PCNA is efficiently loaded on the DNA recombination intermediate to modulate polymerase δ, η, and ζ activities

Biochemical reconstitution of heat-induced mutational processes

Non-enzymatic spontaneous deamination of 5-methylcytosine, producing thymine, is the proposed etiology of cancer mutational signature 1, which is the most predominant signature in all cancers. Here, the proposed mutational process was reconstituted using synthetic DNA and purified proteins. First, single-stranded DNA containing 5-methylcytosine at CpG context was incubated at an elevated temperature to accelerate spontaneous DNA damage. Then, the DNA was treated with uracil DNA glycosylase to remove uracil residues that were formed by deamination of cytosine. The resulting DNA was then used as a template for DNA synthesis by yeast DNA polymerase δ. The DNA products were analyzed by next-generation DNA sequencing, and mutation frequencies were quantified. The observed mutations after this process were exclusively C>T mutations at CpG context, which was very similar to signature 1. When 5-methylcytosine modification and uracil DNA glycosylase were both omitted, C>T mutations were produced on C residues in all sequence contexts, but these mutations were diminished by uracil DNA glycosylase-treatment. These results indicate that the CpG>TpG mutations were produced by the deamination of 5-methylcytosine. Additional mutations, mainly C>G, were introduced by yeast DNA polymerase ζ on the heat-damaged DNA, indicating that G residues of the templates were also damaged. However, the damage on G residues was not converted to mutations with DNA polymerase δ or ε.

Biochemical reconstitution of a major age-related cancer mutational signature by heat-induced spontaneous deamination of 5-methylcytosine residues, repair of uracil residues, and DNA replication

Non-enzymatic spontaneous deamination of 5-methylcytosine, producing thymine, is the proposed etiology of cancer mutational signature 1, which is the most predominant signature in all cancers. Here, the proposed mutational process was reconstituted using synthetic DNA and purified proteins. First, single-stranded DNA containing 5-methylcytosine at CpG context was incubated at an elevated temperature to accelerate spontaneous DNA damage. Then, the DNA was treated with uracil DNA glycosylase to remove uracil residues that were formed by deamination of cytosine. The resulting DNA was then used as a template for DNA synthesis by yeast DNA polymerase δ. The DNA products were analyzed by next-generation DNA sequencing, and mutation frequencies were quantified. The observed mutations after this process were exclusively C>T mutations at CpG context, which was very similar to signature 1. When 5-methylcytosine modification and uracil DNA glycosylase were both omitted, C>T mutations were produced on C residues in all sequence contexts, but these mutations were diminished by uracil DNA glycosylase-treatment. These results indicate that the CpG>TpG mutations were produced by the deamination of 5-methylcytosine. Additional mutations, mainly C>G, were introduced by yeast DNA polymerase ζ on the heat-damaged DNA, indicating that G residues of the templates were also damaged. However, the damage on G residues was not converted to mutations with DNA polymerase δ or ε. These results provide biochemical evidence to support that the majority of mutations in cancers are produced by ordinary DNA replication on spontaneously damaged DNA.

Physical interactions between specifically regulated subpopulations of the MCM and RNR complexes prevent genetic instability

The helicase MCM and the ribonucleotide reductase RNR are the complexes that provide the substrates (ssDNA templates and dNTPs, respectively) for DNA replication. Here, we demonstrate that MCM interacts physically with RNR and some of its regulators, including the kinase Dun1. These physical interactions encompass small subpopulations of MCM and RNR, are independent of the major subcellular locations of these two complexes, augment in response to DNA damage and, in the case of the Rnr2 and Rnr4 subunits of RNR, depend on Dun1. Partial disruption of the MCM/RNR interactions impairs the release of Rad52 -but not RPA-from the DNA repair centers despite the lesions are repaired, a phenotype that is associated with hypermutagenesis but not with alterations in the levels of dNTPs. These results suggest that a specifically regulated pool of MCM and RNR complexes plays non-canonical roles in genetic stability preventing persistent Rad52 centers and hypermutagenesis.

Biochemical analysis of H2O2-induced mutation spectra revealed that multiple damages were involved in the mutational process

Reactive oxygen species (ROS) are a major threat to genomic integrity and believed to be one of the etiologies of cancers. Here we developed a cell-free system to analyze ROS-induced mutagenesis, in which DNA was exposed to H2O2 and then subjected to translesion DNA synthesis by various DNA polymerases. Then, frequencies of mutations on the DNA products were determined by using next-generation sequencing technology. The majority of observed mutations were either C>A or G>A, caused by dAMP insertion at G and C residues, respectively. These mutations showed similar spectra to COSMIC cancer mutational signature 18 and 36, which are proposed to be caused by ROS. The in vitro mutations can be produced by replicative DNA polymerases (yeast DNA polymerase δ and ε), suggesting that ordinary DNA replication is sufficient to produce them. Very little G>A mutation was observed immediately after exposure to H2O2, but the frequency was increased during the 24 h after the ROS was removed, indicating that the initial oxidation product of cytosine needs to be maturated into a mutagenic lesion. Glycosylase-sensitivities of these mutations suggest that the C>A were made on 8-oxoguanine or Fapy-guanine, and that G>A were most likely made on 5-hydroxycytosine modification.

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.

The DHX9 helicase interacts with human DNA polymerase δ4 and stimulates its activity in D-loop extension synthesis

The extension of the invading strand within a displacement loop (D-loop) is a key step in homology directed repair (HDR) of doubled stranded DNA breaks. The primary goal of these studies was to test the hypotheses that 1) D-loop extension by human DNA polymerase δ4 (Pol δ4) is facilitated by DHX9, a 3' to 5' motor helicase, which acts to unwind the leading edge of the D-loop, and 2) the recruitment of DHX9 is mediated by direct protein-protein interactions between DHX9 and Pol δ4 and/or PCNA. DNA synthesis by Pol δ4 was analyzed in a reconstitution assay by the extension of a 93mer oligonucleotide inserted into a plasmid to form a D-loop. Product formation by Pol δ4 was monitored by incorporation of [α-32P]dNTPs into the 93mer primer followed by denaturing gel electrophoresis. The results showed that DHX9 strongly stimulated Pol δ4 mediated D-loop extension. Direct interactions of DHX9 with PCNA, the p125 and the p12 subunits of Pol δ4 were demonstrated by pull-down assays with purified proteins. These data support the hypothesis that DHX9 helicase is recruited by Pol δ4/PCNA to facilitate D-loop synthesis in HDR, and is a participant in cellular HDR. The involvement of DHX9 in HDR represents an important addition to its multiple cellular roles. Such helicase-polymerase interactions may represent an important aspect of the mechanisms involved in D-loop primer extension synthesis in HDR.

Combination Therapy with Trastuzumab and Niraparib: Quantifying Early Proliferative Alterations in HER2+ Breast Cancer Models

HER2-targeted treatments have improved survival rates in HER2+ breast cancer patients, yet poor responsiveness remains a major clinical obstacle. Recently, HER2+ breast cancer cells, both resistant and responsive to HER2-targeted therapies, have demonstrated sensitivity to poly-(ADP-ribose) polymerase (PARP) inhibition, independent of DNA repair deficiencies. This study seeks to describe biological factors that precede cell viability changes in response to the combination of trastuzumab and PARP inhibition. Treatment response was evaluated in HER2+ and HER2- breast cancer cells. Further, we evaluated the utility of 3'-Deoxy-3'-[18F]-fluorothymidine positron emission tomography ([18F]FLT-PET) imaging for early response assessment in a HER2+ patient derived xenograft (PDX) model of breast cancer. In vitro, we observed decreased cell viability. In vivo, we observed decreased inhibition in tumor growth in combination therapies, compared to vehicle and monotherapy-treated cohorts. Early assessment of cellular proliferation corresponds to endpoint cell viability. Standard summary statistics of [18F]FLT uptake from PET were insensitive to early proliferative changes. Meanwhile, histogram analysis of [18F]FLT uptake indicated the potential translatability of imaging proliferation biomarkers. This study highlights the potential of combined trastuzumab and PARP inhibition in HER2+ breast cancer, while demonstrating a need for optimization of [18F]FLT-PET quantification in heterogeneous models of HER2+ breast cancer.

Proliferating cell nuclear antigen inhibitors block distinct stages of herpes simplex virus infection

Proliferating cell nuclear antigen (PCNA) forms a homotrimer that encircles replicating DNA and is bound by DNA polymerases to add processivity to cellular DNA synthesis. In addition, PCNA acts as a scaffold to recruit DNA repair and chromatin remodeling proteins to replicating DNA via its interdomain connecting loop (IDCL). Despite encoding a DNA polymerase processivity factor UL42, it was previously found that PCNA associates with herpes simplex virus type 1 (HSV-1) replication forks and is necessary for productive HSV-1 infection. To define the role that PCNA plays during viral DNA replication or a replication-coupled process, we investigated the effects that two mechanistically distinct PCNA inhibitors, PCNA-I1 and T2AA, have on the HSV-1 infectious cycle. PCNA-I1 binds at the interface between PCNA monomers, stabilizes the homotrimer, and may interfere with protein-protein interactions. T2AA inhibits select protein-protein interactions within the PCNA IDCL. Here we demonstrate that PCNA-I1 treatment results in reduced HSV-1 DNA replication, late gene expression, and virus production, while T2AA treatment results in reduced late viral gene expression and infectious virus production. To pinpoint the mechanisms by which PCNA inhibitors affect viral processes and protein recruitment to replicated viral DNA, we performed accelerated native isolation of proteins on nascent DNA (aniPOND). Results indicate that T2AA inhibits recruitment of the viral uracil glycosylase UL2 and transcription regulatory factors to viral DNA, likely leading to a defect in viral base excision repair and the observed defect in late viral gene expression and infectious virus production. In addition, PCNA-I1 treatment results in decreased association of the viral DNA polymerase UL30 and known PCNA-interacting proteins with viral DNA, consistent with the observed block in viral DNA replication and subsequent processes. Together, we conclude that inhibitors of cellular PCNA block recruitment of key viral and cellular factors to viral DNA to inhibit viral DNA synthesis and coupled processes.

Saccharomyces cerevisiae DNA polymerase IV overcomes Rad51 inhibition of DNA polymerase δ in Rad52-mediated direct-repeat recombination

Saccharomyces cerevisiae DNA polymerase IV (Pol4) like its homolog, human DNA polymerase lambda (Polλ), is involved in Non-Homologous End-Joining and Microhomology-Mediated Repair. Using genetic analysis, we identified an additional role of Pol4 also in homology-directed DNA repair, specifically in Rad52-dependent/Rad51-independent direct-repeat recombination. Our results reveal that the requirement for Pol4 in repeat recombination was suppressed by the absence of Rad51, suggesting that Pol4 counteracts the Rad51 inhibition of Rad52-mediated repeat recombination events. Using purified proteins and model substrates, we reconstituted in vitro reactions emulating DNA synthesis during direct-repeat recombination and show that Rad51 directly inhibits Polδ DNA synthesis. Interestingly, although Pol4 was not capable of performing extensive DNA synthesis by itself, it aided Polδ in overcoming the DNA synthesis inhibition by Rad51. In addition, Pol4 dependency and stimulation of Polδ DNA synthesis in the presence of Rad51 occurred in reactions containing Rad52 and RPA where DNA strand-annealing was necessary. Mechanistically, yeast Pol4 displaces Rad51 from ssDNA independent of DNA synthesis. Together our in vitro and in vivo data suggest that Rad51 suppresses Rad52-dependent/Rad51-independent direct-repeat recombination by binding to the primer-template and that Rad51 removal by Pol4 is critical for strand-annealing dependent DNA synthesis.

The Inability to Disassemble Rad51 Nucleoprotein Filaments Leads to Aberrant Mitosis and Cell Death

The proper maintenance of genetic material is essential for the survival of living organisms. One of the main safeguards of genome stability is homologous recombination involved in the faithful repair of DNA double-strand breaks, the restoration of collapsed replication forks, and the bypass of replication barriers. Homologous recombination relies on the formation of Rad51 nucleoprotein filaments which are responsible for the homology-based interactions between DNA strands. Here, we demonstrate that without the regulation of these filaments by Srs2 and Rad54, which are known to remove Rad51 from single-stranded and double-stranded DNA, respectively, the filaments strongly inhibit damage-associated DNA synthesis during DNA repair. Furthermore, this regulation is essential for cell survival under normal growth conditions, as in the srs2Δ rad54Δ mutants, unregulated Rad51 nucleoprotein filaments cause activation of the DNA damage checkpoint, formation of mitotic bridges, and loss of genetic material. These genome instability features may stem from the problems at stalled replication forks as the lack of Srs2 and Rad54 in the presence of Rad51 nucleoprotein filaments impedes cell recovery from replication stress. This study demonstrates that the timely and efficient disassembly of recombination machinery is essential for genome maintenance and cell survival.

Host translesion polymerases are required for viral genome integrity

Human cells encode up to 15 DNA polymerases with specialized functions in chromosomal DNA synthesis and damage repair. In contrast, complex DNA viruses, such as those of the herpesviridae family, encode a single B-family DNA polymerase. This disparity raises the possibility that DNA viruses may rely on host polymerases for synthesis through complex DNA geometries. We tested the importance of error-prone Y-family polymerases involved in translesion synthesis (TLS) to human cytomegalovirus (HCMV) infection. We find most Y-family polymerases involved in the nucleotide insertion and bypass of lesions restrict HCMV genome synthesis and replication. In contrast, other TLS polymerases, such as the polymerase ζ complex, which extends past lesions, was required for optimal genome synthesis and replication. Depletion of either the polζ complex or the suite of insertion polymerases demonstrate that TLS polymerases suppress the frequency of viral genome rearrangements, particularly at GC-rich sites and repeat sequences. Moreover, while distinct from HCMV, replication of the related herpes simplex virus type 1 is impacted by host TLS polymerases, suggesting a broader requirement for host polymerases for DNA virus replication. These findings reveal an unexpected role for host DNA polymerases in ensuring viral genome stability.

Fanconi anemia: current insights regarding epidemiology, cancer, and DNA repair

Fanconi anemia is a genetic disorder that is characterized by bone marrow failure, as well as a predisposition to malignancies including leukemia and squamous cell carcinoma (SCC). At least 22 genes are associated with Fanconi anemia, constituting the Fanconi anemia DNA repair pathway. This pathway coordinates multiple processes and proteins to facilitate the repair of DNA adducts including interstrand crosslinks (ICLs) that are generated by environmental carcinogens, chemotherapeutic crosslinkers, and metabolic products of alcohol. ICLs can interfere with DNA transactions, including replication and transcription. If not properly removed and repaired, ICLs cause DNA breaks and lead to genomic instability, a hallmark of cancer. In this review, we will discuss the genetic and phenotypic characteristics of Fanconi anemia, the epidemiology of the disease, and associated cancer risk. The sources of ICLs and the role of ICL-inducing chemotherapeutic agents will also be discussed. Finally, we will review the detailed mechanisms of ICL repair via the Fanconi anemia DNA repair pathway, highlighting critical regulatory processes. Together, the information in this review will underscore important contributions to Fanconi anemia research in the past two decades.

Stalling of Eukaryotic Translesion DNA Polymerases at DNA-Protein Cross-Links

DNA-protein cross-links (DPCs) are extremely bulky adducts that interfere with replication. In human cells, they are processed by SPRTN, a protease activated by DNA polymerases stuck at DPCs. We have recently proposed the mechanism of the interaction of DNA polymerases with DPCs, involving a clash of protein surfaces followed by the distortion of the cross-linked protein. Here, we used a model DPC, located in the single-stranded template, the template strand of double-stranded DNA, or the displaced strand, to study the eukaryotic translesion DNA polymerases ζ (POLζ), ι (POLι) and η (POLη). POLι demonstrated poor synthesis on the DPC-containing substrates. POLζ and POLη paused at sites dictated by the footprints of the polymerase and the cross-linked protein. Beyond that, POLζ was able to elongate the primer to the cross-link site when a DPC was in the template. Surprisingly, POLη was not only able to reach the cross-link site but also incorporated 1–2 nucleotides past it, which makes POLη the most efficient DNA polymerase on DPC-containing substrates. However, a DPC in the displaced strand was an insurmountable obstacle for all polymerases, which stalled several nucleotides before the cross-link site. Overall, the behavior of translesion polymerases agrees with the model of protein clash and distortion described above.

Biochemical and photochemical mechanisms that produce different UV-induced mutation spectra

Although UV-induced mutagenesis has been studied extensively, the precise mechanisms that convert UV-induced DNA damage into mutations remain elusive. One well-studied mechanism involves DNA polymerase (Pol) η and ζ, which produces C > T transitions during translesion synthesis (TLS) across pyrimidine dimers. We previously proposed another biochemical mechanism that involves multiple UV-irradiations with incubation in the dark in between. The incubation facilitates spontaneous deamination of cytosine in a pyrimidine dimer, and the subsequent UV irradiation induces photolyase-independent (direct) photoreversal that converts cytosine into monomeric uracil residue. In this paper, we first demonstrate that natural sunlight can induce both mutational processes in vitro. The direct photoreversal was also reproduced by monochromatic UVB at 300 nm. We also demonstrate that post-irradiation incubation in the dark is required for both mutational processes, suggesting that cytosine deamination is required for both the Pol η/ζ-dependent and the photoreversal-dependent mechanisms. Another Y-family polymerase Pol ι also mediated a mutagenic TLS on UV-damaged templates when combined with Pol ζ. The Pol ι-dependent mutations were largely independent of post-irradiation incubation, indicating that cytosine deamination was not essential for this mutational process. Sunlight-exposure also induced C > A transversions which were likely caused by oxidation of guanine residues. Finally, we constructed in vitro mutation spectra in a comparable format to cancer mutation signatures. While both Pol η-dependent and photoreversal-dependent spectra showed high similarities to a cancer signature (SBS7a), Pol ι-dependent mutation spectrum has distinct T > A/C substitutions, which are found in another cancer signature (SBS7d). The Pol ι-dependent T > A/C substitutions were resistant to T4 pyrimidine dimer glycosylase-treatment, suggesting that this mutational process is independent of cis-syn pyrimidine dimers. An updated model about multiple mechanisms of UV-induced mutagenesis is discussed.

Balancing DNA repair to prevent ageing and cancer

DNA damage is a constant stressor to the cell. Persistent damage to the DNA over time results in an increased risk of mutation and an accumulation of mutations with age. Loss of efficient DNA damage repair can lead to accelerated ageing phenotypes or an increased cancer risk, and the trade-off between cancer susceptibility and longevity is often driven by the cell's response to DNA damage. High levels of mutations in DNA repair mutants often leads to excessive cell death and stem cell exhaustion which may promote premature ageing. Stem cells themselves have distinct characteristics that enable them to retain low mutation rates. However, when mutations do arise, stem cell clonal expansion can also contribute to age-related tissue dysfunction as well as heightened cancer risk. In this review, we will highlight increasing DNA damage and mutation accumulation as hallmarks common to both ageing and cancer. We will propose that anti-ageing interventions might be cancer preventative and discuss the mechanisms through which they may act.

The role of DNA damage response in chemo- and radio-resistance of cancer cells: Can DDR inhibitors sole the problem?

Up to now, many improvements have been made in providing more therapeutic strategies for cancer patients. The lack of susceptibility to common therapies like chemo- and radio-therapy is one of the reasons why we need more methods in the field of cancer therapy. DNA damage response (DDR) is a set of mechanisms which identifies DNA lesions and triggers the repair process for restoring DNA after causing an arrest in the cell cycle. The ability of DDR in maintaining the genome stability and integrity can be favorable to cancerous cells which are exposed to radiation therapy or are treated with chemotherapeutic agents. When DDR mechanisms are error-free in cancer cells, they can escape the expected cellular death and display resistance to treatment. In this regard, targeting different components of DDR can help to increase the susceptibility of advanced tumors to chemo- and radio-therapy.

Strand Displacement Activity of PrimPol

Human PrimPol is a unique enzyme possessing DNA/RNA primase and DNA polymerase activities. In this work, we demonstrated that PrimPol efficiently fills a 5-nt gap and possesses the conditional strand displacement activity stimulated by Mn²⁺ ions and accessory replicative proteins RPA and PolDIP2. The DNA displacement activity of PrimPol was found to be more efficient than the RNA displacement activity and FEN1 processed the 5′-DNA flaps generated by PrimPol in vitro.

Polymerase δ promotes chromosomal rearrangements and imprecise double-strand break repair

Recent studies have implicated DNA polymerases θ (Pol θ) and β (Pol β) as mediators of alternative nonhomologous end-joining (Alt-NHEJ) events, including chromosomal translocations. Here we identify subunits of the replicative DNA polymerase δ (Pol δ) as promoters of Alt-NHEJ that results in more extensive intrachromosomal mutations at a single double-strand break (DSB) and more frequent translocations between two DSBs. Depletion of the Pol δ accessory subunit POLD2 destabilizes the complex, resulting in degradation of both POLD1 and POLD3 in human cells. POLD2 depletion markedly reduces the frequency of translocations with sequence modifications but does not affect the frequency of translocations with exact joins. Using separation-of-function mutants, we show that both the DNA synthesis and exonuclease activities of the POLD1 subunit contribute to translocations. As described in yeast and unlike Pol θ, Pol δ also promotes homology-directed repair. Codepletion of POLD2 with 53BP1 nearly eliminates translocations. POLD1 and POLD2 each colocalize with phosphorylated H2AX at ionizing radiation-induced DSBs but not with 53BP1. Codepletion of POLD2 with either ligase 3 (LIG3) or ligase 4 (LIG4) does not further reduce translocation frequency compared to POLD2 depletion alone. Together, these data support a model in which Pol δ promotes Alt-NHEJ in human cells at DSBs, including translocations.

Comparison of High- and Low-LET Radiation-Induced DNA Double-Strand Break Processing in Living Cells

High-linear-energy-transfer (LET) radiation is more lethal than similar doses of low-LET radiation types, probably a result of the condensed energy deposition pattern of high-LET radiation. Here, we compare high-LET α-particle to low-LET X-ray irradiation and monitor double-strand break (DSB) processing. Live-cell microscopy was used to monitor DNA double-strand breaks (DSBs), marked by p53-binding protein 1 (53BP1). In addition, the accumulation of the endogenous 53BP1 and replication protein A (RPA) DSB processing proteins was analyzed by immunofluorescence. In contrast to α-particle-induced 53BP1 foci, X-ray-induced foci were resolved quickly and more dynamically as they showed an increase in 53BP1 protein accumulation and size. In addition, the number of individual 53BP1 and RPA foci was higher after X-ray irradiation, while focus intensity was higher after α-particle irradiation. Interestingly, 53BP1 foci induced by α-particles contained multiple RPA foci, suggesting multiple individual resection events, which was not observed after X-ray irradiation. We conclude that high-LET α-particles cause closely interspaced DSBs leading to high local concentrations of repair proteins. Our results point toward a change in DNA damage processing toward DNA end-resection and homologous recombination, possibly due to the depletion of soluble protein in the nucleoplasm. The combination of closely interspaced DSBs and perturbed DNA damage processing could be an explanation for the increased relative biological effectiveness (RBE) of high-LET α-particles compared to X-ray irradiation.

Role of RadA and DNA Polymerases in Recombination-Associated DNA Synthesis in Hyperthermophilic Archaea

Among the three domains of life, the process of homologous recombination (HR) plays a central role in the repair of double-strand DNA breaks and the restart of stalled replication forks. Curiously, main protein actors involved in the HR process appear to be essential for hyperthermophilic Archaea raising interesting questions about the role of HR in replication and repair strategies of those Archaea living in extreme conditions. One key actor of this process is the recombinase RadA, which allows the homologous strand search and provides a DNA substrate required for following DNA synthesis and restoring genetic information. DNA polymerase operation after the strand exchange step is unclear in Archaea. Working with Pyrococcus abyssi proteins, here we show that both DNA polymerases, family-B polymerase (PolB) and family-D polymerase (PolD), can take charge of processing the RadA-mediated recombination intermediates. Our results also indicate that PolD is far less efficient, as compared with PolB, to extend the invaded DNA at the displacement-loop (D-loop) substrate. These observations coincide with previous genetic analyses obtained on Thermococcus species showing that PolB is mainly involved in DNA repair without being essential probably because PolD could take over combined with additional partners.

Interaction of Deubiquitinase 2A-DUB/MYSM1 with DNA Repair and Replication Factors

The deubiquitination of histone H2A on lysine 119 by 2A-DUB/MYSM1, BAP1, USP16, and other enzymes is required for key cellular processes, including transcriptional activation, apoptosis, and cell cycle control, during normal hematopoiesis and tissue development, and in tumor cells. Based on our finding that MYSM1 colocalizes with γH2AX foci in human peripheral blood mononuclear cells, leukemia cells, and melanoma cells upon induction of DNA double-strand breaks with topoisomerase inhibitor etoposide, we applied a mass spectrometry-based proteomics approach to identify novel 2A-DUB/MYSM1 interaction partners in DNA-damage responses. Differential display of MYSM1 binding proteins significantly enriched after exposure of 293T cells to etoposide revealed an interacting network of proteins involved in DNA damage and replication, including factors associated with poor melanoma outcome. In the context of increased DNA-damage in a variety of cell types in Mysm1-deficient mice, in bone marrow cells upon aging and in UV-exposed Mysm1-deficient skin, our current mass spectrometry data provide additional evidence for an interaction between MYSM1 and key DNA replication and repair factors, and indicate a potential function of 2A-DUB/MYSM1 in DNA repair processes.

Biochemical reconstitution of UV-induced mutational processes

We reconstituted two biochemical processes that may contribute to UV-induced mutagenesis in vitro and analysed the mutational profiles in the products. One process is translesion synthesis (TLS) by DNA polymerases (Pol) δ, η and ζ, which creates C>T transitions at pyrimidine dimers by incorporating two dAMPs opposite of the dimers. The other process involves spontaneous deamination of cytosine, producing uracil in pyrimidine dimers, followed by monomerization of the dimers by secondary UV irradiation, and DNA synthesis by Pol δ. The mutational spectrum resulting from deamination without translesion synthesis is similar to a mutational signature found in melanomas, suggesting that cytosine deamination encountered by the replicative polymerase has a prominent role in melanoma development. However, CC>TT dinucleotide substitution, which is also commonly observed in melanomas, was produced almost exclusively by TLS. We propose that both TLS-dependent and deamination-dependent mutational processes are likely involved in UV-induced melanoma development.

Methylation of UHRF1 by SET7 is essential for DNA double-strand break repair

Ubiquitin-like with PHD and RING finger domains 1 (UHRF1) is a key epigenetic regulator of DNA methylation maintenance and heterochromatin formation. The roles of UHRF1 in DNA damage repair also have been emphasized in recent years. However, the regulatory mechanism of UHRF1 remains elusive. In this study, we showed that UHRF1 is methylated by SET7 and demethylation is catalyzed by LSD1. In addition, methylation of UHRF1 is induced in response to DNA damage and its phosphorylation in S phase is a prerequisite for interaction with SET7. Furthermore, UHRF1 methylation catalyzes the conjugation of polyubiquitin chains to PCNA and promotes homologous recombination for DNA repair. SET7-mediated UHRF1 methylation is also shown to be essential for cell viability against DNA damage. Our data revealed the regulatory mechanism underlying the UHRF1 methylation status by SET7 and LSD1 in double-strand break repair pathway.

Chronic stress reduces spermatogenic cell proliferation in rat testis

Male reproductive dysfunction induced by mental stress and environmental factors has increased greatly in recent years. Previous studies of the male rat reproductive system under stress conditions evaluated changes in physiology and pathophysiology. However, no genome-wide study has been applied to such models. Here we studied the histopathologic changes in testes of rats under different durations of stress and used RNA sequencing (RNA-seq) to investigate the testicular transcriptome and detect differentially expressed genes. Reverse transcription quantitative real-time polymerase chain reaction (RT-qPCR) and immunohistochemistry were used to verify these. Chronic stress resulted in significant histopathologic changes in seminiferous tubules and RNA-seq showed that growing numbers of genes were dysregulated with increasing stress exposure. Gene Ontology (GO) analysis showed that many biological processes of cell proliferation-associated terms were highly significantly enriched among downregulated genes, from chronically stressed groups. Proliferating cell nuclear antigen (PCNA) was used as a key marker of cell proliferation. RT-qPCR and immunohistochemistry indicated that PCNA mRNA and protein expression levels were greatly decreased with prolonged stress, thereby contributing to the attenuation of spermatogenic cell proliferation in the rat testis. This could provide a new scientific basis for the study of male reproductive dysfunction caused by stress.

Ex Vivo Hepatocyte Reprograming Promotes Homology-Directed DNA Repair to Correct Metabolic Disease in Mice After Transplantation

Ex vivo CRISPR/Cas9-mediated gene editing in hepatocytes using homology-directed repair (HDR) is a potential alternative curative therapy to organ transplantation for metabolic liver disease. However, a major limitation of this approach in quiescent adult primary hepatocytes is that nonhomologous end-joining is the predominant DNA repair pathway for double-strand breaks (DSBs). This study explored the hypothesis that ex vivo hepatocyte culture could reprogram hepatocytes to favor HDR after CRISPR/Cas9-mediated DNA DSBs. Quantitative PCR (qPCR), RNA sequencing, and flow cytometry demonstrated that within 24 hours, primary mouse hepatocytes in ex vivo monolayer culture decreased metabolic functions and increased expression of genes related to mitosis progression and HDR. Despite the down-regulation of hepatocyte function genes, hepatocytes cultured for up to 72 hours could robustly engraft in vivo. To assess functionality long-term, primary hepatocytes from a mouse model of hereditary tyrosinemia type 1 bearing a single-point mutation were transduced ex vivo with two adeno-associated viral vectors to deliver the Cas9 nuclease, target guide RNAs, and a 1.2-kb homology template. Adeno-associated viral Cas9 induced robust cutting at the target locus, and, after delivery of the repair template, precise correction of the point mutation occurred by HDR. Edited hepatocytes were transplanted into recipient fumarylacetoacetate hydrolase knockout mice, resulting in engraftment, robust proliferation, and prevention of liver failure. Weight gain and biochemical assessment revealed normalization of metabolic function. Conclusion: The results of this study demonstrate the potential therapeutic effect of ex vivo hepatocyte-directed gene editing after reprogramming to cure metabolic disease in a preclinical model of hereditary tyrosinemia type 1.

Guidelines for DNA recombination and repair studies: Mechanistic assays of DNA repair processes

Genomes are constantly in flux, undergoing changes due to recombination, repair and mutagenesis. In vivo, many of such changes are studies using reporters for specific types of changes, or through cytological studies that detect changes at the single-cell level. Single molecule assays, which are reviewed here, can detect transient intermediates and dynamics of events. Biochemical assays allow detailed investigation of the DNA and protein activities of each step in a repair, recombination or mutagenesis event. Each type of assay is a powerful tool but each comes with its particular advantages and limitations. Here the most commonly used assays are reviewed, discussed, and presented as the guidelines for future studies.

DNA Damage Tolerance Mechanisms Revealed from the Analysis of Immunoglobulin V Gene Diversification in Avian DT40 Cells

DNA replication is an essential biochemical reaction in dividing cells that frequently stalls at damaged sites. Homologous/homeologous recombination (HR)-mediated template switch and translesion DNA synthesis (TLS)-mediated bypass processes release arrested DNA replication forks. These mechanisms are pivotal for replication fork maintenance and play critical roles in DNA damage tolerance (DDT) and gap-filling. The avian DT40 B lymphocyte cell line provides an opportunity to examine HR-mediated template switch and TLS triggered by abasic sites by sequencing the constitutively diversifying immunoglobulin light-chain variable gene (IgV). During IgV diversification, activation-induced deaminase (AID) converts dC to dU, which in turn is excised by uracil DNA glycosylase and yields abasic sites within a defined window of around 500 base pairs. These abasic sites can induce gene conversion with a set of homeologous upstream pseudogenes via the HR-mediated template switch, resulting in templated mutagenesis, or can be bypassed directly by TLS, resulting in non-templated somatic hypermutation at dC/dG base pairs. In this review, we discuss recent works unveiling IgV diversification mechanisms in avian DT40 cells, which shed light on DDT mode usage in vertebrate cells and tolerance of abasic sites.

MiR-584-5p potentiates vincristine and radiation response by inducing spindle defects and DNA damage in medulloblastoma

Despite improvements in overall survival, only a modest percentage of patients survives high-risk medulloblastoma. The devastating side effects of radiation and chemotherapy substantially reduce quality of life for surviving patients. Here, using genomic screens, we identified miR-584-5p as a potent therapeutic adjuvant that potentiates medulloblastoma to radiation and vincristine. MiR-584-5p inhibited medulloblastoma growth and prolonged survival of mice in pre-clinical tumor models. MiR-584-5p overexpression caused cell cycle arrest, DNA damage, and spindle defects in medulloblastoma cells. MiR-584-5p mediated its tumor suppressor and therapy-sensitizing effects by targeting HDAC1 and eIF4E3. MiR-584-5p overexpression or HDAC1/eIF4E3 silencing inhibited medulloblastoma stem cell self-renewal without affecting neural stem cell growth. In medulloblastoma patients, reduced expression of miR-584-5p correlated with increased levels of HDAC1/eIF4E3. These findings identify a previously undefined role for miR-584-5p/HDAC1/eIF4E3 in regulating DNA repair, microtubule dynamics, and stemness in medulloblastoma and set the stage for a new way to treat medulloblastoma using miR-584-5p.

Putting together and taking apart: assembly and disassembly of the Rad51 nucleoprotein filament in DNA repair and genome stability

Homologous recombination is a key mechanism providing both genome stability and genetic diversity in all living organisms. Recombinases play a central role in this pathway: multiple protein subunits of Rad51 or its orthologues bind single-stranded DNA to form a nucleoprotein filament which is essential for initiating recombination events. Multiple factors are involved in the regulation of this step, both positively and negatively. In this review, we discuss Rad51 nucleoprotein assembly and disassembly, how it is regulated and what functional significance it has in genome maintenance.

Dynamaic changes of proliferation and apoptosis in rat retina development

Precise coordination of cell proliferation and apoptosis is essential for proper organ morphogenesis and function during mammalian development. The retina is a part of the central nervous system, with a function in capture and conversion of light particles into nerve signals. However, the dynamical changes of cell proliferation and apoptosis during the retinal development is unclear. Thus, this study was to observe the changes of proliferating cell nuclear antigen expression and apoptosis during the retina development in rats. Rats from different developmental stages (embryonic days 14-20 (E14-20), postnatal days 0-15 (P0-15) and adult (P36)) were used. The eyeballs were removed and fixed immediately in formalin. The tissues were embedded in paraffin. Sections were prepared for immunohistochemical staining and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). PCNA expression appeared in large quantities in rat retina from E14 to P7, decreased obviously since P9, and could not be observed since P15. Apoptotic cells appeared in small quantities since E18. And then apoptotic cells dramatically increased during the retina development, and peaked at P7. As maturation continued, these cells decreased gradually. Apoptotic cells could not be found in the mature retina. Cell proliferation and apoptosis shows orderly and dynamic changes during the retina development, which is essential for the development of normal structure and function of the retina.

NGS-based analysis of base-substitution signatures created by yeast DNA polymerase eta and zeta on undamaged and abasic DNA templates in vitro

Translesion synthesis (TLS) is the mechanism in which DNA polymerases (TLS polymerases) bypass unrepaired template damage with high error rates. DNA polymerase η and ζ (Polη and Polζ) are major TLS polymerases that are conserved from yeast to humans. In this study, we quantified frequencies of base-substitutions by yeast Polη and Polζ on undamaged and abasic templates in vitro. For accurate quantification, we used a next generation sequencing (NGS)-based method where DNA products were directly analyzed by parallel sequencing. On undamaged templates, Polη and Polζ showed distinct base-substitution profiles, and the substitution frequencies were differently influenced by the template sequence. The base-substitution frequencies were influenced mainly by the adjacent bases both upstream and downstream of the substitution sites. Thus we present the base-substitution signatures of these polymerases in a three-base format. On templates containing abasic sites, Polη created deletions at the lesion in more than 50% of the TLS products, but the formation of the deletions was suppressed by the presence of Polζ. Polζ and Polη cooperatively facilitated the TLS reaction over an abasic site in vitro, suggesting that these two polymerases can cooperate in efficient and high fidelity TLS.

The Role of PALB2 in the DNA Damage Response and Cancer Predisposition

The deoxyribonucleic acid (DNA) damage response (DDR) is a major feature in the maintenance of genome integrity and in the suppression of tumorigenesis. PALB2 (Partner and Localizer of Breast Cancer 2 (BRCA2)) plays an important role in maintaining genome integrity through its role in the Fanconi anemia (FA) and homologous recombination (HR) DNA repair pathways. Since its identification as a BRCA2 interacting partner, PALB2 has emerged as a pivotal tumor suppressor protein associated to hereditary cancer susceptibility to breast and pancreatic cancers. In this review, we discuss how other DDR proteins (such as the kinases Ataxia Telangiectasia Mutated (ATM) and ATM- and Rad3-Related (ATR), mediators BRCA1 (Breast Cancer 1)/BRCA2 and effectors RAD51/DNA Polymerase η (Polη) interact with PALB2 to orchestrate DNA repair. We also examine the involvement of PALB2 mutations in the predisposition to cancer and the role of PALB2 in stimulating error-free DNA repair through the FA/HR pathway.

Srs2 promotes synthesis-dependent strand annealing by disrupting DNA polymerase δ-extending D-loops

Synthesis-dependent strand annealing (SDSA) is the preferred mode of homologous recombination in somatic cells leading to an obligatory non-crossover outcome, thus avoiding the potential for chromosomal rearrangements and loss of heterozygosity. Genetic analysis identified the Srs2 helicase as a prime candidate to promote SDSA. Here, we demonstrate that Srs2 disrupts D-loops in an ATP-dependent fashion and with a distinct polarity. Specifically, we partly reconstitute the SDSA pathway using Rad51, Rad54, RPA, RFC, DNA Polymerase δ with different forms of PCNA. Consistent with genetic data showing the requirement for SUMO and PCNA binding for the SDSA role of Srs2, Srs2 displays a slight but significant preference to disrupt extending D-loops over unextended D-loops when SUMOylated PCNA is present, compared to unmodified PCNA or monoubiquitinated PCNA. Our data establish a biochemical mechanism for the role of Srs2 in crossover suppression by promoting SDSA through disruption of extended D-loops.

DOI:http://dx.doi.org/10.7554/eLife.22195.001

Eukaryotic DNA Polymerases in Homologous Recombination

Homologous recombination (HR) is a central process to ensure genomic stability in somatic cells and during meiosis. HR-associated DNA synthesis determines in large part the fidelity of the process. A number of recent studies have demonstrated that DNA synthesis during HR is conservative, less processive, and more mutagenic than replicative DNA synthesis. In this review, we describe mechanistic features of DNA synthesis during different types of HR-mediated DNA repair, including synthesis-dependent strand annealing, break-induced replication, and meiotic recombination. We highlight recent findings from diverse eukaryotic organisms, including humans, that suggest both replicative and translesion DNA polymerases are involved in HR-associated DNA synthesis. Our focus is to integrate the emerging literature about DNA polymerase involvement during HR with the unique aspects of these repair mechanisms, including mutagenesis and template switching.

Unloading of homologous recombination factors is required for restoring double-stranded DNA at damage repair loci

Cells use homology‐dependent DNA repair to mend chromosome breaks and restore broken replication forks, thereby ensuring genome stability and cell survival. DNA break repair via homology‐based mechanisms involves nuclease‐dependent DNA end resection, which generates long tracts of single‐stranded DNA required for checkpoint activation and loading of homologous recombination proteins Rad52/51/55/57. While recruitment of the homologous recombination machinery is well characterized, it is not known how its presence at repair loci is coordinated with downstream re‐synthesis of resected DNA. We show that Rad51 inhibits recruitment of proliferating cell nuclear antigen (PCNA), the platform for assembly of the DNA replication machinery, and that unloading of Rad51 by Srs2 helicase is required for efficient PCNA loading and restoration of resected DNA. As a result, srs2Δ mutants are deficient in DNA repair correlating with extensive DNA processing, but this defect in srs2Δ mutants can be suppressed by inactivation of the resection nuclease Exo1. We propose a model in which during re‐synthesis of resected DNA, the replication machinery must catch up with the preceding processing nucleases, in order to close the single‐stranded gap and terminate further resection.

Acetylation of PCNA Sliding Surface by Eco1 Promotes Genome Stability through Homologous Recombination

During DNA replication, proliferating cell nuclear antigen (PCNA) adopts a ring-shaped structure to promote processive DNA synthesis, acting as a sliding clamp for polymerases. Known posttranslational modifications function at the outer surface of the PCNA ring to favor DNA damage bypass. Here, we demonstrate that acetylation of lysine residues at the inner surface of PCNA is induced by DNA lesions. We show that cohesin acetyltransferase Eco1 targets lysine 20 at the sliding surface of the PCNA ring in vitro and in vivo in response to DNA damage. Mimicking constitutive acetylation stimulates homologous recombination and robustly suppresses the DNA damage sensitivity of mutations in damage tolerance pathways. In comparison to the unmodified trimer, structural differences are observed at the interface between protomers in the crystal structure of the PCNA-K20ac ring. Thus, acetylation regulates PCNA sliding on DNA in the presence of DNA damage, favoring homologous recombination linked to sister-chromatid cohesion.

Characterization of human translesion DNA synthesis across a UV-induced DNA lesion

Translesion DNA synthesis (TLS) during S-phase uses specialized TLS DNA polymerases to replicate a DNA lesion, allowing stringent DNA synthesis to resume beyond the offending damage. Human TLS involves the conjugation of ubiquitin to PCNA clamps encircling damaged DNA and the role of this post-translational modification is under scrutiny. A widely-accepted model purports that ubiquitinated PCNA recruits TLS polymerases such as pol η to sites of DNA damage where they may also displace a blocked replicative polymerase. We provide extensive quantitative evidence that the binding of pol η to PCNA and the ensuing TLS are both independent of PCNA ubiquitination. Rather, the unique properties of pols η and δ are attuned to promote an efficient and passive exchange of polymerases during TLS on the lagging strand.

DOI:http://dx.doi.org/10.7554/eLife.19788.001

Enhancing tumor-targeting monoclonal antibodies therapy by PARP inhibitors

Monoclonal antibodies (mAbs) have become a successful therapeutic approach in cancer. However, some patients do not achieve long-term clinical benefit and most mAbs only exert modest effects as monotherapies. Therefore, combinations with chemotherapy are currently being investigated. Emerging studies have shown a synergistic therapeutic effect of PARP inhibitors and mAbs in cancer. PARP enzymes catalytically cleave β-NAD⁺ and transfer the ADP-ribose moiety to acceptor proteins, modifying their function. In here, we update recent data about the therapeutic effect of the combination of PARP inhibitors with mAbs in cancer treatment and discuss the molecular mechanisms involved in this synergy.

Roles of mutagenic translesion synthesis in mammalian genome stability, health and disease

Most spontaneous and DNA damage-induced nucleotide substitutions in eukaryotes depend on translesion synthesis polymerases Rev1 and Pol ζ, the latter consisting of the catalytic subunit Rev3 and the accessory protein Rev7. Here we review the regulation, and the biochemical and cellular functions, of Rev1/Pol ζ-dependent translesion synthesis. These are correlated with phenotypes of mouse models with defects in Rev1, Rev3 or Rev7. The data indicate that Rev1/Pol ζ-mediated translesion synthesis is important for adaptive immunity while playing paradoxical roles in oncogenesis. On the other hand, by enabling the replication of endogenously damaged templates, Rev1/Pol ζ -dependent translesion synthesis protects stem cells, thereby preventing features of ageing. In conclusion, Rev1/Pol ζ-dependent translesion synthesis at DNA helix-distorting nucleotide lesions orchestrates pleiotropic responses that determine organismal fitness and disease.

Proliferating cell nuclear antigen binds DNA polymerase-β and mediates 1-methyl-4-phenylpyridinium-induced neuronal death

The mechanisms leading to dopaminergic neuronal loss in the substantia nigra of patients with Parkinson disease (PD) remain poorly understood. We recently reported that aberrant DNA replication mediated by DNA polymerase-β (DNA pol-β) plays a causal role in the death of postmitotic neurons in an in vitro model of PD. In the present study, we show that both proliferating cell nuclear antigen (PCNA) and DNA pol-β are required for MPP(+)-induced neuronal death. PCNA binds to the catalytic domain of DNA pol-β in MPP(+)-treated neurons and in post-mortem brain tissues of PD patients. The PCNA-DNA pol-β complex is loaded into DNA replication forks and mediates DNA replication in postmitotic neurons. The aberrant DNA replication mediated by the PCNA-DNA pol-β complex induces p53-dependent neuronal cell death. Our results indicate that the interaction of PCNA and DNA pol-β contributes to neuronal death in PD.

Genetic evidence suggests that Spata22 is required for the maintenance of Rad51 foci in mammalian meiosis

Meiotic nodules are the sites of double-stranded DNA break repair. Rpa is a single-stranded DNA-binding protein, and Rad51 is a protein that assists in the repair of DNA double strand breaks. The localisation of Rad51 to meiotic nodules before the localisation of Rpa in mice introduces the issue of whether Rpa is involved in presynaptic filament formation during mammalian meiosis. Here, we show that a protein with unknown function, Spata22, colocalises with Rpa in meiotic nodules in rat spermatocytes. In spermatocytes of Spata22-deficient mutant rats, meiosis was arrested at the zygotene-like stage, and a normal number of Rpa foci was observed during leptotene- and zygotene-like stages. The number of Rad51 foci was initially normal but declined from the leptotene-like stage. These results suggest that both formation and maintenance of Rpa foci are independent of Spata22, and the maintenance, but not the formation, of Rad51 foci requires Spata22. We propose a possible model of presynaptic filament formation in mammalian meiosis, which involves Rpa and Spata22.

Poly (ADP-ribose) polymerase inhibition enhances trastuzumab antitumour activity in HER2 overexpressing breast cancer

AIM

Poly (ADP-ribose) polymerase (PARP) inhibitors have shown promising results in Breast Cancer (BRCA) deficient breast cancer, but not in molecularly unselected patient populations. Two lines of research in this field are needed: the identification of novel subsets of patients that could potentially benefit from PARP inhibitors and the discovery of suitable targeted therapies for combination strategies.

METHODS

We tested PARP inhibition, alone or combined with the anti-HER2 antibody trastuzumab on HER2+ breast cancer. We used two PARP inhibitors in clinical development, olaparib and rucaparib, as well as genetic downmodulation of PARP-1 for in vitro studies. DNA damage was studied by the formation of γH2AX foci and comet assay. Finally, the in vivo anti-tumour effect of olaparib and trastuzumab was examined in nude mice subcutaneously implanted with BT474 cells.

RESULTS

In a panel of four HER2 overexpressing breast cancer cell lines, both olaparib and rucaparib significantly decreased cell growth and enhanced anti-tumour effects of trastuzumab. Cells exposed to olaparib and trastuzumab had greater DNA damage than cells exposed to each agent alone. Mechanistic exploratory assays showed that trastuzumab downmodulated the homologous recombination protein proliferating cell nuclear antigen (PCNA). Combination treatment in the BT474 xenograft model resulted in enhanced growth inhibition, reduced tumour cell proliferation, and increased DNA damage and apoptosis.

CONCLUSION

Taken together, our results show that PARP inhibition has antitumour effects and increases trastuzumab activity in HER2 overexpressing breast cancer. These findings make this novel combination a promising strategy for clinical development.

Breast cancer proteins PALB2 and BRCA2 stimulate polymerase η in recombination-associated DNA synthesis at blocked replication forks

One envisioned function of homologous recombination (HR) is to find a template for DNA synthesis from the resected 3'-OH molecules that occur during double-strand break (DSB) repair at collapsed replication forks. However, the interplay between DNA synthesis and HR remains poorly understood in higher eukaryotic cells. Here, we reveal functions for the breast cancer proteins BRCA2 and PALB2 at blocked replication forks and show a role for these proteins in stimulating polymerase η (Polη) to initiate DNA synthesis. PALB2, BRCA2, and Polη colocalize at stalled or collapsed replication forks after hydroxyurea treatment. Moreover, PALB2 and BRCA2 interact with Polη and are required to sustain the recruitment of Polη at blocked replication forks. PALB2 and BRCA2 stimulate Polη-dependent DNA synthesis on D loop substrates. We conclude that PALB2 and BRCA2, in addition to their functions in D loop formation, play crucial roles in the initiation of recombination-associated DNA synthesis by Polη-mediated DNA repair.

A small molecule inhibitor of monoubiquitinated Proliferating Cell Nuclear Antigen (PCNA) inhibits repair of interstrand DNA cross-link, enhances DNA double strand break, and sensitizes cancer cells to cisplatin

Small molecule inhibitors of proliferating cell nuclear antigen (PCNA)/PCNA interacting protein box (PIP-Box) interactions, including T2 amino alcohol (T2AA), inhibit translesion DNA synthesis. The crystal structure of PCNA in complex with T2AA revealed that T2AA bound to the surface adjacent to the subunit interface of the homotrimer of PCNA in addition to the PIP-box binding cavity. Because this site is close to Lys-164, which is monoubiquitinated by RAD18, we postulated that T2AA would affect monoubiquitinated PCNA interactions. Binding of monoubiquitinated PCNA and a purified pol η fragment containing the UBZ and PIP-box was inhibited by T2AA in vitro. T2AA decreased PCNA/pol η and PCNA/REV1 chromatin colocalization but did not inhibit PCNA monoubiquitination, suggesting that T2AA hinders interactions of pol η and REV1 with monoubiquitinated PCNA. Interstrand DNA cross-links (ICLs) are repaired by mechanisms using translesion DNA synthesis that is regulated by monoubiquitinated PCNA. T2AA significantly delayed reactivation of a reporter plasmid containing an ICL. Neutral comet analysis of cells receiving T2AA in addition to cisplatin revealed that T2AA significantly enhanced formation of DNA double strand breaks (DSBs) by cisplatin. T2AA promoted colocalized foci formation of phospho-ATM and 53BP1 and up-regulated phospho-BRCA1 in cisplatin-treated cells, suggesting that T2AA increases DSBs. When cells were treated by cisplatin and T2AA, their clonogenic survival was significantly less than that of those treated by cisplatin only. These findings show that the inhibitors of monoubiquitinated PCNA chemosensitize cells by inhibiting repair of ICLs and DSBs.

Length-dependent processing of telomeres in the absence of telomerase

In the absence of telomerase, telomeres progressively shorten with every round of DNA replication, leading to replicative senescence. In telomerase-deficient Saccharomyces cerevisiae, the shortest telomere triggers the onset of senescence by activating the DNA damage checkpoint and recruiting homologous recombination (HR) factors. Yet, the molecular structures that trigger this checkpoint and the mechanisms of repair have remained elusive. By tracking individual telomeres, we show that telomeres are subjected to different pathways depending on their length. We first demonstrate a progressive accumulation of subtelomeric single-stranded DNA (ssDNA) through 5'-3' resection as telomeres shorten. Thus, exposure of subtelomeric ssDNA could be the signal for cell cycle arrest in senescence. Strikingly, early after loss of telomerase, HR counteracts subtelomeric ssDNA accumulation rather than elongates telomeres. We then asked whether replication repair pathways contribute to this mechanism. We uncovered that Rad5, a DNA helicase/Ubiquitin ligase of the error-free branch of the DNA damage tolerance (DDT) pathway, associates with native telomeres and cooperates with HR in senescent cells. We propose that DDT acts in a length-independent manner, whereas an HR-based repair using the sister chromatid as a template buffers precocious 5'-3' resection at the shortest telomeres.