Ex vivo gene therapy with stromal cells transduced with a retroviral vector containing the BMP4 gene completely heals critical size calvarial defect in rats

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Abstract
In order to develop a successful gene therapy system for the healing of bone defects, we developed a murine leukemia virus (MLV)-based retroviral system expressing the human bone morphogenetic protein (BMP) 4 transgene with high transduction efficiency. The bone formation potential of BMP4 transduced cells was tested by embedding 2.5 × 106 transduced stromal cells in a gelatin matrix that was then placed in a critical size defect in calvariae of syngenic rats. Gelatin matrix without cells or with untransduced stromal cells were the two control groups. The defect area was completely filled with new bone in experimental rats after 4 weeks, while limited bone formation occurred in either control group. Bone mineral density (BMD) of the defect in the gene therapy group was 67.8 ± 5.7 mg/cm2 (mean ± s.d., n = 4), which was 119 ± 10% of the control BMD of bone surrounding the defect (57.2 ± 1.5 mg/cm2). In contrast, BMD of rats implanted with untransduced stromal cells was five-fold lower (13.8 ± 7.4 mg/cm2, P < 0.001). Time course studies revealed that there was a linear increase in BMD between 2–4 weeks after inoculation of the critical size defect with 2.5 × 106 implanted BMP4 cells. In conclusion, the retroviral-based BMP4 gene therapy system that we have developed has the potential for regeneration of large skeletal defects.