Excellent translational research in oncology: A journey towards novel and more effective anti-cancer therapies

Comprehensive Cancer Centres (CCCs) serve as critical drivers for improving cancer survival. In Europe, we have developed an Excellence Designation System (EDS) consisting of criteria to assess "excellence" of CCCs in translational research (bench to bedside and back), with the expectation that many European CCCs will aspire to this status.

Frat oncoproteins act at the crossroad of canonical and noncanonical Wnt-signaling pathways

Wnt-signal transduction is critical for development and tissue homeostasis in a wide range of animal species and is frequently deregulated in human cancers. Members of the Frat/GBP family of glycogen synthase kinase 3beta (Gsk3b)-binding oncoproteins are recognized as potent activators of the Wnt/beta-catenin pathway in vertebrates. Here, we reveal a novel, Gsk3b-independent function of Frat converging on the activation of JNK and AP-1. Both these have been used as readouts for the noncanonical Frizzled/PCP pathway, which controls polarized cell movements and the establishment of tissue polarity. We find that Frat synergizes with Diversin, the mammalian homolog of the Drosophila PCP protein diego, in the activation of JNK/AP-1 signaling. Importantly, Frat mutants deficient for binding to Gsk3b retain oncogenic activity in vivo, suggesting that Wnt/beta-catenin-independent events contribute to Frat-induced malignant transformation. The observed activities of Frat are reminiscent of the dual function of Dishevelled in the Wnt/beta-catenin and Frizzled/PCP pathways and suggest that Frat may also function to bridge canonical and noncanonical Wnt pathways.

Retroviral insertional mutagenesis: past, present and future

Retroviral insertion mutagenesis screens in mice are powerful tools for efficient identification of oncogenic mutations in an in vivo setting. Many oncogenes identified in these screens have also been shown to play a causal role in the development of human cancers. Sequencing and annotation of the mouse genome, along with recent improvements in insertion site cloning has greatly facilitated identification of oncogenic events in retrovirus-induced tumours. In this review, we discuss the features of retroviral insertion mutagenesis screens, covering the mechanisms by which retroviral insertions mutate cellular genes, the practical aspects of insertion site cloning, the identification and analysis of common insertion sites, and finally we address the potential for use of somatic insertional mutagens in the study of nonhaematopoietic and nonmammary tumour types.

Genotype-phenotype relationships in a mouse model for human small-cell lung cancer

Lung tumors are usually classified into small-cell lung cancer (SCLC) or non-SCLC (NSCLC) depending on their pathological and histological characteristics. SCLC is defined not only by its characteristic neuroendocrine differentiation, aggressiveness, and metastatic potential, but also by a specific set of genetic aberrations, including the loss of the tumor suppressor genes p53 and Rb1 and the amplification of any member of the Myc family of oncogenes. We have previously described a mouse model of SCLC by somatic conditional disruption of Trp53 and Rb1 genes that closely resembles the human condition. Based on the possibility to study early tumor lesions and to culture and subclone progressed tumors and metastases, we discuss here a strategy to define genotype-phenotype relationships that can explain the underlying biology of lung neuroendocrine tumors. We have found that tumors may be constituted by genetically variant cell populations, which might represent different progression stages. Interestingly, we observed L-myc amplification and Ascl-1 expression in those populations showing neuroendocrine differentiation. Non-neuroendocrine cell populations from the same tumors did not show L-myc amplification nor Ascl-1 expression. We propose that this genetic divergence can play a relevant role in the definition of some phenotypic characteristics like metastasis potential or chemoresistance.

Synergistic tumor suppressor activity of BRCA2 and p53 in a conditional mouse model for breast cancer

Inheritance of one defective BRCA2 allele predisposes humans to breast cancer. To establish a mouse model for BRCA2-associated breast cancer, we generated mouse conditional mutants with BRCA2 and/or p53 inactivated in various epithelial tissues, including mammary-gland epithelium. Although no tumors arose in mice carrying conditional Brca2 alleles, mammary and skin tumors developed frequently in females carrying conditional Brca2 and Trp53 alleles. The presence of one wildtype Brca2 allele resulted in a markedly delayed tumor formation; loss of the wildtype Brca2 allele occurred in a subset of these tumors. Our results show that inactivation of BRCA2 and of p53 combine to mediate mammary tumorigenesis, and indicate that disruption of the p53 pathway is pivotal in BRCA2-associated breast cancer.

Mouse model for lung tumorigenesis through Cre/lox controlled sporadic activation of the K-Ras oncogene

The onset of human lung cancer occurs through sequential mutations in oncogenes and tumor suppressor genes. Mutations in K-Ras play a prominent role in human non-small cell lung cancer. We have developed a mouse lung tumor model in which K-Ras can be sporadically activated through Cre-lox mediated somatic recombination. Adenoviral mediated delivery of Cre recombinase in lung epithelial cells gave rise to rapid onset of tumorigenesis, yielding pulmonary adenocarcinomas with 100% incidence after a short latency. The lung tumor lesions shared many features with human non-small cell lung cancer. Our data show that sporadic expression of the K-Ras oncogene is sufficient to elicit lung tumorigenesis. Therefore this model has many advantages over conventional transgenic models used thus far.

Loss of p16Ink4a confers susceptibility to metastatic melanoma in mice

CDKN2A (INK4a/ARF) is frequently disrupted in various types of human cancer, and germline mutations of this locus can confer susceptibility to melanoma and other tumours. However, because CDKN2A encodes two distinct cell cycle inhibitory proteins, p16INK4a and p14ARF (p19Arf in mice), the mechanism of tumour suppression by CDKN2A has remained controversial. Genetic disruption of Cdkn2a(p19Arf) (hereafter Arf) alone predisposes mice to tumorigenesis, demonstrating that Arf is a tumour-suppressor gene in mice. We mutated mice specifically in Cdkn2a(p16Ink4a) (hereafter Ink4a). Here we demonstrate that these mice, designated Ink4a*/*, do not show a significant predisposition to spontaneous tumour formation within 17 months. Embryo fibroblasts derived from them proliferate normally, are mortal, and are not transformed by oncogenic HRAS. The very mild phenotype of the Ink4a*/* mice implies that the very strong phenotypes of the original Ink4a/ArfDelta2,3 mice were primarily or solely due to loss of Arf. However, Ink4a*/Delta2,3 mice that are deficient for Ink4a and heterozygous for Arf spontaneously develop a wide spectrum of tumours, including melanoma. Treatment of these mice with the carcinogen 7,12-dimethylbenzanthracene (DMBA) results in an increased incidence of melanoma, with frequent metastases. Our results show that, in the mouse, Ink4a is a tumour-suppressor gene that, when lost, can recapitulate the tumour predisposition seen in humans.

Growth inhibition and DNA damage induced by Cre recombinase in mammalian cells

The use of Cre/loxP recombination in mammalian cells has expanded rapidly. We describe here that Cre expression in cultured mammalian cells may result in a markedly reduced proliferation and that this effect is dependent on the endonuclease activity of Cre. Chromosome analysis after Cre expression revealed numerous chromosomal aberrations and an increased number of sister chromatid exchanges. Titration experiments in mouse embryo fibroblasts with a ligand-regulatable Cre-ER(T) show that toxicity is dependent on the level of Cre activity. Prolonged, low levels of Cre activity permit recombination without concomitant toxicity. This urges for a careful titration of Cre activity in conditional gene modification in mammalian cells.

Mice doubly deficient for the Polycomb Group genes Mel18 and Bmi1 reveal synergy and requirement for maintenance but not initiation of Hox gene expression

Polycomb group genes were identified as a conserved group of genes whose products are required in multimeric complexes to maintain spatially restricted expression of Hox cluster genes. Unlike in Drosophila, in mammals Polycomb group (PcG) genes are represented as highly related gene pairs, indicative of duplication during metazoan evolution. Mel18 and Bmi1 are mammalian homologs of Drosophila Posterior sex combs. Mice deficient for Mel18 or Bmi1 exhibit similar posterior transformations of the axial skeleton and display severe immune deficiency, suggesting that their gene products act on overlapping pathways/target genes. However unique phenotypes upon loss of either Mel18 or Bmi1 are also observed. We show using embryos doubly deficient for Mel18 and Bmi1 that Mel18 and Bmi1 act in synergy and in a dose-dependent and cell type-specific manner to repress Hox cluster genes and mediate cell survival of embryos during development. In addition, we demonstrate that Mel18 and Bmi1, although essential for maintenance of the appropriate expression domains of Hox cluster genes, are not required for the initial establishment of Hox gene expression. Furthermore, we show an unexpected requirement for Mel18 and Bmi1 gene products to maintain stable expression of Hox cluster genes in regions caudal to the prospective anterior expression boundaries during subsequent development.

A highly efficient ligand-regulated Cre recombinase mouse line shows that LoxP recombination is position dependent

Conditional gene inactivation using the Cre/loxP system is widely used, but the difficulty in properly regulating Cre expression remains one of the bottlenecks. One approach to regulate Cre activity utilizes a mutant estrogen hormone-binding domain (ERT) to keep Cre inactive unless the non-steroidal estrogen analog 4-hydroxytamoxifen (OHT) is present. Here we describe a mouse strain expressing Cre-ERT from the ubiquitously expressed ROSA26 (R26) locus. We demonstrate efficient temporal and spatial regulation of Cre recombination in vivo and in primary cells derived from these mice. We show the existence of marked differences in recombination frequencies between different substrates within the same cell. This has important consequences when concurrent switching of multiple alleles within the same cell is needed, and highlights one of the difficulties that may be encountered when using reporter mice as indicator strains.

Mouse models for sporadic cancer

Much of the advancement in mouse models for cancer during the past 2 decades can be attributed to our increasing capacity to specifically modify the mouse germ line. The first generations of oncomice and tumor-suppressor gene knockouts are now being succeeded by regulatable or conditional mouse tumor models, which can be utilized more effectively to establish correlations between distinct genetic lesions and specific tumor characteristics and to design and improve therapeutic intervention strategies. In this review we try to give the reader a flavor of how the latest reagents can be utilized.

A novel tetracycline-dependent transactivator with E2F4 transcriptional activation domain

A tetracycline-controlled gene expression system provides a powerful tool to dissect the functions of gene products. However, it often appears difficult to establish cell lines or transgenic animals stably expressing tetracycline-dependent transactivators, possibly as a result of toxicity of the transactivator domains used. In order to overcome this problem, we developed a novel tetracycline-dependent transactivator that works efficiently in mammalian cells. This transactivator is a fusion of the tet reverse repressor mutant and the transcriptional activating domain of human E2F4, which is ubiquitously expressed in vivo. We demonstrate here that this tetracycline-regulated gene expression system provides a two log transcriptional activation in mammalian cells as assessed by northern blot and luciferase analyses. Combining this system with green fluorescent protein reporter systems or microarray gene expression profiling will facilitate the study of gene function.

p27kip1-independent cell cycle regulation by MYC

MYC transcription factors are potent stimulators of cell proliferation. It has been suggested that the CDK-inhibitor p27kip1 is a critical G1 phase cell cycle target of c-MYC. We show here that mouse embryo fibroblasts deficient for both p27kip1 and the related p21cip1 are still responsive to stimulation by c-MYC and can be arrested in G1 by a dominant negative mutant of c-MYC. This growth arrest can be overruled by ectopic expression of E2F or adenovirus E1A, but not by a mutant of E1A defective for binding to retinoblastoma family proteins. We show that fibroblasts with a genetic disruption of all three retinoblastoma family members (pRb, p107 and p130) are unresponsive to a dominant negative c-MYC mutant. These data indicate that p27kip1 is not the only rate limiting cell cycle target of c-MYC and suggest that regulation of E2F is also essential for c-MYC's mitogenic activity.

Mice with a targeted disruption of the Fanconi anemia homolog Fanca

Fanconi anemia (FA) is a hereditary chromosomal instability syndrome with cancer predisposition. Bone marrow failure resulting in pancytopenia is the main cause of death of FA patients. Diagnosis of FA is based on their cellular hypersensitivity to DNA crosslinking agents and chromosome breakages. Somatic complementation experiments suggest the involvement of at least eight genes in FA. The gene for complementation group A (FANCA) is defective in the majority of FA patients. We show here that mice deficient of FANCA: are viable and have no detectable developmental abnormalities. The hematological parameters showed a slightly decreased platelet count and a slightly increased erythrocyte mean cell volume in mice at young age, but this did not progress to anemia. Consistent with the clinical phenotype of FA patients, both male and female mice showed hypogonadism and impaired fertility. Furthermore, embryonic fibroblasts of the knock-out mice exhibited spontaneous chromosomal instability and were hyper-responsive to the clastogenic effect of the crosslinker mitomycin C.

Conditional biallelic Nf2 mutation in the mouse promotes manifestations of human neurofibromatosis type 2

Hemizygosity for the NF2 gene in humans causes a syndromic susceptibility to schwannoma development. However, Nf2 hemizygous mice do not develop schwannomas but mainly osteosarcomas. In the tumors of both species, the second Nf2 allele is inactivated. We report that conditional homozygous Nf2 knockout mice with Cre-mediated excision of Nf2 exon 2 in Schwann cells showed characteristics of neurofibromatosis type 2. These included schwannomas, Schwann cell hyperplasia, cataract, and osseous metaplasia. Thus, the tumor suppressor function of Nf2, here revealed in murine Schwann cells, was concealed in hemizygous Nf2 mice because of insufficient rate of second allele inactivation in this cell compartment. The finding of this conserved function documents the relevance of the present approach to model the human disease.

Induction of medulloblastomas in p53-null mutant mice by somatic inactivation of Rb in the external granular layer cells of the cerebellum

Medulloblastomas are among the most common malignancies in childhood, and they are associated with substantial mortality and morbidity. The molecular pathogenesis as well as the ontogeny of these neoplasms is still poorly understood. We have generated a mouse model for medulloblastoma by Cre-LoxP-mediated inactivation of Rb and p53 tumor suppressor genes in the cerebellar external granular layer (EGL) cells. GFAP-Cre-mediated recombination was found both in astrocytes and in immature precursor cells of the EGL in the developing cerebellum. GFAP-Cre;Rb(LoxP/LoxP);p53(-/- or LoxP/LoxP) mice developed highly aggressive embryonal tumors of the cerebellum with typical features of medulloblastoma. These tumors were identified as early as 7 weeks of age on the outer surface of the molecular layer, corresponding to the location of the EGL cells during development. Our results demonstrate that loss of function of RB is essential for medulloblastoma development in the mouse and strongly support the hypothesis that medulloblastomas arise from multipotent precursor cells located in the EGL.

Mammalian Trithorax and polycomb-group homologues are antagonistic regulators of homeotic development

Control of cell identity during development is specified in large part by the unique expression patterns of multiple homeobox-containing (Hox) genes in specific segments of an embryo. Trithorax and Polycomb-group (Trx-G and Pc-G) proteins in Drosophila maintain Hox expression or repression, respectively. Mixed lineage leukemia (MLL) is frequently involved in chromosomal translocations associated with acute leukemia and is the one established mammalian homologue of Trx. Bmi-1 was first identified as a collaborator in c-myc-induced murine lymphomagenesis and is homologous to the Drosophila Pc-G member Posterior sex combs. Here, we note the axial-skeletal transformations and altered Hox expression patterns of Mll-deficient and Bmi-1-deficient mice were normalized when both Mll and Bmi-1 were deleted, demonstrating their antagonistic role in determining segmental identity. Embryonic fibroblasts from Mll-deficient compared with Bmi-1-deficient mice demonstrate reciprocal regulation of Hox genes as well as an integrated Hoxc8-lacZ reporter construct. Reexpression of MLL was able to overcome repression, rescuing expression of Hoxc8-lacZ in Mll-deficient cells. Consistent with this, MLL and BMI-I display discrete subnuclear colocalization. Although Drosophila Pc-G and Trx-G members have been shown to maintain a previously established transcriptional pattern, we demonstrate that MLL can also dynamically regulate a target Hox gene.

In vivo analysis of Frat1 deficiency suggests compensatory activity of Frat3

The Frat1 gene was first identified as a proto-oncogene involved in progression of mouse T cell lymphomas. More recently, FRAT/GBP (GSK-3beta Binding Protein) family members have been recognized as critical components of the Wnt signal transduction pathway. In an attempt to gain more insight into the function of Frat1, we have generated Frat1-deficient mice in which most of the coding domain was replaced by a promoterless beta-galactosidase reporter gene. While the pattern of LacZ expression in Frat1(lacZ)/+ mice indicated Frat1 to be expressed in various neural and epithelial tissues, homozygous Frat1(lacZ) mice were apparently normal, healthy and fertile. Tissues of homozygous Frat1(lacZ) mice showed expression of a second mouse Frat gene, designated Frat3. The Frat1 and Frat3 proteins are structurally and functionally very similar, since both Frat1 and Frat3 are capable of inducing a secondary axis in Xenopus embryos. The overlapping expression patterns of Frat1 and Frat3 during murine embryogenesis suggest that the apparent dispensability of Frat1 for proper development may be due to the presence of a second mouse gene encoding a functional Frat protein.

Overexpression of Frat1 in transgenic mice leads to glomerulosclerosis and nephrotic syndrome, and provides direct evidence for the involvement of Frat1 in lymphoma progression

The proto-oncogene Frat1 was originally identified as a common site of proviral insertion in transplanted tumors of Moloney murine leukemia virus (M-MuLV)-infected Emu-Pim1 transgenic mice. Contrary to most common insertion sites implicated in mouse T cell lymphomagenesis, retroviral insertional mutagenesis of Frat1 constitutes a relatively late event in M-MuLV-induced tumor development, suggesting that proviral activation of Frat1 contributes to progression of T cell lymphomas rather than their genesis. To substantiate this notion we have generated transgenic mice that overexpress Frat1 in various organs, including lymphoid tissues. Frat1 transgenic mice develop focal glomerulosclerosis and a nephrotic syndrome, but they do not exhibit an increased incidence of spontaneous lymphomas. Conversely, these mice are highly susceptible to M-MuLV-induced lymphomagenesis, and Frat1/Pim1 bitransgenic animals develop lymphomas with increased frequency compared to Pim1 transgenic littermates. These data support a role for Frat1 in tumor progression.

PIM1 reconstitutes thymus cellularity in interleukin 7- and common gamma chain-mutant mice and permits thymocyte maturation in Rag- but not CD3gamma-deficient mice

The majority of lymphomas induced in Rag-deficient mice by Moloney murine leukemia virus (MoMuLV) infection express the CD4 and/or CD8 markers, indicating that proviral insertions cause activation of genes affecting the development from CD4(-)8(-) pro-T cells into CD4(+)8(+) pre-T cells. Similar to MoMuLV wild-type tumors, 50% of CD4(+)8(+) Rag-deficient tumors carry a provirus near the Pim1 protooncogene. To study the function of PIM proteins in T cell development in a more controlled setting, a Pim1 transgene was crossed into mice deficient in either cytokine or T cell receptor (TCR) signal transduction pathways. Pim1 reconstitutes thymic cellularity in interleukin (IL)-7- and common gamma chain-deficient mice. In Pim1-transgenic Rag-deficient mice but notably not in CD3gamma-deficient mice, we observed slow expansion of the CD4(+)8(+) thymic compartment to almost normal size. Based on these results, we propose that PIM1 functions as an efficient effector of the IL-7 pathway, thereby enabling Rag-deficient pro-T cells to bypass the pre-TCR-controlled checkpoint in T cell development.

Bmi-1 collaborates with c-Myc in tumorigenesis by inhibiting c-Myc-induced apoptosis via INK4a/ARF

The bmi-1 and myc oncogenes collaborate strongly in murine lymphomagenesis, but the basis for this collaboration was not understood. We recently identified the ink4a-ARF tumor suppressor locus as a critical downstream target of the Polycomb-group transcriptional repressor Bmi-1. Others have shown that part of Myc's ability to induce apoptosis depends on induction of p19arf. Here we demonstrate that down-regulation of ink4a-ARF by Bmi-1 underlies its ability to cooperate with Myc in tumorigenesis. Heterozygosity for bmi-1 inhibits lymphomagenesis in Emu-myc mice by enhancing c-Myc-induced apoptosis. We observe increased apoptosis in bmi-1(-/-) lymphoid organs, which can be rescued by deletion of ink4a-ARF or overexpression of bcl2. Furthermore, Bmi-1 collaborates with Myc in enhancing proliferation and transformation of primary embryo fibroblasts (MEFs) in an ink4a-ARF dependent manner, by prohibiting Myc-mediated induction of p19arf and apoptosis. We observe strong collaboration between the Emu-myc transgene and heterozygosity for ink4a-ARF, which is accompanied by loss of the wild-type ink4a-ARF allele and formation of highly aggressive B-cell lymphomas. Together, these results reinforce the critical role of Bmi-1 as a dose-dependent regulator of ink4a-ARF, which on its turn acts to prevent tumorigenesis on activation of oncogenes such as c-myc.

Developmental defects and tumor predisposition in Rb mutant mice

Targeted gene disruption in the mouse germline permits the introduction of gene mutations similar to those found in inherited human diseases. New advances in gene targeting that enable cell type specific gene disruption in mice further increases the utility of mouse models to study genetic defects as found in cancer. Here we review the phenotypes observed in mice carrying germline mutated copies of the retinoblastoma tumor suppressor gene. We will illustrate how methods that permit tissue-specific Rb inactivation in mice provide new and more versatile tools to gain insight into the etiology of sporadic cancer.

Pim kinase expression is induced by LTP stimulation and required for the consolidation of enduring LTP

In animals and several cellular models of synaptic plasticity, long-lasting changes in synaptic strength are dependent on gene transcription and translation. Here we demonstrate that Pim-1, a serine/threonine kinase closely related to Pim-2 and Pim-3, is induced in hippocampus in response to stimuli that evoke long-term potentiation (LTP). Mice deficient for Pim-1 show normal synaptic transmission and short-term plasticity. However, they fail to consolidate enduring LTP even though Pim-2 and Pim-3 are constitutively expressed in the hippocampus and Pim-3 expression is similarly induced by synaptic activity. Thus, expression of Pim-1 is required for LTP. Its level of expression and, consequently, its capacity to phosphorylate target proteins in dendritic and nuclear compartments of stimulated neurons might be a determining factor for the establishment of long-lasting changes in synaptic strength.

Schwann cell hyperplasia and tumors in transgenic mice expressing a naturally occurring mutant NF2 protein

Specific mutations in some tumor suppressor genes such as p53 can act in a dominant fashion. We tested whether this mechanism may also apply for the neurofibromatosis type-2 gene (NF2) which, when mutated, leads to schwannoma development. Transgenic mice were generated that express, in Schwann cells, mutant NF2 proteins prototypic of natural mutants observed in humans. Mice expressing a NF2 protein with an interstitial deletion in the amino-terminal domain showed high prevalence of Schwann cell-derived tumors and Schwann cell hyperplasia, whereas those expressing a carboxy-terminally truncated protein were normal. Our results indicate that a subset of mutant NF2 alleles observed in patients may encode products with dominant properties when overexpressed in specific cell lineages.

Identification and characterization of collaborating oncogenes in compound mutant mice

We have used proviral tagging in tumor-prone transgenic mice to identify collaborating oncogenes and genes contributing to tumor progression. This has yielded a series of oncogenes that could be assigned to different complementation groups in transformation: the myc, Pim, Bmi1, and Frat1 complementation groups. Frat1 is involved in tumor progression and appears to function in the Wnt signaling pathway. Overexpression of Fratl confers a growth advantage to transplanted tumor cells in vivo and to cells grown in vitro at high density. Frat1 might exert its activity by impairing the kinase activity of Gsk3beta, which is involved in the degradation of beta-catenin. Pim genes appear to act in tumor initiation and show strong synergism with myc in lymphomagenesis. Overexpression of Pim1 can also overcome some of the proliferative defects caused by defective interleukin signaling supporting a role of Pim1 in cell proliferation. We have applied proviral tagging in compound mutant Emu-myc/Pim1-/-/Pim2-/- mice to identify genes that can complement for the loss of Pim1 and Pim2 and, therefore, are able to synergize with c-myc in lymphomagenesis. A number of new as well as known genes have been found by this "complementation tagging." The latter included c-kit, Tp12, and cyclin D2, suggesting that Pim kinases might act upstream of or parallel to these known proto-oncogenes.

A link between cell cycle and cell death: Bax and Bcl-2 modulate Cdk2 activation during thymocyte apoptosis

Resting thymocytes undergoing apoptosis in response to specific stimuli degrade the cdk inhibitor p27(Kip1) and upregulate Cdk2 kinase activity. Inhibition of Cdk2 kinase activity efficiently blocks cell death via certain apoptosis pathways whereas overexpression of Cdk2 accelerates such cell death, suggesting its involvement in the signal transduction pathways activated by certain apoptotic stimuli. We found that Cdk2 activation during thymocyte apoptosis can be regulated by p53, Bax and Bcl-2. The highly elevated Cdk2 kinase activity in the apoptosing thymocytes is not associated with its canonical cyclins, cyclin E and cyclin A, and requires de novo synthesis of proteins for activation to take place. We therefore propose Cdk2 activation to be a crucial event in distinct pathways of apoptosis and the point at which the cell cycle and cell death pathways interact.

Vaccination with irradiated autologous melanoma cells engineered to secrete human granulocyte-macrophage colony-stimulating factor generates potent antitumor immunity in patients with metastatic melanoma

We conducted a Phase I clinical trial investigating the biologic activity of vaccination with irradiated autologous melanoma cells engineered to secrete human granulocyte-macrophage colony-stimulating factor in patients with metastatic melanoma. Immunization sites were intensely infiltrated with T lymphocytes, dendritic cells, macrophages, and eosinophils in all 21 evaluable patients. Although metastatic lesions resected before vaccination were minimally infiltrated with cells of the immune system in all patients, metastatic lesions resected after vaccination were densely infiltrated with T lymphocytes and plasma cells and showed extensive tumor destruction (at least 80%), fibrosis, and edema in 11 of 16 patients examined. Antimelanoma cytotoxic T cell and antibody responses were associated with tumor destruction. These results demonstrate that vaccination with irradiated autologous melanoma cells engineered to secrete granulocyte-macrophage colony-stimulating factor stimulates potent antitumor immunity in humans with metastatic melanoma.

Flp-mediated tissue-specific inactivation of the retinoblastoma tumor suppressor gene in the mouse

The yeast-derived Flp-frt site-specific DNA recombination system was used to achieve pituitary-specific inactivation of the retinoblastoma (Rb) tumor suppressor gene. Whereas mice carrying only frt sites in both alleles of Rb remain tumor free, tumorigenesis ensues when the Flp recombinase is expressed. The rate of tumorigenesis in these mice depends both on the expression level of the Flp recombinase and on the presence of frt sites in one or both Rb alleles. This permitted a more accurate definition of the consecutive steps in pituitary tumorigenesis. Our study illustrates the potential of this approach for studying sporadic cancer in a defined mouse model.

p107 is a suppressor of retinoblastoma development in pRb-deficient mice

Hemizygosity for the retinoblastoma gene RB in man strongly predisposes to retinoblastoma. In the mouse, however, Rb hemizygosity leaves the retina normal, whereas in Rb-/- chimeras pRb-deficient retinoblasts undergo apoptosis. To test whether concomitant inactivation of the Rb-related gene p107 is required to unleash the oncogenic potential of pRb deficiency in the mouse retina, we inactivated both Rb and p107 by homologous recombination in embryonic stem cells and generated chimeric mice. Retinoblastomas were found in five out of seven adult pRb/p107-deficient chimeras. The retinal tumors showed amacrine cell differentiation, and therefore originated from cells committed to the inner but not the outer nuclear layer. Retinal lesions were already observed at embryonic day 17.5. At this stage, the primitive nuclear layer exhibited severe dysplasia, including rosette-like arrangements, and apoptosis. These findings provide formal proof for the role of loss of Rb in retinoblastoma development in the mouse and the first in vivo evidence that p107 can exert a tumor suppressor function.

Saturating mutagenesis and characterization of a herpesvirus genome using in vivo reconstitution of virus from cloned subgenomic regions

The study of genome structure and gene function is pivotal in understanding the mechanisms of replication, pathogenesis, and virulence of herpesviruses. In this respect, mutagenesis and sequence analysis of genes encoded by the virus are of great importance. However, the herpesvirus genomes are large, with sizes ranging between 120 and over 200 kbp and encoding between 70 and 200 genes (see ref. 1 for a review). This large size hampers handling and systematic mutagenesis of the virus genome using standard modern molecular biology techniques. Most current methods of mutagenesis therefore do not rely on direct modification of the viral genome in vitro but depend on exchange in vivo, by homologous recombination, of a viral gene by a copy of the latter gene that is truncated in vitro by insertion of a marker gene. Mutant virus progeny can be screened or selected for, depending on the marker gene that is used. Commonly used marker genes are thymidine kinase and lacZ. This procedure is generally used, reliable, and has yielded a wealth of information on the function of herpers simplex virus type 1 (HSV-1) encoded genes. However, it requires prior mapping and cloning of every gene to be mutagenized and is therefore less feasible if the virus is a novel or less-well-known herpesvirus.