Effects of XPD mutations on ultraviolet-induced apoptosis in relation to skin cancer-proneness in repair-deficient syndromes.

To understand the relationship between DNA repair, apoptosis, transcription, and cancer-proneness, we have studied the apoptotic response and the recovery of RNA synthesis following ultraviolet C and ultraviolet B irradiation in nucleotide excision repair deficient diploid fibroblasts from the cancer-prone xeroderma pigmentosum (XP) syndrome patients and the non-cancer-prone trichothiodystrophy (TTD) patients. Analysis of four XPD and four TTD/XPD fibroblast strains presenting different mutations on the XPD gene has shown that XPD cells are more sensitive to ultraviolet-induced apoptosis than TTD/XPD cells, and this response seems to be modulated by the type and the location of the mutation on the XPD gene. Moreover, the other xeroderma pigmentosum fibroblast strains analyzed (groups A and C) are more sensitive to undergo apoptosis after ultraviolet irradiation than normal human fibroblasts, showing that the cancer-proneness of xeroderma pigmentosum patients is not due to a deficiency in the ultraviolet-induced apoptotic response. We have also found that cells from transcription-coupled repair deficient XPA, XPD, TTD/XPD, and Cockayne's syndrome patients undergo apoptosis at lower ultraviolet doses than transcription-coupled repair proficient cells (normal human fibroblasts and XPC), indicating that blockage of RNA polymerase II at unrepaired lesions on the transcribed strand is the trigger. Moreover, XPD and XPA cells are more sensitive to ultraviolet-induced apoptosis than trichothiodystrophy and Cockayne's syndrome fibroblasts, suggesting that both cyclobutane pyrimidine dimers and pyrimidine 6-4 pyrimidone on the transcribed strand trigger apoptosis. Finally, we show that apoptosis is directly proportional to the level of inhibition of transcription, which depends on the density of ultraviolet-induced lesions occurring on transcribed sequences.

Relationship between posttranslational modification of transaldolase and catalase deficiency in UV-sensitive repair-deficient xeroderma pigmentosum fibroblasts and SV40-transformed human cells.

Xeroderma Pigmentosum (XP) is a rare recessively inherited human disease associated with a hypersensitivity to ultraviolet radiation. The ultraviolet component of sunlight can initiate and promote the formation of cutaneous tumors as seen in the skin cancer-prone XP patients. Previously, we have found that the low activity of the NADPH-dependent antioxydant enzyme, catalase, which we have observed in XP diploid fibroblasts and SV40-tranformed cells, could be restored by the addition of NADPH. Here we have analyzed transaldolase, which regulates NADPH levels produced by the pentose phosphate pathway in order to examine how it influences the catalase activity regulated in XP and SV40-transformed cells. We find that transaldolase activity is high in XP and SV40-transformed human fibroblasts, whereas transaldolase transcription is unchanged, suggesting that modification of transaldolase activity is due to a posttranslational modification of the protein. Two-dimensional electrophoresis analysis has allowed us to identify a complex set of transaldolase isoforms and to postulate that the phosphorylation of specific isoforms could be correlated with the different enzymatic activities seen. Our results show that high transaldolase activity corresponds to a low catalase activity in SV40-transformed cells and in fibroblasts from XP patients who have a high predisposition to develop skin cancer.

High levels of patched gene mutations in basal-cell carcinomas from patients with xeroderma pigmentosum.

Recently, hptc, a human gene homologous to the Drosophila segment polarity gene patched (ptc), has been implicated in the nevoid basal-cell carcinoma (BCC) syndrome, and somatic mutations of hptc also have been found in sporadic BCCs, the most frequent cancers found in the white population. We have analyzed the hptc gene, postulated to be a tumor suppressor gene, in 22 BCCs from patients with the hyperphotosensitive genodermatosis xeroderma pigmentosum (XP). Patients with XP are deficient in the repair of UV-induced DNA lesions and are characterized by their predisposition to cancers in sun-exposed skin. Analysis using PCR-single-strand conformation polymorphism of the hptc gene identified 19 alterations in 16 of 22 (73%) of the BCCs examined. Only two (11%) deletions of the hptc gene were found in XP BCCs compared with >30% rearrangement observed in non-XP sporadic BCCs, and 17 of 19 (89%) were base substitutions. Among the 17 base substitutions, 11 (65%) were CC --> TT tandem mutations, and 4 (23%) were C --> T substitutions, all targeted at bipyrimidine sites. Hence, a significantly higher number (15 of 19; 79%) of UV-specific alterations are seen in XP tumors, in contrast to non-XP sporadic BCCs. Interestingly, we have found that in 7 of 14 (50%) XP BCCs analyzed, both hptc and the tumor suppressor gene p53 are mutated. Not only have our data indicated the key role played by hptc in the development of BCCs, they also have substantiated the link between unrepaired UV-induced DNA lesions and skin carcinogenesis, as exemplified by the UV-specific alterations of different genes in the same tumors.

p53 mutations in skin and internal tumors of xeroderma pigmentosum patients belonging to the complementation group C.

Fifty-eight skin biopsies and three primary internal tumors from patients affected by the rare hereditary disease xeroderma pigmentosum (XP) were studied by an improved PCR-single strand conformation polymorphism analysis to detect the mutations of the tumor suppressor gene p53. The results from cutaneous XP tumors, including 27 squamous cell carcinomas and 6 basal cell carcinomas, show a very high level (86%) of p53 mutations. The analysis of mutations found in XP skin cancers according to the complementation group of the patients shows that tandem CC-->TT transitions are a characteristic of XP-C patients with a frequency much higher in their skin tumors (85%) compared with tumors in XP patients who do not belong to group C (33%). In all XP-C biopsies, mutations were due to replication of unrepaired DNA lesions on the nontranscribed strand of the p53 gene, substantiating the preferential repair in vivo of the transcribed strand of this gene in human tissues. For the first time, we were able to analyze three primary internal tumors (a neuroendocrine tumor of the thyroid, a gastric adenocarcinoma, and a glioma of the brain) of young XP children. All of them contained one mutation on the p53 gene, which were different from the ones found in the XP skin tumors and could have resulted from unrepaired lesions caused by oxidative damage.

Recovery of the normal p53 response after UV treatment in DNA repair-deficient fibroblasts by retroviral-mediated correction with the XPD gene.

Among the major responses of human cells to DNA damage is accumulation of the p53 tumor suppressor protein, which plays a crucial role as a cell-cycle checkpoint. We have already shown that this response is different in cells from the UV-hypersensitive human syndromes xeroderma pigmentosum (XP) and trichothiodystrophy (TTD), which overlap with each other and arise from mutations in genes involved in nucleotide excision repair. In this paper we report that correction of the repair defect by retroviral-mediated transduction of the wild-type XPD gene in XP-D and TTD/XP-D untransformed primary fibroblasts leads to a normal p53 response in these cells. Thus, the complemented cells, like normal human fibroblasts, require higher UV doses (10 J/m2) for p53 induction than the parental repair-deficient XP-D or TTD/XP-D cells (both mapping at the XPD locus), which accumulate p53 protein at very low UV doses (2.5 and 5 J/m2). The p53 protein levels return to normal 24 h after irradiation when UV-induced lesions have been efficiently repaired by the restored NER activity. These data confirm our earlier results that p53 accumulation following UV treatment is directly related to the presence of unrepaired cyclobutane dimers on the transcribed strand of active genes.

p53 mutations in cutaneous lesions induced in the hairless mouse by a solar ultraviolet light simulator.

We investigated skin lesions induced in hairless SKH:HR1 mice by chronic exposure to a solar ultraviolet light (UV) simulator for alterations of the p53 gene in conserved domains. Mutations of exons 5-8 of the p53 gene in skin lesions were screened in 31 benign skin lesions (hyperplasias), 25 precancerous skin lesions (keratoacanthomas), and 25 malignant skin lesions (squamous cell carcinomas; SCC) by polymerase chain reaction-single-strand conformation polymorphism analysis. Most of the mutations occurred at dipyrimidine sequences located on the nontranscribed strand; the most frequent modifications were C-->T transitions (77%) and CC-->TT tandem mutations (5%); the latter are considered the UV fingerprint. p53 mutations were detected in 3% of the hyperplasias, 12% of the keratoacanthomas, and 52% of the SCCs. Hence, the high frequency of p53 mutations in SCCs compared with keratoacanthomas induced by a solar UV simulator suggested that, in our study, p53 mutations probably occurred as a late event in the skin carcinogenesis progression of SCC. Interestingly, the level of CC-->TT tandem mutations in the SCCs (5%) was similar to that found in SCCs induced in hairless mice by UVB alone. p53 protein was also detected in the different types of skin lesions by immunohistochemical analysis. Thus, our data from hairless mouse skin tumors induced by a solar UV simulator confirmed the major role of UVB-induced DNA damage in skin carcinogenesis and suggested that UVA plays a minor role in bringing about p53 alterations.

Prolonged p53 protein accumulation in trichothiodystrophy fibroblasts dependent on unrepaired pyrimidine dimers on the transcribed strands of cellular genes.

Trichothiodistrophy (TTD), xeroderma pigmentosum (XP), and Cockayne's syndrome (CS) are three distinct human diseases with sensitivity to ultraviolet (UV) radiation affected by mutations in genes involved in nucleotide excision repair (NER). Among the many responses of human cells to UV irradiation, both nuclear accumulation of p53, a tumor suppressor protein, and alterations in cell-cycle checkpoints play crucial roles. The purpose of this study was to define the signals transmitted after UV-C-induced DNA damage, which activates p53 accumulation in TTD/XP-D fibroblasts, and compare this with XP-D cell lines that carry different mutations in the same gene, XPD. Our results showed that p53 was rapidly induced in the nuclei of TTD/XP-D and XP-D fibroblasts in a dose-dependent manner after UV-C irradiation, as seen in XP-A and CS-A fibroblasts, much lower doses being required for the protein accumulation than in normal human fibroblasts, XP variant cells, and XP-C cells. The kinetics of accumulation of p53 and two effector proteins involved in cell-cycle arrest, WAF1 and GADD45, were also directly related to the repair potential of the cells, as in normal human fibroblasts their levels declined after 24 h, the time required for repair of UV-induced lesions, whereas NER-deficient TTD/XP-D cells showed p53, WAF1, and GADD45 accumulation for over 72 h after irradiation. Our results indicate that p53 accumulation followed by transcriptional activation of genes implicated in growth arrest is triggered in TTD/XP-D cells by the persistence of cyclobutane pyrimidine dimers, which are known to block transcription, on the transcribed strands of active genes.

Specific UV-induced mutation spectrum in the p53 gene of skin tumors from DNA-repair-deficient xeroderma pigmentosum patients.

The UV component of sunlight is the major carcinogen involved in the etiology of skin cancers. We have studied the rare, hereditary syndrome xeroderma pigmentosum (XP), which is characterized by a very high incidence of cutaneous tumors on exposed skin at an early age, probably due to a deficiency in excision repair of UV-induced lesions. It is interesting to determine the UV mutation spectrum in XP skin tumors in order to correlate the absence of repair of specific DNA lesions and the initiation of skin tumors. The p53 gene is frequently mutated in human cancers and represents a good target for studying mutation spectra since there are > 100 potential sites for phenotypic mutations. Using reverse transcription-PCR and single-strand conformation polymorphism to analyze > 40 XP skin tumors (mainly basal and squamous cell carcinomas), we have found that 40% (17 out of 43) contained at least one point mutation on the p53 gene. All the mutations were located at dipyrimidine sites, essentially at CC sequences, which are hot spots for UV-induced DNA lesions. Sixty-one percent of these mutations were tandem CC-->TT mutations considered to be unique to UV-induced lesions; these mutations are not observed in internal human tumors. All the mutations, except two, must be due to translesion synthesis of unrepaired dipyrimidine lesions left on the nontranscribed strand. These results show the existence of preferential repair of UV lesions [either pyrimidine dimers or pyrimidine-pyrimidone (6-4) photoproducts] on the transcribed strand in human tissues.

High mutation frequency in ras genes of skin tumors isolated from DNA repair deficient xeroderma pigmentosum patients.

Xeroderma pigmentosum (XP) patients are clinically characterized by a very high incidence of skin cancers on exposed skin, at an early age. XP cells in vitro are strongly deficient in excision-repair and highly mutagenized by UV light. We were, therefore, interested in measuring mutation frequency and in determining mutation spectra in patients' tumors exposed to UV lesions. We chose to look at oncogene activation in skin tumors with the idea that more mutations, particularly of the ras gene family, would be found in XP tumors where lesions remain unrepaired compared to normal individuals. Our results clearly show that more than a 2-fold significantly higher mutation frequency (50%) of the ras genes was found in XP in contrast to control tumors (22%). The majority of the mutations were found at codon 12 of all three ras genes with a preponderance for N-ras in XP samples. The mutation spectra indicate that all mutations found were located opposite pyrimidine-pyrimidine sequences which represent a hot spot for UV-induced DNA lesions. Most of the mutations were of the type expected from studies performed in vitro with model systems. This high mutation frequency in XP was accompanied by a very high level of Ha-ras and c-myc gene amplification and rearrangement. All these data are consistent with a fundamental role of unrepaired UV-induced DNA lesions as an initiating event in human skin tumors on exposed parts of the body.

Comparative study of Epstein-Barr virus and SV40-based shuttle-expression vectors in human repair-deficient cells.

Shuttle vectors and expression vectors have been used in human cells to examine various aspects of DNA repair including effects of DNA damage on mutagenesis, transcription, replication and recombination. A combined shuttle-expression system should provide further advantages for the stable expression of and perhaps selection/rescue strategies for DNA repair genes. We describe 2 such systems. The first is a simian virus 40 (SV40) shuttle system which allows a quasi-stable episomal vector/host relationship in which the shuttle vector may be recovered in extrachromosomal DNA preparations many months after transfection and selection but in which a high proportion of the plasmids rescued in bacteria are heavily mutated and rearranged. Secondly, we describe Epstein-Barr virus-based shuttle-expression vectors which exist as stable, multicopy episomes in human cells. Using a reporter gene and a metal-inducible promoter we have obtained low basal and very high induced expression from episomal vectors in a variety of human cells including xeroderma pigmentosum and ataxia telangiectasia cell lines. This should facilitate many molecular genetic experiments in human cells and may have particular application to molecular cloning, expression and analysis of DNA repair genes.

An immortalized xeroderma pigmentosum, group C, cell line which replicates SV40 shuttle vectors.

We have established and characterized an immortalized xeroderma pigmentosum (XP), group C, cell line. Transformation of the human fibroblasts was carried out with a recombinant plasmid, pLAS-wt, containing SV40 DNA encompassing the entire early region with a defective origin of DNA replication. The transformed XP cell line, XP4PA-SVwt, and the normal transformed fibroblasts AS3-SVwt, both express SV40 T antigen together with enhanced levels of the transformation-associated cellular protein, p53. XP4PA-SVwt retains the XP UV-repair defective phenotype as demonstrated by low levels of unscheduled DNA synthesis and by the reduced survival of irradiated SV40 virus. Analysis of cellular DNA shows a single major, stable, integration site of pLAS-wt in the XP4PA-SVwt cells. The T antigen in these cells supports efficiently the replication of SV40 based shuttle vectors and should prove suitable for the introduction, expression and selection of genes related to DNA repair and to the study of mutagenesis using defined molecular probes.