Ubiquitin COOH-terminal hydrolase L1 deletion is associated with urinary α-klotho deficiency and perturbed phosphate homeostasis.

Loss of ubiquitin COOH-terminal hydrolase L1 (UCHL1), a deubiquitinating enzyme required for neuronal function, led to hyperphosphatemia accompanied by phosphaturia in mice, while calcium homeostasis remained intact. We therefore investigated the mechanisms underlying the phosphate imbalance in Uchl1-/- mice. Interestingly, phosphaturia was not a result of lower renal brush border membrane sodium-phosphate cotransporter expression as sodium-phosphate cotransporter 2a and 2c expression levels was similar to wild-type levels. Plasma parathyroid hormone and fibroblast growth factor 23 levels were not different; however, fibroblast growth factor 23 mRNA levels were significantly increased in femur homogenates from Uchl1-/- mice. Full-length and soluble α-klotho levels were comparable in kidneys from wild-type and Uchl1-/- mice; however, soluble α-klotho was reduced in Uchl1-/- mice urine. Consistent with unchanged components of 1,25(OH)2D3 metabolism (i.e., CYP27B1 and CYP24A1), sodium-phosphate cotransporter 2b protein levels were not different in ileum brush borders from Uchl1-/- mice, suggesting that the intestine is not the source of hyperphosphatemia. Nonetheless, when Uchl1-/- mice were fed a low-phosphate diet, plasma phosphate, urinary phosphate, and fractional excretion of phosphate were significantly attenuated and comparable to levels of low-phosphate diet-fed wild-type mice. Our findings demonstrate that Uchl1-deleted mice exhibit perturbed phosphate homeostasis, likely consequent to decreased urinary soluble α-klotho, which can be rescued with a low-phosphate diet. Uchl1-/- mice may provide a useful mouse model to study mild perturbations in phosphate homeostasis.

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.

Fractional excretion of phosphorus modifies the association between fibroblast growth factor-23 and outcomes.

Fibroblast growth factor-23 (FGF23) induces phosphaturia through its effects on renal tubules. Higher levels of FGF23 associate with cardiovascular disease (CVD) events and all-cause mortality, but it is unknown whether these associations differ by the degree of phosphaturia. Here, we measured serum FGF23 and 24-hour urine fractional excretion of phosphorus (FePi) in 872 outpatients with stable CVD and a mean estimated GFR of 71 ml/min per 1.73 m(2). During an average 7.5 years of follow-up, there were 337 deaths and 199 CVD events. Urinary FePi significantly modified the association of FGF23 with each outcome (P interaction<0.001 for all-cause mortality and P interaction<0.05 for CVD events). In models adjusted for CVD risk factors, kidney function, and PTH, those patients who had FGF23 above the median (≥ 42.3 relative units [RU]/ml) but FePi below the median (<15.7%) had the highest risks of both all-cause mortality (HR=1.98, 95% CI=1.42-2.77) and CVD events (HR=1.92, 95% CI=1.25-2.94) compared with those patients who had low concentrations of FGF23 and low FePi. In summary, associations of FGF23 with mortality and CVD events are stronger in persons with lower FePi independent of PTH and kidney function. In such individuals, the renal tubular response to FGF23 may be suboptimal.

Oncogenic osteomalacia, a rare paraneoplastic syndrome due to phosphate wasting--a case report and review of the literature.

An appropriate phosphate homeostasis is absolutely required for correct bone mineralization and remodeling, for diverse signaling pathways as well as cell membrane formation. Its disequilibrium results in serious complications like hypophosphatemia and excessively reduced fractional tubule phosphate reabsorption (TRP). A rare cause of such a disturbed phosphate balance is tumor-induced osteomalacia (TIO)--a phosphate wasting disorder sometimes associated with certain mesenchymal tumors. These primitive tumors secrete so-called phosphatonins--recently identified factors involved in the regulation of phosphate homeostasis such as the secreted frizzled related protein 4 (sFRP-4), the fibroblast growth factors 7 and 23 (FGF-7/-23), or the matrix extracellular phosphoglycoprotein (MEPE). Progressive muscular weakness and spontaneous bone fractures caused by inadequate osteoid mineralization are the characteristic clinical symptoms, which completely resolve after tumor resection. Here we report a new case of TIO caused by tumor secreted FGF-23 and review the literature to facilitate the correct diagnosis of this rare disorder.

Metabolism of vitamin D3-3H in vitamin D-resistant rickets and familial hypophosphatemia.

The fate of an intravenous dose of tritiated vitamin D(3) was studied in seven normal subjects, four children with vitamin D-resistant rickets, and four adults with a familial history of vitamin D-resistant rickets and persistent hypophosphatemia. An abnormal metabolism of vitamin D in vitamin D-resistant rickets was defined and characterized by a decrease in the plasma fractional turnover rate, a marked increase in plasma water-soluble metabolites, and a relative decrease in the conversion of vitamin D to a polar, biologically active metabolite. Alterations in vitamin D metabolism in the adults with persistent hypophosphatemia were similar but less severe than those of affected children with vitamin D-resistant rickets. It is tentatively concluded that the abnormalities in vitamin D metabolism documented in patients with vitamin D-resistant rickets and familial hypophosphatemia may account for the observed osseous and biochemical changes.

Intronic deletions in the SLC34A3 gene cause hereditary hypophosphatemic rickets with hypercalciuria.

CONTEXT: Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is a rare metabolic disorder, characterized by hypophosphatemia and rickets/osteomalacia with increased serum 1,25-dihydroxyvitamin D [1,25-(OH)(2)D] resulting in hypercalciuria. OBJECTIVE: Our objective was to determine whether mutations in the SLC34A3 gene, which encodes sodium-phosphate cotransporter type IIc, are responsible for the occurrence of HHRH. DESIGN AND SETTING: Mutation analysis of exons and adjacent introns in the SLC34A3 gene was conducted at an academic research laboratory and medical center. PATIENTS OR OTHER PARTICIPANTS: Members of two unrelated families with HHRH participated in the study. RESULTS: Two affected siblings in one family were homozygous for a 101-bp deletion in intron 9. Haplotype analysis of the SLC34A3 locus in the family showed that the two deletions are on different haplotypes. An unrelated individual with HHRH was a compound heterozygote for an 85-bp deletion in intron 10 and a G-to-A substitution at the last nucleotide in exon 7. The intron 9 deletion (and likely the other two mutations) identified in this study causes aberrant RNA splicing. Sequence analysis of the deleted regions revealed the presence of direct repeats of homologous sequences. CONCLUSION: HHRH is caused by biallelic mutations in the SLC34A3 gene. Haplotype analysis suggests that the two intron 9 deletions arose independently. The identification of three independent deletions in introns 9 and 10 suggests that the SLC34A3 gene may be susceptible to unequal crossing over because of sequence misalignment during meiosis.

X-linked hypophosphatemic rickets: a study (with literature review) of linear growth response to calcitriol and phosphate therapy.

Not all children with X-linked hypophosphatemia (XLH) have demonstrated improved linear growth with calcitriol [1,25-(OH)2D3] and inorganic phosphate (Pi) therapy. To assess which factors are associated with a favorable growth response during this treatment, we retrospectively compared demographics and biochemical parameters of bone metabolism to the linear growth patterns of 20 children with XLH who were prepubertal and had not required osteotomy. A total of 15 patients had family histories consistent with XLH; 5 appeared to be sporadic cases. During 3 years of therapy, the growth velocities of 12 patients had been at or above the mean for age (good growers) and those of 8 patients had been below the mean (poor growers). Data from the two groups were contrasted. We found no difference between the good growers and poor growers before or after the 3 year period of therapy in mean age, dietary calcium, calcitriol dose or compliance, or Pi dose or compliance. Both groups increased their mean fasting serum Pi levels with treatment. The TmP/GFR (mean +/- SEM) of the good growers improved with therapy (1.9 +/- 0.2 to 2.6 +/- 0.2 mg/dl, p = 0.01), and their posttreatment value was higher compared to that of the poor growers (2.6 +/- 0.1 versus 2.2 +/- 0.1 mg/dl, p = 0.02). However, their enhanced TmP/GFR was not associated with a reduction in serum iPTH levels (before, 693 +/- 50; after, 688 +/- 76 pg/ml; p = 0.9). The Z test for binomial proportions showed that the group that grew well contained a disproportionate number of girls (10 of 12, p = 0.04). Our findings suggest that calcitriol may exert a direct effect on the renal tubule to improve Pi reclamation in XLH. The observation that heterozygous girls appear to respond better than hemizygous boys to calcitriol and Pi therapy provides evidence for a gene dosage effect in the expression of this X-linked dominant disorder.

Hypophosphatemic vitamin D-resistant rickets: metabolic balance studies in a child receiving 1,25 dihydroxyvitamin D3, phosphate, and ascorbic acid.

A child with hypophosphatemic vitamin D-resistant rickets was treated for three years with the conventional vitamin D-inorganic phosphate supplementation followed by a new therapeutic regimen consisting of 1,25 dihydroxyvitamin D3 (1,25 (OH)2D3) and half of the previous phosphate supplementation. The effectiveness of the two treatment regimens was compared by calcium, phosphate, and magnesium balance techniques and by serial radiological examinations as well as careful height measurements. In addition, the lowering of the urinary pH with ascorbic acid supplementation seems to be associated with improvement in the renal tubular reabsorption of phosphate, but its distinct effect, separate from the rest of the treatment modalities, was not tested in this study. The conventional treatment did not correct the hypophosphatemia and alkaline phosphatase elevation, whereas the 1,25 (OH)2 D3-inorganic phosphate regimen is well tolerated and effective in achieving a sustained normalization of these variables. In addition, the improved growth and healing of rickets further attest to the efficacy of the new treatment.

Femoral abnormalities and vitamin D metabolism in X-linked hypophosphatemic (Hyp and Gy) mice.

X-linked hypophosphatemia is a genetic bone disease in humans and mice. Two closely linked mutations in mice, Hyp and Gy, cause low plasma phosphate and a rachitic and osteomalacic bone disease. Because of the controversy as to whether Gy is a good model for X-linked hypophosphatemia, the phenotypic severity of these two mutations was compared in both sexes and on two genetic backgrounds. The depression in plasma levels of phosphate was similar in all 10-week-old mutant mice. Male Hyp mice and heterozygous female Hyp mice were affected with similar severity in terms of reduced tail growth, shortened femora, reduced femoral mineral content, and abnormal mineral composition of the femoral matrix. In contrast, male Gy mice did not survive on the C57BL/6J background and were more severely affected than female Gy mice on the B6C3H background. The hybrid B6C3H background ameliorated the bone disease compared with the inbred C57BL/6J background for both mutant strains. There was no evidence of change in the plasma levels of 1,25-dihydroxyvitamin D, duodenal level of vitamin D-dependent calcium-binding protein, or urinary level of calcium in these adult mutant mice. In summary, Gy mice have a sexual dimorphism not present in Hyp mice. These two genes may indicate the presence of multiple gene loci in the human disease, with multiple proteins involved in the pathophysiology of the bone disease.

Sirolimus induced phosphaturia is not caused by inhibition of renal apical sodium phosphate cotransporters.

The vast majority of glomerular filtrated phosphate is reabsorbed in the proximal tubule. Posttransplant phosphaturia is common and aggravated by sirolimus immunosuppression. The cause of sirolimus induced phosphaturia however remains elusive. Male Wistar rats received sirolimus or vehicle for 2 or 7 days (1.5mg/kg). The urine phosphate/creatinine ratio was higher and serum phosphate was lower in sirolimus treated rats, fractional excretion of phosphate was elevated and renal tubular phosphate reabsorption was reduced suggesting a renal cause for hypophosphatemia. PTH was lower in sirolimus treated rats. FGF 23 levels were unchanged at day 2 but lower in sirolimus treated rats after 7 days. Brush border membrane vesicle phosphate uptake was not altered in sirolimus treated groups or by direct incubation with sirolimus. mRNA, protein abundance, and subcellular transporter distribution of NaPi-IIa, Pit-2 and NHE3 were not different between groups but NaPi-IIc mRNA expression was lower at day 7. Transcriptome analyses revealed candidate genes that could be involved in the phosphaturic response. Sirolimus caused a selective renal phosphate leakage, which was not mediated by NaPi-IIa or NaPi-IIc regulation or localization. We hypothesize that another mechanism such as a basolateral phosphate transporter may be responsible for the sirolimus induced phosphaturia.

Renal phosphaturia during metabolic acidosis revisited: molecular mechanisms for decreased renal phosphate reabsorption.

During metabolic acidosis (MA), urinary phosphate excretion increases and contributes to acid removal. Two Na(+)-dependent phosphate transporters, NaPi-IIa (Slc34a1) and NaPi-IIc (Slc34a3), are located in the brush border membrane (BBM) of the proximal tubule and mediate renal phosphate reabsorption. Transcriptome analysis of kidneys from acid-loaded mice revealed a large decrease in NaPi-IIc messenger RNA (mRNA) and a smaller reduction in NaPi-IIa mRNA abundance. To investigate the contribution of transporter regulation to phosphaturia during MA, we examined renal phosphate transporters in normal and Slc34a1-gene ablated (NaPi-IIa KO) mice acid-loaded for 2 and 7 days. In normal mice, urinary phosphate excretion was transiently increased after 2 days of acid loading, whereas no change was found in Slc34a1-/- mice. BBM Na/Pi cotransport activity was progressively and significantly decreased in acid-loaded KO mice, whereas in WT animals, a small increase after 2 days of treatment was seen. Acidosis increased BBM NaPi-IIa abundance in WT mice and NaPi-IIc abundance in WT and KO animals. mRNA abundance of NaPi-IIa and NaPi-IIc decreased during MA. Immunohistochemistry did not indicate any change in the localization of NaPi-IIa and NaPi-IIc along the nephron. Interestingly, mRNA abundance of both Slc20 phosphate transporters, Pit1 and Pit2, was elevated after 7 days of MA in normal and KO mice. These data demonstrate that phosphaturia during acidosis is not caused by reduced protein expression of the major Na/Pi cotransporters NaPi-IIa and NaPi-IIc and suggest a direct inhibitory effect of low pH mainly on NaPi-IIa. Our data also suggest that Pit1 and Pit2 transporters may play a compensatory role.

[Phosphate diabetes and Paget's disease: 2 cases]. / Diabète phosphoré et maladie de Paget: 2 observations.

The authors report 2 cases of Paget's disease associated with phosphate diabetes. The hypothesis of the secretion of a phosphaturic substance by Paget's bone cells is discussed.

Increased urinary excretion of cyclic nucleotides in X-linked hypophosphatemic (Hyp) mice.

Hyp mice, a model for human X-linked hypophosphatemia, had elevated urinary cyclic AMP, cyclic GMP, and magnesium excretion compared to normal mice. The data suggest a renal origin of the urinary cyclic nucleotides. No significant differences in plasma cyclic AMP and cyclic GMP were observed between genotypes.

Hereditary hypophosphatemic rickets with hypercalciuria: report of a new kindred.

We report a new kindred of hereditary hypophosphatemic rickets with hypercalciuria. The symptomatic child and several relatives had increased renal phosphate clearance leading to hypophosphatemia, hyperabsorptive hypercalciuria, low PTH and increased 1,25-(OH)2D serum level. However, association with vitamin D deficiency and normal urinary excretion of cyclic AMP might suggest another tubular defect in phosphate transport.

Guide-lines to the treatment of patients with X-linked hypophosphatemic rickets.

The causes of the development of nephrocalcinosis in familial hypophosphatemic rickets (FHR) are reviewed. The treatment combines vitamin D or 1,25 dihydroxyvitamin D and oral phosphate supplementation. Hypercalcaemia and hypercalciuria were thought to cause the renal calcification. On the basis of the data of eighteen patients with familiar hypophosphatemic rickets we have found that the main difference between the treatment of patients having nephrocalcinosis and those with normal renal morphology consisted in the dose of oral phosphate intake. Patients with nephrocalcinosis received significantly higher doses of oral phosphate (130 mg/kg/day versus 70 mg/kg/day, p < 0.01). Correspondingly, their urinary phosphate excretion was also significantly higher (p < 0.01). There was no difference between the two groups with respect of the doses of vitamin D and urinary calcium excretion. It can be concluded, that high concentrations of urinary phosphate can lead to nephrocalcinosis even if urinary calcium concentration is normal. In order to prevent nephrocalcinosis in patients with X-linked hypophosphatemia, the following guide-lines could be recommended: 1) urinary calcium excretion should be kept lower, than the usually allowed < 4 mg/kg/day; 2) oral phosphate supplementation should not exceed 100 mg/kg/day, 3) patients should be encouraged to drink large amounts of water, 4) regular ultrasound controls should be part of the routine follow-up.