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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Science. Author manuscript; available in PMC Apr 19, 2011.
Published in final edited form as:
PMCID: PMC3079436
NIHMSID: NIHMS263632
Mouse Tumor Model for Neurofibromatosis Type 1
Kristine S. Vogel,1* Laura J. Klesse,1* Susana Velasco-Miguel,1* Kimberly Meyers,1* Elizabeth J. Rushing,2 and Luis F. Parada1
1 Center for Developmental Biology, University of Texas Southwestern Medical Center, 6000 Harry Hines Blvd., Dallas, TX 75235–9133, USA
2 Department of Pathology, University of Texas Southwestern Medical Center, 6000 Harry Hines Blvd., Dallas, TX 75235–9133, USA
To whom correspondence should be addressed. parada/at/utsw.swmed.edu
*These authors contributed equally to this work.
Present address: Department of Cell Biology and Anatomy, Louisiana State University Medical Center, New Orleans, LA 70112, USA.
Abstract
Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder characterized by increased incidence of benign and malignant tumors of neural crest origin. Mutations that activate the protooncogene ras, such as loss of Nf1, cooperate with inactivating mutations at the p53 tumor suppressor gene during malignant transformation. One hundred percent of mice harboring null Nf1 and p53 alleles in cis synergize to develop soft tissue sarcomas between 3 and 7 months of age. These sarcomas exhibit loss of heterozygosity at both gene loci and express phenotypic traits characteristic of neural crest derivatives and human NF1 malignancies.
Mutations in tumor suppressor genes are common events in human cancers (1). Individuals with a mutation in one copy of the NF1 gene develop benign cutaneous neurofibromas, plexiform neurofibromas, café-au-lait spots, and axillary freckling (2). Through loss of heterozygosity (LOH) at the NF1 locus, patients with neurofibromatosis type 1 are at increased risk of developing malignancies of neural crest derivatives, including malignant peripheral nerve sheath tumors (MPNSTs), malignant Triton tumors (MTTs), and pheochromocytomas (2, 3). MPNSTs and MTTs arise from plexiform neurofibromas and frequently are associated with mutation or loss of the p53 tumor suppressor gene (2, 4). The protein product of the NF1 gene neurofibromin is a guanosine triphosphatase (GTPase)–activating protein (GAP) that can negatively regulate p21ras signaling (5). Mutations that activate Ras cooperate with mutations that inactivate p53 in a number of transformation assays and models of tumorigenesis (6).
The Nf1 and p53 genes are linked in humans and in mice (7). To determine whether null mutations in Nf1 and p53 cooperate to accelerate tumorigenesis in vivo, we generated a recombinant mouse strain harboring inactivated Nf1 and p53 alleles linked on mouse chromosome 11. We accomplished this with intercrosses of trans-Nf1+/−:p53+/− compound heterozygotes or through crosses with p53+/− heterozygotes. The integrity of the recombinant cis-Nf1:p53 chromosome was established by genomic Southern blot analysis and by polymerase chain reaction (PCR). All progeny of cis-Nf1+/−:p53+/− test crosses to wild-type animals were either compound heterozygotes or entirely wild type, which confirms the integrity of the double-mutant chromosome.
Mice that are heterozygous for the Nf1 mutation alone are at increased risk of developing pheochromocytomas and myeloid leukemias between 18 and 28 months of age (8). Loss of one or both copies of the p53 gene leads to accelerated tumorigenesis. Thus, p53−/− mice develop lymphomas and hemangiosarcomas by 6 months of age, whereas p53+/− mice exhibit a predominance of osteosarcomas that arise later, after 9 months (9). We compared the mortality of trans- and cis-Nf1+/−:p53+/− compound heterozygotes with that of p53+/− and p53−/− mice on a mixed C57BL/6/129sv background (Fig. 1A). In addition, we analyzed survival of the null p53 genotype together with Nf1 heterozygosity (cis-Nf1+/−:p53−/−). Introduction of one null Nf1 allele accelerated tumor formation and mortality in the context of both the p53+/− and the p53−/− backgrounds (Fig. 1A). Both cis- and trans-Nf1:p53 mice developed tumors (primarily sarcomas) and died at 15 and 25 weeks, respectively, whereas the p53+/− mice survived beyond 37 weeks of age. Similarly, Nf1+/−:p53−/− mice began to develop tumors (primarily lymphomas) and die as early as 3 weeks of age, whereas p53−/− mice did not develop lymphomas before 18 weeks. Mice homozygous for the p53 and the Nf1 mutations are embryonic lethal and exhibit a high incidence of exencephaly (10).
Fig. 1
Fig. 1
(A) Mice were maintained in specific pathogen-free conditions and observed daily for evidence of illness or tumor formation. If palpable tumors exceeded 1 cm in diameter or interfered with feeding and grooming, mice were sacrificed. Moribund mice with (more ...)
To determine whether LOH had occurred at both loci in the cis-Nf1:p53 tumors, we used PCR-based assays to identify the wild-type and neo-disrupted alleles (9, 11). As shown in Fig. 1B, 22 of 31 (71%) of the soft tissue sarcomas exhibited LOH at both loci; however, this may be an underestimate because of the difficulties of isolating pure tumor cells from surrounding normal tissue. Lymphomas isolated from trans-Nf1+/−:p53+/− and cis-Nf1+/−:p53−/− mice did not exhibit LOH at the Nf1 locus (Fig. 1B, tumor 4).
To determine whether introduction of the Nf1 mutation altered the tumor spectrum in cis-Nf1:p53 mice, we examined the pathological and phenotypic characteristics of the tumors. Histological analysis revealed that the cis-Nf1+/−:p53+/− mice exhibited a significant incidence of soft tissue sarcomas that appeared to be malignant based on their dedifferentiated morphology, disrupted tissue organization, and increased number of mitotic figures. Of 66 characterized tumors, 51 (77%) were sarcomas, 9 (14%) were lymphomas, 10 (8%) were carcinomas, and 2 were neuroblastomas. We subjected the soft tissue sarcomas to further analyses with antibodies specific for myoglobin, desmin, S100, and smooth muscle markers. On the basis of histopathological criteria and immunohistochemical analysis, we classified 11 tumors as MTTs, 31 as MPNSTs, 2 as leiomyosarcomas, 5 as rhabdomyosarcomas, and 4 as malignant fibrohistiocytomas (Fig. 1C). At least 3 of these malignant tumor types—MTTs, MPNSTs, and leiomysarcomas—occur with increased frequency in human NF1 patients (2). Thus, the presence of the Nf1 mutation, which alone is weakly tumorigenic, accelerates tumorigenesis and alters the tumor spectrum in the context of the p53+/− background.
Although NF1 is generally considered to be a neoplastic disorder of neural crest–derived cells, certain malignancies (rhabdomyosarcomas, leiomyosarcomas) have been associated with a mesodermal, rather than a neuroectodermal, origin (2). To address the origin of NF1 malignancies, we established permanent cell lines from over 70 independent cis-Nf1:p53 tumors. All tumor cell lines tested exhibited LOH at both Nf1 and p53 loci after only two or three passages in vitro to remove contaminating untransformed cells (Fig. 2A). As shown in Fig. 2B, reverse transcriptase (RT)–PCR analysis of representative cis-Nf1:p53 tumor cell lines revealed a spectrum of mRNAs encoding early neural crest and Schwann cell markers (12), including the transcription factor Pax-3, the low-affinity nerve growth factor (NGF) receptor p75, cell adhesion molecules N-CAM and L1, GAP-43, and the calcium binding protein S100. Many of the cell lines expressed mRNAs characteristic of more differentiated Schwann cells, including Krox-20 and the myelin-specific proteins P0 and myelin basic protein (12). In addition to these glial markers, the MTT lines expressed mRNAs characteristic of myogenic differentiation, including desmin, MyoD, SM-22, and calponin (12). Immunocytochemical and immunoblot analyses also demonstrated expression of p75, S100, GFAP, GAP-43, smooth muscle actin (SMA), calponin, neurofilament, and peripherin proteins in many of the cis-Nf1: p53 tumor cell lines (Table 1). Tumor cell lines isolated from p53−/− sarcomas did not express proteins characteristic of Schwann cell or neuronal differentiation in these assays (Table 1). Taken together, these data suggest that cis-Nf1:p53 cell lines derive from a neural crest stem cell that can follow Schwann cell, smooth muscle, or autonomic neuronal differentiation pathways (13). Thus, although the sarcomas that arise in cis-Nf1:p53 mice may differ in precise pathological classification, molecular analyses of the tumor cell lines are consistent with a common neural crest, rather than mesodermal, origin.
Fig. 2
Fig. 2
Expression of neural crest markers and LOH in cis-Nf1:p53 tumor cell lines (19). (A) DNAs from nine representative tumor lines that included MPNST, MTT, RMS, and LMS were assessed for LOH at Nf1 and p53. All tumor cell lines had complete LOH at both genes. (more ...)
Table 1
Table 1
Expression of neural crest, Schwann cell, smooth muscle, and neuronal markers in cis-Nf1: p53 tumor cell lines. Up to 41 tumor cell lines isolated from 10 cis-Nf1:p53 sarcomas were processed for immunocytochemistry. Immunostaining patterns for SMA, S100, (more ...)
Generation of mouse mutants at suspected tumor suppressor loci has provided important information about the underlying mechanisms of tumor formation (14). It has therefore been perplexing that the original knockouts of the Nf1 gene, modeled after the most frequently observed genetic neoplastic disorder in humans, did not afford a useful model for the benign neurofibromas or the malignant neurofibrosarcomas in NF1 patients (8, 11). A possible explanation is that the period of embryonic development, cell differentiation, and growth is significantly reduced in the mouse. This temporal difference may shorten the window of opportunity for acquisition of additional mutations within a given cell or reduce the size of the target cell population. Our data, and those of Cichowski and colleagues (15), indicate that an additional mutation in the p53 tumor suppressor gene is required to predispose Nf1+/− mouse neural crest–derived cells to malignant transformation. Moreover, our molecular and immunochemical analyses provide evidence that NF1-associated rhabdomyosarcomas and leiomyosarcomas may be of neural crest origin and provide a possible explanation for the development of MTTs. Cell lines isolated from MTTs express both Schwann cell and smooth muscle markers, often in the same tumor cell (16). The phenotype of these tumors is consistent with immortalization of a pluripotent neural crest stem cell, which under normal circumstances adopts a glial, smooth muscle, or neuronal fate (13). Throughout development and adulthood, specific combinations of tumor suppressor genes may cooperate to control proliferation, differentiation, and survival in different cell lineages.
Acknowledgments
Supported by NIH grant NS34296 and the National Neurofibromatosis Foundation (L.F.P.) and by a grant from the Cancer Association of Greater New Orleans (K.S.V.). We thank T. Jacks and colleagues for sharing unpublished results, S. Colvin and J. Richardson for early assistance with histopathology, and members of the Parada lab for helpful discussions.
1. Hinds PW, Weinberg RA. Curr Opin Genet Dev. 1994;4:135. [PubMed]Knudson AG. Proc Natl Acad Sci USA. 1993;90:10914. [PubMed]
2. Bernardis V, et al. Digestion. 1999;60:82. [PubMed]Gutmann DH, et al. JAMA. 1997;278:51. [PubMed]Ishikazi Y, et al. Surgery. 1992;111:706. [PubMed]Riccardi VM, Womack JE, Jacks T. Am J Pathol. 1994;145:994. [PubMed]
3. Guha A, et al. Oncogene. 1996;12:507. [PubMed]Legius E, Marchuk DA, Collins FS, Glover TW. Nature Genet. 1993;3:122. [PubMed]Xu W, et al. Genes Chromosomes Cancer. 1992;4:337. [PubMed]
4. Halling KC, et al. Anat Pathol. 1996;106:282. [PubMed]Jhanwar SC, Chen Q, Li FP, Brennan MF, Woodruff JM. Cancer Genet Cytogenet. 1994;78:138. [PubMed]Menon AG, et al. Proc Natl Acad Sci USA. 1990;87:5435. [PubMed]
5. Ballester R, et al. Cell. 1990;63:851. [PubMed]Buchberg AM, Cleveland LS, Jenkins NA, Copeland NG. Nature. 1990;347:291. [PubMed]Martin GA, et al. Cell. 1990;63:843. [PubMed]Xu GF, et al. Cell. 1990;62:599. [PubMed]
6. Eliyahu D, Raz A, Gruss P, Givol D, Oren M. Nature. 1984;312:646. [PubMed]Hundley JE, et al. Mol Cell Biol. 1997;17:723. [PubMed]Kemp CJ, Burns PA, Brown K, Nagase H, Balmain A. Cold Spring Harbor Symp Quant Biol. 1994;54:427. [PubMed]Parada LF, Land H, Weinberg RA, Wolf D, Rotter V. Nature. 1984;312:649. [PubMed]Tanaka M, Omura K, Watanabe Y, Oda Y, Nakanishi I. J Surg Oncol. 1994;57:57. [PubMed]
7. Copeland NG, et al. Science. 1993;262:57. [PubMed]
8. Jacks T, et al. Nature Genet. 1994;7:353. [PubMed]
9. Donehower LA, et al. Nature. 1992;356:215. [PubMed]Jacks T, et al. Curr Biol. 1994;4:1. [PubMed]
10. Vogel KS, Parada LF. Mol Cell Neurosci. 1998;11:19. [PubMed]
11. Brannan CI, et al. Genes Dev. 1994;8:1019. [PubMed]
12. Kioussi C, Gruss P. Trends Genet. 1996;12:84. [PubMed]Stemple DL, Anderson DJ. Cell. 1992;71:973. [PubMed]
13. Morrison SJ, White PM, Zock C, Anderson DJ. Cell. 1999;96:737. [PubMed]Shah NM, Groves AK, Anderson DJ. Cell. 1996;85:331. [PubMed]
14. Fero ML, Randel E, Gurley KE, Roberts JM, Kemp CJ. Nature. 1998;396:177. [PubMed]Giovannini M, et al. Genes Dev. 1999;13:978. [PubMed]Jacks T. Annu Rev Genet. 1996;30:603. [PubMed]Kamijo T, Bodner S, van de Kamp E, Hrandle D, Sherr CJ. Cancer Res. 1999;59:2217. [PubMed]McClatchey AI, et al. Genes Dev. 1998;12:1121. [PubMed]Orlow I, et al. Int J Oncol. 1999;15:17. [PubMed]Prolla TA, et al. Nature Genet. 1998;18:276. [PubMed]Zhu Y, Richardson JA, Parada LF, Graff JM. Cell. 1998;94:703. [PubMed]
15. Cicowski K, et al. Science. 1999;286:2172. [PubMed]
16. Vogel KS. unpublished data.
17. Nf1+/−:p53+/− and Nf1+/−:p53−/− progeny of trans-Nf1+/−:p53+/− crosses were bred with mice that were wild type at both loci. A founder male with the genotype Nf1+/−:p53−/− proved to harbor a stable recombinant chromosome 11 after test crosses to three wild-type females. The recombinant (cis configuration) chromosome was maintained on a mixed C57B6/129sv background, as were mice of other genotypes used for mortality studies. For genotyping, tail DNA was subjected to two separate three-primer PCRs, one for Nf1 (11) and one for p53 (8). Samples of macroscopically recognizable tumor were fixed in 10% buffered formalin, embedded in paraffin, and stained with hematoxylin and eosin. All immunostaining was done at room temperature on a BioTek Solutions Techmate automated immunostainer (Ventana BioTek Systems, Tucson, AZ). Buffers, blocking solutions, secondary antibodies, avidin-biotin complex reagents, chromogen, and hematoxylin counterstain were used as supplied in the Chem-Mate secondary detection kit (Ventana BioTek Systems). Optimum primary antibody dilutions were predetermined with known positive control tissues. A known positive control section was included in each run to ensure proper staining. Paraffin sections were cut at 3 μmm on a rotary microtome and mounted on positively charged glass slides (POP100 capillary gap slides; Ventana BioTek Systems) at pH 6.8. Sections were incubated in unlabeled blocking antibody solution for 5 to 10 min to block nonspecific binding of secondary antibody and then incubated for 25 min with either primary antibody (S100 protein; SMA, 1:400; myoglobin, 1:60,000; desmin, 1:100; DAKO, Carpenteria, CA) or with buffer alone as a negative reagent control. After washing in buffer, sections were incubated for 25 min with biotinylated polyvalent secondary antibody solution (containing goat antibodies to rabbit, mouse, and rat immunoglobulin). After another buffer wash, sections were incubated with three changes, 2.5 min each, of 3% H2O2 to inhibit endogenous tissue peroxidase activity and again washed in buffer. Sections were then incubated for 25 min with freshly prepared horseradish peroxidase–conjugated avidin-biotin complex. Sections were then washed in buffer and incubated with three changes, 5 min each, of a freshly prepared mixture of diaminobenzidine (DAB) and H2O2 in buffer, followed by washing in buffer and then water. Sections were then counterstained with hematoxylin, dehydrated in a graded series of ethanols and xylene, and coverslipped. Slides were reviewed by light microscopy. Positive reactions with DAB were identified as a dark brown reaction product. Sections were photographed on a Nikon Optiphot microscope (Nikon Instruments, Melville, NY).
18. Characterization of soft tissue sarcomas in cis-Nf1:p53 mice. Histopathological examination was performed on all tumors obtained from animals at the termination of the experiment. Soft tissue tumors were classified in accordance with 1994 World Health Organization criteria. Tumor masses were removed under sterile conditions and measured. Small pieces of tumor tissue were removed for histological processing, DNA isolation, and establishment of tumor cell lines. Tumor samples were fixed in either Bouin’s fixative (for hematoxylin and eosin staining) or 10% formalin (for immunohistochemistry) and processed for paraffin embedding and sectioning at 5 to 7 μmm. Tumor sections were immunostained with S100 antibody (anti-S100) (Novocastra), anti-desmin (Signet), anti-α-actin (Boehringer), or anti-myoglobin (Signet) and visualized by the Vectastain Elite ABC peroxidase method (Vector). Tumor DNA was genotyped by three-primer PCRs as described above.
19. Nf1/p53 tumor-derived cell lines were isolated as follows: Overlying skin and hair were removed from the tumor mass and then the tumor mass immersed briefly in Dulbecco’s phosphate-buffered saline and in a solution of penicillin and streptomycin (Gibco). Small pieces of the tumor mass were minced in Dulbecco’s modified Eagle’s medium (DMEM) [supplemented with 10% heat-inactivated fetal calf serum (HIFCS), penicillin and streptomycin, and nonessential amino acids] (Gibco) with watchmaker’s forceps and fine curved scissors. Tumor pieces were allowed to attach to 60-mm plastic tissue culture dishes, and clonal cell lines were established from tumor outgrowths after four to six passages.
20. For immunocytochemistry, tumor cells grown on coverslips in 2% HIFCS/DMEM were fixed in 4% paraformaldehyde and exposed to antibodies diluted overnight at 4°C as follows: p75 (Chemicon), 1:200; c-neu (Santa Cruz), 1:200; GAP-43, 1:250; S100 (Novocastra), 1:200; GFAP (Santa Cruz), 1:200; SMA (Sigma), 1:400; calponin (Sigma), 1:400; neurofilament (Chemicon), 1:200. To visualize bound antibody, we used Vectastain Elite ABC peroxidase kits (Vector Laboratories), specific for goat, rabbit, or mouse primary antibodies, according to the manufacturer’s instructions. We also tested all primary antibodies with Cy3- or fluorescein isothiocyanate–conjugated fluorescent secondary antibodies (Chemicon, Sigma) to determine which method yielded little or no background staining. For immunoblots, proteins were extracted from tumor cells grown in 162-cm2 culture flasks in Nonidet P-40 lysis buffer containing protease inhibitors (Sigma). Insoluble (SMA, GFAP) and soluble (c-neu, S100) fractions were subjected to SDS–polyacrylamide gel electrophoresis on 8% to 10% minigels. After protein transfer, nitrocellulose membranes were blocked with 2% bovine serum albumin in tris-buffered saline and incubated with primary antibody overnight at 4°C. Specific protein bands were visualized with Vectastain Elite ABC peroxidase kits (Vector Laboratories), or with Immun-Star kits (Bio-Rad), according to the manufacturers’ instructions.