View Program - 63rd Annual Drosophila Research Conference

Meier-Gorlin syndrome (MGS) is a rare autosomal recessive disorder characterized by microtia, primordial dwarfism, small ears and skeletal abnormalities. Patients with MGS often carry mutations in the genes encoding the subunits of the Origin Recognition Complex (ORC), components of the pre-replicative complex (pre-RC) and replication machinery. Orc6 is an important component of ORC and has functions in both DNA replication and cytokinesis. A mutation in conserved C-terminal motif of Orc6, Y225 to S, is associated with MGS and impedes the interaction of Orc6 with the core ORC. Recently, a new mutation in Orc6, K23 to E, was identified however, it is localized in the N-terminal domain of the protein. In order to study the functions of new Orc6 mutation in live animal system we used human Orc6 gene or a hybrid Orc6-HD transgene (containing intact human N-terminal TFIIB like domain and Drosophila C-terminus) to rescue the orc6 deletion in Drosophila. The obtained flies survive to the adult stage and allow studies of MGS mutations in humanized animal model system. Using this approach we discovered that unlike previously identified Y225S MGS mutation in Orc6, the K23 to E substitution in the N-terminal TFIIB-like domain does not disrupt the interaction between Orc6 and the rest of the ORC. However, K23E MGS mutation results in a reduced DNA binding ability of the Orc6 protein. The flies carrying MGS mutations are unable to fly and display growth and development defects. Overall, despite having different underlying molecular mechanisms both MGS mutations resulted in similar phenotypes, deficient pre-RC formation and reduced DNA replication.
Our studies revealed the importance of evolutionarily conserved and variable domains of Orc6 protein and allowed the studies of human protein functions and the analysis of the critical amino acids in live animal heterologous system as well as provided novel insights into the mechanisms underlying MGS pathology. We believe that hybrid approach not only open a broad avenue to study new Orc6 mutations for medical and general science purposes but might be useful in other humanized models.

Humanized Drosophila model of the Meier-Gorlin syndrome.

**Humanized Drosophila model of the Meier-Gorlin syndrome.**