Translational regulation is a control point in RUNX2/Cbfa1 gene expression

Runt-related transcription factor-2 (RUNX2)/core binding factor a1 (Cbfa1) is implicated in the regulation of osteoblast differentiation and osteoblast-specific gene expression. Mutations in RUNX2 cause the bone disease cleidocranial dysplasia, which is characterized by multiple skeletal defects. RUNX2 is expressed as two isoforms (type-I and type-II) encoded by two different mRNAs. We report here the detection of both mRNAs in osteoblastic cells and osteoblast precursors as well as nonosteoblastic cells. Surprisingly, however, osteoblast precursors and nonosteoblastic cells express no RUNX2 protein; mature osteoblasts express both isoforms, while less mature osteoblastic cells express only type-I protein. Northern blot analysis of RNA isolated from polysomes and ribonucleoprotein particles demonstrated that RUNX2 mRNA is polysome-associated in osteoblastic cells but polysome-free in osteoblast precursors. These results suggest that (a) RUNX2 mRNAs are expressed but dormant in osteoblast precursors and nonosteoblastic cells, (b) RUNX2 gene expression is controlled at the translational level, and (c) the expression of individual protein isoforms of RUNX2 is differentiation stage specific. Thus, differentiation of cells along the osteoblast lineage appears to be regulated at the level of RUNX2 mRNA translation.