HEXO1 AND HRNASEH2: TWO NUCLEASES REQUIRED FOR MAINTENACE OF GENOME INTEGRITY

Genome integrity is continuously jeopardized by environmental insults, such as radiations, UV light and chemicals, as well as by endogenous factors including byproducts of cellular metabolism such as reactive oxygen species. In addition, various DNA transactions necessary for cell survival and proliferation (replication, recombination and transcription) are potential sources for genome instability. To cope with this situation, cells have evolved mechanisms acting to maintain genome integrity whose failure leads to genome instability, which is the main cause of cancer predisposition and genetic disorders. Nucleases acting in various DNA repair pathways play important roles both in removing altered or mismatched nucleotides and in processing recombination or replication intermediates, thus facilitating subsequent repair steps. In this thesis we studied the roles of two important nucleases (hExo1 and RNase H2) required for maintaining genome stability in human cells. We uncovered a role for hExo1 in the cellular response to UV irradiation in human cells by demonstrating its recruitment at sites of UV damage together with NER factors and its role in the activation of the checkpoint signal transduction cascade in response to UV irradiation. We also gained evidence on the role of RNase H2 in genome stability maintenance by counteracting the misincorporation of ribonucleotides (rNMPs) in the human genome. The presence of rNMPs in DNA molecules has been recently identified as the most common lesion in DNA, and mutations in genes coding for the RNase H2 complex subunits have been reported as the most frequent cause of the Aicardi-Goutières syndrome (AGS), suggesting a possible link between the RNase H2 molecular functions and AGS pathogenesis.