PRDM paralogs are required for Meckel's cartilage formation during mandibular bone development

Mandibular bone development depends on the formation of a cartilaginous anlage Meckel's cartilage derived from neural crest cells (NCC) and intramembranous ossification or direct differentiation of NCCs toward osteoblasts. Wnt/β-catenin signaling drives osteogenic vs chondrogenic differentiation and must be tightly controlled during the differentiation of osteochondroprogenitors. Chromatin remodelers add hierarchal regulation to the activation and repression of crucially timed gene regulatory networks and signaling cascades. In this study, we investigated the function of two chromatin remodelers-histone methyltransferases, PRDM3 and PRDM16 during murine craniofacial development. Conditionally ablating both Prdm3 and Prdm16 in the neural crest lineage using the Wnt1-Cre driver resulted in dramatic craniofacial phenotypes, including a severely hypoplastic mandible with complete absence of Meckel's cartilage at E18.5. Focusing on the Meckel's cartilage and mandibular bone phenotype, histological analysis demonstrated a significant increase in RUNX2+ osteoblast precursors, and loss of SOX9+ chondrogenic cells, suggesting an increase in osteoblast progenitors at the expense of chondrocytes that would otherwise form the Meckel's cartilage. This was not due to alterations in proliferation or apoptosis, as we observed no significant changes in the number of phosphoH3+ or cleaved caspase3+ cells in the mandibular process at E11.5, suggesting lack of NCC-derived chondrocytes is due to a change in NCC osteochondroprogenitor fate decisions. mRNA transcripts and protein abundance of Wnt/β-catenin signaling components were elevated in the mandibular process during initial NCC osteochondroprogenitor condensation events, suggesting PRDM3 and PRDM16 normally restrict expression of Wnt/β-catenin signaling components during NCC-derived osteochondroprogenitor differentiation to promote chondrogenesis and Meckel's cartilage formation. Taken together, PRDM3 and PRDM16 are required for NCC differentiation toward chondrocytes during Meckel's cartilage formation by controlling proper spatiotemporal Wnt/β-catenin transcriptional activity and this process is necessary for morphogenesis of the developing mandible.