Orthotopic murine model of a primary malignant bone tumor in the spine: Functional, bioluminescence, and histological correlations

Object. There is currently no reproducible animal model of human primary malignant bone tumors in the spine to permit laboratory investigation of the human disease. Therefore, the authors sought to adapt their previously developed orthotopic model of spinal metastasis to a model for primary malignant bone tumors of the spine. Methods. A transperitoneal surgical approach was used to implant osteosarcoma (Krib-1) into the L-3 vertebral body of nude mice via a drill hole. Motor function was evaluated daily using the previously validated qualitative key milestones of tail dragging, dorsal stepping, hindlimb sweeping, and paralysis. A subset of these animals was euthanized upon reaching the various milestones, and the spines were removed, sectioned, and stained. The degree of spinal cord compression was correlated with the occurrence of milestones and assessed by a ratio between the neural elements divided by the area of the spinal canal. Another subset of animals received stably transfected Krib-1 cells with the luciferase gene, and bioluminescence was measured at 10, 20, and 30 days postimplantation. Results. Osteosarcoma xenografts grew in all animals according to a reliable and reproducible time course; the mean time for development of behavioral milestones was noted in relation to the day of implantation (Day 1). Tail dragging (Milestone 1) occurred on Day 19.06 (95% CI 16.11-22.01), dorsal stepping (Milestone 2) occurred on Day 28.78 (95% CI 26.79-30.77), hindlimb sweeping (Milestone 3) occurred on Day 35.61 (95% CI 32.9-38.32), and paralysis of the hindlimb (Milestone 4) occurred on Day 41.78 (95% CI 39.31-44.25). These clinically observed milestones correlated with increasing compression of the spinal cord on histological sections. The authors observed a progressive increase in the local bioluminescence (in photons/cm2/sec) of the implanted level over time with a mean of 2.17 (range 0.0-8.61) at Day 10, mean 4.68 (range 1.17-8.52) at Day 20, and mean 5.54 (range 1.22-9.99) at Day 30. Conclusions. The authors have developed the first orthotopic murine model of a primary malignant bone tumor in the spine, in which neurological decline reproducibly correlates with tumor progression as evidenced by pathological confirmation and noninvasive bioluminescence measurements. Although developed for osteosarcoma, this model can be expanded to study other types of primary malignant bone tumors in the spine. This model will potentially allow animal testing of targeted therapies against specific primary malignant tumor types.