A Meier-Gorlin Syndrome Mutation in Orc4 CausesTissue-Specific DNA Replication Defects in Drosophila melanogaster

Meier-Gorlin syndrome is a rare recessive disorder characterized by a number of distinct developmental defects, including primordial dwarfism, small ears, and small or missing patella. Genes encoding members of the origin recognition complex (ORC) and additional proteins essential for DNA replication (CDC6, CDT1, GMNN, CDC45, and MCM5) are mutated in individuals diagnosed with MGS. The primary role of ORC is to license origins during the G1 phase of the cell cycle, but it also plays roles in cilia development, heterochromatin formation, and other cellular processes. Because of its essential role in DNA replication, ORC is required for every cell division during development. Thus, it is unclear how the Meier-Gorlin syndrome mutations in ORC lead to the tissue-specific defects associated with the disease. To address this question, we have used Cas9-mediated genome engineering to generate a Drosophila melanogaster model of individuals carrying a mutation in ORC4. Like the people with Meier-Gorlin syndrome, these flies reach adulthood, but have several tissue-specific defects. Genetic analysis revealed that this allele is a hypomorph and that mutant females are sterile. We demonstrated that this sterility is caused by a failure in DNA replication. By leveraging the well-studied Drosophila system, we showed that a disease-causing mutation in orc4 disrupts DNA replication, and we propose that in individuals with MGS defects arise preferentially in tissues with a high-replication demand.