View Program - 63rd Annual Drosophila Research Conference

rRNA expression and ribosome assembly are becoming appreciated as processes required for proper development. Defects in rRNA synthesis/processing, ribosomal protein expression, and ribosome assembly, all constitute a class of diseases referred to as ribosomopathies. Ribosomopathies are characterized by a tissue-specific requirement for sufficient translational output not being satisfied due to mutations in genes ultimately involved in translation. Several animal models of ribosomopathies have been established to study the molecular and physiological consequences of those mutations. Drosophila is emerging as an excellent model system to study ribosomopathies in various developmental contexts. Here, we establish Drosophila as a model to study the ribosomopathy Treacher Collins Syndrome (TCS). TCS is caused by mutations in several genes, one of which is POLR1D. POLR1D protein forms a heterodimer with POLR1C, which is a prerequisite to the assembly of RNA Pol I and Pol III, which synthesize 47S and 5S rRNAs, respectively. Loss of POLR1D function is thought to affect polymerase assembly, reduce the production of rRNAs, and impair ribosome assembly, which ultimately affects translation. We found that flies hemizygous for an allele of Drosophila POLR1D (dPOLR1D) are pre-adult lethal, and viable larvae exhibit reduced growth, and are arrested in the L1 and L2 instar stages. dPOLR1D mutant larvae have reduced levels of 18S and 28S rRNAs, and this is reflected in heterozygous dPOLR1D adults, which exhibit the bobbed phenotype characteristic of mutants lacking ample rRNA production. Thus, dPOLR1D is required for rRNA expression, which is necessary to complete larval development. We sequenced this allele of dPOLR1D, and identified a missense mutation causing a substitution in an evolutionarily conserved amino acid that has been found to be mutated to a different residue in a single case of non-familial TCS. We found that the “fly mutation” in both dPOLR1D and human POLR1D (hPOLR1D) does not affect binding of POLR1D proteins to their cognate POLR1C proteins; however, the “human mutation” does. This suggests that while different amino acid mutations can both affect development, the molecular mechanism by which this occurs can be different. TCS is a disease of neural crest cells. We performed a preliminary RNAi screen in several cells types of the nervous system, and found that as in humans, dPOLR1D is required in the nervous system to complete development.

The Drosophila ortholog of POLR1D, an RNA Polymerase I & III assembly protein, is required for development