Sobp modulates Six1 transcriptional activation and is required during craniofacial development

Branchio-oto-renal syndrome (BOR) is a disorder characterized by hearing loss, craniofacial and/or renal defects. Mutations in the transcription factor Six1 and its cofactor Eya1, both required for otic development, are linked to BOR. We previously identified Sobp as a potential Six1 cofactor and SOBP mutations in mouse and humans cause otic phenotypes; therefore, we asked whether Sobp interacts with Six1 and thereby may contribute to BOR. Co-IP and immunofluorescence experiments demonstrate that Sobp binds to and co-localizes with Six1 in the cell nucleus. Luciferase assays show that Sobp represses Six1+Eya1 transcriptional activation. Experiments in Xenopus embryos that either knockdown or increase expression show that Sobp is required for formation of ectodermal domains at neural plate stages. In addition, altering Sobp levels disrupts otic vesicle development and causes craniofacial cartilage defects. Expression of Xenopus Sobp containing the human mutation disrupts the pre-placodal ectoderm similar to full-length Sobp, but other changes are distinct. These results indicate that Sobp modifies Six1 function, is required for vertebrate craniofacial development, and identifies Sobp as a potential candidate gene for BOR and other deafness syndromes.