Inbred embryonic stem-cell derived mice

BACKGROUND OF THE INVENTION

[0002] Genetically altered mice harboring specific transgenes or genetically engineered mutations have proven to be extremely useful tools for the analysis of gene function, the study of disease processes, and the discovery and testing of new therapies. However, current methods for generating recombinant mice are cumbersome, time consuming and expensive. A commonly used method of introducing targeted mutations or transgenes into the germ line of mice involves the use of embryonic stem (ES) cells as recipients of an engineered gene. ES cells are pluripotent cells that can be derived directly from the inner cell mass of blastocysts (Evans et al., (1981) Nature 292:154-156; Martin (1981) Proc. Natl. Acad Sci. USA 78:7634-7638; Magnuson et al., (1982) J. Embryo. Exp. Morph. 81:211-217; Doetchman et al., (1988) Dev. Biol. 127:224-227), disaggregated blastocysts (Eistetter, (1989) Dev. Gro. Differ. 31:275-282), and from primordial germ cells (Matsui et al., (1992) Cell 70:841-847; Resnick et al., (1992) Nature 359:550-551). Recombinant genes can be introduced into ES cells using any method suitable for gene transfer into cells, e.g., by transfection, cell fusion, electroporation, microinjection, DNA viruses, and RNA viruses (Johnson et al., (1989) Fetal Ther. 4 (Suppl. 1):28-39). The advantages of using ES cells include their ability to form permanent cell lines in vitro, thus providing an unlimited source of genetic material. Additionally ES cells are the most pluripotent cultured animal cells known. For example, when ES cells are injected into an intact blastocyst cavity or under the zona pellucida, at the blastocyst stage embryo, ES cells are capable of contributing to all somatic tissues including the germ line in the resulting animals. Besides the mouse, as described further below, ES-like cells have been isolated from rat (Iannaccone P M et al (1994) Dev Biol 163:288-292), pig (Chen L R et al. (1999) Theriogenology 52:195-212), bovine (Talbot N C et al. (1995) Mol Reprod Dev 42:35-52), rabbit (Schoonjans L et al. (1996) Mol Reprod Dev 45:439-443), primates (Thomson J A et al. (1995) PNAS 92:7844-7848) and human (Thomson J A et al (1998) Science 282:1145-1147).

[0003] Conventional procedures for the production of genetically engineered mice involve the use of genetically engineered ES cells to create genetically altered chimeric mice by either aggregation with diploid embryos or injection of engineered ES cells into diploid blastocysts, and subsequent introduction of the resulting chimeric embryos into pseudo-pregnant female mice (Capecchi, Science 244:1288-1292 (1989); Capecchi, Trends in Genetics 5:70-76 (1989)). The resulting chimeric mice are then bred to obtain mice that are heterozygous or homozygous for the desired genetic alterations. Despite the high success rate of this approach for generating recombinant mice, this method has serious limitations, particularly for generating large numbers of different recombinant mice carrying alterations in different genes in a high throughput setting, or for combining alterations in multiple genes in the same mouse strain. For example, the creation of a mouse with a homozygous mutation by the above approach requires a step of in vitro ES cell manipulation to target the gene of interest, followed by the production of a chimeric mouse. The chimeric founder animal is then bred to generate heterozygous progeny that are subsequently interbred to create mice homozygous for the desired alteration. Thus, the process of obtaining a homozygous mutant mouse requires at least three mouse generations, or 9 months of breeding time, to generate the desired mouse strain. These manipulations are complicated further in terms of breeding requirements if other mutations or transgenes are incorporated into the desired mutant mouse strain. Due to the lengthy time of breeding and the costs and effort of maintaining animals, it has become highly desirable, particularly in commercial or high throughput settings, to develop alternate methods to routinely generate genetically altered mice that do not require the production of a chimeric mouse intermediate.

[0004] Use of tetraploid embryos instead of diploid embryos for injection or aggregation with genetically altered ES cells is one approach that is used to circumvent the generation of a chimeric mouse intermediate and lengthy breeding steps (Nagy A et al. (1990), Development. 110:815-21; Misra R P et al. (2001) BMC Biotechnology 1:12). This method, termed “tetraploid complementation”, takes advantage of the property that blastomeres may be readily made tetraploid (4N) by electrofusion of a two-cell embryo, and the resulting tetraploid cells have the capacity to replicate and form trophoblast and endoderm of the placenta and extraembryonic membranes, but fail to form fetal structures. On the other hand, ES cells have the capacity form fetal structures, but cannot form trophoblast and extraembryonic endoderm. Consequently, chimeric embryos formed by the introduction, either by injection or aggregation, of ES cells into tetraploid embryos successfully form normal concepti due to the complementary contributions of ES and tetraploid cells (Nagy et al., 1990; Nagy et al., PNAS (1993) 90:8424-8428; and James et al., Dev Biol (1995) 167(1):213-26). Thus, the method of tetraploid complementation appears to hold promise as a facile way to create nonchimeric ES cell-derived mice (‘ES mice’) in one step, without any additional breeding steps. However, in numerous studies attempting to apply the method of tetraploid complementation to the generation of ES mice the results have been highly variable at best, and for the most part viable and fertile ES mice have been produced in disappointingly low yield (Nagy A et al, (1990), supra; Nagy A et al. (1993) supra; Ueda O et al. (1995) Exp Anim. 44:205-10; Wang Z Q et al., (1997) Mech Dev. 62:137-45; PCT publication WO98/06834). In particular, it has proven especially difficult to obtain viable and fertile ES mice via tetraploid complementation using ES cells derived from inbred mouse lines (Nagy et al., 1993, supra, Ueda et al., 1995, supra). Also, in vitro genetic manipulation of ES cells or continued propagation of ES cells for prolonged times appeared to further reduce the efficiency to generate ES mice (Nagy et al., supra). Unfortunately, then, these collective results indicate that it may be commercially impractical to use tetraploid complementation for the routine generation of genetically engineered ES mice from inbred genetic backgrounds, due to extremely low efficiencies. This is especially problematic since inbred lines are the preferred lines to use for comparative studies of biological function, disease processes, or therapeutic efficacy, due to the constancy of genetic background in inbred strains.

[0005] Recently, Eggan et al. (Eggan K et al. (2001) PNAS 98:6209-6214) described a carefully controlled set of experiments comparing the behavior of inbred ES cells with F1 hybrid ES cells for the capacity to generate viable ES mice through tetraploid complementation. The F1 hybrid ES cells in this study were derived from outbred F1 progeny of crosses between different inbred mouse lines, e.g. 129 x C57BL/6, BALB/c x C57BL6, CBA x 129, 129/SvJ x 129/SV-CP and others. Remarkably, this study showed a reproducible and large difference in the capacity of inbred ES cells versus F1 hybrid ES cells to generate viable adult ES mice: inbred ES cells yielded viable adult ES mice at a low frequency of only 0-1.4%, depending on the strain used, whereas F1 hybrid ES cells yielded adult ES mice at an average frequency of 15%. Thus, in this study, inbred ES cells again proved impractical for use in routine generation of ES mice, even though F1 hybrid cells did work at a useful frequency. Although this study did not solve the problem of how to efficiently and routinely generate ES mice from inbred ES cells, it did demonstrate that differences presumably introduced by outbreeding in F1 progeny could somehow increase the efficiency of generation of ES mice by as much as 50-fold. Nonetheless, the complexity and mechanistic basis of this difference caused by outbreeding the F1 hybrid lines remained undefined, and therefore these results did not directly suggest a method to modify the isolation or manipulation of pure inbred ES lines such that inbred ES lines would have increased efficiency for the generation of ES mice via tetraploid complementation.

[0006] Several independent studies have focused on developing methods for improving the isolation and maintenance of undifferentiated pluripotent ES cell lines from a variety of sources. Leukemia Inhibitory Factor (LIF) has long been used as a key component of culture medium for isolation and propagation of undifferentiated ES cell lines (Pease S, et al. (1990) Dev Biol. 141:344-52). LIF is a soluble secreted protein factor that acts through a complex on recipient cells containing two receptors, gp130 and the low affinity LIF receptor, LIF-R. Response of cells to LIF appears to be mediated intracellularly by components of the STAT and MAPK signally pathways (Smith A and Burdon T, WO0015764). For propagation of ES cells LIF is typically provided either by a feeder layer of cells in the culture that express LIF, by preconditioning the culture medium through exposure to cells expressing LIF, or by adding recombinant LIF to the culture medium (Pease et al., supra). In addition to LIF other soluble protein-like factors have been characterized partially which appear to improve the isolation and maintenance of undifferentiated ES cells; Dani et al. have described a factor termed ESRF (Dani et al. (1998) Dev Biol 203:149-162; PCT application WO 9730151), and Schoonjans and Moreadith have described a conditioned medium with improved properties derived from recombinant rabbit fibroblasts expressing rabbit LIF (PCT application WO 0200847). In addition, Burdon et al. have presented data that inhibition of the MAPK signaling pathway, through the addition of ERK inhibitor PD098059 to the culture medium, enhances the growth of undifferentiated ES cells (Burdon et al. (1999) Dev. Biol. 210:30-43; PCT application WO 0015764). However, it has not been shown that such factors, which promote isolation or maintenance of ES cells in culture, can solve the problem of promoting tetraploid complementation of inbred ES cells for the generation of ES mice. For example, LIF has been used to maintain both inbred and F1 hybrid ES cells; yet, in one study, there was a 50-fold greater efficiency in the generation of adult ES mice using the F1 hybrid ES cells compared with the inbred ES cells (Eggan et al., supra). Consequently, there is no apparent direct relationship between factors that improve the isolation and/or maintenance of ES cells in culture compared to factors that might specifically improve the developmental potential of ES cells in tetraploid aggregates.

[0007] Thus, there remains a need in the art for improved and reproducible methods for the generation of genetically altered ES mice from genetically engineered inbred ES cells.

[0008] All references cited herein are incorporated in their entireties.

SUMMARY OF THE INVENTION

[0009] The invention provides an improved and reproducible method for the generation of ES mice from inbred ES cells, without the need to generate a chimeric mouse intermediate. The method comprises the steps of propagating an inbred embryonic stem (ES) cell in a culture medium containing a predetermined amount of a ras/MAPK kinase pathway inhibitor. In preferred embodiments, the ras/MAPK inhibitor inhibits MEK1 and/or MEK2. A preferred MEK inhibitor is PD098095.

[0010] The ras/MAPK-inhibited ES cells that are generated are introduced into a tetraploid embryo by injection into a tetraploid blastocyst or aggregation with a tetraploid morula, to generate an ES cell-complemented tetraploid embryo. The ES cell-complemented tetraploid embryo is transplanted into a female mouse, and viable ES mouse progeny are generated. In a preferred embodiment, the inbred ES cells are recombinant or genetically modified, in that they harbor a defined genetic alteration in their genome, such as a mutation, defined gene knock-out or knock-in, gene replacement and/or conditional knockout, and thus, the resulting ES mice are transgenic.

[0011] The method can be used for the fast production of ES mice strains homozygous for a defined genetic alteration. In this aspect of the invention, male ES cells harboring a defined genetic alteration are propagated under ras/MAPK pathway-inhibiting conditions to produce male (XY) cells and female (XO) cells. Tetraploid embryo complementation is performed with the male (XY) cells to generate viable male (XY) ES mice, and with isolated female (XO) cells to produce viable female (XO) ES mice. The male (XY) ES mice and the female (XO) ES mice are crossed to produce F1 progeny mice that are homozygous for the genetic alteration.

DETAILED DESCRIPTION OF THE INVENTION

[0012] We have discovered that viable and fertile embryonic stem (ES) cell derived-mice (ES mice) can be generated at much higher frequency than the previously described methods of using inbred ES cells and tetraploid complementation, when the inbred ES cells used for the tetraploid complementation have been propagated in a culture medium that contains an inhibitor of the ras/MAPK pathway. Thus, the invention provides an improved and reproducible method for the generation of ES mice from inbred ES cells, without the need to generate a chimeric mouse intermediate.

[0013] Propagation of Inbred Embryonic Stem (ES) Cells

[0014] In the methods of the invention, an inbred ES stem cell is obtained, for example using an available cell line, or by generating primary ES cells using standard methods (see Hogan et al., in Manipulating the Mouse Embryo, CSHL press 1994 pp253-289). For example, male and female mice of inbred strains are allowed to naturally mate, and the ES cells from the blastocysts of fertilized mice are obtained. Any inbred strain can be used; preferred strains include C57BL/6, Balb/c, C3H, CBA, SJL, and 129SvEv/TAC (available from Janvier Le Genest-St-Isle, France and Taconic M&B, Denmark). The ES cells are cultured in medium and under standard conditions suitable for propagation of ES cells (Torres and Kuehn, Laboratory Protocols For Conditional Gene Targeting. (1997) Oxford University Press; Hogan et al. (1994); and Manipulating the Mouse Embryo, 2.edition, Cold Spring Harbor Laboratory Press, NY). Additionally, the culture medium is supplemented with an exogenously added inhibitor of the Raf/MEK/ERK signaling pathway (also referred to herein as the “ras/MAPK pathway”). The ras/MAPK pathway controls the activation of many cellular functions as diverse and (sometimes seemingly contradictory) as cell proliferation, cell-cycle arrest, terminal differentiation and apoptosis (see Murakami M S, Morrison D K., Sci STKE (2001) 99:PE30; and Peyssonnaux C, Eychene A., Biol Cell 2001 Sep;93(1-2):53-62).

[0015] Inhibitors of the ras/MAPK pathway that can be used are known in the art. Examples of small molecule inhibitors include ZM 336372 (N-[5-(e-Dimethylaminobenzamido)-2-methylphenyl]-4-hydroxybenzamide), an inhibitor of c-raf; 5-Iodotubercidin (Cas No. 24386-93-4), an inhibitor of ERK2; PD-98059 (2′-amino-3′-methoxyflavone; Cas No. 167869-21-8), an inhibitor of MEK (Alessi, D. R. et al. (1995) J. Biol. Chem. 270: 27489-27494); and U 0126 (bis[amino[(2-aminophenyl)thio]methylene]butanedinitrile; Cas No. 109511-58-2), also an inhibitor of MEK (Dudley, D. T. et al. (1995) Proc. Natl. Acad. Sci. USA 92: 7686-7689) (all aforementioned ras/MAPK pathway inhibitors are available from BIOMOL Research Labs, Plymouth Meeting, Pa.). An example of a protein inhibitor of the ras/MAPK pathway is Anthrax lethal factor which inhibits MEK (Duesbery N S, Vande Woude G F, J Appl Microbiol (1999) 87(2):289-93; Duesbery N S et al (2001) Proc Natl Acad Sci USA. 98:4089-94). Preferred inhibitors inhibit MEK1 and/or MEK2. A particularly preferred inhibitor is PD-98059.

[0016] Additional Ras/MAPK pathway inhibitors can be identified using available assays. For example, inhibition of MEK can be detected by performing in vitro phosphorylation of an ERK:GST-fusion protein in the presence of various concentrations of a putative MEK inhibitor. Presence of the activated ERK is detected using Western blot and antibodies specific for active ERK (Said et al., Promega Notes, Number 69, 1998, p.6). Typically, small molecule inhibitors are used within the range of 10 nM to 100 mM. Preferred concentrations for PD98059 are in the range of 10-50 μM. Preferred concentrations of U0126 are in the range of 100 nM-10 μM. The optimal concentration of a particular inhibitor can be determined using routine experimentation. The concentration of the ras/MAPK inhibitor may be reduced once cell lines are established.

[0017] The culture medium is supplemented with an “exogenously added” ras/MAPK pathway inhibitor, meaning that the medium is supplemented with the inhibitor in a controlled manner. Typically, this is achieved by adding a known amount of a purified inhibitor to the culture medium to achieve a desired final concentration in the culture medium. In the case of protein inhibitors however, the culture medium may be supplemented with an “exogenously added” ras/MAPK pathway inhibitor by cells that express a recombinant inhibitor (e.g. recombinant feeder cells) in a sufficient amount to achieve ras/MAPK pathway inhibition. Thus, it is apparent from the foregoing examples that the term “exogenously added” does not encompass the situation where feeder cells in a conditioned medium, or the ES cells themselves, secrete an endogenously produced ras/MAPK inhibitor. Inhibitors of the ras/MAPK pathway used in the methods of the invention are typically small molecule or protein inhibitors such as the ones described above, but can also include other inhibitory agents, such as nucleic acid inhibitors (e.g. antisense, RNAi (see PCT WO 01/75164) etc.). The ES cells are cultured in the medium under conditions that promote proliferation. The cells may be passaged multiple times until the desired number of cells is obtained; they may then be used in tetraploid complementation, or frozen and stored for later use. ES cells propagated in the presence of a ras/MAPK pathway inhibitor are referred to herein as “ras/MAPK inhibited ES cells.”

[0018] Tetraploid Complementation

[0019] The ras/MAPK inhibited ES cells may be used for tetraploid complementation without further modification to generate cloned ES mice, or they may first be genetically modified, and then used for the production of transgenic ES mice (discussed further below).

[0020] Tetraploid embryos are generated using known methods (Wang et al., supra; WO98/06834; Eggan et al., supra). As an example, female mice are superovulated and mated. Fertilized zygotes are collected and cultured to obtain two-cell embryos, which are then electrofused to produce one-cell tetraploid embryos. The tetraploid embryos are cultured in vitro to the blastocyst stage. Ras/MAPK inhibited inbred ES cells are introduced into a tetraploid embryo using known methods such as aggregation with a tetraploid morula (Nagy et al., (1990), supra) or injection into a tetraploid blastocyst (Eggan et al., supra). Typically about 10-15 ES cells are introduced into each embryo, resulting what is termed herein as an “ES cell-complemented tetraploid embryo.” Approximately 5-15 ES cell-complemented tetraploid embryos are implanted into recipient female mice, and allowed to develop to a point where they can survive ex-utero. The term “viable ES mouse progeny,” is used herein to refer to an ES mouse generated by the above-described method that can survive at least 2 days after removal from the uterus of its recipient female mouse.

[0021] Transgenic ES Mice

[0022] Prior to tetraploid complementation, the ras/MAPK inhibited ES cells may be genetically modified to produce recombinant ES cells (i.e. that harbor a defined genetic alteration in their genome, such as a mutation, defined gene knock-out or knock-in, gene replacement and/or conditional knockout), and thus, the resulting ES mice are transgenic, which are used to generate transgenic ES mice. Alternatively, the ES cells themselves are derived from a transgenic or recombinant mouse, and thus are already “genetically modified”. As another alternative, primary ES cells or existing ES cell lines are genetically modified prior to ras/MAPK inhibition. Methods for production of recombinant ES cells are well known in the art (Gene Knockout Protocols. In: Methods in Molecular Biology, Vol. 158. (2001) Eds M J Tymms and I. Kola, Humana Press, Totowa, N.J.). Recombinant ES cells harbor altered expression (increased or decreased expression, including lack of expression) of one or more genes. Altered expression of genes in ES cells can be accomplished by gene knock-out, gene knock-in, and targeted mutations.

[0023] In one embodiment, the recombinant ES cells and resulting transgenic animals harbor a gene “knock-out”, having a heterozygous or homozygous alteration in the sequence of an endogenous gene that results in a decrease of gene function, preferably such that gene expression is undetectable or insignificant. Knock-out cells are typically generated by homologous recombination with a vector comprising a transgene having at least a portion of the gene to be knocked out. Typically a deletion, addition or substitution has been introduced into the transgene to functionally disrupt it. The transgene can be a human gene (e.g., from a human genomic clone) but more preferably is an ortholog of the human gene derived from the transgenic host species. For example, a mouse gene is used to construct a homologous recombination vector suitable for altering an endogenous gene in the mouse genome. Detailed methodologies for homologous recombination in mice are available (see Capecchi, Science (1989) 244:1288-1292; Joyner et al., Nature (1989) 338:153-156). Procedures for the production of non-rodent transgenic mammals and other animals are also available (Houdebine and Chourrout, supra; Pursel et al., Science (1989) 244:1281-1288; Simms et al., Bio/Technology (1988) 6:179-183).

[0024] In another embodiment, the recombinant ES cell and the resulting transgenic animals harbor a gene “knock-in”, having an alteration in its genome that results in altered expression (e.g., increased (including ectopic) or decreased expression) of the gene, e.g., by introduction of additional copies of a gene, or by operatively inserting a regulatory sequence that provides for altered expression of an endogenous copy of the gene. Such regulatory sequences include inducible, tissue-specific, and constitutive promoters and enhancer elements. The knock-in can be homozygous or heterozygous.

[0025] ES cells may also be used to produce transgenic nonhuman animals that contain selected systems allowing for regulated expression of a transgene. One example of such a system that may be produced is the cre/loxP recombinase system of bacteriophage P1 (Lakso et al., PNAS (1992) 89:6232-6236; U.S. Pat. No. 4,959,317). If a cre/loxP recombinase system is used to regulate expression of the transgene, animals containing transgenes encoding both the Cre recombinase and a selected protein are required. Such animals can be provided through the construction of “double” transgenic animals, e.g., by mating two transgenic animals, one containing a transgene encoding a selected protein and the other containing a transgene encoding a recombinase. Another example of a recombinase system is the FLP recombinase system of Saccharomyces cerevisiae (O'Gorman et al. (1991) Science 251:1351-1355; U.S. Pat. No. 5,654,182). In a preferred embodiment, both Cre-LoxP and Flp-Frt are used in the same system to regulate expression of the transgene, and for sequential deletion of vector sequences in the same cell (Sun X et al., (2000) Nat Genet 25:83-6).

[0026] The targeting constructs used to produce recombinant ES cells may be produced using standard methods, and preferably comprise the nucleotide sequence to be incorporated into the wild-type (WT) genomic sequence, and one or more selectable markers (Sambrook et al., 1989, Molecular Cloning: A Laboratory Manual, Second edition, CSHL press Cold Spring Harbor, N.Y.; E. N. Glover (eds.), 1985, DNA Cloning: A Practical Approach, Volumes I and II; F. M. Asubel et al, 1994, Current Protocols In Molecular Biology, John Wiley and Sons, Inc.). The targeting construct may be introduced into the host ES cell using any method known in the art, such as microinjection, electroporation, retroviral-mediated transfer, sperm-mediated gene transfer, transfection, and calcium phosphate/DNA co-precipitation, among others. In a preferred embodiment, the targeting construct is introduced into host ES cells by electroporation (Potter H et al. (1994) PNAS 81:7161-7165). The presence of the targeting construct in cells is then detected by identifying cells expressing the selectable marker gene. For example, cells that express an introduced neomycin resistance gene are resistant to the compound G418.

[0027] Transgenic female ES mice derived from male ES cells can be generated for the fast generation of genetically altered inbred mouse strains. Male ES cells harboring genetic alterations can be propagated to generate ras/MAPK inhibited cells as described above. In rare cases, cell divisions lead to non-disjunction, thereby producing ES cells with an XO genotype (ES cells containing an X, but not a Y chromosome) among the progeny of the parental ES cells. ES cells with XY and XO genotypes may be distinguished using Y-specific markers. XY and XO ES cells are then used to produce transgenic ES mice, as described above. In contrast to humans (Turner-Syndrome), XO female mice are fertile. Therefore, ES mice with XY and XO genotypes are then mated, resulting in 25% offspring homozygous for a genetic alteration introduced into the parental male ES cell clone.

[0028] The transgenic ES mice produced using the methods of the invention can be used in a variety of applications, such as in genetic studies to elucidate signaling pathways or to identify additional genes involved in the same pathway that are also involved in disease progression. For example, two different transgenic mice, each harboring an altered gene known to be involved in cancer, can be mated to produce a double transgenic animal. The double trangenic animal is then used to determine the frequency and rate of cancer development. The identification of genes which accelerate malignant progression in a specific tissue, or which induce tumors in other tissues, provides further targets for therapeutic treatment.

[0029] Transgenic animals are also used as animal models of disease and disorders implicating defective gene function. They can also be used in drug development for in vivo testing of candidate therapeutic agents to evaluate compound efficacy and toxicity. The candidate therapeutic agents are administered to a transgenic animal having altered gene function and phenotypic changes are compared with appropriate control animals such as genetically modified animals that receive placebo treatment, and/or animals with unaltered gene expression that receive candidate therapeutic agent. Assays generally require systemic delivery of the candidate modulators, such as by oral administration, injection, etc. Following initial screening, a candidate therapeutic agent that appears promising is further evaluated by administering various concentrations of the compound to the transgenic animals in order to determine an approximate therapeutic dosing range.

EXAMPLES

[0030] The following experimental section and examples are offered by way of illustration and not by way of limitation.

[0031] I. Derivation of Wildtype and Mutant C57BL/6 or 129SvEv/Tac Inbred ES Cell Lines

[0032] ES cell derivation was essentially performed as described by Hogan et al. (Manipulating the mouse embryo, CSHL Press 1994, pp253-89), except that cell culture was performed at 39° C. instead of 37° C. C57BL/6 (Janvier, France), 129SvEv/Tac (Taconic M&B, Denmark) or C57BL/6-APCMin (Jackson Laboratories, USA) male and female mice were mated to obtain blastocysts from fertilized females. The C57BL/6-APCMin mouse strain harbours a spontaneous mutation in the APC tumor suppressor gene (Su L K et al (1992) Science 256:668-670) and provides a genetic model for human hereditary colon cancer. Plug positive females were set aside, and 3 days later blastocysts were isolated by flushing their uteri. The blastocysts were further cultured overnight in CZB medium (Chatot et al. (1990) Biol. Reprod. 42:432-440) and then transferred into 12-well tissue culture plates (1 blastocyst per well), precoated with a monolayer of Mitomycin-C inactivated primary mouse embryonic fibroblasts, in standard ES cell culture medium (“standard conditions”) (Torres and Kuehn, Laboratory protocols for conditional gene targeting, Oxford University Press 1997), or in standard medium supplemented with the MEK inhibitor PD 98059 (NEB Biolabs) (50 micromolar concentration, diluted from a 50 millimolar stock in DMSO stored at −20° C.) or with the MEK inhibitor UO126 (10 micromolar concentration; NEB Biolabs). These cultures were incubated in a tissue culture incubator (Heraeus) for 6 days at 39° C. in a 10% CO2 atmosphere. The outgrown inner cell mass of each blastocyst was isolated with a pipette tip under low power magnification, transferred into a 50 microliter drop of Trypsin solution and incubated at 39° C. for 5-10 minutes. This cell clump was further dissociated by pipetting, the cells were transferred into tissue culture dishes and further propagated at 39° C. using standard ES cell culture conditions (Torres and Kuehn 1997, supra), with and without PD98059 (50 micromolar) or UO126 (10 micromolar). After the initial establishment of ES cell lines propagated in the presence of PD98059 (6 days after the first dissociation) the concentration of PD98059 was reduced to 25 micromolar, the concentration of UO 126 was reduced to 4 micromolar, and the cells were further propagated. The C57BL/6 line Bruce4 (Kontgen et al, (1993) Int Immunol. 5:957-964) was grown under standard conditions at 37° C.

[0033] The sex of the cell lines was determined through Southern blot hybridisation of genomic DNA using a detection probe (pY353) specific to a Y-chromosome specific repeat (Bishop C E and Hatat D. (1987) Nucleic Acids Res 15, 2959-2969).

[0034] For the karyotype analysis of ES cell clones, 4×106 cells were cultured for 1 hour in the presence of 0.2 μg/ml Demicolchicin (Sigma), trypsinised, resuspended in a hypotonic (0.56%) KCl solution and incubated for 8 minutes at room temperature. The swollen cells were sedimented by centrifugation, resuspended in freshly prepared fixative (3:1 mixture of ethanol/glacial acetic acid) and subsequently washed 2 times in fixative. Finally, the cells were resuspended in 0.5 ml fixative and dropped on glass slides precleaned with ethanol/acetic acid (19/1). After air drying, the metaphase spreads were stained for 5 minutes in a 2% solution of Giemsa's stain (Merck), washed in water, and air-dried. The chromosome numbers of 20 suitable metaphase spreads were counted at 1000× magnification under oil immersion.

[0035] These experiments resulted in 4 ES cell lines (2 male, 2 female) from 31 C57BL/6 blastocysts (13% efficiency) and 6 ES lines from 27 129SvEv/Tac blastocysts (22% efficiency) for ES cells grown under standard conditions. For cultures grown in media with PD 98059, 11 ES lines resulted from 18 C57BL/6 blastocysts (61% efficiency) and 9 ES lines were obtained from 12 129SvEv/Tac blastocysts (75% efficiency). From cultures grown in media with UO 126, 32 ES lines were obtained from 40 C57BL/6 blastocysts (80% efficiency). Thus, ES cell lines could be derived at a 5-fold higher efficiency in the presence of the MEK inhibitors PD 98059 or UO 126 (Table 1). Furthermore, 12 ES cell lines were generated from blastocysts of the C57BL/6-APCMin mutant mouse strain (36% efficiency).

1TABLE 1Derivation of ES Cell LinesNumber ofEfficiencyMaleNumber ofestablishedto deriveESFemaleCell lineBlastocystsES linesES lineslinesES linesconventionalC57BL/631413%22129SvEv/Tac27622%51with PD98059C57BL/6181161%82129SvEv.tac12975%54with UO126C57BL/6403280%115C57BL/6331236%48ApcMin

[0036] II. Injection of Inbred ES Cells into Tetraploid Blastocysts/Generation of Inbred ES Mice

[0037] Production of mice by tetraploid embryo complementation has been previously described (Eggan et al. (2001) PNAS 98:6209-6214). Briefly, embryo culture was carried out in microdrops on standard bacterial petri dishes (Falcon) under mineral oil (Sigma). Modified CZB media (Chatot et al, Supra) was used for embryo culture unless otherwise noted. Hepes buffered CZB was used for room temperature operations. After administration of hormones, superovulated B6D2F1 females were mated with B6D2F1 males. Fertilized zygotes were isolated from the oviduct and any remaining cumulus cells were removed with hyluronidase. After overnight culture, two-cell embryos were electrofused to produce one cell tetraploid embryos using a CF150-B cell fusion instrument from BLS (Budapest, Hungary) according to the manufacturers instructions. Embryos that had not undergone membrane fusion within 1 hour were discarded. Embryos were then cultured in vitro to the blastocyst stage. For microinjection, 5-6 blastocysts were placed in a drop of DMEM with 15% FCS under mineral oil. A flat tip, piezo actuated microinjection-pipette with an internal diameter of 12-15 μm was used to inject 15 ES cells into each blastocyst. After recovery, ten injected blastocysts were transferred to each uterine horn of 2.5 days post coitum, pseudopregnant NMRI females that had been mated with vasectomized males. For cesarian derivation, recipient mothers were sacrificed at E 19.5 and pups were quickly removed. Newborns that were alive and respirating were cross-fostered to lactating females. Three days later, the litters were controlled, and pups alive by that time were counted as surviving pups.

[0038] Results: Two C57BL/6 derived ES cell lines (Bruce4 (Koentgen et al., supra) and ESAR-B6-8) established and grown under standard conditions were, as published, very inefficient in producing ES mice (Table 2; 0.67% respirating pups, 0% surviving pups). In contrast, new C57BL/6 ES lines established and grown in the presence of PD98059 produced significantly more alive pups (Table 2; 2.6% respirating pups, 0.57% surviving pups). ES cell lines established from 129SvEv/Tac blastocysts and grown in the presence of PD98059 also produced significantly more surviving pups (4.48%) upon tetraploid complementation as compared to three lines grown under standard conditions (0.85% surviving pups) (Table 2).

2TABLE 2EX Mouse Production From Inbred Cell LinesNumber ofNumber ofNumber ofinjectedrespiratingsurvivingC57BL/6 ESGenetictetraploidpupspupsCell linesbackgroundblastocystsefficiencyefficiencyw/o SubstanceBruce4C57BL/6J22010.45%00.00%ESAR-B6-8C57BL/6J22720.88%00.00%Sum44730.67%00.00%with PD98059ESAR-B6-PD.13C57BL/6J18042.22%00.00%ESAR-B6-PD.3C57BL/6J37282.15%10.27%ESAR-B6-PD.4C57BL/6J329113.34%41.22%Sum881232.61%50.57%129 ESw/o Substance129SvEv/Tac129S6/SvEvTac36851.36%10.27%ESAR-S6-15129S6/SvEvTac13621.47%10.74%ESAR-S6-18129S6/SvEvTac321103.12%51.56%Sum825172.06%70.85%with PD98059ESAR-S6-PD.4129S6/SvEvTac335298.66%154.48%

[0039] III. Production of Genetically Engineered and Mutant Inbred ES Mice

[0040] In order to generate inbred ES mice which harbour genetic alterations we pursued two different strategies. In the first protocol ES cell lines that were established and grown in media with UO 126 (see Example I) and which harbour the C57BL/6-APCMin mutation were injected into tetraploid blastocysts. These injection resulted in 12 respirating pups (1.9% efficiency) (Table 3). In the second protocol the C57BL/6 ES cell line ESAR-B6-PD.4 was genetically modified in a predetermined manner through homologous recombination using a gene targeting vector. This vector (Rosa(lacZ) knock-in) introduces a beta-galactosidase reporter gene in conjunction with a selectable hygromycin resistance gene into the endogenous Rosa26 locus of the ES cell genome (Seibler et al., Nucleic Acids Res. 31, e12, 2003). This gene targeting vector was electroporated into ESAR-B6-PD4 cells exactly as described (Seibler et al., Nucleic Acids Res. 31, e12, 2003) and hygromycin resistant ES cell colonies were selected, isolated and further expanded. The genomic DNA of resistant colonies was isolated and tested by Southern blot analysis for the occurrence of a homologous recombination event in one of the Rosa26 alleles. Cells from one of the recombined ES cell clones (ESAR-B6-PD.4-R9 A-F2) was injected into tetraploid blastocysts. These injections resulted in 4 respirating pups (2.47% efficiency) (Table 3).

3TABLE 3Production of Genetically Engineered and Mutant Inbred ES MiceNumber ofNumber ofinjectedrespiratingC57BL/6 ESGenetictetraploidpupscell linesbackgroundblastocystsefficiencywith UO126ESAR-APC 3 UOC57BL/6 J Apcmin405112.72%ESAR-APC 9 UOC57BL/6 J Apcmin22810.44%Sum633121.90%with PD98059ESAR-B6-PD.4 R9C57BL/6 J16242.47%B-G12

[0041] IV. Production of Female Transgenic ES Mice From Male ES Cells for the Accelerated Generation of Homozygous Mouse Mutants

[0042] Male ES cells harboring genetic alterations are propagated in culture medium with PD098059. In rare cases, cell divisions lead to non-disjunction, thereby producing ES cells with an XO genotype (ES cells containing an X, but not a Y chromosome) among the progeny of the parental ES cells. In a given ES cell culture the frequency of such 39XO cells is about 1-2%. These cells are isolated as pure clones by plating of the population at low density (1000 cells/culture dish) and further culture for 9 days until each single cell has formed a distinct colony of 1 mm size (about 2000 cells). Using a pipette tip several hundred of these colonies are picked and distributed into wells of 96-well microtiter plates, prefilled with 50 microliter Trypsin solution. After a 10 minute incubation period the dissociated cells of each clone are further transferred into 96-well microtiter plates precoated with Mitomycin-C-inactivated embryonic feeder cells and cultured for several days in ES cell culture medium containing PD098059. Upon this initial culture phase the clones of each plate are split and distributed into two 96-well culture plates, one containing a layer of embryonic feeder cells while the other plate is precoated with a gelatine solution. Upon a 3 day expansion phase the culture medium in the plate containing the feeder layer is replaced by freezing medium and the plates are stored as frozen stock at −80° C. The second, gelatin coated plate is cultured for further two days until the ES cells reach confluency. Next, genomic DNA is isolated from these clones, digested with the restriction enzyme EcoRI, separated by agarose gel electrophoresis, and transferred onto nylon membranes by capillary transfer. These Southern blot membranes are hybridized with the Y-chromosome-specific 1.5 kb DNA probe from plasmid pY353 (Bishop, C. E. & Hatat, D. Molecular cloning and sequence analysis of a mouse Y chromosome RNA transcript expressed in the testis. Nucleic Acids Res 15, 2959-2969. (1987)). While the great majority of clones (98-99%) exhibit a strong Y chromosome specific signal, a minority of the clones (1-2%) shows no signal, presumably due to a spontaneous loss of the Y-chromosome which occurred in the progenitor of the cell which formed such a Y-negative colony. The latter clones are recovered from the frozen storage plate and further expanded in culture medium containing PD098059. To confirm that these clones underwent only a loss of the Y chromosome, their karyotype is characterized by chromosome counting of Giemsa stained metaphases as described in Example 1. Clones which exhibit a majority of metaphases with 39 chromosomes are defined as female cell lines with a 39, XO karyotype. These XO ES cells are then used to produce female transgenic ES mice through injection into tetraploid blastocysts, as further described below. Male ES mice with the same genotype as the XO ES females can be produced from the parental male ES cell population which was used to isolate the rare female cells. Male ES mice with a 40XY karyotype and female ES mice with a 39XO karyotype but the same genotype are then mated, resulting in 25% offspring homozygous for the genetic alteration which was introduced into the parental male ES cell line. This procedure to generate homozygous inbred mouse mutants involves only one breeding step and thus saves time as compared to the standard method via chimeric mice, which requires two breeding steps.

Inbred embryonic stem-cell derived mice

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