Tissue-Specific DNA Replication Defects in Drosophila melanogaster Caused by a Meier-Gorlin Syndrome Mutation in Orc4

Role of BicDR in bristle shaft construction, tracheal development, and support of BicD functions

Cell polarization requires asymmetric localization of numerous mRNAs, proteins, and organelles. The movement of cargo towards the minus end of microtubules mostly depends on cytoplasmic dynein motors, which function as multiprotein complexes. In the dynein/dynactin/Bicaudal-D (DDB) transport machinery, Bicaudal-D (BicD) links the cargo to the motor. Here we focus on the role of BicD-related (BicDR) and its contribution to microtubule-dependent transport processes. Drosophila BicDR is required for the normal development of bristles and dorsal trunk tracheae. Together with BicD, it contributes to the organization and stability of the actin cytoskeleton in the not-yet-chitinized bristle shaft and the localization of Spn-F and Rab6 at the distal tip. We show that BicDR supports the function of BicD in bristle development and our results suggest that BicDR transports cargo more locally whereas BicD is more responsible for delivering functional cargo over the long distance to the distal tip. We identified the proteins that interact with BicDR and appear to be BicDR cargo in embryonic tissues. For one of them, EF1γ, we showed that EF1γ genetically interacts with BicD and BicDR in the construction of the bristles.

The expanding genetic and clinical landscape associated with Meier-Gorlin syndrome

High-throughput sequencing has become a standard first-tier approach for both diagnostics and research-based genetic testing. Consequently, this hypothesis-free testing manner has revealed the true breadth of clinical features for many established genetic disorders, including Meier-Gorlin syndrome (MGORS). Previously known as ear-patella short stature syndrome, MGORS is characterized by growth delay, microtia, and patella hypo/aplasia, as well as genital abnormalities, and breast agenesis in females. Following the initial identification of genetic causes in 2011, a total of 13 genes have been identified to date associated with MGORS. In this review, we summarise the genetic and clinical findings of each gene associated with MGORS and highlight molecular insights that have been made through studying patient variants. We note interesting observations arising across this group of genes as the number of patients has increased, such as the unusually high number of synonymous variants affecting splicing in CDC45 and a subgroup of genes that also cause craniosynostosis. We focus on the complicated molecular genetics for DONSON, where we examine potential genotype-phenotype patterns using the first 3D structural model of DONSON. The canonical role of all proteins associated with MGORS are involved in different stages of DNA replication and in addition to summarising how patient variants impact on this process, we discuss the potential contribution of non-canonical roles of these proteins to the pathophysiology of MGORS.

Humanized Drosophila Model of the Meier-Gorlin Syndrome Reveals Conserved and Divergent Features of the Orc6 Protein

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 genes encoding the subunits of the Origin Recognition Complex (ORC), components of the prereplicative complex and replication machinery. Orc6 is an important component of ORC and has functions in both DNA replication and cytokinesis. A mutation in the conserved C-terminal motif of Orc6 associated with MGS impedes the interaction of Orc6 with core ORC. Recently, a new mutation in Orc6 was also identified; however, it is localized in the N-terminal domain of the protein. To study the functions of Orc6, we used the human gene to rescue the orc6 deletion in Drosophila Using this "humanized" Orc6-based Drosophila model of MGS, we discovered that unlike the previous Y225S MGS mutation in Orc6, the K23E substitution in the N-terminal TFIIB-like domain of Orc6 disrupts the protein ability to bind DNA. 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.