EP1185169A2

EP1185169A2 - USE OF TRANSGENIC MICE LACKING JunB EXPRESSION IN THE MYELOID LINEAGE

Info

Publication number: EP1185169A2
Application number: EP00936807A
Authority: EP
Inventors: Emmanuelle Passegu; Wolfram Jochum; Erwin F. Wagner
Original assignee: Boehringer Ingelheim International GmbH
Current assignee: Boehringer Ingelheim International GmbH
Priority date: 1999-05-26
Filing date: 2000-05-25
Publication date: 2002-03-13
Also published as: CA2371979A1; EP1055366A1; JP2003500074A; WO2000072666A3; MXPA01011703A; WO2000072666A2

Abstract

The use of a transgenic animal lacking JunB expression in the myeloid lineage as a model for human chronic myeloid leukemia. The ue of JunB deficient myeloid cells for the identification of drugs that inhibit the proliferation of myeloid cells. A method for identifying a compound as a negative regulator of the inhibitory effect of JunB on cell proliferation, the identified compound being useful as a drug to burst myeloid cell proliferation after chemotherapy and/or radiotherapy.

Description

Use of transgenic mice lacking JunB expression in the myeloid lineage

The present invention relates to the field of myeloid leukemias .

Myelopoiesis, the regulated process which produces granulocytes and monocytes, involves mechanisms such as cell differentiation, progenitor cell proliferation, survival and apoptosis. A number of transcriptional regulators critical to myeloid differentiation have been identified (reviewed in Tenen et al . , 1997), however, less is known about transcription factors which modulate cell numbers in the myeloid lineage by controlling proliferation and cell death.

AP-1 (activating protein-1) is a dimeric transcription factor composed of Jun, Fos or ATF (activating transcription factor) family members that binds to a common DNA site, the AP-1 binding site, and converts extracellular signals into changes in the transcription of many cellular and viral genes (reviewed in Angel and Karin, 1991) . AP-1 activity is modulated by various signals including growth factors, cytokines, tumor promoters, carcinogens and specific oncogenes. AP-1 has been implicated in a number of biological processes such as cell proliferation, cell differentiation and apoptosis. However, analysis of AP-1 functions in vivo and in tissue culture cells has shown that different AP-1 members regulate different target genes and thus execute distinct biological functions in a cell-type specific fashion. All the Jun proteins (c-Jun, JunB and JunD) are similar with respect to their primary structure and their DNA- binding specificity. However, JunB, due to a small number of amino acid changes in its basic-leucine zipper region exhibits a 10-fold weaker DNA-binding activity and a decreased homodimerization property compared to c-Jun. Transfection studies have suggested that JunB is a repressor of AP-1-mediated transactivation since JunB efficiently repressed c-Jun mediated transactivation and transformation most likely due to the preferential formation of inactive c-Jun/JunB dimers (Chiu et al., 1989; Deng and Karin, 1993) . However, in vi tro and in vivo data have demonstrated that JunB can also be a strong transactivator depending on both the interacting partner and the promoter context (Chiu et al . , 1989; Li et al., 1999). It was recently shown that fetuses lacking junB die around day 9.5 of embryonic development due to defective feto-maternal interactions (Schorpp-Kistner et al . , 1999). In contrast, transgenic mice constitutively expressing the mouse junB gene under the control of the human ubiquitin C promoter (Ubi-junB) in many different tissues have no obvious phenotype (Schorpp et al., 1996). Schorpp-Kistner et al. (1999) also showed that the embryonic lethality of junB mutant embryos could be rescued by the Ubi-junB transgene indicating that the lack of junB was solely responsible for the lethal phenotype in the embryo.

JunB has been implicated in the control of proliferation and differentiation of various cell types including myeloid cells (Schlingensiepen et al. 1993; Lord et al., 1990). JunB is constitutively expressed in human mature granulocytes (Mollinedo et al., 1991). Furthermore, JunB expression is strongly induced following terminal differentiation of bone marrow myeloid precursors and established myeloid cell lines such as HL-60, U937 and Ml (Lord et al., 1990;

Mollinedo and Naranjo, 1991; Lord et al . , 1993). It was thus postulated that JunB may play an important role in promoting myeloid differentiation and it was speculated that genetic alterations affecting JunB expression may contribute to leukemogenesis. However, the role of junB has never been established.

Human chronic myeloid leukemia (CML) is a malignant clonal disorder of hematopoietic stem cells characterized by increased proliferation of the myeloid lineage (reviewed in Deininger, 1998; Sawyers, 1999). CML accounts for about 25% of adult leukemias. The principal clinical features include elevated white cell counts in the peripheral blood, hepato-splenomegaly, and myeloid hyperplasia of the bone marrow. The natural course of the disease is progressive. The initial chronic phase is followed by a rapidly fatal blast crisis within three to five years. In 90 to 95% of CML patients, the Philadelphia chromosome is present, a shortened chromosome 22 that arises from a reciprocal translocation, t(9;22) (q34;qll) and serves as a cytogenetic hallmark of the disease. The treatment of CML is largely palliative, since only a small percentage of patients can undergo potentially curative therapeutical procedures such as allogeneic bone marrow or stem cell transplantation. During the chronic phase of CML, cytoreductive therapy using oral chemotherapeutic drugs such as busulfan and hydroxyurea is able to induce haematologic remissions and to maintain the neutrophil count in the normal range to avoid thrombotic complications. Alternatively, interferon alpha can induce haematologic and cytogenetic remissions in patients with chronic phase CML. In contrast, the blastic phase of CML is refractory to most chemotherapeutic drug regimens, although short-term remissions in a small percentage of patients have been obtained. The poor prognosis of blast crisis patients clearly indicates an urgent need for more effective modes of treatment. However, advances along that line require a better understanding of the molecular events which cause or accompany the transition to blast crisis stage of CML.

A number of experimental approaches have been taken to generate animal models for human CML in order to identify mechanisms contributing to leukemogenesis and to ultimately design effective CML therapies interfering with these mechanisms. These approaches can be grouped in three categories:

1. Transfer of human myeloid leukemia cells into SCID mice

Established growth factor-independent human myeloid leukemia cell lines (K562, U937) or peripheral blood/bone marrow cells derived from patients with chronic myeloid leukemia in blast crisis were injected into severe immunodeficient (SCID) mice to induce a CML-like disease (Sawyers 1992, Cesano 1992).

2. Transduction of genes into hematopoietic stem cells using retroviral vectors Using retroviral vectors, wild-type bone marrow cells were transduced with genes encoding hematopoietic growth factors, oncogenes and mutated tumor suppressor genes such as G-CSF (Chang et al., 1989), GM-CSF (Johnson et al., 1989), IL-3 (Chang et al . , 1989; Wong et al., 1989), IL-6 (Hawley et al., 1992), PDGF-B (Yan et al., 1994), v-src (Keller and Wagner, 1989), Bcr-abl (Daley et al., 1990; Elefanty et al . , 1990; Kelliher et al., 1990), v-myc (Bonha et al., 1992), TEL/JAK2 (Schwaller et al . , 1998), N-ras (MacKanzie et al., 1999) and mutant p53 (Shounan et al . , 1997). The genetically modified bone marrow was introduced into lethally irradiated wild-type recipient mice, which developed a myeloproliferative syndrome after various periods of time.

3. Genetically engineered mice

Transgenic mice expressing a CML-associated Bcr-Abl protein under the control of the metallothionein-1 promoter develop acute leukemia at birth (Heisterkamp et al., 1990). Mice lacking ICSBP (Holtschke et al., 1996), Ship (Helgason et al., 1998) or Nf-1 (Largaespada et al., 1996) in the hematopoietic system develop a myeloproliferative syndrome, which in the case of ICSBP mutant and Nf-1 mutant mice can progress to a blast crisis.

It was in object of the invention to provide an alternative animal model for human CML which is particularly suitable to study the transition from chronic phase to blast crisis and to elucidate the role of JunB in tumorigenesis of hematopoetic cells in order to harness these findings for the therapy of leukemias.

The role of junB in the hematopoietic system was studied using junB^~/^_Ubi-junB transgenic mice. These mice were obtained according to the method described by Schorpp-Kistner et al . (1999). In brief, Ubi-junB transgenic mice were crossed to junB mutant heterozygous mice and the transgenic junB mutant heterozygous progenies were intercrossed to generate mice homozygous for the inactivated junB allele that carry the Ubi-junB transgene (Fig. 1). Such junB^_/^~Ubi-junB mice were obtained in the expected Mendelian ratio (Table 1) .

It was surprisingly found in the experiments of the invention that j nB-/-Ubi- unB mice develop a myeloproliferative disease due to lack of JunB expression in the myeloid lineage.

In a first set of experiments, JunB expression was analyzed, at the RNA and protein level, in various hematopoietic lineages derived from junB^~/^"Ubi-junB mice. It was found that all hematopoetic lineages express JunB except for the myeloid lineage (Fig. 2). The silencing of the Ubiquitin C promoter happens at the transcriptional level and in a manner which is strictly myeloid-specific and age-dependent.

Furthermore, the progressive loss of Ubi-junB transgene expression in the myeloid lineage perfectly correlates with the onset of the myeloproliferative disease.

All junB-/^"Ubi-junB mice develop a myeloproliferative disease with complete penetrance by 11 months of age. The myeloproliferative disease was characterized by a progressive and selective enlargement of the granulopoietic compartment in hematopoietic organs (Fig. 3 and Table 2) . In a certain proportion of the junB^~/^~Ubi-junB mice the disease progresses to a fatal blast crisis.

The myeloproliferative disease was found to be cell autonomous, since it is transplantable into wild-type recipient mice (Fig. 4), and specific for the lack of JunB, since it develops in mice reconstituted with junB^{_ ~} fetal liver stem cells. Lack of JunB was found to affect myelopoiesis at the progenitor level, since increased numbers of cycling myeloid progenitors were observed both in vi tro and in vivo (Fig. 5). Increased numbers of myeloid progenitors are therefore likely to cause the myeloproliferative disease. Further in vi tro characterisation revealed that JunB-deficient myeloid progenitors have reduced apoptosis levels, which are independent of the growth factor and cytokine environment, and increased proliferation in the presence of the growth factor GM-CSF.

The experiments of the present invention have thus shown that JunB plays a critical role in vivo during myelopoiesis since its lack results in a myeloid leukemia. They show that JunB acts as a negative regulator of myelopoiesis by controlling both proliferation and survival of the myeloid progenitors without interfering with myeloid maturation.

The myeloproliferative disorder observed in junB^~/^~Ubi-junB mice surprisingly recapitulates essential features of human CML. Absence of JunB expression in the myeloid lineage leads to increased myeloid proliferation and consecutive myeloid hyperplasia. Consistent with the model of multistage tumorigenesis, a certain proportion of junB^~/^~Ubi-junB mice develop blast crisis in the course of the myeloproliferative disease indicating that hyperproliferation predisposes to additional genetic alterations that eventually lead to blastic transformation. The similarities of the myeloproliferative disease in junB^~~/^~Ubi-junB mice with human CML indicates that these mice are a very efficient tool to identify genes that contribute to leukemogenesis, especially blastic transformation.

In a first aspect, the present invention relates to the use of transgenic animals lacking JunB expression in the myeloid lineage as a model for human CML.

Since the technology of gene manipulation is best developed for mice, in a preferred embodiment, the animal is a mouse. However, other mammals like rats, monkeys, pigs, etc. may be used applying methods that allow for equivalent gene manipulation leading to the corresponding phenotype.

Several ways of generating mice that lack junB in the myeloid lineage are available.

According to a first method, the mice may be obtained by transgene complementation of a conventional junB knock-out applying a junB transgene in which junB is under the control of a strong and constitutive promoter that leads to expression during early embryonic development. An example is the human ubiquitin C promoter (Schorpp et al., 1996), which was used in the experiments of the present invention.

In an alternative method, the mice may be generated by conditionally inactivating junB in the myeloid lineage by established genetic approaches. Examples for such a conditional inactivation approach are the well- established Cre/loxP system (Araki et al., 1995) and the Flp/FRT system (Dymecki, 1996) . In the Cre/loxP system, floxed junB knock-in mice are generated by standard ES-cell technology as described by Capecchi (1989), by injecting blastocysts with ES-cells in which the endogenous junB gene was replaced by a floxed junB allele (entire junB coding sequence flanked by two loxP sites) . Floxed junB mice are then crossed to transgenic mice expressing the Cre recombinase in the myeloid lineage, preferably in myeloid progenitor cells. The promoter elements of a number of myeloid-specific genes have been demonstrated to target transgene expression to myeloid cells such as the promoters of the human

HMRP8 gene (Lagasse and Weissmann, 1994; Brown et al., 1997) and the human cathepsin G gene (Grisolano et al., 1997). However, every promoter element which directs CRE recombinase activity to the myeloid lineage is suitable for this approach. A similar strategy can be applied for the Flp/FRT system by flanking junB by FRT sites and by expressing the Flp recombinase under the control of a myeloid specific promoter.

Another method for obtaining mice with the desired phenotype, applies the introduction junB homzygous mutant ES cells into wild-type tetraploid blastocysts as described previously (Wang et al., 1997). This technique is able to rescue the early embryonic lethality of junB mutant embryos and to allow for development of junB mutant embryos up to mid-gestation (Schorpp-Kistner et al., 1999; our own results). Fetal liver cell suspensions containing hematopoietic stem cells are prepared from such E12.5 tetraploid rescued junB mutant fetuses and are used to reconstitute lethally irradiated wild-type recipient mice according to standard procedures. In experiments of the invention we could demonstrate that junB mutant fetal liver cells indeed reconstitute the hematopoietic system of lethally irradiated syngenic recipients and that such recipient mice develop a myeloproliferative disease.

Mice obtained according to these methods, are useful for the following purposes:

Firstly, mice lacking JunB expression in the myeloid lineage can be used as a source for establishing JunB deficient myeloid cell lines as described in Example 6. In the experiments of the invention, myeloid cell lines lacking JunB expression were successfully derived from bone marrow of junB^"/^~Ubi-junB mice (Fig. 6) . Alternatively, such cell lines can also be obtained from any mouse lacking JunB expression in the myeloid lineage.

Thus, the present invention relates to myeloid cells lacking JunB expression.

These cells are suitable tools for (a) the identification of drugs that inhibit the proliferation of myeloid cells and are thus useful for the treatment of CML, in particular during blast crisis, (b) the search for JunB target genes involved in the control of myeloid cell proliferation and differentiation, and (c) the screening for genetic events involved in the transformation of myeloid cells.

An example for conducting screening assays based on this principle is the following.

The JunB deficient myeloid cell lines can be used:

(a) to identify drugs or components that inhibit cell proliferation and induce terminal differentiation.

(b) to identify JunB target genes involved in myeloid cell proliferation and differentiation: For this purpose, artificial controls can be generated by re-introducing junB in the JunB deficient myeloid cell lines. The cells are stably transfected or infected by conventional techniques with a recombinant DNA molecule containing the full-length mouse junB gene fused in frame to any kind of inducible system such as the hormone-binding domain of the human estrogen receptor (ER) or the human progesterone receptor (PR) . The principle of these inducible systems is that, although the chimaeric proteins are constitutively expressed, it is in an inactive state in the absence of the appropriate ligand such as exogenous β-estradiol for the estrogen receptor (Picard, 1994). These pseudo- control cells and junB^~/^"Ubi-junB cells can be subject to analysis for identifying JunB target genes using methods known in the art to determine differential gene and protein expressions (e.g., differential display, RT-PCR analysis, DNA microarray technology, subtractive hybridization or proteome analysis) .

(c) to identify genetic events associated with blastic transformation. Standard techniques for analyzing gene rearrangements and chromosome translocations can be used to search for common transformation events observed in a statistical number of independent JunB deficient myeloid cell lines.

Mice lacking junB expression in the myeloid lineage can be used as an animal model to confirm that a candidate compound identified in a screening assay (a) to the anti-proliferative effect. The compound will be administered to the mouse and the effect of the compound will be monitored by determining its potential therapeutic effect, e.g. by measuring the absolute number of neutrophilic granulocytes in the peripheral blood. Compounds which reduce the number of these cells are candidates as drugs for the treatment of human CML, in particular during the chronic phase.

Mice lacking junB expression in the myeloid lineage that are in blast crisis, can also be used as a source for appropriate material (tissue, cells, DNA, RNA, protein) which is analyzed by appropriate methods known in the art to confirm the in vivo manifestation of events identified in (c) .

Furthermore, in the experiments of the present invention, the role of JunB in controlling cell proliferation was analyzed in vi tro using fibroblasts derived from Ubi-junB transgenic mice (Fig. 7) . Spontaneously immortalized 3T3-fibroblasts expressing high JunB levels have a pronounced proliferation defect due to an extension of the Gl-phase of the cell cycle. These fibroblasts have elevated levels of the cyclin- dependent kinase inhibitor pl6/INK4a resulting in a complete inhibition of the cyclinDl/CDK4-β kinase activity and reduced pRb phosphorylation . Finally, three JunB/AP-1 binding sites were characterized in the mouse pl6 promoter through which JunB transcriptionally regulates plβ expression. These results identify JunB as a negative regulator of cell proliferation and provide a molecular link between growth factor signaling and the regulation of cell cycle progression by cyclin-dependent kinase inhibitors.

These findings combined with the in vivo results described above, allow to consider JunB as a potential tumor suppressor gene that could operate in vivo to cause growth arrest of cells during terminal differentiation processes, e.g. the growth arrest of myeloid cells during terminal differentiation.

Defining the role of JunB in cell cycle control and its mechanism of action provides, inter alia, the basis for methods for identifying drugs that enhance its tumor suppressor function either directly or indirectly. Examples for such drugs are substances increasing the activity of JunB target genes, e.g. plβ a negative regulator of the cell-cycle, or inhibiting the activity of downstream components of the JunB-mediated signal transduction pathway, e.g. the cyclinD/CDK4-6 complex, a positive regulator of the cell-cycle (cyclinDl is repressed by JunB and therefore increased in the absence of JunB; Bakiri et al . , 2000). Further examples of B-Jun target genes are c-Jun, which is repressed by JunB and therefore increased in the absence of JunB (Deng and Karin; 1993) and IL4, which is activated by JunB (Li et al., 1999).

Regulatory sequences of JunB target genes as well as newly discovered JunB target genes can be used to monitor the effect of drugs suppressing the inhibitory effect of JunB on cell proliferation.

An example for conducting screening assays based on this principle is the following:

Suitable test cells are fibroblasts expressing high levels of JunB protein, e.g. fibroblasts derived from Ubi-junB transgenic mice as described in Example 7. The cells are stably transfected, by conventional techniques, with a recombinant DNA molecule containing the regulatory regions of a JunB target gene, e.g. plβ (Soloff et al., 1996; Genbank accession number AF022809) , fused to a reporter gene, according to known methods. Any reporter gene may be used whose expression product is detectable by measuring a signal proportional to its concentration and which can be detected and quantified. Preferably, a reporter gene is employed whose expression can be measured with high sensitivity in an automated assay such as the luciferase gene, the Green Fluorescent Protein (GFP) gene or the LacZ gene. The cells are grown in a suitable medium in the presence or absence of the substance to be tested and are assayed for their effects on (1) proliferation of the cells and (2) reporter gene activity. Proliferation can be measured by standard techniques, e.g. by quant tating the cell number or by measuring the³H-Thymιdιne or desoxybromouridine incorporation.

Substances which exhibit the desired effect will increase proliferation of the test cells and down- regulate reporter gene activity.

In a further step, the identified substances can be tested for their effect on proliferation of any cell type for which JunB has an inhibitory effect on cell proliferation, e.g. on bone marrow-derived primary myeloid cells. Substances which exhibit tne desired effect will increase the proliferation rate of myeloid cells and are useful for the application as human drugs to burst myeloid cell proliferation after, for example, chemotherapy and/or radiotherapy.

Therefore, in a further aspect, the invention relates to pharmaceutical compositions comprising, as an active ingredient, a substance which inhibits JunB activity as a negative regulator of cell proliferation in normal cells. Substances that inhibit JunB activity are useful as agents promoting the regeneration of cells such as myeloid cells in situations of impaired hematopoiesis, e.g. after chemotherapy and/or radiotherapy. They may act according to various mechanisms e.g. by reducing JunB expression, JunB stability, JunB DNA binding and/or transactivation activity.

In a further aspect, the invention relates to pharmaceutical compositions comprising, as an active ingredient, a substance which enhances the activity of JunB as a negative regulator of proliferation of tumor cells. Substances that enhance JunB activity are therefore useful as antitumor agents. They may act according to various mechanisms, e.g. by increasing JunB expression, JunB stability, JunB DNA binding and/or transactivation activity.

Compounds which enhance JunB activity can be identified m a screening assay according to methods known per se (WO 92/13063) , using cells that are transfected with a reporter gene under the control of the regulatory region of the JunB gene. A substance with the desired effect will increase reporter gene expression.

Alternatively, useful compounds can be identified by growing cells that have been transfected with a plasmid containing JunB cDNA to overexpress JunB with in the presence or absence of the test compound, measuring the stability of the expressed JunB, e.g. by means of Methionine Pulse Chase Labeling, and selecting a compound which increases JunB stability.

Brief description of the Figures

Fig. 1

Breeding scheme to obtain unB^~< ^~ mice carrying the Ubi-junB transgene

Fig. 2

A) RT-PCR analysis of expression levels for the endogenous junB gene and the Ubi-junB transgene in various FACS-sorted haemopoietic lineages. B) Western-blot analysis of JunB protein levels in FACS-sorted B cells and myeloid cells.

Fig. 3

FACS analysis of the myeloid compartment from spleen and bone marrow of control and junB^~/^~Ubi-junB mice.

Fig,

A) FACS analysis of the bone marrow myeloid compartment of recipient mice transplanted with control and junB-/-Ubi-junB bone marrow cells.

B) Genotyping of the recipient mice

Fig. 5

Cell cycle profile of a spleenic myeloid population from control and junB-/-Ubi-junB mice.

Fig. 6

A) RT-PCR analysis of expression levels for the endogenous junB gene and the Ubi-junB transgene in myeloid cell lines derived from control and junB^~/^~ Ubi-junB mice.

B) Proliferation rates of control and JunB deficient myeloid cell lines.

Fig. 7

A) Proliferation rates of wild-type and Ubi-junB transgenic 3T3-fibroblasts .

B) Western blot analysis of plβ protein levels in wild-type and Ubi-junB transgenic 3T3-fibroblasts . C) Northern blot analysis of plβ mRNA levels in wild-type and Ubi-junB transgenic 3T3-fibroblasts .

D) plβ transcriptional activity in wild-type, Ubi-junB transgenic and junB-/- 3T3-fibroblasts .

E) JunB/AP-1 binding sites identified in the murine plβ promoter.

Example 1

Generation of junB^~/-Ubi-junB mice

junB^"/""Ubi-junB mice were obtained according to the method described by Schorpp-Kistner et al. (1999). In brief, Ubi-junB transgenic mice were crossed to junB mutant heterozygous mice and the transgenic junB mutant heterozygous progenies were intercrossed to generate mice homozygous for the inactivated junB allele that carry the Ubi-junB transgene. The approach used is schematically outlined in Fig. 1. Such junB-/^~Ubi-junB mice were obtained in the expected Mendelian ratio, which is given in Table 1.

Ubi-junB

junB⁺/⁺ junB⁺/^~ junB-/^"

number 46 97 52

frequency 24% 50% 26% Example 2

Analysis of JunB expression in various hematopoetic lineages

To examine JunB expression in the hematopoietic system of junB-/-Ubi-junB mice, various hematopoietic lineages were FACS-sorted by known methods (Coligan et al . , 1992) according to their cell surface marker constellations. mRNA and protein preparations of these cell populations were obtained according to standard methods (Sa brook et al . , 1989). RT-PCR and protein analysis revealed (Fig. 2) that only myeloid cells (M: Gr-1+/Mac-1+) from junB^~/-Ubi-junB mice were lacking JunB expression, while JunB expression was detected in B lymphocytes (B: CD19+/Ig-M+) , T helper lymphocytes (T: CD4+/CD8-) and erythroid cells

(E: TER-119+) . This expression profile indicates that junB^"/^"Ubi-junB mice display a cell type-specific loss of JunB expression in the myeloid lineage.

Example 3

Analysis of the phenotype of junB^"/-Ubi-junB mice

All junB-/-Ubi-junB mice obtained in Example 1 developed a myeloproliferative disease with complete penetrance by 11 months of age. Hematological analysis of peripheral blood by measuring absolute blood cell counts and evaluating stained blood smears revealed a 10-fold increase in leukocyte numbers in junB^"/-Ubi-junB mice at 6 to 11 months of age. The increase was largely due to elevated numbers of neutrophilic granulocytes. The numbers of other hematopoietic lineages such as erythrocytes, lymphocytes, monocytes and eosinphilic granulocytes were comparable to wild-type controls. However, reduced hemoglobin concentrations indicated the presence of hypochromic anaemia. The results are given in Table 2 (n = number of mice analyzed for each genotype) .

Wild-type Ubi-junB junB^~/^~Ubi- junB

(n= [n=ιo;

(n=ll.

White blood cells 4,9 ± 0,6 6,9 ± 1,3 52,6 ± 9,9

(xlOAμl)

Erythrocytes ,9 ± 0,2 9,5 ± 0,3 9,0 + 0,6

(xlOAμl)

Thrombocytes 1011 ± 75 752 ± 88 1139 ± 155 (xl0³/μl)

Neutrophil 29,8 ± 3,5 26,5 ± 2,0 72,2 ± 3,0 segments (%)

Neutrophil 0,5 ± 0,1 0,8 ± 0,2 2,8 ± 0,4 bands (%)

Lymphocytes (%) 64,1 ± 3,7 67,1 ± 2,0 17,9 ± 2,5

Eosinophils (%) 3,3 ± 0,6 2,7 ± 0,4 3,2 ± 0,9

Monocytes (%) 2,2 ± 0,4 2,9 ± 0,3 2,2 ± 0,4 Gross anatomical analysis of diseased junB-/^_Ubi-junB mice consistently revealed a massive enlargement of the spleen and lymph nodes. Histological examination of spleen sections showed pronounced myeloid hyperplasia indicated by infiltration of the red pulp by myeloid cells of various maturation stages. Histochemical stains for chloroacetate esterase indicated the presence of numerous mature neutrophilic granulocytes.

Stained cytospin preparations of spleen cells confirmed the presence of numerous myeloid cells, mainly mature granulocytes. The bone marrow of junB^"/^~Ubi-junB mice showed increased cellularity and myeloid hyperplasia as determined by the investigation of cytospin preparations and histological sections of bone marrow. Histological analysis of junB-/^_Ubi-junB livers showed infiltration of the portal tracts by mature and immature cells of the granulocyte lineage. FACS analysis of single cell suspensions of bone marrow and spleen cells using antibodies directed to cell type specific surface marker showed a drastic expansion of the myeloid compartment with consecutive reduction of the lymphocyte compartments (Fig. 3).

In 10 to 20% of junB^"/-Ubi-junB mice disease progression to blast crisis was observed as indicated by the infiltration of haemopoietic or non- hematopoietic organs such as heart, lung, kidney and skin by immature blastic cells. Example 4

Bone marrow transplantation experiment to demonstrate that the CML-like disorder is a cell autonomous defect

Myelopoiesis is controlled by cytokines such as IL-3, IL-β, GM-CSF and G-GSF, which are able to stimulate myeloid proliferation and differentiation in vi tro and in vivo . To test whether the myeloproliferative disease of junB-/-Ubi-junB mice is due to a cell autonomous defect, bone marrow cells were transplanted into lethally irradiated wild-type recipients as described

(Keller and Wagner, 1989) . Recipients were followed for the development of the myeloproliferative disease by stained blood smears. Within a follow-up period of 8 months, 85% of recipients developed a disease characterized by elevated neutrophilic granulocyte counts in peripheral blood smears and expansion of the myeloid compartment in spleen and bone marrow as measured by FACS analysis (Fig. 4A) . PCR-based analysis of DNA extracted from bone marrow and spleen cells of recipients indicated complete reconstitution of the recipients by junB^"/-Ubi-junB bone marrow cells (Fig. 4B) .

Example 5

Increased numbers of cycling myeloid progenitors in junB^""/-Ubi-j nB mice

To study whether the expansion of the myeloid compartment was due to increased proliferation of precursor cells within the lineage, the DNA content of myeloid spleen cells (Gr-1+/Mac-1+) was analyzed by FACS using a similar method as in Example 2. In junB-/^"Ubi-junB mice, 13 % of the myeloid cell population was found in S and G2/M phase of the cell cycle, compared to 3 % in control mice, showing increased number of cycling cells in the myeloid population of junB^~/^~Ubi-junB mice (Fig. 5) . This demonstrates increased numbers of cycling myeloid progenitors in vivo in junB-/^_Ubi-junB mice. In addition, increased numbers of myeloid progenitors were found in vi tro in both spleen- and bone marrow-derived cultures from the junB-/-Ubi-junB mice.

Example 6

Derivation of myeloid cell lines lacking JunB expression

Immortalized myeloid cell lines lacking junB expression were generated from junB^"/-Ubi-junB mice. Bone marrow cells were isolated from the long bones of the mice and grown in methylcellulose medium containing a combination of growth factors and cytokines supporting proliferation and differentiation along the granulocytic lineage (e.g., GM-CSF, G-CSF, SCF, IL-3 and IL-6) as described in Liu et al. (1996) . After 7 days, single differentiated colonies were picked from the methylcellulose and grown in liquid culture containing the same cocktail of growth factors and cytokines. After 2-3 weeks, wild-type derived colonies stopped proliferating and terminally differentiated. In contrast, colonies derived from junB^~/-Ubi-junB mice that lack JunB expression kept proliferating and were immortalized (Fig. 6).

Example 7

JunB suppresses 3T3-fibroblasts proliferation by transcriptionally activating expression of the cyclin- dependent kinase inhibitor plβ/INK4a.

To determine the effect of high JunB expression levels on cell proliferation, mouse embryonic fibroblasts were derived from E12.5 Ubi-junB transgenic embryos (Schorpp et al., 1996) and established according to the 3T3-protocol (Todaro and Green, 1963) . All established Ubi-junB transgenic 3T3-fibroblast lines proliferated slowly (Fig. 7A) with a doubling time of 43 ± 2.5 hours, compared to 19.4 ± 1.3 hours in the wild-type counterparts, and saturation densities of only 50% of the wild-type cells.

To identify the mechanism through which JunB impairs fibroblasts proliferation, the expression levels of members of the p21 and plβ families of cyclin-dependent kinase inhibitors (CKIs) were investigated. CKIs are known to negatively regulate cell cycle progression through binding and inhibition of cyclin/CDK complexes (Sherr and Roberts, 1995) . Only the level of the CKI plβ/INK4a was found to be dramatically elevated at both the protein (Fig. 7B) and the mRNA (Fig. 7C) levels in all Ubi-junB transgenic 3T3-fibroblasts tested. To study whether JunB can transactivate the plβ promoter, a 1.2 kb 5 ' -fragment of the mouse plβ promoter was cloned into a luciferase reporter construct (pl6-Luc) and transient transfection experiments were performed with wild-type, Ubi-junB transgenic and junB-/- 3T3-fibroblasts (Fig. 7D) . Wild-type fibroblasts showed a moderate level of pl6-Luc activity consistent with the low level of plβ mRNA and protein expressed. In contrast, Ubi-junB transgenic cells exhibited 3 to 5 times more plβ transcriptional activity whereas in junB-/- cells only half of the activity was detected compared to wild-type cells .

Finaly, we identified three JunB/AP-1 binding sites in the murine plβ promoter (depicted in Fig. 7E) and demonstrated that they are critical for both basal and JunB-mediated plβ expression since their mutation results in both decreased plβ transactivation in wild-type cells and complete inhibition of JunB-mediated plβ transactivation.

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Claims

1. The use of a transgenic animal lacking JunB expression in the myeloid lineage as a model for human chronic myeloid leukemia.

2. The use of claim 1, wherein said animal is a mouse,

3. The use of a transgenic animal defined in claim 1 or 2, said animal being obtainable by transgene complementation of a junB knock-out animal and application of a junB transgene in which junB is under the control of a strong and constitutive promoter that leads to expression during early embryonic development.

4. The use of claim 3, wherein said promotor is the ubiquitin C promotor.

5. The use of a transgenic animal defined in claim 1 or 2, said animal being obtainable by conditional inactivation of junB in the myeloid lineage.

6. The use of a transgenic animal defined in claim 1 or 2, said animal being obtainable by introduction of junB homzygous mutant ES cells into wild-type tetraploid blastocysts

7. JunB deficient myeloid cells obtainable from a transgenic animal defined in claim 1 or 2.

8. The use of JunB deficient myeloid cells for the identification of drugs that inhibit the proliferation of myeloid cells.

9. A method for identifying a compound as a negative regulator of the inhibitory effect of JunB on cell proliferation, wherein cells which have been transfected with a recombinant DNA molecule containing a reporter gene under the control of the regulatory region of a JunB target gene and which express high levels of JunB are grown in the presence of a test compound, the compound A effect on cell proliferation and reporter gene expression is determined and the compound is selected for its ability to increase cell proliferation and to down- regulate reporter gene expression.

10. The method of claim 9, wherein the cells are f broblasts .

11. The method of claim 9 or 10, wherein the reporter gene is under the control of the regulatory region of the human plβ gene.

12. The method of any one of claims 9 to 11, wherein the selected compound is further tested for its ability to increase the proliferation of myeloid cells .

13. A compound identified in a method of any one of claims 9 to 12 for application to an individual after chemotherapy and/or radiotherapy.

4. A pharmaceutical composition comprising, as an active ingredient, a substance which enhances the activity of JunB as a negative regulator of proliferation of tumor cells.