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Genetic instability is a hallmark of both cancer and aging. Homologous recombination (HR) is a prominent DNA repair pathway maintaining genomic integrity. Mutations in many HR genes lead to cancer predisposition. Paradoxically, the consequences of mutations in the pivotal HR player, RAD51, on cancer development remain puzzling. Moreover, in contrast with other HR genes, RAD51 mouse models are not available to experimentally address the role of RAD51 on aging and carcinogenesis, in vivo. Here, we engineered a mouse model with an inducible dominant negative form of RAD51 (SMRad51) that suppresses RAD51-mediated HR without stimulating alternative non-conservative repair pathways. We found that, in vivo expression of SMRad51 did not trigger tumorigenesis, but instead induced premature aging. We propose that these in vivo phenotypes result from the exhaustion of proliferating progenitors submitted to chronic endogenous replication stress resulting from RAD51-mediated HR suppression. Our data underline the importance of the RAD51 activity for progenitors homeostasis, preventing aging, and more generally for the balance between cancer and aging.

Trastuzumab deruxtecan (T-DXd) is an anti-HER2 (human epidermal growth factor receptor 2) antibody-drug conjugate which has previously shown efficacy in patients with HER2-overexpressing and HER2-low metastatic breast cancer (mBC). However, the mechanisms of action and resistance of this drug remain partially unclear. DAISY (NCT04132960) is a phase II, open-label study that included patients with mBC whose disease progressed after at least one line of chemotherapy in the metastatic setting. Patients were enrolled in three cohorts according to HER2 expression determined by immunohistochemistry (IHC); cohort 1: HER2-overexpressing (HER2 IHC 3 + or HER2 IHC 2+/ISH+, n = 72), cohort 2: HER2-low (HER2 IHC2+/ISH- or HER2 IHC 1+, n = 74), and cohort 3: HER2 IHC 0 mBC (n = 40). Patients were treated with T-DXd 5.4 mg/kg every 3 weeks until disease progression or unacceptable toxicity. In the full analysis set population (n = 177), the confirmed objective response rate (ORR) was of 70.6% (95% CI: 58.3–81) in cohort 1, 37.5% (95% CI: 26.4–49.7) in cohort 2, and 29.7% (95% CI: 15.9–47) in cohort 3 (p p = 0.007), and cohort 3 with shorter PFS (adjusted HR: 1.96, 95% IC: 1.21–3.15, p = 0.006) as compared to cohort 2. Exploratory analyses showed that HER2 spatial distribution predicted T-DXd response in patients with HER2-overexpressing mBC and that the transcriptomic response to T-DXd was different according to HER2 expression. No quantitative modulation of tumor microenvironment was observed after 6 to 8 weeks of treatment. Finally, recurrent mutations of the DNA repair gene SLX4 were identified in 20% of samples at resistance (4/20) as compared to 2% in baseline samples (2/88), suggesting that SLX4 mutations could mediate secondary resistance to T-DXd. These data suggest that HER2 is a key determinant of T-DXd efficacy. However, an antitumor activity is also observed in a subgroup of patients without detectable HER2 expression and resistance could be partially mediated by payload sensitivity.

The DNA polymerase zeta (Polζ) plays a critical role in bypassing DNA damage. REV3L, the catalytic subunit of Polζ, is also essential in mouse embryonic development and cell proliferation for reasons that remain incompletely understood. In this study, we reveal that REV3L protein interacts with heterochromatin components including repressive histone marks and localizes in pericentromeric regions through direct interaction with HP1 dimer. We demonstrate that Polζ/REV3L ensures progression of replication forks through difficult‐to‐replicate pericentromeric heterochromatin, thereby preventing spontaneous chromosome break formation. We also find that Rev3l‐deficient cells are compromised in the repair of heterochromatin‐associated double‐stranded breaks, eliciting deletions in late‐replicating regions. Lack of REV3L leads to further consequences that may be ascribed to heterochromatin replication and repair‐associated functions of Polζ, with a disruption of the temporal replication program at specific loci. This is correlated with changes in epigenetic landscape and transcriptional control of developmentally regulated genes. These results reveal a new function of Polζ in preventing chromosome instability during replication of heterochromatic regions., HP1‐mediated targeting of REVL3, the catalytic subunit of translesion synthesis Polζ, influences replication fork progression and epigenetic and transcriptional landscapes of developmentally‐regulated genes.

BackgroundClinical studies have highlighted the efficacy of anti-programmed death 1 (αPD-1) monoclonal antibodies in patients with DNA mismatch repair-deficient (MMRD) tumors. However, the responsiveness of MMRD cancers to αPD-1 therapy is highly heterogeneous, and the origins of this variability remain not fully understood.Methods4T1 and CT26 mouse tumor cell lines were inactivated for the MMRD geneMsh2,leading to a massive accumulation of mutations after serial passages of cells. Insertions/deletion events and mutation load were evaluated by whole exome sequencing. Mice bearing highly mutated MMRD tumor or parental tumors were treated with αPD-1 and tumor volume was monitored. Immune cell type abundance was dynamically assessed in the tumor microenvironment and the blood by flow cytometry. Neutrophils were depleted in mice using αLY6G antibody, and regulatory T (Treg) cell population was reduced with αCD25 or anti-cytotoxic T-lymphocytes-associated protein 4 (αCTLA-4) antibodies. Patients with MMRD tumors treated with immune checkpoint blockade-based therapy were retrospectively identified and neutrophil-to-lymphocyte ratio (NLR) was evaluated and examined for correlation with clinical benefit.ResultsBy recapitulating mismatch repair deficiency in different mouse tumor models, we revealed that elevated circulating tumor-induced neutrophils (TIN) in hypermutated MMRD tumors hampered response to αPD-1 monotherapy. Importantly, depletion of TIN using αLy-6G antibody reduced Treg cells and restored αPD-1 response. Conversely, targeting Treg cells by αCD25 or αCTLA-4 antibodies limited peripheral TIN accumulation and elicited response in αPD-1-resistant MMRD tumors, highlighting a crosstalk between TIN and Treg cells. Thus, αPD-1+αCTLA-4 combination overcomes TIN-induced resistance to αPD-1 in mice bearing MMRD tumors. Finally, in a cohort of human (high microsatellite instability)/MMRD tumors we revealed that early on-treatment change in the NLR ratio may predict resistance to αPD-1 therapy.ConclusionsTIN countered αPD-1 efficacy in MMRD tumors. Since αCTLA-4 could restrict TIN accumulation, αPD-1+αCTLA-4 combination overcomes αPD-1 resistance in hosts with hypermutated MMRD tumors displaying abnormal neutrophil accumulation.

DNA lesions in S phase threaten genome stability. The DNA damage tolerance (DDT) pathways overcome these obstacles and allow completion of DNA synthesis by the use of specialised translesion (TLS) DNA polymerases or through recombination-related processes. However, how these mechanisms coordinate with each other and with bulk replication remain elusive. To address these issues, we monitored the variation of replication intermediate architecture in response to ultraviolet irradiation using transmission electron microscopy. We show that the TLS polymerase η, able to accurately bypass the major UV lesion and mutated in the skin cancer-prone xeroderma pigmentosum variant (XPV) syndrome, acts at the replication fork to resolve uncoupling and prevent post-replicative gap accumulation. Repriming occurs as a compensatory mechanism when this on-the-fly mechanism cannot operate, and is therefore predominant in XPV cells. Interestingly, our data support a recombination-independent function of RAD51 at the replication fork to sustain repriming. Finally, we provide evidence for the post-replicative commitment of recombination in gap repair and for pioneering observations of in vivo recombination intermediates. Altogether, we propose a chronology of UV damage tolerance in human cells that highlights the key role of polη in shaping this response and ensuring the continuity of DNA synthesis.

Background: DNA damage and genomic instability contribute to non-small cell lung cancer (NSCLC) etiology and progression. However, their therapeutic exploitation is disappointing. CTC-derived eXplants (CDX) offer systems for mechanistic investigation of CTC metastatic potency and biology-driven therapeutic testings. We perform in-depth molecular and functional characterization of CDX models and demonstrate that targeting defects in the DNA damage response (DDR) and genome integrity regulators impedes CTC-driven metastasis in NSCLC. Methods: CTCs were enriched from 30 mL blood samples of 56 advanced NSCLC patients and implanted subcutaneously into Nod/Scid-IL2Rγ-/- (NSG) mice. Tumors were palpable within a median of 108 days. Among the four CDX models established, three CDX-derived cell lines (GR-CDXL1, GR-CDXL3, GR-CDXL4) were obtained. CDX and cell lines were characterized by immunofluorescence (IF), immunohistochemistry and whole-exome sequencing (WES). Chromosomal instability (CIN) and DDR activity were evaluated by IF and western blot. Gene expression was quantified by qRT-PCR. Tumorigenic potential of CDX-derived cell lines was assessed in the chick embryo chorioallantoic membrane and NSG mice engrafted intravenously. IC50 was assessed using CellTiter-Glo®. Results: Four CDX models and three CDX-derived cell lines were established from NSCLC CTCs and recapitulated patient tumor histology (available for three patients) and response to platinum-based chemotherapy. WES analysis showed considerable mutational landscape similarity between the CDX (GR-CDXL1, GR-CDXL2, GR-CDXL3, GR-CDXL4), corresponding patient tumor biopsy and/or single CTCs. Truncal alterations in key DDR and genome integrity-related genes were prevalent across models and assessed as therapeutic targets in vitro, in ovo and in vivo. GR-CDXL1 presented homologous recombination deficiency linked to bi-allelic BRCA2 mutation, FANCA deletion and unrepaired DNA lesions post-mitosis. GR-CDXL1 cells were sensitive to PARP inhibitor (PARPi) olaparib, despite chemoresistance, which challenges the current clinical hypothesis claiming that chemosensitive NSCLC patients should respond to PARPi. Targeting CIN through centrosome clustering inhibition in GR-CDXL3 impeded tumor growth in ovo and in vivo. In GR-CDXL4, olaparib sensitivity was dictated by SLFN11 overexpression, which also correlated with increased neuroendocrine marker expression at patient disease progression, suggesting a predictive value of SLFN11 in histological transformation of NSCLC into SCLC. Conclusion: This study unravels distinct DDR profiles as a central mechanism underpinning CTC metastatic potency. Our CDX models provide a robust platform for ex vivo drug testing of DDR-targeted strategies to expand patient categories that may benefit from precision medicine in metastatic NSCLC. Citation Format: Tala Tayoun, Vincent Faugeroux, Marianne Oulhen, Olivier Deas, Judith Michels, Laur Brulle-Soumare, Stefano Cairo, Jean-Yves Scoazec, Virginie Marty, Agathe Aberlenc, David Planchard, Jordi Remon, Santiago Ponce, Benjamin Besse, Patricia Kannouche, Jean-Gabriel Judde, Patrycja Pawlikowska, Françoise Farace. Circulating tumor cell-derived explant models reveal DNA damage response-based therapeutic opportunities in non-small cell lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1951.

Endometrial cancer (EC) harboring heterozygous POLE proofreading inactivating mutations (POLE-exo*) is associated with an increased number of somatic mutations that result in a distinctive anti-tumor immune response. However, the consequences of such POLE mutations in the context of the missing wild-type allele have not yet been described in endometrial tumors. A 72-year-old woman harboring a germline monoallelic frameshift mutation (p.Pro269fsTer26) in POLE was diagnosed with an EC having a somatic heterozygous mutation in the exonuclease domain of POLE (S459F). Targeted gene sequencing revealed an ultramutated phenotype (381 mutations/Mb) in the tumor and a 2-fold excess of mutations on the DNA leading strand. Additionally, we observed a mutational signature similar to the COSMIC signature 10, a higher mutation rate in this tumor than in endometrial tumors with heterozygous POLE-exo*, and an increased number of T lymphocytes. This is the first report of an ultramutated EC harboring a somatic POLE-exo* mutation in association with a germline loss-of-function mutation in this gene. The absence of a wild type POLE allele led to a particularly high mutational burden.

Background: CDX models have emerged as tractable systems to explore mechanisms involved in metastatic progression and tumor-initiating properties of CTCs. However, their development is challenging due to low prevalence of CTCs in patient blood. We report the genomic and functional characterization of 4 NSCLC CDX models and their derived cell lines. We focus on genome and chromosomal instability (CIN) mechanisms operating in the CDX-derived cell lines and decipher relevant therapeutic targets. Methods: CTCs were enriched from 30mL blood samples of 58 advanced NSCLC patients and implanted subcutaneously into Nod/Scid-IL2Rγ-/- (NSG) mice. Tumors were palpable within a median of 108 days. Among the 4 CDX models established, 3 CDX-derived cell lines (GR-CDXL1, GR-CDXL3 and GR-CDXL4) were obtained. Cell lines were characterized by immunofluorescence (IF), immunohistochemistry and whole-exome sequencing (WES). CIN and DNA damage response (DDR) activity were evaluated using IF and western blot. Tumorigenic potential of CDX-derived cell lines was assessed in the chick embryo chorioallantoic membrane (CAM) and NSG mice engrafted intravenously. IC50 was assessed using CellTiter-Glo®. Results: All CDX models had an epithelial phenotype. CDX and cell lines recapitulated the corresponding tumor histological features (available for 3 patients). WES revealed multiple copy number alterations (CNAs) in driver genes implicated in diverse genome instability mechanisms. GR-CDXL1 presented BRCA2 mutation and FANCA promoter deletion, while AKT gain was detected in GR-CDXL3 in addition to whole genome doubling. This led us to investigate the DDR in the cell lines. Homologous recombination deficiency and unrepaired DNA damage post-mitosis were observed in GR-CDXL1. GR-CDXL3 presented numerical CIN and centrosome clustering. GR-CDXL4 showed high levels of DNA damage and CIN. CNA and functional analysis provided a biological rationale for the selection of drug candidates in pharmacological testings on the CDX-derived cell lines. The assays mirrored patients' response to chemotherapy and revealed olaparib efficiency in GR-CDXL1 and GR-CDXL4 cells, which was validated in the CAM. GR-CDXL3 cells were sensitive to phosphoinositide 3-kinase-α inhibitor alpelisib, suggesting tumor dependence on PI3K/AKT pathway. Cell lines were tumorigenic in the CAM and mice. Notably, GR-CDXL3 seeded multiple metastases, which is concordant with its distinguished CIN characteristics and WGD. In ovo and in vivo validation of candidate therapeutic strategies is ongoing and will be presented. Conclusion: This study reveals distinct genome and CIN patterns operating in our CDX-derived cell lines that may play a critical role in CTC seeding capacity. Our CDX models offer new tools for designing therapeutic strategies targeting metastatic progression in NSCLC. Citation Format: Tala Tayoun, Vincent Faugeroux, Marianne Oulhen, Emma Pailler, Olivier Deas, Laura Mezquita, Laura Brulle-Soumare, Stephano Cairo, Jean-Yves Scoazec, Virginie Marty, Agathe Aberlenc, Maud NgoCamus, Claudio Nicotra, David Planchard, Patricia Kannouche, Benjamin Besse, Jean-Gabriel Judde, Patrycja Pawlikowska, Françoise Farace. Patterns and dynamics of genome instability drive metastatic activity in non-small cell lung cancer (NSCLC) circulating tumor cell (CTC)-derived xenograft (CDX) models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 595.

There is a Blood Commentary on this article in this issue.

In the last few years, immunotherapy has revolutionized the oncology landscape by targeting the host immune system. Blocking immune checkpoints such as cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), programmed cell death-1 (PD-1) and its ligand (PD-L1 or B7-H1), has proven its efficacy in several solid cancers. Recently, several clinical studies have demonstrated a significant improvement in clinical response to the anti-PD-1-based immunotherapy in a subset of patients with microsatellite instability-high (MSI-H)/mismatch repair (MMR)-deficient tumors that accumulate short insertion/deletion mutations notably in coding microsatellites regions of the genome. Thus, the responsiveness of MSI cancers to immune checkpoint inhibitors can be explained by the increased rate of putative frameshift peptide neoantigens and the immunogenic tumor microenvironment. However, not all MSI tumors respond to immunotherapy. The current review will summarize how and why MMR deficiency has emerged as an important predictor of sensitivity for immunotherapy-based strategies. We will also discuss tumor-cell intrinsic genetic and immune-related features of MSI tumors that can modulate immune checkpoint blockade response and explain primary and/or acquired resistance to anti-PD-1 therapy. Finally, we will also discuss about emerging scores which can define more precisely the immune context of the tumor microenvironment and thus better evaluate prognosis and predict response to Immune Checkpoint Blockade.

Immune checkpoint inhibitors have demonstrated unprecedented clinical activity in a wide range of cancers. Significant therapeutic responses have recently been observed in patients presenting mismatch repair-deficient (MMRD) tumours. MMRD cancers exhibit a remarkably high rate of mutations, which can result in the formation of neoantigens, hypothesised to enhance the antitumour immune response. In addition to MMRD tumours, cancers mutated in the exonuclease domain of the catalytic subunit of the DNA polymerase epsilon (POLE) also exhibit an ultramutated genome and are thus likely to benefit from immunotherapy. In this review, we provide an overview of recent data on hypermutated tumours, including MMRD and POLE-mutated cancers, with a focus on their distinctive clinicopathological and molecular characteristics as well as their immune environment. We also discuss the emergence of immune therapy to treat these hypermutated cancers, and we comment on the recent Food and Drug Administration approval of an immune checkpoint inhibitor, the programmed cell death 1 antibody (pembrolizumab, Keytruda), for the treatment of patients with metastatic MMRD cancers regardless of the tumour type. This breakthrough represents a turning point in the management of these hypermutated tumours and paves the way for broader strategies in immunoprecision medicine.

Xeroderma pigmentosum variant (XP-V) is a rare genetic disease, characterized by some sunlight sensitivity and predisposition to cutaneous malignancies. We described clinical and genetic features of the largest collection ever published of 23 XPV patients (ages between 21 and 86) from 20 unrelated families. Primary fibroblasts from patients showed normal nucleotide excision repair but UV-hypersensitivity in the presence of caffeine, a signature of the XP-V syndrome. 87% of patients developed skin tumors with a median age of 21 for the first occurrence. The median numbers of basal-cell carcinoma was 13 per patient, six for squamous-cell carcinoma, and five for melanoma. XP-V is due to defects in the translesion-synthesis DNA polymerase Polη coded by the POLH gene. DNA sequencing of POLH revealed 29 mutations, where 12 have not been previously identified, leading to truncated polymerases in 69% of patients. Four missense mutations are correlated with the protein stability by structural modeling of the Polη polymerase domain. There is a clear relationship between the types of missense mutations and clinical severity. For truncating mutations, which lead to an absence of or to inactive proteins, the life-cumulated UV exposure is probably the best predictor of cancer incidence, reinforcing the necessity to protect XP-Vs from sun exposure.

During replication, DNA damage can challenge replication fork progression and cell viability. Homologous Recombination (HR) and Translesion Synthesis (TLS) pathways appear as major players involved in the resumption and completion of DNA replication. How both pathways are coordinated in human cells to maintain genome stability is unclear. Numerous helicases are involved in HR regulation. Among them, the helicase FBH1 accumulates at sites of DNA damage and potentially constrains HR via its anti-recombinase activity. However, little is known about its regulation in vivo. Here, we report a mechanism that controls the degradation of FBH1 after DNA damage. Firstly, we found that the sliding clamp Proliferating Cell Nuclear Antigen (PCNA) is critical for FBH1 recruitment to replication factories or DNA damage sites. We then showed the anti-recombinase activity of FBH1 is partially dependent on its interaction with PCNA. Intriguingly, after its re-localization, FBH1 is targeted for degradation by the Cullin-ring ligase 4-Cdt2 (CRL4(Cdt2))-PCNA pathway via a PCNA-interacting peptide (PIP) degron. Importantly, expression of non-degradable FBH1 mutant impairs the recruitment of the TLS polymerase eta to chromatin in UV-irradiated cells. Thus, we propose that after DNA damage, FBH1 might be required to restrict HR and then degraded by the Cdt2-proteasome pathway to facilitate TLS pathway.

After exposure to DNA-damaging agents that block the progress of the replication fork, monoubiquitination of proliferating cell nuclear antigen (PCNA) mediates the switch from replicative to translesion synthesis DNA polymerases. We show that in human cells, PCNA is monoubiquitinated in response to methyl methanesulfonate and mitomycin C, as well as UV light, albeit with different kinetics, but not in response to bleomycin or camptothecin. Cyclobutane pyrimidine dimers are responsible for most of the PCNA ubiquitination events after UV-irradiation. Failure to ubiquitinate PCNA results in substantial sensitivity to UV and methyl methanesulfonate, but not to camptothecin or bleomycin. PCNA ubiquitination depends on Replication Protein A (RPA), but is independent of ATR-mediated checkpoint activation. After UV-irradiation, there is a temporal correlation between the disappearance of the deubiquitinating enzyme USP1 and the presence of PCNA ubiquitination, but this correlation was not found after chemical mutagen treatment. By using cells expressing photolyases, we are able to remove the UV lesions, and we show that PCNA ubiquitination persists for many hours after the damage has been removed. We present a model of translesion synthesis behind the replication fork to explain the persistence of ubiquitinated PCNA.

Translesion polymerase eta (polη) was characterized for its ability to replicate ultraviolet-induced DNA lesions that stall replicative polymerases, a process promoted by Rad18-dependent PCNA mono-ubiquitination. Recent findings have shown that polη also acts at intrinsically difficult to replicate sequences. However, the molecular mechanisms that regulate its access to these loci remain elusive. Here, we uncover that polη travels with replication forks during unchallenged S phase and this requires its SUMOylation on K163. Abrogation of polη SUMOylation results in replication defects in response to mild replication stress, leading to chromosome fragments in mitosis and damage transmission to daughter cells. Rad18 plays a pivotal role, independently of its ubiquitin ligase activity, acting as a molecular bridge between polη and the PIAS1 SUMO ligase to promote polη SUMOylation. Our results provide the first evidence that SUMOylation represents a new way to target polη to replication forks, independent of the Rad18-mediated PCNA ubiquitination, thereby preventing under-replicated DNA., Translesion synthesis polymerase eta has a well characterized role in replicating past UV-induced DNA lesions and has recently been shown to act at difficult to replicate sequences. Here the authors show that its SUMOylation is required to recruit pol eta at the replication fork and to prevent under-replicated DNA.

The common techniques to study protein-protein proximity in vivo are not well adapted to the capabilities and the expertise of a standard proteomics laboratory, typically based on the use of mass spectrometry. With the aim of closing this gap, we have developed PUB-MS (for proximity utilizing biotinylation and mass spectrometry), an approach to monitor protein-protein proximity, based on biotinylation of a protein fused to a biotin-acceptor peptide (BAP) by a biotin-ligase, BirA, fused to its interaction partner. The biotinylation status of the BAP can be further detected by either Western analysis or mass spectrometry. The BAP sequence was redesigned for easy monitoring of the biotinylation status by LC-MS/MS. In several experimental models, we demonstrate that the biotinylation in vivo is specifically enhanced when the BAP- and BirA-fused proteins are in proximity to each other. The advantage of mass spectrometry is demonstrated by using BAPs with different sequences in a single experiment (allowing multiplex analysis) and by the use of stable isotopes. Finally, we show that our methodology can be also used to study a specific subfraction of a protein of interest that was in proximity with another protein at a predefined time before the analysis.

Replicative DNA polymerases duplicate genomes in a very efficient and accurate mode. However their progression can be blocked by DNA lesions since they are unable to accommodate bulky damaged bases in their active site. In response to replication blockage, monoubiquitination of PCNA promotes the switch between replicative and specialized polymerases proficient to overcome the obstacle. In this study, we characterize novel connections between proteins involved in replication and TransLesion Synthesis (TLS). We demonstrate that PDIP38 (Poldelta interacting protein of 38kDa) directly interacts with the TLS polymerase Poleta. Interestingly, the region of Poleta interacting with PDIP38 is found to be located within the ubiquitin-binding zinc finger domain (UBZ) of Poleta. We show that the depletion of PDIP38 increases the number of cells with Poleta foci in the absence of DNA damage and diminishes cell survival after UV irradiation. In addition, PDIP38 is able to interact directly not only with Poleta but also with the specialized polymerases Rev1 and Polzeta (via Rev7). We thus suggest that PDIP38 serves as a mediator protein helping TLS Pols to transiently replace replicative polymerases at damaged sites.

Background: CDX models are expected to provide crucial information on mechanisms involved in metastatic progression, tumor-initiating properties of CTCs and the development of drug resistance. However, excepted for small-cell lung cancer (SCLC), CDX models are very difficult to develop. Here, we report the establishment, phenotypic and molecular characterization of four NSCLC CDX models and three in vitro cell lines derived from these CDXs. Methods: CTCs were enriched by RosetteSep from 30 ml blood samples and implanted subcutaneously into Nod/Scid-IL2Rγ-/- (NSG) mice. CDXs were phenotypically and genetically characterized by immunofluorescence, immunohistochemistry and whole-exome sequencing (WES). CDX-derived cell lines were established after mouse fibroblast depletion using classical culture medium. Standard conditions were used for IC50 determination. Results: Between January 2014 and June 2017, CTCs from 58 NSCLC patients with advanced metastatic disease were implanted into NSG mice resulting in the establishment of four CDXs. All had an epithelial phenotype. Based on CellSearch® counts, median and mean numbers of engrafted CTCs were 9 and 693 respectively (range, 0-17,694). GR-CDXL1, GR-CDXL2, GR-CDXL3, GR-CDXL4 were established starting from 3500, 35, 330, and 1102 CTCs respectively. Measurable tumors were obtained between 100 and 200 days after CTC implantation and were maintained by successive transplantations in NSG mice. Three in-vitro cell lines were established from GR-CDXL1, GR-CDXL3 and GR-CDXL4 tumors, and expressed an epithelial phenotype and CSC-markers such as ALDH, CD133 and CD90. Immunohistochemistry with epithelial and neuroendocrine markers, TTF1 and Ki67 indicated that CDXs and CDX-derived cell lines were representative of the corresponding patient tumor specimens (available in three patients). WES indicated 86%, 93%, 82% mutational similarity between GR-CDXL2, GR-CDXL3 and GR-CDXL4 and the corresponding tumor biopsies. The mutational similarity of GR-CDXL1, GR-CDXL3 and GR-CDXL4 and their corresponding in vitro cell lines was 24%, 83% and 84% respectively. WES of individual CTCs isolated at the time of CTC implantation is ongoing. In in vitro cytotoxicity assays, CDX-derived cell lines mirrored the patient's responsiveness to cisplatin and paclitaxel chemotherapy. The results of ongoing in vivo drug efficacy assays and of mutational tree analyses reconstructing the phylogenic evolution of tumor biopsies, CTCs, CDX and cell lines will be presented. Conclusion: This study revealed considerable similarities between CDXs and their corresponding patient tumor biopsies. These NSCLC CDX models represent unique tools to identify clonal mutations associated with the tumor-initiating capacity of CTCs and explore the genetic and phenotypic basis of metastasis and drug resistance associated with advanced NSCLC. Citation Format: Vincent Faugeroux, Emma Pailler, Olivier Deas, Judith Michels, Laura Mezquita, Laura Brulle-Soumare, Stefano Cairo, Jean-Yves Scoazec, Virginie Marty, Pauline Queffelec, Maud Ngo-Camus, Claudio Nicotra, David Planchard, Patricia Kannouche, Benjamin Besse, Jean-Gabriel Judde, Françoise Farace. Development and characterization of novel non-small cell lung cancer (NSCLC) circulating tumor cells (CTCs)-derived xenograft (CDX) models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2597.

Human DNA polymerase iota (poliota) is a unique member of the Y-family of specialised polymerases that displays a 5'deoxyribose phosphate (dRP) lyase activity. Although poliota is well conserved in higher eukaryotes, its role in mammalian cells remains unclear. To investigate the biological importance of poliota in human cells, we generated fibroblasts stably downregulating poliota (MRC5-pol iota(KD)) and examined their response to several types of DNA-damaging agents. We show that cell lines downregulating poliota exhibit hypersensitivity to DNA damage induced by hydrogen peroxide (H(2)O(2)) or menadione but not to ethylmethane sulphonate (EMS), UVC or UVA. Interestingly, extracts from cells downregulating poliota show reduced base excision repair (BER) activity. In addition, poliota binds to chromatin after treatment of cells with H(2)O(2) and interacts with the BER factor XRCC1. Finally, green fluorescent protein-tagged poliota accumulates at the sites of oxidative DNA damage in living cells. This recruitment is partially mediated by its dRP lyase domain and ubiquitin-binding domains. These data reveal a novel role of human poliota in protecting cells from oxidative damage.

Xeroderma pigmentosum variant (XP-V) is a rare genetic disease, characterized by sunlight sensitivity and predisposition to cutaneous malignancies. XP-V is caused by a deficiency in DNA polymerase eta (Polη) that plays a pivotal role in translesion synthesis by bypassing UV-induced pyrimidine dimers. Previously we identified a new Polη variant containing two missense mutations, one mutation within the bipartite NLS (T692A) and a second mutation on the stop codon (X714W) leading to a longer protein with an extra 8 amino acids (721 instead of 713 AA). First biochemical analysis revealed that this Polη missense variant was barely detectable by western blot. As this mutant is extremely unstable and is nearly undetectable, a definitive measure of its functional deficit in cells has not been explored. Here we report the molecular and cellular characterization of this missense variant. In cell free extracts, the extra 8 amino acids in the C-terminal of Polη(721) only slightly reduce the bypass efficiency through CPD lesions. In vivo, Polη(721) accumulates in replication factories and interacts with mUb-PCNA albeit at lower level than Polη(wt). XP-V cells overexpressing Polη(721) were only slightly UV-sensitive. Altogether, our data strongly suggest that Polη(721) is functional and that the patient displays a XP-V phenotype because the mutant protein is excessively unstable. We then investigated the molecular mechanisms involved in this excessive proteolysis. We showed that Polη(721) is degraded by the proteasome in an ubiquitin-dependent manner and that this proteolysis is independent of the E3 ligases, CRL4(cdt2) and Pirh2, reported to promote Polη degradation. We then demonstrated that the extra 8 amino acids of Polη(721) do not act as a degron but rather induce a conformational change of the Polη C-terminus exposing its bipartite NLS as well as a sequence close to its UBZ to the ubiquitin/proteasome system. Interestingly we showed that the clinically approved proteasome inhibitor, Bortezomib restores the levels of Polη(721) suggesting that this might be a therapeutic approach to preventing tumor development in certain XP-V patients harboring missense mutations.

International audience; The SLX4 Fanconi anemia protein is a tumor suppres- sor that may act as a key regulator that engages the cell into specific genome maintenance pathways. Here, we show that the SLX4 complex is a SUMO E3 ligase that SUMOylates SLX4 itself and the XPF sub- unit of the DNA repair/recombination XPF-ERCC1 endonuclease. This SLX4-dependent activity is medi- ated by a remarkably specific interaction between SLX4 and the SUMO-charged E2 conjugating enzyme UBC9 and relies not only on newly identified SUMO- interacting motifs (SIMs) in SLX4 but also on its BTB domain. In contrast to its ubiquitin-binding UBZ4 motifs, SLX4 SIMs are dispensable for its DNA inter- strand crosslink repair functions. Instead, while detri- mental in response to global replication stress, the SUMO E3 ligase activity of the SLX4 complex is crit- ical to prevent mitotic catastrophe following common fragile site expression.

Xeroderma pigmentosum (XP) is an autosomal recessive photosensitive disorder with an extremely high incidence of skin cancers. Seven complementation groups, corresponding to seven proteins involved in nucleotide excision repair (NER), are associated with this syndrome. However, in XP variant patients, the disorder is caused by defects in DNA polymerase eta; this error prone polymerase, encoded by POLH, is involved in translesion DNA synthesis (TLS) on DNA templates damaged by ultraviolet light (UV). We constructed a recombinant adenovirus carrying the human POLH cDNA linked to the EGFP reporter gene (AdXPV-EGFP) and infected skin fibroblasts from both XPV and XPA patients. Twenty-four hours after infection, the DNA polymerase eta-EGFP fusion protein was detected by Western blot analysis, demonstrating successful transduction by the adenoviral vector. Protein expression was accompanied by reduction in the high sensitivity of XPV cells to UV, as determined by cell survival and apoptosis-induction assays. Moreover, the pronounced UV-induced inhibition of DNA synthesis in XPV cells and their arrest in S phase were attenuated in AdXPV-EGFP infected cells, confirming that the transduced polymerase was functional. However, over-expression of polymerase eta mediated by AdXPV-EGFP infection did not result in enhancement of cell survival, prevention of apoptosis, or higher rate of nascent DNA strand growth in irradiated XPA cells. These results suggest that TLS by DNA polymerase eta is not a limiting factor for recovery from cellular responses induced by UV in excision-repair deficient fibroblasts.

Translesion synthesis (TLS) is the major pathway by which mammalian cells replicate across DNA lesions. Upon DNA damage, ubiquitination of proliferating cell nuclear antigen (PCNA) induces bypass of the lesion by directing the replication machinery into the TLS pathway. Yet, how this modification is recognized and interpreted in the cell remains unclear. Here we describe the identification of two ubiquitin (Ub)–binding domains (UBM and UBZ), which are evolutionarily conserved in all Y-family TLS polymerases (pols). These domains are required for binding of polη and polι to ubiquitin, their accumulation in replication factories, and their interaction with monoubiquitinated PCNA. Moreover, the UBZ domain of polη is essential to efficiently restore a normal response to ultraviolet irradiation in xeroderma pigmentosum variant (XP-V) fibroblasts. Our results indicate that Ub-binding domains of Y-family polymerases play crucial regulatory roles in TLS.

Human papillomaviruses (HPV) are causative agents in a variety of human diseases; for example over 99% of cervical carcinomas contain HPV DNA sequences. Often in cervical carcinoma the HPV genome is integrated into the host genome resulting in unregulated expression of the viral transforming proteins E6 and E7. Therefore viral integration is a step toward HPV-induced carcinogenesis. Integration of the HPV genome could occur following double-strand DNA breaks that could arise during viral DNA replication. We investigated the fidelity of HPV 16 E1- and E2-mediated DNA replication of non-damaged and UVC-damaged templates in a variety of cell lines with different genetic backgrounds; C33a (derived from an HPV-negative cervical carcinoma), XP30RO (deficient in the by-pass polymerase eta (poleta)), XP30eta (expressing a restored wild-type poleta), XP12RO (nucleotide excision repair defective), and MRC5 (derived from a 14-week-old human fetus). The results demonstrate that the fidelity of E1- and E2-mediated DNA replication is reflective of the genetic background in which the assays are carried out. For example, restoring poleta to the XP30 cell line results in a 3-fold drop in the number of mutants obtained following replication of a UVC-damaged template. A relatively high percentage of the mutant-replicated molecules arise as a result of genetic rearrangement. This is the first time such studies have been carried out with an HPV replication system, and the results are discussed in the context of the HPV life cycle and what is known about HPV genomes in human cancers.

UVC or ionizing radiation of mammalian cells elicits a complex genetic response that allows recovery and cell survival. Kin17 gene, which is highly conserved among mammals, is upregulated during this response. Kin17 gene encodes a 45 kDa protein which binds to DNA and presents a limited similarity with a functional domain of the bacterial RecA protein. Kin17 protein is accumulated in the nucleus of proliferating fibroblasts and forms intranuclear foci. Using expression vectors, we show that overexpression of kin17 protein inhibits cell-cycle progression into S phase. Our results indicate that growth inhibition correlates with disruption of the nuclear morphology which seems to modify the intranuclear network required during the early steps of DNA replication. We report that a mutant encoding a protein deleted from the central domain of kin17 protein enhanced these effects whereas the deletion of the C-terminal domain considerably reduced them. These mutants will be used to elucidate the molecular mechanism by which kin17 protein alters cell growth and DNA replication.

UV-irradiation induces, in mammalian cells, the expressionof a set of genes known as the ‘UV-response’, which maybe reminiscent of the bacterial response, called SOS system.The multifunctional protein RecA controls the expressionof the SOS genes. We report the expression profile of amouse gene conserved among mammals, called Kin17, thatcodes a DNA-binding protein of undetermined biochemicalactivity and which shares epitopes with the bacterial RecAprotein. We demonstrate that the level of Kin17 RNA was5-fold higher in mid-S phase of serum-stimulated BALB/c3T3 fibroblasts than in quiescent cells. Cells in S-phasedisplayed a high level of kin17 protein with a markednuclear localisation. The maximal level of Kin17 RNA wasobserved 18 h after serum stimulation, indicating thatKin17 gene is a new member of the late growth-relatedgenes. The accumulation of kin17 protein during cellproliferation follows the increase in Kin17 RNA and correl-ates with DNA synthesis, which suggests a possible role ofkin17 protein in a transaction related to DNA-replication.In quiescent fibroblasts, a 3-fold increase inKin17 RNAwas seen 13 h after UV irradiation. In parallel, kin17protein accumulated in the nucleus, which suggests thatit might be required after the stress produced by UVirradiation.IntroductionExposure of mammalian cells to UV radiation leads to theformation of DNA-lesions and to structural changes thatperturb DNA metabolism. The constitutive expression of genesinvolved in the nucleotide excision repair system (NER*)allows the efficient removal of the great majority of the UV-induced lesions (1). In addition to the NER system, mammaliancells react to UV-irradiation by inducing the ‘UV-response’,which involves the activation of multiple and complex signaltransduction pathways. The UV-activation of nuclear transcrip-tion factors such as AP-1 and p53 leads to the induction of

International audience; We have developed an approach termed PUB-NChIP (proximity utilizing biotinylation with native ChIP) to purify and study the protein composition of chromatin in proximity to a nuclear protein of interest. It is based on coexpression of (1) a protein of interest, fused with the bacterial biotin ligase BirA, together with (2) a histone fused to a biotin acceptor peptide (BAP), which is specifically biotinylated by BirA-fusion in the proximity of the protein of interest. Using the RAD18 protein as a model, we demonstrate that the RAD18-proximal chromatin is enriched in some H4 acetylated species. Moreover, the RAD18-proximal chromatin containing a replacement histone H2AZ has a different pattern of H4 acetylation. Finally, biotin pulse-chase experiments show that the H4 acetylation pattern starts to resemble the acetylation pattern of total H4 after the proximity of chromatin to RAD18 has been lost.

Replicative DNA polymerases are blocked by damage in the template DNA. To get past this damage, the cell employs specialized translesion synthesis (TLS) polymerases, which have reduced stringency and are able to bypass different lesions. For example, DNA polymerase eta (poleta) is able to carry out TLS past UV-induced cyclobutane pyrimidine dimers. How does the cell bring about the switch from replicative to TLS polymerase? We have shown that, in human cells, when the replication machinery is blocked at DNA damage, PCNA, the sliding clamp required for DNA replication, is mono-ubiquitinated and that this modified form of PCNA has increased affinity for poleta. This provides a mechanism for the polymerase switch. In this Extra-View, we discuss the possible signals that might trigger ubiquitination of PCNA, whether PCNA becomes de-ubiquitinated after TLS has been accomplished and the role of the hREV1 protein in TLS. We point out some apparent differences between mechanisms in Saccharomyces cerevisiae and human cells.

Low numbers of circulating tumor cells (CTCs) have so far limited the establishment of CTC-derived xenografts (CDXs) to improve our understanding of tumor progression, drug resistance mechanisms, and their biological properties. We report the establishment and the phenotypic and molecular characterization of one NSCLC CDX. Blood samples (30 ml) were drawn from 49 NSCLC patients with advanced metastatic disease. CTCs were enriched by RosetteSep, embedded in matrigel, and implanted in the interscapular aspect of NSG mouse (Nod/Scid-IL2Rγ-/-). Mice were followed-up for one year according to ethical regulations. CDX tumours and CDX derived cell lines were phenotypically and molecularly characterized by immunofluorescence, immunohistochemistry, CGHarray, exome sequencing and transcriptome gene expression. CTCs from one NSCLC patient with 750 CTCs detected by CellSearch gave rise to a tumor 5 months after initial murine injection. Histological analysis confirmed the human origin of the tumor and the presence of a poorly differentiated adenocarcinoma consistent with the patient's biopsy. Tumor was positive for EpCAM, EMA, CK8;18, and Ki67, and negative for vimentin. A fraction of cells (25%) from freshly dissociated tumors exhibited ALDH activity. CGH from CDX tumors at passage 1 and 2 shows multiple gene rearrangement, revealing a high degree of genomic instability. Transcriptome analysis of ALDH positive and negative cells is ongoing and should help of identifying a cancer stem cell gene expression signature. Whole-exome sequencing of CDX tumor is ongoing and will be compared to data obtained from single CTCs from the patient. A cell line established in vitro from the CDX model grows in 3D clusters and is tumorigenic in mice. Interestingly, this cell line is positive for cytokeratins, EpCAM, E-cadherin, N-cadherin, vimentin, and expresses multiple cancer stem cell markers including CD166, CD24, CD133 and, ALDH activity. The cell line is hypotetraploid (about 70 chromosomes) and its CGH profile was similar to that of the CDX tumour, revealing a high level of genome instability. By investigating DNA replication process in this cell line, we found that it exhibits a spontaneous enhanced DNA damage signaling associated to an accumulation of DNA double strand breaks mainly in S phase strongly suggesting that the CDX-derived cell line displays hallmarks on replication stress that could explain, at least partially, the genomic instability in the cells. We report a low success rate in the establishment of NSCLC CDX (2%). However one NSCLC CDX model harboring cancer stem cell properties and deficiency of DNA replication maintenance was established. Ongoing work to identify a cancer stem cell signature and characteristics replication stress markers in this CDX model will be presented. This NSCLC CDX model will be useful to test drugs targeting these alterations in vivo and improve our knowledge of drug resistance. Citation Format: Vincent Faugeroux, Olivier Deas, Judith Michels, Jean Gabriel Judde, Stefano Cairo, Philippe Vielh, Virginie Marty, Fanny Billiot, Maud Ngocamus, Benjamin Besse, Patricia Kannouche, Françoise Farace. Establishment and characterization of circulating tumor cell-derived xenografts in non-small cell lung cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2256.

During translesion synthesis (TLS), low-fidelity polymerases of the Y-family polymerases bypass DNA damages that block the progression of conventional processive DNA polymerases, thereby allowing the completion of DNA replication. Among the TLS polymerases, DNA polymerase eta (polη) performs nucleotide incorporation past ultraviolet (UV) photoproducts and is deficient in cancer-prone xeroderma pigmentosum variant (XPV) syndrome. Upon UV irradiation, the DNA sliding clamp PCNA is monoubiquitylated on its conserved Lys-164. This event is considered to facilitate the TLS process in vivo since polη preferentially interacts with monoubiquitylated PCNA through its ubiquitin-binding domain (UBZ) as well as its PCNA interacting peptide (PIP)-box. However, recent observations questioned this model. Therefore, in this study, we re-examined the relative contribution of the regulatory UBZ and PIP domains of polη in response to UVC. We show that simultaneous invalidation of both motifs confers sensitivity to UVC, sensitization by low concentrations of caffeine, prolonged inhibition of DNA synthesis and persistent S phase checkpoint activation, all characteristic features of XPV cells. While each domain is essential for efficient accumulation of polη in replication factories, mutational inactivation of UBZ or PIP motif only confers a slight sensitivity to UVC indicating that, although informative, polη focus analysis is not a reliable tool to assess the polη's ability to function in TLS in vivo. Taken together, these data indicate that PIP and UBZ motifs are not required for recruitment but for retention of polη at sites of stalled replication forks. We propose that this is a way to ensure that a sufficient amount of the protein is available for its bypass function. Environ. Mol. Mutagen., 2012. © 2012 Wiley Periodicals, Inc.

Posttranslational modification of PCNA by ubiquitin plays an important role in coordinating the processes of DNA damage tolerance during DNA replication. The monoubiquitination of PCNA was shown to facilitate the switch between the replicative DNA polymerase with the low-fidelity polymerase eta (η) to bypass UV-induced DNA lesions during replication. Here, we show that in response to oxidative stress, PCNA becomes transiently monoubiquitinated in an S phase- and USP1-independent manner. Moreover, Polη interacts with mUb-PCNA at sites of oxidative DNA damage via its PCNA-binding and ubiquitin-binding motifs. Strikingly, while functional base excision repair is not required for this modification of PCNA or Polη recruitment to chromatin, the presence of hMsh2-hMsh6 is indispensable. Our findings highlight an alternative pathway in response to oxidative DNA damage that may coordinate the removal of oxidatively induced clustered DNA lesions and could explain the high levels of oxidized DNA lesions in MSH2-deficient cells.

DNA polymerase eta (poleta) performs translesion synthesis past ultraviolet (UV) photoproducts and is deficient in cancer-prone xeroderma pigmentosum variant (XP-V) syndrome. The slight sensitivity of XP-V cells to UV is dramatically enhanced by low concentrations of caffeine. So far, the biological explanation for this feature remains elusive. Using DNA combing, we showed that translesion synthesis defect leads to a strong reduction in the number of active replication forks and a high proportion of stalled forks in human cells, which contrasts with budding yeast. Moreover, extensive regions of single-strand DNA are formed during replication in irradiated XP-V cells, leading to an over-activation of ATR/Chk1 pathway after low UVC doses. Addition of a low concentration of caffeine post-irradiation, although inefficient to restore S-phase progression, significantly decreases Chk1 activation and abrogates DNA synthesis in XP-V cells. While inhibition of Chk1 activity by UCN-01 prevents UVC-induced S-phase delay in wild-type cells, it aggravates replication defect in XP-V cells by increasing fork stalling. Consequently, UCN-01 sensitizes XP-V cells to UVC as caffeine does. Our findings indicate that poleta acts at stalled forks to resume their progression, preventing the requirement for efficient replication checkpoint after low UVC doses. In the absence of poleta, Chk1 kinase becomes essential for replication resumption by alternative pathways, via fork stabilization.

Faithful genome transmission requires a network of pathways coordinating DNA replication to DNA repair and recombination. Here, we used molecular combing to measure the impact of homologous recombination (HR) on the velocity of DNA replication forks. We used three hamster cell lines defective in HR either by overexpression of a RAD51 dominant-negative form, or by a defect in the RAD51 paralogue XRCC2 or the breast tumor suppressor BRCA2. Irrespectively of the type or extent of HR alteration, all three cell lines exhibited a similar reduction in the rate of replication-fork progression, associated with an increase in the density of replication forks. Importantly, this phenotype was completely reversed in complemented derivatives of Xrcc2 and Brca2 mutants. These data reveal a novel role for HR, different from the reactivation of stalled replication forks, which may play an important role in genome stability and thus in tumor protection.

Replicative DNA polymerases are blocked at DNA lesions. Synthesis past DNA damage requires the replacement of the replicative polymerase by one of a group of specialised translesion synthesis (TLS) polymerases, most of which belong to the Y-family. Each of these has different substrate specificities for different types of damage. In eukaryotes mono-ubiquitination of PCNA plays a crucial role in the switch from replicative to TLS polymerases at stalled forks. All the Y-family polymerases have ubiquitin binding sites that increase their binding affinity for ubiquitinated PCNA at the sites of stalled forks.

During translesion synthesis past sites of damaged DNA, specialized Y-family polymerases are employed by the cell to replace the high stringency replicative polymerases and synthesize DNA past the damaged site. These polymerases are localized in replication factories during the S phase of the cell cycle. When progress of the replication fork is blocked, the polymerase accessory protein, proliferating cell nuclear antigen (PCNA), becomes ubiquitinated and the monoubiquitinated PCNA has an increased affinity for Y-family DNA polymerase eta (poleta). This chapter describes methods for visualizing the polymerases in replication factories, for analyzing the ubiquitination status of PCNA, and for measuring its interaction with poleta in chromatin extracts.

During translesion synthesis past sites of damaged DNA, specialized Y-family polymerases are employed by the cell to replace the high stringency replicative polymerases and synthesize DNA past the damaged site. These polymerases are localized in replication factories during the S phase of the cell cycle. When progress of the replication fork is blocked, the polymerase accessory protein, proliferating cell nuclear antigen (PCNA), becomes ubiquitinated and the monoubiquitinated PCNA has an increased affinity for Y-family DNA polymerase eta (poleta). This chapter describes methods for visualizing the polymerases in replication factories, for analyzing the ubiquitination status of PCNA, and for measuring its interaction with poleta in chromatin extracts.

DNA polymerases of the Y-family are involved in translesion DNA synthesis past different types of DNA damage. Previous work has shown that DNA polymerases {eta} and {iota} are localised in replication factories during S phase, where they colocalise one-to-one with PCNA. Cells with factories containing these polymerases accumulate after treatment with DNA damaging agents because replication forks are stalled at sites of damage. We now show that DNA polymerase {kappa} (pol{kappa}) has a different localisation pattern. Although, like the other Y-family polymerases, it is exclusively localised in the nucleus, pol{kappa} is found in replication foci in only a small proportion of S-phase cells. It does not colocalise in those foci with proliferating cell nuclear antigen (PCNA) in the majority of cells. This reduced number of cells with pol{kappa} foci, when compared with those containing pol{eta} foci, is observed both in untreated cells and in cells treated with hydroxyurea, UV irradiation or benzo[a]pyrene. The C-terminal 97 amino acids of pol{kappa} are sufficient for this limited localisation into nuclear foci, and include a C2HC zinc finger, bipartite nuclear localisation signal and putative PCNA binding site.

The progress of replicative DNA polymerases along the replication fork may be impeded by the presence of lesions in the genome. One way to circumvent such hurdles involves the recruitment of specialized DNA polymerases that perform limited incorporation of nucleotides in the vicinity of the damaged site. This process entails DNA polymerase switch between replicative and specialized DNA polymerases. Five eukaryotic proteins can carry out translesion synthesis (TLS) of damaged DNA in vitro, DNA polymerases zeta, eta, iota, and kappa, and REV1. To identify novel proteins that interact with hpol eta, we performed a yeast two-hybrid screen. In this paper, we show that hREV1 interacts with hpol eta as well as with hpol kappa and poorly with hpol iota. Furthermore, cellular localization analysis demonstrates that hREV1 is present, with hpol eta in replication factories at stalled replication forks and is tightly associated with nuclear structures. This hREV1 nuclear localization occurs independently of the presence of hpol eta. Taken together, our data suggest a central role for hREV1 as a scaffold that recruits DNA polymerases involved in TLS.

Y-family DNA polymerases are believed to facilitate the replicative bypass of damaged DNA in a process commonly referred to as translesion synthesis. With the exception of DNA polymerase eta (poleta), which is defective in humans with the Xeroderma pigmentosum variant (XP-V) phenotype, little is known about the cellular function(s) of the remaining human Y-family DNA polymerases. We report here that an interaction between human DNA polymerase iota (poliota) and the proliferating cell nuclear antigen (PCNA) stimulates the processivity of poliota in a template-dependent manner in vitro. Mutations in one of the putative PCNA-binding motifs (PIP box) of poliota or the interdomain connector loop of PCNA diminish the binding between poliota and PCNA and concomitantly reduce PCNA-dependent stimulation of poliota activity. Furthermore, although retaining its capacity to interact with poleta in vivo, the poliota-PIP box mutant fails to accumulate in replication foci. Thus, PCNA, acting as both a scaffold and a modulator of the different activities involved in replication, appears to recruit and coordinate replicative and translesion DNA synthesis polymerases to ensure genome integrity.

In humans, inactivation of the DNA polymerase eta gene (pol eta) results in sunlight sensitivity and causes the cancer-prone xeroderma pigmentosum variant syndrome (XP-V). Cells from XP-V individuals have a reduced capacity to replicate UV-damaged DNA and show hypermutability after UV exposure. Biochemical assays have demonstrated the ability of pol eta to bypass cis-syn-cyclobutane thymine dimers, the most common lesion generated in DNA by UV. In most cases, this bypass is error-free. To determine the actual requirement of pol eta in vivo, XP-V cells (XP30RO) were complemented by the wild type pol eta gene. We have used two pol eta-corrected clones to study the in vivo characteristics of mutations produced by DNA polymerases during DNA synthesis of UV-irradiated shuttle vectors transfected into human host cells, which had or had not been exposed previously to UV radiation. The functional complementation of XP-V cells by pol eta reduced the mutation frequencies both at CG and TA base pairs and restored UV mutagenesis to a normal level. UV irradiation of host cells prior to transfection strongly increased the mutation frequency in undamaged vectors and, in addition, especially in the pol eta-deficient XP30RO cells at 5'-TT sites in UV-irradiated plasmids. These results clearly show the protective role of pol eta against UV-induced lesions and the activation by UV of pol eta-independent mutagenic processes.

Y-family DNA polymerases can replicate past a variety of damaged bases in vitro but, with the exception of DNA polymerase eta (poleta), which is defective in xeroderma pigmentosum variants, there is little information on the functions of these polymerases in vivo. Here, we show that DNA polymerase iota (poliota), like poleta, associates with the replication machinery and accumulates at stalled replication forks following DNA-damaging treatment. We show that poleta and poliota foci form with identical kinetics and spatial distributions, suggesting that localization of these two polymerases is tightly co-ordinated within the nucleus. Furthermore, localization of poliota in replication foci is largely dependent on the presence of poleta. Using several different approaches, we demonstrate that poleta and poliota interact with each other physically and that the C-terminal 224 amino acids of poliota are sufficient for both the interaction with poleta and accumulation in replication foci. Our results provide strong evidence that poleta targets poliota to the replication machinery, where it may play a general role in maintaining genome integrity as well as participating in translesion DNA synthesis.

DNA polymerase η carries out translesion synthesis past UV photoproducts and is deficient in xeroderma pigmentosum (XP) variants. We report that polη is mostly localized uniformly in the nucleus but is associated with replication foci during S phase. Following treatment of cells with UV irradiation or carcinogens, it accumulates at replication foci stalled at DNA damage. The C-terminal third of polη is not required for polymerase activity. However, the C-terminal 70 aa are needed for nuclear localization and a further 50 aa for relocalization into foci. Polη truncations lacking these domains fail to correct the defects in XP-variant cells. Furthermore, we have identified mutations in two XP variant patients that leave the polymerase motifs intact but cause loss of the localization domains.

Complete and accurate DNA replication is crucial to transfer correct genetic information to the daughter cells. Various obstacles can interfere with the progression of the replication machinery, threatening genome integrity. Specialized error-prone translesion DNA polymerases (TLS polymerases) assist the replicative polymerases to replicate across DNA lesions. During my PhD I characterized the contribution of TLS pol eta (polη), best known for its role in preventing UV-induced mutagenesis, during unperturbed replication. Polη was shown to promote the stability of the common fragile sites and associates with the replisome in unchallenged S phase. However, the kind of replication barriers requiring pol eta and the consequences of its absence on the replication of these regions were unclear. My results show that polη is recruited at a subset of replication forks all along the S phase and that polη defect modifies the replication timing of genomic regions enriched in large transcribed genes, where transcription-replication conflicts (TRCs) are more likely to occur. Overall, I show that polη recruitment at the replication fork is transcription-dependent, and that pol eta plays a role in the coping with TRCs. Altogether, these results highlight a new role for an error-prone DNA polymerase in protecting the genome stability., La réplication complète et fidèle de l’ADN est cruciale pour transmettre l’information génétique de manière correcte aux cellules filles. Divers obstacles peuvent interférer avec la progression de la machinerie de réplication, et donc menacer l’intégrité du génome. Des ADN polymérases spécialisées, dites translésionnelles (polymérases TLS), assistent les ADN polymérases réplicatives pour la poursuite de la réplication malgré ces lésions. Elles peuvent répliquer de manière fidèle ou non ces entraves, mais sont mutagènes sur des séquences d’ADN non-endommagées. Au cours de ma thèse, j’ai pu caractériser davantage la contribution de l’ADN polymérase TLS eta (polη) au cours de la réplication non-perturbée. Cette polymérase permet principalement de prévenir la mutagénèse induite par les UV. Mais il a également été montré qu’elle promeut la stabilité des sites fragiles communs, et est associée au réplisome durant la phase S non-perturbée. Cependant, la nature des obstacles nécessitant polη et les conséquences de son absence pour la réplication de ces régions restaient à déterminer. Mes résultats montrent que polη est recrutée au niveau d’une fraction des fourches de réplication tout au long de la phase S et que l’absence de pol eta conduit à une modification du timing de réplication de régions génomiques riches en grands gènes transcrits, où les conflits entre réplication et transcription sont potentiellement plus fréquents. Plus généralement, je montre que le recrutement de pol eta à la fourche de réplication dépend de la transcription et qu’elle joue un rôle dans la prise en charge des conflits entre réplication et transcription. Ces résultats mettent en évidence un nouveau rôle de protection de la stabilité du génome pour cette ADN polymérase mutagène.