1053. Novel Exogenously Regulated Stem Cell–Mediated Gene Therapy Platform for Spinal Fusion

Abstract

Cell-mediated gene therapy involving mesenchymal stem cells (MSCs) and bone morphogenetic protein (BMP) genes can provide a local supply of precursor cells and osteoinductive molecules that may promote bone formation. Spinal fusion has become a popular surgical technique for different pathological spinal conditions. We hypothesized that MSCs that overexpress the human (h)BMP-2 gene under the tetracycline (Tet)-controlled expression system (Tet-Off) can induce regulated bone formation leading to spinal fusion. We used a Tet-Off system based on genetically engineered MSCs containing an inducible hBMP-2 expression vector (ptTATop-BMP2) that had a bidirectional promoter (TATA sequence) utilizing elements of the tetracycline regulatory system. The system was regulated by the administration of doxycycline (Dox), a tetracycline analogue. In C3H/HeN mice 2 |[times]| 10^6 genetically engineered MSCs were injected into the paravertebral muscle of the lumbar spine (L2|[ndash]|6). Ten animals served as a negative control group (Dox was added to their drinking water) and 20 animals constituted the experimental group (Dox was supplied for different time periods leading to different duration of hBMP-2 expression). A longitudinal quantitative analysis of vertebrae fusion was performed using micro|[ndash]|computerized tomography (|[mu]|CT) to observe bone formation. In addition, engineered MSCs were injected in a similar manner to transgenic mice expressing the luciferase gene under human osteocalcin promoter. In these mice measuring the luciferase signal using a bioluminescence imaging system can noninvasively and quantitatively monitor osteogenic activity. From Week 1 postinjection the |[mu]|CT analysis demonstrated extensive bone formation leading to vertebrae fusion (Fig. 1A). No bone formation was observed in the control animals, in which hBMP-2 was inhibited. A minimum of 7 days of hBMP-2 expression was required to induce bone formation (Fig. 1B). Detection of luciferase gene expression in the transgenic mice 2 and 4 weeks postinjection indicated an increase in the osteogenic activity in vivo, which could be attributed to the paracrine effect of the engineered MSCs (Fig. 2). Our data demonstrate a regulated system for the induction of spinal fusion. We conclude that exogenously controlled gene therapy for spinal fusion can be achieved using the Tet-regulated hBMP-2 system.