Biologic Characteristics of Fibrous Hamartoma from Congenital Pseudarthrosis of the Tibia Associated with Neurofibromatosis Type 1

Background: Fibrous hamartoma is a key pathologic component of congenital pseudarthrosis of the tibia, a challenging and disabling bone disorder. We investigated the biologic characteristics of fibrous hamartoma cells in order to better understand the pathogenesis of this rare disease. Methods: Fibrous hamartoma tissues were surgically excised at the time of osteosynthesis from seven patients with congenital pseudarthrosis of the tibia associated with neurofibromatosis type 1. Distal tibial periosteum was also harvested as control tissue during tibial derotation osteotomy from two other patients with cerebral palsy and one patient with idiopathic internal tibial torsion. Fibroblast-like cells were enzymatically dissociated and cultured from these tissues. Immunophenotypes were investigated for positive (CD44 and CD105) and negative (CD45 and CD14) mesenchymal lineage cell markers, and the mRNA expressions of bone morphogenetic protein(BMP)-2, BMP-4, and their receptors were assayed by reverse transcription-polymerase chain reaction. After rhBMP-2 treatment, the changes in alkaline phosphatase activity, and in the mRNA expressions of typed collagen (COL1A1), alkaline phosphatase, and osteocalcin genes, were assayed with use of an RNase protection assay. The mRNA expressions of receptor activator of nuclear factor-kappa B ligand (RANKL) and osteoprotegerin (OPG) were quantitatively assayed with use of real-time RT-PCR. Osteoclastic differentiation of RAW(264.7) cells in coculture with fibrous hamartoma cells was evaluated. Results: All fibrous hamartoma and tibial periosteal cells tested were CD44+/CD105+/CD45-/CD14-and expressed the mRNAs of BMP-2, BMP-4 and their receptors. The baseline mRNA expressions of COL1A1, alkaline phosphatase, and osteocalcin genes in the fibrous hamartoma cells were diverse. These gene expressions were upregulated by BMP treatment in tibial periosteal cells but did not change or were downregulated in fibrous hamartoma cells. Fibrous hamartoma cells expressed higher levels of RANKL and lower levels of OPG than did tibial periosteal cells. Coculture with fibrous hamartoma cells enhanced osteoclastic differentiation of RAW264.7 cells. Conclusions: Fibrous hamartoma cells maintain some of the mesenchymal lineage cell phenotypes, but do not undergo osteoblastic differentiation in response to BMP. They are more osteoclastogenic than are tibial periosteal cells. Clinical Relevance: The low osteogenicity and high osteoclastogenicity of fibrous hamartoma cells may play a role in the pathogenesis of congenital pseudarthrosis of the tibia. This should be taken into consideration when developing a treatment protocol for congenital pseudarthrosis of the tibia.