyp mice and fewer apoptotic hypertrophic chondrocytes were detected. These findings suggest that Phex may control mineralization and removal of hypertrophic chondrocytes and cartilage matrix in growth plate by regulating the synthesis and deposition of certain bone matrix proteins and proteases such as MMP-9.

Growth and metabolic control during puberty in girls with X-linked hypophosphataemic rickets

OBJECTIVE

X-linked hypophosphataemic rickets (XLH) results in defective bone mineralization and impaired growth. Treatment with oral phosphate (Pi) and calcitriol improves but does not normalize growth. This study assessed whether pubertal growth and metabolic control contribute to the height deficit.

METHODS

Study included patients with XLH who were treated with Pi-calcitriol from diagnosis to adult height; their hospital records, biochemistry and radiographs were reviewed.

RESULTS

Six females with XLH were included. Their mean peak height velocity and total height gain during puberty were nearly normal despite deteriorating metabolic control.

CONCLUSIONS

In treated girls with XLH, the pubertal growth is nearly normal despite suboptimal metabolic control. The major height loss occurs prior to puberty and is not recovered during the pubertal growth spurt.

Effect of gene dose and parental origin on bone histomorphometry in X-linked Hyp mice

X-linked hypophosphatemia (XLH) is characterized by rickets and osteomalacia and arises from mutations in the Phex and PHEX genes in mice (Hyp) and humans, respectively. The present study was undertaken to examine the effect of gene dose on the skeletal phenotype using a histomorphometric approach. Metrical traits (vertebral length, growth plate thickness, cancellous osteoid volume per bone volume, and cancellous, endocortical, and periosteal osteoid thickness) were compared in caudal vertebrae of mutant female (Hyp/+, Hyp/Hyp) and male (Hyp/Y) mice and their normal female (+/+) and male (+/Y) littermates. Mutant animals had trait values that differed significantly from those of normal animals. However, with the exception of vertebral length and cancellous osteoid thickness, values were not significantly different between the three mutant genotypes. We also examined the effect of gamete-of-origin on histomorphometric parameters in obligate Hyp/+ females derived from male or female transmitting parents. The metrical trait values in both groups of Hyp/+ mice were similar, with the exception of vertebral length and cancellous osteoid volume per bone volume. In summary, we demonstrate that the amount of osteoid per bone volume is similar in the three mutant genotypes and conclude that the extent and magnitude of the mineralization defect is fully dominant and likely not affected by gene dose. The differences in vertebral length in the mutants suggest that rickets and osteomalacia are not the only causes of decreased vertebral growth in Hyp mice and that Phex protein may influence bone growth and mineralization by distinct pathways.

Disorders of phosphate metabolism--pathomechanisms and management of hypophosphataemic disorders

Hypophosphataemia does not necessarily indicate phosphate (Pi) depletion. In acute emergencies such as septicaemia, alkalosis or re-feeding, hypophosphataemia may result from redistribution of Pi from the extracellular to the intracellular space. Hypophosphataemia from true Pi depletion gives rise to skeletal (osteomalacia) and extraskeletal (myopathy, cardiomyopathy) disorders. It is practically never the result of diminished nutritional intake. The most severe syndromes of Pi depletion result from diminished tubular Pi re-absorption and renal Pi wasting. In the differential diagnosis mainly four conditions have to be considered: (i) tumour-associated osteomalacia, (ii) X-linked hypophosphataemia (XLH), (iii) autosomal dominant hypophosphataemia, and (iv) hypercalcaemic renal phosphate wasting. Recent molecular insight has put fibroblast growth factor (FGF-23) into the centre of pathophysiological considerations because of (i) overproduction (tumour-associated osteomalacia) or (ii) hypothetically, accumulation resulting from mutations causing resistance to processing or degradation (autosomal dominant hypophosphataemia) or (iii) loss-of-function of a protease (PHEX) interfering with FGF-23 breakdown (XLH). In oncogenic osteomalacia the treatment of choice is resection of the tumour. Recently, pharmacological treatment has also become possible, i.e. administration of octreotide. XLH and autosomal dominant hypophosphataemia must be managed by oral administration of phosphate and calcitriol. In patients with gastrointestinal intolerance to phosphate or with severely symptomatic bone disease, prolonged intravenous administration of Pi is necessary.

Early treatment improves growth and biochemical and radiographic outcome in X-linked hypophosphatemic rickets

X-Linked hypophosphatemic rickets (XLH) is characterized by hypophosphatemia, rickets, and impaired growth. Despite oral phosphate and 1,25-dihydroxyvitamin D(3) treatment, many patients have suboptimal growth and bone healing. The aim of this study was to assess whether age at treatment onset impacts the outcome. Growth data, biochemistry, and radiographs of 19 well-controlled patients with XLH were analyzed retrospectively. Patients were divided into two groups based on the age at treatment onset (group 1, <1.0 yr; group 2, >or=1.0 yr). The median height z-score was higher in group 1 (n = 8) than in group 2 (n = 11) at treatment onset [-0.4 SD score (SDS) vs. -1.7 SDS; P = 0.001], at the end of the first treatment year (-0.7 SDS vs. -1.8 SDS; P = 0.009), throughout childhood (P > 0.05) and until predicted adult height (-0.2 SDS vs. -1.2 SDS; P = 0.06). The degree of hypophosphatemia was similar in both groups, but serum alkaline phosphatase remained higher in group 2 throughout childhood. Radiographic signs of rickets were more marked in group 2, but even patients with early treatment developed significant skeletal changes of rickets. These data suggest that treatment commenced in early infancy results in improved outcome in patients with XLH, but does not completely normalize skeletal development.

2 alpha-(3-hydroxypropyl)- and 2 alpha-(3-hydroxypropoxy)-1 alpha,25-dihydroxyvitamin D3 accessible to vitamin D receptor mutant related to hereditary vitamin D-resistant rickets

Hereditary vitamin D-resistant rickets (HVDRR) is a genetic disorder caused by mutations in the vitamin D receptor, which lead to resistance to 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)]. We found that the A ring-modified analogues, 2alpha-(3-hydroxypropyl)- and 2alpha-(3-hydroxypropoxy)-1alpha,25(OH)(2)D(3), (O1C3 and O2C3) can bind better than the natural hormone to the mutant VDR (R274A), which similar to the HVDRR mutant, R274L, had lost the hydrogen bond to the 1alpha-hydroxyl group of 1alpha,25(OH)(2)D(3).

Prolonged high-dose phosphate treatment: a risk factor for tertiary hyperparathyroidism in X-linked hypophosphatemic rickets

OBJECTIVE

X-linked hypophosphatemic rickets is characterized by renal phosphate wasting, hypophosphatemia and defective bone mineralization. Treatment with oral phosphate (Pi) and calcitriol improves skeletal changes but associates with secondary hyperparathyroidism and nephrocalcinosis. Tertiary hyperparathyroidism is a rare complication of the treatment. The aim of the present study was to identify treatment-related factors that might be associated with the transition of secondary hyperparathyroidism to tertiary hyperparathyroidism in patients with X-linked hypophosphatemic rickets.

DESIGN

Thirteen patients with X-linked hypophosphatemic rickets and secondary or tertiary hyperparathyroidism were included in the study. Their hospital records were reviewed and compared for onset, duration and dosage of treatment, and for age of diagnosis and degree of secondary hyperparathyroidism.

RESULTS

Two patients developed tertiary hyperparathyroidism and 11 patients secondary hyperparathyroidism during the treatment. Patients with tertiary hyperparathyroidism had, on average, earlier onset and longer duration of treatment, higher dose of Pi and longer duration of treatment with very high Pi doses (> 100 mg/kg/day) compared to the 11 patients with secondary hyperparathyroidism. However, variation of all parameters was great with considerable overlap. Very high S-PTH levels > or = 42 pmol/l were observed in those who later developed tertiary hyperparathyroidism.

CONCLUSIONS

Prolonged very high dose oral Pi treatment is a major risk factor for the development of tertiary hyperparathyroidism in X-linked hypophosphatemic rickets.

A novel nonsense mutation in the ligand binding domain of the vitamin D receptor causes hereditary 1,25-dihydroxyvitamin D-resistant rickets

Hereditary 1,25-dihydroxyvitamin D resistant rickets (HVDRR) is a genetic disorder most often caused by mutations in the vitamin D receptor (VDR). In this report, we present our findings on a young girl who exhibited the typical clinical features of HVDRR with early onset rickets, hypocalcemia, secondary hyperparathyroidism, and elevated serum concentrations of alkaline phosphatase and 1,25-dihydroxyvitamin D [1,25(OH)(2)D(3)]. The patient also had total body alopecia. Fibroblasts from the patient were cultured for analysis of the VDR structure and function. In [3H]1,25(OH)(2)D(3) binding assays, no significant specific binding to the VDR was observed in cytosols from the patient's fibroblasts. The patient's fibroblast were also totally resistant to high doses of 1,25(OH)(2)D(3) as demonstrated by their failure to induce expression of the 24-hydroxylase gene, a marker of 1,25(OH)(2)D(3) activity. DNA sequence analysis of the VDR gene uncovered a unique C to T mutation in exon 8. The mutation changed the codon for glutamine to a premature stop codon at amino acid 317 (Q317X). Restriction enzyme analysis showed that the patient was homozygous for the mutation. Both parents were heterozygous for the mutant allele. In conclusion, we have identified a novel mutation in the VDR, Q317X, as the molecular defect in a patient with HVDRR. The Q317X mutation deletes 110 amino acids of the ligand-binding domain of the VDR and results in the loss of [3H]1,25(OH)(2)D(3) binding and target gene transactivation.

Rickets in childhood

Bone accretion is a two-stage process, with the osteoblasts laying down osteoid, which is then mineralized. Mineralization of osteoid requires vitamin D to be available in its active form (1,25(OH) 2 D); in addition, mineralization also requires normal levels of serum calcium, phosphorus, and alkaline phosphate. Deficiencies of any of these will result in defective mineralization of bone, in which the mineral-osteoid ratio is reduced (a qualitative abnormality). In the juvenile skeleton deficiencies particularly affect enchondral ossification at the growth plates, giving the characteristic clinical and radiological features of rickets. The bone may be soft, leading to deformity. In this article the causes of rickets in childhood are reviewed, together with the clinical and radiological features and strategies for prevention and treatment.

Ossification of the posterior longitudinal ligament in vitamin D-resistant rickets: case report and review of the literature

STUDY DESIGN

A case report of cervical myelopathy caused by ossification of the posterior longitudinal ligament in a patient with vitamin D-resistant rickets is presented together with a review of literature.

OBJECTIVE

To report the diagnosis of ossification of the posterior longitudinal ligament in a white woman with vitamin D-resistant rickets.

SUMMARY OF BACKGROUND DATA

The association between ossification of the posterior longitudinal ligament and untreated vitamin D-resistant rickets has been reported in Japan, but infrequently in white populations. In whites, ossification of the posterior longitudinal ligament is closely associated with diffuse idiopathic skeletal hyperostosis. A clear association between ossification of the posterior longitudinal ligament and vitamin D-resistant rickets in white populations has not yet been established.

METHODS

The medical record and imaging studies of a patient treated at the authors' institution for cervical myelopathy caused by ossification of the posterior longitudinal ligament in the setting of treated vitamin D-resistant rickets were reviewed. A Medline search of the medical literature between 1966-1999 was performed to identify pertinent studies and similar case reports.

RESULTS

The occurrence of spinal stenosis in untreated adults with vitamin D-resistant rickets has been reported in all regions of the spine in Japanese patients. The association between ossification of the posterior longitudinal ligament and untreated vitamin D-resistant rickets was first reported in Japan, where ossification of the posterior longitudinal ligament is endemic. This association may be incidental, because reports on ossification of the posterior longitudinal ligament in whites are not as frequent as in Japanese, reflecting the higher prevalence of this condition in Japan.

CONCLUSION

Ossification of the posterior longitudinal ligament and ossification of the posterior longitudinal ligament associated with deranged calcium or phosphate metabolism may be different pathologic entities sharing a common outcome. Adequate treatment of vitamin D-resistant rickets may not always prevent or reverse ossification of the posterior longitudinal ligament.

Hereditary hypophosphatemic rickets with hypercalciuria is not caused by mutations in the Na/Pi cotransporter NPT2 gene

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH), a renal phosphate (Pi) wasting disease first described in an extended Bedouin kindred, is characterized by hypophosphatemia, elevated serum 1,25-dihydroxyvitamin D levels, hypercalciuria, rickets, and osteomalacia. Correction of all abnormalities, except for renal Pi wasting, can be achieved by oral Pi supplementation. These findings and the demonstration that mice that are homozygous for the disrupted Na/Pi cotransporter gene Npt2 exhibit many of the biochemical features of HHRH suggested that mutations in the human orthologue NPT2 might be responsible for HHRH. The NPT2 gene in affected individuals from the Bedouin kindred and four small families was screened for mutations to test this hypothesis. No putative disease-causing mutation was found. Two single nucleotide polymorphisms (SNP), a silent substitution in exon 7 and a nucleotide substitution in intron 4, were identified, and neither consistently segregated with HHRH in the Bedouin kindred. Linkage analysis indicated that the two NPT2 intragenic SNP as well as five microsatellite markers in the NPT2 gene region were not linked to HHRH in the Bedouin kindred. Therefore, this is evidence to exclude NPT2 as a candidate gene for HHRH in the families that were studied.

A new approach to mRNA in proximal tubule cells of patients with CLCN5 channelopathy

ClC-5 is a chloride channel whose gene mutations have been reported to be associated with X-linked nephrolithiasis (XRN), X-linked recessive hypophosphatemic rickets (XLRH), Dent disease, and idiopathic low-molecular-weight proteinuria (ILMWP) in Japanese children. To establish more efficient screening for CLCN5 abnormalities, we developed a new diagnostic method using reverse transcription and polymerase chain reaction (RT-PCR) of cultured renal tubular cells from the urine of patients. Using this new method, we successfully detected microdeletion of ClC-5 mRNA in a patient and splicing abnormality of the CLCN5 Cl channel.

Osteomalacia in hyp mice is associated with abnormal phex expression and with altered bone matrix protein expression and deposition

To explore how the loss of Phex function contributes to the pathogenesis of osteomalacia, we examined the abnormalities of mineralization, Phex, and bone matrix protein expression occurring in Hyp mice in vivo and in ex vivo bone marrow cell cultures. The results in vivo show that mineralization was decreased significantly in Hyp mouse bone. Phex protein was identifiable in osteoblasts and osteocytes in wild-type mice, but not in Hyp mice. In Hyp mice, osteocalcin, bone sialoprotein, and vitronectin expression were down-regulated, whereas biglycan and fibrillin-1 expression were up-regulated in osteocytes and bone matrix relative to those in their wild-type counterparts. Parallel studies ex vivo demonstrated that cells derived from 18-day Hyp mouse bone marrow cell cultures had a 3'-Phex deletion, no Phex protein expression, decreased alkaline phosphatase activity, collagen deposition, and calcium accumulation, and reduced osteocalcin, bone sialoprotein, and vitronectin at both the protein and messenger RNA levels. Furthermore conditioned medium from Hyp mouse bone marrow cultures could induce analogous defects in bone marrow cell cultures of wild-type cells. These novel findings indicate that there is an intrinsic osteogenic cell differentiation defect in addition to the known hypomineralization of bone in Hyp mice, which may be inducible by an autocrine/paracrine secreted factor. These results suggest that alterations in the Phex gene may control bone matrix mineralization indirectly by regulating the synthesis and deposition of bone matrix proteins.

Autosomal recessive hypophosphataemic rickets with hypercalciuria is not caused by mutations in the type II renal sodium/phosphate cotransporter gene

BACKGROUND

At present the genetic defect for autosomal recessive and autosomal dominant hypophosphataemic rickets with hypercalciuria (HHRH) is unknown. Type II sodium/phosphate cotransporter (NPT2) gene is a serious candidate for being the causative gene in either or both autosomal recessive and autosomal dominant HHRH. In the present study we tested this hypothesis in one autosomal recessive family.

METHODS

The gene structure of human NPT2 is known. We tested the complete open reading frame in the affected siblings by polymerase chain reaction in combination with automatic DNA sequencing for the presence of mutations.

RESULTS

We did not observe disease-causing mutations in the NPT2 gene of the affected siblings. A T855C polymorphism resulting in a histidine to arginine transition was present in the open reading frame of NPT2. The polymorphism was present in both affected as well as unaffected family members.

CONCLUSION

The hypothesis that a defect in the NPT2 gene could be an underlying cause for autosomal recessive HHRH could not be sustained in our study.

Hypophosphatemic rickets accompanying McCune-Albright syndrome: evidence that a humoral factor causes hypophosphatemia

McCune-Albright syndrome (MAS) is sometimes complicated by hypophosphatemia. However, it remains unclear whether a humoral factor is associated with the cause of hypophosphatemia. We isolated cells with mutations of the Gsalpha gene from fibrous bone dysplasia tissues of two MAS patients (MAS cells). Severe combined immunodeficiency (SCID) mice were subjected to experiments using from one of these cells patients. Effects of conditioned media (CM) isolated from MAS cells (MAS-CM) on phosphate transport were investigated by using rat renal slices, the renal cell line OK-B, rat intestinal rings and the human intestinal cell line Caco-2. In addition, the effects of MAS-CM on human sodium-dependent phosphate transporter (NPT2) gene promoter activity expression were investigated in the renal cell line OK-B2400 and were compared with the effects of CM isolated from a patient with oncogenic hypophosphatemic osteomalacia (OHO). MAS cells caused significant hypophosphatemia (P < 0.05) and elevated serum alkaline phosphatase activity (P < 0.05) in SCID mice. The MAS-CM significantly inhibited phosphate uptake in everted intestinal rings (P < 0.01), whereas it had no effect on glucose uptake. The MAS-CM had no effect on either phosphate uptake in the kidney or NPT2 gene promoter activity. In contrast, the CM of the OHO patient significantly inhibited phosphate uptake and NPT2 gene promoter activity. These results indicate that the humoral factor derived from fibrous dysplasia cells of the MAS patient is different to that from OHO patients, because the humoral factor from the MAS patient inhibited phosphate transport not in the kidney but in the intestine.

Morphological and functional features of clasts in low phosphate, vitamin D-deficiency rickets

Focusing on resorption processes, we have extended our previous studies on chondroclasts and osteoclasts in normally developing tissues, using a model of nutritionally induced vitamin D-deficiency rickets. To analyze the resorption process, we investigated the matrix-resorbing cells in this modified and poorly mineralized tissue regarding morphological features and expression of tartrate-resistant acid phosphatase (TRAP) at the subcellular level. Our goal was to test the hypotheses that initiation of resorption is impaired with unmineralized matrix, and that such alterations involve changes in the subcellullar distribution of TRAP, implicating a role for this enzyme in the resorption process. Our results reveal distinctly different morphological appearances of clast-like cells in rickets compared with normal osteoclasts and chondroclasts. Ordinary resorption structures of osteoclasts and chondroclasts at the cell-matrix border, i.e., ruffled borders and clear zones, are profoundly altered in favor of a less well-defined intermediate zone. TRAP distribution at the subcellullar level is also clearly different from that in osteoclasts and chondroclasts from normal rodents, with impaired secretion; consequently, the enzyme is unable to function in the matrix outside the ruffled border. Our ultrastructural observations demonstrate that in rickets, the clasts are incapable of degrading the poorly mineralized cartilage and bone efficiently. Rachitic clasts seem to be recruited to the matrix surface and interaction between cell and matrix is also initiated, but definitive resorption structures at the cell-matrix border are not normally developed. Whether resorption is inhibited by the mere lack of mineral or mineral-associated proteins, or by other mechanisms remains to be settled.

Hypophosphataemic rickets

X-Linked hypophosphataemic rickets (XLH) is frequently associated with short stature even when conventional treatment (1, 25-dihydroxyvitamin D(3) or 1alpha-hydroxyvitamin D(3) plus inorganic phosphate salts) is administered for a long time. The pathogenesis of growth retardation is probably multifactorial. Affected patients usually show normal growth hormone (GH) secretion. In some poorly growing XLH patients, long-term GH treatment associated with conventional therapy improves linear growth. GH treatment also increases phosphate retention but this effect is transient.

Tumor-induced osteomalacia and the regulation of phosphate homeostasis

Tumor-induced osteomalacia (TIO) is a rare and unique syndrome characterized by hypophosphatemia, excessive urinary phosphate excretion, reduced 1,25-dihydroxyvitamin D concentrations, and osteomalacia. Removal of the tumor is associated with a cure of the lesion. Several laboratories have now shown that conditioned medium derived from cultures of such tumors contain a small, heat-sensitive substance ("phosphatonin") of <25,000 daltons that specifically inhibits sodium-dependent phosphate transport in cultured renal proximal tubular epithelia. This substance does not increase cyclic adenosine monophosphate (cAMP) formation in tubular epithelial cells and does not increase cAMP excretion in urine. A substance with similar properties is present in the circulation of patients on hemodialysis. A syndrome with a remarkably similar biochemical phenotype, namely, X-linked hypophosphatemic rickets (XLH), also has a circulating factor with properties similar, if not identical, to those of the tumor-derived factor, "phosphatonin." The molecular defect in XLH has been shown to be due to a mutant endopeptidase, PHEX, whose substrate might be "phosphatonin." Hypophosphatemia and other biochemical abnormalities in TIO are due to excessive production of "phosphatonin" with normal PHEX function, whereas the biochemical abnormalities in XLH are caused by a mutant PHEX enzyme that fails to process "phosphatonin."

Axial osteomalacia with sacroiliitis and moderate phosphate diabetes: report of a case

We report a new case of axial osteomalacia diagnosed in a 51-year-old white Caucasian male, made particular by its association with sacroiliitis, positive HLA-B27 antigen, and also moderate phosphate diabetes responsible for a decreased appendicular bone mass. The diagnosis was suspected when X-ray evaluation showed increased density and coarse trabeculation mainly involving the pelvis and spine. Dual energy X-ray absorptiometry confirmed the elevated bone density at the lumbar spine (T score: +1.92) contrasting with a decreased bone mass at the femoral neck (T score: -2.33). The diagnosis was confirmed by histomorphometry of the iliac crest showing marked thickening of the cortices (2190 microns +/- 0.574, N = 780 +/- 40) and an increased trabecular bone volume (33.24%, N = 14 +/- 3). Osteoid parameters were also markedly increased with an osteoid volume of 2.1% (N = 1.2 +/- 0.5) and a mean osteoid thickness of 28.7 microns (N = 13 +/- 2.5), with a normal bone fluoride content (0.082%, N < 0.10). Bone resorption as assessed on bone biopsy and by the measurement of markers of bone remodeling (serum procollagen type I C-terminal telopeptide and 24 hr urinary cross-laps to creatinine ratio) was increased. This latter finding was not necessarily due to axial osteomalacia and could be the consequence of moderate phosphate diabetes. The patient was treated with calcitriol which was promptly discontinued due to gastrointestinal symptoms and replaced by calcidiol without any significant effect on the low back pain.