Disruption of calvarial ossification in E2f4mutant embryos correlates with increased proliferation and progenitor cell populations

The E2F family of transcription factors, in association with pocket protein family members, are important for regulating genes required for cellular proliferation. The most abundant E2F, E2F4, is implicated in maintaining the G0/G1 cell cycle state via transcriptional repression of genes that encode proteins required for S-phase progression.Here, we investigate E2F4's role in bone development using E2f4germline mutant mice. We find that mutation of E2f4impairs the formation of several bones that arise through intramembranous or endochondral ossification. The most severe defect occurred in the calvarial bones of the skull where we observed a striking delay in their ossification. In vivoand in vitroanalyses established that E2F4 loss did not block the intrinsic differentiation potential of calvarial osteoblast progenitors.  However, our data showed that E2f4 mutation elevated proliferation in the developing calvaria in vivoand it increased the endogenous pool of undifferentiated progenitor cells. These data suggest that E2F4 plays an important role in enabling osteoblast progenitors to exit the cell cycle and subsequently differentiate thereby contributing to the commitment of these cells to the bone lineage.