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    ASBMR 31st Annual Meeting

    Nf1 Loss-of-function in Mesenchymal Stem Cells Impairs Chondrocyte Hypertrophy, Axial Skeleton Development and Remodeling

     Disorders of Bone and Mineral Metabolism (Genetic, Basic, and Trans.)
     Aging and Other Disorders of Bone and Mineral Metabolism (Clinical)
     Bone Acquisition and Pediatric Bone Disease (Clinical)

    Oral Presentations, Presentation Number: 1126
    Session: Concurrent Oral Session 21: Disorders of Bone and Mineral Metabolism III: Phenotype and Rescue Studies
    Sunday, September 13, 2009 3:45 PM - 4:00 PM, Colorado Convention Center, Four Seasons Ballroom 4

    * , UNITED STATES, Caitlyn Whitten, Vanderbilt University, Heather Moss, Vanderbilt University, , UNITED STATES

    Kinases play crucial roles during skeletogenesis, as shown by the vertebral and growth defects observed in mice characterized by constitutive activation of MEK/ERK in mesenchymal cells. ERK constitutive activation is also a hallmark of neurofibromatosis type I (NF1), a congenital disorder caused by mutations in the NF1 gene. NF1 patients exhibit appendicular and axial skeleton dysplasiae leading to bowing tibia and scoliosis. Whether NF1 dystrophic scoliosis is caused by adjacent neurofibromas or a bone cell-autonomous defect is unknown. To determine whether Nf1 loss-of-function in mesenchymal stem cells (MSCs) affects vertebral development and induces dystrophic lesions similar to the ones observed in NF1, we generated conditional mutant mice lacking Nf1 in MSCs (Nf1msc-/-) and analyzed them at embryonic, growing and adult stages. Recombination of Nf1 in MSCs, chondrocytes and osteoblasts was confirmed by PCR. Histological studies in Nf1msc-/- embryos revealed a shortened hypertrophic chondrocyte zone in long bones, a delay of notochord regression and abnormal nucleus pulpusus formation in the vertebral axis. Adult Nf1msc-/- mice were dwarf, displayed bowing tibia and skull asymmetry. Lumbar scoliosis and cervical kyphosis became visible by 3 months of age, progressing to severe axial angulation, low BMD and misshaped vertebrae by 6 months of age. These defects were accompanied by a severe fenestration of cortical bones and decreased trabecular and cortical BV/TV, measured by µCT. Extensive unmineralized matrix was detected in all bone surfaces, despite normal Ph/Ca levels and in agreement with an increase in Osteopontin expression (an inhibitor of mineralization) and increased collagen synthesis by Nf1-/- osteoblasts. Nf1-/- MSCs and chondrocytes were characterized by constitutive ERK activation in vitro. This observation and the similar bone phenotypes between Nf1msc-/- mice and mice characterized by FGFR3 or MEK/ERK gain-of-function suggested that Nf1 acts as a brake on FGFR3 signaling in mesenchymal differentiation process. Together, these results indicate that the vertebral lesions in NF1 can be induced by NF1 loss-of-function in MSCs during development, and may worsen with time due to impaired bone remodeling or healing, independent of the presence of neurofibromas. These results thus identify Nf1 as a critical gene during skeletogenesis, and provide a pre-clinical model to investigate new therapies for the most severe NF1 skeletal manifestations.

    Disclosures: None

    * Presenting Authors(s): , UNITED STATES