A number of DNA repair pathways exist that repair different types of DNA lesion. The molecular basis of these pathways is becoming increasingly well defined. A remaining challenge is to understand how these pathways integrate to maintain genome integrity and cell viability should one pathway fail. Understanding these relationships has important clinical implications. Given that DNA repair pathways are often inactivated in malignant cells, characterising compensatory pathways that maintain cell viability in these cells will identify novel synthetic lethal therapeutic strategies that specifically target tumours. The best established example of this strategy is toxicity of BRCA2 defective breast and ovarian cancer cells to poly (ADPribose) polymerase (PARP) inhibitors (PARPi). This occurs through the inability of brca2-null cells to perform homologous recombination at DNA double strand breaks (DSBs) generated as a consequence of replication forks encountering unrepaired single strand breaks during S-phase. PARPi are currently in clinical trials to treat BRCA-defective breast and ovarian cancers.
However, these agents inhibit several PARPs that regulate different DNA repair pathways. More precisely defining the PARPs that display synthetic lethality with HR will facilitate more refined treatments when PARP-specific inhibitors become available. Further, identifying other repair pathways that exhibit synthetic lethality with PARPi will widen the use of these agents to tumours other than those defective in BRCA2. Finally, identifying novel synthetic lethal relationships between other repair pathways will also broaden the use of this strategy to other categories of tumour. One way to address these questions is to generate transgenic mice which are genetically modified to be deficient in combinations of different DNA repair pathways. However, these experiments require large numbers of animals, particularly if conditional knockout strategies are required. Mice deficient in DNA repair pathways are prone to cancer and often have other phenotypes such as neurological problems, developmental abnormalities and immunodeficiency. Generating mice deficient in multiple pathways compounds these problems and increases the severity of procedures. Although invertebrate model organisms can replace animals, these approaches are often prohibited by the lack of certain human DNA repair components, including PARPs, in the most commonly used models to study DNA repair.
Recently, we identified that the mechanisms of DNA repair in the eukaryotic model organism Dictyostelium are more similar to humans than other genetically tractable invertebrates, including synthetic lethality between PARPs and HR. The overall goal of this research is to exploit Dictyostelium as a non-sentient model that replaces murine models to characterise novel synthetic lethal relationships between DNA repair pathways. Specifically we aim to determine whether PARPi display synthetic lethality with repair pathways other than HR. We will also determine which specific PARP(s) is responsible for this interaction to facilitate the use of inhibitors specific for different members of the PARP family. Finally, we will extend this analysis to identify other combinations of repair pathways whose combined loss leads to lethality and understanding the molecular basis behind this event. Therefore, Dictyostelium will prove an effective replacement model to assess synthetic lethality between different DNA repair pathways. In the long term we believe these studies will make Dictyostelium the model organism of choice for such analysis with the potential to replace the use of mice in other studies
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Kolb AL et al. (2017). Redundancy between nucleases required for homologous recombination promotes PARP inhibitor resistance in the eukaryotic model organism Dictyostelium. Nucleic Acids Res. 45(17):10056–10067. doi: 10.1093/nar/gkx639
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