DNA Recombination, Replication and Genome Stability

Team Publications

Year of publication 2019

Anissia Ait-Saada, Olga Khorosjutina, Jiang Chen, Karol Kramarz, Vladimir Maksimov, J Peter Svensson, Sarah Lambert, Karl Ekwall (2019 Oct 1)

Chromatin remodeler Fft3 plays a dual role at blocked DNA replication forks.

Life science alliance : DOI : e201900433 Learn more
Summary

Here, we investigate the function of fission yeast Fun30/Smarcad1 family of SNF2 ATPase-dependent chromatin remodeling enzymes in DNA damage repair. There are three Fun30 homologues in fission yeast, Fft1, Fft2, and Fft3. We find that only Fft3 has a function in DNA repair and it is needed for single-strand annealing of an induced double-strand break. Furthermore, we use an inducible replication fork barrier system to show that Fft3 has two distinct roles at blocked DNA replication forks. First, Fft3 is needed for the resection of nascent strands, and second, it is required to restart the blocked forks. The latter function is independent of its ATPase activity.

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Sarah Lambert (2019 Mar 3)

Unstable genomes promote inflammation.

Nature : 41-42 : DOI : 10.1038/d41586-019-00510-5 Learn more
Summary

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Hannah L Klein, Giedrė Bačinskaja, Jun Che, Anais Cheblal, Rajula Elango, Anastasiya Epshtein, Devon M Fitzgerald, Belén Gómez-González, Sharik R Khan, Sandeep Kumar, Bryan A Leland, Léa Marie, Qian Mei, Judith Miné-Hattab, Alicja Piotrowska, Erica J Polleys, Christopher D Putnam, Elina A Radchenko, Anissia Ait Saada, Cynthia J Sakofsky, Eun Yong Shim, Mathew Stracy, Jun Xia, Zhenxin Yan, Yi Yin, Andrés Aguilera, Juan Lucas Argueso, Catherine H Freudenreich, Susan M Gasser, Dmitry A Gordenin, James E Haber, Grzegorz Ira, Sue Jinks-Robertson, Megan C King, Richard D Kolodner, Andrei Kuzminov, Sarah Ae Lambert, Sang Eun Lee, Kyle M Miller, Sergei M Mirkin, Thomas D Petes, Susan M Rosenberg, Rodney Rothstein, Lorraine S Symington, Pawel Zawadzki, Nayun Kim, Michael Lisby, Anna Malkova (2019 Jan 7)

Guidelines for DNA recombination and repair studies: Cellular assays of DNA repair pathways.

Microbial cell (Graz, Austria) : 1-64 : DOI : 10.15698/mic2019.01.664 Learn more
Summary

Understanding the plasticity of genomes has been greatly aided by assays for recombination, repair and mutagenesis. These assays have been developed in microbial systems that provide the advantages of genetic and molecular reporters that can readily be manipulated. Cellular assays comprise genetic, molecular, and cytological reporters. The assays are powerful tools but each comes with its particular advantages and limitations. Here the most commonly used assays are reviewed, discussed, and presented as the guidelines for future studies.

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Year of publication 2018

Anissia Ait Saada, Sarah A E Lambert, Antony M Carr (2018 Aug 25)

Preserving replication fork integrity and competence via the homologous recombination pathway.

DNA repair : DOI : S1568-7864(18)30182-4 Learn more
Summary

Flaws in the DNA replication process have emerged as a leading driver of genome instability in human diseases. Alteration to replication fork progression is a defining feature of replication stress and the consequent failure to maintain fork integrity and complete genome duplication within a single round of S-phase compromises genetic integrity. This includes increased mutation rates, small and large scale genomic rearrangement and deleterious consequences for the subsequent mitosis that result in the transmission of additional DNA damage to the daughter cells. Therefore, preserving fork integrity and replication competence is an important aspect of how cells respond to replication stress and avoid genetic change. Homologous recombination is a pivotal pathway in the maintenance of genome integrity in the face of replication stress. Here we review our recent understanding of the mechanisms by which homologous recombination acts to protect, restart and repair replication forks. We discuss the dynamics of these genetically distinct functions and their contribution to faithful mitoticsegregation.

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Year of publication 2017

Ana Teixeira-Silva, Anissia Ait Saada, Julien Hardy, Ismail Iraqui, Marina Charlotte Nocente, Karine Fréon, Sarah A E Lambert (2017 Dec 7)

The end-joining factor Ku acts in the end-resection of double strand break-free arrested replication forks.

Nature communications : 1982 : DOI : 10.1038/s41467-017-02144-5 Learn more
Summary

Replication requires homologous recombination (HR) to stabilize and restart terminally arrested forks. HR-mediated fork processing requires single stranded DNA (ssDNA) gaps and not necessarily double strand breaks. We used genetic and molecular assays to investigate fork-resection and restart at dysfunctional, unbroken forks in Schizosaccharomyces pombe. Here, we report that fork-resection is a two-step process regulated by the non-homologous end joining factor Ku. An initial resection mediated by MRN-Ctp1 removes Ku from terminally arrested forks, generating ~110 bp sized gaps obligatory for subsequent Exo1-mediated long-range resection and replication restart. The mere lack of Ku impacts the processing of arrested forks, leading to an extensive resection, a reduced recruitment of RPA and Rad51 and a slower fork-restart process. We propose that terminally arrested forks undergo fork reversal, providing a single DNA end for Ku binding. We uncover a role for Ku in regulating end-resection of unbroken forks and in fine-tuning HR-mediated replication restart.

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Anissia Ait Saada, Ana Teixeira-Silva, Ismail Iraqui, Audrey Costes, Julien Hardy, Giulia Paoletti, Karine Fréon, Sarah A E Lambert (2017 May 4)

Unprotected Replication Forks Are Converted into Mitotic Sister Chromatid Bridges.

Molecular cell : 398-410.e4 : DOI : 10.1016/j.molcel.2017.04.002 Learn more
Summary

Replication stress and mitotic abnormalities are key features of cancer cells. Temporarily paused forks are stabilized by the intra-S phase checkpoint and protected by the association of Rad51, which prevents Mre11-dependent resection. However, if a fork becomes dysfunctional and cannot resume, this terminally arrested fork is rescued by a converging fork to avoid unreplicated parental DNA during mitosis. Alternatively, dysfunctional forks are restarted by homologous recombination. Using fission yeast, we report that Rad52 and the DNA binding activity of Rad51, but not its strand-exchange activity, act to protect terminally arrested forks from unrestrained Exo1-nucleolytic activity. In the absence of recombination proteins, large ssDNA gaps, up to 3 kb long, occur behind terminally arrested forks, preventing efficient fork merging and leading to mitotic sister chromatid bridging. Thus, Rad52 and Rad51 prevent temporarily and terminally arrested forks from degrading and, despite the availability of converging forks, converting to anaphase bridges causing aneuploidy and cell death.

Free acces : authors.elsevier.com/a/1U~li3vVUP2C0m

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Year of publication 2016

Simon Gemble, Géraldine Buhagiar-Labarchède, Rosine Onclercq-Delic, Denis Biard, Sarah Lambert, Mounira Amor-Guéret (2016 Aug 15)

A balanced pyrimidine pool is required for optimal Chk1 activation to prevent ultrafine anaphase bridge formation.

Journal of cell science : 3167-77 : DOI : 10.1242/jcs.187781 Learn more
Summary

Cytidine deaminase (CDA) deficiency induces an excess of cellular dCTP, which reduces basal PARP-1 activity, thereby compromising complete DNA replication, leading to ultrafine anaphase bridge (UFB) formation. CDA dysfunction has pathological implications, notably in cancer and in Bloom syndrome. It remains unknown how reduced levels of PARP-1 activity and pyrimidine pool imbalance lead to the accumulation of unreplicated DNA during mitosis. We report that a decrease in PARP-1 activity in CDA-deficient cells impairs DNA-damage-induced Chk1 activation, and, thus, the downstream checkpoints. Chemical inhibition of the ATR-Chk1 pathway leads to UFB accumulation, and we found that this pathway was compromised in CDA-deficient cells. Our data demonstrate that ATR-Chk1 acts downstream from PARP-1, preventing the accumulation of unreplicated DNA in mitosis, and, thus, UFB formation. Finally, delaying entry into mitosis is sufficient to prevent UFB formation in both CDA-deficient and CDA-proficient cells, suggesting that both physiological and pathological UFBs are derived from unreplicated DNA. Our findings demonstrate an unsuspected requirement for a balanced nucleotide pool for optimal Chk1 activation both in unchallenged cells and in response to genotoxic stress.

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Year of publication 2015

Simon Gemble, Akshay Ahuja, Géraldine Buhagiar-Labarchède, Rosine Onclercq-Delic, Julien Dairou, Denis S F Biard, Sarah Lambert, Massimo Lopes, Mounira Amor-Guéret (2015 Jul 16)

Pyrimidine Pool Disequilibrium Induced by a Cytidine Deaminase Deficiency Inhibits PARP-1 Activity, Leading to the Under Replication of DNA.

PLoS genetics : e1005384 : DOI : 10.1371/journal.pgen.1005384 Learn more
Summary

Genome stability is jeopardized by imbalances of the dNTP pool; such imbalances affect the rate of fork progression. For example, cytidine deaminase (CDA) deficiency leads to an excess of dCTP, slowing the replication fork. We describe here a novel mechanism by which pyrimidine pool disequilibrium compromises the completion of replication and chromosome segregation: the intracellular accumulation of dCTP inhibits PARP-1 activity. CDA deficiency results in incomplete DNA replication when cells enter mitosis, leading to the formation of ultrafine anaphase bridges between sister-chromatids at “difficult-to-replicate” sites such as centromeres and fragile sites. Using molecular combing, electron microscopy and a sensitive assay involving cell imaging to quantify steady-state PAR levels, we found that DNA replication was unsuccessful due to the partial inhibition of basal PARP-1 activity, rather than slower fork speed. The stimulation of PARP-1 activity in CDA-deficient cells restores replication and, thus, chromosome segregation. Moreover, increasing intracellular dCTP levels generates under-replication-induced sister-chromatid bridges as efficiently as PARP-1 knockdown. These results have direct implications for Bloom syndrome (BS), a rare genetic disease combining susceptibility to cancer and genomic instability. BS results from mutation of the BLM gene, encoding BLM, a RecQ 3′-5′ DNA helicase, a deficiency of which leads to CDA downregulation. BS cells thus have a CDA defect, resulting in a high frequency of ultrafine anaphase bridges due entirely to dCTP-dependent PARP-1 inhibition and independent of BLM status. Our study describes previously unknown pathological consequences of the distortion of dNTP pools and reveals an unexpected role for PARP-1 in preventing DNA under-replication and chromosome segregation defects.

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Saed Mohebi, Sarah A E Lambert, Antony M Carr (2015 Apr 29)

Analyzing the Response to Dysfunction Replication Forks Using the RTS1 Barrier System in Fission Yeast.

Methods in molecular biology (Clifton, N.J.) : 239-59 : DOI : 10.1007/978-1-4939-2596-4_15 Learn more
Summary

The study of how eukaryotic cells overcome problems associated with dysfunctional DNA replication forks is assisted by experimental systems that allow site-specific replication fork arrest. Here we provide protocols for the use of the fission yeast RTS1 replication fork barrier. The RTS1 barrier is a directional, or polar, replication fork barrier that evolved to ensure directional replication of the fission yeast mating-type locus. We have moved the 859 bp RTS1 sequence to another locus in the genome and demonstrated that it arrests replication forks in a dysfunctional confirmation and that replication is restarted within ~20 min by the action of homologous recombination. We describe here the barrier constructs currently available, the methods by which we regulate the activity of the barrier, how to synchronize cells for analysis of replication intermediates by 2D gel electrophoresis, and the use of a replication slippage assay to measure fork fidelity.

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Year of publication 2014

Violena Pietrobon, Karine Fréon, Julien Hardy, Audrey Costes, Ismail Iraqui, Françoise Ochsenbein, Sarah A E Lambert (2014 Oct 14)

The chromatin assembly factor 1 promotes Rad51-dependent template switches at replication forks by counteracting D-loop disassembly by the RecQ-type helicase Rqh1.

PLoS biology : e1001968 : DOI : 10.1371/journal.pbio.1001968 Learn more
Summary

At blocked replication forks, homologous recombination mediates the nascent strands to switch template in order to ensure replication restart, but faulty template switches underlie genome rearrangements in cancer cells and genomic disorders. Recombination occurs within DNA packaged into chromatin that must first be relaxed and then restored when recombination is completed. The chromatin assembly factor 1, CAF-1, is a histone H3-H4 chaperone involved in DNA synthesis-coupled chromatin assembly during DNA replication and DNA repair. We reveal a novel chromatin factor-dependent step during replication-coupled DNA repair: Fission yeast CAF-1 promotes Rad51-dependent template switches at replication forks, independently of the postreplication repair pathway. We used a physical assay that allows the analysis of the individual steps of template switch, from the recruitment of recombination factors to the formation of joint molecules, combined with a quantitative measure of the resulting rearrangements. We reveal functional and physical interplays between CAF-1 and the RecQ-helicase Rqh1, the BLM homologue, mutations in which cause Bloom’s syndrome, a human disease associating genome instability with cancer predisposition. We establish that CAF-1 promotes template switch by counteracting D-loop disassembly by Rqh1. Consequently, the likelihood of faulty template switches is controlled by antagonistic activities of CAF-1 and Rqh1 in the stability of the D-loop. D-loop stabilization requires the ability of CAF-1 to interact with PCNA and is thus linked to the DNA synthesis step. We propose that CAF-1 plays a regulatory role during template switch by assembling chromatin on the D-loop and thereby impacting the resolution of the D-loop.

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Ellen Tsang, Izumi Miyabe, Ismail Iraqui, Jiping Zheng, Sarah A E Lambert, Antony M Carr (2014 Jul 1)

The extent of error-prone replication restart by homologous recombination is controlled by Exo1 and checkpoint proteins.

Journal of cell science : 2983-94 : DOI : 10.1242/jcs.152678 Learn more
Summary

Genetic instability, a hallmark of cancer, can occur when the replication machinery encounters a barrier. The intra-S-phase checkpoint maintains stalled replication forks in a replication-competent configuration by phosphorylating replisome components and DNA repair proteins to prevent forks from catastrophically collapsing. Here, we report a novel function of the core Schizosaccharomyces pombe checkpoint sensor kinase, Rad3 (an ATR orthologue), that is independent of Chk1 and Cds1 (a CHK2 orthologue); Rad3(ATR) regulates the association of recombination factors with collapsed forks, thus limiting their genetic instability. We further reveal antagonistic roles for Rad3(ATR) and the 9-1-1 clamp – Rad3(ATR) restrains MRN- and Exo1-dependent resection, whereas the 9-1-1 complex promotes Exo1 activity. Interestingly, the MRN complex, but not its nuclease activity, promotes resection and the subsequent association of recombination factors at collapsed forks. The biological significance of this regulation is revealed by the observation that Rad3(ATR) prevents Exo1-dependent genome instability upstream of a collapsed fork without affecting the efficiency of recombination-mediated replication restart. We propose that the interplay between Rad3(ATR) and the 9-1-1 clamp functions to fine-tune the balance between the need for the recovery of replication through recombination and the risk of increased genome instability.

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Josée Guirouilh-Barbat, Sarah Lambert, Pascale Bertrand, Bernard S Lopez (2014 Jun 11)

Is homologous recombination really an error-free process?

Frontiers in genetics : 175 : DOI : 10.3389/fgene.2014.00175 Learn more
Summary

Homologous recombination (HR) is an evolutionarily conserved process that plays a pivotal role in the equilibrium between genetic stability and diversity. HR is commonly considered to be error-free, but several studies have shown that HR can be error-prone. Here, we discuss the actual accuracy of HR. First, we present the product of genetic exchanges (gene conversion, GC, and crossing over, CO) and the mechanisms of HR during double strand break repair and replication restart. We discuss the intrinsic capacities of HR to generate genome rearrangements by GC or CO, either during DSB repair or replication restart. During this process, abortive HR intermediates generate genetic instability and cell toxicity. In addition to genome rearrangements, HR also primes error-prone DNA synthesis and favors mutagenesis on single stranded DNA, a key DNA intermediate during the HR process. The fact that cells have developed several mechanisms protecting against HR excess emphasize its potential risks. Consistent with this duality, several pro-oncogenic situations have been consistently associated with either decreased or increased HR levels. Nevertheless, this versatility also has advantages that we outline here. We conclude that HR is a double-edged sword, which on one hand controls the equilibrium between genome stability and diversity but, on the other hand, can jeopardize the maintenance of genomic integrity. Therefore, whether non-homologous end joining (which, in contrast with HR, is not intrinsically mutagenic) or HR is the more mutagenic process is a question that should be re-evaluated. Both processes can be “Dr. Jekyll” in maintaining genome stability/variability and “Mr. Hyde” in jeopardizing genome integrity.

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Mery Pina, Tamara Basta, Tessa E F Quax, Alexandra Joubert, Sonia Baconnais, Diego Cortez, Sarah Lambert, Eric Le Cam, Stephen D Bell, Patrick Forterre, David Prangishvili (2014 Jun 1)

Unique genome replication mechanism of the archaeal virus AFV1.

Molecular microbiology : 1313-25 : DOI : 10.1111/mmi.12630 Learn more
Summary

The exceptional genomic content and genome organization of the Acidianus filamentous virus 1 (AFV1) that infects the hyperthermophilic archaeon Acidianus hospitalis suggest that this virus might exploit an unusual mechanism of genome replication. An analysis of replicative intermediates of the viral genome by two-dimensional (2D) agarose gel electrophoresis revealed that viral genome replication starts by the formation of a D-loop and proceeds via strand displacement replication. Characterization of replicative intermediates using dark-field electron microscopy, in combination with the 2D agarose gel electrophoresis data, suggests that recombination plays a key role in the termination of AFV1 genome replication through the formation of terminal loops. A terminal protein was found to be attached to the ends of the viral genome. The results allow us to postulate a model of genome replication that relies on recombination events for initiation and termination.

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Indiana Magdalou, Bernard S Lopez, Philippe Pasero, Sarah A E Lambert (2014 Jun 1)

The causes of replication stress and their consequences on genome stability and cell fate.

Seminars in cell & developmental biology : 154-64 : DOI : 10.1016/j.semcdb.2014.04.035 Learn more
Summary

Alterations of the dynamics of DNA replication cause genome instability. These alterations known as “replication stress” have emerged as a major source of genomic instability in pre-neoplasic lesions, contributing to cancer development. The concept of replication stress covers a wide variety of events that distort the temporal and spatial DNA replication program. These events have endogenous or exogenous origins and impact globally or locally on the dynamics of DNA replication. They may arise within a short window of time (acute stress) or during each S phase (chronic stress). Here, we review the known situations in which the dynamics of DNA replication is distorted. We have united them in four main categories: (i) inadequate firing of replication origins (deficiency or excess), (ii) obstacles to fork progression, (iii) conflicts between replication and transcription and (iv) DNA replication under inappropriate metabolic conditions (unbalanced DNA replication). Because the DNA replication program is a process tightly regulated by many factors, replication stress often appears as a cascade of events. A local stress may prevent the completion of DNA replication at a single locus and subsequently compromise chromosome segregation in mitosis and therefore have a global effect on genome integrity. Finally, we discuss how replication stress drives genome instability and to what extent it is relevant to cancer biology.

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Martin Dutertre, Sarah Lambert, Aura Carreira, Mounira Amor-Guéret, Stéphan Vagner (2014 Mar 1)

DNA damage: RNA-binding proteins protect from near and far.

Trends in biochemical sciences : 141-9 : DOI : 10.1016/j.tibs.2014.01.003 Learn more
Summary

Recent work, including large-scale genetic and molecular analyses, identified RNA-binding proteins (RBPs) as major players in the prevention of genome instability. These studies show that RBPs prevent harmful RNA/DNA hybrids and are involved in the DNA damage response (DDR), from DNA repair to cell survival decisions. Indeed, specific RBPs allow the selective regulation of DDR genes at multiple post-transcriptional levels (from pre-mRNA splicing/polyadenylation to mRNA stability/translation) and are directly involved in DNA repair. These multiple activities are mediated by RBP binding to mRNAs, nascent transcripts, noncoding RNAs, and damaged DNA. Finally, because DNA damage modifies RBP localization and binding to different RNA/DNA molecules, we propose that upon DNA damage, RBPs coordinately regulate various aspects of both RNA and DNA metabolism.

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