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We studied the transcriptional regulation of NELL-1, a craniosynostosis-related gene. We identitifed three OSE2 elements in the NELL-1 promoter that are directly bound and transactivated by Runx2. Forced expression of Runx2 induces NELL-1 expression in rat calvarial cells. Introduction:We previously reported the upregulation of NELL-1 in human craniosynostosis and the overexpression of Nell-1 in transgenic animals that induced premature suture closure associated with increased osteoblast differentiation. To study the transcriptional regulation of NELL-1, we analyzed the 5Ј flanking region of the human NELL-1 gene. We identified three osteoblast specific binding elements 2 (OSE2) sites (A, B, and C) within 2.2 kb upstream of the transcription start site and further studied the functionality of these sites. Materials and Methods: An area of 2.2 kb and a truncated 325 bp, which lacked the three OSE sites, were cloned into a luciferase reporter gene, and co-transfected with Runx2 expression plasmid. The three OSE2 sites were individually mutated and co-transfected with Runx2 expression plasmid into Saos2 cells. Gel shifts and supershifts with Runx2 antibodies were used to determine specific binding to OSE2 sites. CHIP assays were used to study in vivo binding of Runx2 to the Nell-1 promoter. Runx2 expression plasmid was transfected into wildtype and Runx2 −/− calvarial cells. Nell-1, osteocalcin, and Runx2 expression levels were measured using RT-PCR. Results: Addition of Runx2 dose-dependently increased the luciferase activity in the human NELL-1 promoter-luciferase p2213. The p325 truncated NELL-1 construct showed significantly lower basal level of activity. Nuclear extract from Saos2 cells formed complexes with site A, B, and C probes and were supershifted with Runx2 antibody. Mutation of sites A, B, and C significantly decreased basal promoter activity. Furthermore, mutation of sites B and C had a blunted response to Runx2, whereas mutation of site A had a lesser effect. Runx2 bound to NELL-1 promoter in vivo. Transfection of Runx2 in rat osteoblasts upregulated Nell-1 and Ocn expression, and in Runx2 null calvarial cells, both Nell-1 and Ocn expression were rescued. Conclusions: Runx2 directly binds to the OSE2 elements and transactivates the human NELL-1 promoter. These results suggest that Nell-1 is likely a downstream target of Runx2. These findings may also extend our understanding of the molecular mechanisms governing the pathogenesis of craniosynostosis.

NELL-1, first identified by its overexpression in synostotic cranial sutures, is a novel osteoinductive growth and differentiation factor. In order to further define Nell-1’s role in craniofacial patterning, we characterized defects of the ENU-induced Nell-1 deficient (END) mice, focusing on both intramembranous and endochondral cranial bones. Results showed that calvarial bones of neonatal END mice were reduced in thickness and density, with a phenotype resembling calvarial cleidocraniodysplasia (CCD). In addition, a global reduction in osteoblast markers was observed, including reductions in Runx2, alkaline phosphatase, and osteocalcin. Remarkably, detailed analysis of endochondral bones showed dysplasia as well. The chondrocranium in the END mouse showed enrichment for early, proliferating Sox9+ chondrocytes, while in contrast markers of chondrocytes maturation were reduced. These data suggest that Nell-1 is an important growth factor for regulation of osteochondral differentiation, by regulating both Runx2 and Sox9 expression within the calvarium. In summary, Nell-1 is required for normal craniofacial membranous and endochondral skeletal development.

Previous analysis of tumor-specific constitutional LOH had identified a putative tumor-suppressor gene (LOH18CR) active in osteosarcoma tumorigenesis, which mapped to a subregion of chromosome 18q linked to both familial Paget's disease and FEO. Using 9 new polymorphic loci within the previous minimal region of LOH, we have reduced the minimal region of LOH in osteosarcoma tumors to localize the LOH18CR locus to the distal end of chromosome 18q21.33. This new region is approximately 500 kb and contains at least 7 known genes; however, it excludes 2 previous candidate genes: TNFRSF11A (RANK) and BCL2.