DNA Replication Fork Progression Research Articles

PCNA-binding activity separates RNF168 functions in DNA replication and DNA double-stranded break signaling.

RNF168 orchestrates a ubiquitin-dependent DNA damage response to regulate the recruitment of repair factors, such as 53BP1 to DNA double-strand breaks (DSBs). In addition to its canonical functions in DSB signaling, RNF168 may facilitate DNA replication fork progression. However, the precise role of RNF168 in DNA replication remains unclear. Here, we demonstrate that RNF168 is recruited to DNA replication factories in a manner that is independent of the canonical DSB response pathway regulated by Ataxia-Telangiectasia Mutated (ATM) and RNF8. We identify a degenerate Proliferating Cell Nuclear Antigen (PCNA)-interacting peptide (DPIP) motif in the C-terminus of RNF168, which together with its Motif Interacting with Ubiquitin (MIU) domain mediates binding to mono-ubiquitylatedPCNA at replication factories. An RNF168 mutant harboring inactivating substitutions in its DPIP box and MIU1 domain (termed RNF168 ΔDPIP/ΔMIU1) is not recruited to sites of DNA synthesis and fails to support ongoing DNA replication. Notably, the PCNA interaction-deficient RNF168 ΔDPIP/ΔMIU1 mutant fully rescues the ability of RNF168-/- cells to form 53BP1 foci in response to DNA DSBs. Therefore, RNF168 functions in DNA replication and DSB signaling are fully separable. Our results define a new mechanism by which RNF168 promotes DNA replication independently of its canonical functions in DSB signaling.

HLTF resolves G4s and promotes G4-induced replication fork slowing to maintain genome stability

G-quadruplexes (G4s) form throughout the genome and influence important cellular processes. Their deregulation can challenge DNA replication fork progression and threaten genome stability. Here, we demonstrate an unexpected role for the double-stranded DNA (dsDNA) translocase helicase-like transcription factor (HLTF) in responding to G4s. We show that HLTF, which is enriched at G4s in the human genome, can directly unfold G4s invitro and uses this ATP-dependent translocase function to suppress G4 accumulation throughout the cell cycle. Additionally, MSH2 (a component of MutS heterodimers that bind G4s) and HLTF act synergistically to suppress G4 accumulation, restrict alternative lengthening of telomeres, and promote resistance to G4-stabilizing drugs. In a discrete but complementary role, HLTF restrains DNA synthesis when G4s are stabilized by suppressing primase-polymerase (PrimPol)-dependent repriming. Together, the distinct roles of HLTF in the G4 response prevent DNA damage and potentially mutagenic replication to safeguard genome stability.

Harnessing DNA replication stress to target RBM10 deficiency in lung adenocarcinoma

The splicing factor RNA-binding motif protein 10 (RBM10) is frequently mutated in lung adenocarcinoma (LUAD) (9-25%). Most RBM10 cancer mutations are loss-of-function, correlating with increased tumorigenesis and limiting the efficacy of current LUAD targeted therapies. Remarkably, therapeutic strategies leveraging RBM10 deficiency remain unexplored. Here, we conduct a CRISPR-Cas9 synthetic lethality (SL) screen and identify ~60 RBM10 SL genes, including WEE1 kinase. WEE1 inhibition sensitizes RBM10-deficient LUAD cells in-vitro and in-vivo. Mechanistically, we identify a splicing-independent role of RBM10 in regulating DNA replication fork progression and replication stress response, which underpins RBM10-WEE1 SL. Additionally, RBM10 interacts with active DNA replication forks, relying on DNA Primase Subunit 1 (PRIM1) that synthesizes Okazaki RNA primers. Functionally, we demonstrate that RBM10 serves as an anchor for recruiting Histone Deacetylase 1 (HDAC1) to facilitate H4K16 deacetylation and R-loop homeostasis to maintain replication fork stability. Collectively, our data reveal a role of RBM10 in fine-tuning DNA replication and provide therapeutic arsenal for targeting RBM10-deficient tumors.

Eukaryotic Pif1 helicase unwinds G-quadruplex and dsDNA using a conserved wedge

G-quadruplexes (G4s) formed by guanine-rich nucleic acids induce genome instability through impeding DNA replication fork progression. G4s are stable DNA structures, the unfolding of which require the functions of DNA helicases. Pif1 helicase binds preferentially to G4 DNA and plays multiple roles in maintaining genome stability, but the mechanism by which Pif1 unfolds G4s is poorly understood. Here we report the co-crystal structure of Saccharomyces cerevisiae Pif1 (ScPif1) bound to a G4 DNA with a 5′ single-stranded DNA (ssDNA) segment. Unlike the Thermus oshimai Pif1-G4 structure, in which the 1B and 2B domains confer G4 recognition, ScPif1 recognizes G4 mainly through the wedge region in the 1A domain that contacts the 5′ most G-tetrad directly. A conserved Arg residue in the wedge is required for Okazaki fragment processing but not for mitochondrial function or for suppression of gross chromosomal rearrangements. Multiple substitutions at this position have similar effects on resolution of DNA duplexes and G4s, suggesting that ScPif1 may use the same wedge to unwind G4 and dsDNA. Our results reveal the mechanism governing dsDNA unwinding and G4 unfolding by ScPif1 helicase that can potentially be generalized to other eukaryotic Pif1 helicases and beyond.

The TIMELESS and PARP1 interaction suppresses replication-associated DNA gap accumulation.

TIMELESS (TIM) in the fork protection complex acts as a scaffold of the replisome to prevent its uncoupling and ensure efficient DNA replication fork progression. Nevertheless, its underlying basis for coordinating leading and lagging strand synthesis to limit single-stranded DNA (ssDNA) exposure remains elusive. Here, we demonstrate that acute degradation of TIM at ongoing DNA replication forks induces the accumulation of ssDNA gaps stemming from defective Okazaki fragment (OF) processing. Cells devoid of TIM fail to support the poly(ADP-ribosyl)ation necessary for backing up the canonical OF processing mechanism mediated by LIG1 and FEN1. Consequently, recruitment of XRCC1, a known effector of PARP1-dependent single-strand break repair, to post-replicative ssDNA gaps behind replication forks is impaired. Physical disruption of the TIM-PARP1 complex phenocopies the rapid loss of TIM, indicating that the TIM-PARP1 interaction is critical for the activation of this compensatory pathway. Accordingly, combined deficiency of FEN1 and the TIM-PARP1 interaction leads to synergistic DNA damage and cytotoxicity. We propose that TIM is essential for the engagement of PARP1 to the replisome to coordinate lagging strand synthesis with replication fork progression. Our study identifies TIM as a synthetic lethal target of OF processing enzymes that can be exploited for cancer therapy.

Abstract A011: DPY30 regulates immunoediting by suppressing uncoordinated DNA replication in pancreatic cancer

Abstract DNA replication is a tightly regulated, multi-step process orchestrated by licensing and replication factors that ensure the faithful duplication of the entire genome exclusively during S phase of the cell cycle. Although the mechanisms preventing uncoordinated DNA replication are well characterized, it remains unclear how cells counteract its potentially harmful effects when it does occur, especially in a context of oncogenic stress. Here, we identify DPY30, an understudied component of all COMPASS-like complexes, as a modulator of DNA replication. We found upregulated DPY30 expression in pancreatic ductal adenocarcinoma (PDAC) as well as a correlation between increasing DPY30 expression level and higher tumor grade and poor clinical outcomes. To investigate the role of DPY30 in DNA replication, we generated and characterized human- and mouse-derived PDAC cells silenced or knockout for DPY30 expression in vivo, within a context of increasing immune pressure. ATAC-Seq and ChIP-Seq were used to inform on the epigenetic landscape of PDAC cells silenced for DPY30 or WDR5 expression. Additionally, we conducted unbiased proteomics and transcriptomics analyses to uncover the underlying molecular mechanisms. The canonical function of COMPASS complexes is to regulate histone 3 lysine 4 (H3K4) methylation. Unexpectedly, DPY30 loss in PDAC cells did not affect H3K4 methylation patterns compared to WDR5 loss, the core member of the COMPASS-like complex. Transcriptomic data unveil a dysregulation of genes belonging to DNA replication and repair pathways in DPY30-silenced PDAC cells. This prompted unbiased proteomics analysis, which showed that DPY30 interacts with members of the DNA helicase, possibly stabilizing the DNA replication fork progression. We found that DPY30 overexpression, by preventing Geminin ubiquitylation, strongly stabilizes its expression, which in turn suppresses Cdt1 activity in late S-phase, buffering the risk of uncoordinated DNA replication in G2/M phase. Consistently, DPY30 loss resulted in DNA re-replication and chromosomal instability, as manifested by DNA fiber and metaphasic spread. In vivo, whole-exome sequencing analysis of DPY30-knockout tumors revealed that DPY30 loss markedly increased mutational burden in immunodeficient mice. In syngeneic models, DPY30 loss strongly inhibited tumor growth, prolonged survival, and increased T cell infiltration of tumors. Moreover, the immune system selectively cleared tumor cells with complex karyotypes, and we observed improved anti-tumor efficacy with DPY30 knockdown and anti-PD-1 treatment compared to either condition alone. In summary, our study has unveiled a previously unrecognized role for DPY30 in stabilizing the replisome machinery by preventing excessive DNA replication. In the context of PDAC, DPY30 sustains genome stability, providing a strong rationale for exploring DPY30 or its effector proteins as potential targets and suggesting that patients whose tumors carry low DPY30 expression might be successfully treated with immune checkpoint inhibitors. Citation Format: Francesca Citron, Luigi Perelli, Chiara Balestrieri, Luca Cecchetto, Yanshuo Chu, I-Lin Ho, Li Zhang, Er-Yen Yen, Luis Castillo Montanez, Rutvi Shah, Sergio Attanasio, Sanjana Srinivasan, Charles Dyke, Ko-Chien Chen, Shan Jiang, Jing Pan, Sisi Gao, Huamin Wang, Wantong Yao, Linghua Wang, Mauro Di Pilato, Giannicola Genovese, Andrea Viale, Giulio Draetta. DPY30 regulates immunoediting by suppressing uncoordinated DNA replication in pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: DNA Damage Repair: From Basic Science to Future Clinical Application; 2024 Jan 9-11; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2024;84(1 Suppl):Abstract nr A011.

Chromatin assembly factor-1 preserves genome stability in ctf4Δ cells by promoting sister chromatid cohesion.

Chromatin assembly and the establishment of sister chromatid cohesion are intimately connected to the progression of DNA replication forks. Here we examined the genetic interaction between the heterotrimeric chromatin assembly factor-1 (CAF-1), a central component of chromatin assembly during replication, and the core replisome component Ctf4. We find that CAF-1 deficient cells as well as cells affected in newly-synthesized H3-H4 histones deposition during DNA replication exhibit a severe negative growth with ctf4Δ mutant. We dissected the role of CAF-1 in the maintenance of genome stability in ctf4Δ yeast cells. In the absence of CTF4, CAF-1 is essential for viability in cells experiencing replication problems, in cells lacking functional S-phase checkpoint or functional spindle checkpoint, and in cells lacking DNA repair pathways involving homologous recombination. We present evidence that CAF-1 affects cohesin association to chromatin in a DNA-damage-dependent manner and is essential to maintain cohesion in the absence of CTF4. We also show that Eco1-catalyzed Smc3 acetylation is reduced in absence of CAF-1. Furthermore, we describe genetic interactions between CAF-1 and essential genes involved in cohesin loading, cohesin stabilization, and cohesin component indicating that CAF-1 is crucial for viability when sister chromatid cohesion is affected. Finally, our data indicate that the CAF-1-dependent pathway required for cohesion is functionally distinct from the Rtt101-Mms1-Mms22 pathway which functions in replicated chromatin assembly. Collectively, our results suggest that the deposition by CAF-1 of newly-synthesized H3-H4 histones during DNA replication creates a chromatin environment that favors sister chromatid cohesion and maintains genome integrity.

Trim33 masks a non-transcriptional function of E2f4 in replication fork progression

Replicative stress promotes genomic instability and tumorigenesis but also presents an effective therapeutic endpoint, rationalizing detailed analysis of pathways that control DNA replication. We show here that the transcription factor E2f4 recruits the DNA helicase Recql to facilitate progression of DNA replication forks upon drug- or oncogene-induced replicative stress. In unperturbed cells, the Trim33 ubiquitin ligase targets E2f4 for degradation, limiting its genomic binding and interactions with Recql. Replicative stress blunts Trim33-dependent ubiquitination of E2f4, which stimulates transient Recql recruitment to chromatin and facilitates recovery of DNA synthesis. In contrast, deletion of Trim33 induces chronic genome-wide recruitment of Recql and strongly accelerates DNA replication under stress, compromising checkpoint signaling and DNA repair. Depletion of Trim33 in Myc-overexpressing cells leads to accumulation of replication-associated DNA damage and delays Myc-driven tumorigenesis. We propose that the Trim33-E2f4-Recql axis controls progression of DNA replication forks along transcriptionally active chromatin to maintain genome integrity.

Ruthenium(II) Polypyridyl Complexes as FRET Donors: Structure- and Sequence-Selective DNA-Binding and Anticancer Properties.

Ruthenium(II) polypyridyl complexes (RPCs) that emit from metal-to-ligand charge transfer (MLCT) states have been developed as DNA probes and are being examined as potential anticancer agents. Here, we report that MLCT-emissive RPCs that bind DNA undergo Förster resonance energy transfer (FRET) with Cy5.5-labeled DNA, forming mega-Stokes shift FRET pairs. Based on this discovery, we developed a simple and rapid FRET binding assay to examine DNA-binding interactions of RPCs with diverse photophysical properties, including non-"light switch" complexes [Ru(dppz)2(5,5'dmb)]2+ and [Ru(PIP)2(5,5'dmb)]2+ (dppz = dipyridophenazine, 5,5'dmb = 5,5'-dimethyl-2,2'-bipyridine, PIP = 2-phenyl-imidazo[4,5-f][1,10]phenanthroline). Binding affinities toward duplex, G-quadruplex, three-way junction, and mismatch DNA were determined, and derived FRET donor-acceptor proximities provide information on potential binding sites. Molecules characterized by this method demonstrate encouraging anticancer properties, including synergy with the PARP inhibitor Olaparib, and mechanistic studies indicate that [Ru(PIP)2(5,5'dmb)]2+ acts to block DNA replication fork progression.

Prevalent and dynamic binding of the cell cycle checkpoint kinase Rad53 to gene promoters.

Replication of the genome must be coordinated with gene transcription and cellular metabolism, especially following replication stress in the presence of limiting deoxyribonucleotides. The Saccharomyces cerevisiae Rad53 (CHEK2 in mammals) checkpoint kinase plays a major role in cellular responses to DNA replication stress. Cell cycle regulated, genome-wide binding of Rad53 to chromatin was examined. Under replication stress, the kinase bound to sites of active DNA replication initiation and fork progression, but unexpectedly to the promoters of about 20% of genes encoding proteins involved in multiple cellular functions. Rad53 promoter binding correlated with changes in expression of a subset of genes. Rad53 promoter binding to certain genes was influenced by sequence-specific transcription factors and less by checkpoint signaling. However, in checkpoint mutants, untimely activation of late-replicating origins reduces the transcription of nearby genes, with concomitant localization of Rad53 to their gene bodies. We suggest that the Rad53 checkpoint kinase coordinates genome-wide replication and transcription under replication stress conditions.

NEDDylated Cullin 3 mediates the adaptive response to topoisomerase 1 inhibitors.

DNA topoisomerase 1 (TOP11) inhibitors are mainstays of anticancer therapy. These drugs trap TOP1 on DNA, stabilizing the TOP1-cleavage complex (TOP1-cc). The accumulation of TOP1-ccs perturbs DNA replication fork progression, leading to DNA breaks and cell death. By analyzing the genomic occupancy and activity of TOP1, we show that cells adapt to treatment with multiple doses of TOP1 inhibitor by promoting the degradation of TOP1-ccs, allowing cells to better tolerate subsequent doses of TOP1 inhibitor. The E3-RING Cullin 3 ligase in complex with the BTBD1 and BTBD2 adaptor proteins promotes TOP1-cc ubiquitination and subsequent proteasomal degradation. NEDDylation of Cullin 3 activates this pathway, and inhibition of protein NEDDylation or depletion of Cullin 3 sensitizes cancer cells to TOP1 inhibitors. Collectively, our data uncover a previously unidentified NEDD8-Cullin 3 pathway involved in the adaptive response to TOP1 inhibitors, which can be targeted to improve the efficacy of TOP1 drugs in cancer therapy.

Drosophila SUMM4 complex couples insulator function and DNA replication control.

Asynchronous replication of chromosome domains during S phase is essential for eukaryotic genome function, but the mechanisms establishing which domains replicate early versus late in different cell types remain incompletely understood. Intercalary heterochromatin domains replicate very late in both diploid chromosomes of dividing cells and in endoreplicating polytene chromosomes where they are also underreplicated. Drosophila SNF2-related factor SUUR imparts locus-specific underreplication of polytene chromosomes. SUUR negatively regulates DNA replication fork progression; however, its mechanism of action remains obscure. Here, we developed a novel method termed MS-Enabled Rapid protein Complex Identification (MERCI) to isolate a stable stoichiometric native complex SUMM4 that comprises SUUR and a chromatin boundary protein Mod(Mdg4)-67.2. Mod(Mdg4) stimulates SUUR ATPase activity and is required for a normal spatiotemporal distribution of SUUR in vivo. SUUR and Mod(Mdg4)-67.2 together mediate the activities of gypsy insulator that prevent certain enhancer-promoter interactions and establish euchromatin-heterochromatin barriers in the genome. Furthermore, SuUR or mod(mdg4) mutations reverse underreplication of intercalary heterochromatin. Thus, SUMM4 can impart late replication of intercalary heterochromatin by attenuating the progression of replication forks through euchromatin/heterochromatin boundaries. Our findings implicate a SNF2 family ATP-dependent motor protein SUUR in the insulator function, reveal that DNA replication can be delayed by a chromatin barrier, and uncover a critical role for architectural proteins in replication control. They suggest a mechanism for the establishment of late replication that does not depend on an asynchronous firing of late replication origins.

A genome-wide screen identifies SCAI as a modulator of the UV-induced replicative stress response.

Helix-destabilizing DNA lesions induced by environmental mutagens such as UV light cause genomic instability by strongly blocking the progression of DNA replication forks (RFs). At blocked RF, single-stranded DNA (ssDNA) accumulates and is rapidly bound by Replication Protein A (RPA) complexes. Such stretches of RPA-ssDNA constitute platforms for recruitment/activation of critical factors that promote DNA synthesis restart. However, during periods of severe replicative stress, RPA availability may become limiting due to inordinate sequestration of this multifunctional complex on ssDNA, thereby negatively impacting multiple vital RPA-dependent processes. Here, we performed a genome-wide screen to identify factors that restrict the accumulation of RPA-ssDNA during UV-induced replicative stress. While this approach revealed some expected “hits” acting in pathways such as nucleotide excision repair, translesion DNA synthesis, and the intra-S phase checkpoint, it also identified SCAI, whose role in the replicative stress response was previously unappreciated. Upon UV exposure, SCAI knock-down caused elevated accumulation of RPA-ssDNA during S phase, accompanied by reduced cell survival and compromised RF progression. These effects were independent of the previously reported role of SCAI in 53BP1-dependent DNA double-strand break repair. We also found that SCAI is recruited to UV-damaged chromatin and that its depletion promotes nascent DNA degradation at stalled RF. Finally, we (i) provide evidence that EXO1 is the major nuclease underlying ssDNA formation and DNA replication defects in SCAI knockout cells and, consistent with this, (ii) demonstrate that SCAI inhibits EXO1 activity on a ssDNA gap in vitro. Taken together, our data establish SCAI as a novel regulator of the UV-induced replicative stress response in human cells.

Abstract 2128: Single-cell replication dynamics in genomically unstable cancers

Abstract Background: Single-cell whole genome sequencing (scWGS) methods such as direct library preparation (DLP) provide amplification-free capture of cells in all cycle phases and have enabled rich interrogation into the cell to cell genomic diversity of cancer genomes. Previous DLP-driven clonal evolution studies removed S-phase cells as replicated loci confounded phylogenetic inference from somatic copy number aberrations (CNAs), leaving single-cell replication timing (scRT) as an unstudied property. We thus developed a method that assigned S-phase cells to phylogenetic clones and used such assignments to infer scRT profiles in aneuploid cell lines and tumors. We applied this method to determine the relative proliferation rate between clones and obtain a single-cell picture of DNA replication fork progression for genomically unstable cancers. Methods: S-phase cells were assigned to phylogenetic clones based on copy number profile similarity and subsequently scRT profiles were inferred by normalizing out the effects of clonal, subclonal, and cell-specific CNAs before binarizing each genomic bin as replicated or unreplicated. scRT heterogeneity was quantified via a summary statistic representing the aggregate difference between expected and observed replication times (T-width). The scRT profiles were grouped into clone- and sample-level pseudobulks (aggregations of single cells) to determine genomic regions with differential replication timing. We then assessed the ability of S-phase enrichment to predict clonal expansions on time-series patient-derived xenografts of HER2+ and triple negative breast cancers that were either treatment-naive or exposed to cisplatin. In addition, we studied genetically engineered cell lines with inactivated DNA damage response genes (TP53, BRCA1, BRCA2) and primary high-grade serous ovarian cancer samples to quantify scRT heterogeneity at different states of genomic instability. Results: We analyzed 15 datasets containing over 25,000 cells with ~1/3 of cells in S-phase. Time-series datasets revealed that S-phase enriched clones were highly proliferative in the treatment-naive setting and were most sensitive to cisplatin. T-width values calculated from inferred scRT profiles increased as a function of genomic instability, with the degree of heterogeneity varied between genomic regions. Clone-level pseudobulk replication timing profiles revealed that whole-chromosome aneuploidies were more likely to have unperturbed replication timing compared to complex structural variants such as translocations, inversions, and micronuclei. Conclusions: Our time-series results show that S-phase cells enable the prediction of clone-specific proliferation and chemosensitivity using scWGS data from a single time point. The quantification of scRT heterogeneity across our collection of data implicates dysfunctional DNA replication as both driver and consequence of genomic instability. Citation Format: Adam C. Weiner, Andrew McPherson, Sohrab P. Shah. Single-cell replication dynamics in genomically unstable cancers [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 2128.

Hypomorphic GINS3 variants alter DNA replication and cause Meier-Gorlin syndrome.

The eukaryotic CDC45/MCM2-7/GINS (CMG) helicase unwinds the DNA double helix during DNA replication. The GINS subcomplex is required for helicase activity and is, therefore, essential for DNA replication and cell viability. Here, we report the identification of 7 individuals from 5 unrelated families presenting with a Meier-Gorlin syndrome–like (MGS-like) phenotype associated with hypomorphic variants of GINS3, a gene not previously associated with this syndrome. We found that MGS-associated GINS3 variants affecting aspartic acid 24 (D24) compromised cell proliferation and caused accumulation of cells in S phase. These variants shortened the protein half-life, altered key protein interactions at the replisome, and negatively influenced DNA replication fork progression. Yeast expressing MGS-associated variants of PSF3 (the yeast GINS3 ortholog) also displayed impaired growth, S phase progression defects, and decreased Psf3 protein stability. We further showed that mouse embryos homozygous for a D24 variant presented intrauterine growth retardation and did not survive to birth, and that fibroblasts derived from these embryos displayed accelerated cellular senescence. Taken together, our findings implicate GINS3 in the pathogenesis of MGS and support the notion that hypomorphic variants identified in this gene impaired cell and organismal growth by compromising DNA replication.

MDM2 binds and ubiquitinates PARP1 to enhance DNA replication fork progression.

The MDM2 oncoprotein antagonizes the tumor suppressor p53 by physical interaction and ubiquitination. However, it also sustains the progression of DNA replication forks, even in the absence of functional p53. Here, we show that MDM2 binds, inhibits, ubiquitinates, and destabilizes poly(ADP-ribose) polymerase 1 (PARP1). When cellular MDM2 levels are increased, this leads to accelerated progression of DNA replication forks, much like pharmacological inhibition of PARP1. Conversely, overexpressed PARP1 restores normal fork progression despite elevated MDM2. Strikingly, MDM2 profoundly reduces the frequency of fork reversal, revealed as four-way junctions through electron microscopy. Depletion of RECQ1 or the primase/polymerase (PRIMPOL) reverses the MDM2-mediated acceleration of the nascent DNA elongation rate. MDM2 also increases the occurrence of micronuclei, and it exacerbates camptothecin-induced cell death. In conclusion, high MDM2 levels phenocopy PARP inhibition in modulation of fork restart, representing a potential vulnerability of cancer cells.

DNA Polymerase‐Parental DNA Interaction is Essential for Helicase‐Polymerase Coupling in Bacteriophage T7 DNA Replication

DNA helicase and polymerase work cooperatively at the replication fork to unwind the parental DNA and perform leading strand DNA synthesis. It was believed that the helicase migrates at the forefront of the replication fork and unwinds DNA to provide a template for DNA polymerase. Although extensively investigated, the molecular basis of the helicase‐polymerase coupling is not fully understood. Replisome from bacteriophage T7 has been a model system for estimating mechanisms of DNA replication. Recently, the first structure of the helicase‐polymerase complex at a replication fork was reported with the T7 system. The T7 replisome structure suggested that the helicase and polymerase sandwich the parental DNA, with each enzyme pulling one daughter strand in opposite directions. Interestingly, it is the T7 polymerase but not helicase that carries the parental DNA with a β‐hairpin loop stacking at the fork opening and a positively charged cleft that holds the parental DNA. However, the role of the polymerase‐parental DNA interaction in helicase‐polymerase coupling has not been investigated. Here, we created and characterized T7 polymerases with perturbed β‐hairpin loop and the positively charged cleft. Mutations on both structural elements significantly reduced the strand displacement synthesis by T7 polymerase but had a minor effect on DNA synthesis against linear DNA substrate. Moreover, the mutations eliminate synergistic helicase polymerase binding to the DNA fork substrate and replication fork progression in mobility shift assay and rolling‐circle replication assays. Our biochemical data suggested that polymerase plays a dominant role in helicase‐polymerase coupling and replisome progression.

GAS6/AXL Inhibition Enhances Ovarian Cancer Sensitivity to Chemotherapy and PARP Inhibition through Increased DNA Damage and Enhanced Replication Stress.

Over 80% of women with high-grade serous ovarian cancer (HGSOC) develop tumor resistance to chemotherapy and die of their disease. There are currently no FDA-approved agents to improve sensitivity to first-line platinum- and taxane-based chemotherapy or to PARP inhibitors. Here, we tested the hypothesis that expression of growth arrest-specific 6 (GAS6), the ligand of receptor tyrosine kinase AXL, is associated with chemotherapy response and that sequestration of GAS6 with AVB-S6-500 (AVB-500) could improve tumor response to chemotherapy and PARP inhibitors. We found that GAS6 levels in patient tumor and serum samples collected before chemotherapy correlated with ovarian cancer chemoresponse and patient survival. Compared with chemotherapy alone, AVB-500 plus carboplatin and/or paclitaxel led to decreased ovarian cancer-cell survival in vitro and tumor burden in vivo. Cells treated with AVB-500 plus carboplatin had more DNA damage, slower DNA replication fork progression, and fewer RAD51 foci than cells treated with carboplatin alone, indicating AVB-500 impaired homologous recombination (HR). Finally, treatment with the PARP inhibitor olaparib plus AVB-500 led to decreased ovarian cancer-cell survival in vitro and less tumor burden in vivo. Importantly, this effect was seen in HR-proficient and HR-deficient ovarian cancer cells. Collectively, our findings suggest that GAS6 levels could be used to predict response to carboplatin and AVB-500 could be used to treat platinum-resistant, HR-proficient HGSOC. IMPLICATIONS: GAS6/AXL is a novel target to sensitize ovarian cancers to carboplatin and olaparib. Additionally, GAS6 levels can be associated with response to carboplatin treatment.

Cleavage-free human genome editing

Most gene editing technologies introduce breaks or nicks into DNA, leading to the generation of mutagenic insertions and deletions by non-homologous end-joining repair. Here, we report a new, cleavage-free gene editing approach based on replication interrupted template-driven DNA modification (RITDM). The RITDM system makes use of sequence-specific DLR fusion molecules that are specifically designed to enable localized, temporary blockage of DNA replication fork progression, thereby exposing single-stranded DNA that can be bound by DNA sequence modification templates for precise editing.We evaluate the use of zinc-finger arrays for sequence recognition. We demonstrate that RITDM can be used for gene editing at endogenous genomic loci in human cells and highlight its safety profile of low indel frequencies and undetectable off-target side effects in RITDM-edited clones and pools of cells.

BRCA2 associates with MCM10 to suppress PRIMPOL-mediated repriming and single-stranded gap formation after DNA damage

The BRCA2 tumor suppressor protects genome integrity by promoting homologous recombination-based repair of DNA breaks, stability of stalled DNA replication forks and DNA damage-induced cell cycle checkpoints. BRCA2 deficient cells display the radio-resistant DNA synthesis (RDS) phenotype, however the mechanism has remained elusive. Here we show that cells without BRCA2 are unable to sufficiently restrain DNA replication fork progression after DNA damage, and the underrestrained fork progression is due primarily to Primase-Polymerase (PRIMPOL)-mediated repriming of DNA synthesis downstream of lesions, leaving behind single-stranded DNA gaps. Moreover, we find that BRCA2 associates with the essential DNA replication factor MCM10 and this association suppresses PRIMPOL-mediated repriming and ssDNA gap formation, while having no impact on the stability of stalled replication forks. Our findings establish an important function for BRCA2, provide insights into replication fork control during the DNA damage response, and may have implications in tumor suppression and therapy response.