A RUNX2 reporter is expressed prior to osteochondral differentiation and models metaphyseal dysplasia with maxillary hypoplasia and brachydactyly

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 condensation prior to RUNX2 activation and osteochondral differentiation to chondrocyte and osteoblast lineages. Unlike limb development, the anterior craniofacial skeleton arises from multipotent cranial neural crest cells (cNCCs). To examine the temporal activation of SOX9 and RUNX2 within cNCCs, we utilized a combination of immunofluorescence to detect endogenous proteins and mouse genetic reporters to label SOX9 and RUNX2 expressing cells. We find that RUNX2 is expressed broadly throughout cNCCs 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 Sox9 and Runx2. Although the GFP based Runx2 reporter did not delete any 3'UTR sequences, homozygous Runx2GFP/GFP pups develop postnatal deficiencies in intramembranous and endochondral ossification that correlate with enhanced expression of RUNX2 protein in osteoblasts and hypertrophic chondrocytes. We find that this RUNX2 upregulation leads to compaction of growth plates, facilitates the transition of columnar chondrocytes to hypertrophy, and restricts terminal hypertrophic differentiation. Runx2GFP/GFP phenotypes model the human disorder, Metaphyseal Dysplasia with Maxillary Hypoplasia and Brachydactyly (MDMHB), resulting from RUNX2 enhanced activity due to intragenic duplications. Altogether, this reporter model provides a valuable tool for studying RUNX2 function in early cNCC-derived lineages and highlights the high sensitivity of ossification pathways to RUNX2 dosage.