Transformation of DNA repair-deficient human diploid fibroblasts with a simian virus 40 plasmid

Fibroblasts from patients with xeroderma pigmentosum (XP) complementation groups A, C, D, E, and G, as well as Bloom syndrome (BS) and Fanconi anemia (FA) have been transfected with a plasmid, pSV7, containing the early region of Simian virus 40 (SV40). All of the cultures exhibited cytologic changes characteristic of transformed cells and expressed T-antigen. They also contained integrated copies of DNA derived from the vector, and in several cases, extrachromosomally replicated DNA. Not all of the transfected cultures became immortalized. The transformed xeroderma pigmentosum (XP) cultures retained their UV-sensitive phenotype in all but one case. The BS and FA cell lines retained their characteristic phenotype. All of the cultures, except the BS cells, can be readily transfected with the plasmids, pSV2neo and pSV2gpt.     

Interspecies complementation analysis of xeroderma pigmentosum and UV-sensitive chinese hamster cells

Complementation analysis was performed 24 h after fusion of UV-sensitive CHO cells (CHO 12 RO) with XP cells of complementation groups A, B, C, D, F and G. The parental cells are characterized by low levels of unscheduled DNA synthesis (UDS). In all combinations, the UDS levels observed in heterokaryons were higher than those in parental mutant cells, clearly indicating cooperation of human and Chinese hamster repair functions. In heterokaryons of CHO 12 RO with XP-A and XP-C cells, the UDS values reached about the normal human level, whereas in heterokaryons with XP-B, XP-D and XP-F, UDS was restored at a level approaching that in wild-type CHO cells. The results obtained after fusion of CHO cells with two representative cell strains from the XP-G group, XP 2 BI and XP 3 BR, were inconsistent. Fusion with XP 3 BR cells yielded UDS levels ranging from wild-type Chinese hamster to normal human, whereas fusion with XP 2 BI cells resulted in a slight increase in UDS which even after 48 h remained below the level found in wild-type CHO cells. The occurrence of complementation in these interspecies heterokaryons indicates that the genetic defect in the CHO 12 RO cells is different from the defects in the XP complementation groups tested.     

Transformation and immortalization of diploid xeroderma pigmentosum fibroblasts

Diploid xeroderma pigmentosum (XP) skin fibroblast strains from various XP-complementation groups (B, C, G, and H) were transformed with an origin-defective SV40 early region or with the pSV3 gpt plasmid. In the latter case, transfected cells were selected for their ability to express the dominant xgpt gene. Immortalized cell lines were obtained, from XP-complementation groups C (8CA, 3MA, and 20MA; XP3MA and XP20MA were formerly considered to belong to complementation group I), G (2BI and 3BR), and H (2CS). No immortalized cells could be isolated from complementation group B (11BE). The immortalization frequency of wild-type diploid fibroblasts and diploid cultures from XP patients was not significantly increased by cotransfection with the SV40 early region plus several selected viral and cellular oncogenes. In fact, co-transfection with some of the oncogenes caused a marked decrease of the transformation frequency. The observed immortalization occurred at a frequency of approximately 5 x 10(-8).     

Lack of complementation between xeroderma pigmentosum complementation groups D and H

Summary The construction of permanent hybrid cell lines between xeroderma pigmentosum (XP) cells from different complementation groups allows analysis not only of the degree of repair correction but also of the restoration of biological activity to the UV-irradiated cells. With use of an immortal human cell line (HD2) that expresses excision repair defects typical of XP group D, a series of permanent hybrid cells has been produced with XP cells from groups A to H. Excision repair, as measured by incision analysis and unscheduled DNA synthesis, is restored to normal or near normal levels in crosses involving HD2 and cells from XP groups A, B, C, E, F, G, and I. All these hybrids show complementation for the recovery of normal UV restistance. As expected, hybrids expressing poor incision and hypersensitivity to UV were produced in crosses between HD2 and XPD fibroblasts, but they were also produced without exception when XPH was the partner. In the permanent HD2 x XPD or XPH hybrids, analysis of incision capacity reveals abnormally low activity and therefore that there has been no complementation. The true hybrid nature of HD2 x XPH cells has been confirmed by HL-A and -B tissue typing; moreover, detailed kinetic analysis of incision in these cells shows that the XPH phenotype, rather than the XPD, is expressed, i.e. breaks accumulate at low UV fluence of 1 J/m². To help confirm these findings, another immortal XPD cell line was used in fusions involving HD2, XPH, or XPI. Cells resistant to ultraviolet were produced only with XPI fibroblasts. These data are discussed in terms of whether XPD and H mutations are likely to be allelic with respect to incision.     

Defect in UV-induced unscheduled DNA synthesis in cultured epidermal keratinocytes from xeroderma pigmentosum

DNA repair synthesis in 8 explant-outgrowth cultures of epidermal cells isolated from variant and complementation groups A and E of xeroderma pigmentosum (XP) was examined by measuring unscheduled DNA synthesis (UDS) on autoradiographs. The extents of UDS in XP epidermal cells were compared with those in normal epidermal cells obtained from 26 subjects. In both normal and XP epidermal cells, UDS was induced dose-dependently by radiation at doses of 5-20 J/m2. XP epidermal cells showed various extents of defect in DNA repair depending on the type of XP. In XP-A, the extent of UDS in epidermal cells was very low, being seen in only 3-10% of the normal epidermal cells. But epidermal cells isolated from XP-E and XP-variants exhibited relatively high levels of residual DNA repair; i.e., 69-84% of the control in XP-E and 67-85% in XP-variant. The extents of UDS in XP epidermal cells were almost the same as those in fibroblastic cells isolated from the same specimens.     

Transient correction of excision repair defects in fibroblasts of 9 xeroderma pigmentosum complementation groups by microinjection of crude human cell extracts

Crude extracts from human cells were microinjected into the cytoplasm of cultured fibroblasts from 9 excision-deficient xeroderma pigmentosum (XP) complementation groups. The level of UV-induced unscheduled DNA synthesis (UDS) was measured to determine the effect of the extract on the repair capacity of the injected cells. With a sensitive UDS assay procedure a (transient) increase in UV-induced UDS level was found in fibroblasts from all complementation groups after injection of extracts from repair-proficient (HeLa) or complementing XP cells (except in the case of XP-G), but not after introduction of extracts from cells belonging to the same complementation group. This indicates that the phenotypic correction is exerted by complementation-group-specific factors in the extract, a conclusion that is in agreement with the observation that different levels of correction are found for different complementation groups. The XP-G-correcting factor was shown to be sensitive to proteolytic degradation, suggesting that it is a protein like the XP-A factor.     

Repair of 4-nitroquinoline-1-oxide-induced DNA damage in normal human cells and cells from classical and variant xeroderma pigmentosum

The effect of 4-nitroquinoline-1-oxide (4NQO) upon 3 fibroblast cell lines derived from normal and xeroderma pigmentosum subjects have been compared. Excision-deficient XP cells (XP2BI), complementation group G, are nearly 200-fold more sensitive than normal cells to the lethal effect of 4NQO while XP variants (XP7TA), are 2-fold more sensitive. This cytotoxicity correlates with the levels of unscheduled DNA synthesis performed by the 3 cell lines. 4NQO causes a dose-related inhibition of DNA replication in all cell lines. However, newly replicated DNA synthesised immediately after treatment of cells with 4NQO is slightly smaller in XP7TA variant cells than in normal cells receiving the same dose of 4NQO, but DNA fragments in excision-deficient XP2BI are 50% smaller. It is likely that replicon elongation and joining together of newly replicated DNA fragments is dependent upon the excision of certain 4NQO-induced lesions, possibly normally repaired by a 'short-patch' repair process defective in XP2BI.     

Repair of damaged DNA by extracts from a xeroderma pigmentosum complementation group A revertant and expression of a protein absent in its parental cell line.

Cells derived from individuals with mutations in the xeroderma pigmentosum complementation group A gene (XP-A gene) are hypersensitive to UV light and have a severe defect in nucleotide excision repair of damaged DNA. UV-resistant revertant cell lines can arise from XP-A cells in culture. Cells of one such revertant, XP129, were previously shown to remove (6-4) photoproducts from irradiated DNA, but to have poor repair of cyclobutane pyrimidine dimers. To analyze the biochemical nature of the reversion, whole cell extracts were prepared from the SV40-immortalized fibroblast cell lines XP12RO (an XP-A cell line), the revertant XP129 (derived from XP12RO), and 1BR.3N (from a normal individual). The ability of extracts to carry out repair synthesis in UV-irradiated DNA was examined, and immunoblots were performed using antiserum that recognizes XP-A protein. XP12RO extracts exhibited a very low level of repair and no detectable XP-A protein, but repair activity could be conferred by adding purified XP-A protein to the reaction mixture. XP129 extracts have essentially normal repair synthesis consistent with the observation that most repair of UV-irradiated DNA by extracts appears to occur at (6-4) photoproducts. An XP-A polypeptide of normal size was present in XP129, but in reduced amounts. The results indicate that in XP129 a mutational event has converted the inactive XP12RO XP-A gene into a form which expresses an active XP-A protein.     

Development and characterization of new immortalized human breast cell lines

New human breast cell lines were developed from metastatic breast cancer tissues and normal breast tissues. Primary cultures were initiated from cellular outgrowths of explanted tissues or from mechanically isolated cells in two serum-free media. Cell cultures derived from both cancer and normal tissues were immortalized with pRSV-T plasmid to generate permanent breast cell lines that exhibited an epithelial morphology. Cell lines generated in this study were characterized with respect to morphology, growth rate, karyotype, presence of specific genes, and the expression of epithelial and breast markers. The cell lines expressed the epithelial cell markers, cytokeratins 8 and 18, and retained the capacity to produce human milk fat globulin. They also express the BRCA-1, erbB2, and EGF receptor genes and possess the H-ras, K-ras, and p53 genes. Preliminary data showed that one of the new cancer cell lines was highly sensitive to the cytotoxic action of taxol. It is envisioned that the new breast cell lines will be useful as targets for identification of therapeutic agents against breast cancer and as models for carcinogenesis studies. This revised version was published online in June 2006 with corrections to the Cover Date.     

Suppression of UV-induced apoptosis by the human DNA repair protein XPG

The severe xeroderma pigmentosum/Cockayne syndrome (XP/CS) syndrome is caused by mutations in the XPB, XPD and XPG genes that encode the helicase subunits of TFIIH and the 3' endonuclease of nucleotide excision repair (NER). Because XPB and XPD have been implicated in p53-mediated apoptosis, we examined the possible involvement of XPG in this process. After ultraviolet light (UV) irradiation, primary fibroblasts of XP complementation group G (XP-G) individuals with CS enter apoptosis more readily than other NER-deficient cells, but this is unlinked to unrepaired damage. These XP-G/CS cells accumulate p53 post-UV but they fail to accumulate the 90/92 kDa isoforms of Mdm2 and their cellular distribution of Mdm2 is impaired. Apoptosis levels revert to wild type, Mdm2 90/92 kDa isoforms accumulate, and Mdm2 regains its normal post-UV nuclear location in transduced XP-G/CS cells expressing wild-type XPG, but not an XPG catalytic site mutant. These results suggest that XPG suppresses UV-induced apoptosis and that this suppression, most simply, requires its endonuclease function.     

Human uracil DNA N-glycosidase: studies in normal and repair defective cultured fibroblasts.

Uracil DNA N-glycosidase, an enzyme which participates in the excision of uracil from DNA, was measured in extracts from fibroblasts lines cultured from normal subjects, from several subjects with the genetic disease xeroderma pigmentosum, and from a subject with ataxia telangiectasia. The cell lines representative of complementation groups A and D of xeroderma pigmentosum and of ataxia telangiectasia had roughly the same level of activity as did the normal cells. On the other hand, cells from two xeroderma pigmentosum variants (XP4BE and XP13BE) had roughly half the normal level of activity, and cells from the heterozygous mother of XP4BE had an intermediate level of activity. In spite of these quantitative differences, no systematic alterations in reaction characteristics, apparent Km for substrate, or purification characteristics were noted for enzyme from any of the lines. Thus a causal relationship, if any, between levels of activity and the disease symptoms is equivocal.     

Complementation analysis of xeroderma pigmentosum variants

DNA repair after UV exposure was studied in multinucleate cells, obtained after fusion of excision-defective and variant xeroderma pigmentosum fibroblasts. Optimal fusion conditions were determined, facilitating the measurement of DNA replication in heterokaryons. In unirradiated multikaryons, entry into the S phase was depressed, when compared with unfused cells. The extent of the depression of S phase entry was dependent on the fusion conditions. In heterokaryons obtained after fusion of XP variant (6 different strains) with excision-defective XP (three cell strains from complementation groups A, C and D) both unscheduled DNA synthesis and postreplication repair after UV irradiation were restored to normal levels. In contrast, complementation was not observed after pairwise fusion of the XP variant cell strains. These results suggest that the XP variants comprise a single complementation group, different from complementation groups A, C and D.     

Unscheduled DNA synthesis in xeroderman pigmentosum cells after microinjection of yeast photoreactivity enzyme

Photoreactivating enzyme (PRE) from yeast causes a light-dependent reduction of UV-induced unscheduled DNA synthesis (UDS) when injected into the cytoplasm of repair-proficieint human fibroblasts (Zwetsloot et al., 1985). This result indicates that the exogenous PRE monomerizers UV-induced dimers in these cells competing with the endogenous excision repair. In this paper we present the results of the injection of yeast PRE on (residual) UDS in fibroblasts from different excision-deficient XP-strains representing complementation groups A, C, D, E, F, H and I (all displaying more than 10% of the UDS of wild-type cells) and in fibroblasts from two excision-proficient XP-variant strains.In fibroblasts belonging to complementation groups C, F and I and in fibroblasts from the XP-variant strains UDS was significantly reduced, indicating that pyrimidine dimers in these cells are accessible to and can be monomerized by the injected yeast PRE. The UDS reduction in the XP-variant strains is comparable with the effect in wild-type cells. In cells from complementation groups C, F and I the reduction is less than in wild-type and XP-variant cells. Fibroblasts belonging to groups A, D, E and H did not show any reduction in UDS level after PRE injection and illumination with photoreactivating light. These result give evidence that the genetic repair defect in some XP-strains is probably due to an altered accessibility of the UV-damaged sites.     

Intermolecular plasmid recombination in fibroblasts from humans with DNA damage-processing defects

We have evaluated the ability of immortalized human fibroblasts to recombine transfected plasmid DNA. A number of cell lines from normal individuals and from patients with DNA damage-processing defects were examined. Two plasmid recombination substrates were derived from pSV2neo and contained nonoverlapping deletions in the aminoglycoside phosphotransferase II gene. Intermolecular recombination was assessed by two methods after cotransfection. In a short-term, extrachromosomal recombination assay, low molecular weight DNA was extracted from the human cells 48 h after transfection, and recombinant plasmids were detected by transformation into appropriate indicator bacteria. In a long-term stable recombination assay the fibroblasts were cotransfected and G418-resistant colonies allowed to form. By the former assay all but two cultures were recombination-proficient, whereas all were recombination-proficient by the latter assay. The efficiency of transfection of human cells with plasmids appears to be a major variable affecting recombination. Recombination can be stimulated by uv irradiation of plasmid DNA prior to transfection. Cells from patients with Fanconi anemia, ataxia telangiectasia, and xeroderma pigmentosum complementation groups A, C, D, E, and G are not defective at intermolecular plasmid recombination.     

Repair of DNA damage after exposure to 4-nitroquinoline-1-oxide in heterokaryons derived from xeroderma pigmentosum cells

Xeroderma pigmentosum (XP) cells are dificient in the repair of damage induced by ultraviolet irradiation. Excision-repair-deficient XP cell strains have been classified into 7 distinct complementation groups, according to results of studies on cell fusion and UV irradiation. XP cells are not only abnormally sensitive to UV, but also to a variety of chemical carcinogens, including 4-nitroquinoline-1-oxide (4NQO). Complementation analysis with XP strains from 4 different complementation groups with respect to the repair of 4NQO-induced DNA damage revealed that the classification of the strains into complementation groups with respect to 4NQO-induced repair coincides with the classification based on the repair of UV damage.     

Host-cell reactivation of UV-irradiated and chemically-treated herpes simplex virus-1 by xeroderma pigmentosum, XP heterozygotes and normal skin fibroblasts

The host-cell reactivation of UV-irradiated and N-acetoxy-2-acetylamino-fluorene-treated herpes simplex virus type 1 strain MP was studied in normal and xeroderma pigmentosum human skin fibroblasts. Virus treated with either agent demonstrated lower survival in XP cells from complementation groups A, B, C and D than in normal fibroblasts. The relative reactivation ability of XP cells from the different genetic complementation groups was found to be the same for both irradiated and chemically treated virus. In addition, the inactivation kinetics for virus treated with either agent in the XP variant were comparable to that seen in normal skin fibroblasts. The addition of 2 or 4 mmoles caffeine to the post-infection assay medium had no effect on the inactivation kinetics of virus treated by either agent in the XP variant or in XP cells from the different genetic complementation groups. Treatment of the virus with nitrogen mustard resulted in equivalent survival in normal and XP genetic complementation group D cells. No apparent defect was observed in the ability of XP heterozygous skin fibroblasts to repair virus damaged with up to 100 μg N-acetoxy-2-acetylaminofluorene per ml. These findings indicate that the repair of UV-irradiated and N-acetoxy-2-acetylaminofluorene-treated virus is accomplished by the same pathway or different pathways sharing a common intermediate step and that the excision defect of XP cells plays little if any role in the reactivation of nitrogen mustard treated virus.     

Efficient repair of cyclobutane pyrimidine dimers at mutational hot spots is restored in complemented Xeroderma pigmentosum group C and trichothiodystrophy/xeroderma pigmentosum group D cells

Xeroderma pigmentosum (XP) and trichothiodystrophy (TTD) are rare heritable diseases. Patients suffering from XP and 50% of TTD afflicted individuals are photosensitive and have a high susceptibility to develop skin tumors. One solution to alleviating symptoms of these diseases is to express the deficient cDNAs in patient cells as a form of gene therapy. XPC and TTD/XPD cell lines were complemented using retroviral transfer. Expressed wild-type XPC or XPD cDNAs in these cells restored the survival to UVC radiation to wild-type levels in the respective complementation groups. Although complemented XP cell lines have been studied for years, data on cyclobutane pyrimidine dimer (CPD) repair in these cells at different levels are sparse. We demonstrate that CPD repair is faster in the complemented lines at the global, gene, strand specific, and nucleotide specific levels than in the original lines. In both XPC and TTD/XPD complemented lines, CPD repair on the non-transcribed strand is faster than that for the MRC5SV line. However, global repair in the complemented cell lines and MRC5SV is still slower than in normal human fibroblasts. Despite the slower global repair rate, in the complemented XPC and TTD/XPD cells, almost all of the CPDs at "hotspots" for mutation in the P53 tumor database are repaired as rapidly as in normal human fibroblasts. Such evaluation of repair at nucleotide resolution in complemented nucleotide excision repair deficient cells presents a crucial way to determine the efficient re-establishment of function needed for successful gene therapy, even when full repair capacity is not restored.     

Elevation of dCTP pools in xeroderma pigmentosum variant human fibroblasts alters the effects of DNA repair arrest by arabinofuranosyl cytosine

DNA excision repair inhibition by arabinofuranosyl cytosine (ara-C) or by ara-C/hydroxyurea (HU) was measured in log phase and confluent cultures of normal and xeroderma pigmentosium (XP)-variant human fibroblasts following insult by ultraviolet (UV) light (20 J/m²). Repair inhibition was determined by measuring the accumulation of DNA single-strand breaks/10⁸ daltons following cell culture exposure to ara-C or ara-C/HU in a series of 3 hr. pulses up ro 24 hr. after UV insult. Both normal and XP-variant derived cells showed a wide range of sensitivity to ara-C in log phase cells (0.2–9.4 breaks/10⁸ daltons DNA), although strand break accumulation was constant for each specific cell line. The same cells were more sensitive to ara-C/HU with a 2–14 fold increase in DNA strand breaks depending upon the cell line assayed. In confluent cultures of normal cells, maximum sensitivity to ara-C and ara-C/HU was achieved with similar levels of repair inhibition observed (16.1 and 16.5 breaks/10⁸ daltons, respectively). The same level of repair inhibition was observed in confulent XP-variants receiving ara-C/HU, but was reduced by 62–68% in cells treated with ara-C alone. Ara-C repair arrest was more rapidly reversed by competing concentrations of exogenous deoxycytidine (dCyd) in XP-variant compared to normal cells, especially in confluent cell cultures. In ara-C/HU treated cells, the level of dCyd reversal was reduced in the XP-variant when compared to cells exposed to ara-C alone. However, the same addition of HU had relatively little effect on dCyd reversal in normal cells. The measurements of dNTP levels indicate an elevated level of intracellular deoxycytosine triphosphate in XP-variant vs normal cells. The implications of these results are discussed as they relate to possible excision repair anomalies in the XP-variant.Abbreviations ara-C arabinofuranosul cytosine - dCTP deoxycytosine triphosphate - dCyd deoxycytidine - dNTP deoxynucleoside triphosphate - dT thymidine - HU hydroxyurea - XP xeroderma pigmentosium This research was sponsored jointly by the National Cancer Institute under Interagency Agreement #40-5-63, and the Office of Health and Environment Research, U. S. Department of Energy, under Contract W-7405-eng-26 with the Union Carbide Corporation.