一種利用Cre_loxP系統(tǒng)進行標記基因刪除與靶基因置換的細

1、A Model Study:Simulation of Marker Gene Deletion and Target Gene Replacement Mediated by Cre/lox System in Cell Line *CUI Wen-Tao 1,2,REN Li-Ming 1,HOU Jian 1,ZHANG Ying 1,CHEN Yong-Fu 1,AN Xiao-Rong 1*(1State Key Laboratory for Agrobiotechnology,China Agricultural University,Beijing 100094,China;2I

2、nstitute of Animal Sciences,Chinese Academy of Agricultural Sciences,Beijing 100094,ChinaAbstract The incorporation of site-specific recombination systems can help to overcome bottlenecks in livestock transgenic technology.For evaluating the efficiency of Cre/lox mediated DNA recombination in embryo

3、s and somatic cells,a working model was established using rat mammary carcinoma cell line SHZ-88,aimed at creation of and use repeatedly of selected “friendly loci ”in transgenic livestock.An integration vector pTE-lox 2272-DsRed-lox P-GFP-lox P,which red fluorescence gene DsRed served as the first

4、target gene and green fluorescence gene GFP as marker gene,was constructed for introduction of acceptor loci in genome.At the same time a replacement vector pT-lox 2272-neo-lox P in which Neo coding sequence served as the second target gene was also constructed for replacing DsRed gene.Transgenic ce

5、ll clones were produced by electroporating SHZ-88cell with the integration vector.Cells from three transgenic clones selected randomly were further amplified and were then co-electroporated with the replacement vector as well as cre gene.Analysis of the expression patterns of DsRed and GFP indicated

6、 that among the 1070cell colonies the efficiency on marker GFP deletion was 91.1%and the efficiency on gene replacement was 29.3%.Molecular analysis by PCR and Southern blotting confirmed that the color patterns as expressed by cell colonies could represent the actual molecular events.This working m

7、odel mediated by Cre/lox system should be useful for the improvement of the present animal transgenic technology.Key words lox 2272,lox P,Cre,marker gene deletion,target gene replacement*This work was supported by a grant from Hi-Tech Research and Development Program of China(2002AA206311and 2006AA0

8、2Z148.*Corresponding author.Tel:86-10-62733463,E-mail:xra Received:January 29,2008Accepted:March 10,2008生物化學與生物物理進展Progress in Biochemistry and Biophysics 2008,35(6: 650660研究報告Research PapersAnimal transgenic technology holds considerable promise for livestock industries as well as has important use

9、s for biomedicine industries 1,2.However,a quarter of a century has passed since the first successes in the production of “supper mouse ”and of mice expressing a human gene in their mammary tissue 3,4,the applications of transgenic animal technology have not yet had the impact on industry as predict

10、ed by many people at that time.The main obstacles to prevent the progress of this important technology have been the low efficiency in production of transgenic livestock and of the insertion of foreign genes at a random fashion in genome.To circumvent the difficulties in mass production of transgeni

11、c livestock,methods of high efficiency and low cost were tried by many investigators 59.However,all these methods have their inherent limitations and have not yet been ready for use in commercial livestock production.Almost at the beginning,some authority authors had foreseen the problems in the app

12、lication of transgenic technology to livestock and had envisaged that one possible way to produce transgenic livestock efficiently might be to screen and use repeatedly so called “friendly gene loci ”10.However,mainly for quick returns nobody had ever tried seriously this strategy.More recently,seve

13、ral investigators have re-emphasized on the importance of creating genomic acceptor sites for foreign genes by insertion of pairs of hetero-specific lox sites to appropriate gene loci 11.Cre/lox site-specific recombination system is a high崔文濤等:一種利用Cre/loxP系統(tǒng)進行標記基因刪除與靶基因置換的細胞模型研究2008;35(6efficient to

14、ol for gene insertion/excision12.With the mediation of Cre recombinase any intervening sequence between two lox sites in the same orientation can be excised13.In recent years,several hetero-specific lox sites have been identified and tested14,15, such as lox511,lox5171and lox2272.By flanking a pairs

15、 of hetero-specific lox sites to a foreign gene,it can be replaced by another gene flanked by the same pairs of lox sites after its insertion into the mammalian genome,namely“Recombinase Mediated Cassette Exchange”(RMCE16.This system has been successfully used to modify precisely mice stem cell1719a

16、nd several types of somatic cells20.With the advent in site-specific recombination it is now quite possible that by combining use of the available technologies,for example,in vitro fertilization,somatic cloning with Cre/lox system,“friendly loci”or foreign gene acceptor sites could be identified and

17、 used repeatedly in commercial practice. Aimed at finding“friendly loci”to be used in animal mammary bioreactor,we have designed and constructed a set of vectors which contain two expression cassettes:one for selection of positive transgenic embryos and the other for exchange target gene by RMCE aft

18、er the first target gene has expressed efficiently at a“friendly locus”.In this paper we report an examination of its function in deletion of marker gene and in exchange of target genes using a simulated method.1Materials and methods1.1Construction of integration vectorThe integration vector pTE-lox

19、2272-DsRed-lox P-GFP-lox P,containing both of the red and green fluorescence protein coding sequences,flanked by a pairs of lox2272/lox P and a pairs of lox P/lox P respectively,was constructed by removing required elements from intermediate vectors of pT-lox P-GFP-lox P(kept in the authors!laborato

20、ry,pDsRed1-1 (Clontech,USAand pEGFP-N1(Clontechand then cloned into vector pTE-lox2272-BIB(kept in the authors!laboratoryin sequential.1.2Construction of replacement vectorThe replacement vector pT-lox2272-neo-lox P was constructed in two steps:firstly to create the intermediate plasmid pTE-lox2272-

21、neo-lox P by recombining of the plasmid pTE-lox2272-BIB-lox P-GFP-lox P(constructed in this experimentwith plasmid p-TN containing Neo coding sequence(kept in the authors!laboratory,secondly restriction sites Eco47and Mluwere deleted by PCR amplification and re-cloning into plasmid pMD18-T (Takara,C

22、hina.1.3Cell culture and electroporationSHZ-88cells(Cell Bank,The Chinese Academy of Sciences,Shanghaiwere cultured at37in5% CO2in RPMI1640(Gibco,USAmedium plus10% FCS(Hyclone,USA,100U/ml penicillin(Gibcoand 100mg/L streptomycin(Gibco.For producing of cell colonies carrying integration vector,SHZ-88

23、cells(1×107/0.8ml in PBS were electroporated at200V and960#F with50#g of ApaLI-digested pTE-lox2272-DsRed-lox P-GFP-lox P.At48h after culture,the cell colonies showing both red and green color under a fluorescence microscope(Nikon,Japanwere picked and expanded for the subsequent experiments.For

24、 assay of the efficiencies of Cre mediated DNA sequence deletion and replacement,the above picked cell colonies,which always showed both red and green color even after56passages,were co-electroporated at200V and960#F with20#g of the replacement vector pT-lox2272-neo-lox P and 10#g of Cre expression

25、vector pBS185(Gibcoin their circular forms.At36h after electroporation,the cells were selected with400g/L G418(Gibcofor7 days,and then observed and scored under the fluorescence microscope.Through comparing the proportions of different colored cell clones,the efficiencies of Cre mediated DNA sequenc

26、e deletion and replacement were calculated.1.4Assay by PCR and Southern blotsPrimer G1(Forward5TAGCGCTACTAGTCT-CGG3and G2(Reverse5CTCTCCCATATGGTC-GAC3were used in PCR reactions for assay of the insertion of the integration vector in the genomes of the green and red transgenic clones,for assay of the

27、 red clones resulted from marker deletion and for assay of the colorless clones resulted from gene replacement plus deletion.The conditions of reaction with primer G1and G2were:9440s,5830s,724min,30 cycles.Primer P1(Forward5GAGTGATGAGGTTC-GCAAGA3and P2(Reverse5TAGCGCCGTAA-ATCAATCGA3were used in PCR

28、assay for the integration of Cre coding sequence in the genomes of the red clones resulted from marker deletion and the colorless clones resulted from gene replacement plus deletion.The conditions of reaction with primer P1651生物化學與生物物理進展Prog.Biochem.Biophys.2008;35(6and P2were:9440s,6030s,721min, 30

29、cycles.For Southern blots,Primer R1(Forward5TGC-GCTCCTCCAAGAACGTC3and R2(Reverse5AGCTTCAGGGCCTTGTGGAT3were used for synthesis of probes specific to pTE-lox2272-DsRed-lox P-GFP-lox P and conditions of reaction were:9440s,5630s,7240s,30cycles;Primer N1 (Forward5GGATCGGCCATTGAACAAGA3and N2(Reverse5CAGA

30、AGAACTCGTCAAGAAG3 were also used for the synthesis of probe specific to pT-lox2272-neo-lox P and conditions of reaction were: 9440s,5430s,7240s,30cycles.Six micrograms of genomic DNA was digested with appropriate restriction enzymes,electrophoresed on a 0.9%agarose gel(Sigma,USAand blotted onto a ny

31、lon membrane(Amersham,UK.Southern blots were performed using a DIG DNA Labeling and (RNA blotDetection Kit(Roche,Ger.1.5Data analysisStudent"t-test was used to analyze data and P< 0.05was considered significant statistically.2Results2.1Experiment designFor the easiness of observation of mark

32、er gene deletion and target gene replacement,vector pTE-lox2272-DsRed-lox P-GFP-lox P was constructed and used as a working model to create imitated “friendly loci”in the genome of cells,instead of using the more complex original mammary tissue specific expression vector without a visualized phenoty

33、pe.At the same time vector pT-lox2272-neo-lox P was also constructed for use in RMCE to replace the red fluorescence gene DsRed with neo gene.To introduce Cre recombinase into cells,a commercial expression vector PBS185was used in the electroporation experiments together with plasmid pT-lox2272-neo-

34、lox P.In theory,the outcomes of these experiments should be as follows:upon the integration of pTE-lox2272-DsRed-lox P-GFP-lox P transgenic cells showing both of red and green color would be produced along with colorless none-transgenic cells. The transgenic single cells could then be amplified in c

35、ulture to single clones.However,upon the introduction of pT-lox2272-neo-lox P and pBS185, under the mediation of Cre enzyme,the green fluorescence gene GFP should be deleted by the recombination of the two wild type lox P sites leaving one wild type lox P site in the location.At the same time,the re

36、d fluorescence gene sequence flanked by a lox2272site and a wild type lox P site should be replaced by neo gene flanked by the same combination of hetero-specific lox sites in the replacement vector. Since marker gene GFP was flanked by a pair of wild type lox P sites in the process of gene replacem

37、ent,the extreme3"end lox P site could well be used to react with the lox P site in the replacement vector instead of the upstream lox P site.As a result,GFP sequence would be replaced together with DsRed sequence. These predicted results are illustrated schematically in Figure1.2.2Production of

38、 transgenic cellsFor assay of the efficiencies of Cre mediated DNA sequence deletion and replacement,acceptor sites were created by electroporating SHZ-88cells with linear pTE-lox2272-DsRed-lox P-GFP-lox P DNA. As showed in Figure1,this construct contains two fluorescence genes:a red fluorescence pr

39、otein gene flanked by a lox2272site and a wild type lox P site,and a green fluorescence gene flanked by two wild type lox P sites.Theoretically,transgenic cell clones should show both of red and green fluorescence color under the fluorescence microscope while none-transgenic cells should be colorles

40、s.In our experiment about50% cells showed both red and green fluorescence color under the fluorescence microscope after electroporation and culture for recovery.This could largely attribute to the transient expression of DsRed and GFP genes because both of them preceded by the strong CMV promoter.Ho

41、wever,following growth in culture most cells lost fluorescence color but a few fluorescent cell clones formed.When these cell clones were examined under the fluorescence microscope,all cells in a single clone maintained uniformly their newly acquired properties i.e.showing both of red and green fluo

42、rescent color.These clones represented the original transgenic single cells that had the inputting DNA construct copy or copies integrated into their genomes. These clones were then used for the experiments of marker gene deletion and target gene replacement.One of the cell clones is shown in Figure

43、2.These results confirmed that the predicted acceptor loci had been created in groups of cells.652崔文濤等:一種利用Cre/loxP 系統(tǒng)進行標記基因刪除與靶基因置換的細胞模型研究2008;35(6Fig.2A transgenic cell clone under the fluorescence microscope (10×10The same cell clone is observed under a fluorescence microscope at bright fiel

44、d (a,fluorescence fields with thewavelength of 488nm for GFP detection (band of 558nm for DsRed detection(c.(a(b(cFig.1Schematic representation of marker gene deletion and target gene replacement as explained in text653生物化學與生物物理進展Prog.Biochem.Biophys.2008;35(6Fig.3The representative cell clones obse

45、rved under the fluorescence microscope(10×20after co-electroporationType1:A clone showing colorless resulted from GFP gene deletion and DsRed gene replacement;Type2:A clone showing red color resulted from greenGFP deletion;Type3:A clone showing both of red and green colors came from none Cre me

46、diated reaction.Type1Type2Type3Bright fieldGFP detetion DsRed detetionThe efficiencies of the two types of Cre/lox mediated reactions were estimated by comparing the proportions of different colored cell clones.The efficiency of marker gene deletion was calculated as the percentage of colorless clon

47、es (Type1plus red color clones (Type2in total clones while the2.3Marker gene deletion and target gene replacement in cellsAs described in the previous section of experiment design,red fluorescent protein gene was used as imitated target gene that could be replaced by another target gene (in this exp

48、eriment is neo genewhile green fluorescent protein gene was designed to serve as marker that should be deleted after selection of transgenic embryos (in the present experiment is transgenic cells.For examining the efficiencies of Cre/lox system in these two roles,three transgenic single clones,which

49、 produced both of red and green fluorescence color,were empirically picked up and designated as clone A1,clone A2and clone A3.The above three clones were amplified by culture to enough cell numbers,three cell samples were taken from each of the three clones and then all samples were co-electroporate

50、d by the replacement vector pT-lox 2272-neo-lox P and Cre expression vector pBS185.All together nine co-electroporationexperiments were carried out and for each of the three clones three repeats were set up on independent days.Electroporated cells grew very well under the selection of G418and a lot

51、of cells clones were observed.Observing under the fluorescence microscope,three types of cell clones were found:colorless clones (Type1,red clones (Type2,double colored clones (Type3.The representative images for three types of cell clones were shown in Figure 3.These results signified that two type

52、s of Cre mediated reactions had occurred accurately,leading to marker gene deleted and target gene replaced in part of the cells.This is in good agreement with predicted results in experiment designs,according to which two wild type lox P sites would recombine by themselves and delete the intervenin

53、g GFP whereas the neo gene flanked by a lox 2272site and a wild type lox P site would replace the intervening sequence between the lox 2272site and one of the lox P sites.6541Cells of three clones (A1,A2and A3were co-electroporated with the replacement vector pT-lox 2272-neo-lox P and the Cre expres

54、sion vector pBS185in three repeats for each of the clones in similar conditions.Type1:a clone showing colorless at both wave lengths 488nm and 558nm resulted from GFP gene deletion and DsRed1gene replacement;Type2:a clone showing red color resulted from green GFP deletion;Type3:a clone showing both

55、of red and green colors came from none Cre mediated reaction.Deletion/%=Number of Type1and Type2Number of Total ×100;Replace/%=Number of Type1Number of Total×100Table 1Efficiencies of Cre/lox mediated sequence deletion and replacement 1Cell clone Batch of electroporationType 1Type 2Type 3T

56、otal Deletion %Replace %A11st 34731312089.228.32nd 4182813193.931.33th35771012291.828.7A21st 2359149685.424.02nd 30711111289.326.83th2165129887.821.4A31st 4177912792.932.32nd 4683513496.234.33th4874813093.836.9efficiency of target gene replacement was calculated asthe percentage of colorless clones

57、(Type1in the total.These results are presented in the following Table 1.Data analysis showed that among the total observed 1070cell clones,colorless clones accounted for 29.3%indicating about one third DsRed gene were replaced by neo gene.However,red color clones plus colorless clones were the major

58、ity,accounting for 91.1%of the total clones signifying that deletion of the GFP gene was more efficient than that of gene replacement.Of course these could be attributed to an added effects of two site specific recombination reactions:one between the two wild type lox P sites and the other happened

59、in the process of gene replacement mediated by two lox 2272sites in genome and in replacement vector respectively as well as by the lox P site in the replacement vector and the far 3"end wild type lox P site of the integration vector in genome.However,there were still about 10%clones remaining red &green colors signifying Cre/lox mediated reaction was not performed.The above efficiencies were compa