Generation and characterization of Six2 conditional mice.

Heterozygous deletion of Six2, which encodes a member of sine oculis homeobox family transcription factors, has recently been associated with the frontonasal dysplasia syndrome FND4. Previous studies showed that Six2 is expressed in multiple tissues during craniofacial development in mice, including embryonic head mesoderm, postmigratory frontonasal neural crest cells, and epithelial and mesenchymal cells of the developing palate and nasal structures. Whereas Six2 (-/-) mice exhibited cranial base defects but did not recapitulate frontonasal phenotypes of FND4 patients, Six1 (-/-) Six2 (-/-) double mutant mice showed severe craniofacial defects including midline facial clefting. The complex phenotypes of FND4 patients and of Six1 (-/-) Six2 (-/-) mutant mice indicate that Six2 plays crucial roles in distinct cell types at multiple stages of craniofacial morphogenesis. Here we report generation of mice carrying insertions of a pair of loxP sites flanking exon-1 of the Six2 gene (Six2 (f) allele) using CRISPR/Cas9-mediated genome editing. We show that the Six2 (f) allele functions normally and is effectively inactivated by Cre-mediated recombination in vivo. Furthermore, we show that Six2 (f/f) ;Wnt1-Cre mice recapitulated cranial base defects but not neonatal lethality of Six2 (-/-) mice. These results indicate that Six2 (f/f) mice enable systematic investigation of cell type- and stage-specific Six2 function in development and disease.