LOCALIZATION OF OSTEOPROTEGERIN GENE EXPRESSION IN THE HUMAN TIBIAL PLATEAU AT END STAGE OSTEOARTHRITIS: CORRELATION WITH BONY ADAPTATION BY MICROCT.

Abstract

Osteoarthritis (OA) involves pathology in both articular cartilage and subchondral bone. The osteoprotegerin (OPG)/receptor activator of nuclear factor kappa beta ligand (RANK-L) balance is known to modulate bone turnover. We compared the bony changes in human total knee arthroplasty (TKA) and cadaveric controls. A qualitative increase in subchondral and ligamentous insertional bone mineral density was observed on micro-CT sections of TKA bone compared with cadaveric controls. In-situ hybridization of digoxygenase (DIG)-labelled OPG riboprobes showed selective uptake in osteoblasts but not osteocytes or osteoclasts in TKA bone. Those data suggested that the upregulation of OPG expression by osteoblasts may have precipitated the bony hypertrophy of end-stage OA.

Altered joint mechanics produced by periarticular bone remodelling may precede the cartilage changes of osteoarthritis (OA). Recently, receptor activator of nuclear factor kappa beta (RANK), along with its soluble ligand (RANK-L), have been shown to induce both maturation and activation of bone-degrading osteoclasts. Activation of RANK on osteoclast cells by RANK-L is opposed by another soluble factor, osteoprotegerin (OPG). Thus RANK/OPG balance is important in regulating bone turnover. Here, we compared periarticular bone from patients with end-stage OA undergoing total knee arthroplasty (TKA) with those of cadaveric controls. We assessed bony, histological and molecular changes that are important in the pathogenesis of OA.

Using in-situ hybridization, we found increased staining of digoxygenase (DIG)-labelled OPG in osteoblasts of TKA bone. A corresponding increase in subchondral and insertional bone was seen on micro-CT (μCT) sections from TKA bone in comparison with cadaveric controls. Those changes were accompanied by marked articular cartilage degeneration on histology.

This study is the first of which we are aware that directly assessed the role of OPG in inducing the bony changes seen in human end-stage OA. We used μCT to compare corresponding samples qualitatively from TKA and cadaveric bone. Adjacent sections underwent hybridization of digoxygenase (DIG)-labelled OPG riboprobes to assess gene expression in situ. Finally, samples were stained and analysed for histology.

Bony hypertrophy may be a result of overexpression of OPG that occurs as an important feature of OA pathophysiology.

Funding: This work was supported by a grant from the Hip Hip Hooray Fund of the Canadian Orthopaedic Research Foundation (CORF) and the Wood Professorship in Joint Injury Research. There was no commercial funding for this research project.