The regenerating fin as a model to examine the skeletal defects of Roberts Syndrome

Abstract Roberts Syndrome (RBS) is a human developmental disorder characterized by craniofacial abnormalities, limb malformation and often severe mental retardation. RBS arises from mutations in the cohesin auxiliary factor ESCO2 that targets the SMC3 subunit of the cohesin complex. Mutations in cohesin subunits and a subset of cohesin auxiliary factors gives rise to a related developmental malady termed Cornelia de Lange Syndrome (CdLS). They are collectively termed cohesinopathies since both disorders comprise overlapping phenotypes and the causative genes for both syndromes perform common activities. The underlying cause of CdLS is largely modeled as occurring through transcriptional deregulation. Whereas, the mechanism that underlies RBS, remains unknown. A popular model states that RBS arises due to mitotic failure that leads to elevated levels of apoptosis. My thesis is based on developing a new vertebrate model system for examining RBS-like skeletal defects in order to better understand the mechanisms underlying the disease. Using the zebrafish regenerating fin model, I discovered a transcriptional role of esco2 during fin regeneration. First, my results show that Esco2 contributes to skeletal growth and patterning independent of elevated levels of apoptosis – negating a model for mitotic failure. Second, I provide the first characterization of Esco2-dependent gene expression of a skeletal gene, cx43, which encodes the gap junction connexin subunit required for cell-cell communication and skeletal development in all vertebrates. Mutations in human CX43 gives rise to a skeletal disorder, Oculodentodigital dysplasia (ODDD). My results conceptually link ODDD to cohesinopathies and provide evidence that ESCO2 may play a transcriptional role critical for human development. I next addressed the mechanism through which Esco2 regulates cx43 expression, focusing on the Esco2 target Smc3. My results show that the Smc3 knockdown perturbs bone and tissue growth that recapitulates RBS-type phenotypes in the regenerating fin model. Importantly, Smc3 regulates cx43 expression, similar to that of Esco2. Moreover, Chromatin Immunoprecipitation (ChIP) assays revealed that Smc3 binds to a discrete region of the cx43 promoter, suggesting that Esco2 exerts transcriptional regulation of cx43 through modification of Smc3 bound to the cx43 promoter. These findings unify RBS and CdLS as transcription-based mechanisms.