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

We have reported previously that severe Paget's disease of bone had become less common at our center between 1973 and 1993. Data from several countries support the view that there are important secular trends in the prevalence and severity of Paget's disease. In this paper we describe recent trends in the demography of newly referred patients with Paget's disease to determine if the secular trend toward milder disease has continued. A database of all newly referred patients (n = 1487) with Paget's disease (1973 to 2002 inclusive, 30 years) was examined. Of these, 832 subjects (56%) had scintiscans. Plasma total alkaline phosphatase (total ALP) activity, disease extent on scintiscan, and a derived index of average ALP activity of pagetic bone were used as indices of severity. The number of new referrals with Paget's disease declined sharply from 1994 onward and is currently at half the rate seen 20 years earlier, while the mean age at presentation has progressively increased by 4 years per decade (P < 0.0001). Total ALP at diagnosis, disease extent on scintiscan, and the number of bones involved were all negatively correlated with both date of birth (P < 0.0001) and year of presentation (P < 0.0001), indicating that more recently born and presenting subjects had substantially less severe bone disease. The average activity of pagetic bone was only weakly correlated with year of presentation, but not with year of birth or age at presentation. Although there are a number of potential biases, these data are consistent with a continued secular trend to presentation in older subjects with less extensive skeletal involvement, and a declining prevalence of Paget's disease.

Paget's disease of bone

Paget's disease of bone is common, affecting up to 4% of Australians over the age of 55 years. The incidence of the disease and the severity of newly diagnosed cases appear to be falling, for unknown reasons. The cause of Paget's disease is unknown, but there is a strong genetic influence. Recently, mutations in the sequestosome 1/p62 gene have been identified as a cause of familial Paget's disease and of some apparently sporadic cases of the disease. The disease is often asymptomatic, but can cause bone pain, deformity, fracture and other complications. Paget's disease is eminently treatable. Potent bisphosphonates such as pamidronate, alendronate and risedronate relieve symptoms and may reduce the risk of complications. The Pharmaceutical Benefits Scheme subsidises treatment only for patients with symptomatic disease. A strong case be made for also treating asymptomatic patients with involvement of long bones, vertebrae or base of skull, patients with significant osteolytic lesions, and perhaps all younger patients.

Management of Paget's disease of bone

Paget's disease of bone is a common condition with a strong genetic component, characterized by focal increases in bone turnover, affecting one or more bones throughout the skeleton. Paget's disease can be asymptomatic but is frequently associated with bone pain, bone deformity, pathological fracture, secondary osteoarthritis and deafness. Inhibitors of osteoclastic bone resorption, such as bisphosphonates and calcitonin, suppress bone turnover and improve bone pain in Paget's disease. Many patients also require therapy with analgesics and anti-inflammatory agents, since pain in Paget's disease can arise not only from increased bone turnover but also from complications such as osteoarthritis and nerve compression syndromes, which do not respond well to antiresorptive therapy. Comparative studies have shown that second- and third-generation bisphosphonates, such as tiludronate, alendronate and risedronate, are more effective than etidronate at inhibiting bone turnover in Paget's disease but they have not been found to be significantly more effective in controlling bone pain. Importantly, none of the treatments that are currently available for Paget's disease have been shown to prevent complications such as deafness, fracture or bone deformity, or to alter the natural history of the disease. More research is required to define the long-term effects of antiresorptive treatment on clinical outcomes in Paget's disease, so that clinicians and their patients can make better-informed choices about the risks and benefits of treatment.

Paget's disease-a VDR coactivator disease?

Paget's disease is the most exaggerated example of bone remodeling with increased osteoclastic bone resorption followed by excessive bone formation. One of the earliest findings in our studies of Paget's disease is that pagetic osteoclast (OCL) precursors are hyper-responsive to 1,25-(OH)(2)D(3) and form OCL at concentrations of 1,25-(OH)(2)D(3) that are physiologic rather than pharmacologic. The increased responsivity to 1,25-(OH)(2)D(3) is not due to increased levels of the Vitamin D receptor (VDR) or to increased infinity of 1,25-(OH)(2)D(3) for VDR. We have recently shown using GST-VDR chimeric protein pull-down assays that TAF(II)-17, a member of the TAF(II)-D transcription complex, is increased in OCL precursors from patients with Paget's disease compared to normals. We further showed that TAF(II)-17 can enhance VDR mediated gene transcription and allow formation of the transcription complex at very low levels of 1,25-(OH)(2)D(3). In addition, coactivators of VDR including CPB300 and DRIP205 are also increased in OCL precursors from Paget's patients. These data suggest that the enhanced sensitivity of OCL precursors for 1,25-(OH)(2)D(3) in Paget's disease results from increased expression of coactivators of VDR and suggest that part of the pathophysiology underlying OCL formation in Paget's disease may result from enhanced expression of VDR coactivators.

The mechanism of hearing loss in Paget's disease of bone

OBJECTIVES/HYPOTHESIS

The mechanism of hearing loss (HL) in Paget's disease of bone was investigated. The present study was a systematic, prospective, controlled set of clinical investigations to test the hypothesis that there is a general underlying mechanism of HL in Paget's disease of bone and to gain additional insights into the auditory and otologic dynamics of this disease. Specific questions were 1) whether the mechanism is cochlear or retrocochlear and 2) whether the bone mineral density of the cochlear capsule is related to hearing levels.

STUDY DESIGN

Several double-blinded, cross-sectional, prospective, correlational studies were conducted in a population of elderly human subjects with skull involvement with Paget's disease versus a control population of elderly subjects free of Paget's disease. Demographic and clinical data were recorded. Longitudinal observations were made in subjects under treatment.

METHODS

Subjects were recruited from a Paget's disease clinic. Pure-tone auditory thresholds, word recognition, and auditory brainstem responses (ABRs) were recorded. The dimensions of the internal auditory canals were measured using computed tomographic (CT) images and digital image analysis. The precision, accuracy, and temporal stability of methods to measure the bone mineral density of the cochlear capsule and an adjacent area of nonotic capsule bone were validated and applied. Correlations were sought between hearing levels and cochlear capsule bone mineral density.

RESULTS

ABRs were recorded in 64 ears with radiographically confirmed Paget's disease involving the skull. Responses were absent in eight ears, all of which had elevated high pure-tone thresholds. ABRs were interpreted as normal in 56 ears; none were abnormal. The mid-length diameter and minimum diameter of the internal auditory canal of 68 temporal bones from subjects with Paget's disease were found to have no statistically significant relationship to hearing thresholds. The Pearson product-moment correlation coefficients (age- and sex-adjusted) in the group with Paget's disease involving the temporal bone were -0.63 for left ears and -0.73 for right ears for high-frequency air conduction pure-tone thresholds (mean of 1, 2, and 4 kHz) versus cochlear capsule density. Correlation coefficients (age- and sex-adjusted) between cochlear capsule density and air-bone gap (mean at 0.5 and 1 kHz) for the affected group were -0.67 for left ears and -0.63 for right ears. All correlations between hearing thresholds and cochlear capsule density in pagetic subjects were significant at P <.001. The regressions were consistent throughout the ranges of hearing level. There were no significant correlations between cochlear capsule mean density and hearing level in the volunteer subjects.

CONCLUSIONS

The evidence supports the existence of a general, underlying, cochlear mechanism of pagetic HL that is closely related to loss of bone mineral density in the cochlear capsule. This mechanism accounts well for both the high-frequency sensorineural HL and the air-bone gap. Early identification, radiographic diagnosis of temporal bone involvement, and vigorous treatment with third-generation bisphosponates are important to limit the development and progression of pagetic HL.

Inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia is caused by mutant valosin-containing protein

Inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia (IBMPFD) is a dominant progressive disorder that maps to chromosome 9p21.1-p12. We investigated 13 families with IBMPFD linked to chromosome 9 using a candidate-gene approach. We found six missense mutations in the gene encoding valosin-containing protein (VCP, a member of the AAA-ATPase superfamily) exclusively in all 61 affected individuals. Haplotype analysis indicated that descent from two founders in two separate North American kindreds accounted for IBMPFD in approximately 50% of affected families. VCP is associated with a variety of cellular activities, including cell cycle control, membrane fusion and the ubiquitin-proteasome degradation pathway. Identification of VCP as causing IBMPFD has important implications for other inclusion-body diseases, including myopathies, dementias and Paget disease of bone (PDB), as it may define a new common pathological ubiquitin-based pathway.

Vitamin D receptor, oestrogen receptor- and calcium-sensing receptor genotypes, bone mineral density and biochemical markers in Paget's disease of bone

OBJECTIVES

The significance of genetic polymorphisms in the development of Paget's disease of bone is unclear at present.

METHODS

We analysed the BsmI polymorphism of the vitamin D receptor (VDR) gene, the PvuII and XbaI polymorphisms of the oestrogen receptor-alpha (ER alpha) gene, and the A986S polymorphism of the calcium-sensing receptor (CaSR) gene in 69 pagetic patients and 120 healthy subjects. We also examined the relationship of these polymorphisms with lumbar spine and femoral neck BMD as well as with biochemical parameters (serum alkaline phosphatase, osteocalcin and parathyroid hormone) in Paget's disease.

RESULTS

The XbaI and PvuII genotype distributions of the ER alpha gene were significantly different between patients with Paget's disease and control subjects (P<0.001). Also, the CaSR A986S genotype frequency was significantly different between pagetic patients and controls (P<0.01). No significant effect of gene polymorphisms on BMD or biochemical parameters of bone turnover was observed.

CONCLUSION

Our results suggest that the ER alpha PvuII/XbaI and CaSR A986S polymorphisms may contribute to genetic susceptibility to Paget's disease. However, further studies are required to investigate the underlying pathomechanism and to replicate the associations.

Apoptotic gene expression in Paget's disease: a possible role for Bcl-2

Paget's disease of bone is characterized by an increase in both the size and the number of bone-resorbing osteoclasts. An important regulator of osteoclast activity is the process of apoptosis, and any aberration in this process could lead to increased osteoclasis. Analysis using human apoptosis cDNA expression arrays revealed that the apoptotic suppressor, Bcl-2, showed a marked increase in expression in Pagetic bone. In situ hybridization (ISH) and computer-assisted image analysis confirmed that the levels of Bcl-2 transcripts were significantly (p<0.0001) increased in Pagetic osteoclasts. The Bcl-2:Bax transcript ratios were similarly elevated. These findings were confirmed by immunohistochemistry. The Bcl-2 gene promoter sequence from 20 Pagetic patients and controls was analysed. Single nucleotide mutations were identified in three of the Paget's patients and one of the controls. Luciferase reporter analysis showed that the mutations induced a basal 12-fold increase and hydrogen peroxide-induced 19-fold increase in luciferase expression, compared with the normal construct. It is concluded that in Paget's disease, there is an increase in the expression of genes that are involved in the inhibition of apoptosis, notably Bcl-2. The increase in Bcl-2 may be explained in some patients by mutations in the Bcl-2 gene promoter. These results provide a potential explanation for the dramatic increase in osteoclasis seen in patients with Paget's disease.

Osteoclast diseases

Osteoclasts are the only cells capable of resorbing mineralised bone, dentine and cartilage. Osteoclasts act in close concert with bone forming osteoblasts to model the skeleton during embryogenesis and to remodel it during later life. A number of inherited human conditions are known that are primarily caused by a defect in osteoclasts. Most of these are rare monogenic disorders, but others, such as the more common Paget's disease, are complex diseases, where genetic and environmental factors combine to result in the abnormal osteoclast phenotype. Where the genetic defect gives rise to ineffective osteoclasts, such as in osteopetrosis and pycnodysostosis, the result is the presence of too much bone. However, the phenotype in many osteoclast diseases is a combination of osteosclerosis with osteolytic lesions. In such conditions, the primary defect is hyperactivity of osteoclasts, compensated by a secondary increase in osteoblast activity. Rapid progress has been made in recent years in the identification of the causative genes and in the understanding of the biological role of the proteins encoded. This review discusses the known osteoclast diseases with particular emphasis on the genetic causes and the resulting osteoclast phenotype. These human diseases highlight the critical importance of specific proteins or signalling pathways in osteoclasts.

Paget's disease and fibrous dysplasia

Paget's disease and fibrous dysplasia are benign disorders that can involve the temporal bone and skull base. They commonly lead to otolaryngologic symptoms such as impingement of cranial nerves or the orbit or blockage of the external auditory canal or paranasal sinuses, although they can often be a challenge to diagnose because of their insidious presentation. Their benign nature and common presentation within the difficult-to-access confines of the skull base should lead the clinician to exercise caution in their treatment, reserving surgical intervention for either diagnosis or the relief of symptoms. As a better understanding of the etiology of these conditions develops and new pharmacotherapeutic agents are tested, it is likely that physicians will be able to turn to medical rather than surgical techniques to treat these disorders.

Mathematical model predicts a critical role for osteoclast autocrine regulation in the control of bone remodeling

Bone remodeling occurs asynchronously at multiple sites in the adult skeleton and involves resorption by osteoclasts, followed by formation of new bone by osteoblasts. Disruptions in bone remodeling contribute to the pathogenesis of disorders such as osteoporosis, osteoarthritis, and Paget's disease. Interactions among cells of osteoblast and osteoclast lineages are critical in the regulation of bone remodeling. We constructed a mathematical model of autocrine and paracrine interactions among osteoblasts and osteoclasts that allowed us to calculate cell population dynamics and changes in bone mass at a discrete site of bone remodeling. The model predicted different modes of dynamic behavior: a single remodeling cycle in response to an external stimulus, a series of internally regulated cycles of bone remodeling, or unstable behavior similar to pathological bone remodeling in Paget's disease. Parametric analysis demonstrated that the mode of dynamic behavior in the system depends strongly on the regulation of osteoclasts by autocrine factors, such as transforming growth factor beta. Moreover, simulations demonstrated that nonlinear dynamics of the system may explain the differing effects of immunosuppressants on bone remodeling in vitro and in vivo. In conclusion, the mathematical model revealed that interactions among osteoblasts and osteoclasts result in complex, nonlinear system behavior, which cannot be deduced from studies of each cell type alone. The model will be useful in future studies assessing the impact of cytokines, growth factors, and potential therapies on the overall process of remodeling in normal bone and in pathological conditions such as osteoporosis and Paget's disease.

Relationships between pharmacokinetics and rate of bone turnover after intravenous bisphosphonate (olpadronate) in patients with Paget's disease of bone

Bisphosphonates are the treatment of choice of Paget's disease, but variable responses have been reported, and despite the availability of potent bisphosphonates, biochemical remission is not achieved in a substantial number of patients. This may, in part at least, be because of the influence of pharmacokinetics of bisphosphonates on their pharmacodynamics. That is the response of bone turnover to treatment. To address this issue, we examined the pharmacokinetics and pharmacodynamics of the bisphosphonate olpadronate given intravenously to 75 patients with Paget's disease, using a specific assay for olpadronate concentrations in serum and urine. The skeletal uptake of olpadronate varied greatly among patients and ranged between 10% and 90% of the administered dose. The two major determinants of skeletal uptake were renal function and prevalent rate of bone turnover. Serum and urinary data were well described by a physiology-based four-compartment pharmacokinetic model that takes into account the distribution of the bisphosphonate in the bone and its subsequent elimination. Bone turnover was suppressed to well within the normal range in virtually all patients. This, together with the absence of resolution of effect during 1 year, does not allow the construction of an adequate integrated pharmacokinetic/pharmacodynamic model. However, the pharmacokinetic model, described for the first time in Paget's disease, can accurately simulate the amount of bisphosphonate delivered to the skeleton with different dose regimens as well as the amount still present in bone after 1 year. Such approaches can lead to improved patient care and individualization of treatment of Paget's disease with bisphosphonates.

Corrective osteotomy for deformity in Paget disease

BACKGROUND

Severe deformity resulting from Paget disease is not uncommon. Malalignment of the extremity may lead to intractable pain, mechanical overload of the neighboring joints, limitation of motion and function, and dysmorphic appearance. Although corrective osteotomy has been used to treat osseous deformities, the outcome of corrective osteotomy for long-bone deformities resulting from Paget disease remains largely unknown.

METHODS

The results after twenty-five corrective osteotomies (twenty-two patients), performed between 1975 and 1995, in sixteen tibiae, eight femora, and one radius were evaluated. There were thirteen men and nine women with a mean age of sixty-seven years. The indication for osteotomy was pain in twenty limbs, recurrent stress fractures in three, and limitation of function in two. A variety of osteotomies and fixation methods were used. Two patients underwent simultaneous total hip arthroplasty and proximal femoral osteotomy.

RESULTS

Twenty-three of twenty-five osteotomies healed with an average time to union of six months. Both nonunions were in patients who had been managed with intramedullary fixation. The time to union was significantly shorter in metaphyseal osteotomies fixed with plates than in diaphyseal osteotomies (p < 0.04). There was a substantial improvement in the deformities. Satisfaction was rated excellent or good by fourteen patients, fair by six, and poor by two. Complications included a pin-track infection in two patients, peroneal nerve palsy in one, and loss of fixation following external fixation in one. Disease activity, as measured by serum alkaline phosphatase level, and medical treatment with calcitonin and/or bisphosphonates did not have a significant impact on time to union.

CONCLUSIONS

Corrective osteotomy for the treatment of severe deformity in Paget disease can be challenging and yet rewarding. A higher prevalence of complications was observed following intramedullary nailing and external fixation. Fracture-healing seems to be particularly protracted in diaphyseal osteotomies.

Defective bone remodelling in osteoprotegerin-deficient mice

Previous studies have reported enhanced osteoclastogenesis, increased bone resorption and osteoporosis in osteoprotegerin (OPG)-deficient mice. In the present study, we show that the tibial epiphyses contain abundant, thin trabeculae lined with numerous osteoclasts and cuboidal osteoblasts. The increase in osteoblasts and osteoclasts was associated with a dramatic increase in calcein labelling of the mineralization fronts and replacement of much of the intertrabecular marrow with numerous alkaline phosphatase-positive preosteoblasts. Furthermore, the discrete, linear cement lines seen in wild-type mice were replaced by a randomly oriented meshwork of cement lines that were stained intensely for tartrate-resistant acid phosphatase and osteopontin in the OPG-/- mice. These indices of accelerated bone remodelling in mutant bone were associated with irregular trabecular surfaces, a disorganized collagen matrix interspersed with amorphous ground substance and numerous fissures between old and new bone. In total, these observations indicate that enhanced osteoclastic activity in OPG-/- epiphyses led to a coupled increase in osteoblast differentiation and activity and an increase in bone remodelling. The high bone turnover, disorganized matrix and impaired attachment of new to old bone in the cement lines in OPG-/- mice appear to cause bone fragility.

Outcome of uncemented hip arthroplasty components in patients with Paget's disease

The outcome of uncemented hip arthroplasty in patients with Paget's disease is unknown. The clinical and radiographic records of patients with Paget's disease affecting the hip who had total hip arthroplasty using uncemented components against pagetic bone were reviewed. There were 18 patients (19 hips) with an average age of 71.3 years (range, 54-85 years). Followup averaged 7 years (range, 2-15 years). Surgery often was reported to be demanding technically because of hard sclerotic bone and excessive bleeding in some patients. Estimated blood loss averaged 996 mL (range, 200-2500 mL). Harris hip scores improved significantly. There was clinical and radiographic evidence of bone ingrowth in all patients. Six of 19 hips had heterotopic bone. One hip had severe Brooker Grade IV heterotopic ossification and the patient required excision of the ossification at 5 years. One hip was unstable (subluxating) at the latest followup. There was no revision for component loosening and no recognized cases of clinical or radiographic loosening of any uncemented implants. Results of total hip arthroplasty using uncemented components in patients with Paget's disease are excellent overall, with an extremely low risk of component loosening during the first decade after implantation. These patients may be at higher risk for heterotopic bone formation and increased perioperative blood loss because of hypervascularity of the bone.

Effect of surgical menopause and Paget's disease of bone on the isomerization of type I collagen carboxyterminal telopeptide: evolution after antiresorptive therapy

The objective of this study was to analyze the effect of surgical menopause and Paget's disease of bone, as well as the influence of therapy, on the isomerization of the carboxyterminal telopeptide of type I collagen (CTX). Fourteen women who had undergone surgical menopause and had begun hormone replacement therapy (HRT) after surgery were recruited. Results for these women were compared with those of 29 patients with Paget's disease of bone treated with tiludronate (400 mg/day) for 3 months, and with those of a group of 21 healthy premenopausal women (control group I). In addition, 14 healthy individuals with an age range similar to that of the pagetic patients (control group II) were included in the study. Urine samples were analyzed for levels of nonisomerized and beta-isomerized CTX (alpha-CTX and beta-CTX). Biochemical determinations were performed 3 months after surgical menopause and after 3 and 9 months of HRT, and at baseline, and 1 and 6 months after tiludronate treatment in the pagetic patients. The average levels of alpha-CTX and beta-CTX were higher in patients than in controls. In patients after surgical menopause, because of their greater increase of beta-CTX, the alpha-CTX/beta-CTX ratio was lower than that of control group I (0.881 +/- 0.3 vs 1.515 +/- 0.8; P < 0.05). In contrast, at baseline, pagetic patients showed marked increases in alpha-CTX levels, resulting in a higher alpha-CTX/beta-CTX ratio than that of control group II (2.879 +/- 1.3 vs 0.96 +/- 0.25; P < 0.0001). The average percent decrease in both markers after therapy was similar in both conditions (-60% for alpha-CTX and -44% for beta-CTX after 3 months of HRT in the surgical menopause group, vs -66% for alpha-CTX and -41% for beta-CTX in the pagetic group, 1 month after finishing tiludronate therapy; P, NS), resulting in a significant decrease of the alpha-CTX/beta-CTX ratio in pagetic patients (2.879 +/- 1.3 vs. 1.614 +/- 0.8; P < 0.001). In conclusion, surgical menopause is associated with a decrease in the urinary alpha-CTX/beta-CTX ratio because of the higher increase in the beta-CTX level after menopause. Pagetic patients show an increase in this ratio, compared with the control value, and the ratio decreases after bisphosphonate treatment. The response to therapy was similar in both conditions, with a comparable decrease of both markers. These findings show how bone markers may contribute to the understanding of pathophysiologic mechanisms in bone diseases.

Bisphosphonate therapy for Paget's disease in a patient with hypoparathyroidism: profound hypocalcemia, rapid response, and prolonged remission

Bisphosphonate treatment for severe Paget's disease leads to hypocalcemia followed by a secondary hyperparathyroid response to restore normocalcemia. A case is presented of a 60-year-old woman with polyostotic Paget's disease and postsurgical hypoparathyroidism. In 1993 her Paget's disease--alkaline phosphatase (ALP), 1260 U/liter (35-135 U/liter), and fasting urinary hydroxyproline excretion, 13.7 micromol/liter GF (0.4-1.9 micromol/liter)--was treated with intravenous pamidronate. Symptomatic hypocalcemia followed the first 60-mg dose, requiring large doses of calcium supplementation and calcitriol. Pamidronate therapy to a total dose of 360 mg was followed by rapid and prolonged remission with indices of bone turnover in the normal range within 2 months and persisting for at least 19 months after treatment. In 1999 relapse of Paget's disease--ALP, 511 U/liter (35-135 U/liter), and fasting urinary deoxypyridinoline/creatinine 53.1 micromol/mol (5-27 micromol/mol)--was treated with alendronate, 10 mg daily. Symptomatic hypocalcemia occurred again, requiring increased calcium and calcitriol therapy. Indices of bone turnover were within the normal range 9 weeks after the start of therapy. These responses were significantly more rapid and sustained than those observed in euparathyroid subjects. This case suggests that the lack of parathyroid response may modify the response to bisphosphonates by: (a) increasing intrinsic uptake of bisphosphonate into the pagetic skeleton, allowing response to a smaller dose; (b) increasing duration and severity of hypocalcemia after bisphosphonate therapy; and (c) removing the hyperparathyroid drive to reactivation of pagetic osteoclasts, leading to a prolonged remission. These observations have implications for optimizing bisphosphonate therapy both in Paget's disease and in osteoporosis.

Studies in Paget's disease and their relevance to oncology

Paget's disease and bone metastases in cancer patients share many common properties. Both are characterized by a localized increase in osteoclast (OCL) formation leading to bone resorption. In both Paget's disease and bone metastases the increased OCL formation and the increased osteoclastogenic nature of the bone microenvironment are mediated by common factors, namely interleukin (IL)-6 and RANK ligand (RANKL). Available data suggest that in the case of Paget's disease there is increased RANKL and IL-6 production, and IL-6 enhances the responsivity of the OCL precursors to RANKL, contributing to the elevated numbers of OCLs. In patients with multiple myeloma, 95% to 100% of whom develop bone lesions, both IL-6 and RANKL levels are increased. Bisphosphonates bind locally to the surfaces of the bone undergoing osteoclastic resorption to inhibit this process. Paget's disease has in the past and will continue in the future to provide a model to test the efficacy of bisphosphonates in inhibiting bone resorption. Paget's disease provides an ideal model in which to investigate the efficacy of the new third-generation bisphosphonates in the treatment of bone metastases as well as nonmalignant bone disease.

Osteoclasts formed by measles virus-infected osteoclast precursors from hCD46 transgenic mice express characteristics of pagetic osteoclasts

Pagetic osteoclasts (OCLs) are abnormal in size and contain paramyxoviral-like nuclear inclusions that cross-react with antibodies to measles virus (MV). However, the role that MV infection plays in Paget's disease is unknown, because no animal model of Paget's disease is available. Therefore, we targeted a cellular MV receptor, human CD46 (hCD46), to cells in the OCL lineage in transgenic mice using the mouse tartrate-resistant acid phosphatase (TRAP) gene promoter. In vitro infection of OCL precursors from hCD46 transgenic mice with MV significantly increased OCL formation in bone marrow cultures. The numbers of TRAP-positive mononuclear cells and CFU-GM, the earliest identifiable OCL precursor, were also significantly increased. MV-infected OCLs formed from hCD46 marrow were increased in size, contained markedly increased numbers of nuclei, and had increased bone-resorbing capacity per OCL compared with OCLs formed from marrow of nontransgenic littermates. Furthermore, IL-6 and 24-hydroxylase messenger RNA expression levels were increased in MV-infected hCD46 transgenic mouse bone marrow cultures. Treatment of MV-infected hCD46 marrow cultures with a neutralizing antibody to IL-6 blocked the increased OCL formation seen in these cultures. These data demonstrate that MV infection of OCL precursors results in OCLs that have many features of pagetic OCLs, that the enhanced OCL formation is in part mediated by increased IL-6 expression induced by MV infection, and suggest that the hCD46 transgenic mouse may be a useful model for examining the effects of MV infection on OCL formation in vivo.