Intramyocellular phosphate metabolism in X-linked hypophosphatemic rickets

Intracellular Phosphate and ATP Depletion Measured by Magnetic Resonance Spectroscopy in Patients Receiving Maintenance Hemodialysis

BACKGROUND

The precise origin of phosphate that is removed during hemodialysis remains unclear; only a minority comes from the extracellular space. One possibility is that the remaining phosphate originates from the intracellular compartment, but there have been no available data from direct assessment of intracellular phosphate in patients undergoing hemodialysis.

METHODS

We used phosphorus magnetic resonance spectroscopy to quantify intracellular inorganic phosphate (Pi), phosphocreatine (PCr), and βATP. In our pilot, single-center, prospective study, 11 patients with ESKD underwent phosphorus (³¹P) magnetic resonance spectroscopy examination during a 4-hour hemodialysis treatment. Spectra were acquired every 152 seconds during the hemodialysis session. The primary outcome was a change in the PCr-Pi ratio during the session.

RESULTS

During the first hour of hemodialysis, mean phosphatemia decreased significantly (-41%; P<0.001); thereafter, it decreased more slowly until the end of the session. We found a significant increase in the PCr-Pi ratio (+23%; P=0.001) during dialysis, indicating a reduction in intracellular Pi concentration. The PCr-βATP ratio increased significantly (+31%; P=0.001) over a similar time period, indicating a reduction in βATP. The change of the PCr-βATP ratio was significantly correlated to the change of depurated Pi.

CONCLUSIONS

Phosphorus magnetic resonance spectroscopy examination of patients with ESKD during hemodialysis treatment confirmed that depurated Pi originates from the intracellular compartment. This finding raises the possibility that excessive dialytic depuration of phosphate might adversely affect the intracellular availability of high-energy phosphates and ultimately, cellular metabolism. Further studies are needed to investigate the relationship between objective and subjective effects of hemodialysis and decreases of intracellular Pi and βATP content.

CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER

Intracellular Phosphate Concentration Evolution During Hemodialysis by MR Spectroscopy (CIPHEMO), NCT03119818.

Hereditary hypophosphatemic rickets with hypercalciuria: pathophysiology, clinical presentation, diagnosis and therapy

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH; OMIM: 241530) is a rare autosomal recessive disorder with an estimated prevalence of 1:250,000 that was originally described by Tieder et al. Individuals with HHRH carry compound-heterozygous or homozygous (comp/hom) loss-of-function mutations in the sodium-phosphate co-transporter NPT2c. These mutations result in the development of urinary phosphate (Pi) wasting and hypophosphatemic rickets, bowing, and short stature, as well as appropriately elevated 1,25(OH)₂D levels, which sets this fibroblast growth factor 23 (FGF23)-independent disorder apart from the more common X-linked hypophosphatemia. The elevated 1,25(OH)₂D levels in turn result in hypercalciuria due to enhanced intestinal calcium absorption and reduced parathyroid hormone (PTH)-dependent calcium-reabsorption in the distal renal tubules, leading to the development of kidney stones and/or nephrocalcinosis in approximately half of the individuals with HHRH. Even heterozygous NPT2c mutations are frequently associated with isolated hypercalciuria (IH), which increases the risk of kidney stones or nephrocalcinosis threefold in affected individuals compared with the general population. Bone disease is generally absent in individuals with IH, in contrast to those with HHRH. Treatment of HHRH and IH consists of monotherapy with oral Pi supplements, while active vitamin D analogs are contraindicated, mainly because the endogenous 1,25(OH)₂D levels are already elevated but also to prevent further worsening of the hypercalciuria. Long-term studies to determine whether oral Pi supplementation alone is sufficient to prevent renal calcifications and bone loss, however, are lacking. It is also unknown how therapy should be monitored, whether secondary hyperparathyroidism can develop, and whether Pi requirements decrease with age, as observed in some FGF23-dependent hypophosphatemic disorders, or whether this can lead to osteoporosis.

New perspectives on the biology and treatment of X-linked hypophosphatemic rickets

This article updates the practicing pediatrician's knowledge of the hypophophatemic disorders that may occur in children. The classic X-linked disorder is emphasized. Details of clinical manifestations, the wide spectrum of disease severity, and complications of the disorder in adults are reviewed. Recent research, new genetic findings, and speculations regarding pathophysiology are discussed. A strategy for approaching medical treatment of X-linked hypophosphatemic rickets is provided, together with complications of treatment and treatment after cessation of growth.

Therapeutics of X-linked hypophosphatemic rickets

X-linked hypophosphatemia, the most common form of familial rickets, is conventionally treated with 1,25-dihydroxyvitamin D3 (5-50 ng/kg per day) plus phosphate supplementation (70-100 mg/kg per day). However, nephrocalcinosis is noted in many children treated with this therapy. Whether to treat or not and whether such treatment should be continued into adulthood or in pregnancy are unsettled questions. This article reviews these controversies and provides current recommendations.

1,25-Dihydroxyvitamin D3 does not up-regulate vitamin D receptor messenger ribonucleic acid levels in hypophosphatemic mice

The effect of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) administration on duodenal vitamin D receptor (VDR) mRNA levels in hypophosphatemic (Hyp) mice, a murine homologue of human X-linked hypophosphatemic rickets, was examined. Basal levels of VDR mRNA in Hyp mice were similar to those of normal littermates and, in normal mice, VDR mRNA levels were up-regulated 1.8-2.7-fold after injection of 1 microgram/kg 1,25(OH)2D3. In contrast, no significant change in VDR mRNA was observed in Hyp mice treated with 1,25(OH)2D3. To determine the effect of phosphate repletion on VDR mRNA levels, high-phosphate diet was fed to Hyp mice. Although plasma phosphorus concentration was restored to normal, up-regulation of VDR mRNA was not recovered with phosphate supplementation. These results indicate that the vitamin D-resistance in Hyp mice is not caused by hypophosphatemia, per se, and may result from a fundamental molecular defect in vitamin D action at the intestine which could be related to ineffective up-regulation of VDR mRNA by 1,25(OH)2D3.