Sclerosing bone dysplasias

The group of sclerosing bone dysplasia's is a clinically and genetically heterogeneous group of rare bone disorders which, according to the latest Nosology and classification of genetic skeletal disorders (2015), can be subdivided in three subgroups; the neonatal osteosclerotic dysplasias, the osteopetroses and related disorders and the other sclerosing bone disorders. Here, we give an overview of the most important radiographic and clinical symptoms, the underlying genetic defect and potential treatment options of the different sclerosing dysplasias included in these subgroups.

Unmistakable Morphology? Infantile Malignant Osteopetrosis Resembling Juvenile Myelomonocytic Leukemia in Infants

The initial clinical and hematologic presentation of infantile malignant osteopetrosis may be indistinguishable from that of juvenile myelomonocytic leukemia in infants. Timely radiographic imaging, however, allows straightforward delineation of these 2 severe diseases and facilitates immediate initiation of appropriate therapy.

Dissociation of bone resorption and bone formation in adult mice with a non-functional V-ATPase in osteoclasts leads to increased bone strength

Osteopetrosis caused by defective acid secretion by the osteoclast, is characterized by defective bone resorption, increased osteoclast numbers, while bone formation is normal or increased. In contrast the bones are of poor quality, despite this uncoupling of formation from resorption.To shed light on the effect of uncoupling in adult mice with respect to bone strength, we transplanted irradiated three-month old normal mice with hematopoietic stem cells from control or oc/oc mice, which have defective acid secretion, and followed them for 12 to 28 weeks.Engraftment levels were assessed by flow cytometry of peripheral blood. Serum samples were collected every six weeks for measurement of bone turnover markers. At termination bones were collected for µCT and mechanical testing. An engraftment level of 98% was obtained. From week 6 until termination bone resorption was significantly reduced, while the osteoclast number was increased when comparing oc/oc to controls. Bone formation was elevated at week 6, normalized at week 12, and reduced onwards. µCT and mechanical analyses of femurs and vertebrae showed increased bone volume and bone strength of cortical and trabecular bone.In conclusion, these data show that attenuation of acid secretion in adult mice leads to uncoupling and improves bone strength.

[Bone and joint diseases in children. Abnormal bone mineral density in children]

In recent years, the problem of osteoporosis is increasingly raised in children as well as adults both as a primary disorder and as secondary results to various diseases, medications, and lifestyle issues. On the other hand, molecular mechanisms of pathogenesis have been elucidated in several sclerosing disorders contributing to our understanding of basic biology of bone metabolism. In this chapter, we reviewed the recent progress on the method to evaluate BMD in growing period and etiologies of abnormal BMD in children.

Miscellaneous bone disorders

This chapter deals with a few of the important childhood bone disorders associated with high bone mass as well as conditions associated with fragility fractures and limb deformities, which have not been dealt with in previous chapters. A couple of skeletal dysplasias that can be sometimes be confused with rickets are also dealt with in this chapter. The principal features of the conditions described here are shown in a table. However, a comprehensive review of skeletal dysplasias is beyond the scope of this chapter.

A practical approach to hypocalcaemia in children

Hypocalcaemia is one of the commonest disorders of mineral metabolism seen in children and can be a consequence of several different aetiologies. These include a failure of secretion or action of parathyroid hormone, disorders of vitamin D metabolism and abnormal function of the calcium-sensing receptor. A practical approach to the investigation, diagnosis and subsequent management of hypocalcaemic disorders is presented.

Genetics, pathogenesis and complications of osteopetrosis

Human osteopetrosis is a rare genetic disorder caused by osteoclast failure, which ranges widely in severity. In the most severe forms, deficient bone resorption prevents enlargement of bone cavities, impairing development of bone marrow, leading to hematological failure. Closure of bone foramina causes cranial nerve compression with visual and hearing deterioration. Patients also present with osteosclerosis, short stature, malformations and brittle bones. This form is fatal in infancy, has an autosomal recessive inheritance and is cured with hematopoietic stem cell transplantation, with a rate of success <50% and unsatisfactory rescue of growth and visual deterioration. It relies on loss-of-function mutations of various genes, including the TCIRG1 gene, encoding for the a3 subunit of the H+ATPase and accounting for >50% of cases, the ClCN7 and the OSTM1 genes, which have closely related function and account for approximately 10% of cases, also presenting with neurodegeneration. Further genes are implicated in rare forms with various severities and association with other syndromes and, recently, the RANKL gene has been found to be mutated in a subset of patients lacking osteoclasts. Autosomal recessive osteopetrosis may also have intermediate severity, with a small number of cases due to loss-of-function mutations of the CAII or the PLEKHM1 genes. Dominant negative mutations of the ClCN7 gene cause the so-called Albers-Schönberg disease, which represents the most frequent and heterogeneous form of osteopetrosis, ranging from asymptomatic to intermediate/severe, thus suggesting additional genetic/environmental determinants affecting penetrance. Importantly, recent work has demonstrated that osteoblasts may also contribute to the pathogenesis of the disease, either because they are affected by intrinsic defects, or because their activity may be enhanced by deregulated osteoclasts abundantly present in most forms. Therapy is presently unsatisfactory and effort is necessary to unravel the gene defects yet unrecognized and identify new treatments to improve symptoms and save life.

RGS10-null mutation impairs osteoclast differentiation resulting from the loss of [Ca2+]i oscillation regulation

Increased osteoclastic resorption leads to many bone diseases, including osteoporosis and rheumatoid arthritis. While rapid progress has been made in characterizing osteoclast differentiation signaling pathways, how receptor activator of nuclear factor kappaB (NF-kappaB) ligand (RANKL) evokes essential [Ca2+]i oscillation signaling remains unknown. Here, we characterized RANKL-induced signaling proteins and found regulator of G-protein signaling 10 (RGS10) is predominantly expressed in osteoclasts. We generated RGS10-deficient (RGS10-/-) mice that exhibited severe osteopetrosis and impaired osteoclast differentiation. Our data demonstrated that ectopic expression of RGS10 dramatically increased the sensitivity of osteoclast differentiation to RANKL signaling; the deficiency of RGS10 resulted in the absence of [Ca2+]i oscillations and loss of NFATc1; ectopic NFATc1 expression rescues impaired osteoclast differentiation from deletion of RGS10; phosphatidylinositol 3,4,5-trisphosphate (PIP3) is essential to PLCgamma activation; and RGS10 competitively interacts with Ca2+/calmodulin and PIP3 in a [Ca2+]i-dependent manner to mediate PLCgamma activation and [Ca2+]i oscillations. Our results revealed a mechanism through which RGS10 specifically regulates the RANKL-evoked RGS10/calmodulin-[Ca2+]i oscillation-calcineurin-NFATc1 signaling pathway in osteoclast differentiation using an in vivo model. RGS10 provides a potential therapeutic target for the treatment of bone diseases.

Metabolic bone disease in adolescents: recognition, evaluation, treatment, and prevention

Bone is a metabolically active organ that undergoes constant remodeling. Bone is very important for maintaining homeostasis of calcium and phosphorous in the body. Disorders of bone metabolism may encompass disease processes involving underactive or overactive bone turnover. The spectrum of abnormal bone metabolism includes rickets and osteomalacia; osteopenia and osteoporosis; osteogenesis imperfecta, renal osteodystrophy; osteopetrosis and osteosclerosis; and effect of pharmacologic therapies on bone metabolism. The recognition, evaluation, treatment, and prevention of these conditions will be discussed after a brief description of normal bone metabolism.

Are nonresorbing osteoclasts sources of bone anabolic activity?

Some osteopetrotic mutations lead to low resorption, increased numbers of osteoclasts, and increased bone formation, whereas other osteopetrotic mutations lead to low resorption, low numbers of osteoclasts, and decreased bone formation. Elaborating on these findings, we discuss the possibility that osteoclasts are the source of anabolic signals for osteoblasts. In normal healthy individuals, bone formation is coupled to bone resorption in a tight equilibrium. When this delicate balance is disturbed, the net result is pathological situations, such as osteopetrosis or osteoporosis. Human osteopetrosis, caused by mutations in proteins involved in the acidification of the resorption lacuna (ClC-7 or the a3-V-ATPase), is characterized by decreased resorption in face of normal or even increased bone formation. Mouse mutations leading to ablation of osteoclasts (e.g., loss of macrophage-colony stimulating factor [M-CSF] or c-fos) lead to secondary negative effects on bone formation, in contrast to mutations where bone resorption is abrogated with sustained osteoclast numbers, such as the c-src mice. These data indicate a central role for osteoclasts, and not necessarily their resorptive activity, in the control of bone formation. In this review, we consider the balance between bone resorption and bone formation, reviewing novel data that have shown that this principle is more complex than originally thought. We highlight the distinct possibility that osteoclast function can be divided into two more or less separate functions, namely bone resorption and stimulation of bone formation. Finally, we describe the likely possibility that bone resorption can be attenuated pharmacologically without the undesirable reduction in bone formation.

Chloride and the endosomal-lysosomal pathway: emerging roles of CLC chloride transporters

Several members of the CLC family of Cl- channels and transporters are expressed in vesicles of the endocytotic-lysosomal pathway, all of which are acidified by V-type proton pumps. These CLC proteins are thought to facilitate vesicular acidification by neutralizing the electric current of the H+-ATPase. Indeed, the disruption of ClC-5 impaired the acidification of endosomes, and the knock-out (KO) of ClC-3 that of endosomes and synaptic vesicles. KO mice are available for all vesicular CLCs (ClC-3 to ClC-7), and ClC-5 and ClC-7, as well as its beta-subunit Ostm1, are mutated in human disease. The associated mouse and human pathologies, ranging from impaired endocytosis and nephrolithiasis (ClC-5) to neurodegeneration (ClC-3), lysosomal storage disease (ClC-6, ClC-7/Ostm1) and osteopetrosis (ClC-7/Ostm1), were crucial in identifying the physiological roles of vesicular CLCs. Whereas the intracellular localization of ClC-6 and ClC-7/Ostm1 precluded biophysical studies, the partial expression of ClC-4 and -5 at the cell surface allowed the detection of strongly outwardly rectifying currents that depended on anions and pH. Surprisingly, ClC-4 and ClC-5 (and probably ClC-3) do not function as Cl- channels, but rather as electrogenic Cl--H+ exchangers. This hints at an important role for luminal chloride in the endosomal-lysosomal system.

Cells of the osteoclast lineage as mediators of the anabolic actions of parathyroid hormone in bone

PTH is an anabolic agent used to treat osteoporosis, but its mechanisms of action are unclear. This study elucidated target cells and mechanisms for anabolic actions of PTH in mice during bone growth. Mice with c-fos ablation are osteopetrotic and lack an anabolic response to PTH. In this study, there were no alterations in PTH-regulated osteoblast differentiation or proliferation in vitro in cells from c-fos -/- mice compared with +/+; hence, the impact of osteoclastic cells was further investigated. A novel transplant model was used to rescue the osteopetrotic defect of c-fos ablation. Vertebral bodies (vossicles) from c-fos -/- and +/+ mice were implanted into athymic hosts, and the c-fos -/- osteoclast defect was rescued. PTH treatment to vossicle-bearing mice increased 5-bromo-2'-deoxyuridine (BrdU) positivity in the bone marrow and increased bone area regardless of the vossicle genotype. To inhibit recruitment of osteoclast precursors to wild-type vossicles, stromal derived factor-1 signaling was blocked, which blunted the PTH anabolic response. Treating mice with osteoprotegerin to inhibit osteoclast differentiation also blocked the anabolic action of PTH. In contrast, using c-src mutant mice with a late osteoclast differentiation defect did not hinder the anabolic action, suggesting key target cells reside in the intermediately differentiated osteoclast population in the bone marrow. These results indicate that c-fos in osteoblasts is not critical for PTH action but that cells of the osteoclast lineage are intermediate targets for the anabolic action of PTH.

Modulation of CSF-1-regulated post-natal development with anti-CSF-1 antibody

Colony-stimulating factor-1 (CSF-1) regulates the survival, proliferation and differentiation of macrophages. CSF-1-deficient mice are osteopetrotic due to a lack of osteoclasts, while their tissue macrophage deficiencies and an absence of CSF-1 regulation of CSF-1 receptor-expressing cells in the female reproductive tract contribute to their pleiotropic phenotype. To further understand CSF-1 regulation of macrophages in vivo, we developed a neutralizing anti-mouse CSF-1 antibody which was expressed as a recombinant Fab' fragment and coupled to 40 kDa polyethylene glycol. As developmental regulation by CSF-1 is highest during the early post-natal period, the ability of this anti-CSF-1 reagent to inhibit development was tested by regular subcutaneous injection of mice from post-natal days 0.5-57.5. Antibody treatment decreased growth rate, decreased osteoclast number, induced osteopetrosis, decreased macrophage density in bone marrow, liver, dermis, synovium and kidney and decreased adipocyte size in adipose tissue, thereby inducing phenotypes shared by CSF-1- and CSF-1 receptor-deficient mice. While the antibody blocked macrophage development in some tissues, macrophage densities in other tissues were initially high and were reduced by treatment, proving that the antibody also blocked macrophage maintenance. Since cell surface CSF-1 is sufficient for the maintenance of normal synovial macrophage densities, these studies suggest that anti-CSF-1 Fab'-PEG efficiently neutralizes all three CSF-1 isoforms in vivo, namely the secreted proteoglycan, secreted glycoprotein and cell surface glycoprotein. Since CSF-1 has been shown to enhance chronic disease development in a number of mouse model systems, these studies demonstrate the feasibility of neutralizing CSF-1 effects in these models with an anti-CSF-1 antibody.

[Osteopetrosis with carbonic anhydrase II deficiency: report of 24 cases]

Twenty four patients suffering from osteopetrosis caused by carbonic anhydrase II deficiency are colliged. This pathology seems to be frequent in Tunisia. Mental retardation is present in 52%, 85% of patients have short stature and 25% have optic atrophy. All affected subjects show craniofacial disproportion and dental anomalies. Twenty patients have at least one bone fracture. Metabolic acidosis is constant: it is profound during the first life decade. A severe selective reduction of carbonic anhydrase II in erythrocyte is confirmed in 18 cases. Osteosclerosis and defective skeletal modelling are constant, cerebral calcification can be seen at the scanner approximately at the age of two years and six months. All patients are homozygous for a splice junction mutation in intron 2 of the carbonic anhydrase II gene, this mutation does not seem to protect patients from bone fractures nor induce a severe metabolic acidosis.

Canonical Wnt signaling in differentiated osteoblasts controls osteoclast differentiation

Inactivation of beta-catenin in mesenchymal progenitors prevents osteoblast differentiation; inactivation of Lrp5, a gene encoding a likely Wnt coreceptor, results in low bone mass (osteopenia) by decreasing bone formation. These observations indicate that Wnt signaling controls osteoblast differentiation and suggest that it may regulate bone formation in differentiated osteoblasts. Here, we study later events and find that stabilization of beta-catenin in differentiated osteoblasts results in high bone mass, while its deletion from differentiated osteoblasts leads to osteopenia. Surprisingly, histological analysis showed that these mutations primarily affect bone resorption rather than bone formation. Cellular and molecular studies showed that beta-catenin together with TCF proteins regulates osteoblast expression of Osteoprotegerin, a major inhibitor of osteoclast differentiation. These findings demonstrate that beta-catenin, and presumably Wnt signaling, promote the ability of differentiated osteoblasts to inhibit osteoclast differentiation; thus, they broaden our knowledge of the functions Wnt proteins have at various stages of skeletogenesis.

Interaction between osteoblast and osteoclast: impact in bone disease

The intercellular communication between osteoblasts and osteoclasts is crucial to bone homeostasis. Since Rodan and Martin proposed the control of osteoclasts by osteoblasts in the 1980s, many factors have been isolated from osteoblasts and shown to regulate the differentiation and function of osteoclasts. However, the mechanism by which osteoblasts regulate osteoclasts during bone remodelling is still unclear. On the other hand, it is well accepted that many metabolic bone diseases are associated with the disruption of the communication between osteoblast and osteoclasts. Thus, this review focuses on the cross-talk between osteoblasts and osteoclasts and its impact in bone disease.

Carbonic anhydrase II deficiency: a rare autosomal recessive disorder of osteopetrosis, renal tubular acidosis, and cerebral calcification

The authors present a case of osteopetrosis due to carbonic anhydrase II deficiency. Clinical, laboratory and radiologic findings are reported. The genetics of osteopetrosis are reviewed.

Transient benign osteopetrosis in a calf persistently infected with bovine virus diarrhoea virus

A two-day-old Simmental calf was admitted suffering from a fracture of the right femur. The radiographs showed striking changes in all bones, evident as alter-noting zones of dense and less dense tissue (bone-in-bone) in the right femur and striped densities in the vertebral bodies. A stainless steel plate was used to repair the fracture, which healed well. The calf developed normally but was diagnosed as persistently infected with bovine virus diarrhoea (BVD) virus. It was kept in isolation and examined physically and radiographically during the following 13 months. The radiographic changes diminished during the first three months and at 13 months were barely visible. The animal was euthanatized, and immunohistochemistry revealed BVD virus antigen in numerous tissues. The radiographic abnormalities seen in this case are similar to those of the transient form of osteopetrosis in humans. Osteopetrosis in humans is currently thought to have a genetical cause, whereas it appears to be associated with viral disease in animals.

Infantile osteopetrosis in four Thai infants

Four Thai infants, aged between 4 and 23 months, had progressive abdominal distension, pallor and delayed or regressed developmental milestones, with age at onset of 1 month, 3 months, 4 months and 1 month, respectively. Clinical findings consisted of growth and developmental retardation, anemia, frontal bossing, marked hepatosplenomegaly, and hearing and visual impairment. Laboratory findings revealed moderate anemia, leukocytosis and thrombocytopenia. The radiographic findings comprised generalized sclerosis of all bones, including the cranial base, and obliteration of the medullary canals and trabecular patterns. The first and second patients, who had swelling of the wrist joints and prominent costochondral junctions, had hypophosphatemia, elevated levels of serum alkaline phosphatase, and metaphyseal flaring on their radiographs, which was consistent with infantile osteopetrosis complicated by rickets. After Stoss therapy, there were biochemical and radiological responses suggesting vitamin D deficiency in the first patient, but not in the second. The third patient, who had hypocalcemia, hypophosphatemia and normal levels of serum alkaline phosphatase, received vitamin D at 3000 units per day, without improvement. Despite frequent blood transfusions, all patients continued to deteriorate and were finally lost to follow-up. Rickets should be identified and treated at the onset, because treatment of rickets leads to improvement in well-being and an adequate clinical response to bone marrow transplantation.

Characterization of a novel bipotent hematopoietic progenitor population in normal and osteopetrotic mice

Several reports indicate that osteoclasts and B-lymphocytes share a common progenitor. This study focuses on the characterization of this bipotent progenitor from the bone marrow of the osteopetrotic oc/oc mouse, where the bipotent progenitor population is amplified, and of normal mice.

INTRODUCTION

Osteoclasts have a myelomonocytic origin, but they can also arise in vitro from pro-B-cells, suggesting that a subset of normal pro-B-cells is uncommitted and may reorient into the myeloid lineage representing a B-lymphoid/osteoclastic progenitor. The aim of this study was to characterize this progenitor population.

MATERIALS AND METHODS

The osteopetrotic oc/oc mouse was used as a choice model because it displays an increased number of both osteoclasts and pro-B-cells in the bone marrow. Our results have been confirmed in normal littermates. Bone marrow cells from these animals were analyzed by flow cytometry. After sorting, the cells were cultured under different conditions to assess their differentiation capacity.

RESULTS

Pro-B-cells from oc/oc and normal mice include an unusual biphenotypic population expressing markers from the B-lymphoid (CD19, CD43, CD5) and the myeloid (F4/80) lineages. This population also expresses progenitor markers (CD34 and Flt3) and is uncommitted. After sorting from the oc/oc bone marrow, this population is able to differentiate in vitro into osteoclast-like cells in the presence of RANKL and macrophage colony-stimulating factor (M-CSF), into dendritic-like cells in the presence of granulocyte/macrophage colony-stimulating factor (GM-CSF), interleukin (IL)-4, and TNFalpha, and into immature B-cells when seeded onto ST2 cells in the presence of IL-7.

CONCLUSION

Our results show the existence of a novel bipotent biphenotypic hematopoietic progenitor population present in the bone marrow that has retained the capacity to differentiate into myeloid and B-lymphoid cells.

The cell surface form of colony-stimulating factor-1 is biologically active in bone in vivo

The specific biological function of the cell surface or membrane-bound isoform of colony-stimulating factor-1 (mCSF-1) is not well understood. To help define the role of this isoform in bone, we developed a transgenic mouse in which targeted expression of human mCSF-1 in osteoblasts was achieved under the control of the 2.4-kb rat collagen type I alpha promoter. Bone density, determined by peripheral quantitative computed tomography, was reduced 7% in mCSF-1 transgenic compared with that in wild-type mice. Histomorphometric analyses indicated that the number of osteoclasts in bone (NOc/BPm, NOc/TAR, OcS/BS) was significantly increased in transgenic mice (1.7- to 1.8-fold; P < 0.05 to P < 0.01) compared with that in wild-type animals. Interestingly, the osteoblast-restricted isoform transgene corrected the osteopetrosis seen in CSF-1-deficient op/op mice. Skeletal growth and bone density in op/op mice expressing mCSF-1 in osteoblasts were similar to those in wild-type mice and were dramatically different from those in the unmanipulated op/op animals. The op/op mice expressing mCSF-1 in bone had normal incisor and molar tooth eruption, whereas the op/op mice evidenced the expected failure of tooth eruption. These findings directly support the conclusion that mCSF-1 is functionally active in bone in vivo and is probably an important local source of CSF-1.

Osteopetrosis

Osteopetrosis is a rare skeletal condition characterized by skeletal sclerosis caused by aberrant osteoclast-mediated bone resorption. Three clinically distinct forms of osteopetrosis are recognized--the infantile malignant autosomal recessive form, the intermediate autosomal recessive form, and the adult benign autosomal dominant form. The disease represents a spectrum of clinical variants because of the heterogeneity of genetic defects resulting in osteoclast dysfunction. The pathogenic defects may be intrinsic to either the osteoclast-monocyte lineage or the mesenchymal cells that constitute the microenvironment that supports osteoclast ontogeny and activation. Implicated factors include specific proto-oncogenes, growth factors, and immune regulators. A subset of patients with the intermediate autosomal recessive form has been characterized with carbonic anhydrase II isoenzyme deficiency. Management of patients with osteopetrosis requires a comprehensive approach to characteristic clinical problems including hematologic and metabolic abnormalities, fractures, deformity, back pain, bone pain, osteomyelitis, and neurologic sequelae. Medical treatment of osteopetrosis is based on efforts to stimulate host osteoclasts or provide an alternative source of osteoclasts. Stimulation of host osteoclasts has been attempted with calcium restriction, calcitrol, steroids, parathyroid hormone, and interferon. Bone marrow transplant has been used with cure for infantile malignant osteopetrosis. As osteopetrosis likely represents a spectrum of underlying etiologies resulting in osteoclast dysfunction, effective therapies most likely need to be individualized.

Mitf and Tfe3, two members of the Mitf-Tfe family of bHLH-Zip transcription factors, have important but functionally redundant roles in osteoclast development

The Mitf-Tfe family of basic helix-loop-helix-leucine zipper (bHLH-Zip) transcription factors encodes four family members: Mitf, Tfe3, Tfeb, and Tfec. In vitro, each protein in the family can bind DNA as a homo- or heterodimer with other family members. Mutational studies in mice have shown that Mitf is essential for melanocyte and eye development, whereas Tfeb is required for placental vascularization. Here, we uncover a role for Tfe3 in osteoclast development, a role that is functionally redundant with Mitf. Although osteoclasts seem normal in Mitf or Tfe3 null mice, the combined loss of the two genes results in severe osteopetrosis. We also show that Tfec mutant mice are phenotypically normal, and that the Tfec mutation does not alter the phenotype of Mitf, Tfeb, or Tfe3 mutant mice. Surprisingly, our studies failed to identify any phenotypic overlap between the different Mitf-Tfe mutations. These results suggest that heterodimeric interactions are not essential for Mitf-Tfe function in contrast to other bHLH-Zip families like Myc/Max/Mad, where heterodimeric interactions seem to be essential.

RANK ligand and osteoprotegerin: paracrine regulators of bone metabolism and vascular function

In 1997, investigators isolated a secreted glycoprotein that blocked osteoclast differentiation from precursor cells, prevented osteoporosis (decreased bone mass) when administered to ovariectomized rats, and resulted in osteopetrosis (increased bone mass) when overexpressed in transgenic mice. Since then, the isolation and characterization of the protein named osteoprotegerin (OPG) has stimulated much work in the fields of endocrinology, rheumatology, and immunology. OPG functions as a soluble decoy receptor for receptor activator of nuclear factor-kappaB ligand (RANKL, or OPG ligand) and shares homologies with other members of the tumor necrosis factor receptor superfamily. OPG acts by competing with the receptor activator of nuclear factor-kappaB, which is expressed on osteoclasts and dendritic cells for specifically binding to RANKL. RANKL is crucially involved in osteoclast functions and bone remodeling as well as immune cell cross-talks, dendritic cell survival, and lymph node organogenesis. More recently, emerging evidence from in vitro studies and mouse genetics attributed OPG an important role in vascular biology. In fact, OPG could represent the long sought-after molecular link between arterial calcification and bone resorption, which underlies the clinical coincidence of vascular disease and osteoporosis, which are most prevalent in postmenopausal women and elderly people.

Macrophage colony-stimulating factor and receptor activator NF-kappaB ligand fail to rescue osteoclast-poor human malignant infantile osteopetrosis in vitro

Malignant infantile osteopetrosis (MIOP) is a disease characterized by failure in bone resorption, leading to dense fragile bones with a severely reduced bone marrow cavity. Normal or increased numbers of osteoclasts are present in the common variant of this disease; in such cases, the defect is likely to be inherent to the mature osteoclast and can be cured by bone marrow transplantation. However, MIOP also results from failure of osteoclast formation (osteoclast-poor MIOP). We report on two infants diagnosed with osteoclast-poor MIOP and utilize modern cell culture techniques to investigate the pathogenesis of disease. Peripheral blood mononuclear cells (PBMNCs) from these children were cultured in the presence of recombinant macrophage colony-stimulating factor and receptor activator NF-kappaB ligand for up to 3 weeks. Control cultures included PBMNCs from age-matched children, one of whom had an osteoclast-rich form of MIOP. Formation of osteoclasts (cells coexpressing vitronectin receptor and F-actin rings) occurred in all the control cultures. Significant bone resorption occurred in cultures from PBMNCs of the healthy individuals, whereas almost no bone resorption occurred in the osteoclast-rich MIOP cultures. In contrast, PBMNC cultures from the osteoclast-poor MIOP child formed only very occasional small F-actin ring-positive osteoclasts, which coexpressed vitronectin receptor and cathepsin K, and extremely rare foci of resorption. Because neither macrophage colony-stimulating factor nor receptor activator NF-kappaB ligand rescued the defect in osteoclast differentiation in the two cases of osteoclast-poor MIOP in vitro, there would be little benefit in treating these children with either of these recombinant proteins. Finally, these results demonstrate that this experimental culture model replicates the human osteopetrosis phenotype observed in vivo and should prove useful in analyzing the pathogenesis of the various forms of MIOP.

Association of severe autosomal recessive osteopetrosis and Dandy-Walker syndrome with agenesis of the corpus callosum

A severe form of autosomal recessive osteopetrosis associated with Dandy-Walker syndrome and agenesis of the corpus callosum is reported in a full-term boy born to consanguineous parents. The diagnosis was made shortly after birth. Clinical features were cranio-facial dysmorphy, macrocephaly, hepatosplenomegaly, severe anemia and thrombocytopenia. Skeletal radiographs revealed generalized increase in bone density and abnormal metaphyseal remodeling. Cranial ultrasonogram and computed tomography scan showed Dandy-Walker syndrome, agenesis of corpus callosum and hydrocephalus. The patient rapidly developed severe medullary deficiency and a severe pulmonary infection. He died at the age of 2 months. This association seems extremely rare and was not previously reported in the literature.

Klotho-deficient mice are resistant to bone loss induced by unloading due to sciatic neurectomy

Unloading induces bone loss as seen in experimental animals as well as in space flight or in bed-ridden conditions; however, the mechanisms involved in this phenomenon are not fully understood. Klotho mutant mice exhibit osteopetrosis in the metaphyseal regions indicating that the klotho gene product is involved in the regulation of bone metabolism. To examine whether the klotho gene product is involved in the unloading-induced bone loss, the response of the osteopetrotic cancellous bones in these mice was investigated. Sciatic nerve resection was conducted using klotho mutant (kl/kl) and control heterozygous mice (+/kl) and its effect on bone was examined by micro-computed tomography (microCT). As reported previously for wild-type mice (+/+), about 30% bone loss was induced in heterozygous mice (+/kl) by unloading due to neurectomy within 30 days of the surgery. By contrast, kl/kl mice were resistant against bone loss induced by unloading after neurectomy. Unloading due to neurectomy also induced a small but significant bone loss in the cortical bone of the mid-shaft of the femur in the heterozygous mice; no reduction in the cortical bone was observed in kl/kl mice. These results indicate that klotho mutant mice are resistant against bone loss induced by unloading due to neurectomy in both cortical and trabecular bone and indicate that klotho is one of the molecules involved in the loss of bone by unloading.

Malignant osteopetrosis obscured by maternal vitamin D deficiency in a neonate

A neonate presented with clinical, biochemical, endocrine and radiographic features consistent with vitamin D deficiency rickets of maternal origin. Persistent hypocalcemia and subsequent development of pancytopenia, hemolysis and hepatosplenomegaly prompted further studies that led to the diagnosis of infantile osteopetrosis.

CONCLUSION

Osteopetrosis is an important differential diagnosis of neonatal rickets and is not excluded by low vitamin D levels.

Study of the nonresorptive phenotype of osteoclast-like cells from patients with malignant osteopetrosis: a new approach to investigating pathogenesis

Osteopetrosis manifests as failure of osteoclastic bone resorption. The cause of the disease lies either in the hematopoietic lineage or in the bone marrow stromal microenvironment. It has not been possible to define the cell type involved in the various forms of the human disease because of the inability to form human osteoclasts in vitro. Using the recently described method for generating human osteoclasts from peripheral blood in coculture with rat osteoblastic UMR 106 cells, we demonstrate that a defect lies in the mature osteoclast-like cells in four cases of this disease. Control and osteopetrotic cocultures generated large numbers of osteoclast-like cells (calcitonin and vitronectin receptor positive, and F-actin ring-positive cells) with similar morphology. Bone resorption did not occur in three of the four osteopetrotic cultures. In case 1, in which bone resorption was identified, the area of resorption was negligible compared with the number of osteoclast-like cells in the culture and was detected only by scanning electron microscopy. In contrast, up to 20% of the bone surface in controls was resorbed. The normal and osteopetrotic osteoclast-like cells had a similar phenotype except that two of the osteopetrotic cases did not express CD44 and two expressed CD44 weakly, whereas CD44 was strongly expressed in the controls. This study shows that it is possible to reproduce in vitro the pathological features of human osteopetrosis, and the assay provides a means of acquiring a greater understanding of the pathogenesis of human osteopetrosis.

Human malignant osteopetrosis: pathophysiology, management and the role of bone marrow transplantation

Osteopetrosis is a heterogeneous group of diseases characterized by lack of osteoclast function. Osteopetrosis is found spontaneously in most mammalian species and many transgenic animals have been created, but so far no animal model has been found that genetically corresponds to human malignant autosomal recessive osteopetrosis. The only curative treatment for malignant osteopetrosis is bone marrow transplantation. A review of the literature and preliminary data from IBMTR shows that infants transplanted with marrow from an HLA-identical sibling or unrelated volunteer donor have an actuarian five-year survival with a functioning graft of 50-70%, while those transplanted with a T-cell-depleted mismatched marrow have a very poor survival of only about 10%.

Age-resolving osteopetrosis: a rat model implicating microphthalmia and the related transcription factor TFE3

Microphthalmia (Mi) is a basic helix-loop-helix-leucine zipper (b-HLH-ZIP) transcription factor implicated in pigmentation, mast cells, and bone development. Two dominant-negative mi alleles (mi/mi and Mior/Mior) in mice cause osteopetrosis. In contrast, osteopetrosis has not been observed in a number of recessive mi alleles, suggesting the existence of Mi protein partners important in osteoclast function. An osteopetrotic rat of unknown genetic defect (mib) has been described whose skeletal sclerosis improves dramatically with age and that is associated with pigmentation defects reminiscent of mouse mi alleles. Here we report that this rat strain harbors a large genomic deletion encompassing the 3' half of mi including most of the b-HLH-ZIP region. Osteoclasts from these animals lack Mi protein in contrast to wild-type rat, mouse, and human osteoclasts. Mi is not detectable in primary osteoblasts. In addition TFE3, a b-HLH-ZIP transcription factor related to Mi, was found to be expressed in osteoclasts, but not osteoblasts, and to coimmunoprecipitate with Mi. These results demonstrate the existence of members of a family of biochemically related transcription factors that may cooperate to play a central role in osteoclast function and possibly in age-related osteoclast homeostasis.

Osteopetrosis in mice lacking haematopoietic transcription factor PU.1

Osteoclasts are multinucleated cells and the principal resorptive cells of bone. Although osteoclasts are of myeloid origin, the role of haematopoietic transcription factors in osteoclastogenesis has not been explored. Here we show that messenger RNA for the myeloid- and B-cell-specific transcription factor PU.1 progressively increases as marrow macrophages assume the osteoclast phenotype in vitro. The association between PU.1 and osteoclast differentiation was confirmed by demonstrating that PU.1 expression increased with the induction of osteoclastogenesis by either 1,25-dihydroxyvitamin D3 or dexamethasone. Consistent with the participation of PU.1 in osteoclastogenesis, we found that the development of both osteoclasts and macrophages is arrested in PU.1-deficient mice. Reflecting the absence of osteoclasts, PU.1-/- mice exhibit the classic hallmarks of osteopetrosis, a family of sclerotic bone diseases. These animals were rescued by marrow transplantation, with complete restoration of osteoclast and macrophage differentiation, verifying that the PU.1 lesion is intrinsic to haematopoietic cells. The absence of both osteoclasts and macrophages in PU.1-mutant animals suggests that the transcription factor regulates the initial stages of myeloid differentiation, and that its absence represents the earliest developmental osteopetrotic mutant yet described.

Recent developments in the understanding of the pathophysiology of osteopetrosis

Osteopetrosis is a rare metabolic bone disease characterized by a generalized increase in skeletal mass. It is inherited in a number of mammalian species, including man, and results from a congenital defect in the development or function of the osteoclasts. The consequent impairment of bone resorption prevents formation of bone marrow cavities, causes delayed or absent tooth eruption and results often in abnormally shaped bone. The pathogenetic defect may be intrinsic either to the osteoclast lineage or to the mesenchymal cells that constitute the microenvironment supporting the development and activation of the osteoclasts. In the first example, the disease can be cured by transplantation of hemopoietic cells. In some cases, bone marrow transplantation has also been successful in curing human osteopetrosis. This, together with the variability in the age of onset and severity of clinical aspects, suggests that a multiplicity of genetic mutations may cause the human disease. In recent years the genetic effects of some osteopetrotic mutations have been identified. This new information has been essential for the understanding of osteoclast biology. Colony stimulating factor 1 (CSF-1), the growth factor for cells of the mononuclear phagocytic system, is also essential for the development of osteoclasts. In the osteopetrotic (op) mouse, no biologically active CSF-1 is synthesized due to a point mutation in the coding region of its gene. This leads to an almost complete lack of osteoclast development and to impaired bone resorption. Altered CSF-1 production seems also to be involved in the toothless (tl) rat osteopetrosis. Recently, the mutation responsible for the microphthalmic (mi) mouse osteopetrosis has been identified in the gene encoding a member of the basic-helix-loop-helix-leucine zipper (bHLH-ZIP) protein family of transcription factors. The mi gene product seems to play a role in the fusion process of osteoclast precursor cells. Finally, osteopetrosis has been the result of experimental gene disruption in mice. Targeted disruption of the c-src proto-oncogene encoding a nonreceptor tyrosine kinase leads to a form of osteopetrosis where osteoclasts are present but inactive. This indicates that pp60c-src, localized primarily on ruffled border membranes and vacuoles of the osteoclasts, is important for osteoclastic function. Disruption of the c-fos proto-oncogene, a major component of the AP-1 transcription factor complex, leads to an osteopetrotic phenotype characterized by a complete absence of osteoclasts. The defect is intrinsic to hemopoietic precursors that are unable to progress beyond an early stage of osteoclast differentiation. In humans, deficiency of carbonic anhydrase II has been identified as the primary defect in the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification. A lack of expression of the vacuolar proton pump has been observed in osteoclasts of a patient with craniometaphyseal dysplasia. In conclusion, the disease, although rare, is of great pathophysiological relevance for our understanding of the processes that govern the development and function of osteoclasts.

Mice lacking both macrophage- and granulocyte-macrophage colony-stimulating factor have macrophages and coexistent osteopetrosis and severe lung disease

Mice deficient in granulocyte-macrophage colony-stimulating factor (GM-CSF) and macrophage colony-stimulating factor (M-CSF, CSF-1) were generated by interbreeding GM-CSF-deficient mice generated by gene targeting (genotype GM-/-) with M-CSF-deficient osteopetrotic mice (genotype M-/-, op/op). Mice deficient in both GM-CSF and M-CSF (genotype GM-/-M-/-) are viable and have coexistent features corresponding to mice deficient in either factor alone. Like M-CSF-deficient mice, they have osteopetrosis and are toothless because of failure of incisor eruption. Like GM-CSF-deficient mice, they have a characteristic alveolar-proteinosis-like lung pathology, but it is more severe than that of GM-CSF-deficient mice and is often fatal. In particular, in GM-/-M-/- mice the accumulation of lipo-proteinaceous alveolar material is more marked, and bacterial pneumonic infections are more prevalent and more extensive, particularly involving Gram-negative bacteria. Neutrophilia consistently accompanies pulmonary infections, and some older GM-/-M-/- mice have polycythemia. Survival of GM-/-M-/- mice is significantly reduced compared with mice deficient in either factor alone, and all GM-/-M-/- mice have broncho- or lobar-pneumonia at death. These observations indicate that in vivo, M-CSF is involved in modulating the consequences of GM-CSF deficiency in the lung. Interestingly, GM-/-M-/- mice have circulating monocytes at levels comparable with those in M-CSF-deficient mice and the diseased lungs of all GM-/-M-/- mice contain numerous phagocytically active macrophages, indicating that in addition to GM-CSF and M-CSF, other factors can be used for macrophage production and function in vivo.

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)

Cytokines and growth factors in the regulation of bone remodeling

Osteoporosis and periodontal disease both represent examples of abnormal bone remodeling. As knowledge of the cellular and molecular events in the normal bone remodeling process has accumulated in the last decade, better understanding of the pathophysiology of bone loss associated with periodontal disease and with aging has occurred. This short review does not attempt to include all aspects of this topic but covers specific areas in which there have been recent advances. (1) Observations made in the last few years have indicated that a hierarchy of both receptor and nonreceptor tyrosine kinases may be involved in normal osteoclastic bone resorption and that certain members of these tyrosine kinase families may mediate cytokine effects. Studies in the op/op variant of murine osteopetrosis have shown that normal production of monocyte-macrophage colony-stimulating factor 1 (M-CSF, also called CSF-1) and activation of its receptor (the receptor tyrosine kinase c-fms) are required for normal osteoclast formation. (2) Studies in mice made deficient in nonreceptor tyrosine kinase by gene knockout have shown that expression of this nonreceptor tyrosine kinase is required for normal osteoclast action and ruffled border formation, although not for osteoclast formation. (3) Recent studies have shown that in addition to prostaglandins of the E series, other arachidonic acid metabolites may be involved in normal and abnormal osteoclastic bone resorption. 5-Lipoxygenase metabolites, the leukotrienes, stimulate isolated osteoclasts to form resorption pits as well as cause osteoclastic bone resorption in organ cultures of neonatal mouse calvariae. These compounds, which are unstable in tissue culture media, are readily inhibitable by agents that inhibit 5-lipoxygenase enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo role of macrophage growth factors as delineated using CSF-1 deficient op/op mouse

Total absence of CSF-1 in the op/op mouse leads to a profound and generalized deficiency of macrophages and to osteopetrosis subsequent to the absence of osteoclasts. These observations confirm that CSF-1 is a genuine regulator of macrophage and osteoclast formation in vivo. Further studies in affected animals have shown that the CSF-1 absence variably affects macrophage differentiation stages and different organ macrophage populations, and that functionally competent macrophages are produced in low numbers without CSF-1, presumably under the influence of GM-CSF and IL-3. The op/op mice have increased levels of both endogenous GM-CSF and IL-3, which apparently are not fully able to compensate for the absence of CSF-1. Macrophage deficiencies but not osteoclast deficiencies in the op/op mouse could be completely corrected by exogenous GM-CSF, while exogenous CSF-1 corrects both osteoclast and macrophage deficiencies, but only in those tissues which could be reached by CSF-1 from the circulation. Despite severe quantitative macrophage deficiencies, the op/op mice demonstrate normal in vivo phagocytosis and immune functions suggesting that CSF-1 dependent macrophages do not contribute significantly to those processes in vivo. On the other hand, the op/op mice demonstrate severe secondary deficiencies of TNF-alpha, IL-1 alpha, and G-CSF suggesting that major function of CSF-1 dependent macrophages is the release of monokines.

Circulating macrophage colony-stimulating factor is not reduced in malignant osteopetrosis

Malignant osteopetrosis is a disorder characterized by a deficiency in osteoclast number or function. In one animal model of osteopetrosis, the op/op mouse, macrophage colony-stimulating factor (M-CSF) is absent, and the administration of M-CSF corrects the defects. We evaluated the serum of 13 patients with malignant osteopetrosis by an M-CSF radioimmunoassay to determine if a quantitative M-CSF deficiency existed in these patients. All patients had M-CSF present in levels equal to or higher than control serum. In addition, serum from 6 osteopetrotic patients was tested in a bioassay to determine if the M-CSF present is biologically active, and in all cases there was demonstrable activity in these samples. We provide evidence that deficiency of circulating M-CSF is unlikely to be a major contributor to the etiologic basis for the majority of children with malignant osteopetrosis.

Rickets, osteomalacia, and osteopetrosis

In the field of rickets and osteomalacia, progress has been made mainly in the mapping of vitamin D-dependency rickets or "pseudodeficiency rickets" type I to chromosome 12q14, and the further identification of a variety of abnormalities in the calcitriol receptor complex responsible for hereditary resistance to 1,25-dihydroxyvitamin D. The study of the molecular basis of this latter inherited disorder has important implications for a better understanding of the physiologic role of 1,25-dihydroxyvitamin D. Concerning osteopetrosis, the finding of a reverse transcriptase activity in a patient with the benign form of this disorder opens new perspectives such as the possibility that retroviral infection may be the origin of at least some type(s) of osteopetrosis. Moreover, impairment of macrophage colony-stimulating factor production appears to be a key event in the pathogenesis of the osteopetrotic op/op mutation in rodents.

Retroviral expression in mononuclear blood cells isolated from a patient with osteopetrosis (Albers-Schönberg disease)

We report the presence of reverse transcriptase activity in the supernatant of long-term culture of mononuclear blood cells (monocytes and lymphocytes) isolated from a 27-year-old patient suffering from benign osteopetrosis. The enzyme was purified to homogeneity according to the technique of Chandra and Steel, by chromatography, first on DEAE-cellulose (DE 52) and then on phosphocellulose (P11). After purification, the enzyme was characterized biochemically for its template specificity and ionic requirements. The purified enzyme was able to transcribe poly(rA).(dT)12-18 and poly(rC).(dG)12-18 very efficiently and had a marked preference for Mg2+ ions over Mn2+ ions. The pattern of ionic dependency for this enzyme is similar to that of reverse transcriptases purified from human lymphotropic viruses. The patient was tested and found sero-negative for HIV-1, HIV-2, and HTLV-I and seropositive (immunoglobulin G) for cytomegalovirus. Epstein-Barr virus nuclear antigens (EBNA) were detected in the patient's B lymphocytes. Since reverse transcriptase is the hallmark of retroviruses, we suggest that a retrovirus may be involved in the etiology of osteopetrosis.

Assessment of the influence of NaF or 1,25-(OH)2 vitamin D3 on the proliferative bone effect of an avian osteopetrosis virus, MAV-2(O)

1,25-(OH)2 vitamin D3 (D3) and sodium fluoride (NaF) were given to chicken embryos and newly hatched chickens infected with a slow onset strain of avian osteopetrosis-inducing virus [MAV-2(O)] to determine if either agent influenced MAV-2(O)-induced proliferation of bone. Embryos were administered MAV-2(O) and treated with: 1) up to 240 micrograms NaF or up to 100 ng D3 as embryos; 2) up to 1.8 g NaF/kg or up to 9.5 micrograms D3/kg after hatching: or 3) 240 micrograms NaF as embryos and up to 1.8 g NaF/kg after hatching. Administration of MAV-2(O) alone resulted in expansion of the cortical diameter of bone. Coadministration of NaF or D3 with MAV-2(O) did not influence the change in cortical diameter seen with MAV-2(O) alone at 18 days of incubation, and 3 and 6 weeks after hatching. Increased osteoid relative to bone (hyperosteoidosis), with NaF and MAV-2(O) compared to MAV-2(O) alone, and NaF compared to untreated controls reflected delayed mineralization of osteoid, a known fluoride effect. We conclude that the administration of NaF or D3 did not influence the incidence, severity or time of onset of the MAV-2(O)-induced proliferative changes of bone.