Normal Bone Metabolism
The adult skeleton is continuously renewed throughout life by the process of bone remodelling. Old bone is removed by the osteoclasts whereas new bone is formed in the same location by the osteoblasts. This occurs in an orderly fashion through temporary anatomic structures called basic multicellular units (BMUs). A basic multicellular unit comprises a team of osteoclasts at the front and a team of osteoblasts at the back supported by blood vessels, nerves, and loose connective tissue. Osteoclasts and osteoblasts are derived from different precursors in the bone marrow. Osteoclasts originate from haematopoietic precursors of the monocyte/ macrophage lineage while osteoblasts originate from multipotent mesenchymal stem cells, which give also rise to bone marrow stromal cells, chondrocytes, adipocytes, and muscle cells. The formation and lifespan of bone cells are controlled by mechanical, systemic, and local factors. Important regulators of osteoclast formation and activity belong to a ligand/ receptor/ soluble (decoy) receptor system involving proteins of the TNF receptor superfamily. These are RANK ligand, RANK, and OPG. RANKL is produced by osteocytes and osteoblastic/ stromal cells, reacts with RANK, which is localized in haematopoietic osteoclast precursors, stimulates the formation and activity of osteoclasts, and prolongs their lifespan. OPG is a soluble receptor which counteracts the biological effects of RANKL preventing its binding to RANK and thereby suppressing bone resorption.
Pathology
Paget’s disease of bone is a focal disorder of bone remodelling characterized by an increase in the number and size of osteoclasts in affected sites while the rest of the skeleton remains normal. The typically large osteoclasts, which may contain up to 100 nuclei per cell, induce excessive bone resorption associated with an increased recruitment of osteoblasts to the remodelling sites, resulting in increased bone formation and, hence, an overall increase in the rate of bone turnover. The increase in bone formation is thought to be secondary to the increased rate of bone resorption due to the coupling of the two processes. Some evidence, however, suggests that osteoblastic/ stromal cells might also be primarily affected in Paget’s dis ease and contribute to the increased rate of bone formation. The accelerated rate of bone turnover is responsible for the deposition of bone with disorganized architecture and structural weakness. The bone packets lose their lamellar structure and are replaced by woven bone with a characteristic mosaic pattern while bone marrow is infiltrated by fibrous tissue and blood vessels.
Cell Biology
In clinical studies the likelihood of a bone being affected by Paget’s disease was related to the amount of bone marrow present in that bone, leading to the postulation that the development of bone lesions may be related to specific properties of pagetic bone marrow. In bone marrow cultures from patients with Paget’s disease the rate of formation of osteoclasts and their number is markedly increased, suggesting that intrinsic abnormalities of the bone marrow microenvironment and/ or of osteoclast precursors may contribute to the upregulation of osteoclastogenesis. A number of studies supported these notions and documented two major abnormalities. First, pagetic osteoclasts and their precursors express high levels of osteotropic factors (e.g. IL- 6), a bone resorbing cytokine which has been proposed as a possible paracrine/ autocrine factor contributing to the pathogenesis of the disease. In addition, enhanced expression of RANKL was detected in bone marrow stromal cells from patients with Paget’s disease and might contribute to the increased number of osteoclasts. Second, compared to controls, bone marrow and peripheral cells from patients are hypersensitive to the action of RANKL and calcitriol and there is evidence suggesting that TAFII- 17, a component of the transcription complex that binds vitamin D receptor, may be responsible for the hypersensitivity to calcitriol. Thus, while the molecular characteristics of the cellular abnormalities of the disease are currently understood, the precise mechanism(s) that trigger these changes remain to be elucidated.
Aetiology
Several, not mutually exclusive, hypotheses have been proposed to explain the pathology of the disease, the most relevant being the viral and the genetic hypotheses. Studies of the distribution of bone lesions in patients with Paget’s disease showed that the prob ability of a bone being affected is very similar to the probability of a bone being affected with haematogenous osteomyelitis, suggesting that the disease may be caused by a circulating infectious agent. An infection by a slow virus of the paramyxovirus family (measles virus, respiratory syncytial virus, canine distemper virus) was supported by the detection of nuclear and cytoplasmic inclusions resembling paramyxoviral nucleocapsids in osteoclasts and of measles virus nucleocapsid transcripts in bone marrow and peripheral blood monocytes from patients with the disease. However, paramyxoviral- like structures have also been found in specimens from patients with other bone diseases, questioning the specificity of this finding. In addition, further search for viral presence in the osteoclasts provided conflicting results and in a large cohort of patients with Paget’s disease no evidence supporting an association between the disease and persistent infection with measles or other paramyxoviruses was found. There is, however, good evidence that paramyxoviruses and viral proteins can promote the formation of osteoclasts with features similar to those of pagetic osteoclasts.
In familial aggregation studies the risk of first- degree relatives of patients with Paget’s disease to develop the disorder was seven to ten times greater than the risk of individuals without such relatives. Furthermore, a positive family history has been reported in up to 40% of patients with Paget’s disease. Familial Paget’s dis ease is inherited as an autosomal dominant trait and initial genetic analyses showed evidence of linkage to chromosome 18q21– 22 in some families. This chromosome also contains the locus of the rare disease familial expansile osteolysis, which resembles Paget’s disease and was found to be associated with activating mutations in the gene TNFRSF11A, which encodes RANK, while abnormalities of the same gene are responsible for another rare skeletal disease, expansile skeletal hyperphosphatasia. Subsequent studies, however, failed to detect such mutations in patients with familial or sporadic Paget’s disease. Other abnormal genes that have been identified in diseases with bone phenotypes similar to that of Paget’s disease include TNFRSF11B, which encodes OPG in juvenile Paget’s disease, and VCP, which encodes p97 in the syndrome of inclusion body myopathy associated with Paget’s disease of bone and frontotemporal dementia. All these genetic defects have in common the up- regulation of the NF- kB signal transduction, an essential process in the differentiation and activation of osteo clasts. These genes have also been investigated in patients with familial or sporadic Paget’s disease but no mutations were identified. Analysis of families with Paget’s disease identified further possible loci in other chromosomes indicating genetic heterogeneity. However, studies in different parts of the world identified mutations in the SQSTM1 gene, located on chromosome 5q35, in up to 50% of patients with familial Paget’s disease and up to 10% of those with sporadic disease. SQSTM1 encodes p62, an adaptor protein that binds ubiquitin and is involved in various signalling pathways including the NF- κB pathway. The most common mutation of SQSTM1 associated with Paget’s disease (P329L) has been detected in patients from different European countries suggesting a founder gene defect. In addition, animals overexpressing this mutation in cells of the osteoclast lineage formed more osteo clasts, which were hypersensitive to RANKL but did not develop bone lesions resembling those of Paget’s disease in one study while in another they did. In addition, genetic studies of patients with Paget’s disease and no SQSTM1 mutations identified seven loci that contribute substantially to the risk of developing the disease as well as affecting its severity in combination with mutations of SQSTM1. These variants include genes important in the differentiation and function of osteoclasts (e.g. CSF1) but also genes that have not been implicated in the regulation of bone metabolism.
The functional significance of these polymorphisms is unknown. Whether mutations of genes associated with Paget’s disease are the cause of the disease or whether individuals with a mutation have an increased susceptibility to the disease when exposed to environ mental factors, such as paramyxoviruses, is currently unclear
