Familial vitamin D-resistant rickets. Study of six cases with evaluation of the pathogenetic role of secondary hyperparathyroidism

Renal Phosphate Handling: Independent Effects of Circulating FGF23, PTH, and Calcium

Excess fibroblast growth factor 23 (FGF23), excess PTH, and an increase in extracellular calcium cause hypophosphatemia by lowering the maximum renal phosphate reabsorption threshold (TmP/GFR). We recently reported two cases of X-linked hypophosphatemia (XLH) with severe tertiary hyperparathyroidism who had normalization of TmP/GFR upon being rendered hypoparathyroid following total parathyroidectomy, despite marked excess in both C-terminal FGF23 (cFGF23) and intact FGF23 (iFGF23). We explored the effects of FGF23, PTH, and calcium on TmP/GFR in a cross-sectional study (n = 74) across a spectrum of clinical cases with abnormalities in TmP/GFR, PTH, and FGF23. This comprised three groups: FGF23-dependent hypophosphatemia (n = 27), hypoparathyroidism (HOPT; n = 17), and chronic kidney disease (n = 30). Measurements included TmP/GFR, cFGF23, PTH, ionized calcium, vitamin D metabolites, and bone turnover markers. The combined effect of cFGF23, PTH, and ionized calcium on TmP/GFR was modeled using hierarchical multiple regression and was probed by moderation analysis with PROCESS. Modeling analysis showed independent effects on TmP/GFR by cFGF23, PTH, and ionized calcium in conjunction with a weak but significant effect of the interaction term for PTH and FGF23; probing showed that the effect was most prominent during PTH deficiency. Teriparatide 20 μg daily was self-administered for 28 days by one case of X-linked hypophosphatemia with hypoparathyroidism (XLH-HOPT) to assess the response of TmP/GFR, cFGF23, iFGF23, nephrogenous cyclic adenosine monophosphate (NcAMP), vitamin D metabolites, and bone turnover markers. After 28 days, TmP/GFR was lowered from 1.10 mmol/L to 0.48 mmol/L; this was accompanied by increases in NcAMP, ionized calcium, and bone turnover markers. In conclusion, the effect of FGF23 excess on TmP/GFR is altered by PTH such that the effect is ameliorated by hypoparathyroidism and the effect is augmented by hyperparathyroidism. © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Downregulation of osteoblast Phex expression by PTH

Human/murine X-linked hypophosphatemia is a dominant disorder associated with renal phosphate wasting and defective bone mineralization. This disorder results from mutations in the PHEX/Phex (Phosphate-regulating gene with homologies to endopeptidases on the X chromosome) gene, which is expressed in fully differentiated osteoblasts. The purpose of the present study was to assess whether PTH, a major regulator of bone development and turnover, modulates osteoblastic Phex expression. The effects of different concentrations of PTH (rat fragment 1-34) were determined on Phex mRNA and protein expression in vitro using MC3T3-E1 osteoblastic cells and mouse primary osteoblasts; and in vivo using 45-day-old mice infused for 3 days with PTH. Phex mRNA levels were quantitated on Northern blots by densitometric analysis relative to GAPDH mRNA levels. Phex protein levels were analyzed by immunoprecipitation of 35S-methionine-labeled osteoblast lysates or by immunoblotting of calvaria membrane extracts using a polyclonal rabbit antiserum raised against a mouse Phex carboxy-terminal peptide. Fully differentiated MC3T3-E1 cells were incubated for 4 to 48 h with increasing concentrations of PTH (10(-11) to 10(-7) M). PTH inhibited Phex mRNA expression in both mineralizing and nonmineralizing osteoblast cultures in a dose- and time-dependent manner with a maximal inhibition at 10(-7) M PTH after 24 h (15+/-7% of control levels, n=5, P<0.001). The PTH-mediated downregulation of Phex mRNA levels was associated with corresponding decreases in Phex protein synthesis and suppression at 10(-7) M PTH. Similar results were obtained with primary osteoblasts isolated from newborn mouse calvaria. Consistent with the in vitro findings, continuous PTH infusion to mice elicited decreases in Phex expression in calvaria. The effect of PTH was also assessed on matrix mineralization by mature MC3T3-E1 cells by measuring 45Ca accumulation in cell layers. PTH (10(-7) M) inhibited the initiation (57+/-2% of control levels, n=5, P<0.001) and the progression of matrix mineralization (75+/-1% of control levels, n=5, P<0.001). In summary, PTH inhibits osteoblastic Phex expression in vitro and in vivo. The downregulation of Phex expression by PTH in vitro is associated with inhibition of matrix mineralization, consistent with a role for Phex in bone mineralization.

An unusual giant cell lesion in a child with vitamin D-resistant rickets

This paper reports the presence of a focus of giant cells in a sinus tract associated with an abscessed primary tooth in a patient with vitamin D-resistant rickets. The relevance of this giant cell lesion to the systemic disorder is discussed.

Acute effects of calcitriol and phosphate salts on mineral metabolism in children with hypophosphatemic rickets

We investigated the acute effects of oral administration of 1,25-dihydroxyvitamin D (1,25-(OH)2D) and phosphate on the major mineral metabolism indexes in six children with vitamin D-resistant rickets treated with a long-term regimen of phosphate and calcitriol. Two acute tests were performed in which plasma calcium, phosphate, immunoreactive parathyroid hormone (iPTH) (intact molecule), 25-hydroxyvitamin D (25-OHD), and 1,25-(OH)2D levels were measured: the first after an oral phosphate load (20 mg/kg) was administered after calcitriol had been discontinued for 10 days, and the second after a calcitriol load (0.03 microgram/kg) plus the same phosphate load but with the children receiving the usual combination treatment. There were no significant differences in basal levels of calcium, phosphate, iPTH, 25-OHD, or 1,25-(OH)2D between the two tests, nor were delta percent calcium and 25-OHD values significantly different. The delta percent plasma phosphate concentration at 60 minutes was significantly higher during test 2 than during test 1 (p less than 0.01) and delta percent iPTH concentration at 60 minutes was significantly higher during test 1 than during test 2 (p less than 0.01). In test 2 the iPTH level returned to baseline at 180 minutes. Higher delta percent 1,25-(OH)2D values at 60 minutes were observed in test 2 than in test 1 (p less than 0.01). Furthermore, the delta percent 1,25-(OH)2D levels were still higher at 180 minutes in test 2 than during test 1 (p less than 0.01). Our study indicates that oral calcitriol has an inhibitory effect on iPTH secretion in the hours immediately after oral phosphate administration in children with vitamin D-resistant rickets.

Tumor-induced osteomalacia: long-term follow-up of two patients cured by removal of their tumors

Two patients with mesenchymal tumors had osteomalacia associated with marked hypophosphatemia and renal phosphate wasting. Their serum calcium and parathyroid hormone levels were normal. The first patient also had aminoaciduria and glucosuria in addition to phosphaturia. Both patients were treated with very large doses of vitamin D2 and phosphate without improvement in the osteomalacia or normalization of the serum phosphate. Complete removal of a low-grade fibrosarcoma in the second patient and removal of an ossifying fibroma in the first patient resulted in a cure in both patients despite no further therapy with vitamin D or phosphate. The excessive aminoaciduria and glucosuria of the first patient also remitted. During a 14-year follow-up period, there has been no evidence of tumor recurrence, hypophosphatemia, or osteomalacia in either patient. The levels of 1,25-dihydroxycholecalciferol remain within the normal range in both patients.

Adult hypophosphatemic osteomalacia: report of two cases

Two cases of late hypophosphatemic osteomalacia are described: a male aged 30 who had the disease since he was 22 and a woman of 23 who had the disease since she was 14. Both presented with myopathy and bone pain, and showed hypophosphatemia, hyperglycinuria, reduced tubular phosphate reabsorption (TPR), increased hydroxyprolinuria and normal iPTH and iCT values. Radiologically the male had no Looser's zones and the woman did. Bone biopsy confirmed hypophosphatemic osteomalacia. Both cases were treated with vitamin D and oral phosphate and no improvement was observed. When treatment with 25(OH)D3 was initiated, no improvement was seen and afterwards this was combined with treatment using 1.25(OH)2D3 and from this time on a clinical improvement of the myopathy became evident in both patients. In the woman, healing of the bone lesions occurred at the same time as that of the myopathy, whereas in the male the bone lesions became worse. Healing of the myopathy was only obtained when treatment with 1.25(OH)2D3 was begun. Both patients had reduced values of 2.3 erythrocytic DPG and low level of serum phosphorus when the myopathy was cured, which suggests a lack of effect of 2.3 DPG or serum phosphorus as a cause of the myopathy. Although this had been attributed to a deficiency in the function of 25(OH)D3, the response to 1.25(OH)2D3 and due to the effects of this metabolite on calcium transport in muscle, suggests that the myopathy which occurs in late hypophosphatemic osteomalacia is a result of deficiency or resistance to the muscular effect of this metabolite. We cannot explain the lack of bone healing in the man and further therapeutic studies are required.

Hypohyperparathyroidism: a model for renal osteodystrophy?

A child who presented with features of renal osteodystrophy but with normal renal function is described. Improvement occurred both on large doses of vitamin D and small doses of 1, alpha-hydroxy-vitamin D3 (1, alpha-OHD3). Investigations suggested that the primary defect was an impaired renal response to parathyroid hormone. The relationship between renal osteodystrophy, abnormalities of vitamin D metabolism and hypohyperparathyroidism is discussed and an alternative hypothesis for the development of renal bone disease suggested.

Familial hypophosphatemic rickets: defective transport of inorganic phosphate by intestinal mucosa

Uptake of inorganic phosphate is impared in intestinal mucosa from hemizygous males and heterozygous females with X-linked familial hypophosphatemic rickets. Considerable intrafamilial and interfamilial variation in uptake of inorganic phosphate is observed in affected patients. Uptake by normal mucosa is concentrative and energy-dependent, and is mediated by at least two systems with widely different affinities. These results lend direct support to the thesis that the primary metabolic disturbance in this disease results from impaired transport of inorganic phosphate in kidney and gut.

Loss of a parathyroid hormone-sensitive component of phosphate transport in X-linked hypophosphatemia

Mutant hemizygotes with X-linked hypophosphatemia lack a parathyroid hormone-sensitive component of inorganic phosphate transport in kidney; female heterozygotes retain a variable proportion of this type of transport. The residual mechanism for reabsorption in affected males allows inorganic phosphate efflux from the kidney to urine so that net "secretion" is sometimes observed; the latter is directly proportional to the serum concentration of inorganic phosphate. Calcium acts on the kidney tubule to enhance net reabsorption by this component of inorganic phosphate transport.

Serum parathyroid hormone in X-linked hypophosphatemia

Serum immunoreactive parathyroid hormone(IPTH) is normal in patients with X-linked hypophosphatemic rickets who are not treated with phosphate salts. Phosphate raises IPTH in these patients. Endogenous IPTH does not influence the existing defect in tubular reabsorption of phosphate in male patients.