Runx1 Orchestrates Sphingolipid Metabolism and Glucocorticoid Resistance in Lymphomagenesis.

The three-membered RUNX gene family includes RUNX1, a major mutational target in human leukemias, and displays hallmarks of both tumor suppressors and oncogenes. In mouse models, the Runx genes appear to act as conditional oncogenes, as ectopic expression is growth suppressive in normal cells but drives lymphoma development potently when combined with over-expressed Myc or loss of p53. Clues to underlying mechanisms emerged previously from murine fibroblasts where ectopic expression of any of the Runx genes promotes survival through direct and indirect regulation of key enzymes in sphingolipid metabolism associated with a shift in the "sphingolipid rheostat" from ceramide to sphingosine-1-phosphate (S1P). Testing of this relationship in lymphoma cells was therefore a high priority. We find that ectopic expression of Runx1 in lymphoma cells consistently perturbs the sphingolipid rheostat, whereas an essential physiological role for Runx1 is revealed by reduced S1P levels in normal spleen after partial Cre-mediated excision. Furthermore, we show that ectopic Runx1 expression confers increased resistance of lymphoma cells to glucocorticoid-mediated apoptosis, and elucidate the mechanism of cross-talk between glucocorticoid and sphingolipid metabolism through Sgpp1. Dexamethasone potently induces expression of Sgpp1 in T-lymphoma cells and drives cell death which is reduced by partial knockdown of Sgpp1 with shRNA or direct transcriptional repression of Sgpp1 by ectopic Runx1. Together these data show that Runx1 plays a role in regulating the sphingolipid rheostat in normal development and that perturbation of this cell fate regulator contributes to Runx-driven lymphomagenesis. J. Cell. Biochem. 118: 1432-1441, 2017. © 2016 Wiley Periodicals, Inc.

Sensitivity to myc-induced apoptosis is retained in spontaneous and transplanted lymphomas of CD2-mycER mice.

To study the effects of the Myc oncoprotein in a regulatable in vivo system, we generated lines of transgenic mice in which a tamoxifen inducible Myc fusion protein (c-mycER) is expressed under the control of the CD2 locus control region. Activation of the Myc oncoprotein resulted in both proliferation and apoptosis in vivo. Lines with a high transgene copy number developed spontaneous lymphomas at low frequency, but the tumour incidence was significantly increased with tamoxifen treatment. Surprisingly, we found that cellular sensitivity to Myc-induced apoptosis was retained in tumours from these mice and in most lymphoma cell lines, even when null for p53. Resistance to Myc-induced apoptosis could be conferred on these cells by co-expression of Bcl-2. However, acquired resistance is clearly not an obligatory progression event as sensitivity to apoptosis was retained in transplanted tumours in athymic mice. In conclusion, lymphomas arising in CD2-mycER mice retain the capacity to undergo apoptosis in response to Myc activation and show no phenotypic evidence of the presence of an active dominant inhibitor.

A full-length Cbfa1 gene product perturbs T-cell development and promotes lymphomagenesis in synergy with myc.

The Cbfa1/PEBP2 alpha A/AML3 gene plays an essential role in osteogenesis but is also expressed in the T-cell lineage where it has been implicated in lymphoma development as a target for retroviral insertional mutagenesis. As lymphoma cells with til-1 insertion express at least five distinct Cbfa1 isoforms, it is important to establish which, if any, have intrinsic oncogenic potential. We have generated transgenic mice in which the most abundant lymphoma isoform (G1/p57) is expressed under the control of the CD2 locus control region. Co-precipitation analysis of transgenic thymus revealed high levels of Cbfa1 protein in an abundant complex containing the binding cofactor Cbfb. CD2-Cbfa1-G1 mice displayed abnormal T-cell development, with a pronounced skew towards CD8 SP cells in the thymus and developed a low incidence of spontaneous lymphomas (6% at 12 months) with cells of similar phenotype. Strongly synergistic tumour development was seen when CD2-Cbfa1-G1 mice were crossed with lines carrying myc transgenes (CD2-myc or tamoxifen-regulatable CD2-mycER) and Cbfa1 was found to rescue expression of the CD2-myc transgene in pre-leukaemic mice. However, synergy did not appear to be due to a dominant block of myc-induced apoptosis by Cbfa1 as explanted primary tumours and cell lines from CD2-Cbfa1-G1/CD2-mycER mice showed accelerated death on induction with tamoxifen at similar rates to CD2-mycER controls. Moreover, thymocytes from preleukaemic CD2-Cbfa1-G1 mice showed reduced survival in vitro and increased sensitivity to the inhibitory effects of TGF-beta. This study demonstrates that a full-length Cbf alpha-chain gene can act as an oncogene without fusion to a heterologous protein.

Synergy between a human c-myc transgene and p53 null genotype in murine thymic lymphomas: contrasting effects of homozygous and heterozygous p53 loss.

Activation of the c-myc oncogene and functional loss of the p53 tumour suppressor gene are among the most frequently recorded genetic lesions in neoplasia but their combined effect has not previously been investigated. By breeding together mice transgenic for human c-myc (CD2-myc) and mice carrying an inactive p53 allele (p53-/-) we found that these genetic lesions act synergistically in vivo. Offspring carrying the CD2-myc transgene and the homozygous p53 null mutation (p53-/-/CD2-myc) were viable but developed thymic lymphomas with dramatically increased frequency and reduced latency compared to both parental groups. The tumour phenotype was similar to that previously recorded for CD2-myc mice (predominantly CD3+, CD4+8+) but tumour clonal complexity and metastasis was significantly greater in the p53-/-/CD2-myc mice. In contrast, no significant increase in tumour incidence was seen in p53+/-/CD2-myc vs p53+/+/CD2-myc mice over a 6 month observation period. However, the loss of wild type p53 in a proportion of tumour cells in p53+/-/CD2-myc lymphomas suggests that wild type allele loss can occur as a late progression step rather than an initiating step in these tumours. We suggest that p53 loss of function may collaborate with the CD2-myc transgene at more than one stage in thymic lymphoma development.

Runx2: a novel oncogenic effector revealed by in vivo complementation and retroviral tagging.

The Runx2 (Cbfa1, Pebp2alphaA, Aml3) gene was previously identified as a frequent target for transcriptional activation by proviral insertion in T-cell lymphomas of CD2-MYC transgenic mice. We have recently shown that over-expression of the full-length, most highly expressed Runx2 isoform in the thymus perturbs T-cell development, leads to development of spontaneous lymphomas at low frequency and is strongly synergistic with Myc. To gain further insight into the relationship of Runx2 to other lymphomagenic pathways, we tested the effect of combining the CD2-Runx2 transgene either with a Pim1 transgene (E(mu)-Pim1) or with the p53 null genotype, as each of these displays independent synergy with Myc. In both cases we observed synergistic tumour development. However, Runx2 appeared to have a dominant effect on the tumour phenotype in each case, with most tumours conforming to the CD3(+), CD8(+), CD4(+/-) phenotype seen in CD2-Runx2 mice. Neonatal infection of CD2-Runx2 mice with Moloney murine leukaemia virus (Moloney MLV) also led to a dramatic acceleration of tumour onset. Analysis of known Moloney MLV target genes in these lymphomas showed a high frequency of rearrangement at c-Myc or N-Myc (82%), and a significant number at Pim1 or Pim2 (23%), and at Pal1/Gfi1 (18%). These results indicate that Runx2 makes a distinct contribution to T-cell lymphoma development which does not coincide with any of the oncogene complementation groups previously identified by retroviral tagging.

p53 and tumour viruses: catching the guardian off-guard.

Oncogenic viruses have evolved direct and indirect mechanisms to overcome the tumour suppressor p53. Fortunately, tumour development is limited by the narrow cell tropisms of the viruses concerned and the host immune response. However, such viruses are helping to elucidate the p53 response pathway and may play a future role as novel cancer therapeutic agents.

Fas-independent apoptosis in T-cell tumours induced by the CD2-myc transgene.

Depending on the cellular context, the Myc oncoprotein is capable of promoting cell proliferation or death by apoptosis. These observations suggest that apoptosis in response to deregulated gene expression may represent a natural brake to tumour development. The pathways by which Myc induces apoptosis are as yet poorly characterised although recent observations on rat fibroblasts over-expressing Myc have demonstrated a requirement for the Fas pathway. To investigate the role of Fas in Myc-induced lymphomagenesis we backcrossed CD2-myc mice onto an lpr background. Rates of tumour development and phenotypic properties, including levels of apoptosis were indistinguishable from CD2-myc controls. Further, tumour cell lines derived from mice expressing a regulatable form of Myc showed inducible apoptosis at similar rates regardless of their lpr genotype. These results show that activation of c-myc and loss of Fas do not collaborate in T lymphoma development and that Myc-induced apoptosis in T-cells occurs by Fas-independent pathways.

Recent advances in transgenic technology.

Techniques that allow modification of the mammalian genome have made a considerable contribution to many areas of biological science. Despite these achievements, challenges remain in two principal areas of transgenic technology, namely gene regulation and efficient transgenic livestock production. Obtaining reliable and sophisticated expression that rivals that of endogenous genes is frequently problematic. Transgenic science has played an important part in increasing understanding of the complex processes that underlie gene regulation, and this in turn has assisted in the design of transgene constructs expressed in a tightly regulated and faithful manner. The production of transgenic livestock is an inefficient process compared to that of laboratory models, and the lack of totipotential embryonic stem (ES) cell lines in farm animal species hampers the development of this area of work. This article highlights recent progress in efficient trans gene expression systems, and the current efforts being made to find alternative means of generating transgenic livestock.

Transgenic science.

The ability to manipulate the genome of the whole animal has, for the past 10 years, provided researchers with an alternative route of inquiry into many complex biological processes. Transgenic animals have numerous applications, encompassing a wide range of different disciplines, but they have proved especially useful in the investigation of gene regulation and gene function within the context of the living animal. This review describes the different techniques which have been used to produce transgenic animals and highlights advances which have been achieved using the transgenic approach.

Skewed T-cell receptor Vbeta8.2 expression in transgenic CD2-myc induced thymic lymphoma: a role for antigen stimulation in tumour development?

Transgenic mice expressing the c-myc proto-oncogene under the control of the CD2-dominant control region show stochastic development of mainly clonal thymic lymphoma with long latency, indicating that cooperative events are needed for the development of the fully malignant phenotype. Previous studies have suggested that T-cell receptor-associated signals can contribute to tumour development. We have therefore used this transgenic model of T-cell transformation to determine whether antigen-specific responses could constitute an epigenetic event in lymphomagenesis. The T-cell receptor (TcR) repertoires of lymphoma clones were analysed with a panel of monoclonal antibodies (Abs) recognizing TcR Vbeta chains. The Vbeta repertoire of tumour clones arising in these mice was non-random with overrepresentation of Vbeta8.2 TcR species. The majority of Vbeta8.2+ clones were of a mature CD3+ CD8 single-positive (SP) phenotype. The biased TcR usage, together with a mature cell phenotype is consistent with the hypothesis that TcR-mediated signals cooperate with activated myc during T-cell transformation.

Selection for loss of p53 function in T-cell lymphomagenesis is alleviated by Moloney murine leukemia virus infection in myc transgenic mice.

Thymic lymphomas induced by Moloney murine leukemia virus (MMLV) have provided many examples of oncogene activation, but the role of tumor suppressor pathways in these tumors is less clear. These tumors display little evidence of loss of heterozygosity, and MMLV is only weakly synergistic with the Trp53 null genotype, suggesting that viral lymphomagenesis involves mechanisms which do not require mutational loss of Trp53 function. To explore this relationship in greater depth, we infected CD2-myc transgenic mice with MMLV and examined the role of Trp53 in the genesis of these tumors. Most (19 of 27) of the tumors from MMLV-infected, CD2-myc Trp53(+/-) mice retained the wild-type Trp53 allele in vivo while tumors of uninfected CD2-myc Trp53(+/-) mice invariably showed allele loss from a significant fraction of primary tumor cells. The functional integrity of the Trp53 gene in these tumors was indicated by ongoing allele loss or selection for mutational stabilization during in vitro propagation and by the radiosensitivity of selected Trp53(+/-) tumor cell lines. An inverse correlation was noted between retention of the wild-type Trp53 allele and expression of p19(ARF), providing further evidence of negative-feedback control of the latter by p53. However, expression of p19(ARF) does not appear to be counterselected in the absence of p53, and its integrity in Trp53(+/-) tumors was indicated by its transcriptional upregulation on Trp53 wild-type allele loss in vitro in selected tumor cell lines. The role of MMLV was investigated further by analysis of proviral insertion sites in tumors of CD2-myc transgenic mice sorted for Trp53 genotype. A proportion of tumors showed insertions at Runx2, an oncogene which has been shown to collaborate independently with CD2-myc and with the Trp53 null genotype, and at a novel common integration site (ptl-1) on chromosome 8. Genotypic analysis of the panel of tumors suggested that neither of these integrations is functionally redundant with loss of p53, but it appears that the combination of the MMLV oncogenic program with the CD2-myc oncogene relegates p53 loss to a late step in tumor progression or in vitro culture. While the means by which these tumors preempt the p53 tumor suppressor response remains to be established, this study provides further evidence that irreversible inactivation of this pathway is not a prerequisite for tumor development in vivo.

Moloney murine leukemia virus-induced lymphomas in p53-deficient mice: overlapping pathways in tumor development?

The effect of Moloney murine leukemia virus (MoMLV) infection was examined in mice lacking a functional p53 gene. Virus-infected p53-/- mice developed tumors significantly faster than uninfected p53-/- or virus-infected p53+/+ littermates. However, the degree of synergy between MoMLV and the p53 null genotype was weaker than the synergy between either of these and c-myc transgenes. A similar range of T-cell tumor phenotypes was represented in all p53 genotype groups, including p53-/- mice, which developed thymic lymphomas as the most common of several neoplastic diseases. Lack of p53 was associated with higher rates of metastasis and the ready establishment of tumors in tissue culture. Loss of the wild-type allele was a common feature of tumors in p53+/- mice and was complete in tumor cells in vitro, but this appeared to occur by a mechanism other than proviral insertion at the wild-type allele. A lower average MoMLV proviral copy number was observed in tumors of the p53 null and heterozygote groups, suggesting that the absence of a functional p53 gene reduced the number of steps required to complete the malignant phenotype. Mink cell focus-forming virus-like proviruses were detected in tumors of all infected mice but were relatively rare in p53 null mice. Analysis of c-myc, pim-1, and pal-1 showed that these loci were occupied by proviruses in some cases but at similar frequencies in p53 wild-type and null mice. In conclusion, while inactivation of p53 in the germ line predisposes mice to tumors similar in phenotype to those induced by MoMLV, it appears that virus-induced tumors generally occur without p53 loss. We speculate that a bcl-2-like function carried or induced by MoMLV may underlie this p53-independent pathway.

Tumours derived from HTLV-I tax transgenic mice are characterized by enhanced levels of apoptosis and oncogene expression.

In order to investigate the role that the human T-lymphotropic virus type I (HTLV-I) tax oncogene plays in apoptosis and transformation in vivo, four lines of HTLV-I tax transgenic mice were generated under the regulatory control of the CD3-epsilon promoter-enhancer sequence. These mice develop a variety of phenotypes including mesenchymal tumours, which develop at wound sites, and salivary and mammary adenomas. In situ DNA fragment labelling and immunocytochemical analysis of these tumours reveals that they display enhanced levels of apoptosis, which is associated with elevated levels of Myc, Fos, Jun, and p53 protein expression. Furthermore, double immunofluorescent staining shows that Tax expression and apoptosis co-localize, indicating that Tax expression is closely associated with apoptosis in vivo.

Apparent bypass of negative selection in CD8+ tumours in CD2-myc transgenic mice.

A role for antigen stimulation in lymphoid neoplasia has been postulated and is supported by indirect evidence that suggests that the interaction of antigen with both T cells and B cells may constitute an epigenetic event that can contribute to tumour induction or tumour progression. Using myc-bearing transgenic mice that develop mainly clonal T-cell lymphomas we have investigated the possibility that endogenous antigen-mediated clonal deletion might be overridden in tumorigenesis. CD2-myc transgenic mice were backcrossed on to a CBA/Ca background to ensure Mtv-mediated deletion of V beta 11-expressing T cells in the resultant offspring. Lymphomas arising from these mice were subsequently screened for V beta 11 expression. There was a clear correlation between the age at which mice developed neoplasia and the tumour phenotype. Mice with CD4- CD8+ tumours succumbed to thymic lymphoma at a significantly younger age than mice developing CD4+ CD8+ tumours. A small number of tumours consisted of the 'forbidden' V beta 11 phenotype, showing that cells vulnerable to transformation could escape negative selection. The majority of the V beta 11-positive tumours were CD4- CD8+ and were only observed in mice showing clinical evidence of tumour development at a relatively young age. The phenotype of these cells and the age at which tumours arose suggests that T cells escaping tolerance may be susceptible to transformation.

Long-range effects of retroviral insertion on c-myb: overexpression may be obscured by silencing during tumor growth in vitro.

The c-myb oncogene is a frequent target for retroviral activation in hemopoietic tumors of avian and mammalian species. While insertions can target the gene directly, numerous clusters of retroviral insertion sites have been identified which map close to c-myb and outside the transcription unit in T-lymphomas (Ahi-1, fit-1, and Mis-2) and monocytic and myeloid leukemias (Mml1, Mml2, Mml3, and Epi-1). Previous analyses showed no consistent effect of these insertions on c-myb expression, raising the possibility that other nearby genes were the true targets. In contrast, our analysis of four cell lines established from lymphomas bearing insertions at fit-1 (fti-1) (feline leukemia virus) and Ahi-1 (Moloney murine leukemia virus) shows that these display higher expression levels of c-myb RNA and protein compared to a panel of phenotypically similar cell lines lacking such insertions. An interesting feature of the cell lines with long-range c-myb insertions was that each also carried an activated Myc allele. The potential for oncogenic synergy between Myb and Myc in T-cell lymphoma was confirmed in transgenic mice overexpressing alleles of both genes in the T-cell compartment, lending further credence to the case for c-myb as the major target for long-range activation. In contrast, mapping and analysis of c-myb neighboring genes (HBS1 and FLJ20069) showed that the expression of these genes did not correlate well with the presence of proviral insertions. A possible explanation for the paradoxical behavior of c-myb was provided by one of the murine T-lymphoma lines bearing an insertion at Ahi-1 (p/m16i) that reproducibly down-regulated c-myb RNA and protein to very low levels or undetectable levels on prolonged culture. Our observations implicate c-myb as a key target of upstream and downstream retroviral insertions. However, overexpression may become dispensable during outgrowth in vitro, and perhaps during tumor progression in vivo, providing a potential rationale for the previously observed discordance between retroviral insertion and c-myb expression levels.