Tubular phosphate transport: a comparison between different methods of urine sample collection in FGF23-dependent hypophosphatemic syndromes

OBJECTIVES

Tubular maximum phosphate reabsorption per glomerular filtration rate (TmP/GFR) is used to evaluate renal phosphate reabsorption and it is a useful tool for the differential diagnosis of hypophosphatemic syndromes. TmP/GFR is typically calculated from fasting plasma and second morning void urine samples, obtained 2 h after the first void (TmP/GFR 2 h). The purpose of this study was to evaluate if TmP/GFR calculated from 24 h urine collection (TmP/GFR 24 h) can be used as an alternative for TmP/GFR 2 h in patients with urine phosphate wasting.

METHODS

We enrolled adult patients with X-linked hypophosphatemia (XLH) or tumor-induced osteomalacia (TIO). All patients underwent blood and urine sample collections, to calculate TmP/GFR 24 h and TmP/GFR 2 h.

RESULTS

Twenty patients (17 XLH and 3 TIO), aged 24-78 years, were included. All patients had low TmP/GFR 2 h (0.35 mmol/L, IQR 0.24-0.47 mmol/L) and TmP/GFR 24 h (0.31 mmol/L, IQR 0.22-0.43 mmol/L). The concordance correlation coefficient between TmP/GFR 2 h and TmP/GFR 24 h was 0.86 (95 % CI: 0.69-0.93), with a systematic bias of 0.05 mmol/L (95 % limits of agreement: -0.10 to 0.20). Furthermore, in 70 % (i.e., 14 patients out of 20) and 80 % (i.e., 16 patients out of 20) of cases the difference between TmP/GFR 2 h and TmP/GFR 24 h was within ±30 % and ±35 %, respectively.

CONCLUSIONS

Despite TmP/GFR 2 and 24 h show a relatively suboptimal agreement, the difference between the two parameters appears to be small and not clinically significant in the setting of adult patients with FGF23-dependent urine phosphate wasting and secondary hypophosphatemia.

Unexpected widespread hypophosphatemia and bone disease associated with elemental formula use in infants and children

OBJECTIVE

Hypophosphatemia occurs with inadequate dietary intake, malabsorption, increased renal excretion, or shifts between intracellular and extracellular compartments. We noticed the common finding of amino-acid based elemental formula [EF] use in an unexpected number of cases of idiopathic hypophosphatemia occurring in infants and children evaluated for skeletal disease. We aimed to fully characterize the clinical profiles in these cases.

METHODS

A retrospective chart review of children with unexplained hypophosphatemia was performed as cases accumulated from various centres in North America and Ireland. Data were analyzed to explore any relationships between feeding and biochemical or clinical features, effects of treatment, and to identify a potential mechanism.

RESULTS

Fifty-one children were identified at 17 institutions with EF-associated hypophosphatemia. Most children had complex illnesses and had been solely fed Neocate® formula products for variable periods of time prior to presentation. Feeding methods varied. Hypophosphatemia was detected during evaluation of fractures or rickets. Increased alkaline phosphatase activity and appropriate renal conservation of phosphate were documented in nearly all cases. Skeletal radiographs demonstrated fractures, undermineralization, or rickets in 94% of the cases. Although the skeletal disease had often been attributed to underlying disease, most all improved with addition of supplemental phosphate or change to a different formula product.

CONCLUSION

The observed biochemical profiles indicated a deficient dietary supply or severe malabsorption of phosphate, despite adequate formula composition. When transition to an alternate formula was possible, biochemical status improved shortly after introduction to the alternate formula, with eventual improvement of skeletal abnormalities. These observations strongly implicate that bioavailability of formula phosphorus may be impaired in certain clinical settings. The widespread nature of the findings lead us to strongly recommend careful monitoring of mineral metabolism in children fed EF. Transition to alternative formula use or implementation of phosphate supplementation should be performed cautiously with as severe hypocalcemia may develop.

Fibroblast growth factor 23 is increased in calcium nephrolithiasis with hypophosphatemia and renal phosphate leak

CONTEXT

Nephrolithiasis affects about 10% of the population in industrialized countries, with calcium salts composing more than 80% of renal stones. A significant percentage of patients with calcium nephrolithiasis and normal parathyroid function show hypophosphatemia and reduced renal phosphate reabsorption (i.e. a renal phosphate leak).

OBJECTIVES

The objective of the study was to compare serum levels of fibroblast growth factor 23 (FGF23), a regulator of phosphate homeostasis, in 110 recurrent stone formers with or without renal phosphate leak, six patients affected by X-linked hypophosphatemic rickets, five patients affected by oncogenic osteomalacia, and 60 unrelated healthy controls.

DESIGN

This was a prospective interventional study.

METHODS

Renal phosphate leak was identified based on the occurrence of idiopathic hypophosphatemia [serum phosphate concentration < 2.50 mg/dl (<0.80 mmol/liter)] and reduced renal threshold phosphate concentration [<2.2 mg/liter (<0.70 mmol/liter)].

RESULTS

In 22 stone formers with renal phosphate leak, serum FGF23 concentration was significantly higher as compared with 88 stone formers without renal phosphate leak and with controls [83.3 (65.6-101.1) vs. 32.1 (26.8-37.4) and 24.5 (19.8-29.1) reference units (RU)/ml, respectively]. Stone formers with renal phosphate leak showed lower FGF23, compared with patients with oncogenic osteomalacia and X-linked hypophosphatemic rickets [572.3 (235.9-908.7) RU/ml]. Among stone formers and controls, serum FGF23 concentration displayed a strong inverse association with serum phosphate (r = -0.784, P = 0.009) and the rate of tubular phosphate reabsorption (r = -0.791, P = 0.008).

CONCLUSIONS

In our study population, renal phosphate leak affected 20% of stone formers and was strongly associated with increased serum FGF23 concentration.

Hypophosphaturic mesenchymal tumor of the ethmoid associated with oncogenic osteomalacia

Oncogenic osteomalacia is an uncommon syndrome characterized by bone pain, proximal muscle weakness, hypophosphatemia, hyperphosphaturia, and a low plasma concentration of 1,25-dihydroxy-vitamin D. The disease affects both sexes at around 40 years of age, although it can sometimes affect children and adolescents. Generally, the syndrome is associated with a tumor, usually benign, of mesenchymal origin and is resolved after removal of the tumor; this syndrome can sometimes be associated with malignant tumors. These tumors seem to be histologically heterogeneous and are generally localized in soft tissues and bone. In this article, a case of oncogenic osteomalacia associated with a hypophosphaturic mesenchymal tumor of the ethmoid is reported in a 24-year-old man. After surgical and radical removal of the tumor, the patient noted a decrease in the clinical symptoms and signs.

Role of plasma and urinary calcium and phosphorus measurements in early detection of phosphorus deficiency in very low birthweight infants

AIM

To analyse the role of serum and urinary calcium and phosphorus levels in early detection of mineral deficiency in very low birthweight (VLBW) infants born appropriate (AGA) and small for gestational age (SGA).

METHODS

64 VLBW infants were included in a cohort study and divided into two groups: AGA (n = 30) and SGA infants (n = 34). Then, they were divided according to the presence of radiological signs of metabolic bone disease (MBD): with MBD (n = 21) and without MBD (n = 34). Blood samples and 6 h urine collections were obtained for calcium, phosphorus, alkaline phosphatase activity and creatinine determinations between 3 and 5 wk of life.

RESULTS

There were no biochemical differences between AGA and SGA. Higher values of urinary calcium (MBD = 31.9 +/- 20.2, without MBD = 19.8 +/- 15.4; p = 0.017), calciuria (MBD = 2.3 +/- 0.3, without MBD = 1.4 +/- 0.8; p = 0.037) and alkaline phosphatase activity (MBD = 369 +/- 114, without MBD = 310 +/- 93; p = 0.04) were found in infants who developed MBD. Both groups showed high tubular phosphorus reabsorption indicating mineral deficiency.

CONCLUSION

Serum calcium and phosphorus levels are not good markers in early detection of mineral deficiency. However, the monitoring of calcium urinary levels may be helpful in early detection of mineral deficiency.

Evidence for a PTH-independent humoral mechanism in post-transplant hypophosphatemia and phosphaturia

BACKGROUND

Patients undergoing successful kidney transplantation often manifest overt hypophosphatemia associated with exaggerated phosphaturia during the early post-transplant period (2 weeks to 3 months). The mechanism for this phenomenon has not been fully elucidated. We tested the hypothesis that a circulating serum factor [non-parathyroid hormone (non-PTH)], which operates during chronic renal failure (CRF) to maintain phosphate (Pi) homeostasis, can increase fractional excretion of Pi (FE(PO4)) in normal functioning kidney grafts during the early post-transplant period, thereby causing phosphaturia and hypophosphatemia.

METHODS

Five groups of patients were studied: control subjects (group 1, N = 16), "early" (2 weeks to 1 month) post-transplant patients (group 2, N = 22), "late" (9 to 12 months) post-transplant patients (group 3, N = 14), patients with advanced CRF (glomerular filtration rate = 30 to 40 mL/min; group 4, N = 8), and patients who suffered from end-stage renal failure and were treated by chronic hemodialysis (group 5, N = 14). Group 2 manifested significant hypophosphatemia and phosphaturia when compared with groups 1 and 3 (Pi = 0.9 +/- 0.003 mg/dL, FE(PO4) = 68+/- 5%, P < 0.0005 vs. groups 1 and 3). Sera were taken from each of the five subject groups and applied to the proximal tubular opossum kidney (OK) cells. The activity of Na/Pi-type 4 (that is, OK-specific type II transporter) was evaluated by measuring Na(+)-dependent (32)Pi flux. The expression of Na/Pi type II mRNA and the abundance of Na/Pi protein were determined by Northern and Western blot assays, respectively.

RESULTS

When compared with sera from groups 1 and 3, 10% sera taken from groups 2, 4, and 5 (incubated overnight with OK cells) inhibited (32)Pi flux by 25 to 30% (P < 0.0003). Both Na/Pi mRNA and the expression of Na/Pi protein were markedly augmented under the same conditions (P < 0.05 groups 2, 4, and 5 vs. groups 1 and 3). Time-course analysis revealed that the up-regulation of Na/Pi protein by sera from groups 2, 4, and 5 was observed as early as four hours of incubation, whereas augmented abundance of Na/Pi mRNA was only seen after eight hours of incubation. The addition of PTH (1-34) to sera from groups 2, 4, and 5 abolished the augmented expression of NaPi protein. We labeled OK cell surface membrane proteins with N-hydroxysuccinimide bound to biotin (NHS-SS-biotin). Biotinylated transporters incubated with the different sera were precipitated by strepavidin and identified by Western blot analysis. Cells incubated in sera from group 2 showed increased membrane bound transporter when compared with control sera, whereas the intracellular pool of the transporter was comparable between the two groups.

CONCLUSION

A non-PTH circulating serum factor (possibly phosphatonin) that increases FE(PO4) during CRF is also responsible for phosphaturia and hypophosphatemia in the early period following successful kidney transplantation. The putative factor inactivates Na/Pi activity along with inhibition of the transporter trafficking from the cell membrane into the cytosol.

Dipyridamole decreases renal phosphate leak and augments serum phosphorus in patients with low renal phosphate threshold

It has been shown that an acute infusion of dipyridamole increased renal phosphate reabsorption in rats and humans. A prospective study was performed to determine whether chronic treatment by dipyridamole given orally could decrease renal phosphate leak and increase serum phosphorus in patients with idiopathic low renal phosphate threshold (TmPO4/GFR < 0.77 mM). Sixty-four patients with low TmPO4/GFR were included and treated with dipyridamole (75 mg, 4 times daily) for more than 12 mo. Serum phosphorus, TmPO4/GFR, parathyroid hormone, serum calcium, and 1,25-dihydroxyvitamin D were measured sequentially before treatment, and after 3, 6 to 9, and 12 mo of treatment. Under chronic treatment with dipyridamole, TmPO4/GFR and serum phosphorus significantly increased in 80% of patients within 3 mo, with maximal values reached within 9 mo. This improvement persisted after 12 mo of treatment. In 28 patients, 1,25-dihydroxyvitamin D concentrations were above the normal range (> 42 pg/ml) and normalized in parallel with the increase of serum phosphorus. The 24-h calcium excretion (which was initially increased in patients with high vitamin D concentrations) and urolithiasis decreased under treatment. Ionized serum calcium and parathyroid hormone remained unchanged. After 2 yr, treatment was discontinued in three patients; serum phosphorus and TmPO4/GFR decreased within 1 mo after discontinuation. Dipyridamole at a dose of 75 mg 4 times daily increases low TmPO4/GFR and improves hypophosphatemia in patients with renal phosphate losses and can be used to treat these patients.

Profound hypophosphatemia and isolated hyperphosphaturia in two cases of multiple myeloma

We describe the development of severe hypophosphatemia and urinary phosphate wasting in two patients with multiple myeloma. In both cases, the serum phosphorus was repeatedly less than 1.0 mg/dL despite vigorous replacement, and the calculated fractional excretion of urinary phosphorus was greater than 100%. Neither patient demonstrated other tubular defects typical of Fanconi's syndrome. With treatment of the myeloma, both patients achieved normalization of the serum phosphorus and no longer required phosphorus supplementation. We believe that multiple myeloma should be considered in the differential diagnosis in patients with profound hypophosphatemia, urinary phosphate wasting, and otherwise intact tubular function.