Molecular mechanisms of cleft lip formation in CL/Fr mice

CL/Fr mice have a high incidence of cleft lip and the cleft lip is the result of incomplete fusion between the medial and lateral nasal prominences and the maxillary prominence at about day 11.5 of gestation. However, little is known about the molecular mechanisms that are responsible for the incomplete fusion. We made a molecular pathological investigation using 11.5-day CL/Fr embryos. Five embryos were each examined for real-time polymerase chain reaction (PCR) analysis. During the first palatal formation in normal development, an epithelial seam is formed when the medial and lateral nasal prominences first make contact. Some epithelial cells of the epithelial seam then undergo apoptosis, with remaining cells transforming into a mesenchymal phenotype (epithelial-mesenchymal transition, EMT). Mesenchymal cells of the medial and lateral nasal prominences then merge across the previous boundary of separation. In CL/Fr mice with cleft lip, neither apoptosis nor EMT occurs in the epithelial cells. Increased expression of claudin 6 mRNA is seen in epithelial cells of epithelial seam in cleft lip compared with that in normal embryos. Slug mRNA expression was also significantly reduced whereas noggin was increased in CL/Fr embryos with cleft lip. We suggest that EMT is prevented in CL/Fr mice with cleft lip by increased expression of claudin 6 and coexistent downregulation of slug in cells of the epithelial seam, and these altered concentrations of transcription factors/repressors prevent fusion of the medial and lateral nasal prominences, leading to clefts of the lip.