Proc. Natl. Acad. Sci. USAVol. 93, pp. 5860-5865, June 1996Genetics

Efficient in vivo manipulation of mouse genomic sequences at thezygote stage

(transgenic/Cre recombinase/loxP)

MERJA LAKSO*t$, JOSE G. PICHEL*t, JAMES R. GORMANt§, BRIAN SAUER1, YO OKAMOTO*II, ERIC LEE*,FREDERICK W. ALT§, AND HEINER WESTPHAL*'***Laboratory of Mammalian Genes and Development, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD20892-2790; ILaboratory of Biochemistry and Metabolism, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health,Bethesda, MD 20892-1800; and }Howard Hughes Medical Institute and Department of Pediatrics, The Children's Hospital, Harvard UniversityMedical School, 300 Longwood Avenue, Boston, MA 02115

Contributed by Frederick W. Alt, February 8, 1996

ABSTRACT We describe a transgenic mouse line carry-ing the cre transgene under the control of the adenovirus Ellapromoter that targets expression of the Cre recombinase tothe early mouse embryo. To assess the ability of this recom-binase to excise loxP-flanked DNA sequences at early stages ofof development, we bred EIIa-cre transgenic mice to twodifferent mouse lines carrying laxP-flanked target sequences:(i) a strain with a single gene-targeted neomycin resistancegene flanked by loxP sites and (ii) a transgenic line carryingmultiple transgene copies with internal loxP sites. Matingeither of these loxP-carrying mouse lines to EIIa-cre miceresulted in first generation progeny in which the loxP-flankedsequences had been efficiently deleted from all tissues tested,including the germ cells. Interbreeding of these first genera-tion progeny resulted in efficient germ-line transmission ofthe deletion to subsequent generations. These results demon-strate a method by which oxP-flanked DNA sequences can beefficiently deleted in the early mouse embryo. Potential ap-plications of this approach are discussed, including reductionof multicopy transgene loci to produce single-copy transgeniclines and introduction ofa variety of subtle mutations into thegerm line.

The Cre/loxP system is rapidly becoming a major tool formodification of gene expression in mice (1-7). The approachuses the ability of the Cre recombinase of bacteriophage P1 tocatalyze conservative reciprocal recombination events in mam-malian cells (8). Genes whose function is to be altered carryinserts of 34 bp loxP sequences, the target sites of Cre-mediated recombination in bacteriophage P1. When Cre en-zyme is provided, the sequences between directly repeatedloxP sites are efficiently excised. In its convenient binary form,the experiment uses two strains of mice. One strain carries atransgene that is designed to express Cre enzyme in a tissue ofinterest. The other strain carries loxP sites at a transgeneinsertion or endogenous gene locus. The two strains arecrossed to produce progeny that express Cre in the tissue ofinterest and that have excised chromosomal sequences be-tween the loxP sites. Cre brought in by mating displays aremarkable efficiency of recombination in the developinganimal, as tested in the context of Cre-mediated activation ofa dormant transgene (1, 3) or Cre-mediated inactivation of aresident chromosomal gene (5, 7).The present study addresses the efficiency of Cre-mediated

DNA recombination in the specific context of the zygote stageof embryo development. We have generated transgenic micein which the adenovirus EIIa promoter directs Cre expressionselectively to preimplantation embryos (9). The ability of the

The publication costs of this article were defrayed in part by page chargepayment. This article must therefore be hereby marked "advertisement" inaccordance with 18 U.S.C. §1734 solely to indicate this fact.

EIIa-regulated recombinase activity to excise DNA sequencesin vivo was assessed in two experiments. In the first experiment,we bred EIIa-cre mice to gene-targeted mice homozygous forthe insertion of a loxP-neo-loxP cassette, a constitutively ex-pressed marker commonly used in gene targeting (10, 11). Thesecond experiment was aimed at varying transgene copynumbers at a given insertion site. EIIa-cre mice were matedwith transgenic mice carrying multiple copies of maA-loxP-TAg, a lens-specific cassette in which the loxP site was placedbetween a maA-crystallin promoter sequence and simian virus40 (SV40) sequences encoding the tumor antigens (TAg).Using the Cre/loxP system, we wished to determine whetherchanges in TAg transgene copy number at a fixed genomicintegration site would affect the timing of TAg accumulationand the subsequent patterns of oncogenesis in the differenti-ating lens.Our results show that sequences between loxP sites can be

readily excised from developing mice exposed to Cre activityat an early stage of embryonic development. The resultinggene alterations are genetically fixed and therefore passed onto progeny. In the examples shown, this strategy allowedefficient excision of DNA sequences at a targeted chromo-somal locus, and reduction of the transgene copy number at apreset transgene insertion site.

MATERIALS AND METHODSGeneration of Ela-cre Transgenic Mice. The EIIa-cre plas-

mid was constructed by replacing the human cytomegaloviruspromoter in pBS185 (12) with the adenovirus EIIa promoterfrom pEII-lacZ (9). The construct was microinjected intomouse zygotes for production of transgenic mice (13). Creactivity present in zygotes of the EIIa-cre genotype was mea-sured as follows. EIIa-cre/EIIa-cre homozygous animals weremated with +/maA-loxP-STOP-loxP-TAg heterozygous ani-mals (1, 3) and the presence of lens tumors in double transgenicprogeny, indicative of Cre-mediated excision ofSTOP sequencesand a concomitant activation of SV40 TAg, was scored.

Breeding ofEla-cre Mice with loxP-neo-oxP Gene-TargetedMice. The gene-targeted mice carry a specific gene-targetedmutation that replaces a region 8.5 Kb 3' of the immunoglob-ulin light chain kappa constant region (J.R.G., unpublished

Abbreviations: SV40, simian virus 40; TAg, tumor antigen; E, em-bryonic day; ES cells, embryonic stem cells; maA, murine aA-crystallin gene promoter.tM.L., J.G.P., and J.R.G. contributed equally to this work.tPresent address: Transgenic Unit, Biocenter, University of Helsinki,P.O. Box 56, 00014 Helsinki, Finland.lPresent address: Department of Anatomy and Developmental Biol-ogy, Faculty of Medicine, Kyoto University, Japan.**To whom reprint requests should be addressed at: National Insti-

tutes of Health, Building 6B, Room 413, 6 Center Drive, Bethesda,MD 20892-2790.

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work) with a single neomycin phosphotransferase gene,flanked by loxP sites (loxP-neo-loxP). Female mice homozy-gous for the loxP-neo-loxP locus were bred with male micehomozygous for the EIIa-cre transgene. Deletion of the neogene in F1 offspring was assayed by polymerase chain reaction(PCR) of tail DNA. The following primers were used in theassays of Fig. 1: a, 5'-ggccgctaattccgatcatattc-3'; b, 5'-ccttctt-gacgagttcttctgagg-3'; c, 5'-actaacatttgacaggtggatgc-3'. PCR re-actions were carried out as described (14) and included 200 ngof tail DNA and 2.5 units of AmpliTaq Polymerase (Perkin-Elmer). PCR conditions: 94°C, 2 min, followed by 45 cycles of94°C, 30 sec; 50°C, 45 sec; and 65°C, 90 sec. Products wereseparated by electrophoresis on a 1.5% agarose gel, and werevisualized by staining with ethidium bromide. F1 mice withcomplete deletion of the neo gene in the tail were interbred toobtain F2 mice that were subsequently assayed by PCR of tailDNA for deletion of the neo gene and for the presence orabsence of the cre transgene.

Breeding of Ela-cre Mice with Multicopy (maA-4oP-TAg),Transgenic Mice. The maA-loxP-TAg cassette was derived bydeleting STOP from the previously described construct maA-loxP-STOP-loxP-TAg (1). Multicopy (maA-loxP-TAg)n trans-genic mice were generated by zygote injection of the maA-loxP-TAg cassette. Multicopy (maA-loxP-TAg)n heterozygousmales were mated with homozygous EIIa-cre females to gen-erate single-copy maA-loxP-TAg progeny. Pregnancies weretimed by administration of gonadotrophins to females beforecopulation and embryonic eyes were collected at differentdevelopmental stages for morphological and immunohisto-chemical analyses. The embryonic torsos or extraembryonicmembranes were used as a source of DNA for PCR andSouthern blot analysis. PCR reactions were performed asdescribed (1). For Southern blot analysis, genomicDNA (7 ,ig)was extracted from embryonic torsos or extraembryonic mem-branes or adult mouse tails and digested with EcoRV, KpnI,BglII (no restriction sites within the maA-loxP-TAg sequence),and BamHI (one single restriction site in maA-loxP-TAg andEIIa-cre) (see Fig. 2). The digested DNA was separated in a0.75% agarose gel, transferred to Gene Screen filters (Du-Pont), and hybridized as described (15). A 1169-bp HindIIIfragment from SV40 TAg and a 400-bp fragment of the 5'portion of cre were used as hybridization probes for analysis ofTAg and cre transgene sequences, respectively. Histologicaland immunohistochemical analyses of embryonic mouse eyesections was performed as described (3).

RESULTSCre Encoded by the Ela-cre Transgene Efficiently Deletes

a loxP-neo-loxP Cassette in Vivo. The EIIa-cre transgene wasdesigned in an effort to direct Cre recombinase activity to theone-cell mouse embryo. The EIIa promoter confines the Creaction to a very early stage of preimplantation embryogenesis,most likely to one-cell embryos (9). Cre-mediated recombi-nation of the genome, if indeed taking place in the zygote,should generate a gene alteration that would be present in allcells of the developing animal, including its germ cells, and thatwould be transmitted to progeny. To test whether the productof the EIIa-cre transgene would efficiently delete a singleloxP-X-loxP cassette in vivo, we crossed homozygous EIIa-cremice with homozygous mice carrying a targeted insertion of asingle copy loxP-neo-loxP cassette in the immunoglobulin lightchain kappa locus (J.R.G., unpublished work). Homozygousfemale loxP-neo-loxP mice were selected based on screening oftail DNA by Southern blot analysis (data not shown) and werebred with homozygous male IIa-cre transgenic mice.The F1 offspring, all of which were heterozygous for the

chromosome carrying the EIIa-cre transgene array and also forthe loxP-neo-loxP targeted chromosome, were assayed by PCRand Southern blot analyses of tail DNA for the presence or

absence of the neo gene on the targeted chromosome. ThePCR assay was designed to detect both the loxP-neo-loxPtargeted chromosome (called the NEO allele) and the loxPconfiguration that remains at the targeted locus after Creenzyme has excised the neo gene (the ANEO allele). Repre-sentative results are shown in Fig. 1. In total, 56 F1 mice werescreened. These mice contained a single target for Cre actionin each cell. In each of these mice, the ANEO allele wasdetectable in tail DNA, indicating that all individuals hadundergone Cre-mediated excision of the neo sequence in atleast a portion of cells present in the tail tissue examined. Ofthe 56 F1 tail DNA samples, 28 (50%) showed both the NEOand ANEO PCR products. This finding indicates that a pro-portion of tail cells in this half of the cohort failed to undergoCre-mediated neo excision. Persistence of the undeleted NEOallele in a proportion of cells may reflect Cre action past thezygote stage, resulting in some mosaicism. The remaining 28out of 56 F1 mice (50%) had undergone complete excision ofthe neo sequences as suggested by the fact that we detectedonly the ANEO PCR product. To test whether absence of adetectable loxP-neo-loxP cassette in tail DNA reflected com-plete excision in the germ cells, F1 mice that displayed com-plete neo excision in the tail were selected for breeding toobtain F2 progeny.The neo Excision Is Fixed in the Germ Line and Transmitted

to F2 Mice. Male and female F1 offspring judged by tail DNAPCR as ANEO, having lost the neo gene, were interbred toassay for germ-line transmission of the deletion. To unambig-uously genotype F2 mice as +/+, +/ANEO, or ANEO/ANEO, we assayed for the presence or absence of the untar-geted wild-type (+) allele both by PCR (data not shown) andby Southern blot analyses. In all, 58 F2 individuals derived fromcrosses of four F1 males and nine F1 females were analyzed.Mice of the +/+, +/ANEO, and ANEO/ANEO genotypeswere obtained in roughly Mendelian ratios (11:28:19, or19%:48%:33%). None contained any detectable NEO PCRproduct nor were neo sequences detectable by Southern blotanalysis. PCR was judged to be a more sensitive assay forexcision or retention of the neo sequence because we hadobserved occasional F1 mice that appeared completely deletedfor neo by Southern blot analysis but were still neo positive byPCR, indicating low levels of residual undeleted neo se-quences. To assess whether neo excision observed in F2 tailtissue reflected Cre action in the F1 or in the F2 zygote, F2 micewere assayed by PCR for the presence or absence of theEIIa-cre transgene (Fig. 1). The EIIa-cre transgene was foundto have segregated in roughly Mendelian fashion, with 42 of 58individuals harboring and 16 of 58 lacking this transgene. The16 F2 mice that lacked the EIIa-cre transgene encompassedboth +/ANEO and ANEO/ANEO individuals. These 16 micecarried no detectable neo sequences and must therefore haveinherited the ANEO allele from one or both F1 parents. Thesein turn must have undergone Cre-mediated neo excision duringor before germ cell formation. Our data indicate that F1 micethat showed complete neo deletion in the tail had indeedundergone neo deletion in their germ cells as well.Cre Activity from EIIa-cre Efficiently Reduces Transgene

Copy Number at a Multicopy Integration Site. In the contextof our ongoing study of oncogenesis in a differentiatingepithelial tissue (1, 3, 15,16), the second experiment was aimedat studying the effect of SV40 TAg dosage on oncogenesis inthe developing lens of transgenic mouse embryos. The exper-iment, outlined in Fig. 2, calls for Cre-mediated reduction oftransgene copy number at a multicopy TAg integration site.Excision ofTAg transgene copies is designed to occur betweenthe loxP sites present in each TAg copy. The maA-loxP-TAgtransgene uses the murine aA-crystallin gene promoter (maA)to direct expression ofTAg to the developing lens, resulting inlens oncogenesis (15).A 34-bp loxP site separates the maA andTAg sequences in the transgene construct. We selected for our

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FIG. 1. (A) Restriction map of the targeted region in the immunoglobulin light chain kappa locus. Restriction enzyme sites are representedby letters: B, BamHI; Xh, XhoI; R, EcoRI. The XhoI site is located 8.5 kb 3' of the exon encoding the kappa constant region. (B) Scheme of thetargeted locus with the neo cassette replacing the region between the XhoI site and the EcoRI site of the endogenous locus. Solid triangles flankingthe neo gene indicate loxP sites in direct repeat orientation. Arrows and lowercase letters represent described PCR primers; forward primer a islocated adjacent to the 5' loxP site and forward primer b corresponds to a sequence within neo. Reverse primer c is located 3' of the neo cassette.The ac sequence, amplified between the a and the c primer, is too large to amplify in the assay, whereas be represents a 700-bp neo sequence betweenprimers b and c on the loxP-neo-loxP targeted chromosome. (C) Targeted locus after in vivo Cre-mediated excision of the neo gene. Only the ac

sequence is amplified in this ANEO constellation. (D) PCR assay of tail DNA. (Left) The products of a PCR mix containing primers a, b, and c.The undeleted neo cassette and/or the deleted product will be amplified if present. Lanes 1-4, tail DNA from four Fl offspring carrying a + alleleand variable amounts of NEO and/or ANEO alleles, depending on the efficiency of neo excision; lanes 5-8, DNA from four F2 offspring; lane 9,DNA from +/ANEO ES cells in which the neo gene had been deleted in vitro by transient transfection with a Cre expression vector; lane 10, +/NEOES cells from which the homozygous loxP-neo-loxP mice used in this study were generated (note that the genotype of the mouse represented inlane 6 is +/+ based on data not shown, so no ANEO product is amplified; also note that lanes 5, 7, and 8 represent ANEO/ANEO mice, i.e., onlythe ANEO product is amplified). (Right) The 269-bp PCR amplification products of the cre transgene. Lane 12 represents an F2 mouse not shownat Left, whereas lanes 5-8 show PCR data for the mice analyzed in the corresponding lanes at Left. Location and size of molecular weight markersare shown to the left of each panel. In this panel, as in E, the alleles scored, + (the wild-type allele), N (NEO), or A (ANEO), are indicated belowthe lane numbers. (E) Southern blot analysis of ES cell and tail DNA. BamHI-digested DNA was blotted and probed with a 500-bp XhoI/NcoIfragment located just 5' of the XhoI site in the genomic locus. Lanes 1-4, ES cell DNA; lanes 5-7, offspring of a +/NEO x +/NEO cross; lanes8-11, Fl offspring of a NEO/NEO x EIIacre/EIIa-cre cross; lanes 12-14, F2 offspring of interbred F1 mice (only homozygotes are shown); lanes15-18, thymus DNA from F2 mice. Expected allele sizes are shown to the right.

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FIG. 2. DNA constructs used to generate EIIa-cre and (maA-loxP-TAg), transgenic mice and schematic representation of matings thatresult in maA-loxP-TAg progeny. EIIa-cre mice carry a transgeneencoding the bacteriophage P1 recombinase under the control of anadenovirus EIIa promoter that directs Cre action to the earliest stagesof embryonic development (9). The maA-loxP-TAg transgene con-tains the 34-bp loxP site between the maA-crystallin promoter and theSV40 large TAg gene derived from paA366a-T (16). The (maA4oxP-TAg)n strain carries multiple copies of the transgene in head-to-tailarrangement. The +/(maA-oxP-TAg)n animals have lens tumors anddie young. Crosses of EIIa-cre/EIIa-re females and +/(maA-oxP-TAg), males resulted in a Mendelian distribution of transgenes amongoffspring. Those mice that harbored both the EIIa-cre and themaA-loxP-TAg transgene displayed eye cataracts and had a life spanthat was close to normal. DNA fragments used as probes in Southernblot analyses are shown. B, BamHI restriction site.

study a strain of mice that carries multiple copies of themaA-4oxP-TAg transgene at a single integration site. Lenstumor progression in these (maA-loxP-TAg)n mice is fulmi-nant, and their life span is less than 3 months. In fact, to be ableto propagate the strain, we resorted to ovary transfer fromyoung adult transgenic to nontransgenic females. We crossedmale +/(maA4oxP-TAg), and female EIIa-cre/EIIa-cre miceto obtain zygotes in which the multicopy maA-oxP-TAg sitewould be exposed to Cre activity. If recombination were tooccur at all loxP sites, the number of transgenes present at themulticopy transgene integration site should be reduced to a

single copy. Observing lens development in embryos derivedfrom these zygotes would allow us to describe the phenotypicconsequences of a reduction in oncogene copy number withoutalteration of the chromosomal position of the transgene.Genomic analysis of progeny derived from such crosses

verified that the multicopy mA-loxP-TAg transgene integra-tion site can indeed be reduced to a single-copy site. Animmediate indication of success was the birth of Fl progenythat showed lens cataracts, indicative of TAg oncogenesis, butthat appeared healthy, unlike their +/(maA-loxP-TAg)n par-ents. They stayed healthy for at least 8 months, after whichtime some of them developed lens tumors. Southern blotanalysis of tail DNA gave evidence of efficient, and possiblycomplete, excision of sequences flanked by loxP sites. In Fig.3A, the transgene locus of (maA-loxP-TAg), parents is com-pared with that of F2 progeny of an EIIa-cre/(maA-loxP-TAg)ndouble transgenic mouse. Genomic tail DNA was restrictedwith EcoRV that cuts outside the transgene sequences. Thesize (>30 kb) and intensity (determined by densitometricanalysis) of the bands in lanes 1 and 3 suggest that the(maA-loxP-TAg)n parent locus carries more than 10 copies ofthe transgene, probably in a tandem head-to-tail arrangement(13). By contrast, the size and intensity of the TAg-specificbands in F2 progeny DNA (lanes 5, 6, and 8) corresponds to a

single transgene arrangement. A more extensive analysis, using

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FIG. 3. Southern blot analysis of tail DNA and eye phenotype of(maA4oxP-TAg), and maA-oxP-TAg mice. (A) EcoRV digestsprobed with the 1169-bp HindIII fragment of the TAg plasmid (seeFig. 2). Note that there are no EcoRV sites inside the maA-oxP-TAgtransgene. (B) BamHI digested DNA hybridized with a 400-bp frag-ment of cre sequence (see Fig. 2). Because BamHI cuts once in theEIIa-cre sequence covered by the probe, the blot shows two bands ofhybridizing fragments. Lanes 1 and 3, (maA-oxP-TAg)n DNA; lanes2 and 4, nontransgenic control DNA; lanes 5-8, DNA from F2offspring derived from the progeny of a cross between EIIa-cr/EIIa-cre females and +/(maA-IoxP-TAg)n males. Letters below the lanenumbers refer to the eye phenotype observed in young adults. T,bilateral lens tumors; N, normal clear eyes; C, bilateral lens cataracts.

additional restriction enzymes, confirmed this observation(data not shown). A comparable reduction of transgene copieswas observed in progeny of at least three additional EIIa-cre x(maA-AoxP-TAg)n crosses (data not shown). We conclude thatthe germ cells that gave rise to the F2 progeny analyzed herecarried a single copy of the maA-loxP-TAg transgene. Thedistribution of cre-specific bands of BamH1 restricted tailDNA is consistent with the segregation pattern of a nonlinkedElla-cre allele (Fig. 3B). These results indicate efficient re-combination by the Cre enzyme in EIIa-cre/(maA4oxP-TAg)nearly embryos and show that single-copy maA-oxP-TAg prog-eny lacking the EIIa-cre allele can readily be obtained.

Cre-Mediated Reduction ofmaAaoxP-TAg Transgene CopyNumber Leads to a Delay in Lens Tumorigenicity. The effi-ciency of EIIa-cre-mediated transgene excision allowed us toexamine whether a reduction of maA-loxP-TAg transgenecopy number affects the rate of oncogenesis in the developingembryonic lens. In Fig. 4, we compared multicopy and singlecopy maA-oxP-TAg lenses by studying cell morphology inH&E and in TAg immunostained sections. Our previousstudies have shown that the onset of TAg accumulation in themaA-TAg transgenic lens coincides with embryonic lens celldifferentiation at midgestation (15, 16). In the multicopy(maA-loxP-TAg)n lens observed at day 11.5 of embryonicdevelopment (E11.5) nuclei of posterior epithelial cells thatare about to differentiate into primary fibers readily stain forTAg (Fig. 4A). This nuclear TAg immunostaining is absentfrom the single copy maA-loxP-TAg lens at E11.5 (Fig. 4B),but readily visible at embryonic day E13.5 when lens epithelialcells have already undergone fiber elongation (Fig. 4D). At thisstage of lens development, it appears that the multicopy(maA-oxP-TAg)n lens is filled with largely undifferentiated,TAg transformed cells likely to be tumorigenic (Fig. 4C). Bycontrast, cells in the single copy maA-loxP-TAg lens, especiallyin the posterior hemisphere (Fig. 4D) appear to have under-gone fiber elongation before TAg accumulation and may thusnot be able to participate in the process of lens tumor

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FIG. 4. Histology and TAg immuno-histochemistry of tissue sections derivedfrom embryonic multicopy (maA-oxP-TAg)n and single-copy maA-loxP-TAgeyes. All sections are oriented with theanterior pole of the lens facing the top ofthe page. lu, Lumen of lens vesicle; cb, irisand ciliary body primordia; f, fiber; e, an-terior epithelial layer; n, necrosis. (Bar =

50 i/m.) (A-D) Frozen sections of embry-onic lenses derived from multicopy (maA-loxP-TAg)n (A and C) and single-copymaA-loxP-TAg individuals (B and D), re-spectively. The tissue sections were immu-nostained with an antibody specific forTAg using an indirect peroxidase stainingprocedure. (A and B) E11.5; (C and D)E13.5. The lens vesicle inA shows strongnuclear staining in differentiating cellslocated in the posterior lens hemisphere.There is a barely visible and diffuse TAgsignal in the corresponding area of B. AtE13.5, all lens cells of both the multicopy(C) and the single-copy genotype (D),show strong nuclear TAg staining, withthe exception of the anterior epitheliallayer. Note that the nuclei of cells differ-entiating into primary fibers (f) in theposterior half of the lens displayed in Dshow a weaker and more diffused TAgi'"'' signal. (E-J) Methacrylate-embedded, he-matoxylin/eosin stained sections of trans-genic and normal eyes, derived fromE13.5 (E-G) or E17.5 embryos (H-J). AtE13.5, the multicopy (maA-loxP-TAg)nlens (E) is completely filled with rounded,dividing cells that are poorly differenti-ated, compared with the elongated, post-mitotic fiber cells (f) in the lens of an

P/"14 age-matched normal littermate (F). In the- single-copy maA-loxP-TAg lens (G),

there is an intermediate phenotype ofrounded and dividing cells alternatingwith partially elongated fiber-like (f) cells.

tt At E17.5, necrosis (n) becomes visible inthe center of the multicopy (maAloxP-TAg)n lens (H). The cells surrounding thisnecrotic center are rapidly dividing (thearrowhead points to a mitotic figure). (J)The single-copy maA-loxP-TAg lens atE17.5. Disorganization of lens tissue is lesssevere than in H, but nonetheless quitenoticeable when compared with the nor-mal situation (I).

formation (see also ref. 3). These conclusions are supported bya study of the morphology of hematoxylin/eosin stained E13.5(Fig. 4 E-G) and E17.5 (Fig. 4 H-J) lens sections. Normal lensdevelopment is characterized by orderly lens fiber elongationthroughout these stages of development (Fig. 4 F and I). Bycomparison, cell arrangement is highly disorganized both inthe multicopy (maA-loxP-TAg)n lens (Fig. 4 E and H) and inthe single copy maA-loxP-TAg lens (Fig. 4 G and J). Inaddition, the process of TAg-mediated lens transformation isclearly accelerated in the (maA-loxP-TAg)n lens as evidencedby a nearly uniform population of small, round, rapidly divid-ing cells, whereas the single-copy maA-loxP-TAg lens showsmore protracted proliferation of cells that retain some mor-

phologic features of elongated fiber cells (compare Fig. 4 Ewith G). By E17.5, the (maA-roxP-TAg)n lens showed smallrapidly dividing cells with compacted nuclei that surround anecrotic lens center (Fig. 4H), whereas the maA-loxP-TAg lensstill retained morphological features of fiber elongation (Fig.4J). The difference between the rate ofTAg accumulation andcell proliferation in the multicopy (maA-IoxP-TAg)n and the

single copy maA-loxP-TAg lens strongly suggests a quantita-tive connection between transgene dosage and TAg output.The results presented here extend our earlier investigationsand confirm our conclusions that rapid transformation is theoutcome of an early and rapid accumulation of TAg, whereaslate and slow accumulation leads to a less malignant lensphenotype (3, 16).

DISCUSSIONOur results show that targeted endogenous genes or transgenesbearing loxP sites can be efficiently modified in vivo. Mating ofEIIa-cre mice with gene-altered mice that carry DNA se-

quences flanked by loxP sites readily generates progeny inwhich excision of sequences between the loxP sites has takenplace and is genetically fixed. We can infer from what is knownabout the adenovirus EIIa promoter activity in the mouse (9)that much of this efficient Cre action is confined to the one-cellzygote stage of embryonic development. The efficient excisionthat we observed in cell populations as divergent as germ-line

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Proc. Natl. Acad. Sci. USA 93 (1996) 5865

cells, tail tissue, and lens tissue certainly supports this view andcan be fully explained by Cre activity targeted to the first cellof the developing organism. The frequency and extent ofexcision events may depend on a variety of factors, amongthem the genomic site of Cre action (17) and the length of thesequences to be excised. Use of these EIIa-cre mice to bringabout gene modifications at loxP-targeted loci in a variety ofexperimental settings will allow the general applicability of theprocedure to be further assessed in the near future.The two experiments reported here point to a number of

potential applications. In the first experiment, excision of theneo expression cassette from a targeted gene removes afunctional promoter and a selectable gene product after theyhave served their purpose. This is done to prevent theseelements from exerting potentially unwanted effects on thetargeted tissue. A similar need was realized in earlier exper-iments where direct Cre action on targeted embryonic stem(ES) cells led to the in vitro removal of neo cassettes (4). Ourapproach to accomplish this goal in vivo obviates the need forextended culture of ES cells in order to expose them to Creactivity before germ-line transmission. It also affords directcomparisons between animals that underwent Cre-mediatedgene modification and those that did not.By a slight modification of the conventional breeding strat-

egy for germ-line transmission, in vivo Cre-loxP-mediateddeletion of DNA sequences could be accomplished in amanner that would reduce in vitro ES cell manipulationwithout requiring any extra breeding steps. Conventional genetargeting technology involves injection of gene-targeted EScells into blastocysts to generate chimeras carrying the tar-geted locus. Male chimeras are subsequently crossed withnormal females to test for germ-line transmission of thetargeted allele, and offspring that have transmitted the gene-targeted allele are interbred to produce second generationoffspring homozygous for the targeted locus. If EIIa-cre miceare substituted for the normal females in this strategy, loxP-flanked sequences present at the targeted locus could bedeleted as the male chimeras are tested for germ-line trans-mission. Progeny of crosses between males chimeric for thegene-targeted allele and females carrying the EIIa-cre trans-gene should include offspring that have deleted loxP-flankedsequences at an early embryonic stage, and can transmit theCre-modified targeted allele. These progeny would be inter-bred to obtain a fraction of offspring that lack the EIIa-cretransgene and are homozygous for the Cre-modified allele.Obtaining these mice after two successive crosses wouldprovide an attractive alternative to schemes of Cre-mediatedin vitro excision of loxP-flanked sequences from ES cells.Examples of applications include introduction of a variety ofsubtle mutations into the germ line, replacement of mutatedalleles with corrected sequences, replacement of genes of onespecies with those from another, and gene targeting of cis-acting elements.

In our second experiment, a multicopy gene locus in whichthe TAg transgene was flanked on one side by a loxP site wasreproducibly reduced to one transgene copy by breeding withEIIa-cre transgenic mice. This reduction in TAg copy numberdemonstrated a correlation between transgene copy numberand lens oncogenicity. Production of transgenic mice by cur-rent approaches results in multicopy tandem arrays of trans-genes whose number can only be varied by generating a set offounders that differ by the number of copies they carry.However, they also differ by integration site and this intro-duces an unwanted variable. The crosses between EIIa-cre

mice and strains that contain loxP sites next to a transgene ofchoice allow reproducible and efficient generation of mousestrains that differ by the number of transgene copies which theycarry at a given integration site. This approach has obvious andfar-reaching applications.The system of Cre-mediated gene modification described

here leaves room for technical improvements. For example,the first of the two experiments that we describe would benefitfrom a further increase in Cre efficiency, whereas the secondexperiment would benefit from a graded administration of Creactivity to produce more variation in transgene copy number.Such titration of Cre activity and a resulting spread of trans-gene copy number among progeny has been achieved (unpub-lished work) using direct zygote microinjection with circularplasmid DNA encoding Cre; this goal has been independentlypursued elsewhere (18). The direct microinjection approach,while labor intensive, demonstrates the ability to titrate thedegree of Cre-mediated DNA excision by varying the amountof Cre enzyme delivered. A similar result could conceivably beachieved with the help of a set of EIIa-cre strains that vary intheir ability to deliver Cre activity to the early embryo.Note Added in Proof: A relevant study entitledA cre-transgenic mousestrain for the ubiquitous deletion of loxP-flanked gene segmentsincluding deletion in germ cells has recently been published by F.Schwenk, U. Baron, and K. Rajewsky.

J.R.G. is supported in part by a scholarship from the Life and HealthInsurance Medical Research Fund. F.W.A. was supported by theHoward Hughes Medical Institute and by National Institutes of HealthGrants A.I. 20047 and 31541.

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