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

Osteosarcoma mechanobiology and therapeutic targets

Osteosarcoma (OS) is the one of the most common primary tumors of bone with less than a 20% 5-year survival rate after the development of metastases. OS is highly predisposed in Paget's disease (PD) of bone, and both have common characteristic skeletal features due to rapid bone remodeling. OS prognosis is location dependent which further emphasizes the likely contribution of the bone microenvironment in its pathogenesis. Mechanobiology is the phenomenon when mechanical cues from the changing physical microenvironment of bone are transduced to biological pathways through mechanosensitive cellular components. Mechanobiology-driven therapies have been used for curbing tumor progression by direct alteration of the physical microenvironment or inhibition of metastasis-associated mechanosensitive proteins. This review emphasizes the contribution of mechanobiology to OS progression, and sheds light on current mechanobiology-based therapies and potential new targets for improving disease management. Additionally, the variety of 3D models currently used to study OS mechanobiology are summarized.

The role of autophagy in bone homeostasis

Autophagy is an evolutionarily conserved intracellular process and is considered one of the main catabolism pathways. In the process of autophagy, cells are digested nonselectively or selectively to recover nutrients and energy, so it is regarded as an antiaging process. In addition to the essential role of autophagy in cellular homeostasis, autophagy is a stress response mechanism for cell survival. Here, we review recent literature describing the pathway of autophagy and its role in different bone cell types, including osteoblasts, osteoclasts, and osteocytes. Also discussed is the mechanism of autophagy in bone diseases associated with bone homeostasis, including osteoporosis and Paget's disease. Finally, we discuss the application of autophagy regulators in bone diseases. This review aims to introduce autophagy, summarize the understanding of its relevance in bone physiology, and discuss its role and therapeutic potential in the pathogenesis of bone diseases such as osteoporosis.

TGF-β-activated kinase 1 (TAK1) mediates cadmium-induced autophagy in osteoblasts via the AMPK / mTORC1 / ULK1 pathway

Cadmium (Cd) is one of worldwide environmental pollutants that causes bone homeostasis, which depends on the resorption of bones by osteoclasts and formation of bones by the osteoblasts (OB). However, the Cd toxicity on OB and its mechanism are unclear. Autophagy is an evolutionarily conserved degradation process in which domestic intracellular components are selectively digested for the recycling of nutrients and energy. This process is indispensable for cell homeostasis maintenance and stress responses. Dysregulation at the level of autophagic activity consequently disturbs the balance between bone formation and bone resorption and mediates the onset and progression of multiple bone diseases, including osteoporosis. TAK1 has been recently emerged as an activator of AMPK and hence an autophagy inducer. AMPK is a key molecule that induces autophagy and regulates cellular metabolism to maintain energy homeostasis. Conversely, autophagy is inhibited by mTORC1. In this study, we found that Cd treatment caused the formation of autophagosomes, LC3-II lipidation and p62 downregulation, and the increased autophagic flux, indicating that Cd treatment induced autophagy in OBs. Cd treatment induced TAK1 activation mediated AMPK phosphorylation, which promoted autophagy via phosphorylation of ULK1 at S317. Meanwhile, Cd treatment dramatically decreased mTORC1, S6K1, 4E-BP1, S6, ULK1^S555 and ULK1^S757 phosphorylation, suggesting that mTORC1 activity was inhibited and inactive mTORC1 prevents ULK1 activation by phosphorylating ULK1 at SerS555 and Ser757. Our data strongly suggest that TAK1 mediates AMPK activation, which activates ULK1 by phosphorylating ULK1^S317 and suppressing mTORC1-mediated ULK1^S555 and ULK1^S757 phosphorylation. Our study has revealed a signaling mechanism for TAK1 in Cd-induced autophagy in OBs.

Hesperetin suppresses RANKL-induced osteoclastogenesis and ameliorates lipopolysaccharide-induced bone loss

Destructive bone diseases caused by osteolysis are increasing in incidence. They are characterized by an excessive imbalance of osteoclast formation and activation. During osteolysis, the activation of nuclear factor-κB (NF-κB) and mitogen-activated protein kinase (MAPK) signaling pathways are triggered by receptor activator of NF-κB ligand (RANKL), inflammatory factors, and oxidative stress. Previous studies have indicated that the common flavanone glycoside compound hesperetin exhibits anti-inflammatory and antioxidant activity by inhibition of NF-κB and MAPK signaling pathways. However, the direct relationship between hesperetin and osteolysis remain unclear. In the present study, we investigated the effects of hesperetin on lipopolysaccharide (LPS)-induced osteoporosis and elucidated the related mechanisms. Hesperetin effectively suppressed RANKL-induced osteoclastogenesis, osteoclastic bone resorption, and F-actin ring formation in a dose-dependent manner. It also significantly suppressed the expression of osteoclast-specific markers including tartrate-resistant acid phosphatase, matrix metalloproteinase-9, cathepsin K, c-Fos, and nuclear factor of activated T-cells cytoplasmic 1. Furthermore, it inhibited osteoclastogenesis by inhibiting activation of NF-κB and MAPK signaling, scavenging reactive oxygen species, and activating the nuclear factor E2 p45-related factor 2/heme oxygenase 1 (Nrf2/HO-1) signaling pathway. Consistent with in vitro results, hesperetin effectively ameliorated LPS-induced bone loss, reduced osteoclast numbers, and decreased the RANKL/OPG ratio in vivo. As such, our results suggest that hesperetin may be a great candidate for developing a novel drug for destructive bone diseases such as periodontal disease, tumor bone metastasis, rheumatoid arthritis, and osteoporosis.

[Pathological and metabolic bone diseases: Clinical importance for fracture treatment]

Pathological and metabolic bone diseases are common and relevant occurrences in orthopedics and trauma surgery; however, fractures are often treated as being the illness itself and not seen as the symptom of an underlying bone disease. This is why further diagnostics and systemic treatment options are often insufficiently considered in the routine treatment of fractures. This review focuses on osteoporosis, osteopetrosis, hypophosphatasia and Paget's disease of bone.In patients with osteoporotic vertebral or proximal femur fractures, pharmaceutical treatment to prevent subsequent fractures is an integral part of fracture therapy together with surgical treatment. Osteopetrosis is caused by compromised osteoclastic bone resorption; therefore, even in the face of an elevated bone mass, vitamin D3 supplementation is crucial to avoid clinically relevant hypocalcemia. Unspecific symptoms of the musculoskeletal system, especially together with stress fractures, are typically found in patients suffering from hypophosphatasia. In these patients measurement of alkaline phosphatase shows reduced enzyme activity. Elevated levels of alkaline phosphatase are found in Paget's disease of bone where bisphosphonates are still the treatment of choice.

The ubiquitin-associated (UBA) domain of SCCRO/DCUN1D1 protein serves as a feedback regulator of biochemical and oncogenic activity

Amplification of squamous cell carcinoma-related oncogene (SCCRO) activates its function as an oncogene in a wide range of human cancers. The oncogenic activity of SCCRO requires its potentiating neddylation domain, which regulates its E3 activity for neddylation. The contribution of the N-terminal ubiquitin-associated (UBA) domain to SCCRO function remains to be defined. We found that the UBA domain of SCCRO preferentially binds to polyubiquitin chains in a linkage-independent manner. Binding of polyubiquitin chains to the UBA domain inhibits the neddylation activity of SCCRO in vivo by inhibiting SCCRO-promoted nuclear translocation of neddylation components and results in a corresponding decrease in cullin-RING-ligase-promoted ubiquitination. The results of colony formation and xenograft assays showed a mutation in the UBA domain of SCCRO that reduces binding to polyubiquitin chains, significantly enhancing its oncogenic activity. Analysis of 47 lung and head and neck squamous cell carcinomas identified a case with a frameshift mutation in SCCRO that putatively codes for a protein that lacks a UBA domain. Analysis of data from The Cancer Genome Atlas showed that recurrent mutations cluster in the UBA domains of SCCRO, lose the ability to bind to polyubiquitinated proteins, and have increased neddylation and transformation activities. Combined, these data suggest that the UBA domain functions as a negative regulator of SCCRO function. Mutations in the UBA domain lead to loss of inhibitory control, which results in increased biochemical and oncogenic activity. The clustering of mutations in the UBA domain of SCCRO suggests that mutations may be a mechanism of oncogene activation in human cancers.

Massive accumulation of osteoclastic giant cells in rapid destructive hip disease

The term rapid destructive hip disease (RDHD) designates a rare coxarthropathy characterized by a rapid destruction of the femoral head within a few weeks to several months. The etiology and pathogenesis of RDHD are not understood. The goal of this study was to define the histological and histomorphometric hallmarks of RDHD to provide a basis for a deeper understanding of the underlying mechanisms. Fifteen consecutive RDHD patients (mean age 78.9 ± 6.4 years, 14 females) demonstrated the characteristic radiographic appearance of vanishing femoral head bone stock without signs of reactive anabolic changes. Histologic and histomorphometric analyses revealed severe bone resorption with significantly increased numbers of morphologically altered giant osteoclasts, the formation of giant cell granulomas both in bone marrow and the synovium and significantly increased osteoblast numbers and osteoid as compared to controls with primary osteoarthritis (OA). These results suggest that in RDHD, an unknown stimulus leads to a massive activation of osteoclasts and this in turn provokes a reactive increase in osteoblast activity that is too weak to counteract the rapid destruction of the hip, thus providing a rationale for a prospective systematic search for the stimuli upstream of the massively increased osteoclast activation in RDHD.

Ca2+ in quality control: an unresolved riddle critical to autophagy and mitophagy

Calcium (Ca (2+)) has long been known as a ubiquitous intracellular second messenger, exploited by cells to control processes as diverse as development, proliferation, learning, muscle contraction and secretion. The spatial and temporal patterns of these Ca (2+)-associated signals, as well as their amplitude, is precisely controlled to create gradients of the ion, varying considerably depending on cell type and function. Tuning of intracellular Ca (2+) is achieved in part by the buffering role of mitochondria, whose unperturbed function is essential for maintaining cellular energy balance. Quality of mitochondria is ensured by the process of targeted autophagy or mitophagy, which depends on a molecular cascade driving the catabolic process of autophagy toward damaged or deficient organelles for elimination via the lysosomal pathway. Nonspecific and targeted autophagy are highly regulated processes fundamental to cell growth and tissue homeostasis, allowing resources to be reallocated in nutrient-deprived cells as well as being instrumental in the repair of damaged organelles or the elimination of those in excess. Given the role of Ca (2+) signaling in many fundamental cellular processes requiring precise regulation, the involvement of Ca (2+) in autophagy is still somewhat ill-defined, and only in the past few years has evidence emerged linking the two. This mini-review aims to summarize recent work implicating Ca (2+) as an important regulator of autophagy, outlining a role for Ca (2+) that may be even more critical in the regulation of targeted mitochondrial autophagy.

Impact of cellular autophagy on viruses: Insights from hepatitis B virus and human retroviruses

Autophagy is a protein degradative process important for normal cellular metabolism. It is apparently used also by cells to eliminate invading pathogens. Interestingly, many pathogens have learned to subvert the cell’s autophagic process. Here, we review the interactions between viruses and cells in regards to cellular autophagy. Using findings from hepatitis B virus and human retroviruses, HIV-1 and HTLV-1, we discuss mechanisms used by viruses to usurp cellular autophagy in ways that benefit viral replication.

Genotype-phenotype correlation in juvenile Paget disease: role of molecular alterations of the TNFRSF11B gene

Juvenile Paget disease (JPD) {MIM 239000} is a rare inherited bone disease that affects children. The patients affected with JPD present an altered bone turnover, therefore, show a phenotype characterized by progressive bone deformities, fractures, and short stature. Deletions or missense mutations of the TNFRSN11B gene are common in these children. This gene encodes a soluble protein, the osteoprotegerin, which leads to uncontrolled osteoclastogenesis when mutated. JPD is characterized by a strong genotype-phenotype correlation, so depending on the alteration of the TNFRSN11B gene, the phenotype is variable. This review describes the different clinical features which are characteristic of JPD and the correspondence with the different molecular alterations of the TNFRSN11B gene.

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.

Paget’s disease: clinical update

Paget’s disease is a chronic disorder of bone remodeling, characterized by an abnormal increase of osteoclast and, hence, osteoblast activity. The imbalance of bone turnover results in the formation of unhealthy and fragile bone. It also leads to impairment of adjacent joints and to a risk of various complications. Current research focuses on the elucidation of the etiologic role viral infection and predisposing genetic factors. Paget’s disease is commonly discovered by chance; its suspicion is raised either by high level of alkaline phosphatase or by the X-ray of the pathological bone. Bisphosphonates have proven to be effective in controlling disease activity because they inhibit osteoclast function. Their use is recommended when bone-derived serum alkaline phosphatase is high and/or when disease localizations are highly suspected for the development of complications. Orv. Hetil., 2011, 152, 1337–1346.

Marathon of eponyms: 16 Paget disease of bone

The use of eponyms has long been contentious, but many remain in common use, as discussed elsewhere (Editorial: Oral Diseases. 2009: 15; 185). The use of eponyms in diseases of the head and neck is found mainly in specialties dealing with medically compromised individuals (paediatric dentistry, special care dentistry, oral and maxillofacial medicine, oral and maxillofacial pathology, oral and maxillofacial radiology and oral and maxillofacial surgery) and particularly by hospital-centred practitioners. This series has selected some of the more recognised relevant eponymous conditions and presents them alphabetically. The information is based largely on data available from MEDLINE and a number of internet websites as noted below: the authors would welcome any corrections. This document summarises data about Paget disease of bone.

Epidemiological, clinical, and genetic characteristics of Paget's disease of bone in a rural area of Calabria, Southern Italy

BACKGROUND

The prevalence of Paget's disease of bone (PDB) is unknown in peninsular Southern Italy, although an elevated clinical severity of the disease was reported in patients from Campania.

AIM

This study was performed to evaluate the epidemiological and genetic characteristics of PDB in a rural area of Calabria, the southernmost region in the Italian peninsula.

SUBJECTS AND METHODS

We examined 1068 consecutive pelvic radiographs of patients older than 40 yr referred for any reason to the "Spinelli" Hospital, Belvedere Marittimo, from January 1st 2004 to December 31st 2006. In subjects with radiological findings of pelvic PDB, a 99m Technetium methylene diphosphonate bone scan and the sequence analysis of the sequestosome 1 (SQSTM1) gene were subsequently performed.

RESULTS

In the examined geographic area, the crude radiographic prevalence of pelvic PDB was 0.74% (8/1068; male:female 5:3, mean age 71.6 ± 13.1 yr) whereas the estimated overall prevalence of PDB between 0.82% and 1.21%. PDB patients from Calabria showed clinical characteristics similar to those reported in patients from Campania. The disease was also frequently complicated by osteoarthritis and the right side of the body was more affected than the left. The SQSTM1 gene analysis revealed the presence of a novel missense mutation (M401V) in exon 8 in one subject with a familial and aggressive form of PDB.

CONCLUSION

The study results confirmed that patients with PDB from rural districts of Southern Italy show an earlier onset and an increased clinical severity of the disease that appears mostly independent from the presence of germinal SQSTM1 mutations.

Infiltrating giant cell tumor in a case of Paget's disease of bone

Giant cell tumor (GCT) of the bone, also called osteoclastoma, is a rare complication of Paget's bone disease. We report a patient from Southern Italy who developed a GCT infiltrating the neighboring tissues. The natural history and the therapeutic outcomes of this unique complication of Paget's bone disease are presented.

Paget's disease of bone: histologic analysis of 754 patients

Although Paget's disease of bone (PDB) is the second most common metabolic bone disease, to our knowledge, there is only one quantitative analysis on the histological and especially on the histomorphometric level. Therefore, the aim of this study was to analyze, on the basis of the Hamburg Bone Register, PBD in terms of incidence, skeletal distribution, malignant transformation, and histological and histomorphometric characteristics. Bone biopsies and patient files of 754 cases with histologically proven PDB were reviewed in a retrospective study. Quantitative static histomorphometry was performed on a representative subgroup of 247 biopsies derived from patients with manifestation of PDB at the iliac crest and compared with an age- and sex-matched control group. The peak incidence of PDB was between 70 and 80 yr of age. The majority of monostotic skeletal manifestation was localized at the os ilium, followed by the spine and femur. Histomorphometric results showed a high bone turnover with a significant increase in bone resorption and bone formation indices leading to an increased bone volume. Paget sarcoma was diagnosed in 6 of 754 patients, indicating a malignant transformation in 0.8% of the affected patients. Taken together, our study characterizes PDB in Germany on the basis of one of the largest cohorts of patients with histologically proven PDB. Moreover, for the first time, a quantitative histomorphometric approach was taken for >200 cases, where we could show local high bone mass lesions as a result of an increase of both osteoclast and osteoblast indices.

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.

Management of patients with Paget's disease: a consensus document of the Belgian Bone Club

Paget's disease of bone (PDB) is a potentially crippling condition. Pain, fracture, spinal stenosis, nerve entrapment, vascular steal syndrome, secondary osteoarthritis, bone deformity, dental problems, deafness, excessive bleeding during orthopaedic surgery, rare sarcomatous degeneration, and hypercalcaemia constitute complications that may impair the quality of life. The therapeutic approach varies from symptomatic (analgesics, anti-inflammatory drugs) to more specific drugs such as increasingly potent bisphosphonates. Studies such as the PRISM study should in the future help to determine the superiority or not of aggressive treatment over symptomatic treatment in the prevention of complications. Various oral and/or intravenous (i.v.) bisphosphonates have been tested and are currently on the market. The most recently available nitrogen-containing bisphosphonate, i.v. zoledronic acid, is the most potent therapy available for the treatment of PDB. Its therapeutic efficacy, its long-term effect on biologic activity and its good tolerance currently supports its use as a first-line therapeutic option in patients suffering from PDB.

Pathogenesis and management of Paget's disease of bone

Paget's disease of bone is a common disease characterised by focal areas of increased bone turnover, affecting one or several bones throughout the skeleton. Paget's disease is often asymptomatic but can be associated with bone pain and other complications such as osteoarthritis, pathological fracture, bone deformity, deafness, and nerve compression syndromes. Genetic factors have an important role in this disease, and mutations have been identified in four genes that cause Paget's disease and related syndromes. The most important of these is Sequestosome 1 (SQSTM1), which is a scaffold protein in the nuclear factor kappaB (NFkappaB) signalling pathway. Patients with SQSTM1 mutations have severe Paget's disease of bone and a high degree of penetrance with increasing age. Environmental factors also contribute. Most research has focused on paramyxovirus infection as a possible trigger, but evidence for this notion is conflicting. Other potential triggers include deficiency of dietary calcium and repetitive mechanical loading of the skeleton. Medical management of Paget's disease of bone is based on giving inhibitors of osteoclastic bone resorption, and bisphosphonates are the treatment of first choice. Bisphosphonate therapy is primarily indicated for patients who have bone pain arising from increased metabolic activity in affected bones. Bisphosphonate therapy is highly effective at reducing bone turnover, and it has been shown to heal radiological lesions and restore normal histology; however, the long-term effects of bisphosphonates on disease progression have not been adequately studied. No firm evidence as yet exists to show that bisphosphonates can prevent the development of complications of Paget's disease of bone, and further work is needed to address the effects of treatment on long-term clinical outcome.

Autophagy in the pathogenesis of disease

Autophagy is a lysosomal degradation pathway that is essential for survival, differentiation, development, and homeostasis. Autophagy principally serves an adaptive role to protect organisms against diverse pathologies, including infections, cancer, neurodegeneration, aging, and heart disease. However, in certain experimental disease settings, the self-cannibalistic or, paradoxically, even the prosurvival functions of autophagy may be deleterious. This Review summarizes recent advances in understanding the physiological functions of autophagy and its possible roles in the causation and prevention of human diseases.

An enhancer 20 kilobases upstream of the human receptor activator of nuclear factor-kappaB ligand gene mediates dominant activation by 1,25-dihydroxyvitamin D3

Receptor activator of nuclear factor-kappaB ligand (RANKL) is a TNF-like factor that is both produced by osteoblasts, mesenchymal cells, and activated T cells and required for osteoclast maturation and survival. The gene is up-regulated by the two primary calcemic hormones, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and PTH. Previous studies have indicated that five enhancer regions located significantly upstream of the mouse Rankl transcriptional start site mediate up-regulation by 1,25(OH)2D3 and PTH. The most distal of these, termed mRLD5, is highly conserved in the human gene at -96 kb where it was also shown to be functionally active. Four additional mouse Rankl upstream enhancers are also highly conserved in the human gene at -20, -25, -75, and -87 kb. In the present studies, we characterized the activity of these regions, explored their capacity to mediate the actions of 1,25(OH)2D3, and identified the vitamin D response elements contained within the two most proximal segments. Interestingly, whereas the most distal of the five enhancers is the dominant mediator of 1,25(OH)2D3 activity in the mouse Rankl gene, that role in the human gene is manifested by the most proximal element at -20 kb. Importantly, activity at this region in response to 1,25(OH)2D3 was associated with a significant increase in histone acetylation as well as the enhanced recruitment of RNA polymerase II. Both likely reflect the primary role of this enhancer in human RANKL gene expression. Our studies confirm the complex nature of RANKL regulation and indicate that although the five enhancers are evolutionarily conserved across several species, their relative contributions to RANKL expression in response to 1,25(OH)2D3 may be different.