Many faces of ATM: eighth international workshop on ataxia-telangiectasia

Importance of CNOT8 Deadenylase Subunit in DNA Damage Responses Following Ionizing Radiation (IR)

Background

The Ccr4-Not protein complex (CNOT complex) is a key regulator of gene expression in eukaryotic cells. Ccr4-Not Complex is composed of at least nine conserved subunits in mammalian cells with two main enzymatic activities. CNOT8 is a subunit of the complex with deadenylase activity that interacts transiently with the CNOT6 or CNOT6L subunits. Here, we focused on the role of the human CNOT8 subunit in the DNA damage response (DDR).

Methods

Cell viability was assessed to measure ATP level using a Cell Titer-Glo Luminescence reagent up to 4 days' post CNOT8 siRNA transfection. In addition, expression level of phosphorylated proteins in signalling pathways were detected by western blotting and immunofluorescence microscopy. CNOT8- depleted Hela cells post- 3 Gy ionizing radiation (IR) treatment were considered as a control.

Results

Our results from cell viability assays indicated a significant reduction at 72-hour post CNOT8 siRNA transfection (p= 0.04). Western blot analysis showed slightly alteration in the phosphorylation of DNA damage response (DDR) proteins in CNOT8-depleted HeLa cells following treatment with ionizing radiation (IR). Increased foci formation of γH2AX, RPA, 53BP1, and RAD51 foci was observed after IR in CNOT8-depleted cells compared to the control cells.

Conclusion

We conclude that CNOT8 deadenylase subunit is involved in the cellular response to DNA damage.

The onset of p53-dependent DNA repair or apoptosis is determined by the level of accumulated damaged DNA

The p53 tumor suppressor gene plays an important role in both apoptosis and DNA repair pathways that are pivotal for genomic stability. Here we show that the treatment of cells with low doses of gamma-irradiation or cisplatin resulted in an immediate enhancement of p53-dependent DNA repair, measured by base excision repair (BER) activity. However, treatment of cells with high doses of DNA damaging agents resulted in a reduction in p53-dependent DNA repair and in the induction of p53-dependent apoptosis. Analysis of p53 upstream molecular events suggested that regulation of p53-associated DNA repair is ATM-dependent. Furthermore, we observed that while dephosphorylation of Ser376 at the C-terminus of the p53 protein was associated with enhancement in DNA repair, phosphorylation at the N-terminal Ser15 resulted in the reduction in DNA repair. The latter is also in correlation with an enhancement in the specific DNA binding activity and in the induction of apoptosis. Treatment of cells with a caspase inhibitor, prior to the damaging agent-blocked apoptosis, had no effect on the DNA repair pattern. Taken together, this suggests that the decision of cells to induce a p53-dependent DNA repair or apoptosis is most probably controlled by the level of genotoxic agent introduced to cells.

Characterization of mec1 kinase-deficient mutants and of new hypomorphic mec1 alleles impairing subsets of the DNA damage response pathway

DNA damage checkpoints lead to the inhibition of cell cycle progression following DNA damage. The Saccharomyces cerevisiae Mec1 checkpoint protein, a phosphatidylinositol kinase-related protein, is required for transient cell cycle arrest in response to DNA damage or DNA replication defects. We show that mec1 kinase-deficient (mec1kd) mutants are indistinguishable from mec1Delta cells, indicating that the Mec1 conserved kinase domain is required for all known Mec1 functions, including cell viability and proper DNA damage response. Mec1kd variants maintain the ability to physically interact with both Ddc2 and wild-type Mec1 and cause dominant checkpoint defects when overproduced in MEC1 cells, impairing the ability of cells to slow down S phase entry and progression after DNA damage in G(1) or during S phase. Conversely, an excess of Mec1kd in MEC1 cells does not abrogate the G(2)/M checkpoint, suggesting that Mec1 functions required for response to aberrant DNA structures during specific cell cycle stages can be separable. In agreement with this hypothesis, we describe two new hypomorphic mec1 mutants that are completely defective in the G(1)/S and intra-S DNA damage checkpoints but properly delay nuclear division after UV irradiation in G(2). The finding that these mutants, although indistinguishable from mec1Delta cells with respect to the ability to replicate a damaged DNA template, do not lose viability after UV light and methyl methanesulfonate treatment suggests that checkpoint impairments do not necessarily result in hypersensitivity to DNA-damaging agents.

Early embryonic lethality in PARP-1 Atm double-mutant mice suggests a functional synergy in cell proliferation during development

PARP-1 and ATM are both involved in the response to DNA strand breaks, resulting in induction of a signaling network responsible for DNA surveillance, cellular recovery, and cell survival. ATM interacts with double-strand break repair pathways and induces signals resulting in the control of the cell cycle-coupled checkpoints. PARP-1 acts as a DNA break sensor in the base excision repair pathway of DNA. Mice with mutations inactivating either protein show radiosensitivity and high radiation-induced chromosomal aberration frequencies. Embryos carrying double mutations of both PARP-1 and Atm genes were generated. These mutant embryos show apoptosis in the embryo but not in extraembryonic tissues and die at embryonic day 8.0, although extraembryonic tissues appear normal for up to 10.5 days of gestation. These results reveal a functional synergy between PARP-1 and ATM during a period of embryogenesis when cell cycle checkpoints are not active and the embryo is particularly sensitive to DNA damage. These results suggest that ATM and PARP-1 have synergistic phenotypes due to the effects of these proteins on signaling DNA damage and/or on distinct pathways of DNA repair.

p53 binding protein 1 (53BP1) is an early participant in the cellular response to DNA double-strand breaks

p53 binding protein 1 (53BP1), a protein proposed to function as a transcriptional coactivator of the p53 tumor suppressor, has BRCT domains with high homology to the Saccharomyces cerevisiae Rad9p DNA damage checkpoint protein. To examine whether 53BP1 has a role in the cellular response to DNA damage, we probed its intracellular localization by immunofluorescence. In untreated primary cells and U2OS osteosarcoma cells, 53BP1 exhibited diffuse nuclear staining; whereas, within 5-15 min after exposure to ionizing radiation (IR), 53BP1 localized at discreet nuclear foci. We propose that these foci represent sites of processing of DNA double-strand breaks (DSBs), because they were induced by IR and chemicals that cause DSBs, but not by ultraviolet light; their peak number approximated the number of DSBs induced by IR and decreased over time with kinetics that parallel the rate of DNA repair; and they colocalized with IR-induced Mre11/NBS and gamma-H2AX foci, which have been previously shown to localize at sites of DSBs. Formation of 53BP1 foci after irradiation was not dependent on ataxia-telangiectasia mutated (ATM), Nijmegen breakage syndrome (NBS1), or wild-type p53. Thus, the fast kinetics of 53BP1 focus formation after irradiation and the lack of dependency on ATM and NBS1 suggest that 53BP1 functions early in the cellular response to DNA DSBs.