Modulation of spine fusion with BMP-2, MEK inhibitor (PD0325901), and zoledronic acid in a murine model of NF1 double inactivation.

Abstract

Spine fusion is a common procedure for the treatment of severe scoliosis, a frequent and challenging deformity associated with Neurofibromatosis type 1 (NF1). Moreover, deficiencies in NF1-Ras-MEK signaling affect bone formation and resorption that in turn impacts on spine fusion outcomes. In this study we describe a new model for AdCre virus induction of Nf1 deficiency in the spines of Nf1^flox/flox mice. The virus is delivered locally to the mouse spine in a fusion procedure induced using BMP-2. Systemic adjunctive treatment with the MEK inhibitor (MEKi) PD0325901 and the bisphosphonate zoledronic acid (ZA) were next trialed in this model. AdCre delivery resulted in abundant fibrous tissue (Nf1^null +393%, P < 0.001) and decreased marrow space (Nf1^null -67%, P < 0.001) compared to controls. While this did not significantly impact on the bone volume of the fusion mass (Nf1^null -14%, P = 0.999 n.s.), the presence of fibrous tissue was anticipated to impact on the quality of spine fusion. Multinucleated TRAP + cells were observed in the fibrous tissues seen in Nf1^null spines. In Nf1^null spines, MEKi increased bone volume (+194%, P < 0.001) whereas ZA increased bone density (+10%, P < 0.002) versus BMP-2 alone. Both MEKi and ZA decreased TRAP + cells in the fibrous tissue (MEKi -62%, P < 0.01; ZA -43%, P = 0.054). No adverse effects were seen with either MEKi or ZA treatment including weight loss or signs of illness or distress that led to premature euthanasia. These data not only support the utility of an AdCre-virus induced knockout spine model, but also support further investigation of MEKi and ZA as adjunctive therapies for improving BMP-2 induced spine fusion in the context of NF1.