Vertebrate POLQ and POLbeta cooperate in base excision repair of oxidative DNA damage

PrimPol-mediated repriming elicits gap-filling by template switching and promotes cellular tolerance to cidofovir

A nucleoside analog, Cidofovir (CDV), is used for the treatment of viral diseases such as cytomegalovirus retinitis and herpes virus infection. CDV converts to its active diphosphate metabolite (CDVpp) through cellular kinases and acts as a competitive inhibitor for viral polymerase thereby interfering with viral replication. However, the effect of this drug on the replication of healthy host cells and the mechanisms involved in the cellular tolerance to CDV are yet to be fully understood. In this study, we explored the mechanisms underlying cellular tolerance to CDV by screening mutant cell lines exhibiting hypersensitivity to CDV from a collection of DT40 mutants deficient in various genome maintenance systems. We identified Rad17 and PrimPol as critical factors for CDV tolerance. We found that Rad17 plays a pivotal role in activating intra-S phase checkpoint by the phosphorylation of Chk1, a vital checkpoint mediator. We showed that PrimPol, a factor involved in the release of stalled replication, plays critical roles in CDV tolerance in tandem with Rad17. We found that PrimPol deficient cells showed slower replication on the CDV-incorporated template strand than did wild-type cells, indicating a critical role of PrimPol in the continuous replication fork progression on the CDV-incorporated damaged template. PrimPol releases replication arrest with its DNA-damage bypass function and its repriming function, we thus investigated which PrimPol function is involved in CDV tolerance using the separation of function mutant genes of PRIMPOL. The CDV hypersensitive phenotype of PrimPol deficient cells was restored by PRIMPOLY89D (primase active / reduced polymerase activity), indicating that the repriming function of PrimPol is required for maintaining replication on the CDV-damaged template. Moreover, we found that the number of sister chromatid exchange (SCE) was reduced in PrimPol-deficient cells. These data indicate that gaps generated by PrimPol-mediated repriming on CDV-damaged templates promote post-replicative gap-filing by template switching.

Construction and Validation of a Prognostic Model Based on Mitochondrial Genes in Prostate Cancer

This study attempted to build a prostate cancer (PC) prognostic risk model with mitochondrial feature genes. PC-related MTGs were screened for Cox regression analyses, followed by establishing a prognostic model. Model validity was analyzed via survival analysis and receiver operating characteristic (ROC) curves, and model accuracy was validated in the GEO dataset. Combining risk score with clinical factors, the independence of the risk score was verified by using Cox analysis, followed by generating a nomogram. The Gleason score, microsatellite instability (MSI), immune microenvironment, and tumor mutation burden were analyzed in two risk groups. Finally, the prognostic feature genes were verified through a q-PCR test. Ten PC-associated MTGs were screened, and a prognostic model was built. Survival analysis and ROC curves illustrated that the model was a good predictor for the risk of PC. Cox regression analysis revealed that risk score acted as an independent prognostic factor. The Gleason score and MSI in the high-risk group were substantially higher than in the low-risk group. Levels of ESTIMATE Score, Immune Score, Stromal Score, immune cells, immune function, immune checkpoint, and immunopheno score of partial immune checkpoints in the high-risk group were significantly lower than in the low-risk group. Genes with the highest mutation frequencies in the two groups were SPOP, TTN, and TP53. The q-PCR results of the feature genes were consistent with the gene expression results in the database. The 10-gene model based on MTGs could accurately predict the prognosis of PC patients and their responses to immunotherapy.

Overexpression of human DNA polymerase theta is a biomarker of aggressive and DNA repair-deficient papillary thyroid cancers

BACKGROUND

DNA polymerase theta (POLQ) is an enzyme that repairs double-strand DNA breaks. POLQ is overexpressed in several cancer types, and increased expression is associated with a poor prognosis. Ablating POLQ function in vitro increases drug sensitivity to agents that cause double-strand DNA breaks, including chemotherapies and ionizing radiation. POLQ's role in thyroid cancer remains poorly understood.

METHODS

Expression of POLQ and other genes of interest were analyzed in 513 papillary thyroid cancers (505 primary tumors and 8 metastatic lesions) and 59 normal thyroid samples available in the Cancer Genome Atlas. The Cancer Genome Atlas RNA and DNA sequencing data were queried with the Xena platform. The Recombination Proficiency Score was calculated to assess DNA repair efficiency. Other signaling events associated with thyroid tumorigenesis and clinical outcomes were analyzed. Univariate and multivariate analyses were performed. Treatment with the POLQ inhibitors ART558 and Novobiocin tested the effect of POLQ inhibition on in vitro thyroid cancer growth.

RESULTS

POLQ expression was increased in papillary thyroid cancers compared to normal thyroid tissue (P < .05). POLQ expression levels were inversely correlated with Recombination Proficiency Score levels (P < .05). POLQ expression was highest in tall cell papillary thyroid cancers and in metastases. Higher POLQ expression was also associated with dedifferentiation, BRAF signaling, and shorter progression-free intervals (P < .05). Treatment with POLQ inhibitors decreased in vitro thyroid cancer growth (P < .05).

CONCLUSION

These findings suggest that increased POLQ expression could serve as a valuable clinical marker and a potential therapeutic target in the treatment of thyroid cancer.

RAD18- and BRCA1-dependent pathways promote cellular tolerance to the nucleoside analog ganciclovir

Ganciclovir (GCV) is a clinically important drug as it is used to treat viral infections. GCV is incorporated into the DNA during replication, where it interferes with subsequent replication on GCV-incorporated templates. However, the effects of GCV on the host genome and the mechanisms underlying cellular tolerance to GCV remain unclear. In this study, we explored these mechanisms using a collection of mutant DT40 cells. We identified RAD17/-, BRCA1-/-, and RAD18-/- cells as highly GCV-sensitive. RAD17, a component of the alternative checkpoint-clamp loader RAD17-RFC, was required for the activation of the intra-S checkpoint following GCV treatment. BRCA1, a critical factor for promoting homologous recombination (HR), was required for suppressing DNA double-strand breaks (DSBs). Moreover, RAD18, an E3-ligase involved in DNA repair, was critical in suppressing the aberrant ligation of broken chromosomes caused by GCV. We found that BRCA1 suppresses DSBs through HR-mediated repair and template switching (TS)-mediated damage bypass. Moreover, the strong GCV sensitivity of BRCA1-/- cells was rescued by the loss of 53BP1, despite the only partial restoration in the sister chromatid exchange events which are hallmarks of HR. These results indicate that BRCA1 promotes cellular tolerance to GCV through two mechanisms, TS and HR-mediated repair.

REV3 promotes cellular tolerance to 5-fluorodeoxyuridine by activating translesion DNA synthesis and intra-S checkpoint

The drug floxuridine (5-fluorodeoxyuridine, FUdR) is an active metabolite of 5-Fluorouracil (5-FU). It converts to 5-fluorodeoxyuridine monophosphate (FdUMP) and 5-fluorodeoxyuridine triphosphate (FdUTP), which on incorporation into the genome inhibits DNA replication. Additionally, it inhibits thymidylate synthase, causing dTMP shortage while increasing dUMP availability, which induces uracil incorporation into the genome. However, the mechanisms underlying cellular tolerance to FUdR are yet to be fully elucidated. In this study, we explored the mechanisms underlying cellular resistance to FUdR by screening for FUdR hypersensitive mutants from a collection of DT40 mutants deficient in each genomic maintenance system. We identified REV3, which is involved in translesion DNA synthesis (TLS), to be a critical factor in FUdR tolerance. Replication using a FUdR-damaged template was attenuated in REV3-/- cells, indicating that the TLS function of REV3 is required to maintain replication on the FUdR-damaged template. Notably, FUdR-exposed REV3-/- cells exhibited defective cell cycle arrest in the early S phase, suggesting that REV3 is involved in intra-S checkpoint activation. Furthermore, REV3-/- cells showed defects in Chk1 phosphorylation, which is required for checkpoint activation, but the survival of FUdR-exposed REV3-/- cells was further reduced by the inhibition of Chk1 or ATR. These data indicate that REV3 mediates DNA checkpoint activation at least through Chk1 phosphorylation, but this signal acts in parallel with ATR-Chk1 DNA damage checkpoint pathway. Collectively, we reveal a previously unappreciated role of REV3 in FUdR tolerance.

Dominant roles of BRCA1 in cellular tolerance to a chain-terminating nucleoside analog, alovudine

Alovudine is a chain-terminating nucleoside analog (CTNA) that is frequently used as an antiviral and anticancer agent. Generally, CTNAs inhibit DNA replication after their incorporation into nascent DNA during DNA synthesis by suppressing subsequent polymerization, which restricts the proliferation of viruses and cancer cells. Alovudine is a thymidine analog used as an antiviral drug. However, the mechanisms underlying the removal of alovudine and DNA damage tolerance pathways involved in cellular resistance to alovudine remain unclear. Here, we explored the DNA damage tolerance pathways responsible for cellular tolerance to alovudine and found that BRCA1-deficient cells exhibited the highest sensitivity to alovudine. Moreover, alovudine interfered with DNA replication in two distinct mechanisms: first: alovudine incorporated at the end of nascent DNA interfered with subsequent DNA synthesis; second: DNA replication stalled on the alovudine-incorporated template strand. Additionally, BRCA1 facilitated the removal of the incorporated alovudine from nascent DNA, and BRCA1-mediated homologous recombination (HR) contributed to the progressive replication on the alovudine-incorporated template. Thus, we have elucidated the previously unappreciated mechanism of alovudine-mediated inhibition of DNA replication and the role of BRCA1 in cellular tolerance to alovudine.

CTF18-RFC contributes to cellular tolerance against chain-terminating nucleoside analogs (CTNAs) in cooperation with proofreading exonuclease activity of DNA polymerase ε

Chemotherapeutic nucleoside analogs, such as cytarabine (Ara-C), are incorporated into genomic DNA during replication. Incorporated Ara-CMP (Ara-cytidine monophosphate) serves as a chain terminator and inhibits DNA synthesis by replicative polymerase epsilon (Polε). The proofreading exonuclease activity of Polε removes the misincorporated Ara-CMP, thereby contributing to the cellular tolerance to Ara-C. Purified Polε performs proofreading, and it is generally believed that proofreading in vivo does not need additional factors. In this study, we demonstrated that the proofreading by Polε in vivo requires CTF18, a component of the leading-strand replisome. We found that loss of CTF18 in chicken DT40 cells and human TK6 cells results in hypersensitivity to Ara-C, indicating the conserved function of CTF18 in the cellular tolerance of Ara-C. Strikingly, we found that proofreading-deficient POLE1^D269A/-, CTF18^-/-, and POLE1^D269A/-/CTF18^-/- cells showed indistinguishable phenotypes, including the extent of hypersensitivity to Ara-C and decreased replication rate with Ara-C. This observed epistatic relationship between POLE1^D269A/- and CTF18^-/- suggests that they are interdependent in removing mis-incorporated Ara-CMP from the 3' end of primers. Mechanistically, we found that CTF18^-/- cells have reduced levels of chromatin-bound Polε upon Ara-C treatment, suggesting that CTF18 contributes to the tethering of Polε on fork at the stalled end and thereby facilitating the removal of inserted Ara-C. Collectively, these data reveal the previously unappreciated role of CTF18 in Polε-exonuclease-mediated maintenance of the replication fork upon Ara-C incorporation.

Multifaceted Nature of DNA Polymerase θ

DNA polymerase θ belongs to the A family of DNA polymerases and plays a key role in DNA repair and damage tolerance, including double-strand break repair and DNA translesion synthesis. Pol θ is often overexpressed in cancer cells and promotes their resistance to chemotherapeutic agents. In this review, we discuss unique biochemical properties and structural features of Pol θ, its multiple roles in protection of genome stability and the potential of Pol θ as a target for cancer treatment.

Resistance to DNA repair inhibitors in cancer

The DNA damage response (DDR) represents a complex network of proteins which detect and repair DNA damage, thereby maintaining the integrity of the genome and preventing the transmission of mutations and rearranged chromosomes to daughter cells. Faults in the DDR are a known driver and hallmark of cancer. Furthermore, inhibition of DDR enzymes can be used to treat the disease. This is exemplified by PARP inhibitors (PARPi) used to treat cancers with defects in the homologous recombination DDR pathway. A series of novel DDR targets are now also under pre-clinical or clinical investigation, including inhibitors of ATR kinase, WRN helicase or the DNA polymerase/helicase Polθ (Pol-Theta). Drug resistance is a common phenomenon that impairs the overall effectiveness of cancer treatments and there is already some understanding of how resistance to PARPi occurs. Here, we discuss how an understanding of PARPi resistance could inform how resistance to new drugs targeting the DDR emerges. We also discuss potential strategies that could limit the impact of these therapy resistance mechanisms in cancer.

POLQ suppresses genome instability and alterations in DNA repeat tract lengths

DNA polymerase theta (POLQ) is a principal component of the alternative non-homologous end-joining (ANHEJ) DNA repair pathway that ligates DNA double-strand breaks. Utilizing independent models of POLQ insufficiency during telomere-driven crisis, we found that POLQ - /- cells are resistant to crisis-induced growth deceleration despite sustaining inter-chromosomal telomere fusion frequencies equivalent to wild-type (WT) cells. We recorded longer telomeres in POLQ - / - than WT cells pre- and post-crisis, notwithstanding elevated total telomere erosion and fusion rates. POLQ - /- cells emerging from crisis exhibited reduced incidence of clonal gross chromosomal abnormalities in accordance with increased genetic heterogeneity. High-throughput sequencing of telomere fusion amplicons from POLQ-deficient cells revealed significantly raised frequencies of inter-chromosomal fusions with correspondingly depreciated intra-chromosomal recombinations. Long-range interactions culminating in telomere fusions with centromere alpha-satellite repeats, as well as expansions in HSAT2 and HSAT3 satellite and contractions in ribosomal DNA repeats, were detected in POLQ - / - cells. In conjunction with the expanded telomere lengths of POLQ - /- cells, these results indicate a hitherto unrealized capacity of POLQ for regulation of repeat arrays within the genome. Our findings uncover novel considerations for the efficacy of POLQ inhibitors in clinical cancer interventions, where potential genome destabilizing consequences could drive clonal evolution and resistant disease.

Next-Generation Sequencing-Based Analysis of the Roles of DNA Polymerases ν and θ in the Replicative Bypass of 8-Oxo-7,8-dihydroguanine in Human Cells

DNA polymerase (Pol) ν and Pol θ are two specialized A-family DNA polymerases that function in the translesion synthesis of certain DNA lesions. However, the biological functions of human Pols ν and θ in cellular replicative bypass of 8-oxo-7,8-dihydroguanine (8-oxoG), an important carcinogenesis-related biomarker of oxidative DNA damage, remain unclear. Herein, we showed that depletion of Pols ν and θ in human cells could cause an elevated hypersensitivity to oxidative stress induced by hydrogen peroxide. Using next-generation sequencing-based lesion bypass and mutagenesis assay, we further demonstrated that Pols ν and θ had important roles in promoting translesion synthesis of 8-oxoG in human cells. We also found that the depletion of Pol ν, but not Pol θ, caused a substantial reduction in G → T mutation frequency for 8-oxoG. These findings provided novel insights into the involvement of A-family DNA polymerases in oxidative DNA damage response.

TEB/POLQ plays dual roles in protecting Arabidopsis from NO-induced DNA damage

Nitric oxide (NO) is a key player in numerous physiological processes. Excessive NO induces DNA damage, but how plants respond to this damage remains unclear. We screened and identified an Arabidopsis NO hypersensitive mutant and found it to be allelic to TEBICHI/POLQ, encoding DNA polymerase θ. The teb mutant plants were preferentially sensitive to NO- and its derivative peroxynitrite-induced DNA damage and subsequent double-strand breaks (DSBs). Inactivation of TEB caused the accumulation of spontaneous DSBs largely attributed to endogenous NO and was synergistic to DSB repair pathway mutations with respect to growth. These effects were manifested in the presence of NO-inducing agents and relieved by NO scavengers. NO induced G2/M cell cycle arrest in the teb mutant, indicative of stalled replication forks. Genetic analyses indicate that Polθ is required for translesion DNA synthesis across NO-induced lesions, but not oxidation-induced lesions. Whole-genome sequencing revealed that Polθ bypasses NO-induced base adducts in an error-free manner and generates mutations characteristic of Polθ-mediated end joining. Our experimental data collectively suggests that Polθ plays dual roles in protecting plants from NO-induced DNA damage. Since Polθ is conserved in higher eukaryotes, mammalian Polθ may also be required for balancing NO physiological signaling and genotoxicity.

Synthetic Lethality Targeting Polθ

Research studies regarding synthetic lethality (SL) in human cells are primarily motivated by the potential of this phenomenon to be an effective, but at the same time, safe to the patient's anti-cancer chemotherapy. Among the factors that are targets for the induction of the synthetic lethality effect, those involved in DNA repair seem to be the most relevant. Specifically, when mutation in one of the canonical DNA double-strand break (DSB) repair pathways occurs, which is a frequent event in cancer cells, the alternative pathways may be a promising target for the elimination of abnormal cells. Currently, inhibiting RAD52 and/or PARP1 in the tumor cells that are deficient in the canonical repair pathways has been the potential target for inducing the effect of synthetic lethality. Unfortunately, the development of resistance to commonly used PARP1 inhibitors (PARPi) represents the greatest obstacle to working out a successful treatment protocol. DNA polymerase theta (Polθ), encoded by the POLQ gene, plays a key role in an alternative DSB repair pathway-theta-mediated end joining (TMEJ). Thus, it is a promising target in the treatment of tumors harboring deficiencies in homologous recombination repair (HRR), where its inhibition can induce SL. In this review, the authors discuss the current state of knowledge on Polθ as a potential target for synthetic lethality-based anticancer therapies.

DNA polymerase θ promotes CAG•CTG repeat expansions in Huntington's disease via insertion sequences of its catalytic domain

Huntington's disease (HD), a neurodegenerative disease characterized by progressive dementia, psychiatric problems, and chorea, is known to be caused by CAG repeat expansions in the HD gene HTT. However, the mechanism of this pathology is not fully understood. The translesion DNA polymerase θ (Polθ) carries a large insertion sequence in its catalytic domain, which has been shown to allow DNA loop-outs in the primer strand. As a result of high levels of oxidative DNA damage in neural cells and Polθ's subsequent involvement in base excision repair of oxidative DNA damage, we hypothesized that Polθ contributes to CAG repeat expansion while repairing oxidative damage within HTT. Here, we performed Polθ-catalyzed in vitro DNA synthesis using various CAG•CTG repeat DNA substrates that are similar to base excision repair intermediates. We show that Polθ efficiently extends (CAG)n•(CTG)n hairpin primers, resulting in hairpin retention and repeat expansion. Polθ also triggers repeat expansions to pass the threshold for HD when the DNA template contains 35 repeats upward. Strikingly, Polθ depleted of the catalytic insertion fails to induce repeat expansions regardless of primers and templates used, indicating that the insertion sequence is responsible for Polθ's error-causing activity. In addition, the level of chromatin-bound Polθ in HD cells is significantly higher than in non-HD cells and exactly correlates with the degree of CAG repeat expansion, implying Polθ's involvement in triplet repeat instability. Therefore, we have identified Polθ as a potent factor that promotes CAG•CTG repeat expansions in HD and other neurodegenerative disorders.

Polymerase θ Coordinates Multiple Intrinsic Enzymatic Activities during DNA Repair

The POLQ gene encodes DNA polymerase θ, a 2590 amino acid protein product harboring DNA-dependent ATPase, template-dependent DNA polymerase, dNTP-dependent endonuclease, and 5'-dRP lyase functions. Polymerase θ participates at an essential step of a DNA double-strand break repair pathway able to join 5'-resected substrates by locating and pairing microhomologies present in 3'-overhanging single-stranded tails, cleaving the extraneous 3'-DNA by dNTP-dependent end-processing, before extending the nascent 3' end from the microhomology annealing site. Metazoans require polymerase θ for full resistance to DNA double-strand break inducing agents but can survive knockout of the POLQ gene. Cancer cells with compromised homologous recombination, or other DNA repair defects, over-utilize end-joining by polymerase θ and often over-express the POLQ gene. This dependency points to polymerase θ as an ideal drug target candidate and multiple drug-development programs are now preparing to enter clinical trials with small-molecule inhibitors. Specific inhibitors of polymerase θ would not only be predicted to treat BRCA-mutant cancers, but could thwart accumulated resistance to current standard-of-care cancer therapies and overcome PARP-inhibitor resistance in patients. This article will discuss synthetic lethal strategies targeting polymerase θ in DNA damage-response-deficient cancers and summarize data, describing molecular structures and enzymatic functions.

Depletion of DNA Polymerase Theta Inhibits Tumor Growth and Promotes Genome Instability through the cGAS-STING-ISG Pathway in Esophageal Squamous Cell Carcinoma

Simple Summary

DNA polymerase theta, encoded by the human POLQ gene, is upregulated in several cancers and is associated with poor clinical outcomes. The importance of POLQ, however, has yet to be elucidated in esophageal cancer. In this study, we explored the functional impacts of POLQ and looked into its underlying mechanisms. POLQ was overexpressed in esophageal squamous cell carcinoma (ESCC) tumors associated with unfavorable prognosis and contributed to malignant phenotypes by promoting genome stability, suggesting that targeting polymerase theta may provide a potential therapeutic approach for improving ESCC management.

Abstract

Overexpression of the specialized DNA polymerase theta (POLQ) is frequent in breast, colon and lung cancers and has been correlated with unfavorable clinical outcomes. Here, we aimed to determine the importance and functional role of POLQ in esophageal squamous cell carcinoma (ESCC). Integrated analysis of four RNA-seq datasets showed POLQ was predominantly upregulated in ESCC tumors. High expression of POLQ was also observed in a cohort of 25 Hong Kong ESCC patients and negatively correlated with ESCC patient survival. POLQ knockout (KO) ESCC cells were sensitized to multiple genotoxic agents. Both rH2AX foci staining and the comet assay indicated a higher level of genomic instability in POLQ-depleted cells. Double KO of POLQ and FANCD2, known to promote POLQ recruitment at sites of damage, significantly impaired cell proliferation both in vitro and in vivo, as compared to either single POLQ or FANCD2 KOs. A significantly increased number of micronuclei was observed in POLQ and/or FANCD2 KO ESCC cells. Loss of POLQ and/or FANCD2 also resulted in the activation of cGAS and upregulation of interferon-stimulated genes (ISGs). Our results suggest that high abundance of POLQ in ESCC contributes to the malignant phenotype through genome instability and activation of the cGAS pathway.

Translesion Synthesis or Repair by Specialized DNA Polymerases Limits Excessive Genomic Instability upon Replication Stress

DNA can experience "replication stress", an important source of genome instability, induced by various external or endogenous impediments that slow down or stall DNA synthesis. While genome instability is largely documented to favor both tumor formation and heterogeneity, as well as drug resistance, conversely, excessive instability appears to suppress tumorigenesis and is associated with improved prognosis. These findings support the view that karyotypic diversity, necessary to adapt to selective pressures, may be limited in tumors so as to reduce the risk of excessive instability. This review aims to highlight the contribution of specialized DNA polymerases in limiting extreme genetic instability by allowing DNA replication to occur even in the presence of DNA damage, to either avoid broken forks or favor their repair after collapse. These mechanisms and their key regulators Rad18 and Polθ not only offer diversity and evolutionary advantage by increasing mutagenic events, but also provide cancer cells with a way to escape anti-cancer therapies that target replication forks.

DNA-damage tolerance through PCNA ubiquitination and sumoylation

DNA-damage tolerance (DDT) is employed by eukaryotic cells to bypass replication-blocking lesions induced by DNA-damaging agents. In budding yeast Saccharomyces cerevisiae, DDT is mediated by RAD6 epistatic group genes and the central event for DDT is sequential ubiquitination of proliferating cell nuclear antigen (PCNA), a DNA clamp required for replication and DNA repair. DDT consists of two parallel pathways: error-prone DDT is mediated by PCNA monoubiquitination, which recruits translesion synthesis DNA polymerases to bypass lesions with decreased fidelity; and error-free DDT is mediated by K63-linked polyubiquitination of PCNA at the same residue of monoubiquitination, which facilitates homologous recombination-mediated template switch. Interestingly, the same PCNA residue is also subjected to sumoylation, which leads to inhibition of unwanted recombination at replication forks. All three types of PCNA posttranslational modifications require dedicated conjugating and ligation enzymes, and these enzymes are highly conserved in eukaryotes, from yeast to human.

Targeting the DNA Repair Enzyme Polymerase θ in Cancer Therapy

Targeted cancer therapies represent a milestone towards personalized treatment as they function via inhibition of cancer-specific alterations. Polymerase θ (POLQ), an error-prone translesion polymerase, also involved in DNA double-strand break (DSB) repair, is often upregulated in cancer. POLQ is synthetic lethal with various DNA repair genes, including known cancer drivers such as BRCA1/2, making it essential in homologous recombination-deficient cancers. Thus, POLQ represents a promising target in cancer therapy and efforts for the development of POLQ inhibitors are actively underway with first clinical trials due to start in 2021. This review summarizes the journey of POLQ from a backup DNA repair enzyme to a promising therapeutic target for cancer treatment.

The HelQ human DNA repair helicase utilizes a PWI-like domain for DNA loading through interaction with RPA, triggering DNA unwinding by the HelQ helicase core

Genome instability is a characteristic enabling factor for carcinogenesis. HelQ helicase is a component of human DNA maintenance systems that prevent or reverse genome instability arising during DNA replication. Here, we provide details of the molecular mechanisms that underpin HelQ function-its recruitment onto ssDNA through interaction with replication protein A (RPA), and subsequent translocation of HelQ along ssDNA. We describe for the first time a functional role for the non-catalytic N-terminal region of HelQ, by identifying and characterizing its PWI-like domain. We present evidence that this domain of HelQ mediates interaction with RPA that orchestrates loading of the helicase domains onto ssDNA. Once HelQ is loaded onto the ssDNA, ATP-Mg²⁺ binding in the catalytic site activates the helicase core and triggers translocation along ssDNA as a dimer. Furthermore, we identify HelQ-ssDNA interactions that are critical for the translocation mechanism. Our data are novel and detailed insights into the mechanisms of HelQ function relevant for understanding how human cells avoid genome instability provoking cancers, and also how cells can gain resistance to treatments that rely on DNA crosslinking agents.

Impairment of Pol β-related DNA base-excision repair leads to ovarian aging in mice

The mechanism underlying the association between age and depletion of the human ovarian follicle reserves remains uncertain. Many identified that impaired DNA polymerase β (Pol β)-mediated DNA base-excision repair (BER) drives to mouse oocyte aging. With aging, DNA lesions accumulate in primordial follicles. However, the expression of most DNA BER genes, including APE1, OGG1, XRCC1, Ligase I, Ligase α, PCNA and FEN1, remains unchanged during aging in mouse oocytes. Also, the reproductive capacity of Pol β+/- heterozygote mice was impaired, and the primordial follicle counts were lower than that of wild type (wt) mice. The DNA lesions of heterozygous mice increased. Moreover, the Pol β knockdown leads to increased DNA damage in oocytes and decreased survival rate of oocytes. Oocytes over-expressing Pol β showed that the vitality of senescent cells enhances significantly. Furthermore, serum concentrations of anti-Müllerian hormone (AMH) indicated that the ovarian reserves of young mice with Pol β germline mutations were lower than those in wt. These data show that Pol β-related DNA BER efficiency is a major factor governing oocyte aging in mice.

POLQ suppresses interhomolog recombination and loss of heterozygosity at targeted DNA breaks

Interhomolog recombination (IHR) occurs spontaneously in somatic human cells at frequencies that are low but sufficient to ameliorate some genetic diseases caused by heterozygous mutations or autosomal dominant mutations. Here we demonstrate that DNA nicks or double-strand breaks (DSBs) targeted by CRISPR-Cas9 to both homologs can stimulate IHR and associated copy-neutral loss of heterozygosity (cnLOH) in human cells. The frequency of IHR is 10-fold lower at nicks than at DSBs, but cnLOH is evident in a greater fraction of recombinants. IHR at DSBs occurs predominantly via reciprocal end joining. At DSBs, depletion of POLQ caused a dramatic increase in IHR and in the fraction of recombinants exhibiting cnLOH, suggesting that POLQ promotes end joining in cis, which limits breaks available for recombination in trans These results define conditions that may produce cnLOH as a mutagenic signature in cancer and may, conversely, promote therapeutic correction of both compound heterozygous and dominant negative mutations associated with genetic disease.

Base Excision DNA Repair Deficient Cells: From Disease Models to Genotoxicity Sensors

Base excision DNA repair (BER) is a vitally important pathway that protects the cell genome from many kinds of DNA damage, including oxidation, deamination, and hydrolysis. It involves several tightly coordinated steps, starting from damaged base excision and followed by nicking one DNA strand, incorporating an undamaged nucleotide, and DNA ligation. Deficiencies in BER are often embryonic lethal or cause morbid diseases such as cancer, neurodegeneration, or severe immune pathologies. Starting from the early 1980s, when the first mammalian cell lines lacking BER were produced by spontaneous mutagenesis, such lines have become a treasure trove of valuable information about the mechanisms of BER, often revealing unexpected connections with other cellular processes, such as antibody maturation or epigenetic demethylation. In addition, these cell lines have found an increasing use in genotoxicity testing, where they provide increased sensitivity and representativity to cell-based assay panels. In this review, we outline current knowledge about BER-deficient cell lines and their use.

DNA polymerase theta (Polθ) - an error-prone polymerase necessary for genome stability

Mammalian cells have evolved multiple pathways to repair DNA double strand breaks (DSBs) and ensure genome stability. In addition to non-homologous end-joining (NHEJ) and homologous recombination (HR), cells evolved an error-prone repair pathway termed microhomology-mediated end joining (MMEJ). The mutagenic outcome of MMEJ derives from the activity of DNA polymerase theta (Polθ) - a multidomain enzyme that is minimally expressed in normal tissue but overexpressed in tumors. Polθ expression is particularly crucial for the proliferation of HR deficient cancer cells. As a result, this mutagenic repair emerged as an attractive target for cancer therapy, and inhibitors are currently in pre-clinical development. Here, we review the multifunctionality of this enigmatic polymerase, focusing on its role during DSB repair in mammalian cells and its impact on cancer genomes.

NAD+ consumption by PARP1 in response to DNA damage triggers metabolic shift critical for damaged cell survival

DNA damage signaling is critical for the maintenance of genome integrity and cell fate decision. Poly(ADP-ribose) polymerase 1 (PARP1) is a DNA damage sensor rapidly activated in a damage dose- and complexity-dependent manner playing a critical role in the initial chromatin organization and DNA repair pathway choice at damage sites. However, our understanding of a cell-wide consequence of its activation in damaged cells is still limited. Using the phasor approach to fluorescence lifetime imaging microscopy and fluorescence-based biosensors in combination with laser microirradiation, we found a rapid cell-wide increase of the bound NADH fraction in response to nuclear DNA damage, which is triggered by PARP-dependent NAD+ depletion. This change is linked to the metabolic balance shift to oxidative phosphorylation (oxphos) over glycolysis. Inhibition of oxphos, but not glycolysis, resulted in parthanatos due to rapid PARP-dependent ATP deprivation, indicating that oxphos becomes critical for damaged cell survival. The results reveal the novel prosurvival response to PARP activation through a change in cellular metabolism and demonstrate how unique applications of advanced fluorescence imaging and laser microirradiation-induced DNA damage can be a powerful tool to interrogate damage-induced metabolic changes at high spatiotemporal resolution in a live cell.

Complex genetic interactions between DNA polymerase β and the NHEJ ligase

Mammalian cells possess multiple pathways for repairing various types of DNA damage. Although the molecular mechanisms of each DNA repair pathway have been analyzed by biochemical analysis and cell biological analysis, interplay between different pathways has not been fully elucidated. In this study, using human Nalm-6-mutant cell lines, we analyzed the relationship between the base excision repair factor DNA polymerase β (POLβ) and DNA ligase IV (LIG4), which is essential for DNA double-strand break (DSB) repair by non-homologous end-joining (NHEJ). We found that cells lacking both POLβ and LIG4 grew significantly more slowly than either single mutant, indicating cooperative functions of the two proteins in normal cell growth. To further investigate the genetic interaction between POLβ and LIG4, we examined DNA damage sensitivity of the mutant cell lines. Our results suggested that NHEJ acts as a backup pathway for repairing alkylation damage (when converted into DSBs) in the absence of POLβ. Surprisingly, despite the critical role of POLβ in alkylation damage repair, cells lacking POLβ exhibited increased resistance to camptothecin (a topoisomerase I inhibitor that induces DNA single-strand breaks), irrespective of the presence or absence of LIG4. A LIG4-independent increased resistance associated with POLβ loss was also observed with ionizing radiation; however, cells lacking both POLβ and LIG4 were more radiosensitive than either single mutant. Taken together, our findings provide novel insight into the complex interplay between different DNA repair pathways.

POLQ plays a key role in the repair of CRISPR/Cas9-induced double-stranded breaks in the moss Physcomitrella patens

Double-stranded breaks can be repaired by different mechanisms such as homologous recombination (HR), classical nonhomologous end joining (C-NHEJ) and alternative end joining (Alt-EJ). Polymerase Q (POLQ) has been proposed to be the main factor involved in Alt-EJ-mediated DNA repair. Here we describe the role of POLQ in DNA repair and gene targeting in Physcomitrella patens. The disruption of the POLQ gene does not influence the genetic stability of P. patens nor its development. The polq mutant shows the same sensitivity as wild-type towards most of the genotoxic agents tested (ultraviolet (UV), methyl methanesulfonate (MMS) and cisplatin) with the notable exception of bleomycin for which it shows less sensitivity than the wild-type. Furthermore, we show that POLQ is involved in the repair of CRISPR-Cas9-induced double-stranded breaks in P. patens. We also demonstrate that POLQ is a potential competitor and/or inhibitor of the HR repair pathway. This finding has a consequence in terms of genetic engineering, as in the absence of POLQ the frequency of gene targeting is significantly increased and the number of clean two-sided HR-mediated insertions is enhanced. Therefore, the control of POLQ activity in plants could be a useful strategy to optimize the tools of genome engineering for plant breeding.

Expanded Substrate Scope of DNA Polymerase θ and DNA Polymerase β: Lyase Activity on 5'-Overhangs and Clustered Lesions

DNA polymerase θ (Pol θ) is a multifunctional enzyme with double-strand break (DSB) repair, translesion synthesis, and lyase activities. Pol θ lyase activity on ternary substrates containing a 5'-dRP that are produced during base excision repair of abasic sites (AP) is weak compared to that of DNA polymerase β (Pol β), a polymerase integrally involved in base excision repair. This led us to explore whether Pol θ utilizes its lyase activity to remove 5'-dRP and incise abasic sites from alternative substrates that might be produced during DNA damage and repair. We found that Pol θ exhibited lyase activity on abasic lesions near DSB termini and on clustered lesions. To calibrate the Pol θ activity, Pol β reactivity was examined with the same substrates. Pol β excised 5'-dRP from within a 5'-overhang 80 times faster than did Pol θ. Pol θ and Pol β also incised AP within clustered lesions but showed opposite preferences with respect to the polarity of the lesions. AP lesions in 5'-overhangs were typically excised by Pol β 35-50 times faster than those in a duplex substrate but 15-20-fold more slowly than 5'-dRP in a ternary complex. This is the first report of Pol θ exhibiting lyase activity within an unincised strand. These results suggest that bifunctional polymerases may exhibit lyase activity on a greater variety of substrates than previously recognized. A role in DSB repair could potentially be beneficial, while the aberrant activity exhibited on clustered lesions may be deleterious because of their conversion to DSBs.

Mechanistic Insight through Irreversible Inhibition: DNA Polymerase θ Uses a Common Active Site for Polymerase and Lyase Activities

DNA polymerase θ (Pol θ) is a multifunctional enzyme. It is nonessential in normal cells, but its upregulation in cancer cells correlates with cellular resistance to oxidative damage and poor prognosis. Pol θ possesses polymerase activity and poorly characterized lyase activity. We examined the Pol θ lyase activity on various abasic sites and determined that the enzyme is inactivated upon attempted removal of the oxidized abasic site commonly associated with C4'-oxidation (pC4-AP). Covalent modification of Pol θ by the DNA lesion enabled determination of the primary nucleophile (Lys2383) responsible for Schiff base formation in the lyase reaction. Unlike some other base excision repair polymerases, Pol θ uses a single active site for polymerase and lyase activity. Mutation of Lys2383 significantly reduces both enzyme activities but not DNA binding. Demonstration that Lys2383 is required for polymerase and lyase activities indicates that this residue is an Achilles heel for Pol θ and suggests a path forward for designing inhibitors of this attractive anticancer target.

Differential micronucleus frequency in isogenic human cells deficient in DNA repair pathways is a valuable indicator for evaluating genotoxic agents and their genotoxic mechanisms

The micronucleus (MN) test has become an attractive tool both for evaluating the genotoxicity of test chemicals because of its ability to detect clastogenic and aneugenic events and for its convenience. As the MN assay has been mostly performed using only DNA repair-proficient mammalian cells, we believed that the comparison of the MN frequency between DNA repair-proficient and -deficient human cells may be an excellent indicator for detecting the genotoxic potential of test chemicals and for understanding their mode of action. To address this issue, the following five genes encoding DNA-damage-response (DDR) factors were disrupted in the TK6 B cell line, a human cell line widely used for the MN test: FANCD2, DNA polymerase ζ (REV3), XRCC1, RAD54, and/or LIG4. Using these isogenic TK6 cell lines, the MN test was conducted for four widely-used DNA-damaging agents: methyl methanesulfonate (MMS), hydrogen peroxide (H₂ O₂ ), γ-rays, and mitomycin C (MMC). The frequency of micronuclei in the double strand break repair-deficient RAD54^-/- /LIG4^-/- cells after exposure to γ-rays, H₂ O₂ , MMS and MMC was 6.2-7.5 times higher than that of parental wild-type TK6 cells. The percentages of cells exhibiting micronuclei in the base excision repair- and single strand break repair-deficient XRCC1^-/- cells after exposure to H₂ O₂ , MMC and MMS were all ∼5 times higher than those of wild-type cells. In summary, a supplementary MN assay using the combination of RAD54^-/- /LIG4^-/- , XRCC1^-/- and wild-type TK6 cells is a promising method for detecting the genotoxic potential of test chemicals and their mode of action. Environ. Mol. Mutagen., 2018. © 2018 Wiley Periodicals, Inc.

The dominant role of proofreading exonuclease activity of replicative polymerase ε in cellular tolerance to cytarabine (Ara-C)

Chemotherapeutic nucleoside analogs, such as Ara-C, 5-Fluorouracil (5-FU) and Trifluridine (FTD), are frequently incorporated into DNA by the replicative DNA polymerases. However, it remains unclear how this incorporation kills cycling cells. There are two possibilities: Nucleoside analog triphosphates inhibit the replicative DNA polymerases, and/or nucleotide analogs mis-incorporated into genomic DNA interfere with the next round of DNA synthesis as replicative DNA polymerases recognize them as template DNA lesions, arresting synthesis. To address the first possibility, we selectively disrupted the proofreading exonuclease activity of DNA polymerase ε (Polε), the leading-strand replicative polymerase in avian DT40 and human TK6 cell lines. To address the second, we disrupted RAD18, a gene involved in translesion DNA synthesis, a mechanism that relieves stalled replication. Strikingly, POLE1^exo−/− cells, but not RAD18^−/− cells, were hypersensitive to Ara-C, while RAD18^−/− cells were hypersensitive to FTD. gH2AX focus formation following a pulse of Ara-C was immediate and did not progress into the next round of replication, while gH2AX focus formation following a pulse of 5-FU and FTD was delayed to the next round of replication. Biochemical studies indicate that human proofreading-deficient Polε-exo⁻ holoenzyme incorporates Ara-CTP, but subsequently extend from this base several times less efficiently than from intact nucleotides. Together our results suggest that Ara-C acts by blocking extension of the nascent DNA strand and is counteracted by the proofreading activity of Polε, while 5-FU and FTD are efficiently incorporated but act as replication fork blocks in the subsequent S phase, which is counteracted by translesion synthesis.

Somatic APC mosaicism and oligogenic inheritance in genetically unsolved colorectal adenomatous polyposis patients

Germline variants in the APC gene cause familial adenomatous polyposis. Inherited variants in MutYH, POLE, POLD1, NTHL1, and MSH3 genes and somatic APC mosaicism have been reported as alternative causes of polyposis. However, ~30-50% of cases of polyposis remain genetically unsolved. Thus, the aim of this study was to investigate the genetic causes of unexplained adenomatous polyposis. Eight sporadic cases with >20 adenomatous polyps by 35 years of age or >50 adenomatous polyps by 55 years of age, and no causative germline variants in APC and/or MutYH, were enrolled from a cohort of 56 subjects with adenomatous colorectal polyposis. APC gene mosaicism was investigated on DNA from colonic adenomas by Sanger sequencing or Whole Exome Sequencing (WES). Mosaicism extension to other tissues (peripheral blood, saliva, hair follicles) was evaluated using Sanger sequencing and/or digital PCR. APC second hit was investigated in adenomas from mosaic patients. WES was performed on DNA from peripheral blood to identify additional polyposis candidate variants. We identified APC mosaicism in 50% of patients. In three cases mosaicism was restricted to the colon, while in one it also extended to the duodenum and saliva. One patient without APC mosaicism, carrying an APC in-frame deletion of uncertain significance, was found to harbor rare germline variants in OGG1, POLQ, and EXO1 genes. In conclusion, our restrictive selection criteria improved the detection of mosaic APC patients. In addition, we showed for the first time that an oligogenic inheritance of rare variants might have a cooperative role in sporadic colorectal polyposis onset.

Filling gaps in translesion DNA synthesis in human cells

During DNA replication, forks may encounter unrepaired lesions that hamper DNA synthesis. Cells have universal strategies to promote damage bypass allowing cells to survive. DNA damage tolerance can be performed upon template switch or by specialized DNA polymerases, known as translesion (TLS) polymerases. Human cells count on more than eleven TLS polymerases and this work reviews the functions of some of these enzymes: Rev1, Pol η, Pol ι, Pol κ, Pol θ and Pol ζ. The mechanisms of damage bypass vary according to the lesion, as well as to the TLS polymerases available, and may occur directly at the fork during replication. Alternatively, the lesion may be skipped, leaving a single-stranded DNA gap that will be replicated later. Details of the participation of these enzymes are revised for the replication of damaged template. TLS polymerases also have functions in other cellular processes. These include involvement in somatic hypermutation in immunoglobulin genes, direct participation in recombination and repair processes, and contributing to replicating noncanonical DNA structures. The importance of DNA damage replication to cell survival is supported by recent discoveries that certain genes encoding TLS polymerases are induced in response to DNA damaging agents, protecting cells from a subsequent challenge to DNA replication. We retrace the findings on these genotoxic (adaptive) responses of human cells and show the common aspects with the SOS responses in bacteria. Paradoxically, although TLS of DNA damage is normally an error prone mechanism, in general it protects from carcinogenesis, as evidenced by increased tumorigenesis in xeroderma pigmentosum variant patients, who are deficient in Pol η. As these TLS polymerases also promote cell survival, they constitute an important mechanism by which cancer cells acquire resistance to genotoxic chemotherapy. Therefore, the TLS polymerases are new potential targets for improving therapy against tumors.

miR-139-5p Modulates Radiotherapy Resistance in Breast Cancer by Repressing Multiple Gene Networks of DNA Repair and ROS Defense

Radiotherapy is essential to the treatment of most solid tumors and acquired or innate resistance to this therapeutic modality is a major clinical problem. Here we show that miR-139-5p is a potent modulator of radiotherapy response in breast cancer via its regulation of genes involved in multiple DNA repair and reactive oxygen species defense pathways. Treatment of breast cancer cells with a miR-139-5p mimic strongly synergized with radiation both in vitro and in vivo, resulting in significantly increased oxidative stress, accumulation of unrepaired DNA damage, and induction of apoptosis. Several miR-139-5p target genes were also strongly predictive of outcome in radiotherapy-treated patients across multiple independent breast cancer cohorts. These prognostically relevant miR-139-5p target genes were used as companion biomarkers to identify radioresistant breast cancer xenografts highly amenable to sensitization by cotreatment with a miR-139-5p mimetic.Significance: The microRNA described in this study offers a potentially useful predictive biomarker of radiosensitivity in solid tumors and a generally applicable druggable target for tumor radiosensitization. Cancer Res; 78(2); 501-15. ©2017 AACR.

In Vitro Bypass of Thymidine Glycol by DNA Polymerase θ Forms Sequence-Dependent Frameshift Mutations

Unrepaired DNA lesions block replication and threaten genomic stability. Several specialized translesion polymerases, including polymerase θ (Pol θ), contribute to replicative bypass of these lesions. The role of Pol θ in double-strand break repair is well-understood, but its contribution to translesion synthesis is much less so. We describe the action of Pol θ on templates containing thymidine glycol (Tg), a major cytotoxic, oxidative DNA lesion that blocks DNA replication. Unrepaired Tg lesions are bypassed in human cells by specialized translesion polymerases by one of two distinct pathways: high-fidelity bypass by the combined action of Pol κ and Pol ζ or weakly mutagenic bypass by Pol θ. Here we report that in vitro bypass of Tg by Pol θ results in frameshift mutations (deletions) in a sequence-dependent fashion. Steady-state kinetic analysis indicated that one- and two-nucleotide deletions are formed 9- and 6-fold more efficiently, respectively, than correct, full-length bypass products. Sequencing of in vitro bypass products revealed that bypass preference decreased in the following order on a template where all three outcomes were possible: two-nucleotide deletion > correct bypass > one-nucleotide deletion. These results suggest that bypass of Tg by Pol θ results in mutations opposite the lesion, as well as frameshift mutations.

ALC1/CHD1L, a chromatin-remodeling enzyme, is required for efficient base excision repair

ALC1/CHD1L is a member of the SNF2 superfamily of ATPases carrying a macrodomain that binds poly(ADP-ribose). Poly(ADP-ribose) polymerase (PARP) 1 and 2 synthesize poly(ADP-ribose) at DNA-strand cleavage sites, promoting base excision repair (BER). Although depletion of ALC1 causes increased sensitivity to various DNA-damaging agents (H₂O₂, UV, and phleomycin), the role played by ALC1 in BER has not yet been established. To explore this role, as well as the role of ALC1’s ATPase activity in BER, we disrupted the ALC1 gene and inserted the ATPase-dead (E165Q) mutation into the ALC1 gene in chicken DT40 cells, which do not express PARP2. The resulting ALC1^-/- and ALC1^-/E165Q cells displayed an indistinguishable hypersensitivity to methylmethane sulfonate (MMS), an alkylating agent, and to H₂O₂, indicating that ATPase plays an essential role in the DNA-damage response. PARP1^-/- and ALC1^-/-/PARP1^-/- cells exhibited a very similar sensitivity to MMS, suggesting that ALC1 and PARP1 collaborate in BER. Following pulse-exposure to H₂O₂, PARP1^-/- and ALC1^-/-/PARP1^-/- cells showed similarly delayed kinetics in the repair of single-strand breaks, which arise as BER intermediates. To ascertain ALC1’s role in BER in mammalian cells, we disrupted the ALC1 gene in human TK6 cells. Following exposure to MMS and to H₂O₂, the ALC1^-/- TK6 cell line showed a delay in single-strand-break repair. We therefore conclude that ALC1 plays a role in BER. Following exposure to H₂O_2,ALC1^-/- cells showed compromised chromatin relaxation. We thus propose that ALC1 is a unique BER factor that functions in a chromatin context, most likely as a chromatin-remodeling enzyme.

Role of specialized DNA polymerases in the limitation of replicative stress and DNA damage transmission

Replication stress is a strong and early driving force for genomic instability and tumor development. Beside replicative DNA polymerases, an emerging group of specialized DNA polymerases is involved in the technical assistance of the replication machinery in order to prevent replicative stress and its deleterious consequences. During S-phase, altered progression of the replication fork by endogenous or exogenous impediments induces replicative stress, causing cells to reach mitosis with genomic regions not fully duplicated. Recently, specific mechanisms to resolve replication intermediates during mitosis with the aim of limiting DNA damage transmission to daughter cells have been identified. In this review, we detail the two major actions of specialized DNA polymerases that limit DNA damage transmission: the prevention of replicative stress by non-B DNA replication and the recovery of stalled replication forks.

The A-Rule and Deletion Formation During Abasic and Oxidized Abasic Site Bypass by DNA Polymerase θ

DNA polymerase θ (Pol θ) is implicated in various cellular processes including double-strand break repair and apurinic/apyrimidinic site bypass. Because Pol θ expression correlates with poor cancer prognosis, the ability of Pol θ to bypass the C4'-oxidized abasic site (C4-AP) and 2-deoxyribonolactone (L), which are generated by cytotoxic agents, is of interest. Translesion synthesis and subsequent extension by Pol θ past C4-AP or L and an abasic site (AP) or its tetrahydrofuran analogue (F) was examined. Pol θ conducts translesion synthesis on templates containing AP and F with similar efficiencies and follows the "A-rule," inserting nucleotides in the order A > G > T. Translesion synthesis on templates containing C4-AP and L is less efficient than AP and F, and the preference for A insertion is reduced for L and absent for C4-AP. Extension past all abasic lesions (AP, F, C4-AP, and L) was significantly less efficient than translesion synthesis and yielded deletions caused by the base one or two nucleotides downstream from the lesion being used as a template, with the latter being favored. These results suggest that bypass of abasic lesions by Pol θ is highly mutagenic.

Analysis of DNA polymerase ν function in meiotic recombination, immunoglobulin class-switching, and DNA damage tolerance

DNA polymerase ν (pol ν), encoded by the POLN gene, is an A-family DNA polymerase in vertebrates and some other animal lineages. Here we report an in-depth analysis of pol ν–defective mice and human cells. POLN is very weakly expressed in most tissues, with the highest relative expression in testis. We constructed multiple mouse models for Poln disruption and detected no anatomic abnormalities, alterations in lifespan, or changed causes of mortality. Mice with inactive Poln are fertile and have normal testis morphology. However, pol ν–disrupted mice have a modestly reduced crossover frequency at a meiotic recombination hot spot harboring insertion/deletion polymorphisms. These polymorphisms are suggested to generate a looped-out primer and a hairpin structure during recombination, substrates on which pol ν can operate. Pol ν-defective mice had no alteration in DNA end-joining during immunoglobulin class-switching, in contrast to animals defective in the related DNA polymerase θ (pol θ). We examined the response to DNA crosslinking agents, as purified pol ν has some ability to bypass major groove peptide adducts and residues of DNA crosslink repair. Inactivation of Poln in mouse embryonic fibroblasts did not alter cellular sensitivity to mitomycin C, cisplatin, or aldehydes. Depletion of POLN from human cells with shRNA or siRNA did not change cellular sensitivity to mitomycin C or alter the frequency of mitomycin C-induced radial chromosomes. Our results suggest a function of pol ν in meiotic homologous recombination in processing specific substrates. The restricted and more recent evolutionary appearance of pol ν (in comparison to pol θ) supports such a specialized role.

The work described here fills a current gap in the study of the 16 known DNA polymerases in vertebrate genomes. Until now, experiments with genetically disrupted mice have been reported for all but pol ν, encoded by the POLN gene. To intensively analyze the role of mammalian pol ν we generated multiple Poln-deficient murine models. We discovered that Poln is uniquely upregulated during testicular development and that it is enriched in spermatocytes. This, and phylogenetic analysis indicate a testis-specific function. We observed a modest reduction in meiotic recombination at a recombination hotspot in Poln-deficient mice. Pol ν has been suggested to function in DNA crosslink repair. However, we found no increased DNA crosslink sensitivity in Poln-deficient mice or POLN-depleted human cells. This is a major difference from some previous findings, and we support our conclusion by multiple experimental approaches, and by the very low or absent expression of functional pol ν in mammalian somatic cells. The present work represents the first description and comprehensive analysis of mice deficient in pol ν, and the first thorough phenotypic analysis in human cells.

Microhomology-mediated end joining: new players join the team

DNA double-strand breaks (DSBs) are the most deleterious type of DNA damage in cells arising from endogenous and exogenous attacks on the genomic DNA. Timely and properly repair of DSBs is important for genomic integrity and survival. MMEJ is an error-prone repair mechanism for DSBs, which relies on exposed microhomologous sequence flanking broken junction to fix DSBs in a Ku- and ligase IV-independent manner. Recently, significant progress has been made in MMEJ mechanism study. In this review, we will summarize its biochemical activities of several newly identified MMEJ factors and their biological significance.

Co-inhibition of pol θ and HR genes efficiently synergize with cisplatin to suppress cisplatin-resistant lung cancer cells survival

Cisplatin exert its anticancer effect by creating intrastrand and interstrand DNA cross-links which block DNA replication and is a major drug used to treat lung cancer. However, the main obstacle of the efficacy of treatment is drug resistance. Here, we show that expression of translesion synthesis (TLS) polymerase Q (POLQ) was significantly elevated by exposure of lung cancer cells A549/DR (a cisplatin-resistant A549 cell line) to cisplatin. POLQ expression correlated inversely with homologous recombination (HR) activity. Co-depletion of BRCA2 and POLQ by siRNA markedly increased sensitivity of A549/DR cells to cisplatin, which was accompanied with impairment of double strand breaks (DSBs) repair reflected by prominent cell cycle checkpoint response, increased chromosomal aberrations and persistent colocalization of p-ATM and 53BP1 foci induced by cisplatin. Thus, co-knockdown of POLQ and HR can efficiently synergize with cisplatin to inhibit A549/DR cell survival by inhibiting DNA DSBs repair. Similar results were observed in A549/DR cells co-depleted of BRCA2 and POLQ following BMN673 (a PARP inhibitor) treatment. Importantly, the sensitization effects to cisplatin and BMN673 in A549/DR cells by co-depleting BRCA2 and POLQ was stronger than those by co-depleting BRCA2 and other TLS factors including POLH, REV3, or REV1. Our results indicate that there is a synthetic lethal relationship between pol θ-mediated DNA repair and HR pathways. Pol θ may be considered as a novel target for lung cancer therapy.

Rev1 is a base excision repair enzyme with 5'-deoxyribose phosphate lyase activity

Rev1 is a member of the Y-family of DNA polymerases and is known for its deoxycytidyl transferase activity that incorporates dCMP into DNA and its ability to function as a scaffold factor for other Y-family polymerases in translesion bypass events. Rev1 also is involved in mutagenic processes during somatic hypermutation of immunoglobulin genes. In light of the mutation pattern consistent with dCMP insertion observed earlier in mouse fibroblast cells treated with a base excision repair-inducing agent, we questioned whether Rev1 could also be involved in base excision repair (BER). Here, we uncovered a weak 5′-deoxyribose phosphate (5′-dRP) lyase activity in mouse Rev1 and demonstrated the enzyme can mediate BER in vitro. The full-length Rev1 protein and its catalytic core domain are similar in their ability to support BER in vitro. The dRP lyase activity in both of these proteins was confirmed by NaBH₄ reduction of the Schiff base intermediate and kinetics studies. Limited proteolysis, mass spectrometry and deletion analysis localized the dRP lyase active site to the C-terminal segment of Rev1's catalytic core domain. These results suggest that Rev1 could serve as a backup polymerase in BER and could potentially contribute to AID-initiated antibody diversification through this activity.

DNA Polymerase θ: A Unique Multifunctional End-Joining Machine

The gene encoding DNA polymerase θ (Polθ) was discovered over ten years ago as having a role in suppressing genome instability in mammalian cells. Studies have now clearly documented an essential function for this unique A-family polymerase in the double-strand break (DSB) repair pathway alternative end-joining (alt-EJ), also known as microhomology-mediated end-joining (MMEJ), in metazoans. Biochemical and cellular studies show that Polθ exhibits a unique ability to perform alt-EJ and during this process the polymerase generates insertion mutations due to its robust terminal transferase activity which involves template-dependent and independent modes of DNA synthesis. Intriguingly, the POLQ gene also encodes for a conserved superfamily 2 Hel308-type ATP-dependent helicase domain which likely assists in alt-EJ and was reported to suppress homologous recombination (HR) via its anti-recombinase activity. Here, we review our current knowledge of Polθ-mediated end-joining, the specific activities of the polymerase and helicase domains, and put into perspective how this multifunctional enzyme promotes alt-EJ repair of DSBs formed during S and G2 cell cycle phases.

DNA polymerase θ (POLQ), double-strand break repair, and cancer

DNA polymerase theta (pol θ) is encoded in the genomes of many eukaryotes, though not in fungi. Pol θ is encoded by the POLQ gene in mammalian cells. The C-terminal third of the protein is a family A DNA polymerase with additional insertion elements relative to prokaryotic homologs. The N-terminal third is a helicase-like domain with DNA-dependent ATPase activity. Pol θ is important in the repair of genomic double-strand breaks (DSBs) from many sources. These include breaks formed by ionizing radiation and topoisomerase inhibitors, breaks arising at stalled DNA replication forks, breaks introduced during diversification steps of the mammalian immune system, and DSB induced by CRISPR-Cas9. Pol θ participates in a route of DSB repair termed "alternative end-joining" (altEJ). AltEJ is independent of the DNA binding Ku protein complex and requires DNA end resection. Pol θ is able to mediate joining of two resected 3' ends harboring DNA sequence microhomology. "Signatures" of Pol θ action during altEJ are the frequent utilization of longer microhomologies, and the insertion of additional sequences at joining sites. The mechanism of end-joining employs the ability of Pol θ to tightly grasp a 3' terminus through unique contacts in the active site, allowing extension from minimally paired primers. Pol θ is involved in controlling the frequency of chromosome translocations and preserves genome integrity by limiting large deletions. It may also play a backup role in DNA base excision repair. POLQ is a member of a cluster of similarly upregulated genes that are strongly correlated with poor clinical outcome for breast cancer, ovarian cancer and other cancer types. Inhibition of pol θ is a compelling approach for combination therapy of radiosensitization.

A putative Leishmania DNA polymerase theta protects the parasite against oxidative damage

Leishmania infantum is a protozoan parasite that is phagocytized by human macrophages. The host macrophages kill the parasite by generating oxidative compounds that induce DNA damage. We have identified, purified and biochemically characterized a DNA polymerase θ from L. infantum (LiPolθ), demonstrating that it is a DNA-dependent DNA polymerase involved in translesion synthesis of 8oxoG, abasic sites and thymine glycol lesions. Stably transfected L. infantum parasites expressing LiPolθ were significantly more resistant to oxidative and interstrand cross-linking agents, e.g. hydrogen peroxide, cisplatin and mitomycin C. Moreover, LiPolθ-overexpressing parasites showed an increased infectivity toward its natural macrophage host. Therefore, we propose that LiPolθ is a translesion synthesis polymerase involved in parasite DNA damage tolerance, to confer resistance against macrophage aggression.

miR-20b downregulates polymerases κ and θ in XP-V tumor cells

XP-V is a subtype of Xeroderma pigmentosum diseases with typical pigmentation and cancers in sun-exposed regions. The present study investigated the role of microRNA-20b (miR-20b) in the imbalance of polymerase expression levels in XP-V tumor cells. Following software prediction results, certain miRNAs were chosen as candidate regulators for the observed imbalance in polymerases in XP-V tumor cells. Reverse transcription-quantitative polymerase chain reaction and western blot were used to test candidate miRNAs for their ability to reduce the expression of these polymerases. A luciferase reporter assay was used to further verify the western blot results. Polymerases κ and θ were expressed at lower levels in XP-V tumor cells compared to normal control cells. A positive correlation was demonstrated between miR-20b and polymerases κ and θ. It was also demonstrated that a proportion of miRNAs had no effect on polymerases κ and θ, despite the software predicting that these miRNAs would target these two polymerases. Therefore, miR-20b may be responsible for the low expression levels of polymerase κ and θ in XP-V tumor cells, which accelerated mismatch in DNA replication repairing.

Linking DNA polymerase theta structure and function in health and disease

DNA polymerase theta (Pol θ) is an error-prone A-family polymerase that is highly conserved among multicellular eukaryotes and plays multiple roles in DNA repair and the regulation of genome integrity. Studies conducted in several model organisms have shown that Pol θ can be utilized during DNA interstrand crosslink repair and during alternative end-joining repair of double-strand breaks. Recent genetic and biochemical studies have begun to elucidate the unique structural features of Pol θ that promote alternative end-joining repair. Importantly, Pol θ-dependent end joining appears to be important for overall genome stability, as it affects chromosome translocation formation in murine and human cell lines. Pol θ has also been suggested to act as a modifier of replication timing in human cells, though the mechanism of action remains unknown. Pol θ is highly upregulated in a number of human cancer types, which could indicate that mutagenic Pol θ-dependent end joining is used during cancer cell proliferation. Here, we review the various roles of Pol θ across species and discuss how these roles may be relevant to cancer therapy.

Conserved overlapping gene arrangement, restricted expression, and biochemical activities of DNA polymerase ν (POLN)

DNA polymerase ν (POLN) is one of 16 DNA polymerases encoded in vertebrate genomes. It is important to determine its gene expression patterns, biological roles, and biochemical activities. By quantitative analysis of mRNA expression, we found that POLN from the zebrafish Danio rerio is expressed predominantly in testis. POLN is not detectably expressed in zebrafish embryos or in mouse embryonic stem cells. Consistent with this, injection of POLN-specific morpholino antisense oligonucleotides did not interfere with zebrafish embryonic development. Analysis of transcripts revealed that vertebrate POLN has an unusual gene expression arrangement, sharing a first exon with HAUS3, the gene encoding augmin-like complex subunit 3. HAUS3 is broadly expressed in embryonic and adult tissues, in contrast to POLN. Differential expression of POLN and HAUS3 appears to arise by alternate splicing of transcripts in mammalian cells and zebrafish. When POLN was ectopically overexpressed in human cells, it specifically coimmunoprecipitated with the homologous recombination factors BRCA1 and FANCJ, but not with previously suggested interaction partners (HELQ and members of the Fanconi anemia core complex). Purified zebrafish POLN protein is capable of thymine glycol bypass and strand displacement, with activity dependent on a basic amino acid residue known to stabilize the primer-template. These properties are conserved with the human enzyme. Although the physiological function of pol ν remains to be clarified, this study uncovers distinctive aspects of its expression control and evolutionarily conserved properties of this DNA polymerase.

Structures of human DNA polymerases ν and θ expose their end game

Although the two B-family human DNA polymerases, pol δ and pol ε, are responsible for the bulk of nuclear genome replication, at least 14 additional polymerases have roles in nuclear DNA repair and replication. In this issue, newly reported crystal structures of two specialized A-family polymerases, pol δ and pol ε, expose these enzymes’ strategies for handling aberrant DNA ends.

Abacavir, an anti-HIV-1 drug, targets TDP1-deficient adult T cell leukemia

Adult T cell leukemia (ATL) is an aggressive T cell malignancy caused by human T cell leukemia virus type 1 (HTLV-1) and has a poor prognosis. We analyzed the cytotoxic effects of various nucleoside analog reverse transcriptase inhibitors (NRTIs) for HIV-1 on ATL cells and found that abacavir potently and selectively kills ATL cells. Although NRTIs have minimal genotoxicities on host cells, the therapeutic concentration of abacavir induced numerous DNA double-strand breaks (DSBs) in the chromosomal DNA of ATL cells. DSBs persisted over time in ATL cells but not in other cell lines, suggesting impaired DNA repair. We found that the reduced expression of tyrosyl-DNA phosphodiesterase 1 (TDP1), a repair enzyme, is attributable to the cytotoxic effect of abacavir on ATL cells. We also showed that TDP1 removes abacavir from DNA ends in vitro. These results suggest a model in which ATL cells with reduced TDP1 expression are unable to excise abacavir incorporated into genomic DNA, leading to irreparable DSBs. On the basis of the above mechanism, we propose abacavir as a promising chemotherapeutic agent for ATL.