Abstract
Cleft palate and orofacial clefts are common congenital anomalies whose etiology results from deficient fusion of the bilateral processes of the first pharyngeal arch and/or the frontonasal process. Craniofacial clefting can result from genetic and teratogenic disruptions affecting the growth and development of the first pharyngeal arch. Many of the molecular pathways regulating normal and abnormal growth and development of the first arch have been studied; however, the role of microRNA (miRNA) in mammalian palatal development has not been elucidated. miRNA are a class of short 19-21 nt noncoding RNA whose function is in the repression and attenuation of translation of their target mRNA. The function of specific miRNA is known in many developmental events and disease processes, and they are estimated to regulate up to one third of human genes. We hypothesized that miRNA are essential to mammalian orofacial development, and as such, a disruption of miRNA biogenesis will disrupt established orofacial and palatal developmental processes. To test this hypothesis a conditional Dicer knockout mouse model was used. In this model, floxed Dicer alleles were knocked out in the first pharyngeal arch precursor tissues of the palate. Embryos lacking Dicer in their palatal precursors experienced craniofacial developmental defects that included several missing viscerocranial and chondrocranial bones and a complete cleft of the secondary palate. The developmental defects of the conditional Dicer knockout were similar to defects seen as a result of aberrant cranial neural crest cell migration and differentiation. Sox9 is a molecular marker of cranial neural crest cells and participates in their maintenance and ability to respond to chemotactic signals during migration. Investigation of Sox9 expression, between E9.5 and E12.5, showed ectopic Sox9 expression in the Dicer knockout embryos between E9.5 and E11.5, in the rhombencephalon and midbrain; and decreased Sox9 expression in the maxillary process at E12.5, indicating that improper development or migration of cranial neural crest cells may be responsible for the viscerocranial malformations. These results demonstrate the importance of miRNA in mammalian palatogenesis and viscerocranial morphogenesis.