Tibial pseudarthrosis causes substantial morbidity in patients with NF1. We studied tibial pseudarthrosis tissue from patients with NF1 and found elevated levels of ß-catenin compared to unaffected bone. To elucidate the role of ß-catenin in fracture healing, we used a surgically induced tibial fracture model in conditional knockout (KO) Nfl (Nf1flox/flox) mice. When treated with a Cre-expressing adenovirus (Ad-Cre), there was a localized knockdown of Nf1 in the healing fracture and a subsequent development of a fibrous pseudarthrosis. Consistent with human data, elevated ß-catenin levels were found in the murine fracture sites. The increased fibrous tissue at the fracture site was rescued by local treatment with a Wnt antagonist, Dickkopf-1 (Dkk1). The murine pseudarthrosis phenotype was also rescued by conditional Ctnnb1 gene inactivation. The number of CFU-Os, a surrogate marker of undifferentiated mesenchymal cells able to differentiate to osteoblasts, correlated with the capacity to form bone at the fracture site. Our findings indicate that the protein level of ß-catenin must be precisely regulated for normal osteoblast differentiation. An up-regulation of ß-catenin in NF1 causes a shift away from osteoblastic differentiation resulting in a pseudarthrosis in vivo. These results support the notion that pharmacological modulation of ß-catenin can be used to treat pseudarthrosis in patients with NF1.—Ghadakzadeh, S., Kannu, P., Whetstone, H., Howard A., Alman, B. A. ß-catenin modulation in neurofibromatosis type 1 bone repair: therapeutic implications.

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