Sca-1⁺ hematopoietic cell-based gene therapy with a modified FGF-2 increased endosteal/trabecular bone formation in mice

This study assessed the feasibility of using an ex vivo stem cell antigen-1-positive (Sca-1+) cell-based systemic fibroblast growth factor-2 (FGF-2) gene therapy to promote endosteal bone formation. Sca-1+ cells were used because of their ability to home to, and engraft into, the bone marrow cavity. The human FGF-2 gene was modified to increase protein secretion and stability by adding the bone morphogeneic protein (BMP)-2/4 hybrid signal sequence and by mutating two key cysteines. Retro-orbital injection of Sca-1+ cells transduced with a Moloney leukemia virus (MLV)-based vector expressing the modified FGF-2 gene into sub-lethally irradiated W41/W41 recipient mice resulted in long-term engraftment, more than 100-fold elevation in serum FGF-2 level, increased serum bone-formation markers, and massive endosteal bone formation. In recipient mice showing very high serum FGF-2 levels (>2,000 pg/ml), this enhanced endosteal bone formation was so robust that the marrow space was filled with bony tissues and insufficient calcium was available for the mineralization of all the newly formed bone, which led to secondary hyperparathyroidism and osteomalacia. These adverse effects appeared to be dose related. In conclusion, this study provided compelling test-of-principle evidence for the feasibility of using an Sca-1+ cell-based ex vivo systemic FGF-2 gene therapy strategy to promote endosteal bone formation.