Generation of a new six1-null line in Xenopus tropicalis for study of development and congenital disease.

The vertebrate Six (Sine oculis homeobox) family of homeodomain transcription factors plays critical roles in the development of several organs. Six1 plays a central role in cranial placode development, including the precursor tissues of the inner ear, as well as other cranial sensory organs and the kidney. In humans, mutations in SIX1 underlie some cases of Branchio-oto-renal (BOR) syndrome, which is characterized by moderate-to-severe hearing loss. We utilized CRISPR/Cas9 technology to establish a six1 mutant line in Xenopus tropicalis that is available to the research community. We demonstrate that at larval stages, the six1-null animals show severe disruptions in gene expression of putative Six1 target genes in the otic vesicle, cranial ganglia, branchial arch, and neural tube. At tadpole stages, six1-null animals display dysmorphic Meckel's, ceratohyal, and otic capsule cartilage morphology. This mutant line will be of value for the study of the development of several organs as well as congenital syndromes that involve these tissues.