IgH enhancer-mediated deregulation of N-myc gene expression in transgenic mice: generation of lymphoid neoplasias that lack c-myc expression

Discovery platform for inhibitors of IgH gene enhancer activity

Immunoglobulin heavy chain (IgH) translocations are common and early oncogenic events in B cell and plasma cell malignancies including B cell non-Hodgkin's lymphoma (NHL) and multiple myeloma (MM). IgH translocations bring oncogenes into close proximity with potent enhancer elements within the IgH locus, leading to oncogene up-regulation. As IgH enhancer activity is tightly controlled by B cell lineage-specific signaling and transcriptional networks, we hypothesized that IgH enhancers are potentially druggable targets/elements. To test this, we developed a molecular imaging-based high-throughput screening platform for discovering inhibitors of IgH enhancer-driven transcriptional activity. As proof of concept, we identified a low micromolar potency molecule (compound 30666) that inhibited immunoglobulin production by MM cells and blocked expression of an array of IgH translocation-induced oncogenes (CCND1, FGFR3/MMSET, and MYC) in MM and NHL cell lines. Prolonged exposure to 30666 significantly reduced the viability of IgH translocation-positive NHL and MM cells, but was less effective against cells lacking IgH translocations. Compound 30666 exhibited suitable pharmacological properties, including metabolic stability in liver microsomes and oral bioavailability in mice, and demonstrated preclinical anti-MM activity in a plasmacytoma mouse model. Our work suggests that IgH enhancers are attractive and potentially druggable targets for IgH translocation driven malignancies.

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.

N-MYC Oncogene Amplification: A Consequence of Genomic Instability in Human Neuroblastoma

Increase of the dosage of cellular oncogenes by DNA amplification is a frequent genetic alteration of cancer cells and arises as the consequence of genomic instability. The presence of amplified cellular oncogenes is usually signaled by conspicuous chromosomal abnormalities "double minutes," or "homogeneously staining chromosomal regions." Some human cancers carry a specific amplified oncogene at high incidence. In neuroblastomas, which are tumors of the peripheral nervous system that arise from primitive neuroectodermal cells derived from neural crest, the amplification of the gene N-MYC has been associated with aggressively growing cancers and is an indicator for poor prognosis. N-MYC amplification is of predictive value for iden tifying neuroblastoma patients who either require specific therapeutic regimens or who do not benefit from chemotherapy. The Neuroscientist 1:277-285, 1995

Triplication of a 21q22 region contributes to B cell transformation through HMGN1 overexpression and loss of histone H3 Lys27 trimethylation

Down syndrome confers a 20-fold increased risk of B cell acute lymphoblastic leukemia (B-ALL)¹ and polysomy 21 is the most frequent somatic aneuploidy amongst all B-ALLs². Yet, the mechanistic links between chr.21 triplication and B-ALL remain undefined. Here we show that germline triplication of only 31 genes orthologous to human chr.21q22 confers murine progenitor B cell self-renewal in vitro, maturation defects in vivo, and B-ALL with either BCR-ABL or CRLF2 with activated JAK2. Chr.21q22 triplication suppresses H3K27me3 in progenitor B cells and B-ALLs, and “bivalent” genes with both H3K27me3 and H3K4me3 at their promoters in wild-type progenitor B cells are preferentially overexpressed in triplicated cells. Strikingly, human B-ALLs with polysomy 21 are distinguished by their overexpression of genes marked with H3K27me3 in multiple cell types. Finally, overexpression of HMGN1, a nucleosome remodeling protein encoded on chr.21q22^3–5, suppresses H3K27me3 and promotes both B cell proliferation in vitro and B-ALL in vivo.

Chromosomal location targets different MYC family gene members for oncogenic translocations

The MYC family of cellular oncogenes includes c-Myc, N-myc, and L-myc, which encode transcriptional regulators involved in the control of cell proliferation and death. Accordingly, these genes become aberrantly activated and expressed in specific types of cancers. For example, c-Myc translocations occur frequently in human B lymphoid tumors, while N-myc gene amplification is frequent in human neuroblastomas. The observed association between aberrations in particular MYC family genes and specific subsets of malignancies might reflect, at least in part, tissue-specific differences in expression or function of a given MYC gene. Since c-Myc and N-myc share substantial functional redundancy, another factor that could influence tumor-specific gene activation would be mechanisms that target aberrations (e.g., translocations) in a given MYC gene in a particular tumor progenitor cell type. We have previously shown that mice deficient for the DNA Ligase4 (Lig4) nonhomologous DNA end-joining factor and the p53 tumor suppressor routinely develop progenitor (pro)-B cell lymphomas that harbor translocations leading to c-Myc amplification. Here, we report that a modified allele in which the c-Myc coding sequence is replaced by N-myc coding sequence (NCR allele) competes well with the wild-type c-Myc allele as a target for oncogenic translocations and amplifications in the Lig4/p53-deficient pro-B cell lymphoma model. Tumor onset, type, and cytological aberrations are similar in tumors harboring either the wild-type c-Myc gene or the NCR allele. Our results support the notion that particular features of the c-Myc locus select it as a preferential translocation/amplification target, compared to the endogenous N-myc locus, in Lig4/p53-deficient pro-B cell lymphomas.

The Myc target gene JPO1/CDCA7 is frequently overexpressed in human tumors and has limited transforming activity in vivo

MYC is frequently overexpressed in human cancers, but the downstream events contributing to tumorigenesis remain incompletely understood. MYC encodes an oncogenic transcription factor, of which target genes presumably contribute to cellular transformation. Although Myc regulates about 15% of genes and combinations of target genes are likely required for tumorigenesis, we studied in depth the expression of the Myc target gene, JPO1/CDCA7, in human cancers and its ability to provoke tumorigenesis in transgenic mice. JPO1/CDCA7 is frequently overexpressed in human cancers, and in particular, its expression is highly elevated in chronic myelogenous leukemia blast crisis as compared with the chronic phase. In murine lymphoid tissues, ectopic human JPO1/CDCA7 expression resulted in a 2-fold increased risk of lymphoid malignancies at 1 year. The transgene, which was driven by the H2-K promoter, exhibited leaky expression in nonlymphoid tissues such as kidney. We observed a significant increased incidence of transgenic animal solid tumors, which were not seen in littermate controls. These observations suggest that JPO1/CDCA7 may contribute to Myc-mediated tumorigenesis.

Artemis and p53 cooperate to suppress oncogenic N-myc amplification in progenitor B cells

The nonhomologous DNA end-joining (NHEJ) pathway contains six known components, including Artemis, a nuclease mutated in a subset of human severe combined immunodeficient patients. Mice doubly deficient for the five previously analyzed NHEJ factors and p53 inevitably develop progenitor B lymphomas harboring der(12)t(12;15) translocations and immunoglobin heavy chain (IgH)/c-myc coamplification mediated by a breakage-fusion-bridge mechanism. In this report, we show that Artemis/p53-deficient mice also succumb reproducibly to progenitor B cell tumors, demonstrating that Artemis is a tumor suppressor in mice. However, the majority of Artemis/p53-deficient tumors lacked der(12)t(12;15) translocations and c-myc amplification and instead coamplified IgH and N-myc through an intra- or interchromosome 12 breakage-fusion-bridge mechanism. We discuss this finding in the context of potential implications for mechanisms that may target IgH locus translocations to particular oncogenes.

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.

Effects of MYCN antisense oligonucleotide administration on tumorigenesis in a murine model of neuroblastoma

BACKGROUND

Human MYCN (hMYCN) oncogene amplification is a powerful predictor of treatment failure in childhood neuroblastoma, and dysregulation of hMYCN protein expression appears to be critically involved in the pathogenesis of this disease. We used hMYCN antisense (AS) oligonucleotides to investigate, both in vitro and in vivo, the therapeutic potential of inhibiting hMYCN expression.

METHODS

We transiently transfected human neuroblastoma IMR-32 cells, which have an amplified hMYCN gene, with fluorescently labeled hMYCN AS or scrambled (SCR) control oligonucleotides and used fluorescence-activated cell sorting to enrich for cell populations containing different levels of the oligonucleotides. We used fluorescence immunocytochemistry or reverse transcription polymerase chain reaction to assay gene expression levels and trypan blue exclusion to assay growth inhibition in the cell populations. We examined the effects of continuous treatment for 6 weeks with AS or SCR oligonucleotides via subcutaneously implanted microosmotic pumps on tumor growth in a transgenic mouse model of hMYCN-induced neuroblastoma (n = 20 mice per group). All statistical tests were two-sided.

RESULTS

IMR-32 cells treated with AS oligonucleotides had approximately half as much hMYCN protein and cell proliferation as either SCR oligonucleotide-transfected or mock-transfected controls; the differences were statistically significant. Transgenic mice treated with AS oligonucleotides had lower tumor incidence and statistically significantly lower tumor mass than SCR-treated or untreated control mice. Compared with control treatments, AS oligonucleotide treatment in vitro and in vivo was associated with decreased expression of hMYCN and putative hMYCN target genes but not with that of closely related genes. Several AS oligonucleotide-treated mice developed tumors contralateral to the site of oligonucleotide administration, whereas SCR oligonucleotide-treated or untreated mice displayed bilateral tumor growth.

CONCLUSIONS

Decreased expression of hMYCN protein is achievable with the use of AS oligonucleotide treatment, even in the presence of hMYCN oncogene amplification. Antisense strategies targeting the hMYCN oncogene in vivo decrease mouse neuroblastoma tumorigenesis. Investigation of their clinical effect in children with neuroblastoma is warranted.

Epstein-Barr virus nuclear antigen-1 and Myc cooperate in lymphomagenesis

The lymphomagenic action of myc genes in conjunction with Epstein-Barr virus nuclear antigen-1 (EBNA-1) have been examined using transgenic mice in several separate tests. Synergy between Myc and EBNA-1 in lymphomagenesis was revealed in a cross breed study where co-expression of transgenic myc and EBNA-1 led to a tumor latency period reduced significantly in some crosses. In the resulting bitransgenic tumors, expression of the Emu-myc genes was not affected by EBNA-1 expression. MoMLV was utilized as a transposon tag to activate cellular oncogenes by infection of EmuEBNA-1 mice. Rearrangement at the c-myc locus in B cell tumors from these mice again suggests a cooperative action between myc and EBNA-1. Tumors arising in EmuEBNA-1 mice typically showed a trisomy of chromosome 15, upon which the c-myc locus resides. Bitransgenic tumors (EBNA-1/c-myc) did not show trisomy 15. This raises the possibility that amplification of c-myc is factorial in the selection of trisomy 15 in these tumors. These data indicate that myc and EBNA-1 act cooperatively and are not redundant in lymphomagenesis. Expression of EBNA-1 by EBV may provide a selection pressure in addition to translocation of the c-myc locus in the genesis of endemic Burkitt's lymphoma (BL).

Regulation of signaling in B cells through the phosphorylation of Syk on linker region tyrosines. A mechanism for negative signaling by the Lyn tyrosine kinase

The B cell antigen receptor (BCR) is coupled to the mobilization of Ca(2+) by the protein-tyrosine kinase, Syk. Syk, recruited to the clustered BCR, becomes phosphorylated on three tyrosines (Tyr-317, Tyr-342, and Tyr-346) located within the linker region that separates the C-terminal catalytic domain from the N-terminal tandem Src homology 2 domains. Phosphorylation within the linker region can be either activating or inhibitory to Ca(2+) mobilization depending on the sites that are modified. Syk that is not phosphorylated on linker region tyrosines couples the BCR to Ca(2+) mobilization through a phosphoinositide 3-kinase-dependent pathway. The phosphorylation of Tyr-342 and -346 enhances the phosphorylation and activation of phospholipase C-gamma and the early phase of Ca(2+) mobilization via a phosphoinositide 3-kinase-independent pathway. The phosphorylation of Tyr-317 strongly dampens the Ca(2+) signal. In cells that lack the Src family kinase, Lyn, the phosphorylation of the inhibitory Tyr-317 is suppressed leading to elevated production of inositol 1,4,5-trisphosphate and an amplified Ca(2+) signal. This provides a novel mechanism by which Lyn functions as an inhibitor of BCR-stimulated signaling. Thus, Syk and Lyn combine to determine the pathway through which the BCR is coupled to Ca(2+) mobilization as well as the magnitude and duration of the Ca(2+) flux.

Recent advances in cancer research: mouse models of tumorigenesis

Over the past 20 years, cancer research has gained major insights into the complexity of tumor development, in particular into the molecular mechanisms that underlie the progressive transformation of normal cells into highly malignant derivatives. It is estimated that the transformation of a normal cell to a malignant tumor cell is dependent upon a small number of genetic alterations, estimated to be within the range of four to seven rate-limiting events. Critical events in the evolution of neoplastic disease include the loss of proliferative control, the failure to undergo programmed cell death (apoptosis), the onset of neoangiogenesis, tissue remodeling, invasion of tumor cells into surrounding tissue and, finally, metastatic dissemination of tumor cells to distant organs. In patients, the molecular analysis of these multiple steps is hampered by the unavailability of tumor biopsies from all tumor stages. In contrast, mouse models of tumorigenesis allow the reproducible isolation of all tumor stages, including normal tissue, which are then amenable to pathological, genetic and biochemical analyses and, hence, have been instrumental in investigating cancer-related genes and their role in carcinogenesis. In this review, we discuss mouse tumor models that have contributed substantially to the identification and characterization of novel tumor pathways. In particular, we focus on transgenic and knockout mouse models that closely mimic human cancer and thus can be used as model systems for cancer research.

Allele-specific loss or imbalance of chromosomes 9, 15, and 16 in B-cell tumors from interspecific F1 hybrid mice carrying Emu-c-myc or N-myc transgenes

Mice carrying an immunoglobulin enhancer (Emu-) linked c- or N-myc transgene develop fatal monoclonal or oligoclonal pre-B or B-cell lymphomas. This indicates that, beside the Emu-activated myc gene, additional genetic changes are required for tumor development. To trace these additional changes, we carried out a genome-wide search for loss of heterozygosity (LOH) and allelic imbalance (AI). This was done at 53 microsatellite markers in a panel of 34 lymphomas and four plasmacytomas from c- or N-myc transgene carrying (BALB/c x Mus spretus)F1 hybrids. An additional 43 lymphomas and three plasmacytomas from non-transgenic F1 mice were also investigated. Losses of one or more spretus-derived chromosome 9 markers were detected in 19 of 23 (83%) of the lymphomas, but in none of the four plasmacytomas that developed in N-myc F1 mice. No LOH-9 was found in any of the 11 lymphomas from Emu-c-myc F1 mice and only in 1 of 46 (2%) tumors derived from non-transgenic (BALB/c x spretus)F1 hybrid controls. These results suggest that a gene on spretus chromosome 9 confers resistance to the development of N-myc but not c-myc-induced lymphomas. AI of chromosome 15 markers (AI-15) was detected in 57 of 77 (74%) lymphomas and in 5 of 7 (72%) plasmacytomas, independently of the transgenic status and the mode of induction. All of the lymphomas and plasmacytomas with AI-15 revealed a relative gain of the spretus-derived D15Mit6 allele (located at 13.7 cM from the centromere), together with a gain of the BALB/c allele of the more distal (29.6 cM) D15Mit64 marker, suggesting somatic recombination. LOH in the region close to c-myc was detected in a proportion of tumors with AI-15. The observation of complex genetic alterations includes somatic recombination, AI and LOH involving chromosome 15 in tumors induced by a myc transgene. This indicates that at least two genes in addition to c-myc on this chromosome can be involved in lymphoma development.

N-myc can functionally replace c-myc in murine development, cellular growth, and differentiation

Members of the myc family of cellular oncogenes have been implicated as transcriptional regulators in pathways that govern cellular proliferation and death. In addition, N-myc andc-myc are essential for completion of murine embryonic development. However, the basis for the evolutionary conservation ofmyc gene family has remained unclear. To elucidate this issue, we have generated mice in which the endogenous c-myccoding sequences have been replaced with N-myc coding sequences. Strikingly, mice homozygous for this replacement mutation can survive into adulthood and reproduce. Moreover, when expressed from the c-myc locus, N-myc is similarly regulated and functionally complementary to c-myc in the context of various cellular growth and differentiation processes. Therefore, themyc gene family must have evolved, to a large extent, to facilitate differential patterns of expression.

N-myc can functionally replace c-myc in murine development, cellular growth, and differentiation

Members of the myc family of cellular oncogenes have been implicated as transcriptional regulators in pathways that govern cellular proliferation and death. In addition, N-myc and c-myc are essential for completion of murine embryonic development. However, the basis for the evolutionary conservation of myc gene family has remained unclear. To elucidate this issue, we have generated mice in which the endogenous c-myc coding sequences have been replaced with N-myc coding sequences. Strikingly, mice homozygous for this replacement mutation can survive into adulthood and reproduce. Moreover, when expressed from the c-myc locus, N-myc is similarly regulated and functionally complementary to c-myc in the context of various cellular growth and differentiation processes. Therefore, the myc gene family must have evolved, to a large extent, to facilitate differential patterns of expression.

Inhibition of Signaling Through the B Cell Antigen Receptor by the Protooncogene Product, c-Cbl, Requires Syk Tyrosine 317 and the c-Cbl Phosphotyrosine-Binding Domain

The Syk protein-tyrosine kinase couples the B cell Ag receptor (BCR) to intracellular biochemical pathways. Syk becomes phosphorylated on multiple tyrosine residues upon receptor cross-linking. Tyrosine 317 is a site of phosphorylation located within the linker region of Syk that separates the amino-terminal, tandem pair of SH2 domains from the carboxyl-terminal catalytic domain. The amino acid sequence surrounding phosphotyrosine 317 matches the consensus sequence for recognition by the phosphotyrosine-binding (PTB) domain of the protooncogene product, c-Cbl. The overexpression of c-Cbl in DT40 B cells inhibits Ag receptor-mediated activation of the NF-AT transcription factor. The ability of overexpressed c-Cbl to inhibit signaling requires both Syk tyrosine 317 and a functional c-Cbl PTB domain. Mutant forms of Syk lacking tyrosine 317 exhibit an enhanced ability to couple the BCR to pathways leading to the activation of both NF-AT and Elk-1. Coimmunoprecipitation experiments indicate that Syk phosphotyrosine 317 and the c-Cbl PTB domain enhance, but are not required for, all interactions between these two proteins. In unstimulated cells, c-Cbl and Syk can be isolated in a complex that also contains tubulin. A mutant form of Syk lacking tyrosine at position 317 exhibits an enhanced ability to interact with a diphosphopeptide modeled on the immunoreceptor tyrosine-based activation motif of the CD79a component of the Ag receptor. These studies indicate that c-Cbl may contribute to the regulation of BCR signaling by modulating the ability of Syk to associate with the BCR and couple the receptor to intracellular signaling pathways.

Mutational analysis of transgenic mouse B cell lymphomas: indication of a Trp53-independent pathway in tumor progression

Deregulated myc, bcl-2 and/or TP53 gene expression is associated with non-Hodgkin's B cell lymphomas (B-NHLs). Emu-N-myc transgenic mice that misexpress N-myc protein and carry a non-disrupted bcl-2 gene develop indolent B cell lymphomas reminiscent of the B-NHL, follicular lymphoma. Tumors from mice with end-stage disease exhibited discrete, nodular lesions as well as areas of diffuse tumor likely due to coalescence of enlarged follicles. Tumor DNAs were screened for mutations in the Trp53 gene, the murine homologue of the TP53 gene, which participates in B cell differentiation and survival. By PCR-based sequence analyses, we determined there were no mutations in exons 5-8, the common sites of TP53 mutation in B-NHLs. These findings suggested that disease progression in our novel murine lymphoma model may proceed via a Trp53-independent pathogenetic pathway.

Transgenic models of lymphoid neoplasia and development of a pan-hematopoietic vector

The pathways to lymphoid neoplasia have been explored in a number of transgenic models. Because B lymphoid malignancies often involve translocation of an oncogene (e.g. myc, bcl-2, cyclin D1) to an immunoglobulin locus, resulting in its deregulated expression, the consequences of oncogene overexpression in lymphocytes can be evaluated with transgenes driven by an immunoglobulin regulatory element, such as an enhancer from the IgH locus. Mice bearing such transgenes have provided insight into the preneoplastic state, including alterations in the control of cellular proliferation, differentiation or apoptosis. They have also allowed studies on oncogene cooperation in vivo and the modulating effect of genetic background. Briefly reviewed here are the models studied in the authors' laboratories. Mice bearing myc and bcl-2 transgenes have received most attention but others studied include abl, ras, cyclin D1 and bmi-1 oncogenes. Also discussed is a new transgenic vector that should facilitate transgenic approaches to non-lymphoid leukemias. The vector bears elements from the promoter region of the vav gene, which is expressed almost exclusively in hematopoietic cells. It has proven capable of driving transgene expression throughout the hematopoietic compartment, including progenitor cells and their precursors. This novel vector should aid studies on many aspects of hematopoiesis, including the modeling of leukemogenesis.

MYC oncogenes and human neoplastic disease

c-myc, N-myc and L-myc are the three members of the myc oncoprotein family whose role in the pathogenesis of many human neoplastic diseases has received wide empirical support. In this review, we first summarize data, derived mainly from non-clinical studies, indicating that these oncoproteins actually serve quite different roles in vivo. This concept necessarily lies at the heart of the basis for the observation that the deregulated expression of each MYC gene is reproducibly associated with only certain naturally occurring malignancies in humans and that these genes are not interchangeable with respect to their aberrant functional consequences. We also review evidence implicating each of the above MYC genes in specific neoplastic diseases and have attempted to identify unresolved questions which deserve further basic or clinical investigation. We have made every attempt to review those diseases for which significant and confirmatory evidence, based on studies with primary tumor material, exists to implicate MYC members in their causation and/or progression.

An intact NF-kappa B signaling pathway is required for maintenance of mature B cell subsets

Members of the NF-kappaB/Rel transcription factor family are expressed constitutively during B cell development and are further induced by mitogen activation. Mice harboring germline disruptions in individual NF-kappaB subunits exhibit distinct defects in B lymphocyte activation and survival. However, the role of NF-kappaB in the production and maintenance of B cell subsets has been difficult to dissect in these knockout animals due to functional impairment of other immune cells. To directly address the cell autonomous requirements for NF-kappaB in humoral immune compartments, transgenic mice were generated that express a transdominant form of Ikappa-Balpha in B lineage cells. Whereas expression of the inhibitor had only modest effects on basal or LPS-induced levels of NF-kappaB, transgenic B cells were significantly impaired for cellular proliferation and NF-kappaB induction in response to B cell receptor (BCR) crosslinking. Furthermore, the trans-dominant inhibitor produced a dose-dependent reduction in the population of mature splenic B cells. This cellular defect was more pronounced in long-lived B lymphocyte subsets that recirculate to the adult bone marrow. Together, these results indicate that BCR-mediated signaling must maintain NF-kappaB levels above a stringent threshold for proper regulation of B cell homeostasis.

Phosphorylation- and activation-independent association of the tyrosine kinase Syk and the tyrosine kinase substrates Cbl and Vav with tubulin in B-cells

Aggregation of the B-cell antigen receptor leads to the activation of the 72-kDa Syk protein-tyrosine kinase and the phosphorylation of tubulin on tyrosine. To explore the requirement of Syk catalytic activity for tubulin phosphorylation, tubulin was isolated from cytosolic fractions from anti-IgM-activated B-cells (DT40) that lacked endogenous Syk and immunoblotted with anti-phosphotyrosine antibodies. Tubulin was not tyrosine-phosphorylated in Syk- B-cells. Phosphorylation could be restored by the expression of wild-type, but not catalytically inactive, Syk. However, both catalytically inactive and wild-type Syk were capable of constitutive association with tubulin, indicating that tubulin phosphorylation is not required for this interaction. Anti-phosphotyrosine antibody immunoblotting of proteins adsorbed to colchicine-agarose revealed the presence of three major tubulin-associated phosphoproteins of 110, 90, and 74 kDa, the phosphorylation of which was dependent on Syk expression. The proteins of 110 and 90 kDa were identified as Cbl and Vav, two proto-oncogene products known to become prominently phosphorylated following receptor engagement. Both proteins were shown to be constitutively associated with tubulin.

Terminal Deoxynucleotidyl Transferase Expression During Neonatal Life Alters DH Reading Frame Usage and Ig-Receptor-Dependent Selection of V Regions

During neonatal life, Ig diversity is limited in many respects. The absence of terminal deoxynucleotidyl transferase (TdT) expression with the consequent lack of nontemplated addition during the neonatal period, coupled with the predominant usage of a single DH reading frame (RF), leads to severe limitations of diversity in the CDR3 region of Ig heavy (H) chains. The neonatal Ig H chain repertoire is also characterized by restricted VH usage, with predominant expression of certain VH segments, such as VH81x, that are rarely evident during adult life. In this report, we examine the effect of enforced TdT expression on the neonatal repertoire of VH81xDJH rearrangements. We find that TdT synthesis abrogates DH RF bias during the fetal/neonatal period through a Ig-receptor-independent mechanism. These findings suggest that DH RF bias during neonatal life is determined largely by homology-directed joining. We also find that TdT synthesis alters the selection of productively rearranged VH81xDJH alleles in the neonatal spleen through a Ig-receptor-dependent mechanism. Analysis of predicted CDR3 amino acid sequences indicates that positive selection of VH81x-encoded H chains is correlated with the presence of a consensus sequence immediately adjacent to the VH segment. These data support the hypothesis that the CDR3 region is critical in determining the ability of VH81x-encoded H chains to form functional receptors that support positive selection of B lymphocytes. Together, our results demonstrate that TdT can indirectly influence the Ig repertoire by influencing both receptor-dependent and receptor-independent selection processes.

Syk- and Lyn-Dependent Phosphorylation of Syk on Multiple Tyrosines Following B Cell Activation Includes a Site That Negatively Regulates Signaling

The Syk protein tyrosine kinase is an essential component of the B cell Ag receptor signaling pathway. Syk is phosphorylated on tyrosine following B cell activation. However, the sites that are modified and the kinases responsible for these modifications have yet to be determined. To approach this problem, we used a mapping strategy based on the electrophoretic separation of peptides on alkaline polyacrylamide gels to identify the tryptic phosphopeptides derived from metabolically labeled Syk. In this work, we report that Syk from activated B cells is phosphorylated principally on six tyrosines: one located between the tandem SH2 domains (Tyr130); three in the linker region (Tyr317, Tyr342, and Tyr346); and two in the catalytic domain (Tyr519 and Tyr520). The linker region sites are the primary targets of the Src family protein tyrosine kinase, Lyn, and include a site that negatively (Tyr317) regulates receptor signaling. Efficient phosphorylation of the catalytic domain and inter-SH2 domain tyrosines is catalyzed primarily by Syk itself, but only occurs to an appreciable extent in cells that express Lyn. We propose that these sites are phosphorylated following the binding of Syk to immunoreceptor tyrosine-based activation motif.

High-level DNA amplifications are common genetic aberrations in B-cell neoplasms

Gene amplification is one of the molecular mechanisms resulting in the up-regulation of gene expression. In non-Hodgkin's lymphomas, such gene amplifications have been identified rarely. Using comparative genomic hybridization, a technique that has proven to be very sensitive for the detection of high-level DNA amplifications, we analyzed 108 cases of B-cell neoplasms (42 chronic B-cell leukemias, 5 mantle cell lymphomas, and 61 aggressive B-cell lymphomas). Twenty-four high-level amplifications were identified in 13% of the patients and mapped to 15 different genomic regions. Regions most frequently amplified were bands Xq26-28, 2p23-24, and 2p14-16 as well as 18q21 (three times each). Amplification of several proto-oncogenes and a cell cycle control gene (N-MYC (two cases), BCL2, CCND2, and GLI) located within the amplified regions was demonstrated by Southern blot analysis or fluorescence in situ hybridization to interphase nuclei of tumor cells. These data demonstrate that gene amplifications in B-cell neoplasms are much more frequent than previously assumed. The identification of highly amplified DNA regions and genes included in the amplicons provides important information for further analyses of genetic events involved in lymphomagenesis.

Syk activation and dissociation from the B-cell antigen receptor is mediated by phosphorylation of tyrosine 130

Syk (p72(syk)) is a 72-kDa cytoplasmic protein-tyrosine kinase that serves as an essential component of the signal transduction machinery coupled to the B-cell antigen receptor. Syk is recruited to the receptor when it is cross-linked and, in response, becomes tyrosine-phosphorylated and activated before it dissociates from the receptor and appears in the cytoplasm. To begin to explore how tyrosine phosphorylation affects Syk activation and receptor binding, Tyr-130, which is localized within the Syk inter-Src homology 2 domain region, was substituted with Phe or Glu. Substitution of Tyr-130 with Phe enhanced the binding of Syk to the receptor and increased receptor-mediated protein tyrosine phosphorylation, while substitution with Glu greatly reduced this interaction. Replacement of Tyr-130 with Glu also increased the basal activity of the kinase, while replacement with Phe decreased its activity and uncoupled kinase activation from receptor engagement. These data suggest that the phosphorylation of Tyr-130 normally plays an important role in mediating both the activation of Syk and its release from the antigen receptor.

Some characteristics of neoplastic cell transformation in transgenic mice

The role of the expression of different cellular genes and viral oncogenes in malignant cell transformation is discussed. We pay special attention to the role of the genes for growth factors and their receptors and homeobox genes in oncogenesis. Based on both the literature and our own data, specific features of tumors developed in transgenic mice are discussed. All of these data are used to analyze current theories of multistep oncogenesis and the stochastic component in this process. We suggest that all known evidence about the mechanisms of oncogenesis be used in studying the problem at various structural and functional levels in an organism. The chapter shows that transgenic mice are a most suitable model for studying various aspects of malignant transformation from the molecular to the organismal and populational levels.

Detection and partial purification of a cruciform-resolving activity (X-solvase) from nuclear extracts of mouse B-cells

We have identified a cruciform-resolving enzyme (X-solvase) in nuclear extracts from mouse B-cells, called EMX1, by using an exonuclease-resistant cruciform DNA as a substrate. The cruciform was a 104-nt oligonucleotide that spontaneously adopted a branched conformation with four arms, each arm protected by a terminal loop of five T residues. A ligatable nick was left in one arm. After ligation, the covalently closed substrate was used to follow an 1800-fold purification of the mouse X-solvase (EMX1) from crude nuclear extracts by chromatography on DEAE-cellulose, MonoQ and heparin-Sepharose. The purest fractions containing EMX1 show high specificity for cruciform DNA. The cleavage pattern is indistinguishable from that found in the same substrates after treatment with endonuclease VII from phage T4 or endonuclease X3 from the yeast Saccharomyces cerevisiae. EMX1 and yeast endonuclease X3 were also found to be sensitive to anti-(endonuclease VII) antibodies which inhibited their reactions with cruciform DNAs in vitro.

Juxtaposition of N-myc and Ig kappa through a reciprocal t(6;12) translocation in a mouse plasmacytoma

Nearly all mouse plasmacytomas (MPCs) carry an Ig/myc translocation. Any one of the three Ig loci may participate, while the myc contribution has been limited to c-myc, excluding other members of the myc gene family. The same is true for the other two known Ig/myc translocation-carrying tumors, Burkitt's lymphoma and rat immunocytoma. It is believed that the Ig/myc juxtaposition plays a decisive, rate limiting role in the genesis of the three tumors, acting through the constitutive activation of myc that makes it refractory to normal regulation. Here we describe the molecular analysis of a unique t(6;12)(CI;B) translocation that we previously identified in an exceptional MPC that expressed N-myc but not c-myc. We now show that the translocation led to the juxtaposition of N-myc and Ig kappa. This is the first case of an Ig/myc-carrying tumor that involves N-myc rather than c-myc. These findings suggest that the translocation may already have occurred at the pro- or pre-B cell stage at which N-myc is open for transcription. According to this interpretation, constitutive activation of N-myc would suppress the expression of c-myc, but would not interfere with the differentiation of the pro-B cell into a fully mature plasma cell. Its tumorigenic influence would become manifest only at the time when the cell would normally be programmed to leave the cycling compartment, in connection with its terminal differentiation.

Molecular requirements for rapid plasmacytoma and pre-B lymphoma induction by Abelson murine leukemia virus in myc-transgenic mice

Mutants and fusion products of the c-abl gene were used to define some of the molecular requirements for rapid plasmacytoma (PC) and pre-B-lymphoma induction in pristane-treated N-myc transgenic BALB/c mice. A-MuLV induced PCs in 21 of 25 mice with a mean post-pristane latency period of 46 +/- 9 days, compared to 134 +/- 25 days in controls exposed to pristane alone. delta XB, a mutant of type IV c-abl with a deletion of the SH3 domain, was equally effective in inducing PCs in 7 of 7 mice with a latency period of 49 +/- 7 days, indicating that gag sequences are not required for rapid PC induction. The delta XB delta Nar mutant that carried a large C-terminal deletion in addition showed only a negligible activity, if any, suggesting that PC acceleration requires the C-terminal domain in the same way as lymphoid transformation and in contrast to fibroblast transformation. BCR-ABL fusion constructs encoding an 185-kDa protein as in acute leukemia, or a 210-kDa protein as in chronic myelocytic leukemia (CML), did not accelerate pristane-induced PC development in the N-myc transgenic mice, in contrast to their known ability to immortalize lymphoid cells in vitro. Only one of 14 non-transgenic littermates developed a pre-B lymphoma after A-MuLV infection, and none of 10 normal littermates infected with delta XB virus developed a construct-carrying tumor. This result suggests that PC acceleration is due to co-operative interaction of the N-myc transgene and activated abl. Infection of N-myc transgenic bone marrow or spleen cells with A-MuLV in vitro led to the outgrowth of pre-B lymphomas after transplantation to pristane-treated BALB/c recipients. The lymphoma-inducing activity of A-MuLV depends on its high titer, since diluted A-MuLV or the lower-titered delta XB induced only PCs under the same conditions. The v-abl, delta XB and BCR-ABL-carrying viruses generated immortalized lymphoblastoid lines in vitro, regardless of the presence of the N-myc transgene, suggesting that lymphoid transformation is a direct function of appropriate abl sequences in contrast to PC acceleration.

Regulation of gene expression at early stages of B-cell differentiation

The phenotype of B lymphocytes at various stages of differentiation is, in part, controlled at the transcriptional level. Recently, a number of B-cell lineage and stage-specific transcription factors have been identified as candidate determinants for the developmental regulation of gene expression in B lymphocytes.

Defects in heart and lung development in compound heterozygotes for two different targeted mutations at the N-myc locus

Two types of mutant allele, one leaky and one null, have been generated by gene targeting at the N-myc locus in embryonic stem cells and the phenotypes of mice homozygous for these mutations have been described. These mutations have shown that N-myc has a number of functions during development, including a role in branching morphogenesis in the lung, which manifests itself at birth in mice homozygous for the leaky allele, and roles in the development of the mesonephric tubules, the neuroepithelium, the sensory ganglia, the gut and the heart, which become evident at midgestation in embryos homozygous for the null allele. In an attempt to define roles for N-myc at other stages of development, we have combined the two types of N-myc mutant allele in a compound heterozygote that as a result contains approximately 15% of normal levels of N-Myc protein. Compound heterozygotes died during gestation at a time intermediate to the times of death of embryos homozygous for either mutation individually, and their death appeared to result from cardiac failure stemming from hypoplasia of the compact subepicardial layer of the myocardium. Investigation of the expression pattern of N-myc and various markers of differentiation in wild-type and compound heterozygote mutant hearts has suggested that N-myc may function in maintaining the proliferation and/or preventing the differentiation of compact layer myocytes. This study illustrates the importance of generating different mutations at a given locus to elucidate fully the function of a particular gene during development.

L-myc and N-myc in hematopoietic malignancies

The myc proto-oncogenes encode nuclear DNA-binding phosphoproteins which regulate cell proliferation and differentiation. The c-myc gene is implicated in hematopoietic malignancies on the basis of its frequent deregulation in naturally occurring leukemias and lymphomas. Recent evidence suggests that also the N-myc and L-myc genes may have a role in normal and malignant hematopoiesis. N-myc and to a certain degree L-myc can substitute for c-myc in transformation assays in vitro, and their overexpression can block the differentiation of leukemia cell lines. Immunoglobulin heavy chain enhancer (IgH) -driven overexpression of N-myc or L-myc genes cause lymphatic and myeloid tumors, respectively, in transgenic mice. Furthermore, the L-myc and N-myc genes are expressed in several human leukemias and leukemia cell lines, L-myc predominantly in myeloid and N-myc both in myeloid and in some lymphoid leukemias. All N/L-myc positive leukemias and leukemia cell lines coexpress the c-myc gene, thus exemplifying a lack of negative cross-regulation between the different myc genes in leukemia cells. Taken together, these data suggest that L-myc and N-myc may participate in the growth regulation of hematopoietic cells.

Chimeric homeobox gene E2A-PBX1 induces proliferation, apoptosis, and malignant lymphomas in transgenic mice

Expression of the homeobox fusion gene E2A-PBX1 under control of the immunoglobulin heavy chain enhancer efficiently induced malignancies in transgenic mice. All animals died before 5 months of age with lymphomas that demonstrated phenotypes consistent with transitional intermediate thymocytes (CD4+/CD8+/CD3med). E2A-PBX1 also markedly altered lymphoid development in pretumorous animals, reducing the number of thymocytes and bone marrow B lineage progenitors to 20% of normal levels. In spite of the observed reductions in lymphoid cells, premalignant animals contained significantly increased numbers of cycling thymocytes, but a higher proportion was also undergoing apoptosis, suggesting that increased cell death resulted in the marked lymphopenias. These data indicate that the chimeric homeodomain protein E2A-PBX1 paradoxically induces both proliferation and apoptosis in lymphoid cells, suggesting an in vivo association between nuclear oncogene-induced cell cycle progression and programed cell death.

Rearrangement of the N-myc oncogene in acute leukemias: a case report

Rearrangements of the N-myc oncogene have not been described in the literature. The authors present a case of ANLL with the N-myc gene rearranged out of 16 ANLLs and 11 ALLs. While the proportion of the blast cells in the case reported was 85%, the rearranged gene gave a hybridization band which was less intense than that of the germline band, making it probable that this gene rearrangement is present in a leukemia subclone. The c-myc oncogene as well as Ig and TcR genes were in germline configurations. The presence of the rearranged N-myc in the reported case could imply an alternative mechanism of mutation of this oncogene in the pathogenesis of hematological malignancies.

In vitro differentiation of a Hodgkin's disease derived cell line

We have examined the Hodgkin's disease derived cell line Co in terms of its capacity to differentiate in vitro. Co cells show the characteristics of immature T cells and express CD3 molecules in the cytoplasm. On activation with 12-O-tetradecanoylphorbol-13-acetate (TPA) these cells express the CD3 antigen and the T cell receptor alpha beta (TCR alpha beta) on the cell surface. Surface expression of the activation marker CD25 (IL2 receptor) was also greatly increased, whereas CD4 and CD8 levels were not altered. Supernatants of TPA-stimulated Co cells contained the cytokines IL2, IL3, IL4 and IL8, whereas these cytokines were not detected in the supernatants of untreated cells. Different subclones of the Co cell line differed in their response to TPA with respect to the induced CD3 and TCR expression. Our data demonstrate that a Hodgkin's disease derived cell line can be induced to differentiate in vitro from a pre-T cell phenotype towards a more mature T cell. It is possible that similar processes may occur in Hodgkin's disease in vivo.

An exceptional mouse plasmacytoma with a new kappa/N-myc [T(6; 12) (C1; B)] translocation expresses N-myc but not c-myc

Mouse plasmacytomas (MPC) carry one of three reciprocal translocations that juxtapose c-myc to one of the three immunoglobulin (Ig) loci. Here we describe an exceptional MPC, induced by pristane oil and Abelson (A-MuLV) virus. It does not carry any of the three c-myc/Ig translocations, but contains a previously unknown reciprocal T(6;12) translocation affecting the bands known to carry the IgK (6C/1) and N-myc (12B) loci, respectively. Northern blot analysis showed high N-myc but no c-myc expression. This is consistent with the constitutive activation of N-myc by a juxtaposition of the IgK and N-myc loci. Reciprocal translocation in B-cell derived tumors are believed to involve the Ig loci by the action of some enzyme that participates in the physiological rearrangement of the Ig loci. Only transcriptionally active chromatin regions are accessible to such recombinases (Alt et al. 1987). N-myc is not expressed in B-cells, but it is transcriptionally active during the early pro- and pre-B cell stage, whereafter it and the surrounding chromatin region becomes inactive (Smith et al. 1992). It is therefore most likely that the N-myc/Kappa translocation has arisen at an early stage of B-cell differentiation. This would imply that the myc/Ig translocations do not block B-cell differentiation. They also reaffirm the functional equivalence of N- and c-myc in relation to B-cell carcinogenesis, as shown by our previous work on tumor induction in N-myc transgenic mice (Wang et al. 1992).

Molecular biology in the diagnosis and prognosis of solid and lymphoid tumors

The application of molecular biology to the study of human malignancies has led to tremendous gains in our understanding of their pathogenesis. Although their practical applications are still somewhat limited at this point, the use of molecular diagnostic tools is likely to grow at a very rapid rate as newer and more accurate prognostic markers are identified. The availability of reliable prognostic markers should allow earlier intervention in patients with aggressive disease but exhibiting only limited extent of disease at the time of initial diagnosis. Early intervention in such cases could realistically increase the probability of cure, since highly aggressive tumor cells are more likely to be eliminated by early institution of cytotoxic chemotherapy (4). The p53 tumor suppressor gene clearly represents the most promising potential prognostic marker at present, because of both the multiple phenotypic alterations caused by different p53 mutations and the high frequency of p53 mutations which have been observed in a variety of human cancers. Other prognostic markers related to oncogenes and tumor suppressor genes are almost certain to follow. Validation of new prognostic markers requires a knowledge of both histopathologic diagnostic criteria as well as the consequences for the patient of each diagnosis. There is bound to be some "shake-out" in the field of molecular diagnostics just as there was with other recently introduced techniques such as immunohistochemistry and flow cytometry which were found to provide additional useful information for some tumors and not for others. Since the clinical-pathologic studies needed for verification of putative prognostic markers require relatively long periods of follow up, progress in this area will almost certainly lag behind the ability of molecular biologists to identify new and potentially useful prognostic markers. Our collective ability to reap tangible gains in the clinical arena from our heavy investments in molecular biology and biotechnology depends to a large extent on open channels of communication between clinical and basic scientists. As our ever-increasing insights into oncogenic processes spawn new diagnostic and prognostic markers, our priorities should remain focused on those areas which are inadequately addressed by current methods, and we should avoid the technological trap of devising redundant solutions which increase the expense, but not the efficiency of patient care.

Transgenic models of tumor development

Numerous cancer-prone strains of mice have been created by the introduction of candidate tumor-promoting genes into fertilized eggs. Each transgenic strain is predisposed to develop specific types of tumors, but they usually arise stochastically because of the need for spontaneous mutation of genes that collaborate with the introduced oncogene. These mice are providing insights into the effects of individual oncogenes on cellular proliferation, differentiation, and viability, as well as on oncogene cooperativity. Their predisposed state imposes sensitivity to viral and chemical carcinogenesis, and the mice should prove valuable in tests of potential carcinogens, therapies, and preventive measures.

Preferential development of pre-B lymphomas with drastically down-regulated N-myc in the E mu-ret transgenic mice

We established one transgenic mouse line which developed pre-B leukemic lymphomas by introducing ret cDNA driven by the SV40 promoter and the mouse immunoglobulin (Ig) enhancer. Lymphomas developed not only in the lymph nodes and the spleen but also in the thymus between the ages of 7 and 21 weeks. Analyses of cell surface phenotypes and Ig gene rearrangement revealed that these tumors were surface IgM-B220+ pre-B lymphomas. The rearrangement pattern of the Ig heavy chain locus indicated that the tumor cells were mono- or oligoclonal. Northern blot analysis showed that the ret transgene was expressed at a high level not only in the tumors but also in the prelymphomatous lymphoid tissues. We found that the expression of N-myc was dramatically down-regulated in the tumor cells, while the expression of c-myc was rather stable. Further experiments demonstrated that ret gene product did not directly down-regulate the expression of N-myc in transformed pre-B cell lines by in vitro transfection assay. From these results, we conclude that under the control of Ig enhancer, the ret transgene affected B lymphocytes at the early maturation stage as a prerequisite for transformation, preferentially generating a unique maturation stage of pre-B lymphomas whose N-myc expression was developmentally down-regulated.