Co-transfer of human X-linked markers into murine somatic cells via isolated metaphase chromosomes

Research progress on the biological basis of Traditional Chinese Medicine syndromes of gastrointestinal cancers

Gastrointestinal cancers account for 11.6 % of all cancers, and are the second most frequently diagnosed type of cancer worldwide. Traditional Chinese medicine (TCM), together with Western medicine or alone, has unique advantages for the prevention and treatment of cancers, including gastrointestinal cancers. Syndrome differentiation and treatment are basic characteristics of the theoretical system of TCM. TCM syndromes are the result of the differentiation of the syndrome and the basis of treatment. Genomics, transcriptomics, proteomics, metabolomics, intestinal microbiota, and serology, generated around the central law, are used to study the biological basis of TCM syndromes in gastrointestinal cancers. This review summarizes current research on the biological basis of TCM syndrome in gastrointestinal cancers and provides useful references for future research on TCM syndrome in gastrointestinal cancers.

Transgenesis and Genome Engineering: A Historical Review

Our ability to modify DNA molecules and to introduce them into mammalian cells or embryos almost appears in parallel, starting from the 1970s of the last century. Genetic engineering techniques rapidly developed between 1970 and 1980. In contrast, robust procedures to microinject or introduce DNA constructs into individuals did not take off until 1980 and evolved during the following two decades. For some years, it was only possible to add transgenes, de novo, of different formats, including artificial chromosomes, in a variety of vertebrate species or to introduce specific mutations essentially in mice, thanks to the gene-targeting methods by homologous recombination approaches using mouse embryonic stem (ES) cells. Eventually, genome-editing tools brought the possibility to add or inactivate DNA sequences, at specific sites, at will, irrespective of the animal species involved. Together with a variety of additional techniques, this chapter will summarize the milestones in the transgenesis and genome engineering fields from the 1970s to date.

Monochromosomal Hybrids and Chromosome Transfer: A Functional Approach for Gene Identification

Functional complementation of cellular defects has been a valuable approach for localizing causative genes to specific chromosomes. The complementation strategy was followed by positional cloning and characterization of genes for their biological relevance. We herein describe strategies used for the construction of monochromosomal hybrids and their applications for cloning and characterization of genes related to cell growth, cell senescence and DNA repair. We have cloned RNaseT2, GluR6 (glutamate ionotropic receptor kainate type subunit 2-GRIK2) and protein tyrosine phosphatase, receptor type K (PTPRK) genes using these strategies.

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.

Long-range structure of H-ras 1-selected transgenomes

We have used chromosome-mediated gene transfer (CMGT) and whole cell fusion to derive human-mouse hybrid cells carrying reduced human chromosomes 11, by selecting for expression of the transforming H-ras 1 oncogene. To realize the full potential of these somatic cell genetic techniques as resources for enriched DNA probe isolation and the fine structure mapping of chromosomes, the nature of any molecular rearrangements that may accompany the process of DNA transfer must be understood. We have analyzed the long-range structure of our transgenomes by pulsed field gel electrophoresis (PFGE) and show here that, whereas during cell fusion several megabase pairs (Mb) of DNA can be transferred intact, multiple rearrangements of DNA accompany CMGT even in transgenomes where other methods of analysis gave no indication of such molecular scrambling.

Construction of microcell hybrid panel containing different neo gene insertions in mouse chromosome 17 used for chromosome-mediated gene transfer

A panel of four microcell hybrids representing different sites of insertion of the exogenous neo gene into mouse chromosome 17 has been constructed. These constructions were based on a cotransfer of mouse chromosome 17 and neomycin resistance generated in a stepwise procedure involving (1) random insertion of the neo gene into a primary cell hybrid containing mouse chromosome 17 in a hamster cell background, (2) microcell-mediated chromosome transfer (MMCT) to segregate mouse and hamster chromosomes, and (3) identification of the mouse chromosome containing cells using a novel cell dotting procedure for mass screening at the cell colony level by molecular hybridization. Using this panel of four microcell hybrids for chromosome mediated gene transfer (CMGT), we obtained one transformant containing a chromosome fragment derived from the t-complex region located on mouse chromosome 17. It is concluded that the specific chromosome based procedure used here to generate CMGT transfectants may provide a general means to produce large numbers of transfectants containing megabase fragments covering, in principle, all regions of a given chromosome.

Number and size of human X chromosome fragments transferred to mouse cells by chromosome-mediated gene transfer

Labeled probes of unique-sequence human X chromosomal deoxyribonucleic acid, prepared by two different procedures, were used to measure the amount of human X chromosomal deoxyribonucleic acid in 12 mouse cell lines expressing human hypoxanthine phosphoribosyltransferase after chromosome-mediated gene transfer. The amount of X chromosomal deoxyribonucleic acid detected by this procedure ranged from undetectable levels in the three stable transformants and some unstable transformants examined to about 20% of the human X chromosome in two unstable transformants. Reassociation kinetics of the X chromosomal probe with deoxyribonucleic acid from the two unstable transformants containing 15 to 20% of the human X chromosome indicate that a single copy of these sequences is present. In one of these lines, the X chromosomal sequences exist as multiple fragments which were not concordantly segregated when the cells were selected for loss of hprt.

Investigation of chromosome-mediated gene transfer using the HPRT region of the human X chromosome as a model

A panel of over 50 hybrid cells containing varying portions of the long arm of the human X chromosome have been obtained by chromosome-mediated gene transfer (CMGT) of human chromosomes to mouse cells deficient in HPRT. This panel is used to investigate the size and integrity of transfected human chromosome fragments and also to examine the effect of including a selectable DNA plasmid in the transfection mix. Chromosomal rearrangements are found to be generated in the chromosome transfer process, and the human X centromeric region is detected in the transfected cells at an unusually high frequency. Extensive lengths of X chromosome DNA are transferred intact, suggesting potential uses of CMGT in cloning large genes and loci for which only the chromosomal map position is known.

Molecular and physical arrangements of human DNA in HRAS1-selected, chromosome-mediated transfectants

We used mitotic chromosomes isolated from a human EJ bladder carcinoma cell line for morphological transformation of mouse C127 cells. These chromosome-mediated transformants were analyzed for cotransfer of markers syntenic with c-Ha-ras-1 on human chromosome 11. We also used cloned, dispersed human DNA repeats, in a general mapping strategy, to quantitate the amounts and molecular state of human DNA transferred along with the activated c-Ha-ras-1 gene. In situ hybridization was used to visualize the physical state of the transfected human chromatin. The combined use of these various techniques revealed the occurrence of both chromosomal and DNA rearrangements. However, our analysis also demonstrated that, in general, very substantial lengths of DNA are transferred intact. Closely linked markers are likely to cosegregate. Therefore, these transformants should be invaluable sources for the complete molecular cloning of isolated fragments of the short arm of human chromosome 11.

HRAS1-selected, chromosome-mediated transformants vary in phenotype in vitro and tumorigenic potential in vivo

Transfection of mouse C127 cells with mitotic chromosomes isolated from a human EJ bladder carcinoma cell line gave rise, at high frequency, to foci of transformed cells. Independent, HRAS1-selected chromosome-mediated transformants displayed distinctive cellular morphologies in monolayer culture and colony-forming abilities in low-melting-point agarose. Subcutaneous inoculation of neonatally thymectomized, Ara-C-protected, total-body-irradiated CBA mice was used to compare the tumorigenic potential of each transformant. Significant quantitative and qualitative differences in tumorigenicity were found between transformants which correlated with differences in malignant phenotype observed in vitro. The sensitivity of the tumorigenicity assay is such that rare transformation events can be selected directly in vivo.

Chromosome mediated gene transfer of six DNA markers linked to the cystic fibrosis locus on human chromosome seven

The DNA probes met and pJ3.11 are derived from loci on chromosome seven that are closely linked to, and probably flanking, the gene mutation causing cystic fibrosis (CF). We have shown that mitotic chromosomes from the cell line MNNG-HOS, which contains an activated met oncogene, can induce morphological transformation of mouse NIH-3T3 cells. Southern analysis of isolated transfectant cell lines with cloned dispersed repetitive human DNA sequences as probes demonstrated that several lines of transformed NIH 3T3 cells had stabley incorporated large segments of chromosome seven DNA. Southern blot analysis also demonstrated the presence of met, pJ3.11 and several other single copy sequences that had been previously localised to chromosome 7 within the transgenomes. In this way a further four genetic markers were shown to be physically linked to met, and thus to CF. These probes may prove useful in confirming the order of loci around CF and in the prenatal diagnosis of this common autosomal recessive disease.

Molecular and physical arrangements of human DNA in HRAS1-selected, chromosome-mediated transfectants

We used mitotic chromosomes isolated from a human EJ bladder carcinoma cell line for morphological transformation of mouse C127 cells. These chromosome-mediated transformants were analyzed for cotransfer of markers syntenic with c-Ha-ras-1 on human chromosome 11. We also used cloned, dispersed human DNA repeats, in a general mapping strategy, to quantitate the amounts and molecular state of human DNA transferred along with the activated c-Ha-ras-1 gene. In situ hybridization was used to visualize the physical state of the transfected human chromatin. The combined use of these various techniques revealed the occurrence of both chromosomal and DNA rearrangements. However, our analysis also demonstrated that, in general, very substantial lengths of DNA are transferred intact. Closely linked markers are likely to cosegregate. Therefore, these transformants should be invaluable sources for the complete molecular cloning of isolated fragments of the short arm of human chromosome 11.

Rapid and quantitative detection of unique sequence donor DNA in extracts of cultured mammalian cells: an aid to chromosome mapping

A rapid and highly sensitive method for screening the human DNA content of hybrid or transfected mammalian cells is described. Transfectants containing as little as 200 kb of otherwise undefined human DNA can be readily detected in a background of mouse chromatin. At the highest stringency, single-copy sequences can be detected. Large numbers of independent gene-transfer products are easily screened, making the method ideally suited to the identification of rare, but otherwise unselectable, events. The method does not rely upon the expression of the gene sequence of interest; the sole proviso is the availability of an appropriate DNA probe for the chromosomal region or locus of interest.

Isolation and regional mapping of random X sequences from distal human X chromosome

Chromosome-mediated gene transfer (CMGT) lines were shown to be convenient donors of genomic sequences from specific regions of the genome adjacent to selectable markers. Two libraries were prepared from CMGT lines carrying sequences spanning the long arm of the human X chromosome from HPRT (Xq26) to G6PD (Xq28). A series of 22 CMGT lines sharing the same selectable marker (HPRT) were used in conjunction with five standard translocation hybrids to provide fine-resolution regional mapping of the nonrepetitive X specific probes isolated from the libraries. The order of three human recombinant sequences with respect to known X-linked markers is: PGK (Xq13), 05-02 (DXS78); HPRT (Xq26), 07-03 (DXS79); surface antigen S11 (Xq27), 07-14 (DXS80); and G6PD (Xq28).

Chromosome-mediated transfer of murine alleles for hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and ouabain resistance into human cell lines

Genetic drug-resistance markers were transferred via purified metaphase chromosomes from mouse L cells into the human fibrosarcoma line HT1080 and HeLa S3 cells. Interspecific chromosome-mediated transfer of hypoxanthine-guanine phosphoribosyl transferase (HGPRT; EC 2.4.2.8) from mouse L cells into HGPRT- HT1080 cells occurred at a frequency of approximately 1 x 10(-7). The presence of the mouse allele for HGPRT in transferent isolates was confirmed by isoelectric focusing. Transfer of ouabain resistance from mouse L cells to HT1080 and HeLa S3 cells occurred at an average frequency of approximately 4 x 10(-7). Expression of the mouse trait in transferent isolates was confirmed by their ability to withstand doses of ouabain which would be lethal to spontaneous ouabain-resistant mutants of the human cells but not to mouse L cells, ouabain-resistant transferents of human cells showed 10(4)-to greater than 10(5)-fold enhanced drug resistance, characteristic of either wild-type or mutant alleles, respectively, from ouabain-resistant donor L cells. Unstable expression of the transferred phenotypes in the absence of selection was seen in some isolates, but expression was lost at slow rates.

Short-term, high-efficiency expression of transfected DNA

We have achieved high-efficiency uptake and expression of foreign DNA in mouse Ltk- cells by modifying the DEAE-dextran-mediated transfection method of McCutchan and Pagano (J. Natl. Cancer Inst. 42:351-357, 1968) to include an initial incubation at elevated pH followed by a shock treatment with dimethyl sulfoxide. Up to 80% of mouse Ltk- cells transfected with the herpes simplex virus thymidine kinase gene expressed thymidine kinase as measured by autoradiography.

Cotransfer and phenotypic stabilisation of syntenic and asyntenic mink genes into mouse cells by chromosome-mediated gene transfer

By means of metaphase chromosomes, the genes for mink thymidine kinase (TK) and hypoxanthine-phosphoribosyltransferase (HPRT) were transferred to mutant mouse cells, LMTK-, A9 (HPRT-) and teratocarcinoma cells, PCC4-aza 1 (HPRT-). Eighteen colonies were isolated from LMTK- (series A), 9 from A9 (series B) and none from PCC4-aza 1. The transformed clones contained mink TK or HPRT. Analysis of syntenic markers in series B demonstrated that one clone contained mink glucose-6-phosphate dehydrogenase (G6PD) and the other alpha-galactosidase; in series A, nine clones contained mink galactokinase (GALK) and six mink aldolase C (ALDC). Analysis of 12 asyntenic markers located in ten mink chromosomes showed the presence of only aconitase-1 (ACON1) (the marker of mink chromosome 12) in three clones of series A. The clones lost mink ACON1 between the fifth to tenth passages. Cytogenetic analysis established the presence of a fragment of mink chromosome 8 in eight clones of series A, but not in series B. The clones of series A lost mink TK together with mink GALK and ALDC during back-selection; in B, back-selection retained mink G6PD. No stable TK+ phenotype was detected in clones with a visible fragment of mink chromosome 8. Stability analysis demonstrated that about half of the clones of series B have stable HPRT+ phenotype whereas only three clones of series A have stable TK+ phenotype. It is suggested that the recipient cells, LMTK- and A9, differ in their competence for genetic transformation and integration of foreign genes.

Short-term, high-efficiency expression of transfected DNA

We have achieved high-efficiency uptake and expression of foreign DNA in mouse Ltk- cells by modifying the DEAE-dextran-mediated transfection method of McCutchan and Pagano (J. Natl. Cancer Inst. 42:351-357, 1968) to include an initial incubation at elevated pH followed by a shock treatment with dimethyl sulfoxide. Up to 80% of mouse Ltk- cells transfected with the herpes simplex virus thymidine kinase gene expressed thymidine kinase as measured by autoradiography.

The use of cell surface antigens to characterize and select for fragments of human chromosomes retained by interspecies hybrids

We have used a mouse cell transformant generated by human chromosome-mediated gene transfer (CMGT) to explore the use of cell surface antigens in the identification of fragments of human chromosomes retained by somatic cell hybrids. The transformed line, 21-30b, contained an intact rear-ranged human chromosome, and could be shown by isozyme analysis to contain genetic material from chromosomes 9 and X. By using the transformant as an immunogen in mice, it was also possible to produce antiserum to human-specific surface antigens. Using genetically characterized human X rodent hybrid lines, the genes controlling expression of these antigens could be localized to 11per----11p13, segregating concordantly with surface antigen S3. These conclusions were possible despite the fact that the presence of chromosome 11 in the transformant was not detectable by the presence of chromosome specific isozyme LDH-A or surface antigens W6/34 and 4F2. Finally, the fluorescence-activated cell sorter (FACS) was used to fractionate the transformant cells into antigen positive and negative subpopulations. This resulted in the isolation and characterization of four additional chromosome rearrangements involving interspecies chromosome translocations. This work demonstrates the value of chromosome-specific surface antigens and the FACS in the evaluation of human chromosome fragments retained by interspecies hybrids.

Transforming ras genes from human melanoma: a manifestation of tumour heterogeneity?

Variability in the phenotype of cells comprising individual tumours is a striking feature of animal and human cancer and is generally referred to as tumour heterogeneity. Studies of clonally derived cell populations from tumours that originated presumably from a single transformed cell have shown that tumours are made up of cells that differ in a variety of traits, including drug resistance, antigen expression and metastatic potential. The origin and maintenance of tumour heterogeneity are unclear, but mutational and epigenetic mechanisms are thought to be involved. Here we report the results of a search for transforming genes in human melanoma which have raised the possibility that ras gene activation follows the same variable pattern as other traits involved in tumour heterogeneity. DNA from 4 of 30 melanoma cell lines yielded transforming ras genes in the NIH/3T3 assay. Of five cell lines originating from separate metastatic deposits of a single patient, only one contained activated ras, indicating heterogeneity in ras activation in this case and suggesting that ras activation was not involved in tumour initiation or maintenance in this patient.

Expression of a preproinsulin-beta-galactosidase gene fusion in mammalian cells

As an approach to the study of mammalian gene expression, the promoters and translation initiation regions of the rat preproinsulin II and the simian virus 40 early genes were fused to the structural gene of Escherichia coli beta-galactosidase, a sensitive probe for gene expression. These fusions were introduced into COS-7 cells, a simian virus 40 large tumor-antigen-producing monkey kidney cell line, where they directed the synthesis of enzymatically active hybrid beta-galactosidase proteins. Conditions for transfection were varied to optimize the expression of beta-galactosidase activity in the transfected cells. The pH optimum of this activity was found to be 7.0, the same as that of native E. coli beta-galactosidase and distinct from the major lysosomal "acid" beta-galactosidase. The fused preproinsulin-beta-galactosidase was further characterized by gel electrophoresis of nondenatured cell extracts stained by a fluorogenic substrate and by immunoprecipitation and gel electrophoresis of 3H-labeled cell proteins. These results all indicate that fully active tetrameric beta-galactosidase hybrids can be produced in mammalian cells. The expression of preproinsulin-beta-galactosidase activity was measured in the presence of high glucose, insulin, dexamethasone, or epidermal growth factor but no regulatory changes were observed.

Expression and stabilization of microinjected plasmids containing the herpes simplex virus thymidine kinase gene and polyoma virus DNA in mouse cells

To observe the effects of polyoma virus DNA on the expression of the herpes simplex virus (HSV) thymidine kinase (TK) gene early after transfer into TK-deficient mouse cells and the subsequent development of stable TK-positive transformants, we constructed a series of recombinant plasmids containing the herpes simplex virus TK gene joined with various segments of the polyoma virus genome and microinjected them into the nuclei or cytoplasm of LTK-A cells (TK(-), APRT(-)). The frequency of nucleus-injected cells expressing TK after 1 day, measured by autoradiography of cells incubated with [(3)H]thymidine, increased approximately 30-fold when the plasmids contained the polyoma virus origin of replication. The origin includes sequences with homology to the simian virus 40 origin of replication and adjoining sequences, including a recently defined transcription-enhancing sequence. After microinjection of a single origin-containing plasmid molecule per cell, TK expression was detected in approximately 50% of the injected cells. When a larger number of origin-containing plasmid molecules were injected per cell, all cells showed early TK activity. When the entire polyoma virus early region was present, neighboring uninjected cells became TK positive. When plasmids were injected into the cell cytoplasm, approximately 400 times as many molecules per cell were needed to cause early TK activity. The frequency of stable transformation observed 2 weeks after nuclear injection of 10 to 20 polyoma virus origin-containing plasmid molecules per cell was at least 2 orders of magnitude greater than with plasmids containing the TK gene alone. The greatest enhancement of stable TK transformation was obtained with plasmids containing the origin alone, when the maximum frequency of stable transformation was 5%. The addition of the coding regions for the small and medium T antigens or the entire early region significantly decreased TK transformation frequency in a copy-dependent fashion. The timing of stabilization of TK-positive transformation was analyzed by releasing hypoxanthine-aminopterin-thymidine selection pressure at various times after microinjection, culturing the cells in nonselective medium, and assaying for TK activity. Stabilization was found to occur between 3 and 6 days after nuclear injection. Cells injected with a plasmid containing the origin and the early region were examined for expression of the large T antigen with polyoma virus antitumor serum and immunofluorescent staining. The expression of the large T antigen was clearly associated with a cytopathic effect. TK-positive clones observed 2 weeks after injection of the plasmid were uniformly T antigen negative. Cytotoxicity may be the result of plasmid replication and toxic levels of T antigen or TK. In addition, expression of the large T antigen may block stabilization by preventing the integration of origin-containing plasmid molecules.

Expression and stabilization of microinjected plasmids containing the herpes simplex virus thymidine kinase gene and polyoma virus DNA in mouse cells

To observe the effects of polyoma virus DNA on the expression of the herpes simplex virus (HSV) thymidine kinase (TK) gene early after transfer into TK-deficient mouse cells and the subsequent development of stable TK-positive transformants, we constructed a series of recombinant plasmids containing the herpes simplex virus TK gene joined with various segments of the polyoma virus genome and microinjected them into the nuclei or cytoplasm of LTK-A cells (TK(-), APRT(-)). The frequency of nucleus-injected cells expressing TK after 1 day, measured by autoradiography of cells incubated with [(3)H]thymidine, increased approximately 30-fold when the plasmids contained the polyoma virus origin of replication. The origin includes sequences with homology to the simian virus 40 origin of replication and adjoining sequences, including a recently defined transcription-enhancing sequence. After microinjection of a single origin-containing plasmid molecule per cell, TK expression was detected in approximately 50% of the injected cells. When a larger number of origin-containing plasmid molecules were injected per cell, all cells showed early TK activity. When the entire polyoma virus early region was present, neighboring uninjected cells became TK positive. When plasmids were injected into the cell cytoplasm, approximately 400 times as many molecules per cell were needed to cause early TK activity. The frequency of stable transformation observed 2 weeks after nuclear injection of 10 to 20 polyoma virus origin-containing plasmid molecules per cell was at least 2 orders of magnitude greater than with plasmids containing the TK gene alone. The greatest enhancement of stable TK transformation was obtained with plasmids containing the origin alone, when the maximum frequency of stable transformation was 5%. The addition of the coding regions for the small and medium T antigens or the entire early region significantly decreased TK transformation frequency in a copy-dependent fashion. The timing of stabilization of TK-positive transformation was analyzed by releasing hypoxanthine-aminopterin-thymidine selection pressure at various times after microinjection, culturing the cells in nonselective medium, and assaying for TK activity. Stabilization was found to occur between 3 and 6 days after nuclear injection. Cells injected with a plasmid containing the origin and the early region were examined for expression of the large T antigen with polyoma virus antitumor serum and immunofluorescent staining. The expression of the large T antigen was clearly associated with a cytopathic effect. TK-positive clones observed 2 weeks after injection of the plasmid were uniformly T antigen negative. Cytotoxicity may be the result of plasmid replication and toxic levels of T antigen or TK. In addition, expression of the large T antigen may block stabilization by preventing the integration of origin-containing plasmid molecules.

Chromosome-mediated gene transfer of HPRT and APRT in an intraspecific human cell system

Chromosome-mediated transfer of genes between human cell lines was accomplished using HeLa cells as chromosome donors and HT1080 fibrosarcoma lines as recipients. This report describes the intraspecific transfer of two genetic markers, hypoxanthine-guanine-phosphoribosyltransferase (HPRT+) and adenine phosphoribosyltransferase (APRT+). The isolation and characterization of the necessary enzyme-deficient (HPRT- and APRT-) recipient HT1080 cell lines are also described. The chromosome-mediated gene transfer was carried out using a modification of the procedure of Miller and Ruddle, including treatment of the donor chromosomes with calcium phosphate and subsequent exposure of the recipient cells of dimethyl sulfoxide. In experiments to optimize this procedure for HT1080 cell recipients, we found that a brief (2-min) exposure to high DMSO concentration (20%) was effective for enhancing transfer efficiencies in this system. Transfer frequencies (transferents per recipient cells assayed) averaged approximately 1 x 10(-6) for HPRT+ and were greater than 2 x 10(-6) for APRT+.

Adaptation of a retrovirus as a eucaryotic vector transmitting the herpes simplex virus thymidine kinase gene

We investigated the feasibility of using retroviruses as vectors for transferring DNA sequences into animal cells. The thymidine kinase (tk) gene of herpes simplex virus was chosen as a convenient model. The internal BamHI fragments of a DNA clone of Moloney leukemia virus (MLV) were replaced with a purified BamHI DNA segment containing the tk gene. Chimeric genomes were created carrying the tk insert in both orientations relative to the MLV sequence. Each was transfected into TK- cells along with MLV helper virus, and TK+ colonies were obtained by selection in the presence of hypoxanthine, aminopterin, and thymidine (HAT). Virus collected from TK+-transformed, MLV producer cells passed the TK+ phenotype to TK- cells. Nonproducer cells were isolated, and TK+ transducing virus was subsequently rescued from them. The chimeric virus showed single-hit kinetics in infections. Virion and cellular RNA and cellular DNA from infected cells were all shown to contain sequences which hybridized to both MLV- and tk-specific probes. The sizes of these sequences were consistent with those predicted for the chimeric virus. In all respects studied, the chimeric MLV-tk virus behaved like known replication-defective retroviruses. These experiments suggest great general applicability of retroviruses as eucaryotic vectors.

Adaptation of a retrovirus as a eucaryotic vector transmitting the herpes simplex virus thymidine kinase gene

We investigated the feasibility of using retroviruses as vectors for transferring DNA sequences into animal cells. The thymidine kinase (tk) gene of herpes simplex virus was chosen as a convenient model. The internal BamHI fragments of a DNA clone of Moloney leukemia virus (MLV) were replaced with a purified BamHI DNA segment containing the tk gene. Chimeric genomes were created carrying the tk insert in both orientations relative to the MLV sequence. Each was transfected into TK- cells along with MLV helper virus, and TK+ colonies were obtained by selection in the presence of hypoxanthine, aminopterin, and thymidine (HAT). Virus collected from TK+-transformed, MLV producer cells passed the TK+ phenotype to TK- cells. Nonproducer cells were isolated, and TK+ transducing virus was subsequently rescued from them. The chimeric virus showed single-hit kinetics in infections. Virion and cellular RNA and cellular DNA from infected cells were all shown to contain sequences which hybridized to both MLV- and tk-specific probes. The sizes of these sequences were consistent with those predicted for the chimeric virus. In all respects studied, the chimeric MLV-tk virus behaved like known replication-defective retroviruses. These experiments suggest great general applicability of retroviruses as eucaryotic vectors.

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.

A new era in mammalian gene mapping: somatic cell genetics and recombinant DNA methodologies

Mammalian gene mapping techniques are now sufficiently advanced to contribute significantly to prenatal diagnosis and to human molecular genetics. Restriction fragment mapping can be used to place polymorphic genetic markers at random sites within the genome, and these sites used to assign genes responsible for disease conditions to a chromosomal region. Somatic cell genetic techniques can then be applied to saturate that region with additional restriction fragment markers, some of which will be closely linked to the disease gene. Closely linked restriction fragment markers, especially flanking pairs of markers, can act as predictors for the transmission of defective genes to offspring. A series of tightly linked flanking restriction markers might in addition contribute to the eventual isolation and cloning of the disease gene itself.

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.

Lipochromosome mediated gene transfer: identification and probable specificity of localization of human chromosomal material and stability of the transferents

Using lipochromosomes (phospholipid-entrapped chromosomes) were have transferred the human HGPRT gene into HGPRT deficient mouse cells (A9) with a frequency of approximately 1 x 19(-5) (Mukherjee et al. Proc. Natl. Acad. Sci. USA 75: 1361-1365; 1978). Two other genes located on the long arm of the human X-chromosome were also expressed two independently derived populations of transferents (A9/GT3 and A9/GT4). We report here the chromosomal and enzymatic composition of human HGPRT-positive clones from each subpopulation analyzed in detail with alkaline Giemsa-11 staining. All the clones expressed human PGK and HGPRT, but one (A9/GT4C6) lacked human G6PD. In each of four clone examined microscopically, a small piece of presumptive human chromatin was visible in the karyotypes of most cells. The chromatin fragment was free or attached in each cell of an individual clone. When integrated, the human chromosomal fragment in each clone appeared associated with the centromere of the same telocentric A9 chromosome (No. 6 Q-banding). These data suggest that: (a)substantial human chromosomal fragments can be transferred into recipient cell using the lipochromosome technique; (b) clones from human HGPRT positive A9 transferent subpopulations may or may not possess other human X-linked markers: (c) the stability of lipochromosomally transferred genes varied from clone to clone and stability is generally poor in the absence of continuous selection pressure (e.g., HAT); (d) when multiple X-linked human genes were transferred to mouse cells a cytologically detectable human chromosomal fragment was identified free or attached to a host chromosome; and (e) integration of transferred human chromosomal material into mouse chromosomes may occur at preferential site(s) in the recipient genome.

Number and size of human X chromosome fragments transferred to mouse cells by chromosome-mediated gene transfer

Labeled probes of unique-sequence human X chromosomal deoxyribonucleic acid, prepared by two different procedures, were used to measure the amount of human X chromosomal deoxyribonucleic acid in 12 mouse cell lines expressing human hypoxanthine phosphoribosyltransferase after chromosome-mediated gene transfer. The amount of X chromosomal deoxyribonucleic acid detected by this procedure ranged from undetectable levels in the three stable transformants and some unstable transformants examined to about 20% of the human X chromosome in two unstable transformants. Reassociation kinetics of the X chromosomal probe with deoxyribonucleic acid from the two unstable transformants containing 15 to 20% of the human X chromosome indicate that a single copy of these sequences is present. In one of these lines, the X chromosomal sequences exist as multiple fragments which were not concordantly segregated when the cells were selected for loss of hprt.

Assignment of a human genetic locus to chromosome 5 which corrects the heat sensitive lesion associated with reduced leucyl-tRNA synthetase activity in ts025Cl Chinese hamster cells

A heat-sensitive (hs) leucyl-tRNA synthetase (leuRS) deficient CHO mutant, ts025Cl, was fused with human leukocytes and hybrids isolated in HAT medium at the nonpermissive temperature. Nineteen heat-resistant (hr) and 14 hs subclones were isolated from four independent primary hybrids and tested for the expression of 24 human isozymes which have been assigned to 17 human chromosomes. Four hr independent subclones and three hs independent subclones were analyzed for the presence of human chromosomes. A pattern of concordant segregation was noted for the hr phenotype, human hexosaminidase B (EC 3.2.1.30) and human chromosome 5. Based on these results, we have defined the human genetic locus which corrects the heat-sensitive lesion in ts025Cl as hr025Cl and have assigned this locus to human chromosome 5. Two hr hybrids exhibited leuRS activity 2.5 and 4 times the leuRS activity of ts025Cl but a wild-type level of activity was not restored. One hs hybrid had only 73% of the leuRS activity exhibited by ts025Cl while another hs hybrid had 1.8 times the leuRS activity of ts025Cl.

DNA-mediated cotransfer of unlinked mammalian cell markers into mouse L cells

Purified DNA from three different types of mammalian cells was precipitated with calcium phosphate and added to mouse L cells deficient in thymidine kinase (TK). Donor DNA was prepared from three cell lines: (a) mouse cells transfected with UV-inactivated herpes simplex virus (HSV) type 1, or a purified fragment of HSV carrying the TK gene (b) human HeLa cells, and (c( CHO, a cell line derived from Chinese hamster ovaries. Several hypoxanthine-aminopterin-thymidine resistant colonies were isolated from each experiment. The origin of the TK that is expressed in these cells was studied by polyacrylamide gel electrohporesis, isoelectric focusing, or heat stability. The TK in all instances was of the donor origin. To determine the extent of gene transfer we have assayed the CHO and HeLa DNA transfectants for galactokinase (GALK), a marker closely linked to TK, and 25 other isozymes representing a large number of different chromosomes. No cotransfer of GALK was observed, indicating that the size of the transferred DNA segment is limited. We observed that, in one instance, esterase-D, an unlinked marker of Chinese hamster origin, was transferred along with TK. These experiments indicate that nonselected markers can be transferred by this method, although at a low efficiency.

Chromosome-mediated gene transfer results in two classes of unstable transformants

The human thymidine kinase gene has been transferred from HeLa S3 cells to mouse LM(TK-) cells via isolated metaphase chromosomes. Efficient transfer of the thymidine kinase gene (1.8 X 10(-5) colonies per recipient cell) was obtained when the donor chromosomes were precipitated with calcium phosphate and the recipient cells were treated with 10% (vol/vol) dimethyl sulfoxide. Thirty-five independent cell lines were analyzed in detail. Cytologically detectable donor chromosome fragments were observed in 14% of the cell lines. Many of the transformed cell lines were also found to express the human genes for galactokinase (23% of the transformed cell lines) and procollagen type I (69% of the transformed cell lines), which are syntenic to thymidine kinase on human chromosome 17. On the basis of stability analyses, three classes of transformed cell lines were defined and characterized. One class of transformants was stable, showing no loss of the transferred phenotype in the absence of selection. A second group of transformants was unstable, losing the thymidine kinase phenotype at a rate of 1.5-2.5% per day. This group of transformants was found to possess large donor chromosome fragments (macrotransgenomes) and relatively low levels of donor gene activity. The third group of transformants lost the thymidine kinase phenotype rapidly, at a rate of 6-10% per day. These cell lines contained small, cytologically undetectable transgenomes (microtransgenomes) and overexpressed the transferred thymidine kinase gene.

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

The conditions necessary to achieve high frequency transfer of the thymidine kinase and dihydrofolate reductase genes from hamster cells into mouse cells were investigated. Of the parameters examined, the length of adsorption time, input gene dosage, and treatment with dimethylsulfoxide (DMSO) were found to significantly alter the transfer frequency using either metaphase chromosomes or purified DNA as the transfer vehicle. With the mouse cell line as a recipient, the optimal adsorption period for DNA or chromosomes from MtxRIII cells was found to vary from 8 to 16 h in those experiments where the recipient cells were subsequently treated with DMSO. Without DMSO, similar frequencies could be obtained by extending the period of adsorption. Increasing the dosage of DNA or chromosomes resulted in an almost linear increase in the number of transformants. The optimal conditions for transfer did not significantly differ for the two genes studied. On the average, the optimal conditions yielded 1.5 x 10(3) transformants per 10(7) recipient cells with chromosomes; with DNA an average of only 60 transformants were observed. In general, DNA transformants grown in the absence of methotrexate were unstable; whereas, under the same conditions about 20% of the transformants from the chromosome experiments were stable.

Cotransfer of linked eukaryotic genes and efficient transfer of hypoxanthine phosphoribosyltransferase by DNA-mediated gene transfer

The efficiency of DNA-mediated transfer of the gene (hprt) for hypoxanthine phosphoribosyltransferase (HPRT; IMP: pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) is dependent upon the recipient cell used. hprt has been transferred into mouse TG8 or Chinese hamster CHTG49 cells at a high frequency, similar to the frequency of the gene (tk) for thymidine kinase (TK; ATP:thymidine 5'-phosphotransferase, EC 2.7.1.21) transfer into mouse LMTK- cells (i.e., 10(-6)). In contrast, the frequency of transfer of hprt into mouse A9 cells was about two orders of magnitude less. The identification of efficient recipient cells for hprt transfer permits the use of DNA-mediated transfer as a bioassay for the gene. Cotransfer of the linked tk gene and the gene (galk) for galactokinase (ATP: D-galactose 1-phosphotransferase, EC 2.7.1.6) to LMTK- cells has been detected once among 87 tk transferrents. This suggests that the distance between the tk and galk genes in the Chinese hamster genome may be smaller than was previously thought. Significant differences between chromosome-mediated and DNA-mediated gene transfer were observed with respect to both the size of the transferred functional genetic fragment and the recipient cell specificity.

Gene transfer and gene mapping in mammalian cells in culture

The ability to transfer mammalian genes parasexually has opened new possibilities for gene mapping and fine structure mapping and offers great potential for contributing to several aspects of mammalian biology, including gene expression and genetic engineering. The DNA transferred has ranged from whole genomes to single genes and smaller segments of DNA. The transfer of whole genomes by cell fusion forms cell hybrids, which has promoted the extensive mapping of human and mouse genes. Transfer, by cell fusion, of rearranged chromosomes has contributed significantly to determining close linkage and the assignment of genes to specific chromosomal regions. Transfer of single chromosomes has been achieved utilizing microcells fused to recipient cells. Metaphase chromosomes have been isolated and used to transfer single-to-multigenic DNA segments. DNA-mediated gene transfer, simulating bacterial transformation, has achieved transfer of single-copy genes. By utilizing DNA cleaved with restriction endonucleases, gene transfer is being empolyed as a bioassay for the purification of genes. Gene mapping and the fate of transferred genes can be examined now at the molecular level using sequence-specific probles. Recently, single genes have been cloned into eucaryotic and procaryotic vectors for transfer into mammalian cells. Moreover, recombinant libraries in which entire mammalian genomes are represented collectively are a rich new source of transferable genes. Methodology for transferring mammalian genetic information and applications for mapping mammalian genes is presented and prospects for the future discussed.

Transformation of the gene for hypoxanthine phosphoribosyltransferase

Purified DNA from wild-type Chinese ovary (CHO) cells has been used to transform three hypoxanthine phosphoribosyltransferase (HPRT) deficient murine cell mutants to the enzyme positive state. Transformants appeared at an overall frequency of 5 x 10(-8) colonies/treated cell and expressed CHO HPRT activity as determined by electrophoresis. One gene recipient, B21, was a newly isolated mutant of LMTK- deficient in both HPRT and thymidine kinase (TK) activities. Transformation of B21 to HPRT+ occurred at 1/5 the frequency of transformation to TK+; the latter was, in turn, an order of magnitude lower than that found in the parental LMTK- cells, 3 x 10(-6). Thus both clonal and marker-specific factors play a role in determining transformability. The specific activity of HPRT in transformant extracts ranged from 0.5 to 5 times the CHO level. The rate of loss of the transformant HPRT+ phenotype, as measured by fluctuation analysis, was 10(-4)/cell/generation. While this value indicates stability compared to many gene transferents, it is much greater than the spontaneous mutation rate at the indigenous locus. The ability to transfer the gene for HPRT into cultured mammalian cells may prove useful for mutational and genetic mapping studies in this well-studied system.

DNA-mediated gene transfer of a circular plasmid into murine cells

We have used DNA-mediated gene transfer to introduce a recombinant plasmid containing the human beta-globin gene (H beta 1) into cells of a mouse tissue culture line, Ltk-. DNA isolated from independent transfer lines was analyzed by restriction endonuclease digestion, gel electrophoresis, modified Southern blotting, and filter hybridization using H beta 1 as a probe. H beta 1 sequences were present in 80% of the lines at 1-30 copies per cell. Many of the lines gave a hybridization pattern indicative of H beta 1 sequences integrated into high molecular weight DNA. DNA from three cell lines, digested with several restriction enzymes, produced a pattern providing evidence for the presence of circular H beta 1 molecules in the murine recipient cells.

Somatic cell genetic analysis of transgenome integration

The site of association of the human transgenome and host murine chromosomes was determined in several subclones of a stable human/mouse transformed cell line. Chromosomes were transferred from each of three transformed subclones into Chinese hamster recipient cells, and selection was applied for the expression of human transgenome-encoded HPRT. A series of trispecific microcell hybrids was isolated and characterized for each subclone. Evidence is presented that, within a given transformed subclone, only a single host (murine) chromosome was associated with the human transgenome. This contrasts with previous results which utilized a newly stabilized transformed cell line as the microcell donor and in which a variety of chromosomal sites of association existed. The results presented here support the view that the heterogeneity of transgenome association (integration) sites in newly stabilized transformants was due to the fact that these populations were multiclonal mixtures resulting from independent stabilization events. The initial heterogeneity in the population was subsequently reduced upon prolonged cultivation, as a subset of the original population became predominant.

Passage of phenotypes of chemically transformed cells via transfection of DNA and chromatin

DNA was prepared from 15 different mouse and rat cell lines transformed by chemical carcinogens in vitro and in vivo. These DNAs were applied to NIH3T3 mouse fibroblast cultures by using the calcium phosphate transfection technique. DNAs of five donor lines were able to induce foci on the recipient monolayers. Ten other donor DNAs yielded few or no foci. DNAs from control, nontransformed parental cell lines induced few or no foci. Chromosomes were transfected from one donor whose naked DNA was unable to induce foci, and morphologic transformation of recipients was observed. These experiments prove that in five of these cell lines the chemically induced phenotype is encoded in DNA, and the sequences specifying the transformed phenotype behave as a dominant allele in the NIH3T3 recipient cells. The sequences encoding the transformation are likely found on a single fragment of DNA.