Abstract:
A method for sustained expression of an exogenous gene forms a circular episomal plasmid to solve the problem of transient expression of baculovirus in the transduced mammalian cells caused by the dilution and degradation when mammalian cells replicate and also prevents dysfunctional cell metabolism.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for sustained expression of an exogenous gene, and more particularly in sustained expression of an exogenous gene by using baculovirus. 
         [0003]    2. Description of the Prior Art 
         [0004]    Gene delivery vectors are classified into non-viral vectors and viral vectors. Non-viral methods of introducing an exogenous gene include various chemical agents and physical methods, e.g. polymer or liposome-mediated transfection or electroporation. However, these methods have following disadvantages including not applicable to various cell types, especially the primary cells; taking huge amount of purified exogenous gene; severely damaging the cell membranes and impacting the cellular physiology; and most of all, low transfection efficiency. 
         [0005]    Viruses possess a number of advantages as an exogenous gene delivery vector, that own high infection efficiency in a wide variety of cell types and tissues, and are provided with an ability of target specificity and enhanced expression of the exogenous gene in vitro and in vivo. Current viral vectors including retroviruses, lentivirus, adenoviruses, adeno-associated viruses and herpesvirus have been employed for gene therapy. 
         [0006]    Although the above-mentioned vectors had certain degrees of success in particular application, there are still a variety of obstacles that cannot be surmounted. Plasmid delivery is considered less efficient than viral vectors. Retroviruses integrate viral genes into host cell chromosome resulting in the insertional mutagenesis in host cells or other dysfunction. The French team (2002) found that 9× linked-SCID patients are cured by using retroviruses as gene delivery vector, but 2 patients were found with leukemia; therefore, there are some safety concerns in adopting retroviruses as gene delivery vectors. 
         [0007]    As for adenoviruses, adenoviruses do not integrate viral genes into the host cells but may have safety issues concerning the elicitation of immune responses; in addition, there was a death report in a gene therapy experiment employing adenovirus (1999). Recently, a gutless vector has been developed for mitigating immune responses; however, it needs a helper virus for co-infection in the process of gutless vector production, resulting in more complexity of purifying viral particles. Though adeno-associcated viruses (AAV) is safer and enables longer gene expression time, adeno-associcated viruses can not deliver large genes because of their own 3.5-4 kb cloning capacity limit, and AAV production can not easily be scaled up due to complicated preparation procedure. Furthermore, adeno-associated viruses are found to integrate viral genes into the host cell chromosome and induce hepatoma in neonatal mice at high viral dosage. Due to these drawbacks, the lack of an ideal vector has been impeding successful application of gene therapy. 
         [0008]    On the other hand, baculovirus ( Autographa californica  multiple nucleopolyhedrovirus) is a DNA virus that infects insects as the natural hosts, and has been widely employed for recombinant protein production. However, baculovirus is found to transduce various mammalian cells efficiently, e.g. BHK, CHO, CV-1, HeLa, 293, Cos7, neural cells, human fibroblasts and pancreatic cells, and can drive the expression of exogenous gene under promoters that are active in mammalian cells, e.g. SV-40 (Simian Virus 40) or CMV-IE (cytomegalovirus immediate-early) promoter, without any visible cytopathic effect (CPE). (1995). Besides, it is recently found that baculovirus can transduce bone marrow mesenchymal stem cells with transduction efficiency up to 87%. 
         [0009]    Baculovirus vector has following advantages: (1) baculovirus does not replicate and induce cellular toxicity after transducing mammalian cells. Its DNA does not integrate into the host chromosome and is degraded with time; (2) there are no pre-existing antibodies against baculovirus in mammals since the natural hosts of baculovirus are insect cells; (3) baculovirus genome (˜130 kb) is much larger than other viral vectors and therefore is able to carry larger and more exogenous genes (at least 38 kb) into cells; (4) baculovirus production can be scaled up by infecting the natural hosts (insect cells) in simple operating procedures; and (5) baculovirus are classified into insect virus and can be operated in bio-safety 1 laboratories. Features (1) to (3) make baculovirus an effective and safer gene delivery vector, features (4) and (5) make baculovirus scaled up in a safe and easy way. These features altogether make baculovirus a good gene therapy vector in mammalian cells. Thanks to these features, baculovirus has hence been developed as a candidate for exogenous gene vector, but a number of roadblocks need to be lifted. One major hurdle is transient expression nature (&lt;7 days) due to its inability to replicate in mammalian cells and has a large genome with multiple recognition sites for many restriction endonucleases. Since the genome of baculovirus cannot replicate in mammalian cells, it is usually degraded or diluted soon after transduction, leading to loss of transgene expression over time. 
         [0010]    In general, there are two ways for sustained expression and replication of an exogenous gene in cells. First, the exogenous gene is integrated into host chromosome for the exogenous gene to be replicated with the cellular chromosome. As mentioned above, this method might easily interfere with the normal gene function of cells. 
         [0011]    The other method for sustained expression and replication of an exogenous gene is to form a replicable episomal plasmid, wherein the episomal plasmid carries the exogenous gene and includes an origin of replication sequence that is able to allow the episomal plasmid to self-replicate and stably express the exogenous gene in mammalian cells. The episomal plasmid prevents dysfunctional cell metabolism without integrating into the cellular chromosome. 
         [0012]    To sum up, the primary goal of this invention to develop a baculovirus vector that allows for the formation of a replicable episomal plasmid for sustained expression of exogenous genes in mammalian cells. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention is directed to provide a method for sustained expression of an exogenous gene, which forms a circular episomal plasmid to solve the problem of transient expression of baculovirus in the transduced mammalian cell caused by the dilution and degradation when mammalian cells replicate and also to prevent dysfunctional cell metabolism. 
         [0014]    According to an embodiment, a method for sustained expression of an exogenous gene includes providing a first recombinant baculovirus comprising a recombinase gene; providing a second recombinant baculovirus comprising two recombinase recognition sites and an exogenous gene cassette flanked by the recombinase recognition sites, wherein the exogenous gene cassette comprises an origin of replication sequence; and co-transducing with the first recombinant baculovirus and the second recombinant baculovirus into a mammalian cell, wherein a recombinase is expressed by the recombinase gene and identifies the recombinase recognition sites for site-specific recombination to form a circular episomal plasmid comprising the exogenous gene cassette. 
         [0015]    Other advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The foregoing aspects and many of the accompanying advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0017]      FIG. 1  is a flow chart schematically illustrating a preferred embodiment of the present invention; 
           [0018]      FIG. 2   a  to  FIG. 2   c  is a diagram schematically illustrating recombinant baculovirus according to an embodiment of the present invention; 
           [0019]      FIG. 3  is a diagram schematically illustrating an embodiment of the present invention; and 
           [0020]      FIG. 4   a  to  FIG. 6   b  are diagrams schematically illustrating the experimental outcomes of an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    Referring to  FIG. 1 ,  FIG. 2   a,  and  FIG. 2   b;    FIG. 1  is a flow chart schematically illustrating a preferred embodiment of the present invention;  FIG. 2   a  and  FIG. 2   b  are diagrams schematically illustrating a first recombinant baculovirus  1  and a second recombinant baculovirus  2  according to a preferred embodiment of the present invention. 
         [0022]    At step S 11 , a first recombinant baculovirus  1  is provided, wherein the first recombinant baculovirus  1  includes a recombinase gene  11  and a first promoter  12  positioned upstream the recombinant gene  11 . 
         [0023]    At step S 12 , a second recombinant baculovirus  2  is provided, wherein the second recombinant baculovirus  2  includes two recombinase recognition sites  21  and an exogenous gene cassette  23  flanked by the recombinase recognition sites  21 . The exogenous gene  23  includes a nuclear antigen gene  241 , an origin of replication sequence  242 , and a second promoter  243 . A nuclear antigen is expressed by the nuclear antigen  241  and binds to the origin of replication sequence  242 . 
         [0024]    According to a preferred example of the present invention, the recombinase comprises a flipase recombination enzyme (FLP), wherein the corresponding recombinase recognition site is a flipase recognition target (Frt). For site-specific recombination, a site-specific recombinase, the FLP, recognizes the specific recombinase recognition sites and catalyzes highly efficient recombination between the Frt sequences. The FLP/Frt site-specific recombination occurs frequently, and the Frt sequence has not been discovered in certain model animals (e.g.  C. elegans, Drosophila,  and mice) and human genome; therefore, FLP/Frt site-specific recombination is specific in mammalian cells. 
         [0025]    According to another example, the recombinase comprises a cyclization recombination recombinase (CRE), wherein the corresponding recombinase recognition site is a locus of X-over P1 (loxp). 
         [0026]    Furthermore, the episomal plasmid has to include an origin of replication sequence for the sustained replication and stable presence of transgenes in the cell. According to an example of the present invention, the origin of replication sequence  242  comprises a scaffold/matrix attachment region (S/MAR) element isolated from the interferon β-gene, that does not require any virally encoded trans-acting factors for episome replication. Therefore, an origin of replication sequence is sufficient for the sustained replication and stable presence of transgenes in the cell. 
         [0027]    In another embodiment, the second recombinant baculovirus  2  further comprises a nuclear antigen gene, and a nuclear antigen is expressed by the nuclear antigen gene  241  and binds to the origin of replication sequence  242 . For example, the origin of replication sequence  242  includes Epstein-Barr virus origin for stable episomal replication (EBV oriP), wherein the corresponding nuclear antigen is an EBV nuclear antigen 1 (EBNA-1). The oriP/EBNA-1 system includes the origin of replication sequence and the nuclear antigen adopted from the EB virus and drives one replication of the episomal plasmid when the cell replicates by the EBNA-1 interacts with OriP to distribute the episomal plasmids evenly to daughter cells for equivalent and stable presence of episomal plasmids in the cells during cell mitosis. According to another example of the present invention, the origin of replication sequence  242  comprises a Simian Virus 40 (SV40) origin of replication sequence, wherein the corresponding nuclear antigen is a SV40 large T antigen. 
         [0028]    In a preferred embodiment of the present invention, the exogenous gene cassette  23  further includes a transgene  251  and a third promoter  252  positioned upstream the transgene  251 . 
         [0029]    The aforementioned promoters are used for enhancing gene expression, and therefore any one of the promoters presence is optional; any one of the promoters direction is the same as the gene to be enhanced; and the promoters may be constitutive or inducible. For example, the first promoter  12 , the second promoter  243 , and the third promoter  252  may include a cytomegalovirus immediate early (CMV-IE) promoter or a SV40 promoter. 
         [0030]    Next, at step S 13 , the first recombinant baculovirus  1  and the second recombinant baculovirus  2  are co-transduced to a mammalian cell. A recombinase is expressed by the recombinase gene  11  in the mammalian cell and identifies the recombinase recognition sites  21  for site-specific recombination to form a circular episomal plasmid comprising the exogenous gene cassette  23 . 
         [0031]    Referring to  FIG. 2   b,  the transgene  251  may comprise a reporter gene, e.g. an enhanced green fluorescence protein (EGFP), or a functional target gene. 
         [0032]    The transgene  251  may comprise an antibiotics resistant gene, e.g. a neomycin resistant gene, used for increasing stability of the exogenous gene in the mammalian cell in the presence of an antibiotic. Therefore, an antibiotic may be added to provide a selective pressure for increasing stability of the exogenous gene cassette  23  of episomal plasmid in the mammalian cell. 
         [0033]    Referring to  FIG. 2   c,  a second recombinant baculovirus  2 ′ according to an example of the present invention is constructed for analyzing a percentage of the formation and expression of the circular episomal plasmid. The second recombinant baculovirus  2 ′ is different from the second recombinant baculovirus  2  in the position of the third promoter  252 , wherein the third promoter  252  of the second recombinant baculovirus  2 ′ would be in the same direction with the transgene  251  as a result of site-specific recombination. In the present embodiment, the transgene  251  comprises a reporter gene, e.g. an egfp. Prior to the formation of the circular episomal plasmid, the third promoter  252  is not in the same direction as the egfp gene and hence unable to drive the expression of EGFP; after FLP-mediated circularization, the third promoter  252  is in the same direction with the egfp gene and drives the expression of EGFP. Therefore, a percentage of the fluorescent mammalian cell is detected to determine a percentage of the mammalian cell harboring the newly formed episome. 
         [0034]    To sum up, the present invention provides a novel baculovirus as the genetic vector for safe and efficient gene delivery, which delivers the circular episomal plasmid by FLP-mediated recombination and prolongs the expression of exogenous gene by an origin of replication sequence. 
         [0035]    The following descriptions of specific embodiments of the present invention have been presented for purposes of illustrations and description. They are not intended to be exclusive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents. 
         [0036]      FIG. 3  is a diagram schematically illustrating an embodiment of the present invention, wherein the first recombinant baculovirus (denoted as Bac-FLP) comprises an FLP gene (the recombinase gene  11 ) and a P CMV-IE  (CMV-IE promoter, the first promoter  12 ); the second recombinant baculovirus (denoted as BacCON-CE) harbors the exogenous gene cassette  23  flanked by Frt (the recombinase recognition site  21 ). To facilitate the monitoring of initial transgene expression and genuine gene delivery efficiency of BacCON-CE, the vector that expresses the transgene  251  includes an egfp gene (the reporter gene) under the control of a P CMV-IE  promoter and a Neor (neomycin resistant gene, the antibiotics resistant gene) with a P SV40  (SV40 promoter). The cassette also comprises EBNA-1 (the nuclear antigen gene  241 ), oriP (the origin of replication sequence  242 ), and P CMV-IE  (the second promoter  243 ). 
         [0037]    HEK293 cells are co-transduced with Bac-FLP (MOI  100 ) and BacCON-CE (MOI  400 ) and cultured in the presence or absence of 50 μg/ml G418. FLP is expressed by the FLP gene which is driven by the P CMV-IE  in Bac-FLP and identifies the Frt sites of BacCON-CE for site-specific recombination. After recombination, the exogenous gene cassette is thus separated from the BacCON-CE genome, and the Frt sequences are self-ligated to form a circular episomal plasmid, pFC-OEE. The circular episomal plasmid is replicated once when the cell replicates, so that it is evenly distributed to daughter cells for stable presence and sustained expression of the exogenous gene cassette in the cells. 
         [0038]    As shown in  FIG. 4   a  and  FIG. 4   b,  the cells are transduced with BacCON-CE (MOI  400 ) alone or with a typical baculovirus vector (Bac-CE) that transiently expresses egfp under the CMV-IE promoter (MOI  400 ). Under these transduction conditions, the % GFP+ cells approach 99-100% initially, but the GFP expression mediated by Bac-CE transduction underwent a rapid decay, with ≈16% and less than 5% cells emitting fluorescence at 14 and 20 dpt, respectively. By defining the time point at which the % GFP+ cells dropped below 5% as the expression duration, BacCON-CE transduction alone merely prolonged the expression to 28 days, but significantly elevated the mean fluorescence intensity (MFI). In the absence of G418, co-transduction with BacCON-CE and BacFLP considerably elevate the % GFP+ cells at 14 and 21 dpt to ≈66% and ≈50% with concurrent enhanced MFI. More importantly, the co-transduction extends the expression duration to at least  48  days. EGFP is expressed in the cells co-transduced with Bac-FLP and BacCON-CE for a prolonged period of time. 
         [0039]    With the addition of G418, the insertion of Neor gene in the cassette resulted in even more remarkable enhancement in gene expression level and duration, as 82%, 78%, 42% and 36% of transduced cells are EGFP-positive at 14, 21, 49 and 63 dpt, respectively, and the mean FI remained higher than 1000 throughout the culture period. Thus the duration of gene expression is extended to at least 63 days. Therefore, the present invention provides a method for sustained expression of an exogenous gene in cells. 
         [0040]    Furthermore, a polymerase chain reaction (PCR) is performed to validate the generation of the episomal plasmid. A pair of primers are designed on the basis of Frt sequence (Frt-F: 5′-CGTGTACGGTGGGAGGTCTAT-3′ and Frt-R:5′-GCCCT TGCTCACCATGGT-3′) since the ligated Frt sequence is present in the plasmid pFC-OEE after recombination. As mentioned above, the exogenous gene cassette is separated from the baculovirus genome, and the Frt sequences are self-ligated to form a circular plasmid after FLP-catalyzed recombination; therefore, the PCR is thus performed to validate the formation and presence of the circular episomal plasmid. 
         [0041]    As mentioned above, the present invention provides a method for sustained expression of an exogenous gene in mammalian cells, which confers longer expression time than typical baculovirus vectors. A quantitative polymerase chain reaction (Q-PCR) is performed to validate whether the persistent fluorescence is an artifact of slow EGFP protein decay in the co-transduced cells or stemmed from the long-term maintenance of episomal plasmid pCON-CE. The primers are designed on the basis of Frt sequences: forward primer 5′-GGATCCATGATAAATTTAATTATTGATG-3′ and reverse primer 5′-GGTACCTTATTTAGTATATTTTAAGTG-3′ since the ligated Frt sequence is present in plasmid pCON-CE after recombination. All data were normalized against those at 2 dpt to yield the relative copy number.  FIG. 5   a  depicts that after single transduction with Bac-CE or co-transduction with BacCON-CE and BacFLP, baculoviral genome as probed by the primers targeting endogenous gp64 gene is rapidly degraded and virtually disappeared at 21 dpt. In contrast, the episomal plasmid pCON-CE as probed by the primers targeting the recombined Frt sites decayed at a slower rate even without the addition of G418, decreasing to 1.5% of the original episomal DNA at 21 dpt, and remained detectable until 45 dpt. By defining the time point at which the episomal DNA drops below 0.1% as the expression duration. Note that the data are obtained with internal control gene GAPDH, thus the decay of relative gene copy number arise from the dilution as well as degradation of genes in the cells. Strikingly, co-trasnduction with BacCON-CE and BacFLP, in conjunction with G418 addition, gave arise to stable maintenance of the episomal plasmid, which remain higher than 3.5% at 21 dpt and persist for at least 63 days. Under the same condition, the number of baculoviral genome still decay steadily with time albeit at a slow decay rate, and drop to 0.2% at 63 dpt. In conclusion, the sustained expression of the exogenous gene is caused by the stable presence of the episomal plasmid and may be further enhanced by adding an antibiotic to the incubating environment of the cells. 
         [0042]    Conventional transfection method usually results in poor gene delivery into primary cell lines or mesenchymal stem cells. The present invention would test the efficiency of episomal plasmid delivery by transducing various kinds of cells and provide a novel delivery method in addition to conventional transfection method. Cells are transduced in accordance with the mentioned transduction procedure, and the percentage and fluorescence intensity of GFP+ cells are analyzed at second day post-transduction. As illustrated in  FIG. 6   a  and  FIG. 6   b,  the % GFP+ cells exceed 36% in all cell lines even for the difficult-to-transfect cell lines HepG2 and HuH-7. The % GFP+ cells are the highest in BHK cells (83%) and remain high in primary chondrocytes (77%) and MSCs (48%). Again, the trend in mean FI concurred with that in the % GFP+ cells. It should be noted that baculovirus can enter these cells at efficiencies higher than 80%, thus the disparity in the % GFP+ cells and mean FI reflect the differences in the recombination efficiencies in these cell types. 
         [0043]    Since the FLP/Frt-based baculovirus system enables efficient plasmid formation within cells, the episome delivery efficiency via baculovirus-mediated transduction or lipid-mediated trasnfection are then compared. The cells are seeded (≈5×10 5  cells/well) and cultured overnight followed by co-transduction with BacFLP (MOI  100 ) and BacFC-OEE (MOI  400 ), or transfection with a control plasmid (≈6.5×10 11  copies/well) by lipofectamine™ 2000. With this design, 800-1100 times more egfp gene is introduced by transfection (≈6.5×10 11  copies) than by transduction (≈6.0-8.0×10 8  copies from 2.0×10 8  pfu per well, by assuming that 1 pfu is equivalent to 3-4 baculovirus particles). wherein the resultant % GFP+ cells are significantly higher in the transduced (42%, 48% and 76% for HuH-7, MSC and chondrocytes, respectively) than in the transfected cells (9%, 3% and 7% for HuH-7, MSC and chondrocytes, respectively) (not shown), demonstrating the significantly more efficient transgene expression mediated by baculovirus carried plasmids. 
         [0044]    To sum up, a method of sustained expression of an exogenous gene according to the present invention effectively transduces various kinds of cells and improves the expression time of baculovirus to more than 60 days. The present invention has following advantages in gene delivery.
   1. Efficiency: baculovirus is an efficient gene delivery vector; therefore high transduction efficiency and generation efficiency of episomal plasmid is achieved by adopting baculovirus to transduce mammalian cells.   2. Cost-saving: conventional transfection agents used in plasmid transfection, e.g. a liposome (which is expensive), is not a necessity for the present invention.   3. Time-saving: baculovirus is easily scaled up to obtain high-titer virus with advantages of saving operation time and cost.   
 
         [0048]    While the invention is susceptible to various modifications and alternative forms, a specific example thereof has been shown in the drawings and is herein described in detail. It should be understood, however, that the invention is not to be limited to the particular form disclosed, but to the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims.