DNA double-strand breaks (DSB) with chemically modified end-termini are the most significant lesions resulting from radio/chemotherapeutic intervention of cancer and non homologous end-joining (NHEJ) factor Artemis nuclease has been implicated in the repair of such breaks. To examine whether the resolution of terminally blocked DNA DSBs is the biologically relevant function of Artemis, Artemis deficient fibroblasts were stably complemented with wild type or an endonuclease deficient D165N mutant Artemis. Physiological levels of wild type (WT) Artemis completely restored DSB repair proficiency and resistance to γ-radiation, bleomycin, and neocarzinostatin. Cells expressing the D165N mutants remained as chemo/radiosensitive and as repair deficient as parental cells, with persistent γ H2AX and 53BP1 foci that increased in size 6-18 hour post irradiation. These persistent foci co-localized with DNA double strand break repair factor Mre11 and also with promyelocytic leukemia protein (PML). Further, in vitro studies have revealed that DNA-PK dependent Artemis endonucleolytic activity may play a role in the repair of commonly found oxidative base damage; 8-oxoguanine (8-oxoG), a hallmark of complex DSBs. However, majority of DNA DSBs are repaired in an Artemis independent manner, and recently discovered, DNA end-specific nuclease, Metnase is a candidate enzyme for repair of such breaks. To study the role of Metnase in resolution of 3ʹ-blocked termini, several substrates mimicking such breaks were constructed. A 3ʹ-phosphoglycolate moiety on longer overhangs (4 and 6 bases) altered specificity and stimulated Metnase-mediated cleavage of the terminal 3 nucleotides. However, an 8-oxoG residue at the single-strand/double-strand border did not affect specificity or extent of cleavage. Metnase preferentially cleaved ssDNA-overhang of a partially duplex substrate, and the cleavage increased with increase in length of 3ʹ-overhangs. A D483A mutation in Metnase completely abrogated Metnase cleavage activity towards DNA ends. These results suggest that Metnase may resolve oxidatively damaged DNA ends to facilitate repair while Artemis is required for the resolution of more complex DNA DSBs that persist for longer times and are not amenable to repair by other NHEJ factors.

Charles University in Prague Faculty of Pharmacy in Hradec Králové Department of Pharmacology & Toxicology Student: Eva Troppová Supervisor: PharmDr. Marie Vopršalová, CSc. Specialized supervisor: Prof. Dr. Antje Gohla Title of diploma thesis: Establishment of screening methods to find new regulators of the activity of phosphoglycolate phosphatase This work deals with the siRNA-based genomic screening for the modification of phosphoglycolate phosphatase (PGP) activity. 235 proteins were affected by transient transfection of siRNAs in vitro. Each siRNA was used in duplicates and the control was carried out by two control siRNAs. After downregulation of protein by siRNA PGP activity was evaluated, whether any modifications of PGP activity have occurred. PGP was the main research target. The main goal of this study before the screening was to set up a method, to create a reliable protocol. The whole study was 96 plate well. It was necessary to find the right conditions to measure PGP activity in two cell types (HEK AD 293 and Hep G2). Subsequently, optimal conditions were set up to influence expression of the protein. The method was optimalized using PGP siRNAs and 2 types of transfection reagents were tested. During our study the following methods were used: PGP activity assay, Bicinchoninic acid...