Abstract:
The present invention relates to a recombinant vector comprising MJ1 gene coding an integrase derived from  Enterococcus  temperate bacteriophage φ FC1 and an integration method using the vector. More particularly, the present invention relates to a recombinant vector comprising MJ1 gene which can make a site-specific integration in the human cell independently without other factors but not cause an excision and an integration method using the vector. Therefore, the present invention can be very useful in gene therapy in mammalian animal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of Korean Patent Application No. 10-2004-0107077, filed on Dec. 16, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a recombinant vector containing MJ1 gene coding an integrase derived from  Enterococcus  temperate bacteriophage φ FC1 and an integration method using the vector. More particularly, the present invention relates to a recombinant vector containing MJ1 gene which can make a site-specific integration in the human cell independently without other factors but not cause an excision and an integration method using the vector. 
     2. Description of Related Art 
       Enterococcus faecalis  is a gram-positive anaerobic bacterium that normally occurs in the intestines of most animal from cockroaches to humans.  E.faecalis  KBL703 strain and its temperate phage φ FC1 was first isolated in our laboratory from a culture of the lysogenic strain of KBL703 following induction by UV irradiation (Kim et al., (1994) Mol. Cell., 4, 155-158). Bacteriophage φ FC1 has double-stranded DNA genome of approximately 40.5 Kbp, icosahedral head and a sheathless noncontractile tail. Phage φ FC1 integration into the host chromosome occurs by a site-specific mechanism. A gene that encodes a site-specific integrase, MJ1, and that is upstream of the attP site has been identified. MJ1 encodes a 465-amino-acid polypeptide with similarity in its N-terminal domain to site-specific integrases. Analysis of the DNA sequences around the attP region identified two predicted bacterium-phage junction regions (attL and attR). The corresponding bacterial attachment site (attB) was deduced from the sequences of these regions (Kim et al., (1996) Biochem. Mol. Biol, 29, 448-45412). 
     Integrase MJ1 mediates unidirectional site-specific integration between two DNA recognition sequences which is the phage attachment site, attP, and the bacterial attachment site, attB. To accomplish integration, temperate phage ΦFC1 encodes an integrase MJ1 that mediates integration between attP and attB. In 290 bp attB sequences, attB site has 3 bp conserved core sequences, which overlapped with 3 bp conserved core sequences of 730 bp attP. Centering around core sequences, attP and attB share their sequences in half, which become attL and attR and 3 bp core sequences is repeated (Yang et al., (2002) J. Bacteriol., 184, 1859-1864). 
     In gene therapy, vectors are the vehicles that are used to transfer the gene of interest to the target cells. Unfortunately, there is no such thing as a ‘good universal vector,’ all of the vectors that are currently available have both advantages and disadvantages. For example, one viral vector might be able to enter target cells very efficiently but once there invokes a strong immune response, resulting in that cell being killed by the immune system, so begin to have problem with safety in host. Accordingly, it is clear that the previously distinct boundaries between viral and non-viral vectors are becoming increasingly blurred. From these reasons, development of safe integration system into implored gene is very important. However, cruder methods prevail for placement of the introduced gene into the genome, random integration often being state of the art. Lack of control over the position of introduced DNA results in unpredictable gene expression and potentially undesirable mutagenesis of important genes. A better solution would be a method that produces efficient site-specific integration into safe locations in the target genome. In these cases, conservative site-specific integration is important in genetic engineering strategies. 
     Enzymes of the site-specific recombination also share high specificity, and, in addition, they act with greater efficiency. Some recombinases function with no requirement for cofactors, permitting their activity in foreign cellular environments. Recombinases such as Cre and FLP perform both integration and excision with the same target sites (Sauer, B. (1994) Curr Opin. Biotechnol., 5, 521-527). Therefore, although these recombinases efficiently perform integration in mammalian cell, the net integration frequency that they mediate is low because of the excisive back reaction. To express the desired gene stably and efficiently at specific site, problems, such as low integration efficiency and stability, must be solved. 
     The present inventors have identified that integrase MJ1 from φFC1 of  enterococcus faecalis  is a good example in the efficiency and stability. 
     SUMMARY OF THE INVENTION 
     The present invention provides a recombinant vector containing MJ1 gene encoding integrase having an amino acid sequence of SEQ ID No. 2, wherein the integrase mediates site-specific integration but not mediates excision in animal cell. 
     In the present recombinant vector according to claim  1 , the MJ1 gene may have any base sequence capable of encoding integrase having an amino acid sequence of SEQ ID No. 2. Preferably, the MJ1 gene has a base sequence of SEQ ID No. 1 and the recombinant vector has a plasmid map shown in  FIG. 1C . 
     The present invention provides a method of site-specific integration in animal cell, which comprises co-transfecting the animal cell with a recombinant vector containing MJ1 gene encoding integrase having an amino acid sequence of SEQ ID No. 2, a recombinant vector containing attB gene and a recombinant vector containing attP gene. 
     In the present method, the recombinant vectors may be any recombinant vector which has MJ1 gene, attB gene or attP gene. Preferably, the MJ1 gene has a base sequence of SEQ ID No. 1 and the recombinant vector containing MJ1 gene has a plasmid map shown in  FIG. 1C . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
       The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1A  is a plasmid map of pcB vector which comprises 290 bp attB gene and CMV promoter. 
         FIG. 1B  is a plasmid map of pGP(−) vector which comprises 739 bp attP gene and includes not CMV promoter but GFP reporter gene. 
         FIG. 1C  is a plasmid map of pcMJ1 vector which comprises MJ1 gene of the present invention. 
         FIG. 1D  is a plasmid map of pcL vector which comprises 624 bp attL gene and CMV promoter. 
         FIG. 1E  is a plasmid map of pGR(−) vector which comprises 404 bp attR gene and includes not CMV promoter but GFP reporter gene. 
         FIG. 1F  is a plasmid map of pREC-1 vector which is formed by integration due to co-transfection with PcB, pGP(−) and pcMJ1. 
         FIG. 1G  is a plasmid map of pREC-E vector which may be formed by exision due to co-transfection with PcL, pGR(−) and pcMJ1. 
         FIG. 2  is an image showing MJ1 expression in 293T cell. 
         FIG. 3  is an image of confocal microscope showing integration results in 293T cell. 
         FIG. 4A  is a FACS data showing GFP expression without MJ1. (Y-axis is cell number and X-axis is GFP expression level) 
         FIG. 4B  is a FACS data showing GFP expression with MJ1. (Y-axis is cell number and X-axis is GFP expression level) 
         FIG. 4C  is a graph showing quantized numerical values of  FIG. 4A and 4B . 
         FIG. 5  is an image of confocal microscope showing excision results in 293T cell. 
         FIG. 6  is a schematic diagram showing nested deletion of attP and attB. 
         FIG. 7  is a graph showing integration efficiency due to size of attP and attB site. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the present invention will be described in greater detail. 
     In the present invention, it is shown that integrase MJ1 of φFC1 from  enterococcus faecalis  also functions in human cells as well as  E.coli . On extrachromosomal vector system, each plasmid which has attP, attB and integrase MJ1 coding sequence was constructed and cotransfected into human cell line HEK 293T ( FIG. 1 ). The site-specific integration and excision reaction in trans, in the presence or absence of MJ1, was monitored by green fluorescent protein (GFP) as a reporter. Plasmids that carry attB (pcB) gene and integrase MJ1 coding gene (pcMJ1) have cytomegalovirus (CMV) promoter. However, plasmid (pGP(−)) harboring both attP and GFP couldn&#39;t express GFP because it does not have CMV promoter, so recombinant plasmid (pREC-I) could not be created. In this way, site-specific excision reaction could be observed when attP and attB is replaced with attL and attR. 
     In addition, it was confirmed that MJ1 integrase functions in human cell without any accessory factors on an extrachromosomal vector based system, and efficiency of integrase MJ1 was the better for no excision. Also, several hundred base pair of att site could be reduced to about 50 bp. 
     In the present invention, attP means the attachment site of the bacteriophage (739 bp) and attB means the attachment site of the bacterium (290 bp). And, attL and attR mean the left region (404 bp) and the right region (624 bp) which are created by site-specific recombination between attP and attB. 
     To clone the attB, a genomic DNA of the indicator strain of bacteriophage φFC,  Enterococcus faccalis  KBL707 strain (Accession No. KFCC 12177) deposited with Korean Culture Center of Microorganisms (KCCM), was used as a template of PCR. As for attL and attR, a genomic DNA of KBL703 strain (Kim et al. Mol. Cells 4; 155-158) of which chromosome was integrated with bacteriophage φFC was used as a template of PCR. As for attP, pFE1 which is inserted with 7.7 kb of φFC1 DNA (Kim and Y. W. 1999. Genetic studies of bacteriophage FC1 from  Enterococcus faecalis . PH. D. Thesis. Korea University. Korea) was used as a template of PCR. 
     In the present invention, the base sequence of MJ1 gene was shown in SEQ ID No. 1 and the amino acid sequence of MJ1 integrase was shown in SEQ ID No. 2. Further, the base sequence of the attP gene was shown in SEQ ID No. 3 and the base sequence of the attB was shown in SEQ ID No.4. 
     Detection of minimal size at attP and attB site is valuable for taking advantage of efficiency strategy for gene therapy. Full length of 739 bp attP and 290 bp attB, already reported, are so long to utilize of useful gene therapy. Therefore nested deletion of attP and attB was performed based on MJ1 binding site at att to find the minimal size of aft site. These results demonstrate that efficient minimal size of attP and attB is 54 (160-233 of SEQ ID No. 3) and 48 bp (66-113 of SEQ ID No.4), and that the size is enough functional to integration by MJ1. 
     In the concrete examples of the present invention, the bacterium and bacteriophage were cultured, the plasmids containing MJ1 gene, attP or attB site were constructed, human fetus kidney cell line 293 was cotransfected with the plasmids, fluorescence activity of GFP reporter was measured, RT-PCR of MJ1 gene was performed in animal cell, FACS analysis was carried out, and integration of attP and attB and excision of attL and attR was confirmed. 
     EXAMPLES  
     The present invention now will be described in greater detail by means of the following examples. The following examples are for illustrative purpose and are not intended to limit the scope of the invention. 
     Example 1  
     Culture of Bacterium and Bacteriophage 
       Enterococcus faecalis  KBL703 strain and KBL707 strain (Accession No. KFCC 12177) were cultured in Todd Hewitt broth (THB: Difco Co. U.S.A) at 37° C. without shaking.  E.coli  (DH5α) was cultured in LB at 37° C. with shaking. Temperate bacteriophage φFC1 was inducted from  Enterococcus faecalis  KBL703 with UV-radiation and purified. The bacteria strains and plasmids used in this invention was listed in the below Table 1. 
     
       
         
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Strains or Plasmids 
                 Characteristics 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 
                   Enterococcus 
                 
                 KBL703 
                 Lysogenic strain of φFC1 phage 
               
               
                 
                   faccalis 
                 
                 KBL707 
                 Indicator strain of φFC1phage 
               
               
                 
                   Escherichia 
                 
                 DH5α 
                 supE44, ΔlacU169, hsdR17, recA1, 
               
               
                 
                   coli 
                 
                   
                 endA1, gyrA96, thi-1, relA1 
               
               
                 Plasmid 
                 pcDNA3 
                 mammalian cell expression vector, Ampr, 
               
               
                   
                   
                 Neor, replicative ori of Col1, promoter 
               
               
                   
                   
                 of CMV, T7, Sp6, SV40 
               
               
                   
                 pEGFP- 
                 mammalian cell expression vector, Kanr, 
               
               
                   
                 N1 
                 Neor, replicative ori of pUC, promoter of 
               
               
                   
                   
                 CMV, HSV, TK, SV40e, GEP exp 
               
               
                   
                 pG(−) 
                 pEGFP-N1 deleting PCMV 
               
               
                   
                 pGP(+) 
                 pEGFP-N1 carrying 739 bp attP fragment 
               
               
                   
                 pGP(−) 
                 pG(−) carrying 739 bp attP fragment 
               
               
                   
                 pcB 
                 pcDNA3 carrying 290 bp attB fragment 
               
               
                   
                 pGR(−) 
                 pG(−) carrying 404 bp attR fragment 
               
               
                   
                 pGR(+) 
                 pEGFP-N1 carrying 404 attR fragment 
               
               
                   
                 pcL 
                 pcDNA3 carrying 624 bp attL fragment 
               
               
                   
                 pcMJ1 
                 pcDNA3 carrying mj1 
               
               
                   
               
             
          
         
       
     
     Example 2  
     Construction of Plasmids for Transfection to Animal Cell 
     To synthesize the MJ1 intergrase gene of about 1500 bp, attB gene of 290 bp and attL gene of 347 bp, PCR reactions were performed using DNA of bacteriophage cPFC1 and genomic DNA of KBL707 and KBL703 as templates and primers shown in the below Table 2. The PCR products were subcloned between BamHI and EcoRI sites or HindIII and BamHI sites of pcDNA3 (Invitrogen Carsbad, Calif.) to construct pcMJ1 ( FIG. 1C ), pcB ( FIG. 1A ) and pcL ( FIG. 1D ). 
     Plasmid pG(−) was created by deleting its small NdeI-BgIII fragment from pEGFP-N1 (ClonTech, Palo Alto, Calif.) and blunt-end self ligation using klenow treatment. Plasmids pGP(−) ( FIG. 1B ) and pGR(−) ( FIG. 1E ) were made by subcloning the attP and attR PCR products amplified with primers shown in the below Table 2. Synthetic oligonucleotides used as primers in this invention are listed in Table 2. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Sense 
                 Antisense 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 MJ1 
                 CGGGATCCATGAAACGTGCAGCATTG 
                 CGGAATTCACCGAATGCATGTTCGTA 
                   
               
               
                   
               
               
                 attP 
                 CGGAATTCACCGAATGCATGTTCGTA 
                 GCGTTAACTGCCAATATAGC 
               
               
                   
               
               
                 attB 
                 CGGCCATTGAATTAGGGTGTCGAAT 
                 CGGATTGCCAGATGGATGAT 
               
               
                   
               
               
                 attL 
                 CGCAAGCTTGAAACGTTAAAAACTTTTAAT 
                 CGGGATCCCGGCCATTGAATTAGGGTGTC 
               
               
                   
               
               
                 attR 
                 CGCAAGCTTCGGATTGCCAGATGGATGATT 
                 CGGGATCCGAAGATCTTGTTCTCGAGCAT 
               
               
                   
               
               
                 MJ1RT 
                 GGAAATCAATTAGGGCTTTT 
                 TCGTTATGCTTCTTGGAAAT 
               
               
                   
               
               
                 GAPDH 
                 CATCTCTGCCCCCTCTGCTG 
                 CGACGCCTGCTTCACCACCT 
               
               
                   
               
             
          
         
       
     
     The sequences disclosed above in Table 2 are identified as follows: 
     MJ1, Sense, is SEQ ID NO:5; attP, Sense, is SEQ ID NO:7; attB, Sense, is SEQ ID NO:9; attL, Sense, is SEQ ID NO:11; attR, Sense, SEQ ID NO:13; MJ1RT, Sense, SEQ ID NO:15; GAPDH, Sense, SEQ ID NO:17; MJ1, Antisense, is SEQ ID NO:6; attP, Antisense, is SEQ ID NO:8; attB, Antisense, is SEQ ID NO:10; attL, Antisense, is SEQ ID NO:12; attR, Antisense, SEQ ID NO:14; MJ1RT, Antisense, SEQ ID NO:16; and GAPDH, Antiense, SEQ ID NO:18. 
     Example 3  
     Transfection and Fluorescence Microscopy 
     Human embryonic kidney 293T cells (American Type Culture Collection, Manassas, Va.) were grown in Dulbecco&#39;s modified Eagle&#39;s medium (DMEM) supplemented with 10% heat-inactivated fetal bovine serum (FBS) and 100 U/ml penicillin in 100-mm plates. Cells were split into eight six-well culture plates or 60-mm plates and grown, until they were about 50% confluent. Plasmids prepared in Example 2 that carry attP, attB and MJ1 coding sequence were transfected in equimolar amounts (6 μg or 3 μg of total DNA) by using the calcium phosphate method. 
     The transfected cells were grown on cover slip placed in a six- well culture plates. After 72 h incubation the cells were examined under the confocal fluorescence microscope (Zeiss) for the expression of GFP. The efficiency of transformation was determined by the percentage of fluorescing cells in the transfected cells. 
     Example 4  
     Reverse Transcription-PCR (RT-PCR) 
     Total RNA was extracted from pcMJ1 transfected 293T cell using TRIzol reagent (Invitrogen). Reverse transcription and PCR amplification were performed using SUPERSCRIPT II RNaseH-reverse transcriptase (Invitrogen) and pfu DNA polymerase (Promega). The level of expression of GAPDH gene served as internal control and the sequences of the primers were described in Table 2. PCR was performed for 25 cycles at 94° C. for 5 min, 94° C. for 20 s, 52° C. for 30 s, and 68° C. for 40 s. The PCR products were analyzed on a 1.5% agarose gel and stained with ethidium bromide for visualization. As shown in  FIG. 2 , expression level of MJ1 mRNA was elevated considerably as compared to that of the internal control gene GAPDH. The PCR generated a 320 bp product with specific primer pair for the MJ1 coding region ( FIG. 2 ). RT-PCR showed that integrase MJ1 is ectopically expressed in human cell. 
     Example 5  
     Fluorescent-Activated Cell Sorting (FACS) Analysis 
     FACS was performed for the expression of GFP. 72 hr post-transfection, cells were harvested, resuspended with 3 ml of 5 mM EDTA/PBS and washed twice, and then fixed in 300 μl of 0.1% BSA and 0.05% NaN3/PBS. FACS was performed on a FACScan analysis (Becton Dickinson Immunocytometry Systems) using the CellQuest program (Becton Dickinson). 
     Below, the experimental result which verifies the result of the above-mentioned examples of this invention is disclosed. 
     Experimental Example 1  
     Site-specific Integration or Excision 
     Different constructs could monitor either the integration reaction (attP×attB) or the excision (attL×attR). To determine effect of MJ1 on the reaction of integration, 293T cells were cotransfected with plasmids pcB and pGP(−), together with pcMJ1 or without pcMJ1. After 72 hr, cells were observed by confocal microscope. In the case of plasmid pREC-1 ( FIG. 1F ) created by MJ1-mediated integration, expression of GFP only occurred ( FIG. 3A ). In contrast, GFP expression was not observed in the cell cotransfected without MJ1 expectably ( FIG. 3B ). To show that each cases was not owing to promoter, cells that were transfected with plasmid pGP(+) or pGP(−) with or without CMV promoter, respectively, were also observed by confocal microscope ( FIGS. 5C and 5D ). As expected, GFP expression was not investigated in cell transfected with only pGP(−). To search integration efficiency, the 293T cells were transfected with plasmids pcB and pGP(−) together with or without MJ1 expressing plasmid pcMJ1 and 72 hr after transfection the cells underwent a FACS analysis. Histogram plots that depict the fluorescence of transfected cells in the absence or presence of MJ1 are shown in  FIGS. 4A and 4B . Cells transfected with plasmids that carry att sites along, in the absence of MJ1, showed a fluorescence of 396 cells ( FIG. 4A ). In contrast, in the presence of MJ1 the cell number of fluorescence was 1237 in the integration reaction, so their gap came up to about 4 times as shown  FIG. 4C . 
     In the case of excision, plasmids that carry attR (pGR(−)) and attL (pGL) instead of pGP(−) and pcB were cotransfected into 293T cell. As a result, GFP expression was not observed in contrast to integration ( FIG. 5A ). To prove that the result was not caused by promoter, cells transfected with plasmid pGR(+) or pGR(−), with or without CMV promoter, respectively, were observed by confocal microscope. pGR(+), but not pGR(−) showed GFP activity ( FIGS. 5B and 5C ). This result means that putative recombinant plasmid pREC-E ( FIG. 1G ) was not created by MJ1. It is shown that MJ1-catalyzed excision does not occur between attL and attR. 
     It is demonstrated that MJ1 mediates site-specific integration between attP and attB efficiently in human cell on an extrachromosomal vector base, but MJ1-catalyzed excision does not occur between attR and attL. 
     Experimental Example 2  
     Determination of Minimal Size of attB and attP 
     Before this invention, no information about minimal size of attB and attP sites on which the MJ1 functions had been reported. To minimal functional sizes for these att sites, site-specific integration assay system described above was performed using nested set of deletion clones of attB and attP ( FIG. 6 ). Each plasmid that carries deleted site bearing 3 bp conserved core sequences was made by nested deletion centering around the site known for MJ1 binding. Short double-stranded adaptor molecules containing att sites of various lengths were created by annealing single-stranded oligonucleotides. These deleted sites were used to replace the full-length att sites in the pGP(−) and pcB plasmids, the plasmids were cotransfected with or without pcMJ1 and integration efficiencies were determined by FACS analysis. 
     As a result, integration efficiency owing to size of attP and attB site was indicated as  FIG. 7 . The discrepancy of cell that transfected with or without plasmid pcMJ1 was graphed to % numerical value. The integration efficiency was not been decreased in proportion to reduction attP and attB site. However, the efficiency of integration was gradually decreased from integration efficiency between 54 bp attP and 48 bp attB. In the final analysis, almost integration functions were lacked between 50 bp attP and 44 bp attB. These results demonstrate that efficient minimal size of attP and attB is 54 (160-233 of SEQ ID No. 3) and 48 bp (66-113 of SEQ ID No. 4), and that the size is enough functional to integration by MJ1. 
     Integrase MJ1 has lots of advantage to construct gene transfer system than any other viral vector or existing gene transfer system. 
     First, MJ1 does not need any requirements for  E.faecalis -specific cofactor for site-specific integration. In contrast, λ integrase requires IHF for both type of recombination and IHF is required in many types of recombination, including that of phage HK022 and HP1 (Dorgai et al., (1998) J. Mol. Biol., 277, 1059-1070). This invention demonstrates that site-specific integration in human cell occurred between attP and attB with only integrase MJ1 without any cofactor as observing GFP expression. 
     Second, unlike other integrases (recombinases) from phage which mediate both integration and excision, integrase MJ1 was shown to only have integration function. It was reported that integrase (recombinase) from phage HK022 could perform both integrative (attP/attB) and excisive (attL/attR) recombination in mouse NIH3T3 cells (Kolot et al., (1999) Mol. Biol. Rep., 26, 207-213). In this invention, MJ1 only mediates the integration function, and that integrated gene was not excised from host genome, so desired gene could be expressed continuously. 
     Third, it was shown that 48-bp and 54-bp minimal sizes for the attB and attP sites enabled MJ1 to mediate the integration. It appears that this integration may be done at naturally occurring pseudo-att sites in eukaryotic genomes having significant similarity to native att sequences (Thyagarajan et al., (2000) Gene, 244). The expected rarity of good matches with att sites may limit recombination to a small number of chromosomal pseudo-att sites, which may produce usable integration frequencies at endogenous locations in the chromosomes. 
     Besides these advantages, MJ1-mediated site-specific integration system is profitable for development of gene transfer system. The probability that has an effect on expression of essential genes or/and on interference of host cell mechanism was decreased certainly because this system get inserted gene to integrate into pseudo-site on host genomic DNA. Also integrase from phage could transfer more than ten kilo-base pair of insertion gene into target site, allowing for the several tens of kb-sized of phage congestion, because phages can transfer integrated their whole phage genome into bacterial genome using its integrase by nature.