Genetic transformation of somatic cells. VI. Transfer of the trait of the rate of loss of transformant phenotype by means of DNA from cells containing the thymidine kinase gene of the herpes simplex virus

Using dot-hybridization with thymidine kinase gene (tk gene) of Herpes simplex virus type 1 (HSV 1) of DNA preparations obtained from isolated metaphase chromosomes and lysate fractions of metaphase cells, which presumably contain smaller particles compared to metaphase chromosomes, it has been shown that the tk gene of HSV 1 is localized in chromosomes of cells of transformant clones unstable in TK+-phenotype. The DNA isolated from the metaphase chromosomes from cells of transformant clones is 1.5- or 2-fold more efficient in transforming TK-Chinese hamster cells than is the total high molecular weight DNA from the same cells. Upon transformation of TK- cells by the high molecular weight DNA from the tk gene of HSV 1-containing clones, varying in the rate of the loss of TK+-phenotypes, the character "rate of the loss of transformant phenotype" is transferred together with the tk gene of HSV 1 in 22% of cases. Cells of rerevertant clones, produced from TK- subclones of transformant clones, display the rate of the loss of transformant phenotype characteristic of cells of parental TK+-clones. A comparison of the results allows a conclusion that DNA sequences, determining the character "rate of the loss of transformant phenotype", are linked tightly with the transforming DNA proper containing the tk gene of HSV 1, but are not localized inside such a DNA.     

Genetic transformation of somatic cells. VIII. The effect of the DNA methylation inhibitor 5-azacytidine on the stability of thymidine kinase-negative and -positive phenotypes of transformant clone cells

A study was made of the effect of an DNA methylation inhibitor 5-azacytidine (azaC) on the frequency of reversion to a thymidine kinase-positive (TK+) phenotype in 5-bromodeoxy-uridine (BrdU)-resistant subclones obtained from clones of Chinese hamster cells transformed by thymidine kinase gene (tk-gene) of Herpes simplex virus type 1 (HSV1). It is shown that in 8 of 15 BrdU-resistant subclones azaC increases 2-1000-fold the frequency of reversion to TK+ phenotype. Variations in the inducibility of reversions to TK+ phenotype indicate that the DNA methylation associated with TK- phenotype affects but differently tk gene of HSV1. Cultivation of TK+ cells of transformant clones in the presence of azaC may lead to stabilization (or decrease in the rate of the loss) of TK+ phenotype, or may not influence the stability of transformant phenotype. The reaction of TK+ cells of transformant clones depends both on genetically determined rate of the loss of TK+ phenotype, and on the structure of transforming DNA introduced to cells. A conclusion is drawn that the TK- phenotype of transformant clone cells arises due to processes which are not associated with methylation of tk gene of HSV1 in spite of the fact that such a methylation may later stabilize significantly the TK- phenotype.     

Genetic transformation of somatic cells. VII. The loss of the transformant phenotype is accompanied by both stable and unstable changes in DNA

From 6 clones of Chinese hamster cells varying in the rate of the loss of transformant phenotype and containing a thymidine kinase gene (tk-gene) of Herpes simplex virus type 1 (HSV1), 25 subclones negative in thymidine kinase (TK-) were isolated on a medium with 50 micrograms/ml 5-bromodeoxyuridine (BrdU). A study was made of the frequency of spontaneous reversions to the TK+ phenotype in cell populations of BrdU-resistant subclones, and of the transforming activity (upon transformation of TK- cells of A238 clone to the TK+ phenotype) of DNA preparations from a row BrdU-resistant subclones. In 7 of 11 BrdU-resistant subclones the TK- phenotype is associated with changes reducing significantly the transforming activity of DNA. Some of these alterations are stable and undergo no spontaneous reversion, while the other ones are unstable, being reversed or suppressed at a high frequency. BrdU-resistant subclones produced from clones more stable in transformant phenotype are on the whole more stable in the TK- phenotype than BrdU-resistant subclones from the clones with the high rate of the loss of the TK+ phenotype.     

Genetic transformation of somatic cells. IV. The rate of loss of transformant phenotype depends on the structure of the transforming DNA and changes when the cells are treated with the tumor promotor 12-o-tetradecanoyl-phorbol-13-acetate

Analysis was made of the phenotype stability of some clones of thymidine kinase deficient (TK-) Chinese hamster cells transformed by thymidine kinase gene (TK-gene) of Herpes simplex virus type (HSV 1). The presence of a fragment of human satellite DNA III in the plasmid DNA carrying the TK-gene of HSV 1 reduced notably the rate of the loss of TK+-phenotype, and the treatment of the cells with a tumour promoter--12-o-tetradecanoyl-phorbol-13-acetate--immediately after transformation destabilizes TK+-phenotype of transformant clones. Removal of the eukaryotic carrier DNA for the plasmid DNA without the TK-gene of HSV 1 destabilizes the clone transformant phenotype. Changes in the structure of the plasmid DNA containing no TK-gene of HSV 1 and introduced into cells simultaneously with TK-gene containing plasmids affects the rate of the loss of TK+-phenotype transformed cells.     

Genetic transformation of somatic cells. IX. The loss of the transformant phenotype is accompanied by rearrangements in the plasmid DNA containing the thymidine kinase gene of the herpes virus

As demonstrated by dot-hybridization, the cells of HT-subclones isolated from the cells of transformant clones cultured on a non-selective medium differ significantly in the number of copies of thymidine kinase gene (tk-gene) of Herpes simplex virus (HSV1). Since the cells of transformant clones lose thymidine kinase-positive (TK+) phenotype during cultivation, this data are indicative of high frequence rearrangements in the region of transforming DNA as responsible for the transformant phenotype nonstability. These rearrangements, among other things, induce alterations in the number of copies of tk gene of HSV1. The analysis of cells of subclones isolated on a medium containing 5-bromodeoxyuridine (BrdU) shows that the number of copies of tk gene of HSV1 decreases as compared to the cells of parental clones. The decrease in the number of copies of tk gene of HSV1 in a row of BrdU-resistant subclones is accompanied by simultaneous increase in the number of sequences of pBR325 plasmide DNA to which tk gene of HSV1 is linked covalently in the pST826 plasmide introduced into cells of transformant clones. This evidence implies a most complex nature of transforming DNA rearrangements reducing the number of copies of tk gene of HSV1 due possibly to a genetic correction. The analysis of results permits a hypothesis that instability of cells in transformant phenotype may be determined by the genetic instability of insertion type. The rate of the loss of transformant phenotype depends on the frequency of rearrangements in the transforming DNA locus.     

Genetic transformation of somatic cells. II. An analysis of the status of the plasmid nucleotide sequences in chromosomal DNA and the thymidine kinase activity in transformant clone cells

Chinese hamster A238 TK- -cells were transformed with plasmids (derivatives of pBR325) containing thymidine kinase (TK) gene of Herpes simplex virus type 1 (HSV1). The results of dot- and blot-hybridization indicate the presence of pBR325 sequences in the chromosomal fractions of DNA in the transformant clones. These sequences are probably tandemly arranged, and each cluster contains 25--50 copies. SV40 sequences cloned in pBR325 were introduced into the Chinese hamster cells by co-transformation with TK-gene of HSV1-containing plasmid DNA, and all the co-transformant clones selected for TK+-phenotype were shown by hydridization to contain 3V40 DNA fragments. Isoelectrofocusing in polyacrylamide gel shows that thymidine kinase from TK+-transformant clones is of viral type (isoelectric point 7), in contrast to the cellular enzyme (coded by chromosomal gene) having alkaline isoelectric point (pH 9). The results suggest that the true TK+-transformant cells are selected by the procedure used in this study.     

Stability of the transformants obtained by phage particle-mediated gene transfer

Recombinant lambda phage DNA, encapsulated in phage particles and coprecipitated with calcium phosphate, efficiently transforms cultured mammalian cells without a requirement for carrier DNA. The present paper analyzes the stability of the transformants obtained by the phage transfer method. lambda phage particles containing recombinant DNA that includes the thymidine kinase (TK) gene of herpes simplex virus type 1 as a selective marker were introduced into Ltk- cells deficient in TK activity, and TK+ transformants were selected in HAT medium. To test the stability of the TK+ phenotype of the transformants, seven individual transformant clones were isolated, cultured in HAT selective medium and then in non-selective medium for various lengths of time. After such culture, transformants were allowed to develop colonies in both selective and non-selective medium. For all seven transformant clones, the numbers of colonies obtained in the two types of medium were almost identical, irrespective of whether or not each transformant clone had been previously cultured for 15 to 50 days in non-selective medium. This result suggests that most transformants obtained by the phage transfer method maintain the TK+ phenotype stably, for at least 50 days, when grown in non-selective medium.     

Genetic transformation of somatic cells. V. Inheritability of the rate of loss of the trait and the stabilization of the transformant phenotype

Subclones were isolated both on selective and nonselective medium from the Chinese hamster cells transformed by thymidine kinase gene (TK-gene) of Herpes simplex virus (HSV-1) and varying in the rate of the loss of transformant phenotype. The study of the stability of thymidine kinase-positive (TK+) phenotype in cell populations the subclones shows that the nonstability and the rate of the loss of transformant phenotype are the characters that are inherited in the cell generations. Durable cultivation on a HAT-selective medium may lead to a complete or partial (expressed as a reduced rate of the loss of the character) stabilization of TK+-phenotype of the cells of transformant clones. The rate of stabilization of TK+-phenotype may differ depending on the structure of transforming DNA introduced into cells of transformant clones.     

Genetic transformation of somatic cells. III. An analysis of the status of the plasmid nucleotide sequences in the extrachromosomal DNA of transformant clone cells and the rescue of extrachromosomal molecules of the plasmid DNA

Extrachromosomal DNAs from TK+ transformant clones of A238 Chinese hamster cells isolated after the treatment with plasmid pST826 containing thymidine kinase gene (TK-gene) of Herpes simplex virus (HSV1) and 1.8 kb insert of human satellite III DNA (HSIII) were studied by hybridization technique. In two TK+-clones (2T301 and 2T16) large quantities of rearranged plasmid DNA molecules were found. Electron microscopy show in clone 2T301 the presence of circular DNAs with average length being 4.64 +/- 0.27 kb. These molecules were rescued by retransformation into E. coli and analysed by restriction mapping and hybridization. All of them contain deletions spanning the entire TK gene of HSV1 and pBR325 sequences situated just downstream from the ORI of replication. The origin of extra-replicating circular DNA in 2T301 clone is discussed.     

Carrier DNA affects the integration of HSV-1 TK gene in the genome of L929TK- cells.

L929TK- cells were cotransfected with DNA mixtures containing tk gene of HSV-1, plasmids carrying LTR of MoMLV or RSV and carrier DNA of salmon sperm or chromosomal DNA of recipient cells. Selection of TK+ transformants was conducted in DMEM supplemented with HAT. Plasmids carrying LTR sequences of MoMLV or RSV retroviruses showed enhancing effect on the frequency of TK+ transformation. Southern blot analysis of chromosomal DNA of TK+ transformants demonstrated in clones deriving from cotransfections of tk gene and carrier DNA of L929TK- cells multiple copies of tk gene integrated into several genomic sites of host. Single copies of tk gene integrated into different sites of host genome occurred in chromosomal DNA of TK+ clones deriving from cotransfections of tk gene and carrier DNA of salmon sperm. Cells cotransfected with tk gene and plasmids carrying LTR sequences of MoMLV or RSV formed three dimensional colonies in semisolid agar medium. No effect of carrier DNA on the morphology of TK+ transformant clones was noticed.     

Imaging expression of cytosine deaminase-herpes virus thymidine kinase fusion gene (CD/TK) expression with [124I]FIAU and PET

Double prodrug activation gene therapy using the Escherichia coli cytosine deaminase (CD)-herpes simplex virus type 1 thymidine kinase (HSV1-tk) fusion gene (CD/TK) with 5-fluorocytosine (5FC), ganciclovir (GCV), and radiotherapy is currently under evaluation for treatment of different tumors. We assessed the efficacy of noninvasive imaging with [124I]FIAU (2'-fluoro-2'-deoxy-1-beta-D-arabinofuranosyl-5-iodo-uracil) and positron emission tomography (PET) for monitoring expression of the CD/TK fusion gene. Walker-256 tumor cells were transduced with a retroviral vector bearing the CD/TK gene (W256CD/TK cells). The activity of HSV1-TK and CD subunits of the CD/TK gene product was assessed in different single cell-derived clones of W256CD/TK cells using the FIAU radiotracer accumulation assay in cells and a CD enzyme assay in cell homogenates, respectively. A linear relationship was observed between the levels of CD and HSV1-tk subunit expression in corresponding clones in vitro over a wide range of CD/TK expression levels. Several clones of W256CD/TK cells with significantly different levels of CD/TK expression were selected and used to produce multiple subcutaneous tumors in rats. PET imaging of HSV1-TK subunit activity with [124I]FIAU was performed on these animals and demonstrated that different levels of CD/TK expression in subcutaneous W256CD/TK tumors can be imaged quantitatively. CD expression in subcutaneous tumor sample homogenates was measured using a CD enzyme assay. A comparison of CD and HSV1-TK subunit enzymatic activity of the CD/TK fusion protein in vivo showed a significant correlation. Knowing this relationship, the parametric images of CD subunit activity were generated. Imaging with [124I]FIAU and PET could provide pre- and posttreatment assessments of CD/TK-based double prodrug activation in clinical gene therapy trials.     

Genetic transformation of somatic cells. I. Clone of Chinese hamster cells defective in thymidine kinase and characterized by high transformation efficiency

A characteristics is given of clone A238 of the Chinese hamster cells deficient in thymidine kinase (TK). The isolation procedure is described. Upon transformation with the aid of DNA of plasmids, containing thymidine kinase gene (tk-gene) of Herpes simplex virus type 1 (HSV1) clone A238 cells show frequency (7.10(-5) and efficiency (130 TK+ colonies per 1 microgram of plasmid DNA) compatible with those of mouse line LMtk- cells. Modified transformation and selection conditions of clone A238 cells expressing TK-gene of HSV1 are demonstrated. A simple method is described for discriminating somatic cells, expressing either their proper or a virus TK-gene according to the cloning efficiency of cells on the HAT medium containing thymidine in concentration 100 micrograms/ml. It is shown that at the fixed total DNA concentrations a complete replacement of the eukaryotic carrier DNA for the plasmid DNA, containing no tg gene of HSV1, decreases but only insignificantly the frequency and efficiency of transformation.     

Virus Type-Specific Thymidine Kinase in Cells Biochemically Transformed by Herpes Simplex Virus Types 1 and 2

Transformation of mouse cells (Ltk(-)) and human cells (HeLa Bu) from a thymidine kinase (TK)-minus to a TK(+) phenotype (herpes simplex virus [HSV]-transformed cells) has been induced by infection with ultraviolet-irradiated HSV type 2 (HSV-2), as well as by HSV type 1 (HSV-1). Medium containing methotrexate, thymidine, adenine, guanosine, and glycine was used to select for cells able to utilize exogenous thymidine. We have determined the kinetics of thermal inactivation of TK from cells lytically infected with HSV-1 or HSV-2 and from HSV-1- and HSV-2-transformed cells. Three hours of incubation at 41 C produces a 20-fold decrease in the TK activity of cell extracts from HSV-2-transformed cells and Ltk(-) cells lytically infected with HSV-2. The same conditions produce only a twofold decrease in the TK activities from HSV-1-transformed cells and cells lytically infected with HSV-1. This finding supports the hypothesis that an HSV structural gene coding for TK has been incorporated in the HSV-transformed cells.     

I型单纯疱疹病毒胸苷激酶真核表达载体的构建及其表达鉴定

为构建单纯疱疹病毒I型胸苷激酶(HSV1TK)的真核表达载体pcDNA3.1-EGFP/HSV1TK,鉴定其在真核细胞中的表达和功能.以pORF-HSV1TK为模板,PCR扩增的目的基因HSV1TK片段与pMD18-T载体相连接构建重组克隆pMD18-T/HSV1TK.再双酶切出HSV1TK片段,插入pcDNA3.1-EGFP多克隆位点,构建pcDNA3.1-EGFP/ HSV1TK真核表达载体并进行酶切、测序鉴定[1].分别用荧光显微镜观察和RT-PCR方法检测脂质体介导pcDNA3.1-EGFP/ HSV1TK在卵巢癌细胞SKOV3的表达;分别用MTT法和光镜检测胸苷激酶/丙氧鸟苷(HSV1TK/GCV)系统对SKOV3体外杀伤作用及旁观者效应.结果表明,重组载体酶切鉴定结果与预期结果一致,基因序列与GenBank上报道的HSV1TK基因序列完全一致.荧光显微镜观察转染后的细胞发出绿色荧光;RT-PCR结果表明HSV1TK基因能在SKOV3内有效表达.MTT和光镜结果显示转染HSV1TK基因的SKOV3细胞,加入前体药物丙氧鸟苷(GCV)处理后对其有明显的杀伤作用和旁观者效应.成功构建的真核表达载体pcDNA3.1-EGFP/ HSV1TK能在SKOV3细胞中稳定表达,且HSV1TK对卵巢癌细胞株SKOV3体外有强大的杀伤作用和旁观者效应.     

Helper activity in antigen-specific antibody production mediated by CD4+ human cytotoxic T cell clones directed against herpes simplex virus

Three HSV type 1 (HSV-1) and HSV type 2 (HSV-2) common ("HSV-type common") and three HSV-1 specific CTL clones, which were CD3+, CD4+, CD8-, 4B4+, and 2H4-, were established. These clones proliferated in response to stimulation with HSV in the presence of autologous APC. The HSV type specificity of the proliferative response was identical with that of the cytotoxic activity of the clones. The cytotoxic activity and the proliferative response were both inhibited by addition of anti-HLA-DR mAb to the culture. After culture of these CTL clones with autologous B cells and macrophages followed by HSV Ag stimulation, anti-HSV antibody was detected in the culture supernatant. The HSV type specificity of the helper function for antibody production was identical with that of the cytotoxicity, i.e., HSV-type common clones, upon stimulation with either HSV-1, or HSV-2, and HSV-1-specific clones, upon stimulation with HSV-1 but not with HSV-2, showed helper activity for anti-HSV antibody production by autologous B cells. Moreover, it was found that these clones produced humoral factors which help autologous B cells to produce antibody. The helper factors were produced by T cell clones in an HSV-type-specific manner. These data suggest that some CD4+ T cells can simultaneously manifest both specific cytotoxicity and helper activity for Ag-specific antibody production by B cells, and that these multifunctional T cells might play an important role in protection against viral infection.     

Transfer of the herpes simplex thymidine kinase gene from human cells to mouse cells by means of metaphase chromosomes.

Thymidine kinase (TK)-deficient human cells were infected with ultraviolet light-inactivated Herpes simplex virus type 1, and "transformed" cells that expressed Herpes TK activity were isolated. Purified metaphase chromosomes were isolated from the transformed human line and incubated with TK-deficient mouse cells. TK+ cells were selected, and it was shown that these cells were gene transferents which expressed Herpes TK activity, identical to that found in the transformed human cells. The gene transferents contained no intact human chromosomes. When removed from selective pressure, the gene transferents rapidly lost the TK+ phenotype. However, upon continued growth in nonselective medium, a subpopulation in which the TK+ phenotype had become more stabilized appeared. These results suggest that the Herpes gene for thymidine kinase has integrated into the genome of the HSV-transformed human cells and that it can be transferred to other cells by means of purified metaphase chromosomes.     

A herpes simplex virus 1 integration site in the mouse genome defined by somatic cell genetic analysis.

Transfection experiments with HSV 1 in which one uses herpes simplex virus (HSV) thymidine kinase (TK) as a selectable prototrophic marker yield two classes of transformed cells: stable and unstable. In this report, we test the hypothesis that the stability phenotype can be explained by virus genome integration into a recipient cell chromosome. The method of analysis is by means of somatic cell genetics. We have isolated a series of microcell hybrids between a TK- Chinese hamster cell line and a transformed mouse cell line expressing the TK encoded by HSV 1. Several of the hybrid lines contain a single murine chromosome and express only the viral TK. Karyotypic analysis of these hybrids and of TK- derivatives generated by BrdUrd counterselection reveals that the TK+ phenotype is correlated with the presence of the terminal portion of the long arm of a specific murine chromosome. Results of extensive isozyme analyses of the hybrids and their TK- segregants fully corroborate the karyologic data. The results are consistent with the hypothesis that the viral tk gene is covalently integrated into this chromosomal region which itself does not appear to carry the endogenous murine tk locus. Other more complicated models are discussed. Our findings also show that somatic cell genetics can be used to localize viral integration sites in host chromosomes with high resolution.     

Biochemical transfer of single-copy eucaryotic genes using total cellular DNA as donor.

Previous studies from our laboratories have demonstrated the feasibility of transferring the thymidine kinase (tk) gene from restriction endonuclease-generated fragments of herpes simplex virus (HSV) DNA to cultured mammalian cells. In this study, high molecular weight DNA from cells containing only one copy of the HSV gene coding for tk was successfully used to transform L+K-cells to the tk+ phenotype. The acquired phenotype was demonstrated to be donor-derived by analysis of the electrophoretic mobility of the tk activity, and the presence of HSV DNA sequences in the recipient cells was demonstrated. In companion experiments, we used high molecular weight DNA derived from tissues and cultured cells of a variety of species to transfer tk activity. The tk+ mouse cells transformed with human DNA were shown to express human type tk activity as determined by isoelectric focusing.     

Phenotypic switching in cells transformed with the herpes simplex virus thymidine kinase gene

Biochemical transformation of Ltk- cells with the herpes simplex virus thymidine kinase (tk) gene resulted in numerous TK+ colonies that survived selection in hypoxanthine-aminopterin-thymidine medium. Many of these TK+ cell lines switched phenotypes and reverted to the TK- state. In this report, we describe the biological and biochemical characteristics of three TK- revertant lines. One (K1B5) transiently expressed TK in the presence of bromodeoxyuridine, which selects for the TK- phenotype. Another TK- sibling (K1B6n) expressed TK only after removal from bromodeoxyuridine-containing medium. The last variant (K1B6me) lost the ability to switch to the TK+ phenotype, although it maintained the herpes simplex virus sequences coding for TK. Loss of the ability of K1B6me cells to express TK was correlated with extensive methylation of the sequence recognized by the restriction endonuclease HpaII (pCpCpGpG). After these cells were treated with 5-azacytidine, they regained the ability to clone in hypoxanthine-aminopterin-thymidine medium and reexpressed virus tk mRNA and enzyme. In addition, the HpaII sites that were previously shown to be refractile to enzyme digestion were converted to a sensitive state, demonstrating that they were no longer methylated.