A RUNX2 GFP reporter is expressed prior to osteochondral differentiation and models Metaphyseal Dysplasia with Maxillary Hypoplasia and Brachydactyly (MDMHB).

SOX9 and RUNX2 are lineage defining transcription factors that drive differentiation of chondrocyte and osteoblast lineages respectively from osteochondral progenitors. In limb development, these progenitors are specified first by SOX9 expression required for mesenchymal stem cell (MSC) condensation prior to RUNX2 activation and osteochondral differentiation to chondrocyte and osteoblast lineages. Unlike limb development, the anterior craniofacial skeleton arises from cranial neural crest (cNCC) stem cells. To examine the temporal activation of SOX9 and RUNX2 within cNCCs, we utilized a combination of immunofluorescence to detect endogenous proteins and genetic reporters to label SOX9 and RUNX2 expressing cells. We find that RUNX2 is expressed broadly throughout cNCC stem cells of the first branchial arch that will give rise to developing mandibular tissue at a timepoint prior to osteochondral lineage determination. Substantial SOX9 expression is activated subsequently within differentiating chondrocytes. These findings were validated by fluorescent reporters inserted in the 3' untranslated regions (3'UTRs) of Summary: We have developed a novel mouse model for a human disorder resulting from excessive RUNX2, a transcription factor required for bone formation. We find that RUNX2 turns on early within facial stem cells in a pattern unique from limb development. Excessive RUNX2 is particularly detrimental to bone growth in juvenile development after birth.