Patent Publication Number: US-2003229912-A1

Title: Non-human transgenic mammals and their use as a disease model

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     [0001] This application claims benefit of the filing date of U.S. application Ser. No. 60/375,407, filed Apr. 26, 2002, which is incorporated herein in its entirety by reference. 
    
    
     
       BACKGROUND OF THE INVENTION  
       FIELD OF THE INVENTION  
       [0002] The present invention is in the field of oncology, in particular in the field of animal cancer models.  
       [0003] Rhabdomyosarcoma is the most common soft-tissue sarcoma of childhood and accounts for 4-8% of all pediatric malignancies 1 . Rhabdomyosarcomas are histologically classified as embryonal, alveoar, or pleomorphic 2 . Although rhabdomyosarcomas display rhabdomyoblastic differentiation, the cell type of origin has not been identified yet. The molecular pathogenesis of human rhabdomyosarcoma is poorly understood. Most rhabdomyosarcoma of the alveolar subtype carry the characteristic t(2;13)(q35;q14) and t(1;13)(p36;q14) translocations which result in the formation of the chimeric PAX3-FKHR and PAX7-FKHR transcription factors 3-5 . In addition, the TP53 tumor suppressor gene, among others like MDM2, CDKN2A (p16 INK4 ) and CDK4 (reviewed in reference 6), is frequently mutated in sporadic embryonal rhabdomyosarcoma 7,8 . A function for the TP53 pathway in rhabdomyosarcoma tumorigenesis is further indicated by the observation that rhabdomyosarcoma is the most common sarcoma type in patients with hereditary predisposition to cancer due to germ-line mutations in the TP53 gene 9-11 . Rhabdomyosarcomas also develop in p53 heterozygous and homozygous mutant mice, although with low penetrance and long latency 12,13 . Mutated TP53 may function through an inhibition of MYOD function, thereby blocking differentiation 14 , or by loss of the normal transcriptional repression of IGF-2 by wildtype TP53 15 .  
       [0004] Evading apoptosis is an important mechanism in the pathogenesis of rhabdomyosarcoma. Besides enhancing cellular proliferation and invasion 33 , the PAX3-FKHR fusion protein of alveloar rhabdomyosarcoma protects tumor cells against apoptosis 34,35 , most likely by induction of BCL-XL 36 , c-MET 37,38  and other regulators of myogenic differentiation 39,40 . Furthermore, IGF2 which has important functions in normal skeletal muscle development and acts as a survival factor in various tumor types, is overexpressed in the majority of human rhabdomyosarcomas 41,42 . Overexpression in rhabdomyosarcomas is achieved either by loss of the maternal IGF-2 allele, which is normally silenced by imprinting, and duplication of the paternal allele 43,44 , or by loss of imprinting of the maternal allele 45 . Recently, IGF2 overexpression has also been linked to inactivation of the tumor suppressor Patched (Ptc), the receptor of sonic hedgehog (shh). Mice heterozygous for a targeted mutation of the Ptc gene develop embryonal rhabdomyosarcoma with low incidence 46 . In these tumors, Igf2 and the transcription factor Gli-1, a mediator of Ptc signaling, are overexpressed suggesting that Ptc negatively regulates Igf2 expression and genetic evidence indicates that rhabdomyosarcoma development in Ptcl± mice depends on Igf2 gene expression 47 .  
       [0005] The Fos protein is a major component of the AP-1 transcription factor, which regulates various biological processes by converting extracellular signals into changes in the expression of specific target genes 16 . Mice lacking Fos develop osteopetrosis due to the lack of osteoclasts, the resorbing cell type of the bone 17-19 . Consistent with its original identification as the transforming activity of the mouse osteosarcoma viruses FBJ-MuSV and FRB-MuSV 20 , ectopic expression of Fos in embryonal stem cells and transgenic mice results in oncogenic transformation and the development of chondrosarcoma and osteosarcoma, respectively 21,22 . In addition, Fos mediates both pro- and anti-apoptotic signals and regulates invasive growth and angiogenesis during tumorigenesis 23-26 .  
       SUMMARY OF THE INVENTION  
       [0006] While rhabdomyosarcoma is the most common soft-tissue sarcoma of childhood, animal models for rhabdomyosarcoma are rare and the application of available models is limited by low tumor incidence or long latency periods (references 12,46,48-50).  
       [0007] Therefore, it was an object of the invention to elucidate the molecular mechanisms that contribute to the formation of rhabdomyosarcoma in order to provide a model system that is useful in developing therapies for this disease.  
       [0008] In the experiments of the invention, it was surprisingly found that mice carrying germline mutations in both the p53 and Fos genes develop rhabdomyosarcoma with high penetrance and short latency. Re-expression of Fos in p53; Fos mutant rhabdomyosarcoma cells leads to increased spontaneous apoptosis suggesting that the disruption of the pro-apoptotic functions of Fos contributes to rhabdomyosarcoma development. These results suggest that mutations in the p53 and Fos genes cooperate in the development of rhabdomyosarcoma and that p53; Fos mutant mice may be a potent mouse model for human rhabdomyosarcoma. They further indicate that Fos, in addition to its functions in oncogenic transformation and tumor progression, may also be a tumor suppressor.  
       [0009] Thus, the present invention relates to non-human mammals that carry germline mutations in both the p53 and the Fos genes and their use as a model for rhabdomyosarcoma. (In the following, these transgenic animals are referred to as “p53/Fos double knockout” animals.)  
       [0010] In related aspects, the invention relates to methods for generating p53/Fos double knockout non-human mammals as well as cell lines and tissues obtained from such animals.  
     
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
     [0011]FIG. 1: Rhabdomyosarcoma development in p53/Fos double knockout mice.  
     [0012]FIGS. 2 a - 2   f:  Histological analysis of rhabdomyosarcomas in p53/Fos double knockout mice.  
     [0013]FIGS. 3 a - 3   d:  Histological analysis of muscle tissue in p53/Fos double knockout mice.  
     [0014]FIGS. 4 a - 4   d:  Fos-induced apoptosis in RMS cell lines. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0015] In the animals of the invention, both the p53 and the Fos genes are mutated such that the animals do not express the functional gene products.  
     [0016] A “mutation” is a detectable change in the genetic material in the animal, which is transmitted to the animal&#39;s progeny. A mutation is usually a change in one or more deoxyribonucleotides, the modification being obtained by, for example, adding, deleting, inverting, or substituting for nucleotides.  
     [0017] Preferably, the genome of the transgenic non-human mammal comprises one or more deletions in one or more exons of the genes and further comprises a heterologous selectable marker gene.  
     [0018] The term “knockout” characterizes a mutation of a nucleic acid sequence which significantly reduces, in particular suppresses or destroys expression of the biologically active encoded polypeptide. For example, “Fos knock-out animal” refers to an animal in which the expression Fos has been reduced or suppressed by the introduction of a recombinant nucleic acid molecule that disrupts at least a portion of the genomic DNA sequence encoding Fos.  
     [0019] In principle, knockout animals may have one or both copies of the gene sequence of interest disrupted. In the latter case, in which a homozygous disruption is present, the mutation is termed a “null” mutation. In the case where only one copy of the nucleic acid sequence of interest is disrupted, the knockout animal is termed a “heterozygous knockout animal”. The p53/Fos double knockout animals of the present invention are preferably homozygous for the disruption of both the p53 and the Fos genes.  
     [0020] Techniques for obtaining the transgenic animals of the invention are well known in the art; the techniques for introducing foreign DNA sequences into the mammalian germ line were originally developed in mice 51 . One route of introducing foreign DNA into a germ line entails the direct microinjection of linear DNA molecules into a pronucleus of a fertilized one-cell egg. Microinjected eggs are subsequently transferred into the oviducts of pseudopregnant foster mothers and allowed to develop. About 25% of the progeny mice inherit one or more copies of the micro-injected DNA 52 . Currently, the most frequently used techniques for generating chimeric and transgenic animals are based on genetically altered embryonic stem cells or embryonic germ cells. Techniques suitable for obtaining transgenic animals have been amply described, e.g. see references 53,54,55,58. A suitable technique for obtaining completely ES cell derived transgenic non-human animals is described in WO 98/06834.  
     [0021] To generate the animals of the invention, in the first step, transgenic animals are generated that lack either p53 or Fos function. Such animals can be obtained by standard gene targeting methods as described above, preferably by using ES cells. In the next step, the transgenics are intercrossed to obtain p53/Fos double knockout mice.  
     [0022] Thus, in a further aspect, the invention relates to a method for producing a p53/Fos double knockout non-human mammal comprising the steps  
     [0023] i. providing an embryonic stem cell from the relevant animal species comprising an intact p53 gene and an embryonic stem cell from the relevant animal species comprising an intact Fos gene,  
     [0024] ii. providing a targeting vector capable of disrupting the p53 gene and a targeting vector capable of disrupting the Fos gene upon homologous recombination,  
     [0025] iii. introducing said targeting vectors into said ES cells under conditions where the intact p53 gene or the intact Fos gene, respectively, and the relevant targeting vector undergo homologous recombination to produce a mutant p53 and a mutant Fos gene respectively,  
     [0026] iv. introducing the ES cells carrying a disrupted p53 gene or Fos gene, respectively, into separate blastocysts,  
     [0027] v. implanting the blastocysts into the uterus of pseudopregnant females,  
     [0028] vi. delivering animals from said females, identifying animals that carry the mutant allele and breeding them to homozygosity,  
     [0029] vii. intercrossing homozygous p53−/− and Fos−/− animals, selecting for p53/Fos double knockout animals and breeding them.  
     [0030] A “targeting vector” is a vector comprising sequences that can be inserted into the gene to be disrupted, e.g., by homologous recombination.  
     [0031] The targeting vector generally has a 5′ flanking region and a 3′ flanking region homologous to segments of the gene of interest, surrounding a foreign DNA sequence to be inserted into the gene. For example, the foreign DNA sequence may encode a selectable marker, such as an antibiotics resistance gene. Examples for suitable selectable markers are the neomycin resistance gene (NEO) and the hygromycin β-phosphotransferase gene. The 5′ flanking region and the 3′flanking region are homologous to regions within the gene surrounding the portion of the gene to be replaced with the unrelated DNA sequence. DNA comprising the targeting vector and the native gene of interest are contacted under conditions that favor homologous recombination. For example, the targeting vector and native gene sequence of interest can be used to transform embryonic stem (ES) cells, in which they can subsequently undergo homologous recombination.  
     [0032] Proper homologous recombination can be confirmed by Southern blot analysis of restriction endonuclease digested DNA using, as a probe, a non-disrupted region of the gene. Since the native gene will exhibit a restriction pattern different from that of the disrupted gene, the presence of a disrupted gene can be determined from the size of the restriction fragments that hybridize to the probe.  
     [0033] In an animal obtained in step vi, the extent of the contribution of the ES cells that contain the disrupted p53 or the Fos gene to the somatic tissues of the transgenic animal can be determined visually by choosing animal strains for the source of the ES cells and blastocyst that have different coat colors.  
     [0034] In a preferred embodiment, the double knockout animals of the invention are mice. In other embodiments of this invention, the animals are rats, guinea pigs, rabbits, or dogs.  
     [0035] In another aspect, the present invention relates to the use of a p53/Fos double knockout animal, in particular a mouse, as a disease model for rhabdomyosarcoma.  
     [0036] In the p53/Fos double knockout mice, rhabdomyosarcomas develop at defined sites with short and reproducible latency.  
     [0037] In a further embodiment, the present invention relates to cells and tissues that carry mutations in both the p53 and the Fos genes. The cells can be primary cells or established cell lines obtained from the transgenic animals of the present invention according to routine methods, i.e. by isolating and disintegrating tissue, in particular tumor tissue, from the p53/Fos double knockout animal and passaging the cells.  
     [0038] Such cells and tissues derived from the animals of the invention, in which the activity of p53 and Fos has been reduced or abolished, are useful in in vitro methods relating to rhabdomyosarcoma and the functional analysis of p53 and Fos in other cell types, e.g. osteoclasts and epithelial cells of the mammary gland or other organs.  
     [0039] In a further aspect, the invention relates to a method for determining whether a compound has therapeutic potential as a drug for the treatment of rhabdomyosarcoma and related disorders, wherein a candidate compound is administered to rhabdomyosarcoma bearing p53/Fos double knockout animals and the ability of the compound to inhibit tumor growth is correlated with its utility as a drug for the treatment of rhabdomyosarcoma.  
     [0040] The test compound can be administered to the non-human p53/Fos double knockout animal in a variety of ways, e.g. orally, in a suitable formulation, by parenteral injection, subcutaneous, intramuscular, or intra-abdominal injection, infusion, ingestion, suppository administration, and skin-patch application. The effect of the compound on the tumor can be determined using methods well known to a person of ordinary skill in the art by analysing the incidence of tumor development, the growth of the tumor, the histopathological features of the tumor, the proliferation of the tumor cells and the apoptosis of the tumor cells.  
     [0041] In a method for screening compounds useful in rhabdomyosarcoma therapy, alternatively to administering the test compound to a p53/Fos double knockout animal, cells derived from such animal are incubated with the compound. A compound that has the ability to inhibit proliferation of the cells is a drug candidate and may be further assessed with regard to its therapeutic benefit for the treatment of rhabdomyosarcoma; e.g. by administering it to a p53/Fos double knockout animal as described above.  
     [0042] Cell lines derived from the p53/Fos double knockout animals are further useful to dissect the physiological and biochemical functions of p53 and Fos.  
     [0043] In a further embodiment, the present invention relates to cell lines derived from the p53/Fos double knockout animal into which a wildtype or modified p53 or a wildtype or modified Fos function has been reintroduced, i.e. that have been transfected with a wild type or modified p53 and/or Fos cDNA according to known methods. Such cell lines can be used for the analysis of p53 and Fos functions in biological processes, e.g. cell differentiation, proliferation, apoptosis, and invasive growth.  
     [0044] In the Examples, the following materials and methods were used:  
     [0045] a) Histological Analysis  
     [0046] Tissues were immersion-fixed in 4% buffered formalin, embedded in paraffin, sectioned and stained with haematoxylin/eosin using standard histological techniques. For immunohistochemistry, antigen retrieval with citrate buffer was performed and tissue sections were incubated with primary antibody: desmin (clone Y-20; Santa Cruz), MyoD (clone 5.8A; DAKO), myogenin (F5D; DAKO), Ki-67 (clone TEC3; DAKO). Immunoreactivity was detected with biotinylated secondary antibodies and the biotin/streptavidin-horseradish peroxidase system (DAKO; Vector) using diaminobenzidine (DAB) as a chromogen.  
     [0047] b) Generation of Rhabdomyosarcoma Cell Lines, Immunofluorescence Staining  
     [0048] Tumor tissues from p53/Fos double knockout mice were mechanically disaggregated and explanted into petri dishes. Pools of cells from three independently isolated tumors were established as cell lines. Experiments were performed at passage 5-8 in culture. For all experiments cells were cultured in DMEM containing 10% fetal calf serum except for differentiation into myotubes which was performed in DMEM containing 5% horse serum. Cell lines were characterized by indirect immunofluorescence staining applying standard techniques using the following primary antibodies: desmin (clone Y-20; Santa Cruz), MyoD (clone 5.8A; DAKO) and α-actinin (Sigma), (reference 56).  
     [0049] c) Retroviral Gene Transfer, Proliferation and Apoptosis Assays  
     [0050] p53 and Fos cDNAs were cloned into pBabe retroviral vectors containing an IRES-EGFP cassette to monitor expression. Cell lines at passage 6 were infected with viruses and selected with 2.5 ug/ml Puromycin according to standard procedures. Expression of p53 and Fos was verified by Northern blot and Western blot analysis. For proliferation assays, 0.5×10 5  cells were plated into 6-well dishes and counted every 24 hours for 6 consecutive days. Duplicates were counted for each cell line and time point. For apoptosis assays, cells were plated in duplicates at 0.5×10 5  cells per 6-well dish and apoptosis was measured after 48 hrs in culture using the Annexin-V cell death detection kit (Roche) according to the manufacturer&#39;s instructions. Representative results from at least two independent experiments for one cell line (pfl) are shown. Similar data were obtained with all three cell lines analyzed.  
     [0051] The experiments of the invention show that mice lacking both p53 and Fos develop high grade sarcomas of the orbital and facial region with almost complete penetrance at 6 months of age. Histologically, these tumors are pleomorphic, express markers of myogenic differentiation, and have a high proliferative activity resembling the embryonal variant of human rhabdomyosarcoma. p53 homozygous and heterozygous mutant mice display enhanced tumor susceptibility and develop a broad range of tumors including rhabdomyosarcoma with low frequency 12,13,28 . The experiments of the invention suggest that the absence of Fos enhances tumorigenesis in p53 mutant mice. This effect appears to be specific to the myogenic lineage since p53; Fos mutant mice otherwise displayed the same tumor spectrum as p53 mutant mice, as only malignant lymphomas were observed in p53; Fos mutant mice up to 6 months of age. Although genetic alterations of the TP53 gene are frequent in sporadic human rhabdomyosarcoma 7,8  and rhabdomyosarcoma is the most common sarcoma type in Li-Fraumeni syndrome 9,10 , a hereditary predisposition to cancer in patients who carry germ line mutations in the TP53 gene 11 , rhabdomyosarcomas develop only with low incidence and long latency in p53 heterozygous and homozygous mutant mice 12,13 . In the absence of Fos, the development of this tumor type was specifically enhanced and accelerated in both p53 heterozygous and homozygous mutant mice. These results suggest that Fos, in addition to its oncogenic potential, may also be a tumor suppressor, at least in cooperation with p53, since tumors have not been observed in Fos single mutant mice 17,18 .  
     [0052] Previous studies have demonstrated that Fos is essential for osteoclast differentiation, acts as an oncogene in vitro and in vivo, mediates transforming signals, and controls invasive growth and angiogenesis during tumor progression 16 . Furthermore, several observations suggest a role for Fos in myogenic differentiation. Fos expression is modulated during myoblast differentiation in vitro 29  and ectopic Fos impairs skeletal muscle differentiation by inhibiting myoblast withdrawal from the cell cycle and fiber fusion during terminal differentiation 29,30 . Consistently, Fos negatively regulates the expression of the myogenic factors MyoD1 and myogenin 29-31 . The data that the formation of rhabdomyosarcomas in p53 mutant mice is drastically enhanced in the absence of Fos suggest that Fos, although being dispensible for muscle development in vivo, may have a function in the suppression of tumorigenesis in the myogenic lineage. To study possible molecular mechanisms, Fos was re-introduced into p53; Fos mutant rhabdomyosarcoma cell lines. Ectopic Fos did not alter cell cycle progression but led to increased cell death of tumor cells suggesting that changes in apoptosis contribute to rhabdomyosarcoma formation in p53; Fos mutant mice. The increase in apoptosis upon Fos re-expression is consistent with previous observations that Fos can have a pro-apoptotic function in various cell types including neurons, retinal photoreceptors, prostatic epithelial cells and B lymphocytes (reviewed in reference 16). It is conceivable that, in the absence of the pro-apoptotic p53 protein, the additional inactivation of Fos may render tumor precursor cells resistent to apoptosis. This may indicate that p53 and Fos act in two different apoptotic pathways, although Fos has been shown to be a target of transcriptional regulation by p53 32 , and that inactivation of two different apoptotic pathways is sufficient for tumor formation in the myogenic lineage.  
     [0053] In the experiments of the invention, a possible role for Fos inactivation in human rhabdomyosarcomas was investigated. PCR analysis of cDNAs prepared from 6 human rhabdomyosarcoma cells lines and 36 primary rhabdomyosarcomas of alveoloar and embryonal type indicated that Fos expression is not completely abolished in human rhabdomyosarcomas. Subsequent sequence analysis revealed wild-type Fos sequences in all tumors (data not shown). These results suggest that mutations in the coding regions of the Fos gene have no major role in the tumorigenesis of human rhabdomyosarcoma. However, it cannot be excluded that changes in Fos expression levels or mutations in Fos target genes might contribute to the development of human rhabdomyosarcomas.  
     EXAMPLE 1  
     [0054] Generation of p53/Fos Double Knockout Mice  
     [0055] p53/Fos double knockout mice were generated, genotyped and phenotypically characterized according to the principle of the methods described for p53 and Fos mutant mouse strains, respectively 13,17 .  
     [0056] More specifically, the mice were obtained as follows:  
     [0057] Targeting vectors were constructed that are capable of disrupting the p53 gene or the Fos gene, respectively, upon homologous recombination, as described in reference 13 or 17. The vectors were introduced, by means of electroporation, into embryonic stem cells derived from a mouse comprising an intact p53 gene and embryonic stem cells derived from a mouse comprising an intact Fos gene. Selection and Southern blot analysis to confirm the desired recombination event were performed as described in references 13 and 17.  
     [0058] Heterozygous ES cells were microinjected into blastocysts, and the blastocysts implanted into the uterus of pseudopregnant female C57BL/6 mice. Animals were delivered from the females, chimeric animals were identified and bred to wildtype animals. From the offspring, heterozygous p53± and Fos± mice were selected as described in references 13 and 17 and the heterozygous mice were intercrossed to generate homozygous p53−/− and Fos−/− mice. These mice were obtained at the expected Mendelian number of 25%.  
     [0059] Homozygous p53−/− and Fos−/− animals were intercrossed to generate p53±; Fos± animals. These mice were mated to p53−/− mice to obtain p53/−/−; Fos animals, which were intercrossed to generate p53/Fos double knockout animals, which were obtained from this cross at the expected Mendelian number of 25%.  
     [0060] p53/Fos double knockout mice were maintained on a mixed (129 Sv×C57BL/6) genetic background. Mice were inspected once a week for morbidity and obvious tumor formation. Necropsies were performed on moribund mice.  
     EXAMPLE 2  
     [0061] p53/Fos Double Knockout Mice Develop Rhabdomyosarcoma  
     [0062] To study possible cooperative effects between germline mutations in the p53 and Fos genes, p53−/− and Fos−/− mice were intercrossed to generate p53/Fos compound-mutant mice. These mice were obtained at almost Mendelian numbers and were phenotypically indistinguishable from Fos−/− mice (data not shown). However, p53−/−; Fos−/− mice started to develop tumors of the facial and orbital regions at 10 weeks of age and tumor penetrance was 93% at 25 weeks of age (FIG. 1). Tumors at these sites also grew in p53±; Fos−/− mice, although with lower penetrance, but were not observed in p53−/−, Fos± and p53±;Fos± mice (FIG. 1). Histological analysis of tumors revealed broad anastomosing fascicles of tumor cells with small areas of necrosis and invasive growth into adjacent muscle and fat (FIGS. 2 a - 2   b ). Tumor cells were polygonal or elongated and had pleomorphic nuclei and abundant eosinophilic cytoplasm which displayed cross-striations in few neoplastic cells (FIG. 2 c ). Mitotic figures were numerous and staining for the cell cycle associated Ki-67 antigen revealed nuclear labeling in many tumor cells indicating high proliferative activity (FIG. 2 d ). Immunoreactivity for makers of myogenic differentiation including desmin, myogenin, and MyoD1 was observed in most tumor cells (FIGS. 2 e - 2   f ). Tumor cell morphology and maker profile were diagnostic of the embryonal type of rhabdomyosarcoma. These data suggest that p53/Fos compound-mutant mice develop embryonal rhabdomyosarcoma with high penetrance and short latency in the orbital and facial regions.  
     EXAMPLE 3  
     [0063] Normal Muscle Development in p53/Fos Double Knockout Mice  
     [0064] To analyse whether inactivation of the p53 and Fos genes interferes with normal muscle development, histological analysis of orbital, facial and femoral muscles was performed in 6 to 10-week-old, tumor-free p53−/−; Fos−/− mice. No morphological abnormalities were found and comparable numbers of satellite cells and Ki-67 reactive cells were present in muscles derived from p53−/−; Fos−/− and p53±; Fos± mice (FIGS. 3 a - 3   d,  data not shown). Therefore, muscle tumor development in p53/Fos compound-mutant mice does not reflect a general defect in muscle cell differentiation and increased numbers of progenitor cells.  
     EXAMPLE 4  
     [0065] Fos Regulates Apoptosis in Rhabdomyosarcoma Cells in Vitro  
     [0066] Enhanced rhabdomyosarcoma formation in Fos-deficient p53 mutant mice suggested that Fos might regulate cell differentiation, proliferation or apoptosis in the myogenic lineage. To address and distinguish between these possibilities, three tumor cell lines were established. All cell lines showed characteristic myoblast morphology and expressed the muscle-specific markers MyoD and desmin (FIG. 4 a ). When grown at high density and in medium containing 5% horse serum, cells fused to form differentiated myotubes with cytoplasmic cross-striations, visualized by α-actinin staining (FIG. 4 b ). These results provide additional evidence for the rhabdomyoblastic phenotype of the tumors in p53−/−; Fos−/− mice.  
     [0067] To study the molecular mechanisms underlying tumor development, the effects of re-expressing p53 and Fos in double mutant cell lines were analyzed by using retroviral gene transfer. Expression of p53 resulted in massive cell death and no cell lines could be established (data not shown). This is in agreement with p53 function to induce cell cycle arrest and apoptosis when over-expressed in cultured cells 27 . In contrast, the inventors were able to generate p53−/−; Fos−/− rhabdomyosarcoma cell lines re-expressing Fos at intermediate levels, as judged by Northern blot and Western blot analysis Surprisingly, cell lines expressing Fos grew significantly more slowly than non-infected or empty-vector controls, resulting in a 30-50% decrease in cell numbers after  6  days in culture (FIG. 4 c,  data not shown).  
     [0068] Whereas cell cycle analysis did not reveal significant changes in cell cycle progression (data not shown), a 2 to 3-fold increase in the number of apoptotic cells upon ectopic Fos expression was found (FIG. 4 d ). These data show that re-expression of Fos in p53/Fos -deficient muscle tumor cells leads to increased cell death without affecting cell proliferation and suggest that changes in apoptosis contribute to muscle tumor development in p53−/−; Fos−/− mice.  
     EXAMPLE 5  
     [0069] Use of p53/Fos Double Knockout Mice as a Rhabdomyosarcoma Model  
     [0070] p53/Fos double knockout mice in a mixed (129 Sv×C57BL/6) genetic background develop rhabdomyosarcomas with complete penetrance. When the tumors are established, treatment with the test compound is started (5-8 mice per test group). The test compounds are administered at various concentrations. The test compound is administered to the test animals in a variety of ways, e.g. orally, in a suitable formulation, by parenteral injection, subcutaneous, intramuscular, or intra-abdominal injection, infusion, ingestion, suppository administration, and skin-patch application. The effect of the compound on the tumor is determined using generally known methods, in particular, by measuring the tumor volume every other week, by histological analysis of tumor tissue harvested at various time points during the experiment, by assessing tumor cell proliferation by immunohistochemical staining for the Ki-67 antigen (reference 57) and by determining the number of apoptotic tumor cells using the TUNEL in situ end-labeling technique (reference 57).  
     EXAMPLE 6  
     [0071] Use of p53/Fos Double Knockout Rhabdomyosarcoma Cell Lines  
     [0072] Tumor cells obtained from p53/Fos double knockout rhabdomyosarcomas are injected subcutaneously in the dorsal flank region of nude mice. When the grafts are established after 7 days, treatment with the test compound is started (8-10 mice per test group). The test compounds are administered at various concentrations. The test compound is administered to the test animal in a variety of ways, e.g. orally, in a suitable formulation, by parenteral injection, subcutaneous, intramuscular, or intra-abdominal injection, infusion, ingestion, suppository administration, and skin-patch application. The effect of the compound on the tumor is determined using known methods, e.g. by measuring the tumor volume every other day, by histological analysis of tumor tissue harvested at various time points during the experiment, by assessing tumor cell proliferation by immunohistochemical staining for the Ki-67 antigen (reference 57) and by determining the number of apoptotic tumor cells using the TUNEL in situ end-labeling technique (reference 57).  
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