Nucleotide excision repair DNA synthesis by excess DNA polymerase beta: a potential source of genetic instability in cancer cells

Silencing DNA Polymerase β Induces Aneuploidy as a Biomarker of Poor Prognosis in Oral Squamous Cell Cancer

Most patients with oral squamous cell cancer (OSCC) have a locally advanced stage at diagnosis. The treatment strategies are diverse, including surgery, radiotherapy and chemotherapy. Despite multimodality treatment, the response rate is unsatisfactory. DNA repair and genetic instability are highly associated with carcinogenesis and treatment outcomes in oral squamous cell cancer, affecting cell growth and proliferation. Therefore, focusing on DNA repair and genetic instability interactions could be a potential target for improving the outcomes of OSCC patients. DNA polymerase-β (POLB) is an important enzyme in base excision repair and contributes to gene instability, leading to tumorigenesis and cancer metastasis. The aim of our study was to confirm POLB regulates the growth of OSCC cells through modulation of cell cycle and chromosomal instability. We analyzed a tissue array from 133 OSCC patients and discovered that low POLB expression was associated with advanced tumor stage and poor overall survival. In multivariate Cox proportional hazards regression analysis, low POLB expression and advanced lymph node status were significantly associated with poor survival. By performing in vitro studies on model cell lines, we demonstrated that POLB silencing regulated cell cycles, exacerbated mitotic abnormalities and enhanced cell proliferation. After POLB depletion, OSCC cells showed chromosomal instability and aneuploidy. Thus, POLB is an important maintainer of karyotypic stability in OSCC cells.

Replication Fork Remodeling and Therapy Escape in DNA Damage Response-Deficient Cancers

Most cancers have lost a critical DNA damage response (DDR) pathway during tumor evolution. These alterations provide a useful explanation for the initial sensitivity of tumors to DNA-targeting chemotherapy. A striking example is dysfunctional homology-directed repair (HDR), e.g., due to inactivating mutations in BRCA1 and BRCA2 genes. Extensive efforts are being made to develop novel targeted therapies exploiting such an HDR defect. Inhibitors of poly(ADP-ribose) polymerase (PARP) are an instructive example of this approach. Despite the success of PARP inhibitors, the presence of primary or acquired therapy resistance remains a major challenge in clinical oncology. To move the field of precision medicine forward, we need to understand the precise mechanisms causing therapy resistance. Using preclinical models, various mechanisms underlying chemotherapy resistance have been identified. Restoration of HDR seems to be a prevalent mechanism but this does not explain resistance in all cases. Interestingly, some factors involved in DNA damage response (DDR) have independent functions in replication fork (RF) biology and their loss causes RF instability and therapy sensitivity. However, in BRCA-deficient tumors, loss of these factors leads to restored stability of RFs and acquired drug resistance. In this review we discuss the recent advances in the field of RF biology and its potential implications for chemotherapy response in DDR-defective cancers. Additionally, we review the role of DNA damage tolerance (DDT) pathways in maintenance of genome integrity and their alterations in cancer. Furthermore, we refer to novel tools that, combined with a better understanding of drug resistance mechanisms, may constitute a great advance in personalized diagnosis and therapeutic strategies for patients with HDR-deficient tumors.

Structural insights into the promutagenic bypass of the major cisplatin-induced DNA lesion

The cisplatin-1,2-d(GpG) (Pt-GG) intrastrand cross-link is the predominant DNA lesion generated by cisplatin. Cisplatin has been shown to predominantly induce G to T mutations and Pt-GG permits significant misincorporation of dATP by human DNA polymerase β (polβ). In agreement, polβ overexpression, which is frequently observed in cancer cells, is linked to cisplatin resistance and a mutator phenotype. However, the structural basis for the misincorporation of dATP opposite Pt-GG is unknown. Here, we report the first structures of a DNA polymerase inaccurately bypassing Pt-GG. We solved two structures of polβ misincorporating dATP opposite the 5'-dG of Pt-GG in the presence of Mg2+ or Mn2+. The Mg2+-bound structure exhibits a sub-optimal conformation for catalysis, while the Mn2+-bound structure is in a catalytically more favorable semi-closed conformation. In both structures, dATP does not form a coplanar base pairing with Pt-GG. In the polβ active site, the syn-dATP opposite Pt-GG appears to be stabilized by protein templating and pi stacking interactions, which resembles the polβ-mediated dATP incorporation opposite an abasic site. Overall, our results suggest that the templating Pt-GG in the polβ active site behaves like an abasic site, promoting the insertion of dATP in a non-instructional manner.

Oxidative DNA Damage Modulates DNA Methylation Pattern in Human Breast Cancer 1 (BRCA1) Gene via the Crosstalk between DNA Polymerase β and a de novo DNA Methyltransferase

DNA damage and base excision repair (BER) are actively involved in the modulation of DNA methylation and demethylation. However, the underlying molecular mechanisms remain unclear. In this study, we seek to understand the mechanisms by exploring the effects of oxidative DNA damage on the DNA methylation pattern of the tumor suppressor breast cancer 1 (BRCA1) gene in the human embryonic kidney (HEK) HEK293H cells. We found that oxidative DNA damage simultaneously induced DNA demethylation and generation of new methylation sites at the CpGs located at the promoter and transcribed regions of the gene ranging from -189 to +27 in human cells. We demonstrated that DNA damage-induced demethylation was mediated by nucleotide misincorporation by DNA polymerase β (pol β). Surprisingly, we found that the generation of new DNA methylation sites was mediated by coordination between pol β and the de novo DNA methyltransferase, DNA methyltransferase 3b (DNMT3b), through the interaction between the two enzymes in the promoter and encoding regions of the BRCA1 gene. Our study provides the first evidence that oxidative DNA damage can cause dynamic changes in DNA methylation in the BRCA1 gene through the crosstalk between BER and de novo DNA methylation.

Enhanced Activity of Variant DNA Polymerase β (D160G) Contributes to Cisplatin Therapy by Impeding the Efficiency of NER

Cisplatin, commonly used in a variety of cancer treatments, induces apoptosis in cancer cells by causing lethal DNA damage. Several DNA repair pathways participate in regulation of cisplatin treatment, leading to cisplatin sensitivity or resistance in cancer cells. DNA polymerase β (pol β), a key protein involved in base excision repair, confers a response to cisplatin therapy that is dependent on polymerase activity. Pol β D160G mutation with enhanced polymerase activity, previously identified in clear cell renal cell carcinoma, enhances the sensitivity of human cancer cells and mouse xenografts to cisplatin by limiting the efficiency of nucleotide excision repair (NER). Notably, the D160G mutation impedes the recruitment of XPA to cisplatin-induced sites of DNA damage, leading to unrepaired damage and further inducing cell death. Molecular architecture analysis indicated that the D160G mutation alters protein-DNA interactions and the surface electrostatic properties of the DNA-binding regions, resulting in greater DNA affinity and polymerase activity compared with wild-type pol β. Collectively, these results indicate that enhancing pol β activity impedes the efficiency of NER and provide a promising adjuvant therapeutic strategy for cisplatin chemotherapy. IMPLICATIONS: Our studies demonstrate that polβ D160G mutation with enhanced polymerase activity impedes NER efficiency during the repair of cisplatin-induced DNA damage, leading to increased cisplatin sensitivity in cancer cells.

Mutagenic Replication of the Major Oxidative Adenine Lesion 7,8-Dihydro-8-oxoadenine by Human DNA Polymerases

Reactive oxygen species attack DNA to produce 7,8-dihyro-8-oxoguanine (oxoG) and 7,8-dihydro-8-oxoadenine (oxoA) as major lesions. The structural basis for the mutagenicity of oxoG, which induces G to T mutations, is well understood. However, the structural basis for the mutagenic potential of oxoA, which induces A to C mutations, remains poorly understood. To gain insight into oxoA-induced mutagenesis, we conducted kinetic studies of human DNA polymerases β and η replicating across oxoA and structural studies of polβ incorporating dTTP/dGTP opposite oxoA. While polη readily bypassed oxoA, it incorporated dGTP opposite oxoA with a catalytic specificity comparable to that of correct insertion, underscoring the promutagenic nature of the major oxidative adenine lesion. Polη and polβ incorporated dGTP opposite oxoA ∼170-fold and ∼100-fold more efficiently than that opposite dA, respectively, indicating that the 8-oxo moiety greatly facilitated error-prone replication. Crystal structures of polβ showed that, when paired with an incoming dTTP, the templating oxoA adopted an anti conformation and formed Watson-Crick base pair. When paired with dGTP, oxoA adopted a syn conformation and formed a Hoogsteen base pair with Watson-Crick-like geometry, highlighting the dual-coding potential of oxoA. The templating oxoA was stabilized by Lys280-mediated stacking and hydrogen bonds. Overall, these results provide insight into the mutagenic potential and dual-coding nature of the major oxidative adenine lesion.

MiR-149 sensitizes esophageal cancer cell lines to cisplatin by targeting DNA polymerase β

Human DNA polymerase β (polβ) is a small, monomeric protein essential for short‐patch base excision repair (BER). polβ plays an important role in the regulation of chemotherapy sensitivity in tumour cells. In this study, we determined that the expression levels of polβ mRNA and miR‐149 in tumour tissues were significantly higher than in adjacent non‐tumour tissues. We also found that the expression level of miR‐149 in EC tumour tissues was inverse to that of polβ expression. Bioinformatics analysis and dual‐luciferase reporter assay predicted that miR‐149 negatively regulates polβ expression by directly binding to its 3′UTR. CCK‐8 assay indicated that miR‐149 could enhance the anti‐proliferative effects of cisplatin in EC1 and EC9706 cell lines. Flow cytometry, caspase 3/7 activity, and immunofluorescence microscopy results indicated that miR‐149 could enhance the apoptotic effects of cisplatin in EC1 and EC9706 cell lines. We also showed that the expression of polβ lacking the 3′UTR sequence could override the proliferative and apoptotic functions of miR‐149, suggesting that miR‐149 negatively regulates polβ expression by binding to its 3′UTR. Surface plasmon resonance results also showed that miR‐149 could bind with wild‐type polβ. In addition, we identified a new variant of polβ (C1134G). In conclusion, this study confirms that miR‐149 may enhance the sensitivity of EC cell lines to cisplatin by targeting polβ, and that miR‐149 may be unable to regulate the C1134G variant of polβ. Based on these findings, potential drugs could be developed with a focus on enhanced sensitivity of EC patients to chemotherapy.

Translesion DNA Synthesis in Cancer: Molecular Mechanisms and Therapeutic Opportunities

The genomic landscape of cancer is one marred by instability, but the mechanisms that underlie these alterations are multifaceted and remain a topic of intense research. Cellular responses to DNA damage and/or replication stress can affect genome stability in tumors and influence the response of patients to therapy. In addition to direct repair, DNA damage tolerance (DDT) is an element of genomic maintenance programs that contributes to the etiology of several types of cancer. DDT mechanisms primarily act to resolve replication stress, and this can influence the effectiveness of genotoxic drugs. Translesion DNA synthesis (TLS) is an important component of DDT that facilitates direct bypass of DNA adducts and other barriers to replication. The central role of TLS in the bypass of drug-induced DNA lesions, the promotion of tumor heterogeneity, and the involvement of these enzymes in the maintenance of the cancer stem cell niche presents an opportunity to leverage inhibition of TLS as a way of improving existing therapies. In the review that follows, we summarize mechanisms of DDT, misregulation of TLS in cancer, and discuss the potential for targeting these pathways as a means of improving cancer therapies.

Overexpression of A613T and G462T variants of DNA polymerase β weakens chemotherapy sensitivity in esophageal cancer cell lines

BACKGROUND

Human DNA polymerase β (polβ) is a small monomeric protein that is essential for short-patch base excision repair. It plays an important role in regulating the sensitivity of tumor cells to chemotherapy.

METHODS

We evaluated the mutation of polβ in a larger cohort of esophageal cancer (EC) patients by RT-PCR and sequencing analysis. The function of the mutation was evaluated by CCK-8, in vivo tumor growth, and flow cytometry assays.

RESULTS

There are 229 patients with the polβ mutation, 18 patients with A613T mutation, 12 patients with G462T mutation among 538 ECs. Analysis results of survival time showed that EC patients with A613T, G462T mutation had a shorter survival than the others (P < 0.05). CCK-8 and flow cytometry assays results showed the A613T, G462T EC9706 cells were less sensitive than WT cells to 5-FU and cisplatin (P < 0.05). Experiments results in vivo showed that the tumor sizes of A613T and G462T group were larger than WT and polβ^-/- groups (P < 0.05).

CONCLUSIONS

In this study, we discovered A to T point mutation at nucleotide 613 (A613T) and G to T point mutation at nucleotide 462 (G462T) in the polβ gene through 538 EC patients cohort study. A613T and G462T variant of DNA polymerase β weaken chemotherapy sensitivity of esophageal cancer.

G648C variant of DNA polymerase β sensitizes esophageal cancer to chemotherapy

Human DNA polymerase β (polβ) is a small monomeric protein that is essential for short-patch base excision repair. It plays an important role in regulating the sensitivity of tumor cells to chemotherapy. We have previously identified a G to C point mutation at nucleotide 648 (G648C) of polβ in esophageal cancer (EC). In this study, we evaluated the mutation of polβ in a larger cohort of EC patients by RT-PCR and sequencing analysis. The function of the mutation was evaluated by MTT, in vivo tumor growth, and flow cytometry assays. The G648C mutation occurred in 15 (3.45 %) of 435 EC patients. In addition, patients with this mutation had significantly longer survival time than those without, following postoperative chemotherapy. Cell lines with G648C mutation in polβ gene were more sensitive to treatment with 5-fluorouracil and cisplatin than those with wild-type polβ. These results suggest that polβ gene with G648C mutation in surgically resected esophagus may be clinically useful for predicting responsiveness to chemotherapy in patients with EC. The polβ gene alteration may serve as a prognostic biomarker for EC.

Bypass of a 5',8-cyclopurine-2'-deoxynucleoside by DNA polymerase β during DNA replication and base excision repair leads to nucleotide misinsertions and DNA strand breaks

5',8-Cyclopurine-2'-deoxynucleosides including 5',8-cyclo-dA (cdA) and 5',8-cyclo-dG (cdG) are induced by hydroxyl radicals resulting from oxidative stress such as ionizing radiation. 5',8-cyclopurine-2'-deoxynucleoside lesions are repaired by nucleotide excision repair with low efficiency, thereby leading to their accumulation in the human genome and lesion bypass by DNA polymerases during DNA replication and base excision repair (BER). In this study, for the first time, we discovered that DNA polymerase β (pol β) efficiently bypassed a 5'R-cdA, but inefficiently bypassed a 5'S-cdA during DNA replication and BER. We found that cell extracts from pol β wild-type mouse embryonic fibroblasts exhibited significant DNA synthesis activity in bypassing a cdA lesion located in replication and BER intermediates. However, pol β knock-out cell extracts exhibited little DNA synthesis to bypass the lesion. This indicates that pol β plays an important role in bypassing a cdA lesion during DNA replication and BER. Furthermore, we demonstrated that pol β inserted both a correct and incorrect nucleotide to bypass a cdA at a low concentration. Nucleotide misinsertion was significantly stimulated by a high concentration of pol β, indicating a mutagenic effect induced by pol β lesion bypass synthesis of a 5',8-cyclopurine-2'-deoxynucleoside. Moreover, we found that bypass of a 5'S-cdA by pol β generated an intermediate that failed to be extended by pol β, resulting in accumulation of single-strand DNA breaks. Our study provides the first evidence that pol β plays an important role in bypassing a 5',8-cyclo-dA during DNA replication and repair, as well as new insight into mutagenic effects and genome instability resulting from pol β bypassing of a cdA lesion.

Clinical significance of a point mutation in DNA polymerase beta (POLB) gene in gastric cancer

Gastric cancer (GC) is a major cause of global cancer mortality. Genetic variations in DNA repair genes can modulate DNA repair capability and, consequently, have been associated with risk of developing cancer. We have previously identified a T to C point mutation at nucleotide 889 (T889C) in DNA polymerase beta (POLB) gene, a key enzyme involved in base excision repair in primary GCs. The purpose of this study was to evaluate the mutation and expression of POLB in a larger cohort and to identify possible prognostic roles of the POLB alterations in GC. Primary GC specimens and their matched normal adjacent tissues were collected at the time of surgery. DNA, RNA and protein samples were isolated from GC specimens and cell lines. Mutations were detected by PCR-RFLP/DHPLC and sequencing analysis. POLB gene expression was examined by RT-PCR, tissue microarray, Western blotting and immunofluorescence assays. The function of the mutation was evaluated by chemosensitivity, MTT, Transwell matrigel invasion and host cell reactivation assays. The T889C mutation was detected in 18 (10.17%) of 177 GC patients. And the T889C mutation was associated with POLB overexpression, lymph nodes metastases and poor tumor differentiation. In addition, patients with- the mutation had significantly shorter survival time than those without-, following postoperative chemotherapy. Furthermore, cell lines with T889C mutation in POLB gene were more resistant to the treatment of 5-fluorouracil, cisplatin and epirubicin than those with wild type POLB. Forced expression of POLB gene with T889C mutation resulted in enhanced cell proliferation, invasion and resistance to anticancer drugs, along with increased DNA repair capability. These results suggest that POLB gene with T889C mutation in surgically resected primary gastric tissues may be clinically useful for predicting responsiveness to chemotherapy in patients with GC. The POLB gene alteration may serve as a prognostic biomarker for GC.

Structural basis for the inefficient nucleotide incorporation opposite cisplatin-DNA lesion by human DNA polymerase β

Human DNA polymerase β (polβ) has been suggested to play a role in cisplatin resistance, especially in polβ-overexpressing cancer cells. Polβ has been shown to accurately albeit slowly bypass the cisplatin-1,2-d(GpG) (Pt-GG) intramolecular cross-link in vitro. Currently, the structural basis for the inefficient Pt-GG bypass mechanism of polβ is unknown. To gain structural insights into the mechanism, we determined two ternary structures of polβ incorporating dCTP opposite the templating Pt-GG lesion in the presence of the active site Mg(2+) or Mn(2+). The Mg(2+)-bound structure shows that the bulky Pt-GG adduct is accommodated in the polβ active site without any steric hindrance. In addition, both guanines of the Pt-GG lesion form Watson-Crick base pairing with the primer terminus dC and the incoming dCTP, providing the structural basis for the accurate bypass of the Pt-GG adduct by polβ. The Mn(2+)-bound structure shows that polβ adopts a catalytically suboptimal semiclosed conformation during the insertion of dCTP opposite the templating Pt-GG, explaining the inefficient replication across the Pt-GG lesion by polβ. Overall, our studies provide the first structural insights into the mechanism of the potential polβ-mediated cisplatin resistance.

Involvement of DNA polymerase β overexpression in the malignant transformation induced by benzo[a]pyrene

OBJECTIVE

To explore the relationship between DNA polymerase β (pol β) overexpression and benzo[a]pyrene (BaP) carcinogenesis.

METHODS

Firstly, mouse embryonic fibroblasts that express wild-type level of DNA polymerase β (pol β cell) and high level of pol β (pol β oe cell) were treated by various concentrations of BaP to determine genetic instability induced by BaP under differential expression levels of pol β. Secondly, malignant transformation of pol β cells by low concentration of BaP (20 μM) was determined by soft agar colony formation assay and transformation focus assay. Thirdly, the mRNA and protein levels of BaP-transformed pol β cells (named pol β-T cells) was measured by reverse transcriptase-polymerase chain reaction (RT-PCR) and western blot, and the genetic instability of these cells were examined by HPRT gene mutation assay and random amplified polymorphic DNA (RAPD) assay.

RESULTS

Pol β cells were successfully transformed into malignant pol β-T cells by an exposure to low concentration of BaP for 6 months. Pol β-T cells exhibited increased levels of pol β gene expression, HPRT gene mutation frequency and polymorphisms of RAPD products that were comparable to those of pol β oe cells.

CONCLUSION

Pol β overexpression and its-associated genetic instability may play a key role in BaP carcinogenesis.

Biological and therapeutic relevance of nonreplicative DNA polymerases to cancer

Apart from surgical approaches, the treatment of cancer remains largely underpinned by radiotherapy and pharmacological agents that cause damage to cellular DNA, which ultimately causes cancer cell death. DNA polymerases, which are involved in the repair of cellular DNA damage, are therefore potential targets for inhibitors for improving the efficacy of cancer therapy. They can be divided, according to their main function, into two groups, namely replicative and nonreplicative enzymes. At least 15 different DNA polymerases, including their homologs, have been discovered to date, which vary considerably in processivity and fidelity. Many of the nonreplicative (specialized) DNA polymerases replicate DNA in an error-prone fashion, and they have been shown to participate in multiple DNA damage repair and tolerance pathways, which are often aberrant in cancer cells. Alterations in DNA repair pathways involving DNA polymerases have been linked with cancer survival and with treatment response to radiotherapy or to classes of cytotoxic drugs routinely used for cancer treatment, particularly cisplatin, oxaliplatin, etoposide, and bleomycin. Indeed, there are extensive preclinical data to suggest that DNA polymerase inhibition may prove to be a useful approach for increasing the effectiveness of therapies in patients with cancer. Furthermore, specialized DNA polymerases warrant examination of their potential use as clinical biomarkers to select for particular cancer therapies, to individualize treatment for patients.

BCR-ABL1 kinase inhibits uracil DNA glycosylase UNG2 to enhance oxidative DNA damage and stimulate genomic instability

Tyrosine kinase inhibitors (TKIs) revolutionized the treatment of chronic myeloid leukemia in chronic phase (CML-CP). Unfortunately, 25% of TKI-naive patients and 50-90% of patients developing TKI-resistance carry CML clones expressing TKI-resistant BCR-ABL1 kinase mutants. We reported that CML-CP leukemia stem and progenitor cell populations accumulate high amounts of reactive oxygen species, which may result in accumulation of uracil derivatives in genomic DNA. Unfaithful and/or inefficient repair of these lesions generates TKI-resistant point mutations in BCR-ABL1 kinase. Using an array of specific substrates and inhibitors/blocking antibodies we found that uracil DNA glycosylase UNG2 were inhibited in BCR-ABL1-transformed cell lines and CD34(+) CML cells. The inhibitory effect was not accompanied by downregulation of nuclear expression and/or chromatin association of UNG2. The effect was BCR-ABL1 kinase-specific because several other fusion tyrosine kinases did not reduce UNG2 activity. Using UNG2-specific inhibitor UGI, we found that reduction of UNG2 activity increased the number of uracil derivatives in genomic DNA detected by modified comet assay and facilitated accumulation of ouabain-resistant point mutations in reporter gene Na(+)/K(+)ATPase. In conclusion, we postulate that BCR-ABL1 kinase-mediated inhibition of UNG2 contributes to accumulation of point mutations responsible for TKI resistance causing the disease relapse, and perhaps also other point mutations facilitating malignant progression of CML.

HAX-1 promotes the chemoresistance, invasion, and tumorigenicity of esophageal squamous carcinoma cells

BACKGROUND

HAX-1 is an anti-apoptotic factor and regulates the expression of DNA pol β. Interestingly, DNA polymerase pol β is overexpressed in esophageal squamous cell carcinoma (ESCC). However, the functional role of HAX-1 in ESCC remains unclear.

AIMS

To investigate the role of HAX-1 in chemoresistance, invasion, and tumorigenicity of ESCC.

METHODS

Lentivirus-mediated overexpression or knockdown of HAX-1 was employed to establish ESCC EC9706 cell lines that expressed HAX-1 at different levels. The biological behaviors of these engineered cells were characterized in vitro and in vivo using a xenograft nude mice model. In addition, HAX-1 and pol β expression in the tumor tissues was detected by RT-PCR and immunohistochemistry.

RESULTS

HAX-1 overexpression promoted cell proliferation and resistance against cisplatin, increased cell invasion and suppressed apoptosis along with increased pol β expression. Conversely, HAX-1 knockdown inhibited the malignant phenotypes of EC9706 cells. The xenograft nude mice model demonstrated that HAX-1 overexpression or depletion led to increased or decreased tumor growth in vivo, respectively. Furthermore, a positive correlation of HAX-1 and pol β expression in the tumor tissues was observed.

CONCLUSIONS

HAX-1 promotes the proliferation, chemoresistance, invasion, and tumorigenicity of ESCC, and this is correlated with increased poly β expression. HAX-1 may represent a potential target to overcome the resistance and metastasis of ESCC.

Aberrant expression of alternative DNA polymerases: a source of mutator phenotype as well as replicative stress in cancer

The cell life span depends on a subtle equilibrium between the accurate duplication of the genomic DNA and less stringent DNA transactions which allow cells to tolerate mutations associated with DNA damage. The physiological role of the alternative, specialized or TLS (translesion synthesis) DNA polymerases could be to favor the necessary "flexibility" of the replication machinery, by allowing DNA replication to occur even in the presence of blocking DNA damage. As these alternative DNA polymerases are inaccurate when replicating undamaged DNA, the regulation of their expression needs to be carefully controlled. Evidence in the literature supports that dysregulation of these error-prone enzymes contributes to the acquisition of a mutator phenotype that, along with defective cell cycle control or other genome stability pathways, could be a motor for accelerated tumor progression.

The evolution of transcription-associated biases of mutations across vertebrates

Background

The interplay between transcription and mutational processes can lead to particular mutation patterns in transcribed regions of the genome. Transcription introduces several biases in mutational patterns; in particular it invokes strand specific mutations. In order to understand the forces that have shaped transcripts during evolution, one has to study mutation patterns associated with transcription across animals.

Results

Using multiple alignments of related species we estimated the regional single-nucleotide substitution patterns along genes in four vertebrate taxa: primates, rodents, laurasiatheria and bony fishes. Our analysis is focused on intronic and intergenic regions and reveals differences in the patterns of substitution asymmetries between mammals and fishes. In mammals, the levels of asymmetries are stronger for genes starting within CpG islands than in genes lacking this property. In contrast to all other species analyzed, we found a mutational pressure in dog and stickleback, promoting an increase of GC-contents in the proximity to transcriptional start sites.

Conclusions

We propose that the asymmetric patterns in transcribed regions are results of transcription associated mutagenic processes and transcription coupled repair, which both seem to evolve in a taxon related manner. We also discuss alternative mechanisms that can generate strand biases and involves error prone DNA polymerases and reverse transcription. A localized increase of the GC content near the transcription start site is a signature of biased gene conversion (BGC) that occurs during recombination and heteroduplex formation. Since dog and stickleback are known to be subject to rapid adaptations due to population bottlenecks and breeding, we further hypothesize that an increase in recombination rates near gene starts has been part of an adaptive process.

Characterization of DNA polymerase beta splicing variants in gastric cancer: the most frequent exon 2-deleted isoform is a non-coding RNA

DNA repair polymerase beta (Pol beta) gene variants are frequently associated with tumor tissues. In this study a search for Pol beta mutants and splice variants was conducted in matched normal and tumor gastric tissues and blood samples from healthy donors. No tumor associated mutations were found while a variety of alternative Pol beta splicing variants were detected with high frequency in all the specimens analysed. Quantitative PCR of the Pol beta variant lacking exon 2 (Ex2Delta) and the isoforms with exon 11 skipping allowed to clarify that these variants are not tumor- neither tissue-specific and their levels vary greatly among different individuals. The most frequent Ex2Delta variant was further characterized. We clearly demonstrated that this variant does not encode protein, as detected by both western blotting and immunofluorescence analysis of human AGS cells expressing HA-tagged Ex2Delta. The lack of translation was confirmed by comparing the DNA gap-filling capacity and alkylation sensitivity of wild type and Pol beta null murine fibroblasts expressing the human Ex2Delta variant. We showed that the Ex2Delta transcript is polyadenylated and its half-life is significantly longer than that of the wild type mRNA as inferred by treating AGS cells with actinomycin D. Moreover, we found that it localizes to polyribosomes suggesting a role as post-transcriptional regulator. This study identifies a new type of DNA repair variants that do not give rise to functional proteins but to non-coding RNAs that could either modulate target mRNAs or represent unproductive splicing events.

A novel function of adenomatous polyposis coli (APC) in regulating DNA repair

Prevailing literature suggests diversified cellular functions for the adenomatous polyposis coli (APC) gene. Among them a recently discovered unique role of APC is in DNA repair. The APC gene can modulate the base excision repair (BER) pathway through an interaction with DNA polymerase beta (Pol-beta) and flap endonuclease 1 (Fen-1). Taken together with the transcriptional activation of APC gene by alkylating agents and modulation of BER activity, APC may play an important role in carcinogenesis and chemotherapy by determining whether cells with DNA damage survive or undergo apoptosis. In this review, we summarize the evidence supporting this novel concept and suggest that these results will have implications for the development of more effective strategies for chemoprevention, prognosis and chemotherapy of certain types of tumors.

Exogenous 8-oxo-dG is not utilized for nucleotide synthesis but enhances the accumulation of 8-oxo-Gua in DNA through error-prone DNA synthesis

7,8-Dihydro-8-oxoguanine (8-oxo-Gua) and its nucleoside in cytosol are derived from the repair of oxidative DNA and the cleanup of oxidatively damaged DNA precursors, respectively. While the harmful effects of 8-oxo-Gua present in DNA have been studied extensively, few have reported its effects on cytosolic function. Our previous study showed that the addition of 8-oxo-dG to culture media caused an accumulation of 8-oxo-Gua in nuclear DNA in several leukemic cells including KG-1, which lack 8-oxoguanine glycosylase 1 (OGG1) activity due to mutational loss. However, the mechanism underlying 8-oxo-Gua level increases in DNA has not been addressed. In this study, we elucidated the metabolic fate of 8-oxo-Gua-containing nucleotide and the effect of exogenous 8-oxo-dG on DNA synthesis in KG-1 cells. We found that 8-oxo-dGMP was rapidly dephosphorylated to 8-oxo-dG rather than phosphorylated to 8-oxo-dGDP, thus indicating that 8-oxo-Gua-containing molecule is not used as a substrate for DNA synthesis in KG-1 cells. In fact, radiolabeled 8-oxo-dG was incubated but radioactivity was not detected in nuclear DNA of KG-1 cells, showing that 8-oxo-dG is not directly incorporated into DNA. Interestingly, the activity of DNA polymerase beta, which synthesize DNA with low fidelity increased in KG-1 cells treated with 8-oxo-dG, whereas the expression of DNA polymerase alpha decreased. In addition, the accumulation of 8-oxo-Gua in KG-1 DNA was completely inhibited by a specific inhibitor of DNA polymerase beta. Thus, our findings address that the insertion of 8-oxo-dG into KG-1 DNA is not due to the direct incorporation of exogenous 8-oxo-dG, but rather to the inaccurate incorporation of endogenous 8-oxo-dGTP by DNA polymerase beta. It further suggests that 8-oxo-dG in the cytosol may function as an active molecule itself and perturb the well-defined DNA synthesis.

An intrinsic 5′-deoxyribose-5-phosphate lyase activity in DNA polymerase beta from Leishmania infantum supports a role in DNA repair

Leishmania infantum is a parasitic protozoan which infects humans. This paper reports the expression in Escherichia coli and purification of the L. infantum gene product (AF182167), as well as its characterization as a DNA polymerase beta (Polbeta)-like, template-dependent DNA repair enzyme, with a metal preference for Mn2+ over Mg2+. As is the case with mammalian Polbeta and DNA polymerase lambda (Pollambda), L. infantum DNA polymerase beta (Li Polbeta) prefers gapped-DNA substrates having a 5'-phosphate end, in agreement with its role in DNA repair reactions. Purified Li Polbeta also displayed a 5'-deoxyribose-5-phosphate (dRP) lyase activity, consistent with a beta-elimination mechanism. The concerted action of dRP lyase and DNA polymerization activities of Li Polbeta on a uracil-containing DNA suggests its participation in "single-nucleotide" base excision repair (BER). Analysis of Li Polbeta DNA polymerization activity at different stages of the L. infantum infective cycle supports a role for Li Polbeta in nuclear DNA repair after the oxidative damage occurring inside the macrophage.

Transcriptional upregulation of DNA polymerase beta by TEIF

The overexpression of DNA polymerase beta (beta-pol) has been identified in lots of human cancers, but the mechanism has seldom been investigated. Telomerase transcriptional element-interacting factor (TEIF) can bind to hTERT promoter, stimulating its transcription and telomerase activities. Here, we report that TEIF could also enhance the expression of beta-pol at transcription level. TEIF could specifically activate transcription of beta-pol promoter, but not that of DNA polymerase alpha or delta promoter. The responsible sequences for binding of TEIF were revealed as GC-rich elements dispersing from +19 to -29 nt of beta-pol promoter, which due to mutations caused decreasing in binding of TEIF and apparent losing of transactivation activity. The in vivo interaction between TEIF and beta-pol promoter was identified by chromatin immunoprecipitation assay. Besides, ectopic expression of TEIF in HeLa cells could upregulate both levels of endogenous beta-pol mRNA and protein, and consequently increases resistance to the oxidative stress of H2O2. The data may provide new clue to the elucidation of beta-pol overexpression in cancers and also a functional link between beta-pol and telomerase.

The overexpression of specialized DNA polymerases in cancer

Specialized DNA polymerases are required to bypass DNA damage lesions that would otherwise cause replication arrest and cell death. When operating on non-canonical templates, such as undamaged DNA or on non-cognate lesions, these polymerases exhibit considerably reduced fidelity, resulting in the generation of mutations. Ectopic overexpression of these polymerases can also lead to an increased mutation rate and an enhanced capability of DNA repair, suggesting that they could potentially act as oncogenes if they were overexpressed in cancers. Here, we examine expression patterns of DNA polymerases in matched normal and tumor samples from a diverse range of tissues. As well as investigating the specialized polymerases beta, lambda, iota and kappa, we also investigate the expression of the replicative polymerases alpha, delta and epsilon. The data presented provide evidence for the overexpression of specialized polymerases in tumors, with more than 45% of the 68 tumor samples studied demonstrating greater than two-fold enhanced expression of at least one specialized polymerase. Of particular note, DNA polymerase beta (pol beta) was found to be overexpressed at both the mRNA and protein level in approximately one third of all tumor types studied, with overexpression being particularly frequent in uterus, ovary, prostate and stomach samples. Pols lambda, and iota were also found to be overexpressed to a significant extent in a range of tumor types, albeit less frequently than pol beta. In contrast, pol kappa was rarely found to be overexpressed in tumors but was found to be commonly underexpressed in many samples. Downregulation of pol beta expression by siRNA resulted in an increased sensitivity to the chemotherapeutic agent cisplatin, suggesting a role for this polymerase in providing tolerance to cisplatin-induced damage. These observations suggest that specialised DNA polymerases, and particularly pol beta, could be considered both as caretaker genes altered during tumorigenesis, and as potential drug targets to sensitise tumors to chemotherapy.

Modulation of cellular response to cisplatin by a novel inhibitor of DNA polymerase beta

DNA polymerase beta (Pol beta) is an error-prone enzyme whose up-regulation has been shown to be a genetic instability enhancer as well as a contributor to cisplatin resistance in tumor cells. In this work, we describe the isolation of new Pol beta inhibitors after high throughput screening of 8448 semipurified natural extracts. In vitro, the selected molecules affect specifically Pol beta-mediated DNA synthesis compared with replicative extracts from cell nuclei. One of them, masticadienonic acid (MA), is particularly attractive because it perturbs neither the activity of the purified replicative Pol delta nor that of nuclear HeLa cell extracts. With an IC50 value of 8 microM, MA is the most potent of the Pol beta inhibitors found so far. Docking simulation revealed that this molecule could substitute for single-strand DNA in the binding site of Pol beta by binding Lys35, Lys68, and Lys60, which are the main residues involved in the interaction Pol beta/single-strand DNA. Selected inhibitors also affect the Pol beta-mediated translesion synthesis (TLS) across cisplatin adducts; MA was still the most efficient. Therefore, masticadienonic acid sensitized the cisplatin-resistant 2008C13*5.25 human tumor cells. Our data suggest that molecules such as masticadienonic acid could be suitable in conjunction with cisplatin to enhance anticancer treatments.

Decreased Gene Expression Responsible for Post-Ultraviolet DNA Repair Synthesis in Aging: A Possible Mechanism of Age-Related Reduction in DNA Repair Capacity

A reduction in the post-ultraviolet (UV) DNA repair capacity is associated with aging. To clarify the mechanism of this change, we examined the DNA repair capacity of skin fibroblasts from healthy donors of different ages by the two methods: host cell reactivation (HCR) assay and ELISA of cyclobutane pyrimidine dimers and pyrimidine-pyrimidone (6-4) photoproducts. In HCR assay, cells from elderly donors exhibited significant declines in the ability to restore transfected reporter DNA damaged by UV light. In contrast, the ability to remove DNA damage declined little with age in ELISA. These results imply that the age-sensitive step took place after the damage excision in nucleotide excision repair (NER). The mRNA expression of DNA repair synthesis-related genes (DNA polymerase delta, replication factor C, and proliferating cell nuclear antigen) were markedly decreased in the cells from multiple elderly subjects compared with those from young subjects. Further, the protein level of DNA polymerase delta1, a catalytic subunit of the pivotal factor in repair synthesis, correlated with the mRNA level. These findings suggest that the reduced post-UV DNA repair capacity in aging results from an impairment in the latter step of NER by the decreased expression of factors in repair synthesis.

Up-Regulation of the Error-Prone DNA Polymerase κ Promotes Pleiotropic Genetic Alterations and Tumorigenesis

It is currently widely accepted that genetic instability is key to cancer development. Many types of cancers arise as a consequence of a gradual accumulation of nucleotide aberrations, each mutation conferring growth and/or survival advantage. Genetic instability could also proceed in sudden bursts leading to a more drastic upheaval of structure and organization of the genome. Genetic instability, as an operative force, will produce genetic variants and the greater the instability, the larger the number of variants. We report here that the overexpression of human DNA polymerase κ, an error-prone enzyme that is up-regulated in lung cancers, induces DNA breaks and stimulates DNA exchanges as well as aneuploidy. Probably as the result of so many perturbations, excess polymerase κ favors the proliferation of competent tumor cells as observed in immunodeficient mice. These data suggest that altered regulation of DNA metabolism might be related to cancer-associated genetic changes and phenotype.

Atypical protein kinase C stimulates nucleotide excision repair activity

Nucleotide excision repair (NER) deals with bulky DNA damages. However, the regulation of this process is still unclear. Here, we show that both cell resistance to genotoxic agents that generate DNA lesions corrected by NER and in vitro NER activity are correlated with atypical protein kinase C (PKC) zeta expression levels. Moreover, repair intermediates are produced and eliminated more rapidly in UV-irradiated PKCzeta-overexpressing cells. The expression levels of XPC and hHR23B, two NER proteins, are correlated with PKCzeta expression. Altogether, these results strongly suggest that PKCzeta could act as a modulator of NER activity by regulating the expression of XPC/hHR23B heterodimer.

‘Knock down’ of DNA polymerase β by RNA interference: recapitulation of null phenotype

DNA polymerase beta (pol beta) is the major DNA polymerase involved in the base excision repair (BER) pathway in mammalian cells and, as a consequence, BER is severely compromised in cells lacking pol beta. Pol beta null (-/-) mouse embryos are not viable and pol beta null cells are hypersensitive to alkylating agents. Using RNA interference (RNAi) technology in mouse cells, we have reduced the pol beta protein and mRNA to undetectable levels. Pol beta knockdown cell lines display a pattern of hypersensitivity to DNA damaging agents similar to that observed in pol beta null cells. Generation of pol beta knock down cells makes it possible to combine the pol beta null phenotype with deficiencies in other DNA repair proteins, thereby helping to elucidate the role of pol beta and its interactions with other proteins in mammalian cells.

DNA polymerase beta overexpression stimulates the Rad51-dependent homologous recombination in mammalian cells

Overexpression of DNA polymerase beta (polbeta), an error-prone DNA repair enzyme, has been shown to result in mutagenesis, aneuploidy and tumorigenesis. To further investigate the molecular basis leading to cancer-associated genetic changes, we examined whether the DNA polbeta could affect homologous recombination (HR). Using mammalian cells carrying an intrachromosomal recombination marker we showed that the DNA polbeta overexpression increased the HR mostly by enhancing gene conversion. Concomitantly, we observed the generation of DNA strand breaks as well as a DNA polbeta-dependent formation of Rad51 foci. The stimulation of HR was abolished by the coexpression of a dominant negative form of Rad51, suggesting that the Rad51 was involved in the increased HR events. The expression of different DNA polbeta mutants lacking polymerase activity did not result in HR stimulation, indicating that the DNA synthesis activity of DNA polbeta was related to this phenotype. These results provide new insights into the molecular mechanisms of the genetic instability observed in DNA polbeta overexpressing tumour cells.

DNA polymerase beta interacts with TRF2 and induces telomere dysfunction in a murine mammary cell line

DNA polymerase beta (Polbeta) is a DNA repair protein that functions in base excision repair and meiosis. The enzyme has deoxyribose phosphate lyase and polymerase activity, but it is error prone because it bears no proofreading activity. Errors in DNA repair can lead to the accumulation of mutations and consequently to tumorigenesis. Polbeta expression has been found to be higher in tumors, and deregulation of its expression has been found to induce chromosomal instability, a hallmark of tumorigenesis, but the underlying mechanisms are unclear. In the present study, we have investigated whether ectopic expression of Polbeta influences the stability of chromosomes in a murine mammary cell line. The results demonstrate a telomere dysfunction phenotype: an increased rate of telomere loss and chromosome fusion, suggesting that ectopic expression of Polbeta leads to telomere dysfunction. In addition, Polbeta interacts with TRF2, a telomeric DNA binding protein. Colocalization of the two proteins occurs at nontelomeric sites and appears to be influenced by the change in the status of the telomeric complex.

Evidence of finely tuned expression of DNA polymerase beta in vivo using transgenic mice

DNA polymerase (Pol) is an error-prone repair DNA polymerase that has been shown to create genetic instability and tumorigenesis when overexpressed by only 2-fold in cells, suggesting that a rigorous regulation of its expression may be essential in vivo. To address this question, we have generated mice which express a transgene (Tg) bearing the Pol cDNA under the control of the ubiquitous promoter of the mouse H-2K gene from the major histocompatibility complex. These mice express the Tg only in thymus, an organ which normally contains the most abundant endogenous Pol mRNA and protein, supporting the idea of a tight regulation of Pol in vivo. Furthermore, we found no tumor incidence, suggesting that the single Pol overexpression event is not sufficient to initiate tumorigenesis in vivo.

The D246V mutant of DNA polymerase beta misincorporates nucleotides: evidence for a role for the flexible loop in DNA positioning within the active site

DNA polymerase beta, a member of the X family of DNA polymerases, is known to be involved in base excision repair. A key to determining the biochemical properties of this DNA polymerase is structure-function studies of site-specific mutants that result in substitution of particular amino acids at critical sites. In a previous genetic screen, we identified three 3'-azido-2',3'-dideoxythymidine 5'-triphosphate-resistant mutants, namely E249K, D246V, and R253M, of polymerase beta in the flexible loop of the palm domain. In this work, we perform an extensive kinetic analysis to investigate the role of the D246V mutant on polymerase fidelity. We find that D246V misincorporates T opposite template bases G and C. The mechanistic basis of misincorporation appears to be altered DNA positioning within the active site. We provide evidence that the fidelity of D246V is greatly affected by the base that is 5' of the templating base. We propose that the Asp residue at position 246 helps to maintain the proper positioning of the DNA within the polymerase active site and maintains the fidelity of polymerase beta. Altogether, the results suggest that the flexible loop domain of polymerase beta plays a major role in its fidelity.

p210 BCR/ABL kinase regulates nucleotide excision repair (NER) and resistance to UV radiation

Both clinical and experimental evidence illustrate that p190 and p210 BCR/ABL oncogenic tyrosine kinases induce resistance to DNA damage and confer an intrinsic genetic instability. Here, we investigated whether BCR/ABL expression could modulate nucleotide excision repair (NER). We found that ectopic expression of p210 BCR/ABL in murine lymphoid BaF3 cell line inhibited NER activity in vitro, promoting hypersensitivity of these cells to ultraviolet (UV) treatment and facilitating a mutator phenotype. However, expression of p210 BCR/ABL in human and murine myeloid cell lines and primary bone marrow cells resulted in the increased NER activity and resistance to UV irradiation. The ABL tyrosine kinase inhibitor STI571 reversed these effects, showing that p210 BCR/ABL tyrosine kinase activity is responsible for deregulation of NER. Hypoactivity of NER in p210 BCR/ABL-positive lymphoid cells was accompanied by the decreased interaction between proliferating cell nuclear antigen (PCNA) and xeroderma pigmentosum group B (XPB); conversely, this interaction was enhanced in p210 BCR/ABL-positive myeloid cells. p190 BCR/ABL did not affect NER in lymphoid and myeloid cells. In summary, our study suggests that p210 BCR/ABL reduced NER activity in lymphoid cells, leading to hypersensitivity to UV and mutagenesis. In contrast, p210 BCR/ABL expression in myeloid cells facilitated NER and induced resistance to UV.

DNA polymerase beta can incorporate ribonucleotides during DNA synthesis of undamaged and CPD-damaged DNA

Overexpression of the error-prone DNA polymerase beta (Pol beta) has been found to increase spontaneous mutagenesis by competing with the replicative polymerases during DNA replication. Here, we investigate an additional mechanism potentially used by Pol beta to enhance genetic instability via its ability to incorporate ribonucleotides into DNA. By using an in vitro primer extension assay, we show that purified human and calf thymus Pol beta can synthesize up to 8-mer long RNA. Moreover, Pol beta can efficiently incorporate rCTP opposite G in the absence of dCTP and, to a lesser extent, rATP opposite T in the absence of dATP and rGTP opposite C in the absence of dGTP. Recently, Pol beta was shown to catalyze in vitro translesion replication of a thymine cyclobutane pyrimidine dimer (CPD). Here, we investigate if ribonucleotides could be incorporated opposite the CPD damage and modulate the efficiency of the bypass process. We find that all four rNTPs can be incorporated opposite the CPD lesion, and that this process affects translesion synthesis. We discuss how incorporation of ribonucleotides into DNA may contribute to the high frequency of mutagenesis observed in Pol beta up-regulating cells.

An age-related increase in resistance to DNA damage-induced apoptotic cell death is associated with development of DNA repair mechanisms

Neurons in the developing brain die via apoptosis after DNA damage, while neurons in the adult brain are generally resistant to these insults. The basis for this resistance is a matter of conjecture. We report here that cerebellar granule neurons (CGNs) in culture lose their competence to die in response to DNA damage as a function of time in culture. CGNs at either 1 day in vitro (DIV) or 7 DIV were treated with the DNA damaging agents camptothecin, UV or gamma-irradiation and neuronal survival measured. The younger neurons were effectively killed by these agents, while the older neurons displayed a significant resistance to killing. Neuronal survival did not change with time in culture when cells were treated with C2-ceramide or staurosporine, agents which do not target DNA. The resistance to UV irradiation developed over time in culture and was not due to changes in mitotic rate. Increases in DNA strand breakage, up-regulation of the levels of both p53 and its phosphorylated form and nuclear translocation of p53 were equivalent in both older and younger neurons, indicating a comparable p53 stress response. In addition, we show that treatment of older neurons with pharmacological inhibitors of distinct components of the DNA repair machinery promotes the accumulation of DNA damage and sensitizes these cells to the toxic effects of UV exposure. These data demonstrate that older neurons appear to be more proficient in DNA repair in comparison to their younger counterparts, and that this leads to increased survival after DNA damage.

A role for DNA polymerase beta in mutagenic UV lesion bypass

We report here that DNA polymerase beta (pol beta), the base excision repair polymerase, is highly expressed in human melanoma tissues, known to be associated with UV radiation exposure. To investigate the potential role of pol beta in UV-induced genetic instability, we analyzed the cellular and molecular effects of excess pol beta. We firstly demonstrated that mammalian cells overexpressing pol beta are resistant and hypermutagenic after UV irradiation and that replicative extracts from these cells are able to catalyze complete translesion replication of a thymine-thymine cyclobutane pyrimidine dimer (CPD). By using in vitro primer extension reactions with purified pol beta, we showed that CPD as well as, to a lesser extent, the thymine-thymine pyrimidine-pyrimidone (6-4) photoproduct, were bypassed. pol beta mostly incorporates the correct dATP opposite the 3'-terminus of both CPD and the (6-4) photoproduct but can also misinsert dCTP at a frequency of 32 and 26%, respectively. In the case of CPD, efficient and error-prone extension of the correct dATP was found. These data support a biological role of pol beta in UV lesion bypass and suggest that deregulated pol beta may enhance UV-induced genetic instability.

Erratic expression of DNA polymerases by beta-amyloid causes neuronal death

An ectopic reentrance into the cell cycle with ensuing DNA replication is required for neuronal apoptosis induced by beta-amyloid. Here, we investigate the repertoire of DNA polymerases expressed in beta-amyloid-treated neurons, and their specific role in DNA synthesis and apoptosis. We show that exposure of cultured cortical neurons to beta-amyloid induces the expression of DNA polymerase-beta, proliferating cell nuclear antigen, and the p49 and p58 subunits of DNA primase. Induction requires the activity of cyclin-dependent kinases. The knockdown of the p49 primase subunit prevents beta-amyloid-induced neuronal DNA synthesis and apoptosis. Similar effects are observed by knocking down DNA polymerase-beta or by using dideoxycytidine, a preferential inhibitor of this enzyme. Thus, the reparative enzyme DNA polymerase-beta unexpectedly mediates a large component of de novo DNA synthesis and apoptotic death in neurons exposed to beta-amyloid. These data indicate that DNA polymerases become death signals when erratically expressed by differentiated neurons.

Long-patch base excision repair of apurinic/apyrimidinic site DNA is decreased in mouse embryonic fibroblast cell lines treated with plumbagin: involvement of cyclin-dependent kinase inhibitor p21Waf-1/Cip-1

Molecular interactions among cell cycle and DNA repair proteins have been described, but the impact of many of these interactions on cell cycle control and DNA repair remains unclear. The cyclin-dependent kinase inhibitor, p21, is known to be involved in DNA damage-induced cell cycle arrest and blocking DNA replication and repair. Participation of p21 has been implicated in nucleotide excision repair. However, the role of p21 in the base excision repair (BER) pathway has not been thoroughly studied. In the present investigation, we treated isogenic mouse embryonic fibroblast (MEF) cell lines containing wild-type (MEF-polbeta) or DNA polymerase beta (polbeta) gene-knockout (MEFpolbetaKO) with oxidative DNA-damaging agent, plumbagin, and examined its effect on p21 levels and BER activity. Plumbagin treatment caused a S-G(2)/M phase arrest and cell death of both MEF cell lines, induced p21 levels, and decreased p21-mediated long-patch (LP) BER by blocking DNA ligase activity in the polbeta-dependent pathway and by blocking both FEN1 and DNA ligase activity in polbeta-independent pathway. These findings suggest that plumbagin induced p21 levels play a regulatory role in cell cycle arrest, apoptosis, and polbeta-dependent and -independent LP-BER pathways in MEF cells.

Deregulated DNA polymerase beta strengthens ionizing radiation-induced nucleotidic and chromosomal instabilities

DNA polymerase beta (Pol beta) is an error-prone enzyme which has been found to be overexpressed in several human tumors. By using a couple of recombinant CHO cells differing only from the exogenous expression of Pol beta, we showed here that cells overexpressing Pol beta are much more sensitive to IR treatments by increasing apoptosis. We also found that the surviving cells displayed an hypermutator phenotype which could be explained by different pathways involving Pol beta, such as (i) an increased capacity to incorporate into DNA the mutagenic dGTP analog, 8-oxo-dGTP, one of the most abundant purine-derived nucleotides exposed to gamma-irradiation, (ii) the induction of IR-induced DNA breaks and (iii) accumulation of chromosome aberrations induced by radiation. Alteration of Pol beta expression in irradiated cells thus appears to strengthen both cell death and genetic changes associated with a malignant phenotype. These data provide new insights into the cellular response to radiations and the associated carcinogenic consequences.

Involvement of DNA polymerase beta in DNA replication and mutagenic consequences

Overexpression in mammalian cells of the error-prone DNA polymerase beta (Pol beta) has been found to increase the spontaneous mutagenesis. Here, we investigated a possible mechanism used by Pol beta to be a genetic instability enhancer: its interference in replicative DNA synthesis, which is normally catalysed by the DNA polymerases alpha, delta and epsilon. By taking advantage of the ability to incorporate efficiently into DNA the chain terminator ddCTP as well as the oxidised nucleotide 8-oxo-dGTP, we show here that purified Pol beta can compete with the replicative DNA polymerases during replication in vitro of duplex DNA when added to human cell extracts. We found that involvement of Pol beta lowers replication fidelity and results in a modified error-specificity. Furthermore, we demonstrated that involvement of Pol beta occurred during synthesis of the lagging strand. These in vitro data provide one possible explanation of how overexpression of the enzyme could perturb the genetic instability in mammalian cells. We discuss these findings within the scope of the up-regulation of Pol beta in many cancer cells.

Enhanced expression and activity of DNA polymerase beta in human ovarian tumor cells: impact on sensitivity towards antitumor agents

DNA polymerase beta, one of the most inaccurate DNA synthesizing enzymes, has been shown to confer genetic instability when up-regulated in cells, a situation found in several human cancers. Here, we demonstrated that enhanced activity and expression of this enzyme occur in the human ovarian tumor 2008/C13*5.25 cells, which are resistant to the antitumor agent cisplatin and hypersensitive to 6-thioguanine. We found that translesion synthesis across platinated DNA crosslinks as well as increased incorporation into DNA of 6-thioguanine took place in the 2008/C13*5.25 cells compared to the parental 2008 cells. Such features being molecular signatures of DNA polymerase beta, these findings suggest that deregulation of its expression in cancer cells may contribute to the modulation of the response to antitumor treatments and therefore to tumor progression.

DNA polymerase beta imbalance increases apoptosis and mutagenesis induced by oxidative stress

Oxidative stress has been proposed to be one of the major causes leading to the accumulation of mutation that is associated with the initiation and progression of cancers. Elevated expression of DNA polymerase beta, an event found in many human tumors, has been shown to generate a mutator phenotype. Here, we demonstrated that overexpression of DNA polymerase beta strengthens the mutagenicity of oxidative damages, concomitantly with a higher cellular sensitivity and increased apoptosis. Deregulated expression of DNA polymerase beta could represent a predisposition factor for mutagenic effects of oxidative stress and thus have implication in the generation and/or evolution of cancer.

Mechanisms of resistance to cisplatin

The use of cisplatin in cancer chemotherapy is limited by acquired or intrinsic resistance of cells to the drug. Cisplatin enters the cells and its chloride ligands are replaced by water, forming aquated species that react with nucleophilic sites in cellular macromolecules. The presence of the cisplatin adducts in DNA is thought to trigger cell cycle arrest and apoptosis. Knowledge of the mechanism of action of cisplatin has improved our understanding of resistance. Decreased intracellular concentration due to decreased drug uptake, increased reflux or increased inactivation by sulfhydryl molecules such as glutathione can cause resistance to cisplatin. Increased excision of the adducts from DNA by repair pathways or increased lesion bypass can also result in resistance. Finally, altered expression of regulatory proteins involved in signal transduction pathways that control the apoptotic pathway can also affect sensitivity to the drug. An improved understanding of the mechanisms of resistance operative in vivo has identified targets for intervention and may increase the utility of cisplatin for the treatment of cancer.