Alteration of mineral crystallinity and collagen cross-linking of bones in osteopetrotic toothless (tl/tl) rats and their improvement after treatment with colony stimulating factor-1

A common feature of various types of mammalian osteopetroses is a marked increase in bone mass accompanied by spontaneous bone fractures. The toothless (tl/tl) rat osteopetrotic mutation is characterized by drastically reduced bone resorption due to a profound deficiency of osteoclasts and their precursors. An altered bone morphology has also been observed. The mutants cannot be cured by bone marrow transplantation, but skeletal defects are greatly reduced after treatment with colony stimulating factor 1 (CSF-1). The objectives of this study were to characterize mineral and collagen matrices in cancellous and compact bone isolated from long bones of 6-week-old normal littermates, tl/tl osteopetrotic mutants and mutants (tl/tl) treated with CSF-1. There were no differences in bone mineral content, but a significant decrease in the crystallinity of mineral evaluated by the method based on electron paramagnetic resonance spectrometry was observed in all bones of tl/tl mutants as compared to that of controls. Within the collagen matrix, slight decreases in the labile cross-links, but significant increases in the content of the stable cross-links, pyridinoline, and deoxypyridinoline, were observed in both cancellous and compact bone of osteopetrotic mutants. In tl/tl mutants treated with human recombinant CSF-1, the normalization of the crystallinity of bone mineral as well as collagen cross-links was found. Our results indicate that remodeling of bone matrix in tl/tl mutants is highly suppressed, but that after treatment with CSF-1, this activity recovers significantly. Taken together, these data provide further support for the hypothesis that CSF-1 is an essential factor for normal osteoclast differentiation and bone remodelling.

Macrophage differentiation and granulomatous inflammation in osteopetrotic mice (op/op) defective in the production of CSF-1

Since the osteopetrotic (op/op) mouse was demonstrated to have a mutation within the coding region of the CSF-1 gene itself, it serves as a model for investigating the differentiation mechanism of macrophage populations in the absence of functional CSF-1. The op/op mice were severely monocytopenic and showed marked reduction and abnormal differentiation of tissue macrophages. Osteoclasts as well as marginal metallophilic macrophages and marginal zone macrophages in the spleen were absent. Most of the tissue macrophages were reduced in number and ultrastructurally immature. However, the degree of reduction in numbers of macrophages in the mutant mice was variable among tissues, suggesting that the heterogeneity of macrophages was generated by their different dependency on CSF-1. After daily CSF-1 injection, the numbers of monocytes, tissue macrophages, and osteoclasts were remarkably increased, and the macrophages showed morphological maturation. However, the numbers of macrophages in the ovary, uterus, and synovial membrane were not increased. In the bone marrow, macrophage precursors detected by monoclonal antibody ER-MP58 proliferated and differentiated into preosteoclasts and osteoclasts. In the spleen, marginal metallophilic macrophages and marginal zone macrophages developed slowly. In this manner, CSF-1 plays an important role in the development, proliferation, and differentiation of certain tissue macrophage populations and osteoclasts. In the op/op mice, Kupffer cells proliferated, transformed into epithelioid cells and multinucleated giant cells, and participated in glucan-induced granuloma formation. In CSF-1-treated op/op mice, the process of granuloma formation was similar to that in normal littermates due to increased monocytopoiesis and monocyte influx into the granulomas. These results indicate that CSF-1 is a potent inducer of the development and differentiation of CSF-1-dependent monocyte/macrophages, and that CSF-1-independent macrophages also play an important role in granuloma formation.

Thrombocytopenia in the toothless (osteopetrotic) rat and its rescue by treatment with colony-stimulating factor-1

Osteopetrosis in toothless (tl) rats is characterized by reductions in bone resorption, osteoclasts, and macrophages, resistance to cure by bone marrow transplantation, and skeletal improvement after treatment with colony-stimulating factor-1 (CSF-1). Reductions in skeletal osteocalcin tl rats, together with the recent demonstration of osteocalcin expression in platelets and its possible role in bone turnover, prompted us to examine whether this rat mutation is associated with altered platelet numbers. Our prediction of a thrombocytopenia was confirmed by examination of tl rats, in which a profound reduction (32%) in platelets was accompanied by a significant elevation (62%) in megakaryocytes (MKC) compared to normal littermates. Light and transmission electron microscopy confirmed increases in both number and size of MKC in mutants without morphologic abnormalities of circulating platelets. CSF-1 treatment (10(6) U/48 hours for 10 days) of mutants restored platelet numbers to those found normal littermates and increased osteoclasts and the frequency of MKC in numbers. Preliminary studies of another mutation the rat, osteopetrosis (op), revealed a similar reduction (33%) in platelets. These data demonstrate the coexistence of osteopetrosis and thrombocytopenia in two osteopetrotic rat mutations and an increase in osteoclasts and platelets in one mutation after CSF-1 treatment. Together, these data suggest a potential functional interaction of MKC and osteoclasts in bone turnover.

Neonatal changes of osteoclasts in osteopetrosis (op/op) mice defective in production of functional macrophage colony-stimulating factor (M-CSF) protein and effects of M-CSF on osteoclast development and differentiation

In mice homozygous for the osteopetrosis (op) mutation, loss of osteoclasts in the postnatal period and their development, differentiation, and maturation following daily M-CSF administration in adult life were investigated. Histochemical, immunohistochemical, and ultrastructural approaches, as well as [3H]thymidine autoradiography, clarified the role of M-CSF on osteoclast development and differentiation. In op/op mice osteoclasts appeared normal at birth. However, osteoclast numbers were reduced within a few days after birth, and osteoclasts were undetectable by 3-4 days of age. In adult op/op mice there were no multinuclear osteoclasts; however, small numbers of mononuclear cells (so-called 'preosteoclasts') were observed on the endosteal surface of bone. These preosteoclasts expressed tartrate-resistant acid phosphatase and showed ultrastructural features of immature osteoclasts. After daily M-CSF administration in op/op mice, osteoclasts developed from the fusion of preosteoclasts and osteoclasts numbers increased to the levels of normal littermates at 3 days. Autoradiographic analysis with [3H]thymidine revealed no labeling in osteoclasts and preosteoclasts. In the mutant mice, M-CSF administration induced numerical increases of monocytes, promonocytes, and earlier precursor cells in bone marrow, ER-MP12- or, ER-MP58-positive granulocyte/macrophage colony-forming cells (GM-CFCs). Among these macrophage precursors, ER-MP58-positive cells were considered preosteoclast precursors, and possessed marked proliferative potential. These data suggest that an ER-MP58-positive cell subpopulation of GM-CFCs proliferates in response to M-CSF, differentiates into preosteoclasts which fuse with each other to develop into mature osteoclasts.

Effects of vitamin D binding protein-macrophage activating factor (DBP-MAF) infusion on bone resorption in two osteopetrotic mutations

Osteopetrosis is a heterogeneous group of bone diseases characterized by an excess accumulation of bone and a variety of immune defects. Osteopetrosis (op) and incisors absent (ia) are two nonallelic mutations in the rat which demonstrated these skeletal defects as a result of reduced bone resorption. Osteopetrotic (op) rats have severe sclerosis as a result of reduced numbers of osteoclasts which are structurally abnormal. The sclerosis in ia rats is not as severe as in op mutants; they have elevated numbers of osteoclasts, but they are also morphologically abnormal, lacking a ruffled border. Both of these mutations have defects in the inflammation-primed activation of macrophages. They demonstrate independent defects in the cascade involved in the conversion of vitamin D binding protein (DBP) to a potent macrophage activating factor (DBP-MAF). Because this factor may also play a role in the pathogenesis of osteoclastic dysfunction, the effects of ex vivo-generated DBP-MAF were evaluated on the skeletal system of these two mutations. Newborn ia and op rats and normal littermate controls were injected with DBP-MAF or vehicle once every 4 days from birth until 2 weeks of age, at which time bone samples were collected to evaluate a number of skeletal parameters. DBP-MAF treated op rats had an increased number of osteoclasts and the majority of them exhibited normal structure. There was also reduced bone volume in the treated op animals and an associated increased cellularity of the marrow spaces. The skeletal sclerosis was also corrected in the ia rats; the bone marrow cavity size was significantly enlarged and the majority of the osteoclasts appeared normal with extensive ruffled borders.

CSF-1 treatment promotes angiogenesis in the metaphysis of osteopetrotic (toothless, tl) rats

It has recently been shown that following treatment with colony-stimulating factor-1 (CSF-1) the osteopetrotic condition in toothless (tl) rats greatly improves and growth is accelerated. We have examined the effects of such treatment on the microvasculature of the distal femoral chondro-osseous junction, a site where bone growth in length is coordinated with angiogenesis. Vascular casts and ultrastructural analyses of this region showed that, compared to untreated normal rats, untreated mutants showed little bone growth or angiogenesis. When mutants were treated with CSF-1 angiogenesis was markedly accelerated. These data show a remarkable effect of this growth factor on angiogenesis in this osteopetrotic mutation. Whether this effect of CSF-1 on angiogenesis is direct or indirect is not known and indicates that its effects on the normal microvasculature deserve further study.

Colony-stimulating factor-1 injections improve but do not cure skeletal sclerosis in osteopetrotic (op) mice

The osteopetrotic (op) mutation in mice is characterized by general skeletal sclerosis; reduced numbers of osteoclasts, macrophages, and monocytes; and failure to be cured by bone marrow transplantation. This mutation has been shown to result from an absence of colony-stimulating factor-1 (CSF-1) and reported to be cured by treatment with CSF-1. Contrary to previous reports, we have noted persistent metaphyseal sclerosis in op mice treated with CSF-1 at doses above physiological concentrations of circulating CSF-1. We pursued this observation by quantitating osteoclasts and macrophages in the first 500 microns (area A) and the subsequent 1000 microns (area B) in the proximal tibial metaphysis using tartrate-resistant acid phosphatase and F4/80 as cell markers. In untreated normal mice, osteoclasts and macrophages were found in areas A (9.1 and 13.8 cells/1000 microns2) and B (4.1 and 9.4 cells/1000 microns2), respectively. In untreated mutants, osteoclasts and macrophages as percentages of normal were, respectively, 0% and 2% (area A) and 30% and 13% (area B). After CSF-1 treatment (0.15, 0.3, 0.5, and 1.0 x 10(6) U/day) for 28 days, marrow cavity size and numbers of osteoclasts and macrophages increased significantly in area B. However, area A remained sclerotic, with few macrophages (3% to 20%), and although osteoclast numbers were normal, their distribution was not, being absent in subepiphyseal sites. High CSF-1 gene expression occurs at bone modeling sites, co-localizes with osteoblasts, and temporally correlates with their differentiation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of 1,25-dihydroxyvitamin D3 on bone resorption and natural immunity in osteopetrotic (ia) rats

Osteopetrois is an inherited bone disease characterized by an excessive accumulation of bone throughout the skeleton. The disease in the ia (incisors absent) rat is the result of reduced bone resorption caused by defective, although numerous osteoclasts. In addition to the bone defects, ia rats have suppressed natural immunity, even though these animals have excessive numbers of natural killer (NK) cells. The osteopetrotic condition also appears to have an associated abnormality in vitamin D metabolism. Because 1,25-dihydroxyvitamin D3[1,25-(OH)2D3] stimulates bone resorption and has a role in the immunoregulation of NK cells, mutant and normal rats were infused with 1,25-(OH)2D3 for 14 days in an attempt to correct the defects in this mutant. Serum levels of osteocalcin, 25-OHD3, and 1,25-(OH)2D3, as well as NK function and parameters of bone resorption, were evaluated after the infusion period. Serum levels of osteocalcin and 1,25-(OH)2D3 were elevated in both ia and normal rats treated with 1,25-(OH)2D3. Serum 25-OHD3 levels were significantly reduced in the treated animals. The elevated percentage of NK cells normally found in ia rats was reduced to normal in the treated mutants, and NK cell function was elevated to normal levels of lytic activity. The percentage of NK cells and NK function remained unchanged in the treated normal rats. The bone marrow cavity size was significantly increased in the 1,25-(OH)2D3-treated mutants, as was the percentage of osteoclasts exhibiting normal morphology. Radiographically, the mutant bones were less dense.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructure of macrophages and dendritic cells in osteopetrosis (op) mutant mice lacking macrophage colony-stimulating factor (M-CSF/CSF-1) activity

The ultrastructural features of macrophages and dendritic cells of mice homozygous for osteopetrosis (op/op) mutation were studied. The mutant mice are characterized by defective differentiation of osteoclasts, monocytes, and tissue macrophages due to the lack of functional macrophage colony stimulating factor (M-CSF/CSF-1) activity. In op/op mice, tissue macrophages were reduced in number and smaller than in normal littermates. Macrophages in op/op mice showed various degrees of phagocytosis but the development of intracytoplasmic organelles and microvillous projections was poor. After administration of CSF-1 daily for 2 weeks, macrophages in op/op mice developed lysosomes and microvillous projections. In the thymic medulla, T-cell zone of lymph nodes, splenic white pulp and epidermis of the op/op mice, the number of dendritic cells was similar to that in normal littermates and the dendritic cells developed a tubulovesicular system typical of interdigitating cells. Birbeck granules in epidermal Langerhans cells were detected in unmanipulated op/op mice, op/op mice injected with CSF-1, and normal littermates or control mice. However, in untreated op/op mice, dendritic cells projected shorter cytoplasmic processes than in normal littermates, normal control mice and CSF-1 injected op/op mice. These results indicate that the differentiation and maturation of tissue macrophages are mediated by CSF-1, but the dendritic cell differentiation is controlled by other factor(s) than CSF-1, most probably by GM-CSF.

Congenital erythropoietic porphyria: skeletal manifestations and effect of pamidronate treatment

Congenital erythropoietic porphyria (CEP) is a rare disorder of heme biosynthesis. Skeletal abnormalities have been described in patients with this disease. We report a 25-year-old woman with osteodystrophy from CEP. On examination, mild hepatosplenomegaly, multiple hyperpigmented scars, hypertrichosis, erythrodontia and red coloration of urine were found. Biochemical studies showed increased serum levels of alkaline phosphatase, fasting and total 24-h urinary calcium excretion. Serum 250H vitamin-D concentration was low due to avoidance of sun exposure. Skeletal radiographs disclosed marked vertical and horizontal trabecular pattern and biconcavity of most of the dorsal and lumbar vertebral bodies. Several round sclerotic lesions (1-3 cm in diameter) were seen in the skull, pelvis and one lumbar vertebrae. The sclerotic lesions were augmented in size and number compared to X-rays obtained 8 years before. Bone mineral density (evaluated by DEXA) was markedly reduced at the spine and moderately diminished at the proximal femur and total skeleton. Treatment for 11 months with pamidronate (and the addition of hydrochlorotiazide for the last 6 months) reduced to normal values the serum levels of alkaline phosphatase and fasting urinary calcium. The 24-h urinary excretion of calcium and hydroxyproline were also decreased. The BMD increased in all the skeletal areas with presumably hyperactive bone marrow: spine, head, ribs and pelvis (and total skeleton), but did not change at the extremities and diminished at the femoral neck. Patients with CEP may present osteodystrophy characterized by sclerotic lesions and osteopenia, most likely due to accelerated bone turnover in areas of active bone marrow.(ABSTRACT TRUNCATED AT 250 WORDS)

Failure of calcitriol treatment in a patient with malignant osteopetrosis

In an attempt to stimulate bone resorption, a 10-week-old infant with malignant infantile osteopetrosis was treated with high doses of calcitriol, a potent bone resorption stimulatory agent, combined with a low calcium diet to prevent hypercalcaemia. Although calcitriol administration was initiated at this very young age, our patient did not show any clinical, radiological, or histological improvement. Despite reports of positive results of this treatment in the literature, our patient did not reveal any signs of bone resorption. She eventually died from the complications of osteopetrosis at the age of 6 months after 88 days of therapy.

Osteopetrosis. The pharmaco-physiologic basis of therapy

Medical treatments of osteopetrosis have attempted to improve hematologic function, reduce the osteosclerotic condition, and/or improve immune function. Prednisone therapy has improved hematologic function in some patients, but has not resulted in a reduction in bone mass. Calcium deficient diets have limited further sclerosis in some patients. High-dose calcitriol and parathormone infusions have stimulated osteoclastic activity. In some patients, high-dose calcitriol has resulted in clinical improvement. Newer treatments, such as interferon gamma and macrophage colony stimulating factor, may alter the osteoclastic and immune defects by stimulating cellular formation and function. These therapies, alone or in combination, ameliorate but do not cure the osteopetrotic condition.

The skeletal effects of colony-stimulating factor-1 in toothless (osteopetrotic) rats: persistent metaphyseal sclerosis and the failure to restore subepiphyseal osteoclasts

Toothless (tl), one of four osteopetrotic mutations in the rat, is characterized by few osteoclasts, undetectable bone resorption, and failure of correction by bone marrow transplantation. We recently reported that CSF-1 treatment improves these skeletal problems but that metaphyseal sclerosis persists. In the present study we demonstrate that optimal reduction of the skeletal sclerosis in tl rats following CSF-1 treatment has lower and upper dosage thresholds and that skeletal sclerosis returns after CSF-1 withdrawal. Osteoclasts increase significantly in tl rats after CSF-1 treatment, but compared to untreated normal littermates, histochemical staining for characteristic enzymes and osteoclast number is reduced and no osteoclasts appear in the subepiphyseal areas of long bones. These data are interpreted to mean that there are dosage limits to the beneficial skeletal effects of CSF-1, that persistent sclerosis is related to the failure to restore subepiphyseal osteoclasts, that osteoclast or progenitor populations may be deficient or differ in their responses to CSF-1, and that the defect in tl rats is not merely lack of a circulating, biologically active form of CSF-1.

Combination macrophage-colony stimulating factor and interferon-gamma administration ameliorates the osteopetrotic condition in microphthalmic (mi/mi) mice

Malignant osteopetrosis is a fatal congenital bone disorder characterized by defective osteoclastic function. Death frequently occurs within the first decade of life. The precise molecular defect(s) that causes osteopetrosis is not known. The possibility that osteoclasts, like macrophages, are controlled by interactions with cytokines suggests that these agents may provide a means of increasing osteoclastic function. Macrophage-colony stimulating factor (M-CSF), a cytokine known to enhance macrophage and osteoclast generation, and recombinant human interferon-gamma (rIFN), a cytokine known to stimulate superoxide generation by white cells, were administered to microphthalmic (mi/mi) mice in an attempt to improve the osteopetrotic condition. Each cytokine was administered separately and in combination to neonatal mi/mi mice for 7 consecutive d. Bone turnover, osteoclast numbers, superoxide generation by white cells, and hematocrit were assessed. rIFN, M-CSF, and a combination of the cytokines stimulates oxygen-derived free radical production by white cells and increased bone resorption. rIFN resulted in a reduction in the number of osteoclasts. This reduction in number was ameliorated by M-CSF. M-CSF alone and in combination with rIFN resulted in improved hematopoietic function, increased weight gain, and increased physical activity of the affected mutants.

[Early infantile malignant osteopetrosis. Experiences with drug therapy]

The autosomal recessive form of osteopetrosis becomes manifest in early infancy and often is lethal during the first decade of life. In an infant diagnosed at the age of eight weeks, a therapy with Calcitriol (Rocaltrol) was initiated with the intention to stimulate the osteoclast activity and to improve the bone marrow insufficiency. During therapy, a massive diarrhoea has been observed as potential and previously undescribed side effect of Calcitriol. Therefore the therapy had to be discontinued. A simultaneous trial with Ca-depleted diet did not show any effect. In the absence of a suitable donor, a bone-marrow transplantation could not be performed. The patient died at the age of five month.

The effects of colony-stimulating factor-1 on tooth eruption in the toothless (osteopetrotic) rat in relation to the critical periods for bone resorption during tooth eruption

The toothless (tl) rat is an osteopetrotic mutation characterized by a generalized skeletal sclerosis, reduced bone resorption, few osteoclasts and a total absence of erupted teeth. This mutation is not cured by bone marrow transplants from normal littermates. It is known that the skeletal defects in tl rats are greatly improved after treatment with colony-stimulating factor-1 (CSF-1). This investigation concerns the effects of CSF-1 on the development and eruption of the dentition of tl rats. Untreated tl rats had no erupted teeth by 56 days after birth, and the roots of incisors and molars were severely distorted by compression against bone. The apex of the mandibular incisor did not extend past the first molar and continued growth of its apical end produced odontoma-like masses consisting of distorted dentine and enamel matrices. In addition, few osteoclasts were seen on alveolar bone surfaces surrounding the developing teeth. Mutants given CSF-1 were characterized by delayed eruption of all molars and sometimes incisors. The incidence of incisor eruption was related inversely to the age at which CSF-1 treatment began. Molars of treated tl rats had well-developed roots similar to those in normal rats. Treated mutants had numerous osteoclasts in alveolar bone and well-developed haemopoietic marrow spaces in the mandible. Histochemical staining for both tartrate-resistant acid phosphatase and tartrate-resistant acid ATPase was reduced or negligible in osteoclasts of untreated tl rats, heavy in normal osteoclasts and of intermediate intensity in CSF-1-treated mutants.(ABSTRACT TRUNCATED AT 250 WORDS)

Administration of colony stimulating factor-1 corrects some macrophage, dental, and skeletal defects in an osteopetrotic mutation (toothless, tl) in the rat

The toothless (tl/tl) mutation in the rat results in a paucity of osteoclasts and osteopetrosis that cannot be corrected by bone marrow transplantation. In the present study we demonstrate that tl/tl rats also have profound deficiencies of femoral, peritoneal, and pleural cavity macrophages. Furthermore, the macrophage colony stimulating activity of post-endotoxin sera from tl/tl rats is substantially reduced, suggesting that, as in the case of the op mutation in mice, the basis of the tl mutation is a deficiency of the macrophage growth factor, colony stimulating factor-1 (CSF-1). Consistent with this suggestion, treatment of tl/tl rats from birth for up to six weeks with CSF-1 reduced the osteopetrosis, increased body weight, and permitted tooth eruption. In addition, CSF-1 treatment induced large numbers of osteoclasts in tl/tl bones and macrophages in the peritoneal cavity and bone marrow. Persistence of metaphyseal sclerosis, however, indicated that the disease was not totally corrected by this treatment. These studies indicate that the basis of the tl mutation is most likely another CSF-1 deficiency, and further emphasize the role of this growth factor in osteoclast differentiation.

Imaging rounds #110. Osteopetrosis

The following case is presented to illustrate the roentgenographic and clinical findings of a condition of interest to the orthopaedic surgeon. Initial history, physical findings, and roentgenographic examinations are found on this page. The next page presents the final clinical and roentgenographic differential diagnosis.

Correction by CSF-1 of defects in the osteopetrotic op/op mouse suggests local, developmental, and humoral requirements for this growth factor

Mice that are mutant at the op locus have a severe deficiency of mononuclear phagocytes due to an inactivating mutation in the CSF-1 (macrophage colony-stimulating factor, M-CSF) gene. op/op mice are toothless, possessing skeletal abnormalities, a low body weight, and compromised fertility; they are osteopetrotic due to a deficiency of osteoclasts. The congenital osteopetrosis, toothless phenotype, osteoclast deficit, and the defects in splenic and femoral macrophages were corrected by routes of administration of human recombinant CSF-1 that maintained normal circulating CSF-1 concentrations. Early restoration of circulating CSF-1 was required for rescue of the toothless phenotype, but only partially restored body weight. In contrast, the deficiencies of pleural and peritoneal cavity macrophages and the reduced female fertility were not corrected by restoration of circulating CSF-1. These results suggest that although circulating CSF-1 is required for osteoclast and macrophage production, local synthesis and action of the growth factor are important for certain target cell populations.

Treatment of congenital osteopetrosis in the rabbit with high-dose 1,25-dihydroxyvitamin D

Osteopetrosis is a congenital metabolic bone disease characterized by skeletal sclerosis resulting from defective osteoclast-mediated bone resorption. Osteopetrosis has been described in several animal species (mouse, rat, and rabbit) and in children. Bone marrow transplantation, originally shown to reverse the skeletal sclerosis in some animal mutations, has been effective in curing osteopetrosis in some children. Unfortunately, not all children with osteopetrosis are candidates for or respond to bone marrow transplantation. Recent studies have shown that several animal mutations and some children inheriting osteopetrosis have significantly elevated serum levels of 1,25-(OH)2D. Based on the possibility that there may be a resistance to 1,25-(OH)2D, high-dose calcitriol therapy has been used to treat some children and stimulated some parameters of resorption. In this study, we have examined the effects of high-dose calcitriol therapy on various serum and skeletal parameters in the osteopetrotic rabbit. Mutant rabbits and normal littermates were given continuous infusions of calcitriol via subcutaneously implanted osmotic minipumps for 2 weeks at a dose of 0.5, 2.5, or 25 micrograms/kg/per day. Untreated mutant rabbits are hypocalcemic and hypophosphatemic in the presence of elevated serum 1,25-(OH)2 levels in comparison with their normal littermates. Calcitriol infusions resulted in dose-dependent increases in circulating 1,25-(OH)2D levels in both normal and mutant rabbits. However, evaluation of other serum parameters and the skeletal response demonstrated significant differences between osteopetrotic and normal rabbits. At the highest dose, normal animals rapidly became hypercalcemic and osteoporotic, accompanied by weight loss and a failure to thrive; mutants remained hypocalcemic and osteopetrotic but did not exhibit the deleterious physical effects seen in treated normal littermates. Although the number of osteoclasts increased in both mutants and normals, osteoclast phenotype in the former remained abnormal. These data indicate that although very high levels of circulating 1,25-(OH)2D were achieved in osteopetrotic mutants, activation of osteoclast-mediated bone resorption with subsequent improvement of skeletal sclerosis was not observed.

Juvenile osteopetrosis: effects on blood and bone of prednisone and a low calcium, high phosphate diet

Four children with juvenile osteopetrosis are described who were treated with a combination of prednisone and a low calcium, high phosphate diet. One of the children, treated as a neonate, achieved complete clinical and radiological remission from the disease after nine months, at which point treatment was stopped. There have been no signs of recurrence for two years. Two who did not start treatment until over 24 months of age have shown a good clinical and radiological response but have remained on treatment for six years. The fourth child started treatment at 6 months and showed a good clinical response, but x ray films showed no change nine months later. He was then lost to follow up, stopped treatment, and died two years later of a septicaemia. These patients provide further evidence for the efficacy of steroids in juvenile osteopetrosis, and the combination with the low calcium, high phosphate diet described offers a potentially effective alternative treatment to marrow transplantation, both for the haematological and skeletal complications of the disorder.