TWIST inactivation reduces CBFA1/RUNX2 expression and DNA binding to the osteocalcin promoter in osteoblasts

The Saethre-Chotzen (SC) syndrome is characterized by increased osteogenesis and premature fusion of cranial sutures, resulting from mutations in TWIST, a basic helix-loop-helix transcription factor. The molecular target genes for Twist in osteoblasts are however unknown. We report here that TWIST haploinsufficiency in mutant osteoblasts reduces mRNA and protein levels for CBFA1/RUNX2, a specific osteoblast transcription factor, during both osteoblast cell growth and in vitro osteogenesis. Moreover, this is associated with altered expression of major osteoblast-specific genes. Electrophoretic mobility shift assay (EMSA) showed reduced-binding ability of Cbfa1 to its target OSE2 element in the osteocalcin promoter in mutant osteoblasts. By contrast, TWIST inactivation does not hamper Cbfa1 binding on a similar upstream element present in the alpha1(I) collagen promoter in mutant osteoblasts. This provides the first evidence that TWIST inactivation alters CBFA1/RUNX2 expression and Cbfa1 binding ability to the osteocalcin promoter, indicating that CBFA1/RUNX2 is a target gene for TWIST in human osteoblasts.