Paget's disease of bone possible viral basis

Dual-specificity tyrosine phosphorylation-regulated kinase 2 regulates osteoclast fusion in a cell heterotypic manner

Monocyte fusion into osteoclasts, bone resorbing cells, plays a key role in bone remodeling and homeostasis; therefore, aberrant cell fusion may be involved in a variety of debilitating bone diseases. Research in the last decade has led to the discovery of genes that regulate osteoclast fusion, but the basic molecular and cellular regulatory mechanisms underlying the fusion process are not completely understood. Here, we reveal a role for Dyrk2 in osteoclast fusion. We demonstrate that Dyrk2 down regulation promotes osteoclast fusion, whereas its overexpression inhibits fusion. Moreover, Dyrk2 also promotes the fusion of foreign-body giant cells, indicating that Dyrk2 plays a more general role in cell fusion. In an earlier study, we showed that fusion is a cell heterotypic process initiated by fusion-founder cells that fuse to fusion-follower cells, the latter of which are unable to initiate fusion. Here, we show that Dyrk2 limits the expansion of multinucleated founder cells through the suppression of the fusion competency of follower cells.

OPG/RANKL system imbalance in a case of hepatitis C-associated osteosclerosis: the pathogenetic key?

Hepatitis C-associated osteosclerosis (HCAO) is an impressive example of acquired diffuse osteosclerosis in adults, recently described in ten patients infected with hepatitis C virus (HCV). Its hallmark is a painful and generalized increase of bone mass. Bone biopsies show enhanced accretion rate, usually without histological abnormalities. The HCAO pathogenesis is hitherto unknown. HCV might induce a slow bone cell infection and the production of bone growth factors, such as insulin-like growth factors. Recently, receptor activator of nuclear factor-kappaB (RANK), its ligand (RANKL), and soluble decoy receptor osteoprotegerin (OPG) have been identified as a pivotal cytokine system in the bone remodeling control. We describe the 11th case of HCAO. Notably, the patient's bone biopsy showed the presence of a high number of OPG-positive osteoblasts, a slight increase of RANKL-positive stromal cells, and a dramatic reduction of the osteoclasts. Moreover, OPG serum levels were increased. These findings reported here for the first time are consistent with a pathogenetic role of the OPG/RANKL system imbalance in HCAO.

Variable disease severity associated with a Paget's disease predisposition gene

We have recently identified a region on the long arm of chromosome 18 that carries a predisposition gene for Paget's disease using linkage analysis. This region was explored because of earlier studies demonstrating the presence of a gene for another bone disorder, familial expansile osteolysis (FEO) within this region. FEO has many similarities to Paget's disease including osteoclast abnormalities and viral-like nuclear inclusions. Therefore, it was proposed that FEO and Paget's disease are disorders resulting from mutations at the same locus. For our linkage study, we utilized a large kindred with a high incidence of Paget's disease. The propositus in this family had polyostotic disease symptoms beginning at age 31. During the process of characterizing this family, four other family members were diagnosed with Paget's disease. One of the proband's siblings was asymptomatic but had high normal serum alkaline phosphatase levels; Paget's disease was identified only after bone scanning (at age 50). This implies that the predisposition gene does not consistently cause severe disease and raises a question concerning the mechanism of predisposition: Does the predisposition gene affect the age of onset and/or the severity of disease? To further explore this question, two individuals from this kindred, under age 30 and carrying the affected haplotype, were evaluated for early onset Paget's disease. No evidence of disease was observed after thorough evaluation. This implies that the age of onset is highly variable for this locus, indicating variable expression of disease in individuals carrying the same mutation.

Genetic linkage of Paget disease of the bone to chromosome 18q

Paget disease is a common bone disease characterized by abnormal osteoclasts that are large, multinucleated, and overactive and that contain paramyxovirus-like nuclear inclusions. There is evidence for a major genetic component to Paget disease, with up to 40% of patients having affected first-degree relatives; however, the locus (loci) and gene(s) involved are unknown. Another bone disorder, familial expansile osteolysis (FEO), although extremely rare, also is characterized by similar osteoclast abnormalities but has an earlier age at onset and a more aggressive clinical progression. The causative gene for FEO has been localized to a region of human chromosome 18q. On the basis of the presence of similar clinical findings and of viral-like nuclear inclusions in osteoclasts, we hypothesized that FEO and Paget disease are allelic versions of the same locus. Therefore, a large kindred with a high incidence of Paget disease was examined to determine if Paget disease was linked to genetic markers in the same region of chromosome 18 as that for FEO. Our analysis yielded a two-point LOD score of 3.40, with the genetic marker D18S42, a marker tightly linked to the FEO locus. This demonstrates that the gene(s) responsible for FEO and that for Paget disease are either closely linked or the same locus.