Rickets and the pathogenesis of impaired tubular transport of phosphate and other solutes

Tubular Dysfunction Mimicking Dent's Disease in 2 Infants Born with Extremely Low Birth Weight

Two preterm infants, with extremely low birth weight born at gestational weeks 24 and 25, showed generalized proximal tubular dysfunction during their stay in the neonatal intensive care unit, including glucosuria, low molecular weight proteinuria, phosphaturia, uricosuria, enzymuria (elevated urine N-acetyl-β-D-glucosaminidase), panaminoaciduria, and hypercalciuria, associated with renal calcification. Renal tubular acidosis was not present in either patient. DNA mutation analysis for Dent's disease, performed in patient 1, was negative. Although both patients had rickets of prematurity, tubular dysfunction persisted after its resolution. Patient 2, who had severe chronic lung disease, also had elevated serum creatinine, proteinuria, and hypertension, suggesting glomerular damage. In patient 1, low molecular weight proteinuria, enzymuria, panaminoaciduria, hypercalciuria, and renal calcification were still present at the age of 8 years. In patient 2, tubular dysfunction resolved except for β2 microglobulinuria at the age of 5 years. While a reduced nephron number resulting in focal segmental glomerulosclerosis is well-known, generalized proximal tubular dysfunction can also occur in infants born preterm and/or with extremely low birth weight.

Interactions of vitamin D and the proximal tubule

Severe vitamin D deficiency (reduction in serum 25(OH)D concentration) in infants and children can cause features of the Fanconi syndrome, including phosphaturia, glycosuria, aminoaciduria, and renal tubular acidosis. This indicates that vitamin D and its metabolites influence proximal tubule function. Filtered 25(OH)D bound to vitamin D binding protein (DBP) is endocytosed by megalin-cubilin in the apical membrane. Intracellular 25(OH)D is metabolized to 1,25(OH)2D or calcitroic acid by 1-α-hydroxylase or 24-hydroxylase in tubule cell mitochondria. Bone-produced fibroblast growth factor 23 (FGF23) bound to Klotho in tubule cells and intracellular phosphate concentrations are regulators of 1-α-hydroxylase activity and cause proximal tubule phosphaturia. Aminoaciduria occurs when amino acid transporter synthesis is deficient, and 1,25(OH)2D along with retinoic acid up-regulate transporter synthesis by a vitamin D response element in the promoter region of the transporter gene. This review discusses evidence gained from studies in animals or cell lines, as well as from human disorders, that provide insight into vitamin D-proximal tubule interactions.

Differential regulation of TauT by calcitriol and retinoic acid via VDR/RXR in LLC-PK1 and MCF-7 cells

The interaction between taurine and the absorption of fat-soluble -vitamins, such as vitamin A and D, has been an interesting topic in the field of -nutrition science, because taurine-conjugated bile acid optimizes fat and fat-soluble vitamin absorption. However, whether the hormone calcitriol (1,25-dihydroxyvitamin D(3)) and retinoic acid regulate the expression of the TauT gene is unknown. In this study, we test the hypothesis that the TauT gene is regulated by vitamin D(3) (VD(3)) and retinoic acid (RA) via activation of the vitamin D receptor (VDR) and retinoic acid receptor (RXR). Taurine uptake, Western blotting, gene reporter assay, and immunohistochemical analysis of TauT, VDR, and RXR were used in VD(3)- and/or RA-treated LLC-PK1 and MCF-7 cells. We demonstrated that VD(3) alone had little effect on TauT expression in both LLC-PK1 and MCF-7 cells. Expression of TauT was significantly increased by RA, which was synergized by the addition of VD(3) after RXR activation in LLC-PK1 cells. In contrast, expression of TauT was significantly decreased by the combination of VD(3) and RA in MCF-7 cells. Regulation of TauT by VD(3)/RA appears to occur at the transcriptional level, as determined by a reporter gene assay of the TauT promoter. Immunohistochemical study showed that VDR and RXR were activated by VD(3) and RA, respectively, in both LLC-PK1 and MCF-7 cells. The activated VDR and RXR also colocated in nuclei of both cells, suggesting that a VDR/RXR complex is involved in the transcriptional regulation of TauT. Our results show that expression of TauT is differentially regulated by VD(3) and RA via formation of VDR and RXR complexes in the nuclei in a cell type-dependent manner.

Mild phosphate diabetes in adults

Phosphate diabetes has been considered as rare and to occur almost exclusively in children. Upon examination of adult patients with rheumatic or kidney diseases it has, however, been found that the combination of hypophosphataemia and hyperphosphaturia is not so rare. This paper deals with 24 adult patients of this type, whom we have found during 6 months. Their mean serum phosphorus concentration was 0.7 mmol/l (range 0.5--0.8). Mean phosphate clearance was 31 ml/min/1.73 m2 (range 16--51). The diagnoses were myalgia, dorsalgia (n = 7), papillitis calcificans (n = 5), prostatitis or prostate accretions (n = 4), dizziness (n = 2), kidney stones, tubular defect, interstitial nephritis, medullary sponge kidney (1 case each), two patients had transplanted kidneys. Asthenia was a common additional diagnosis. The patients' complaints have been pain in the muscles, joints, bones (18 cases), tiredness (10 cases), dizziness (8 cases), shakyness, numbness, burning sensation (7 cases), tenderness in the muscles and bones ("the princess-on-the-pea syndrome") (7 cases). The most common findings upon examination were bone tenderness (13 cases), reduced manual power (8 cases), positive Romberg test (3 cases), slight muscle atrophy (2 cases), waddling gait (2 cases). The most common findings encountered in the laboratory, besides hypophosphataemia and hyperphosphaturia, were high pH in the urine, hyperaminoaciduria, and phosphate crystals in dried urine.

Hypophosphatemic rickets: easy to diagnose, difficult to treat

Hypophosphatemic rickets (HR) has generated a lot of interest in recent times. There is need to recognize this disorder and differentiate it from the more common nutritional rickets because the therapy is different. It is also important to emphasize that a detailed clinical examination with pedigree analysis and easily available biochemical tests are adequate to establish the diagnosis in most cases. This report presents three families with hypophosphatemic rickets. Interestingly, many of these patients had a mixed picture of HR and nutritional rickets. Their important features are described with special emphasis on early initiation of treatment with oral phosphate and stringent monitoring of renal functions to prevent development of irreversible renal insufficiency.

Skeletal manifestations of moderate phosphate diabetes

Six patients, with vertebral osteoporosis and reflex sympathetic algodystrophy syndrome of the lower limbs, due to moderate diabetes, are presented. Osteoporosis was documented by low CT scan bone density and moderate decrease of bone trabecular volume. Histomorphometric studies found a mild increase of resorption areas. All patients had unremarkable serum phosphorus level (mean: 0.91 +/- 0.13) but increase of urinary phosphorus excretion was documented by phosphate clearance higher than 20ml/mm, phosphate tubular reabsorption lower than 80% and TmPo4/GFR lower than 0.8mmol/l. Phosphate clearance and phosphate tubular reabsorption are studied in two control groups. Mild phosphate diabetes, of unknown incidence and prevalence (need for prospective studies) may be the vector of osteoporosis, vertebral and peripheral. The diagnosis of PD requires determination of phosphate clearance, phosphate tubular reabsorption, TmPo4/GFR and these tests may be useful in the diagnostic work up of bone demineralization disorders. We thought that osteoporosis could be the result of progressive dissolution of bone apatite crystals necessary to maintain normal or sub-normal blood phosphate level in spite of the phosphate diabetes.

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)

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.

Does growth hormone influence the severity of phosphopenic rickets?

The severity of bone disease in phosphopenic rickets is not correlated with serum phosphate levels. In order to determine whether growth hormone (GH) secretion may influence rachitic changes, we evaluated the 24 h integrated concentration of growth hormone (IC-GH) in five children with phosphopenic rickets. Two patients with marked clinical and roentgenographic rachitic abnormalities had normal IC-GH levels. In contrast, three patients with low IC-GH levels had mild rachitic changes. We suggest that the level of spontaneous GH secretion may be one factor which influences the severity of phosphopenic 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.

The human glycine receptor: a new probe that is linked to the X-linked hypophosphatemic rickets gene

We undertook linkage analysis in four large North Carolina kindreds with X-linked hypophosphatemic rickets (HYP) using a newly defined polymorphic probe, derived from the 5' untranslated portion of the human glycine receptor (GLR). Two-point linkage analysis established linkage between GLR and HYP [Z(theta) = 7.91 at theta = 0.07] and confirmed linkage between HYP and DXS41 [Z(theta) = 8.31 at theta = 0.06] and DXS43 [Z(theta) = 5.94 at theta = 0.05]. Additionally, we found GLR tightly linked to DXS43 [Z(theta) = 5.40 at theta = 0.0]. Multipoint analysis indicated that GLR is on the telomeric side of HYP with a map order of Xpcen-DXS41-HYP-(GLR/DSX43).

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.

A histomorphometric determination of iliac bone remodeling in patients with recurrent renal stone formation and idiopathic hypercalciuria

33 normocalcemic patients (22 males and 11 females) aged 20-68 years with recurrent renal stone formation and idiopathic hypercalciuria were compared to 33 approximately sex- and age-matched normal controls. Quantitative histomorphometric analysis of iliac crest biopsies were performed after intravital tetracycline double labeling in the patients and in 30 sex- and age-matched normal controls. No difference was found between patients and controls in albumin adjusted serum calcium levels. Serum phosphorus was significantly reduced (p less than 0.01) in the patient group whereas the urinary phosphorus/creatinine ratio was increased (p less than 0.01). The serum calcium phosphate product (S-CaxS-P) was significantly reduced in the patients (p less than 0.05). As expected, the urinary calcium/creatinine ratio was higher in the patient group than in the controls (p less than 0.001). Serum parathyroid hormone was normal. The histomorphometric analysis revealed signs of a moderate mineralization defect (reduced adjusted appositional rate (p less than 0.05), prolonged mineralization lag time (p less than 0.05) and prolonged formation (p less than 0.05)), and an increased extension of eroded surfaces (P less than 0.05) in the patients. The amount of trabecular bone and the balance between the thickness of bone resorbed and later formed per remodeling cycle and all other histomorphometric parameters were found normal in the patients. The combined histomorphometric and biochemical data are best explained by a primary renal phosphate leak leading to hypophosphataemia and a slight mineralization defect. The hypercalciuria may be explained by an enhanced renal production of 1.25-dihydroxyvitamin D secondary to the reduced serum levels of phosphorus. No signs of secondary or primary hyperparathyroidism were observed.

Evaluating the child with short stature

Short stature is a common pediatric problem that requires us to decide whether a child's small size represents only normal variation or indicates the presence of an underlying disease. In a population of children two standard deviations (SD) below the mean for height (below the third percentile), about 20 per cent may be expected to have pathologic short stature with the remaining 80 per cent about equally divided between familial short stature and constitutional growth delay. In contrast, most children three SD below the mean for height have pathologic short stature. Set forth in this article is an orderly approach to identify normal variants of short stature and to investigate the causes of pathologic short stature.

Healing of bone disease in X-linked hypophosphatemic rickets/osteomalacia. Induction and maintenance with phosphorus and calcitriol

Although conventional therapy (pharmacologic doses of vitamin D and phosphorus supplementation) is usually successful in healing the rachitic bone lesion in patients with X-linked hypophosphatemic rickets, it does not heal the coexistent osteomalacia. Because serum 1,25-dihydroxyvitamin D levels are inappropriately low in these patients and high calcitriol concentrations may be required to heal the osteomalacia, we chose to treat five affected subjects with high doses of calcitriol (68.2 +/- 10.0 ng/kg total body weight/d) and supplemental phosphorus (1-2 g/d) performing metabolic studies and bone biopsies before and after 5-8 mo of this therapy in each individual. Of these five patients, three (aged 13, 13, and 19 yr) were receiving conventional treatment at the inception of the study and therefore showed base-line serum phosphorus concentrations within the normal range. The remaining two untreated patients (aged 2 and 37 yr) displayed characteristic hypophosphatemia before calcitriol therapy. All five patients demonstrated serum calcitriol levels in the low normal range (22.5 +/- 3.2 pg/ml), impaired renal phosphorus conservation (tubular maximum for the reabsorption of phosphate per deciliter of glomerular filtrate, 2.13 +/- 0.20 mg/dl), and osteomalacia on bone biopsy (relative osteoid volume, 14.4 +/- 1.7%; mean osteoid seam width, 27.7 +/- 3.7 micron; mineral apposition rate, 0.46 +/- 0.12 micron/d). On high doses of calcitriol, serum 1,25-dihydroxyvitamin D levels rose into the supraphysiologic range (74.1 +/- 3.8 pg/ml) with an associated increment in the serum phosphorus concentration (2.82 +/- 0.19 to 3.78 +/- 0.32 mg/dl) and improvement of the renal tubular maximum for phosphate reabsorption (3.17 +/- 0.22 mg/dl). The serum calcium rose in each patient while the immunoactive parathyroid hormone concentration measured by three different assays remained within the normal range. Most importantly, repeat bone biopsies showed that high doses of calcitriol and phosphorus supplements had reversed the mineralization defect in all patients (mineral apposition rate, 0.88 +/- 0.04 micron/d) and consequently reduced parameters of bone osteoid content to normal (relative osteoid volume, 4.1 +/- 0.7%; mean osteoid seam width, 11.0 +/- 1.0 micron). Complications (hypercalcemia and hypercalciuria) ensued in four of these five patients within 1-17 mo of documented bone healing, necessitating reduction of calcitriol doses to a mean of 1.6 +/- 0.2 micrograms/d (28 +/- 4 ng/kg ideal body weight per day). At follow-up bone biopsy, these four subjects continued to manifest normal bone mineralization dynamics (mineral apposition rate, 0.88 +/-0.10 micrometer/d) on reduced doses of 1.25-dihydroxyvitamin D with phosphorus supplements (2 g/d) for a mean of 21.3 +/- 1.3 mo after bone healing was first documented. Static histomorphometric parameters also remained normal (relative osteoid volume, 1.5 +/- 0.4%; mean osteoid seam width, 13.5 +/- 0.8 micrometer). These data indicate that administration of supraphysiologic amounts of calcitriol, in conjunction with oral phosphorus, results in complete healing of vitamin D resistant osteomalacia in patients with X-linked hypophosphatemic rickets. Although complications predictably require calcitriol dose reductions once healing is achieved, continued bone healing can be maintained for up to 1 yr with lower doses of 1,25-dihydroxyvitamin D and continued phosphorus supplementation.

Hereditary hypophosphatemic rickets with hypercalciuria

We studied a new hereditary syndrome of hypophosphatemic rickets and hypercalciuria in six affected members of one kindred. In all patients, the manifestations of disease began in early childhood. The characteristic features are rickets, short stature, increased renal phosphate clearance (the ratio between the maximal tubular reabsorption rate for phosphorus and the glomerular filtration rate [TmP/GFR] is 2 to 4 S.D. below the age-related mean), hypercalciuria (8.6 mg of urinary calcium per kilogram of body weight per 24 hours vs. the upper normal value of 4.0), normal serum calcium levels, increased gastrointestinal absorption of calcium and phosphorus, an elevated serum concentration of 1,25-dihydroxyvitamin D (390 +/- 99 pg per milliliter vs. the upper normal value of 110), and suppressed parathyroid function (an immunoreactive parathyroid hormone level of 0.33 +/- 0.1 ng per milliliter and a cyclic AMP level of 1.39 +/- 0.12 nmol per deciliter of glomerular filtrate vs. the lower normal values of 0.3 and 1.5, respectively). Long-term phosphate supplementation as the sole therapy resulted in reversal of all clinical and biochemical abnormalities except the decreased TmP/GFR. We propose that the pivotal defect in this syndrome is a renal phosphate leak resulting in hypophosphatemia with an appropriate elevation of 1,25-dihydroxyvitamin D levels, which causes increased calcium absorption, parathyroid suppression, and hypercalciuria. This syndrome may represent one end of a spectrum of hereditary absorptive hypercalciuria. Our observations support the importance of phosphate as a mediator in controlling 1,25-dihydroxyvitamin D production in human beings.

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.

Nonosteomalacic osteopathy associated with chronic hypophosphatemia

We studied bone histomorphometry in 19 patients with chronic hypophosphatemia related to an idiopathic renal phosphate wasting and without histological osteomalacia. Nine patients had renal lithiasis (group 1), three had radiological osteoporosis (group 2), and seven had lumbar pain (group 3). In the whole group of 19 patients, serum phosphate levels were low (24.9 +/- 2.1 mg/l), calcium in blood was normal, calcium in urine was increased, and iPTH was low. Histomorphometric data showed decreased osteoblastic surfaces with normal resorption surfaces, normal osteoid volume and calcification front. There was no correlation between serum phosphate level and histomorphometric parameters. There was no statistical difference between the data of the 3 groups of hypophosphatemic patients. We concluded that chronic hypophosphatemia in the adult doses not always lead to osteomalacia but to an unusual osteopathy characterized by an osteopenia due to an isolated decrease in bone formation. The respective importance of phosphate deficiency and of decreased iPTH level in the pathogenesis of this osteopathy is uncertain.

Long-term treatment of familial hypophosphatemic rickets with oral phosphate and 1 alpha-hydroxyvitamin D3

Combined treatment with oral phosphate and 1 alpha (OH)D3 was carried out in nine children with familial hypophosphatemic rickets. All nine had positive responses over a four- to six-year period as judged by healing of rickets, change in growth rate, decrease in alkaline phosphatase activity, and symptomatic improvement. In two patients therapy was stopped for a short time because of hypercalcemia. In one patient in whom therapy was effective there was a significant reduction in creatinine clearance which necessitated cessation of treatment. The results of this study suggest that combined treatment with 1 alpha(OH)D3 and oral phosphate is an effective form of therapy for this condition, but that the balancing of these two modalities of therapy in each patient is essential if hypercalcemia and hypercalciuria, on the one hand, and secondary hyperparathyroidism, on the other, are to be avoided. A simple means of balancing these therapeutic modalities is suggested.

The etiology of enamel hypoplasia: a unifying concept

In a study of children with chronic disorders of calcium and phosphate homeostasis, enamel hypoplasia was found in hereditary vitamin D-dependency rickets and in hypoparathyroidism, conditions characterized by hypocalcemia, and was not found in X-linked hypophosphatemic rickets, a condition in which the plasma calcium concentration is normal. The occurrence of enamel hypoplasia bore no relation to the plasma phosphate concentration. Enamel hypoplasia has also been reported in other pediatric disorders in which hypocalcemia is a major sign (for example, vitamin D deficiency, prematurity, and neonatal tetany). The existence of enamel hypoplasia in a hypoparathyroid or rachitic patient, when correlated with the chronology of enamel mineralization, helps to establish the time of onset of hypocalcemia. The observations led us to the hypothesis that a low serum calcium concentration during enamel formation is a specific determinant of enamel hypoplasia. This hypothesis may be relevant to the etiology of linear enamel hypoplasia, an endemic lesion of primary teeth in children of many Third World countries that predisposes the teeth to dental caries. The hypothesis may therefore be relevant also in explaining the prevalence of caries in the primary teeth of children in many underdeveloped countries.

Autosomal hypophosphataemic bone disease responds to 1,25-(OH)2D3

We diagnosed non X-linked hypophosphataemic bone disease in a 38-month-old girl. Findings included: genu varum, shortened stature, fasting hypophosphataemia (2.3-2.5 mg/100 ml; 0.74-0.81 mmol/l), diminished theoretical renal threshold for phosphate (TmP/GFR), and osteomalacia without rickets. One patient (the father) had fasting hypophosphataemia (2.3-2.7 mg/100 ml; 0.74-0.87 mmol/l) and low TmP/GFR without osteomalacia or shortened stature. Treatment of the girl with 1,25-(OH)2D3 (1 microgram a day) raised the level of serum phosphorus, improved tubular reabsorption of phosphate, and healed the bone deformity; this combination of responses is not present in X-linked hypophosphataemia. There was no correction of hypophosphataemia or TmP/GFR with 1,25-(OH)2D3 treatment (1-3 micrograms a day) in the father.

Evaluation of a role for 1,25-dihydroxyvitamin D3 in the pathogenesis and treatment of X-linked hypophosphatemic rickets and osteomalacia

Although a defect in renal transport of phosphate seems well established as the primary abnormality underlying the pathogenesis of X-linked hypophosphatemic rickets and osteomalacia, several observations indicate that renal phosphate wasting and hypophosphatemia cannot solely account for the spectrum of abnormalities characteristic of this disease. Thus, in the present study, we investigated the potential role of abnormal vitamin D metabolism in the pathogenesis of this disorder and the effect of 1,25-dihydroxyvitamin D(3) therapy on both the biochemical abnormalities characteristic of this disease and the osteomalacia. Four untreated patients, ages 14-30 yr, had normocalcemia (9.22+/-0.06 mg/dl); hypophosphatemia (2.25+/-0.11 mg/dl); a decreased renal tubular maximum for the reabsorption of phosphate per liter of glomerular filtrate (2.12+/-0.09 mg/dl); normal serum immunoreactive parathyroid hormone concentration; negative phosphate balance; and bone biopsy evidence of osteomalacia. The serum 25-hydroxyvitamin D(3) concentration was 33.9+/-7.2 ng/ml and, despite hypophosphatemia, the serum level of 1,25-dihydroxyvitamin D(3) was not increased, but was normal at 30.3+/-2.8 pg/ml. These data suggested that abnormal homeostasis of vitamin D metabolism might be a second defect central to the phenotypic expression of X-linked hypophosphatemic rickets/osteomalacia. This hypothesis was supported by evaluation of the long-term response to pharmacological amounts of 1,25-dihydroxyvitamin D(3) therapy in three subjects. The treatment regimen resulted in elevation of the serum 1,25-dihydroxyvitamin D levels to values in the supraphysiological range. Moreover, the serum phosphate and renal tubular maximum for the reabsorption of phosphate per liter of glomerular filtrate increased towards normal whereas the phosphate balance became markedly positive. Most importantly, however, repeat bone biopsies revealed that therapy had positively affected the osteomalacic component of the disease, resulting in normalization of the mineralization front activity. Indeed, a central role for 1,25-dihydroxyvitamin D(3) in the mineralization of the osteomalacic bone is suggested by the linear relationship between the serum level of this active vitamin D metabolite and the mineralization front activity. We, therefore, suggest that a relative deficiency of 1,25-dihydroxyvitamin D(3) is a factor in the pathogenesis of X-linked hypophosphatemic rickets and osteomalacia and may modulate the phenotypic expression of this disease.

Aminoaciduria--its relationship to vitamin D and parathyroid hormone

Early reports of patients with metabolic bone diseases such as nutritional osteomalacia, Fanconi syndrome, indicated an association with aminoaciduria. This association has since been described in osteomalacia of G. I. or hepatic origin, secondary to anticonvulsant therapy, tumors, and chronic renal failure. Aminoaciduria also occurs in primary hyperparathyroidism. In nutritional osteomalacia, vitamin D deficiency was thought to be responsible for the renal tubular abnormality, since it responded to treatment with vitamin D. However, since the description of aminoaciduria in hyperparathyroidism, the literature has been divided concerning the etiology of aminoacidura in conditions associated with abnormal vitamin D metabolism because secondary hyperparathyroidism often occurs in these conditions. Recently, some cases of Fanconi syndrome and a case of tumor-associated osteomalacia have been described with low or absent plasma 1,25-dihydroxycholecalciferol levels, normal serum PTH, and aminoaciduria. In one of these cases, and more recently in patients with chronic renal failure, it has been demonstrated that treatment with 1,25(OH)2D3 can improve amino acid transport independently from changes in serum PTH levels. 1,25(OH)2D3 therefore normally opposes the aminoaciduric effect of PTH. This is an agreement with observations which demonstrate that 1,25(OH)2D3 also opposes the phosphaturic action of parathyroid hormone.

Autosomal dominant Fanconi syndrome with early renal failure

The "idiopathic" Fanconi syndrome occurs mostly sporadically, occasionally as an autosomal recessive trait. However, few instances of autosomal dominant inheritance have been reported. We described a father and son with the Fanconi syndrome, ie, with renal glycosuria, generalized aminoaciduria, phosphaturia, metabolic acidosis, and bone disease. No other causes of the Fanconi syndrome were found. Both father and son developed end stage renal disease. Aminoaciduria in excess of that seen in renal insufficiency is shown by comparison with published data for amino acid excretion in uremia. Renal transplantation in the father has improved kidney function with no evidence of Fanconi syndrome. This family is unique in that there are no other reports of autosomal dominant Fanconi syndrome with progression to early renal failure.

Recent advances in calcium metabolism. II. Disorders of calcium homeostasis

Abnormalities of calcium and mineral metabolism are described in relation to the differential diagnosis, clinical characteristics, diagnostic procedures, and treatment of infants and children with hypocalcemia, hypercalcemia, rickets, chronic renal insufficiency, and other disorders of calcium metabolism. Understanding of the basic pathogenesis of each abnormality of calcium homeostasis is essential for the rational management of affected patients.