NF1 (neurofibromatosis type 1) is a relatively common genetic disease which may be characterized by the presence of scoliosis and altered bone metabolism, amongst its many orthopaedic manifestations. Traditionally, spine fusion procedures have been used to correct and limit the progression of this deformity. NF1 bone healing at the tibia and spine feature impaired bone anabolism, excessive catabolism, and fibrosis. Fibrotic tissue in the tibia and between the vertebrae can lead to pseudarthrosis, which can require substantive clinical intervention. In particular, complications associated with the spine can represent a significant source of morbidity in this population, often presenting with persistent deformity, pain, and hardware failure. Revision procedures, themselves a source of morbidity, are often required when a primary procedure has failed. This thesis explores systematic approaches to modelling deficient NF1 spine healing and treatment to improve outcomes in this patient population.

A murine model of posterolateral fusion using rhBMP-2 (bone morphogenetic protein-2) was developed to test a range of pharmacological interventions. This model was first applied to Nf1 heterozygous mice and was reproducible and reliable. Nf1^+/- mice exhibited a mild orthopaedic phenotype with increased osteoclasts on histology. Treatment with the bisphosphonate Zoledronic acid (ZA) increased the bone volume of the fusion masses in both control and Nf1^+/- mice, though the improvement was larger in controls.

Several studies have shown that tibial pseudarthroses can be associated with a localized double inactivation of the Nf1 gene, and we speculated that this could underlie local lesions in the spine. To recapitulate this, we utilized a model where a Cre-expressing adenovirus induced local double inactivation in Nf1^flox/flox mice. This was then applied to the established spine fusion model. Consistent with the clinical presentation of spinal pseudarthrosis, a limited amount of rhBMP-2 bone was formed and substantive fibrous tissue was present. Targeted treatments with pharmaceutical agents were next trialled in this model. The MEK inhibitor PD0325901 increased bone volume in all groups while ZA increased bone density. In summary, this model represents a robust platform upon which to test targeted interventions to reduce the fibroproliferative phenotype of NF1.

A second goal of this research project was to investigate the cellular contributors to spine fusion in general, which could be used to design new treatments both for NF1 and non-NF1 spine fusion. To accomplish this, a murine genetic model of lineage tracking was employed. This featured Tie2-Cre:​Ai9 and αSMA-creERT2:​Col2.3- GFP:​Ai9 reporter mice. Spine fusion operations were performed in these mice, and the distribution of lineage-labelled cells were traced using fluorescence. Notably, Tie2 lineage cells co-labelled with TRAP positive cells, suggesting a primary contribution to the osteoclasts but not osteoblasts of the fusion mass. Conversely, αSMA lineage cells co-labelled with Col2.3-GFP expressing osteoblasts, suggesting new bone primarily arises from mesenchymal cells with negligible input from endothelial cells undergoing transdifferentiation.

In conclusion, treatment of scoliosis remains a challenge in individuals with NF1. The development of a fibroproliferative model of spine fusion in an Nf1 deficient mouse represents a robust platform upon which to test targeted interventions to improve outcomes in NF1. Additionally, advancements in genetic modeling of human disease in animals may provide new models in which to investigate this process.