Molecular pathways of calcium pyrophosphate deposition disease

Calcium pyrophosphate deposition disease (CPDD) is a common type of arthritis defined by the presence of calcium pyrophosphate (CPP) crystals in articular cartilage. While CPDD typically affects elderly patients in a sporadic fashion, it also occurs prematurely in familial patterns. Studies of familial forms of this disease present exciting opportunities to identify novel therapeutic targets for this currently untreatable arthritis. We recently confirmed that a mutation in the stop codon of TNFRSF11B causes early onset CPDD. TNFRSF11B codes for osteoprotegerin (OPG). OPG is a decoy receptor for Receptor Activator of Nuclear Factor Kappa B Ligand (RANKL). RANKL promotes osteoclast formation. Our preliminary data clearly demonstrate that recombinant mutant OPG (OPGmt) displays inefficient inhibition of RANKL resulting in excess osteoclastogenesis in vitro. Our genetically engineered mice carrying OPGmt have osteopenia and premature arthritis mirroring the effects of OPGmt in humans. We have also found that osteoclast conditioned media potently stimulates chondrocyte pyrophosphate (PPi) production, a necessary process for CPP crystal formation. The purpose of this work is to investigate the conceptually innovative hypothesis that OPGmt produces CPDD by increasing osteoclastogenesis in subchondral bone. We propose that excess osteoclasts in subchondral bone stimulate cartilage PPi production and that high cartilage-derived PPi levels target pre-osteoblasts to exaggerate subchondral osteoclastogenesis seen in the presence of OPGmt. The scientific premise of this work stems from careful observations of the phenotype of patients with OPGmt and strong preliminary data. In Aim 1 we will use Opgmt/+ and Opgmt/mt knock-in mice to comprehensively determine the role of Opgmt in arthritis pathogenesis in mice and determine if disease can be prevented by blocking RANKL. In Aim 2, we will employ in vitro and in vivo models to investigate the role of high PPi levels in promoting OPGmt-induced arthritis and elucidate the underlying mechanisms. This work is the first to identify the OPG/RANKL/RANK pathway in CPDD and to implicate subchondral bone as a primary target tissue in this disease. This proposal includes a novel mouse model that will serve as the basis for further mechanistic and pre-clinical studies in CPDD. The shared clinical features of patients with the OPG mutation and those with age-related CPDD and the existence of available drugs which target these pathways support rapid translation of this work.