Expression of the chromosomal mouse Beta maj-globin gene cloned in SV40

Efficient electroporation in primary cells with PEDOT:PSS electrodes

Precise and efficient delivery of macromolecules into cells enhances basic biology research and therapeutic applications in cell therapies, drug delivery, and personalized medicine. While pulsed electric field electroporation effectively permeabilizes cell membranes to deliver payloads without the need for toxic chemical or viral transduction agents, conventional bulk electroporation devices face major challenges with cell viability and heterogeneity due to variations in fields generated across cells and electrochemistry at the electrode-electrolyte interface. Here, we introduce the use of microfabricated electrodes based on the conducting polymer poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS), which substantially increases cell viability and transfection efficiency. As a proof of concept, we demonstrate the enhanced delivery of Cas9 protein, guide RNA, and plasmid DNA into cell lines and primary cells. This use of PEDOT:PSS enables rapid modification of difficult-to-transfect cell types to accelerate their study and use as therapeutic platforms.

The paths toward non-viral CAR-T cell manufacturing: A comprehensive review of state-of-the-art methods

Although CAR-T cell therapy has emerged as a game-changer in cancer immunotherapy several bottlenecks limit its widespread use as a front-line therapy. Current protocols for the production of CAR-T cells rely mainly on the use of lentiviral/retroviral vectors. Nevertheless, according to the safety concerns around the use of viral vectors, there are several regulatory hurdles to their clinical use. Large-scale production of viral vectors under "Current Good Manufacturing Practice" (cGMP) involves rigorous quality control assessments and regulatory requirements that impose exorbitant costs on suppliers and as a result, lead to a significant increase in the cost of treatment. Pursuing an efficient non-viral method for genetic modification of immune cells is a hot topic in cell-based gene therapy. This study aims to investigate the current state-of-the-art in non-viral methods of CAR-T cell manufacturing. In the first part of this study, after reviewing the advantages and disadvantages of the clinical use of viral vectors, different non-viral vectors and the path of their clinical translation are discussed. These vectors include transposons (sleeping beauty, piggyBac, Tol2, and Tc Buster), programmable nucleases (ZFNs, TALENs, and CRISPR/Cas9), mRNA, plasmids, minicircles, and nanoplasmids. Afterward, various methods for efficient delivery of non-viral vectors into the cells are reviewed.

The current landscape of nucleic acid therapeutics

The increasing number of approved nucleic acid therapeutics demonstrates the potential to treat diseases by targeting their genetic blueprints in vivo. Conventional treatments generally induce therapeutic effects that are transient because they target proteins rather than underlying causes. In contrast, nucleic acid therapeutics can achieve long-lasting or even curative effects via gene inhibition, addition, replacement or editing. Their clinical translation, however, depends on delivery technologies that improve stability, facilitate internalization and increase target affinity. Here, we review four platform technologies that have enabled the clinical translation of nucleic acid therapeutics: antisense oligonucleotides, ligand-modified small interfering RNA conjugates, lipid nanoparticles and adeno-associated virus vectors. For each platform, we discuss the current state-of-the-art clinical approaches, explain the rationale behind its development, highlight technological aspects that facilitated clinical translation and provide an example of a clinically relevant genetic drug. In addition, we discuss how these technologies enable the development of cutting-edge genetic drugs, such as tissue-specific nucleic acid bioconjugates, messenger RNA and gene-editing therapeutics.

How Simian Virus 40 Hijacks the Intracellular Protein Trafficking Pathway to Its Own Benefit … and Ours

Viruses efficiently transfer and express their genes in host cells and evolve to evade the host's defense responses. These properties render them highly attractive for use as gene delivery vectors in vaccines, gene, and immunotherapies. Among the viruses used as gene delivery vectors, the macaque polyomavirus Simian Virus 40 (SV40) is unique in its capacity to evade intracellular antiviral defense responses upon cell entry. We here describe the unique way by which SV40 particles deliver their genomes in the nucleus of permissive cells and how they prevent presentation of viral antigens to the host's immune system. The non-immunogenicity in its natural host is not only of benefit to the virus but also to us in developing effective SV40 vector-based treatments for today's major human diseases.

Small DNA tumor viruses: large contributors to biomedical sciences

Studies of the small DNA tumor viruses (the polyomaviruses, the adenoviruses and the papillomaviruses) have led to fundamental discoveries that have advanced our understanding of basic mammalian cell molecular biology processes such as transcription and DNA replication, uncovered pathways and genes often perturbed in human cancer, and identified bona fide human cancer viruses. In this article we examine the many contributions that have come from the small DNA tumor virus field and provide a recounting of some of the major landmark discoveries.

A concise peer into the background, initial thoughts and practices of human gene therapy

The concept of human gene therapy came on the heels of fundamental discoveries on the nature and working of the gene. However, realistic prospects to correct the underlying cause of recessive genetic disorders through the transfer of wild-type alleles of defective genes had to wait for the arrival of recombinant DNA technology. These techniques permitted the isolation and insertion of genes into the first recombinant delivery systems. The realization that viruses are natural gene carriers provided inspiration for gene therapy and, as engineered vectors, viruses became prominent gene delivery vehicles. Nonetheless, when put in the context of human and non-human primate studies, all vectors fell short of success regardless of their viral or non-viral origin. Recognition of issues such as inefficient gene transfer and short-lived or scant expression in the relevant cell type(s) prompted researchers to refine and develop several gene delivery systems, in particular those based on retroviruses, adeno-associated viruses and adenoviruses. Concomitantly, available technology was deployed to tackle disorders that require few genetically corrected cells to attain therapy.

Template structural requirements for transcription in vivo by RNA polymerase II

Purified simian virus 40 (SV40) DNA is reconstituted into chromatin and transcribed by endogenous RNA polymerase II when microinjected into nuclei of Xenopus laevis oocytes. We have correlated the kinetics of chromatin reconstitution with that of accumulation of virus-specific RNA in this system. A delay of approximately 3 h was found in the appearance of appreciable numbers of both fully supercoiled molecules and transcriptionally active templates. SV40 mini-chromosomes, isolated from virus-infected monkey cells with 0.2 M NaCl, also exhibited this lag in onset of transcriptional activity when microinjected into oocytes. These findings indicate that neither purified SV40 DNA nor SV40 DNA containing a full complement of nucleosomes can function as a template for transcription in vivo before association with appropriate cellular nonhistone chromosomal factors has taken place. In addition, the gradual degradation of linear SV40 DNA in oocytes was not sufficient to account for the fact that it was much less transcriptionally active than circular SV40 DNA. Taken together, these results indicate that the conformational state of the DNA can affect its ability to function as a template for transcription in vivo by RNA polymerase II. In contrast, transcription by RNA polymerase III of purified, circularized cloned DNAs encoding genes for 5S rRNA was detectable long before the injected DNAs had time to reconstitute into chromatin. Therefore, the template structural requirements for transcription in vivo by RNA polymerases II and III are different.

Template structural requirements for transcription in vivo by RNA polymerase II

Purified simian virus 40 (SV40) DNA is reconstituted into chromatin and transcribed by endogenous RNA polymerase II when microinjected into nuclei of Xenopus laevis oocytes. We have correlated the kinetics of chromatin reconstitution with that of accumulation of virus-specific RNA in this system. A delay of approximately 3 h was found in the appearance of appreciable numbers of both fully supercoiled molecules and transcriptionally active templates. SV40 mini-chromosomes, isolated from virus-infected monkey cells with 0.2 M NaCl, also exhibited this lag in onset of transcriptional activity when microinjected into oocytes. These findings indicate that neither purified SV40 DNA nor SV40 DNA containing a full complement of nucleosomes can function as a template for transcription in vivo before association with appropriate cellular nonhistone chromosomal factors has taken place. In addition, the gradual degradation of linear SV40 DNA in oocytes was not sufficient to account for the fact that it was much less transcriptionally active than circular SV40 DNA. Taken together, these results indicate that the conformational state of the DNA can affect its ability to function as a template for transcription in vivo by RNA polymerase II. In contrast, transcription by RNA polymerase III of purified, circularized cloned DNAs encoding genes for 5S rRNA was detectable long before the injected DNAs had time to reconstitute into chromatin. Therefore, the template structural requirements for transcription in vivo by RNA polymerases II and III are different.

Recombinant technique and gonadotropins production: new era in reproductive medicine

OBJECTIVE

To review current knowledge regarding recombinant DNA technology and its safety and efficacy in relation to recombinant gonadotropin production.

DATA IDENTIFICATION AND SELECTION

Studies that relate specifically to recombinant DNA technology, method of laboratory production, and the clinical aspects of using recombinant gonadotropins were identified through literature and Medline searches.

RESULTS

Recent developments in recombinant DNA technology have resulted in a rapidly expanding range of new diagnostic and therapeutic opportunities. This technology paves the way to the identification, isolation, cloning, and production of specific proteins. Recently, recombinant human gonadotropins became available for clinical use. The pharmacokinetics, receptor availability, pharmacodynamics, and safety were studied extensively and the drugs were found to be identical if not superior to urinary gonadotropins that have been used in reproductive medicine for the last 30 years. It is clear today that the use of recombinant human gonadotropins is expected to provide better batch-to-batch consistency, steady supply, and most importantly, a purified compound with high specific activity, which accounts for >99% of the preparation's protein content, allowing SC administration.

CONCLUSION

There is no doubt that recombinant gonadotropins produced by genetic engineering technology are here to stay and will represent an important treatment modality in various fertility disturbances.

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.

Bovine papilloma virus deoxyribonucleic acid: a novel eucaryotic cloning vector

A novel eucaryotic vector derived from the transforming region of bovine papilloma virus was established and demonstrated to be highly effective for introducing foreign genes into animal cells. The foreign deoxyribonucleic acid (DNA) is replicated and actively transcribed as an episome, and the transcripts are translated into an authentic gene product. We have constructed a DNA hybrid molecule, BPV69T-rI1, containing the transforming region of bovine papilloma virus DNA and the rat preproinsulin gene I (rI1), and used it to transform susceptible mouse cells. DNA hybridization analysis has demonstrated the presence of multiple unintegrated copies of hybrid DNA molecules, with the bovine papilloma virus 1 DNA segment and the rI1 gene covalently linked in selected transformed cell lines. S1 nuclease analysis revealed the presence of a correctly spliced coding segment of the preproinsulin transcript similar or identical in its electrophoretic mobility to that of messenger ribonucleic acid produced in rat insulinoma cells. Significant levels of a protein immunoreactive with anti-insulin serum were detected by radioimmunoassay in the culture medium of transformed cells. Immunoprecipitation analysis in conjunction with competitive binding to bovine proinsulin established the identity of the protein as that of rat proinsulin.

Cell-type specific processing of neuroendocrine precursor proteins using vaccinia recombinants

The cell-type specific processing of human pro-enkephalin was determined using a vaccinia recombinant (VV:hPE). The results show that all cell types infected with VV:hPE efficiently synthesize pro-enkephalin following cleavage of the signal peptide after Ala24. In addition, pro-enkephalin is shown to undergo N-linked glycosylation as well as other post-translational modifications. However, only one cell line. AtT-20, was able to efficiently cleave pro-enkephalin to smaller peptides including Met-enkephalin. Some results have been previously reported (Science 232, 1641-1643).

Expression of the Rous sarcoma virus env gene from a simian virus 40 late-region replacement vector: effects of upstream initiation codons

Expression of the Rous sarcoma virus envelope gene (env) from a simian virus 40 (SV40) late-region replacement vector is dependent on the position of env within the SV40 late-region sequences. The difference in expression levels appeared to be due to differences in the efficiency with which the env-specific transcripts were translated, because transcription levels from different constructions were similar. Deletion of the nucleotides encoding the agnoprotein initiator codon, located upstream of the env sequences in the poorly expressed construct, resulted in high levels of env expression. The agnoprotein initiator codon and overlapping open reading frame thus act as strong barriers to further ribosome scanning and prevent initiation at the env AUG codon. We conclude that AUG codons present in the late region of SV40 can reduce expression of inserted genes positioned downstream. Nevertheless, intrinsic properties of the gene may determine its ultimate level of expression.

Vectors for expression and amplification of cDNA in mammalian cells: expression of rat phenylalanine hydroxylase

We have constructed two recombinant plasmid vectors for direct expression and amplification of cDNA in mammalian cells. Each vector carries two dominant selectable markers (the bacterial neo gene and the mouse DHFR gene), a promoter sequence (viral LTR in pAV009/A+, and sheep metallothionein promoter in pMT010/A+), a polyadenylation signal sequence, and a Bam HI site to allow insertion of cDNA. We have used these vectors to prepare recombinant clones for the expression of rat phenylalanine hydroxylase (PH) in LTK- cells. Selection of transformants with neomycin followed by selection of the transformants in methotrexate led to a 30- to 60-fold amplification of the DHFR marker and co-amplification of the PH cDNA, with a corresponding increase in the level of PH mRNA and enzyme polypeptide. The expressed enzyme has a subunit molecular weight of 50,000 which corresponds to the W- allele of rat liver PH. PH activity was detected in the transfected cells by enzymatic measurement of the conversion of [14C]phenylalanine to [14C]tyrosine, and by growth of these cells in a tyrosine-free culture medium. Expression of rat PH in cell culture should facilitate the analysis of the biochemical properties of this enzyme.

New host cell system for regulated simian virus 40 DNA replication

Transformed monkey cell lines (CMT and BMT) that inducible express simian virus 40 (SV40) T antigen from the metallothionein promoter have been isolated and characterized. Immunoprecipitation of pulse-labeled T antigen demonstrates a 5- to 12-fold increase in the rate of synthesis on addition of heavy-metal inducers to the culture medium. Radioimmunoassay of cell extracts indicates the accumulation of three- to fourfold more total T antigen after 2 days of induction by comparison with uninduced controls. A direct correlation was found between the level of T-antigen synthesis and the extent of SV40 DNA replication in inducible cells. Inducible BMT cells expressing a low basal level of T antigen were efficiently transformed by a vector carrying the neomycin resistance marker and an SV40 origin of replication. These vector sequences were maintained in an episomal form in most G418-resistant cell lines examined and persisted even in the absence of biochemical selection. Extensive rearrangements were observed only if the vector contained bacterial plasmid sequences. Expression of a protein product under the control of the SV40 late promoter in such vectors was increased after heavy-metal-dependent amplification of the template. These results demonstrate the ability of BMT cells to maintain a cloned eucaryotic gene in an amplifiable episomal state.

Tissue restricted and stage specific transcription is maintained within 411 nucleotides flanking the 5' end of the chicken alpha-skeletal actin gene

alpha-skeletal actin message levels have been shown to be tightly regulated in chicken primary myoblast cultures. To test for gene elements required for muscle cell specific expression, DNA sequences containing the 5'-flanking regions of the chicken alpha-skeletal actin, beta-cytoplasmic actin, and the histone H2b genes were linked to the coding sequences of the chloramphenicol acetyltransferase gene and transfected into myogenic and non-myogenic cells. In contrast to beta-actin CAT hybrids, the alpha-skeletal actin CAT constructions displayed restricted CAT expression in transfected non-myogenic cells. We showed that a 411 nucleotide fragment flanking the 5' end of of the alpha-skeletal actin gene was responsible for a 9-15 fold increase in CAT enzymatic activity during myoblast fusion, versus only a transient 2 fold rise for the beta-actin and histone flanking sequences. These results indicate that DNA sequences within 411 bp of the 5' terminus of the alpha-skeletal actin gene influenced its cell type and stage specific expression.

Expression of the late gene of simian virus 40 under the control of the simian virus 40 early-region promoter in monkey and mouse cells

We constructed a recombinant plasmid (pVNR4) with the simian virus 40 (SV40) early promoter positioned 30 nucleotides upstream from the major SV40 late transcription initiation site at residue 325. After transfection of the recombinant plasmid DNA into COS and mouse L cells, the transcripts of the SV40 late region were analyzed by S1 nuclease and primer extension analysis. The following are the principal findings. (i) The 16S and 19S late RNAs used the characteristic wild-type splice; no detectable levels of 19S unspliced RNA were observed. (ii) The majority of the late RNAs were heterogeneous and initiated in the early region (upstream and downstream from the Hogness-Goldberg sequence), and a minor population initiated at residue 325, the principal 5' terminus of the wild-type late RNA. (iii) During SV40 lytic infection there was a shift in initiation sites used to transcribe the early region from sites that are downstream to sites which are upstream (up RNA) of the origin of DNA replication. We observed that unlike lytic infection, T antigen and viral DNA replication were not needed for the appearance of up RNA in mouse L cells. (iv) In mouse L cells late RNAs were made, and the residue 325 5' end was utilized in the absence of T antigen or DNA replication. (v) In COS cells we found down RNA and up RNA transcribed from the extrachromosomally replicating plasmid but only down RNA produced by the integrated SV40 genome.

Cosmid vectors for high efficiency DNA-mediated transformation and gene amplification in mammalian cells: studies with the human growth hormone gene

We have constructed two new recombinant cosmid vectors that can be used for direct expression and amplification of genomic DNA in mammalian cells. The vectors allow cloning of DNA fragments up to 40 kb in size. Each carries two dominant selectable markers: the bacterial neo gene and the mouse DHFR gene. In the first vector, pCV001, the neo and DHFR genes are regulated by the SV40 early promoter, and in the second, pAVCV007, by the avian sarcoma virus LTR promoter. The neo gene served as a dominant marker for the selection of transformants in all mammalian cell types, and we demonstrate here that the LTR promoter significantly improved the efficiency of DNA-mediated transformation of a human cell line. We isolated the human growth hormone genes from genomic libraries prepared in these cosmid vectors and used these recombinant cosmids for direct transfections of cultured cells. Selection of transformants in increasing concentrations of methotrexate led to the outgrowth of resistant cell populations carrying amplified copies of the DHFR marker. A 40-1000-fold coamplification of the hGH genes was observed in the different transfected cell lines, along with a corresponding increase in transcription and translation activity of the hGH gene. Gene amplification could be achieved in both DHFR deficient or normal cell lines. High level expression of a cloned gene mediated by gene amplification should facilitate characterization of DNA sequences, as well as isolation of specific gene products for biochemical, functional, and pharmacological studies.

Gene transfer into mouse embryos

Gene transfer into the murine genome was accomplished nearly a decade ago by use of chimeras and teratocarcinomas; however, the low frequencies of transfer into the germ line and other difficulties stemming from mosaicism and karyotypic abnormalities in chimeric mice have limited the general usefulness of this procedure in achieving transformation in mammalian embryos. The introduction of cloned genes into teratocarcinoma cells, selection for a mutant phenotype, and transfer of those cells into mouse embryos holds some promise as a technique to employ mouse chimeras for gene transfer into mice. Infection with animal viruses and retroviral vectors provides another way to introduce exogenous DNA into mouse embryos. Infection with Mo-MuLV has been utilized to characterize the relationship between sites of integration and gene function in developing and adult mice. Gene transfer by microinjection of cloned recombinant DNA has been used by many laboratories for the transfer of DNAs into mouse embryos. The factors affecting transformation frequencies and sites of integration are unknown at present, although it seems that integration is not strictly mediated by homology-dependent events. Many genes have been introduced into mouse embryos by these procedures and many of these are expressed at high levels in appropriate tissues. No realistic possibility exists at the present time for the utilization of embryo gene transfer in the medical field for the correction of genetic defects for several reasons. First, in order to effectively provide "gene therapy" it would be necessary to determine the genotype of each recipient egg, a technical impossibility. The genetic diseases that would be amenable to germ line intervention are recessive diseases and there would be only a 25% chance of any one embryo derived from heterozygous parents being a homozygous recessive. Moreover, it would be impossible to distinguish the normal from abnormal embryos. Second, the frequencies of transformation are so low as to exclude work on human beings on ethical grounds. Third, the parameters effecting chromosomal integration sites and gene expression have not been fully characterized. Until it becomes experimentally possible to target the newly introduced DNA into expressable chromosomal sites and actively replace or supplement defective genes, the possibility of gene therapy through manipulation of embryos is remote. Yet, efforts to provide gene therapy in somatic tissues have been promising, leading to expression of a modified phenotype (Anderson, 1984). In contrast to embryo gene therapy, gene therapy in somatic tissues would not lead to germ line propagation of the manipulated genotype.(ABSTRACT TRUNCATED AT 400 WORDS)

New host cell system for regulated simian virus 40 DNA replication

Transformed monkey cell lines (CMT and BMT) that inducible express simian virus 40 (SV40) T antigen from the metallothionein promoter have been isolated and characterized. Immunoprecipitation of pulse-labeled T antigen demonstrates a 5- to 12-fold increase in the rate of synthesis on addition of heavy-metal inducers to the culture medium. Radioimmunoassay of cell extracts indicates the accumulation of three- to fourfold more total T antigen after 2 days of induction by comparison with uninduced controls. A direct correlation was found between the level of T-antigen synthesis and the extent of SV40 DNA replication in inducible cells. Inducible BMT cells expressing a low basal level of T antigen were efficiently transformed by a vector carrying the neomycin resistance marker and an SV40 origin of replication. These vector sequences were maintained in an episomal form in most G418-resistant cell lines examined and persisted even in the absence of biochemical selection. Extensive rearrangements were observed only if the vector contained bacterial plasmid sequences. Expression of a protein product under the control of the SV40 late promoter in such vectors was increased after heavy-metal-dependent amplification of the template. These results demonstrate the ability of BMT cells to maintain a cloned eucaryotic gene in an amplifiable episomal state.

Expression of human choriogonadotropin in monkey cells using a single simian virus 40 vector

We have inserted the cDNAs coding for both polypeptide subunits, alpha and beta, of human choriogonadotropin (hCG) into a single simian virus 40 expression vector in such a way that they replace the viral VP2 and VP1 coding sequences, respectively. Monkey cells infected with this virus and the appropriate helper virus produce dimeric hCG. hCG produced in this system was shown to chromatograph identically to standard hCG preparations on gel filtration and to be biologically active in the mouse uterine weight assay.

Nuclear localization of poliovirus capsid polypeptide VP1 expressed as a fusion protein with SV40-VP1

The poliovirus cDNA fragment coding for capsid polypeptide VP1 was inserted between the EcoRI and BamHI sites of SV40 DNA, generating a chimaeric gene in which the sequence of the 302 amino acids (aa) of poliovirus capsid polypeptide VP1 was placed downstream from that of the 94 N-terminal aa of SV40 capsid polypeptide VP1. The resulting defective, hybrid virus, SV40-delta 1 polio, was propagated in CV1 cells using an early SV40 mutant, am404, as a helper. Cells doubly infected by SV40-delta 1 polio and am404 expressed a 50-kDal fusion protein which was specifically immunoprecipitated by polyclonal and/or monoclonal antibodies raised against poliovirus capsids or against poliovirus polypeptide VP1. Examination of the infected cells by immunofluorescence after staining with anti-poliovirus VP1 immune sera revealed that the fusion protein was mostly located in the intra- and perinuclear space of the cells, in contrast to the exclusively intracytoplasmic location of genuine poliovirus VP1 polypeptide that was observed in poliovirus-infected cells. This suggests that the N-terminal part of the SV40-VP1 polypeptide could contain an important sequence element acting as a migration signal for the transport of proteins from the cytoplasm to the nucleus.

Transfer of genes into hematopoietic cells using recombinant DNA viruses

The ability of recombinant DNA viruses to transfer genes into hematopoietic cells has been explored. A recombinant simian virus 40 (SV40) in which the early region had been replaced with the chloramphenicol acetyltransferase (CAT) gene driven by the promoter from Rous sarcoma virus (RSV), was constructed. This virus transferred the CAT gene more efficiently into mouse and human bone marrow cells and into the K562, MEL, and WEHI hematopoietic tissue culture cell lines, than the classical calcium phosphate DNA transfer procedure, as shown by assay for CAT activity 48 hr after infection. Recombinant SV40 virions were also shown to be capable of stably transforming Chinese hamster ovary cells by use of an early region recombinant containing the methotrexate-resistant dihydrofolate reductase (DHFR) gene driven by the RSV promoter. The entire DHFR transcriptional unit could be detected in the genome of transformed cells that were also shown to be resistant to methotrexate. A recombinant adenovirus stock containing the neomycin-resistance gene driven by the SV40 early promoter was used to infect the K562 and MEL hematopoietic cell lines to resistance to the antibiotic G418. Transformation frequency was 10- to 100-fold higher than that obtained with calcium phosphate-precipitated DNA. Most or all of the recombinant adenovirus genome was integrated as 1-3 copies in the transformed cells. These studies show the feasibility of using DNA viruses for introduction of new genetic material into hematopoietic cells.

SV40-alpha-globulin hybrid minichromosomes. Differences in DNase I hypersensitivity of promoter and enhancer sequences

The structure of minichromosomes from an SV40 recombinant carrying a functional mouse alpha-globulin gene in its late coding region was studied by DNase I treatment of isolated nuclei from infected cells. Ten prominent DNase-I-hypersensitive sites were detected in the inserted globin fragment, six in the region upstream from the transcription initiation site and four within the gene itself. Corresponding sites were not seen in naked DNA, nor in minichromosomes without this insertion. However this hypersensitivity pattern differs from that observed for mouse alpha-globin in Friend erythroleukemia cells before or after induction. The recombinant minichromosome also displayed eight hypersensitive sites, indistinguishable from those seen in unsubstituted SV40, in the viral sequences containing the origin of replication and the regulatory signals for early and late viral transcription. The cleavage sites in SV40 occur predominently in the origin-promotor-enhancer region, whereas those in the globin fragment were more dispersed and less pronounced. This suggests that the specific local opening of this region in the viral chromatin is not due solely to the presence of active promoters, but is associated with the function of the enhancer element.

Transduction of the Chinese hamster ovary aprt gene by herpes simplex virus

The Chinese hamster ovary adenine phosphoribosyl transferase gene (aprt) was reengineered to be flanked by sequences from the thymidine kinase (tk) gene of herpes simplex virus. This construct was cotransfected with DNA from herpes simplex virus type 1, and after 3 days, virus was harvested and Tk- plaques were selected after the virus was plated on Tk- cells in the presence of bromodeoxycytosine. Recombinant viruses were identified by dot-blot hybridization, and the arrangement of aprt and tk sequences were determined by Southern blot hybridization. Analysis of the recombinants revealed that acquisition of aprt sequences resulted from insertional inactivation of the tk locus as a consequence of homology-based recombination. Recombination was precise, as evidenced by the failure to detect plasmid sequences or the synthetic restriction endonuclease sites that bounded the mutant tk gene in the aprt-tk construct. Infection of Aprt- mouse or Chinese hamster ovary cells with UV-irradiated virus and selection in medium containing azaserine and adenine resulted in the survival of numerous colonies that stably express the aprt gene. Transformed cells synthesized an aprt mRNA that is identical to wild-type mRNA as determined by Northern blot and S1 nuclease analyses. Cells lytically infected with the recombinant virus do not appear to transcribe the aprt gene. Thus, infected cells differentiate between virus and foreign promoters even when a cellular gene is cis to the virus chromosome.

Construction and functional characterization of polyomavirus genomes that separately encode the three early proteins

Modified polyomavirus genomes that individually encode the large and small T proteins were constructed by exchanging restriction endonuclease fragments between cDNA copies of the respective mRNAs and cloned genomic DNA. The efficacies of the new constructs, and that of the middle T protein gene described previously (R. Treisman , U. Novak, J. Favaloro , and R. Kamen , Nature [London] 292:595-600, 1981), were demonstrated with simian virus 40 (SV40)-polyomavirus recombinants in which part or all of the SV40 late region was replaced with the modified polyomavirus early genes. Each of the three recombinant viruses induced the synthesis of only the expected polyomavirus early protein in infected CV-1 cells. The rates of synthesis of large, middle, and small T proteins were ca. 1.5, 4.0, and 9.0 times the rate of synthesis of SV40 large T protein, respectively. The deletion of introns had no detrimental effect on mRNA biogenesis. Indeed, a further polyomavirus-SV40 recombinant, containing wild-type polyomavirus early region DNA, expressed an aberrant 58,000-dalton form of the middle T protein which we believe to result from utilization of a cryptic splice site. Immunofluorescence studied with monkey cells infected by the recombinant viruses allowed us to determine the cellular locations of the polyomavirus early proteins. Overproduction of the middle T protein did not result in a corresponding overproduction of the middle T protein-associated tyrosine phosphokinase activity.

General method for production and selection of infectious vaccinia virus recombinants expressing foreign genes

The production and selection of infectious vaccinia virus recombinants expressing foreign genes was facilitated by the construction of plasmid vectors. These vectors contain all or part of the vaccinia virus thymidine kinase (TK) gene interrupted by multiple unique restriction endonuclease sites placed adjacent to the TK promoter or another promoter translocated within the TK gene. The insertion of a continuous coding sequence for a foreign protein at one of the unique restriction endonuclease sites juxtaposes the transcriptional start site of a vaccinia promoter and the translational start site of a foreign gene. After transfection of vaccinia virus-infected cells with such plasmids, homologous recombination occurs between the vaccinia virus sequences flanking the chimeric gene and the same sequences within the virus genome. Recombinants formed in this manner have the chimeric gene inserted within the body of the vaccinia virus TK gene under control of a vaccinia virus promoter. Since recombinants have an interrupted TK gene, they are selected on the basis of their TK- phenotype and then checked for the presence and expression of the foreign gene. Infectious recombinant viruses expressing the procaryotic enzyme chloramphenicol acetyltransferase were constructed to optimize the system. The absence of chloramphenicol acetyltransferase activity in uninfected cells or in cells infected with wild-type vaccinia virus and the availability of a sensitive and quantitative enzyme assay allowed an estimation of the relative strengths of various promoter constructs. The expression of chloramphenicol acetyltransferase was detected within 1 h after infection of cells with recombinant virus, reflecting the early nature of the promoters used.

Gene therapy

A number of techniques are available for insertion of new genetic information into mammalian cells. some of these have been used successfully for genetic modification of germ line cells and somatic cells of living animals. Some of these techniques may be applicable to treatment of some of the genetic diseases of man, once problems related to the control of expression of introduced genes are solved.

Introduction of purified genes into mammalian cells

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

Analysis of gene expression using simian virus 40 vectors

Extensive studies on the DNA tumor virus Simian Virus 40 (SV40) have provided a wealth of information regarding the genome organization, regulation of viral gene expression, and the mechanism of DNA replication. SV40 can grow lytically in permissive monkey cells or the viral DNA can integrate into the host genome of nonpermissive rodent cells causing morphological transformation. The viral DNA exists as a minichromosome within the nuclei of lytically infected cells and, as a consequence of DNA replication, there is a significant amplification of the viral genome during infection. These properties suggested that SV40 could be developed as a transducing vector to introduce exogenous DNA into mammalian cells and to express this foreign DNA during the SV40 infectious cycle. In this article the properties of SV40 virus vectors and SV40 hybrid plasmid vectors are described and contrasted.

Isolation of a genomic clone for bovine pancreatic trypsin inhibitor by using a unique-sequence synthetic DNA probe

Unique-sequence synthetic DNA probes, based on the known amino acid sequence of bovine pancreatic trypsin inhibitor, were constructed from oligodeoxynucleotides. In genomic Southern blot experiments, these probes were shown to hybridize specifically to discrete restriction fragments. A synthetic probe also was used to isolate a cloned BPTI gene from a bovine genomic library. DNA sequence analysis of this clone indicated that the BPTI coding region was neither preceded by a start codon nor immediately followed by a termination codon. This suggests that the mature form of BPTI may be produced through proteolytic processing from a larger polypeptide precursor.

Versatile cosmid vectors for the isolation, expression, and rescue of gene sequences: studies with the human alpha-globin gene cluster

We have developed a series of cosmids that can be used as vectors for genomic recombinant DNA library preparations, as expression vectors in mammalian cells for both transient and stable transformations, and as shuttle vectors between bacteria and mammalian cells. These cosmids were constructed by inserting one of the SV2-derived selectable gene markers--SV2-gpt, SV2-DHFR, and SV2-neo--in cosmid pJB8. High efficiency of genomic cloning was obtained with these cosmids and the size of the inserts was 30-42 kilobases. We isolated recombinant cosmids containing the human alpha-globin gene cluster from these genomic libraries. The simian virus 40 DNA in these selectable gene markers provides the origin of replication and enhancer sequences necessary for replication in permissive cells such as COS 7 cells and thereby allows transient expression of alpha-globin genes in these cells. These cosmids and their recombinants could also be stably transformed into mammalian cells by using the respective selection systems. Both of the adult alpha-globin genes were more actively expressed than the embryonic zeta-globin genes in these transformed cell lines. Because of the presence of the cohesive ends of the Charon 4A phage in the cosmids, the transforming DNA sequences could readily be rescued from these stably transformed cells into bacteria by in vitro packaging of total cellular DNA. Thus, these cosmid vectors are potentially useful for direct isolation of structural genes.

Expression vehicles used in recombinant DNA technology

We survey cloning vehicles whose function is to carry and express a gene in host cells including Escherichia coli, Saccharomyces cerevisiae and mammalian cells. In E. coli these include vehicles based on the lac operon, the trp operon, the rho leftward operon, and the recA gone; open reading frame cloning vehicles are also discussed, as are steps that can be taken to extrude a gene product from the cell and the use of plasmids with runaway replication. In S. cerevisiae we discuss vehicles based on the PGK gene, the ADH1 gene, the acid phosphatase gene and the GAL1-GAL10 gene cluster. In mammalian cells we discuss vehicles based on SV40 promoters, the metallothionein gene, retroviral LTR promoters, bovine papilloma virus and vaccinia virus.

Vaccinia virus: a selectable eukaryotic cloning and expression vector

Foreign DNA was inserted into two nonessential regions of the vaccinia virus genome by homologous recombination in cells infected with virus and transfected with plasmids containing the foreign DNA elements flanked by vaccinia virus DNA. Thymidine kinase-negative (TK-) recombinants were selected after inserting foreign DNA into the coding region of the TK gene of wild-type vaccinia virus; TK+ recombinants were selected after inserting the herpesvirus TK gene into TK- mutants of vaccinia virus. For TK+ expression, it was necessary to insert a 275-base-pair DNA fragment containing the initiation site and sequences upstream of an early vaccinia virus transcript next to the coding sequences of the herpesvirus gene. The unique ability of the herpesvirus TK to phosphorylate 125I-labeled deoxycytidine provided independent confirmation of gene expression. These studies demonstrate the use of vaccinia virus as a selectable cloning and expression vector, confirm the map location of the vaccinia virus TK gene, and provide initial information regarding the location of vaccinia virus transcriptional regulatory sequences.

Construction of a mammalian transducing vector from the genome of Moloney murine leukemia virus

A 0.9-kilobase DNA fragment from the genome of Moloney murine leukemia virus, including the viral long terminal repeat, was covalently linked to the herpes simplex virus I thymidine kinase (tk) gene whose promoter was previously removed. The hybrid DNA structure was introduced into the chromosome of tk- mouse cells at single copy numbers, via transfection procedures. Cells expressing the newly introduced tk gene were identified by the HAT selection procedure and analyzed for tk- and moloney murine leukemia virus-specific DNA and RNA sequences by blot hybridization procedures. Expression of the tk gene is dependent on function(s) provided in cis by the viral DNA fragment. Vectors derived from this region are termed rGag (rG) vectors.

In vivo assembly of regularly spaced nucleosomes on mouse beta maj-globin DNA cloned in an SV40 recombinant

Short-term transformation of HeLa cells with an SV40 recombinant carrying a 7.0-Kb mouse genomic globin DNA was studied. It was found that 48 h after transfection the donor DNA was present in the cell nucleus at high copy episomal numbers with the globin gene in regularly spaced nucleosomal form and transcribed into 9S poly A+ RNA. These mini-chromosomes can be isolated in sufficient quantity to allow further biochemical and electron microscopic studies.

Altered utilization of splice sites and 5' termini in late RNAs produced by leader region mutants of simian virus 40

We compared the 5' termini and splices of the late 16S and 19S RNAs synthesized by wild-type simian virus 40 and five mutants containing deletions in their late leader region. All mutants produced more unspliced 19S RNA than did wild-type virus, and in two mutants, unspliced 19S RNA constituted more than 60% of the total 19S species. The other three mutants each utilized predominantly a different one of the three spliced species of 19S mRNA. All mutants also produced decreased quantities of 16S mRNA, indicating that they may be defective for splicing both late RNAs. None of the 5' termini of the 16S and 19S RNAs made by the five mutants predominated as in those made by the wild type. Some of the mutant 5' termini were the same as those used by the wild type, whereas others were different. Although present, the major 5'-end positions used by the wild type were frequently not used as major sites by the mutants. In addition, mutants with very similar deletion endpoints synthesized RNAs with different 5' ends. Thus, downstream mutations have a pronounced effect on the location of 5' ends of the late RNAs, and there is no obvious involvement of a measuring function in the placement of 5' ends. For all mutants and wild-type virus, the 5' termini used for 16S and 19S RNAs showed major differences, with some degree of correlation found between the 5' ends and the internal splices of specific mRNA species. A model for the regulation of simian virus 40 late gene expression is presented to explain these findings.

Genetic transformation of Drosophila with transposable element vectors

Exogenous DNA sequences were introduced into the Drosophila germ line. A rosy transposon (ry1), constructed by inserting a chromosomal DNA fragment containing the wild-type rosy gene into a P transposable element, transformed germ line cells in 20 to 50 percent of the injected rosy mutant embryos. Transformants contained one or two copies of chromosomally integrated, intact ry1 that were stably inherited in subsequent generations. These transformed flies had wild-type eye color indicating that the visible genetic defect in the host strain could be fully and permanently corrected by the transferred gene. To demonstrate the generality of this approach, a DNA segment that does not confer a recognizable phenotype on recipients was also transferred into germ line chromosomes.

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.

Construction of poxviruses as cloning vectors: insertion of the thymidine kinase gene from herpes simplex virus into the DNA of infectious vaccinia virus

We have constructed recombinant vaccinia viruses containing the thymidine kinase gene from herpes simplex virus. The gene was inserted into the genome of a variant of vaccinia virus that had undergone spontaneous deletion as well as into the 120-megadalton genome of the large prototypic vaccinia variant. This was accomplished via in vivo recombination by cotransfection of eukaryotic tissue culture cells with cloned BamHI-digested thymidine kinase gene from herpes simplex virus containing flanking vaccinia virus DNA sequences and infectious rescuing vaccinia virus. Pure populations of the recombinant viruses were obtained by replica filter techniques or by growth of the recombinant virus in biochemically selective medium. The herpes simplex virus thymidine kinase gene, as an insert in vaccinia virus, is transcribed in vivo and in vitro, and the fidelity of in vivo transcription into a functional gene product was detected by the phosphorylation of 5-[125I]iodo-2'-deoxycytidine.