Ultraviolet mutational spectrum in a shuttle vector propagated in xeroderma pigmentosum lymphoblastoid cells and fibroblasts

High γ-radiation sensitivity is associated with increased gastric cancer risk in a Chinese Han population: a case-control analysis

Hypersensitivity to radiation exposure has been suggested to be a risk factor for the development of several malignancies, but not including gastric cancer. In this case-control study, radiation sensitivity as measured by chromatid breaks per cell (b/c) was examined in cultured peripheral blood lymphocytes (PBLs) from 517 patients with gastric cancer and 525 healthy controls. Our results showed that b/c values were significantly higher in cases than in controls (Mean [SD], 0.47 [0.20] vs. 0.34 [0.17]; P<0.001). Using the 50^th percentile value for controls (0.34 b/c) as the cutoff point, unconditional logistic regression analysis revealed that γ–radiation-sensitive individuals were at significantly higher risk for gastric cancer (adjusted odds ratio [OR] 2.01, 95% confidence interval [CI] 1.49–3.13). Quartile stratification analysis indicated a dose-response relationship between γ-radiation sensitivity and gastric cancer risk (P for trend <0.001). When using the subjects in first quartile of b/c values as reference, the adjusted ORs and corresponding CIs for the subjects in second, third, and fourth quartiles were 1.48 (0.91–2.17), 2.42 (1.76–3.64), and 3.40 (2.11–5.29), respectively. The γ-radiation sensitivity was related to age and smoking status. In addition, a clear joint effect on cancer risk was found between γ-Radiation sensitivity and smoking status. The risk for ever smokers with high sensitivity was higher than those for never smokers with high sensitivity and ever smokers with low sensitivity (OR [CI], 4.67 [2.31–6.07] vs. 2.14 [1.40–3.06] vs. 2.42 [1.57–3.95], respectively). No significant interaction was found between both factors (P for interaction  = 0.42). We conclude that chromatid radiosensitivity is associated with gastric cancer susceptibility in a Chinese population.

Reduced DNA repair capacity for removing tobacco carcinogen-induced DNA adducts contributes to risk of head and neck cancer but not tumor characteristics

PURPOSE

Although cigarette smoking and alcohol use are known risk factors for squamous cell carcinoma of head and neck (SCCHN), only a few exposed individuals develop this disease, suggesting an individual susceptibility. In this study, we investigated the associations between genetically determined DNA repair capacity (DRC) for removing tobacco-induced DNA adducts and risk of SCCHN and tumor characteristics.

EXPERIMENTAL DESIGN

We measured DRC in cultured T lymphocytes using the host-cell reactivation assay in a hospital-based case-control study of 744 SCCHN patients and 753 age-, sex-, and ethnicity-matched cancer-free controls recruited from The University of Texas M.D. Anderson Cancer Center.

RESULTS

Patients with SCCHN had significantly lower mean DRC (8.84% +/- 2.68%) than controls (9.97% +/- 2.61%; P < 0.0001), and the difference accounted for approximately 2-fold increased risk of SCCHN [adjusted odds ratio (OR), 1.91; 95% confidence interval (CI), 1.52-2.40] after adjustment for other covariates. Compared with the highest DRC quartile of controls, this increased risk was dose dependent (second highest quartile: OR, 1.40; 95% CI, 0.99-1.98; third quartile: OR, 1.87; 95% CI, 1.34-2.62; and fourth quartile: OR, 2.76; 95% CI, 1.98-3.84, respectively; P(trend) < 0.0001). We also assessed the performance of DRC in risk prediction models by calculating the area of under the receiver operating characteristic curve. The addition of DRC to the model significantly improved the sensitivity of the expanded model. However, we did not find the association between DRC and tumor sites and stages.

CONCLUSION

DRC is an independent susceptibility biomarker for SCCHN risk but not a tumor marker.

Benzo[a]pyrene diol epoxide-induced 9p21 aberrations associated with genetic predisposition to bladder cancer

CDKN2A, 9p21, encodes two alternatively spiked, functionally distinct, tumor-suppressor proteins, P16INK4A and P14ARF, which play active roles in the RB1 and TP53 pathways, respectively. Deletion of 9p is one of the most frequent genomic alterations in bladder cancer. In addition, alterations of 9p21 and P16 are frequently seen in the epithelial cells of chronic smokers. This pilot study evaluated whether 9p21 aberrations induced by exposure in vitro to benzo[a]pyrene diol epoxide (BPDE), the metabolic product of benzo[a]pyrene, a constituent of tobacco smoke, were more common in the peripheral blood lymphocytes of 61 bladder cancer patients compared to 64 matched controls. Our hypothesis was that 9p21 sensitivity to BPDE reflects the susceptibility of a specific locus to damage from carcinogens in tobacco smoke. We found that BPDE-induced chromosome band 9p21 aberrations were significantly higher in lymphocytes of bladder cancer cases (24.97 +/- 5.26 per 1,000) than in controls (20.72 +/- 4.51 per 1,000; P < 0.0001). However, no difference was observed for CEP9, a control locus. After adjustment for age, sex, ethnicity, and smoking status, 9p BPDE sensitivity had an odds ratio (OR) of 9.01 [95% confidence interval (95% CI) 3.75, 21.67] for bladder cancer. We further observed a gradient of elevated bladder cancer risk associated with increasing chromosomal damage. The adjusted ORs for subjects in the second, third, and highest quartiles of BPDE-induced 9p21 aberrations relative to the first quartile were 0.48 (0.04, 5.69), 5.14 (1.12, 23.59), and 21.51 (4.75, 97.34), respectively, providing increasing dose-response evidence of the locus-specific alterations. Thus, 9p21 may be a molecular target for BPDE-induced damage in bladder cancer cases.

Specific chromosome aberrations in peripheral blood lymphocytes are associated with risk of bladder cancer

Specific chromosome aberrations in peripheral blood lymphocytes (PBLs) in chromosomes 9 and 11 may be associated with bladder cancer. To investigate this hypothesis, in this study, we used a whole-chromosome painting technique to detect chromosomal aberrations in PBLs from 100 patients with bladder cancer and 100 matched controls. We also used a locus-specific fluorescence in situ hybridization technique to study 9p21 and cyclin D1 gene (CCND1) aberrations in PBLs of 10 patients and 10 controls and in tumor tissues of 38 additional cases. The chromosome-painting analysis showed that there were more aberrations of chromosomes 9 and 11 in bladder cancer patients than in controls. When categorized by type, the number of deletions of 9p and of translocations of chromosome 11 was significantly higher in patients than in controls (P < 0.05). Stratified analysis showed a larger odds ratio (OR) for bladder cancer in individuals with either a 9p deletion or a chromosome 11 translocation/amplification and an even larger OR in individuals with both aberrations. Using locus-specific analysis, we found that 9p21 aberrations occurred more frequently in bladder cancer patients (12.1 per 1,000 interphase cells) than in controls (6.4 per 1,000 interphase cells, P < 0.05); CCND1 translocation and amplification were observed only in bladder cancer patients. Tumor tissue analysis showed that aberrations of 9p21 (40.0 per 100 interphase cells) and CCND1 (43.8 per 100 interphase cells) were very common. Thus, we concluded that 9p deletions and translocations of chromosome 11, especially at 9p21 and CCND1, are associated with bladder cancer.

Modification of lung cancer susceptibility by green tea extract as measured by the comet assay

Green tea is widely consumed throughout the world and is known to possess various beneficial properties that may affect carcinogen metabolism, free radical scavenging, or formation of DNA adducts. Therefore, it is plausible that green tea extract may modify BPDE-induced DNA damage. In this report, we utilized the comet assay to (1) evaluate BPDE-induced DNA damage as a potential marker of cancer susceptibility and (2) assess the ability of green tea to modify BPDE-induced DNA damage. DNA damage in individual comet cells was quantified by (1) visually measuring the proportion of cells exhibiting migration versus those without and (2) the length of damaged DNA migration (comet tail). We detected a dose-response between BDPE concentration and mean comet tail length in EBV-immortalized lymphoblastiod (lymphoid) cell lines. As the concentration of BPDE increased from 0.5 to 3 microM, the length of the mean comet tail length increased proportionally in the 3590P (derived from a healthy subject) and 3640P (derived from a patient with head and neck cancer) cell lines. In separate experiments using lymphoid cells from 21 lung cancer cases and 12 healthy subjects, the mean comet tail length was significantly higher in the lung cancer cases (80.19 +/- 15.55) versus the healthy subjects (59.94 +/- 14.23) (P < 0.01). Similar findings were observed when analyzing the mean percentage of comet induced cells (84.57 +/- 8.85 and 69.04 +/- 12.50, respectively) (P < 0.01). When green tea extract was added in conjunction with BPDE, there was a notable reduction of the mean comet tail length (13.29 +/- 0.97) as compared to BPDE treatment alone (80.19 +/- 15.55) (P < 0.01) in lung cancer cases. There were no statistical differences between the baseline (no treatments) (12.74 +/- 0.63) and the green tea extract treatment (13.06 +/- 0.97) (P = 0.21). These data suggest the modification of lung cancer susceptibility by the green tea extract. Similar results were observed for the percentage of induced comet cells and the statistical trends were similar for the 12 healthy subjects. This preliminary study demonstrated that the detection of BPDE-induced DNA damage via the comet assay may be a useful biologic marker of lung cancer susceptibility. The differential effects in BPDE-induced DNA damage between lung cancer cases and healthy subjects suggests predisposed cancer susceptibility to lung cancer risk. This reports also demonstrated the chemopreventive effects of green tea extract on BPDE-induced DNA damage. These observations provide further support for the application of the comet assay in molecular epidemiologic studies.

An exploratory analysis of multiple mutation spectra

The last decade has witnessed a remarkable increase in the number of mutations identified both in human disease-related genes and mutation reporter genes including those in mammalian cells and transgenic animals. This has led to the curation of a number of computerised databases, which make mutation data freely available for analysis. A primary interest of both the clinical researcher and the genetic toxicologist is determination of location and types of mutation within a gene of interest. Collections of mutation data observed for a disease-related gene or, for a gene exposed to a particular chemical, permits discovery of regions of sequence along the gene prone to mutagenesis and may provide clues to the origin of a mutation. The principal tool for visualising the distribution pattern of mutant data along a gene is the mutation spectrum: the distribution and frequency of mutations along a nucleotide sequence. In genetic toxicology, the current wealth of mutation data available allows us to construct many mutation spectra of interest to investigate the mutagenic mechanisms and mutational sites for one or a group of mutagens. Using the multivariate statistical methods principal components analysis (PCA) and cluster analysis (CA) we have tested the ability of these methods to establish the underlying patterns within and between 60 UV-induced, mitomycin C-induced and spontaneous mutations in the supF gene. The spectra were derived from human, monkey and mouse cells including both repair efficient and repair deficient cell lines. We demonstrate and support the successful application of multivariate statistical methods for exploring large sets of mutation spectra to reveal underlying patterns, groupings and similarities. The methods clearly demonstrate how different patterns of spontaneous and UV-induced supF mutation spectra can result from variation in plasmid, culture medium, species origin of cell line and whether mutations arose in vivo or in vitro.

Mutation spectra in supF: approaches to elucidating sequence context effects

Shuttle vectors carrying the supF suppressor tRNA gene were originally developed for mutagenesis experiments in primate and human cells. Since then, the supF gene has been used as a mutation reporter in other mammalian cells, yeast, Escherichia coli, and transgenic mice. The widespread use of the vector for studies of many DNA reactive agents has produced a large database of mutation spectra. These provide primary information on the kinds and distribution of mutations provoked by many agents and, in many instances, allow comparisons between related agents or the same agent in different cell backgrounds. In this review we will discuss some of these data with a primary focus on the interpretation of UV mutation spectra. We will also describe our development and application of custom supF marker genes as an approach to studying the effect of sequence context on mutation hotspots and cold spots. Our studies suggest that C-C photoproducts are not mutagenic in certain sequence contexts in which T-C photoproducts are mutation hotspots. In addition, we have found several examples of sequence context effects acting as much as 80 bases away from the site of mutation. We will consider some of the problems raised by these studies and the possible resolution of some of them offered by the newly discovered family of damage bypass DNA polymerases.

The fidelity of DNA polymerase beta during distributive and processive DNA synthesis

During base excision repair, DNA polymerase beta fills 1-6-nucleotide gaps processively, reflecting a contribution of both its 8- and 31-kDa domains to DNA binding. Here we report the fidelity of pol beta during synthesis to fill gaps of 1, 5, 6, or >300 nucleotides. Error rates during distributive synthesis by recombinant rat and human polymerase (pol) beta with a 390-base gap are similar to each other and to previous values with pol beta purified from tissues. The base substitution fidelity of human pol beta when processively filling a 5-nucleotide gap is similar to that with a 361-nucleotide gap, but "closely-spaced" substitutions are produced at a rate at least 60-fold higher than for distributive synthesis. Base substitution fidelity when filling a 1-nucleotide gap is higher than when filling a 5-nucleotide gap, suggesting a contribution of the 8-kDa domain to the dNTP binding pocket and/or a difference in base stacking or DNA structure imposed by pol beta. Nonetheless, 1-nucleotide gap filling is inaccurate, even generating complex substitution-addition errors. Finally, the single-base deletion error rate during processive synthesis to fill a 6-nucleotide gap is indistinguishable from that of distributive synthesis to fill a 390-nucleotide gap. Thus the mechanism of processivity by pol beta does not allow the enzyme to suppress template misalignments.

The molecular basis of xeroderma pigmentosum

BACKGROUND

Xeroderma pigmentosum is an extremely rare, autosomal recessive disease characterized by a more than 1000-fold increase in nonmelanoma skin cancer. Individuals with this disease can be divided into eight complementation groups: A-G and V for variant. Each one represents a different genetic defect in DNA repair.

OBJECTIVE

To review the molecular basis of xeroderma pigmentosum.

RESULTS

Deficiencies in various gene products in the nucleotide excision repair pathway cause xeroderma pigmentosum in complementation groups A-G. The molecular basis of the variant group remains to be elucidated.

CONCLUSIONS

Research into the genetic defects underlying xeroderma pigmentosum have led to an increased understanding of nucleotide excision repair.

UV-induced mutagenesis of human p53 in a vector replicated in Saccharomyces cerevisiae

Mutation of the p53 tumor suppressor gene is the most common genetic alteration identified to date in human cancers. Similarities of p53 mutations found in human cancers with those induced in experimental systems have been interpreted as evidence supporting a causative role for environmental carcinogens in certain tumor types. We have developed and validated a method for generation of mutation spectra and measurement of mutation frequency directly on human p53 cDNA in a vector following treatment with mutagens and replication in yeast. Mutants that had lost the DNA binding/transcription activation function of p53 were detected by yeast colony color, isolated, and sequenced. UV light was used to characterize and validate the system, and a dose-dependent increase in mutation frequency was seen following exposure of the plasmid to increasing doses of UV, resulting in an 18-fold increase over the spontaneous frequency (3.2 x 10(-4)) at the highest level tested (300 J/m2). Sequence analysis of p53 in the mutants revealed that the types of mutations induced were similar to those obtained in previous studies of UV mutagenesis in other model systems, and the types and positions of mutations were also similar to those found in human skin tumors. This experimental system will be useful in further evaluation of the importance of environmental agents as risk factors for cancer.

Lack of gene- and strand-specific DNA repair in RNA polymerase III-transcribed human tRNA genes

UV light induces DNA lesions which are removed by nucleotide excision repair. Genes transcribed by RNA polymerase II are repaired faster than the flanking chromatin, and the transcribed strand is repaired faster than the coding strand. Transcription-coupled repair is not seen in RNA polymerase I-transcribed human rRNA genes. Since repair of genes transcribed by RNA polymerase III has not been analyzed before, we investigated DNA repair of tRNA genes after irradiation of human fibroblasts with UVC. We studied the repair of UV-induced cyclobutane pyrimidine dimers at nucleotide resolution by ligation-mediated PCR. A single-copy gene encoding selenocysteine tRNA, a tRNA valine gene, and their flanking sequences were analyzed. Protein-DNA footprinting showed that both genes were occupied by regulatory factors in vivo, and Northern blotting and nuclear run-on analysis of the tRNA indicated that these genes were actively transcribed. We found that both genes were repaired slower than RNA polymerase II-transcribed genes. No major difference between repair of the transcribed and the coding DNA strands was detected. Transcribed sequences of the tRNA genes were not repaired faster than flanking sequences. Indeed, several sequence positions in the 5' flanking region of the tRNA(Val) gene were repaired more efficiently than the gene itself. These results indicate that unlike RNA polymerase II, RNA polymerase III has no stimulatory effect on DNA repair. Since tRNA genes are covered by the regulatory factor TFIIIC and RNA polymerase III, these proteins may actually inhibit the DNA's accessibility to repair enzymes.

Use of an infectious Simian virus 40-based shuttle vector to analyse UV-induced mutagenesis in monkey cells

SV40 based shuttle vectors able to be packaged as pseudovirions have been used either as naked DNA or as pseudovirus to analyse the mutation frequency and the UV-induced mutation spectra obtained after transfection or infection of COS7 monkey cells. The frequency of supF spontaneous mutants was similar whatever the state of the vector, indicating that the transfection step is not responsible for the high spontaneous mutation frequency when using shuttle vectors. Nevertheless the UV-induced mutation frequency of the supF gene was higher when transfected DNA was replicated into COS7 cells than when pseudovirus infection was performed. The UV induced mutation spectra was basically similar in both situations but a new hot-spot at nucleotide 110 was obtained after pseudovirus infection. UV-pretreated and control COS7 cells were infected with untreated or UV-damaged pi SVPC7 shuttle virus and the survival and the supF mutation frequency were analysed in the progeny. The survival of UV-damaged pseudovirus replicated in 10 J/m2 UV-pretreated cells was 2-fold higher than in untreated cells. This increase in the survival was accompanied by a slight enhancement in the number of supF mutants.

Effects of vacuum UV and UVC radiation on dry Escherichia coli plasmid pUC19. II. Mutational specificity at the lacZ gene

The mutational spectra at the lacZ gene, induced either by vacuum UV at 160 nm or UVC at 254 nm in vacuum-dried preparations of Escherichia coli plasmid pUC19 DNA, have been characterized from 72 E. coli-propagated mutants by DNA sequencing. In plasmids irradiated in vacuum, vacuum UV is five times more mutagenic than UVC. In the UV-induced mutants, base substitutions largely predominate, with GC-->AT (G, guanine; C, cytosine; A, adenine; T, thymine) transitions being the most abundant type of base change for vacuum UV (61%) and UVC (47%). Most of the GC-->AT transitions appear to occur at dipyrimidine sites, which are located at the non-transcribed DNA strand. Some, but not all, hot spots for GC-->AT transitions are identical for vacuum UV and UVC. Frameshifts, resulting from a loss of the thymine residue, are specific for UVC (22%), and were not detected after treatment with vacuum UV. They occur predominantly at thymine runs of the transcribed DNA strand. Only a few deletions were detected following irradiation with vacuum UV (7.5%) and UVC (2%); however, their frequency is not enhanced compared with the spontaneous mutation spectrum. The data confirm the important role of base substitution mutations in UV-induced mutagenesis, which is not only valid for the UVC range, but extends towards the vacuum UV range.

Xeroderma pigmentosum and related disorders: examining the linkage between defective DNA repair and cancer

Xeroderma pigmentosum, Cockayne syndrome, the xeroderma pigmentosum-Cockayne syndrome complex, and trichothiodystrophy cells have defects in DNA repair and are associated with clinical and cellular hypersensitivity to ultraviolet radiation (UV). Familial dysplastic nevus syndrome cells have UV hypermutability. Although xeroderma pigmentosum and dysplastic nevus syndrome have markedly increased cancer risk. Cockayne syndrome and trichothiodystrophy do not. At the molecular level, these disorders are associated with several different genetic defects as evidenced by the existence of multiple overlapping complementation groups. Recent progress has been made in identifying the chromosomal location and cloning the defective genes in these disorders. Using plasmid shuttle vectors we have shown abnormal repair and mutagenesis of DNA damaged by 254-nm (UVC) or 295-nm (UVB) radiation or the chemical carcinogen aflatoxin in cells from patients with xeroderma pigmentosum. Although xeroderma pigmentosum cells are defective in repair of all photoproducts, Cockayne syndrome cells appear to be defective in repair of cyclobutane dimers and have normal repair of nondimer photoproducts. DNS cells have post UV plasmid hypermutability. These diseases may serve as models for examining molecular mechanisms of carcinogenesis in humans.

A potential laboratory test for dysplastic nevus syndrome: ultraviolet hypermutability of a shuttle vector plasmid

The diagnosis of the melanoma-prone disorder dysplastic nevus syndrome (DNS) is based currently on a combination of clinical and histopathologic examinations of patients. To develop a potential laboratory test for DNS, we utilized the observation that an ultraviolet light (UV)-treated mutagenesis plasmid shuttle vector has an abnormally increased frequency of mutations after transfection into lymphoblastoid cells from a patient with familial DNS. pSP189 (containing the bacterial suppressor tRNA gene supF as a marker for mutations and a gene for ampicillin resistance for selection) was treated with UV and transfected into familial DNS, xeroderma pigmentosum complementation group A (XP-A), and normal lymphoblastoid cells by electroporation or diethylaminoethyl (DEAE) dextran. Untreated plasmid pZ189K (containing a gene for kanamycin resistance) was co-transfected as an internal standard to reduce the variability of plasmid survival measurements. After 2 d, plasmids were extracted, used to transform an indicator strain of Escherichia coli, and assayed on plates containing ampicillin or kanamycin. Counting light blue or white colonies (containing mutated supF in the plasmid) and blue colonies (with wild type supF) permitted measurement of the plasmid survival and mutation frequency. Transfection by electroporation or DEAE dextran resulted in abnormally reduced survival of UV-treated plasmid after passage through the XP-A but normal survival in the three DNS lines. Transfection of UV-treated plasmid by DEAE dextran yielded a greater hypermutability with the familial DNS lines than by electroporation. These results suggest that pSP189 UV hypermutability with normal UV survival using DEAE dextran transfection may form the basis of a potential laboratory assay for familial DNS.

Evidence for nontargeted mutagenesis in a monkey kidney cell line and analysis of its sequence specificity using a shuttle-vector plasmid

Intact pZ189 DNA was allowed to replicate in monkey kidney vero cells that had been pretreated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The E. coli MBM7070 was transfected with replicated plasmid, and those with mutations in the supF gene were identified. The frequency of mutants that did not contain recognizable changes in the electrophoretic mobility of the plasmid DNA was scored. The frequency of such mutants was 12.2 x 10(-4) (43/35376) and 6.2 x 10(-4) (22/35712) in mutants derived from cells pretreated with 0.2 mumoles/l and 2 mumoles/l MNNG respectively; these values represent an increase of 5.8- and 2.9-fold over the spontaneous mutation frequency of 2.1 x 10(-4) (10/47741) (p < 0.01). Sequence analysis of the supF genes of these mutants showed that 89% (24/27) of base substitutions occurred at G.C base pairs; 59% of the base substitutions (16/27) were transversions, and 41% (11/27) were transitions. The types of base substitutions were predominantly G.C-->T.A and G.C-->A.T. 48% of base substitutions occurred at 6 sites of the supF gene; 4 of these sites consist of 5'-TTNN where N is G or C. Base substitutions never previously reported were found, namely, T-->C at 61, G-->T at 70, G-->T at 99, and G-->C at 103 were found; these have never been reported up to now. In addition, 2 of the 5 frameshifts occurred in the region 99-105 of the supF gene (GGTGGGG), suggesting that this region is a hot spot for nontargeted frameshifts. These results strongly suggest that nontargeted mutagenesis can occur in mammalian cells and shows that the spectrum of mutations induced differs from that of spontaneous and targeted mutations.

Ultraviolet hypermutability of a shuttle vector propagated in xeroderma pigmentosum variant cells

Patients with the variant form of xeroderma pigmentosum (XP) have clinical XP including a high frequency of skin cancer but, in contrast to the other forms of XP, have normal post-ultraviolet (UV) DNA excision repair and nearly normal post-UV survival. However, like excision repair-deficient XP cells, the XP variant cells are UV hypermutable. We used a UV-treated plasmid shuttle vector, pZ189, to examine the DNA repair defect in lymphoblastoid cells from an XP variant patient, XPPHBE, and a normal control. Plasmid repair, mutagenesis, and replication occur within transfected cells in a process dependent on the cells' repair capacity. With the XP variant cells post-UV, plasmid survival was normal with but there was an abnormally increased post-UV plasmid mutation frequency. Sequence analysis of the mutated plasmids revealed an increased frequency of plasmids with single base substitution mutations with the XP variant cells. As in earlier studies with UV mutagenesis, there was a predominance of G:C-->A:T base substitution mutations with plasmids recovered from both cell lines. The frequency of G:C-->C:G transversions was significantly higher with plasmids recovered from the XP variant cells than from normal cells. The location of mutations in the marker gene was non-random with different mutagenic hotspots found in plasmids recovered from the XP variant cells and from the normal cells. This study suggests that plasmid UV hypermutability in the presence of normal UV survival may be related to the increased UV skin cancer susceptibility of XP variant patients.

Ultraviolet-induced mutations in Cockayne syndrome cells are primarily caused by cyclobutane dimer photoproducts while repair of other photoproducts is normal

We compared the contribution to mutagenesis in Cockayne syndrome (CS) cells of the major class of UV photoproducts, the cyclobutane pyrimidine dimer, to that of other DNA photoproducts by using the mutagenesis shuttle vector pZ189. Lymphoblastoid cell lines from the DNA repair-deficient disorders CS and xeroderma pigmentosum (XP) and a normal line were transfected with UV-treated pZ189. Cyclobutane dimers were selectively removed before transfection by photoreactivation (PR), leaving nondimer photoproducts intact. After UV exposure and replication in CS and XP cells, plasmid survival was abnormally reduced and mutation frequency was abnormally elevated. After PR, plasmid survival increased and mutation frequency in CS cells decreased to normal levels but remained abnormal in XP cells. Sequence analysis of > 200 mutant plasmids showed that with CS cells a major mutational hot spot was caused by unrepaired cyclobutane dimers. These data indicate that with both CS and XP cyclobutane dimers are major photoproducts generating reduced plasmid survival and increased mutation frequency. However, unlike XP, CS cells are proficient in repair of nondimer photoproducts. Since XP but not CS patients have a high frequency of UV-induced skin cancers, our data suggest that prevention of UV-induce skin cancers is associated with proficient repair of nondimer photoproducts.

Protein oxidative damage is associated with life expectancy of houseflies

The objective of this study was to test some of the predictions of the oxidative-stress hypothesis of aging, which postulates that aging is causally associated with the molecular damage inflicted by reactive oxygen species. Protein carbonyl content was used as an index of molecular oxidative modifications. The carbonyl content was found to be associated with the physiological age or life expectancy of flies rather than with their chronological age. Exposure of flies to sublethal hyperoxia (100% oxygen) irreversibly enhanced the carbonyl content of the flies and decreased their rate of oxygen consumption. Results of this study indicate that protein carbonyl content may be a biomarker of aging and support the general concept that oxidative stress may be a causal factor in the aging process.

Human genetic instability syndromes: single gene defects with increased risk of cancer

The experimental findings of the last 5 years are reviewed for the genetic instability syndromes: Xeroderma pigmentosum, Fanconi's anaemia, Ataxia telangiectasia and Bloom's syndrome. In these autosomal recessive genetic diseases, single gene defects lead to genetic instability, increased mutation rates and cancer. Deficiencies in the ability to effectively repair DNA lesions have been suggested for all of these syndromes. The status of characterization of these DNA repair defects is presented and the possible mechanisms of lesion fixation as mutation are discussed. The four known human genes whose mutation leads to inherited genetic instability are described.

In vivo evidence that UV-induced C-->T mutations at dipyrimidine sites could result from the replicative bypass of cis-syn cyclobutane dimers or their deamination products

The major mutations induced by UV light are C-->T transitions at dipyrimidines and arise from the incorporation of A opposite the C of dipyrimidine photoproducts. The incorporation of A has most often been explained by the known preference of a polymerase to do so opposite noninstructional DNA damage such as an abasic site (A rule). There are also mechanisms that suppose, however, that cis-syn dipyrimidine photodimers are instructional. In one such mechanism (tautomer bypass), the incorporation of A is directed by the tautomer of a C of a dimer that is equivalent in base-pairing properties to U [Person et al. (1974) Genetics 78, 1035-1049]. In another mechanism (deamination bypass), the incorporation of A is directed by a U of a dimer that results from the deamination of the C of a dimer [Taylor & O'Day (1990) Biochemistry 29, 1624-1632]. The viability of these mechanisms was tested by obtaining the mutation spectrum of a TU dimer in Escherichia coli by application of a standard method for site-directed mutagenesis. To this end, a 41-mer containing a site-specific TU dimer was constructed via ligation of a dimer-containing decamer that was produced by triplet-sensitized irradiation and used to prime DNA synthesis on a uracil-containing (+) strand of an M13 clone containing a double mismatch opposite the dimer. The reaction mixture was used to transfect a uracil glycosylase proficient, photoproduct repair deficient E. coli host, and all progeny phage weakly hybridizing to the parental (+) or (-) strands were sequenced. Under non-SOS conditions the TU dimer almost completely blocked replication, while under SOS conditions it directed the incorporation of two As with much higher specificity (96%) than would an abasic site. The implications of these results to the mechanism of the UV-induced TC-->TT mutation, and by extension to the CT-->TT, CC-->TC, CC-->CT, and the tandem CC-->TT mutations, are discussed.

Modulation of ultraviolet light mutational hotspots by cellular stress

Stressful treatments of cells provoke broad, transient, changes in cellular physiology and gene expression. In addition to these effects, DNA-damaging agents often induce permanent change in the form of mutations. Mutational patterns in target genes typically show hotspots and coldspots, the molecular basis of which appears to lie in the sequence context of the particular site. We determined the mutational pattern in an ultraviolet light-modified (in vitro) marker gene in a shuttle vector passaged through repair deficient (xeroderma pigmentosum) cells and compared it with patterns obtained from cells exposed to stress imposed by a DNA-damaging agent or a calcium ionophore. We found that the mutational hotspot pattern was altered by both stress treatments. We conclude that the cellular environment can influence the probability of mutagenesis at specific sites and propose that some of these effects on mutagenesis are mediated by alterations in cellular calcium levels.

Ultraviolet mutagenesis in human lymphocytes: the effect of cellular transformation

We have assessed the role of cellular transformation in ultraviolet (uv)-induced mutagenic events in human cells. To maintain uniformity of genetic background and to eliminate the effect of DNA repair, primary nontransformed lymphocytes (T-cells) and Epstein-Barr virus-transformed lymphocytes (B-cells) from one patient (XP12Be) with the DNA repair-deficient disorder xeroderma pigmentosum (group A) were transfected with the mutagenesis shuttle vector pZ189. Parallel control experiments were performed with primary, nontransformed lymphocytes from a normal individual and with a repair-proficient Epstein-Barr virus-transformed lymphocyte line (KR6058). pZ189 was treated with uv and introduced into the four cell lines by electroporation. Plasmid survival and mutations inactivating the marker supF suppressor tRNA gene in the recovered pZ189 were scored by transforming an indicator strain of Escherichia coli. Plasmid survival was reduced and mutation frequency elevated equally with both XP-A cell lines compared to both normal cell lines. Base sequence analysis of more than 250 independent plasmids showed that while the G:C----A:T base substitution mutation was found in at least 60% of plasmids with single or tandem mutations with all four cell lines, the frequency with the transformed XP-A (93%) cells was significantly higher (P less than 0.01) than that with the nontransformed XP-A cells (77%). In addition, with the transformed XP-A cells, there were significantly fewer plasmids with transversions and with mutations at a transversion hotspot (base pair 134) than with plasmids recovered from nontransformed XP-A cells. Interleukin-2 and phytohemagglutinin (used to maintain growth of the nontransformed lymphocytes) treatment of transformed XP12Be cells did not change overall plasmid survival or mutation frequency, but increased the transversion frequency and induced a mutational hotspot (at base pair 159), while another mutational hotspot (at base pair 123) disappeared. Thus we have demonstrated that in repair-deficient human cells, cellular transformation, while not affecting overall postuv plasmid survival and mutation frequency, does increase the susceptibility to G:C----A:T transition mutations, a type of mutation associated with uv-induced neoplasia.

UV-induced base substitution mutations in a shuttle vector plasmid propagated in group C xeroderma pigmentosum cells

To assess the contribution to mutagenesis of human DNA repair defects, the UV-irradiated shuttle vector plasmid pZ189 was propagated in fibroblasts derived from a xeroderma pigmentosum (XP) patient in DNA repair complementation group C. In comparison to results with DNA repair-proficient human cells (WI-38 VA13), UV-irradiated pZ189 propagated in the XP-C (XP4PA(SV)) cells showed fewer surviving plasmids and a higher frequency of mutated plasmids. Base sequence analysis of 67 mutated plasmids recovered from the XP-C cells revealed similar classes of point mutations and mutation spectrum, and a higher frequency of G:C to A:T transitions along with a lower frequency of transversions among plasmids with single or tandem mutations compared to plasmids recovered from the normal line. Most single-base substitution mutations (83%) occurred at G:C base pairs in which the 5'-adjacent base of the cytosine was thymine or cytosine. These results indicate that the DNA repair defects in XP-C, in comparison to data previously reported for XP-A, XP-D and XP-F, result in different UV survival and mutation frequency but in similar types of base substitution mutations.