Loss of Irx1 function in mice leads to neonatal lethality and teeth defects

Objectives: Dental defects, such as tooth agenesis and enamel hypoplasia, are among the most common developmental anomalies in human. Initiation, morphogenesis, and differentiation are the three fundamental processes for tooth development, which result in sequential cellular processes such as proliferation, differentiation, migration, and secretion, leading to the formation of a functional tooth. Although the process and mechanism of dental development is well established, it remains elusive that how some transcription factors control the cell differentiation and migration. Iroquois homeobox 1 (Irx1), as a novel DNA homeobox binding transcription factor, is highly expressed during tooth development with unknown function.
Methods: X-gal staining, H & E staining, immunostaining, gel shift assay, chIP assay
Results: Irx1 is only expressed in outer enamel epithelium (OEE) and Stratum intermedium (SI) layer of tooth, which makes it an ideal candidate to study the function of OEE and SI during amelogenesis. Knocking out Irx1 in mice leads to neonatal lethality and pulmonary immaturity. Irx1-deficient mice show delayed lung maturation characterized by defective surfactant proteins secretion and shorter lower incisors in E18.5 embryos. The teeth in Irx1-/- embryos are shorter too. CIdU and IdU incorporation assays showed that cell migration is down-regulated in Irx1^-/- lower incisor. Gel shift assay and chromatin immunoprecipitation assay showed that Pitx2 binds to the promoter region of Irx1 and regulates the transcriptional activity of Irx1. RNA-seq analysis on the craniofacial tissues at E14.5 from wild type and Irx1-/- embryos shows that about 300 genes are down-regulated and about 70 genes are upregulated, suggesting that Irx1 dominantly acts as a transcription activator during tooth development.
Conclusions: Irx1 regulates cell differentiation and migration during tooth development. Pitx2 acts as a upstream regulator for Irx1, and such regulation is disrupted by the presence of Sox2.