Parameters governing the transfer of the genes for thymidine kinase and dihydrofolate reductase into mouse cells using metaphase chromosomes or DNA

Phage particle-mediated gene transfer to cultured mammalian cells

Recombinant phage particles carrying the thymidine kinase (TK) gene of herpes simplex virus type 1, coprecipitated with calcium phosphate, efficiently transformed mouse Ltk- cells to the TK+ phenotype. The conditions necessary to achieve high efficiency of transfer of the TK gene by phage particle-mediated gene transfer were investigated. Of the parameters examined, the pH of the buffer used for coprecipitation of phage particles with calcium phosphate, the length of time of coprecipitation, and the length of the adsorption period were found to alter the transfer efficiency significantly. The optimal pH was 6.87 at 25 degrees C. The other optimal values for these parameters were as follows: coprecipitation time, 7 to 20 min; adsorption time, 18 to 30 h. Treatment with dimethyl sulfoxide, glycerol, or sucrose did not enhance gene transfer. The optimal conditions yielded about 1 transformant per 10(5) phage particles per 10(6) cells without carrier DNA. An increase in the dosage of phage particles, up to at least 5 x 10(7) phage particles per 100-mm dish, resulted in a linear increase in the number of transformants. Addition of carrier phage, up to 10(10) phage particles per dish, did not significantly affect the number of transformants.

Phage particle-mediated gene transfer to cultured mammalian cells

Recombinant phage particles carrying the thymidine kinase (TK) gene of herpes simplex virus type 1, coprecipitated with calcium phosphate, efficiently transformed mouse Ltk- cells to the TK+ phenotype. The conditions necessary to achieve high efficiency of transfer of the TK gene by phage particle-mediated gene transfer were investigated. Of the parameters examined, the pH of the buffer used for coprecipitation of phage particles with calcium phosphate, the length of time of coprecipitation, and the length of the adsorption period were found to alter the transfer efficiency significantly. The optimal pH was 6.87 at 25 degrees C. The other optimal values for these parameters were as follows: coprecipitation time, 7 to 20 min; adsorption time, 18 to 30 h. Treatment with dimethyl sulfoxide, glycerol, or sucrose did not enhance gene transfer. The optimal conditions yielded about 1 transformant per 10(5) phage particles per 10(6) cells without carrier DNA. An increase in the dosage of phage particles, up to at least 5 x 10(7) phage particles per 100-mm dish, resulted in a linear increase in the number of transformants. Addition of carrier phage, up to 10(10) phage particles per dish, did not significantly affect the number of transformants.

Control of phosphatidylserine synthase II activity in Chinese hamster ovary cells

Phosphatidylserine (PtdSer) in Chinese hamster ovary (CHO) cells is synthesized through the action of PtdSer synthase (PSS) I and II, which catalyzes the exchange of L-serine with the base moiety of phosphatidylcholine and phosphatidylethanolamine, respectively. The PtdSer synthesis in a CHO cell mutant, PSA-3, which lacks PSS I but has normal PSS II activity, was almost completely inhibited by the addition of PtdSer to the culture medium, like that in the wild-type CHO-K1 cells. In contrast, the PtdSer synthesis in a PSS II-overproducing stable transformant of CHO-K1, K1/wt-pssB, was reduced by only 35% upon addition of PtdSer. The serine exchange activity in a membrane fraction of K1/wt-pssB cells was not inhibited by PtdSer at all, whereas those of PSA-3 and CHO-K1 cells were inhibited by >95%. These results indicated that PSS II activity in PSA-3 and CHO-K1 cells is inhibited by exogenous PtdSer and that overproduction of PSS II leads to the loss of normal control of PSS II activity by exogenous PtdSer. Although overproduced PSS II in K1/wt-pssB cells was not normally controlled by exogenous PtdSer, K1/wt-pssB cells cultivated without exogenous PtdSer exhibited a normal PtdSer biosynthetic rate similar to that in CHO-K1 cells. In contrast to K1/wt-pssB cells, another stable transformant of CHO-K1, K1/R97K-pssB, which overproduces R97K mutant PSS II, exhibited a approximately 4-fold higher PtdSer biosynthetic rate compared with that in CHO-K1 cells. These results suggested that for maintenance of a normal PtdSer biosynthetic rate, the activity of overproduced wild-type PSS II in K1/wt-pssB cells is depressed by an as yet unknown post-translational mechanisms other than those for the exogenous PtdSer-mediated inhibition and that Arg-97 of PSS II is critical for this depression of overproduced PSS II activity. When the cDNA-directed wild-type and R97K mutant PSS II activities were expressed at nonoverproduction levels in a PSS I- and PSS II-defective mutant of CHO-K1 cells, expression of the mutant PSS II activity but not that of the wild-type PSS II activity induced the PtdSer-resistant PtdSer biosynthesis. This suggested that Arg-97 of PSS II is critical also for the exogenous PtdSer-mediated inhibition of PSS II.

Genetic evidence that phosphatidylserine synthase II catalyzes the conversion of phosphatidylethanolamine to phosphatidylserine in Chinese hamster ovary cells

Phosphatidylserine (PS) in mammalian cells is synthesized through the exchange of free L-serine with the base moiety of phosphatidylcholine or phosphatidylethanolamine (PE). The serine base exchange in Chinese hamster ovary (CHO) cells is catalyzed by at least two enzymes, PS synthase (PSS) I and II. A PSS I-lacking mutant of CHO-K1 cells, PSA-3, which exhibits approximately 2-fold lower serine base exchange activity than CHO-K1, is defective in the conversion of phosphatidylcholine to PS but has the ability to convert PE to PS. The PSA-3 mutant requires exogenous PS or PE for cell growth. In the present study, from PSA-3 mutant cells, we isolated a mutant, named PSB-2, with a further decrease in the serine base exchange activity. The activity in the homogenate of PSB-2 mutant cells was approximately 10% that of PSA-3 mutant cells and approximately 5% that of CHO-K1 cells. The PSB-2 mutant exhibited an approximately 80% reduction in the PSS II mRNA level relative to that in PSA-3 mutant and CHO-K1 cells. These results showed that the PSB-2 mutant is defective in PSS II. Like the PSA-3 mutant, the PSB-2 mutant grew well in medium supplemented with PS. However, in the medium supplemented with PE, the PSB-2 mutant was incapable of growth, in contrast to the PSA-3 mutant. In the medium with exogenous PE, the PSB-2 mutant was defective in PS biosynthesis, whereas the PSA-3 mutant synthesized a normal amount of PS. A metabolic labeling experiment with exogenous [32P]PE revealed that the PSB-2 mutant was defective in the conversion of exogenous PE to PS. This defect and the growth and PS biosynthetic defects of the PSB-2 mutant cultivated with exogenous PE were complemented by the PSS II cDNA. In addition, the cDNA of the other PS synthase, PSS I, was shown not to complement the defect in the conversion of exogenous PE to PS of the PSB-2 mutant, implying that PSS I negligibly contributes to the conversion of PE to PS in CHO-K1 cells. These results indicated that PSS II is critical for the growth and PS biosynthesis of PSA-3 mutant cells cultivated with exogenous PE and suggested that most of the PS formation from PE in CHO-K1 cells is catalyzed by PSS II.

Control of phosphatidylserine biosynthesis through phosphatidylserine-mediated inhibition of phosphatidylserine synthase I in Chinese hamster ovary cells

Phosphatidylserine (PtdSer) synthesis in Chinese hamster ovary (CHO) cells occurs through the exchange of L-serine with the base moiety of phosphatidylcholine or phosphatidylethanolamine. The synthesis is depressed on the addition of PtdSer to the culture medium. A CHO cell mutant named mutant 29, whose PtdSer biosynthesis is highly resistant to this depression by exogenous PtdSer, has been isolated from CHO-K1 cells. In the present study, the PtdSer-resistant PtdSer biosynthesis in the mutant was traced to a point mutation in the PtdSer synthase I gene, pssA, resulting in the replacement of Arg-95 of the synthase by lysine. Introduction of the mutant pssA cDNA, but not the wild-type pssA cDNA, into CHO-K1 cells induced the PtdSer-resistant PtdSer biosynthesis. In a cell-free system, the serine base-exchange activity of the wild-type pssA-transfected cells was inhibited by PtdSer, but that of the mutant pssA-transfected cells was resistant to the inhibition. Like the mutant 29 cells, the mutant pssA-transfected cells grown without exogenous PtdSer exhibited an approximately 2-fold increase in the cellular PtdSer level compared with that in CHO-K1 cells, although the wild-type pssA-transfected cells did not exhibit such a significant increase. These results indicated that the inhibition of PtdSer synthase I by PtdSer is essential for the maintenance of a normal PtdSer level in CHO-K1 cells and that Arg-95 of the synthase is a crucial residue for the inhibition.

Cloning of a Chinese hamster ovary (CHO) cDNA encoding phosphatidylserine synthase (PSS) II, overexpression of which suppresses the phosphatidylserine biosynthetic defect of a PSS I-lacking mutant of CHO-K1 cells

Phosphatidylserine (PtdSer) in mammalian cells is synthesized through the exchange of free L-serine for the polar head group (base) of preexisting phospholipid. We previously showed the presence of two different enzymes catalyzing the serine base exchange in Chinese hamster ovary (CHO) cells and isolated the cDNA of one of the enzymes, PtdSer synthase (PSS) I, which also catalyzes the exchange of the base moiety of phospholipid(s) for ethanolamine and choline. In this study, we cloned a CHO cDNA, designated as pssB, which encodes a protein exhibiting 32% amino acid sequence identity with CHO PSS I. Introduction of the pssB cDNA into CHO-K1 cells resulted in striking increases in both the serine and ethanolamine base exchange activities. In contrast to the PSS I cDNA, the pssB cDNA was incapable of increasing the choline base exchange activity. The expression of the pssB gene in Sf9 insect cells also results in striking increases in both serine and ethanolamine base exchange activities. The pssB cDNA was found to transform a PtdSer-auxotrophic PSS I-lacking mutant of CHO-K1 cells to PtdSer prototrophy. The PtdSer content of the resultant transformant grown without exogenous PtdSer for 2 days was 4-fold that of the mutant and similar to that of CHO-K1 cells, indicating that the pssB cDNA complemented the PtdSer biosynthetic defect of the PSS I-lacking mutant. These results suggested that the pssB cDNA encoded the second PtdSer synthase PSS II, which catalyzed the serine and ethanolamine base exchange, but not the choline base exchange.

Stable transfection of Acanthamoeba

The promoter activity of an Acanthamoeba polyubiquitin gene was analyzed in its homologous system. A modified calcium phosphate transfection method using a neomycin marker vector was developed to achieve highly efficient transfection of the Acanthamoeba polyubiquitin gene into Acanthamoeba cells. In this transfection procedure, the calcium phosphate-DNA complex was formed gradually in the medium during incubation with cells and precipitated on the cells. The crucial factors for obtaining efficient transfection were the pH (6.95) of the transfection buffer used for the calcium phosphate precipitation and the amount (25 micrograms/96-well tissue culture plate) and form (circular) of transfecting DNA. Under these conditions, Acanthamoeba isolate 1B6 was transfected at an efficiency of about 40% with the constructed vector pOPSBU, a pOP13CAT-based polyubiquitin gene incorporated neomycin resistance vector. Acanthamoeba polyphaga was transfected at an efficiency of about 10% with this vector. Transfection of both Acanthamoeba strains appeared to result in low copy plasmid integration (about two copies per cell are suggested). The chloramphenicol acetyltransferase (CAT) assays showed that the promoter of the Acanthamoeba polyubiquitin gene in the constructed vector was especially strong in A. polyphaga, thus the pOPSBU-Acanthamoeba system may be useful for the construction of cDNA expression libraries, as well as for the expression of cloned genes.

DNA topoisomerase II inhibitors enhance random integration of transfected vectors into human chromosomes

To study the involvement of DNA topoisomerase (topo) II on nonhomologous (illegitimate) recombination, we examined the effect of topo II inhibitors on random integration of exogenous vectors into human chromosomes. We transfected human cell lines PA1, HeLa and EJ-1 with linearized plasmid pSV2neo by electroporation, treated with topo II inhibitors and determined the frequency of Geneticin-resistant (G418r) colonies. We found that three topo II inhibitors, etoposide (VP-16), ICRF-193 and amsacrine (m-AMSA), greatly enhanced the frequency of G418r colonies. These effects were maximally expressed by as little as 12 hrs treatment with the drugs. Similar enhancements were found with different vectors (closed-circular and linear), different cell types, or by different transfection methods (calcium precipitation and lipofection). In contrast, the inhibitor treatments did not affect the transient expression of chloramphenicol acetyltransferase and beta-galactosidase activity following transfection with pSV2CAT and pCH110, respectively. Southern blot analysis revealed that the integration pattern of transfected pSV2neo into PA1 chromosomes was random and not characteristic for each inhibitor. These results suggest that topo II inhibitors directly act at a nonhomologous recombination reaction, promoting the integration process of transfected vectors into human chromosomes. We discuss the enhancement mechanism with a special emphasis on DNA strand breaks induced by the inhibitors.

An early history of gene transfer and therapy

The term "gene therapy" was coined to distinguish it from the Orwellian connotations of "human genetic engineering," which, in turn, was derived from the term "genetic engineering." Genetic engineering was first used at the Sixth International Congress of Genetics held in 1932 and was taken to mean "the application of genetic principles to animal and plant breeding." Once the basics of molecular genetics and gene transfer in bacteria were established in the 1960s, gene transfer into animals and humans using either viral vectors and/or genetically modified cultured cells became inevitable. Despite the early exposition of the concept of gene therapy, progress awaited the advent of recombinant DNA technology. The lack of trustworthy techniques did not stop many researchers from attempting to transfer genes into cells in culture, animals, and humans. Viral genomes were used for the development of the first relatively efficient methods for gene transfer into mammalian cells in culture. In the late 1970s, early transfection techniques were combined with selection systems for cultured cells and recombinant DNA technology. With the development of retroviral vectors in the early 1980s, the possibility of efficient gene transfer into mammalian cells for the purpose of gene therapy became widely accepted.

En bloc incorporation of coatomer subunits during the assembly of COP-coated vesicles

The cDNA encoding epsilon-COP, the 36-kD subunit of coatomer, was cloned from a bovine liver cDNA library and sequenced. Immunoblotting with an anti-epsilon-COP antibody showed that epsilon-COP exists in COP-coated vesicles as well as in the cytosolic coatomer. Using the cloned cDNA, recombinant His6- tagged epsilon-COP was overexpressed in cultured Chinese hamster ovary (CHO) cells, from which metabolically radiolabeled coatomer was purified by taking advantage of the His6 tag. Radiolabeled coatomer was employed to establish that all the subunits of the coatomer enter coated vesicles as an intact unit.

Regulation of G418 selection efficiency by cell-cell interaction in transfection

We attempted to establish the optimum conditions for the calcium phosphate (CaPO4) precipitation protocol by counting G418 resistant (G418r) colonies after transfection of pSV2-neo DNA into BALB 3T3 cells. The amount and molecular size of carrier DNA, number of plating cells, treatment period of DNA-CaPO4 precipitates and expression time of G418 selection were found to be important factors in the induction of G418r colonies. Six G418r clones were derived from BALB 3T3, NIH 3T3 and FRSK cells, and cocultured with G418 sensitive (G418s) parent cells in G418 medium. The colony formation capacity of all G418r cell clones decreased with the increasing number of plated G418s cells. Cell-cell contact appeared to be necessary to reduce the colony formation of G418r cells, and contact-dependent G418r cell killing was probably not related to gap junction formation. Contact-mediated cell killing is a likely explanation for the observation that induction of G418r colonies is often reduced under conditions of high-density plating, long treatment of DNA-CaPO4 precipitates, and long expression time of G418 selection. These results suggest that in some instances transfection efficiency using pSV2-neo DNA should be carefully evaluated because culture conditions can mask the induction of G418r colonies.

The activated human c-Ha-ras-1 oncogene as a mutagen

The induction of gene mutations and chromosome aberrations by plasmid pEJ6.6 carrying the activated c-Ha-ras-1 oncogene from human bladder carcinoma was studied in cultured Chinese hamster cells. Both an increase in the frequency of hypoxanthine-phosphoribosyltransferase-deficient (HPRT-) mutants and chromosome aberrations was observed after pEJ6.6 transfection as compared to control series (pBR322). In order to define whether it is the oncogene which is responsible for the mutagenic effect of pEJ6.6, a derivative of c-Ha-ras-1 carrying a deletion in its coding region was constructed. As shown in all experiments, the frequency of HPRT- mutants after treatment with pEJ6.6 plasmid exceeded that in control dishes treated by pEJ6.6 plasmid with an inactivated oncogene. The effect was rather weak but statistically significant. Thus, the results of experiments carried out show that the mutagenic activity of pEJ6.6 plasmid is chiefly determined by its oncogene. The role of the mutagenic effects of activated oncogenes in malignant transformation is discussed.

DNA alterations at proto-oncogene loci and their clinical significance in operable non-small cell lung cancer

DNA from tumor samples of 54 patients with operable non-small cell lung cancer (NSCLC) was analyzed to determine whether proto-oncogene alterations could be correlated with the clinical behavior of lung cancer. Among seven proto-oncogenes tested, changes in the copy number of Ha-ras, c-myc and c-raf-1 were found in only seven tumors. Most of them were epidermoid carcinomas without lymph node involvement (N0). In spite of a localized disease with complete surgical resection, six of these patients relapsed within a mean disease-free interval (DFI) of only 6.5 months. There is a significant correlation between DNA alterations at proto-oncogene loci and clinical relapse within 12 months of surgical resection (P less than 0.025).

Glucocorticoid repression and basal regulation of the epoxide hydrolase promoter

Through a series of promoter deletions and gene transfer experiments we have examined the basal regulation and glucocorticoid-mediated repression of the rat epoxide hydrolase gene. Three regions of the 5' flanking sequence were found to influence the basal level of promoter function in H4IIE hepatoma cells. Region A (-891 to -355 bp) contains an apparent repressor of epoxide hydrolase expression, while regions B (-271 to -171 bp) and C (-141 to -85) were found to contain important sequences required for optimal promoter activity. Previous work has demonstrated that dexamethasone represses epoxide hydrolase transcription by approximately 50% in isolated rat liver nuclei, and, in this study, we have demonstrated that the ability of the epoxide hydrolase promoter to drive CAT expression is similarly repressed in H4IIE cells treated with 1 microM dexamethasone. Furthermore, the level of endogenous epoxide hydrolase mRNA is decreased by 70-88% in nontransfected H4IIE cells treated with dexamethasone. Interestingly, promoter activity was not decreased by dexamethasone in COS cells, which lack glucocorticoid receptors. The current data show that sequences from -42 to +110 bp are sufficient to support the dexamethasone response, and, furthermore, they suggest that repression may not require direct interaction of the ligand-receptor complex with the promoter region.

Development of new cell lines for animal cell biotechnology

Mammalian cell culture has been an important technique in laboratory-scale experimentation for many decades. Developments in large-scale culture have been due to the need to grow large numbers of cells to support the growth of viruses for vaccine production, and more recently, for growing hybridoma cells as a source of monoclonal antibody. Increasingly, however, pharmaceutical products such as hormones, enzymes, growth factors, and clotting factors are being produced from cell lines which have been manipulated by recombinant DNA techniques. It is clear, therefore, that the high cost of growing mammalian cells on a large scale does not necessarily prohibit their use for biotechnology, and indeed there is considerable evidence to suggest that animal cell biotechnology will continue to be a major growth area in the future.

Chromosomal recombination and breakage associated with instability in mouse centrometric satellite DNA

A mouse L cell line containing the centromeric insertion of herpes thymidine kinase genes (tk) was previously shown to undergo a high frequency of DNA rearrangement at the site of tk insertion. Analysis of TK- revertants had demonstrated that DNA rearrangements were usually associated with DNA deletion and were always mediated by intrachromosomal recombinations. In this study, we further analyzed several TK+ subclones to examine the mode of DNA rearrangements in the absence of negative selection pressure. In two clones, LC2-3F and LC2-3E17, rearrangements were accompanied by DNA amplification and were mediated by intrachromosomal recombination. In subclone LC2-3E17-19, we further detected perturbations in the pattern of centromeric heterochromatization. This was associated with chromosome instability, as evidenced by chromosome breakage at the centromere. The analysis of three other sibling clones, LC2-3, LC2-6 and LC2-15, further suggests that reciprocal recombination events may play a role in such centromeric rearrangements. These results suggest that DNA rearrangements in the centromere may be mediated by a number of different mechanisms, and generally do not affect chromosome stability except when accompanied by changes in the pattern of heterochromatization.

Efficient DNA-mediated gene transfer into primary cultures of adult rat hepatocytes

An efficient, simple, and reproducible DNA-mediated gene transfer procedure has been developed for primary cultures of adult rat hepatocytes. Calcium phosphate-DNA precipitate is formed in complete culture medium during 5 hr incubation with cells. Unabsorbed precipitate is then washed out, and 40 hr later gene expression is measured. Under optimal conditions, up to 20-25% of cells in cultures transfected with the beta-galactosidase (lacZ) gene stain positively for this activity, and cells transfected with the chloramphenicol acetyl transferase (CAT) gene, fused to a strong promoter, express CAT activities of 10-14 nmoles/min per mg protein. Five conditions were optimized based on transfection efficiency, CAT expression, and cell viability. (i) Medium composition: the presence of protein, such as fetal bovine serum or bovine serum albumin, in the medium was essential. (ii) Cell substratum: tissue culture plastic was superior to calf skin collagen and Matrigel. (iii) Cell density: 0.5-1.0 X 10(6) cells/60-mm dish were superior to higher densities. (iv) Duration of exposure to calcium phosphate-DNA: 5-8 hr was better than shorter or longer times. (v) Length of time hepatocytes were maintained in culture before initiating transfection: 2-3 days was superior to earlier times. This procedure was successful with reporter genes linked to three different eukaryotic promoters. These included a chimeric promoter containing the polycyclic aromatic hydrocarbon-responsive enhancer of the cytochrome P450c gene (CYP1A1), which was shown to confer upon the CAT gene responsiveness to polycyclic aromatic hydrocarbons comparable to that of the native P450c gene. This transfection procedure should be of considerable use for the study of liver-specific gene expression in primary hepatocyte cultures.

The histidine residue of codon 715 is essential for function of elongation factor 2

Several mutant cDNAs of elongation factor 2 (EF-2) were constructed by site-directed mutagenesis and their products expressed in mouse cells were investigated. Amino acid substitution for the histidine residue of codon 715, which is modified post-translationally to diphthamide, resulted in non-functional EF-2 and this substitution did not render EF-2 resistant to Pseudomonas aeruginosa exotoxin A, which inactivates EF-2 transferring ADP-ribose to the diphthamide residue. These non-functional EF-2s with replacements of the histidine-715 residue showed various extents of inhibition of protein synthesis by competing with functional EF-2 in vivo. These results suggest that histidine-715 is essential for the translocase activity of EF-2 and that the region around diphthamide functions in recognition of, and/or binding to ribosomes. Substitution of proline for the alanine-713 residue and substitution of glutamine for the glycine-717 residue converted EF-2 to partially toxin-resistant forms. Two-dimensional gel analysis with fragment A of diphtheria toxin of these toxin-resistant EF-2s revealed that their ADP-ribosylations by toxin were much less than that of wild-type EF-2.

Two types of mouse FM3A cell mutants deficient in 5-aminoimidazole-4-carboxamide ribonucleotide transformylase and their transformants isolated by human chromosome-mediated gene transfer

We isolated three adenine auxotrophic mutants (Ade1, Ade2 and Ade3) of mouse FM3A cells deficient in 5-aminoimidazole-4-carboxamide ribotide transformylase (EC 2.1.2.3) activity. Ade1 and Ade3 but not Ade2 also lacked inosinicase (EC 3.5.4.10) activity. While Ade2 and Ade3 complemented each other, Ade1 complemented neither Ade2 nor Ade3, suggesting that two complementation groups exist in these mutants. We introduced human genes into the Ade2 and Ade3 cells by chromosome-mediated gene transfer. All the transformants tested were found to produce the human transformylase and inosinicase, and identical DNA bands containing human Alu sequences were detected in the transformants of Ade2 and Ade3. These mutants seem to have arisen by mutation in the same gene or adjacent genes, since only human chromosome 2 was capable of rescuing the genetic defects in all these mutants.

High-frequency deletion event at aprt locus of CHO cells: detection and characterization of endpoints

Two mechanisms are implicated in generating recessive drug resistance mutants at the adenine phosphoribosyltransferase (aprt) locus of Chinese hamster ovary (CHO) cells, one of which is a spontaneous high-frequency deletion of the entire gene. We have isolated and mapped a 19-kb fragment carrying aprt and its flanking sequences. A Southern blot study of 198 independent deletion mutants revealed that two different mutants have one of their breakpoints within the 19-kb region analyzed. One of these has an upstream breakpoint which could be narrowed down to a 4-kb fragment containing repetitive sequences. The other mutant has a breakpoint within a 410-bp sequence located 8.5 kb downstream of the aprt gene and which carries several elements similar to those signaling V-(D)-J joining in immunoglobulin and T-cell receptor gene rearrangements. In each case the other breakpoint lay outside of the analyzed region. These results support the previous indications that the deletions created by this spontaneous event are large.

Identification of microinjected cells using biotinylated antibodies and Strep-avidin-conjugated horseradish peroxidase

Results from experiments using needle microinjection of cells are often compromised by an inability to readily demonstrate which cells within a population have been injected. The technique described here allows the unambiguous identification of cells that have been successfully microinjected. Sequential incubation of fixed cells with biotinylated anti-immunoglobulin antibodies, followed by horseradish peroxidase (HRP)-conjugated Strep-avidin and HRP substrate, provides a sensitive assay for identification of cells containing trace amounts of immunoglobulins. This allows direct correlation to the presence of injected molecules of effects on cell morphology, the ability to enter into DNA synthesis, or expression of specific genes. By a variety of criteria, nonspecific immunoglobulins do not adversely affect cellular processes when injected by themselves or in the presence of other proteins known to have biological effects when injected, such as cAMP-dependent protein kinase and the ras oncogene protein.

Gene transfer from targeted liposomes to specific lymphoid cells by electroporation

Large unilamellar liposomes, coated with protein A and encapsulating the gene that confers resistance to mycophenolic acid, were used as a model system to demonstrate gene transfer into specific lymphoid cells. Protein A, which selectively recognizes mouse IgG2a antibodies, was coupled to liposomes to target them specifically to defined cell types coated with IgG2a antibody. Protein A-coated liposomes bound human B lymphoblastoid cells preincubated with a mouse IgG2a anti-HLA monoclonal antibody but failed to adhere to cells challenged with an irrelevant (anti-H-2) antibody of the same isotype or to cells incubated in the absence of antibody. Transfection of target cells bound to protein A-coated liposomes was achieved by electroporation. This step was essential since only electroporated cells survived in a selective medium containing mycophenolic acid. Transfection efficiency with electroporation and targeted liposomes was as efficient as conventional procedures that used unencapsulated plasmids free in solution but, in the latter case, cell selectivity is not possible. This technique provides a methodology for introducing defined biological macromolecules into specific cell types.

Active ras and myc oncogenes can be compatible, but Sv40 large T antigen is specifically suppressed with normal differentiation of mouse embryonic stem cells

The pathobiological effects of oncogenes on normal differentiation of mouse embryonic stem cells from 4-day embryos were examined by introducing active ras, myc, and SV40 large T genes, all driven by mouse metallothionein I enhancer and promoter. Stem cell clones R5, M3, and T2 for ras, myc, and SV40 T genes, respectively, were particularly chosen for analyses because of their higher levels of transgene expression and their diploid chromosomal constitutions. These stem cells were then introduced into host 4-day embryos and the embryos were allowed to develop in the uterus of foster mothers. The stem cells colonized the tissues as extensively as the parent cells and gave rise to adult chimera with no apparent loss or abnormality of the embryos. The active ras and myc oncogenes introduced were expressed not only in the stem cells, but also in the developing embryos and in a variety of tissues of adult chimeras. However, although T antigen was originally expressed in the stem cells, it was not expressed in either developing embryos or tissues of adult chimeras. Induced by retinoic acid treatment in vitro or by subcutaneous grafting, this suppression of T-gene expression was also confirmed in differentiated progeny cells from several stem cell clones expressing T antigen. Permanent lines of fibroblast-like cells could be established at higher frequency from primary cultures of tissues of chimera, subcutaneous differentiated cells, and in vitro differentiated cells derived from T2 cells, and all these clones reexpressed T antigen. The results suggest that active myc and ras genes can be compatible with normal differentiation of the stem cells, but the expression of T antigen is specifically suppressed with recognition of its coding domain.

Isolation and characterization of ras-transfected BALB/3T3 clone showing morphological transformation by 12-O-tetradecanoyl-phorbol-13-acetate

The transformation frequency of mouse BALB/3T3 cells was significantly enhanced after transfection with an activated ras oncogene (v-Ha-ras) followed by treatment with a tumor promoter, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), suggesting that the ras oncogene acted as an initiator in two-stage carcinogenesis. A cell clone (Bhas42) containing the ras oncogene was isolated from the ras-transfected BALB/3T3 cells. Bhas42 cells were flat and showed contact inhibition, but the addition of TPA to quiescent Bhas42 cultures resulted in a dramatic change of cell morphology to spindle shape, doubling of the cell population, and increased DNA synthesis.

Identification of human gene complementing ts AlS9 mouse L-cell defect in DNA replication following DNA-mediated gene transfer

The temperature-sensitive (ts) mouse L-cell, ts AlS9, is defective in a gene required for nuclear DNA replication early in the S phase of the cell cycle. Human DNA sequences were introduced into ts AlS9 cells together with the plasmid pSV2neo, which can confer resistance to the drug geneticin. Cotransformants, expressing both the plasmid-derived neomycin gene and the transferred human AlS9 gene, were selected for growth in the presence of the drug at the nonpermissive temperature (npt). The resulting transformants retained a common set of human-specific Alu repetitive DNA sequences. These are likely to be accommodated within, or in proximity to, the transferred human AlS9 gene. The results obtained provide the basis for cloning human genes required for DNA replication.

The aprt heterozygote/hemizygote system for screening mutagenic agents allows detection of large deletions

Frequencies of mutation at the hprt and aprt loci in various CHO cell lines were measured after exposure of the cells to ionizing radiation. In D423 and AA8-16, which are aprt+/- heterozygotes, the ratio of hprt- mutants to aprt- mutants ranged from 0.11 to 0.36. In D422 and AA8-5, which are aprt+/0 cell lines in which only one copy of the gene and its flanking sequences is present these ratios were greater than 5. In contrast, chemical mutagenesis generated mutations at both loci, in all cell lines, at equal frequencies. Southern blot analysis of DNA from hprt- and aprt- mutants of one of the aprt+/- heterozygous lines showed some apparently unaltered genes, some rearrangements and some complete deletions of hprt among hprt- mutants, but only complete deletions of aprt-linked sequences among aprt- mutants. These results strongly suggest that X-ray-induced mutational events are frequently larger than 40 kb (the length of the hprt gene) and that the difference among the frequencies observed at the two loci in the two types of cell lines were due to the presence of essential sequences close the respective target genes. The combined use of these cell lines in screening environmental mutagens should allow qualitative as well as quantitative analysis of the mutagenic potential of environmental agents.

Inactivation of DNA-mediated transformation of hamster cells by gamma-rays and deoxyribonuclease I

DNA damage has been induced in the mammalian expression vector pSV2-gpt by irradiation with X-rays or treatment with deoxyribonuclease I (DNAase I) under controlled conditions in vitro. The biological effect of such treatment was assessed by stable gene expression in Chinese hamster ovary (CHO) cells using DNA-mediated gene transfer. Induction of DNA double-strand breaks (dsbs), resulting from the interaction of independently-induced single-strand breaks (ssbs) under the present conditions, was measured by agarose gel electrophoresis of the treated vector. The correlation between radiation-induced gene inactivation and dsb induction mediated by OH radicals suggests that a dsb in the gene is a major inactivating lesion in this system. Individual radiation-induced ssbs and nucleotide damage are produced much more frequently than dsbs under these conditions, but the majority of these lesions do not appear to inactivate the gpt gene. DNAase I treatment, giving only simple 5' P + 3' OH breaks in the vector DNA, gave a correlation of approximately 1.5 dsb in the gpt gene per inactivating event, confirming little repair of dsbs in this system. Inactivation of the gpt gene without appreciable formation of dsbs was found, however, when the vector was irradiated at high dose rate in the presence of the OH-radical scavenger KBr. The nature of non-break damage causing inactivation requires further study.

Factors influencing efficiency and reproducibility of polybrene-assisted gene transfer

A systematic investigation of factors influencing the efficiency of polybrene-assisted gene transfer for both transient and stable foreign gene expression was carried out utilizing NIH 3T3 fibroblasts as prototypic recipients for the plasmid expression vectors pSV2cat and pSV2neo. While transfection cocktail composition and cell density, in addition to polybrene exposure conditions and exogenous DNA concentration, each played an important role, the key determinant to achieving excellent transfection efficiency proved to be the DMSO treatment regimen. Under optimal conditions, the yield of colonies resistant to the neomycin analog, G418, increased linearly at the rate of 10 clones/ng of input (native form I pSV2neo) DNA up to a plasmid concentration of 50 ng, whereupon the dose-response for colony recovery became semilogarithmic. The incidence of stable transformants was doubled by linearization of the vector DNA, whereas the addition of carrier DNA to the transfection cocktail was without effect until present at concentrations above 10-fold molar excess, at which point the efficacy of gene transfer declined rapidly. Combined Southern and dot-blot analyses of transformed cell DNA demonstrated that the polybrene-DMSO procedure led to the stable integration of relatively few copies of the marker gene in each transformant; the actual number varied from 1-3 to 10-15 per host genome, depending on the concentration of pSV2neo DNA added. The potential for the adaptation of this DNA transfection procedure for general use with other mammalian cell types, as well as its technical strengths and weaknesses, is discussed.

High-efficiency transformation of mammalian cells by plasmid DNA

We describe a simple calcium phosphate transfection protocol and neo marker vectors that achieve highly efficient transformation of mammalian cells. In this protocol, the calcium phosphate-DNA complex is formed gradually in the medium during incubation with cells and precipitates on the cells. The crucial factors for obtaining efficient transformation are the pH (6.95) of the buffer used for the calcium phosphate precipitation, the CO2 level (3%) during the incubation of the DNA with the cells, and the amount (20 to 30 micrograms) and the form (circular) of DNA. In sharp contrast to the results with circular DNA, linear DNA is almost inactive. Under these conditions, 50% of mouse L(A9) cells can be stably transformed with pcDneo, a simian virus 40-based neo (neomycin resistance) marker vector. The NIH3T3, C127, CV1, BHK, CHO, and HeLa cell lines were transformed at efficiencies of 10 to 50% with this vector and the neo marker-incorporated pcD vectors that were used for the construction and transduction of cDNA expression libraries as well as for the expression of cloned cDNA in mammalian cells.

Plasmidal maintenance of composite DNA derived from polyoma related plasmid, L factor

Recently, we reported a multicopy mammalian plasmid with a structure related to polyoma. The plasmid, named L factor, was found at a high copy number (5,000 or more per cell) in a subclone derived from mouse L cells. We attempted to utilize L factor as a plasmid vector for mammalian cells. A series of composite DNA consisting of L factor and a foreign (herpes simplex virus tk) were constructed. These DNA could be established as plasmids after transfection to several mouse cell lines, although the copy number of the re-established plasmids was considerably less than that observed for the original subclone. The composite DNA maintained the structure of the original DNA after prolonged culture and the copy number remained constant even with no selective pressure. A composite DNA, with no DNA sequence corresponding to polyoma T antigen, could also be established as a plasmid in a mouse L cell line in which polyoma T antigen is expressed. The potential use of the plasmid is discussed.

Poly-L-ornithine-mediated transformation of mammalian cells

Poly-L-ornithine has been used to introduce DNA and RNA into mammalian cells in culture. Ornithine-mediated DNA transfer has several interesting and potentially useful properties. The procedure is technically straightforward and is easily applied to either small or large numbers of recipient cells. The efficiency of transformation is high. Under optimal conditions, 1 to 2% of recipient mouse L cells take up and continue to express selectable marker genes. DNA content of transformants can be varied reproducibly, yielding cells with just one or two copies of the new gene under one set of conditions, while under a different set of conditions 25 to 50 copies are acquired. Cotransformation and expression of physically unlinked genes occur at high efficiency under conditions favoring multiple-copy transfer. Polyornithine promotes gene transfer into cell lines other than L cells. These include Friend erythroleukemia cells and NIH 3T3 cells. Both are transformed about 1 order of magnitude more efficiently by this procedure than by standard calcium phosphate products. However, the method does not abolish the large transformation efficiency differences between these cell lines that have been observed previously by other techniques. (vi) mRNA synthesized in vitro was also introduced into cells by this method. The RNA was translated resulting in a transient accumulation of the protein product.

Use of electroporation for high-molecular-weight DNA-mediated gene transfer

Electroporation was used to introduce high-molecular-weight DNA into murine hematopoietic cells and NIH3T3 cells. CCRF-CEM cells were stably transfected with SV2NEO plasmid and the genomic DNA from G-418-resistant clones (greater than 65 kb) was introduced into mouse bone marrow and NIH3T3 cells by electroporation. NEO sequences and expression were detected in the hematopoietic tissues of lethally irradiated mice, with 24% of individual spleen colonies expressing NEO. The frequency of genomic DNA transfer into NIH3T3 cells was 0.25 X 10(-3). Electroporation thus offers a powerful mode of gene transfer not only of cloned genes but also of high-molecular-weight DNA into cells.

High-efficiency transformation of mammalian cells by plasmid DNA

We describe a simple calcium phosphate transfection protocol and neo marker vectors that achieve highly efficient transformation of mammalian cells. In this protocol, the calcium phosphate-DNA complex is formed gradually in the medium during incubation with cells and precipitates on the cells. The crucial factors for obtaining efficient transformation are the pH (6.95) of the buffer used for the calcium phosphate precipitation, the CO2 level (3%) during the incubation of the DNA with the cells, and the amount (20 to 30 micrograms) and the form (circular) of DNA. In sharp contrast to the results with circular DNA, linear DNA is almost inactive. Under these conditions, 50% of mouse L(A9) cells can be stably transformed with pcDneo, a simian virus 40-based neo (neomycin resistance) marker vector. The NIH3T3, C127, CV1, BHK, CHO, and HeLa cell lines were transformed at efficiencies of 10 to 50% with this vector and the neo marker-incorporated pcD vectors that were used for the construction and transduction of cDNA expression libraries as well as for the expression of cloned cDNA in mammalian cells.

Poly-L-ornithine-mediated transformation of mammalian cells

Poly-L-ornithine has been used to introduce DNA and RNA into mammalian cells in culture. Ornithine-mediated DNA transfer has several interesting and potentially useful properties. The procedure is technically straightforward and is easily applied to either small or large numbers of recipient cells. The efficiency of transformation is high. Under optimal conditions, 1 to 2% of recipient mouse L cells take up and continue to express selectable marker genes. DNA content of transformants can be varied reproducibly, yielding cells with just one or two copies of the new gene under one set of conditions, while under a different set of conditions 25 to 50 copies are acquired. Cotransformation and expression of physically unlinked genes occur at high efficiency under conditions favoring multiple-copy transfer. Polyornithine promotes gene transfer into cell lines other than L cells. These include Friend erythroleukemia cells and NIH 3T3 cells. Both are transformed about 1 order of magnitude more efficiently by this procedure than by standard calcium phosphate products. However, the method does not abolish the large transformation efficiency differences between these cell lines that have been observed previously by other techniques. (vi) mRNA synthesized in vitro was also introduced into cells by this method. The RNA was translated resulting in a transient accumulation of the protein product.

Modulation of tk expression in mouse pericentromeric heterochromatin

We have obtained a mouse transformant cell line containing two herpes viral thymidine kinase (tk) genes integrated in pericentromeric heterochromatin. Restriction analysis of tk- revertant and tk+ rerevertant derivatives suggest that one of the two tk genes is repressed in tk- cells, but is reactivated in tk+ rerevertants. The results of Northern analysis indicated that repression-activation is probably controlled at the transcriptional level. To examine the molecular basis for this repression, we cloned the tk gene from a tk- revertant cell line. Then, using the cloned tk gene as donor DNA to select for tk+ transformants, we found that it has a transfection efficiency indistinguishable from the viral tk gene. This indicates that repression is probably not mediated via any DNA sequence changes within the tk gene. The results of further studies by restriction analysis, azacytidine treatments, and secondary DNA transfection assays demonstrated that tk repression is associated with changes in DNA methylation. Surprisingly, derepression of the tk gene was accompanied by rearrangements in the flanking DNA. The latter result suggests that the flanking DNA may exert cis effects on tk gene expression. Additional studies with this system may provide insights into the molecular basis underlying position effects in heterochromatin.

A simple and efficient liposome method for transfection of DNA into mammalian cells grown in suspension

For a highly efficient plasmid transfection into mammalian cells grown in suspension, DNA was entrapped in liposomes prepared by the phosphatidylserine calcium-induced fusion method. Employing this technique, a transfection efficiency of about 2% was achieved, with 22 tk+-transformants obtained from 10(3) of mouse mammary carcinoma FM3Atk- cells transfected with a plasmid carrying the thymidine kinase (tk+) gene of the Herpes simplex virus. As compared with a previous report [Ayusawa et al., J. Biol. Chem. 258 (1983) 48-53], this transfection method was more than four orders of magnitude higher than the calcium phosphate method used for FM3Atk- cells. It was shown that the tk+ gene was integrated into the chromosomal DNA and was expressed in all the tk+-transformant clones tested. The described method could be applied to various types of DNA and cells, including those grown as monolayers.

Modulation of tk expression in mouse pericentromeric heterochromatin

We have obtained a mouse transformant cell line containing two herpes viral thymidine kinase (tk) genes integrated in pericentromeric heterochromatin. Restriction analysis of tk- revertant and tk+ rerevertant derivatives suggest that one of the two tk genes is repressed in tk- cells, but is reactivated in tk+ rerevertants. The results of Northern analysis indicated that repression-activation is probably controlled at the transcriptional level. To examine the molecular basis for this repression, we cloned the tk gene from a tk- revertant cell line. Then, using the cloned tk gene as donor DNA to select for tk+ transformants, we found that it has a transfection efficiency indistinguishable from the viral tk gene. This indicates that repression is probably not mediated via any DNA sequence changes within the tk gene. The results of further studies by restriction analysis, azacytidine treatments, and secondary DNA transfection assays demonstrated that tk repression is associated with changes in DNA methylation. Surprisingly, derepression of the tk gene was accompanied by rearrangements in the flanking DNA. The latter result suggests that the flanking DNA may exert cis effects on tk gene expression. Additional studies with this system may provide insights into the molecular basis underlying position effects in heterochromatin.

Regulated expression of a Drosophila melanogaster heat shock locus after stable integration in a Drosophila hydei cell line

DNA-mediated cotransformation has been used to transfer a Drosophila melanogaster heat shock locus into cultured Drosophila hydei cells by use of the copia-based selectable vector pCV2gpt and of pMH10A, a cloned 87A7 heat shock locus encoding a mutant heat shock protein (hsp). Transformed lines contain between 50 and 200 copies of both plasmids, each separately organized as a head-to-tail concatemer which is stably maintained in the transformed lines. Exposure of the cotransformants to heat shock temperatures induces the regulated expression of the hsp RNA and the mutant hsp in all the lines analyzed.

High frequency DNA rearrangements associated with mouse centromeric satellite DNA

A DNA transformed mouse cell line, generated by the microinjection of a pBR322 plasmid containing the herpes thymidine kinase (tk) gene, was observed to exhibit a high frequency of DNA rearrangement at the site of exogenous DNA integration. The instability in this cell line does not appear to be mediated by the tk inserts or the immediately adjacent mouse DNA, but instead may be a consequence of the larger host environment at the chromosomal site of tk insertion. Results obtained from restriction analysis, in situ chromosome hybridizations, and cesium chloride density-gradient fractionations indicate that the tk inserts are organized as a single cluster of direct and inverted repeats embedded within pericentromeric satellite DNA. To determine the molecular identity of the flanking host sequences, one of the mouse-tk junction fragments was cloned, and subsequent restriction and sequence analyses revealed that this DNA fragment consists almost entirely of classical mouse satellite DNA. On the basis of these observations, we suggest that the instability in this cell line may reflect the endogenous instability or fluidity of satellite DNA.

Regulated expression of a Drosophila melanogaster heat shock locus after stable integration in a Drosophila hydei cell line

DNA-mediated cotransformation has been used to transfer a Drosophila melanogaster heat shock locus into cultured Drosophila hydei cells by use of the copia-based selectable vector pCV2gpt and of pMH10A, a cloned 87A7 heat shock locus encoding a mutant heat shock protein (hsp). Transformed lines contain between 50 and 200 copies of both plasmids, each separately organized as a head-to-tail concatemer which is stably maintained in the transformed lines. Exposure of the cotransformants to heat shock temperatures induces the regulated expression of the hsp RNA and the mutant hsp in all the lines analyzed.

Novel approach for restriction mapping repetitive DNA elements using DNA transformation

We demonstrated that DNA transformation can be used to determine the linkage relationship between DNA restriction fragments in mouse genomic DNA. Using this experimental approach, we obtained linkage information which enabled us to construct a restriction map for the multiple thymidine kinase (tk) gene inserts present in a mouse L-cell line. This restriction map included cutting sites for seven restriction enzymes spanning a distance of over 10 kb. It revealed that the tk inserts in this cell line are arranged in a complex array consisting of direct and inverted repeats. In light of these results, we suggest that this approach will be particularly useful for restriction mapping DNA sequences that are repetitive as such DNA may be difficult to characterize by conventional methods alone.

Gene transfer method for transient gene expression, stable transformation, and cotransformation of suspension cell cultures

A new method was developed to study transient gene expression, stable transformation, and cotransformation in suspension cells, such as mouse myeloma and erythroleukemia cells. This method involves attachment of cells to a concanavalin A-coated tissue culture dish, treatment of cells with DEAE-dextran to adsorb plasmid DNA to the attached cells, and finally treatment with a 40% solution of polyethylene glycol to facilitate the uptake of DNA by the cells. Plasmids pSV2cat and pSV2neo were used as markers to optimize the conditions for transient gene expression and stable transformation, respectively, of mouse myeloma and erythroleukemia cells. This method was successfully used to obtain cotransformants of mouse myeloma cells.

Gene transfer method for transient gene expression, stable transformation, and cotransformation of suspension cell cultures

A new method was developed to study transient gene expression, stable transformation, and cotransformation in suspension cells, such as mouse myeloma and erythroleukemia cells. This method involves attachment of cells to a concanavalin A-coated tissue culture dish, treatment of cells with DEAE-dextran to adsorb plasmid DNA to the attached cells, and finally treatment with a 40% solution of polyethylene glycol to facilitate the uptake of DNA by the cells. Plasmids pSV2cat and pSV2neo were used as markers to optimize the conditions for transient gene expression and stable transformation, respectively, of mouse myeloma and erythroleukemia cells. This method was successfully used to obtain cotransformants of mouse myeloma cells.