Direct and indirect regulation of SIX1+EYA transcriptional activity by PA2G4, MCRS1, and SOBP

Branchio-oto-renal (BOR) syndrome is an autosomal dominant condition characterized by variable malformations including hearing loss and renal dysfunction. Variants in SIX1 or its activating co-factor EYA1 are causative in about 50% of patients. Some patients carrying BOR variants also present with craniosynostosis, indicating that cranial skeletal dysmorphologies could be an under-diagnosed feature. To date, most studies on the role of SIX1 have focused on its role in the cranial placode-based development of the inner ear, whereas its role in the neural crest cells of the mandibular arch, which will give rise to the jaws and middle ear ossicles, is less well characterized. Here, we present novel expression profiles of three putative SIX1 co-factors (PA2G4, MCRS1, and SOBP) in the developing mouse first pharyngeal arch and tooth. SIX1 colocalizes with PA2G4, MCRS1, and SOBP within the oral domain of the mandibular arch and during odontogenesis, although each exhibits a distinct expression pattern. Functional analyses revealed that SOBP binds SIX1, EYA1, and EYA2 and represses both SIX1 + EYA1 and SIX1 + EYA2 transcriptional activity, whereas MCRS1 binds only SIX1 and selectively represses SIX1 + EYA2 activity. In contrast, PA2G4 does not bind SIX1, yet modulates SIX1 + EYA2 activity. We further show that SIX1 is required for proper expression of Pa2g4, Mcrs1, and Sobp in the mouse mandibular arch. Collectively, these results demonstrate that regulation of SIX1 + EYA transcriptional activity is highly context dependent, occurs through both direct and indirect mechanisms, and differs between SIX1 + EYA1 and SIX1 + EYA2 complexes. These findings reveal species-specific differences and uncover a level of regulatory complexity not previously identified in Xenopus studies.