Pim1 cooperates with E2a-Pbx1 to facilitate the progression of thymic lymphomas in transgenic mice

Metabolic protein kinase signalling in neuroblastoma

BACKGROUND

Neuroblastoma is a paediatric malignancy of incredibly complex aetiology. Oncogenic protein kinase signalling in neuroblastoma has conventionally focussed on transduction through the well-characterised PI3K/Akt and MAPK pathways, in which the latter has been implicated in treatment resistance. The discovery of the receptor tyrosine kinase ALK as a target of genetic alterations in cases of familial and sporadic neuroblastoma, was a breakthrough in the understanding of the complex genetic heterogeneity of neuroblastoma. However, despite progress in the development of small-molecule inhibitors of ALK, treatment resistance frequently arises and appears to be a feature of the disease. Moreover, since the identification of ALK, several additional protein kinases, including the PIM and Aurora kinases, have emerged not only as drivers of the disease phenotype, but also as promising druggable targets. This is particularly the case for Aurora-A, given its intimate engagement with MYCN, a driver oncogene of aggressive neuroblastoma previously considered 'undruggable.'

SCOPE OF REVIEW

Aided by significant advances in structural biology and a broader understanding of the mechanisms of protein kinase function and regulation, we comprehensively outline the role of protein kinase signalling, emphasising ALK, PIM and Aurora in neuroblastoma, their respective metabolic outputs, and broader implications for targeted therapies.

MAJOR CONCLUSIONS

Despite massively divergent regulatory mechanisms, ALK, PIM and Aurora kinases all obtain significant roles in cellular glycolytic and mitochondrial metabolism and neuroblastoma progression, and in several instances are implicated in treatment resistance. While metabolism of neuroblastoma tends to display hallmarks of the glycolytic "Warburg effect," aggressive, in particular MYCN-amplified tumours, retain functional mitochondrial metabolism, allowing for survival and proliferation under nutrient stress. Future strategies employing specific kinase inhibitors as part of the treatment regimen should consider combinatorial attempts at interfering with tumour metabolism, either through metabolic pathway inhibitors, or by dietary means, with a view to abolish metabolic flexibility that endows cancerous cells with a survival advantage.

Mutational Profile and Clonal Evolution of Relapsed/Refractory Diffuse Large B-Cell Lymphoma

Primary refractory/relapsed diffuse large B-cell lymphoma (rrDLBCL) is an unresolved issue for DLBCL treatment and new treatments to overcome resistance is required. To explore the genetic mechanisms underlying treatment resistance in rrDLBCL and to identify candidate genes, we performed targeted deep sequencing of 430 lymphoma-related genes from 58 patients diagnosed with rrDLBCL. Genetic alterations found between the initial biopsy and biopsy at recurrence or refractory disease were investigated. The genes most frequently altered (> 20%) were (in decreasing order of frequency) CDKN2A, PIM1, CD79B, TP53, MYD88, MYC, BTG2, BTG1, CDKN2B, DTX1, CD58, ETV6, and IRF4. Genes mutation of which in pretreatment sample were associated with poor overall survival included NOTCH1, FGFR2, BCL7A, BCL10, SPEN and TP53 (P < 0.05). FGFR2, BCL2, BCL6, BCL10, and TP53 were associated with poor progression-free survival (P < 0.05). Most mutations were truncal and were maintained in both the initial biopsy and post-treatment biopsy with high dynamics of subclones. Immune-evasion genes showed increased overall mutation frequency (CD58, B2M) and variant allele fraction (CD58), and decreased copy number (B2M, CD70) at the post-treatment biopsy. Using the established mutational profiles and integrative analysis of mutational evolution, we identified information about candidate genes that may be useful for the development of future treatment strategies.

Targeted therapies in pancreatic cancer: Promises and failures

Pancreatic ductal adenocarcinoma (PDAC) is an incidence rate nearly equal to its mortality rate. The poor prognosis of the disease can be explained by the absence of effective biomarkers for screening and early detection, together with the aggressive behavior and resistance to the currently available chemotherapy. The therapeutic failure can also be attributed to the inter-/intratumor genetic heterogeneity and the abundance of tumor stroma that occupies the majority of the tumor mass. Gemcitabine is used in the treatment of PDAC; however, the response rate is less than 12%. A recent phase III trial revealed that the combination of oxaliplatin, irinotecan, fluorouracil, and leucovorin could be an option for the treatment of metastatic PDAC patients with good performance status, although these approaches can result in high toxicity level. Further investigations are required to develop innovative anticancer agents that either improve gemcitabine activity, within novel combinatorial approaches or acts with a better efficacy than gemcitabine. The aim of the current review is to give an overview of preclinical and clinical studies targeting key dysregulated signaling pathways in PDAC.

PIM1 induces cellular senescence through phosphorylation of UHRF1 at Ser311

PIM1 is a proto-oncogene, encoding a serine/threonine protein kinase that regulates cell proliferation, survival, differentiation and apoptosis. Previous reports suggest that overexpression of PIM1 can induce cellular senescence. However, the molecular mechanism underlying this process is not fully understood. Here we report that UHRF1 is a novel substrate of PIM1 kinase, which could be phosphorylated at Ser311 and therefore promoted to degradation. Our data demonstrates that PIM1 destabilizes UHRF1, leading to DNA hypomethylation, which consequently results in genomic instability, increased p16 expression and subsequent induction of cellular senescence. Taken together, our results suggest that down-regulation of UHRF1 is an important mechanism of PIM1-mediated cellular senescence.

Identification of cooperative genes for E2A-PBX1 to develop acute lymphoblastic leukemia

E2A-PBX1 is a chimeric gene product detected in t(1;19)-bearing acute lymphoblastic leukemia (ALL) with B-cell lineage. To investigate the leukemogenic process, we generated conditional knock-in (cKI) mice for E2A-PBX1, in which E2A-PBX1 is inducibly expressed under the control of the endogenous E2A promoter. Despite the induced expression of E2A-PBX1, no hematopoietic disease was observed, strongly suggesting that additional genetic alterations are required to develop leukemia. To address this possibility, retroviral insertional mutagenesis was used. Virus infection efficiently induced T-cell, B-cell, and biphenotypic ALL in E2A-PBX1 cKI mice. Inverse PCR identified eight retroviral common integration sites, in which enhanced expression was observed in the Gfi1, Mycn, and Pim1 genes. In addition, it is of note that viral integration and overexpression of the Zfp521 gene was detected in one tumor with B-cell lineage; we previously identified Zfp521 as a cooperative gene with E2A-HLF, another E2A-involving fusion gene with B-lineage ALL. The cooperative oncogenicity of E2A-PBX1 with overexpressed Zfp521 in B-cell tumorigenesis was indicated by the finding that E2A-PBX1 cKI, Zfp521 transgenic compound mice developed B-lineage ALL. Moreover, upregulation of ZNF521, the human counterpart of Zfp521, was found in several human leukemic cell lines bearing t(1;19). These results indicate that E2A-PBX1 cooperates with additional gene alterations to develop ALL. Among them, enhanced expression of ZNF521 may play a clinically relevant role in E2A fusion genes to develop B-lineage ALL.

Genetically engineered mouse models of human B-cell precursor leukemias

B-cell precursor acute lymphoblastic leukemias (pB-ALLs) are the most frequent type of malignancies of the childhood, and also affect an important proportion of adult patients. In spite of their apparent homogeneity, pB-ALL comprises a group of diseases very different both clinically and pathologically, and with very diverse outcomes as a consequence of their biology, and underlying molecular alterations. Their understanding (as a prerequisite for their cure) will require a sustained multidisciplinary effort from professionals coming from many different fields. Among all the available tools for pB-ALL research, the use of animal models stands, as of today, as the most powerful approach, not only for the understanding of the origin and evolution of the disease, but also for the development of new therapies. In this review we go over the most relevant (historically, technically or biologically) genetically engineered mouse models (GEMMs) of human pB-ALLs that have been generated over the last 20 years. Our final aim is to outline the most relevant guidelines that should be followed to generate an “ideal” animal model that could become a standard for the study of human pB-ALL leukemia, and which could be shared among research groups and drug development companies in order to unify criteria for studies like drug testing, analysis of the influence of environmental risk factors, or studying the role of both low-penetrance mutations and cancer susceptibility alterations.

Pim kinases in hematological malignancies: where are we now and where are we going?

The proviral insertion in murine (PIM) lymphoma proteins are a serine/threonine kinase family composed of three isoformes: Pim-1, Pim-2 and Pim-3. They play a critical role in the control of cell proliferation, survival, homing and migration. Recently, overexpression of Pim kinases has been reported in human tumors, mainly in hematologic malignancies. In vitro and in vivo studies have confirmed their oncogenic potential. Indeed, PIM kinases have shown to be involved in tumorgenesis, to enhance tumor growth and to induce chemo-resistance, which is why they have become an attractive therapeutic target for cancer therapy. Novel molecules inhibiting Pim kinases have been evaluated in preclinical studies, demonstrating to be effective and with a favorable toxicity profile. Given the promising results, some of these compounds are currently under investigation in clinical trials. Herein, we provide an overview of the biological activity of PIM-kinases, their role in hematologic malignancies and future therapeutic opportunities.

Genetic Modeling of PIM Proteins in Cancer: Proviral Tagging and Cooperation with Oncogenes, Tumor Suppressor Genes, and Carcinogens

The PIM proteins, which were initially discovered as proviral insertion sites in Moloney-murine leukemia virus infection, are a family of highly homologous serine/threonine kinases that have been reported to be overexpressed in hematological malignancies and solid tumors. The PIM proteins have also been associated with metastasis and overall treatment responses and implicated in the regulation of apoptosis, metabolism, the cell cycle, and homing and migration, which makes these proteins interesting targets for anti-cancer drug discovery. The use of retroviral insertional mutagenesis and refined approaches such as complementation tagging has allowed the identification of myc, pim, and a third group of genes (including bmi1 and gfi1) as complementing genes in lymphomagenesis. Moreover, mouse modeling of human cancer has provided an understanding of the molecular pathways that are involved in tumor initiation and progression at the physiological level. In particular, genetically modified mice have allowed researchers to further elucidate the role of each of the Pim isoforms in various tumor types. PIM kinases have been identified as weak oncogenes because experimental overexpression in lymphoid tissue, prostate, and liver induces tumors at a relatively low incidence and with a long latency. However, very strong synergistic tumorigenicity between Pim1/2 and c-Myc and other oncogenes has been observed in lymphoid tissues. Mouse models have also been used to study whether the inhibition of specific PIM isoforms is required to prevent carcinogen-induced sarcomas, indicating that the absence of Pim2 and Pim3 greatly reduces sarcoma growth and bone invasion; the extent of this effect is similar to that observed in the absence of all three isoforms. This review will summarize some of the animal models that have been used to understand the isoform-specific contribution of PIM kinases to tumorigenesis.

Murine models of acute leukemia: important tools in current pediatric leukemia research

Leukemia remains the most common diagnosis in pediatric oncology and, despite dramatic progress in upfront therapy, is also the most common cause of cancer-related death in children. Much of the initial improvement in outcomes for acute lymphoblastic leukemia (ALL) was due to identification of cytotoxic agents that are active against leukemia followed by the recognition that combination of these cytotoxic agents and prolonged therapy are essential for cure. Recent data demonstrating lack of progress in patients for whom standard chemotherapy fails suggests that the ability to improve outcome for these children will not be dramatically impacted through more intensive or newer cytotoxic agents. Thus, much of the recent research focus has been in the area of improving our understanding of the genetics and the biology of leukemia. Although in vitro studies remain critical, given the complexity of a living system and the increasing recognition of the contribution of leukemia extrinsic factors such as the bone marrow microenvironment, in vivo models have provided important insights. The murine systems that are used can be broadly categorized into syngeneic models in which a murine leukemia can be studied in immunologically intact hosts and xenograft models where human leukemias are studied in highly immunocompromised murine hosts. Both of these systems have limitations such that neither can be used exclusively to study all aspects of leukemia biology and therapeutics for humans. This review will describe the various ALL model systems that have been developed as well as discuss the advantages and disadvantages inherent to these systems that make each particularly suitable for specific types of studies.

The PIM family of serine/threonine kinases in cancer

The proviral insertion site in Moloney murine leukemia virus, or PIM proteins, are a family of serine/threonine kinases composed of three different isoforms (PIM1, PIM2, and PIM3) that are highly evolutionarily conserved. These proteins are regulated primarily by transcription and stability through pathways that are controlled by Janus kinase/Signal transducer and activator of transcription, JAK/STAT, transcription factors. The PIM family proteins have been found to be overexpressed in hematological malignancies and solid tumors, and their roles in these tumors were confirmed in mouse tumor models. Furthermore, the PIM family proteins have been implicated in the regulation of apoptosis, metabolism, cell cycle, and homing and migration, which has led to the postulation of these proteins as interesting targets for anticancer drug discovery. In the present work, we review the importance of PIM kinases in tumor growth and as drug targets.

A small molecule inhibitor of Pim protein kinases blocks the growth of precursor T-cell lymphoblastic leukemia/lymphoma

The serine/threonine Pim kinases are up-regulated in specific hematologic neoplasms, and play an important role in key signal transduction pathways, including those regulated by MYC, MYCN, FLT3-ITD, BCR-ABL, HOXA9, and EWS fusions. We demonstrate that SMI-4a, a novel benzylidene-thiazolidine-2, 4-dione small molecule inhibitor of the Pim kinases, kills a wide range of both myeloid and lymphoid cell lines with precursor T-cell lymphoblastic leukemia/lymphoma (pre-T-LBL/T-ALL) being highly sensitive. Incubation of pre-T-LBL cells with SMI-4a induced G1 phase cell-cycle arrest secondary to a dose-dependent induction of p27(Kip1), apoptosis through the mitochondrial pathway, and inhibition of the mammalian target of rapamycin C1 (mTORC1) pathway based on decreases in phospho-p70 S6K and phospho-4E-BP1, 2 substrates of this enzyme. In addition, treatment of these cells with SMI-4a was found to induce phosphorylation of extracellular signal-related kinase1/2 (ERK1/2), and the combination of SMI-4a and a mitogen-activated protein kinase kinase 1/2 (MEK1/2) inhibitor was highly synergistic in killing pre-T-LBL cells. In immunodeficient mice carrying subcutaneous pre-T-LBL tumors, treatment twice daily with SMI-4a caused a significant delay in the tumor growth without any change in the weight, blood counts, or chemistries. Our data suggest that inhibition of the Pim protein kinases may be developed as a therapeutic strategy for the treatment of pre-T-LBL.

High-throughput insertional mutagenesis screens in mice to identify oncogenic networks

Retroviral insertional mutagenesis screens have been used for many years as a tool for cancer gene discovery. In recent years, completion of the mouse genome sequence as well as improved technologies for cloning and sequencing of retroviral insertions have greatly facilitated the retrieval of more complete data sets from these screens. The concomitant increase of the size of the screens allows researchers to address new questions about the genes and signalling networks involved in tumour development. In addition, the development of new insertional mutagenesis tools such as DNA transposons enables screens for cancer genes in tissues that previously could not be analysed by retroviral insertional mutagenesis.

Structural analysis identifies imidazo[1,2-b]pyridazines as PIM kinase inhibitors with in vitro antileukemic activity

Much attention has recently been focused on PIM kinases as potential targets for the treatment of hematopoietic malignancies and some solid cancers. Using protein stability shift assays, we identified a family of imidazo[1,2-b]pyridazines to specifically interact with and inhibit PIM kinases with low nanomolar potency. The high-resolution crystal structure of a PIM1 inhibitor complex revealed that imidazo[1,2-b]pyridazines surprisingly interact with the NH(2)-terminal lobe helix alphaC rather than with the kinase hinge region. Thus, the identified inhibitors are ATP competitive but not ATP mimetic compounds, explaining their enhanced selectivity with respect to conventional type I kinase inhibitors. One of the identified imidazo[1,2-b]pyridazines (K00135) was further tested in several hematopoietic cellular systems. First, K00135 dose-dependently impaired survival of murine Ba/F3 cells that have been rendered cytokine independent by overexpression of human PIMs. Second, K00135 impaired survival and clonogenic growth of a panel of human acute leukemia cells. Third, exposure of K00135 significantly suppressed in vitro growth of leukemic blasts from five acute myelogenous leukemia patients but not of normal umbilical cord blood mononuclear cells. In vitro kinase assays and immunoblotting using lysates from human MV4;11 leukemic cells showed inhibition of phosphorylation of known PIM downstream targets, such as BAD and eukaryotic translation initiation factor 4E-binding protein 1, by K00135. Taken together, we report a family of small molecules that selectively interact and block PIM kinases and could serve as a lead to develop new targeted antileukemic therapeutics.

Genetic alterations in intrahepatic cholangiocarcinoma as revealed by degenerate oligonucleotide primed PCR-comparative genomic hybridization

Intrahepatic cholangiocarcinoma (ICC), a malignant neoplasm of the biliary epithelium,is usually fatal because of difficulty in early diagnosis and lack of availability of effective therapy. The genetic mechanisms involved in the development of ICC are not well understood and only a few cytogenetic studies of ICC have been published. Recently, technique of degenerate oligonucleotide primed (DOP)-PCR comparative genomic hybridization (CGH) permits genetic imbalances screening of the entire genome using only small amounts of tumor DNA. In this study chromosomal aberrations in 33 Korean ICC were investigated by DOP-PCR CGH. The common sites of copy number increases were 20q (67%), 17 (61%), 11q11-q13 (42%), 8p12- qter (39%), 18p (39%), 15q22-qter (36%), 16p (36%), 6p21 (30%), 3q25-qter (27%), 1q41-qter (24%), and 5p14-q11.2 (24%). DNA amplification was identified in 16 carcinomas (48%). The frequent sites of amplification were 20q, 17p, 17q23-qter, and 7p. The most frequent sites of copy number decreases were 1p32-pter (21%) and 4q (21%). The recurrent chromosomal aberrations identified in this study provide candidate regions involved in the tumorigenesis and progression of ICC.

Transgenic Mice as an In Vivo Model of Lymphomagenesis

This review covers multiple data obtained on genetically modified mice that help to elucidate various intricate molecular mechanisms of lymphomagenesis in humans. We are in a "golden age" of mouse genetics. The mouse is by far the most accessible mammalian system physiologically similar to humans. Transgenic mouse models have illuminated how different genes contribute to human lymphomagenesis. Multiple experiments with transgenic mice have not only confirmed the data obtained for human lymphomas but also gave additional evidence for the role of some genes and cooperative participation of their products in the development of human lymphomas. Genes and gene networks detected on transgenic mice can successfully serve as molecular targets for tumor therapy. This review demonstrates the extraordinary possibilities of transgenic technology, which is presently one of the readily available, efficient, and accurate tools to solve the problem of cancer.

Detection of genetic alterations in Korean ovarian carcinomas by degenerate oligonucleotide primed polymerase chain reaction-comparative genomic hybridization

Chromosomal aberrations in 22 Koreans with ovarian carcinomas were investigated by degenerate oligonucleotide primed-polymerase chain reaction comparative genomic hybridization. The common sites of copy number increases were 20q (90%), 17q23 approximately qter (86%), 8q22 approximately qter (68%), 3q25 approximately qter (59%), 6p21 (59%), 11q13 (54%), 16p (40%), 2q31 approximately qter (36%), 7q (36%), 14q31 (36%), 15q24 approximately qter (36%), and 1q32 approximately qter (31%). DNA amplification was identified in 18 carcinomas (82%). The frequent sites of amplification were 20q13.2 approximately qter, 8q24.1, 17q23 approximately qter, 3q25 approximately qter, and 6p21. The most frequent sites of copy number decreases were 4q21 approximately q31 (54%), 5q13 approximately q21 (50%), and 13q14 approximately q21 (45%). The recurrent gains and losses of chromosomal regions identified in this study provide candidate regions that may contain oncogenes or tumor suppressor genes, respectively.

EBNA3A association with RBP-Jkappa down-regulates c-myc and Epstein-Barr virus-transformed lymphoblast growth

Epstein-Barr virus nuclear antigen protein 3A (EBNA3A) is one of four EBNAs (EBNA-2, EBNALP, EBNA3A, and EBNA3C) through the cellular DNA sequence-specific transcription factor RBP-Jkappa/CBF-1/CSL and are essential for conversion of primary B lymphocytes to lymphoblastoid cell lines (LCLs). In the present study, we investigated the effects of EBNA3A on EBNA2 activation of transcription in the IB4 LCL by conditionally overexpressing EBNA3A three- to fivefold. EBNA3A overexpression increased EBNA3A association with RBP-Jkappa, did not change EBNA3C association with RBP-Jkappa or EBNA or LMP1 expression, decreased EBNA2 association with RBP-Jkappa, decreased c-myc expression, and caused G(0)/G(1) growth arrest with prolonged viability. Expression of the fusion protein MycERTM in cells with conditional EBNA3A overexpression restored cell cycle progression and caused apoptosis. In contrast, MycER in the same cells without EBNA3A overexpression enhanced cell proliferation and did not increase apoptosis. These data indicate that EBNA3A overexpression inhibits protection from c-myc-induced apoptosis. In assays of EBNA2- and RBP-Jkappa-dependent transcription, EBNA3A amino acids 1 to 386 were sufficient for repression equivalent to that by wild-type EBNA3A, amino acids 1 to 124 were unimportant, amino acids 1 to 277 were insufficient, and a triple alanine substitution within the EBNA3A core RBP-Jkappa binding domain was a null mutation. In reverse genetic experiments with IB4 LCLs, the effects of conditional EBNA3A overexpression on c-myc expression and proliferation did not require amino acids 524 to 944 but did require amino acids 278 to 524 as well as wild-type sequence in the core RBP-Jkappa binding domain. The dependence of EBNA3A effects on the core RBP-Jkappa interaction domain and on the more C-terminal amino acids (amino acids 278 to 524) required for efficient RBP-Jkappa association strongly implicates RBP-Jkappa in c-myc promoter regulation.

Estrogen-dependent E2a/Pbx1 myeloid cell lines exhibit conditional differentiation that can be arrested by other leukemic oncoproteins

The molecular pathways of normal myeloid differentiation, as well as the mechanisms by which oncogenes disrupt this process, remain poorly understood. A major limitation in approaching this problem has been the lack of suitable cell lines that exhibit normal, terminal, and synchronous differentiation in the absence of endogenous oncoproteins and in response to physiologic cytokines, and whose differentiation can be arrested by ectopically expressed human oncoproteins. This report describes clonal, granulocyte-macrophage colony-stimulating factor-dependent myeloid cell lines that exhibit these properties. The cell lines were established by conditional immortalization of primary murine marrow progenitors with an estrogen-regulated E2a/Pbx1-estrogen receptor fusion protein. Clones were identified that proliferated as immortalized blasts in the presence of estrogen, and that exhibited granulocytic, monocytic, or bipotential (granulocytic and monocytic) differentiation on estrogen withdrawal. Differentiation was normal and terminal as evidenced by morphology, cell surface markers, gene expression, and functional assays. The differentiation of the cells could be arrested by heterologous oncoproteins including AML1/ETO, PML/RARalpha, PLZF/RARalpha, Nup98/HoxA9, and other Hox proteins. Furthermore, the study examined the effects of cooperating oncoproteins such as Ras or Bcr/Abl, which allowed for both factor-independent proliferation and differentiation, or Bcl-2, which permitted factor-independent survival but not proliferation. These myeloid cell lines provide tools for examining the biochemical and genetic pathways that accompany normal differentiation as well as a system in which to dissect how other leukemic oncoproteins interfere with these pathways.

A carboxy-terminal deletion mutant of Notch1accelerates lymphoid oncogenesis in E2A-PBX1transgenic mice

PBX1 is a proto-oncogene that plays important roles in pattern formation during development. It was discovered as a fusion with the E2A gene after chromosomal translocations in a subset of acute leukemias. The resulting E2a-Pbx1 chimeric proteins display potent oncogenic properties that appear to require dimerization with Hox DNA binding partners. To define molecular pathways that may be impacted by E2a-Pbx1, a genetic screen consisting of neonatal retroviral infection was used to identify genes that accelerate development of T-cell tumors in E2A-PBX1 transgenic mice. Retroviral insertions in the Notch1 gene were observed in 88% of tumors arising with a shortened latency. Among these, approximately half created a NotchIC allele, encoding the intracellular, signaling portion of Notch1, suggesting a synergistic interaction between the Notch and E2a-Pbx1 pathways in oncogenesis. The remaining proviral insertions involvingNotch1 occurred in a more 3′ exon, resulting in truncating mutations that deleted the carboxy-terminal region ofNotch1 containing negative regulatory sequences (Notch1ΔC). In contrast toNotchIC, forced expression ofNotch1ΔC in transgenic mice did not perturb thymocyte growth or differentiation. However, mice transgenic for both the E2A-PBX1 and Notch1ΔC genes displayed a substantially shortened latency for tumor development compared with E2A-PBX1 single transgenic mice. These studies reveal a novel mechanism for oncogenic activation ofNotch1 and demonstrate a collaborative relationship between 2 cellular oncogenes that also contribute to cell fate determination during embryonic development.

A carboxy-terminal deletion mutant of Notch1accelerates lymphoid oncogenesis in E2A-PBX1transgenic mice

PBX1 is a proto-oncogene that plays important roles in pattern formation during development. It was discovered as a fusion with the E2A gene after chromosomal translocations in a subset of acute leukemias. The resulting E2a-Pbx1 chimeric proteins display potent oncogenic properties that appear to require dimerization with Hox DNA binding partners. To define molecular pathways that may be impacted by E2a-Pbx1, a genetic screen consisting of neonatal retroviral infection was used to identify genes that accelerate development of T-cell tumors in E2A-PBX1 transgenic mice. Retroviral insertions in the Notch1 gene were observed in 88% of tumors arising with a shortened latency. Among these, approximately half created a NotchIC allele, encoding the intracellular, signaling portion of Notch1, suggesting a synergistic interaction between the Notch and E2a-Pbx1 pathways in oncogenesis. The remaining proviral insertions involvingNotch1 occurred in a more 3′ exon, resulting in truncating mutations that deleted the carboxy-terminal region ofNotch1 containing negative regulatory sequences (Notch1ΔC). In contrast toNotchIC, forced expression ofNotch1ΔC in transgenic mice did not perturb thymocyte growth or differentiation. However, mice transgenic for both the E2A-PBX1 and Notch1ΔC genes displayed a substantially shortened latency for tumor development compared with E2A-PBX1 single transgenic mice. These studies reveal a novel mechanism for oncogenic activation ofNotch1 and demonstrate a collaborative relationship between 2 cellular oncogenes that also contribute to cell fate determination during embryonic development.

Release from apoptosis correlates with tumor progression in the AKR lymphoma

Disturbance of apoptosis is an established factor in tumorigenesis. The role of apoptosis in tumor progression is not yet clear. In the present study we compared the tendency to spontaneous apoptosis (and the proliferative capacity) of tumor cells derived from primary (PT) and metastatic tumor (MT) cells of several AKR lymphoma variants. Apoptosis-related gene expression was also compared. Our results indicate that release from apoptosis has a role in the tumor progression of this T cell lymphoma. At the cellular level, a markedly lower apoptotic tendency was observed in MT than in PT cells. The existence of macrophages only in PT also supports the presence of apoptotic cells in local but not in MTs. By contrast, proliferative capacity does not determine tumor aggressiveness in this system. At the molecular level, we found a higher staining intensity for bcl-2 in MT than in PT cells, suggesting that bcl-2 might be responsible for the reduced apoptosis in MT compared to PT cells. Evidence for p53 overexpression was found in the MT cells of one of the variants but in none of the PT. Comparison of Fas receptor, unexpectedly showed an increased expression in MT versus PT cells, possibly indicating resistance to Fas-induced apoptosis in the MT cells.

EB-1, a tyrosine kinase signal transduction gene, is transcriptionally activated in the t(1;19) subset of pre-B ALL, which express oncoprotein E2a-Pbx1

The t(1;19) translocation of pre-B cell acute lymphocytic leukemia (ALL) produces E2a-Pbx1, a chimeric oncoprotein containing the transactivation domains of E2a joined to the homeodomain protein, Pbx1. E2a-Pbx1 causes T cell and myeloid leukemia in mice, blocks differentiation of cultured myeloid progenitors, and transforms fibroblasts through a mechanism accompanied by aberrant expression of tissue-specific and developmentally-regulated genes. Here we investigate whether aberrant gene expression also occurs specifically in the t(1;19)-containing subset of pre-B cell ALL in man. Two new genes, EB-1 and EB-2, as well as Caldesmon were transcriptionally activated in each of seven t(1;19) cell lines. EB-1 expression was extremely low in marrow from patients having pre-B ALL not associated with the t(1;19), and elevated more than 100-fold in marrow from patients with pre-B ALL associated with the t(1;19). Normal EB-1 expression was strong in brain and testis, the same tissues exhibiting the highest levels of PBX1 expression. EB-1 encodes a signaling protein containing a phosphotyrosine binding domain homologous to that of dNumb developmental regulators and two SAM domains homologous to those in the C-terminal tail of Eph receptor tyrosine kinases. We conclude that aberrant expression of tissue-specific genes is a characteristic of t(1;19) pre-B ALL, as was previously found in fibroblasts transformed by E2a-Pbx1. Potentially, EB-1 overexpression could interfere with normal signaling controlling proliferation or differentiation.

Reassessing the role of C-MYB in tumorigenesis

Hematopoietic tumors in both humans and mice frequently up-regulate expression of the c-myb gene, but it is unclear whether this is a cause or a consequence of the leukemic state. Recent results placing super-activation of the c-Myb protein at the bottom of a kinase-activated signal transduction pathway indicate that it may be a downstream effector of transformation induced by other oncogenes. The relationship between c-Myb and the serine-threonine kinase pim-1, its immediate activator, is discussed, together with the possibility that c-Myb, like pim-1, may be able to synergize with c-Myc to induce tumors.

Establishment and characterization of a mouse strain (TLL) that spontaneously develops T-cell lymphomas/leukemia

In this study, a mouse strain (TLL) that spontaneously develops T-cell lymphomas/leukemia with an early onset and high incidence was established and characterized. All tumors analyzed were found to express the alpha,beta T-cell receptor, and the majority of them had a mature, CD3+CD4+CD8- immunophenotype. In a few cases, tumors with a more immature CD3+CD4+CD8+ phenotype were isolated. Expanded phenotyping using a broad panel of lymphocyte differentiation markers confirmed the mature T-cell phenotype of the tumors. Histologic and cell cycle analysis of the tumors revealed an aggressive lymphoblastic malignancy with a very high proliferation rate and widespread engagement of bone marrow and lymphoid as well as nonlymphoid organs. Thus, the TLL mouse strain represents a unique model for the analysis of the oncogenesis and progression of mature T-cell tumors and for the development of therapeutic measures to combat such tumors.