The role of rRNA transcription and ribosome biogenesis in neural crest progenitors and stem cells during embryonic and postnatal craniofacial development

ABSTRACT Craniofacial anomalies are the most common malformations present at birth and comprise up to 1/3 of all congenital defects. Most of the craniofacial cartilage, bone and connective tissue are derived from neural crest cells (NCC), a migratory stem and progenitor cell population born during early embryogenesis. The majority of craniofacial disorders are therefore thought to be a result of disruptions in NCC development3. In order to improve the prognosis for individuals affected with craniofacial anomalies, and to develop potential preventative therapies, there is a critical need for a deeper understanding of the fundamental mechanisms that govern the formation, migration and differentiation of NCCs during craniofacial morphogenesis4. Recent work in our lab demonstrated that RNA Polymerase I (Pol I)-mediated ribosomal RNA (rRNA) transcription, which is the rate limiting step in ribosome biogenesis, is elevated in NCCs compared to other cell types5, 6. This is necessary to meet NCC’s higher protein translational requirements and underpins their high rate of proliferation, epithelial to mesenchymal transformation, and metabolically expensive migration properties5. In support of this idea, NCC- specific disruption of Pol I function leads to increased NCC death and craniofacial anomalies characteristic of congenital craniofacial disorders such as Treacher Collins syndrome and Acrofacial Dysostosis-Cincinnati type5, 7, 8. Additionally, evidence from other studies supports elevated requirements for ribosomes and translational capacity in order for stem cells to acquire new fates, suggesting that ribosome biogenesis also plays a critical role during the differentiation stage of NCC development. Based on our previous findings I hypothesize that increased rRNA transcription and ribosome biogenesis are essential for both NCC-derived progenitor and stem cell maintenance and differentiation during embryonic and also juvenile phases of craniofacial development. To test this hypothesis, I will genetically delete Polr1a, the catalytic core of Pol I, in tamoxifen-inducible global and tissue-specific mouse lines. Then through phenotypic and cellular analyses, in combination with transcriptomic and proteomic approaches, I will define the mechanisms driving the anatomical anomalies that result from disruptions in Pol I function. My results will provide new knowledge on the roles of rRNA transcription and ribosome biogenesis in craniofacial progenitor and stem cell populations during embryonic and postnatal development. The findings can potentially uncover new therapies for preventing or ameliorating ribosomopathies, and improve clinical outcomes for craniofacial patients, especially when it comes to corrective surgeries, through improved postnatal growth predictions.