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

Fracture Patterns and Mortality in Osteopetrosis: A 7-year Retrospective Analysis from Türkiye's National Registry

BACKGROUND

The aim of this study is to determine the demographic data, fracture treatment methods, and medical treatments of patients diagnosed with osteopetrosis in the national registry.

METHODS

Patients with International Classification of Diseases (ICD)-10 code Q78.2 for osteopetrosis between January 1, 2016 and April 11, 2023 were retrospectively reviewed. Data on sex, age at time of diagnosis, fracture history, mortality, and use of medications were evaluated for all patients. In addition, open reduction and internal fixation, closed reduction and internal fixation, closed reduction and casting, and conservative treatment methods were noted. The number of patients requiring deformity surgery was determined. The incidence and prevalence of osteopetrosis were also calculated in this cross-sectional study.

RESULTS

A total of 476 patients diagnosed with osteopetrosis were identified. The mean age at time of diagnosis of these patients was 5.79 ± 5.43 years. A total of 101 patients died. As the age at diagnosis decreased, the mortality rate of the patients increased with statistical significance ( P <0.001). A total of 192 fractures were seen in 121 osteopetrosis patients in this study. Femur fractures were most common among these patients with osteopetrosis. A history of fracture was statistically significantly less common in patients using a combination of vitamin D + calcium compared with patients not using such medication ( P <0.001). In this 7-year cross-sectional study, the incidence was found to be 1 in 416,000 and the prevalence was 0.00199% in the population under 18 years of age.

CONCLUSION

Younger age at diagnosis is associated with higher mortality in patients with osteopetrosis. In addition, the combination of vitamin D and calcium were associated with lower fracture incidence.

LEVEL OF EVIDENCE

Prognostic Level II.

Interferon Gamma-1b Does Not Increase Markers of Bone Resorption in Autosomal Dominant Osteopetrosis

In autosomal dominant osteopetrosis type 2 (ADO2) CLCN7 mutations cause impaired osteoclast function. Severe consequences include skeletal fragility despite high bone mass, osteomyelitis, osteonecrosis, bone marrow failure, and severe cranial nerve impingement. There is no effective medical treatment for ADO2. We recruited subjects with ADO2 into a 14-week, open-label, pilot clinical trial of interferon gamma-1b. Doses were titrated based on tolerability and if fasting serum C-telopeptide (CTX) was <25% above baseline at week 8, targeting doses of 100 µg/m² three times a week. The primary outcomes were change from baseline in CTX and N-telopeptide/creatinine ratio (NTX/Cr) at week 14. Secondary outcomes included changes in urine calcium/creatinine ratio, bone formation markers and tolerability. Nine adults and three children were recruited. Severe manifestations of ADO2 included histories of fractures (100%), osteomyelitis (16.7%), vision loss (50%), and anemia (58.3%). Baseline CTX and NTX/Cr were generally low-normal. Procollagen type I N-terminal propeptide was elevated or in the upper-normal range in 11 of 12 (91.6%) subjects. Elevations of aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) were common. One subject withdrew due to rash. Five subjects achieved doses of 50 µg/m² 3 days a week, while six reached the full dose of 100 µg/m² 3 days a week. Only 3 of 11 (27.3%) completing subjects achieved the primary outcome of increasing CTX ≥25% above baseline at week 14. The mean ± SD change from baseline in CTX at week 14 was +2.2% ± 43.2%, p = 0.86). Likewise, there was no significant change in NTX/Cr (mean change -2.1%, p = 0.81). Interferon gamma-1b was poorly tolerated. Most subjects had adverse events, and the Mental Health and Mental Component Scales of the SF-36v2 health survey declined slightly (p < 0.05). Over 14 weeks, interferon gamma-1b failed to significantly increase bone turnover markers in ADO2 and was poorly tolerated. Consequently, interferon gamma-1b is unlikely to be effective for decreasing bone mass in ADO2. © 2019 American Society for Bone and Mineral Research.

Hypophosphatemia and the development of rickets in osteopetrotic (op/op) mice

Our previous work has shown that op/op mice hyperabsorb dietary calcium in the vitamin D-deficient state and shunt that calcium into bone. Under these conditions, the op/op mice are hypocalcemic. The purpose of this study was to examine calcium metabolism and bone mineralization in vitamin D-deficient op/op mice. First, the op/op mice and their normal littermates were placed on a vitamin D-deficient, low phosphorus diet to limit bone mineralization. Under these circumstances, op/op mice survived, even when calcium was also removed from the diet. If the diet contained phosphate, op/op mice died from hypocalcemic tetany when calcium was also removed from the diet. Furthermore, serum calcium levels became similar to wild type in the op/op mice administered the vitamin D-deficient, low phosphorus diet, and op/op mice were able to increase serum calcium in response to 1,25-dihydroxyvitamin D3. The op/op mice developed rickets when their serum phosphorus level was too low to support bone mineralization. The op/op mice became hypophosphatemic on regimens in which normal mice were able to maintain normal serum phosphorus levels. It appears that the op/op mouse simply requires a higher dietary calcium and phosphorus level to prevent rickets and hypocalcemic tetany since the bone is not available as a source of these minerals. However, the ability of the op/op mouse to mineralize bone at low serum calcium and phosphorus levels remains unexplained.

The heterogeneity of the osteopetroses reflects the diversity of cellular influences during skeletal development

Experimental studies of the mammalian osteopetroses, characterized by generalized skeletal sclerosis, have illuminated a variety of mechanisms by which bone resorption can be reduced. We review recent data implicating a diverse group of growth factors, proto-oncogenes, and immune regulators that can influence skeletal development and account for the heterogeneity of the osteopetroses. Furthermore, similar studies are likely to continue to provide for improved clinical management of both osteopetrotic children and the localized and generalized osteopenias.

Interdependence of skeletal sclerosis and elevated circulating levels of 1,25-dihydroxyvitamin D in osteopetrotic (op and tl) rats

Osteopetrosis describes a heterogeneous group of inherited, metabolic bone disorders characterized by reduced bone resorption which coexists with elevated circulating levels of 1,25-dihydroxyvitamin D [1,25(OH)2D]. To determine whether or not skeletal sclerosis and high concentrations of 1,25(OH)2D are interdependent, this study used two distinct, nonallelic osteopetrotic mutations in the rat, osteopetrosis (op) and toothless (tl). The op rat is a mutation in which skeletal sclerosis can be cured (mutant) or induced (normal) following the transfer of normal or mutant osteoclast progenitors, respectively. Although these procedures are ineffective in rats of tl stock, infusions of pharmacological doses of macrophage colony-stimulating factor (CSF-1) can stimulate bone resorption and eliminate most of the excess skeletal matrix in tl mutants. This study examined the effects of cure/induction in neonatal mutant/normal rats of op stock and CSF-1 infusions in mutant rats of tl stock on skeletal (bone resorption) and serum [1,25(OH)2D] parameters as a function of time after treatment. Osteopetrotic mutants transplanted (cured) with normal spleen cells demonstrated cellular changes in osteoclast phenotype within 2-3 days followed by histologic and radiographic evidence for increased bone resorption that culminated in a normal appearance of the skeleton by 4 weeks. The markedly elevated serum levels of 1,25(OH)2D observed in untreated mutants fell significantly in transplanted mutants by the end of the first week and were similar to those in normal littermates at 3 and 4 weeks. Normal littermates transplanted (induced) with mutant spleen cells showed a progressive increase in skeletal sclerosis paralleled by significant increases in circulating levels of 1,25(OH)2D.(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)

Mineral metabolism in infants with malignant osteopetrosis: heterogeneity in plasma 1,25-dihydroxyvitamin D levels and bone histology

A group of 16 infants, 2 weeks to 11 months old, with malignant osteopetrosis were investigated to examine their vitamin D metabolism and parathyroid function. Bone biopsies from 6 children were studied by light microscopic histomorphometry and by electron microscopy. Considerable heterogeneity existed among the patients with respect to the parameters reflecting mineral metabolism and with respect to the histological manifestations of the disease. The most constant findings were as follows. Immunoreactive parathyroid hormone (iPTH) was elevated in all children, except in 1 patient who had tubular acidosis, and plasma calcium was low or normal, suggesting skeletal resistance to PTH. Plasma 1,25-dihydroxyvitamin D [1,25-(OH)2D] was not constantly elevated and appeared to depend on plasma phosphorus, as both parameters were negatively correlated (r = 0.704, p less than 0.01). Osteoblast activity, as evaluated by circulating alkaline phosphatase and osteocalcin and osteoblast number, measured for 6 children by bone histology, were not increased, despite hyperparathyroidism, suggesting PTH resistance or defective osteoblasts. Osteoclasts could be detected in 5 of the 6 children who had a biopsy. Osteoclast number (5.7-13.3% of bone surface) was normal or mildly increased, and marrow spaces were relatively well developed in 4 patients, whereas 1 child had markedly increased osteoclast number (28.3% of bone surface) and reduced marrow cavities. These 5 children received transplants, and engraftment occurred in all, except in the "hyperosteoclastic" patient. Further studies are necessary to establish the prognostic significance of this histologic feature.

Congenitally osteosclerotic (os/os) rabbits are not cured by bone marrow transplantation from normal littermates

The osteopetrotic (os) rabbit is a lethal mutation of autosomal recessive inheritance characterized by hypocalcemia, hypophosphatemia, fibrosis of marrow spaces, and ultrastructural abnormalities in both osteoclasts and osteoblasts. Procedures involving the transplantation of cells from normal hemopoietic tissues, which are sources of osteoclast precursors, are known to cure osteopetrosis in several mutations including some children. We tested the ability of transplanted bone marrow and/or spleen from normal littermates to reverse the skeletal sclerosis in os rabbits. Treatment of 15 neonatal mutants consisted of immunosuppression by whole-body irradiation followed by transplantation of normal bone marrow and/or spleen cell suspensions. This treatment failed to prolong life span or to cure osteopetrosis judged radiographically and histologically for up to 3 weeks posttreatment, the longest time of survival. These data indicate that transplantation of stem cells from multiple hemopoietic tissues, procedures known to cure osteopetrosis in other mutations, is not effective in the os rabbit. These results support the hypothesis that the skeletal microenvironment is not capable of supporting the development and function of normal osteoclasts in this mutation.

Decreased thyroid hormone-stimulated oxygen consumption and glucose uptake in mononuclear blood cells from patients with autosomal dominant osteopetrosis type I

Nine patients, from four different families, with autosomal dominant osteopetrosis were investigated. They all had roentgenological type I disease, characterized by universal, symmetrical osteosclerosis and enlarged thickness of the cranial vault. All patients appeared clinically euthyroid. Thyroxine (T4) and tri-iodothyronine (T3) induced oxygen consumption and glucose uptake were studied in vitro in mononuclear blood cells from patients and control persons. Unstimulated oxygen consumption from patients and controls did not differ, and no difference in unstimulated glucose uptake was observed. The increase in T4 and T3 stimulated oxygen consumption was significantly lower in cells from patients with osteopetrosis (T4: 0.007 +/- 0.004 mumol/mg DNA per h, T3: 0.011 +/- 0.004 mumol/mg DNA per h) compared with controls (T4: 0.017 +/- 0.003 mumol/mg DNA per h, T3: 0.023 +/- -0.013 mumol/mg DNA per h; p less than 0.05, p less than 0.05). Cellular glucose uptake after T4 and T3 stimulation was significantly lower in patients (T4: 0.032 +/- 0.017 mmol/l per mg DNA per h, T3: 0.02 +/- 0.017 mmol/l per mg DNA per h) compared with controls (T4: 0.09 +/- 0.017 mmol/l per mg DNA per h, T3: 0.08 +/- 0.01 mmol/l per mg DNA per h; p less than 0.05, p less than 0.01). The reduced oxygen consumption and glucose uptake indicate thyroid hormone resistance which may be of pathogenetic importance for the development of autosomal dominant osteopetrosis type I.

Effects of 1,25 dihydroxyvitamin D on osteoclast number and cytochemistry in normal and osteopetrotic (os) rabbits

Osteoclast-mediated bone resorption is increased in response to 1,25 dihydroxyvitamin D (1,25[OH]2D or calcitriol). Osteopetrosis is a metabolic bone disease characterized by defective, osteoclast-mediated bone resorption, which co-exists with elevated serum 1,25-(OH)2D levels in some osteopetrotic children and animals. We examined the effects of high doses of calcitriol on osteoclast number and cytochemistry in both normal and osteopetrotic (os) rabbits. Calcitriol was continuously infused at doses of 0.5, 2.5, or 25 micrograms/kg/day via subcutaneously implanted osmotic minipumps for a period of 7 days. Following treatment, the proximal tibial metaphyses were processed for histomorphometric and cytochemical analyses. Sections were stained for tartrate-resistant acid phosphatase (TrAP) or acid ATPase (TraATPase). Osteoclasts were significantly reduced in untreated os rabbits compared with age-matched normal littermates between birth and 3 weeks of age (41-46% of normal). Whereas most normal osteoclasts (85%) stained heavily for TrAP or TraATPase, less than half of os osteoclasts were heavily stained for these acid hydrolases. Infusions of 1,25(OH)2D resulted in elevations of osteoclast numbers in both normal and os rabbits, but the number of osteoclasts remained significantly lower in mutants than in normal littermates at any given dose. Calcitriol infusions also resulted in a significant increase in the percentage of os osteoclasts staining heavily for TrAP and TraATPase. These results suggest that in response to 1,25(OH)2D normal osteoclasts increase their production of acid hydrolases before increasing cell numbers and that, in spite of high levels of endogenous calcitriol, os rabbits can respond to exogenous 1,25(OH)2D as evidenced by increased osteoclast number and cytochemical staining, even though these osteoclasts fail to resorb the excess skeletal matrix.(ABSTRACT TRUNCATED AT 250 WORDS)

Defective osteoclast differentiation and function in the osteopetrotic (os) rabbit

We tested the ability of normal osteoclast progenitors found in neonatal liver and bone marrow to develop into functional osteoclasts when co-cultured with metatarsals from newborn osteopetrotic rabbits; the latter inherit an osteoclast incompetence resistant to cure by bone marrow transplantation. This system, developed by Burger and colleagues, has been shown to produce normal, functional osteoclasts when used with normal metatarsals. Our study tested the competence of the mutant skeletal microenvironment for differentiation of normal osteoclasts. Mutant and normal metatarsals were cultured alone or with normal liver, spleen, or bone marrow for up to 14 days. All normal cultures possessed a marrow cavity and contained numerous osteoclasts with cytochemical characteristics (tartrate-resistant acid phosphatase) of active cells. Mutant metatarsals co-cultured with normal spleen, liver, or bone marrow failed to develop a marrow cavity (evidence in itself of reduced bone resorption) and had osteoclasts reduced in both numbers and cytochemically detectable activity. Similar metatarsal cultures of an osteopetrotic rat mutation (incisors--absent) curable by bone-marrow transplantation exhibited marrow cavity development in mutant metatarsals co-cultured with normal spleen. These data suggest that the skeletal environment of osteopetrotic rabbits contains an inhibitor or lacks a promoter of osteoclast differentiation and function.

Relationship of abnormalities in dental and skeletal development in the osteopetrotic (os) rabbit

Osteopetrosis is a metabolic bone disease characterized by reduced bone resorption of heterogenous cause. The rabbit mutation is lethal and exhibits ultrastructural aberrations in osteoclasts and osteoblasts together with hypocalcemia, hypophosphatemia and failure to be cured by bone marrow transplantation. We have studied dental abnormalities in mutants from birth to 3 wk using radiographic, cytologic and autoradiographic methods. Radiographs show hypoplasia of most teeth in mutants. The maxillary incisor is smaller and more curved and the mandibular incisor thin and straight compared to normal littermates. The first 3 molars in both arches are unerupted and of distorted shape while the last 2 are less affected. Microscopically areas of ankylosis of mutant incisors and the first 3 molars were commonly encountered even at birth. Osteoclasts were numerous. Autoradiograms of 3H-proline incorporation showed strong periosteal and weak endosteal labeling of bone in both mutants and normal littermates. Dentin labeling in mutant incisors and molars was not less than that in normal rabbits and sites of ankylosis in mutants exhibited labeling. These data indicate that osteopetrotic rabbits exhibit major aberrations in shape and eruption of incisors and most molars and that these effects are not due to lack of dentin formation. Early ankylosis, perhaps secondary to congenital reduction of bone resorption, appears to be the major cause of the dental abnormalities which are less severe in the youngest (posterior) teeth.

Osteoclast biology in the osteopetrotic (op) rat

Osteopetrosis is a metabolic bone disease characterized by reduced bone resorption. From experimental studies of various osteopetrotic mutations has emerged the hypothesis that each is unique with respect to mechanisms whereby osteoclast development and/or function are reduced. The osteopetrotic (op) mutation in the rat was discovered in Fatty/ORL stock over a decade ago. The paucity of data about osteoclast biology in this mutation prompted this study of cytological, cytochemical, and ultrastructural features of osteoclasts. In op rats, osteoclasts are significantly reduced in number, but are larger and more vacuolated than in normal littermates. Mutant osteoclasts can form ruffled borders and clear zones, but their ability to fragment and excavate bone surfaces is greatly impaired. Cytoplasmic vacuoles in op osteoclasts are randomly distributed and greatly enlarged, and they stain weakly for two cytochemical characteristics of osteoclasts, tartrate-resistant acid phosphatase and acid ATPase. These findings suggest that an abnormality in the lysosomal/vacuolar system, an important component of the resorptive mechanism, may be involved in the interception of osteoclast function in this mutation.

Osteoclast biology: lessons from mammalian mutations

Major contributions to and confirmations of osteoclast biology have been made by experimental investigations of the osteopetrotic mutations in mammals. Congenital osteopetrosis is a bone disease characterized by a generalized increase in skeletal mass due to decreased osteoclast function. Abnormalities of skeletal growth and the failures of marrow cavity development and tooth eruption are secondary to reduced bone resorption of heterogeneous cause. Elucidation of pathogenetic pathways and unraveling of the cell biology of the osteoclast have proceeded hand-in-hand. This is illustrated by the variable differentiation and activation of osteoclasts among mutations and by demonstrations that the disease in certain animals and children can be cured by providing competent stem cells for osteoclasts via bone marrow transplantation. Congenital absence of carbonic anhydrase II (CA II) in children results in a syndrome that included osteopetrosis because osteoclasts are unable to function in the absence of CA II. The resistance of all mutations to the hypercalcemic effects of parathyroid hormone and recent reports of elevated blood levels of 1,25 dihydroxyvitamin D have broadened the scope of pathogenetic possibilities for osteopetrosis and regulatory possibilities for osteoclasts. Immunological effects including reductions in natural killer cell activity, superoxide and interleukin-2 production make osteopetrotic mutants potential models for studying the role of the immune system in osteoclast biology. Furthermore, coexistence of osteopetrosis with rickets and osteoblast abnormalities and the failure of cell transplants to cure the disease in some mutations illustrate the utility of the osteopetroses for exploring the role of matrix as mentor in osteoclast biology. Thus, understanding congenital osteopetrosis and osteoclast biology are likely to continue together.