Multiple Myeloma (MM) remains the second most common hematologic malignancy occurring in adults, which affects primarily the skeletal system. Current therapies, which include chemotherapy, radiotherapy, autologouos stem cell transplantation and in some cases surgery, have the extended median survival between 3 and 10 years. However, MM is still incurable and therefore improving current therapies or developing novel ones to extend survival rate will be highly beneficial for patient management.

The long term goal of this study is to develop a novel therapeutic approach to the treatment of MM using genetically-engineered mesenchymal stem cells (MSC) for inhibiting osteoclast activity by stable expression of osteoprotegin (OPG). Studies have shown that osteoclast activity is increased in myeloma patients through increased expression of Receptor Activator of Nuclear Factor Kappa B Ligand (RANKL) leading to RANKL/RANK signaling, resulting in osteoclast activation and ultimately bone resorption. Osteoprotegerin is a soluble decoy receptor for RANKL, and is decreased in expression in myeloma patients possibly because of a marked decrease in osteoblast and or MSC, which produce OPG. Thus, we hypothesize that therapy with MSC, overexpressing OPG, will greatly decrease osteolytic damage and reduce morbidity. Despite the potential of OPG in inhibiting osteoclast activation, OPG also binds to tumor related apoptosis-inducing ligand (TRAIL) making MM cells resistant to apoptosis.

In order to eliminate TRAIL binding while still possessing RANKL binding, we have created mutant OPG constructs by site-directed mutagenesis based on interactive domain identification, and superimposing structural models of TRAIL, OPG and DR5, the death receptor that binds to TRAIL. The mutant OPGs were produced in HEK 293 cells for characterization of the five potential mutants and their TRAIL binding ability. One of the mutants abolished TRAIL binding while possessing RANKL binding as determined by TRAIL assay and osteoclast assay respectively. Finally, to achieve biphasic effects of MSC-OPG therapy with anti-tumor activity, the regenerative MSC therapy will be tested in combination with chemotherapy and anti-angiogenic therapy. These studies are performed in preclinical mouse models of MM. Currently, other OPG mutants are being characterized for their RANKL and TRAIL binding properties which will be confirmed via the assays mentioned and immunoprecipitation. Thus far, we conclude that using site-directed mutagenesis targeted at the N-terminal of OPG effectively retains RANKL binding and abolishes TRAIL binding.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3391. doi:10.1158/1538-7445.AM2011-3391