FUNCTIONAL MODULATION OF BASE EXCISION REPAIR (BER) BY NON-CANONICAL DNA REPAIR ENZYMES: THE CASE OF NUCLEOPHOSMIN

The base excision DNA repair pathway (BER) is an essential cellular process that deals with small lesions originating from oxidation and alkylation of DNA, which occurs spontaneously, as a result of the intracellular metabolism, or after exposure to genotoxins (e.g. anti-tumor drugs). Unfortunately, dysregulation of BER enzymes is observed in some tumors and can foster cancer cells survival upon DNA damage caused by chemo- and radio-therapy. Recent findings highlighted an unexpected complexity of BER. Apparently unrelated protein components are now listed as novel modulators of the pathway; in addition, the individuation of BER factors within nucleoli opens questions on potential roles for BER proteins beyond DNA repair. The interaction between the apurinic/apyrimidinic endonuclease 1 (APE1) and nucleophosmin (NPM1) is a perfect example of the BER complexity. As the main AP-endonuclease in mammalians, APE1 is central to BER; yet, through the association with NPM1, APE1 accumulates within nucleoli in tumor cells. Here it possibly takes part to RNA processing, a function that is somehow unrelated to the canonical APE1 role in BER. Furthermore, the APE1/NPM1 interaction modulates both the BER and the nucleolar roles of APE1, being essential for the proliferation of cancer cells. On the other hand, despite the growing body of evidence linking NPM1 and the DNA damage response, it is not clear whether NPM1 could play a direct role in the modulation of the overall BER. My PhD project was aimed at the thorough characterization of the APE1/NPM1 association and of the general role of NPM1 in BER. A first part of this thesis will describe the development of small molecules able to disrupt the APE1/NPM1 interaction in tumor cells as novel tools for investigation and for translational purposes. The second part of this work will focus on the characterization of NPM1 as modulator of the BER. This thesis work led to the individuation of low molecular weight compounds targeting the APE1/NPM1 interaction and displaying anti-tumor properties. In addition, I demonstrate here for the first time the implication of NPM1 as multi-level modulator of the BER pathway, providing new insights into the role of BER proteins within nucleoli.