Dominant negative rat DNA polymerase beta mutants interfere with base excision repair in Saccharomyces cerevisiae

HeLa Cells Containing a Truncated Form of DNA Polymerase Beta are More Sensitized to Alkylating Agents than to Agents Inducing Oxidative Stress

The present study was aimed at determining the effects of alkylating and oxidative stress inducing agents on a newly identified variant of DNA polymerase beta (polβ Δ208-304) specific for ovarian cancer. Pol β Δ208-304 has a deletion of exons 11-13 which lie in the catalytic part of enzyme. We compared the effect of these chemicals on HeLa cells and HeLa cells stably transfected with this variant cloned into in pcDNAI/neo vector by MTT, colony forming and apoptosis assays. Polβ Δ208-304 cells exhibited greater sensitivity to an alkylating agent and less sensitivity towards H2O2 and UV when compared with HeLa cells alone. It has been shown that cell death in Pol β Δ208-304 transfected HeLa cells is mediated by the caspase 9 cascade. Exon 11 has nucleotidyl selection activity, while exons 12 and 13 have dNTP selection activity. Hence deletion of this part may affect polymerizing activity although single strand binding and double strand binding activity may remain same. The lack of this part may adversely affect catalytic activity of DNA polymerase beta so that the variant may act as a dominant negative mutant. This would represent clinical significance if translated into a clinical setting because resistance to radiation or chemotherapy during the relapse of the disease could be potentially overcome by this approach.

Functional analyses of human DNA repair proteins important for aging and genomic stability using yeast genetics

Model systems have been extremely useful for studying various theories of aging. Studies of yeast have been particularly helpful to explore the molecular mechanisms and pathways that affect aging at the cellular level in the simple eukaryote. Although genetic analysis has been useful to interrogate the aging process, there has been both interest and debate over how functionally conserved the mechanisms of aging are between yeast and higher eukaryotes, especially mammalian cells. One area of interest has been the importance of genomic stability for age-related processes, and the potential conservation of proteins and pathways between yeast and human. Translational genetics have been employed to examine the functional roles of mammalian proteins using yeast as a pliable model system. In the current review recent advancements made in this area are discussed, highlighting work which shows that the cellular functions of human proteins in DNA repair and maintenance of genomic stability can be elucidated by genetic rescue experiments performed in yeast.

Potentiation of temozolomide cytotoxicity by inhibition of DNA polymerase beta is accentuated by BRCA2 mutation

Base excision repair (BER) plays a critical role in the repair of bases damaged by oxidative metabolism or alkylating agents, such as those commonly used in cancer therapy. Incomplete BER generates intermediates that require activation of homology-dependent DNA repair to resolve. We investigated the effects of lithocholic acid (LCA), an inhibitor of the key BER enzyme DNA polymerase beta (pol beta), in cells deficient in expression of the homology-dependent repair factor BRCA2. In vitro studies show that LCA suppresses the DNA polymerase and 5'-deoxyribose phosphate lyase activities of DNA pol beta by preventing the formation of a stable pol beta-DNA complex, reducing BER effectiveness. Cytotoxicity assays based on colony formation revealed that LCA exhibits synergism with the alkylating agent temozolomide, which engages BER through DNA methylation, and that the degree of synergism is increased in cells lacking functional BRCA2. BRCA2-deficient cells also showed heightened susceptibility to both LCA and temozolomide individually. The potentiation of temozolomide cytotoxicity by LCA owes to the conversion of single-stranded DNA breaks generated through incomplete BER of methylated nucleotides into double-stranded breaks during DNA replication, as indicated by gammaH2AX immunofluorescence. Death seems to be induced in cotreated cells through an accumulation of persistent double-stranded DNA breaks. Mutations of the BRCA2 gene have been extensively characterized and are present in various cancers, implying that inhibition of BER may offer a means to augment tumor selectivity in the use of conventional cancer therapies.

Ischemic preconditioning in the rat brain enhances the repair of endogenous oxidative DNA damage by activating the base-excision repair pathway

The development of ischemic tolerance in the brain, whereby a brief period of sublethal 'preconditioning' ischemia attenuates injury from subsequent severe ischemia, may involve the activation of multiple intracellular signaling events that promote neuronal survival. In this study, the potential role of inducible DNA base-excision repair (BER), an endogenous adaptive response that prevents the detrimental effect of oxidative DNA damage, has been studied in the rat model of ischemic tolerance produced by three episodes of ischemic preconditioning (IP). This paradigm of IP, when applied 2 and 5 days before 2-h middle cerebral artery occlusion (MCAO), significantly decreased infarct volume in the frontal-parietal cortex 72 h later. Correlated with this protective effect, IP markedly attenuated the nuclear accumulations of several oxidative DNA lesions, including 8-oxodG, AP sites, and DNA strand breaks, after 2-h MCAO. Consequently, harmful DNA damage-responsive events, including NAD depletion and p53 activation, were reduced during postischemic reperfusion in preconditioned brains. The mechanism underlying the decreased DNA damage in preconditioned brain was then investigated by measuring BER activities in nuclear extracts. Beta-polymerase-mediated BER activity was markedly increased after IP, and this activation occurred before (24 h) and during the course of ischemic tolerance (48 to 72 h). In similar patterns, the activities for AP site and 8-oxodG incisions were also upregulated after IP. The upregulation of BER activities after IP was likely because of increased expression of repair enzymes beta-polymerase, AP endonuclease, and OGG1. These results suggest that the activation of the BER pathway may contribute to IP-induced neuroprotection by enhancing the repair of endogenous oxidative DNA damage after ischemic injury.

Inducible repair of oxidative DNA lesions in the rat brain after transient focal ischemia and reperfusion

To determine the role of oxidative DNA damage and repair in brain injury after focal ischemia and reperfusion, the authors investigated DNA base damage and DNA base excision repair (BER) capacity, the predominant repair mechanism for oxidative DNA lesions, in the rat model of temporary middle cerebral artery occlusion. Contents of 8-hydroxyl-2'-deoxyguanosine (8-oxodG) and apurinic/apyrimidinic abasic site (AP site), hallmarks of oxidative DNA damage, were quantitatively measured in nuclear DNA extracts from brains 0.25 to 72 hours after 1 hour of middle cerebral artery occlusion. In parallel to the detection of DNA lesions, the capacity for 8-oxodG- or AP site-dependent DNA repair synthesis was measured in nuclear protein extracts using specific in vitro DNA repair assays. After postischemic reperfusion, the levels of 8-oxodG and AP sites were markedly increased in ischemic tissues. In frontal/parietal cortex, regions that survived ischemia, 8-oxodG and AP sites were efficiently repaired during reperfusion. However, in the caudate, a region that was destined to infarct, the DNA lesions were poorly repaired. In consistent with the patterns of endogenous lesion repair, a markedly induced and long-lasting (at least 72 hours) BER activity was detected in the cortex but not in the caudate after ischemia. The induced BER activity in ischemic cortex was attributed to the upregulation of gene expression and activation of selective BER enzymes, particularly DNA polymerase-beta and OGG1. These results strongly suggest that inducible DNA BER constitutes an important endogenous mechanism that protects brain against ischemia-induced oxidative neuronal injury.

Variants of DNA polymerase Beta extend mispaired DNA due to increased affinity for nucleotide substrate

DNA polymerase beta offers an attractive system to study the biochemical mechanism of polymerase-dependent mutagenesis. Variants of DNA polymerase beta, Y265F and Y265W, were analyzed for misincorporation efficiency and mispair extension ability, relative to wild-type DNA polymerase beta. Our data show that the fidelity of the mutant polymerases is similar to wild-type enzyme on a one-nucleotide gapped DNA substrate. In contrast, with a six-nucleotide gapped DNA, the mutant proteins are slightly more accurate than the wild-type enzyme. The mutagenic potential of Y265F and Y265W is more pronounced when encountering a mispaired DNA substrate. Here, both variants can extend a G:G mispair quite efficiently, and Y265F can also extend a T:G mispair. The kinetic basis of the increased mispair extension efficiency is due to an improved ability to bind to the incoming nucleotide. Y265W extends the G:G mispair even with an incorrect nucleotide substrate. Overall, our results demonstrate that the Y265 hinge residue is important for stabilizing the architecture of the nucleotide binding pocket of DNA polymerase beta, and that alterations of this residue can have significant impacts upon the fidelity of DNA synthesis.

The role of DNA polymerase beta in determining sensitivity to ionizing radiation in human tumor cells

Lethal lesions after ionizing radiation are thought to be mainly unrepaired or misrepaired DNA double-strand breaks, ultimately leading to lethal chromosome aberrations. However, studies with radioprotectors and repair inhibitors indicate that single-strand breaks, damaged nucleotides or abasic sites can also influence cell survival. This paper reports on studies to further define the role of base damage and base excision repair on the radiosensitivity of human cells. We retrovirally transduced human tumor cells with a dominant negative form of DNA polymerase beta, comprising the 14 kDa DNA-binding domain of DNA polymerase beta but lacking polymerase function. Radiosensitization of two human carcinoma cell lines, A549 and SQD9, was observed, achieving dose enhancement factors of 1.5-1.7. Sensitization was dependent on expression level of the dominant negative and was seen in both single cell clones and in unselected virally transduced populations. Sensitization was not due to changes in cell cycle distribution. Little or no sensitization was seen in G(1)-enriched populations, indicating cell cycle specificity for the observed sensitization. These results contrast with the lack of effect seen in DNA polymerase beta knockout cells, suggesting that polDN also inhibits the long patch, DNA polymerase beta-independent repair pathway. These data demonstrate an important role for BER in determining sensitivity to ionizing radiation and might help identify targets for radiosensitizing tumor cells.

The contribution of endogenous sources of DNA damage to the multiple mutations in cancer

There is increasing evidence that most human cancers contain multiple mutations. By the time a tumor is clinically detectable it may have accumulated tens of thousands of mutations. In normal cells, mutations are rare events occurring at a rate of 10(-10) mutations per nucleotide per cell per generation. We have argued that the mutation rates exhibited by normal human cells are insufficient to account for the large number of mutations found in human cancers, and therefore, that an early event in tumorigenesis is the development of a mutator phenotype. In normal cells, spontaneous and induced DNA damage is balanced by multiple pathways for DNA repair, and most DNA damage is repaired without error. However, in tumor cells this balance may be shifted such that damage overwhelms the repair capacity, resulting in the accumulation of multiple mutations. Our hypothesis is that multiple random mutations occur during carcinogenesis. The sequential mutations that are observed in some human tumors result from selective events required for tumor progression. We consider the possibility that endogenous sources of DNA damage, in particular oxidative DNA damage, may contribute to genomic instability and to a mutator phenotype in some tumors. Endogenous and environmental sources of reactive oxygen species (ROS) are abundant. In tumor cells, antioxidant or DNA repair capacity may be insufficient to compensate for the production of ROS, and these endogenous ROS may be capable of damaging DNA and inducing mutations in critical DNA stability genes. The possibility that oxidative DNA damage could be a significant source of the genomic instability characteristic of human cancers is exciting, because it may be feasible to modulate the extent of oxidative damage through antioxidant therapy. The use of antioxidants to reduce the extent of molecular damage by ROS could delay the progression of cancer.

Base excision repair in yeast and mammals

Base excision repair (BER), as initiated by at least seven different DNA glycosylases or by enzymes that cleave DNA at abasic sites, executes the repair of a wide variety of DNA damages. Many of these damages arise spontaneously because DNA interacts with the cellular milieu, and so BER profoundly influences spontaneous mutation rates. In addition, BER provides significant protection against the toxic and mutagenic effects of DNA damaging agents present in the external environment, and as such is likely to prevent the adverse health effects of such agents. BER pathways have been studied in a wide variety of organisms (including yeasts) and here we review how these varied studies have shaped our current view of human BER.

Overexpression of enzymes that repair endogenous damage to DNA

A significant contribution to human mutagenesis and carcinogenesis may come from DNA damage of endogenous, rather than exogenous, origin. Efficient repair mechanisms have evolved to cope with this. The main repair pathway involved in repair of endogenous damage is DNA base excision repair. In addition, an important contribution is given by O6-alkylguanine DNA alkyltranferase, that repairs specifically the miscoding base O6-alkylguanine. In recent years, several attempts have been carried out to enhance the efficiency of repair of endogenous damage by overexpressing in mammalian cells single enzymatic activities. In some cases (e.g. O6-alkylguanine DNA alkyltransferase or yeast AP endonuclease) this approach has been successful in improving cellular protection from endogenous and exogenous mutagens, while overexpression of other enzymatic activities (e.g. alkyl N-purine glycosylase or DNA polymerase beta) were detrimental and even produced a genome instability phenotype. The reasons for these different outcomes are analyzed and alternative enzymatic activities whose overexpression may improve the efficiency of repair of endogenous damage in human cells are proposed.

Significance of multiple mutations in cancer

There is increasing evidence that in eukaryotic cells, DNA undergoes continuous damage, repair and resynthesis. A homeostatic equilibrium exists in which extensive DNA damage is counterbalanced by multiple pathways for DNA repair. In normal cells, most DNA damage is repaired without error. However, in tumor cells this equilibrium may be skewed, resulting in the accumulation of multiple mutations. Among genes mutated are those that function in guaranteeing the stability of the genome. Loss of this stability results in a mutator phenotype. Evidence for a mutator phenotype in human cancers includes the frequent occurrence of gene amplification, microsatellite instability, chromosomal aberrations and aneuploidy. Current experiments have centered on two mechanisms for the generation of genomic instability, one focused on mutations in mismatch repair genes resulting in microsatellite instability, and one focused on mutations in genes that are required for chromosomal segregation resulting in chromosomal aberrations. This dichotomy may reflect only the ease by which these manifestations can be identified. Underlying both pathways may be a more general phenomenon involving the selection for mutator genes during tumor progression. During carcinogenesis there is selection for cells harboring mutations that can overcome adverse conditions that limit tumor growth. These mutations are produced by direct DNA damage as well as secondarily as a result of mutations in genes that cause a mutator phenotype. Thus, as tumor progression selects for cells with specific mutations, it also selects for cancer cells harboring mutations in genes that normally function in maintaining genetic instability.

An integration of old and new perspectives of mammalian meiotic sterility

The many events of meiotic prophase can now be viewed as a series of specialized incidents that are monitored by meiotic checkpoints, some of which are similar to their mitotic counterparts, and some of which are probably unique to meiosis. This shift in perspective means that meiotic sterility in mammals must be reexamined and viewed as the result of errors subject to meiotic checkpoint controls. Like their mitotic counterparts, the meiotic checkpoints detect defects and halt normal progression until these mistakes can be repaired. Some of these checkpoints utilize mitotic checkpoint proteins, others may involve meiotic-specific proteins, or splice forms. If repair is impossible, the checkpoints then either trigger immediate apoptosis or cause an arrest of meiotic progression followed by eventual cell death. If a sufficient number of spermatocytes are involved, either alternative results in sterility. Identification of these meiotic checkpoints and delineation of the signal transduction cascades involved has only just begun. While yeast, or other model organisms, may provide clues to some of these pathways, others appears to have arisen during vertebrate evolution. The study of mammalian meiosis has entered a new era and the foundations are being laid for a growing understanding of the many problems that may contribute to sterility.

The E249K mutator mutant of DNA polymerase beta extends mispaired termini

The DNA polymerase beta mutant enzyme, which is altered from glutamic acid to lysine at position 249, exhibits a mutator phenotype in primer extension assays and in the herpes simplex virus-thymidine kinase (HSV-tk) forward mutation assay. The basis for this loss of accuracy was investigated by measurement of misincorporation fidelity in single turnover conditions. For the four misincorporation reactions investigated, the fidelity of the E249K mutant was not significantly different from wild type, implying that the mutator phenotype was not caused by a general inability to distinguish between correct and incorrect bases during the incorporation reaction. However, the discrimination between correct and incorrect substrates by the E249K enzyme occurred less during the conformational change and chemical steps and more during the initial binding step, compared with pol beta wild type. This implies that the E249K mutation alters the kinetic mechanism of nucleotide discrimination without reducing misincorporation fidelity. In a missing base primer extension assay, we observed that the mutant enzyme produced mispairs and extended them. This indicates that the altered fidelity of E249K could be due to loss of discrimination against mispaired primer termini. This was supported by the finding that the E249K enzyme extended a G:A mispair 8-fold more efficiently than wild type and a C:T mispair 4-fold more efficiently. These results demonstrate that an enhanced ability to extend mispairs can produce a mutator phenotype and that the Glu-249 side chain of DNA polymerase beta is critical for mispair extension fidelity.

The Tyr-265-to-Cys mutator mutant of DNA polymerase beta induces a mutator phenotype in mouse LN12 cells

DNA polymerase beta functions in both base excision repair and meiosis. Errors committed by polymerase beta during these processes could result in mutations. Using a complementation system, in which rat DNA polymerase beta substitutes for DNA polymerase I of Escherichia coli, we previously isolated a DNA polymerase beta mutant in which Tyr-265 was altered to Cys (Y265C). The Y265C mutant is dominant to wild-type DNA polymerase beta and possesses an intrinsic mutator activity. We now have expressed the wild-type DNA polymerase and the Y265C mutator mutant in mouse LN12 cells, which have endogenous DNA polymerase beta activity. We demonstrate that expression of the Y265C mutator mutant in the LN12 cells results in an 8-fold increase in the spontaneous mutation frequency of lambdacII mutants compared with expression of the wild-type protein. Expression of Y265C results in at least a 40-fold increase in the frequency of deletions of three bases or more and a 7-fold increase in point mutations. Our results suggest that the mutations we observe in vivo result directly from the action of the mutator polymerase. To our knowledge, this is the first demonstration of a mutator phenotype resulting from expression of a DNA polymerase mutator mutant in mammalian cells. This work raises the possibility that variant polymerases may act in a dominant fashion in human cells, leading to genetic instability and carcinogenesis.

3'-Azido-3'-deoxythymidine-resistant mutants of DNA polymerase beta identified by in vivo selection

We developed an in vivo selection to identify 3'-azido-3'-deoxythymidine (AZT)-resistant mutants of rat DNA polymerase beta (pol beta). The selection utilizes pol beta's ability to substitute for Escherichia coli DNA polymerase I (pol I) in the SC18-12 strain, which lacks active pol I. pol beta allows SC18-12 cells to grow, but they depend on pol beta activity, so inhibition of pol beta by AZT kills them. We screened a library of randomly mutated pol beta cDNA for complementation of the pol I defect in the presence of AZT, and identified AZT-resistant mutants. We purified two enzymes with nonconservative mutations in the palm domain of the polymerase. The substitutions D246V and R253M result in reductions in the steady-state catalytic efficiency (Kcat/Km) of AZT-TP incorporation. The efficiency of dTTP incorporation was unchanged for the D246V enzyme, indicating that the substantial decrease in AZT-TP incorporation is responsible for its drug resistance. The R253M enzyme exhibits significantly higher Km(dTTP) and Kcat(dTTP) values, implying that the incorporation reaction is altered. These are the first pol beta mutants demonstrated to exhibit AZT resistance in vitro. The locations of the Asp-246 and Arg-253 side chains indicate that substrate specificity is influenced by residues distant from the nucleotide-binding pocket.

Functional analysis of human MutSalpha and MutSbeta complexes in yeast

Mismatch repair (MMR) is initiated when a heterodimer of hMSH2*hMSH6 or hMSH2*hMSH3 binds to mismatches. Here we perform functional analyses of these human protein complexes in yeast. We use a sensitive genetic system wherein the rate of single-base deletions in a homopolymeric run in the LYS2 gene is 10 000-fold higher in an msh2 mutant than in a wild-type strain. Expression of the human proteins alone or in combination does not reduce the mutation rate of the msh2 strain, and expression of the individual human proteins does not increase the low mutation rate of a wild-type strain. However, co-expression of hMSH2 and hMSH6 in wild-type yeast increases the mutation rate 4000-fold, while co-expression of hMSH2 and hMSH3 elevates the rate 5-fold. Analysis of cell extracts indicates that the proteins are expressed and bind to mismatched DNA. The results suggest that hMutSalpha and hMutSbeta complexes form, bind to and prevent correction of replication slippage errors in yeast. Expression of hMSH6 with hMSH2 containing a proline substituted for a conserved Arg524 eliminates the mutator effect and reduces mismatch binding. The analogous mutation in humans is associated with microsatellite instability, defective MMR and cancer, illustrating the utility of the yeast system for studying human disease alleles.

The Pol beta-14 dominant negative rat DNA polymerase beta mutator mutant commits errors during the gap-filling step of base excision repair in Saccharomyces cerevisiae

We demonstrated recently that dominant negative mutants of rat DNA polymerase beta (Pol beta) interfere with repair of alkylation damage in Saccharomyces cerevisiae. To identify the alkylation repair pathway that is disrupted by the Pol beta dominant negative mutants, we studied the epistatic relationship of the dominant negative Pol beta mutants to genes known to be involved in repair of DNA alkylation damage in S. cerevisiae. We demonstrate that the rat Pol beta mutants interfere with the base excision repair pathway in S. cerevisiae. In addition, expression of one of the Pol beta dominant negative mutants, Pol beta-14, increases the spontaneous mutation rate of S. cerevisiae whereas expression of another Pol beta dominant negative mutant, Pol beta-TR, does not. Expression of the Pol beta-14 mutant in cells lacking APN1 activity does not result in an increase in the spontaneous mutation rate. These results suggest that gaps are required for mutagenesis to occur in the presence of Pol beta-14 but that it is not merely the presence of a gap that results in mutagenesis. Our results suggest that mutagenesis can occur during the gap-filling step of base excision repair in vivo.

Different DNA polymerases are involved in the short- and long-patch base excision repair in mammalian cells

Mammalian cells possess two distinct pathways for completion of base excision repair (BER): the DNA polymerase beta (Pol beta)-dependent short-patch pathway (replacement of one nucleotide), which is the main route, and the long-patch pathway (resynthesis of 2-6 nucleotides), which is PCNA-dependent. To address the issue of how these two pathways share their role in BER the ability of Pol beta-defective mammalian cell extracts to repair a single abasic site constructed in a circular duplex plasmid molecule was tested in a standard in vitro repair reaction. Pol beta-deficient extracts were able to perform both BER pathways. However, in the case of the short-patch BER, the repair kinetics was significantly slower than with Pol beta-proficient extracts, while the efficiency of the long-patch synthesis was unaffected by the loss of Pol beta. The repair synthesis was fully dependent on PCNA for the replacement of long patches. These data give the first evidence that in cell extracts DNA polymerases other than Pol beta are specifically involved in the long-patch BER. These DNA polymerases are also able to perform short-patch BER in the absence of PCNA, although less efficiently than Pol beta. These findings lead to a novel model whereby the two BER pathways are characterized by different protein requirements, and a functional redundancy at the level of DNA polymerases provides cells with backup systems.

Methyl methanesulfonate-induced hprt mutation spectra in the Chinese hamster cell line CHO9 and its xrcc1-deficient derivative EM-C11

The Chinese hamster cell mutant EM-C11, which is hypersensitive to the cell killing effects of alkylating agents compared to its parental line CHO9, has been used to study the impact of base excision repair on the mutagenic effects of DNA methylation damage. This cell line has a defect in the xrcc1 gene. XRCC1 can interact with DNA polymerase-beta, thereby suppressing strand displacement, and DNA ligase III, both of which have been implicated in base excision repair. XRCC1 may, therefore, allow efficient ligation of single-strand breaks generated during base excision repair. Both EM-C11 and CHO9 cells were treated with methyl methanesulfonate (MMS), a DNA-methylating agent reacting predominantly with nitrogen atoms generating adducts which are substrates for the base excision repair pathway. EM-C11 cells are much more sensitive to the cytotoxic effects of MMS than CHO9: for EM-C11, the dose of MMS inducing 10% survival is 6-fold lower than that for CHO9. In contrast, mutation induction at the hprt locus following MMS is similar in EM-C11 and CHO9. Molecular analysis of hprt gene mutations showed that although the largest class of hprt mutations, both in EM-C11 and CHO9 cells, consisted of GC > AT transitions, most likely caused by O6-methylguanine, the size of this class was smaller in EM-C11. The fraction of deletion mutants in EM-C11, however, was twice as large as that found in CHO9 cells. These results suggest that reduced ligation efficiency of single-strand breaks generated during base excision repair, as result of a defect in XRCC1, may lead to the formation of deletions.

Evidence for a role for DNA polymerase beta in mammalian meiosis

DNA polymerase beta (pol beta) is an enzyme possessing both polymerase and deoxyribose phophatase activities. Although pol beta is not believed to participate in the replication of genomic DNA, several studies have indicated a role for pol beta in DNA repair. The high level of expression of pol beta in mouse and rat testes raises the possibility that pol beta participates in mammalian meiosis. Using antibody localization, we detect foci that stain with pol beta antisera at discrete sites along homologous chromosomes as they synapse and progress through prophase of meiosis I. These data suggest that pol beta participates in meiotic events associated with synapsis and recombination.

A genetic system to identify DNA polymerase beta mutator mutants

DNA polymerase beta (pol beta) is a 39-kDa protein that functions in DNA repair processes in mammalian cells. As a first step toward understanding mechanisms of polymerase fidelity, we developed a genetic method to identify mammalian pol beta mutator mutants. This screen takes advantage of a microbial genetics assay and the ability of rat pol beta to substitute for Escherichia coli DNA polymerase I in DNA replication in vivo. Using this screen, we identified 13 candidate pol beta mutator mutants. Three of the candidate mutator mutants were further characterized in vivo and shown to confer an increased spontaneous mutation frequency over that of wild-type pol beta to our bacterial strain. Purification and subsequent analysis of one of our putative mutator proteins, the pol beta-14 protein, showed that it possesses intrinsic mutator activity in four different assays that measure the fidelity of DNA synthesis. Therefore, residue 265, which is altered in pol beta-14 and another of our mutant proteins, pol beta-166, is probably critical for accurate DNA synthesis by pol beta. Thus, our genetic method of screening for pol beta mutator mutants is useful in identifying active mammalian DNA polymerase mutants that encode enzymes that catalyze DNA synthesis with altered fidelity compared with the wild-type pol beta enzyme.

DNA repair functions in heterologous cells

Our genetic information is constantly challenged by exposure to endogenous and exogenous DNA-damaging agents, by DNA polymerase errors, and thereby inherent instability of the DNA molecule itself. The integrity of our genetic information is maintained by numerous DNA repair pathways, and the importance of these pathways is underscored by their remarkable structural and functional conservation across the evolutionary spectrum. Because of the highly conserved nature of DNA repair, the enzymes involved in this crucial function are often able to function in heterologous cells; as an example, the E. coli Ada DNA repair methyltransferase functions efficiently in yeast, in cultured rodent and human cells, in transgenic mice, and in ex vivo-modified mouse bone marrow cells. The heterologous expression of DNA repair functions has not only been used as a powerful cloning strategy, but also for the exploration of the biological and biochemical features of numerous enzymes involved in DNA repair pathways. In this review we highlight examples where the expression of DNA repair enzymes in heterologous cells was used to address fundamental questions about DNA repair processes in many different organisms.