An essential role for REV3 in mammalian cell survival: absence of REV3 induces p53-independent embryonic death

Disruption of DNA polymerase ζ engages an innate immune response

In mammalian cells, specialized DNA polymerase ζ (pol ζ) contributes to genomic stability during normal DNA replication. Disruption of the catalytic subunit Rev3l is toxic and results in constitutive chromosome damage, including micronuclei. As manifestations of this genomic stress are unknown, we examined the transcriptome of pol ζ-defective cells by RNA sequencing (RNA-seq). Expression of 1,117 transcripts is altered by ≥4-fold in Rev3l-disrupted cells, with a pattern consistent with an induction of an innate immune response. Increased expression of interferon-stimulated genes at the mRNA and protein levels in pol ζ-defective cells is driven by the cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS)-signaling partner stimulator of interferon genes (STING) pathway. Expression of key interferon-stimulated chemokines is elevated in basal epithelial mouse skin cells with a disruption of Rev3l. These results indicate that the disruption of pol ζ may simultaneously increase sensitivity to genotoxins and potentially engage parts of the innate immune response, which could add an additional benefit to targeting pol ζ in cancer therapies.

A large intermediate domain of vertebrate REV3 protein is dispensable for ultraviolet-induced translesion replication

DNA polymerase ζ (pol ζ) is involved in translesion replication (translesion synthesis, TLS) and plays an essential role in embryogenesis. In adults, pol ζ triggers mutation as a result of error-prone TLS and causes carcinogenesis. The catalytic subunit of pol ζ, REV3, is evolutionarily conserved from yeast and plants to higher eukaryotes. However, the structures are notably different: unlike that in yeast REV3, a large intermediate domain is inserted in REV3 of humans and mice. The domain is mostly occupied with noncommittal structures (random coil…etc.); therefore, its role and function are yet to be resolved. Previously, we reported deficient levels of ultraviolet (UV)-induced TLS in fibroblasts derived from the Rev3-knockout mouse embryo (Rev3KO-MEF). Here, we constructed a mouse Rev3-expressing plasmid with a deleted intermediate domain (532-1793 a.a,) and transfected it into Rev3KO-MEF. The isolated stable transformants showed comparable levels of UV-sensitivity and UV-TLS activity to those in wild-type MEF, detected using an alkaline sucrose density gradient sedimentation. These results indicate that the intermediate domain is nonessential for UV-induced translesion replication in cultured mouse cells.

Translesion Synthesis in Plants: Ultraviolet Resistance and Beyond

Plant genomes sustain various forms of DNA damage that stall replication forks. Translesion synthesis (TLS) is one of the pathways to overcome stalled replication in which specific polymerases (TLS polymerase) perform bypass synthesis across DNA damage. This article gives a brief overview of plant TLS polymerases. In Arabidopsis, DNA polymerase (Pol) ζ, η, κ, θ, and λ and Reversionless1 (Rev1) are shown to be involved in the TLS. For example, AtPolη bypasses ultraviolet (UV)-induced cyclobutane pyrimidine dimers in vitro. Disruption of AtPolζ or AtPolη increases root stem cell death after UV irradiation. These results suggest that AtPolζ and ATPolη bypass UV-induced damage, prevent replication arrest, and allow damaged cells to survive and grow. In general, TLS polymerases have low fidelity and often induce mutations. Accordingly, disruption of AtPolζ or AtRev1 reduces somatic mutation frequency, whereas disruption of AtPolη elevates it, suggesting that plants have both mutagenic and less mutagenic TLS activities. The stalled replication fork can be resolved by a strand switch pathway involving a DNA helicase Rad5. Disruption of both AtPolζ and AtRAD5a shows synergistic or additive effects in the sensitivity to DNA-damaging agents. Moreover, AtPolζ or AtRev1 disruption elevates homologous recombination frequencies in somatic tissues. These results suggest that the Rad5-dependent pathway and TLS are parallel. Plants grown in the presence of heat shock protein 90 (HSP90) inhibitor showed lower mutation frequencies, suggesting that HSP90 regulates mutagenic TLS in plants. Hypersensitivities of TLS-deficient plants to γ-ray and/or crosslink damage suggest that plant TLS polymerases have multiple roles, as reported in other organisms.

Eukaryotic DNA polymerase ζ

This review focuses on eukaryotic DNA polymerase ζ (Pol ζ), the enzyme responsible for the bulk of mutagenesis in eukaryotic cells in response to DNA damage. Pol ζ is also responsible for a large portion of mutagenesis during normal cell growth, in response to spontaneous damage or to certain DNA structures and other blocks that stall DNA replication forks. Novel insights in mutagenesis have been derived from recent advances in the elucidation of the subunit structure of Pol ζ. The lagging strand DNA polymerase δ shares the small Pol31 and Pol32 subunits with the Rev3-Rev7 core assembly giving a four subunit Pol ζ complex that is the active form in mutagenesis. Furthermore, Pol ζ forms essential interactions with the mutasome assembly factor Rev1 and with proliferating cell nuclear antigen (PCNA). These interactions are modulated by posttranslational modifications such as ubiquitination and phosphorylation that enhance translesion synthesis (TLS) and mutagenesis.

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

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

Translesion DNA synthesis and mutagenesis in eukaryotes

The structural features that enable replicative DNA polymerases to synthesize DNA rapidly and accurately also limit their ability to copy damaged DNA. Direct replication of DNA damage is termed translesion synthesis (TLS), a mechanism conserved from bacteria to mammals and executed by an array of specialized DNA polymerases. This chapter examines how these translesion polymerases replicate damaged DNA and how they are regulated to balance their ability to replicate DNA lesions with the risk of undesirable mutagenesis. It also discusses how TLS is co-opted to increase the diversity of the immunoglobulin gene hypermutation and the contribution it makes to the mutations that sculpt the genome of cancer cells.

REV1 and DNA polymerase zeta in DNA interstrand crosslink repair

DNA interstrand crosslinks (ICLs) are covalent linkages between two strands of DNA, and their presence interferes with essential metabolic processes such as transcription and replication. These lesions are extremely toxic, and their repair is essential for genome stability and cell survival. In this review, we will discuss how the removal of ICLs requires interplay between multiple genome maintenance pathways and can occur in the absence of replication (replication-independent ICL repair) or during S phase (replication-coupled ICL repair), the latter being the predominant pathway used in mammalian cells. It is now well recognized that translesion DNA synthesis (TLS), especially through the activities of REV1 and DNA polymerase zeta (Polζ), is necessary for both ICL repair pathways operating throughout the cell cycle. Recent studies suggest that the convergence of two replication forks upon an ICL initiates a cascade of events including unhooking of the lesion through the actions of structure-specific endonucleases, thereby creating a DNA double-stranded break (DSB). TLS across the unhooked lesion is necessary for restoring the sister chromatid before homologous recombination repair. Biochemical and genetic studies implicate REV1 and Polζ as being essential for performing lesion bypass across the unhooked crosslink, and this step appears to be important for subsequent events to repair the intermediate DSB. The potential role of Fanconi anemia pathway in the regulation of REV1 and Polζ-dependent TLS and the involvement of additional polymerases, including DNA polymerases kappa, nu, and theta, in the repair of ICLs is also discussed in this review.

The roles of DNA polymerase ζ and the Y family DNA polymerases in promoting or preventing genome instability

Cancer cells display numerous abnormal characteristics which are initiated and maintained by elevated mutation rates and genome instability. Chromosomal DNA is continuously surveyed for the presence of damage or blocked replication forks by the DNA Damage Response (DDR) network. The DDR is complex and includes activation of cell cycle checkpoints, DNA repair, gene transcription, and induction of apoptosis. Duplicating a damaged genome is associated with elevated risks to fork collapse and genome instability. Therefore, the DNA damage tolerance (DDT) pathway is also employed to enhance survival and involves the recruitment of translesion DNA synthesis (TLS) polymerases to sites of replication fork blockade or single stranded DNA gaps left after the completion of replication in order to restore DNA to its double stranded form before mitosis. TLS polymerases are specialized for inserting nucleotides opposite DNA adducts, abasic sites, or DNA crosslinks. By definition, the DDT pathway is not involved in the actual repair of damaged DNA, but provides a mechanism to tolerate DNA lesions during replication thereby increasing survival and lessening the chance for genome instability. However this may be associated with increased mutagenesis. In this review, we will describe the specialized functions of Y family polymerases (Rev1, Polη, Polι and Polκ) and DNA polymerase ζ in lesion bypass, mutagenesis, and prevention of genome instability, the latter due to newly appreciated roles in DNA repair. The recently described role of the Fanconi anemia pathway in regulating Rev1 and Polζ-dependent TLS is also discussed in terms of their involvement in TLS, interstrand crosslink repair, and homologous recombination.

Aberrant DNA replication in cancer

Genomic instability plays an important role in cancer susceptibility, though the mechanics of its development remain unclear. An often-stated hypothesis is that error-prone phenotypes in DNA replication or aberrations in translesion DNA synthesis lead to genomic instability and cancer. Mutations in core DNA replication proteins have been identified in human cancer, although DNA replication is essential for cell proliferation and most mutations eliminating this function are deleterious. With recent developments in this field we review and discuss the possible involvement of DNA replication proteins in carcinogenesis.

Inhibition of REV3 expression induces persistent DNA damage and growth arrest in cancer cells

REV3 is the catalytic subunit of DNA translesion synthesis polymerase ζ. Inhibition of REV3 expression increases the sensitivity of human cells to a variety of DNA-damaging agents and reduces the formation of resistant cells. Surprisingly, we found that short hairpin RNA-mediated depletion of REV3 per se suppresses colony formation of lung (A549, Calu-3), breast (MCF-7, MDA-MB-231), mesothelioma (IL45 and ZL55), and colon (HCT116 +/-p53) tumor cell lines, whereas control cell lines (AD293, LP9-hTERT) and the normal mesothelial primary culture (SDM104) are less affected. Inhibition of REV3 expression in cancer cells leads to an accumulation of persistent DNA damage as indicated by an increase in phospho-ATM, 53BP1, and phospho-H2AX foci formation, subsequently leading to the activation of the ATM-dependent DNA damage response cascade. REV3 depletion in p53-proficient cancer cell lines results in a G(1) arrest and induction of senescence as indicated by the accumulation of p21 and an increase in senescence-associated β-galactosidase activity. In contrast, inhibition of REV3 expression in p53-deficient cells results in growth inhibition and a G(2)/M arrest. A small fraction of the p53-deficient cancer cells can overcome the G(2)/M arrest, which results in mitotic slippage and aneuploidy. Our findings reveal that REV3 depletion per se suppresses growth of cancer cell lines from different origin, whereas control cell lines and a mesothelial primary culture were less affected. Thus, our findings indicate that depletion of REV3 not only can amend cisplatin-based cancer therapy but also can be applied for susceptible cancers as a potential monotherapy.

Caffeine abolishes the ultraviolet-induced REV3 translesion replication pathway in mouse cells

When a replicative DNA polymerase stalls upon encountering a photoproduct on the template strand, it is relieved by other low-processivity polymerase(s), which insert nucleotide(s) opposite the lesion. Using an alkaline sucrose density gradient sedimentation technique, we previously classified this process termed UV-induced translesion replication (UV-TLS) into two types. In human cancer cells or xeroderma pigmentosum variant (XP-V) cells, UV-TLS was inhibited by caffeine or proteasome inhibitors. However, in normal human cells, the process was insensitive to these reagents. Reportedly, in yeast or mammalian cells, REV3 protein (a catalytic subunit of DNA polymerase ζ) is predominantly involved in the former type of TLS. Here, we studied UV-TLS in fibroblasts derived from the Rev3-knockout mouse embryo (Rev3KO-MEF). In the wild-type MEF, UV-TLS was slow (similar to that of human cancer cells or XP-V cells), and was abolished by caffeine or MG-262. In 2 cell lines of Rev3KO-MEF (Rev3^−/−p53^−/−), UV-TLS was not observed. In p53KO-MEF, which is a strict control for Rev3KO-MEF, the UV-TLS response was similar to that of the wild-type. Introduction of the Rev3 expression plasmid into Rev3KO-MEF restored the UV-TLS response in selected stable transformants. In some transformants, viability to UV was the same as that in the wild-type, and the death rate was increased by caffeine. Our findings indicate that REV3 is predominantly involved in UV-TLS in mouse cells, and that the REV3 translesion pathway is suppressed by caffeine or proteasome inhibitors.

Translesion DNA synthesis in the context of cancer research

During cell division, replication of the genomic DNA is performed by high-fidelity DNA polymerases but these error-free enzymes can not synthesize across damaged DNA. Specialized DNA polymerases, so called DNA translesion synthesis polymerases (TLS polymerases), can replicate damaged DNA thereby avoiding replication fork breakdown and subsequent chromosomal instability.

We focus on the involvement of mammalian TLS polymerases in DNA damage tolerance mechanisms. In detail, we review the discovery of TLS polymerases and describe the molecular features of all the mammalian TLS polymerases identified so far. We give a short overview of the mechanisms that regulate the selectivity and activity of TLS polymerases. In addition, we summarize the current knowledge how different types of DNA damage, relevant either for the induction or treatment of cancer, are bypassed by TLS polymerases. Finally, we elucidate the relevance of TLS polymerases in the context of cancer therapy.

DNA polymerases and cancer

There are 15 different DNA polymerases encoded in mammalian genomes, which are specialized for replication, repair or the tolerance of DNA damage. New evidence is emerging for lesion-specific and tissue-specific functions of DNA polymerases. Many point mutations that occur in cancer cells arise from the error-generating activities of DNA polymerases. However, the ability of some of these enzymes to bypass DNA damage may actually defend against chromosome instability in cells, and at least one DNA polymerase, Pol ζ, is a suppressor of spontaneous tumorigenesis. Because DNA polymerases can help cancer cells tolerate DNA damage, some of these enzymes might be viable targets for therapeutic strategies.

DNA polymerase zeta (pol zeta) in higher eukaryotes

Most current knowledge about DNA polymerase zeta (pol zeta) comes from studies of the enzyme in the budding yeast Saccharomyces cerevisiae, where pol zeta consists of a complex of the catalytic subunit Rev3 with Rev7, which associates with Rev1. Most spontaneous and induced mutagenesis in yeast is dependent on these gene products, and yeast pol zeta can mediate translesion DNA synthesis past some adducts in DNA templates. Study of the homologous gene products in higher eukaryotes is in a relatively early stage, but additional functions for the eukaryotic proteins are already apparent. Suppression of vertebrate REV3L function not only reduces induced point mutagenesis but also causes larger-scale genome instability by raising the frequency of spontaneous chromosome translocations. Disruption of Rev3L function is tolerated in Drosophila, Arabidopsis, and in vertebrate cell lines under some conditions, but is incompatible with mouse embryonic development. Functions for REV3L and REV7(MAD2B) in higher eukaryotes have been suggested not only in translesion DNA synthesis but also in some forms of homologous recombination, repair of interstrand DNA crosslinks, somatic hypermutation of immunoglobulin genes and cell-cycle control. This review discusses recent developments in these areas.

RAD18 and poly(ADP-ribose) polymerase independently suppress the access of nonhomologous end joining to double-strand breaks and facilitate homologous recombination-mediated repair

The Saccharomyces cerevisiae RAD18 gene is essential for postreplication repair but is not required for homologous recombination (HR), which is the major double-strand break (DSB) repair pathway in yeast. Accordingly, yeast rad18 mutants are tolerant of camptothecin (CPT), a topoisomerase I inhibitor, which induces DSBs by blocking replication. Surprisingly, mammalian cells and chicken DT40 cells deficient in Rad18 display reduced HR-dependent repair and are hypersensitive to CPT. Deletion of nonhomologous end joining (NHEJ), a major DSB repair pathway in vertebrates, in rad18-deficient DT40 cells completely restored HR-mediated DSB repair, suggesting that vertebrate Rad18 regulates the balance between NHEJ and HR. We previously reported that loss of NHEJ normalized the CPT sensitivity of cells deficient in poly(ADP-ribose) polymerase 1 (PARP1). Concomitant deletion of Rad18 and PARP1 synergistically increased CPT sensitivity, and additional inactivation of NHEJ normalized this hypersensitivity, indicating their parallel actions. In conclusion, higher-eukaryotic cells separately employ PARP1 and Rad18 to suppress the toxic effects of NHEJ during the HR reaction at stalled replication forks.

Loss of DNA polymerase zeta causes chromosomal instability in mammalian cells

Rev3L encodes the catalytic subunit of DNA polymerase zeta (pol zeta) in mammalian cells. In yeast, pol zeta helps cells bypass sites of DNA damage that can block replication enzymes. Targeted disruption of the mouse Rev3L gene causes lethality midway through embryonic gestation, and Rev3L-/- mouse embryonic fibroblasts (MEFs) remain in a quiescent state in culture. This suggests that pol zeta may be necessary for tolerance of endogenous DNA damage during normal cell growth. We report the generation of mitotically active Rev3L-/- MEFs on a p53-/- genetic background. Rev3L null MEFs exhibited striking chromosomal instability, with a large increase in translocation frequency. Many complex genetic aberrations were found only in Rev3L null cells. Rev3L null cells had increased chromosome numbers, most commonly near pentaploid, and double minute chromosomes were frequently found. This chromosomal instability associated with loss of a DNA polymerase activity in mammalian cells is similar to the instability associated with loss of homologous recombination capacity. Rev3L null MEFs were also moderately sensitive to mitomycin C, methyl methanesulfonate, and UV and gamma-radiation, indicating that mammalian pol zeta helps cells tolerate diverse types of DNA damage. The increased occurrence of chromosomal translocations in Rev3L-/- MEFs suggests that loss of Rev3L expression could contribute to genome instability during neoplastic transformation and progression.

DNA polymerases and somatic hypermutation of immunoglobulin genes

Somatic hypermutation of immunoglobulin variable genes, which increases antibody diversity, is initiated by the activation-induced cytosine deaminase (AID) protein. The current DNA-deamination model posits that AID deaminates cytosine to uracil in DNA, and that mutations are generated by DNA polymerases during replication or repair of the uracil residue. Mutations could arise as follows: by DNA replicating past the uracil; by removing the uracil with a uracil glycosylase and replicating past the resulting abasic site with a low-fidelity polymerase; or by repairing the uracil and synthesizing a DNA-repair patch downstream using a low-fidelity polymerase. In this review, we summarize the biochemical properties of specialized DNA polymerases in mammalian cells and discuss their participation in the mechanisms of hypermutation. Many recent studies have examined mice deficient in the genes that encode various DNA polymerases, and have shown that DNA polymerase H (POLH) contributes to hypermutation, whereas POLI, POLK and several other enzymes do not have major roles. The low-fidelity enzyme POLQ has been proposed as another candidate polymerase because it can efficiently bypass abasic sites and recent evidence indicates that it might participate in hypermutation.

Roles of DNA Polymerases in Replication, Repair, and Recombination in Eukaryotes

The functioning of the eukaryotic genome depends on efficient and accurate DNA replication and repair. The process of replication is complicated by the ongoing decomposition of DNA and damage of the genome by endogenous and exogenous factors. DNA damage can alter base coding potential resulting in mutations, or block DNA replication, which can lead to double-strand breaks (DSB) and to subsequent chromosome loss. Replication is coordinated with DNA repair systems that operate in cells to remove or tolerate DNA lesions. DNA polymerases can serve as sensors in the cell cycle checkpoint pathways that delay cell division until damaged DNA is repaired and replication is completed. Eukaryotic DNA template-dependent DNA polymerases have different properties adapted to perform an amazingly wide spectrum of DNA transactions. In this review, we discuss the structure, the mechanism, and the evolutionary relationships of DNA polymerases and their possible functions in the replication of intact and damaged chromosomes, DNA damage repair, and recombination.

Immortalized mouse cell lines that lack a functional Rev3 gene are hypersensitive to UV irradiation and cisplatin treatment

The catalytic subunit of polymerase zeta is encoded from the Rev3 gene. The enzyme is conserved through eukaryotic evolution and its main function appears to be translesion synthesis (TLS) over damaged bases that stall DNA replication. In non-vertebrate cells, inactivation of polymerase zeta results in a moderate hypersensitivity to DNA damage but no proliferative defect in the absence of exogenous damage. Mouse embryos that lack Rev3 however have a severe growth defect and are aborted at midgestation. This has suggested that polymerase zeta may be involved in vital processes in mammalian cells. Here we describe the establishment of immortalized mouse fibroblast cell lines that lack a functional Rev3 gene. These were established from homozygously Rev3-targeted mouse embryos that were also heterozygously targeted at the p53 locus, but the cell lines lost the wild type p53 allele during transformation. Cell lines in which the Rev3 gene is targeted on both alleles grow more slowly than control lines and the deficiency is also associated with an increased frequency of cells at the G2/M phase of the cell cycle and augmented apoptosis. Targeted cells are hypersensitive to UV irradiation and cisplatin treatment and arrest at the S or G2/M phase of the cell cycle if exposed to these treatments. Thus, although vital for murine embryonic development, polymerase zeta activity is not essential for continuous proliferation of transformed mammalian cells that lack p53. It does, however, appear to play an important role in allowing mammalian cells to tolerate DNA damage.

DNA polymerase theta is preferentially expressed in lymphoid tissues and upregulated in human cancers

DNA polymerase theta (Pol theta) is a recently identified family A polymerase that contains an intrinsic helicase domain. Drosophila Pol theta mutants are hypersensitive to bifunctional DNA crosslinking agents and exhibit an elevated frequency of spontaneous chromosomal aberrations, suggesting a role for Pol theta in repair of DNA interstrand crosslinks and in the general maintenance of genome stability. To investigate a possible involvement of Pol theta in tumorigenesis, we have examined its expression in various normal and malignant tissues. Paired tumor and adjacent nontumorous tissues from patients with lung (n = 27), stomach (n = 28) and colon (n = 26) cancer, as well as 26 normal human tissues, were examined for Pol theta expression by RT-PCR, Northern or Western blot analysis. Pol theta was predominantly expressed in primary lymphoid organs including the fetal liver, thymus and bone marrow where lymphocyte progenitors undergo V(D)J rearrangements of their antigen receptor genes. In addition, Pol theta expression was upregulated in germinal center B cells, in which class switch recombination of the immunoglobulin genes occurs. Examination of Pol theta expression in matched cancer specimens revealed that Pol theta was barely detectable in the nontumorous tissues but was upregulated in 17 of 27 (63%) lung, 11 of 28 (39%) stomach and 20 of 26 (77%) colon cancers. Moreover, patients with high levels of Pol theta expression had a significantly poorer clinical outcome compared with those expressing low levels of Pol theta. These results implicate that Pol theta may have a specialized function in lymphocytes and that its overexpression may contribute to tumor progression.

Error-prone DNA polymerases: when making a mistake is the only way to get ahead

Cells have high-fidelity polymerases whose task is to accurately replicate the genome, and low-fidelity polymerases with specialized functions. Although some of these low-fidelity polymerases are exceptional in their ability to replicate damaged DNA and restore the undamaged sequence, they are error prone on undamaged DNA. In fact, these error-prone polymerases are sometimes used in circumstances where the capacity to make errors has a selective advantage. The mutagenic potential of the error-prone polymerases requires that their expression, activity, and access to undamaged DNA templates be regulated. Here we review these specialized polymerases with an emphasis on their biological roles.

DNA polymerase ζ: new insight into eukaryotic mutagenesis and mammalian embryonic development

Information about the mechanisms that generate mutations in eukaryotes is likely to be useful for understanding human health concerns, such as genotoxicity and cancer. Eukaryotic mutagenesis is largely the outcome of attacks by endogenous and environmental agents. Except for DNA repair, cell cycle checkpoints and DNA damage avoidance, cells have also evolved DNA damage tolerance mechanism, by which lesion-targeted mutation might occur in the genome during replication by specific DNA polymerases to bypass the lesions (translesion DNA synthesis, TLS), or mutation on undamaged DNA templates (untargeted mutation) might be induced. DNA polymerase ζ (pol ζ), which was found firstly in budding yeast Saccharomyces cerevisiae and consists of catalytic subunit scRev3 and stimulating subunit scRev7, has received more attention in recent years. Pol ζ is a member of DNA polymerase δ subfamily, which belongs to DNA polymerase B family, and exists in almost all eukaryotes. Human homolog of the scRev3 gene is located in chromosome region 6q21, and the mouse equivalent maps to chromosome 10, distal to the c-myb gene and close to the Macs gene. Alternative splicing, upstream out-of frame ATG can be found in yeast scRev3, mouse and human homologs. Furthermore, the sequence from 253-323 immediate upstream of the AUG initiator codon has the potential to form a stem-loop hairpin secondary structure in REV3 mRNA, suggesting that human REV3 protein may be expressed at low levels in human cells under normal growth conditions. The functional domain analysis showed that yeast Rev3-980 tyrosine in conserved region II is at the polymerase active site. Human REV3 amino acid residues 1776-2195 provide a REV7 binding domain, and REV7 amino acid residues 1-211 provide a bind domain for REV1, REV3 and REV7 itself. More interestingly, REV7 interacts with hMAD2 and therefore might function in the cell cycle control by affecting the activation of APC (anaphase promoting complex). Currently it has been known that pol ζ is involved in most spontaneous mutation, lesion-targeted mutation via TLS, chemical carcinogen induced untargeted mutation and somatic hypermutation of antibody genes in mammalian. In TLS pathway, pol ζ acts as a "mismatch extender" with combination of other DNA polymerases, such as pol ι. Unlike in yeast, it was found that pol ζ also functioned in mouse embryonic development more recently. It was hypothesized that the roles of pol ζ in TLS and cell cycle control might contribute to mouse embryonic lethality.

Rev1 is essential for DNA damage tolerance and non-templated immunoglobulin gene mutation in a vertebrate cell line

The majority of DNA damage-induced mutagenesis in the yeast Saccharomyces cerevisiae arises as a result of translesion replication. This process is critically dependent on the deoxycytidyl transferase Rev1p, which forms a complex with the subunits of DNA polymerase zeta, Rev3p and Rev7p. To examine the role of Rev1 in vertebrate mutagenesis and the DNA damage response, we disrupted the gene in DT40 cells. Rev1-deficient DT40 grow slowly and are sensitive to a wide range of DNA-damaging agents. Homologous recombination repair is likely to be intact as basal and damage induced sister chromatid exchange and immunoglobulin gene conversion are unaffected. How ever, the mutant cells show a markedly reduced level of non-templated immunoglobulin gene mutation, indicating a defect in translesion bypass. Furthermore, ultraviolet exposure results in marked chromosome breakage, suggesting that replication gaps created in the absence of Rev1 cannot be efficiently repaired by recombination. Thus, Rev1-dependent translesion bypass and mutagenesis is likely to be a trade-off for the ability to complete replication of a damaged template and thereby maintain genome integrity.