Temporally distinct post-replicative repair mechanisms fill PRIMPOL-dependent ssDNA gaps in human cells

Trans-lesion synthesis and mismatch repair pathway crosstalk defines chemoresistance and hypermutation mechanisms in glioblastoma

Almost all Glioblastoma (GBM) are either intrinsically resistant to the chemotherapeutical drug temozolomide (TMZ) or acquire therapy-induced mutations that cause chemoresistance and recurrence. The genome maintenance mechanisms responsible for GBM chemoresistance and hypermutation are unknown. We show that the E3 ubiquitin ligase RAD18 (a proximal regulator of TLS) is activated in a Mismatch repair (MMR)-dependent manner in TMZ-treated GBM cells, promoting post-replicative gap-filling and survival. An unbiased CRISPR screen provides an aerial map of RAD18-interacting DNA damage response (DDR) pathways deployed by GBM to tolerate TMZ genotoxicity. Analysis of mutation signatures from TMZ-treated GBM reveals a role for RAD18 in error-free bypass of O6mG (the most toxic TMZ-induced lesion), and error-prone bypass of other TMZ-induced lesions. Our analyses of recurrent GBM patient samples establishes a correlation between low RAD18 expression and hypermutation. Taken together we define molecular underpinnings for the hallmark tumorigenic phenotypes of TMZ-treated GBM.

BRCA2 promotes genomic integrity and therapy resistance primarily through its role in homology-directed repair

Tumor suppressor BRCA2 functions in homology-directed repair (HDR), the protection of stalled replication forks, and the suppression of replicative gaps, but their relative contributions to genome integrity and chemotherapy response are under scrutiny. Here, we report that mouse and human cells require a RAD51 filament stabilization motif in BRCA2 for fork protection and gap suppression but not HDR. In mice, the loss of fork protection/gap suppression does not compromise genome stability or shorten tumor latency. By contrast, HDR deficiency increases spontaneous and replication stress-induced chromosome aberrations and tumor predisposition. Unlike with HDR, fork protection/gap suppression defects are also observed in Brca2 heterozygous cells, likely due to reduced RAD51 stabilization at stalled forks/gaps. Gaps arise from PRIMPOL activity, which is associated with 5-hydroxymethyl-2'-deoxyuridine sensitivity due to the formation of SMUG1-generated abasic sites and is exacerbated by poly(ADP-ribose) polymerase (PARP) inhibition. However, HDR proficiency has the major role in mitigating sensitivity to chemotherapeutics, including PARP inhibitors.

Exploring RAD18-dependent replication of damaged DNA and discontinuities: A collection of advanced tools

DNA damage tolerance (DDT) pathways mitigate the effects of DNA damage during replication by rescuing the replication fork stalled at a DNA lesion or other barriers and also repair discontinuities left in the newly replicated DNA. From yeast to mammalian cells, RAD18-regulated translesion synthesis (TLS) and template switching (TS) represent the dominant pathways of DDT. Monoubiquitylation of the polymerase sliding clamp PCNA by HRAD6A-B/RAD18, an E2/E3 protein pair, enables the recruitment of specialized TLS polymerases that can insert nucleotides opposite damaged template bases. Alternatively, the subsequent polyubiquitylation of monoubiquitin-PCNA by Ubc13-Mms2 (E2) and HLTF or SHPRH (E3) can lead to the switching of the synthesis from the damaged template to the undamaged newly synthesized sister strand to facilitate synthesis past the lesion. When immediate TLS or TS cannot occur, gaps may remain in the newly synthesized strand, partly due to the repriming activity of the PRIMPOL primase, which can be filled during the later phases of the cell cycle. The first part of this review will summarize the current knowledge about RAD18-dependent DDT pathways, while the second part will offer a molecular toolkit for the identification and characterization of the cellular functions of a DDT protein. In particular, we will focus on advanced techniques that can reveal single-stranded and double-stranded DNA gaps and their repair at the single-cell level as well as monitor the progression of single replication forks, such as the specific versions of the DNA fiber and comet assays. This collection of methods may serve as a powerful molecular toolkit to monitor the metabolism of gaps, detect the contribution of relevant pathways and molecular players, as well as characterize the effectiveness of potential inhibitors.

APOBEC3A induces DNA gaps through PRIMPOL and confers gap-associated therapeutic vulnerability

Mutation signatures associated with apolipoprotein B mRNA editing catalytic polypeptide-like 3A/B (APOBEC3A/B) cytidine deaminases are prevalent across cancers, implying their roles as mutagenic drivers during tumorigenesis and tumor evolution. APOBEC3A (A3A) expression induces DNA replication stress and increases the cellular dependency on the ataxia telangiectasia and Rad3-related (ATR) kinase for survival. Nonetheless, how A3A induces DNA replication stress remains unclear. We show that A3A induces replication stress without slowing replication forks. We find that A3A induces single-stranded DNA (ssDNA) gaps through PrimPol-mediated repriming. A3A-induced ssDNA gaps are repaired by multiple pathways involving ATR, RAD51, and translesion synthesis. Both ATR inhibition and trapping of poly(ADP-ribose) polymerase (PARP) on DNA by PARP inhibitor impair the repair of A3A-induced gaps, preferentially killing A3A-expressing cells. When used in combination, PARP and ATR inhibitors selectively kill A3A-expressing cells synergistically in a manner dependent on PrimPol-generated gaps. Thus, A3A-induced replication stress arises from PrimPol-generated ssDNA gaps, which confer a therapeutic vulnerability to gap-targeted DNA repair inhibitors.

PRIMPOL ensures robust handoff between on-the-fly and post-replicative DNA lesion bypass

The primase/polymerase PRIMPOL restarts DNA synthesis when replication is arrested by template impediments. However, we do not have a comprehensive view of how PRIMPOL-dependent repriming integrates with the main pathways of damage tolerance, REV1-dependent 'on-the-fly' lesion bypass at the fork and PCNA ubiquitination-dependent post-replicative gap filling. Guided by genome-wide CRISPR/Cas9 screens to survey the genetic interactions of PRIMPOL in a non-transformed and p53-proficient human cell line, we find that PRIMPOL is needed for cell survival following loss of the Y-family polymerases REV1 and POLη in a lesion-dependent manner, while it plays a broader role in promoting survival of cells lacking PCNA K164-dependent post-replicative gap filling. Thus, while REV1- and PCNA K164R-bypass provide two layers of protection to ensure effective damage tolerance, PRIMPOL is required to maximise the effectiveness of the interaction between them. We propose this is through the restriction of post-replicative gap length provided by PRIMPOL-dependent repriming.

BRCA1/2 Haploinsufficiency: Exploring the Impact of Losing one Allele

Since their discovery in the late 20th century, significant progress has been made in elucidating the functions of the tumor suppressor proteins BRCA1 and BRCA2. These proteins play vital roles in maintaining genome integrity, including DNA repair, replication fork protection, and chromosome maintenance. It is well-established that germline mutations in BRCA1 and BRCA2 increase the risk of breast and ovarian cancer; however, the precise mechanism underlying tumor formation in this context is not fully understood. Contrary to the long-standing belief that the loss of the second wild-type allele is necessary for tumor development, a growing body of evidence suggests that tumorigenesis can occur despite the presence of a single functional allele. This entails that heterozygosity in BRCA1/2 confers haploinsufficiency, where a single copy of the gene is not sufficient to fully suppress tumor formation. Here we provide an overview of the findings and the ongoing debate regarding BRCA haploinsufficiency. We further put out the challenges in studying this topic and discuss its potential relevance in the prevention and treatment of BRCA-related cancers.

Genome maintenance meets mechanobiology

Genome stability is key for healthy cells in healthy organisms, and deregulated maintenance of genome integrity is a hallmark of aging and of age-associated diseases including cancer and neurodegeneration. To maintain a stable genome, genome surveillance and repair pathways are closely intertwined with cell cycle regulation and with DNA transactions that occur during transcription and DNA replication. Coordination of these processes across different time and length scales involves dynamic changes of chromatin topology, clustering of fragile genomic regions and repair factors into nuclear repair centers, mobilization of the nuclear cytoskeleton, and activation of cell cycle checkpoints. Here, we provide a general overview of cell cycle regulation and of the processes involved in genome duplication in human cells, followed by an introduction to replication stress and to the cellular responses elicited by perturbed DNA synthesis. We discuss fragile genomic regions that experience high levels of replication stress, with a particular focus on telomere fragility caused by replication stress at the ends of linear chromosomes. Using alternative lengthening of telomeres (ALT) in cancer cells and ALT-associated PML bodies (APBs) as examples of replication stress-associated clustered DNA damage, we discuss compartmentalization of DNA repair reactions and the role of protein properties implicated in phase separation. Finally, we highlight emerging connections between DNA repair and mechanobiology and discuss how biomolecular condensates, components of the nuclear cytoskeleton, and interfaces between membrane-bound organelles and membraneless macromolecular condensates may cooperate to coordinate genome maintenance in space and time.

DNA replication and replication stress response in the context of nuclear architecture

The DNA replication process needs to be coordinated with other DNA metabolism transactions and must eventually extend to the full genome, regardless of chromatin status, gene expression, secondary structures and DNA lesions. Completeness and accuracy of DNA replication are crucial to maintain genome integrity, limiting transformation in normal cells and offering targeting opportunities for proliferating cancer cells. DNA replication is thus tightly coordinated with chromatin dynamics and 3D genome architecture, and we are only beginning to understand the underlying molecular mechanisms. While much has recently been discovered on how DNA replication initiation is organised and modulated in different genomic regions and nuclear territories-the so-called "DNA replication program"-we know much less on how the elongation of ongoing replication forks and particularly the response to replication obstacles is affected by the local nuclear organisation. Also, it is still elusive how specific components of nuclear architecture participate in the replication stress response. Here, we review known mechanisms and factors orchestrating replication initiation, and replication fork progression upon stress, focusing on recent evidence linking genome organisation and nuclear architecture with the cellular responses to replication interference, and highlighting open questions and future challenges to explore this exciting new avenue of research.

FBH1 deficiency sensitizes cells to WEE1 inhibition by promoting mitotic catastrophe

WEE1 kinase phosphorylates CDK1 and CDK2 to regulate origin firing and mitotic entry. Inhibition of WEE1 has become an attractive target for cancer therapy due to the simultaneous induction of replication stress and inhibition of the G2/M checkpoint. WEE1 inhibition in cancer cells with high levels of replication stress results in induction of replication catastrophe and mitotic catastrophe. To increase potential as a single agent chemotherapeutic, a better understanding of genetic alterations that impact cellular responses to WEE1 inhibition is warranted. Here, we investigate the impact of loss of the helicase, FBH1, on the cellular response to WEE1 inhibition. FBH1-deficient cells have a reduction in ssDNA and double strand break signaling indicating FBH1 is required for induction of replication stress response in cells treated with WEE1 inhibitors. Despite the defect in the replication stress response, FBH1-deficiency sensitizes cells to WEE1 inhibition by increasing mitotic catastrophe. We propose loss of FBH1 is resulting in replication-associated damage that requires the WEE1-dependent G2 checkpoint for repair.

Role of Translesion DNA Synthesis in the Metabolism of Replication-associated Nascent Strand Gaps

Translesion DNA synthesis (TLS) is a DNA damage tolerance pathway utilized by cells to overcome lesions encountered throughout DNA replication. During replication stress, cancer cells show increased dependency on TLS proteins for cellular survival and chemoresistance. TLS proteins have been described to be involved in various DNA repair pathways. One of the major emerging roles of TLS is single-stranded DNA (ssDNA) gap-filling, primarily after the repriming activity of PrimPol upon encountering a lesion. Conversely, suppression of ssDNA gap accumulation by TLS is considered to represent a mechanism for cancer cells to evade the toxicity of chemotherapeutic agents, specifically in BRCA-deficient cells. Thus, TLS inhibition is emerging as a potential treatment regimen for DNA repair-deficient tumors.

The proofreading exonuclease of leading-strand DNA polymerase epsilon prevents replication fork collapse at broken template strands

Leading-strand DNA replication by polymerase epsilon (Polϵ) across single-strand breaks (SSBs) causes single-ended double-strand breaks (seDSBs), which are repaired via homology-directed repair (HDR) and suppressed by fork reversal (FR). Although previous studies identified many molecules required for hydroxyurea-induced FR, FR at seDSBs is poorly understood. Here, we identified molecules that specifically mediate FR at seDSBs. Because FR at seDSBs requires poly(ADP ribose)polymerase 1 (PARP1), we hypothesized that seDSB/FR-associated molecules would increase tolerance to camptothecin (CPT) but not the PARP inhibitor olaparib, even though both anti-cancer agents generate seDSBs. Indeed, we uncovered that Polϵ exonuclease and CTF18, a Polϵ cofactor, increased tolerance to CPT but not olaparib. To explore potential functional interactions between Polϵ exonuclease, CTF18, and PARP1, we created exonuclease-deficient POLE1exo-/-, CTF18-/-, PARP1-/-, CTF18-/-/POLE1exo-/-, PARP1-/-/POLE1exo-/-, and CTF18-/-/PARP1-/- cells. Epistasis analysis indicated that Polϵ exonuclease and CTF18 were interdependent and required PARP1 for CPT tolerance. Remarkably, POLE1exo-/- and HDR-deficient BRCA1-/- cells exhibited similar CPT sensitivity. Moreover, combining POLE1exo-/- with BRCA1-/- mutations synergistically increased CPT sensitivity. In conclusion, the newly identified PARP1-CTF18-Polϵ exonuclease axis and HDR act independently to prevent fork collapse at seDSBs. Olaparib inhibits this axis, explaining the pronounced cytotoxic effects of olaparib on HDR-deficient cells.

Mechanisms of synthetic lethality between BRCA1/2 and 53BP1 deficiencies and DNA polymerase theta targeting

A synthetic lethal relationship exists between disruption of polymerase theta (Polθ), and loss of either 53BP1 or homologous recombination (HR) proteins, including BRCA1; however, the mechanistic basis of these observations are unclear. Here we reveal two distinct mechanisms of Polθ synthetic lethality, identifying dual influences of 1) whether Polθ is lost or inhibited, and 2) the underlying susceptible genotype. Firstly, we find that the sensitivity of BRCA1/2- and 53BP1-deficient cells to Polθ loss, and 53BP1-deficient cells to Polθ inhibition (ART558) requires RAD52, and appropriate reduction of RAD52 can ameliorate these phenotypes. We show that in the absence of Polθ, RAD52 accumulations suppress ssDNA gap-filling in G2/M and encourage MRE11 nuclease accumulation. In contrast, the survival of BRCA1-deficient cells treated with Polθ inhibitor are not restored by RAD52 suppression, and ssDNA gap-filling is prevented by the chemically inhibited polymerase itself. These data define an additional role for Polθ, reveal the mechanism underlying synthetic lethality between 53BP1, BRCA1/2 and Polθ loss, and indicate genotype-dependent Polθ inhibitor mechanisms.

Nuclear actin polymerization rapidly mediates replication fork remodeling upon stress by limiting PrimPol activity

Cells rapidly respond to replication stress actively slowing fork progression and inducing fork reversal. How replication fork plasticity is achieved in the context of nuclear organization is currently unknown. Using nuclear actin probes in living and fixed cells, we visualized nuclear actin filaments in unperturbed S phase and observed their rapid extension in number and length upon genotoxic treatments, frequently taking contact with replication factories. Chemically or genetically impairing nuclear actin polymerization shortly before these treatments prevents active fork slowing and abolishes fork reversal. Defective fork remodeling is linked to deregulated chromatin loading of PrimPol, which promotes unrestrained and discontinuous DNA synthesis and limits the recruitment of RAD51 and SMARCAL1 to nascent DNA. Moreover, defective nuclear actin polymerization upon mild replication interference induces chromosomal instability in a PRIMPOL-dependent manner. Hence, by limiting PrimPol activity, nuclear F-actin orchestrates replication fork plasticity and is a key molecular determinant in the rapid cellular response to genotoxic treatments.

Two independent DNA repair pathways cause mutagenesis in template switching deficient Saccharomyces cerevisiae

Upon DNA replication stress, cells utilize the postreplication repair pathway to repair single-stranded DNA and maintain genome integrity. Postreplication repair is divided into 2 branches: error-prone translesion synthesis, signaled by proliferating cell nuclear antigen (PCNA) monoubiquitination, and error-free template switching, signaled by PCNA polyubiquitination. In Saccharomyces cerevisiae, Rad5 is involved in both branches of repair during DNA replication stress. When the PCNA polyubiquitination function of Rad5 s disrupted, Rad5 recruits translesion synthesis polymerases to stalled replication forks, resulting in mutagenic repair. Details of how mutagenic repair is carried out, as well as the relationship between Rad5-mediated mutagenic repair and the canonical PCNA-mediated mutagenic repair, remain to be understood. We find that Rad5-mediated mutagenic repair requires the translesion synthesis polymerase ζ but does not require other yeast translesion polymerase activities. Furthermore, we show that Rad5-mediated mutagenic repair is independent of PCNA binding by Rev1 and so is separable from canonical mutagenic repair. In the absence of error-free template switching, both modes of mutagenic repair contribute additively to replication stress response in a replication timing-independent manner. Cellular contexts where error-free template switching is compromised are not simply laboratory phenomena, as we find that a natural variant in RAD5 is defective in PCNA polyubiquitination and therefore defective in error-free repair, resulting in Rad5- and PCNA-mediated mutagenic repair. Our results highlight the importance of Rad5 in regulating spontaneous mutagenesis and genetic diversity in S. cerevisiae through different modes of postreplication repair.

Non-B DNA structures as a booster of genome instability

Non-canonical secondary structures (NCSs) are alternative nucleic acid structures that differ from the canonical B-DNA conformation. NCSs often occur in repetitive DNA sequences and can adopt different conformations depending on the sequence. The majority of these structures form in the context of physiological processes, such as transcription-associated R-loops, G4s, as well as hairpins and slipped-strand DNA, whose formation can be dependent on DNA replication. It is therefore not surprising that NCSs play important roles in the regulation of key biological processes. In the last years, increasing published data have supported their biological role thanks to genome-wide studies and the development of bioinformatic prediction tools. Data have also highlighted the pathological role of these secondary structures. Indeed, the alteration or stabilization of NCSs can cause the impairment of transcription and DNA replication, modification in chromatin structure and DNA damage. These events lead to a wide range of recombination events, deletions, mutations and chromosomal aberrations, well-known hallmarks of genome instability which are strongly associated with human diseases. In this review, we summarize molecular processes through which NCSs trigger genome instability, with a focus on G-quadruplex, i-motif, R-loop, Z-DNA, hairpin, cruciform and multi-stranded structures known as triplexes.

Extrachromosomal Telomeres Derived from Excessive Strand Displacements

Alternative Lengthening of Telomeres (ALT) is a telomere maintenance mechanism mediated by break-induced replication (BIR), evident in approximately 15% of human cancers. A characteristic feature of ALT cancers is the presence of C-circles, circular single-stranded telomeric DNAs composed of C-rich sequences. Despite the fact that extrachromosomal C-rich single-stranded DNAs (ssDNAs), unique to ALT cells, are considered potential precursors of C-circles, their generation process remains undefined. Here, we introduce a highly sensitive method to detect single stranded telomeric DNA, called 4SET (Strand-Specific Southern-blot for Single-stranded Extrachromosomal Telomeres) assay. Utilizing 4SET, we are able to capture C-rich single stranded DNAs that are near 200 to 1500 nucleotides in size. Both linear C-rich ssDNAs and C-circles are abundant in the fractions of cytoplasm and nucleoplasm, which supports the idea that linear C-rich ssDNA accumulation may indeed precede C-circle formation. We also found that C-rich ssDNAs originate during Okazaki fragment processing during lagging strand DNA synthesis. The generation of C-rich ssDNA requires CST-PP (CTC1/STN1/TEN1-PRIMASE-Polymerase alpha) complex-mediated priming of the C-strand DNA synthesis and subsequent excessive strand displacement of the C-rich strand mediated by the DNA Polymerase delta and the BLM helicase. Our work proposes a new model for the generation of C-rich ssDNAs and C-circles during ALT-mediated telomere elongation.

Freedom to err: The expanding cellular functions of translesion DNA polymerases

Translesion synthesis (TLS) DNA polymerases were originally described as error-prone enzymes involved in the bypass of DNA lesions. However, extensive research over the past few decades has revealed that these enzymes play pivotal roles not only in lesion bypass, but also in a myriad of other cellular processes. Such processes include DNA replication, DNA repair, epigenetics, immune signaling, and even viral infection. This review discusses the wide range of functions exhibited by TLS polymerases, including their underlying biochemical mechanisms and associated mutagenicity. Given their multitasking ability to alleviate replication stress, TLS polymerases represent a cellular dependency and a critical vulnerability of cancer cells. Hence, this review also highlights current and emerging strategies for targeting TLS polymerases in cancer therapy.

Trans-Lesion Synthesis and Mismatch Repair Pathway Crosstalk Defines Chemoresistance and Hypermutation Mechanisms in Glioblastoma

Almost all Glioblastoma (GBM) are either intrinsically resistant to the chemotherapeutical drug temozolomide (TMZ) or acquire therapy-induced mutations that cause chemoresistance and recurrence. The genome maintenance mechanisms responsible for GBM chemoresistance and hypermutation are unknown. We show that the E3 ubiquitin ligase RAD18 (a proximal regulator of TLS) is activated in a Mismatch repair (MMR)-dependent manner in TMZ-treated GBM cells, promoting post-replicative gap-filling and survival. An unbiased CRISPR screen provides a new aerial map of RAD18-interacting DNA damage response (DDR) pathways deployed by GBM to tolerate TMZ genotoxicity. Analysis of mutation signatures from TMZ-treated GBM reveals a role for RAD18 in error-free bypass of O6mG (the most toxic TMZ-induced lesion), and error-prone bypass of other TMZ-induced lesions. Our analyses of recurrent GBM patient samples establishes a correlation between low RAD18 expression and hypermutation. Taken together we define novel molecular underpinnings for the hallmark tumorigenic phenotypes of TMZ-treated GBM.

Trans-Lesion Synthesis and Mismatch Repair Pathway Crosstalk Defines Chemoresistance and Hypermutation Mechanisms in Glioblastoma

Almost all Glioblastoma (GBM) are either intrinsically resistant to the chemotherapeutical drug temozolomide (TMZ) or acquire therapy-induced mutations that cause chemoresistance and recurrence. The genome maintenance mechanisms responsible for GBM chemoresistance and hypermutation are unknown. We show that the E3 ubiquitin ligase RAD18 (a proximal regulator of TLS) is activated in a Mismatch repair (MMR)-dependent manner in TMZ-treated GBM cells, promoting post-replicative gap-filling and survival. An unbiased CRISPR screen provides a new aerial map of RAD18-interacting DNA damage response (DDR) pathways deployed by GBM to tolerate TMZ genotoxicity. Analysis of mutation signatures from TMZ-treated GBM reveals a role for RAD18 in error-free bypass of O6mG (the most toxic TMZ-induced lesion), and error-prone bypass of other TMZ-induced lesions. Our analyses of recurrent GBM patient samples establishes a correlation between low RAD18 expression and hypermutation. Taken together we define novel molecular underpinnings for the hallmark tumorigenic phenotypes of TMZ-treated GBM.

Multi-step processing of replication stress-derived nascent strand DNA gaps by MRE11 and EXO1 nucleases

Accumulation of single stranded DNA (ssDNA) gaps in the nascent strand during DNA replication has been associated with cytotoxicity and hypersensitivity to genotoxic stress, particularly upon inactivation of the BRCA tumor suppressor pathway. However, how ssDNA gaps contribute to genotoxicity is not well understood. Here, we describe a multi-step nucleolytic processing of replication stress-induced ssDNA gaps which converts them into cytotoxic double stranded DNA breaks (DSBs). We show that ssDNA gaps are extended bidirectionally by MRE11 in the 3'-5' direction and by EXO1 in the 5'-3' direction, in a process which is suppressed by the BRCA pathway. Subsequently, the parental strand at the ssDNA gap is cleaved by the MRE11 endonuclease generating a double strand break. We also show that exposure to bisphenol A (BPA) and diethylhexyl phthalate (DEHP), which are widespread environmental contaminants due to their use in plastics manufacturing, causes nascent strand ssDNA gaps during replication. These gaps are processed through the same mechanism described above to generate DSBs. Our work sheds light on both the relevance of ssDNA gaps as major determinants of genomic instability, as well as the mechanism through which they are processed to generate genomic instability and cytotoxicity.

UBC13-mediated template switching promotes replication stress resistance in FBH1-deficient cells

The proper resolution of DNA damage during replication is essential for genome stability. FBH1, a UvrD, helicase plays crucial roles in the DNA damage response. FBH1 promotes double strand break formation and signaling in response to prolonged replication stress to initiate apoptosis. Human FBH1 regulates RAD51 to inhibit homologous recombination. A previous study suggested that mis-regulation of RAD51 may contribute to replication stress resistance in FBH1-deficient cells, but the underlying mechanism remains unknown. Here, we provide direct evidence that RAD51 promotes replication stress resistance in FBH1-deficient cells. We demonstrate inhibition of RAD51 using the small molecule, B02, partially rescues double strand break signaling in FBH1-deficient cells. We show that inhibition of only the strand exchange activity of RAD51 rescues double strand break signaling in FBH1 knockout cells. Finally, we show that depletion of UBC13, a E2 protein that promotes RAD51-dependent template switching, rescues double strand break formation and signaling sensitizing FBH1-deficient cells to replication stress. Our results suggest FBH1 regulates template switching to promote replication stress sensitivity.

A non-proteolytic release mechanism for HMCES-DNA-protein crosslinks

The conserved protein HMCES crosslinks to abasic (AP) sites in ssDNA to prevent strand scission and the formation of toxic dsDNA breaks during replication. Here, we report a non-proteolytic release mechanism for HMCES-DNA-protein crosslinks (DPCs), which is regulated by DNA context. In ssDNA and at ssDNA-dsDNA junctions, HMCES-DPCs are stable, which efficiently protects AP sites against spontaneous incisions or cleavage by APE1 endonuclease. In contrast, HMCES-DPCs are released in dsDNA, allowing APE1 to initiate downstream repair. Mechanistically, we show that release is governed by two components. First, a conserved glutamate residue, within HMCES' active site, catalyses reversal of the crosslink. Second, affinity to the underlying DNA structure determines whether HMCES re-crosslinks or dissociates. Our study reveals that the protective role of HMCES-DPCs involves their controlled release upon bypass by replication forks, which restricts DPC formation to a necessary minimum.

The multifaceted functions of homologous recombination in dealing with replication-associated DNA damages

The perturbation of DNA replication, a phenomena termed "replication stress", is a driving force of genome instability and a hallmark of cancer cells. Among the DNA repair mechanisms that contribute to tolerating replication stress, the homologous recombination pathway is central to the alteration of replication fork progression. In many organisms, defects in the homologous recombination machinery result in increased cell sensitivity to replication-blocking agents and a higher risk of cancer in humans. Moreover, the status of homologous recombination in cancer cells often correlates with the efficacy of anti-cancer treatment. In this review, we discuss our current understanding of the different functions of homologous recombination in fixing replication-associated DNA damage and contributing to complete genome duplication. We also examine which functions are pivotal in preventing cancer and genome instability.

Spontaneous mutagenesis in human cells is controlled by REV1-Polymerase ζ and PRIMPOL

Translesion DNA synthesis (TLS) facilitates replication over damaged or difficult-to-replicate templates by employing specialized DNA polymerases. We investigate the effect on spontaneous mutagenesis of three main TLS control mechanisms: REV1 and PCNA ubiquitylation that recruit TLS polymerases and PRIMPOL that creates post-replicative gaps. Using whole-genome sequencing of cultured human RPE-1 cell clones, we find that REV1 and Polymerase ζ are wholly responsible for one component of base substitution mutagenesis that resembles homologous recombination deficiency, whereas the remaining component that approximates oxidative mutagenesis is reduced in PRIMPOL-/- cells. Small deletions in short repeats appear in REV1-/-PCNAK164R/K164R double mutants, revealing an alternative TLS mechanism. Also, 500-5,000 bp deletions appear in REV1-/- and REV3L-/- mutants, and chromosomal instability is detectable in REV1-/-PRIMPOL-/- cells. Our results indicate that TLS protects the genome from deletions and large rearrangements at the expense of being responsible for the majority of spontaneous base substitutions.

An ATR-PrimPol pathway confers tolerance to oncogenic KRAS-induced and heterochromatin-associated replication stress

Activation of the KRAS oncogene is a source of replication stress, but how this stress is generated and how it is tolerated by cancer cells remain poorly understood. Here we show that induction of KRASG12V expression in untransformed cells triggers H3K27me3 and HP1-associated chromatin compaction in an RNA transcription dependent manner, resulting in replication fork slowing and cell death. Furthermore, elevated ATR expression is necessary and sufficient for tolerance of KRASG12V-induced replication stress to expand replication stress-tolerant cells (RSTCs). PrimPol is phosphorylated at Ser255, a potential Chk1 substrate site, under KRASG12V-induced replication stress and promotes repriming to maintain fork progression and cell survival in an ATR/Chk1-dependent manner. However, ssDNA gaps are generated at heterochromatin by PrimPol-dependent repriming, leading to genomic instability. These results reveal a role of ATR-PrimPol in enabling precancerous cells to survive KRAS-induced replication stress and expand clonally with accumulation of genomic instability.

CSB Regulates Pathway Choice in Response to DNA Replication Stress Induced by Camptothecin

Topoisomerase inhibitor camptothecin (CPT) induces fork stalling and is highly toxic to proliferating cells. However, how cells respond to CPT-induced fork stalling has not been fully characterized. Here, we report that Cockayne syndrome group B (CSB) protein inhibits PRIMPOL-dependent fork repriming in response to a low dose of CPT. At a high concentration of CPT, CSB is required to promote the restart of DNA replication through MUS81-RAD52-POLD3-dependent break-induced replication (BIR). In the absence of CSB, resumption of DNA synthesis at a high concentration of CPT can occur through POLQ-LIG3-, LIG4-, or PRIMPOL-dependent pathways, which are inhibited, respectively, by RAD51, BRCA1, and BRCA2 proteins. POLQ and LIG3 are core components of alternative end joining (Alt-EJ), whereas LIG4 is a core component of nonhomologous end joining (NHEJ). These results suggest that CSB regulates fork restart pathway choice following high-dosage CPT-induced fork stalling, promoting BIR but inhibiting Alt-EJ, NHEJ, and fork repriming. We find that loss of CSB and BRCA2 is a toxic combination to genomic stability and cell survival at a high concentration of CPT, which is likely due to accumulation of ssDNA gaps, underscoring an important role of CSB in regulating the therapy response in cancers lacking functional BRCA2.

ATR protects ongoing and newly assembled DNA replication forks through distinct mechanisms

The ATR kinase safeguards genomic integrity during S phase, but how ATR protects DNA replication forks remains incompletely understood. Here, we combine four distinct assays to analyze ATR functions at ongoing and newly assembled replication forks upon replication inhibition by hydroxyurea. At ongoing forks, ATR inhibitor (ATRi) increases MRE11- and EXO1-mediated nascent DNA degradation from PrimPol-generated, single-stranded DNA (ssDNA) gaps. ATRi also exposes template ssDNA through fork uncoupling and nascent DNA degradation. Electron microscopy reveals that ATRi reduces reversed forks by increasing gap-dependent nascent DNA degradation. At new forks, ATRi triggers MRE11- and CtIP-initiated template DNA degradation by EXO1, exposing nascent ssDNA. Upon PARP inhibition, ATRi preferentially exacerbates gap-dependent nascent DNA degradation at ongoing forks in BRCA1/2-deficient cells and disrupts the restored gap protection in BRCA1-deficient, PARP-inhibitor-resistant cells. Thus, ATR protects ongoing and new forks through distinct mechanisms, providing an extended view of ATR's functions in stabilizing replication forks.

Conservation of the insert-2 motif confers Rev1 from different species with an ability to disrupt G-quadruplexes and stimulate translesion DNA synthesis

In some organisms, the replication of G-quadruplex (G4) structures is supported by the Rev1 DNA polymerase. We previously showed that residues in the insert-2 motif of human Rev1 (hRev1) increased the affinity of the enzyme for G4 DNA and mediated suppression of mutagenic replication near G4 motifs. We have now investigated the conservation of G4-selective properties in Rev1 from other species. We compared Rev1 from Danio rerio (zRev1), Saccharomyces cerevisiae (yRev1), and Leishmania donovani (lRev1) with hRev1, including an insert-2 mutant form of hRev1 (E466A/Y470A or EY). We found that zRev1 retained all of the G4-selective prowess of the human enzyme, but there was a marked attenuation of G4 binding affinity for the EY hRev1 mutant and the two Rev1 proteins lacking insert-2 (yRev1 and lRev1). Perhaps most strikingly, we found that insert-2 was important for disruption of the G4 structure and optimal stimulation of processive DNA synthesis across the guanine-rich motif by DNA polymerase kappa (pol κ). Our findings have implications for how Rev1 might contribute to G4 replication in different species spanning the evolutionary tree - signaling the importance of selection for enzymes with robust G4-selective properties in organisms where these non-B DNA structures may fulfill taxa-specific physiological functions.

Chronic treatment with ATR and CHK1 inhibitors does not substantially increase the mutational burden of human cells

DNA replication stress (RS) entails the frequent slow down and arrest of replication forks by a variety of conditions that hinder accurate and processive genome duplication. Elevated RS leads to genome instability, replication catastrophe and eventually cell death. RS is particularly prevalent in cancer cells and its exacerbation to unsustainable levels by chemotherapeutic agents remains a cornerstone of cancer treatments. The adverse consequences of RS are normally prevented by the ATR and CHK1 checkpoint kinases that stabilize stressed forks, suppress origin firing and promote cell cycle arrest when replication is perturbed. Specific inhibitors of these kinases have been developed and shown to potentiate RS and cell death in multiple in vitro cancer settings. Ongoing clinical trials are now probing their efficacy against various cancer types, either as single agents or in combination with mainstay chemotherapeutics. Despite their promise as valuable additions to the anti-cancer pharmacopoeia, we still lack a genome-wide view of the potential mutagenicity of these new drugs. To investigate this question, we performed chronic long-term treatments of TP53-depleted human cancer cells with ATR and CHK1 inhibitors (ATRi, AZD6738/ceralasertib and CHK1i, MK8776/SCH-900776). ATR or CHK1 inhibition did not significantly increase the mutational burden of cells, nor generate specific mutational signatures. Indeed, no notable changes in the numbers of base substitutions, short insertions/deletions and larger scale rearrangements were observed despite induction of replication-associated DNA breaks during treatments. Interestingly, ATR inhibition did induce a slight increase in closely-spaced mutations, a feature previously attributed to translesion synthesis DNA polymerases. The results suggest that ATRi and CHK1i do not have substantial mutagenic effects in vitro when used as standalone agents.

Dynamic de novo heterochromatin assembly and disassembly at replication forks ensures fork stability

Chromatin is dynamically reorganized when DNA replication forks are challenged. However, the process of epigenetic reorganization and its implication for fork stability is poorly understood. Here we discover a checkpoint-regulated cascade of chromatin signalling that activates the histone methyltransferase EHMT2/G9a to catalyse heterochromatin assembly at stressed replication forks. Using biochemical and single molecule chromatin fibre approaches, we show that G9a together with SUV39h1 induces chromatin compaction by accumulating the repressive modifications, H3K9me1/me2/me3, in the vicinity of stressed replication forks. This closed conformation is also favoured by the G9a-dependent exclusion of the H3K9-demethylase JMJD1A/KDM3A, which facilitates heterochromatin disassembly upon fork restart. Untimely heterochromatin disassembly from stressed forks by KDM3A enables PRIMPOL access, triggering single-stranded DNA gap formation and sensitizing cells towards chemotherapeutic drugs. These findings may help in explaining chemotherapy resistance and poor prognosis observed in patients with cancer displaying elevated levels of G9a/H3K9me3.

Detection of endogenous translesion DNA synthesis in single mammalian cells

Translesion DNA synthesis (TLS) is an evolutionarily conserved process that cells activate to tolerate DNA damage. TLS facilitates proliferation under DNA damage conditions and is exploited by cancer cells to gain therapy resistance. It has been so far challenging to analyze endogenous TLS factors such as PCNA^mUb and TLS DNA polymerases in single mammalian cells due to a lack of suitable detection tools. We have adapted a flow cytometry-based quantitative method allowing detection of endogenous, chromatin-bound TLS factors in single mammalian cells, either untreated or exposed to DNA-damaging agents. This high-throughput procedure is quantitative, accurate, and allows unbiased analysis of TLS factors' recruitment to chromatin, as well as occurrence of DNA lesions with respect to the cell cycle. We also demonstrate detection of endogenous TLS factors by immunofluorescence microscopy and provide insights into TLS dynamics upon DNA replication forks stalled by UV-C-induced DNA damage.

The Adaptive Mechanisms and Checkpoint Responses to a Stressed DNA Replication Fork

DNA replication is a tightly controlled process that ensures the faithful duplication of the genome. However, DNA damage arising from both endogenous and exogenous assaults gives rise to DNA replication stress associated with replication fork slowing or stalling. Therefore, protecting the stressed fork while prompting its recovery to complete DNA replication is critical for safeguarding genomic integrity and cell survival. Specifically, the plasticity of the replication fork in engaging distinct DNA damage tolerance mechanisms, including fork reversal, repriming, and translesion DNA synthesis, enables cells to overcome a variety of replication obstacles. Furthermore, stretches of single-stranded DNA generated upon fork stalling trigger the activation of the ATR kinase, which coordinates the cellular responses to replication stress by stabilizing the replication fork, promoting DNA repair, and controlling cell cycle and replication origin firing. Deregulation of the ATR checkpoint and aberrant levels of chronic replication stress is a common characteristic of cancer and a point of vulnerability being exploited in cancer therapy. Here, we discuss the various adaptive responses of a replication fork to replication stress and the roles of ATR signaling that bring fork stabilization mechanisms together. We also review how this knowledge is being harnessed for the development of checkpoint inhibitors to trigger the replication catastrophe of cancer cells.

FBH1 deficiency sensitizes cells to WEE1 inhibition by promoting mitotic catastrophe

WEE1 kinase phosphorylates CDK1 and CDK2 to regulate origin firing and mitotic entry. Inhibition of WEE1 has become an attractive target for cancer therapy due to the simultaneous induction of replication stress and inhibition of the G2/M checkpoint. WEE1 inhibition in cancer cells with high levels of replication stress results in induction of replication catastrophe and mitotic catastrophe. To increase potential as a single agent chemotherapeutic, a better understanding of genetic alterations that impact cellular responses to WEE1 inhibition is warranted. Here, we investigate the impact of loss of the helicase, FBH1, on the cellular response to WEE1 inhibition. FBH1-deficient cells have a reduction in ssDNA and double strand break signaling indicating FBH1 is required for induction of replication stress response in cells treated with WEE1 inhibitors. Despite the defect in the replication stress response, FBH1-deficiency sensitizes cells to WEE1 inhibition by increasing mitotic catastrophe. We propose loss of FBH1 is resulting in replication-associated damage that requires the WEE1-dependent G2 checkpoint for repair.

A ribosomal gene panel predicting a novel synthetic lethality in non-BRCAness tumors

Poly (ADP-ribose) polymerase (PARP) inhibitors are one of the most exciting classes of targeted therapy agents for cancers with homologous recombination (HR) deficiency. However, many patients without apparent HR defects also respond well to PARP inhibitors/cisplatin. The biomarker responsible for this mechanism remains unclear. Here, we identified a set of ribosomal genes that predict response to PARP inhibitors/cisplatin in HR-proficient patients. PARP inhibitor/cisplatin selectively eliminates cells with high expression of the eight genes in the identified panel via DNA damage (ATM) signaling-induced pro-apoptotic ribosomal stress, which along with ATM signaling-induced pro-survival HR repair constitutes a new model to balance the cell fate in response to DNA damage. Therefore, the combined examination of the gene panel along with HR status would allow for more precise predictions of clinical response to PARP inhibitor/cisplatin. The gene panel as an independent biomarker was validated by multiple published clinical datasets, as well as by an ovarian cancer organoids library we established. More importantly, its predictive value was further verified in a cohort of PARP inhibitor-treated ovarian cancer patients with both RNA-seq and WGS data. Furthermore, we identified several marketed drugs capable of upregulating the expression of the genes in the panel without causing HR deficiency in PARP inhibitor/cisplatin-resistant cell lines. These drugs enhance PARP inhibitor/cisplatin sensitivity in both intrinsically resistant organoids and cell lines with acquired resistance. Together, our study identifies a marker gene panel for HR-proficient patients and reveals a broader application of PARP inhibitor/cisplatin in cancer therapy.

DNA Combing versus DNA Spreading and the Separation of Sister Chromatids

DNA combing and DNA spreading are two central approaches for studying DNA replication fork dynamics genome-wide at single-molecule resolution by distributing labeled genomic DNA on coverslips or slides for immunodetection. Perturbations in DNA replication fork dynamics can differentially affect either leading or lagging strand synthesis, for example in instances where replication is blocked by a lesion or obstacle on only one of the two strands. Thus, we sought to investigate whether the DNA combing and/or spreading approaches are suitable for resolving adjacent sister chromatids during DNA replication, thereby enabling the detection of DNA replication dynamics within individual nascent strands. To this end, we developed a thymidine labeling scheme that discriminates between these two possibilities. Our data suggests that DNA combing resolves single chromatids, allowing the detection of strand-specific alterations, whereas DNA spreading does not. These findings have important implications when interpreting DNA replication dynamics from data obtained by these two commonly used techniques.

Inhibitors of Rho kinases (ROCK) induce multiple mitotic defects and synthetic lethality in BRCA2-deficient cells

The trapping of Poly-ADP-ribose polymerase (PARP) on DNA caused by PARP inhibitors (PARPi) triggers acute DNA replication stress and synthetic lethality (SL) in BRCA2-deficient cells. Hence, DNA damage is accepted as a prerequisite for SL in BRCA2-deficient cells. In contrast, here we show that inhibiting ROCK in BRCA2-deficient cells triggers SL independently from acute replication stress. Such SL is preceded by polyploidy and binucleation resulting from cytokinesis failure. Such initial mitosis abnormalities are followed by other M phase defects, including anaphase bridges and abnormal mitotic figures associated with multipolar spindles, supernumerary centrosomes and multinucleation. SL was also triggered by inhibiting Citron Rho-interacting kinase, another enzyme that, similarly to ROCK, regulates cytokinesis. Together, these observations demonstrate that cytokinesis failure triggers mitotic abnormalities and SL in BRCA2-deficient cells. Furthermore, the prevention of mitotic entry by depletion of Early mitotic inhibitor 1 (EMI1) augmented the survival of BRCA2-deficient cells treated with ROCK inhibitors, thus reinforcing the association between M phase and cell death in BRCA2-deficient cells. This novel SL differs from the one triggered by PARPi and uncovers mitosis as an Achilles heel of BRCA2-deficient cells.

Polymerase iota (Pol ι) prevents PrimPol-mediated nascent DNA synthesis and chromosome instability

Recent studies have described a DNA damage tolerance pathway choice that involves a competition between PrimPol-mediated repriming and fork reversal. Screening different translesion DNA synthesis (TLS) polymerases by the use of tools for their depletion, we identified a unique role of Pol ι in regulating such a pathway choice. Pol ι deficiency unleashes PrimPol-dependent repriming, which accelerates DNA replication in a pathway that is epistatic with ZRANB3 knockdown. In Pol ι-depleted cells, the excess participation of PrimPol in nascent DNA elongation reduces replication stress signals, but thereby also checkpoint activation in S phase, triggering chromosome instability in M phase. This TLS-independent function of Pol ι requires its PCNA-interacting but not its polymerase domain. Our findings unravel an unanticipated role of Pol ι in protecting the genome stability of cells from detrimental changes in DNA replication dynamics caused by PrimPol.

BRCA2 promotes genomic integrity and therapy resistance primarily through its role in homology-directed repair

Highlights

Gap suppression requires BRCA2 C-terminal RAD51 binding in mouse and human cells Brca2 heterozygosity in mice results in fork protection and gap suppression defects Gap suppression mitigates sensitivity to hmdU, but only when HDR is unperturbedHDR deficiency is the primary driver of chemotherapeutic sensitivity.

eTOC blurb

Lim et al . report that gap suppression as well as fork protection require BRCA2 stabilization of RAD51 filaments in human and mouse cells but have minimal impact on genome integrity, oncogenesis, and drug resistance. BRCA2 suppression of PRIMPOL-mediated replication gaps confers resistance to the nucleotide hmdU, incorporation of which leads to cytotoxic abasic sites.This effect is diminished when HDR is abrogated.

Summary

Tumor suppressor BRCA2 functions in homology-directed repair (HDR), protection of stalled replication forks, and suppression of replicative gaps. The relative contributions of these pathways to genome integrity and chemotherapy response are under scrutiny. Here, we report that mouse and human cells require a RAD51 filament stabilization motif in BRCA2 for both fork protection and gap suppression, but not HDR. Loss of fork protection and gap suppression do not compromise genome instability or shorten tumor latency in mice or cause replication stress in human mammary cells. By contrast, HDR deficiency increases spontaneous and replication stress-induced chromosome aberrations and tumor predisposition. Unlike with HDR, fork protection and gap suppression defects are also observed in Brca2 heterozygous mouse cells, likely due to reduced RAD51 stabilization at stalled forks and gaps. Gaps arise from PRIMPOL activity, which is associated with sensitivity to 5-hydroxymethyl-2’-deoxyuridine due to the formation of abasic sites by SMUG1 glycosylase and is exacerbated by poly(ADP-ribose) polymerase inhibition. However, HDR deficiency ultimately modulates sensitivity to chemotherapeutics, including PARP inhibitors.

Assessment of DNA fibers to track replication dynamics

DNA replication is a complex and tightly regulated process that must proceed accurately and completely if the cell is to faithfully transmit genetic material to its progeny. Organisms have thus evolved complex mechanisms to deal with the myriad exogenous and endogenous sources of replication impediments to which the cell is subject. These mechanisms are of particular relevance to cancer biology, given that such "replication stress" frequently foreshadows genome instability during cancer pathogenesis, and that many traditional chemotherapies and a number of precision medicines function by interfering with the progress of DNA replication. Visualization of the progress and dynamics of DNA replication in living cells was historically a major challenge, neatly surmounted by the development of DNA fiber assays that utilize the fluorescent detection of halogenated nucleotides to track replication forks at single-molecule resolution. This methodology has been widely applied to study the dynamics of unperturbed DNA replication, as well as the cellular responses to various replication stress scenarios. In recent years, subtle modifications to DNA fiber assays have facilitated assessment of the stability of nascent DNA at stalled replication forks, as well as the detection of single-stranded DNA gaps and their subsequent filling by error-prone polymerases. Here, we present and discuss several iterations of the fiber assay and suggest methodologies for the analysis of the data obtained.

Replication fork plasticity upon replication stress requires rapid nuclear actin polymerization

Cells rapidly respond to replication stress actively slowing fork progression and inducing fork reversal. How replication fork plasticity is achieved in the context of nuclear organization is currently unknown. Using nuclear actin probes in living and fixed cells, we visualized nuclear actin filaments in unperturbed S phase, rapidly extending in number and thickness upon genotoxic treatments, and taking frequent contact with replication factories. Chemically or genetically impairing nuclear actin polymerization shortly before these treatments prevents active fork slowing and abolishes fork reversal. Defective fork plasticity is linked to reduced recruitment of RAD51 and SMARCAL1 to nascent DNA. Conversely, PRIMPOL gains access to replicating chromatin, promoting unrestrained and discontinuous DNA synthesis, which is associated with increased chromosomal instability and decreased cellular resistance to replication stress. Hence, nuclear F-actin orchestrates replication fork plasticity and is a key molecular determinant in the rapid cellular response to genotoxic treatments.

Revisiting mutagenesis at non-B DNA motifs in the human genome

Non-B DNA structures formed by repetitive sequence motifs are known instigators of mutagenesis in experimental systems. Analyzing this phenomenon computationally in the human genome requires careful disentangling of intrinsic confounding factors, including overlapping and interrupted motifs and recurrent sequencing errors. Here, we show that accounting for these factors eliminates all signals of repeat-induced mutagenesis that extend beyond the motif boundary, and eliminates or dramatically shrinks the magnitude of mutagenesis within some motifs, contradicting previous reports. Mutagenesis not attributable to artifacts revealed several biological mechanisms. Polymerase slippage generates frequent indels within every variety of short tandem repeat motif, implicating slipped-strand structures. Interruption-correcting single nucleotide variants within short tandem repeats may originate from error-prone polymerases. Secondary-structure formation promotes single nucleotide variants within palindromic repeats and duplications within direct repeats. G-quadruplex motifs cause recurrent sequencing errors, whereas mutagenesis at Z-DNAs is conspicuously absent.

Roles of trans-lesion synthesis (TLS) DNA polymerases in tumorigenesis and cancer therapy

DNA damage tolerance and mutagenesis are hallmarks and enabling characteristics of neoplastic cells that drive tumorigenesis and allow cancer cells to resist therapy. The 'Y-family' trans-lesion synthesis (TLS) DNA polymerases enable cells to replicate damaged genomes, thereby conferring DNA damage tolerance. Moreover, Y-family DNA polymerases are inherently error-prone and cause mutations. Therefore, TLS DNA polymerases are potential mediators of important tumorigenic phenotypes. The skin cancer-propensity syndrome xeroderma pigmentosum-variant (XPV) results from defects in the Y-family DNA Polymerase Pol eta (Polη) and compensatory deployment of alternative inappropriate DNA polymerases. However, the extent to which dysregulated TLS contributes to the underlying etiology of other human cancers is unclear. Here we consider the broad impact of TLS polymerases on tumorigenesis and cancer therapy. We survey the ways in which TLS DNA polymerases are pathologically altered in cancer. We summarize evidence that TLS polymerases shape cancer genomes, and review studies implicating dysregulated TLS as a driver of carcinogenesis. Because many cancer treatment regimens comprise DNA-damaging agents, pharmacological inhibition of TLS is an attractive strategy for sensitizing tumors to genotoxic therapies. Therefore, we discuss the pharmacological tractability of the TLS pathway and summarize recent progress on development of TLS inhibitors for therapeutic purposes.

Replication gap suppression depends on the double-strand DNA binding activity of BRCA2

Replication stress (RS) is a major source of genomic instability and is intrinsic to cancer cells. RS is also the consequence of chemotherapeutic drugs for treating cancer. However, adaptation to RS is also a mechanism of resistance to chemotherapy. BRCA2 deficiency results in replication stress in human cells. BRCA2 protein's main functions include DNA repair by homologous recombination (HR) both at induced DNA double-strand breaks (DSB) and spontaneous replicative lesions. At stalled replication forks, BRCA2 protects the DNA from aberrant nucleolytic degradation and is thought to limit the appearance of ssDNA gaps by arresting replication and via post-replicative HR. However, whether and how BRCA2 acts to limit the formation of ssDNA gaps or mediate their repair, remains ill-defined. Here, we use breast cancer variants affecting different domains of BRCA2 to shed light on this function. We demonstrate that the N-terminal DNA binding domain (NTD), and specifically, its dsDNA binding activity, is required to prevent and repair/fill-in ssDNA gaps upon nucleotide depletion but not to limit PARPi-induced ssDNA gaps. Thus, these findings suggest that nucleotide depletion and PARPi trigger gaps via distinct mechanisms and that the NTD of BRCA2 prevents nucleotide depletion-induced ssDNA gaps.

Leveraging the replication stress response to optimize cancer therapy

High-fidelity DNA replication is critical for the faithful transmission of genetic information to daughter cells. Following genotoxic stress, specialized DNA damage tolerance pathways are activated to ensure replication fork progression. These pathways include translesion DNA synthesis, template switching and repriming. In this Review, we describe how DNA damage tolerance pathways impact genome stability, their connection with tumorigenesis and their effects on cancer therapy response. We discuss recent findings that single-strand DNA gap accumulation impacts chemoresponse and explore a growing body of evidence that suggests that different DNA damage tolerance factors, including translesion synthesis polymerases, template switching proteins and enzymes affecting single-stranded DNA gaps, represent useful cancer targets. We further outline how the consequences of DNA damage tolerance mechanisms could inform the discovery of new biomarkers to refine cancer therapies.

Replication fork uncoupling causes nascent strand degradation and fork reversal

Genotoxins cause nascent strand degradation (NSD) and fork reversal during DNA replication. NSD and fork reversal are crucial for genome stability and are exploited by chemotherapeutic approaches. However, it is unclear how NSD and fork reversal are triggered. Additionally, the fate of the replicative helicase during these processes is unknown. We developed a biochemical approach to study synchronous, localized NSD and fork reversal using Xenopus egg extracts and validated this approach with experiments in human cells. We show that replication fork uncoupling stimulates NSD of both nascent strands and progressive conversion of uncoupled forks to reversed forks. Notably, the replicative helicase remains bound during NSD and fork reversal. Unexpectedly, NSD occurs before and after fork reversal, indicating that multiple degradation steps take place. Overall, our data show that uncoupling causes NSD and fork reversal and elucidate key events that precede fork reversal.

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.

RAD18 opposes transcription-associated genome instability through FANCD2 recruitment

DNA replication is a vulnerable time for genome stability maintenance. Intrinsic stressors, as well as oncogenic stress, can challenge replication by fostering conflicts with transcription and stabilizing DNA:RNA hybrids. RAD18 is an E3 ubiquitin ligase for PCNA that is involved in coordinating DNA damage tolerance pathways to preserve genome stability during replication. In this study, we show that RAD18 deficient cells have higher levels of transcription-replication conflicts and accumulate DNA:RNA hybrids that induce DNA double strand breaks and replication stress. We find that these effects are driven in part by failure to recruit the Fanconi Anemia protein FANCD2 at difficult to replicate and R-loop prone genomic sites. FANCD2 activation caused by splicing inhibition or aphidicolin treatment is critically dependent on RAD18 activity. Thus, we highlight a RAD18-dependent pathway promoting FANCD2-mediated suppression of R-loops and transcription-replication conflicts.

RAD51 paralogs: Expanding roles in replication stress responses and repair

Mammalian RAD51 paralogs are essential for cell survival and are critical for RAD51-mediated repair of DNA double-strand breaks (DSBs) by homologous recombination (HR). However, the molecular mechanism by which RAD51 paralogs participate in HR is largely unclear. Germline mutations in RAD51 paralogs are associated with breast and ovarian cancers and Fanconi anemia-like disorder, underscoring the crucial roles of RAD51 paralogs in genome maintenance and tumor suppression. Despite their discovery over three decades ago, the essential functions of RAD51 paralogs in cell survival and genome stability remain obscure. Recent studies unravel DSB repair independent functions of RAD51 paralogs in replication stress responses. Here, we highlight the recent findings that uncovered the novel functions of RAD51 paralogs in replication fork progression, its stability, and restart and discuss RAD51 paralogs as a potential therapeutic target for cancer treatment.

Mechanisms of PARP1 inhibitor resistance and their implications for cancer treatment

The discovery of synthetic lethality as a result of the combined loss of PARP1 and BRCA has revolutionized the treatment of DNA repair-deficient cancers. With the development of PARP inhibitors, patients displaying germline or somatic mutations in BRCA1 or BRCA2 were presented with a novel therapeutic strategy. However, a large subset of patients do not respond to PARP inhibitors. Furthermore, many of those who do respond eventually acquire resistance. As such, combating de novo and acquired resistance to PARP inhibitors remains an obstacle in achieving durable responses in patients. In this review, we touch on some of the key mechanisms of PARP inhibitor resistance, including restoration of homologous recombination, replication fork stabilization and suppression of single-stranded DNA gap accumulation, as well as address novel approaches for overcoming PARP inhibitor resistance.

POLθ processes ssDNA gaps and promotes replication fork progression in BRCA1-deficient cells

Polymerase theta (POLθ) is an error-prone DNA polymerase whose loss is synthetically lethal in cancer cells bearing breast cancer susceptibility proteins 1 and 2 (BRCA1/2) mutations. To investigate the basis of this genetic interaction, we utilized a small-molecule inhibitor targeting the POLθ polymerase domain. We found that POLθ processes single-stranded DNA (ssDNA) gaps that emerge in the absence of BRCA1, thus promoting unperturbed replication fork progression and survival of BRCA1 mutant cells. A genome-scale CRISPR-Cas9 knockout screen uncovered suppressors of the functional interaction between POLθ and BRCA1, including NBN, a component of the MRN complex, and cell-cycle regulators such as CDK6. While the MRN complex nucleolytically processes ssDNA gaps, CDK6 promotes cell-cycle progression, thereby exacerbating replication stress, a feature of BRCA1-deficient cells that lack POLθ activity. Thus, ssDNA gap formation, modulated by cell-cycle regulators and MRN complex activity, underlies the synthetic lethality between POLθ and BRCA1, an important insight for clinical trials with POLθ inhibitors.