Dynamic regulation and requirement for ribosomal RNA transcription during mammalian development

Abstract

Ribosomal RNA (rRNA) transcription by RNA Polymerase I (Pol I) is a critical rate-limiting step in ribosome biogenesis, which is essential for cell survival. Despite its global function, disruptions in ribosome biogenesis cause tissue-specific birth defects called ribosomopathies which frequently affect craniofacial development. Here, we present a cellular and molecular mechanism to explain the susceptibility of craniofacial development to disruptions in Pol I transcription. We show that Pol I subunits are highly expressed in the neuroepithelium and neural crest cells (NCC), which generate most of the craniofacial skeleton. High expression of Pol I subunits sustains elevated rRNA transcription in NCC progenitors, which supports their high tissue-specific levels of protein translation, but also makes NCC particulalry sensitive to rRNA synthesis defects. Underpinning these findings, NCC-specific deletion of Pol I subunits Polr1a, Polr1c, and associated factor Tcof1 in mice cell-autonomously diminishes rRNA synthesis, which causes an imbalance between rRNA and ribosomal proteins. This leads to increased ribosomal protein binding to Mdm2 and concomitantly diminished Mdm2 binding to p53. Consequently, p53 protein accumulates, resulting in NCC apoptosis and craniofacial anomalies. Furthermore, compound mutations in Pol I subunits and associated factors specifically exacerbates the craniofacial anomalies characteristic of the ribosomopathies Treacher Collins Syndrome and Acrofacial Dysostosis Cincinnati Type. Our novel results therefore demonstrate the dynamic spatiotemporal requirement for rRNA transcription during mammalian cranial NCC development and corresponding tissue-specific threshold sensitivities to disruptions in rRNA transcription in the pathogenesis of craniofacial congenital diseases.

“Beyond transcription: How post‐transcriptional mechanisms drive neural crestEMT”

“…Once transcribed, mRNAs are processed, shuttled, translated, and eventually degraded, with each of these stages in the mRNA's life tightly regulated by cellular trans-acting factors that include RNAbinding proteins and small RNAs (Moore, 2005). The ultimate function of the mRNA-to serve as a template for protein synthesis-can be modulated by the longevity of the transcript, which cells are able to exploit to quickly influence gene expression (Ross, 1995) Chick, Mouse, Zebrafish, Human cell lines (Falcon et al, 2022;Prakash et al, 2019;Noack Watt et al, 2016;Valdez et al, 2004) through a variety of RNA-binding protein and small non-coding RNA (e.g., microRNA)-mediated mechanisms.…”

“…A major rate-limiting step in ribosome biogenesis is rRNA transcription via RNA Polymerase I (Chaillou et al, 2014;Moss & Stefanovsky, 1995). Interestingly, murine and avian delaminating and migrating neural crest cells have been shown in separate studies to exhibit increased rRNA transcription compared to non-migrating cells, with a correlation to increased protein synthesis (Falcon et al, 2022;Prakash et al, 2019). Further, known ribosomopathies (e.g., Treacher…”

“…encodes for Treacle, a transcriptional activator of rRNA acting via direct binding to Upstream Binding Factor (UBF) (Valdez et al, 2004), while the latter two are subunits of RNA polymerases I and III (Noack Watt et al, 2016). Neural crest-specific knockouts of the Treacher Collins syndrome-associated genes have specifically been shown to disrupt both rRNA transcription and total protein synthesis during neural crest migration, resulting in tumor suppressor protein p53 stabilization and cell death (Falcon et al, 2022). Consistent with an inhibitory role during EMT, expression of p53 is negatively correlated with cranial neural crest EMT in the chick and represses Snai2 (Rinon et al, 2011).…”

Bop1 is required to establish precursor domains of craniofacial tissues

“…It is well established that when mutated, factors that are involved in ribosome biogenesis can lead to significant craniofacial dysmorphologies in experimental models and human patients (Falcon et al, 2022;Griffin et al, 2015;Schreiner et al, 2022;Zhou et al, 2014). However, there is evidence that some of the proteins known to be necessary for ribosomal synthesis also have other cellular functions, particularly during early development (Bugner et al, 2011;Gessert et al, 2007;Tecza et al, 2011).…”