[What do we know after ten years of genetic research into calcium kidney stones?]

Genetic studies of calcium kidney stones have so far assessed single candidate genes by testing linkage disequilibrium or association between a locus and stone disease. They showed the possible involvement of the calciumsensing receptor gene, vitamin D receptor gene, and bicarbonate-sensitive adenylate cyclase gene. In addition to research in humans, the study of different strains of knock-out mice let us include the gene of phosphate reabsorption carrier NPT2, caveolin-1, protein NHERF-1 modulating calcium and urate reabsorption, osteopontin and Tamm-Horsfall protein among the possible determinants. However, the interactions between genes and also between environmental factors and genes are generally considered fundamental in calcium stone formation. Thus, the genetic studies carried out to date have not led to a significant growth of the knowledge about the causes of calcium kidney stones, even though they have allowed us to assess the size of the problem and define criteria to address it. Further knowledge of the causes of calcium stones may be obtained using the instruments that modern biotechnology and bioinformatics have made available to researchers.

Renal osteodystrophy and vascular calcification

Chronic kidney disease (CKD) is characterized by phosphate retention and reduced synthesis of 1.25(OH)2-vitamin D stimulating parathyroid hyperplasia. These changes cause a complex osteopathy, defined as renal osteodystrophy, and vascular calcification. Renal osteodystrophy increases the risk of fracture and causes deformities and disability. Vascular calcification occurs in a large proportion of hemodialysis patients and is a marker of arteriopathy. Calcifying arteriopathy induces arterial stiffness and contributes to the high cardiovascular mortality and morbidity among CKD patients. Vascular calcification results from a process of local bone formation induced by osteoblast-like cells developing in the vascular wall from resident cells. Osteoblast differentiation of resident vascular cells may be mediated by metabolic factors and may be induced by high concentrations of phosphate. Therefore, phosphate retention appears as the most detrimental factor affecting arteries in CKD patients. There is no specific therapy to revert soft tissue calcification, but calcification must be prevented in the early stages of CKD.

Resveratrol inhibits cell growth in a human cholangiocarcinoma cell line

BACKGROUND/AIMS

Cholangiocarcinoma is a devastating tumour with a poor prognosis. An efficient therapy is unavailable in unoperable patients and new drugs are widely sought for and required. Resveratrol (RES) is a natural molecule with a reported anticancer effect, evaluated on different tumour cell lines. We tested the efficacy of RES on a cholangiocarcinoma cell line for the first time.

METHODS

We used the human SK-ChA-1 cell line, cultured in the classical two-dimensional model and in the three-dimensional spheroids. After RES exposure morphology, cell viability (colony-forming assay), lactate dehydrogenase (LDH), alkaline phosphatase (ALP) and cancer antigen (CA) 19-9 medium releases, cellular transglutaminase activity, karyotype and cell cycle were evaluated.

RESULTS

Resveratrol inhibited cell growth in both the cell culture systems used (from -15 to -80% vs untreated controls) and induced a 40-fold increase of LDH and ALP activities in the culture medium. Also, transglutaminase (TG) activity increased in the cell lysates, together with a cell cycle perturbation characterised by an accumulation in the G(1)/S phase. Karyotype and CA 19-9 expression were not influenced by the treatment.

CONCLUSIONS

The observed cytotoxic effect of RES on the human cholangiocarcinoma SK-ChA-1 cell line cultured two- and three-dimensionally suggests to further analyse its chemotherapic/chemopreventive possibilities for this kind of cancer.

Resveratrol impairs the formation of MDA-MB-231 multicellular tumor spheroids concomitant with ceramide accumulation

Resveratrol exerts a drastic growth inhibitory effect on human breast cancer MDA-MB-231 cells grown both in vitro and in vivo. Here we show that resveratrol affects the aggregation properties of MDA-MB-231 cells into multicellular tumor spheroids (MCTSs), in association with induction of de novo synthesis of ceramide. After 9 days of 3D growth in the presence of resveratrol (64 microM), MDA-MB-231 cells formed significantly smaller MCTSs. Further, cells from these aggregates failed to form colonies. Addition of resveratrol (64 microM) to preformed MDA-MB-231 MCTSs caused no significant size change, consistent with lack of ceramide induction. Only some apoptotic blebs were found on the MCTSs surface. Altogether these findings indicate that resveratrol might be effective for prevention of breast cancer cell growth.

The role of CSA in the response to oxidative DNA damage in human cells

Cockayne syndrome (CS) is a rare genetic disease characterized by severe growth, mental retardation and pronounced cachexia. CS is most frequently due to mutations in either of two genes, CSB and CSA. Evidence for a role of CSB protein in the repair of oxidative DNA damage has been provided recently. Here, we show that CSA is also involved in the response to oxidative stress. CS-A human primary fibroblasts and keratinocytes showed hypersensitivity to potassium bromate, a specific inducer of oxidative damage. This was associated with inefficient repair of oxidatively induced DNA lesions, namely 8-hydroxyguanine (8-OH-Gua) and (5'S)-8,5'-cyclo 2'-deoxyadenosine. Expression of the wild-type CSA in the CS-A cell line CS3BE significantly decreased the steady-state level of 8-OH-Gua and increased its repair rate following oxidant treatment. CS-A cell extracts showed normal 8-OH-Gua cleavage activity in an in vitro assay, whereas CS-B cell extracts were confirmed to be defective. Our data provide the first in vivo evidence that CSA protein contributes to prevent accumulation of various oxidized DNA bases and underline specific functions of CSB not shared with CSA. These findings support the hypothesis that defective repair of oxidative DNA damage is involved in the clinical features of CS patients.

Interleukin-1 gene polymorphisms and gastric cancer risk in a high-risk Italian population

OBJECTIVES

Host genetic factors, including the IL1 gene cluster, play a key role in determining the long-term outcome of Helicobacter pylori infection. The aim of the study was to investigate the relationship between selected IL1 loci polymorphisms and gastric cancer risk in an Italian population.

METHODS

In a case-control study we compared the IL1B-31 and IL1B+3954 biallelic and IL1RN pentaallelic variable number of tandem repeats (VNTR) polymorphisms in 185 gastric cancer patients and 546 controls randomly sampled from the general population of an area at high gastric cancer risk (Tuscany, Central Italy).

RESULTS

Genotype frequencies of the IL1B-31 T/C, IL1B+3954 C/T, and IL1RN polymorphisms among our population controls were in Hardy-Weinberg equilibrium. In multivariate analyses, no increase in gastric cancer risk was observed for the IL1B-31*C- and IL1B+3954*T- carriers; a significant 50% increase emerged for IL1RN*2 allele carriers (OR = 1.49; 95% CI: 1.01-2.21). Analyses based on combined genotypes showed also that the association with IL1RN*2 allele was limited to two-variant allele carriers who were also homozygous for the IL1B-31*T allele (OR = 2.23; 95% CI: 1.18-4.23) with a statistically significant interaction between these two genotypes (p= 0.043). Haplotype analysis showed an increased risk for the haplotype IL1RN*2/IL1B-31*T.

CONCLUSIONS

Our results suggest that host genetic factors (such as the IL1RN and the IL1B-31 polymorphisms) interact in the complex process of gastric carcinogenesis in this high-risk Italian population. Overall, this effect appears more modest than previously reported in other populations, supporting the hypothesis that other still-to-be-defined factors are important in gastric carcinogenesis. These findings might be due to a haplotype effect.

The base excision repair: mechanisms and its relevance for cancer susceptibility

Base damage or loss occurs at high frequency in the cells (almost 10(4) bases are damaged and hydrolysed per cell per day). DNA repair is fundamental to maintain genomic integrity. Base excision repair (BER) is the main mechanism by which cells correct various types of damaged DNA bases generated either by endogenous or exogenous factors. The widely accepted model for BER mechanism involves five sequential reactions: (i) base removal; (ii) incision of the resulting abasic site; (iii) processing of the generated termini at the strand break; (iv) DNA synthesis, and (v) ligation. In this review, we will briefly summarise the biochemistry of each BER step and will concentrate on the biological relevance of BER as inferred from in vitro and in vivo studies. This information will be the basis for speculation on the potential role of malfunction of BER in human pathology.

8-Oxoguanine DNA damage: at the crossroad of alternative repair pathways

Radical oxygen species (ROS) generate various modified DNA bases. Among them 8-oxo-7,8-dihydroguanine (8oxoG) is the most abundant and seems to play a major role in mutagenesis and in carcinogenesis. 8oxoG is removed from DNA by the specific glycosylase OGG1. An additional post-replication repair is needed to correct the 8oxoG/A mismatches that are produced by persistent 8oxoG residues. This review is focused on the mechanisms of base excision repair (BER) of this oxidized base. It is shown that, in vitro, efficient and complete repair of 8oxoG/C pairs requires a core of four proteins, namely OGG1, APE1, DNA polymerase (Pol) beta, and DNA ligase I. Repair occurs predominantly by one nucleotide replacement reactions (short-patch BER) and Pol beta is the polymerase of election for the resynthesis step. However, alternative mechanisms can act on 8oxoG residues since Pol beta-null cells are able to repair these lesions. 8oxoG/A mismatches are repaired by human cell extracts via two BER events which occur sequentially on the two strands. The removal of the mismatched adenine is followed by preferential insertion of a cytosine leading to the formation of 8oxoG/C pairs which are then corrected by OGG1-mediated BER. Both repair events are inhibited by aphidicolin, suggesting that a replicative DNA polymerase is involved in the repair synthesis step. We propose that Pol delta/epsilon-mediated BER (long-patch BER) is the mode of repair when lesions persist or are formed at replication. Finally, we address the issues of the relative contribution of the two BER pathways to oxidative damage repair in vivo and the possible role of BER gene variants as cancer susceptibility genes.

Characterization of the ultraviolet B and X-ray response of primary cultured epidermal cells from patients with disseminated superficial actinic porokeratosis

BACKGROUND

Disseminated superficial actinic porokeratosis (DSAP) is the most common porokeratosis and is characterized by multiple keratotic lesions which tend to occur at sun-exposed sites. A mild hypersensitivity to X-rays has been reported for DSAP-derived fibroblasts and frequent over-expression of p53 has been found in lesional epidermis.

OBJECTIVES

In order to clarify whether genome maintenance mechanisms might be compromised in this disease the following approaches were undertaken: (i) primary cultured keratinocytes and fibroblasts from DSAP patients were characterized for ultraviolet (UV) B and X-ray response; (ii) 15 lesions were studied for p53 mutations, and (iii) the differentiation status of DSAP-derived keratinocytes was evaluated.

METHODS

Primary cultures of keratinocytes and fibroblasts were established from lesional and nonlesional skin biopsies of two subjects with DSAP. p53 mutations were analysed by DNA sequencing of the conserved region of the TP53 gene. Differentiation was evaluated both in stratified epithelial sheets from confluent keratinocyte cultures and in organotypic skin cultures.

RESULTS

The cytotoxic and apoptotic response to UVB or X-irradiation was similar in DSAP-derived keratinocytes and fibroblasts when compared with normal cells. Two of 15 lesions examined presented p53 mutations located at nondipyrimidine sites. A strikingly decreased expression of filaggrin was observed both in reconstructed epidermis and in reconstructed skin.

CONCLUSIONS

The UVB and X-ray response of DSAP-derived keratinocytes and fibroblasts indicates that the actinic character of this skin pathology is not due to radiation hypersensitivity. In agreement with this finding, mutations in the p53 gene, which are often associated with UV-related skin carcinogenesis, were rarely detected in DSAP lesions and were not UV-specific. Reconstructed epidermis and reconstructed skin models successfully reproduced the main features of this genodermatosis, showing that DSAP-derived keratinocytes bear an inherent defect in the terminal differentiation programme.

Influence of DNA torsional rigidity on excision of 7,8-dihydro-8-oxo-2'-deoxyguanosine in the presence of opposing abasic sites by human OGG1 protein

The human protein OGG1 (hOGG1) targets the highly mutagenic base 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG) and shows a high specificity for the opposite DNA base. Abasic sites can arise in DNA in close opposition to 8-oxodG either during repair of mismatched bases (i.e. 8-oxodG/A mismatches) or, more frequently, as a consequence of ionizing radiation exposure. Bistranded DNA lesions may remain unrepaired and lead to cell death via double-strand break formation. In order to explore the role of damaged-DNA dynamics in recognition/excision by the hOGG1 repair protein, specific oligonucleotides containing an 8-oxodG opposite an abasic site, at different relative distances on the complementary strand, were synthesized. Rotational dynamics were studied by means of fluorescence polarization anisotropy decay experiments and the torsional elastic constant as well as the hydrodynamic radius of the DNA fragments were evaluated. Efficiency of excision of 8-oxodG was tested using purified human glycosylase. A close relation between the twisting flexibility of the DNA fragment and the excision efficiency of the oxidative damage by hOGG1 protein within a cluster was found.

Reconstitution of the base excision repair pathway for 7,8-dihydro-8-oxoguanine with purified human proteins

In mammalian cells, repair of the most abundant endogenous premutagenic lesion in DNA, 7,8-dihydro-8-oxoguanine (8-oxoG), is initiated by the bifunctional DNA glycosylase OGG1. By using purified human proteins, we have reconstituted repair of 8-oxoG lesions in DNA in vitro on a plasmid DNA substrate containing a single 8-oxoG residue. It is shown that efficient and complete repair requires only hOGG1, the AP endonuclease HAP1, DNA polymerase (Pol) beta and DNA ligase I. After glycosylase base removal, repair occurred through the AP lyase step of hOGG1 followed by removal of the 3'-terminal sugar phosphate by the 3'-diesterase activity of HAP1. Addition of PCNA had a slight stimulatory effect on repair. Fen1 or high concentrations of Pol beta were required to induce strand displacement DNA synthesis at incised 8-oxoG in the absence of DNA ligase. Fen1 induced Pol beta strand displacement DNA synthesis at HAP1-cleaved AP sites differently from that at gaps introduced by hOGG1/HAP1 at 8-oxoG sites. In the presence of DNA ligase I, the repair reaction at 8-oxoG was confined to 1 nt replacement, even in the presence of high levels of Pol beta and Fen1. Thus, the assembly of all the core proteins for 8-oxoG repair catalyses one major pathway that involves single nucleotide repair patches.

The mechanism of switching among multiple BER pathways

To preserve genomic beta DNA from common endogenous and exogenous base and sugar damage, cells are provided with multiple base excision repair (BER) pathways: the DNA polymerase (Pol) beta-dependent single nucleotide BER and the long-patch (2-10 nt) BER that requires PCNA. It is a challenge to identify the factors that govern the mechanism of switching among these pathways. One of these factors is the type of DNA damage induced in DNA. By using different model lesions we have shown that base damages (like hypoxanthine and 1, N6-ethenoadenine) excised by monofunctional DNA glycosylases are repaired via both single-nucleotide and long-patch BER, while lesions repaired by a bifunctional DNA glycosylase (like 7,8-dihydro-8-oxoguanine) are repaired mainly by single-nucleotide BER. The presence of a genuine 5' nucleotide, as in the case of cleavage by a bifunctional DNA glycosylase-beta lyase, would then minimize the strand displacement events. Another key factor in the selection of the BER branch is the relative level of cellular polymerases. While wild-type embryonic mouse fibroblast cell lines repair abasic sites predominantly via single-nucleotide replacement reactions (80% of the repair events), cells homozygous for a deletion in the Pol beta gene repair these lesions exclusively via long-patch BER. Following treatment with methylmethane sulfonate, these mutant cells accumulate DNA single-strand breaks in their genome in keeping with the fact that repair induced by monofunctional alkylating agents goes predominantly via single-nucleotide BER. Since the long-patch BER is strongly stimulated by PCNA, the cellular content of this cell-cycle regulated factor is also extremely effective in driving the repair reaction to either BER branch. These findings raise the interesting possibility that different BER pathways might be acting as a function of the cell cycle stage.

Risk factors for basal cell carcinoma in a Mediterranean population: role of recreational sun exposure early in life

OBJECTIVE

To investigate the role of pigmentary traits, different patterns of sun exposure, artificial sources of UV radiation, and lifestyle-related factors on the risk of basal cell carcinoma (BCC) in a Mediterranean population from central-southern Italy.

DESIGN

Hospital-based case-control study.

SETTING

A referral dermatological hospital in Rome, Italy.

PATIENTS

A convenience sample of 166 case patients with histologically confirmed BCC and 158 cancer-free control subjects with minor dermatological conditions observed between March 1995 and June 1997.

RESULTS

In the multivariate analysis, the mean number of weeks per year spent at the beach before the age of 20 years was significantly associated with BCC. A dose-response trend was found for subjects who had spent 3 to 4 (odds ratio, 1.8; 95% confidence interval, 0.8-4.4), 5 to 8 (odds ratio, 3.7; 95% confidence interval, 1.5-9.0), or more than 8 (odds ratio, 4.5; 95% confidence interval, 1.9-10.5) weeks per year at the beach (P =.01 for trend). There was a significant association with the presence of actinic keratoses or solar lentigines, whereas no effect was found for skin type, history of sunburns, exposure to nonsolar UV radiation, and lifestyle-related habits such as cigarette smoking, alcohol consumption, and coffee drinking. Subjects reporting a family history of skin cancer had an extremely increased risk of BCC.

CONCLUSION

The definite association with recreational sun exposure during childhood and adolescence and the strong relation with family history of skin cancer suggest that genetic predisposition and peculiar exposure patterns to UV radiation are key independent risk factors for the development of BCC in a southern European population.

DNA polymerase beta is required for efficient DNA strand break repair induced by methyl methanesulfonate but not by hydrogen peroxide

The most frequent DNA lesions in mammalian genomes are removed by the base excision repair (BER) via multiple pathways that involve the replacement of one or more nucleotides at the lesion site. The biological consequences of a BER defect are at present largely unknown. We report here that mouse cells defective in the main BER DNA polymerase beta (Pol beta) display a decreased rate of DNA single-strand breaks (ssb) rejoining after methyl methanesulfonate damage when compared with wild-type cells. In contrast, Pol beta seems to be dispensable for hydrogen peroxide-induced DNA ssb repair, which is equally efficient in normal and defective cells. By using an in vitro repair assay on single abasic site-containing circular duplex molecules, we show that the long-patch BER is the predominant repair route in Pol beta-null cell extract. Our results strongly suggest that the Pol beta-mediated single nucleotide BER is the favorite pathway for repair of N-methylpurines while oxidation-induced ssb, likely arising from oxidized abasic sites, are the substrate for long-patch BER.

Multiple mutations and frameshifts are the hallmark of defective hPMS2 in pZ189-transfected human tumor cells

Two HeLa variants defective in the mismatch repair protein hPMS2 were isolated by selection for methylation tolerance. Neither variant expressed detectable hPMS2 protein as determined by western blotting. Cell extracts were defective in correcting a single base mispair and were unable to perform mismatch repair-dependent processing of a methylated DNA substrate. Correction of the repair defect and restoration of sensitivity to a methylating agent was achieved by introducing a wild-type copy of chromosome 7 on which the hPMS2 gene is located. Loss of hPMS2 function in the HeLa variants was associated with a 5-fold increase in mutation frequency in the supF gene of the pZ189 shuttle vector. Wild-type levels of mutagenesis were restored by the transferred chromosome 7. Comparisons of mutational spectra identified multiple base substitutions, frameshifts and, to a lesser extent, single base pair changes as the types of mutation which are selectively increased in a hPMS2-defective background. The location of multiple mutations and frameshifts indicates that misalignment-mediated mutagenesis could underlie most of these events. Thus the mutator phenotype associated with loss of hPMS2 most likely arises because of the failure to correct replication slippage errors. Our data also suggest that a considerable fraction of mutagenic intermediates are recognized by the hMutSbeta complex and processed via the hMLH1/hPMS2 heterodimer.

UV mutation signature in tumor suppressor genes involved in skin carcinogenesis in xeroderma pigmentosum patients

Molecular analysis of p53 and patched (PTCH), two candidate tumor suppressor genes for non-melanocytic skin cancer, was performed in skin tumors from six patients affected by the cancer-prone disease xeroderma pigmentosum (XP). UV-specific p53 mutations were detected at a frequency of 38-50% in all the tumor types analysed, including melanomas. Additional analysis of PTCH mutations in the subset of eight basal call carcinomas (BCC) revealed a very high mutation frequency of this gene (90%) which exceeded that detected in the p53 gene in the same tumors (38%). PTCH mutations were predominantly UV-specific C>T transitions. This mutation pattern is different from that reported in BCC from normal donors where PTCH mutation frequency is 27% and mutations are frequently deletions and insertions. These findings suggest that PTCH mutations represent an earlier event in BCC development than p53 alterations and that the inability of XP patients to repair UV-induced PTCH mutations might significantly contribute to the early and frequent appearance of BCC observed in these patients.

Long patch base excision repair with purified human proteins. DNA ligase I as patch size mediator for DNA polymerases delta and epsilon

Among the different base excision repair pathways known, the long patch base excision repair of apurinic/apyrimidinic sites is an important mechanism that requires proliferating cell nuclear antigen. We have reconstituted this pathway using purified human proteins. Our data indicated that efficient repair is dependent on six components including AP endonuclease, replication factor C, proliferating cell nuclear antigen, DNA polymerases delta or epsilon, flap endonuclease 1, and DNA ligase I. Fine mapping of the nucleotide replacement events showed that repair patches extended up to a maximum of 10 nucleotides 3' to the lesion. However, almost 70% of the repair synthesis was confined to 2-4-nucleotide patches and DNA ligase I appeared to be responsible for limiting the repair patch length. Moreover, both proliferating cell nuclear antigen and flap endonuclease 1 are required for the production and ligation of long patch repair intermediates suggesting an important role of this complex in both excision and resynthesis steps.

Factors that influence the DNA repair capacity of normal and skin cancer-affected individuals

DNA repair capacity (DRC) was studied in 49 patients affected by basal cell carcinoma (BCC) and 68 cancer-free controls belonging to a larger case-control population enrolled for studying BCC risk factors. DRC was measured in the subjects' peripheral blood lymphocytes by using a host-cell reactivation assay that measures cellular activation of a reporter gene irradiated with UV light. A statistically significant age-related decline in DRC was observed in the controls from 20 to 70 years of age but not in the BCC cases. When the DRC values of the BCC patients and controls were compared by age, young BCC cases (age, < or =40 year) repaired less than the controls, although the difference was not statistically significant. Conversely, older BCC patients (age, >40 years) presented an enhanced repair capacity (P < 0.001) as compared with their controls. The search for possible factors associated with the high repair rate of elderly BCC cases revealed that both target cell physiology and life-style habits may affect host DNA repair. Smoking was the variable that explained most of the increase in DRC among older patients. The understanding of how these factors affect host DRC will be relevant for a correct use of this biomarker.

The type of DNA glycosylase determines the base excision repair pathway in mammalian cells

The base excision repair (BER) of modified nucleotides is initiated by damage-specific DNA glycosylases. The repair of the resulting apurinic/apyrimidinic site involves the replacement of either a single nucleotide (short patch BER) or of several nucleotides (long patch BER). The mechanism that controls the selection of either BER pathway is unknown. We tested the hypothesis that the type of base damage present on DNA, by determining the specific DNA glycosylase in charge of its excision, drives the repair of the resulting abasic site intermediate to either BER branch. In mammalian cells hypoxanthine (HX) and 1,N6-ethenoadenine (epsilonA) are both substrates for the monofunctional 3-methyladenine DNA glycosylase, the ANPG protein, whereas 7,8-dihydro-8-oxoguanine (8-oxoG) is removed by the bifunctional DNA glycosylase/beta-lyase 8-oxoG-DNA gly- cosylase (OGG1). Circular plasmid molecules containing a single HX, epsilonA, or 8-oxoG were constructed. In vitro repair assays with HeLa cell extracts revealed that HX and epsilonA are repaired via both short and long patch BER, whereas 8-oxoG is repaired mainly via the short patch pathway. The preferential repair of 8-oxoG by short patch BER was confirmed by the low efficiency of repair of this lesion by DNA polymerase beta-deficient mouse cells as compared with their wild-type counterpart. These data fit into a model where the intrinsic properties of the DNA glycosylase that recognizes the lesion selects the branch of BER that will restore the intact DNA template.

Mammalian base excision repair by DNA polymerases delta and epsilon

Two distinct pathways for completion of base excision repair (BER) have been discovered in eukaryotes: the DNA polymerase beta (Pol beta)-dependent short-patch pathway that involves the replacement of a single nucleotide and the long-patch pathway that entails the resynthesis of 2-6 nucleotides and requires PCNA. We have used cell extracts from Pol beta-deleted mouse fibroblasts to separate subfractions containing either Pol delta or Pol epsilon. These fractions were then tested for their ability to perform both short- and long-patch BER in an in vitro repair assay, using a circular DNA template, containing a single abasic site at a defined position. Remarkably, both Pol delta and Pol epsilon were able to replace a single nucleotide at the lesion site, but the repair reaction is delayed compared to single nucleotide replacement by Pol beta. Furthermore, our observations indicated, that either Pol delta and/or Pol epsilon participate in the long-patch BER. PCNA and RF-C, but not RP-A are required for this process. Our data show for the first time that Pol delta and/or Pol epsilon are directly involved in the long-patch BER of abasic sites and might function as back-up system for Pol beta in one-gap filling reactions.

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.

Mutation spectra resulting from carcinogenic exposure: from model systems to cancer-related genes

The events leading to cancer are complex and interactive. Alteration of cancer genes, such as the tumor suppressor gene p53, plays a central role in this process. Analysis of the frequency, type and site of mutations in important cancer-related genes may provide clues to the identification of etiological factors and sources of exposure. In this chapter we have selected a few examples of environmental human carcinogens and have attempted to use the knowledge of their mechanisms of mutagenesis, as derived from in vitro cell systems, as a key to understanding the complexity of p53 mutation spectra in tumors arising at the putative target organ. The analysis will focus on environmental exposure to UV radiation. The examples of tobacco smoke, dietary aflatoxin and vinyl chloride will be also briefly discussed.