. Runx2 is essential for the differentiation of osteoblasts from mesenchymal cells, and the forced expression of Runx2 transdifferentiates fibroblasts into osteoblasts. Moreover, Runx22/2 mice completely lack osteoblasts, lamellar bone and marrow cavities, i.e., the characteristic of affected regions in OPLL, throughout their bodies. However, the pathophysiological role of Runx2 in the development of OPLL has remained unknown. In this study, we used ENPP1ttw/ttw mice, a mouse model of OPLL, and Runx2 mutant mice to investigate the role of Runx2 in OPLL. We found that Runx2 is induced prior to the formation of ectopic bone in OPLL and that Runx2 haploinsufficiency ameliorates OPLL-associated ectopic calcification. Materials and Methods Animals Enpp1ttw/ttw mice were obtained from the Central Institute for Experimental Animals. Runx2+/2 mice have been described previously. We housed all mice under a 12-hr light/dark cycle with ad libitum access to standard food and water. We determined the genotypes of the mice by polymerase chain The Role of Runx2 in the Development of OPLL reaction. All PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/22212322 animal experiments were performed with the approval of the Animal Study Committee of Tokyo Medical and Dental University and conformed to all relevant guidelines and laws. Histological Aalysis For histological examination, after dissection, the tissue samples were fixed immediately in 4% paraformaldehyde/phosphatebuffered saline, then dehydrated with gradually increasing concentrations of ethanol and embedded in paraffin. After fixation, the tissue samples from the adult mice were decalcified in 20% EDTA for two weeks before being embedded in paraffin. For LacZ staining, the spines from heterozygous Runx2- mice were fixed in 0.2% paraformaldehyde at room temperature for 30 minutes, and then stained overnight in X-Gal solution, as previously described. Immunohistochemical staining using antibody against Runx2 was performed using the avidin-biotinperoxidase complex method with the ABC Rabbit IgG Kit, as previously described. The antiRunx2 antibodies 
have been described previously. In situ hybridization was performed using 35S-labeled riboprobes and the standard protocol, as described previously. Micro-computed Tomography Analysis We obtained three-dimensional images of the cervical spine using micro-computed tomography. Each spine was placed in a plastic tube, and images were reconstructed from 750 projections. Ectopic ossification was quantitatively analyzed using bone analysis software. Statistical Analysis All data are presented as the mean 6 s.d.. We performed the statistical analyses using the Student’s t-test. Differences were considered statistically significant when P,.05. The results are representative of more than four individual experiments. Results As an initial measure of the contribution of Runx2 to ligament development, we analyzed the expression of Runx2 in the prospective ligaments of spine including posterior ligament of the vertebrae at the atlanto-occipital area of mouse embryos using three different experimental techniques. First, we performed an in situ hybridization analysis using Runx2 as a probe. At birth, Runx2 was expressed in vertebrae and at the edge of the vertebrae, which corresponds to a order R-547 future ligament. Next, we took advantage of the LacZ allele that was inserted into the Runx2 locus by performing LacZ staining of spines isolated from heterozygous Runx2 mice to monitor the expression of Runx2 in developing mouse skeletons a
