Dissecting the role of Six1 and its co-factors during calvarial bone and suture development

ABSTRACT Variants in the transcription factor SIX1 or its co-factor EYA1 are known underlying genetic causes of Branchio-oto-renal syndrome (BOR), an autosomal dominant disease that results in hearing loss and kidney defects. Recently, a clinical study reported craniosynostosis (CS) in individuals carrying SIX1 BOR variants, including 5’ variants (p.Q11X and p.Q22X) that are predicted to lead to haploinsufficiency; these data suggest that CS may be an undiagnosed defect in BOR. If left untreated, CS can be associated with distortion of skull shape, increased intracranial pressure, and/or brain damage. As defects in the calvarial bone osteoprogenitor cells (OPC) before and/or after birth may lead to CS via increased bone deposition in the cranial sutures, in this application, I plan to address a major knowledge gap regarding Six1 function: What is its role in the development of the calvarial bones? To detect changes in bone development caused by Six1 loss and haploinsufficiency, I will quantitatively analyze head morphology using µCT images and tissue formation using histological analyses (Aim 1). To verify if Six1 and its co-factors have a role the specification and differentiation of OPCs, I will assess gene expression in vivo using RNAscope and qPCR (Aim 1) and in vitro using neural crest-derived mesenchymal precursors and OPCs (Aim 2). Lastly, I will perform single cell RNA-seq and RNAscope to identify cell populations in the supraorbital arch mesenchyme (that gives rise to the rudiments for parietal and frontal bones) that are affected by Six1 loss and haploinsufficiency (Aim 3). Results from this application will shift the paradigm of Six1 function as a cranial placode, neural and muscle transcriptional factor by providing the first direct evidence linking it to normal calvarial development and the pathogenesis of CS. This application will establish Six1-het mice as a new model for craniofacial disease, and will help elucidate the mechanisms by which Six1 variants lead to CS. It will also provide training in soft skills, funding and collaborations required for my next career step. Finally, this training will allow me to bring my extensive knowledge of Six1 transcriptional function in mandible and otic development to a new area of clinically relevant research. Consequently, my research may ultimately prove crucial for CS patient diagnosis and care.