Evaluation of the role of RANK and OPG genes in Paget's disease of bone

Association between Osteoprotegerin and Charcot Neuroarthropathy: a systematic review

AIMS

Osteoprotegerin (OPG) has been associated with Charcot Neuroarthropathy (CN); however, three studied OPG polymorphisms (1181C > G, 245A > C and 950 T > C) have yielded conflicting results. Therefore, this meta-analysis was conducted to determine the difference in serum OPG concentrations between healthy controls and diabetics with and without CN and the effect OPG polymorphisms have on CN development.

METHODS

PubMed, LILAC, SCOPUS, and EBSCO databases and retrieved publications' bibliographies were searched for studies that examined for OPG and CN. Depending on the heterogeneity, fixed or random effects were used to calculate the pooled odds ratio (OR) or standard difference in means (SDM) with 95% confidence intervals (95%CI) for 5 genetic models (heterozygous, homozygous, dominant, recessive, and allelic) and serum concentrations, respectively.

RESULTS

Seven publications (12 studies) demonstrated that serum OPG concentrations were more elevated in subjects with CN (SDM = 0.719, 95%CI = 0.555-0.883, p < 0.001). When CN was compared to healthy controls or diabetics, the difference was more prominent for healthy controls (SDM = 1.043, 95%CI = 0.676-1.409, p < 0.001) than diabetics (SDM = 0.639, 95%CI = 0.456-0.821, p < 0.001) and the SDM difference was significant (p = 0.013). Using 6 publications (9 studies), neither the 1181C > G or the 950 T > C polymorphisms showed any significant associations for any genetic model. For the 245A > C polymorphism, only the homozygous genetic model showed a significant association between the polymorphism and CN (OR = 2.850, 95%CI: 1.051-7.729, p = 0.040).

CONCLUSIONS

Here, we determined a potential correlation between the CN and serum OPG concentrations and that only the CC genotype of the 245A > C polymorphism showed an increased risk of developing CN.

Molecular Signaling Pathways and Essential Metabolic Elements in Bone Remodeling: An Implication of Therapeutic Targets for Bone Diseases

Bone is one of the dynamic tissues in the human body that undergoes continuous remodelling through subsequent actions of bone cells, osteoclasts, and osteoblasts. Several signal transduction pathways are involved in the transition of mesenchymal stem cells into osteoblasts. These primarily include Runx2, ATF4, Wnt signaling and sympathetic signalling. The differentiation of osteoclasts is controlled by M-CSF, RANKL, and costimulatory signalling. It is well known that bone remodelling is regulated through receptor activator of nuclear factor-kappa B ligand followed by binding to RANK, which eventually induces the differentiation of osteoclasts. The resorbing osteoclasts secrete TRAP, cathepsin K, MMP-9 and gelatinase to digest the proteinaceous matrix of type I collagen and form a saucer-shaped lacuna along with resorption tunnels in the trabecular bone. Osteoblasts secrete a soluble decoy receptor, osteoprotegerin that prevents the binding of RANK/RANKL and thus moderating osteoclastogenesis. Moreover, bone homeostasis is also regulated by several growth factors like, cytokines, calciotropic hormones, parathyroid hormone and sex steroids. The current review presents a correlation of the probable molecular targets underlying the regulation of bone mass and the role of essential metabolic elements in bone remodelling. Targeting these signaling pathways may help to design newer therapies for treating bone diseases.

The role of genetic factors and monocyte-to-osteoclast differentiation in the pathogenesis of Charcot neuroarthropathy

Charcot neuroarthropathy is a chronic, progressive condition of the skeletal system that affects some patients with diabetic neuropathy. It results in progressive destruction of bones of the foot and disorganisation of pedal joints and ligaments. Effective prevention and treatment for Charcot neuroarthropathy remain a challenge. Currently, there are no reliable repeatable markers to identify patients with diabetes who are at higher risk of developing Charcot neuroarthropathy. The pathogenesis underlying the development of Charcot neuroarthropathy also remains unclear. In this review, we provide an overview of the history, prevalence, symptoms, risk factors, diagnostics and treatment of Charcot neuroarthropathy. We also discuss the potential for OPG and RANKL gene variants to act as predictive markers for the development of Charcot neuroarthropathy. Finally, we summarise the latest research on the role of monocyte-to-osteoclast differentiation in the development of acute Charcot neuroarthropathy.

Impact of history of periodontitis on gene expression of bone-related factors in young patients

Although dental implants and bone regenerative procedures are important approaches for the reestablishment of esthetics and function in young patients with a history of generalized aggressive periodontitis (GAP), no predictable outcomes have been reported, and the host osteo-immunoinflammatory response may play a relevant role in this context. In view of the lack of molecular investigations into the bone tissue condition of young patients with periodontitis, the aim of this study was to evaluate the gene expression of bone-related factors in this population. Bone biopsies were obtained from the posterior mandible in 16 individuals previously diagnosed with GAP and on periodontal support therapy and from 17 periodontally healthy (PH) patients. The gene expression of tumor necrosis factor (TNF)-α, transforming growth factor (TGF)-β, receptor activator of the NF-κB ligand (RANKL), osteoprotegerin (OPG), osteocalcin (OC), bone sialoprotein (BSP), and type I collagen (COL-I), important biomarkers of bone turnover, was evaluated by qRT-PCR. Lower TGF-β and OPG mRNA levels were observed in GAP patients compared to PH individuals (p ≤ 0.05). There were no between-group differences in levels of TNF-α, BSP, RANKL, OC, or COL-I mRNA (p>0.05). In young adults, a history of periodontal disease can negatively modulate the gene expression of important bone-related factors in alveolar bone tissue. These molecular outcomes may contribute to the future development of therapeutic approaches to benefit bone healing in young patients with history of periodontitis via modulation of osteo-immuno-inflammatory biomarkers.

Influence of single nucleotide polymorphisms (SNPs) in genetic susceptibility towards periprosthetic osteolysis

Wear debris-induced inflammatory osteolysis remains a significant limiting factor for implant replacement surgeries. Hence, a comprehensive understanding of the complex network of cellular and molecular signals leading to these inflammatory responses is required. Both macrophages and monocytes have a critical role in the instigation of the inflammatory reaction to wear debris but differ in the extent to which they induce cytokine expression in patients. Lately, single nucleotide polymorphisms (SNPs) have been associated with genetic susceptibility among individual patients with implant failure. Studies have shown that SNPs in key pro-inflammatory cytokines and their receptors are associated with osteolytic susceptibility. Likewise, SNPs within several genes involved in the regulation of bone turnover have also been found to be associated with wear debris induced osteolysis. It is presumed that SNP variance might play a decisive role in the activation and signaling of macrophages, osteoblasts, chondrocytes, fibroblasts and other cells involved in inflammatory bone loss. Understanding the extent to which SNPs exist among genes that are responsible for inflammatory bone loss may provide potential targets for developing future therapeutic interventions. Herein, we attempt to summarize the various susceptible genes with possible SNP variance that could contribute to the severity of periprosthetic osteolysis in patients with implants.

Candida albicans Elicits Pro-Inflammatory Differential Gene Expression in Intestinal Peyer's Patches

The details of how gut-associated lymphoid tissues such as Peyer's patches (PPs) in the small intestine play a role in immune surveillance, microbial differentiation and the mucosal barrier protection in response to fungal organisms such as Candida albicans are still unclear. We particularly focus on PPs as they are the immune sensors and inductive sites of the gut that influence inflammation and tolerance. We have previously demonstrated that CD11c⁺ phagocytes that include dendritic cells and macrophages are located in the sub-epithelial dome within PPs sample C. albicans. To gain insight on how specific cells within PPs sense and respond to the sampling of fungi, we gavaged naïve mice with C. albicans strains ATCC 18804 and SC5314 as well as Saccharomyces cerevisiae. We measured the differential gene expression of sorted CD45⁺ B220⁺ B-cells, CD3⁺ T-cells and CD11c⁺ DCs within the first 24 h post-gavage using nanostring nCounter^® technology. The results reveal that at 24 h, PP phagocytes were the cell type that displayed differential gene expression. These phagocytes were able to sample C. albicans and discriminate between strains. In particular, strain ATCC 18804 upregulated fungal-specific pro-inflammatory genes pertaining to innate and adaptive immune responses. Interestingly, PP CD11c⁺ phagocytes also differentially expressed genes in response to C. albicans that were important in the protection of the mucosal barrier. These results highlight that the mucosal barrier not only responds to C. albicans, but also aids in the protection of the host.

A FKBP5 mutation is associated with Paget's disease of bone and enhances osteoclastogenesis

Paget's disease of bone (PDB) is a common metabolic bone disease that is characterized by aberrant focal bone remodeling, which is caused by excessive osteoclastic bone resorption followed by disorganized osteoblastic bone formation. Genetic factors are a critical determinant of PDB pathogenesis, and several susceptibility genes and loci have been reported, including SQSTM1, TNFSF11A, TNFRSF11B, VCP, OPTN, CSF1 and DCSTAMP. Herein, we report a case of Chinese familial PDB without mutations in known genes and identify a novel c.163G>C (p.Val55Leu) mutation in FKBP5 (encodes FK506-binding protein 51, FKBP51) associated with PDB using whole-exome sequencing. Mutant FKBP51 enhanced the Akt phosphorylation and kinase activity in cells. A study of osteoclast function using FKBP51^V55L KI transgenic mice proved that osteoclast precursors from FKBP51^V55L mice were hyperresponsive to RANKL, and osteoclasts derived from FKBP51^V55L mice displayed more intensive bone resorbing activity than did FKBP51^WT controls. The osteoclast-specific molecules tartrate-resistant acid phosphatase, osteoclast-associated receptor and transcription factor NFATC1 were increased in bone marrow-derived monocyte/macrophage cells (BMMs) from FKBP51^V55L mice during osteoclast differentiation. However, c-fos expression showed no significant difference in the wild-type and mutant groups. Akt phosphorylation in FKBP51^V55L BMMs was elevated in response to RANKL. In contrast, IκB degradation, ERK phosphorylation and LC3II expression showed no difference in wild-type and mutant BMMs. Micro-CT analysis revealed an intensive trabecular bone resorption pattern in FKBP51^V55L mice, and suspicious osteolytic bone lesions were noted in three-dimensional reconstruction of distal femurs from mutant mice. These results demonstrate that the mutant FKBP51^V55L promotes osteoclastogenesis and function, which could subsequently participate in PDB development.

Prevalence of polymorphisms in OPG, RANKL and RANK as potential markers for Charcot arthropathy development

Charcot arthropathy is one of the most serious complications of diabetic foot syndrome that leads to amputation of the affected limb. Since there is no cure for Charcot arthropathy, early diagnosis and implementation preventive care are the best available treatment. However, diagnosis is hindered by obscure clinical picture of the disease and lack of molecular markers for its early detection. Results of recent research suggest that OPG-RANKL-RANK axis regulating bone metabolism can be associated with Charcot arthropathy and that SNPs in OPG gene are associated with the disease. Here we report the results of comprehensive analysis of ten SNPs in OPG, RANKL and RANK genes in 260 subjects divided into diabetes, neuropathy and Charcot arthropathy groups. Besides genotype analysis we performed linkage disequilibrium and hierarchical clustering to obtain information about correlation between SNPs. Our results show that OPG 245T/G (rs3134069) and OPG 1217C/T (rs3102734) polymorphisms co-occur in patients with Charcot arthropathy (r2 = 0.99). Moreover, hierarchical clustering revealed a characteristic profile of all SNPs in Charcot arthropathy and neuropathy, which is distinct from control group. Our results suggest that analysis of multiple SNPs can be used as potential marker of Charcot arthropathy and provide insight into possible molecular mechanisms of its development.

Explaining RANKL inhibition by OPG through quantum biochemistry computations and insights into peptide-design for the treatment of osteoporosis

Quantum biochemistry computations are applied to precisely describe important protein–protein interactions, providing a basis for the design of inhibitory peptides against osteoporosis.

Genetics of Paget's disease of bone

Paget's disease of bone (PDB) is a common metabolic bone disease characterised by focal areas of increased bone turnover, which primarily affects people over the age of 55 years. Genetic factors have a fundamental role in the pathogenesis of PDB and are probably the main predisposing factor for the disease. The genetic contribution to PDB susceptibility ranges from rare pathogenic mutations in the single gene SQSTM1 to more common, small effect variants in at least seven genetic loci that predispose to the disease. These loci have additive effects on disease susceptibility and interact with SQSTM1 mutations to affect disease severity, making them a potentially useful tool in predicting disease risk and complication and in managing treatments. Many of these loci harbour genes that have important function in osteoclast differentiation such as CSF1, DCSTAMP and TNFRSF11A. Other susceptibility loci have highlighted new molecular pathways that have not been previously implicated in regulation of bone metabolism such as OPTN, which was recently found to negatively regulate osteoclast differentiation. PDB-susceptibility variants exert their effect either by affecting the protein coding sequence such as variants found in SQSTM1 and RIN3 or by influencing gene expression such as those found in OPTN and DCSTAMP. Epidemiological studies indicate that environmental triggers also have a key role in PDB and interact with genetic factors to influence manifestation and severity of the disease; however, further studies are needed to identify these triggers.

Paget's disease of bone-genetic and environmental factors

Paget's disease of bone is generally diagnosed in individuals aged >50 years, usually manifests in one or several bones and is initiated by osteoclast-induced osteolytic lesions. Subsequently, over a period of many years, osteoblastic activity can result in sclerosis and deformation of bone. The prevalence of Paget's disease is highest in the UK and in countries where a large number of residents have ancestors from the UK. Currently, in many countries, the prevalence of the disorder has decreased. A considerable number of affected patients have a family history of Paget's disease and the disorder has an autosomal dominant pattern of inheritance but with incomplete penetrance. A large number of mutations in SQSTM1 (which encodes sequestosome-1; also known as ubiquitin-binding protein p62) seem to account for the susceptibility to develop Paget's disease in some families; the involvement of other genes is currently under investigation. In addition to a genetic cause, environmental factors have been proposed to have a role in the pathogenesis of Paget's disease. Although most evidence has been presented for measles virus as an aetiologic factor, some studies have not confirmed its involvement. The decreasing incidence of Paget's disease, which could be attributed to measles vaccination along with the measles virus nucleocapsid protein induction of Paget's disease lesions in transgenic mice, supports an aetiologic role of the virus.

Paget's disease of bone: an osteoimmunological disorder?

Osteoimmunology represents a large area of research resulting from the cross talk between bone and immune systems. Many cytokines and signaling cascades are involved in the field of osteoimmunology, originating from various cell types. The RANK/receptor activator of nuclear factor Kappa-B ligand (RANKL)/osteoprotegerin (OPG) signaling has a pivotal role in osteoimmunology, in addition to proinflammatory cytokines such as tumor necrosis factor-α, interleukin (IL)-1, IL-6, and IL-17. Clinically, osteoimmunological disorders, such as rheumatoid arthritis, osteoporosis, and periodontitis, should be classified according to their pattern of osteoimmunological serum biomarkers. Paget’s disease of bone is a common metabolic bone disorder, resulting from an excessively increased bone resorption coupled with aberrant bone formation. With the exception of the cellular responses to measles virus nucleocapsid protein and the interferon-gamma signature, the exact role of the immune system in Paget’s disease of bone is not well understood. The cytokine profiles, such as the increased levels of IL-6 and the interferon-gamma signature observed in this disease, are also very similar to those observed in other osteoimmunological disorders. As a potential osteoimmunological disorder, the treatment of Paget’s disease of bone may also benefit from progress made in targeted therapies, in particular for receptor activator of nuclear factor Kappa-B ligand and IL-6 signaling inhibition.

SNP rs2073618 of the osteoprotegerin gene is associated with diabetic retinopathy in Slovenian patients with type 2 diabetes

Recent studies indicate that osteoprotegerin (OPG) acts as an important regulatory molecule in the vasculature. Also, a strong association was observed between circulation OPG and microvascular complication. By considering the possible role of OPG in diabetic retinopathy (DR) we examined two of the most studied polymorphisms of the OPG genes rs2073618 (located in exon I) and rs3134069 (located in the promoter region) and their relation to DR in Slovenian patients with type 2 diabetes. Logistic regression analysis demonstrated that the carriers of the CC genotype had a 2.2 higher risk for DR than those with either the CG genotype or the GG genotype (codominant model for rs2073618). Furthermore, the combined effect of single nucleotide polymorphisms (SNPs) rs2073618 and rs3134069 on the DR was stronger than that of each SNP alone. The odds ratio (OR) for individuals with CC genotype (rs2073618) and AA genotype (rs3134069) compared with carriers of CG/GG (rs2073618) + AA (rs3134069) was 2.54 (95% CI = 1.26–5.13, P = 0.01). To conclude, these results indicate that SNPs in the OPG gene may be implicated in the pathogenesis of DR.

Role of integrin-linked kinase in osteosarcoma progression

Although integrin-linked kinase (ILK) has been suggested to play a role in the tumorigenesis of a number of human epithelial carcinomas, little is known of its role in musculoskeletal sarcoma. The authors studied ILK expression by immunohistochemistry using osteosarcoma prechemotherapy specimens from 56 patients, and investigated the prognostic implications of the findings obtained. It was found that ILK overexpression was significantly correlated with the presence of distant metastasis (p = 0.008) and that it was an independent prognostic factor for both poor overall survival and poor event-free survival (p = 0.015 and 0.010, respectively). During a transfection experiment conducted by transfecting osteosarcoma cells with ILK siRNA, VEGF concentrations were measured using an ELISA kit, and then compared with those of untransfected controls to evaluate its angiogenic effects. In addition, apoptotic percentages were measured by Annexin-V flow cytometry, and invasive properties were evaluated by measuring the numbers of non-migrating cells in a Boyden chamber. It was found that ILK downregulation significantly decreased angiogenesis, increased apoptosis, and decreased invasiveness of osteosarcoma cells. These results show that ILK is a promising prognostic factor in osteosarcoma and a novel potential therapeutic target for the treatment of osteosarcoma.

Giant cell tumor occurring in familial Paget's disease of bone: report of clinical characteristics and linkage analysis of a large pedigree

Neoplastic degeneration represents a rare but serious complication of Paget's disease of bone (PDB). Although osteosarcomas have been described in up to 1% of PDB cases, giant cell tumors are less frequent and mainly occur in patients with polyostotic disease. We recently characterized a large pedigree with 14 affected members of whom four developed giant cell tumors at pagetic sites. The high number of affected subjects across multiple generations allowed us to better characterize the clinical phenotype and look for possible susceptibility loci. Of interest, all the affected members had polyostotic PDB, but subjects developing giant cell tumors showed an increased disease severity with a reduced clinical response to bisphosphonate treatment and an increased prevalence of bone pain, deformities, and fractures. Together with an increased occurrence of common pagetic complications, affected patients of this pedigree also evidenced a fivefold higher prevalence of coronary artery disease with respect to either the unaffected family members or a comparative cohort of 150 unrelated PDB cases from the same geographical area. This association was further enhanced in the four cases with PDB and giant cell tumors, all of them developing coronary artery disease before 60 years of age. Despite the early onset and the severe phenotype, PDB patients from this pedigree were negative for the presence of SQSTM1 or TNFRSF11A mutations, previously associated with enhanced disease severity. Genome-wide linkage analysis identified six possible candidate regions on chromosomes 1, 5, 6, 8, 10, and 20. Because the chromosome 8 and 10 loci were next to the TNFRSF11B and OPTN genes, we extended the genetic screening to these two genes, but we failed to identify any causative mutation at both the genomic and transcription level, suggesting that a different genetic defect is associated with PDB and potentially giant cell tumor of bone in this pedigree.

Paget's Disease of Bone: A Review of Epidemiology, Pathophysiology and Management

Paget's disease of bone is a common disorder which may affect one or many bones. Although many patients are asymptomatic, a variety of symptoms and complications may occur. Fortunately, effective pharmacologic therapy, primarily with potent bisphosphonates, is now available to treat patients with complications or symptoms. This review of Paget's disease of bone will include epidemiology and pathophysiology, complications and clinical findings, indications for treatment, and the drugs currently available to treat this condition.

Pathogenesis of Paget disease of bone

Paget disease of bone (PDB) is a common disease characterized by focal areas of increased and disorganized bone turnover. Some patients are asymptomatic, whereas others develop complications such as pain, osteoarthritis, fracture, deformity, deafness, and nerve compression syndromes. PDB is primarily caused by dysregulation of osteoclast differentiation and function, and there is increasing evidence that this is due, in part, to genetic factors. One of the most important predisposing genes is SQSTM1, which harbors mutations that cause osteoclast activation in 5-20 % of PDB patients. Seven additional susceptibility loci for PDB have been identified by genomewide association studies on chromosomes 1p13, 7q33, 8q22, 10p13, 14q32, 15q24, and 18q21. Although the causal variants remain to be discovered, three of these loci contain CSF1, TNFRSF11A, and TM7SF4, genes that are known to play a critical role in osteoclast differentiation and function. Environmental factors are also important in the pathogenesis of PDB, as reflected by the fact that in many countries the disease has become less common and less severe over recent years. The most widely studied environmental trigger is paramyxovirus infection, but attempts to detect viral transcripts in tissues from patients with PDB have yielded mixed results. Although our understanding of the pathophysiology of PDB has advanced tremendously over the past 10 years, many questions remain unanswered, such as the mechanisms responsible for the focal nature of the disease and the recent changes in prevalence and severity.

A nonsynonymous TNFRSF11A variation increases NFκB activity and the severity of Paget's disease

Mutations in the SQSTM1 gene were identified as a common cause of Paget's disease of bone (PDB) but experimental evidence demonstrated that SQSTM1 mutation is not sufficient to induce PDB in vivo. Here, we identified two nonsynonymous single nucleotide polymorphisms (SNPs) (C421T, H141Y and T575C, V192A) in the TNFRSF11A gene, associated with PDB and with the severity of phenotype in a large population of 654 unrelated patients that were previously screened for SQSTM1 gene mutations. The largest effect was found for the T575C variant, yielding an odds ratio of 1.29 (p = 0.003), with the C allele as the risk allele. Moreover, an even more significant p-value (p = 0.0002) was observed in the subgroup of patients with SQSTM1 mutation, with an odds ratio of 1.71. Interestingly, patients with the C allele also showed an increased prevalence of polyostotic disease (68%, 53%, and 51% in patients with CC, CT, and TT genotypes, respectively; p = 0.01), as well as an increased number of affected skeletal sites (2.9, 2.5, and 2.0 in patients with CC, CT, and TT genotypes, respectively, p = 0.008). These differences increased when analyses were restricted to cases with SQSTM1 mutation. In human cell lines, cotrasfection with mutated SQSTM1 and TNFRSF11A(A192) produced a level of activation of NFκB signaling greater than cotrasfection with wild-type SQSTM1 and TNFRSF11A(V192), confirming genetics and clinical evidences. These results provide the first evidence that genetic variation within the OPG/RANK/RANKL system influences the severity of PBD in synergistic action with SQSTM1 gene mutations.

Indications for a genetic association of a VCP polymorphism with the pathogenesis of sporadic Paget's disease of bone, but not for TNFSF11 (RANKL) and IL-6 polymorphisms

Paget's disease of bone (PDB) is, after osteoporosis, the second most common metabolic bone disorder in the elderly Caucasian population. Mutations in the sequestosome 1 gene (SQSTM1) are responsible for the etiology of PDB in a subset of patients, but the disease pathogenesis in the remaining PDB patients is still unknown. Therefore association studies investigating the relationship between genetic polymorphisms and sporadic PDB have been performed in order to find the susceptibility polymorphisms. In this paper, we sought to determine whether polymorphisms in 3 functional candidate genes play a role in the development of sporadic PDB: TNFSF11 (receptor activator of nuclear factor κB ligand, RANKL), VCP (valosin-containing protein) and IL-6 (interleukin 6). Analyzing 9 tag SNPs and 2 multi-marker tests (MMTs) in TNFSF11, 3 tag SNPs and 1 MMT in VCP and 8 tag SNPs in IL-6 in a population of 196 Belgian patients with sporadic PDB and 212 Belgian control individuals revealed that one VCP SNP (rs565070) turned out to be associated with PDB in this Belgian study population (p=5.5×10(-3)). None of the tag SNPs or MMTs selected for TNFSF11 or IL-6 was associated with PDB. Still, replication of our findings in the VCP gene in other populations is important to confirm our results. However, when combining data of VCP with those from other susceptible gene regions from previous association studies (i.e. TNFRSF11A, CSF1, OPTN and TM7SF4), independent effect of each gene region was found and the cumulative population attributable risk is 72.7%.

Emerging strategies and therapies for treatment of Paget's disease of bone

Paget's disease of bone (PDB) is a progressive monostotic or polyostotic metabolic bone disease characterized by focal abnormal bone remodeling, with increased bone resorption and excessive, disorganized, new bone formation. PDB rarely occurs before middle age, and it is the second most frequent metabolic bone disorder after osteoporosis, affecting up to 3% of adults over 55 years of age. One of the most striking and intriguing clinical features is the focal nature of the disorder, in that once the disease is established within a bone, there is only local spread within that bone and no systemic dissemination. Despite many years of intense research, the etiology of PDB has still to be conclusively determined. Based on a detailed review of genetic and viral factors incriminated in PDB, we propose a unifying hypothesis from which we can suggest emerging strategies and therapies. PDB results in weakened bone strength and abnormal bone architecture, leading to pain, deformity or, depending on the bone involved, fracture in the affected bone. The diagnostic assessment includes serum total alkaline phosphatase, total body bone scintigraphy, skull and enlarged view pelvis x-rays, and if needed, additional x-rays. The ideal therapeutic option would eliminate bone pain, normalize serum total alkaline phosphatase with prolonged remission, heal radiographic osteolytic lesions, restore normal lamellar bone, and prevent recurrence and complications. With the development of increasingly potent bisphosphonates, culminating in the introduction of a single intravenous infusion of zoledronic acid 5 mg, these goals of treatment are close to being achieved, together with long-term remission in almost all patients. Based on the recent pathophysiological findings, emerging strategies and therapies are reviewed: ie, pulse treatment with zoledronic acid; denosumab, a fully human monoclonal antibody directed against RANK ligand; tocilizumab, an interleukin-6 receptor inhibitor; odanacatib, a cathepsin K inhibitor; and proteasome and Dickkopf-1 inhibitors.

Genetic variation in the TNFRSF11A gene encoding RANK is associated with susceptibility to Paget's disease of bone

RANK (receptor activator of nuclear factor-κB), encoded by TNFRSF11A, is a key protein in osteoclastogenesis. TNFRSF11A mutations cause Paget's disease of bone (PDB)-like diseases (ie, familial expansile osteolysis, expansile skeletal hyperphosphatasia, and early-onset PDB) and an osteoclast-poor form of osteopetrosis. However, no TNFRSF11A mutations have been found in classic PDB, neither in familial nor in isolated cases. To investigate the possible relationship between TNFRSF11A polymorphisms and sporadic PDB, we conducted an association study including 32 single-nucleotide polymorphisms (SNPs) in 196 Belgian sporadic PDB patients and 212 control individuals. Thirteen SNPs and 3 multimarker tests (MMTs) turned out to have a p value of between .036 and 3.17 × 10(-4) , with the major effect coming from females. Moreover, 6 SNPs and 1 MMT withstood the Bonferroni correction (p < .002). Replication studies were performed for 2 nonsynonymous SNPs (rs35211496 and rs1805034) in a Dutch and a British cohort. Interestingly, both SNPs resulted in p values ranging from .013 to 8.38 × 10(-5) in both populations. Meta-analysis over three populations resulted in p = .002 for rs35211496 and p = 1.27 × 10(-8) for rs1805034, again mainly coming from the female subgroups. In an attempt to identify the underlying causative SNP, we performed functional studies for the coding SNPs as well as resequencing efforts of a 31-kb region harboring a risk haplotype within the Belgian females. However, neither approach resulted in significant evidence for the causality of any of the tested genetic variants. Therefore, further studies are needed to identify the real cause of the increased risk to develop PDB shown to be present within TNFRSF11A.

Genetic aspects of the Paget's disease of bone: concerns on the introduction of DNA-based tests in the clinical practice. Advantages and disadvantages of its application

BACKGROUND

A large amount of genetic studies have clearly demonstrated the existence of a genetic susceptibility to Paget's disease of bone (PDB). Although the disease is genetically heterogeneous, the SQSTM1/p62 gene, encoding a protein with a pathophysiological role in both osteoclast differentiation and activity, has been found worldwide to harbour germline mutations in most of the PDB patients from geographically distant populations originating from different areas of Europe, both in sporadic and familial cases.

MATERIALS AND METHODS

Thus, SQSTM1/p62 gene mutations may confer an increased lifetime risk of developing PDB.

RESULTS

Several different genotype-phenotype analyses have shown a high penetrance for such mutations. These results suggest the opportunity to perform genetic testing in affected individuals and then, after the identification of a SQSTM1/p62 gene germline mutation, in their relatives as a real and concrete strategy to increase the diagnostic sensitivity in most of the asymptomatic mutant carriers. However, it is of note to underlie that an incomplete penetrance for SQSTM1/p62 gene mutations has also been reported.

CONCLUSIONS

In light of all these contradictory evidences, a review on whether, when and why apply the DNA test to those subjects, its interpretation and clinical application is necessary. In fact, a growing number of preventive care options are now available to affected patients and families and the process of systematically assessing risk is becoming increasingly important for both patients and physicians.

Genome-wide association study identifies variants at CSF1, OPTN and TNFRSF11A as genetic risk factors for Paget's disease of bone

Paget’s disease of bone (PDB) is a common disorder with a strong genetic component characterised by focal increases in bone turnover which in some cases is caused by SQSTM1 mutations. To identify additional susceptibility genes we performed a genome wide association study in 750 PDB cases without SQSTM1 mutations and 1002 controls and identified three candidate loci for the disease which were replicated in an independent set of 500 cases and 535 controls. The strongest signal was with rs484959 on 1p13 close to the CSF1 gene (P = 5.38 × 10⁻²⁴) and significant associations were also observed with rs1561570 on 10p13 within the OPTN gene (P = 6.09 × 10⁻¹³) and with rs3018362 on 18q21 close to the TNFRSF11A gene (P = 5.27 × 10⁻¹³). These studies provide new insights into the pathogenesis of PDB and identify OPTN, CSF1 and TNFRSF11A as novel candidate genes for disease susceptibility.

Association between osteoprotegerin gene polymorphism and bone mineral density in patients with adolescent idiopathic scoliosis

Generalized low bone mass and osteopenia have been reported in the axial and peripheral skeleton of adolescent idiopathic scoliosis (AIS) patients. Recently, many studies have shown that gene polymorphisms are related to osteoporosis. However, no studies have linked the association between gene polymorphisms and bone mass of AIS. Therefore, this study examined the association between the bone mass and RANKL, RANK, and OPG gene polymorphisms in 198 girls diagnosed with AIS. OPG 163 A --> G, 209 G --> A, 245 T --> G, and 1181 G --> C polymorphisms; RANK 421 C --> T and 575 C --> T polymorphisms; and RANKL rs12721445 and rs2277438 polymorphisms, as well as the bone mineral density at the lumbar spine (LSBMD) and femoral neck (FNBMD) were analyzed. The 163 A --> G, 209 G --> A, and 245 T --> G polymorphisms in the OPG gene were in complete linkage. No RANK 421 C --> T and 575 C --> T polymorphisms or RANKL rs12711445 polymorphism were observed. There was a significant association between the OPG gene 1181 G --> C polymorphism and LSBMD. LSBMD in AIS with the CC genotype was found to be significantly higher than in AIS with the GC (P < 0.05) or GG (P < 0.01) genotype. However, there was no significant association between LSBMD or FNBMD and the OPG gene 245 T --> G polymorphism or the RANKL rs2277438 polymorphism. These results suggest that the OPG gene 1181 G --> C polymorphism is associated with LSBMD in girls with AIS.

Therapy for osteosarcoma: where do we go from here?

Osteosarcoma is the most common malignant primary bone tumor in children and adolescents. Current optimal treatment for osteosarcoma consists of multi-agent chemotherapy and aggressive surgical resection of all sites of disease involvement. The current national and international cooperative trial for patients with newly diagnosed osteosarcoma builds upon the backbone of cisplatin, doxorubicin, and methotrexate. This protocol is designed to clarify whether (i) the addition of ifosfamide and etoposide to postoperative chemotherapy with cisplatin, doxorubicin, and methotrexate improves the event-free survival and overall survival for patients with resectable osteosarcoma and a poor histologic response to 10 weeks of preoperative chemotherapy; and (ii) the addition of pegylated interferon-alpha-2b as maintenance therapy after postoperative chemotherapy with cisplatin, doxorubicin, and methotrexate improves the event-free survival and overall survival for patients with resectable osteosarcoma and a good histologic response to 10 weeks of preoperative chemotherapy. However, the optimal treatment strategy (or strategies) for patients with relapsed or metastatic disease has yet to be defined. This remains one of the persistent challenges in the treatment of osteosarcoma. Recent therapeutic advances have focused on circumventing chemotherapy resistance mechanisms, incorporation of non-classical agents into upfront therapy, targeting of the tumor micro-environment, and investigating the role of novel delivery mechanisms. In patients with localized disease the 5-year survival rate is at least 70%; patients with metastatic or recurrent disease have <20% chance of long-term survival despite aggressive therapies. These figures have changed little in the past 2 decades. This review focuses on the current therapy for osteosarcoma, and highlights emerging strategies that will hopefully change the outlook for patients with this disease.

Pathogenesis of Paget's disease of bone

Paget's disease of bone is a common condition characterised by increased and disorganised bone turnover which can affect one or several bones throughout the skeleton. These abnormalities disrupt normal bone architecture and lead to various complications such as bone pain osteoarthritis, pathological fracture, bone deformity, deafness, and nerve compression syndromes. Genetic factors play an important role in PDB and mutations or polymorphisms have been identified in four genes that cause classical Paget's disease and related syndromes. These include TNFRSF11A, which encodes RANK, TNFRSF11B which encodes osteoprotegerin, VCP which encodes p97, and SQSTM1 which encodes p62. All of these genes play a role in the RANK-NFkappaB signalling pathway and it is likely that the mutations predispose to PDB by disrupting normal signalling, leading to osteoclast activation. Although Paget's has traditionally be considered a disease of the osteoclast there is evidence that stromal cell function and osteoblast function are also abnormal, which might account for the fact that the disease is associated with increased bone formation as well as resorption. Environmental factors also contribute to Paget's disease. Most research has focused on paramyxovirus infection as a possible environmental trigger but evidence in favour of the involvement of viruses in the disease remains conflicting. Other factors which have been implicated as possible disease triggers include mechanical loading, dietary calcium and environmental toxins. Further work will be required to identify additional genetic variants that predispose to Paget's disease and to determine how the causal mutations and predisposing polymorphisms interact with environmental factors to influence bone cell function and cause the focal bone lesions that are characteristic of the disease.

Current status and perspectives regarding the treatment of osteo-sarcoma: chemotherapy

Osteosarcoma is the most common primary bone tumor in childhood and adolescence. The use of combination chemotherapy and surgery enables long-term survival in approximately 60-70% of cases. However, the necessity for surgery, the poor prognosis of patients with metastatic or recurrent disease (long-term survival in only about 20% of cases), and the lack of establishment of second-line chemotherapy suggest that improvements in chemotherapy are desperately needed. Currently, in an effort to extend the protocol with the chemotherapy drugs that already exist, high-dose chemotherapy with/without autologous peripheral blood stem cell transplantation, and tumor-targeted drug delivery systems are under investigation. Future drug developments will no doubt lie in the direction of immunotherapy and anti-angiogenic therapy, as well as the use of cytotoxic drugs. Identifying the genes and signal transduction pathways responsible for the development of osteosarcoma or for the occurrence of malignancy in cases of osteosarcoma will undoubtedly lead to the identification of pathway-specific agents, or possible gene therapy. Furthermore, as increased light is shed on the character of osetoblastic differentiation in osteosarcoma, this will certainly give rise to new treatments utilizing differentiation therapy. This article reviews the current status and perspectives regarding the treatment of osteosarcoma in terms of chemotherapy.

Osteosarcoma development and stem cell differentiation

Osteosarcoma is the most common nonhematologic malignancy of bone in children and adults. The peak incidence occurs in the second decade of life, with a smaller peak after age 50. Osteosarcoma typically arises around the growth plate of long bones. Most osteosarcoma tumors are of high grade and tend to develop pulmonary metastases. Despite clinical improvements, patients with metastatic or recurrent diseases have a poor prognosis. Here, we reviewed the current understanding of human osteosarcoma, with an emphasis on potential links between defective osteogenic differentiation and bone tumorigenesis. Existing data indicate osteosarcoma tumors display a broad range of genetic and molecular alterations, including the gains, losses, or arrangements of chromosomal regions, inactivation of tumor suppressor genes, and the deregulation of major signaling pathways. However, except for p53 and/or RB mutations, most alterations are not constantly detected in the majority of osteosarcoma tumors. With a rapid expansion of our knowledge about stem cell biology, emerging evidence suggests osteosarcoma should be regarded as a differentiation disease caused by genetic and epigenetic changes that interrupt osteoblast differentiation from mesenchymal stem cells. Understanding the molecular pathogenesis of human osteosarcoma could ultimately lead to the development of diagnostic and prognostic markers, as well as targeted therapeutics for osteosarcoma patients.

Association between osteoprotegerin (OPG), receptor activator of nuclear factor-kappaB (RANK), and RANK ligand (RANKL) gene polymorphisms and circulating OPG, soluble RANKL levels, and bone mineral density in Korean postmenopausal women

OBJECTIVE

To investigate the association between osteoprotegerin (OPG), receptor activator of nuclear factor-kappaB (RANK), and RANK ligand (RANKL) gene polymorphisms and circulating OPG, soluble RANKL (sRANKL) levels, and bone mineral density (BMD) in Korean postmenopausal women.

DESIGN

The OPG gene A163G, G209A, T245G, and G1181C polymorphisms, the RANK gene C421T and C575T polymorphisms, and the RANKL rs12721445 and rs2277438 polymorphisms were analyzed in 385 Korean postmenopausal women. Levels of serum OPG, soluble RANKL, osteocalcin, C-telopeptide of type I collagen, parathyroid hormone, calcium, and phosphorus and BMD at the lumbar spine and femoral neck were measured.

RESULTS

The A163G, G209A, and T245G polymorphisms in the OPG gene were in complete linkage. The RANK C421T and C575T polymorphisms and the RANKL rs12711445 polymorphism were not observed. An association with BMD was found only for the OPG G1181C polymorphism, and BMD at the lumbar spine in women with the CC genotype was significantly higher than in women with the GC or GG genotype, with a C allele dose effect. In itself, the RANKL rs2277438 polymorphism was not related to BMD, but by combining the RANKL genotypes with the GC genotypes of the OPG G1181C polymorphism, the association with BMD at the lumbar spine became significant. No significant differences in the levels of any biochemical marker among genotypes of these polymorphisms were found.

CONCLUSIONS

The OPG gene G1181C polymorphism, alone and in combination with the RANKL rs2277438 polymorphism, was identified as a genetic factor associated with BMD of the lumbar spine in Korean women.

Identification of sex-specific associations between polymorphisms of the osteoprotegerin gene, TNFRSF11B, and Paget's disease of bone

We studied the role of TNFRSF11B polymorphisms on the risk to develop Paget's disease of bone in a Belgian study population. We observed no association in men, but a highly significant association was found in women, and this was confirmed in a population from the United Kingdom.

INTRODUCTION

Juvenile Paget's disease has been shown to be caused by mutations in TNFRSF11B encoding osteoprotegerin. Although mutations in this gene have never been found in patients with typical Paget's disease of bone (PDB), there are indications that polymorphisms in TNFRSF11B might contribute to the risk of developing PDB.

MATERIALS AND METHODS

We recruited a population of 131 Belgian patients with sporadic PDB and 171 Belgian controls. By means of the HapMap, we selected 17 SNPs that, in combination with four multimarker tests, contain most information on common genetic variation in TNFRSF11B. To replicate the findings observed in the Belgian study population, genotyping data of SNPs generated in a UK population were reanalyzed.

RESULTS

In our Belgian study population, associations were found for two SNPs (rs11573871, rs1485286) and for one multimarker test involving rs1032129. When subsequently analyzing men and women separately, these associations turned out to be driven by women (56 cases, 78 controls). In addition, three other tagSNPs turned out to be associated in women only. These were rs2073617 (C950T), rs6415470, and rs11573869. Reanalysis of genotyping data from a UK study population indicated that the associations found for C950T and C1181G were also exclusively driven by women (146 cases, 216 controls). Meta-analysis provided evidence for risk increasing effects of the T allele of C950T and the G allele of C1181G in the female population (p = 0.002 and 0.003, respectively). The haplotypes formed by the SNPs associated in the Belgian population were also distributed differentially between female cases and controls.

CONCLUSIONS

We showed for the first time that SNPs influencing the risk to develop PDB could be sex-specific. Further research is necessary to identify the causative variants in TNFRSF11B and to elucidate the molecular pathogenic mechanism.

Sporadic hyperphosphatasia syndrome featuring periostitis and accelerated skeletal turnover without receptor activator of nuclear factor-kappaB, osteoprotegerin, or sequestosome-1 gene defects

CONTEXT

A middle-aged woman with recent-onset painful swollen fingers and widespread periostitis, elevated serum alkaline phosphatase (ALP) activity and erythrocyte sedimentation rate, and accelerated skeletal turnover was found not to have mutations in the gene sequences for exon 1 of receptor activator of nuclear factor-kappaB (RANK), osteoprotegerin (OPG), or sequestosome-1.

INTRODUCTION

Hyperphosphatasia refers to disorders that feature elevated serum ALP activity (hyperphosphatasemia) usually from excesses of the bone isoform of ALP. Such conditions include familial expansile osteolysis, expansile skeletal hyperphosphatasia, and a familial form of early-onset Paget's disease of bone (PDB2), all from constitutive activation of RANK, and juvenile Paget's disease from OPG deficiency.

PATIENT AND METHODS

A 38-yr-old woman developed painful swollen fingers and achy bones after an episode of unexplained pericarditis and restrictive lung disease. Sequence analysis of exon 1 of TNFRSF11A encoding RANK, TNFRSF11B encoding OPG, and SQSTM1 encoding sequestosome-1 searched for mutations responsible for familial expansile osteolysis, expansile skeletal hyperphosphatasia, or PDB2, juvenile Paget's disease, or Paget's disease of bone (PDB), respectively.

RESULTS

Serum ALP and osteocalcin and urinary hydroxyproline were increased. Radiographs showed widespread, symmetric hyperostosis in the limbs where bone scintigraphy demonstrated enhanced radionuclide uptake. Iliac crest histology revealed accelerated skeletal turnover. No mutations were detected in the three genes examined. Three years of therapy with 70 mg alendronate orally once weekly improved symptoms, radiographic abnormalities, and biochemical markers.

CONCLUSIONS

Our patient manifested a unique, sporadic hyperphosphatasia syndrome. Unexplained, transient inflammation seemed to cause her pericarditis, restrictive lung disease, and periostitis with accelerated skeletal turnover that responded well to antiinflammatory drugs and alendronate therapy.

Contribution of genetic factors to the pathogenesis of Paget's disease of bone and related disorders

Paget's disease of bone (PDB) is a common condition with a strong genetic component that is characterized by focal increases in bone turnover, leading to bone deformity, pathological fractures, and various other complications. Several rare disorders have also been described that show phenotypic overlap with PDB. Genome-wide searches have identified several susceptibility loci for PDB and PDB-like disorders, and mutations that cause these disorders have now been identified in four genes, all of which are involved in the RANK-NF-kappaB signaling pathway. Mutations in SQSTM1, which encodes an important scaffold protein in this pathway, have been found to be a common cause of classical PDB. Thus far, all disease-causing mutations in SQSTM1 affect the ubiquitin-associated (UBA) domain of the gene product and cause loss of ubiquitin binding. The rare PDB-like disorders of familial expansile osteolysis, early-onset familial PDB, and expansile skeletal hyperphosphatasia are caused by duplication mutations in exon 1 of the TNFRSF11A gene, which encodes the RANK receptor. This gene does not seem to be involved in the pathogenesis of classical PDB. Inactivating mutations in the TNFRSF11B gene, which encodes osteoprotegerin, cause juvenile PDB, and TNFRSF11B polymorphisms seem to increase the risk of classical PDB. The rare syndrome of hereditary inclusion body myopathy, PDB, and frontotemporal dementia (IBMPFD) is caused by mutations in the VCP gene, which is involved in regulating I-kappaB degradation by the proteasome. The disease-causing mutations in VCP cluster in and around a domain involved in ubiquitin binding. Whereas SQSTM1 has emerged as an important gene for classical PDB, most kindreds with familial PDB do not carry SQSTM1 mutations, indicating that additional genes for PDB remain to be discovered. In light of the molecular defects that have been identified thus far, it seems likely that these genes will also be involved in the RANK-NF-kappaB signaling pathway or its interactions with the ubiquitin-proteasome system.

Identification and molecular characterization of a novel splice-site mutation (G1205C) in the SQSTM1 gene causing Paget's disease of bone in an extended American family

Paget's disease of bone (PDB) is a common late-onset bone disorder characterized by focal areas of abnormal bone remodeling. Positional cloning efforts resulted in the identification of seven genetic loci (PDB1-7) with putative involvement in the pathogenesis of PDB. Meanwhile, the PDB-causing gene from the PDB3 region on chromosome 5q35 has been identified as the SQSTM1 gene. All mutations identified in this gene so far are located in or close to the ubiquitin-associated (UBA) domain of the protein. In 2001, we reported genotyping results of genetic markers located in the PDB3 region in an extended American family, indicating the involvement of the PDB3 locus. Here, we report the identification of a novel mutation (G1205C) in the SQSTM1 gene in this family. The G1205C mutation is located in the splice donor site of intron 7 and reverse-transcription polymerase chain reaction experiments showed that the presence of the C allele results in the production of two abnormal mRNA transcripts. Translation of the first transcript would result in a protein that lacks amino acids 351-388, including 26 amino acids of the second PEST domain in addition to two amino acids of the UBA domain. The second mutant mRNA transcript could result in a truncated protein (390X) that lacks almost the complete UBA domain. PDB mutations that disrupt the function of the PEST domain of SQSTM1 have not been reported before, so probably the pathogenic effect of both transcripts resides in the disruption of the ubiquitin-binding properties of the protein.

Evaluation of RANK/RANKL/OPG gene polymorphisms in aggressive periodontitis

BACKGROUND AND OBJECTIVE

Aggressive periodontitis (AgP) is a specific type of periodontal disease that is characterized by rapid attachment loss and bone destruction. While attempting to identify genetic polymorphisms associated with AgP, previous research has focused on candidate genes that may be involved in immune responses to microbial infections. In this study, the focus was on single nucleotide polymorphisms (SNPs) in the key mediators of osteoclast differentiation and activation, which involve receptor activator of nuclear factor-kappaB (RANK), RANK ligand (RANKL) and osteoprotegrin (OPG), in the Japanese population. The aim of this study was to evaluate the association of RANK/RANKL/OPG gene polymorphisms with AgP in the Japanese population.

MATERIAL AND METHODS

We examined 99 patients with AgP and 89 controls from the Japanese population to explore the possibility of RANK/RANKL/OPG loci as candidate regions associated with the disease. All exons and relevant exon-intron boundaries of these three candidate genes were amplified by polymerase chain reaction (PCR) using 19 primers, followed by direct sequencing. The polymorphisms were identified by comparing the sequences obtained from 48 subjects.

RESULTS

We identified 27 SNPs in RANK, including 10 novel SNPs and seven SNPs each in both RANKL and OPG. A pairwise linkage disequilibrium analysis using the r2 statistic showed that some SNP pairs from the three loci are in tight linkage disequilibrium.

CONCLUSION

An association analysis with allelotypes showed that SNPs identified in the RANK/RANKL/OPG genes have no significant association with AgP in the Japanese population.

Chemotherapy resistance in osteosarcoma: current challenges and future directions

For patients with osteosarcoma, the use of chemotherapy has improved survival from 11% with surgical resection alone in the 1960s, to 70% by the mid-1980s. However, survival has since plateaued, despite advances in anticancer therapy. Elucidation of the mechanisms of chemoresistance and implementation of strategies to overcome chemoresistance will likely be pivotal to improving survival. In this review, the focus is on the current understanding of the mechanisms of resistance to the most commonly used agents in the treatment of osteosarcoma and the methods employed to overcome chemotherapy resistance.

A novel mutation (K378X) in the sequestosome 1 gene associated with increased NF-kappaB signaling and Paget's disease of bone with a severe phenotype

Sequestosome 1/p62 (p62) mutations are associated with PDB; however, there are limited data regarding functional consequences. We report a novel mutation in exon 7 (K378X) in a patient with polyostotic Paget's disease of bone. p62 mutants increased NF-kappaB activation and significantly potentiated osteoclast formation and bone resorption in human primary cell cultures.

INTRODUCTION

Sequestosome 1/p62 (p62) mutations are associated with Paget's disease of bone (PDB); however, there are limited data regarding functional consequences. One report has linked the common P392L mutation in the p62 ubiquitin binding associated (UBA) domain with increases in NF-kappaB activity, a transcription factor essential for osteoclastogenesis. To further clarify the functional impact of p62 mutations associated with PDB, we assessed the effect of p62 mutation (a novel mutation: K378X, and previously reported mutations: P392L and E396X) on RANK-induced NF-kappaB activation and compared this with the effect of wildtype p62. In addition, we studied the effect of p62 mutation on osteoclast formation and bone resorption.

MATERIALS AND METHODS

We performed co-transfection experiments with expression plasmids for p62 (wildtype or mutated) and RANK and an NF-kappaB luciferase reporter gene. Luciferase activities were recorded after addition of luciferin to cellular lysates. RAW(264.7) cells stably expressing enhanced green fluorescent protein (EGFP)-tagged p62 (wildtype, K378X, or P392L) or EGFP alone were assessed for changes in cell proliferation. Additionally, these cells were stimulated with RANKL to produce osteoclast-like cells (OLCs). Primary human monocytes collected from the K378X-affected patient and a control subject were stimulated to form OLCs and bone resorption data were obtained.

RESULTS

The novel mutation introduces a premature stop codon in place of Lys-378 and thereby eliminates the entire p62 UBA domain; this and two additional natural mutations (P392L, E396X) increased NF-kappaB activation compared with wildtype p62. Wildtype p62 consistently inhibited NF-kappaB activation compared with empty vector. UBA mutations (K378X and P392L) significantly increased the number of OLCs formed in response to RANKL and also the number of nuclei of the OLCs. K378X-affected human monocytes formed more OLCs with more nuclei and increased bone resorption compared with control monocytes.

CONCLUSIONS

Our data show that mutation of the p62 UBA domain results in increased activation of NF-kappaB and osteoclast formation and function compared with wildtype p62. These results may partially explain the mechanism by which p62 mutation contributes to the pathogenesis of PDB.

Mechanisms of Disease: genetics of Paget's disease of bone and related disorders

Paget's disease of bone (PDB) is a common disorder in which focal abnormalities of increased bone turnover lead to complications such as bone pain, deformity, pathological fractures, and deafness. PDB has a strong genetic component and several susceptibility loci for the disease have been identified by genome-wide scans. Mutations that predispose individuals to PDB and related disorders have been identified in four genes. The rare PDB-like syndromes of familial expansile osteolysis, early-onset familial PDB, and expansile skeletal hyperphosphatasia are caused by insertion mutations in TNFRSF11A, which encodes receptor activator of nuclear factor (NF)kappaB (RANK)-a critical regulator of osteoclast function. Inactivating mutations in TNFRSF11B, which encodes osteoprotegerin (a decoy receptor for RANK ligand) cause idiopathic hyperphosphatasia, and polymorphisms in this gene seem to increase the risk for classical PDB. Mutations of the sequestosome 1 gene (SQSTM1), which encodes an important scaffold protein in the NFkappaB pathway, are a common cause of classical PDB. The rare syndrome of hereditary inclusion body myopathy, PDB, and fronto-temporal dementia is caused by mutations in the valosin-containing protein (VCP) gene. This gene encodes VCP, which has a role in targeting the inhibitor of NFkappaB for degradation by the proteasome. Several additional genes for PDB remain to be discovered, and it seems likely that they will also involve the RANK-NFkappaB signaling pathway or components of the proteasomal processing of this pathway, underscoring the critical importance of this signaling pathway in bone metabolism and bone disease.

Paget's Disease of Bone and Genetic Disorders of RANKL/OPG/RANK/NF- B Signaling

Identification of the RANKL/OPG/RANK/NF-kB (receptor activator of nuclear factor kappa-B ligand / osteoprotegerin) signaling pathway as the major regulatory system for osteoclastogenesis began with discovery of these ligands and receptors in the tumor necrosis factor (TNF) superfamily. Subsequently, genetically altered mice revealed physiologic roles for these proteins in bone biology. However, full appreciation of their significance for the human skeleton came from clinical characterization of several extremely rare, heritable disorders followed by discovery of their genetic bases. Familial expansile osteolysis (FEO) is an autosomal dominant disorder featuring constitutive activation of RANK due to an 18-bp tandem duplication in its gene (TNFRSF11A). A similar, 27-bp duplication causes what has been called a familial form of early-onset Paget's disease of bone (PDB2). Expansile skeletal hyperphosphatasia (ESH) is allelic to FEO and PDB2 and involves a 15-bp tandem duplication in TNFRSF11A. Autosomal recessive inheritance of deactivating mutations of the gene encoding OPG (TNFRSF11B) causes most cases of juvenile Paget disease. These disorders feature high bone turnover, deafness during early childhood, "idiopathic external lysis" of adult teeth, and sometimes focal lesions in appendicular bones that mimic active PDB. Biochemical markers indicate rapid skeletal remodeling. In FEO, osteolysis progresses to fat-filled bone rather than to osteosclerosis. Antiresorptive therapy with bisphosphonates can be effective for each disorder.

Genetic susceptibility to hip arthroplasty failure--association with the RANK/OPG pathway

The OPG/RANK/RANKL system has been implicated in the biological cascade of events initiated by particulate wear debris and bacterial infection resulting in periprosthetic bone loss around total hip arthroplasties (THA). Individual responses to such stimuli may be dictated by genetic variation caused by single nucleotide polymorphisms (SNPs). Case control study of the osteoprotegerin and RANK genes for possible association with deep sepsis or aseptic loosening. All patients were Caucasian and had had a cemented Charnley THA and polyethylene acetabular cup. Cases consisted of 91 patients with early aseptic loosening and 71 patients with deep infection. Controls were 150 clinically and radiologically well-fixed THAs. DNA samples were genotyped using Taqman allelic discrimination. The A allele (p<0.001) and genotype A/A (p<0.001) for the OPG-163 SNP were associated with aseptic failure. Additionally, the RANK +575 (C/T SNP) T allele (p=0.004) and T/T genotype (p=0.008) frequencies were associated with aseptic failure. Comparing the septic group with the controls, the frequency of the A allele (p<0.001) and the genotype A/A (p<0.001) for the OPG-163 SNP were statistically significant. Aseptic loosening and deep infection of THA may be under the influence of susceptibility genes. SNP markers may serve as predictors of implant survival.

Genetics of Paget's disease of bone

Paget's disease of bone is a chronic bone disease that affects up to 3% of Caucasians older than 55 years. The cause of Paget's disease is unknown but involves genetic factors. Familial cases display an autosomal dominant pattern of inheritance with incomplete penetrance. Genetic heterogeneity has been demonstrated and eight potential susceptibility loci identified. There is sound evidence incriminating Sequestosome 1 (SQSTM1) on the long arm of chromosome 5 (5q35-qter), of which nine mutations have been described in Paget's disease of bone. These mutations are located in exons 7 and 8, which encode a highly conserved ubiquitin-binding domain. The prevalence of SQSTM1 mutations is about 10% in France. Tests for SQSTM1 mutations should be done in patients with Paget's disease of bone, even where the family history is negative. Detection of a mutation allows evaluation of family members to ensure early diagnosis of the disease before complications develop.

TNFRSF11B gene variants and bone mineral density in postmenopausal women in Malta

A number of polymorphisms in various genes have been identified and associated with bone mineral density (BMD) and with an increased risk of osteoporosis.

OBJECTIVE

In this study, three single nucleotide polymorphisms (SNPs) within the TNFRSF11B gene were studied for association with an increased risk of osteoporosis in postmenopausal Maltese women (n=126).

METHODOLOGY

Analysis was performed by PCR restriction fragment length polymorphism (RFLP) while BMD at the lumbar spine, femoral neck, Ward's triangle and trochanter was measured by DEXA.

RESULTS

No significant association was observed between genotypes and BMD for all polymorphisms studied within this gene. Homozygotes CC (T(950)-C) were observed to have the highest BMD at all anatomical sites although statistical significance was not reached when comparing the three genotypes. A statistical significant difference was observed in the distribution of genotype frequencies for this polymorphism between normal individuals and those that were either osteopenic or osteoporotic at one or both anatomical sites, with the TT genotype associated more frequently with low BMD. The T(950)-C and G(1181)-C polymorphisms were in strong linkage disequilibrium with each other but not with the A(163)-G polymorphism further upstream in the OPG promoter. Statistical significance was reached when constructing haplotypes, where the A-T-G haplotype was found to be more frequent in individuals with low BMD.

CONCLUSIONS

These results indicate the possible role of TNFRSF11B gene variants in postmenopausal bone loss in women in Malta.

Segregation of a M404V mutation of the p62/sequestosome 1 (p62/SQSTM1) gene with polyostotic Paget's disease of bone in an Italian family

Mutations of the p62/Sequestosome 1 gene (p62/SQSTM1) account for both sporadic and familial forms of Paget's disease of bone (PDB). We originally described a methionine-->valine substitution at codon 404 (M404V) of exon 8, in the ubiquitin protein-binding domain of p62/SQSTM1 gene in an Italian PDB patient. The collection of data from the patient's pedigree provided evidence for a familial form of PDB. Extension of the genetic analysis to other relatives in this family demonstrated segregation of the M404V mutation with the polyostotic PDB phenotype and provided the identification of six asymptomatic gene carriers. DNA for mutational analysis of the exon 8 coding sequence was obtained from 22 subjects, 4 PDB patients and 18 clinically unaffected members. Of the five clinically ascertained affected members of the family, four possessed the M404V mutation and exhibited the polyostotic form of PDB, except one patient with a single X-ray-assessed skeletal localization and one with a polyostotic disease who had died several years before the DNA analysis. By both reconstitution and mutational analysis of the pedigree, six unaffected subjects were shown to bear the M404V mutation, representing potential asymptomatic gene carriers whose circulating levels of alkaline phosphatase were recently assessed as still within the normal range. Taken together, these results support a genotype-phenotype correlation between the M404V mutation in the p62/SQSTM1 gene and a polyostotic form of PDB in this family. The high penetrance of the PDB trait in this family together with the study of the asymptomatic gene carriers will allow us to confirm the proposed genotype-phenotype correlation and to evaluate the potential use of mutational analysis of the p62/SQSTM1 gene in the early detection of relatives at risk for PDB.

Genetic polymorphisms of OPG, RANK, and ESR1 and bone mineral density in Korean postmenopausal women

To evaluate the effects of genetic polymorphisms of OPG, RANK, and ESR1, which regulate osteoclastogenesis, on bone mineral density (BMD), a cross-sectional study was conducted in 650 Korean postmenopausal women. BMDs of the distal radius and the calcaneus were measured by dual energy X-ray absorptiometry (DXA). Genetic polymorphisms of OPG 163 A > G, 1181 G > C; RANK 421 C > T, 575 T > C; and ESR1 1335 C > T, 2142 G > A were determined by matrix-assisted laser desorption/ionization-time of flight (MALDI-ToF) mass spectrometry. The differences between the BMDs of the genotypes of OPG, RANK, and ESR1 were analyzed by multiple linear regression model adjusted for age and body mass index. Women with the OPG 1181 CC genotype had higher BMDs at the distal radius (7%) and calcaneus (10%) than those with the GG genotype; and these differences were statistically significant (P = 0.001 and P = 0.007, respectively). A significant association was also observed between RANK 575 T > C and calcaneus BMD (P for trend = 0.017). No significant association was observed between BMDs and the polymorphisms of ESR1. The association between OPG 1181 G > C and BMD was profound in subjects with the RANK 575 TT or ESR1 2142 GG genotypes; women with OPG 1181 CC had higher BMDs at the distal radius (11%) and calcaneus (11%) than those with OPG 1181 GG only in women with RANK 575 TT genotype (P = 0.002 and P = 0.021, respectively). These results suggest that OPG genetic polymorphisms, especially with the RANK 575 TT or ESR1 2142 GG genotypes, are related to low BMD in postmenopausal Korean women.

Genetics of Paget's disease of bone

PDB (Paget's disease of bone) is a common condition characterized by focal increases in bone turnover affecting one or more sites throughout the skeleton. Genetic factors are important in the pathogenesis of PDB and many families have been described where PDB is inherited in an autosomal-dominant fashion. Several candidate loci for susceptibility to PDB and related syndromes have been identified by genome-wide scans and recent evidence suggests that mutations in genes that encode components of the RANK [receptor activator of NF-κB (nuclear factor-κB)]/NF-κB signalling pathway play an important role in the pathogenesis of this group of diseases. Insertion mutations in the TNFRSF11A gene encoding RANK have been identified as the cause of familial expansile osteolysis, some cases of early onset PDB and expansile skeletal hyperphosphatasia. Inactivating mutations in the TNFRSF11B gene that encodes OPG (osteoprotegerin) have been found to cause the syndrome of juvenile PDB. Polymorphisms in OPG also appear to increase the risk of developing PDB. The most important causal gene for classical PDB is Sequestosome 1 (SQSTM1), which is a scaffold protein in the NF-κB signalling pathway, and mutations affecting the UBA (ubiquitin-associated) domain of this protein occur in between 20–50% of familial and 10–20% of sporadic PDB cases. The rare syndrome of IBMPFD (inclusion body myopathy, PDB and fronto-temporal dementia) is due to mutations in the VCP gene and these also cluster in the domain of VCP that interacts with ubiquitin, suggesting a common disease mechanism with SQSTM1-mediated PDB.