Characterization of Fly POLR1D Mutations in a Clinically Relevant Residue Associated with Treacher Collins Syndrome (TCS)

Abstract

Treacher Collins Syndrome (TCS) is an extreme craniofacial disorder caused by mutations in genes important for RNA polymerase I (Pol I) transcription. Pol I is a multi‐subunit complex that transcribes ribosomal DNA (rDNA) to synthesize ribosomal RNA (rRNA). Two Pol I subunits often mutated in TCS are POLR1D and POLR1C which form a heterodimer that is important for Pol I complex assembly. Using fruit flies (D. melanogaster) as a model system, we mapped a fly POLR1D mutation that displays a severe developmental arrest phenotype in a clinically relevant residue where in a TCS patient this residue is mutated from a glycine residue to a bulky, negatively charged glutamic acid residue. In the fly mutant collection, this glycine residue is mutated to a bulkier but positively charged arginine residue. We used a simple in vitro co‐expression system to examine the biochemical impact of these mutations on fly and human POLR1D and POLR1C heterodimer formation in vitro. We also complement our in vitro work with a chimeric yeast (S. cerevisiae) model in vivo where together we found that in the same mutated residue, there is a defect that impacts heterodimer formation and Pol I and III complex integrity. Our results show that fly POLR1D is a suitable model system for human TCS mutants.