Modelling ADP-ribosyltransferases as therapeutic targets in cancer therapy

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The genome contains a blueprint to produce all the machinery required for cells to function. As such, it is important to maintain the integrity of its constituent DNA. Unfortunately cells are exposed to agents that damage DNA. For example, ionizing radiation (IR) coverts water into reactive oxygen species (ROS) that introduces breaks into the DNA, or alters the bases that encode for proteins. Therefore, cells have evolved multiple mechanisms to repair different types of DNA damage, and these pathways are increasingly well defined. However, a remaining challenge is to understand how these pathways integrate to allow cells to survive IR and other mutagenic agents when one repair pathways fails. Understanding this redundancy has important implications for health and wellbeing. For example, DNA repair pathways are often inactivated in cancer cells and the relative sensitivity of these cells to IR and other DNA damaging agents is exploited in radio/chemotherapy. Defining compensatory pathways that allow cancer cells to tolerate these agents will facilitate the development of drugs that inhibit these pathways and further sensitize cancers to radio/chemotherapy. Importantly, ROS are produced naturally in cells. Therefore, this knowledge will also provide insights into how cancer cells can be sensitized to this endogenous DNA damage, circumventing the requirement for radio/chemotherapy and thus eliminating the inherent toxicity and unwanted side effects of these treatments.An example of such a strategy is inhibition of proteins known as ADP-ribosyltransferases (ARTs) that sense DNA breaks and chemically modify proteins at the DNA damage sites to promote repair. ART inhibitors (ARTi) are currently being developed in the clinic to treat breast and ovarian cancers with defects in the ability to repair DNA breaks by homologous recombination (HR). Importantly, treatment with ARTi is most promising in combination with IR or other mutagens. However, many questions remain regarding how and why ARTi kill HR-defective cancers that, if resolved, will improve the efficacy of these agents. For example, cells contain multiple ARTs that respond to different types of DNA damage and we have identified a significant degree of redundancy between ARTs. Understanding these relationships will facilitate the development of more specific ARTi that refine treatment strategies. Further, whilst ARTi kill certain tumour cells, it is important to establish how they impact on DNA integrity in non-cancer cells, especially in combination with IR. Finally, whilst certain cancers are highly sensitive to ARTi, they rapidly adapt to treatment with these agents. Identifying genes that, when de-regulated, render cancer cells refractory to ARTi treatment will provide potential targets that when inhibited will overcome this resistance.DNA repair pathways function in a similar manner in a wide variety of organisms. Therefore, a powerful approach to address these questions is to exploit the ease of experimentation in relatively simple model organisms and extend the findings to humans. Unfortunately, this approach is hampered by the lack of certain DNA repair proteins in the most commonly used model organisms to study DNA repair. Recently, however, we initiated a study of DNA repair in the microorganism Dictyostelium and have established that it contains a number of DNA repair proteins, including ARTs, absent in other model organisms. Therefore, Dictyostelium will prove an important model to investigate selected DNA repair pathways and redundancy. The overall objectives of this proposal are to exploit the advantages of Dictyostelium and human cells to address the following:i) How do multiple ARTs regulate compensatory repair mechanisms following DNA damage?ii) What is the impact of long term ART inhibition on genome stability in the presence or absence of agents used in radiotherapy?iii) How do HR-defective cells become refractory to ARTi?