DNA replication stress: Causes, resolution and disease

AbstractDNA replication is a fundamental process of the cell that ensures accurate duplication of the genetic information and subsequent transfer to daughter cells. Various pertubations, originating from endogenous or exogenous sources, can interfere with proper progression and completion of the replication process, thus threatening genome integrity. Coordinated regulation of replication and the DNA damage response is therefore fundamental to counteract these challenges and ensure accurate synthesis of the genetic material under conditions of replication stress. In this review, we summarize the main sources of replication stress and the DNA damage signaling pathways that are activated in order to preserve genome integrity during DNA replication. We also discuss the association of replication stress and DNA damage in human disease and future perspectives in the field

Parallel genome-wide screens identify synthetic viable interactions between the BLM helicase complex and Fanconi anemia.

Maintenance of genome integrity via repair of DNA damage is a key biological process required to suppress diseases, including Fanconi anemia (FA). We generated loss-of-function human haploid cells for FA complementation group C (FANCC), a gene encoding a component of the FA core complex, and used genome-wide CRISPR libraries as well as insertional mutagenesis to identify synthetic viable (genetic suppressor) interactions for FA. Here we show that loss of the BLM helicase complex suppresses FANCC phenotypes and we confirm this interaction in cells deficient for FA complementation group I and D2 (FANCI and FANCD2) that function as part of the FA I-D2 complex, indicating that this interaction is not limited to the FA core complex, hence demonstrating that systematic genome-wide screening approaches can be used to reveal genetic viable interactions for DNA repair defects

Map of synthetic rescue interactions for the Fanconi anemia DNA repair pathway identifies USP48

Genome Instability and Cance

Map of synthetic rescue interactions for the Fanconi anemia DNA repair pathway identifies USP48

Defects in DNA repair can cause various genetic diseases with severe pathological phenotypes. Fanconi anemia (FA) is a rare disease characterized by bone marrow failure, developmental abnormalities and increased cancer risk that is caused by defective repair of DNA interstrand crosslinks (ICLs). By performing genome-wide loss-of-function screens across a panel of human haploid isogenic FA-defective cells (FANCA, FANCC, FANCG, FANCI, FANCD2), we identified the deubiquitylating enzyme USP48 as synthetic viable for FA gene deficiencies. Thus, as compared to FA-defective cells alone, FA-deficient cells additionally lacking USP48 are less sensitive to genotoxic stress induced by ICL agents and display enhanced, BRCA1-dependent, clearance of DNA damage. Consequently, USP48 inactivation reduces chromosomal instability of FA-defective cells. Our results highlight a role for USP48 in controlling DNA repair and suggest it as a potential target that could be therapeutically exploited for FA

Chronic p53-independent p21 expression causes genomic instability by deregulating replication licensing

The cyclin-dependent kinase inhibitor p21WAF1/CIP1 (p21) is a cell-cycle checkpoint effector and inducer of senescence, regulated by p53. Yet, evidence suggests that p21 could also be oncogenic, through a mechanism that has so far remained obscure. We report that a subset of atypical cancerous cells strongly expressing p21 showed proliferation features. This occurred predominantly in p53-mutant human cancers, suggesting p53-independent upregulation of p21 selectively in more aggressive tumour cells. Multifaceted phenotypic and genomic analyses of p21-inducible, p53-null, cancerous and near-normal cellular models showed that after an initial senescence-like phase, a subpopulation of p21-expressing proliferating cells emerged, featuring increased genomic instability, aggressiveness and chemoresistance. Mechanistically, sustained p21 accumulation inhibited mainly the CRL4–CDT2 ubiquitin ligase, leading to deregulated origin licensing and replication stress. Collectively, our data reveal the tumour-promoting ability of p21 through deregulation of DNA replication licensing machinery—an unorthodox role to be considered in cancer treatment, since p21 responds to various stimuli including some chemotherapy drugs

Inactivation of the ATMIN/ATM pathway protects against glioblastoma formation

Glioblastoma multiforme (GBM) is the most aggressive human primary brain cancer. Using a Trp53-deficient mouse model of GBM, we show that genetic inactivation of the Atm cofactor Atmin, which is dispensable for embryonic and adult neural development, strongly suppresses GBM formation. Mechanistically, expression of several GBM-associated genes, including Pdgfra, was normalized by Atmin deletion in the Trp53-null background. Pharmacological ATM inhibition also reduced Pdgfra expression, and reduced the proliferation of Trp53-deficient primary glioma cells from murine and human tumors, while normal neural stem cells were unaffected. Analysis of GBM datasets showed that PDGFRA expression is also significantly increased in human TP53-mutant compared with TP53-wild-type tumors. Moreover, combined treatment with ATM and PDGFRA inhibitors efficiently killed TP53-mutant primary human GBM cells, but not untransformed neural stem cells. These results reveal a new requirement for ATMIN-dependent ATM signaling in TP53-deficient GBM, indicating a pro-tumorigenic role for ATM in the context of these tumors

Effect of partial and complete supression of the pimer ARF - double strand DNA break response pathway in cancer development: role in therapeutic approach

The DNA damage response (DDR) pathway and ARF function act as barriers of human cancer development. It has been considered that the DDR and ARF exert this function independently of each other. However, a few studies propose that ARF’s activity is positively regulated by the DDR pathway. Examining this hypothesis we performed a series of experiments using molecular techniques such as immunoblotting, immunohistochemistry, immunofluorescence, immunoprecipitation, Real Time Reverse Transcritpion polymerase chain reaction (RT-PCR) in biological material from cell culture or histological samples, as well as ectopic protein expression through plasmid transfections, proteomic analyses, ribosome RNA biogenesis assay and xenografts of human cancer cells. We surprisingly found that ATM suppressed, in a transcription-independent manner, ARF protein levels and activity. Specifically, ATM activated protein phosphatase 1 (PP1). PP1 antagonized Nek2-dependent phosphorylation of nucleophosmin (NPM), liberating ARF from NPM and rendering it susceptible to degradation by the ULF E3-ubiquitin ligase. In human clinical samples, loss of ATM expression correlated with increased ARF levels and in xenograft and tissue culture models, inhibition of ATM stimulated the tumour-suppressive effects of ARF. The importance of the proposed mechanism can be exploited through a therapeutic approach, especially in cases of tumours bearing loss of p53. In such tumours, DDR may act in favour of the tumour cells, since the major effector of the antiumour barriers of apoptosis and senescence is absent, but ATM inhbition could boost ARF’s tumour-suppressive function, contributing to an anti-tumour response

The roles of p27Kip1 and DNA damage signalling in the chemotherapy-induced delayed cell cycle checkpoint

DNA lesions trigger the DNA damage response (DDR) machinery, which protects genomic integrity and sustains cellular survival. Increasing data underline the significance of the integrity of the DDR pathway in chemotherapy response. According to a recent work, persistent exposure of A549 lung carcinoma cells to doxorubicin induces an initial DDR-dependent checkpoint response, followed by a later DDR-independent, but p27Kip1-dependent one. Prompted by the above report and to better understand the involvement of the DDR signaling after chemotherapeutic stress, we examined the potential role of the canonical DDR pathway in A549 cells treated with doxorubicin. Exposure of A549 cells, prior to doxorubicin treatment, to ATM, ATR and DNA-PKcs inhibitors either alone or in various combinations, revealed that the earlier documented two-step response was DDR-dependent in both steps. Notably, inhibition of both ATM and ATR or selective inhibition of ATM or DNA-PKcs resulted in cell-cycle re-entry despite the increased levels of p27Kip1 at all time points analyzed. We further investigated the regulation of p27Kip1 protein levels in the particular setting. Our results showed that the protein status of p27Kip1 is mainly determined by p38-MAPK, whereas the role of SKP2 is less significant in the doxoroubicin-treated A549 cells. Cumulatively, we provide evidence that the DNA damage signaling is responsible for the prolonged cell cycle arrest observed after persistent chemotherapy-induced genotoxic stress. In conclusion, precise identification of the molecular mechanisms that are activated during the chemotherapeutic cycles could potentially increase the sensitization to the therapy applied. © 2010 The Authors. Journal compilation © 2010 Foundation for Cellular and Molecular Medicine/Blackwell Publishing Ltd

Loss of p14ARF confers resistance to heat shock- and oxidative stress-mediated cell death by upregulating β-catenin

The p14ARF is a key tumor suppressor induced mainly by oncogenic stimuli. Although p14ARF does not seem to respond to DNA damage, there are very few data regarding its role in other forms of stress, such as heat shock (HS) and oxidative stress (OS). Here, we report that suppression of p14ARF increased resistance to cell death when cells were treated with H2O2 or subjected to HS. In this setting, protection from cell death was mediated by elevated levels and activity of β-catenin, as downregulation of β-catenin alleviated the protective role of p14 ARF silencing. Moreover, Hsp70 was shown to regulate β-catenin protein levels by interacting with p14ARF, suggesting that Hsp70, p14ARF and β-catenin form a regulatory network. This novel pathway triggers cell death signals when cells are exposed to HS and OS. © 2010 UICC