Juvenile Paget's disease: the second reported, oldest patient is homozygous for the TNFRSF11B "Balkan" mutation (966_969delTGACinsCTT), which elevates circulating immunoreactive osteoprotegerin levels

Biochemical characterization of a disease-causing human osteoprotegerin variant

Recently, a human mutation of OPG was identified to be associated with familial forms of osteoarthritis. This missense mutation (c.1205A =  > T; p.Stop402Leu) occurs on the stop codon of OPG, which results in a 19-residue appendage to the C-terminus (OPG+19). The biochemical consequence of this unusual sequence alteration remains unknown. Here we expressed OPG+19 in 293 cells and the mutant OPG was purified to homogeneity by heparin affinity chromatography and size exclusion chromatography. We found that in sharp contrast to wildtype OPG, which mainly exists in dimeric form, OPG+19 had a strong tendency to form higher-order oligomers. To our surprise, the hyper-oligomerization of OPG+19 had no impact on how it binds cell surface heparan sulfate, how it inhibits RANKL-induced osteoclastogenesis and TRAIL-induced chondrocytes apoptosis. Our data suggest that in biological contexts where OPG is known to play a role, OPG+19 functions equivalently as wildtype OPG. The disease-causing mechanism of OPG+19 likely involves an unknown function of OPG in cartilage homeostasis and mineralization. By demonstrating the biochemical nature of this disease-causing OPG mutant, our study will likely help elucidating the biological roles of OPG in cartilage biology.

Juvenile Paget's Disease From Heterozygous Mutation of SP7 Encoding Osterix (Specificity Protein 7, Transcription Factor SP7)

Juvenile Paget's disease (JPD) became in 1974 the commonly used name for ultra-rare heritable occurrences of rapid bone remodeling throughout of the skeleton that present in infancy or early childhood as fractures and deformity hallmarked biochemically by marked elevation of serum alkaline phosphatase (ALP) activity (hyperphosphatasemia). Untreated, JPD can kill during childhood or young adult life. In 2002, we reported that homozygous deletion of the gene called tumor necrosis factor receptor superfamily, member 11B (TNFRSF11B) encoding osteoprotegerin (OPG) explained JPD in Navajos. Soon after, other bi-allelic loss-of-function TNFRSF11B defects were identified in JPD worldwide. OPG inhibits osteoclastogenesis and osteoclast activity by decoying receptor activator of nuclear factor κ-B (RANK) ligand (RANKL) away from its receptor RANK. Then, in 2014, we reported JPD in a Bolivian girl caused by a heterozygous activating duplication within TNFRSF11A encoding RANK. Herein, we identify mutation of a third gene underlying JPD. An infant girl began atraumatic fracturing of her lower extremity long-bones. Skull deformity and mild hearing loss followed. Our single investigation of the patient, when she was 15 years-of-age, showed generalized osteosclerosis and hyperostosis. DXA revealed a Z-score of +5.1 at her lumbar spine and T-score of +3.3 at her non-dominant wrist. Biochemical studies were consistent with positive mineral balance and several markers of bone turnover were elevated and included striking hyperphosphatasemia. Iliac crest histopathology was consistent with rapid skeletal remodeling. Measles virus transcripts, common in classic Paget's disease of bone, were not detected in circulating mononuclear cells. Then, reportedly, she responded to several months of alendronate therapy with less skeletal pain and correction of hyperphosphatasemia but had been lost to our follow-up. After we detected no defect in TNFRSF11A or B, trio exome sequencing revealed a de novo heterozygous missense mutation (c.926C>G; p.S309W) within SP7 encoding the osteoblast transcription factor osterix (specificity protein 7, transcription factor SP7). Thus, mutation of SP7 represents a third genetic cause of JPD.

RANKL biology: bone metabolism, the immune system, and beyond

Receptor activator of NF-κB (RANK) ligand (RANKL) induces the differentiation of monocyte/macrophage-lineage cells into the bone-resorbing cells called osteoclasts. Because abnormalities in RANKL, its signaling receptor RANK, or decoy receptor osteoprotegerin (OPG) lead to bone diseases such as osteopetrosis, the RANKL/RANK/OPG system is essential for bone resorption. RANKL was first discovered as a T cell-derived activator of dendritic cells (DCs) and has many functions in the immune system, including organogenesis, cellular development. The essentiality of RANKL in the bone and the immune systems lies at the root of the field of "osteoimmunology." Furthermore, this cytokine functions beyond the domains of bone metabolism and the immune system, e.g., mammary gland and hair follicle formation, body temperature regulation, muscle metabolism, and tumor development. In this review, we will summarize the current understanding of the functions of the RANKL/RANK/OPG system in biological processes.

Juvenile Paget's disease with compound heterozygous mutations in TNFRSF11B presenting with recurrent clavicular fractures and a mild skeletal phenotype

Juvenile Paget's disease (JPD) is a rare recessively-inherited bone dysplasia. The great majority of cases described to date have had homozygous mutations in TNFRSF11B, the gene encoding osteoprotegerin. We describe a boy who presented with recurrent clavicular fractures following minor trauma (8 fractures from age 2 to 11). He was of normal height and despite mild lateral bowing of the thighs and anterior bowing of the shins he remained physically active. Abnormal modelling was noted in ribs and humeri on clavicular radiographs, and a skeletal survey at the age of 7 showed generalised diaphyseal expansion of the long bones with thickening of the periosteal and endosteal surfaces of the cortices. On biochemical evaluation, serum alkaline phosphatase was noted to be persistently elevated. The diagnosis of JPD was confirmed by the finding of compound heterozygous mutations in TNFRSF11B: a maternally-inherited A>G missense mutation at position 1 of the first amino acid codon (previously reported) and a paternally-inherited splice acceptor site mutation in intron 3 at a highly conserved position (not previously reported). Bioinformatics analysis suggested both mutations were disease-causing. Compound heterozygote mutations in TNFRSF11B causing JPD have been previously reported only once - in a boy who also had a relatively mild skeletal phenotype. The milder features may lead to delay in diagnosis and diagnostic confusion with other entities, but the extraskeletal features of JPD may nonetheless develop.

Rare Inherited forms of Paget's Disease and Related Syndromes

Several rare inherited disorders have been described that show phenotypic overlap with Paget's disease of bone (PDB) and in which PDB is a component of a multisystem disorder affecting muscle and the central nervous system. These conditions are the subject of this review article. Insertion mutations within exon 1 of the TNFRSF11A gene, encoding the receptor activator of nuclear factor kappa B (RANK), cause severe PDB-like disorders including familial expansile osteolysis, early-onset familial PDB and expansile skeletal hyperphosphatasia. The mutations interfere with normal processing of RANK and cause osteoclast activation through activation of nuclear factor kappa B (NFκB) independent of RANK ligand stimulation. Recessive, loss-of-function mutations in the TNFRSF11B gene, which encodes osteoprotegerin, cause juvenile PDB and here the bone disease is due to unopposed activation of RANK by RANKL. Multisystem proteinopathy is a disorder characterised by myopathy and neurodegeneration in which PDB is often an integral component. It may be caused by mutations in several genes including VCP, HNRNPA1, HNRNPA2B1, SQSTM1, MATR3, and TIA1, some of which are involved in classical PDB. The mechanisms of osteoclast activation in these conditions are less clear but may involve NFκB activation through sequestration of IκB. The evidence base for management of these disorders is somewhat limited due to the fact they are extremely rare. Bisphosphonates have been successfully used to gain control of elevated bone remodelling but as yet, no effective treatment exists for the treatment of the muscle and neurological manifestations of MSP syndromes.

Juvenile Paget disease

Juvenile Paget disease (JPD) is a rare disorder, mainly caused by mutations in the gene TNFRSF11B that encodes osteoprotegerin (OPG). Loss of OPG action causes generalized, extremely rapid bone turnover. The clinical manifestations are both skeletal - progressive skeletal deformity that develops in childhood - and extra-skeletal, including hearing loss, retinopathy, vascular calcification and internal carotid artery aneurysm formation. The severity of the phenotype seems to be related to the severity of TNFRSF11B gene deactivation. JPD is characterized biochemically by very high alkaline phosphatase activity, as well as other bone turnover markers. Bisphosphonates are commonly used to reduce the greatly accelerated bone turnover and can ameliorate the skeletal phenotype, if started early enough in childhood and continued at least until growth is complete. Limited evidence from patients treated with recombinant OPG or denosumab also provided favorable results. Recombinant OPG would represent a replacement treatment, but it is unavailable for clinical use. It seems that life-long treatment with anti-resorptives is required, since the disease is reactivated after treatment discontinuation. An international collaborating network for the continuous registration and follow-up of JPD patients could be helpful in the future.

Loss of Functional Osteoprotegerin: More Than a Skeletal Problem

INTRODUCTION

Juvenile Paget's disease (JPD), an ultra-rare, debilitating bone disease due to loss of functional osteoprotegerin (OPG), is caused by recessive mutations in TNFRFSF11B. A genotype-phenotype correlation spanning from mild to very severe forms is described.

AIM

This study aimed to describe the complexity of the human phenotype of OPG deficiency in more detail and to investigate heterozygous mutation carriers for clinical signs of JPD.

PATIENTS

We investigated 3 children with JPD from families of Turkish, German, and Pakistani descent and 19 family members (14 heterozygous).

RESULTS

A new disease-causing 4 bp-duplication in exon 1 was detected in the German patient, and a microdeletion including TNFRFSF11B in the Pakistani patient. Skeletal abnormalities in all affected children included bowing deformities and fractures, contractures, short stature and skull involvement. Complex malformation of the inner ear and vestibular structures (2 patients) resulted in early deafness. Patients were found to be growth hormone deficient (2), displayed nephrocalcinosis (1), and gross motor (3) and mental (1) retardation. Heterozygous family members displayed low OPG levels (12), elevated bone turnover markers (7), and osteopenia (6). Short stature (1), visual impairment (2), and hearing impairment (1) were also present.

CONCLUSION

Diminished OPG levels cause complex changes affecting multiple organ systems, including pituitary function, in children with JPD and may cause osteopenia in heterozygous family members. Diagnostic and therapeutic measures should aim to address the complex phenotype.

Auricular ossification: A newly recognized feature of osteoprotegerin-deficiency juvenile Paget disease

We report auricular ossification (AO) affecting the elastic cartilage of the ear as a newly recognized feature of osteoprotegerin (OPG)-deficiency juvenile Paget disease (JPD). AO and auricular calcification refer interchangeably to rigid pinnae, sparing the ear lobe, from various etiologies. JPD is a rare Mendelian disorder characterized by elevated serum alkaline phosphatase activity accompanied by skeletal pain and deformity from rapid bone turnover. Autosomal recessive transmission of loss-of-function mutations within TNFRSF11B encoding OPG accounts for most JPD (JPD1). JPD2 results from heterozygous constitutive activation of TNFRSF11A encoding RANK. Other causes of JPD remain unknown. In 2007, we reported a 60-year-old man with JPD1 who described hardening of his external ears at age 45 years, after 4 years of treatment with bisphosphonates (BPs). Subsequently, we noted rigid pinnae in a 17-year-old boy and 14-year-old girl, yet pliable pinnae in a 12-year-old boy, each with JPD1 and several years of BP treatment. Cranial imaging indicated cortical bone within the pinnae of both teenagers. Radiologic studies of our three JPD patients without mutations in TNFRSF11B showed normal auricles. Review of the JPD literature revealed possible AO in several reports. Two of our JPD1 patients had experienced difficult tracheal intubation, raising concern for mineralization of laryngeal elastic cartilage. Thus, AO is a newly recognized feature of JPD1, possibly exacerbated by BP treatment. Elastic cartilage at other sites in JPD1 might also ossify, and warrants investigation.

Juvenile Paget's disease with heterozygous duplication within TNFRSF11A encoding RANK

Mendelian disorders of RANKL/OPG/RANK signaling feature the extremes of aberrant osteoclastogenesis and cause either osteopetrosis or rapid turnover skeletal disease. The patients with autosomal dominant accelerated bone remodeling have familial expansile osteolysis, early-onset Paget's disease of bone, expansile skeletal hyperphosphatasia, or panostotic expansile bone disease due to heterozygous 18-, 27-, 15-, and 12-bp insertional duplications, respectively, within exon 1 of TNFRSF11A that encodes the signal peptide of RANK. Juvenile Paget's disease (JPD), an autosomal recessive disorder, manifests extremely fast skeletal remodeling, and is usually caused by loss-of-function mutations within TNFRSF11B that encodes OPG. These disorders are ultra-rare. A 13-year-old Bolivian girl was referred at age 3years. One femur was congenitally short and curved. Then, both bowed. Deafness at age 2years involved missing ossicles and eroded cochleas. Teeth often had absorbed roots, broke, and were lost. Radiographs had revealed acquired tubular bone widening, cortical thickening, and coarse trabeculation. Biochemical markers indicated rapid skeletal turnover. Histopathology showed accelerated remodeling with abundant osteoclasts. JPD was diagnosed. Immobilization from a femur fracture caused severe hypercalcemia that responded rapidly to pamidronate treatment followed by bone turnover marker and radiographic improvement. No TNFRSF11B mutation was found. Instead, a unique heterozygous 15-bp insertional tandem duplication (87dup15) within exon 1 of TNFRSF11A predicted the same pentapeptide extension of RANK that causes expansile skeletal hyperphosphatasia (84dup15). Single nucleotide polymorphisms in TNFRSF11A and TNFRSF11B possibly impacted her phenotype. Our findings: i) reveal that JPD can be associated with an activating mutation within TNFRSF11A, ii) expand the range and overlap of phenotypes among the Mendelian disorders of RANK activation, and iii) call for mutation analysis to improve diagnosis, prognostication, recurrence risk assessment, and perhaps treatment selection among the monogenic disorders of RANKL/OPG/RANK activation.

Novel homozygous mutations in the osteoprotegerin gene TNFRSF11B in two unrelated patients with juvenile Paget's disease

Most patients with juvenile Paget's disease (JPD) are homozygous for mutations in the gene TNFRSF11B that result in deficiency of osteoprotegerin (OPG) - a key regulator of bone turnover. So far, about 10 different OPG mutations have been described. The current study presents two novel OPG mutations in JPD patients. Patient 1 was diagnosed at the age of 9months when he presented with inability to sit up, slow growth, marked bone pain and very high levels of serum alkaline phosphatase. Patient 2 presented a milder phenotype. He was initially diagnosed with osteogenesis imperfecta, and although he had numerous fractures and bone deformity, he was still independently mobile at the age of 19years, when a diagnosis of JPD was confirmed. Sequence analysis of DNA samples from the patients determined two novel homozygous mutations in TNFSRF11B. Patient 1 (severe phenotype) had a large (245-251kbp) homozygous deletion beginning in intron 1 that resulted in loss of 4 of the 5 exons of TNFSRF11B, including the whole ligand-binding domain. Patient 2 had a homozygous missense mutation resulting in a Thr>Pro change in exon 2 of TNFSRF11B that is predicted to disrupt the OPG ligand-binding domain. Taken in conjunction with other published cases, these results are consistent with the hypothesis that the most severe phenotypes in JPD are seen in patients with major gene deletions or mutations affecting cysteine residues in the ligand-binding domain.

Denosumab treatment for juvenile Paget's disease: results from two adult patients with osteoprotegerin deficiency ("Balkan" mutation in the TNFRSF11B gene)

CONTEXT

Most patients with juvenile Paget's disease (JPD) have homozygous loss-of-function mutations in the TNFRSF11B gene resulting in osteoprotegerin deficiency. Because recombinant osteoprotegerin is not available for clinical use, an alternative therapeutic approach could be denosumab, which acts on the same pathway.

MAIN OBJECTIVE

The aim was to study the effect of denosumab on bone turnover markers in two adult patients with JPD ("Balkan" mutation) previously treated with calcitonin and bisphosphonates.

SETTING

The study was conducted at two tertiary hospitals in Greece.

PATIENTS

Patient 1 (a 36-year-old woman) developed a severe and long-term hypocalcemia after a single dose (3.5 mg) of zoledronic acid. Her bone disease remained active despite treatment. Patient 2 (a 67-year-old man) had satisfactorily controlled bone disease with only intermittent risedronate treatment during the last 10 years, but suffered from progressive loss of hearing and vision. Low doses (20-40 mg) of denosumab every 3-6 months were administered in both patients.

RESULTS

Bone markers (including total and bone-specific alkaline phosphatase, procollagen I N-terminal peptide, and osteocalcin) were reduced to normal levels in both patients, with nadir observed 2-4 months after each denosumab injection. Retinal and hearing involvement remained unchanged, but patient 2 developed a rapid progression of cataract in the right eye.

CONCLUSIONS

Low-dose denosumab every 3-6 months for about 2 years in two patients with JPD successfully controlled their bone disease. The long-term effect of denosumab on the nonskeletal complications remains to be elucidated.

Juvenile paget's disease in an Iranian kindred with vitamin D deficiency and novel homozygous TNFRSF11B mutation

Juvenile Paget's disease (JPD) is a rare heritable osteopathy characterized biochemically by markedly increased serum alkaline phosphatase (ALP) activity emanating from generalized acceleration of skeletal turnover. Affected infants and children typically suffer bone pain and fractures and deformities, become deaf, and have macrocranium. Some who survive to young adult life develop blindness from retinopathy engendered by vascular microcalcification. Most cases of JPD are caused by osteoprotegerin (OPG) deficiency due to homozygous loss-of-function mutations within the TNFRSF11B gene that encodes OPG. We report a 3-year-old Iranian girl with JPD and craniosynostosis who had vitamin D deficiency in infancy. She presented with fractures during the first year-of-life followed by bone deformities, delayed development, failure-to-thrive, and pneumonias. At 1 year-of-age, biochemical studies of serum revealed marked hyperphosphatasemia together with low-normal calcium and low inorganic phosphate and 25-hydroxyvitamin D levels. Several family members in previous generations of this consanguineous kindred may also have had JPD and vitamin D deficiency. Mutation analysis showed homozygosity for a unique missense change (c.130T>C, p.Cys44Arg) in TNFRSF11B that would compromise the cysteine-rich domain of OPG that binds receptor activator of NF-κB ligand (RANKL). Both parents were heterozygous for this mutation. The patient's serum OPG level was extremely low and RANKL level markedly elevated. She responded well to rapid oral vitamin D repletion followed by pamidronate treatment given intravenously. Our patient is the first Iranian reported with JPD. Her novel mutation in TNFRSF11B plus vitamin D deficiency in infancy was associated with severe JPD uniquely complicated by craniosynostosis. Pamidronate treatment with vitamin D sufficiency can be effective therapy for the skeletal disease caused by the OPG deficiency form of JPD.

The Role of Inflammatory Cytokines, the RANKL/OPG Axis, and the Immunoskeletal Interface in Physiological Bone Turnover and Osteoporosis

Although it has long been recognized that inflammation, a consequence of immune-driven processes, significantly impacts bone turnover, the degree of centralization of skeletal and immune functions has begun to be dissected only recently. It is now recognized that formation of osteoclasts, the bone resorbing cells of the body, is centered on the key osteoclastogenic cytokine, receptor activator of NF- κ B ligand (RANKL). Although numerous inflammatory cytokines are now recognized to promote osteoclast formation and skeletal degradation, with just a few exceptions, RANKL is now considered to be the final downstream effector cytokine that drives osteoclastogenesis and regulates osteoclastic bone resorption. The biological activity of RANKL is moderated by its physiological decoy receptor, osteoprotegerin (OPG). New discoveries concerning the sources and regulation of RANKL and OPG in physiological bone turnover as well as under pathological (osteoporotic) conditions continue to be made, opening a window to the complex regulatory processes that control skeletal integrity and the depth of integration of the skeleton within the immune response. This paper will examine the interconnection between bone turnover and the immune system and the implications thereof for physiological and pathological bone turnover.

Bone effects of vitamin D - Discrepancies between in vivo and in vitro studies

Vitamin D was discovered as an anti-rachitic agent, but even at present, there is no direct evidence to support the concept that vitamin D directly stimulates osteoblastic bone formation and mineralization. It appears to be paradoxical, but vitamin D functions in the process of osteoclastic bone resorption. In 1952, Carlsson reported that administration of vitamin D(3) to rats fed a vitamin D-deficient, low calcium diet raised serum calcium levels. Since the diet did not contain appreciable amounts of calcium, the rise in serum calcium was considered to be derived from bone. Since then, this assay has been used as a standard bioassay for vitamin D compounds. Osteoclasts, the cells responsible for bone resorption, develop from hematopoietic cells of the monocyte-macrophage lineage. Several lines of evidence have shown that the active form of vitamin D(3), 1α,25-dihydroxyvitamin D(3) [1α,25(OH)(2)D(3)] is one of the most potent inducers of receptor activator of NF-κB ligand (RANKL), a key molecule for osteoclastogenesis, in vitro. In fact, 1α,25(OH)(2)D(3) strongly induced osteoclast formation and bone resorption in vitro. Nevertheless, 1α,25(OH)(2)D(3) and its prodrug, Alfacalcidol (1α-hydroxyvitamin D(3)) have been used as therapeutic agents for osteoporosis since 1983, because they increase bone mineral density and reduce the incidence of bone fracture in vivo. Furthermore, a new vitamin D analog, Eldecalcitol [2β-(3-hydroxypropoxy)-1α,25(OH)(2)D(3)], has been approved as a new drug for osteoporosis in Japan in January 2011. Interestingly, these beneficial effects of in vivo administration of vitamin D compounds are caused by the suppression of osteoclastic bone resorption. The present review article describes the mechanism of the discrepancy of vitamin D compounds in osteoclastic bone resorption between in vivo and in vitro.

TNFRSF11B computational development network construction and analysis between frontal cortex of HIV encephalitis (HIVE) and HIVE-control patients

BACKGROUND

TNFRSF11B computational development network construction and analysis of frontal cortex of HIV encephalitis (HIVE) is very useful to identify novel markers and potential targets for prognosis and therapy.

METHODS

By integration of gene regulatory network infer (GRNInfer) and the database for annotation, visualization and integrated discovery (DAVID) we identified and constructed significant molecule TNFRSF11B development network from 12 frontal cortex of HIVE-control patients and 16 HIVE in the same GEO Dataset GDS1726.

RESULTS

Our result verified TNFRSF11B developmental process only in the downstream of frontal cortex of HIVE-control patients (BST2, DGKG, GAS1, PDCD4, TGFBR3, VEZF1 inhibition), whereas in the upstream of frontal cortex of HIVE (DGKG, PDCD4 activation) and downstream (CFDP1, DGKG, GAS1, PAX6 activation; BST2, PDCD4, TGFBR3, VEZF1 inhibition). Importantly, we datamined that TNFRSF11B development cluster of HIVE is involved in T-cell mediated immunity, cell projection organization and cell motion (only in HIVE terms) without apoptosis, plasma membrane and kinase activity (only in HIVE-control patients terms), the condition is vital to inflammation, brain morphology and cognition impairment of HIVE. Our result demonstrated that common terms in both HIVE-control patients and HIVE include developmental process, signal transduction, negative regulation of cell proliferation, RNA-binding, zinc-finger, cell development, positive regulation of biological process and cell differentiation.

CONCLUSIONS

We deduced the stronger TNFRSF11B development network in HIVE consistent with our number computation. It would be necessary of the stronger TNFRSF11B development function to inflammation, brain morphology and cognition of HIVE.

Function of OPG as a traffic regulator for RANKL is crucial for controlled osteoclastogenesis

The amount of the receptor activator of NF-κB ligand (RANKL) on the osteoblastic cell surface is considered to determine the magnitude of the signal input to osteoclast precursors and the degree of osteoclastogenesis. Previously, we have shown that RANKL is localized predominantly in lysosomal organelles, but little is found on the osteoblastic cell surface, and consequently, the regulated subcellular trafficking of RANKL in osteoblastic cells is important for controlled osteoclastogenesis. Here we have examined the involvement of osteoprotegerin (OPG), which is currently recognized as a decoy receptor for RANKL, in the regulation of RANKL behavior. It was suggested that OPG already makes a complex with RANKL in the Golgi apparatus and that the complex formation is necessary for RANKL sorting to the secretory lysosomes. It was also shown that each structural domain of OPG is indispensable for exerting OPG function as a traffic regulator. In particular, the latter domains of OPG, whose physiologic functions have been unclear, were indicated to sort RANKL molecules to lysosomes from the Golgi apparatus. In addition, the overexpression of RANK-OPG chimeric protein, which retained OPG function as a decoy receptor but lost the function as a traffic regulator, inhibited endogenous OPG function as a traffic regulator selectively in osteoblastic cells and resulted in the upregulation of osteoclastogenic ability despite the increased number of decoy receptor molecules. Conclusively, OPG function as a traffic regulator for RANKL is crucial for regulating osteoclastogenesis at least as well as that as a decoy receptor.

Bisphosphonate inhibits bone turnover in OPG(-/-) mice via a depressive effect on both osteoclasts and osteoblasts

Osteoclast differentiation and functioning are strictly controlled by RANKL expressed on osteoblast membrane surfaces, but whether osteoclasts exert control over osteoblasts remains unclear. In the present study, we examined the effect of an osteoclast inhibitor, a bisphosphonate (BP), on the response of maxillary bone to mechanical stress in a high-turnover osteoporosis model (OPG(-/-) mice, a model of juvenile Paget disease). Mechanical stress was induced by use of orthodontic elastics to move the maxillary first molar. BP was administered once per day beginning 5 days before elastic insertion. Relative to wild type (WT), in the OPG(-/-) mice tooth movement distance was greater, resorption of the interradicular septum occurred to a greater extent, the osteoclast count was higher, and serum alkaline phosphatase (ALP) was higher. However, administration of BP to OPG(-/-) mice reduced tooth movement distance, increased bone volume at the interradicular septum, decreased the osteoclast count, and reduced serum ALP. BP administration also caused a temporal shift in peak Runx2 staining in OPG(-/-) mice, such that the overall staining time course was similar to that observed for WT mice. We conclude that BP administration not only inhibited osteoclast activity in OPG(-/-) mice but also systemically and locally inhibited osteoblast activity. It is possible that osteoclasts are able to exert some negative control over osteoblasts.

Pathophysiological roles of osteoprotegerin (OPG)

Osteoprotegerin (OPG) is a secreted glycoprotein central to bone turnover via its role as a decoy receptor for the receptor activator of nuclear factor kappaB ligand (RANKL) and has traditionally been linked to a number of bone-related diseases. However, there is additional evidence that OPG can promote cell survival by inhibiting TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. As a result, a number of in vitro, in vivo and clinical studies have been performed assessing the role of OPG in tumourigenesis. Similar studies have been performed regarding vascular pathologies, resulting from observations of expression and regulation of OPG in the vasculature. This review aims to provide an update on this area and assess the potential protective or detrimental role of OPG in both vascular pathologies and tumourigenesis.