Paget's disease of bone in New Zealand: continued decline in disease severity

Decline in clinical severity of Paget's disease of bone: Comparison between a contemporary cohort and a historical cohort

BACKGROUND

European and Australian studies have reported a decrease in the prevalence, incidence and clinical severity of Paget's disease of bone (PDB). There are no studies on the current clinical characteristics of PDB in Quebec, Canada.

AIMS

The purpose of this study was to describe the characteristics of unrelated patients with PDB diagnosed after the year 2000 in our region and to compare them to a historical cohort diagnosed before 2000.

METHODS

In this retrospective descriptive cohort study, socio-demographic data and clinical characteristics for the contemporary cohort were collected from electronic medical records of patients with PDB followed at our university hospital. For the historical cohort, the same data were collected from the research files of PDB participants in our research program. Inclusion criteria were: age > 18 years, having PDB diagnosed by a rheumatologist, and being followed in our hospital. Exclusion criteria were: having a relative with PDB participating in this study. Variables were reported as mean, standard deviation, frequency and percentage. Chi-square tests were used to compare categorical variables. Continuous values were compared with Wilcoxon-Mann-Whitney tests. Unadjusted p-values and adjusted p-values with the Bonferroni correction method were calculated. A p-value <0.05 was considered statistically significant.

RESULTS

Among the 195 patients with PDB in the contemporary cohort, 53.3 % were men, 60.5 % had monostotic involvement, 14.2 % were symptomatic at diagnosis. In comparison to the historical cohort of 173 patients, patients in the contemporary cohort were older at diagnosis (68.7  10.7 vs. 58.5  10.1; p < 0.0001) and had less family history of PDB (13.8 % vs. 33.6 %; p = 0.0024). They also had lower total alkaline phosphatase levels at diagnosis (118.0 (85.0-184.0)) vs. 184.0 (115.0-312.0)); p = 0.0006), a lower pagetic bone number (1.0 (1.0-3.0) vs. 2.0 (1.0-5.0); p < 0.0001), lower pagetic bone fractures (6.7 % vs. 36.7 %; p = 0.0078) and lower bone deformities (13.0 % vs. 54.0 %; p < 0.0001). There was no significant difference for pagetic bone pain (52.0 % vs. 52.6 %; p = 1.0000), percentage of patients who had orthopedic surgery related to PDB complications (8.8 % vs. 28.6 %; p = 1.0000), secondary osteoarthritis (43.0 % vs. 51.6 %; p = 1.0000), and hearing impairment (51.9 % vs. 61.1 %; p = 0.1000).

CONCLUSION

The contemporary cohort is characterized by an older age at diagnosis, a majority of monostotic disease and fewer complications of PDB. This decline in clinical severity of PDB in Quebec is consistent with studies reported in other countries.

CT Assessment of Otic Capsule Bone Density in Paget's Disease of the Temporal Bone and Its Relationship With Hearing Loss

OBJECTIVES

To study computed tomography findings in Paget's disease of temporal bone (PDTB) and analyze the relations between otic capsule bone mineral density values measured in Hounsfield Units (HU) and hearing loss (HL).

STUDY DESIGN

Observational case-control study.

SETTING

Tertiary referral center.

PATIENTS

Radiographically confirmed PDTB cases and control group.

INTERVENTION

Diagnostic.

MAIN OUTCOME MEASURES

Hearing thresholds and computed tomography bone density values.

RESULTS

Twenty-three ears in the case group (PDTB) and 27 control ears were included. In the PDTB group, HL was found in 87% of the ears (43% mixed) and an air-conduction threshold of 50.7 dB (SD = 19.8). In the control group, 48% of the ears showed HL (7% mixed) and an air-conduction threshold of 34.5 dB (SD = 20.6) was found; the difference was statistically significant (p < 0.05). Measurements of bone density (HU) in the otic capsule (regions of interest [ROI] 1 and 2) and in the petrous bone (ROI 3) were significantly lower (p < 0.05) in the PDTB group than in controls.The PDTB group presented a significant association between otic capsule bone density in ROI 1 and mean otic capsule density with air and bone-conduction thresholds (p < 0.05). In controls, no association was observed between any density value and audiometric thresholds.

CONCLUSION

PDTB patients showed more frequent HL, lower thresholds, and a higher proportion of mixed HL than controls. Bone density (HU) was decreased in all ROIs in PDTB patients in comparison with controls. Bone density in the otic capsule was associated with HL in PDTB patients, but no association was observed between bone density and HL in controls.

Genetic Determinants of Paget's Disease of Bone

PURPOSE OF REVIEW

To provide an overview of the role of genes and loci that predispose to Paget's disease of bone and related disorders.

RECENT FINDINGS

Studies over the past ten years have seen major advances in knowledge on the role of genetic factors in Paget's disease of bone (PDB). Genome wide association studies have identified six loci that predispose to the disease whereas family based studies have identified a further eight genes that cause PDB. This brings the total number of genes and loci implicated in PDB to fourteen. Emerging evidence has shown that a number of these genes also predispose to multisystem proteinopathy syndromes where PDB is accompanied by neurodegeneration and myopathy due to the accumulation of abnormal protein aggregates, emphasising the importance of defects in autophagy in the pathogenesis of PDB. Genetic factors play a key role in the pathogenesis of PDB and the studies in this area have identified several genes previously not suspected to play a role in bone metabolism. Genetic testing coupled to targeted therapeutic intervention is being explored as a way of halting disease progression and improving outcome before irreversible skeletal damage has occurred.

Incidence of Paget's Disease of Bone in the UK: Evidence of a Continuing Decline

OBJECTIVES

To characterise the incidence of clinically diagnosed Paget's disease of bone in the UK during 1999-2015 and to determine variation in incidence of disease by age, sex, geography and level of deprivation.

METHODS

Incident cases of Paget's disease occurring between 1999-2015 were identified from primary care records. Overall crude incidence and incidence stratified by age and sex was calculated each year from 1999-2015. Direct age- and sex-standardised incidence was also calculated. We used Poisson regression to look at variation in incidence by deprivation and UK region.

RESULTS

3,592 incident cases of Paget's disease were identified between 1999-2015. Incidence increased with age and at all ages was greater in men than women. In women and men, respectively, crude incidence increased from 0.037 and 0.074 per 10 000 population per year among those aged 45-49 years to 3.7 and 6.3 per 10 000 population per year among those 85 years and older. Overall standardised incidence reduced from 0.75/10 000 person-years in 1999-0.20/10 000 person-years in 2015. After adjustment for age and sex, incidence was >30% higher in the most, compared with least deprived quintile of deprivation. There was evidence of geographic variation, with highest incidence in the North West of England, which persisted after adjustment for age, sex, and level of deprivation.

CONCLUSION

The incidence of clinically diagnosed Paget's disease has continued to decrease since 1999. The reason for the decline in incidence remains unknown though the rapidity of change points to an alteration in one or more environmental determinants.

The evaluation and treatment of Paget's disease of bone

Paget's disease of bone is a localized skeletal disorder, which is more common in England and in countries to which the English migrated. In recent decades, the prevalence in most countries has decreased. A family history of the disorder is present in approximately 15% of patients. Patients may be asymptomatic and may be diagnosed accidently as a consequence of an elevated serum alkaline phosphatase level or a finding on an x-ray or nuclear bone scan. The diagnosis is made by x-ray but nuclear bone scans define the extent of the disease. Salmon calcitonin and bisphosphonate drugs have proven effective, but by far, the most effective therapy is a single 5 mg intravenous infusion of zoledronic acid. This can normalize alkaline phosphatase levels for up to 6.5 years. A variety of gene mutations may predispose individuals to develop the disease but environmental factors such as measles virus likely play an important role.

Familial Paget's disease of bone: Long-term follow-up of unaffected relatives with and without Sequestosome 1 mutations

OBJECTIVE

Familial Paget's disease of bone is inherited as an autosomal-dominant trait and mutations in the sequestosome 1 (SQSTM1) gene have been reported with variable frequency in patients with familial disease. The natural history, however, of the disease in family members with or without SQSTM1 mutations is unknown.

METHODS

To address this question, we investigated members of families with Paget's disease identified and genotyped in 2000 in The Netherlands without clinical, biochemical or radiological signs of Paget's disease. Seventy-five subjects, median age 56 years (range 44-93), with or without SQSTM1 mutations participated in the present study. Medical history was obtained and clinical examination and laboratory investigations were performed in all. When serum biochemical markers of bone turnover were increased, skeletal scintigraphy with SPECT-CT was performed.

RESULTS

After a mean period of 15.9 ± 0.32 (SD) years no subject without SQSTM1 mutations (either from positive or negative families) developed Paget's disease. Of 14 carriers of SQSTM1 mutations, Paget's disease of the pelvis was diagnosed in a 74-year old asymptomatic woman.

CONCLUSION

The incidence of new Paget's disease in SQSTM1 positive subjects was 7.1% and no mutation-negative subject developed the disease within 16 years of follow-up. Subjects without SQSTM1 mutations can be reassured whereas mutation carriers should consider screening. Our findings should be confirmed in other populations as currently unknown environmental factors that might be involved in the development of the disease may differ.

Recent advances in understanding and managing Paget's disease

Paget’s disease is a condition which continues to challenge and surprise. The dramatic fall in its incidence over the last three decades has been an enormous surprise, as is the capacity of a single infusion of the potent bisphosphonate, zoledronate, to produce biochemical remission in 90% of patients, remissions which usually persist for many years and raise the possibility of a cure in some patients. However, challenges in its management remain. The trials carried out in Paget’s disease have almost always had biochemical indices as their primary endpoints. From these studies, we also know that bone pain is relieved, quality of life improved, bone histology normalised, and radiological lesions healed. Thus, disease progression is halted. Studies have not been powered to assess whether clinically important endpoints such as fracture and the need for joint replacement surgery are diminished, although these complications are well established as part of the natural history of the condition. Since disease progression is prevented by potent bisphosphonates, it is likely that disease complications will also be prevented. Zoledronate also reduces the frequency of follow-up needed and therefore provides a very cost-effective intervention in those who have symptomatic disease or are at risk of complications.

Diagnosis and Management of Paget's Disease of Bone in Adults: A Clinical Guideline

An evidence-based clinical guideline for the diagnosis and management of Paget's disease of bone (PDB) was developed using GRADE methodology, by a Guideline Development Group (GDG) led by the Paget's Association (UK). A systematic review of diagnostic tests and pharmacological and nonpharmacological treatment options was conducted that sought to address several key questions of clinical relevance. Twelve recommendations and five conditional recommendations were made, but there was insufficient evidence to address eight of the questions posed. The following recommendations were identified as the most important: 1) Radionuclide bone scans, in addition to targeted radiographs, are recommended as a means of fully and accurately defining the extent of metabolically active disease in patients with PDB. 2) Serum total alkaline phosphatase (ALP) is recommended as a first-line biochemical screening test in combination with liver function tests in screening for the presence of metabolically active PDB. 3) Bisphosphonates are recommended for the treatment of bone pain associated with PDB. Zoledronic acid is recommended as the bisphosphonate most likely to give a favorable pain response. 4) Treatment aimed at improving symptoms is recommended over a treat-to-target strategy aimed at normalizing total ALP in PDB. 5) Total hip or knee replacements are recommended for patients with PDB who develop osteoarthritis in whom medical treatment is inadequate. There is insufficient information to recommend one type of surgical approach over another. The guideline was endorsed by the European Calcified Tissues Society, the International Osteoporosis Foundation, the American Society of Bone and Mineral Research, the Bone Research Society (UK), and the British Geriatric Society. The GDG noted that there had been a lack of research on patient-focused clinical outcomes in PDB and identified several areas where further research was needed. © 2019 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals Inc.

Paget's disease of bone

Paget's disease is a chronic focal high turnover bone disorder that is primarily present in middle-aged or older adults. It seems to be restricted to humans and has no clear parallels with other diseases. Although much has been learnt about its pathology and epidemiology, and treatment is now highly effective we still lack a complete understanding of its etiology and biology. This review focusses on the natural history of the disorder, in particular its changing epidemiology, recent discoveries about its genetic basis and current approaches to diagnosis and treatment. While there is strong evidence for genetic predisposition to Paget's disease, there is also compelling evidence that it is becoming less prevalent, the age of patients at presentation is increasing and that the extent of skeletal involvement is diminishing, implying that there is an important, but as yet unidentified, environmental factor in its etiology. Contemporary patients are typically elderly and have few bones involved. Treatment with potent intravenous bisphosphonates provides prolonged remission and many will require only once in a lifetime treatment.

Dental Manifestations of Pediatric Bone Disorders

PURPOSE OF REVIEW

Several bone disorders affecting the skeleton often are manifest in the maxillofacial region. This review presents the most common bone disorders in children and their dental-oral manifestations: fibrous dysplasia, Paget's disease, osteogenesis imperfecta, renal osteodystrophy, hypophosphatasia, and osteoporosis. The specific intraoral characteristics will reviewed in detail.

RECENT FINDINGS

Recent studies confirmed the close relationship between the mandible and the maxilla with the most prevalent systemic bone disorders in children. This review will help practitioners to integrate the oral health into the systemic health and improve the multidisciplinary approach of pediatric patients between medicine and dentistry.

The Changing Presentation of Paget's Disease of Bone in Australia, A High Prevalence Region

Studies from several countries suggest that the incidence of Paget's disease of bone (PDB) and the severity of newly diagnosed cases are declining. The aim of this study was to examine secular changes in clinical presentation of PDB in Australia, which historically had the highest prevalence outside the United Kingdom. The participants were 293 patients (61% male) diagnosed between 1956 and 2013 with details recorded in the database of the Paget's Disease Research Group of Western Australia. The mean age at diagnosis was 62 years (range 28-90); 26% of participants had a family history of PDB and 11% had Sequestosome 1 (SQSTM1) mutations. After adjustment for covariates (SQSTM1 mutation status, family history, country of birth, smoking and dog exposure), there was a significant positive relationship between year of diagnosis and age at diagnosis (P < 0.001) and significant negative relationships between year of diagnosis and both pre-treatment total plasma alkaline phosphatase activity (ALP) and number of involved bones (P < 0.001 for each). Patients with SQSTM1 mutations had more extensive disease (P < 0.001) and higher pre-treatment ALP (P = 0.013). In subgroup analyses, relationships between year of diagnosis and each of age at diagnosis, number of involved bones and ALP were similar in patients with sporadic or familial disease, and in patients with and without SQSTM1 mutations. We conclude that the severity of PDB in Western Australia has declined over recent decades. This is likely to reflect altered exposure to one or more environmental agents involved in pathogenesis.

Antibody Response to Paramyxoviruses in Paget's Disease of Bone

Paget's disease of bone (PDB) is a common skeletal disorder characterised by focal abnormalities of increased and disorganised bone turnover. Genetic factors play a central role in the pathogenesis of PDB but environmental factors also contribute. Measles virus (MV), respiratory syncytial virus (RSV) and canine distemper virus (CDV) have all been implicated as potential disease triggers but the data are conflicting. Since chronic paramyxovirus infection with measles is known to be accompanied by increased production of antiviral antibodies, we have analysed circulating concentrations of antibodies to MV, CDV, and RSV as well as mumps, rubella and varicella zoster virus (VZV) in 463 patients with PDB and 220 aged and gender-matched controls. We also studied the relation between viral antibody concentrations and various markers of disease severity and extent in 460 PDB patients. A high proportion of cases and controls tested positive for antiviral antibodies but there was no significant difference in circulating antibody concentrations between PDB cases and controls for MV, CDV, RSV, rubella or VZV. However, mumps virus antibody levels were significantly higher in the PDB cases (mean ± SD = 3.1 ± 0.84 vs. 2.62 ± 0.86. p < 0.001). There was no association between disease severity and circulating antibody concentrations to any of the viruses. In conclusion, we found no evidence to suggest that PDB is associated with abnormalities of immune response to measles or other paramyxoviruses, although there was evidence of a greater antibody response to mumps. The results do not support that hypothesis that PDB is associated with a persistent infection with measles or other paramyxoviruses.

Durability of Response to Zoledronate Treatment and Competing Mortality in Paget's Disease of Bone

There has been a marked secular trend in recent decades toward patients with Paget's disease presenting at a greater age and having less extensive skeletal involvement. Over a similar time frame more potent bisphosphonates with a long duration of effect have been developed, raising the prospect of many patients needing only once in a lifetime treatment. We studied a cohort of 107 patients who had been treated with intravenous zoledronate for the first time at a mean age of 76 years. Sequential measurements of the bone turnover marker procollagen-1 NT-peptide (P1NP) were made for up to 10 years. By 9 years, 64% showed some loss of zoledronate effect (defined as a doubling of P1NP from the nadir value after treatment), but only 14% had a biochemical relapse (defined as a P1NP value >80 μg/L). The mortality rate was substantially greater than the relapse rate-by 10 years more than half the cohort had died (p < 0.0001). We conclude that for the majority of older people with Paget's disease a single intravenous infusion of zoledronate will provide disease suppression for the remainder of their lives. © 2016 American Society for Bone and Mineral Research.

Clinical and Genetic Advances in Paget's Disease of Bone: a Review

Paget’s disease of bone (PDB) is the second most common metabolic bone disorder, after osteoporosis. It is characterised by focal areas of increased and disorganised bone turnover, coupled with increased bone formation. This disease usually appears in the late stages of life, being slightly more frequent in men than in women. It has been reported worldwide, but primarily affects individuals of British descent. Majority of PDB patients are asymptomatic, but clinical manifestations include pain, bone deformity and complications, like pathological fractures and deafness. The causes of the disease are poorly understood and it is considered as a complex trait, combining genetic predisposition with environmental factors. Linkage analysis identified SQSTM1, at chromosome 5q35, as directly related to the disease. A number of mutations in this gene have been reported, pP392L being the most common variant among different populations. Most of these variants affect the ubiquitin-associated (UBA) domain of the protein, which is involved in autophagy processes. Genome-wide association studies enlarged the number of loci associated with PDB, and further fine-mapping studies, combined with functional analysis, identified OPTN and RIN3 as causal genes for Paget’s disease. A combination of risk alleles identified by genome-wide association studies led to the development of a score to predict disease severity, which could improve the management of the disease. Further studies need to be conducted to elucidate other important aspects of the trait, such as its focal nature and the epidemiological changes found in some populations. In this review, we summarize the clinical characteristics of the disease and the latest genetic advances to identify susceptibility genes. We also list current available treatments and prospective options.

The Implications of the Sequestosome 1 Mutation P392L in Patients with Paget's Disease in a United States Cohort

Paget's disease of bone (PDB) is associated with a germline mutation in Sequestosome1/p62 (SQSTM1) found in ≤16 % of sporadic cases worldwide, and in 19-46 % of those studied with familial PDB. The P392L is the most prevalent mutation identified to date. This mutation by itself does not confer PDB or define the phenotype of PDB in a given person. Environmental determinants remain elusive, although increasing age of the individual, other gene polymorphisms in the context of SQSTM1 mutations, and measles virus have been implicated. Measles exposure has been unexamined in this context. The goal of this study is to compare the background history and phenotype of patients with PDB carrying the SQSTM1 P392L mutation to those patients without. Focusing on age, ancestry, P329L mutation, family history, measles exposure, distribution of PDB, and age of onset, we examined outcomes at 10 years. We postulated that aging may play a role in defining phenotype, and that this may become more visible in a well-characterized cohort. This is an observational study focused on a cohort of patients with PDB drawn from the New England Registry in whom environmental and family history has been catalogued, linked to radiographic data. Of the 217 persons who were enrolled in the Registry, 42 (19 %) responded to a letter inviting them to participate in testing for the presence of the measles antibody, and in genetic testing for the P392L mutation. The mean age of the cohort in 2001 was 70 years (range 55-79); 27 were men (64 %). The measles antibody was found in all cases tested. Nine patients had the P392L mutation (21 %), 2 with familial PDB. In these persons, early diagnosis of disease and spinal stenosis marked the male phenotype only. European ancestry was noted in the minority of those with P392L mutation. Most deaths recorded occurred in the ninth decade of life or later. Spinal stenosis emerges as a prominent phenotype in SQSTM1 P392L-positive men with aging. In these 42 patients with PDB from the New England Registry, most do not carry the SQSTM1 P392L mutation, and many do not have European ancestry. Exposure to measles was confirmed in the majority.

Evolution of Paget's disease of bone in adults inheriting SQSTM1 mutations

CONTEXT

The cause of Paget's disease of bone (PDB) is unknown, but genetic factors, particularly SQSTM1 mutations, and environmental factors are important.

OBJECTIVE

To investigate the development of PDB in asymptomatic relatives carrying SQSTM1 mutations to determine whether a secular trend towards a less severe phenotype is evident, and to estimate prospectively the rate at which PDB emerged in this genetically susceptible population.

DESIGN

We recruited first-degree relatives of patients with PDB [33 adult offspring (mean age 45) and 1 sibling] with a familial SQSTM1 mutation. We determined the presence of PDB with skeletal scintiscans and confirmatory radiographs. Those negative for PDB on the initial scan were investigated again a mean 5·1 years later.

RESULTS

The initial skeletal scintiscan demonstrated PDB in six subjects; 26 of the remaining 28 unaffected subjects had a second scintiscan, with two new cases of monostotic PDB diagnosed in 134 patient-years of follow-up. In the total eight adult offspring diagnosed with PDB, the age of diagnosis was greater, by at least 10 years, than that in the 21 probands with clinically identified PDB (P = 0·005). In adult offspring who were older at the time of skeletal scintigraphy than their affected parents were at the time of clinical diagnosis, the difference was even more marked (P < 0·001). In adult offspring with PDB, the disease was significantly less extensive than in their affected parent, as judged by alkaline phosphatase and disease extent (P < 0·003).

CONCLUSION

These findings suggest a substantial gene-environment interaction: the emergence of PDB in offspring inheriting SQSTM1 mutations is delayed by at least a decade, has a substantially attenuated phenotype and occurs at a low rate between the (mean) ages of 45 and 50 years. The nature of the environmental factor is unknown.

Paget's Disease of Bone among Various Ethnic Groups

Paget's disease of bone (PDB) is a relatively benign disease common among many European populations, including those in the UK, Italy and Spain. However, it appears to be rare among Scandinavians and non-European immigrants living in Europe. The prevalence among Asian populations may be underestimated because a large number of reported cases were discovered incidentally. There is a need for surveys addressing the prevalence rate and consequences of PDB to be carried out in various parts of the world, particularly Asia.

Clinical presentation of Paget's disease: evaluation of a contemporary cohort and systematic review

Paget's disease of bone (PDB) has become less common over recent decades but it is unclear if this has impacted on clinical presentation. Here we evaluated the presenting features of PDB in a contemporary cohort of UK patients and conducted a systematic review of studies in which the presenting features had been reported. The case series comprised 88 patients referred to a specialist clinic between 2005 and 2013. Bone pain was the most common presenting feature occurring in 73.8% of patients. Others included bone deformity (18.1%), deafness (7.9%) and pathological fracture (5.7%). The disease was asymptomatic in 22% of cases. Antiresorptive treatment was given for pain in 34 cases and 61.7% of patients responded. Patients with a shorter disease duration were more likely to respond (p = 0.047). In the systematic review, bone pain was the most common presenting feature (52.2% of cases) followed by deformity (21.5%), deafness (8.9%) and fracture (8.5%). Time trend analysis in subjects of European descent showed that fracture was less common in studies performed during the past 25 years as compared with older studies (5.5 vs. 10.8% p < 0.001) whereas pain was more common (54.3 vs. 48.3%, p = 0.003). While changes in the mode of presentation of PDB have occurred over recent years, many patients present with complications such as fracture and deformity. Further research is required to determine if early detection and therapeutic intervention might be of value in preventing the morbidity associated with this common disease.

Genetics of Paget's disease of bone

Paget's disease of bone (PDB) is a common condition, which is characterised by focal areas of increased and disorganized bone remodeling. Genetic factors play an important role in the disease. In some cases, Paget's disease is inherited in an autosomal dominant manner and the most common cause for this is a mutation in the SQSTM1 gene. Other familial cases have been linked to the OPTN locus on Chromosome 10p13 and still other variants have been identified by genome wide association studies that lie within or close to genes that play roles in osteoclast differentiation and function. Mutations in TNFRSF11A, TNFRSF11B and VCP have been identified in rare syndromes with PDB-like features. These advances have improved understanding of bone biology and the causes of PDB. The identification of genetic markers for PDB also raises the prospect that genetic profiling could identify patients at high risk of developing complications, permitting enhanced surveillance and early therapeutic intervention.

Pathobiology of Paget's Disease of Bone

Paget's disease of bone is characterized by highly localized areas of increased bone resorption accompanied by exuberant, but aberrant new bone formation with the primary cellular abnormality in osteoclasts. Paget's disease provides an important paradigm for understanding the molecular mechanisms regulating both osteoclast formation and osteoclast-induced osteoblast activity. Both genetic and environmental etiologies have been implicated in Paget's disease, but their relative contributions are just beginning to be defined. To date, the only gene with mutations in the coding region linked to Paget's disease is sequestosome-1 (SQSTM1), which encodes the p62 protein, and these mutations lead to elevated cytokine activation of NF-B in osteoclasts but do not induce a "pagetic osteoclast" phenotype. Further, genetic mutations linked to Paget's appear insufficient to cause Paget's disease and additional susceptibility loci or environmental factors may be required. Among the environmental factors suggested to induce Paget's disease, chronic measles (MV) infection has been the most studied. Expression of the measles virus nucleocapsid gene (MVNP) in osteoclasts induces pagetic-like osteoclasts and bone lesions in mice. Further, mice expressing both MVNP in osteoclasts and germline mutant p62 develop dramatic pagetic bone lesions that were strikingly similar to those seen in patients with Paget's disease. Thus, interactions between environmental and genetic factors appear important to the development of Paget's disease. In this article we review the mechanisms responsible for the effects of mutant p62 gene expression and MVNP on osteoclast and osteoblast activity, and how they may contribute to the development of Paget's disease of bone.

Republished: Paget's disease of bone: clinical review and update

Paget's disease (PD) is a focal disorder of bone remodelling that occurs commonly in older people. In this article, we review clinical aspects of PD with an emphasis on recent findings. The epidemiology of PD appears to be changing rapidly, with several groups in different parts of the world reporting a marked reduction in the prevalence and incidence of PD, as well as in the severity of disease seen by clinicians. These findings seem most likely to be caused by changes in exposure to unknown environmental factors that have a role in the development of PD. However, genetic factors are also important. Mutations in SQSTM1 occur in 25-50% of familial PD. Genotype-phenotype relationships are present, as PD develops at an earlier age and is more extensive and severe in those with SQSTM1 mutations, and these findings are more pronounced in those with truncating mutations. However, the prevalence of PD in adults with SQSTM1 mutations is uncertain, and it is not known how such mutations might cause PD. Ultimately, if the cause of PD is determined, it seems likely that it will include both genetic and environmental factors. Lastly, clinical trials have shown that potent bisphosphonates are highly effective treatments for active PD, and reduce pain, improve quality of life, normalise bone turnover and heal lytic lesions on radiographs. They can also induce sustained remission that persists for many years.

Common susceptibility alleles and SQSTM1 mutations predict disease extent and severity in a multinational study of patients with Paget's disease

Paget's disease of bone (PDB) has a strong genetic component. Here, we investigated possible associations between genetic variants that predispose to PDB and disease severity. Allelic variants identified as predictors of PDB from genome-wide association studies were analyzed in 1940 PDB patients from the United Kingdom, Italy, Western Australia, and Spain. A cumulative risk allele score was constructed by adding the variants together and relating this to markers of disease severity, alone and in combination with SQSTM1 mutations. In SQSTM1-negative patients, risk allele scores in the highest tertile were associated with a 27% increase in disease extent compared with the lowest tertile (p < 0.00001) with intermediate values in the middle tertile (20% increase; p = 0.0007). The effects were similar for disease severity score, which was 15% (p = 0.01) and 25% (p < 0.00001) higher in the middle and upper tertiles, respectively. Risk allele score remained a significant predictor of extent and severity when SQSTM-positive individuals were included, with an effect size approximately one-third of that observed with SQSTM1 mutations. A genetic risk score was developed by combining information from both markers, which identified subgroups of individuals with low, medium, and high levels of severity with a specificity of 70% and sensitivity of 55%. Risk allele scores and SQSTM1 mutations both predict extent and severity of PDB. It is possible that with further refinement, genetic profiling may be of clinical value in identifying individuals at high risk of severe disease who might benefit from enhanced surveillance and early intervention.

Paget's disease of bone: clinical review and update

Paget's disease (PD) is a focal disorder of bone remodelling that occurs commonly in older people. In this article, we review clinical aspects of PD with an emphasis on recent findings. The epidemiology of PD appears to be changing rapidly, with several groups in different parts of the world reporting a marked reduction in the prevalence and incidence of PD, as well as in the severity of disease seen by clinicians. These findings seem most likely to be caused by changes in exposure to unknown environmental factors that have a role in the development of PD. However, genetic factors are also important. Mutations in SQSTM1 occur in 25-50% of familial PD. Genotype-phenotype relationships are present, as PD develops at an earlier age and is more extensive and severe in those with SQSTM1 mutations, and these findings are more pronounced in those with truncating mutations. However, the prevalence of PD in adults with SQSTM1 mutations is uncertain, and it is not known how such mutations might cause PD. Ultimately, if the cause of PD is determined, it seems likely that it will include both genetic and environmental factors. Lastly, clinical trials have shown that potent bisphosphonates are highly effective treatments for active PD, and reduce pain, improve quality of life, normalise bone turnover and heal lytic lesions on radiographs. They can also induce sustained remission that persists for many years.

Epidemiology of Paget's disease of bone: a systematic review and meta-analysis of secular changes

CONTEXT

Several studies have suggested that the prevalence and severity of PDB have fallen in recent years. The magnitude of this trend and its globalization have not been well established.

OBJECTIVE

The objective of this study is to estimate the pooled magnitude of the changes in the prevalence of PDB and as a secondary objective, to make up a world atlas of PDB prevalence.

METHODS

A systematic review of English and non-English articles using MEDLINE (1946 to 2013) and EMBASE (1980 to 2013) was the method used. Search terms included epidemiology, incidence, prevalence, cohort studies, osteitis deformans or Paget's disease of bone. Studies with incidence and/or prevalence rate for PDB were included. Two authors independently extracted the data using predefined data fields and quality assessment. A pooled analysis based on random-effects models was carried out for secular trends.

RESULTS

Twenty-eight articles documented the prevalence of PDB; four articles the incidence and two articles the rate of new referrals. The prevalence of PDB varied greatly between the different countries, from 0.00028% in Japan to 5.4% in the UK. There were available data on changes in prevalence from two different surveys over two different time frames in Europe and New Zealand. In all but one city (Turin), a drop in the prevalence of PDB was recorded (pooled OR 0.64; 95% CI 0.45-0.91).

CONCLUSION

The incidence and prevalence rates of PDB vary widely between populations but both have decreased in most regions over recent years. The changes are heterogeneous however and within countries, the largest changes have been in areas that previously had a high prevalence. The reasons for these changes remain unclear at present but are likely to be due to an interaction between genetic factors and environmental triggers which may differ in different regions.

Secular changes in Paget's disease: contrasting changes in the number of new referrals and in disease severity in two neighboring regions of Spain

SUMMARY

We studied the changes in the number of new referrals with Paget's disease of bone (PDB) and severity of PDB in a high prevalence focus and its neighboring region. Referral of patients changed only in the high prevalence focus. The severity of PDB decreased in both regions. These results could suggest the effects of an environmental influence on disease activity.

INTRODUCTION

The prevalence and severity of PDB have decreased in several countries over recent years. We previously reported a high radiological prevalence of PDB in Vitigudino. Here we sought to determine if secular changes in the number of new referrals and severity of PDB had occurred over recent years.

METHODS

We studied 280 patients with clinically diagnosed PDB who were evaluated at a regional referral center for metabolic bone disease between 1986 and 2009. Changes in the number of new referrals were calculated by relating these data to the number of subjects at risk as determined by population registers. Trends in disease severity were analyzed with alkaline phosphatase (ALP) activity and disease extent on scan.

RESULTS

Referrals from the Vitigudino region increased substantially between 1986 and 2003 but fell markedly between 2004 and 2009, although by this time there had been depopulation of the region due to emigration. No significant changes in the rates of referral occurred in the remainder of Salamanca. ALP activity and disease extent decreased in Salamanca, but only ALP activity decreased in Vitigudino. Referrals rate and severity of PDB in Vitigudino were greater than in the remainder of Salamanca.

CONCLUSIONS

Referral of patients with clinically diagnosed PDB has remained stable for most of Salamanca during the past 24 years, but substantial changes have been observed in Vitigudino. In agreement with other reports, the severity of PDB has decreased in both regions consistent with the effects of an environmental influence on disease activity.

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.

Implications of a new radiological approach for the assessment of Paget disease

In the present study, we quantified the proportion of skeletal involvement of Paget disease of bone (PDB) not captured by an abdominal X-ray. We also analyzed extension and severity over time and tested the hypothesis that X-rays from selected areas could replace bone scans for mapping PBD. We examined whole skeletal (99m)TC-MDP bone scans from 208 consecutive untreated patients. Pagetic bones included in an abdominal X-ray were delimited; disease extension and activity were calculated using Coutris's index as well as Renier's index and serum alkaline phosphatase (AP) values, respectively. The study period (1965-2007) was divided into quartiles according to the date of the diagnosis. The percentage of patients with PDB captured by an abdominal X-ray was 79 % (95 % CI 74-85 %). In the last quartile vs. the first quartile, PDB was diagnosed at a more advanced age (67 ± 11 vs. 57 ± 9 years, respectively), with a lower median extension (4 vs. 7) and similar median activity (32 vs. 35) but less activity through median AP values (183 vs. 485 UI/L). The skeletal locations to X-ray in order to capture up to 93 % of PDB extension were the abdomen, skull with facial bones, and both tibias. In conclusion, one-fifth of patients are underdiagnosed when assessing prevalence of PDB by an X-ray of the abdomen, and there is a secular trend to presentation in older patients with a decreasing extension of the disease. A set of X-rays that includes abdomen, skull with facial bones, and both tibias provides a reliable alternative to bone scans.

Paget's Disease. A Case Report

Paget's disease (PD) is a common focal progressive osteometabolic disorder characterised by a disturbance in bone modelling and remodelling, because of an increase in osteoblastic and osteoclastic activity. It is a condition of unknown aetiology affecting approximately 3% of the population over 40 years of age and, approximately 10% of those over the age of 85 years. It is most common in Northern Europe and Australia and is rare in Asia and Africa. We describe the case of a 71-year-old man diagnosed with PD confined to the sacrum. After laboratory test, an imaging study with radiography, scintigraphy, computed tomography (CT) and magnetic resonance imaging (MRI) was performed disclosing findings compatible with Paget's disease in middle pathologic phase. The diagnosis was confirmed at biopsy. The structural modification of the sacrum with spongiosa rarefaction, thickening of bone and intact bone cortical, confirmed by CT, are tipical of an intermediate phase of PD. This was also supported by signal MRI changes showing substitution of the red by the fat medulla, visualized by FS sequences. Once the treatment for the bone disease was established, the patient no longer complained of pain. Special attention should be paid to male and elderly patients with pain in the lumbar spine because of the potential risk cancer development (21). The radiologist must be attentive to the possible presentations and complications of PD, even in uncommon sites, trying whenever possible to correlate the radiological features with the patient's clinical symptoms.

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.

Paget's disease in patients of Asian descent in New Zealand

Paget's disease is considered to be uncommon in Asian people, but we have noted a recent increase in the number of Asian patients referred to our clinic, on a background of an apparently decreasing prevalence in the population of European descent. In this article, we report clinical and epidemiological features of patients of Asian descent with Paget's disease, referred between 1973 and 2010 to the Auckland Paget's disease clinic. No Asian patients were referred before 1993, but 14 have presented between 1993 and 2010, with a median of 8 years between arrival into New Zealand and time of diagnosis. The patients were predominantly of south Asian origin. None of the 8 patients tested carried a mutation in exon 8 of the sequestosome 1 (SQSTM1) gene. The number of new Paget's disease referrals expressed as a proportion of the potentially at-risk Asian population in the Auckland region (derived from census data) was 1/10(4) in 2006 to 2011. Amongst Europeans, the corresponding value decreased from 10/10(4) in 1986 to 1991 to 2/10(4) in 2006 to 2010. The increased number of people of Asian descent diagnosed with Paget's disease in the Auckland region has paralleled the increasing size of the local Asian population. The continuing decline of Paget's disease in the European population, in conjunction with the emergence of the disease in the Asian population, supports the view that an environmental determinant to the disease exists and that Asians are not genetically protected. It also implies that the apparent reduction in Paget's disease prevalence in western cities is unlikely to be explicable by the rising Asian population of these cities.

Familial Paget disease and SQSTM1 mutations in New Zealand

Genetic factors play an important role in the pathogenesis of Paget disease of bone (PDB). SQSTM1 is the most important disease-associated gene identified to date. We investigated the relationship of family history, phenotype, and SQSTM1 mutation status in New Zealand (a country with a high prevalence of PDB) in patients with a family history and/or a severe phenotype. We studied 61 unrelated subjects with familial PDB. Family history was subclassified into three groups according to the closeness of the relationship. We also studied a fourth group of 19 unrelated patients defined by early onset and/or severe disease but no family history. The PDB phenotype was defined according to age, alkaline phosphatase activity, and disease extent on scintiscan at the time of diagnosis. Mutations in exon 8 of SQSTM1 were detected by screening of genomic DNA. Four different mutations were identified; the ubiquitous P392L mutation and the truncating mutation E396X accounted for 89% of cases. Overall 26% of patients with familial PBD in New Zealand had disease-associated mutations in the SQSTM1 gene. Mutations were most prevalent (60%) in those with a parent or sibling and at least one other relative affected (P < 0.002). The severity of the phenotype was significantly related to SQSTM1 mutation status but not the strength of the family history (P < 0.005). SQSTM1 mutations were found in 10.5% of patients with early onset and/or severe disease but no family history.

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.

A point mutation in the ubiquitin-associated domain of SQSMT1 is sufficient to cause a Paget's disease-like disorder in mice

Mutations of SQSTM1 occur in about10% of patients with Paget's disease of bone (PDB), but it is unclear whether they play a causal role or regulate susceptibility to an environmental trigger. Here we show that mice with a proline to leucine mutation at codon 394 of mouse sqstm1 (P394L), equivalent to the P392L SQSTM1 mutation in humans, develop a bone disorder with remarkable similarity to PDB. The P394L mutant mice developed focal bone lesions with increasing age and by 12 months, 14/18 (77%) heterozygotes and 20/21 (95%) homozygotes had lesions, compared with 0/18 (0%) wild-type littermates (P< 0.001). Lesions predominantly affected the lower limbs in an asymmetric manner and were characterized by focal increases in bone turnover, with increased bone resorption and formation, disruption of the normal bone architecture and accumulation of woven bone. Osteoclasts within lesions were larger and more nucleated than normal and some contained nuclear inclusions similar to those observed in human PDB. Osteoclast precursors from P394L mutant mice had increased sensitivity to RANKL in vitro resulting in the generation of osteoclasts that were larger and more nucleated than those generated from wild-type littermates. There was increased expression of sqstm1, autophagy-related gene 5 (atg5) and light chain 3 gene (lc3) in osteoclast precursors and increased LC3-II protein levels in Bafilomycin-treated osteoclasts from P394L mutant mice compared with wild-type suggesting dysregulation of autophagy and enhanced autophagosome formation. These studies demonstrate that SQSTM1 mutations can cause a PDB-like skeletal disorder in the absence of an additional trigger and provide a new disease model for PDB.

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.

The role of surgery and adjuvants to survival in Pagetic osteosarcoma

BACKGROUND

Osteosarcoma is a rare complication of Paget's disease with a very poor prognosis. Treatment is controversial: the older age of the patients affected by Paget's disease may limit the use of chemotherapy and axial involvement may limit the practicality of surgery.

QUESTIONS/PURPOSES

The purposes of this study are (1) to report the survival in patients treated for osteosarcoma in Paget's disease; (2) to identify correlations between type of treatment and survival comparing our data with those in the literature; (3) to determine if the extent of Paget's disease and risk of malignant transformation are associated; (4) to assess if prognosis is related with site; and (5) to identify the variations of histologic subtypes of these osteosarcomas.

METHODS

We retrospectively reviewed the medical records of 26 patients treated between 1961 and 2006 who had bone sarcoma arising from a site of Paget's disease. Twenty two of the 26 patients had surgery. In six surgery only was performed; three had surgery, adjuvant chemotherapy, and radiotherapy; one surgery and radiotherapy; 12 underwent surgery and chemotherapy, adjuvant in 10 patients and neoadjuvant in two; two had only radiotherapy and two had only chemotherapy. We performed survival analyses between various combinations of treatment.

RESULTS

At last followup four patients had no evidence of disease (NED) at a minimum followup of 42.6 months (mean, 139 months; range, 42.6-257.4 months) and 22 died with disease (DWD) at a minimum time of 1 month (mean, 20.2 months; range, 1-84 months). One of the six patients (11%) treated with surgery only had NED at 10 years; the other five died from disease at a mean of 30 months. Three of 12 patients (25%) treated with surgery and chemotherapy are NED at a mean followup of 12 years; nine died of disease at a mean of 24 months. All patients treated without surgery died at a mean of 7.5 months (range, 1-13.7 months).

CONCLUSIONS

Despite improvements in surgery and medical treatments the prognosis remains poor in patients with Paget's sarcoma.

SQSTM1 gene analysis and gene-environment interaction in Paget's disease of bone

Even though SQSTM1 gene mutations have been identified in a consistent number of patients, the etiology of Paget's disease of bone (PDB) remains in part unknown. In this study we analyzed SQSTM1 mutations in 533 of 608 consecutive PDB patients from several regions, including the high-prevalence area of Campania (also characterized by increased severity of PDB, higher number of familial cases, and peculiar phenotypic characteristics as giant cell tumor). Eleven different mutations (Y383X, P387L, P392L, E396X, M401V, M404V, G411S, D423X, G425E, G425R, and A427D) were observed in 34 of 92 (37%) and 43 of 441 (10%) of familial and sporadic PDB patients, respectively. All five patients with giant cell tumor complicating familial PDB were negative for SQSTM1 mutations. An increased heterogeneity and a different distribution of mutations were observed in southern Italy (showing 9 of the 11 mutations) than in central and northern Italy. Genotype-phenotype analysis showed only a modest reduction in age at diagnosis in patients with truncating versus missense mutations, whereas the number of affected skeletal sites did not differ significantly. Patients from Campania had the highest prevalence of animal contacts (i.e., working or living on a farm or pet ownership) without any difference between patients with or without mutation. However, when familial cases from Campania were considered, animal contacts were observed in 90% of families without mutations. Interestingly, a progressive age-related decrease in the prevalence of animal contacts, as well as a parallel increase in the prevalence of SQSTM1 mutations, was observed in most regions except in the subgroup of patients from Campania. Moreover, patients reporting animal contacts showed an increased number of affected sites (2.54 +/- 2.0 versus 2.19 +/- 1.9, p < .05) over patients without animal contacts. This difference also was evidenced in the subgroup of patients with SQSTM1 mutations (3.84 +/- 2.5 versus 2.76 +/- 2.2, p < .05). Overall, these data suggest that animal-related factors may be important in the etiology of PDB and may interact with SQSTM1 mutations in influencing disease severity.

Paget's disease of bone

Paget's disease of bone is a focal bone disorder that is common among older people of Western European descent. It is an unusual disorder, for although we now have safe and highly effective treatment, there are many aspects of its pathogenesis and natural history that we do not yet understand. Recent years have seen significant advances in the understanding of its epidemiology, genetics and molecular biology, but an integrated view that incorporates all these aspects remains elusive. In this review we examine some of the outstanding problems, the solutions to which seem likely to change our understanding of bone cell biology.