Quantification of expression of linked cloned genes in a simian virus 40-transformed xeroderma pigmentosum cell line

Ex vivo Analysis of DNA Repair Capacity of Human Peripheral Blood Mononuclear Cells by a Modified Host Cell Reactivation Assay

The ability of humans to repair DNA damages decreases with increasing age. In order to be able to repair daily occurring DNA damages, it becomes more and more important to preserve repair capability of cells with aging. The preservation of DNA repair processes contributes to preventing DNA mutations and subsequently the onset of age-related diseases such as cancer. For the determination of DNA repair of human cells, mostly in vitro cell cultures are used. However, an ex vivo approach can provide a more accurate result compared with in vitro cell cultures, since the DNA repair ability is measured directly without the influence of prolonged culture time. Published protocols use in vitro cultured cells with a single reporter plasmid or a luciferase reporter. Our modified host cell reactivation assay enables the measurement of DNA repair capacity (nucleotide excision repair) of ex vivo isolated human peripheral blood mononuclear cells (PBMCs). For this purpose, PBMCs are isolated out of human anticoagulated blood by density gradient centrifugation. Directly after isolation, the PBMCs are co-transfected with two plasmids, one being previously damaged by UVC irradiation and one remaining undamaged. PBMCs are incubated for 24 h and subsequently analyzed by fluorescence activated cell sorting (FACS). The ability of cells to repair the DNA damages leads to a functional reactivation of the reporter gene. The assay presented here provides a solution to determine human DNA repair capacity ex vivo directly out of the human body. Furthermore, it can be used to research the ex vivo influence of different substances on DNA repair capacity of humans.

A modified fluorimetric host cell reactivation assay to determine the repair capacity of primary keratinocytes, melanocytes and fibroblasts

BACKGROUND

The Host Cell Reactivation Assay (HCRA) is widely used to identify circumstances and substances affecting the repair capacity of cells, however, it is restricted by the transfection procedure used and the sensitivity of the detection method. Primary skin cells are particularly difficult to transfect, and therefore sensitive methods are needed to detect any variations due to the cell-type or inter-individual differences or changes induced by diverse substances.A sensitive and repeatable method to detect the repair capacity of skin cells would be useful in two different aspects: On the one hand, to identify substances influencing the repair capacity in a positive manner (these substances could be promising ingredients for cosmetic products) and on the other hand, to exclude the negative effects of substances on the repair capacity (this could serve as one step further towards replacing or at least reducing animal testing).

RESULTS

In this paper, we present a rapid and sensitive assay to determine the repair capacity of primary keratinocytes, melanocytes and fibroblasts based on two wave-length Green Fluorescent Protein (GFP) and DsRed reporter technology in order to test different substances and their potential to influence the DNA repair capacity. For the detection of plasmid restoration, we used FACS technology, which, in comparison to luminometer technology, is highly sensitive and allows single cell based analysis.The usefulness of this assay and studying the repair capacity is demonstrated by the evidence that DNA repair is repressed by Cyclosporin A in fibroblasts.

CONCLUSIONS

The methodology described in this paper determines the DNA repair capacity in different types of human skin cells. The described transfection protocol is suitable for the transfection of melanocytes, keratinocytes and fibroblasts, reaching efficacies suitable for the detection of the restored plasmids by FACS technology. Therefore the repair capacity of different cell types can be compared with each other. The described assay is also highly flexible, and the activity of other repair mechanisms can be determined using modifications of this method.

Use of a simian virus 40-based shuttle vector to analyze enhanced mutagenesis in mitomycin C-treated monkey cells

When monkey cells were treated with mitomycin C 24 h before transfection with UV-irradiated pZ189 (a simian virus 40-based shuttle vector), there was a twofold increase in the frequency of mutations in the supF gene of the vector. These results suggest the existence of an enhancible mutagenesis pathway in mammalian cells. However, DNA sequence analysis of the SupF- mutants suggested no dramatic changes in the mechanisms of mutagenesis due to mitomycin C treatment of the cells.

Use of a simian virus 40-based shuttle vector to analyze enhanced mutagenesis in mitomycin C-treated monkey cells

When monkey cells were treated with mitomycin C 24 h before transfection with UV-irradiated pZ189 (a simian virus 40-based shuttle vector), there was a twofold increase in the frequency of mutations in the supF gene of the vector. These results suggest the existence of an enhancible mutagenesis pathway in mammalian cells. However, DNA sequence analysis of the SupF- mutants suggested no dramatic changes in the mechanisms of mutagenesis due to mitomycin C treatment of the cells.

Random isolation of gene activator elements from the human genome

Long-range-acting gene activator elements were randomly isolated from the human genome by functional selection. HeLa cells were transfected with an enhancer trap, a plasmid containing an enhancerless xanthine-guanosine phosphoribosyltransferase (gpt) gene transcribed from the simian virus 40 early promoter, and stably transformed GPT+ cells were selected. From several transformants, human DNA sequences flanking the enhancer trap were cloned. Two gene activators (GA1 and GA2) were found in the cloned human DNAs. GA1 and GA2 showed strong enhancer activity both in a stable transformation assay and in a transient expression assay. They had functional properties similar to those of other known enhancers: GA1 and GA2 activated the expression of a linked gene over distances of at least 5 kilobases both upstream and downstream in an orientation-independent fashion. GA1 may be required for the initial establishment of gene activation but was not essential for the maintenance of active expression. GA1 and GA2 were active not only in HeLa cells but also in other types of human cells, such as neuroblastoma cells. This indicates a limited but relatively broad cell type specificity. The HeLa genome contains multiple copies of GA1, while GA2 exists once in the genome.

Random isolation of gene activator elements from the human genome

Long-range-acting gene activator elements were randomly isolated from the human genome by functional selection. HeLa cells were transfected with an enhancer trap, a plasmid containing an enhancerless xanthine-guanosine phosphoribosyltransferase (gpt) gene transcribed from the simian virus 40 early promoter, and stably transformed GPT+ cells were selected. From several transformants, human DNA sequences flanking the enhancer trap were cloned. Two gene activators (GA1 and GA2) were found in the cloned human DNAs. GA1 and GA2 showed strong enhancer activity both in a stable transformation assay and in a transient expression assay. They had functional properties similar to those of other known enhancers: GA1 and GA2 activated the expression of a linked gene over distances of at least 5 kilobases both upstream and downstream in an orientation-independent fashion. GA1 may be required for the initial establishment of gene activation but was not essential for the maintenance of active expression. GA1 and GA2 were active not only in HeLa cells but also in other types of human cells, such as neuroblastoma cells. This indicates a limited but relatively broad cell type specificity. The HeLa genome contains multiple copies of GA1, while GA2 exists once in the genome.

Restricted ultraviolet mutational spectrum in a shuttle vector propagated in xeroderma pigmentosum cells

A shuttle vector plasmid, pZ189, carrying a bacterial suppressor tRNA marker gene, was treated with ultraviolet radiation and propagated in cultured skin cells from a patient with the skin-cancer-prone, DNA repair-deficient disease xeroderma pigmentosum and in repair-proficient cells. After replication in the human cells, progeny plasmids were purified. Plasmid survival and mutations inactivating the marker gene were scored by transforming an indicator strain of Escherichia coli carrying a suppressible amber mutation in the beta-galactosidase gene. Plasmid survival in the xeroderma pigmentosum cells was less than that of pZ189 harvested from repair-proficient human cells. The point-mutation frequency in the 150-base-pair tRNA marker gene increased up to 100-fold with ultraviolet dose. Sequence analysis of 150 mutant plasmids revealed that mutations were infrequent at potential thymine-thymine dimer sites. Ninety-three percent of the mutant plasmids from the xeroderma pigmentosum cells showed G X C----A X T transitions, compared to 73% in the normal cells (P less than 0.002). There were significantly fewer transversions (P less than 0.002) (especially G X C----T X A) and multiple base substitutions (P less than 0.00001) than when pZ189 was passaged in repair-proficient cells. The subset of mutational changes that are common to ultraviolet-treated plasmids propagated in both repair-proficient and xeroderma pigmentosum skin cells may be associated with the development of ultraviolet-induced skin cancer in humans.

One pyrimidine dimer inactivates expression of a transfected gene in xeroderma pigmentosum cells

We have developed a host cell reactivation assay of DNA repair utilizing UV-treated plasmid vectors. The assay primarily reflects cellular repair of transcriptional activity of damaged DNA measured indirectly as enzyme activity of the transfected genes. We studied three plasmids (pSV2cat, 5020 base pairs; pSV2catSVgpt, 7268 base pairs; and pRSVcat, 5027 base pairs) with different sizes and promoters carrying the bacterial cat gene (CAT, chloramphenicol acetyltransferase) in a construction that permits cat expression in human cells. All human simian virus 40-transformed cells studied expressed high levels of the transfected cat gene. UV treatment of the plasmids prior to transfection resulted in differential decrease in CAT activity in different cell lines. With pSV2catSVgpt, UV inactivation of CAT expression was greater in the xeroderma pigmentosum group A and D lines (D0 = 56 J X m-2) than in the other human cell lines tested (normal, ataxia-telangiectasia, Lesch-Nyhan, retinoblastoma)(D0 = 680 J X m-2)(D0 is the dose that reduces the percentage of CAT activity by 63% along the exponential portion of the dose-response curve). The D0 of the CAT inactivation curve was 50 J X m-2 for pSV2cat and for pRSVcat in the xeroderma pigmentosum group A cells. The similarity of the D0 data in the xeroderma pigmentosum group A cells for three plasmids of different size and promoters implies they all have similar UV-inactivation target size. UV-induced pyrimidine dimer formation in the plasmids was quantified by assay of the number of UV-induced T4 endonuclease V-sensitive sites. In the most sensitive xeroderma pigmentosum cells, with all three plasmids, one UV-induced pyrimidine dimer inactivates a target of about 2 kilobases, close to the size of the putative CAT mRNA.