Correlation of gene expression and transformation frequency with the presence of an enhancing sequence in the transforming DNA

The use of integrating DNA vectors to analyse the molecular defects in ionising radiation-sensitive mutants of mammalian cells including ataxia telangiectasia

Integrating DNA vectors, encoding selectable recombinant genes, were used to assess rejoining and recombination in wild-type mammalian cells and their ionising radiation-sensitive mutants. To provide a simple model of an important radiation-induced lesion - the DNA double-strand break - the vectors were cut with restriction endonucleases at specific single sites. If these breaks were made in the coding sequence of a selectable gene, the fidelity of the rejoin/recombination process could be measured by survival of vector-transformed cells in selective medium. Rejoining was assessed using vectors without internal homologies, while recombination was measured using pairs of fragments or deletion vectors carrying homologous regions. Initial experiments were made with vectors carrying a single selectable gene but, to overcome potential artefacts, 2-gene vectors were then constructed where one gene acts as a linked marker and (unbroken) control for the other (broken) gene. Available data are reviewed to show that, compared to their respective wild-type counterparts: (1) an ataxia telangiectasia (A-T) cell line and the hamster irs1 mutant show a consistent reduction in the fidelity of rejoining double-strand breaks (while the hamster mutants irs2, irs3, xrs series, and EM9 show wild-type fidelity); (2) the hamster EM9 mutant shows a reduction in ability to recombine homologous vector fragments (while the A-T line and probably the xrs mutants show show wild-type abilities); and (3) the xrs mutants show a reduction in overall transformation frequency with vector DNA, whether broken or not, while the other mutants tested show approximately wild-type frequencies. A critical account of the techniques and data is given, together with speculations on the molecular nature of the processes which are defective in these mutants, leading to radiosensitivity.

Examination of vectors with two dominant, selectable genes for DNA repair and mutation studies in mammalian cells

A series of vectors with two dominant selectable genes was constructed for repair and mutation studies following transfer into mammalian cells. The recombinant genes (SV-gpt and HSVtk-neo) were placed in different relative orientations and positions in the vectors. These variables were shown to affect transformation frequency of cells by the vectors especially where one of the genes had a relatively weak expression, modelled by truncating the promoter of the HSVtk-neo gene. The use of two-gene vectors to assess DNA repair was investigated by cutting the SV-gpt gene with a restriction endonuclease and monitoring correct rejoining by selecting for gene activity after transfer into various cell types. In such experiments, selection was first applied for the undamaged HSVtk-neo gene to eliminate transfer artefacts, followed by counterselection for the activity of the damaged SV-gpt gene. The measured frequency of correct rejoining of the damaged gene was found to vary both with the vector construct and with the recipient cell species (Chinese hamster V79 or human transformed fibroblasts). Despite this variation, correct rejoining was found to be consistently lower in radiosensitive (ataxia telangiectasia) human cells than in wild-type human cells, irrespective of the vector construct. In these experiments, some of the transformed cell colonies showed 'sectoring' on exposure to the counterselection, suggesting a slow determination of the fate of transferred DNA. For mutation studies a V79 cell clone carrying a single copy of one of these two-gene vectors was identified and shown to be stably integrated. Mutations of the SV-gpt gene in these cells were isolated while maintaining selection for the HSVtk-neo gene, to attempt to limit mutational loss of the total integrated sequence and provide at least one identifiable junction for analysis of deletion events. Spontaneous and X-ray-induced mutants were identified with a variety of genetic changes, as shown by Southern analysis, from presumed point mutations to deletions and rearrangements of the vector sequence. Rescue of integrated two-gene vector sequences from transformed cells, by recloning in E. coli, was shown to be feasible; thus alterations in transferred DNA can be analysed in detail.

Transformation of UV-hypersensitive Chinese hamster ovary cell mutants with UV-irradiated plasmids

Transfection of UV-hypersensitive, DNA repair-deficient Chinese hamster ovary (CHO) cell lines and parental, repair-proficient CHO cells with UV-irradiated pHaprt-1 or pSV2gpt plasmids resulted in different responses by recipient cell lines to UV damage in transfected DNA. Unlike results that have been reported for human cells, UV irradiation of transfecting DNA did not stimulate the genetic transformation of CHO recipient cells. In repair-deficient CHO cells, proportionally fewer transformants were produced with increasing UV damage than in repair-proficient cells in transfections with the UV-irradiated hamster adenine phosphoribosyltransferase (APRT) gene contained in plasmid pHaprt-1. However, transfection of CHO cells with UV-irradiated pSV2gpt resulted in neither decline in transformation frequencies in repair-deficient cell lines relative to repair-proficient cells nor stimulation of genetic transformation by UV damage in the plasmid. Blot hybridization analysis of DNA samples isolated from transformed cells showed no dramatic changes in copy number or arrangement of transfected plasmid DNA with increasing UV dose. We conclude that the responses of recipient cells to UV-damaged transfecting plasmids depend both on the type of recipient cell and the characteristics of the genetic sequence used for transfection.

The expression of integrated plasmid DNA depends on copy number

The effect of copy number, integration site, and enhancers on the expression of stably integrated exogenous DNA was examined in Chinese hamster cells. Three similar plasmids were constructed with the mouse beta maj-globin promoter fused to the galK gene either with no enhancer or with the SV40 or Harvey sarcoma virus (HaSV) enhancer. Eighteen stable cell lines were obtained and characterized with respect to plasmid copy number and galactokinase activity. At copy numbers of four or less, the enhancers showed detectable activity and a DNase I hypersensitive site was present. Above four copies, gene activity decreased as the copy number increased, the enhancer sequences were apparently inactive, and the DNase I hypersensitive site disappeared. These data suggest that, at least in this model system, when exogenous DNA is integrated as multiple head-to-tail copies, the entire multigene unit expresses poorly and inappropriately. When the same exogenous DNA integrates as a single (or low number) copy, expression appears to be relatively normal as judged by enhancer stimulation and DNase I hypersensitivity.

Comparison of the transcriptional properties of the Friend and Moloney retrovirus long terminal repeats: importance of tandem duplications and of the core enhancer sequence

The effects of sequence differences within the long terminal repeats (LTRs) of various murine retroviruses on transcription are examined by linking genetically engineered recombinant LTRs to the protein coding region of the herpes simplex virus thymidine kinase (TK) gene and assaying TK expression in tissue culture fibroblasts. The Goldberg/Hogness box region and the enhancer region are examined independently. We find that the Friend and Moloney Goldberg/Hogness boxes (TATAAAA and AATAAAA, respectively) give similar results in this assay, whereas differences between the sequences in the enhancer region have a marked effect. Tandem duplications increase the transcription level of the LTR. A single nucleotide transition in the core enhancer sequence has as large an effect on transcription as the presence of a tandem duplication: thymidine in the fifth position of the core enhancer (TCTGTGGTAAG) leads to a much higher transcriptional activity than cytidine. The LTRs have been implicated by others in the oncogenic potential of murine retroviruses, which is perhaps dependent on the transcriptional properties endowed in part by the core enhancer and tandem duplications.

Shuttling of integrated vectors from mammalian cells to E. coli is mediated by head-to-tail multimeric inserts

With the aim of producing nonviral shuttle vectors for mammalian cells, we have constructed mouse mitochondrial DNA derivatives comprising the xanthine-guanine phosphoribosyltransferase gene as a selectable marker. Complete or subcomplete mitochondrial genomes were inserted into the plasmid pBB3 and transferred into hepatoma cells in order to generate, in vivo, new recombinant molecules. A second- and a third-generation vector, p12.2b and p delta respectively, were thus isolated for their ability to shuttle from mammalian cells to recA+ E. coli. Transfection of rodent fibroblasts and hepatoma cells showed that, contrary to our expectations, p12.2b and p delta are not self-replicating episomes; their shuttling from mammalian cells to recA+ E. coli is mediated by tandem integrated copies. The relevant property of p12.2b and p delta is a ubiquitous propensity to form head-to-tail multimeric structures when they integrate into mammalian host chromosomes. This ability is missing in pBB3 and appears only following the insertion of various mitochondrial or nuclear DNA fragments into the plasmid. These data are discussed in terms of homologous recombination and shuttling of integrated vectors.