SL3-3 Murine Leukemia Virus Research Articles

Gene Expression Profiling of Murine T-Cell Lymphoblastic Lymphoma Identifies Deregulation of S-Phase Initiating Genes.

AbstractAbstract 2395Human T-cell lymphoblastic lymphomas (T-LBLs) are neoplasms of immature T-cells and constitute a group of rare, heterogeneous and clinically very aggressive tumors. The molecular pathogenesis that contributes to T-LBL development is not fully elucidated. Since murine T-LBLs are histopathologically and phenotypically comparable to human T-LBLs, mouse models of T-LBLs are ideal to obtain additional insight into the mechanism of T-LBL development in humans. When injected into newborn mice of the NMRI inbred strain, the SL3-3 murine leukemia virus (MLV) induces various types of hematological malignancies, including T-LBLs. The oncogenic effects of the SL3-3 MLV are caused by integration of the viral genome into the host cell DNA through multiple rounds of infection, and subsequent deregulation of nearby cellular genes – a process defined as insertional mutagenesis. If the integration occurs near or within a gene of importance for cancer development, the cell in which the virus has integrated, may gain a growth advantage, eventually leading to malignant transformation and development of a full blown tumor. Screening the murine genome for resulting integration sites in the end-stage tumors, is therefore an efficient method for identifying genes involved in murine and potentially also human T-cell lymphomagenic processes.In a search for genes and pathways implicated in T-cell lymphoblastic lymphoma (T-LBL) development, we used a murine lymphoma model, where mice of the NMRI inbred strain were inoculated with mutants of SL3-3 MLV. The mutants were affected in the glucocorticoid response element and an overlapping E-box of the viral enhancer in the long termial releat. By performing integration analysis on 19 and global gene expression profiling on 22 of the resulting T-LBL tumors, we determined both the effect of the retroviral integrations on the summarized expression of the nearby genes, and the deregulated pathways in the tumors. Fifty two different genes were identified within a 10 kb distance of the retroviral integrations, whereof 15 were specifically involved in G1/S phase transition. Gene expression dot-plots showed an activating effect of the retrovirus on Mr1, Stx6, Cask and Sh3gl3. Gene expression profiling identified increased expression of genes involved in the minichromosome maintenance (Mcm) and origin of recognition (Orc) pathway as well as downregulation in negative regulators of G1/S transition, indicating that murine T-LBLs have increased S-phase initiation. In conclusion, both the integration analysis and patterns of mRNA expression identified by gene expression profiling in the mouse models of T-LBL strongly indicate that genes involved in G1/S phase transition and/or S-phase initiation are deregulated suggesting similar mechanisms to be of importance in human T-LBL pathogenesis. Disclosures:No relevant conflicts of interest to declare.

Impact of Growth Factor Independence 1 in Human T-Cell Lymphomas; Pathogenic Potential Identified by Insertional Mutagenesis in a Murine T-Cell Lymphoma Model.

Abstract 5047 IntroductionThe transcriptional repressor and oncogene Growth factor independence 1 (Gfi1) has a major oncogenic potential and is aberrantly expressed in murine lymphomas and several human cancers. Gfi1 is a key regulator of stem cell quiescence and plays a significant role in T-cell development, lineage commitment, and influences development of maturate granulocytes and monocytes. The genomic locus on murine chromosome 5 encoding Gfi1 is a frequent integration locus activated in T-cell lymphomas induced by the SL3-3 Murine Leukemia Virus (MLV) as well as other MLVs, indicating that Gfi1 is essential in development of these tumors. In the SL3-3 induced T-cell lymphoma model, retroviral insertions in the Gfi1 3'UTR have been demonstrated to decouple microRNA-mediated posttranscriptional regulation of protein expression (Dabrowska et al, 2009) further supporting its role in lymphomagenesis. In human cancers, Gfi1 protein expression has been observed in HTLV-1 induced ATLL and SCLC but no knowledge exists on how Gfi1 contributes to initiating and maintaining human T-cell lymphomas. MethodsGfi1 gene and protein expression patterns were determined in precursor and mature human T-cell lymphomas by real time PCR and Western blot analysis. Furthermore, localization and expression patterns of the Gfi1 protein was determined in the human T-cell lymphomas by immunohistochemical staining with Gfi1 antibodies and compared to similar staining of MLV induced tumors. ResultsOur results demonstrated that Gfi1 mRNA and protein levels vary significantly among the human T-cell lymphomas, and do not always show a direct proportional pattern. Thus, Gfi1 mRNA expression can be relatively high without resulting in a corresponding high protein expression, suggesting that a microRNA mediated posttranscriptional regulation exists in some tumors but may be disrupted in others. Furthermore, an additional Gfi1 protein variant was identified in one of the T-cell lymphoma entities. Immunohistochemical staining demonstrated varying Gfi1 protein expression in both nucleus and cytoplasm in the T-cell lymphomas and different distributions of the protein within the tumor and tumor cells were observed among samples. Staining of normal human tonsil demonstrated Gfi1 protein to be localized in the cytoplasm. We hypothesise that regulation of Gfi1 may include shuttling between cytoplasm and nucleus and that lymphomagenesis enables unlimited nuclear access. ConclusionOur data shows that deregulated Gfi1 expression plays a major role in the development of MLV induced lymphomas and strongly indicates that retroviral insertional mutagenesis in murine models of human NHLs can be used to identify new genes involved in lymphomagenesis and, by use of functional assays, their impact on human lymphomas can be evaluated. DisclosuresNo relevant conflicts of interest to declare.

Loss of MicroRNA Targets in the 3′ Untranslated Region as a Mechanism of Retroviral Insertional Activation of Growth Factor Independence 1

The non-oncogene-bearing retrovirus SL3-3 murine leukemia virus induces strictly T-cell lymphomas with a mean latency of 2 to 4 months in mice of the NMRI-inbred (NMRI-i) strain. By high-throughput sequencing of retroviral tags, we have identified the genomic region carrying the transcriptional repressor and oncogene growth factor independence 1 (Gfi1) as a frequent target for SL3-3 in the NMRI-i mouse genome. Twenty-four SL3-3 insertions were identified within a 1-kb window of the 3' untranslated region (3'UTR) of the Gfi1 gene, a clustering pattern unique for this lymphoma model. Expression analysis determined that the Gfi1 gene was transcriptionally activated by SL3-3 insertions, and an upregulation of Gfi1 protein expression was detected for tumors harboring insertions in the Gfi1 3'UTR. Here we provide data in support of a mechanism by which retroviral insertions in the Gfi1 3'UTR decouple microRNA-mediated posttranscriptional regulation.

Activation of the brain-specific neurogranin gene in murine T-cell lymphomas by proviral insertional mutagenesis

Neurogranin (Nrgn) is a highly expressed brain-specific protein, which sequesters calmodulin at low Ca 2+-levels. We report here on retroviral activation of the Nrgn gene in tumors induced by the T-cell lymphomagenic SL3-3 murine leukemia virus. We have performed a systematic expression analysis of Nrgn in various mouse tissues and SL3-3 induced T-cell tumors. This demonstrated that insertional activation of Nrgn increased RNA and protein expression levels to that observed in brain. Furthermore, elevated Nrgn expression was also observed in some T-cell tumors with no detected provirus integrations into this genomic region. The presented data demonstrate that Nrgn can be produced at high levels outside the brain, and suggest a novel oncogenic role in T-cell lymphomas in mice.

Activation of an oncogenic microRNA cistron by provirus integration

Retroviruses can cause tumors when they integrate near a protooncogene or tumor suppressor gene of the host. We infected >2,500 mice with the SL3-3 murine leukemia virus; in 22 resulting tumors, we found provirus integrations nearby or within the gene that contains the mir-17-92 microRNA (miRNA) cistron. Using quantitative real-time PCR, we showed that expression of miRNA was increased in these tumors, indicating that retroviral infection can induce expression of oncogenic miRNAs. Our results demonstrate that retroviral mutagenesis can be a potent tool for miRNA discovery.

Gene Expression Profiling Identifies Oncogenic Molecular Pathways in T-Lymphoma Induction by SL3-3 Retrovirus Insertional Mutagenesis in Inbred NMRI Mice.

The non-oncogene bearing retrovirus SL3-3 murine leukemia virus (MLV) induces T-cell lymphomas with a remarkably consistent immunophenotype when injected into newborn inbred NMRI (NMRI-i) mice, making it, at present, the most homogeneous murine model of MLV induced T-lymphomas1. The oncogenic effects of SL3-3 are caused by proviral insertional mutagenesis of the host genome in or near genes of major importance for lymphoma/leukemia development. Determination of SL3-3 integration sites in the NMRI-i genome therefore provides an efficient whole genome screening method for identifying genes involved in murine and potentially also human T-cell lymphomagenic processes. Here, the impact of integration sites on latency, tumor immunophenotype, clonality, and gene expression profiles reflecting the affected molecular pathways involved in the carcinogenesis was analyzed. So far, a total of 22 retroviral integration sites were identified in thymic (14) and mesenteric lymph node tumors (8) from 10 SL3-3-infected NMRI-i mice. The majority of tumors were CD3+CD4+CD8− (2 thymus tumors were CD3+CD4+CD8+) and TCRβ clonality was demonstrated for all tumors by Southern analysis. Gene expression profiling using 60-mer oligonucleotide microarrays representing 10.000 different genes was performed in triplicate for 3 thymic tumors and a pool of 3 normal thymuses from uninfected NMRI-i mice. Unsupervised clustering of microarray data, two tailed student t-tests and significance analysis of microarrays (SAM) showed the T-cell thymus tumors to be more closely related to each other than to normal thymus tissues. The most common integration sites identified in thymic T-cell tumors were in or close to growth factor independence-1 (Gfi-1) and G1/S-specific cyclin D3 (CCND3), both involved in regulation of cell cycle progression in T cells. The two thymic tumors with CCND3 integrations contained homogenous CD3+CD4+CD8− single positive populations. The transcript levels determined by microarray of CCND3 were for both tumors 2-fold higher that what was observed in normal thymuses. D cyclins and cyclin-dependent kinases (CDKs) regulate the G1/S checkpoint and the interaction partner of CCND3, CDK6 was also up-regulated as compared to the normal thymus tissue. Similar expression features for CCND3 and CDK6 were observed for the third thymus tumor with, at present, unknown integration sites. Thus, for tumors with CCND3 integrations a promoted S phase entry leading irreversibly to cell division in the tumor cells can be suggested as further supported by the observation of decreased expression levels of the negative regulator of CDK2, p27KIP1 and increased expression of mRNA encoding G1 to S phase transition 2 (Gspt2). Notably, CCND3 is required for T-cell receptor (TCR) dependent expansion of transformed murine T-lymphocytes. Among other genes with a differential up-regulated expression in T-cell derived thymus tumors with SL3-3 integration in CCND3 were members of the TCR signaling cascade (Zap70, Fyn, VAV2, VAV3 and RAC2) and oncognes (Rel and Ect2) illustrating some of the transformation processes occurring in the malignant T-cells. In summary, this murine model of T-lymphomas enables a specific coupling of common integrations sites, like CCND3 or Gfi-1 to their effects on downstream molecular pathways.

Radiation Leukemia Virus Common Integration at the Kis2 Locus: Simultaneous Overexpression of a Novel Noncoding RNA and of the Proximal Phf6 Gene

Retroviral tagging has been used extensively and successfully to identify genes implicated in cancer pathways. In order to find oncogenes implicated in T-cell leukemia, we used the highly leukemogenic radiation leukemia retrovirus VL3 (RadLV/VL3). We applied the inverted PCR technique to isolate and analyze sequences flanking proviral integrations in RadLV/VL3-induced T lymphomas. We found retroviral integrations in c-myc and Pim1 as already reported but we also identified for the first time Notch1 as a RadLV common integration site. More interestingly, we found a new RadLV common integration site that is situated on mouse chromosome X (XA4 region, bp 45091000). This site has also been reported as an SL3-3 and Moloney murine leukemia virus integration site, which strengthens its implication in murine leukemia virus-induced T lymphomas. This locus, named Kis2 (Kaplan Integration Site 2), was found rearranged in 11% of the tumors analyzed. In this article, we report not only the alteration of the Kis2 gene located nearby in response to RadLV integration but also the induction of the expression of Phf6, situated about 250 kbp from the integration site. The Kis2 gene encodes five different alternatively spliced noncoding RNAs and the Phf6 gene codes for a 365-amino-acid protein which contains two plant homology domain fingers, recently implicated in the Börjeson-Forssman-Lehmann syndrome in humans. With the recent release of the mouse genome sequence, high-throughput retroviral tagging emerges as a powerful tool in the quest for oncogenes. It also allows the analysis of large DNA regions surrounding the integration locus.

Analysis of Wild-Type and Mutant SL3-3 Murine Leukemia Virus Insertions in the c- myc Promoter during Lymphomagenesis Reveals Target Site Hot Spots, Virus-Dependent Patterns, and Frequent Error-Prone Gap Repair

The murine leukemia retrovirus SL3-3 induces lymphomas in the T-cell compartment of the hematopoetic system when it is injected into newborn mice of susceptible strains. Previously, our laboratory reported on a deletion mutant of SL3-3 that induces T-cell tumors faster than the wild-type virus (S. Ethelberg, A. B. Sorensen, J. Schmidt, A. Luz, and F. S. Pedersen, J. Virol. 71:9796-9799, 1997). PCR analyses of proviral integrations in the promoter region of the c-myc proto-oncogene in lymphomas induced by wild-type SL3-3 [SL3-3(wt)] and the enhancer deletion mutant displayed a difference in targeting frequency into this locus. We here report on patterns of proviral insertions into the c-myc promoter region from SL3-3(wt), the faster variant, as well as other enhancer variants from a total of approximately 250 tumors. The analysis reveals (i) several integration site hot spots in the c-myc promoter region, (ii) differences in integration patterns between SL3-3(wt) and enhancer deletion mutant viruses, (iii) a correlation between tumor latency and the number of proviral insertions into the c-myc promoter, and (iv) a [5'-(A/C/G)TA(C/G/T)-3'] integration site consensus sequence. Unexpectedly, about 12% of the sequenced insertions were associated with point mutations in the direct repeat flanking the provirus. Based on these results, we propose a model for error-prone gap repair of host-provirus junctions.

Mutation of All Runx (AML1/Core) Sites in the Enhancer of T-Lymphomagenic SL3-3 Murine Leukemia Virus Unmasks a Significant Potential for Myeloid Leukemia Induction and Favors Enhancer Evolution toward Induction of Other Disease Patterns

SL3-3 murine leukemia virus is a potent inducer of T-lymphomas in mice. Using inbred NMRI mice, it was previously reported that a mutant of SL3-3 with all enhancer Runx (AML1/core) sites disrupted by 3-bp mutations (SL3-3dm) induces predominantly non-T-cell tumors with severely extended latency (S. Ethelberg, J. Lovmand, J. Schmidt, A. Luz, and F. S. Pedersen, J. Virol. 71:7273-7280, 1997). By use of three-color flow cytometry and molecular and histopathological analyses, we have now performed a detailed phenotypic characterization of SL3-3- and SL3-3dm-induced tumors in this mouse strain. All wild-type induced tumors had clonal T-cell receptor beta rearrangements, and the vast majority were CD3(+) CD4(+) CD8(-) T-lymphomas. Such a consistent phenotypic pattern is unusual for murine leukemia virus-induced T-lymphomas. The mutant virus induced malignancies of four distinct hematopoietic lineages: myeloid, T lymphoid, B lymphoid, and erythroid. The most common disease was myeloid leukemia with maturation. Thus, mutation of all Runx motifs in the enhancer of SL3-3 severely impedes viral T-lymphomagenicity and thereby discloses a considerable and formerly unappreciated potential of this virus for myeloid leukemia induction. Proviral enhancers with complex structural alterations (deletions, insertions, and/or duplications) were found in most SL3-3dm-induced T-lymphoid tumors and immature myeloid leukemias but not in any cases of myeloid leukemia with maturation, mature B-lymphoma, or erythroleukemia. Altogether, our results indicate that the SL3-3dm enhancer in itself promotes induction of myeloid leukemia with maturation but that structural changes may arise in vivo and redirect viral disease specificity to induction of T-lymphoid or immature myeloid leukemias, which typically develop with moderately shorter latencies.

Triple basepair changes within and adjacent to the conserved YY1 motif upstream of the U3 enhancer repeats of SL3-3 murine leukemia virus cause a small but significant shortening of latency of T-lymphoma induction

A highly conserved sequence upstream of the transcriptional enhancer in the U3 of murine leukemia viruses (MLVs) was reported to mediate negative regulation of their expression. In transient expression studies, negative regulation was reported to be conferred by coexpression of the transcription factor YY1, which binds to a motif in the upstream conserved region (UCR). To address the function of the UCR and its YY1-motif in an in vivo model of MLV–host interactions we introduced six consecutive triple basepair mutations into this region of the potent T-lymphomagenic SL3-3 MLV. We report that all mutants have retained their replication competence and that they all, like the SL3-3 wild type (wt), induce T-cell lymphomas when injected into newborn mice of the SWR strain. However, all mutants induced disease with slightly shorter latency periods than the wt SL3-3, suggesting that the YY1 motif as well as its immediate context in the UCR have a negative effect on the pathogenicity of the virus. This result may have implications for the design of retroviral vectors.

Disruption of hematopoiesis and thymopoiesis in the early premalignant stages of infection with SL3-3 murine leukemia virus.

A time course analysis of SL3-3 murine leukemia virus (SL3) infection in thymus and bone marrow of NIH/Swiss mice was performed to assess changes that occur during the early stages of progression to lymphoma. Virus was detectable in thymocytes, bone marrow, and spleen as early as 1 to 2 weeks postinoculation (p.i.). In bone marrow, virus infection was detected predominantly in immature myeloid or granulocytic cells. Flow cytometry revealed significant reductions of the Ter-119(+) and Mac-1(+) populations, and significant expansions of the Gr-1(+) and CD34(+) populations, between 2 and 4 weeks p.i. Analysis of colony-forming potential confirmed these findings. In the thymus, SL3 replication was associated with significant disruption in thymocyte subpopulation distribution between 4 and 7 weeks p.i. A significant thymic regression was observed just prior to the clonal outgrowth of tumor cells. Proviral long terminal repeats (LTRs) with increasing numbers of enhancer repeats were observed to accumulate exclusively in the thymus during the first 8 weeks p.i. Observations were compared to the early stages of infection with a virtually nonpathogenic SL3 mutant, termed SL3DeltaMyb5, which was shown by real-time PCR to be replication competent. Comparison of SL3 with SL3DeltaMyb5 implicated certain premalignant changes in tumorigenesis, including (i) increased proportions of Gr-1(+) and CD34(+) bone marrow progenitors, (ii) a significant increase in the proportion of CD4(-) CD8(-) thymocytes, (iii) thymic regression prior to tumor outgrowth, and (iv) accumulation of LTR enhancer variants. A model in which disrupted bone marrow hematopoiesis and thymopoiesis contribute to the development of lymphoma in the SL3-infected animal is discussed.

Tissue distribution and timing of appearance of polytropic envelope recombinants during infection with SL3-3 murine leukemia virus or its weakly pathogenic SL3DeltaMyb5 mutant.

A time course analysis was performed to identify the sites of formation and timing of appearance of polytropic recombinant viruses following infection of NIH/Swiss mice with the murine retrovirus SL3-3 murine leukemia virus (SL3) or with a weakly pathogenic mutant termed SL3DeltaMyb5. The results indicated that (i) polytropic recombinant viruses occur initially in the thymus of SL3-infected animals, (ii) the timing of appearance of polytropic recombinants in bone marrow is not consistent with their participation in the previously reported formation of transplantable tumor-forming cells at 3 to 4 week postinoculation, and (iii) the efficient generation of recombinant virus is correlated with efficient tumor induction.

The kissing-loop motif is a preferred site of 5' leader recombination during replication of SL3-3 murine leukemia viruses in mice.

A panel of mouse T-cell lymphomas induced by SL3-3 murine leukemia virus (MLV) and three primer binding site mutants thereof (A. H. Lund, J. Schmidt, A. Luz, A. B. Sorensen, M. Duch, and F. S. Pedersen, J. Virol. 73:6117-6122, 1999) were analyzed for the occurrence of recombination between the exogenous input virus and endogenous MLV-like sequences within the 5' leader region. Evidence of recombination within the region studied was found in 14 of 52 tumors analyzed. Sequence analysis of a approximately 330-bp fragment of 44 chimeric proviruses, encompassing the U5, the primer binding site, and the upstream part of the 5' untranslated region, enabled us to map recombination sites, guided by distinct scattered nucleotide differences. In 30 of 44 analyzed sequences, recombination was mapped to a 33-nucleotide similarity window coinciding with the kissing-loop stem-loop motif implicated in dimerization of the diploid genome. Interestingly, the recombination pattern preference found in replication-competent viruses from T-cell tumors is very similar to the pattern previously reported for retroviral vectors in cell culture experiments. The data therefore sustain the hypothesis that the kissing loop, presumably via a role in RNA dimer formation, constitutes a hot spot for reverse transcriptase-mediated recombination in MLV.

Tandemization of a subregion of the enhancer sequences from SRS 19-6 murine leukemia virus associated with T-lymphoid but not other leukemias.

Most simple retroviruses induce tumors of a single cell type when infected into susceptible hosts. The SRS 19-6 murine leukemia virus (MuLV), which originated in mainland China, induces leukemias of multiple cellular origins. Indeed, infected mice often harbor more than one tumor type. Since the enhancers of many MuLVs are major determinants of tumor specificity, we tested the role of the SRS 19-6 MuLV enhancers in its broad disease specificity. The enhancer elements of the Moloney MuLV (M-MuLV) were replaced by the 170-bp enhancers of SRS 19-6 MuLV, yielding the recombinants DeltaMo+SRS(+) and DeltaMo+SRS(-) M-MuLV. M-MuLV normally induces T-lymphoid tumors in all infected mice. Surprisingly, when neonatal mice were inoculated with DeltaMo+SRS(+) or DeltaMo+SRS(-) M-MuLV, all tumors were of T-lymphoid origin, typical of M-MuLV rather than SRS 19-6 MuLV. Thus, the SRS 19-6 MuLV enhancers did not confer the broad disease specificity of SRS 19-6 MuLV to M-MuLV. However, all tumors contained DeltaMo+SRS M-MuLV proviruses with common enhancer alterations. These alterations consisted of tandem multimerization of a subregion of the SRS 19-6 enhancers, encompassing the conserved LVb and core sites and adjacent sequences. Moreover, when tumors induced by the parental SRS 19-6 MuLV were analyzed, most of the T-lymphoid tumors had similar enhancer alterations in the same region whereas tumors of other lineages retained the parental SRS 19-6 MuLV enhancers. These results emphasize the importance of a subregion of the SRS 19-6 MuLV enhancer in induction of T-cell lymphoma. The relevant sequences were consistent with crucial sequences for T-cell lymphomagenesis identified for other MuLVs such as M-MuLV and SL3-3 MuLV. These results also suggest that other regions of the SRS 19-6 MuLV genome contribute to its broad leukemogenic spectrum.

Replication and pathogenicity of primer binding site mutants of SL3-3 murine leukemia viruses.

Retroviral reverse transcription is primed by a cellular tRNA molecule annealed to an 18-bp primer binding site sequence. The sequence of the primer binding site coincides with that of a negatively acting cis element that mediates transcriptional silencing of murine leukemia virus (MLV) in undifferentiated embryonic cells. In this study we test whether SL3-3 MLV can replicate stably using tRNA primers other than the cognate tRNAPro and analyze the effect of altering the primer binding site sequence to match the 3' end of tRNA1Gln, tRNA3Lys, or tRNA1,2Arg in a mouse pathogenicity model. Contrary to findings from cell culture studies of primer binding site-modified human immunodeficiency virus type 1 and avian retroviruses, our findings were that SL3-3 MLV may stably and efficiently replicate with tRNA primers other than tRNAPro. Although lymphoma induction of the SL3-3 Lys3 mutant was significantly delayed relative to that of the wild-type virus, molecular tumor analysis indicated that all the primer binding site-modified viruses induce T-cell lymphomas similar to those induced by the wild-type virus in terms of frequencies of genomic rearrangements within the T-cell receptor beta-chain, the immunoglobulin kappa light chain, and the c-myc locus. Whereas none of the mutants were found to revert to tRNAPro primer utilization, in two tumors resulting from the injection of the SL3-3 Lys3 mutant the primer binding site was altered to match that of a new primer species, tRNA1,2Lys. In addition, recombination with endogenous viruses resulting in the generation of recombinant viruses carrying a glutamine primer binding site was detected in the majority of the tumors induced by the SL3-3 Lys3 mutant as well as in two tumors induced by wild-type SL3-3 and the SL3-3 Arg1,2 mutant.

B-Cell lymphoma induction by akv murine leukemia viruses harboring one or both copies of the tandem repeat in the U3 enhancer.

Akv is an endogenous, ecotropic murine leukemia virus (MuLV) of the AKR strain. It has served as a prototype nonpathogenic or weakly pathogenic reference virus for studies of closely related potent lymphomagenic viruses such as the T-lymphomagenic SL3-3. We here report that Akv and an Akv mutant (Akv1-99) with only one copy of the 99-bp transcriptional enhancer induce malignant lymphomas with nearly 100% incidence and mean latency periods of 12 months after injection into newborn NMRI mice. Molecular analysis of tumor DNA showed that the majority of the tumors were of the B-cell type. Sequence analysis of proviral transcriptional enhancers in DNA of B-cell lymphomas revealed conservation of the enhancer sequence, as well as a lack of sequence duplications of the Akv1-99 variant, while the repeat copy number in Akv was subject to fluctuations. In support of a B-cell specificity of the Akv enhancer, a murine plasmacytoma cell line was found to sustain three- to fivefold-higher transient transcriptional activity upon the Akv and Akv1-99 enhancers than upon the enhancer of the T-lymphomagenic SL3-3 MuLV. Thus, the overall picture is that Akv MuLV possesses a B- lymphomagenic potential and that the second copy of the 99-bp sequence seems to be of minor importance for this potential. However, in one animal the lymphomas induced by Akv1-99 were of the T-cell type. Among the 24 tumors analyzed only this one harbored a clonal proviral integration in the c-myc locus. This provirus had undergone a duplication of a 113-bp sequence of the enhancer region, partly overlapping with the 99-bp repeat of Akv, as well as a few single nucleotide alterations within and outside the repeats. Taken together with previous studies, our results suggest that T- versus B-lymphomagenic specificity of the enhancer is governed by more than one nucleotide difference and that alterations in binding sites for transcription factors of the AML1 and nuclear-factor-1 families may contribute to this specificity.

The mouse H-2A region influences the envelope gene structure of tumor-associated murine leukemia viruses.

C57BL/10 (B10) strains congenic at the mouse major histocompatibility locus (H-2) were injected with a modified ecotropic SL3-3 murine leukemia virus (MuLV) to determine the effect of the H-2 genes on the envelope gene structure of recombinant MuLVs. All tested strains rapidly developed T-cell lymphomas, and recombinant proviruses were detected in the tumor DNAs by Southern blot. The B10.D2 (H-2d), B10.Br (H-2k), B10.Q (H-2q), and B10.RIII (H-2r) strains exhibited a TI phenotype in which almost all tumors contained type I recombinants. These recombinants characteristically acquire envelope gene sequences from the endogenous polytropic viruses but retain the 5' p15E (TM) gene sequences from the ecotropic virus. The parental B10 (H-2b) strain, however, had a novel phenotype that was designated NS for nonselective. Only 30% of the B10 tumors had detectable type I recombinants, whereas a proportion of the others appeared to contain type II recombinants that lacked the type I-specific ecotropic p15E gene sequences. Studies of other B10 congenic strains with hybrid H-2 loci and selected F1 animals revealed that the NS phenotype was regulated by a dominant gene(s) that mapped to the A region of H-2b. These results demonstrate that a host gene within the major histocompatibility complex can influence the genetic evolution of pathogenic retroviruses in vivo.

An SL3-3 murine leukemia virus enhancer variant more pathogenic than the wild type obtained by assisted molecular evolution in vivo.

SL3-3 is a highly T-lymphomagenic murine retrovirus in which the transcriptional enhancer is a major oncogenic determinant. Here, we describe an SL3-3 enhancer variant that induced T-cell lymphomas in all inoculated mice with a shorter latency period than wild-type SL3-3. The enhancer repeat region of this variant contains two deletions encompassing the nuclear factor 1 binding sites in addition to an additional intact enhancer repeat element. Tumors induced by this variant were T-cell lymphomas, as indicated by T-cell receptor rearrangements, and contained the input provirus enhancer regions. The variant was the result of mutation of specific transcription factor binding sites in the viral enhancer, isolation of rare second-site enhancer variants from the resulting induced tumors, and subsequent restoration of the original first-site mutations of one such variant. We have termed this process assisted molecular evolution.

Increased lymphomagenicity and restored disease specificity of AML1 site (core) mutant SL3-3 murine leukemia virus by a second-site enhancer variant evolved in vivo.

SL3-3 is a highly T-lymphomagenic murine retrovirus. The major genetic determinant of disease is the transcriptional enhancer, which consists of a repeated region with densely packed binding sites for several transcription factors, including AML1 (also known as core binding factor and polyoma enhancer-binding protein 2) and nuclear factor 1 (NF1). Previously, we examined the enhancer structure of proviruses from murine tumors induced by SL3-3 with mutated AML1 (core) sites and found a few cases of second-site alterations. These consisted of deletions involving the NF1 sites and alterations in overall number of repeat elements, and they conferred increased enhancer strength in transient transcription assays. We have now tested the pathogenicity of a virus harboring one such second-site variant enhancer in inbred NMRI mice. It induced lymphomas with a 100% incidence and a significantly shorter latency than the AML1 mutant it evolved from. The enhancer structure thus represents the selection for a more tumorigenic virus variant during the pathogenic process. Sequencing of provirus from the induced tumors showed the new enhancer variant to be genetically stable. Also, Southern blotting showed that the tumors induced by the variant were T-cell lymphomas, as were the wild-type-induced lymphomas. In contrast, tumors induced by the original core/AML1 site I-II mutant appeared to be of non-T-cell origin and several proviral genomes with altered enhancer regions could be found in the tumors. Moreover, reporter constructs with the new tumor-derived variant could not be transactivated by AML1 in cotransfection experiments as could the wild type. These results emphasize the importance of both core/AML1 site I and site II for the pathogenic potential of SL3-3 and at the same time show that second-site alterations can form a viral variant with a substantial pathogenic potential although both AML1 sites I and II are nonfunctional.

Stability of AML1 (core) site enhancer mutations in T lymphomas induced by attenuated SL3-3 murine leukemia virus mutants.

Murine retrovirus SL3-3 is highly T lymphomagenic. Its pathogenic properties are determined by the transcriptional enhancer of the U3 repeat region which shows preferential activity in T cells. Within the U3 repeats, the major determinant of T-cell specificity has been mapped to binding sites for the AML1 transcription factor family (also known as the core binding factor [CBF], polyomavirus enhancer binding protein 2 [PEBP2], and SL3-3 enhancer factor 1 [SEF-1]). SL3-3 viruses with AML1 site mutations have lost a major determinant of T-cell-specific enhancer function but have been found to retain a lymphomagenic potential, although disease induction is slower than for the SL3-3 wild type. To compare the specificities and mechanisms of disease induction of wild-type and mutant viruses, we have examined lymphomas induced by mutant viruses harboring transversions of three consecutive base pairs critical to AML1 site function (B. Hallberg, J. Schmidt, A. Luz, F. S. Pedersen, and T. Grundström. J. Virol. 65:4177-4181, 1991). Our results show that the mutated AML1 sites are genetically stable during lymphomagenesis and that ecotropic provirus numbers in DNA of tumors induced by wild-type and mutant viruses fall within the same range. Moreover, proviruses were found to be integrated at the c-myc locus in similar proportions of wild-type and mutant SL3-3-induced tumors, and the mutated AML1 sites of proviruses at c-myc are unaltered. In some cases, however, including one c-myc-integrated provirus, a single-base pair change was detected in a second, weaker AML1 binding site. By DNA rearrangement analysis of the T-cell receptor beta-locus, tumors induced by the AML1 site mutants are found to be of the T-cell type. Thus, although the AML1 site mutants have weakened T-cell-specific enhancers they are T-lymphomagenic, and wild-type- and mutant-virus-induced tumor DNAs are similar with respect to the number of overall ecotropic and c-myc-integrated clonal proviruses. The SL3-3 wild-type and AML1 site mutant viruses may therefore induce disease by similar mechanisms.