Mesenchymal stem cells expressing osteoprotegerin variants inhibit osteolysis in a murine model of multiple myeloma

The current treatment options for multiple myeloma (MM) osteolytic lesions are mainly combinations of chemotherapy and other small-molecule inhibitors, but toxic side effects still remain a major concern. Studies have shown that osteoclast activity is enhanced in MM patients through increased expression of receptor activator of nuclear factor kappa B ligand (RANKL), triggering RANK signaling on osteoclast precursors, which results in aggressive bone resorption. Furthermore, osteoprotegerin (OPG), a decoy receptor for RANKL, and the osteogenic potential of mesenchymal stem cells (MSCs) are significantly decreased in myeloma patients with multiple bone lesions. Thus, the use of OPG as a therapeutic molecule would greatly decrease osteolytic damage and reduce morbidity. However, in addition to inhibiting osteoclast activation, OPG binds to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), thereby rendering the tumor cells resistant to TRAIL-induced apoptosis and limiting the use of OPG for therapy. The present study developed a bone-disseminated myeloma disease model in mouse and successfully tested a cell therapy approach using MSCs, genetically engineered to express OPG variants that retain the capacity to bind RANKL, but do not bind TRAIL. Our results of skeletal remodeling following this regenerative stem cell therapy with OPG variants indicated a significant protection against myeloma-induced osteolytic bone damage in areas of major myeloma skeletal dissemination, suggesting the potential of this therapy for treating osteolytic damage in myeloma patients.