Human Molecular Genetics Advance Access originally published online on May 17, 2007
Human Molecular Genetics 2007 16(14):1661-1675; doi:10.1093/hmg/ddm114
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Filamin B mutations cause chondrocyte defects in skeletal development
1 Department of Neurology, 2 Department of Radiology and 3 Department of Hematology and Oncology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA 02115, USA, 4 Department of Radiology, University of California at San Francisco, CA 94305, USA, 5 Hospital for Special Surgery, Weill Medical College of Cornell University, New York, NY 10021, USA and 6 Howard Hughes Medical Institute, Program in Biological and Biomedical Sciences, Harvard Medical School, Boston, MA 02115, USA
* To whom correspondence should be addressed at: HIM 858, Beth Israel Deaconess Medical Center, 77 Avenue Louis Pasteur, Boston, MA 02115, USA. Tel: +1 6176672699; Fax: +1 6176670800; Email: vsheen{at}bidmc.harvard.edu
Received January 9, 2007; Accepted April 26, 2007
Filamin B (FLNB) is a cytoplasmic protein that regulates the cytoskeletal network by cross-linking actin, linking cell membrane to the cytoskeleton and regulating intracellular signaling pathways responsible for skeletal development (Stossel, T.P., Condeelis, J., Cooley, L., Hartwig, J.H., Noegel, A., Schleicher, M. and Shapiro, S.S. (2001) Filamins as integrators of cell mechanics and signalling. Nat. Rev. Mol. Cell Biol., 2, 138–145). Mutations in FLNB cause human skeletal disorders [boomerang dysplasia, spondylocarpotarsal (SCT), Larsen, and atelosteogenesis I/III syndromes], which are characterized by disrupted vertebral segmentation, joint formation and endochondral ossification [Krakow, D., Robertson, S.P., King, L.M., Morgan, T., Sebald, E.T., Bertolotto, C., Wachsmann-Hogiu, S., Acuna, D., Shapiro, S.S., Takafuta, T. et al. (2004) Mutations in the gene encoding filamin B disrupt vertebral segmentation, joint formation and skeletogenesis. Nat. Genet., 36, 405–410; Bicknell, L.S., Morgan, T., Bonafe, L., Wessels, M.W., Bialer, M.G., Willems, P.J., Cohn, D.H., Krakow, D. and Robertson, S.P. (2005) Mutations in FLNB cause boomerang dysplasia. J. Med. Genet., 42, e43]. Here we show that Flnb deficient mice have shortened distal limbs with small body size, and develop fusion of the ribs and vertebrae, abnormal spinal curvatures, and dysmorphic facial/calvarial bones, similar to the human phenotype. Characterization of the mutant mice demonstrated increased apoptosis along the bone periphery of the distal appendages, consistent with reduced bone width. No changes in the initial proliferative rate of chondrocytes were observed, but the progressive differentiation of chondrocyte precursors was impaired, consistent with reduced bone length. The extracellular matrix appeared disrupted and phosphorylated ß1-integrin (a collagen receptor and Flnb binding partner) expression was diminished in the mutant growth plate. Like integrin-deficient chondrocytes, adhesion to the ECM was decreased in Flnb(–/–) chondrocytes, and inhibition of ß1-integrin in these cells led to further impairments in cell spreading. These data suggest that disruption of the ECM-ß1-integrin-Flnb pathway contributes to defects in vertebral and distal limb development, similar to those seen in the human autosomal recessive SCT due to Flnb mutations.