449. Sca-1+ Cell-Based Gene Therapy with a Modified Fibroblast Growth Factor 2 (FGF2) Increased Endosteal/Trabecular Bone Formation in Mice

A major problem in bone-wasting diseases such as osteoporosis is the loss of endosteal bone that results in reduced bone strength and increased fracture risk. Recent advances in gene therapy raise the possibility that ex vivo gene therapy with a bone-forming gene may be used to enhance endosteal bone formation. However, a successful strategy requires that the transduced cells remain at the endosteal bone sites. Sca-1+ cells are a desirable cell vehicle because of their ability to home and engraft into the bone marrow cavity. This study assessed the feasibility of an ex vivo Sca-1+ cell-based systemic FGF2 gene therapy to promote endosteal bone formation. To maximize our chances of demonstrating a biological effect, the human FGF2 transgene was modified to increase protein secretion and stability by adding the BMP2/4 hybrid signal sequence and mutating two key cysteines in the FGF2 molecule. Retro-orbital injection of Sca-1+ cells transduced with the MLV-FGF2 vector into sub-lethally irradiated stem cell deficient (W41/W41) mice resulted in long-term engraftment, marked elevation in serum FGF2, and dose-related endosteal bone formation. Ten weeks post transplantation, serum FGF2 levels in mice transplanted with MLV-GFP-transduced Sca-1+ cells (controls) averaged 36|[plusmn]|6 pg/mL, whereas the serum FGF2 level in mice transplanted with FGF2-expressing cells was significantly increased but highly variable (2502|[plusmn]|2459 pg/mL, p<0.02). Longitudinal femur sections stained with Mallory's stain for collagen from 3 mice representing low, intermediate, and high serum FGF2 (Fig. 1) show dose-dependent effects of endosteal bone formation with large amounts of newly formed bone evident in the intermediate and high serum FGF2 mice. pQCT of femurs revealed significant increases in total volumetric bone mineral density (vBMD) (14%, p<0.05) and trabecular vBMD (250%, p<0.001) in the FGF2 group compared to controls. In mice showing very high serum FGF2 (>4,000 pg/mL), endosteal bone formation was so robust that the marrow space was completely filled with bony tissues. There was insufficient calcium available for mineralization of the newly formed bone, leading to hypocalcemia, secondary hyperparathyroidism, and osteomalacia. However, these undesirable side effects appeared to be dose-related. Thus, it may be possible to minimize adverse effects while maximizing efficacy by optimizing transgene expression. In conclusion, these data demonstrate the massive power of our approach to target and deliver a systemic gene therapy to the endosteal bone surface, and provide compelling test-of-principle evidence for the feasibility of a Sca-1+ cell-based ex vivo systemic FGF2 gene therapy to promote endosteal bone formation.