ATR signalling after DNA damage : from the discovery of a new target to the reevaluation of its function in primary chronic lymphocytic leukaemia cells

Deoxycytidine kinase (dCK) catalyses the first and rate limiting step of the deoxynucleoside salvage pathway that supplies cells with dNTPs for DNA synthesis. dCK is also required for the activation of several antiviral and anticancer nucleoside analogues and plays a crucial role in their therapeutic efficacy. Given these important functions, deciphering the mechanisms that control dCK activity is of major interest for both fundamental scientists and clinician researchers. Our group demonstrated that dCK is a phosphoprotein whose activity is increased by phosphorylation of the Ser-74 residue. Moreover, it was established that increase in dCK activity in response to several genotoxic treatments, including ionizing radiation (IR), UV-C, chemotherapeutic nucleoside analogues and DNA polymerase inhibitors, was related to an increase in Ser-74 phosphorylation. These observations suggested that Ser-74 could be phosphorylated by a DNA damage-activated protein kinase, possibly ATM, ATR or DNA-PK. Two different groups identified ATM, a transducer of the DNA damage response (DDR), as the protein kinase responsible for Ser-74 phosphorylation and dCK activation following ionizing radiation (IR), a DNA double-strand break (DSB) inducer. The first objective of this thesis consisted in investigating whether ATM was also involved in dCK activation in response to other types of DNA damage, such as induced by UV-C light that causes replication stress and induces single-stranded DNA (ssDNA). Using ATM-deficient lymphoblastoid cell lines, a selective ATM inhibitor or ATM siRNA, we demonstrated that ATM was not involved in dCK activation induced by UV-C, aphidicolin or cladribine. Surprisingly, we also found that ATM was not essential for dCK activation following IR, contrary to what was stated in the literature. On the other hand, a selective inhibitor of ATR, the kinase that is activated in response to ssDNA, as well as ATR siRNA prevented dCK activation by these agents, though not IR, and also reduced basal dCK activity. Further analyses, including time-course experiments, revealed that ATR, which is also activated after IR at later time points, was responsible for IR-induced dCK activation in cells in which ATM was deficient or inhibited. We concluded that ATR controls basal dCK activity and dCK activation in response to agents that induce ssDNA and that ATM activates dCK in response to agents that induce DSBs. However, as the repair of DSBs includes a resection step that induces the formation of ssDNA and thus activates ATR, ATR can activate dCK independent of ATM in IR-treated ATM-deficient or -inhibited cells.