Catalytic and non-catalytic roles of DNA polymerase κ in the protection of human cells against genotoxic stresses

DNA polymerase κ suppresses inflammation and inflammation-induced mutagenesis and carcinogenic potential in the colon of mice

BACKGROUND

Chronic inflammation induces DNA damage and promotes cell proliferation, thereby increasing the risk of cancer. DNA polymerase κ (Pol κ), involved in translesion DNA synthesis, counteracts mutagenesis induced by inflammation in the colon of mice. In the present study, we examined whether Pol κ suppressed inflammation-induced colon tumorigenesis by treating inactivated Polk knock-in (Polk^-/-) mice with dextran sulfate sodium (DSS), an inducer of colon inflammation.

RESULTS

Male and female Polk^-/- and Polk^+/+ mice were administered 2% DSS in drinking water for six consecutive days, succeeded via a recovery period of 16 days, followed by 2% DSS for another two days. DSS treatment strongly induced colitis, and the severity of colitis was higher in Polk^-/- mice than in Polk^+/+ mice. The mice were sacrificed after 19 weeks from the initiation of the first DSS treatment and subjected to pathological examination and mutation analysis. DSS treatment induced colonic dysplasia, and the multiplicity of dysplasia was higher in Polk^-/- mice than in Polk^+/+mice. Some of the dysplasias in Polk^-/- mice exhibited β-catenin-stained nucleus and/or cytoplasm. Mutation frequencies in the gpt reporter gene were increased by DSS treatment in Polk^-/- mice, and were higher than those in Polk^+/+ mice.

CONCLUSIONS

Pol κ suppresses inflammation and inflammation-induced dysplasia as well as inflammation-induced mutagenesis. The possible mechanisms by which Pol κ suppresses colitis- and colitis-induced dysplasia are discussed.

A catalytic-independent function of human DNA polymerase Kappa controls the stability and abundance of the Checkpoint Kinase 1

DNA polymerase kappa (Pol κ) has been well documented thus far for its specialized DNA synthesis activity during translesion replication, progression of replication forks through regions difficult to replicate, restart of stalled forks and replication checkpoint efficiency. Pol κ is also required for the stabilization of stalled forks although the mechanisms are poorly understood. Here we unveiled an unexpected role for Pol κ in controlling the stability and abundance of Chk1, an important actor for the replication checkpoint and fork stabilization. We found that loss of Pol κ decreased the Chk1 protein level in the nucleus of four human cell lines. Pol κ and not the other Y-family polymerase members is required to maintain the Chk1 protein pool all along the cell cycle. We showed that Pol κ depletion affected the protein stability of Chk1 and protected it from proteasome degradation. Importantly, we also observed that the fork restart defects observed in Pol κ-depleted cells could be overcome by the re-expression of Chk1. Strikingly, this new function of Pol κ does not require its catalytic activity. We propose that Pol κ could contribute to the protection of stalled forks through Chk1 stability.

Translesion DNA Synthesis and Carcinogenesis

Tens of thousands of DNA lesions are formed in mammalian cells each day. DNA translesion synthesis is the main mechanism of cell defense against unrepaired DNA lesions. DNA polymerases iota (Pol ι), eta (Pol η), kappa (Pol κ), and zeta (Pol ζ) have active sites that are less stringent toward the DNA template structure and efficiently incorporate nucleotides opposite DNA lesions. However, these polymerases display low accuracy of DNA synthesis and can introduce mutations in genomic DNA. Impaired functioning of these enzymes can lead to an increased risk of cancer.

Three Human Pol ι Variants with Impaired Polymerase Activity Fail to Rescue H2O2 Sensitivity in POLI-Deficient Cells

Human Y-family DNA polymerase (pol) ι is involved in translesion DNA synthesis (TLS) and base excision repair (BER) of oxidative DNA damage. Genetic variations may alter the function of pol ι and affect cellular susceptibility to oxidative genotoxic agents, but their effects remain unclear. We investigated the impacts of 10 human missense germline variations on pol ι function by biochemical and cell-based assays. Both polymerase and deoxyribose phosphate (dRP) lyase activities were determined utilizing recombinant pol ι (residues 1-445) proteins. The K209Q, K228I, and Q386R variants showed 4- to 53-fold decreases in specificity constants (k_cat/K_m) for dCTP insertion opposite G and 8-oxo-7,8-dihydroguanine compared to the wild-type. The R126C and K345E variants showed wild-type-like polymerase activity, although these two variants (as well as the R209Q, K228I, and Q386R variants) showed greater than 6-fold decreases in dRP lyase activity compared to the wild-type. A CRISPR/Cas9-mediated POLI knockout conferred higher sensitivity to H₂O₂ in human embryonic kidney (HEK293) cells. Exogenous expression of the full-length wild-type, R126C, and K345E variants fully rescued the H₂O₂ sensitivity in POLI-deficient cells, while full-length R209Q, K228I, and Q386R variants did not rescue the sensitivity. Our results indicate that the R126C and K345E variants (having wild-type-like polymerase activity, albeit impaired in dRP lyase activity) could fully rescue the H₂O₂ sensitivity in POLI-deficient cells, while the R209Q, K228I, and Q386R variants, all impaired in polymerase and dRP lyase activity, failed to rescue the sensitivity, indicating the relative importance of TLS-related polymerase function of pol ι rather than its BER-related dRP lyase function in protection from oxidative stress. The possibility exists that the hypoactive pol ι variants increase the individual susceptibility to oxidative genotoxic agents.

Regulation of the error-prone DNA polymerase Polκ by oncogenic signaling and its contribution to drug resistance

The DNA polymerase Polκ plays a key role in translesion synthesis, an error-prone replication mechanism. Polκ is overexpressed in various tumor types. Here, we found that melanoma and lung and breast cancer cells experiencing stress from oncogene inhibition up-regulated the expression of Polκ and shifted its localization from the cytoplasm to the nucleus. This effect was phenocopied by inhibition of the kinase mTOR, by induction of ER stress, or by glucose deprivation. In unstressed cells, Polκ is continually transported out of the nucleus by exportin-1. Inhibiting exportin-1 or overexpressing Polκ increased the abundance of nuclear-localized Polκ, particularly in response to the BRAF^V600E-targeted inhibitor vemurafenib, which decreased the cytotoxicity of the drug in BRAF^V600E melanoma cells. These observations were analogous to how Escherichia coli encountering cell stress and nutrient deprivation can up-regulate and activate DinB/pol IV, the bacterial ortholog of Polκ, to induce mutagenesis that enables stress tolerance or escape. However, we found that the increased expression of Polκ was not excessively mutagenic, indicating that noncatalytic or other functions of Polκ could mediate its role in stress responses in mammalian cells. Repressing the expression or nuclear localization of Polκ might prevent drug resistance in some cancer cells.

Effects of DNA polymerase kappa and mismatch repair on dose-responses of chromosome aberrations induced by three oxidative genotoxins in human cells

Genotoxic carcinogens are regulated under the policy that there is no threshold or safe dose. It has been pointed out, however, that self-defense mechanisms, such as detoxification, DNA repair, and error-free translesion synthesis, may protect chromosome DNA from genotoxic insults, thereby constituting practical threshold. In this study, we examined dose responses of chromosome aberrations induced by three oxidative genotoxins, that is, hydrogen peroxide (H₂ O₂ ), menadione and paraquat, with or without DNA polymerase kappa (Polκ) activities and mismatch repair capacities in human cells. Polκ is involved in translesion synthesis across DNA damage and mismatch repair is responsible for correction of base-base mismatch in DNA. Polκ activity of the cells was inactivated either by point mutations in the catalytically essential amino acids (catalytically dead or CD) or by deletion of the POLK gene (knockout or KO). In the absence of mismatch repair, frequencies of chromosome aberrations induced by H₂ O₂ and menadione were not significantly different among CD, KO, and the wild type (WT) cells. In the presence of mismatch repair, however, cytotoxicity and clastogenicity were enhanced and Polκ modulated the sensitivity of the cells. No-observed-genotoxic-effect-levels (NOGELs) for H₂ O₂ and menadione were CD = KO < WT cells. In contrast, the sensitivities of the cells to paraquat were not significantly affected by the status of mismatch repair or Polκ activity. The results suggest that mismatch repair and Polκ coordinately modulate NOGELs for the clastogenicity of H₂ O₂ and menadione and also that DNA lesion(s) responsible for paraquat-induced chromosome aberrations are different from those induced by H₂ O₂ and menadione. Environ. Mol. Mutagen. 61:193-199, 2020. © 2019 Wiley Periodicals, Inc.

Mammalian DNA Polymerase Kappa Activity and Specificity

DNA polymerase (pol) kappa is a Y-family translesion DNA polymerase conserved throughout all domains of life. Pol kappa is special6 ized for the ability to copy DNA containing minor groove DNA adducts, especially N²-dG adducts, as well as to extend primer termini containing DNA damage or mismatched base pairs. Pol kappa generally cannot copy DNA containing major groove modifications or UV-induced photoproducts. Pol kappa can also copy structured or non-B-form DNA, such as microsatellite DNA, common fragile sites, and DNA containing G quadruplexes. Thus, pol kappa has roles both in maintaining and compromising genomic integrity. The expression of pol kappa is altered in several different cancer types, which can lead to genome instability. In addition, many cancer-associated single-nucleotide polymorphisms have been reported in the POLK gene, some of which are associated with poor survival and altered chemotherapy response. Because of this, identifying inhibitors of pol kappa is an active area of research. This review will address these activities of pol kappa, with a focus on lesion bypass and cellular mutagenesis.

Inhibition of Human DNA Polymerases Eta and Kappa by Indole-Derived Molecules Occurs through Distinct Mechanisms

Overexpression of human DNA polymerase kappa (hpol κ) in glioblastoma is associated with shorter survival time and resistance to the alkylating agent temozolomide (TMZ), making it an attractive target for the development of small-molecule inhibitors. We previously reported on the development and characterization of indole barbituric acid-derived (IBA) inhibitors of translesion DNA synthesis polymerases (TLS pols). We have now identified a potent and selective inhibitor of hpol κ based on the indole-aminoguanidine (IAG) chemical scaffold. The most promising IAG analogue, IAG-10, exhibited greater inhibitory action against hpol κ than any other human Y-family member, as well as pols from the A-, B-, and X-families. Inhibition of hpol κ by IAG analogues appears to proceed through a mechanism that is distinct from inhibition of hpol η based on changes in DNA binding affinity and nucleotide insertion kinetics. By way of comparison, both IAG and IBA analogues inhibited binary complex formation by hpol κ and ternary complex formation by hpol η. Decreasing the concentration of enzyme and DNA in the reaction mixture lowered the IC₅₀ value of IAG-10 to submicromolar values, consistent with inhibition of binary complex formation for hpol κ. Chemical footprinting experiments revealed that IAG-10 binds to a cleft between the finger, little finger, and N-clasp domains on hpol κ and that this likely disrupts the interaction between the N-clasp and the TLS pol core. In cell culture, IAG-10 potentiated the antiproliferative activity and DNA damaging effects of TMZ in hpol κ-proficient cells but not in hpol κ-deficient cells, indicative of a target-dependent effect. Mutagenic replication across alkylation damage increased in hpol κ-proficient cells treated with IAG-10, while no change in mutation frequency was observed for hpol κ-deficient cells. In summary, we developed a potent and selective small-molecule inhibitor of hpol κ that takes advantage of structural features unique to this TLS enzyme to potentiate TMZ, a standard-of-care drug used in the treatment of malignant brain tumors. Furthermore, the IAG scaffold represents a new chemical space for the exploration of TLS pol inhibitors, which could prove useful as a strategy for improving patient response to genotoxic drugs.

DNA polymerase kappa counteracts inflammation-induced mutagenesis in multiple organs of mice

In vitro studies indicate that DNA polymerase kappa (Polκ) is able to accurately and efficiently perform DNA synthesis using templates containing various types of DNA damage, including benzo[a]pyrene (BP)-induced N² -deoxyguanosine adducts. In this study, we examined sensitivity of inactivated Polk knock-in (Polk^-/- ) mice to BP carcinogenicity in the colon by administering an oral dose of BP plus dextran sulfate sodium (DSS), an inflammation causing promoter of carcinogenesis. Although colon cancer was successfully induced by BP plus DSS, there was no significant difference in tumor incidence or multiplicity between Polk^-/- and Polk^+/+ mice. Malignant lymphoma was induced in thymus by the treatment only in Polk^-/- mice, but it lacked statistical significance. Mutant frequencies (MFs) in the gpt reporter gene were strongly enhanced in colon; almost to the same extent in both types of mice. Micronucleus formation in bone marrow at the high dose of BP and DNA adducts in colon and lung was not significantly different between two types of mice. Surprisingly, however, Polk^-/- mice exhibited significantly higher MFs in colon and lung than did Polk^+/+ mice when they were treated with DSS alone. The most prominent mutation induced by DSS treatment was G:C to C:G transversion, whose specific MF in proximal colon was 30 times higher in Polk^-/- than in Polk^+/+ mice. DSS alone did not enhance MF at all in Polk^+/+ mice. The results indicate that Polκ does not suppress BP-induced mutagenesis and carcinogenesis in the colon, but counteracts inflammation-induced mutagenesis in multiple organs. Environ. Mol. Mutagen. 60:320-330, 2019. © 2019 Wiley Periodicals, Inc.

Opposing roles of Y-family DNA polymerases in lipid peroxide mutagenesis at the hisG46 target in the Ames test

DNA polymerases play a key role in mutagenesis by performing translesion DNA synthesis (TLS). The Y-family of DNA polymerases comprises several evolutionarily conserved families, specializing in TLS of different DNA adducts. Exocyclic etheno and propano DNA adducts are among the most common endogenous DNA lesions induced by lipid peroxidation reactions triggered by oxidative stress. We have investigated the participation of two enterobacterial representatives of the PolIV and PolV branches of Y-family DNA polymerases in mutagenesis by two model lipid peroxidation derived genotoxins, glyoxal and crotonaldehyde. Mutagenesis by the ethano adduct (glyoxal-derived) and the propano adduct (crontonaldehyde-derived) at the GC target in the Ames test depended exclusively on PolV type DNA polymerases such as PolRI. In contrast, PolIV suppressed glyoxal and, even more, crotonaldehyde mutagenesis, as detected by enzyme overexpression and gene knockout approaches. We propose that DNA polymerase IV, which is the mammalian DNA polymerase κ ortholog, acts as a housekeeper protecting the genome from lipoxidative stress.

Limited ability of DNA polymerase kappa to suppress benzo[a]pyrene-induced genotoxicity in vivo

DNA polymerase kappa (Polk) is a specialized DNA polymerase involved in translesion DNA synthesis. To understand the protective roles against genotoxins in vivo, we established inactivated Polk knock-in gpt delta (inactivated Polk KI) mice that possessed reporter genes for mutations and expressed inactive Polk. In this study, we examined genotoxicity of benzo[a]pyrene (BP) to determine whether Polk actually suppressed BP-induced genotoxicity as predicted by biochemistry and in vitro cell culture studies. Seven-week-old inactivated Polk KI and wild-type (WT) mice were treated with BP at doses of 5, 15, or 50 mg/(kg·day) for three consecutive days by intragastric gavage, and mutations in the colon and micronucleus formation in the peripheral blood were examined. Surprisingly, no differences were observed in the frequencies of mutations and micronucleus formation at 5 or 50 mg/kg doses. Inactivated Polk KI mice exhibited approximately two times higher gpt mutant frequency than did WT mice only at the 15 mg/kg dose. The frequency of micronucleus formation was slightly higher in inactivated Polk KI than in WT mice at the same dose, but it was statistically insignificant. The results suggest that Polk has a limited ability to suppress BP-induced genotoxicity in the colon and bone marrow and also that the roles of specialized DNA polymerases in mutagenesis and carcinogenesis should be examined not only by in vitro assays but also by in vivo mouse studies. We also report the spontaneous mutagenesis in inactivated Polk KI mice at young and old ages. Environ. Mol. Mutagen. 58:644-653, 2017. © 2017 Wiley Periodicals, Inc.

Hypersensitivity of mouse embryonic fibroblast cells defective for DNA polymerases η, ι and κ to various genotoxic compounds: Its potential for application in chemical genotoxic screening

Genotoxic agents cause modifications of genomic DNA, such as alkylation, oxidation, bulky adduct formation, and strand breaks, which potentially induce mutations and changes to the structure or number of genes. Majority of point mutations are generated during error-prone bypass of modified nucleotides (translesion DNA synthesis, TLS); however, when TLS fails, replication forks stalled at lesions eventually result in more lethal effects, formation of double-stranded breaks (DSBs). Here we compared sensitivities to various compounds among mouse embryonic fibroblasts derived from wild-type and knock-out mice lacking one of the three Y-family TLS DNA polymerases (Polη, Polι, and Polκ) or all of them (TKO). The compounds tested in this study include genotoxins such as methyl methanesulfonate (MMS) and nongenotoxins such as ammonium chloride. We found that TKO cells exhibited the highest sensitivities to most of the tested genotoxins, but not to the non-genotoxins. In order to quantitatively evaluate the hypersensitivity of TKO cells to different chemicals, we calculated ratios of half-maximal inhibitory concentration for WT and TKO cells. The ratios for 9 out of 10 genotoxins ranged from 2.29 to 5.73, while those for 5 nongenotoxins ranged from 0.81 to 1.63. Additionally, the two markers for DNA damage, ubiquitylated proliferating cell nuclear antigen and γ-H2AX after MMS treatment, were accumulated in TKO cells more greatly than in WT cells. Furthermore, following MMS treatment, TKO cells exhibited increased frequency of sister chromatid exchange compared with WT cells. These results indicated that the hypersensitivity of TKO cells to genotoxins resulted from replication fork stalling and subsequent DNA double-strand breaks, thus demonstrating that TKO cells should be useful for evaluating chemical genotoxicity.

DNA polymerase kappa protects human cells against MMC-induced genotoxicity through error-free translesion DNA synthesis

Background

Interactions between genes and environment are critical factors for causing cancer in humans. The genotoxicity of environmental chemicals can be enhanced via the modulation of susceptible genes in host human cells. DNA polymerase kappa (Pol κ) is a specialized DNA polymerase that plays an important role in DNA damage tolerance through translesion DNA synthesis. To better understand the protective roles of Pol κ, we previously engineered two human cell lines either deficient in expression of Pol κ (KO) or expressing catalytically dead Pol κ (CD) in Nalm-6-MSH+ cells and examined cytotoxic sensitivity against various genotoxins. In this study, we set up several genotoxicity assays with cell lines possessing altered Pol κ activities and investigated the protective roles of Pol κ in terms of genotoxicity induced by mitomycin C (MMC), a therapeutic agent that induces bulky DNA adducts and crosslinks in DNA.

Results

We introduced a frameshift mutation in one allele of the thymidine kinase (TK) gene of the KO, CD, and wild-type Pol κ cells (WT), thereby establishing cell lines for the TK gene mutation assay, namely TK+/- cells. In addition, we formulated experimental conditions to conduct chromosome aberration (CA) and sister chromatid exchange (SCE) assays with cells. By using the WT TK+/- and KO TK+/- cells, we assayed genotoxicity of MMC. In the TK gene mutation assay, the cytotoxic and mutagenic sensitivities of KO TK+/- cells were higher than those of WT TK+/- cells. MMC induced loss of heterozygosity (LOH), base pair substitutions at CpG sites and tandem mutations at GpG sites in both cell lines. However, the frequencies of LOH and base substitutions at CpG sites were significantly higher in KO TK+/- cells than in WT TK+/- cells. MMC also induced CA and SCE in both cell lines. The KO TK+/- cells displayed higher sensitivity than that displayed by WT TK+/- cells in the SCE assay.

Conclusions

These results suggest that Pol κ is a modulating factor for the genotoxicity of MMC and also that the established cell lines are useful for evaluating the genotoxicity of chemicals from multiple endpoints in different genetic backgrounds of Pol κ.

Electronic supplementary material

The online version of this article (doi:10.1186/s41021-016-0067-3) contains supplementary material, which is available to authorized users.

Six Germline Genetic Variations Impair the Translesion Synthesis Activity of Human DNA Polymerase κ

DNA polymerase (pol) κ efficiently catalyzes error-free translesion DNA synthesis (TLS) opposite bulky N²-guanyl lesions induced by carcinogens such as polycyclic aromatic hydrocarbons. We investigated the biochemical effects of nine human nonsynonymous germline POLK variations on the TLS properties of pol κ, utilizing recombinant pol κ (residues 1-526) enzymes and DNA templates containing an N²-CH₂(9-anthracenyl)G (N²-AnthG), 8-oxo-7,8-dihydroguanine (8-oxoG), O⁶-methyl(Me)G, or an abasic site. In steady-state kinetic analyses, the R246X, R298H, T473A, and R512W variants displayed 7- to 18-fold decreases in k_cat/K_m for dCTP insertion opposite G and N²-AnthG, with 2- to 3-fold decreases in DNA binding affinity, compared to that of the wild-type, and further showed 5- to 190-fold decreases in k_cat/K_m for next-base extension from C paired with N²-AnthG. The A471V variant showed 2- to 4-fold decreases in k_cat/K_m for correct nucleotide insertion opposite and beyond G (or N²-AnthG) compared to that of the wild-type. These five hypoactive variants also showed similar patterns of attenuation of TLS activity opposite 8-oxoG, O⁶-MeG, and abasic lesions. By contrast, the T44M variant exhibited 7- to 11-fold decreases in k_cat/K_m for dCTP insertion opposite N²-AnthG and O⁶-MeG (as well as for dATP insertion opposite an abasic site) but not opposite both G and 8-oxoG, nor beyond N²-AnthG, compared to that of the wild-type. These results suggest that the R246X, R298H, T473A, R512W, and A471V variants cause a general catalytic impairment of pol κ opposite G and all four lesions, whereas the T44M variant induces opposite lesion-dependent catalytic impairment, i.e., only opposite O⁶-MeG, abasic, and bulky N²-G lesions but not opposite G and 8-oxoG, in pol κ, which might indicate that these hypoactive pol κ variants are genetic factors in modifying individual susceptibility to genotoxic carcinogens in certain subsets of populations.

The role of DNA polymerase ζ in translesion synthesis across bulky DNA adducts and cross-links in human cells

Translesion DNA synthesis (TLS) is a cellular defense mechanism against genotoxins. Defects or mutations in specialized DNA polymerases (Pols) involved in TLS are believed to result in hypersensitivity to various genotoxic stresses. Here, DNA polymerase ζ (Pol ζ)-deficient (KO: knockout) and Pol ζ catalytically dead (CD) human cells were established and their sensitivity towards cytotoxic activities of various genotoxins was examined. The CD cells were engineered by altering the DNA sequence encoding two amino acids essential for the catalytic activity of Pol ζ, i.e., D2781 and D2783, to alanines. Both Pol ζ KO and CD cells displayed a prolonged cell cycle and higher incidence of micronuclei formation than the wild-type (WT) cells in the absence of exogenous genotoxic treatments, and the order of abnormality was CD>KO>WT cells. Both KO and CD cells exhibited higher sensitivity towards the killing effects of benzo[a]pyrene diol epoxide, mitomycin C, potassium bromate, N-methyl-N'-nitro-N-nitrosoguanidine, and ultraviolet C irradiation than WT cells, and there were no differences between the sensitivities of KO and CD cells. Interestingly, neither KO nor CD cells were sensitive to the cytotoxic effects of hydrogen peroxide. Since KO and CD cells displayed similar sensitivities to the genotoxins, we employed only KO cells to further examine their sensitivity to other genotoxic agents. KO cells were more sensitive to the cytotoxicity of 4-nitroquinoline N-oxide, styrene oxide, cisplatin, methyl methanesulfonate, and ethyl methanesulfonate than WT cells. However, the KO cells displayed sensitivity camptothecin, etoposide, bleomycin, hydroxyurea, crotonealdehyde, and methylglyoxal in a manner similar to the WT cells. Our results suggest that Pol ζ plays an important role in the protection of human cells by carrying out TLS across bulky DNA adducts and cross-links, but has no or limited role in the protection against strand-breaks in DNA.

A single aspartate mutation in the conserved catalytic site of Rev3L generates a hypomorphic phenotype in vivo and in vitro

Rev3, the catalytic subunit of yeast DNA polymerase ζ, is required for UV resistance and UV-induced mutagenesis, while its mammalian ortholog, REV3L, plays further vital roles in cell proliferation and embryonic development. To assess the contribution of REV3L catalytic activity to its in vivo function, we generated mutant mouse strains in which one or two Ala residues were substituted to the Asp of the invariant catalytic YGDTDS motif. The simultaneous mutation of both Asp (ATA) phenocopies the Rev3l knockout, which proves that the catalytic activity is mandatory for the vital functions of Rev3L, as reported recently. Surprisingly, although the mutation of the first Asp severely impairs the enzymatic activity of other B-family DNA polymerases, the corresponding mutation of Rev3 (ATD) is hypomorphic in yeast and mouse, as it does not affect viability and proliferation and moderately impacts UVC-induced cell death and mutagenesis. Interestingly, Rev3l hypomorphic mutant mice display a distinct, albeit modest, alteration of the immunoglobulin gene mutation spectrum at G-C base pairs, further documenting its role in this process.

The Polymerase Activity of Mammalian DNA Pol ζ Is Specifically Required for Cell and Embryonic Viability

DNA polymerase ζ (pol ζ) is exceptionally important for maintaining genome stability. Inactivation of the Rev3l gene encoding the polymerase catalytic subunit causes a high frequency of chromosomal breaks, followed by lethality in mouse embryos and in primary cells. Yet it is not known whether the DNA polymerase activity of pol ζ is specifically essential, as the large REV3L protein also serves as a multiprotein scaffold for translesion DNA synthesis via multiple conserved structural domains. We report that Rev3l cDNA rescues the genomic instability and DNA damage sensitivity of Rev3l-null immortalized mouse fibroblast cell lines. A cDNA harboring mutations of conserved catalytic aspartate residues in the polymerase domain of REV3L could not rescue these phenotypes. To investigate the role of REV3L DNA polymerase activity in vivo, a Rev3l knock-in mouse was constructed with this polymerase-inactivating alteration. No homozygous mutant mice were produced, with lethality occurring during embryogenesis. Primary fibroblasts from mutant embryos showed growth defects, elevated DNA double-strand breaks and cisplatin sensitivity similar to Rev3l-null fibroblasts. We tested whether the severe Rev3l^-/- phenotypes could be rescued by deletion of DNA polymerase η, as has been reported with chicken DT40 cells. However, Rev3l^-/-Polh^-/- mice were inviable, and derived primary fibroblasts were as sensitive to DNA damage as Rev3l^-/-Polh^+/+ fibroblasts. Therefore, the functions of REV3L in maintaining cell viability, embryonic viability and genomic stability are directly dependent on its polymerase activity, and cannot be ameliorated by an additional deletion of pol η. These results validate and encourage the approach of targeting the DNA polymerase activity of pol ζ to sensitize tumors to DNA damaging agents.

Translesion synthesis allows DNA replication to occur in the presence of damaged DNA. This process is mediated by low-fidelity DNA polymerases (such as pol ζ or pol η) that maintain genomic stability. The action of these polymerases is crucial to limit cancer. In mice, complete deletion of DNA pol ζ leads to embryonic lethality, and conditional deletion enhances tumorigenesis. Pol ζ is a large protein with many domains that interact with other essential proteins and maintain the structural integrity of pol ζ. It is not known if the polymerase activity of pol ζ mediates its essential activities. Using a cell culture complementation system and in vivo knock-in mice, our work shows that pol ζ–mediated maintenance of genomic stability in the presence of DNA damage is absolutely dependent on its DNA polymerase activity. Others have demonstrated in chicken cells that co-deletion of pol ζ and pol η rescues the pol ζ-dependent phenotypes, but our work in mice and in mouse cell culture does not support that conclusion. These results demonstrate the physiological importance of pol ζ polymerase activity, and show that employing small-molecule inhibitors of the polymerase reaction is a valid strategy for sensitizing tumor cells to chemotherapeutic agents.