Hyperparathyroidism and increased fractional excretion of phosphate predict allograft loss in long-term kidney transplant recipients

BACKGROUND

After kidney transplantation, fibroblast growth factor-23 (FGF-23) normally returns to baseline within 1 year whereas hyperparathyroidism persists in most kidney transplant (KT) recipients. As a result, serum phosphate remains relatively low in association with increased serum calcium and urinary phosphate excretion when compared to chronic kidney disease patients. The relationship between mineral metabolism and outcomes in long-term KT recipients has not been extensively studied. This study investigated whether the alteration in mineral metabolism influenced graft survival in long-term KT recipients.

METHODS

This study included 273 KT recipients after 1 year of transplantation. Mineral parameters were obtained at the time of enrolment and patients were followed prospectively for an average of 71 months.

RESULTS

Graft loss (death-censored) occurred in 41 (15%) patients. In univariate analysis, deceased donor transplantation, decreased serum albumin and estimated glomerular filtration rate, increased serum phosphate, parathyroid hormone (PTH), FGF-23 and fractional excretion of phosphate (FePi) predicted future allograft loss. After adjustments for cardiovascular disease risk factors, donor type, dialysis vintage, serum albumin and allograft function, only increased PTH and FePi remained associated with the outcome. Relationships between increased serum phosphate and FGF-23 with graft survival were lost after adjustments. Adjusted survival curves revealed the association between PTH > 90 pg/mL and FePi > 20% with worse graft survival.

CONCLUSIONS

Hyperparathyroidism and increased FePi predicted allograft loss in long-term KT recipients.

Osteopetrosis, osteopetrorickets and hypophosphatemic rickets differentially affect dentin and enamel mineralization

Osteopetrosis (OP) is an inherited disorder of defective bone resorption, which can be accompanied by impaired skeletal mineralization, a phenotype termed osteopetrorickets (OPR). Since individuals with dysfunctional osteoclasts often develop osteomyelitis of the jaw, we have analyzed, if dentin and enamel mineralization are differentially affected in OP and OPR. Therefore, we have applied non-decalcified histology and quantitative backscattered electron imaging (qBEI) to compare the dental phenotypes of Src(-/-), oc/oc and Hyp(-/0) mice, which serve as models for OP, OPR and hypophosphatemic rickets, respectively. While both, Src(-/-) and oc/oc mice, were characterized by defects of molar root formation, only oc/oc mice displayed a severe defect of dentin mineralization, similar to Hyp(-/0) mice. Most importantly, while enamel thickness was not affected in either mouse model, the calcium content within the enamel phase was significantly reduced in oc/oc, but not in Src(-/-) or Hyp(-/0) mice. Taken together, these data demonstrate that dentin and enamel mineralization are differentially affected in Src(-/-) and oc/oc mice. Moreover, since defects of dental mineralization may trigger premature tooth decay and thereby osteomyelitis of the jaw, they further underscore the importance of discriminating between OP and OPR in the respective individuals.

Growth in X-linked hypophosphatemic rickets

Growth failure appears frequently in children with X-linked hypophosphatemic rickets (XLHR) due to hypophosphatemia, disease severity, body disproportion, and primary bone abnormality. Recombinant human growth hormone (rhGH) increases phosphate tubular reabsorption and phosphate level in blood and, thus, constitutes an attractive but controversial therapy in short children with XLHR, those efficacy was demonstrated in small uncontrolled series. Our aim was to report our experience regarding growth in XLHR. Twenty-seven children with XLHR--20 girls, seven boys--diagnosed at a median (md) of 1.46 years of age, (range 0.39-8.5 years), were studied at 10.12 years of age (1.58-18.56), md (range). All received oral treatment with phosphate and calcitriol. At the first visit, grouped Z-height was -1; (-4.58; 0.54) md (range). After 5 years' follow-up (0.92-15.6), Z-height was -0.91 (- 4.56; 0.17), not different from that at baseline (P = 0.465). In 16 children entirely controlled in our program upon presentation, a "catch up" phenomenon after the rickets had healed (P = 0.823) or throughout the long-term was not observed (P = 0.995). Eight patients had a Z-height </= -2SD at the last visit, and impaired linear growth was associated with age >2 years at diagnosis, male gender and non-adherence to treatment. Four children, all boys, received rhGH, and in two cases with sufficient follow up stature normalized. No rhGH side effects were observed, and phosphate and calcitriol doses remained stable. Linear growth failure appeared in a third of XLHR children. Efforts need to be made to reduce the age of diagnosis and to improve adherence to treatment. Treatment with rhGH should be considered early, after the rickets has been controlled, in those patients with impaired growth or delayed diagnosis.

Evaluation of stature development during childhood and adolescence in individuals with familial hypophosphatemic rickets

This review was conducted to study the diagnosis, treatment, and growth progression in infants and adolescents with familial hypophosphatemic rickets. The bibliographic search was carried out utilizing the electronic databases MEDLINE, OVID, and LILACS and by direct research within the last 15 years using the keywords rickets, familial hypophosphatemia, vitamin D deficiency, stature growth, childhood, and adolescence. Article selection was done by comparing the evaluation of the growth in patients with familial hypophosphatemic rickets, including the variables that might affect them, for possible future therapeutic proposals. It is concluded that the most significant fact in the treatment of familial hypophosphatemic rickets in infancy was the magnitude of the final stature. The use of growth hormone can be helpful in these patients. However, research reporting treatments with the use of the growth hormone for rickets are controversial. The majority of the authors agree that treatment using vitamin D and phosphate enables some statural growth in cases of early diagnosis, reflecting a better prognosis.

Material matters: a mechanostat-based perspective on bone development in osteogenesis imperfecta and hypophosphatemic rickets

This perspective paper presents a hypothesis that links abnormalities of bone material with densitometric findings in two congenital metabolic bone disorders, osteogenesis imperfecta type I (OI) and X-linked hypophosphatemic rickets (XLH). Analyses of iliac bone samples from OI patients have shown that material bone density is elevated and that the bone material is abnormally stiff in this disorder. Therefore, a given mechanical load on an OI bone will generate a smaller than normal deformation. This in turn should lead osteocytes, the putative mechanosensing cells, to systematically underestimate the prevailing mechanical forces. According to the mechanostat model, bone strength should then be adapted to the underestimated mechanical loads, which means that bone architecture and mass remain below requirements. Available densitometric studies are in accordance with this hypothesis. In XLH, a mild mineralization defect persists despite treatment. This mineralization defect should lead to soft bone material. In analogy to the above model for OI, mechanical loads should be overestimated, resulting in increased densitometric parameters of bone strength. Indeed, lumbar spine areal bone mineral density is usually elevated in such patients.

"Phosphatonins" and the regulation of phosphorus homeostasis

Phosphate ions are critical for normal bone mineralization, and phosphate plays a vital role in a number of other biological processes such as signal transduction, nucleotide metabolism, and enzyme regulation. The study of rare disorders associated with renal phosphate wasting has resulted in the discovery of a number of proteins [fibroblast growth factor 23 (FGF-23), secreted frizzled related protein 4 (sFRP-4), matrix extracellular phosphoglycoprotein, and FGF 7 (FGF-7)] that decrease renal sodium-dependent phosphate transport in vivo and in vitro. The "phosphatonins," FGF-23 and sFRP-4, also inhibit the synthesis of 1alpha,25-dihydroxyvitamin D, leading to decreased intestinal phosphate absorption and further reduction in phosphate retention by the organism. In this review, we discuss the biological properties of these proteins, alterations in their concentrations in various clinical disorders, and their possible physiological role.

Hypophosphatémies génétiquement déterminées chez l’adulte

Hereditary hypophosphatemic rickets groups together X-linked hypophosphatemic rickets (XLH), autosomal dominant hypophosphatemic rickets (ADHR) and hereditary hypophosphatemic rickets with hypercalciuria (HHRH, autosomal recessive). Clinical and biological characteristics and treatment depend on specific etiology. Mutations causing hereditary hypophosphatemic rickets involve PHEX located on Xp11.22 for XLH and FGF-23 located on 12p13 for ADHR. The gene involved in HHRH remains unknown: candidates may encode proteins that modulate phosphate transporter expression or activity. Others forms of rickets must be ruled out: acquired hypophosphatemia due to oncogenic osteomalacia, X-linked recessive hypophosphatemic rickets or Dent's disease, and hereditary 1, 25-dihydroxyvitamin D-resistant rickets with a defect either in the 1-alpha-hydroxylase gene (pseudo-vitamin D deficiency rickets, PDDR) or in the vitamin D receptor (hereditary vitamin D-resistant rickets, HVDRR).

Co-existence of X-linked hypophosphatemic rickets (XLH) and primary hyperparathyroidism: case report and review of the literature

X-linked hypophosphatemic rickets (XLH) is a dominant disorder characterized by hypophosphatemia due to impaired renal tubular reabsorption of inorganic phosphate. Cardinal manifestations include defective calcification of cartilage and bone, growth retardation and resistance to phosphorus and vitamin D therapy. Although secondary hyperparathyroidism (HPT) is a common complication of treatment, autonomous HPT is rare, especially in the absence of previous phosphate therapy. We report a case of an adult untreated male XLH patient with primary HPT and give a brief review of the literature regarding the prevalence and pathophysiology of this complication.

Role of matrix extracellular phosphoglycoprotein in the pathogenesis of X-linked hypophosphatemia

X-linked hypophosphatemia (XLH), a disorder characterized by hypophosphatemia, impaired skeletal mineralization, and aberrant regulation of 1, 25(OH)(2)D(3), is caused by inactivating mutations of Phex, which results in the accumulation of putative phosphaturic factors, called phosphatonins. Matrix extracellular phosphoglycoprotein (Mepe) is a proposed candidate for phosphatonin. The authors found that Hyp mice had increased expression of the MEPE and another phosphaturic factor, Fgf23. To establish MEPE's role in the pathogenesis of the XLH, Mepe-deficient mice were back-crossed onto the Hyp mouse homologue of XLH and phenotypes of wild-type, Mepe(-/-), Hyp, and Mepe(-/-)/Hyp mice were examined. Transfer of Mepe deficiency onto the Phex-deficient Hyp mouse background failed to correct hypophosphatemia and aberrant serum 1,25(OH)(2)D(3) levels. Increased Fgf23 levels in Hyp mice were not affected by superimposed Mepe deficiency. In addition, Mepe-deficient Hyp mice retained bone mineralization defects in vivo, characterized by decreased bone mineral density, reduced mineralized trabecular bone volume, lower flexural strength, and histologic evidence of osteomalacia; however, cultures of Hyp-derived bone marrow stromal cells in the absence of Mepe showed improved mineralization and normalization of osteoblast gene expression profiles observed in cells derived from Mepe-null mice. These results demonstrate that MEPE elevation in Hyp mice does not contribute to the hypophosphatemia associated with inactivating Phex mutations and is therefore not phosphatonin.

FGF23 and disorders of phosphate homeostasis

It is well known that fibroblast growth factor (FGF) family members are associated with embryonic development and are critical for basic metabolic functions. This review will focus upon fibroblast growth factor-23 (FGF23) and its roles in disorders associated with phosphate handling. The discovery that mutations in FGF23 were responsible for the isolated renal phosphate wasting disorder autosomal dominant hypophosphatemic rickets (ADHR) has ascribed novel functions to the FGF family. FGF23 circulates in the bloodstream, and animal models demonstrate that FGF23 controls phosphate and Vitamin D homeostasis through the regulation of specific renal proteins. The ADHR mutations in FGF23 produce a protein species less susceptible to proteolytic processing. X-linked hypophosphatemic rickets (XLH), tumor-induced osteomalacia (TIO), and fibrous dysplasia of bone (FD) are disorders involving phosphate homeostasis that share phenotypes with ADHR, indicating that FGF23 may be a common denominator for the pathophysiology of these syndromes. Our understanding of FGF23 will help to develop novel therapies for phosphate wasting disorders, as well as for disorders of increased serum phosphate, such as tumoral calcinosis, a rare disorder, and renal failure, a common disorder.

Renal phosphate--wasting disorders in childhood

The purpose of this review paper is to provide a summary of recent developments in the childhood disorders of renal phosphate-wasting, with particular emphasis on two of the hereditary conditions, X-linked hypophosphatemia (XLH) and autosomal dominant hypophosphatemic rickets (ADHR), as they are distinguished from the paraneoplastic syndrome, oncogenic hypophosphatemic osteomalacia (OHO). An overview of the clinical manifestations, pathogenesis, diagnosis, and treatment of these conditions, with attention to newly-discovered genetic and hormonal signatures, will be discussed. The information may be invaluable in the complex diagnosis and successful treatment of OHO and hereditary hypophosphatemias of childhood.

Non-azotemic refractory rickets in Indian children

In order to examine the etiology of refractory rickets, we evaluated the case records of patients presenting between 1990 and 2002. Subjects with impaired renal functions were excluded. Of 131 patients, 25.9 % each had hypophosphatemic rickets and distal renal tubular acidosis (RTA), 19.6 %vitamin D dependent rickets (VDDR), 11.3 % proximal RTA, 9.1 % liver disease and 6.1 % malabsorption. A significant proportion of patients with VDDR and proximal RTA showed deformities in the first year of life, whereas those with distal RTA and hypophosphatemic rickets presented later. Patients with hypophosphatemic rickets had predominant involvement of lower limbs; hypercalciuria was found in 4. Distal RTA was associated with marked rickets and normal levels of alkaline phosphatase. Hypophosphatemia and low tubular reabsorption of phosphate, though characteristic of hypophosphatemic rickets, was also seen in patients with VDDR (19.2%) and distal RTA (17.6 %). Our findings suggest that application and interpretation of appropriate investigations are useful in determining the cause of non-azotemic refractory rickets allowing initiation of specific therapy.

Transgenic mice overexpressing human fibroblast growth factor 23 (R176Q) delineate a putative role for parathyroid hormone in renal phosphate wasting disorders

Fibroblast growth factor 23 (FGF23) is a recently characterized protein likely involved in the regulation of serum phosphate homeostasis. Increased circulating levels of FGF23 have been reported in patients with renal phosphate-wasting disorders, but it is unclear whether FGF23 is the direct mediator responsible for the decreased phosphate transport at the proximal renal tubules and the altered vitamin D metabolism associated with these states. To examine this question, we generated transgenic mice expressing and secreting from the liver human FGF23 (R176Q), a mutant form that fails to be degraded by furin proteases. At 1 and 2 months of age, mice carrying the transgene recapitulated the biochemical (decreased urinary phosphate reabsorption, hypophosphatemia, low serum 1,25-dihydroxyvitamin D(3)) and skeletal (rickets and osteomalacia) alterations associated with these disorders. Unexpectantly, marked changes in parameters of calcium homeostasis were also observed, consistent with secondary hyperparathyroidism. Moreover, in the kidney the anticipated alterations in the expression of hydroxylases associated with vitamin D metabolism were not observed despite the profound hypophosphatemia and increased circulating levels of PTH, both major physiological stimuli for 1,25-dihydroxyvitamin D(3) production. Our findings strongly support the novel concept that high circulating levels of FGF23 are associated with profound disturbances in the regulation of phosphate and vitamin D metabolism as well as calcium homeostasis and that elevated PTH levels likely also contribute to the renal phosphate wasting associated with these disorders.

Phosphatonin washout in Hyp mice proximal tubules: evidence for posttranscriptional regulation

X-linked hypophosphatemia is the most common inherited form of rickets. It is characterized by renal phosphate wasting, leading to hypophosphatemia and an inappropriately normal or low serum level of 1,25(OH)2 vitamin D. Previous studies have pointed to a circulating factor or phosphatonin-inhibiting phosphate transport by decreasing mRNA of the proximal tubule NaP(i) cotransporter NaPi-2A. The present study examined the hypothesis that there was also posttranscriptional regulation of the NaPi-2A cotransporter in Hyp mice proximal tubules and whether the phosphate transport defect in Hyp mice persisted when they were studied in vitro. We found that the rate of phosphate transport in Hyp mice was <50% that in C57/B6 control mice. While phosphate transport remained stable during incubation with time in C57/B6 mice proximal tubules, it increased from 0.46 +/- 0.47 to 1.83 +/- 0.40 pmol x mm(-1) x min(-1) in Hyp proximal tubules (P < 0.01) consistent with phosphatonin washout in Hyp proximal tubules perfused in vitro. This time-dependent increase in phosphate transport was still observed in the presence of cycloheximide. There was also a reduction of proximal tubule apical NaPi-2A expression from Hyp mice compared with C57/B6 mice using single-tubule immunohistochemistry. Using immunohistochemistry, we demonstrate an increase in apical expression of the NaPi-2A transporter in proximal tubules perfused in vitro in Hyp mice even in the presence of bath cycloheximide. The increase in apical expression of the NaPi-2A transporter in proximal tubules perfused in vitro in Hyp mice was blocked by colchicine. These data are consistent with a rapidly reversible posttranscriptional defect in Hyp mice causing a reduction in phosphate transport.

The genomic response of the mouse kidney to low-phosphate diet is altered in X-linked hypophosphatemia

The mechanism for the renal adaptation to low-phosphate diets is not well understood. Whether the Hyp mutation of the Phex gene blocks this adaptation is also not clear. To gain further insight into this, 5-wk-old normal and Hyp mice were fed a control (1.0% P) or low-phosphate diet (0.03% P) for 3-5 days. Renal RNA was hybridized to Affymetrix U74Av2 microarrays (5 arrays/group). Of the 5,719 detectable genes on each array, 290 responded significantly (P < 0.01) to low-phosphate diet in normal mice. This was reduced significantly (P < 0.001) to 7 in the Hyp mice. This suggested that the adaptations of the normal kidney to a low-phosphate environment were blocked by the Hyp mutation. The Npt2 phosphate transporter, vitamin D 1alpha- and 24-hydroxylases, and calbindins D9K and D28K responded in the expected fashion. Genes with significant (P < 0.05) diet-by-genotype interaction were analyzed by GenMAPP and MAPPFinder. This revealed a cluster of differentially expressed genes associated with microtubule-based processes. Most alpha- and beta-tubulins and most kinesins had responses to low-phosphate diet in normal mice which were abolished or reversed in Hyp mice. In summary, renal adaptation to low-phosphate diet involved changes in the mRNA expression of specific genes. Disruption of these responses in Hyp mice may contribute to their abnormal phosphate homeostasis.

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.

Rickets

Nutritional (vitamin D deficiency) rickets is now a rarity in the industrial world because of food fortification. Most cases result from hereditary inborn errors of vitamin D metabolism or end-organ unresponsiveness as is exemplified in this case of hypophosphatemic rickets.

Deformity correction of knee and leg lengthening by Ilizarov method in hypophosphatemic rickets: outcomes and significance of serum phosphate level

The authors evaluated 14 patients with hypophosphatemic rickets who underwent correction of a knee deformity along with a leg lengthening by the Ilizarov method. Deformity correction alone was performed in 8 femora and 4 tibiae-fibulae, and concomitant deformity correction and limb lengthening (>1.0 cm) in 9 femora and 19 tibiae-fibulae. The healing index correlated with the biochemical parameters. Knee deformities were satisfactorily corrected in all patients except one. There was a statistically significant negative correlation between the healing index and the serum phosphate level: those who had a serum phosphate level higher than 2.5 mg/dL showed a relatively rapid regenerate bone healing compared with those with less than 2.5 mg/dL. The authors conclude that a serum phosphate level of 2.5 mg/dL as a cut-off point should be considered in deciding whether deformity correction alone or with a concomitant leg lengthening should be undertaken.

Ontogeny of Phex/PHEX protein expression in mouse embryo and subcellular localization in osteoblasts

PHEX, a phosphate-regulating gene with homologies to endopeptidases on the X chromosome, is mutated in X-linked hypophosphatemia (XLH) in humans and mice (Hyp). Although recent observations indicate that Phex protein is expressed primarily in bone and may play an important role in osteoblast function and bone mineralization, the pattern of the Phex protein expression in the developing skeleton and its subcellular localization in osteoblasts remain unknown. We examined the ontogeny of the Phex protein in the developing mouse embryo and its subcellular localization in osteoblasts using a specific antibody to the protein. Immunohistochemical staining of mouse embryos revealed expression of Phex in osteogenic precursors in developing vertebral bodies and developing long bones on day 16 postcoitum (pc) and thereafter. Calvaria from day 18 pc mice showed Phex epitopes in osteoblasts. No Phex immunoreactivity was detected in lung, heart, hepatocytes, kidney, intestine, skeletal muscle, or adipose tissue of mouse embryos. Interestingly, embryonic mouse skin showed moderate amounts of Phex immunostaining. In postnatal mice, Phex expression was observed in osteoblasts and osteocytes. Moderate expression of Phex was seen in odontoblasts and slight immunoreactivity was observed in ameloblasts. Confocal microscopy revealed the presence of immunoreactive PHEX protein in the Golgi apparatus and endoplasmic reticulum of osteoblasts from normal mice and in osteoblasts from Hyp mice transduced with a human PHEX viral expression vector. PHEX protein was not detected in untransduced Hyp osteoblasts. These data indicate that Phex protein is expressed in osteoblasts and osteocytes during the embryonic and postnatal periods and that within bone, Phex may be a unique marker for cells of the osteoblast/osteocyte lineage.

The effect of treatment on growth and deformity in hypophosphatemic vitamin D-resistant rickets

Vitamin D-resistant rickets is characterized by short stature, lower extremity deformities, and defective mineralization of bone. While basic biochemical defects vary among involved individuals, all patients show a failure of the proximal tubule of the kidney to resorb inorganic phosphate. Laboratory findings consist of low serum phosphorus, elevated alkaline phosphatase, and abnormal serum calcium. Previously recommended treatment programs of high doses of vitamin D have effected some roentgenographic improvement in the rachitic lesions, but no related increase in height or severity of deformities has been associated with significant complications resulting from vitamin D toxicity. Daily administration of low doses of vitamin D and oral phosphates has more recently been suggested to be beneficial in promoting growth and preventing deformities. Thirteen children with documented vitamin D-resistant rickets were treated with oral phosphate and low doses of vitamin D for a mean of five years, and followed for a mean of ten years. Partial roentgenographic resolution of rachitic lesions was similar to those who received vitamin D alone. While the majority had consistently lowered alkaline phosphatases, no patient had consistently normal phosphate levels. No patient exceeded the third percentile in height. One half required osteotomies of the lower extremities. All osteotomies (eight) performed before maturity required revision, whereas those done after maturity (12) did not. Since no apparent clinical roentgenographic benefit can be documented by the addition of oral phosphate to low doses of vitamin D, we would not recommend continually doing so at this time.

The autosomal dominant hypophosphatemic rickets (ADHR) gene is a secreted polypeptide overexpressed by tumors that cause phosphate wasting

The gene mutated in autosomal dominant hypophosphatemic rickets (ADHR), a phosphate wasting disorder, has been identified as FGF-23, a protein that shares sequence homology with fibroblast growth factors (FGFs). Patients with ADHR display many of the clinical and laboratory characteristics that are observed in patients with oncogenic hypophosphatemic osteomalacia (OHO), a disorder thought to arise by the secretion of a phosphate wasting factor from different mesenchymal tumors. In the present studies, we therefore investigated whether FGF-23 is a secreted factor and whether it is abundantly expressed in OHO tumors. After transient transfection of OK-E, COS-7, and HEK293 cells with the plasmid encoding full-length FGF-23, all three cell lines efficiently secreted two protein species into the medium that were approximately 32 and 12 kDa upon SDS-PAGE and subsequent Western blot analysis using an affinity-purified polyclonal antibody to FGF-23. Furthermore, Northern blot analysis using total RNA from five different OHO tumors revealed extremely high levels of FGF-23 mRNA, and Western blot analysis of extracts from a sixth tumor detected the 32 kDa FGF-23 protein species. In summary, FGF-23, the gene mutated in ADHR, is a secreted protein and its mRNA is abundantly expressed by several different OHO tumors. Our findings indicate that FGF-23 may be a candidate phosphate wasting factor, previously designated "phosphatonin".

Effect of growth hormone treatment on final height, phosphate metabolism, and bone mineral density in children with X-linked hypophosphatemic rickets

OBJECTIVE

To evaluate the effect of growth hormone treatment on final height, phosphate metabolism, bone markers, and bone mineral density in children with X-linked hypophosphatemic rickets.

STUDY DESIGN

Six patients (aged 7.8 +/- 1.8 years; height z score, -3.4 +/- 0.5) received conventional treatment (1,25-dihydroxyvitamin D(3) plus phosphate salts) combined with growth hormone (0.6-0.9 IU/kg per week, subcutaneously) (group A); 6 patients (aged 7.9 +/- 2.5 years; height z score, -2.1 +/- 0.6, P <.01 with respect to group A) received only conventional treatment (group B).

RESULTS

Final height z score significantly improved in group A (-2.4 +/- 0.5, P <.03 with respect to the value at entry), whereas it did not change in group B (-2.8 +/- 0.5, P = NS). At final height, degree of body disproportion was similar between the groups (group A, 1.3 +/- 0.2; group B, 1.2 +/- 0.1; P = NS). Phosphate retention, bone markers, and radial bone mineral density increased only in group A. No adverse effects were observed.

CONCLUSIONS

In poorly growing patients with X-linked hypophosphatemic rickets, growth hormone therapy combined with conventional treatment improves final height, phosphate retention, and radial bone mineral density, but it does not influence degree of body disproportion.

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.

[Deviations in the axes of the lower extremities]

Deviations in the axes can be physiological or pathological. During growth, the axes of the lower extremities undergo significant changes. The femoral neck-shaft-angle decreases from 150 degrees on average at birth to 120 degrees on average at the end of growth. The lower leg is bowed in a varus direction at birth. After a child begins to walk, the knee axis deviates to a slight valgus. This goes back to normal before the age of ten. Pathologic deviations of the axis are rare. They occur in congenital deformations, after trauma or infection, or as a result of metabolic diseases. In association with coxa vara pseudarthrosis of the femoral neck usually develops. Therefore this deformation has to be treated. Correction is best made with a Y-shaped osteotomy according to Pauwels. At the lower leg, a deviation of 15 degrees from the normal axis is usually an indication for operative treatment. In children and adolescents, corrections are best made with (monolateral) external fixators. They allow early weight bearing, which is especially desirable in bilateral corrections.

Disorders of phosphate metabolism

Correct identification of the disorders of hypophosphatemia and hyperphosphatemia is important for determining therapy. Further research will provide insights into normal phosphate homeostasis, a complex and fascinating process.

Final height of Japanese patients with X-linked hypophosphatemic rickets: effect of vitamin D and phosphate therapy

X-linked hypophosphatemic rickets (XLH) is one of the most common causes of rickets in infancy and childhood. Combination therapy of vitamin D and phosphate is generally used for patients with XLH. Effect of treatment of vitamin D and phosphate during childhood on final height of XLH has to be elucidated in Japanese. There have been only three Caucasian studies on final height of XLH with treatment since childhood. Purpose of this study is to report adult height and therapeutic effect of 22 Japanese participants (5 males, 17 females) with XLH who were treated with phosphate (33-200 mg/kg/day as phosphorus divided into 3 or 4 doses) and vitamin D (vitamin D2 or 1alpha-hydroxyvitamin D3) for more than five years and evaluate effect of the treatment on the final height retrospectively. Final height (FHt) for all participants was -1.69+/-11.11 SD. FHt (-1.69+/-1.11 SD) was significantly higher than height at the initiation of treatment (-2.38+/-0.88 SD) for all participants (P<0.01). In conclusion, combination therapy of vitamin D and phosphate improved final height of Japanese patients with XLH as is similar to previous Caucasian studies.

A trial of growth hormone therapy in well-controlled hypophosphataemic rickets

OBJECTIVE

Conventional therapy of hypophosphataemic rickets (HR) with oral phosphate and calcitriol does not always result in normal linear growth. Recombinant human growth hormone (rhGH) offers theoretical advantages as an adjunctive therapy. We aimed to determine the effects of adjunctive rhGH therapy in children with well-controlled HR.

PATIENTS

In this report, 5 prepubertal children (aged 3.5-10.9 years) with well-controlled HR on conventional therapy were given adjunctive standard dose rhGH therapy for one year.

DESIGN AND MEASUREMENTS

Height, growth velocity, metabolic markers of calcium and phosphate metabolism, body composition, bone mineral density, wrist and knee X-rays, and renal sonography were assessed at regular intervals. Height and growth velocities were also calculated 12 months after ceasing rhGH therapy.

RESULTS

After 12 months therapy with rhGH, no significant biochemical or radiological benefits were observed. A significant increase in height SD score was observed (P = 0.023), but this was not associated with any increase in the growth velocity SD score and appears to have been due to catch-up growth caused by conventional therapy alone. When rhGH therapy was ceased, no significant decreases in mean height SD or growth velocity SD scores were observed.

CONCLUSIONS

In well-controlled hypophosphataemic rickets patients receiving conventional therapy, adjunctive therapy with standard dose rhGH offers no benefits in linear growth or rachitic disease markers.

The effect of osteotomy on bowing and height in children with X-linked hypophosphatemia

The clinical records of 40 patients with X-linked hypophosphatemia who were treated and followed for at least 36 months were examined retrospectively. The patients were divided into those treated with medication only (group A) and those treated with medication and surgery (group B). At follow-up, significant improvement in bowing angles was noted for group B patients compared with those treated medically. There was, however, no improvement in the height of children in group A, and there was a significant decrease in height among the children treated surgically (group B). Neither height nor bowing angles of femurs or tibias at presentation predicted the need for future osteotomies. However, children who eventually needed surgery were found to be obese at presentation significantly more frequently than children who did not require osteotomy.

X-linked hypophosphatemia: normal renal function despite medullary nephrocalcinosis 25 years after transient vitamin D2-induced renal azotemia

Nephrocalcinosis (NC) detected by ultrasound is a recognized abnormality for some patients with X-linked hypophosphatemia (XLH) who received vitamin D2 and inorganic phosphate therapy, but is commonly observed in XLH patients treated with 1,25-dihydroxyvitamin D3 and inorganic phosphate supplementation. Nevertheless, long-term follow-up of kidney function in XLH patients with NC detected ultrasonographically has not been reported. We investigated two women with XLH, ages 31 (patient 1) and 39 (patient 2) years, each of whom had suffered at least one documented episode of vitamin D2-induced hypercalcemia and renal azotemia during childhood. Patient 2 had also been treated with inorganic phosphate. No medications for XLH had been taken during adulthood. Renal ultrasonography at our institution demonstrated marked bilateral medullary NC in both women. No other explanation was found for their NC that apparently occurred several decades earlier from medical therapy for XLH. Detailed studies (including creatinine clearance, beta2-microglobulin excretion, and fasting urinary osmolality and acidification) revealed no impairment of kidney function in either patient. Our findings indicate that subradiographic medullary NC acquired during medical therapy for XLH may persist for decades, but with no adverse renal sequelae. Definitive (long-term) assessment of kidney function in the XLH population with NC, however, will be necessary to fully understand the risk of current medical treatment for this most common heritable form of rickets.

Diagnosis of X-linked hypophosphatemic vitamin D resistant rickets

Recent advances in the pathophysiology and diagnosis of X-linked hypophosphatemic rickets (XLH) are reviewed. The recent discovery of the gene that is responsible for XLH especially enables us to understand the mechanism of hypophosphatemia in XLH. Laboratory and radiological findings are important for the diagnosis. However, dental abnormalities such as spontaneous dental abscess and a defect of dentin maturation are also notable findings.

Cardiovascular abnormalities in patients with X-linked hypophosphatemia

Treatment for X-linked hypophosphatemia (XLH; vitamin D metabolites and phosphate salts) may result in hypercalcemia, hypercalciuria, nephrocalcinosis, and hyperparathyroidism. Cardiovascular abnormalities occur in association with these complications, but have not been reported in XLH. We hypothesized that such abnormalities may occur in XLH and evaluated cardiovascular status in 13 patients with this disease. All patients were asymptomatic and had normal cardiovascular physical examinations and Holter studies. Serum calcium and creatinine clearance were normal in all. However, all patients had mild to moderate nephrocalcinosis. Left ventricular hypertrophy was diagnosed by electrocardiogram in three and by ultrasonography in seven children. Baseline blood pressure (BP) was normal (mean +/- SD, 116 +/- 15/74 +/- 6 mm Hg). During exercise stress testing, systolic BP increased in all patients, but the maximal systolic pressure was less than that in healthy age- and sex-matched controls (156 +/- 20 vs. 175 +/- 23; P = 0.002, by t test). An abnormal increase in diastolic BP occurred at all levels of work load in XLH patients; their peak exercise diastolic BP was 91 +/- 12 vs. 72 +/- 6 mm Hg in controls (P < 0.0001, by t test). Whether these abnormal findings are primary defects in XLH or represent complications of treatment is unclear. Patients with XLH should be monitored closely for the development of hypertension and left ventricular hypertrophy. Investigation of the mechanisms involved and establishment of therapeutic guidelines are indicated.

New insights into X-linked hypophosphatemia

X-linked hypophosphatemia is a heritable form of rickets characterized biochemically by phosphaturia and abnormal bioactivation of vitamin D. Recent advances include the observation, using kidney cells from the X-linked hypophosphatemia mouse model (Hyp), that in-vitro renal phosphate transport is normal yet bone mineralization may be intrinsically abnormal. Of special interest is the identification of a gene (PEX) that is mutated in X-linked hypophosphatemic patients.

Regulation of renal phosphate handling: recent findings

Our knowledge of renal phosphate handling and its regulation has expanded in the past 2 years because of the molecular identification of sodium-phosphate cotransport systems. New tools (molecular probes and antibodies) have allowed dissection out of some of the molecular and cellular mechanisms underlying the adaptation of phosphate transport to dietary content, the phosphaturic effect of parathyroid hormone or glucocorticoids and the renal phosphate leak in hypophosphataemic rickets.

24R,25-dihydroxyvitamin D3 promotes bone formation without causing excessive resorption in hypophosphatemic mice

To clarify the differences in the action of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and 24,25-(OH)2D3 in hypophosphatemic (Hyp) mice, a model for familial X-linked hypophosphatemic rickets in humans, we carried out histomorphometric examinations of the effects of these agents in the lumbar vertebra of these mice. The Hyp mice received 1-1000 micrograms/kg.day 24,25-(OH)2D3, 0.01-0.1 micrograms/kg.day 1,25.(OH)2D3, or vehicle alone given daily for 28 days by ip injection. Histomorphometrically, 1,25-(OH)2D3 and 24,25-(OH)2D3 showed similar effects on bone formation. The parameters of bone formation, mineralized bone volume/bone volume, mineral apposition rate, and bone formation rate/bone surface, were improved to a similar extent in a dose-dependent manner by 1,25-(OH)2D3 and 24,25-(OH)2D3, but there were remarkable differences in the indexes of the bone resorption between these two metabolites. In 24,25-(OH)2D3-treated Hyp mice, osteoclast number/bone perimeter and osteoclast surface/bone surface, the parameters of bone resorption, increased to control levels and did not change according to the dose of 24,25-(OH)2D3. However, in 1,25-(OH)2D3-treated Hyp mice, these values increased remarkably, exceeding the control level. That is, 24,25-(OH)2D3 normalized bone resorption in the rachitic mice, whereas 1,25-(OH)2D3 caused excessive stimulation of bone resorption. This qualitative difference between the two compounds contributes to the superior effects exerted by 24,25-(OH)2D3 in improving the bone lesion in Hyp mice. At doses from 1-1000 micrograms/kg.day, 24,25-(OH)2D3 had dose-dependent effects in increasing bone formation without promoting excessive bone resorption, as shown by histomorphometric analysis.

Serum insulin-like growth factor binding protein-3 in the hypophosphatemic mouse: decreased activity and abnormal modulation by dietary phosphate

The hypophosphatemic mouse, the murine homologue of X-linked hypophosphatemia, is characterized by renal defects in phosphate reabsorption and 1,25-dihydroxy vitamin D3 (1,25(OH)2D3) production and by an osteoblast dysfunction. In view of the potential importance of insulin-like growth factors (IGFs) in the regulation of these processes and the role of IGF-binding proteins (IGFBPs) as modulators of IGF action, we asked whether Hyp mice have alterations in IGFs or IGFBPs. Using specific radioimmunoassays and Western ligand blot analysis, we evaluated serum levels of IGFs (IGF-1 and IGF-II) and IGFBPs, respectively, in normal and Hyp mice. We also examined the effect of dietary phosphatase on these parameters. Serum levels of IGF-1 and IGF-II in Hyp mice were not significantly different from those in normal mice, but IGFBP-3 levels were significantly lower (70% of normal, p < 0.05) in the mutant strain. The other IGFBP species appear unchanged. Phosphate supplementation normalized serum phosphate levels in Hyp mice and elicited a significant decrease in serum IGF-I levels (23%, p < 0.05) and a further deduction in IGFBP-3 (22%, p < 0.02). Phosphate deprivation induced hypophosphatemia IGF-II. The present results indicate that the low serum IGFBP-3 activity in Hyp mice is not related to hypophosphatemia per se. Based on the documented effects of parathyroid hormone (PTH) on IGF-I and IGFBP-3, we propose that the secondary hyperparathyroidism displayed by Hyp mice and its exacerbation by phosphate supplementation may contribute to low IGFBP-3 levels in control Hyp mice and to the decreases in serum IGF-I and IGFBP-3 in phosphate-supplemented Hyp mice.

Effect of abnormal mineralization on the mechanical behavior of X-linked hypophosphatemic mice femora

The Hyp mouse is an established animal model of X-linked hypophosphatemia, one of the most common genetic forms of metabolic bone disease in humans. This study describes the first determination of whole bone mechanical behavior in the heterozygous male and female Hyp mouse. Femora from 12-week-old mice were tested in torsion. The contribution of structural and material properties to mechanical behavior was determined by geometrical evaluation prior to testing and by analysis of the diaphyseal mineral after testing. The male and female Hyp femora were found to undergo significantly more angular deformation at failure than the same sex normal femora (82.49 +/- 24.37 vs. 22.63 +/- 8.02 rad/m [corrected] for the females and 128.90 +/- 37.05 vs. 22.79 +/- 7.24 rad/m [corrected] for the males) and to have a significantly lower structural stiffness (0.373 +/- 0.130 x 10(-3) vs. 1.33 +/- 0.380 x 10(-3) [corrected] [N-m/(rad/m)] for the females and 0.167 +/- 0.104 x 10(-3) vs. 1.60 +/- 0.502 x 10(-3) [corrected] [N-m/(rad/m)] for the males). The male Hyp femora had a significantly lower failure torque than male normal femora (1.58 +/- 0.62 x 10(-2) vs. 3.44 +/- 1.57 x 10(-2) N-m). Because the polar movement of inertia, a geometrical property that affects torsional behavior, was not significantly different between the Hyp femora and the same sex normals, differences in mechanical behavior were attributed to material properties.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of familial hypophosphatemic rickets on dental development: a controlled, longitudinal study

Familial or X-linked hypophosphatemic rickets (XLHR) is the most common type of rickets in developed countries today. While the dental manifestations of rickets are well reported, there is little information regarding its relationship to dental development and other dental anomalies. This investigation studied the rate of dental development and associated dental anomalies in 19 XLHR subjects compared with 38 race-, age-, and sex-matched control children. The results showed that in both XLHR and control children, no significant differences existed in dental age compared with the respective chronological age, indicating that rickets did not affect the rate of dental development. Longitudinal growth curves of seven XLHR and matched control children substantiated that relationships of dental to chronological ages were comparable in both groups. Male XLHR subjects showed significantly increased tendency for dental taurodontism with mean Crown-Body (CB):Root (R) ratio of 1.1 compared with 1.0 in females and 0.8 in controls (P < 0.02). Male XLHR children also showed significantly increased prevalence (50%) of ectopic permanent canines compared with control children (8%, P < 0.01).

Adult-onset idiopathic phosphate diabetes. II. Time-course of clinical, laboratory test, and bone mineral density abnormalities under combined phosphate and calcitriol therapy

STUDY OBJECTIVE

to investigate the time-course of clinical, laboratory test, and bone mineral density abnormalities in patients with mild phosphate diabetes treated for at least one year with calcitriol, 0.5 to 1.5 micrograms, and oral phosphate, 788 to 2300 mg per day, in three divided doses.

PATIENTS AND METHODS

we studied eight patients with mild phosphate diabetes defined as a rate for tubular reabsorption of phosphate of less than 18% with a maximal rate for tubular reabsorption of phosphate (Tm) of less than 0.77 in the absence of any detectable cause of secondary tubular disease. Treatment efficacy was evaluated on the basis of pain severity, pain-related functional disability, serum phosphate and calcium levels, maximal rate for tubular reabsorption of phosphate, and dual-photon absorptiometry-measured bone mineral density.

RESULTS

three patients experienced complete relief of pain and fatigue and were able to resume their normal activities. Partial relief was seen in two other patients. The three remaining patients had no response to treatment. Renal colic occurred in one patient. None of the patients developed hypercalcemia.