MRE11 and EXO1 nucleases degrade reversed forks and elicit MUS81-dependent fork rescue in BRCA2-deficient cells

The breast cancer susceptibility proteins BRCA1 and BRCA2 have emerged as key stabilizing factors for the maintenance of replication fork integrity following replication stress. In their absence, stalled replication forks are extensively degraded by the MRE11 nuclease, leading to chemotherapeutic sensitivity. Here we report that BRCA proteins prevent nucleolytic degradation by protecting replication forks that have undergone fork reversal upon drug treatment. The unprotected regressed arms of reversed forks are the entry point for MRE11 in BRCA-deficient cells. The CtIP protein initiates MRE11-dependent degradation, which is extended by the EXO1 nuclease. Next, we show that the initial limited resection of the regressed arms establishes the substrate for MUS81 in BRCA2-deficient cells. In turn, MUS81 cleavage of regressed forks with a ssDNA tail promotes POLD3-dependent fork rescue. We propose that targeting this pathway may represent a new strategy to modulate BRCA2-deficient cancer cell response to chemotherapeutics that cause fork degradation

Impact of ATR/CHK1 haploinsufficiencies and Topoisomerase 1 on replication and Common Fragile Sites induction.

Les Sites Fragiles Communs (SFCs) sont des points de cassures chromosomiques récurrents survenant suite à un stress réplicatif. La majorité d'entre eux ont été identifiés suite à un traitement à l'Aphidicoline (APC), un inhibiteur des ADN polymérases, ce qui explique pourquoi l'étude de leur fragilité est majoritairement basée sur des perturbations de la réplication. Parmi les facteurs impliqués dans la fragilité, ATR (Ataxia Telangiectasia and Rad3 Related), une kinase engagée dans la signalisation des fourches de réplication bloquées, et sa cible CHK1 (Checkpoint Kinase 1), induisent l'apparition de cassures aux SFCs lorsqu'ils sont déplétés dans la cellule. Cependant, ces gènes sont difficiles à étudier car leur déplétion totale est létale pour les cellules. Nous avons donc choisi de développer un modèle basé sur l'utilisation de la lignée MSI de cancer colorectal HCT116de laquelle nous avons isolé des clones haploinsuffisants pour ATR ou CHK1.Ces mutations sont naturelles et sont d'ailleurs dans les cancers MSI. Aujourd'hui, de nouvelles données montrent que la transcription semble aussi être impliquée dans l'induction de certains SFCs. Pour étudier ce mécanisme, nous nous sommes servis de cellules déficientes pour la Topoisomérase 1 (Topo1) grâce à un shARN inductible. Cette protéine est impliquée dans la gestion des interférences entre les machineries de réplication et de transcription et son inhibition est à l'origine d'une forte instabilité chromosomique.Nos études démontrent tout d'abord que l'haploinsuffisance d'ATR ou de CHK1 retrouvée dans les cancers MSI génère des défauts de réplication, des problèmes de checkpoints ainsi que des cassures ADN et en particulier au niveau des SFCs. Une déficience en Topo1 induit aussi l'apparition de problèmes de réplication et une plus grande fragilité ADN, mais elle est aussi capable d'induire des cassures au niveau de certains SFCs, connus pour être sensible à des défauts de réplication. Une étude plus approfondie du SFC le plus fréquemment induit, FRA3B, montre qu'il est sensible à la fois à des défauts dans la voie ATR/CHK1, au stress réplicatif induits par l'aphidicoline mais aussi à la déficience en Topo1. Nos travaux suggèrent que l'haploinsuffisance d'ATR et CHK1 peut favoriser la cancérogenèse à travers l'induction des SFCs et que tous les SFCs n'ont pas les mêmes mécanismes d'induction, laissant la porte ouverte pour l’identification de nouveaux SFCs.Common Fragile Sites (CFSs) are recurrent chromosomal breakpoints occurring when cells are exposed to replicative stress. Most CFSs have been identified following Aphidicolin (APC) treatment, an inhibitor of DNA polymerase and therefore the working model to explain their fragility is indeed mainly based on perturbation of replication. Among the key actors involved in fragility, ATR (Ataxia Telangiectasia and Rad3 Related), a kinase involved in stalled replication fork signaling, and its target CHK1 (Checkpoint Kinase 1), lead to enhanced chromosome fragility when transiently depleted in cells. However, ATR and CHK1 are difficult to study since their complete depletion is lethal for the cells. We have developed a colorectal cancer HCT116 MSI model harboring ATR or CHK1 haploinsufficient mutations found in MSI cancers. Transcription also seems to be involved in the induction of SFCs. To investigate this mechanism, we created topoisomerase 1 (Topo1) deficient cells with inducible shRNA. This protein is involved in interference between replication and transcription and its inhibition is associated with high chromosomal instability. Our studies indicate that ATR or CHK1 haploinsufficiency found in MSI cancers causes replication and checkpoints defects and induces DNA break particularly at CFSs. Topo1 deficiency is responsible of replication defects and DNA fragility and it induces breaks at some CFSs already known to be sensitive to replication defects. Moreover, a precise study of the most induced CFSs, FRA3B, shows that it is sensitive to defects in ATR/CHK1 pathway, to replicative stress induced by aphidicolin but also to Topo1 deficiency. Our results suggest that ATR and CHK1 haploinsufficiency can promote carcinogenesis through the induction of CFSs. Futhermore, we suggest that all CFSs do not rely on the same induction mechanisms, letting us to postulate that new CFSs are still to be identified

Tax impairs DNA replication forks and increases DNA breaks in specific oncogenic genome regions

International audienceBackground: Human T-cell leukemia virus type 1 (HTLV-I) is a human retrovirus associated with adult T-cell leukemia (ATL), an aggressive CD4 T-cell proliferative disease with dismal prognosis. The long latency preceding the development of the disease and the low incidence suggests that the virus itself is not sufficient for transformation and that genetic defects are required to create a permissive environment for leukemia. In fact, ATL cells are characterized by profound genetic modifications including structural and numerical chromosome alterations. Results: In this study we used molecular combing techniques to study the effect of the oncoprotein Tax on DNA replication. We found that replication forks have difficulties replicating complex DNA, fork progression is slower, and they pause or stall more frequently in the presence of Tax expression. Our results also show that Tax-associated replication defects are partially compensated by an increase in the firing of back-up origins. Consistent with these effects of Tax on DNA replication, an increase in double strand DNA breaks (DDSB) was seen in Tax expressing cells. Tax-mediated increases in DDSBs were associated with the ability of Tax to activate NF-kB and to stimulate intracellular nitric oxide production. We also demonstrated a reduced expression of human translesion synthesis (TLS) DNA polymerases Pol-H and Pol-K in HTLV-I-transformed T cells and ATL cells. This was associated with an increase in DNA breaks induced by Tax at specific genome regions, such as the c-Myc and the Bcl-2 major breakpoints. Consistent with the notion that the non-homologous end joining (NHEJ) pathway is hyperactive in HTLV-I-transformed cells, we found that inhibition of the NHEJ pathway induces significant killing of HTLV-I transformed cells and patient-derived leukemic ATL cells. Conclusion: Our results suggest that, replication problems increase genetic instability in HTLV-I-transformed cells. As a result, abuse of NHEJ and a defective homologous repair (HR) DNA repair pathway can be targeted as a new therapeutic approach for the treatment of adult T-cell leukemia