Deoxyribonucleic acid-mediated gene transfer in mammalian cells: molecular analysis of unstable transformants and their progression to stability

Large domains of heterochromatin direct the formation of short mitotic chromosome loops

During mitosis chromosomes reorganise into highly compact, rod-shaped forms, thought to consist of consecutive chromatin loops around a central protein scaffold. Condensin complexes are involved in chromatin compaction, but the contribution of other chromatin proteins, DNA sequence and histone modifications is less understood. A large region of fission yeast DNA inserted into a mouse chromosome was previously observed to adopt a mitotic organisation distinct from that of surrounding mouse DNA. Here, we show that a similar distinct structure is common to a large subset of insertion events in both mouse and human cells and is coincident with the presence of high levels of heterochromatic H3 lysine nine trimethylation (H3K9me3). Hi-C and microscopy indicate that the heterochromatinised fission yeast DNA is organised into smaller chromatin loops than flanking euchromatic mouse chromatin. We conclude that heterochromatin alters chromatin loop size, thus contributing to the distinct appearance of heterochromatin on mitotic chromosomes.

Nonviral gene therapy: the promise of genes as pharmaceutical products

Although most research on gene therapy has focused on the use of recombinant viruses to deliver genes to cells in vivo, progress also has been made toward developing nonviral, pharmaceutical formulations of genes for in vivo human therapy. Various methods for nonviral gene therapy have been proposed. Some approaches are aimed at developing "artificial viruses" that attempt to mimic the process of viral infection using synthetic materials. Others apply the theory and methods of advanced, particulate drug delivery to deliver DNA to select somatic targets. These approaches employ DNA complexes containing lipid, protein, peptide, or polymeric carriers as well as ligands capable of targeting the DNA complex to cell-surface receptors on the target cell and ligands for directing the intracellular trafficking of DNA to the nucleus. Nonviral systems have been used to deliver genes to the lung, liver, endothelium, epithelium, and tumor cells and have been shown to be generally safe. More than a dozen clinical trials are currently underway using nonviral systems for disease indications including cystic fibrosis and cancer. Future advances in nonviral systems will be based on an emerging appreciation of the biological constraints on the fate and function of DNA within the body and within the cell.

Cloning and integration of DNA fragments in human cells via the inverted terminal repeats of the adeno-associated virus 2 genome

In current systems for molecular cloning of eukaryotic genes, bacterial cells are routinely utilized as intermediate hosts. We investigated the possibility of using a viral system for cloning DNA fragments independent of bacterial cell usage. In this report, we provide an alternative approach for molecular cloning of DNA fragments in eukaryotic cells by utilizing the inverted terminal repeats (ITRs) of the genome of a nonpathogenic human parvovirus, the adeno-associated virus 2 (AAV). We constructed a series of chimeric linear duplex DNA molecules, ranging in length from 1.8 to 7.2 kb, containing the cruciform structures of AAV-ITRs at both ends. These 'no-end' (NE) DNA structures, when transfected into adenovirus-infected human cells in the presence of AAV replication proteins (Rep), underwent DNA replication. Furthermore, in the presence of AAV capsid proteins (Cap), all replicated DNA molecules of less than 5.0 kb were packaged into mature, biologically active AAV progeny virions. When a chimeric NE DNA (NE-neo) containing a gene (neo) encoding resistance to neomycin was transfected into human cells, neoR clones could be readily isolated in the presence of G418 (Geneticin). Southern-blot analysis of genomic DNA of several independently isolated neoR clones suggested stable integration of the NE-neo DNA into the host chromosomal DNA. AAV-ITRs, therefore, offer an alternative system for molecular cloning, as well as packaging of DNA fragments in mammalian cells independent of bacterial cell usage.

Human papillomavirus type 6b DNA required for initiation but not maintenance of transformation of C127 mouse cells

We describe the transformation of C127 mouse fibroblasts with human papillomavirus type 6b (HPV-6b) DNA, which is associated primarily with benign tumors of the human genital tract. The major transformed phenotype of the HPV-6b-transfected cells lines, which had been G418 selected, pooled, and maintained without subsequent selection, was tumorigenicity in nude mice. We found that, unlike that reported for other HPVs or papovaviruses, the transformed phenotype was expressed after a delay, in which the cells had undergone extensive culture passages (about 20 passages or 100 generations). Interestingly, the HPV-6b DNA had become reduced or nondetectable in copy number in the cells by the time the transformed phenotype was expressed and in most of the tumors induced by the cells in nude mice, indicating that high levels of HPV-6b DNA were not required for maintenance of the transformed phenotype. Clonal cell lines gave similar results. When continued G418 selection was used to maintain high-copy-number HPV-6b DNA, the cells were tumorigenic, indicating that high levels of HPV-6b DNA did not suppress tumorigenesis. These studies suggest that HPV-6b DNA initiates transformation of C127 cells but is dispensable for expression or maintenance of the transformed phenotype. Transformation by HPV-6b DNA in vitro may provide insights into the HPV type-specific association with benign versus malignant lesions in vivo and may elucidate some of the oncogenic processes involved in tumor progression.

Transformation of Neurospora crassa by an integrative transforming plasmid is not enhanced by ribosomal DNA sequences

Two integrative transforming plasmids of Neurospora crassa that differed only by the presence of almost all of a ribosomal DNA repeat unit on one plasmid were constructed. The plasmids were used to test the target concentration hypothesis which states that the transformation frequency is proportional to the number of genomic copies of a homologous sequence located on the transforming plasmid. Since there are approx. 200 copies of the rDNA sequences in the genome, the target concentration hypothesis would have been proved if the transformation frequency was 200-fold higher for the rDNA-containing plasmid compared with the plasmid without rDNA. The results indicated no difference in the transformation for the two plasmids, thereby providing no support for the hypothesis. The target concentration hypothesis has been proved for yeast, and thus mechanisms different from that responsible for integrative transformation in yeast must operate in N. crassa, perhaps including non-homologous recombination events.

Cytosine methylation in the EcoRI site of active and inactive herpesvirus thymidine kinase promoters

The herpesvirus thymidine kinase (tk) gene integrated in the human cell line, 2.1-a, can be inactivated by limited de novo methylation. All these TK- clones show partial EcoRI digestion of the recognition site (cGAATTCg) in the tk promoter in contrast to complete digestion of this site in the original cell line. Studies on well-defined substrates prepared in vitro showed that methylation of one cytosine in the EcoRI recognition sequence resulted in partial and methylation of both cytosines in severe inhibition of digestion by EcoRI. This characteristic was used to determine whether no, one or both cytosines in the EcoRI site of the tk promoter were methylated in various TK- clones derived from 2.1-a and in TK+ clones re-expressing the gene after 5-azacytidine treatment. A high correlation was found between inactivity of the tk gene and methylation of only one of the two cytosines in the EcoRI recognition site. The results also show that the tk promoter can be active despite the presence of a methylated cytosine.

De novo methylation as major event in the inactivation of transfected herpesvirus thymidine kinase genes in human cells

Spontaneous inactivation of integrated thymidine kinase genes was studied in three human cell lines, one with multiple copies and two with a single copy of a transfected shuttle plasmid containing two selectable genes: the HSV tk gene and the Eco gpt gene. Selection for gpt expression prevented the isolation of TK- mutants which are the result of plasmid loss. Under these conditions TK- clones were isolated with a frequency of 5.10(-6) both with the cell line containing 5 or 6 copies of the tk gene and with one of the two cell lines containing one copy of this gene. This inactivity of the tk gene was associated with de novo methylation as the number of HAT-resistant (TK+) clones strongly increased after growth of the TK- derivatives in the presence of the demethylating agent, 5-azacytidine. Digestion with methylation-sensitive restriction enzymes revealed two different patterns of DNA methylation in the genomic DNA of TK- variants. In the TK- derivatives of the cell line containing multiple copies of the tk gene many HpaII restriction sites in the gene copies were insensitive to digestion. These HpaII sites were, however, not methylated in TK- variants of the cell line containing one copy of the plasmid, and methylated CpGs could be detected only with EcoRI which recognizes the cGAATTCg sequence in the tk promoter region. With the other of the two single-copy TK+ cell lines no TK- mutants were obtained, suggesting that the position of a gene in the genome is an important factor in determining the frequency and the extent of de novo methylation. Additionally, we observed that remethylation is an even more efficient process of gene inactivation as TK+ clones reactivated with 5-azacytidine can become TK- again at a 100-fold higher rate than the original TK+ cell line.

Complementation of the DNA-repair defect in a CHO mutant by human DNA that lacks highly abundant repetitive sequences

Recently, two human DNA-repair genes have been cloned which complement the defects in complementation groups 1 and 2 of the CHO mutants which are sensitive to ultraviolet light and deficient in the incision step of excision repair. Here we report human gene transfer-mediated complementation of a group 4 CHO mutant sensitive to ultraviolet light and mitomycin C (MMC). The transfectants generated by transfecting human DNA into the repair-deficient cell line demonstrate the repair-proficient phenotype, as they have wild-type levels of resistance to UV light and MMC and are competent in performing the incision step of excision repair in response to UV irradiation. 3 of the 8 transfectants isolated display no detectable human repetitive sequences, while the other 5 contain varying amounts of human repetitive DNA. As the evidence suggests that all of the transfectants are repair-proficient as a result of the uptake of human DNA, we conclude that the human gene that complements the repair defect in group 4 CHO mutants contains no highly abundant human repetitive sequences. This imposes the necessity of developing cloning strategies involving the identification of sequences that flank the gene.

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.

Single-strand breaks can lead to complex configurations of plasmid DNA in vitro

DNA species which migrate extremely slowly in agarose gel electrophoresis may be formed from plasmid DNA containing radiation-induced single-strand breaks (ssbs). Postirradiation heat treatment in low ionic strength buffer and subsequent incubation with Mg2+ strongly enhanced the formation of these species. Electron micrographs taken after such treatment show numerous complex configurations containing DNA material from several plasmid molecules. Less extreme formation of slowly migrating DNA occurred without postirradiation heat treatment or Mg2+ incubation when the DNA was co-precipitated with calcium phosphate in a physiologically balanced buffer and incubated under conditions used for DNA-mediated gene transfer. The data suggest that homologous pairing between single-stranded regions formed in relation to ssbs may contribute to cohesion between different molecules. The significance of the cohesion process for gene transfer experiments and cellular radiation effects is discussed.

Induction of cell surface insulin antigenicity in fibroblasts transfected with islet genomic DNA

Ltk- cells were transfected with total genomic DNA obtained from rat islets or from insulinoma cells mixed with DNA tagged with fluorescent dye. Fluorescence-activated cell sorter (FACS) was used for initial selection of successful transfectants, monitoring fluorescent DNA incorporated into the cell. For subsequent selections the cells were treated with anti-insulin antiserum labeled with fluorescent dye and selected by FACS. Beta-cell DNA, but not DNA from murine leukemic cells, induced the appearance of cell surface insulin antigenicity in fibroblasts. This phenotypic expression was transient. Together with our previous demonstration of the induction of insulin secretion in Ltk- cells [(1986) Biochem. Biophys. Res. Commun. 136, 638-644], these results indicate that beta-cell-specific characteristics can be transfected to non-endocrine cells by genomic DNA transfection.

Induction of insulin release in fibroblasts transfected with genomic rat islet DNA

L cells (tk-) were cotransfected with total genomic rat islet DNA and a plasmid containing thymidine kinase gene (ptk). Transfectants were tested for their ability to release insulin into the medium. At least 10% of the colonies contained immunoreactive insulin (IRI) during the initial two weeks. The insulin secreted competed linearly with rat insulin in RIA, the majority of the insulin antigenicity comigrating with rat insulin on G-50 Sephadex chromatography. With continuing propagation the IRI activity diminished; however 3 selected cultures demonstrated increased secretion of IRI following stimulation with glucose. These findings indicate that glucose-induced insulin secretion can be obtained in non-beta cells; however the frequency of success was below one stable transfection for every 5 X 10(8) Ltk- cells exposed to the transfection procedure.

Regulation of herpes simplex virus 1 genes: alpha gene sequence requirements for transient induction of indicator genes regulated by beta or late (gamma 2) promoters

This laboratory reported earlier that chimeric genes consisting of the structural sequences of the thymidine kinase (TK) gene fused to the promoter-regulatory domains of late (gamma 2) genes were regulated as bonafide gamma 2 genes when resident in the herpes simplex virus 1 genome but could not be differentiated from beta genes when introduced by transfection and stably integrated into the environment of the host genome (S. Silver and B. Roizman, Mol. Cell. Biol. 5, 518-528, 1985). We report here that beta-TK and the chimeric gamma 2-TK gene transfected into TK- baby hamster kidney (BHKtk-) were induced by alpha 4 and alpha 0 but not by the other alpha genes. Specifically: Both TK genes were induced by cotransfection with DNA fragments carrying an intact alpha 4 or an intact alpha 0 gene, but not by fragments carrying only the promoter-regulatory domain or the structural sequences of the alpha 4 gene or intact alpha 22, alpha 27, and alpha 47 genes. An alpha 4 gene carrying a 2700-bp deletion in its 3' coding sequence also induced both genes, although less efficiently. RNA homologous to the alpha 4 gene recovered from the cytoplasm of cells transfected with either the intact or truncated alpha 4 gene mapped to the bonafide site of transcription initiation of the alpha 4 gene. RNA homologous to the chimeric TK gene extracted from the cytoplasm of cells transfected with both gamma 2-TK and the alpha 4 gene was transcribed from the bonafide gamma 2 gene capping site fused to the TK gene. These results indicate that the alpha 4 gene and the alpha 0 gene are each capable of inducing the expression of both beta and gamma 2 genes resident in the environment of the cellular genome, that the active site responsible for induction is located near the N terminus of the alpha 4 protein, and reinforce the conclusion that gamma 2 genes resident in the environment of the host cell cannot be used to identify the authentic determinants of gamma 2 gene regulation by currently available tests.

Recovery of hormonal regulation in protein kinase defective adrenal cells through DNA-mediated gene transfer

A cAMP-resistant mutant (Kin-8) isolated from Y1 mouse adrenocortical tumor cells harbors a specific lesion in the regulatory subunit of the type 1 cAMP-dependent protein kinase. This mutant also is resistant to the effects of corticotropin and cAMP on steroidogenesis, growth and morphology, suggesting an obligatory role for the protein kinase in regulation of adrenocortical functions. In this study, the cAMP-resistant phenotype of the Kin-8 mutant was reverted by transformation with DNA from cAMP-responsive Y1 cells, and the biochemical basis of the transformation was explored. Initially, Y1 mouse adrenocortical tumor cells were evaluated for their competence as recipients in DNA-mediated transformation experiments, by measuring their ability to incorporate and express a bacterial gene (neo) encoding resistance to neomycin. Y1 cells were transfected with the plasmid pSV2-neo (an SV40-neo hybrid vector designed for expression in animal cells) and screened for resistance to the neomycin analog, G418. Neomycin-resistant transformants were recovered from Y1 cells at a frequency of approximately one per 10(3) cells per 10 micrograms of DNA, and had specific neo sequences integrated into their high molecular weight (mw) DNA. The Y1 mutant, Kin-8, then was transformed with pSV2-neo DNA plus high mw DNA prepared from cAMP-responsive Y1 cells. Cells competent for transformation were recovered by selective growth in the neomycin analog G418, and these transformants were screened for recovery of morphological responses to cAMP. Several colonies capable of rounding up in the presence of cAMP were recovered after transformation with DNA from Y1 cells. These transformants also recovered the ability to round up in the presence of corticotropin, and were able to respond to both corticotropin and cAMP with increased steroidogenesis. Transformants generated from either Y1 or Kin-8 cells were unstable. Y1 cells lost resistance to neomycin when grown in the absence of G418 at a frequency of 4% per generation. Similarly, Kin-8 transformants lost their sensitivity to cAMP in subsequent culture passages. In some of the cAMP-responsive transformants, cAMP-dependent protein kinase activity was recovered and approached the activity seen in cAMP-responsive Y1 cells. The recovery of a normal protein kinase by transformation appeared to have been sufficient to reverse the cAMP-resistant phenotype of Kin-8 cells. In other cAMP-responsive transformants, protein kinase activity was not appreciably affected by cAMP.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of X-rays and ultraviolet light on DNA-mediated gene transfer in mammalian cells

The uptake, expression and genomic integration of exogenous DNA during DNA-mediated gene transfer are poorly understood in mammalian cells. We studied the effects of ionizing radiation and u.v. light treatments on recipient cells during gene transfer experiments. We found that both X-rays and u.v. light stimulate pSV2-gpt DNA transfer into V79 Chinese hamster cells and they are equally effective for an equi-cytotoxic dose. This result was observed with irradiation both before and after the period of DNA precipitate overlay of the recipient cells. The stimulation of DNA transfer was approximately proportional to dose for both types of radiation. The effect was significantly enhanced using chronic, rather than acute, radiation treatments. The optimal expression time to observe stimulation of DNA transfer, however, differs for the two radiation types. A possible model for DNA-mediated gene transfer, incorporating this result, is discussed.

DNA-mediated gene transfer into human diploid fibroblasts derived from normal and ataxia-telangiectasia donors: parameters for DNA transfer and properties of DNA transformants

We investigate the feasibility of DNA-mediated gene transfer into human diploid fibroblasts derived from patients with the radiation sensitive syndrome ataxia-telangiectasia (A-T) and from a normal donor. Although they are markedly different in their growth characteristics, both normal and A-T strains give similar frequencies for DNA transfer in a model system using the recombinant plasmid pSV2 -gpt. pSV2 -gpt DNA transformants arise with a frequency between 10(-5) and 10(-4) per viable cell. Analysis of such transformants, although possible, is severely handicapped by the limited clonal life span of diploid human cells. Despite these problems it may be concluded that diploid human fibroblasts are competent recipients for DNA-mediated gene transfer and the putative repair deficiency of A-T does not markedly effect the efficiency of this process.

Herpes simplex virus thymidine kinase gene is stably maintained and expressed in cells transformed by protoplast fusion

We examined a series of transformed cell lines resulting from transfer of the herpes simplex virus type 1 thymidine kinase gene to Ltk- cells by protoplast fusion gene transfer. We show that multiple copies of the transforming plasmid DNA, ranging from a minimum of two to greater than 20, were present in one or at most a few integration sites in each cell line. The TK+ phenotype was stable in five independent transformed cell lines after growth in nonselective medium for over a year. Transforming plasmid DNA was stable in one cell line containing from two to five copies after a year of growth in nonselective medium. In another cell line initially containing about 20 copies, the transforming DNA became rearranged soon after growth to mass culture, resulting in a decrease to two to five copies which then remained stably maintained. This suggests that TK+ transformants resulting from protoplast fusion are stable when the input DNA has integrated in a relatively low copy number.

Introduction of purified genes into mammalian cells

There are a number of methods to introduce genes into mammalian cells. These include cell hybridization, chromosome-mediated and DNA-mediated gene transfer. DNA-mediated transfer can be achieved by direct microinjection methods or by indirect methods. The DNA enters the nucleus and is expressed in a high proportion of cells transiently. The DNA then becomes integrated into host cell DNA at random sites resulting in more stably expressing transformants. A number of genes for which selection systems exist can be introduced into mammalian cells. Nonselectable genes can also be introduced into cells by either ligating them to a selectable gene or by mixing them with carrier DNA and a selectable gene. If an amplifiable gene sequence is introduced into cells, it and other genes in its proximity can be coamplified. Amplification of the genes can also be achieved by the use of appropriate viral vectors and recipient cells. The foreign genes are expressed in the recipient cells if they contain the appropriate recognition signals for initiation and termination of transcription. Transfection systems are thus permitting identification of DNA sequences which have a regulatory role in gene expression. The identification of transcriptional signal sequences has formed the basis for construction of appropriate molecules which would permit expression of genes which cannot normally be expressed in mammalian cells (e.g., bacterial genes). The foreign genes are not only expressed in the recipient cells but they can also be subject to regulation in the appropriate environment. This observation is paving the way for identification of regulatory sequences. The foreign DNA sequences integrated into the host genome can be recovered by a variety of methods. Such methods permit isolation of genes which code for a selectable gene product.

Coordinate modulation of transfected HSV thymidine kinase and human globin genes

We have shown that high-frequency phenotypic switching of a transfected gene is associated with alterations in chromatin structure. To examine this phenomenon further, a plasmid containing HSV thymidine kinase and human alpha- and gamma-globin genes was transfected into mouse L cells. All three genes were expressed through utilization of their individual promoters. One of these cell lines was capable of switching to its TK- phenotype at high frequencies (8%-10%). The revertants (TK-) had no TK or globin transcripts, while the rerevertants (TK+) expressed all three genes at their original levels. We conclude that genes introduced into cells by ligated cotransfection can be regulated coordinately and that the unit of this regulated expression can be at least 20 kb long.

Events preceding stable integration of SV40 genomes in a human cell line

We have examined the organization of integrated SV40 sequences in an uncloned population of a transformed human fibroblast cell line. Somatic cell hybrids between mouse B82 cells and human GM847 cells were examined for SV40 T-antigen expression and individual human chromosome presence. This analysis revealed that a functional SV40 genome is located on human chromosome 7. Restriction endonuclease digestion followed by blot hybridization of the parental human cell line revealed that it contains multiple normal and defective SV40 copies integrated into the host genome in tandem. A similar analysis of several T-ag+ hybrid cell lines indicated that the integrated viral sequences in different hybrid cell lines (thus in different cells of the original population) are very closely related but not always identical. Analysis of subclones of GM847 also revealed such differences. Based upon these results, we postulate that following the initial integration event, viral as well as the flanking host DNA sequences become unstable and are subject to deletions and rearrangements. This short-lived structural instability is followed by highly stable integration of SV40 which is maintained in these cells or their hybrid derivatives for at least hundreds of cell generations.

High-efficiency transfer of DNA into eukaryotic cells by protoplast fusion

Protoplast fusion is a highly efficient method for effecting gene transfer to cells in culture resulting in stable transformation at high frequency. A number of cell lines have been used successfully as recipients. There is no need to isolate and purify the DNA, which not only saves time and effort but eliminates steps that cause nicking or breaking of large cloned inserts. The high-frequency transformation achievable by protoplast fusion should make this procedure useful for studies on gene expression and for screening cloned genomic libraries for genes that can be expressed in recipient cells.

Modulation of transfected gene expression mediated by changes in chromatin structure

We examined the molecular basis of modulation of transfected herpes simplex virus (HSV) thymidine kinase (tk) gene in mouse fibroblasts. We observed that one tk+ cell line is capable of reversion to a tk- phenotype, and rereverts to a tk+ phenotype at high frequencies (5%). The revertants lacked tk enzyme activity and tk-specific transcripts. We detected no differences in the organization of foreign DNA or in the CpG methylation patterns in the revertants and rerevertants. We probed the chromatin structure of the revertants and rerevertants, and found the tk sequences in the rerevertants to be more highly sensitive to digestion with DNAase I than the corresponding revertant cells. We conclude that the high frequency switching of gene expression we observed is mediated by changes in chromatin structure, and that this may reflect the behavior of the host sequences at the site of foreign DNA integration.

Inhibition by interferon of biochemical transformation induced by cloned herpesvirus thymidine kinase genes

To learn whether interferon could prevent the biochemical transformations induced by cloned herpesvirus thymidine kinase (TK) genes, LM(TK-) mouse fibroblast cultures were pretreated for 24 h with 2.4-40 international units (I.U.)/ml mouse alpha + beta interferon, and subsequently transformed to the TK+ phenotype with recombinant plasmids containing the herpes simplex virus type 1 (HSV-1) TK gene (pAGO and pMH110) and the marmoset herpesvirus (MarHV) TK gene (pMAR035). Mouse alpha + beta interferon inhibited transformation and the inhibition was interferon dose-dependent. Transformation was also inhibited when LM(TK-) cells were pretreated for 2-5 h with 40 I.U./ml interferon. Maximal inhibitions of TK+ colony formation were observed following a 9-20 h pretreatment period with interferon. In contrast, 40 I.U./ml interferon treatment for 20 h did not reduce the rate or extent of LM(TK-) cell growth. Experiments in which cultures were first treated with plasmid pAGO and only afterwards treated with interferon also showed that, as the interferon concentration used, interferon did not inhibit the outgrowth of transformated colonies. Enzyme assays showed that pretreatment with interferon inhibited the induction of TK activity in cells that had been transfected with pAGO DNA.

DNA-mediated gene transfer in Chinese hamster ovary cells: clonal variation in transfer efficiency

Thymidine kinase-deficient Chinese hamster ovary (CHO) cells were genetically transformed with the BamHI restriction fragment encoding the thymidine kinase gene of herpes simplex virus (HSV-tk). We have observed considerable clonal variation among independent CHO sublines with respect to transformation competence for the DNA-mediated gene transfer of HSV-tk. Transformation frequencies greater than or equal to 3 X 10(-4) were observed consistently in one subline, with a transformation efficiency of approximately 1 transformant per ng viral gene. The frequency and efficiency of transformation we observed in this system are at least 10-fold greater than those previously reported for DNA-mediated transformation of CHO cells by HSV-tk. All of the CHO HSV-tk+ transformants examined were stable for the transferred genotype in the absence of selection, and all showed evidence of co-transformation by unselected plasmid pBR322 sequences.

Transfer of genes to Chinese hamster ovary cells by DNA-mediated transformation

We have transferred DNa to Chinese hamster ovary (CHO) cells by DNA-mediated transformation. CHO tk- cells were transformed with the clones gene for herpes simplex virus thymidine kinase (HSV-tk) and were found to have a 50-fold lower frequency of transformation than mouse Ltk- cells at the same DNA dosage. By altering the amount of tk gene and carrier DNA present, frequencies of up to 5 x 10(-5) were obtained. CHO HSV-tk+ transformants were very stable, and in several clones the HSV-tk gene copies integrated in higher-molecular-weight DNA. These cells also exhibited cotransformation for unselected markers. CHO lines were also transformed at a frequency of 10(-4) with the bacterial gene Ecogpt in a SV40-pBR322 vector. CHO tk-cells could be transformed at a frequency of 10(-7) with cellular DNA isolated from CHO tk+ cells. CHO cells offer a well-defined genetic system within which to transfer either cloned or whole cellular DNAs.

The site of integration of the herpes simplex virus type 1 thymidine kinase gene in human cells transformed by an HSV-1 DNA fragment

To analyze the site of integration of the herpes simplex virus type I (HSV-I) thymidine kinase (TK) gene in biochemically transformed human cells, TK-HeLa-(BU25) cells were transformed to the TK+ phenotype by a cloned, 2 kbp Pvull fragment of HSV-I DNA. The transformed cells [HeLa(BU25)/TF pAGO PP3] were fused with mouse LM(TK-) cells, and human-mouse somatic cell hybrid clones (LH PP3 clones 1, 2, 3, 5 and 6) were isolated in HATG-ouabain selective medium. The HeLa(BU25)/TF pAGO PP3 cells and the LH PP3 hybrid clones expressed HSV-I specific TK activity and a herpesvirus-associated nuclear antigen, and contained herpesvirus nucleotide sequences. Molecular hybridization experiments were carried out to map the HSV-I and flanking cellular nucleotide sequences in the biochemically transformed cells. These experiments demonstrated that the HSV-I nucleotide sequences were integrated at a single site, and that the same cellular nucleotide sequences flanked the viral DNA in transformed HeLa(BU25)/TF pAGO PP3 and LH PP3 clone 5 cells. TK- revertant subclones isolated by growing the LH PP3 clone 5 cells in BrdUrd (and diphtheria toxin) failed to form colonies in HATG medium, but retained HSV-I nucleotide sequences. Isozyme analyses on 21 gene-enzyme systems representing 21 human chromosomes revealed that all of the LH PP3 clonal lines expressed human hexosaminidase B, which has been assigned to chromosome 5, and all were sensitive to diphtheria toxin, which is also a marker for chromosome 5. Chromosome analyses showed that chromosome 5 was the nly human chromosome present in mitoses of LH PP3 clone 5 cells and that human chromosome 5 was present in most of the mitoses of LH PP3 clone 1, 2, 3, and 6 cells. The latter clones also contained 1 or 2 additional human chromosomes in some of the cells. As expected from the molecular hybridization analyses, TK- revertants of LH PP3 clone 5 cells retained portions of chromosome 5 and expressed human hexosaminidase B. The results indicate that HSV-I nucleotide sequences were stably integrated in the biochemically transformed cells, most likely in human chromosome 5.

Formation of infectious progeny virus after insertion of herpes simplex thymidine kinase gene into DNA of an avian retrovirus

We have prepared several infectious stocks of an avian retrovirus, spleen necrosis virus, containing the herpes simplex virus type 1 thymidine kinase (tk) gene. The viruses were produced after cotransfection of chicken cells with DNA from recombinants between cloned spleen necrosis virus and tk DNAs and DNA of cloned reticuloendotheliosis virus strain A. removal of sequences in the tk gene for the end of tk mRNA increased a thousand fold the yield of infectious recombinant virus. Infection of chicken or rat tk- cells with the recombinant virus transformed them to a tk+ phenotype.

Enhancing the efficiency of DNA-mediated gene transfer in mammalian cells

We have investigated several of the experimental factors that affect calcium phosphate-DNA-mediated gene transfer of thymidine kinase (tk) into mouse LM tk- Cl 1D cells using unfractionated DNA from both Chinese hamster ovary cells and L6 rat myoblasts. Increases in the length of exposure to DNA (24 h) and the expression time (48 h) before selection result in a 20-fold enhancement in the efficiency of transformation. These modifications yield frequencies up to 35 HATR colonies/20 microgram tk"NA/10(6) recipient cells. Exposure to dimethyl sulfoxide enhances transformation efficiencies slightly for short DNA exposure times, but has no effect when optimal DNA exposure times are used. Several other variations in our standard transformation protocol were also examined: these include the concentration and size of the DNA and exposure to low concentrations of the nonionic detergent, Tween-80. We have also isolated and characterized a subclone of Cl 1D that is a high-efficiency recipient for the tk+ marker. Segregation analysis reveals that the majority of the TK+ transformants derived from this subclone are stable, in contrast to those derived from the DL 1D parent. The combination of improved methodology and the high-efficiency recipient subclone permits DNA-mediated transformation for tk at frequencies on the order of 10(-4) transformants per recipient cell.

Mechanisms and applications of DNA-mediated gene transfer in mammalian cells - a review

The ability of mammalian cells to take up exogenously added DNA and to express genes included on that DNA has been well documented. DNA-mediated gene transfer (DMGT) potentially is a useful technique for the elucidation of many of the factors that control gene expression, and for the purification and isolation of mammalian genes. Before many of the benefits can be realized, however, a more detailed understanding of the organization, intracellular location, and expression of transferred genes will be needed. Recent studies have begun to characterize the DMGT process. Selected genes become linked to other exogenously added DNA during or subsequent to transfer and persist in the nuclei of recipient cells as part of large molecules called transgenomes. Transgenomes initially are maintained unstably and are lost from the population with first order kinetics. After a variable number of generations in culture, subpopulations arise that maintain the transferred genes stably. In these "stable" cells the transgenome is associated with a recipient cell chromosome, although the particular chromosome differs in independent "stable" lines. Mixture of an excess of specific nonselectable genes with the selected gene prior to transfer results in the inclusion of the nonselected genes in the transgenomes present in most cells that survive selection. This finding demonstrates the feasibility of introducing virtually any purified gene into mammalian cells. Recently microinjection of DNA directly into the nuclei of cells has been demonstrated. This technique greatly increases the frequency of gene transfer and significantly expands the number of cell types that can be genetically transformed.