Pierre J. Marie

Vitamin D dependency: Replacement therapy with calcitriol

Nine patients with vitamin D-dependency type I were studied. We observed that treatment with large doses of vitamin D altered the phenotypic expression of the disease, thus making a delayed diagnosis difficult. At the time of entry, eight children had hypocalcemia, and seven had hypophosphatemia. Elevated serum immunoreactive parathyroid hormone and low (less than 3 SD from control mean) 1 alpha,25-dihydroxyvitamin D values were constant findings, with no vitamin D deficiency. Despite the elevated serum iPTH, three children had normal urinary phosphate excretion and five had normal urinary cAMP excretion. In the five children tested before treatment, there was no significant change in renal phosphate excretion during an acute parathyroid hormone infusion, although in all a significant rise of urinary cAMP occured. Treatment with calcitriol (0.25 to 2 microgram/day) returned all the biochemical values to normal within four months. In two patients, both supplemented with vitamin D, histomorphometric analysis of iliac crest biopsies revealed severe osteomalacia. After nine and ten months of treatment with calcitriol, there was histologic evidence for improvement of bone mineralization. Since calcitriol requirements may vary during the course of treatment, careful monitoring of biochemical variables is essential.

Relation between hypomineralized periosteocytic lesions and bone mineralization in vitamin D-resistant rickets

SummaryHypomineralized periosteocytic lesions (HPL) are peculiar bone features associated with osteomalacia in vitamin D-resistant rickets (VDRR). To determine whether HPL result from defective bone mineralization, tetracycline double-labeled cortical bone specimens from VDRR children were analyzed before (n=13) and after treatment with phosphate supplements combined with vitamin D2 (Pi+D, n=20) or 1,25-dihydroxyvitamin D3 (Pi+1,25) (n=22). On microradiographs of undecalcified sections, the percentage of osteocytes with HPL was determined separately in the old interstitial bone, the well calcified resting osteons, and in the less mineralized growing osteons. Using histomorphometric methods, dynamic parameters of cortical bone mineralization were assessed on the same samples. In untreated patients, HPL were more prominent in the less calcified bone areas, in accordance with the reported decrease with bone aging. In contrast to therapy with Pi+D, Pi+1,25 reduced HPL frequency in all cortical areas in correlation with improvement of dynamic parameters of bone mineralization. In addition, HPL frequency progressively decreased with the duration of treatment, further demonstrating that the lesion resulted in part from the defective cortical bone mineralization. However, HPL frequency was unrelated to the severity of osteomalacia in untreated children and the lesion persisted in more than 20% of young osteocytes despite complete correction of bone mineralization parameters. The data support the hypothesis that an abnormal osteocytic function, which may be part of the VDRR phenotype, is involved along with overall impaired bone mineralization in the etiology of the periosteocytic lesion.

Continuous infusion of 1,25-dihydroxyvitamin D3 stimulates bone turnover in the normal young mouse.

SummaryThe effects of continuous administration of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] on mineral and bone metabolism have been examined in the normal mouse. Four doses (0.05–0.25µg/kg/day) of 1,25(OH)2D3 were infused continuously for 4 weeks in 21-day-old intact animals. Mineral and skeletal changes were evaluated by analytical methods and by histomorphometric analysis of endosteal bone formation and resorption parameters. All doses of 1,25(OH)2D3 increased the fractional osteoclastic surface and the osteoclast number in conjunction with increased hydroxyproline excretion. 1,25(OH)2D3 induced a dose-dependent elevation of the calcification rate, reduction of the mean osteoid seam thickness, and shortening of the mineralization lag time. In addition, there was a dose-related increase in the extent of tetracycline double-labeled osteoid surface and a concurrent rise in the fractional osteoblastic surface associated with elevated serum alkaline phosphatase levels. Increased bone formation appeared to have been balanced by increased bone resorption since the trabecular bone volume remained unchanged.Except at the highest dose given, serum calcium and phosphate concentrations remained normal in spite of increased bone mobilization and presumably enhanced intestinal absorption of minerals. Urinary cAMP and TmP/GFR remained normal, suggesting that parathormone secretion was not altered.The results show that continuous 1,25(OH)2D3 infusion in the young mouse produces a dose-dependent stimulation of bone mineralization rate in response to increased osteoclastic bone resorption. The data indicate that 1,25(OH)2D3 can regulate bone turnover as well as mineral homeostasis in the young mouse.

Bone response to phosphate and vitamin d metabolites in the hypophosphatemic male mouse.

SummaryThe hypophosphatemic male mouse (Hyp/y), the proposed model for human vitamin D-resistant rickets (VDRR), is characterized by chronic hypophosphatemia, dwarfism, and rachitic and osteomalacic bone lesions. We have reported that treatment of Hyp/y mice with phosphate salts (Pi) heals rickets but does not correct the defective endosteal bone mineralization. In an attempt to cure osteomalacia, mutant male animals were treated with Pi combined with 25-hydroxyvitamin D3 (25OHD3, 1 µg/kg/day), 24,25-dihydroxyvitamin D3 [24,25(OH)2D3, 0.5 µg/kg/day], or 1,25-dihydroxyvitamin D3 [1,25(OH)2D3, 0.05–0.25 µg/kg/day] infused constantly for 3 weeks. The biochemical and skeletal effects of treatment were assessed by analytical methods and bone histomorphometry. The results show that only 1,25(OH)2D3 produced a dose-dependent elevation of serum calcium and phosphorus, and greatly improved bone mineralization at doses high enough to increase serum calcium and phosphorus concentrations within or above the normal range. Better improvement of bone mineralization was obtained when Pi was combined to 1,25(OH)2D3. In conjunction with the correction of hypocalcemia, Pi+1,25(OH)2D3 suppressed the stimulation of bone turnover induced by Pi supplementation. The results show that, as in VDRR children, 1,25(OH)2D3 produces beneficial effects on bone lesions in Hyp/y mice, mainly through enhancement of mineral availability. However, the persistence of osteomalacia despite correction of serum mineral concentrations suggests that there is a specific bone cell resistance to mineral and/or hormonal influences in Hyp/y mice.

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.

Influence of magnesium supplementation on bone turnover in the normal young mouse.

SummaryThe effect of magnesium (Mg) supplementation on bone metabolism has been studied in the normal young mouse. Weanling male mice were given Mg-supplemented drinking water (5 mM or 32 mM Mg) for 4 weeks. Mineral and skeletal changes were assessed by biochemical methods and by histomorphometric analysis of endosteal bone formation and resorption parameters evaluated on tetracycline double-labeled, undecalcified caudal vertebrae. Magnesium supplementation increased serum and urinary Mg concentrations and bone Mg content. Both the calcification rate and the extent of tetracycline double-labeled osteoid surface increased progressively in Mg-treated mice, whereas the mineralization lag time was shortened. The osteoblastic surface was reduced, leading to a fall in osteoid surface. Stimulation of bone mineralization was associated with a rise in extracellular calcium (Ca) and phosphorus (P) concentrations whereas serum 25-OHD and 1,25(OH)2D levels remained normal. The Mg supplementation increased the number of acid phosphatase stained chondroclasts and osteoclasts and the extent of resorbing surface showing histochemically stained osteoclasts. Although urinary OH-proline increased above normal, Ca, P, and cyclic adenylic acid (cAMP) excretion and phosphate concentration (TmP/GFR) remained normal, suggesting that parathyroid hormone (PTH) secretion was not altered. The trabecular bone volume remained normal, showing that the increased bone resorption was balanced by the stimulated bone mineralization. The results show that Mg supplementation influenced both bone formation and resorption in the young mouse, and that the stimulation of bone mineralization was the result of increased extracellular mineral availability. On the other hand, stimulation of osteoclastic bone resorption appeared to occur independently of PTH or of increased 1,25(OH)2D production. Therefore, this study suggests that Mg may controlin vivo bone metabolism by directly influencing bone resorbing cells activity.

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.

Bone histomorphometry in asymptomatic adults with hereditary hypophosphatemic vitamin D-resistant osteomalacia

Vitamin D-resistant rickets (VDRR) in adults is characterized by low serum phosphorus and osteomalacia. Despite the disappearance of rickets after the closure of epiphyses, some adults with VDRR present with symptomatic bone disease while other are asymptomatic. In order to test the presumption that asymptomatic adults no longer have active bone disease, we have compared bone histology in 10 symptom-free adults to 6 age-comparable symptomatic adults presenting with bone pain and persistent deformities. Both groups had similar low serum phosphorus and increased serum alkaline phosphatase values. Serum calcium, parathyroid hormone, and vitamin D metabolite concentrations were not different in the two groups. Histomorphometric study of bone formation and resorption was made on undecalcified sections of iliac crest bone biopsies obtained after in vivo single or dual tetracycline labeling. Bone histology revealed that both groups of patients had comparable osteomalacia, as evidenced by increased amount of osteoid tissue, prolonged mineralization lag time, and reduced bone formation rate. Despite the presence of osteomalacia, the trabecular calcified bone volume was within or above normal values in the two groups, implying a remodeling imbalance between the rates of bone resorption and formation. The data show that despite the absence of symptoms and the disappearance of rickets, adults with VDRR still have active bone disease characterized by moderate to severe osteomalacia. The normal to increased trabecular bone mass implies that the occurrence of painful symptoms results from factors other than trabecular osteopenia. These observations thus lead one to question the utility of active medical treatment with vitamin D and/or phosphate in asymptomatic adults with VDRR.

Inadequate Bone Response to Phosphate and Vitamin D in Familial Hypophosphatemic Rickets (FHR)

With the almost complete disappearance of vitamin D deficiency in the North-American continent, most cases of rickets now encountered on the pediatric wards may be attributed to one of the several states of vitamin D resistance. The most common of them is familial hypophosphatemic rickets (FHR) a condition usually transmitted as a sex-linked dominant trait. It is characterized by a hypophosphatemia present at birth, bone changes resembling vitamin D deficiency rickets and growth retardation. Bony deformities of the lower limbs are frequent but myopathy is strikingly absent (1).

Renal Osteodystrophy in Children Treated with 1,25-Dihydroxy-Cholecalciferol [1,25-(OH)2D3]: Histologic Bone Studies

ABSTRACT. Eleven uremic children with osteodystrophy aged 3 to 17 years were studied during administration of l, 25‐(OH)2D, for periods up to 21 months. Nine children presented with pure hyperparathyroidism, one with osteomalacia and one with mixed bone disease. Bone biopsies were performed before initiation of therapy and after 6 to 21 months of treatment following double tetracycline labeling. Skeletal lesions were improved but not cured in 5 of 9 children with hyperparathyroidism. In three instances lesions remained unchanged and worsened in one. No significant change was observed in the child with osteomalacia. Moderate improvement was noted in the patient with mixed bone disease. The propensity to develop hypercalcemia was the major factor associated with treatment failure since it precluded administration of adequate amounts of medication. Therapy with l, 25‐(OH)2D3 was associated with a spectacular improvement in growth velocity in two of six children under age twelve.