Serum parathyroid hormone in X-linked hypophosphatemia

Hyperparathyroidism in Patients With X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is characterized by increased activity of circulating FGF23 resulting in renal phosphate wasting and abnormal bone mineralization. Hyperparathyroidism may develop in XLH patients; however, its prevalence, pathogenesis, and clinical presentation are not documented. This observational study (CNIL 171036 v 0) recruited XLH adult patients in a single tertiary referral center. Each patient was explored in standardized conditions and compared with two healthy volunteers, matched for sex, age, and 25-OH vitamin D concentrations. The primary endpoint was the proportion of patients with hyperparathyroidism. The secondary endpoints were the factors influencing serum parathyroid hormone (PTH) concentrations and the prevalence of hypercalcemic hyperparathyroidism. Sixty-eight patients (51 women, 17 men) were enrolled and matched with 136 healthy volunteers. Patients had higher PTH concentrations compared with healthy controls (53.5 ng/L, interquartile range [IQR] 36.7-72.7 versus 36.0 ng/L, IQR 27.7-44.0, p < .0001). Hyperparathyroidism was observed in 17 patients of 68 (25%). In patients, a positive relationship between PTH and calcium concentrations and a negative relationship between PTH and phosphate concentrations were observed. Seven (10%) patients (3 premenopausal women, 1 postmenopausal woman, and 3 men) were diagnosed with hypercalcemic hyperparathyroidism. All underwent parathyroid surgery, with consecutive normalization of calcium and PTH concentrations. Hyperparathyroidism is a frequent complication in XLH adult patients. Disruption of the physiological regulation of PTH secretion contributes to parathyroid disease. Early-onset hypercalcemic hyperparathyroidism can be effectively and safely cured by surgical resection. © 2020 American Society for Bone and Mineral Research.

Complications of Phosphate and Vitamin D Treatment in X-Linked Hypophosphataemia

Conventional treatment of X-linked hypophosphataemia (XLH) consists in the oral administration of phosphate plus calcitriol supplements. Although this therapy has reduced bone deformities and even achieved adequate patient growth, overtreatment or low adherence could lead to subsequent consequences that may compromise the efficacy of the therapy. Some of the complications associated with phosphate and vitamin D treatment are abdominal discomfort, diarrhoea, hypokalaemia, hyperparathyroidism, hypercalcaemia or hypercalciuria, nephrocalcinosis or nephrolithiasis, and ectopic calcifications. Therefore, constant multidisciplinary monitoring of patients with XLH is necessary to prevent the manifestation of these complications and to deal with them as soon as they appear. The main objective of this article is to review the main complications arising from conventional treatment of XLH and how to deal with them.

The phosphate regulating hormone fibroblast growth factor-23

Over the last decade, the regulation of phosphate (Pi) homeostasis has been under intense investigation. By utilizing modern biochemical and genetic tools, the pathophysiological mechanisms behind several known hereditary and acquired hypo- and hyperphosphatemic diseases have been clarified. The results of these efforts have opened new insights into the causes of Pi dysregulation and hereby also the physiological mechanisms determining Pi homeostasis. Although several potential Pi-regulating proteins have been discovered and investigated, current data strongly argues for fibroblast growth factor-23 (FGF23), a hormonal factor produced in bone, as a particularly important regulator of Pi homeostasis. In this article, we review the discovery of the FGF23 protein, as well as its biochemistry, localization of production, receptor specificity and mechanisms of action.

The journey from vitamin D-resistant rickets to the regulation of renal phosphate transport

In 1937, Fuller Albright first described two rare genetic disorders: Vitamin D resistant rickets and polyostotic fibrous dysplasia, now respectively known as X-linked hypophosphatemic rickets (XLH) and the McCune-Albright syndrome. Albright carefully characterized and meticulously analyzed one patient, W.M., with vitamin D-resistant rickets. Albright subsequently reported additional carefully performed balance studies on W.M. In this review, which evaluates the journey from the initial description of vitamin D-resistant rickets (XLH) to the regulation of renal phosphate transport, we (1) trace the timeline of important discoveries in unraveling the pathophysiology of XLH, (2) cite the recognized abnormalities in mineral metabolism in XLH, (3) evaluate factors that may affect parathyroid hormone values in XLH, (4) assess the potential interactions between the phosphate-regulating gene with homology to endopeptidase on the X chromosome and fibroblast growth factor 23 (FGF23) and their resultant effects on renal phosphate transport and vitamin D metabolism, (5) analyze the complex interplay between FGF23 and the factors that regulate FGF23, and (6) discuss the genetic and acquired disorders of hypophosphatemia and hyperphosphatemia in which FGF23 plays a role. Although Albright could not measure parathyroid hormone, he concluded on the basis of his studies that showed calcemic resistance to parathyroid extract in W.M. that hyperparathyroidism was present. Using a conceptual approach, we suggest that a defect in the skeletal response to parathyroid hormone contributes to hyperparathyroidism in XLH. Finally, at the end of the review, abnormalities in renal phosphate transport that are sometimes found in patients with polyostotic fibrous dysplasia are discussed.

Tertiary hyperparathyroidism attributable to long-term oral phosphate therapy

OBJECTIVE

To report a rare case of tertiary hyperparathyroidism (HPT) as a result of long-term oral phosphate therapy.

METHODS

We present a case report, with a focus on clinical manifestations and biochemical findings during the course of tertiary HPT, and discuss the pathophysiologic features of this disorder and the therapeutic strategies.

RESULTS

A 35-year-old woman, 22 years after the initial diagnosis of familial hypophosphatemic rickets and initiation of treatment with phosphate and vitamin D, underwent assessment for recurrent symptomatic kidney stones, bone pain, and fatigue. Laboratory studies performed 10 months before this presentation showed findings consistent with secondary HPT. Examination was notable for short stature, and pertinent laboratory results were as follows: intact parathyroid hormone 602 pg/mL, calcium 10.9 mg/dL, and phosphorus 3.6 mg/dL. Tertiary HPT was diagnosed, and she underwent subtotal parathyroidectomy and transcervical thymectomy. Postoperatively, she had hypocalcemia and was treated with calcitriol, phosphate, and calcium carbonate; the last agent was discontinued when the serum calcium normalized. Despite multiple dosage alterations in the phosphate and calcitriol therapy, the patient had recurrent tertiary HPT and another kidney stone (treated by lithotripsy). Three years after the subtotal parathyroidectomy, treatment consisted of cinacalcet, calcitriol, and elemental phosphate.

CONCLUSION

Long-term follow-up of patients with tertiary HPT is critical, with careful dosage adjustments in phosphate and vitamin D therapy and monitoring of serum levels of phosphorus, calcium, and parathyroid hormone.

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.

FGF-23 in patients with end-stage renal disease on hemodialysis

BACKGROUND

Fibroblast growth factor (FGF)-23 is a recently identified circulating factor which causes renal phosphate wasting disorders. Although the mechanism of regulation of FGF-23 secretion is unknown, plasma FGF-23 level may be regulated or affected by serum phosphate levels because of its hypophosphatemic effect.

METHODS

We tested the hypothesis that plasma FGF-23 levels may be increased in hyperphosphatemia in patients with end-stage renal disease (ESRD) on maintenance hemodialysis. We measured plasma FGF-23 levels in 158 male uremic patients on maintenance hemodialysis. Plasma samples were obtained before starting dialysis sessions to determine FGF-23 levels by enzyme-linked immunosorbent assay (ELISA).

RESULTS

Plasma FGF-23 level exhibited significant and positive correlations with inorganic phosphate, intact parathyroid hormone (PTH), corrected calcium, and duration of hemodialysis on simple regression analyses. All these associations remained significant in multiple regression analyses.

CONCLUSION

Serum phosphate, calcium, and intact PTH could be regulators of FGF-23 levels in uremic patients on maintenance hemodialysis. Our results may provide new insights into the pathophysiologic effects of FGF-23 on calcium-phosphate homeostasis.

Prolonged high-dose phosphate treatment: a risk factor for tertiary hyperparathyroidism in X-linked hypophosphatemic rickets

OBJECTIVE

X-linked hypophosphatemic rickets is characterized by renal phosphate wasting, hypophosphatemia and defective bone mineralization. Treatment with oral phosphate (Pi) and calcitriol improves skeletal changes but associates with secondary hyperparathyroidism and nephrocalcinosis. Tertiary hyperparathyroidism is a rare complication of the treatment. The aim of the present study was to identify treatment-related factors that might be associated with the transition of secondary hyperparathyroidism to tertiary hyperparathyroidism in patients with X-linked hypophosphatemic rickets.

DESIGN

Thirteen patients with X-linked hypophosphatemic rickets and secondary or tertiary hyperparathyroidism were included in the study. Their hospital records were reviewed and compared for onset, duration and dosage of treatment, and for age of diagnosis and degree of secondary hyperparathyroidism.

RESULTS

Two patients developed tertiary hyperparathyroidism and 11 patients secondary hyperparathyroidism during the treatment. Patients with tertiary hyperparathyroidism had, on average, earlier onset and longer duration of treatment, higher dose of Pi and longer duration of treatment with very high Pi doses (> 100 mg/kg/day) compared to the 11 patients with secondary hyperparathyroidism. However, variation of all parameters was great with considerable overlap. Very high S-PTH levels > or = 42 pmol/l were observed in those who later developed tertiary hyperparathyroidism.

CONCLUSIONS

Prolonged very high dose oral Pi treatment is a major risk factor for the development of tertiary hyperparathyroidism in X-linked hypophosphatemic rickets.

Positional cloning of the PEX gene: new insights into the pathophysiology of X-linked hypophosphatemic rickets

X-linked hypophosphatemic rickets (HYP) is the most common form of hereditary renal phosphate wasting. The hallmarks of this disease are isolated renal phosphate wasting with inappropriately normal calcitriol concentrations and a mineralization defect in bone. Studies in the Hyp mouse, one of the murine models of the human disease, suggest that there is an approximately 50% decrease in both message and protein of NPT-2, the predominant sodium-phosphate cotransporter in the proximal tubule. However, human NPT-2 maps to chromosome 5q35, indicating that it is not the disease gene. Positional cloning studies have led to the identification of a gene, PEX, which is responsible for the disorder. Further studies have led to identification of the murine Pex gene, which is mutated in the murine models of the disorder. These studies, in concert with other studies, have led to improved understanding of the pathophysiology of HYP and a new appreciation for the complexity of normal phosphate homeostasis.

Renal Na(+)-phosphate cotransporter gene expression in X-linked Hyp and Gy mice

The X-linked Hyp and Gy mutations are murine homologues of X-linked hypophosphatemia (XLH), a dominant disorder of phosphate (Pi) homeostasis characterized by growth retardation, rickets, hypophosphatemia and decreased renal tubular maximum for Pi reabsorption relative to glomerular filtration rate (Tmp/GFR). In Hyp and Gy mice, the decrease in Tmp/GFR is associated with a reduction in renal brush-border membrane (BBM) Na(+)-Pi cotransport that can be ascribed to a decrease in renal-specific, Na(+)-Pi cotransporter (NPT2) mRNA and protein abundance. Although renal NPT2 gene expression is reduced in Hyp and Gy mice, the NPT2 gene does not map to the X chromosome. These findings exclude NPT2 as a candidate gene for murine and human X-linked hypophosphatemias and suggest that genes at the Hyp, Gy and XLH (HYP) loci are involved in regulation of NPT2 gene expression. Both Hyp and Gy mice respond to low Pi diet with an increase in BBM Na(+)-Pi cotransport, NPT2 mRNA and protein. The increase in NPT2 protein in Pi-depleted mice far exceeds the increase in NPT2 mRNA, suggesting that translational or post-translational mechanisms are involved in the adaptive process. NPT2 protein is localized to the apical surface of the proximal tubule, where immunostaining in both normal and Hyp mice is increased in response to low Pi diet. Pi-deprived Hyp and Gy mice fail to show an increase in Tmp/GFR, indicating that adaptation at the BBM is not sufficient for the overall increase in Tmp/GFR in response to low Pi diet.

Parental origin of mutant allele does not explain absence of gene dose in X-linked Hyp mice

The expectation for a gene dose effect in an X-linked phenotype is that the corresponding metrical trait in heterozygous females will lie between values for affected hemizygous males and unaffected males and females. We made sequential measurements (at 30, 60, 90, 120 and 150 days) of serum phosphate concentration and tail length in mice with X-linked hypophosphatemia (genotypes: Hyp/Y, Hyp/+ and Hyp/Hyp) and in their normal litter-mates (genotypes: +/Y, +/+). We also measured renal mitochondrial 25-hydroxyvitamin D3-24-hydroxylase (24-hydroxylase) activity in 5 to 7-month-old mice fed control and low phosphate diets and representing all five genotypes. The animals were obtained by controlled breeding under uniform environmental conditions. The mutant animals all had uniformly and significantly lower serum phosphate levels, shorter tail length and higher 24-hydroxylase activity relative to unaffected litter-mates. There was no evidence of a gene dose effect because values were not significantly different among the three mutant genotypes. We also studied the influence of gamete of origin on serum phosphate, tail length and renal mitochondrial 24-hydroxylase activity in the Hyp/+ offspring of affected males (Hyp/Y) or affected females (Hyp/+ or Hyp/Hyp). We found no effect on the distribution of trait values. We conclude that parental origin of mutant allele does not explain the absence of a gene dose effect in Hyp mice.

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.

X-linked hypophosphatemia. A phenotype in search of a cause

XLH is an important disease, it is the subject of several classic articles in the medical sciences (Scriver et al., 1991), and it has been an important stimulus to study renal hypophosphatemias and how they are involved in rickets and osteomalacia (Scriver, 1974; Scriver and Tenenhouse, 1991). Renal transport is the major determinant of phosphate homeostasis in mammals and it is unlikely that this important biochemical parameter would have been left by evolution to a single renal transport system. Together physiologists and geneticists found that the mammalian kidney has several gene products dedicated to phosphate transport. That has implications for biochemists in search of a membrane protein to clone and explain XLH, for example. Let us suppose the transporter affected in XLH is cloned. Will it be the product of the XLH (or Hyp or Gy) locus? One will not know until the transporter gene is mapped. There is no question of the X-chromosome locus product being protein kinase C for example, since it maps to autosomes. But where does one start in the search for the X-chromosome locus? With the elusive putative diffusible factor or with the transporter, or perhaps with an enzyme in vitamin D hormone metabolism? Which goes to say that it is necessary to know the phenotype to arrive at the right locus. Or is it? Sufficient physical mapping of region Xp22.31-p21.3 will eventually lead to positional cloning of the Hyp gene. What will it be?(ABSTRACT TRUNCATED AT 250 WORDS)

Crosstransplantation of kidneys in normal and Hyp mice. Evidence that the Hyp mouse phenotype is unrelated to an intrinsic renal defect

Although deranged phosphate transport is the fundamental abnormality in X-linked hypophosphatemic (XLH) rickets, it remains unknown if this defect is the consequence of an intrinsic kidney abnormality or aberrant production of a humoral factor. To discriminate between these possibilities, we examined phosphate homeostasis in normal and Hyp mice, subjected to renal crosstransplantation. We initially evaluated the effects of uninephrectomy on the indices of phosphate metabolism that identify the mutant biochemical phenotype. No differences were found in the serum phosphorus concentration, fractional excretion of phosphate (FEP), or tubular reabsorption of phosphate per milliliter of glomerular filtrate (TRP) in uninephrectomized normal and Hyp mice, compared with sham-operated controls. Subsequently, single kidneys from normal or Hyp mice were transplanted into normal and Hyp mouse recipients. Normal mice transplanted with normal kidneys and Hyp mice engrafted with mutant kidneys exhibited serum phosphorus, FEP, and TRP no different from those of uninephrectomized normal and Hyp mice, respectively. However, engraftment of normal kidneys in Hyp mice and mutant kidneys in normal mice affected neither serum phosphorus (4.69 +/- 0.31 and 8.25 +/- 0.52 mg/dl, respectively) nor FEP and TRP of the recipients. These data indicate that the Hyp mouse phenotype is neither corrected nor transferred by renal transplantation. Further, they suggest that the phosphate transport defect in Hyp mice, and likely X-linked hypophosphatemia, is the result of a humoral factor, and is not an intrinsic renal abnormality.

X-linked hypophosphataemia: a homologous phenotype in humans and mice with unusual organ-specific gene dosage

XLH (X-linked hypophosphataemia, gene symbol HYP, McKusick 307800, 307810) and its murine counterparts (Hyp and Gy) map to a conserved segment on the X-chromosome (Xp 22.31-p.21.3, human; distal X, mouse). Gene dosage has received relatively little attention in the long history of research on this disease, which began over 50 years ago. Bone and teeth are sites of the principal disease manifestations in XLH (rickets, osteomalacia, interglobular dentin). Newer measures of quantitative XLH phenotypes reveal gene dose effects in bone and teeth with heterozygous values distributed between those in mutant hemizygotes and normal homozygotes. On the other hand, serum phosphate concentrations (which are low in the mutant phenotype and thereby contribute to bone and tooth phenotypes) do not show gene dosage. In Hyp mice serum values in mutant hemizygotes, mutant homozygotes and heterozygotes are similar. Phosphate homeostasis reflects its renal conservation. Renal absorption of phosphate on a high-affinity, Na+ ion-gradient coupled system in renal brush border membrane is impaired and gene dosage is absent at this level; the mutant phenotype is fully dominant. Synthesis and degradation of 1,25(OH)2D are also abnormal in XLH (and Hyp), but gene dosage in these parameters has not yet been measured. An (unidentified) inhibitory trans-acting product of the X-linked locus, affecting phosphate transport and vitamin D metabolism, acting perhaps through cytosolic protein kinase C, could explain the renal phenotype. But why would it have a normal gene dose effect in bone and teeth? Since the locus may have duplicated (to form Hyp and Gy), and shows evidence of variable expression in different organs (inner ear, bone/teeth, kidney), it may have been recruited during evolution to multiple functions.

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.

Conserved loci on the X chromosome confer phosphate homeostasis in mice and humans

Several genes expressed in kidney and other tissues determine phosphate homeostasis in extracellular fluid. The major form of inherited hypophosphatemia in humans involves an X-linked locus (HPDR, Xp22.31-p21.3). It has two murine homologues (Hyp and Gy) which map to closely-linked but separate loci (crossover value 0.4%-0.8%). Both murine mutations impair Na(+)-phosphate cotransport in renal brush border membrane; an associated renal disorder of 1,25-dihydroxyvitamin D3 (1,25(OH)2D) metabolism has been characterized in Hyp mice. Whereas experiments with cultured Hyp renal epithelium indicate that the gene is expressed in kidney, studies showing the development of the mutant renal phenotype in normal mice parabiosed to Hyp mice implicate a circulating factor; these findings can be reconciled if the humoral factor is of renal origin. The gene dose effect of HPDR, Hyp and Gy on serum phosphorus values is consistently deviant and heterozygotes resemble affected hemizygotes. The deviant effect is also seen on renal phosphate transport; all mutant females (Hyp/Hyp and Hyp/+) have similar phenotypes. On the other hand, there is a normal gene dose effect of HPDR in mineralized tissue; tooth PRATIO (pulp area/tooth area) values for heterozygotes are distributed between those for affected males and normals. The tooth data imply that the X chromosome locus is expressed in both renal and non-renal cells. The polypeptide product of the X chromosome gene(s) is still unknown.

Familial hypophosphataemic rickets: experience with 24 children from Kuwait

Between 1982 and 1988, familial hypophosphataemic rickets (FHR) was diagnosed in 24 children, in nine during screening of the families of index patients. The average annual incidence was 0.2/1000 live births. There were 16 boys and 8 girls in 10 families, of which nine had more than one affected child. Their ages at the onset of the disease ranged between 10 months and 14 years (mean 6.9 yrs). Growth retardation and bowing of the legs were the most prominent features, observed in all index patients and in four of the patients diagnosed by screening. Treatment with 1 alpha-hydroxyvitamin D3 and phosphates was associated with acceleration of growth in all children, healing of rickets in 21, and normalization of the serum phosphate in 22. Two children with late diagnosis are now older than 16 years with a final height below the 3rd centile. Three more pubertal children are also shorter than the 3rd centile. In areas where nutritional rickets is common, FHR is likely to be missed and the treatment delayed with grave consequences; in particular, growth retardation and bone deformity.

Adaptation to Pi deprivation of cell Na-dependent Pi uptake: a widespread process

Phosphate enters kidney proximal tubular cells through an apical sodium-phosphate cotransport; this activity (Vmax) increases during phosphate deprivation (Kidney Int. 18: 36-47, 1980). This study investigated the mechanism of phosphate uptake and its adaptation to phosphate deprivation in cultured cells from different origins (kidney, LLC-PK1 and MDCK cells; liver, Fao cells; heart, myocyte primary cultures). All cells exhibited a sodium-dependent phosphate uptake that was reduced (greater than 75%) by external sodium substitution and inhibited by ouabain (35%) and 2,4-dinitrophenol or KCN (80%). Phosphate deprivation (exposure to phosphate-free medium) increased sodium-dependent phosphate uptake by 1.8- to 5.8-fold and decreased cell inorganic phosphate and ATP contents (70-80 and 17-30%, respectively). The stimulation of phosphate uptake resulted from an increase in Vmax without change in Km and was dependent on gene transcription and protein synthesis because it was inhibited by cycloheximide and 3-deoxyadenosine. Thus a deprivation-stimulated, sodium-dependent phosphate transport was demonstrated in cells originating from distal kidney tubules, liver, and heart. The findings suggest that in hypophosphatemic diseases, impairment of renal proximal phosphate reabsorption might be only one expression of a widespread alteration of cell phosphate regulation.

Vitamin D metabolism and phosphate transport in developing kidney: effect of diet and mutation

In order to obtain a better understanding of the molecular mechanisms involved in phosphate reabsorption and vitamin D hormone production by mammalian kidney, we have devoted our efforts to the study of a mutant mouse model (Hyp). Studies from our laboratory have demonstrated that Na(+)-dependent phosphate transport is significantly reduced in renal brush border membrane vesicles derived from Hyp mice and that the regulation of the renal mitochondrial enzymes which metabolize 25-hydroxyvitamin D3 (25-OH-D3) is impaired in the mutant strain. The demonstration of abnormal phosphate transport and 25-OH-D3 metabolism in proximal tubule cells derived from Hyp kidney after 6-8 days in culture indicates that the mutant renal phenotype is independent of circulating factors and, therefore, intrinsic to the kidney. However, the precise relationship between these two proximal tubular abnormalities is poorly understood. Because the Hyp mutation segregates as a Mendelian trait, it is very likely that one mutant gene is responsible for the biochemical and clinical phenotype. Several hypotheses are put forth to explain the nature of the primary mutation in the Hyp mouse.

Development of hypercalcemic hyperparathyroidism after long-term phosphate supplementation in hypophosphatemic osteomalacia. Report of two cases

Orally administered phosphate supplements are the mainstay of therapy for hypophosphatemic osteomalacia of diverse causes and are generally believed to be free from harmful side effects. Two cases are reported, however, in which long-term therapy (14 and 10 years, respectively) resulted in hypercalcemic hyperparathyroidism associated with surgically proved adenomatous hyperplasia. This complication occurred despite concomitant treatment with pharmacologic doses of vitamin D. Thus, long-term oral phosphate therapy can produce tertiary hyperparathyroidism in susceptible patients.

Impaired phosphorus conservation and 1,25 dihydroxyvitamin D generation during phosphorus deprivation in familial hypophosphatemic rickets

The pathogenesis of familial hypophosphatemic rickets (FHR) is incompletely understood. We therefore examined the effects of acute dietary phosphorus deprivation to see whether renal phosphate conservation and increased 1,25 dihydroxyvitamin D [1,25(OH)2D] plasma levels, which normally follow restriction of phosphorus intake, could be induced in patients with FHR. Six healthy male volunteers (age 26 +/- 3 yr) and seven male patients with FHR (age 24 +/- 3 yr) were placed on a low phosphorus diet supplemented with aluminum hydroxide and studied over a 4-d period. The patients with FHR excreted more than five times as much phosphorus per day at the conclusion of the study than did the controls (176 +/- 61 mg/24 h vs. 33 +/- 11 mg/h). In the normal subjects, maximum tubular reabsorptive capacity for phosphorus/glomerular filtration rate (TmP/GFR) rose progressively during phosphorus deprivation, and the rise from base line was more than two times greater than that seen in patients with FHR. Immunoreactive parathyroid hormone levels and nephrogenous cyclic AMP were initially normal in both groups and no change was seen in either group with phosphorus deprivation. In the normal subjects, 1,25(OH)2D levels rose progressively over the 96 h of the study (49 +/- 3 to 63 +/- 6 pg/ml, P less than 0.05), while mean circulating 1,25(OH)2D in the patients with FHR did not change (34 +/- 3 to 29 +/- 3 pg/ml). The changes in individual plasma 1,25(OH)2D levels correlated strongly with the change in individual nephrogenous cyclic AMP measurements in the patients with FHR (r = +0.93), while no such correlation was observed in the normal subjects. These data demonstrate a defective renal response to phosphorus deprivation in patients with FHR including a qualitatively abnormal response in 1,25(OH)2D generation.

Serum 1,25-dihydroxyvitamin D concentration in hypophosphatemic vitamin D-resistant rickets

Fasting serum 1 alpha, 25-dihydroxyvitamin D [1,25-(OH)2D] levels were measured in 3 groups of hypophosphatemic vitamin D-resistant rickets (VDRR) patients; those untreated; those treated with vitamin D and phosphate; and those treated with 1,25-(OH)2D3 and phosphate. In the untreated patients, the mean 1,25-(OH)2D level was higher than in our age-matched control group. Except for one at 66 pg/ml, individual values were however within normal limits. Long term vitamin D2 therapy was accompanied by a slight but significant decrease in 1,25-(OH)2D concentrations; nonetheless the levels remained within the normal range. In the third group of patients, the concentration of 1,25-(OH)2D rose to supranormal levels when sampling was done 1-3 hours after administration of the hormone, decreasing rapidly to levels below that of normal subjects when the specimens were collected 12-24 hours later. Our data show that an alteration of the vitamin D activation pathway is unlikely to be part of the pathogenic mechanism underlying the VDRR condition.

Histomorphometric study of bone remodeling in hypophosphatemic vitamin D-resistant rickets

Static and dynamic histomorphometric parameters were evaluated on undecalcified iliac crest bone biopsies obtained from eight children with untreated hypophosphatemic vitamin D resistant rickets (VDRR) in an attempt to evaluate whether a primary metabolic bone defect contributes to the skeletal disorder observed in that disease. When compared to normal age-matched controls the trabecular calcified bone volume was not decreased and there was no evidence of excessive osteoclastic resorption. Both trabecular and cortical bone envelopes had an excess of osteoid tissue and a decreased extent of the mineralization front. Dual tetracycline labeling revealed a decrease in the osteoblastic calcification rate and a marked prolongation of the mineralization lag time and of the formation period. In the intracortical Haversian system the birthrate of new Basic Multicellular remodeling Units (BMU) was markedly reduced, leading to a marked depression of the bone formation rate at the whole tissue level. The combination of the decreased birthrate of new BMU and the prolonged formation period appears to be characteristic of the disease. These results indicate that abnormal differentiation and function of the osteoblast contribute to the osteomalacic lesion present in VDRR. Defective mineralization and impaired osteoblastic function might be the consequence of the chronic hypophosphatemic state. However, the existence of a primary disorder of the bone cell line cannot be excluded as an explanation of the defective recruitment and function of the bone forming cells.

Stimulation of cortical bone mineralization and remodeling by phosphate and 1,25-dihydroxyvitamin D in vitamin D-resistant rickets

Besides rachitic and osteomalacic bone lesions specific disturbances of intracortical bone remodeling have been described in children with vitamin D-resistant rickets (VDRR). The effects of phosphate and 1,25-dihydroxyvitamin D3 [Pi + 1,25(OH)2D] on the abnormal cortical bone remodeling were assessed by static and dynamic histomorphometric analysis of dual labeled undecalcified iliac crest bone biopsies obtained from 12 young VDRR children. Bone mineralization was markedly improved as shown by reduction of the osteoid thickness, shortening of the mineralization lag time and of the osteon calcification period. In conjunction with improved bone mineralization the extent of dual labeled bone surface was increased together with the osteoblast population, indicating that normal bone calcification requires the presence of osteoblasts. At the tissue level the birthrate of new Basic Multicellular remodeling Units (BMU) was clearly enhanced; while at the cellular level, the low calcification rate remained unchanged in most cases. The data show that treatment with Pi + 1,25(OH)2D stimulates the bone turnover in young patients with VDRR by inducing creation of new BMU after restoration of bone mineralization. Unlike the increased recruitment of new BMU caused by treatment, the persistence of a low calcification rate may reflect the existence of a primary osteoblast defect in some VDRR patients.

Hormone-sensitive adenylate cyclase along the nephron of genetically hypophosphatemic mice

The response of the adenylate cyclase (AC) activity to PTH and calcitonin was measured along the nephron of normal (N) and mutant hypophosphatemic (Hyp) mice of the C 57 BL/6J strain, using in vitro single tubule AC microassay. In each experiment, a Hyp mouse was paired to a N mouse from the same litter. In the presence of PTH (10 U/ml), AC activities (femtomoles cAMP per millimeter of tubule per 30-min incubation) were reduced in the proximal convoluted tubule of Hyp mice as compared to N mice in all experiments (448 +/- (SEM) 46 vs. 831 +/- 79, N = 4, P less than 0.01). Some decrease in AC response to PTH also was noted in the cortical portion of the thick ascending limb of the loop of Henle (476 +/- 70 in Hyp mice vs. 719 +/- 83 in N mice, N = 4, P = NS). The Hyp and N AC responses to PTH were similar in the "bright" and "granular" portions of the distal convoluted tubule (1524 +/- 177 in Hyp mice and 1538 +/- 228 in N mice, N = 4). The other segments tested were not responsive to PTH (except the pars recta of the proximal tubule). In the presence of salmon calcitonin (10 ng/ml), a striking 5- to 12-fold increase in AC activity of the "bright" and "granular" portions of the distal convoluted tubule was observed in each Hyp mouse as compared to its paired N control (2434 +/- 618 vs. 399 +/- 56, N = 6, P less than 0.01). The AC response to calcitonin was also increased, though to a lesser extnet (Hyp/N = 1.8) in the "light" portion of the distal tubule (590 +/- 60 in Hyp and 352 +/- 36 in N mice, P less than 0.01). Other segments of the mouse nephron were also observed to contain calcitonin-sensitive AC, but the responses were of limited magnitude only and were not statistically different in Hyp and N mice. Dose-response curves showed that the decrease of the response to PTH in the proximal tubule as well as the increase of the response to calcitonin in the distal tubule were present in Hyp mice for the whole range of hormone concentrations tested. In both structures, the apparent Km for the cyclase activation by the hormone was similar in the Hyp and its paired N mouse.

Hypophosphatemia: mouse model for human familial hypophosphatemic (vitamin D-resistant) rickets

A new dominant mutation in the laboratory mouse, hypophosphatemia (gene symbol Hyp), has been identified. The Hyp gene is located on the X-chromosome and maps at the distal end. Mutant mice are characterized by hypophosphatemia, bone changes resembling rickets, diminished bone ash, dwarfism, and high fractional excretion of phosphate anion (low net tubular reabsorption). Phosphate supplementation of the diet from wearning prevents the appearance of severe skeletal abnormalities. The hypophosphatemic male mouse resembles human males with X-linked hypophosphatemia and the Hyp gene is presemably homologous with the X-linked human gene. The mouse model should facilitate study of the defect in transport of plasma inorganic phosphate anion.

Phosphorus deficiency, parathyroid hormone and bone resorption in the growing rat

Sham-operated and parathyroidectomized (PTX) rats were divided into two pair-fed groups, one on a normal mineral intake (0.5% Ca, 0.3% P), the other on a regimen low in phosphorus (0.5% Ca, 0.03% P). P depletion led to a drop in plasma P and urine P, a rise in plasma Ca and a marked rise in urine Ca, a drop in serum magnesium and a rise in urine Mg. The changes were more pronounced in the PTX animals, but final values were the same in both groups. Parallel bone-seeking isotope (85Sr, 177Lu, 237Np) studies in nonablated animals revealed an increase in the urinary nuclide output and in the urine/tibia ratio in P-deficient animals. Normal and primary bone osteocytes decreased and enlarged osteocytes increased as a result of P deficiency; osteoclasts and osteoblasts also increased. Bone composition showed a drop in ash content and a rise in water, with a light decrease in both Ca and P, and a corresponding rise in hydroxyproline and nitrogen in the P-deficient animals. The results are interpreted to mean that P-deficiency in the young growing rat leads to an increase in bone resorption which occurs also in the absence of parathyroid hormone (PTH). The fact that final values were similar in the control and PTX P-deficient animals suggests that steady-state regulation can also occur without PTH. Because P-deficiency leads to rapid hypercalcemia and rapid marked hypercalciuria, there may exist a mechanism for phosphate regulation which would then supersede Ca homeostasis. The change in serum and urine Mg levels may reflect a decrease in tubular Ca and Mg reabsorption associated with P-deficiency.

Recessive hypophosphataemic rickets, and possible aetiology of the 'vitamin D-resistant' syndrome

Two out of 3 children of a first-cousin marriage presented with severe rickets in infancy and are now adult. Their disease has shown continued activity, marked resistance to treatment with vitamin D, early fusion of cranial sutures, greatly increased bone density, nerve deafness, and life-long hypophosphataemia unaffected by treatment. Both parents and a third sib were normal clinically and biochemically. Blood grouping supported both stated paternity and parental consanguinity. Genetics of this unique disease can only be explained satisfactorily on the basis of autosomal recessive transmission, a mode not previously reported in primary hypophosphataemia. This homozygous disease resembles an exaggerated form of common X-linked rickets, though it is caused by a different gene. Although rare, it is particularly important because of the information it provides on the pathogenesis of both forms. We propose that both syndromes may result from multiple target organ unresponsiveness to 1,25-dihydroxycholecalciferol in intestine, kidney, bone, and parathyroid gland.

Parathyroid function tests with EDTA infusions in infancy and childhood

To determine the functional capabilities of the parathyroid glands, 17 EDTA infusions were given to 11 children (ages 1 month to 12 years) and to two mothers of four of the children. Serum ionized Ca fell from 4.1 mg/dl to 3.4 mg/dl. Excessive parathyroid hormone responses were elicited during seven of nine EDTA infusions in five children and in one adult with hypophosphatemic rickets, during the active phase of rickets. In four of five subjects with problems related to hypercalcemia, borderline low or undetectable PTH responses were elicited. Three relatively normal PTH responses were obtained, two in an infant after phosphate-induced hypocalcemic tetany was corrected, and one in a child with a malabsorption syndrome. The renal tubular reabsorption of phosphate was inversely related and the urinary cyclic AMP excretion was positively related to the PTH response. Thus EDTA infusions in infants and children might be useful in the identification of hyper-, normo-, or hypoparathyroid states and would be of value in defining the functional condition of the parathyroid glands in children with deranged Ca or P metabolism.

The assay of 1alpha,25-dihydroxyvitamin D3: physiologic and pathologic modulation of circulating hormone levels

A sensitive radioreceptor assay for 1alpha,25-dihydroxyvitamin D3 (1alpha,25-(OH)2D3) is utilized to quantitate the circulating concentration of this sterol in experimental animals and humans. When weanling rats are grown for 2 weeks on low calcium or low phosphate diets, limited availability of either ion elicits a five-fold increase in the plasma level of 1alpha,25-(OH)2D3. The enhancement of 1alpha,25-(OH)2D3 in calcium deficiency is dependent upon the presence of the parathyroid and/or thyroid glands, which is consistent with parathyroid hormone (PTH) mediation of this effect. In contrast, the response to phosphate deficiency is independent of these glands and may result from a direct action of low phosphate on the renal synthesis of 1alpha,25-(OH)2D3. Studies in humans indicate that the normal level of 1alpha,25-(OH)2D is 2.1--4.5 ng/100 ml plasma. Patients with chronic renal failure have markedly lower circulating 1alpha,25-(OH)2D and this kidney hormone is undetectable in anephric subjects, but returns to normal within 1 day after successful renal transplantation. Hypoparathyroidism and pseudohypoparathyroidism are associated with reduced plasma 1alpha,25-(OH)2D while patients with primary hyperparathyroidism have significantly elevated sterol hormone levels. Thus, from measurements in rats and humans, it appears that circulating 1alpha,25-(OH)2D3 is regulated by PTH and/or phosphate and that abnormal plasma 1alpha,25-(OH)2D3 is a part of the pathophysiology of renal osteodystrophy and parathyroid disorders.

Regulation of circulating parathyroid hormone levels: normal physiology and consequences in disorders of mineral metabolism

A review of the chemistry, the biosynthesis, the regulation of the secretion and the metabolism of parathyroid hormone (PTH) provides a background to problems related to the determination of immunoreactive PTH levels in the blood of normal subjects and of patients suffering from disorders of mineral metabolism. Immunoreactive PTH measurements have to be interpreted in the light of the immunoheterogeneity encountered and the occurrence of several circulating PTH species with different molecular weights and immunological properties.