AB0087 DLX5 AND DLX6 PROMOTES THE COMMITMENT OF MSC TO OSTEOBLASTIC LINEAGE AND CORTICAL BONE FORMATION
Authors: Camille Blandin, Morgane Bourmaud, Eris Hay, Giovanni Levi, Martine Cohen Solal
Journal: Annals of the Rheumatic Diseases (2019)

<h3>Background:</h3> Osteoporosis which affects 200 million women worldwide is the consequence of an imbalance of low anabolism to high catabolism, causing a risk of fracture. Impaired anabolism involved reduced osteoblast differentiation. The osteoblast differentiation is mediated by transcription factors, including Dlx5 and Dlx6. Dlx5 is known to have a role in osteoblast/osteoclast couple and as a promotor of osteoblast lineage commitment [1, 2]. Thus, Dlx5 is a transcriptional actor of Runx2 [3], a key element of osteoblastic differentiation. <h3>Objectives:</h3> The goal of this project is to expand our knowledge about bone formation and cellular precursors of osteoblasts, focusing on Dlx5 and Dlx6. <h3>Methods:</h3> We analyze the kinetic expression of Dlx5, Dlx6 and osteoblastic markers during the osteoblastic differentiation from murine osteoblastic progenitor derived from calvaria and bone marrow. Same analysis was carried out in osteoblastic precursors from control cells, Dlx5/Dlx6 cells with <i>ex vivo</i> recombination or from KO mice in parallel to human bone marrow cells. We analyzed the bone phenotype of mutated mice in the absence of expression of Dlx5 and Dlx6 under Osx promoter. <h3>Results:</h3> Dlx5 and Dlx6 increases at D7 during osteoblastic differentiation in the murine bone marrow and then was stable to D21. The absence of Dlx5/6 in cells derived from calvaria and bone marrow resulted in decreased levels of osteocalcin and alkaline phosphatase. <i>Dlx5/6</i><i><sup>fl</sup><sup>/fl</sup></i><i> Osx-Cre</i> mice were lethal. Dlx5/6 <sup>fl/+</sup> Osx Cre mice does not affect cortical and trabecular parameters at 6 weeks but had a significant lower cortical thickness and also lower Tb. BV/TV and Tb. Th along with a lower BMD at 3 months in both sexes. Moreover, periosteal volume was also lower in mutated mice. The skulls revealed a lack of sutures closures and dental abnormalities at 6 weeks and 3 months in both sexes. <h3>Conclusion:</h3> The deletion of these transcription factors under the action of the Osterix promoter generates lethality, in favor of an essential role in bone development. Heterozygous mutation show impaired bone acquisition during growth. To obtain a total deletion of Dlx5 and Dlx6 in osteoblastic precursor cells, a new murine model of conditional induced deletion is generated. Dlx5 and Dlx6 promote osteoblastic differentiation with an effect on late bone markers, in favor of a role in terminal differentiation. Analysis <i>in vitro</i> of Dlx5 and Dlx6 will be confirmed by ongoing <i>in vivo</i> experimentation. <h3>References</h3> [1] - Samee N, Geoffroy V, Marty C, Schiltz C, Vieux-Rochas M, Levi G, et al. Dlx5, a positive regulator of osteoblastogenesis, is essential for osteoblast-osteoclast coupling. Am J Pathol2008;173(3):773-80. [2] - Samee N, Geoffroy V, Marty C, Schiltz C, Vieux-Rochas M, ClementLacroix P, et al. Increased bone resorption and osteopenia in Dlx5 heterozygous mice. J Cell Biochem2009;107(5):865-72. [3] - Hassan MQ, Tare RS, Lee SH, Mandeville M, Morasso MI, Javed A, et al. BMP2 commitment to the osteogenic lineage involves activation of Runx2 by DLX3 and a homeodomain transcriptional network. J Biol Chem2006;281(52):40515-26. <h3>Acknowledgement:</h3> With the support of the french rheumatology society <h3>Disclosure of Interests:</h3> Camille Blandin: None declared, morgane Bourmaud: None declared, eris Hay: None declared, giovanni Levi: None declared, martine Cohen Solal Speakers bureau: Amgen and Lilly