Enhanced BMP signaling in cranial neural crest cells induces aberrant chondrogenesis by upregulating Tbx20 expression during craniofacial development.

Bone morphogenetic proteins (BMPs) are critical for craniofacial development. We previously reported that cranial neural crest cell (CNCC)-specific enhanced BMP signaling through the ALK2 receptor causes ectopic cartilage formation in the face during mouse embryonic development. However, the downstream effectors triggering this ectopic chondrogenesis remain unclear. Here, we investigated the targets of BMP signaling responsible for ectopic cartilage formation. A microarray analysis using CNC-derived ectomesenchymal cells from the first branchial arches identified T-box transcription factor 20 (Tbx20) as the top candidate gene in CNC-specific gain-of-function ALK2 mouse embryos. This prompted us to hypothesize that enhanced BMP signaling increases Tbx20 expression, which triggers ectopic cartilage formation in the craniofacial region. To examine whether Tbx20 overexpression in CNCCs alters craniofacial development, we utilized a Cre-LoxP system to augment Tbx20 expression in a neural crest-specific manner in mice. CNCC-specific overexpression of Tbx20 led to neonatal death with severe craniofacial defects, such as orofacial clefts and exencephaly. Interestingly, aberrant chondrogenesis was observed in the posterior frontal (PF) suture, a structure derived from CNCCs, suggesting that augmented Tbx20 expression triggers ectopic cartilage formation in the PF suture. This study reveals that enhanced BMP-Tbx20 signaling in CNCCs causes aberrant chondrogenesis in the PF suture during craniofacial development.