Selection for c-myc integration sites in polyclonal T-cell lymphomas

Apobec-mediated retroviral hypermutation in vivo is dependent on mouse strain

Replication of the complex retrovirus mouse mammary tumor virus (MMTV) is antagonized by murine Apobec3 (mA3), a member of the Apobec family of cytidine deaminases. We have shown that MMTV-encoded Rem protein inhibits proviral mutagenesis by the Apobec enzyme, activation-induced cytidine deaminase (AID) during viral replication in BALB/c mice. To further study the role of Rem in vivo, we have infected C57BL/6 (B6) mice with a superantigen-independent lymphomagenic strain of MMTV (TBLV-WT) or a mutant strain that is defective in Rem and its cleavage product Rem-CT (TBLV-SD). Compared to BALB/c, B6 mice were more susceptible to TBLV infection and tumorigenesis. Furthermore, unlike MMTV, TBLV induced T-cell tumors in B6 μMT mice, which lack membrane-bound IgM and conventional B-2 cells. At limiting viral doses, loss of Rem expression in TBLV-SD-infected B6 mice accelerated tumorigenesis compared to TBLV-WT in either wild-type B6 or AID-knockout mice. Unlike BALB/c results, high-throughput sequencing indicated that proviral G-to-A or C-to-T mutations were unchanged regardless of Rem expression in B6 tumors. However, knockout of both AID and mA3 reduced G-to-A mutations. Ex vivo stimulation showed higher levels of mA3 relative to AID in B6 compared to BALB/c splenocytes, and effects of agonists differed in the two strains. RNA-Seq revealed increased transcripts related to growth factor and cytokine signaling in TBLV-SD-induced tumors relative to TBLV-WT-induced tumors, consistent with another Rem function. Thus, Rem-mediated effects on tumorigenesis in B6 mice are independent of Apobec-mediated proviral hypermutation.

Apobec-Mediated Retroviral Hypermutation In Vivo is Dependent on Mouse Strain

Replication of the complex retrovirus mouse mammary tumor virus (MMTV) is antagonized by murine Apobec3 (mA3), a member of the Apobec family of cytidine deaminases. We have shown that MMTV-encoded Rem protein inhibits proviral mutagenesis by the Apobec enzyme, activation-induced cytidine deaminase (AID) during viral replication in BALB/c mice. To further study the role of Rem in vivo , we have infected C57BL/6 (B6) mice with a superantigen-independent lymphomagenic strain of MMTV (TBLV-WT) or a mutant strain (TBLV-SD) that is defective in Rem and its cleavage product Rem-CT. Unlike MMTV, TBLV induced T-cell tumors in µMT mice, indicating that mature B cells, which express the highest AID levels, are not required for TBLV replication. Compared to BALB/c, B6 mice were more susceptible to TBLV infection and tumorigenesis. The lack of Rem expression accelerated B6 tumorigenesis at limiting doses compared to TBLV-WT in either wild-type B6 or AID-deficient mice. However, unlike proviruses from BALB/c mice, high-throughput sequencing indicated that proviral G-to-A or C-to-T changes did not significantly differ in the presence and absence of Rem expression. Ex vivo stimulation showed higher levels of mA3 relative to AID in B6 compared to BALB/c splenocytes, but effects of agonists differed in the two strains. RNA-Seq revealed increased transcripts related to growth factor and cytokine signaling in TBLV-SD-induced tumors relative to those from TBLV-WT, consistent with a third Rem function. Thus, Rem-mediated effects on tumorigenesis in B6 mice are independent of Apobec-mediated proviral hypermutation.

Exploring the link between viruses and cancer in companion animals: a comprehensive and comparative analysis

Currently, it is estimated that 15% of human neoplasms globally are caused by infectious agents, with new evidence emerging continuously. Multiple agents have been implicated in various forms of neoplasia, with viruses as the most frequent. In recent years, investigation on viral mechanisms underlying tumoral transformation in cancer development and progression are in the spotlight, both in human and veterinary oncology. Oncogenic viruses in veterinary medicine are of primary importance not only as original pathogens of pets, but also in the view of pets as models of human malignancies. Hence, this work will provide an overview of the main oncogenic viruses of companion animals, with brief notes of comparative medicine.

Mouse Mammary Tumor Virus (MMTV) and MMTV-like Viruses: An In-depth Look at a Controversial Issue

Since its discovery as a milk factor, mouse mammary tumor virus (MMTV) has been shown to cause mammary carcinoma and lymphoma in mice. MMTV infection depends upon a viral superantigen (sag)-induced immune response and exploits the immune system to establish infection in mammary epithelial cells when they actively divide. Simultaneously, it avoids immune responses, causing tumors through insertional mutagenesis and clonal expansion. Early studies identified antigens and sequences belonging to a virus homologous to MMTV in human samples. Several pieces of evidence fulfill a criterion for a possible causal role for the MMTV-like virus in human breast cancer (BC), though the controversy about whether this virus was linked to BC has raged for over 40 years in the literature. In this review, the most important issues related to MMTV, from its discovery to the present days, are retraced to fully explore such a controversial issue. Furthermore, the hypothesis of an MMTV-like virus raised the question of a potential zoonotic mouse–man transmission. Several studies investigate the role of an MMTV-like virus in companion animals, suggesting their possible role as mediators. Finally, the possibility of an MMTV-like virus as a cause of human BC opens a new era for prevention and therapy.

A Protein Antagonist of Activation-Induced Cytidine Deaminase Encoded by a Complex Mouse Retrovirus

Complex human-pathogenic retroviruses cause high morbidity and mortality worldwide, but resist antiviral drugs and vaccine development due to evasion of the immune response. A complex retrovirus, mouse mammary tumor virus (MMTV), requires replication in B and T lymphocytes for mammary gland transmission and is antagonized by the innate immune restriction factor murine Apobec3 (mA3). To determine whether the regulatory/accessory protein Rem affects innate responses to MMTV, a splice-donor mutant (MMTV-SD) lacking Rem expression was injected into BALB/c mice. Mammary tumors induced by MMTV-SD had a lower proviral load, lower incidence, and longer latency than mammary tumors induced by wild-type MMTV (MMTV-WT). MMTV-SD proviruses had many G-to-A mutations on the proviral plus strand, but also C-to-T transitions within WRC motifs. Similarly, a lymphomagenic MMTV variant lacking Rem expression showed decreased proviral loads and increased WRC motif mutations relative to those in wild-type-virus-induced tumors, consistent with activation-induced cytidine deaminase (AID) mutagenesis in lymphoid cells. These mutations are typical of the Apobec family member AID, a B-cell-specific mutagenic protein involved in antibody variable region hypermutation. In contrast, mutations in WRC motifs and proviral loads were similar in MMTV-WT and MMTV-SD proviruses from tumors in AID-insufficient mice. AID was not packaged in MMTV virions. Rem coexpression in transfection experiments led to AID proteasomal degradation. Our data suggest that rem specifies a human-pathogenic immunodeficiency virus type 1 (HIV-1) Vif-like protein that inhibits AID and antagonizes innate immunity during MMTV replication in lymphocytes.IMPORTANCE Complex retroviruses, such as human-pathogenic immunodeficiency virus type 1 (HIV-1), cause many human deaths. These retroviruses produce lifelong infections through viral proteins that interfere with host immunity. The complex retrovirus mouse mammary tumor virus (MMTV) allows for studies of host-pathogen interactions not possible in humans. A mutation preventing expression of the MMTV Rem protein in two different MMTV strains decreased proviral loads in tumors and increased viral genome mutations typical of an evolutionarily ancient enzyme, AID. Although the presence of AID generally improves antibody-based immunity, it may contribute to human cancer progression. We observed that coexpression of MMTV Rem and AID led to AID destruction. Our results suggest that Rem is the first known protein inhibitor of AID and that further experiments could lead to new disease treatments.

Lessons Learned from Mouse Mammary Tumor Virus in Animal Models

Mouse mammary tumor virus (MMTV), which was discovered as a milk-transmitted, infectious, cancer-inducing agent in the 1930s, has been used as an animal model for the study of retroviral infection and transmission, antiviral immune responses, and breast cancer and lymphoma biology. The main target cells for MMTV infection in vivo are cells of the immune system and mammary epithelial cells. Although the host mounts an immune response to the virus, MMTV has evolved multiple means of evading this response. MMTV causes mammary tumors when the provirus integrates into the mammary epithelial and lymphoid cell genome during viral replication and thereby activates cellular oncogene expression. Thus, tumor induction is a by-product of the infection cycle. A number of important oncogenes have been discovered by carrying out MMTV integration site analysis, some of which may play a role in human breast cancer.

Novel candidate cancer genes identified by a large-scale cross-species comparative oncogenomics approach

Comparative genomic hybridization (CGH) can reveal important disease genes but the large regions identified could sometimes contain hundreds of genes. Here we combine high-resolution CGH analysis of 598 human cancer cell lines with insertion sites isolated from 1,005 mouse tumors induced with the murine leukemia virus (MuLV). This cross-species oncogenomic analysis revealed candidate tumor suppressor genes and oncogenes mutated in both human and mouse tumors, making them strong candidates for novel cancer genes. A significant number of these genes contained binding sites for the stem cell transcription factors Oct4 and Nanog. Notably, mice carrying tumors with insertions in or near stem cell module genes, which are thought to participate in cell self-renewal, died significantly faster than mice without these insertions. A comparison of the profile we identified to that induced with the Sleeping Beauty (SB) transposon system revealed significant differences in the profile of recurrently mutated genes. Collectively, this work provides a rich catalogue of new candidate cancer genes for functional analysis.

Role of SV40 integration site at chromosomal interval 1q21.1 in immortalized CRL2504 cells

We have applied a functional gene transfer strategy to show the importance of viral integration site in cellular immortalization. The large tumor antigen of SV40 is capable of extending the cellular life span by sequestering tumor suppressor proteins pRB and p53 in virus-transformed human cells. Although SV40 large T antigen is essential, it is not sufficient for cellular immortalization, suggesting that additional alterations in cellular genes are required to attain infinite proliferation. We show here that the disruption of human chromosomal interval at 1q21.1 by SV40 integration can be an essential step for cellular immortalization. The transfer of a 150-kb bacterial artificial chromosome (BAC) clone, RP364B14, corresponding to viral integration site in CRL2504 cells, reverted their immortal phenotype. Interestingly, the BAC transfer clones of CRL2504 cells displayed characteristics of either senescence as shown by beta-galactosidase activity or apoptosis as revealed by positive staining with M30 CytoDEATH antibody. The SV40 integration at 1q21.1, in the vicinity of epidermal differentiation complex (EDC) genes, resulted in the down-regulation of the filaggrin (FLG) gene that is part of the EDC. FLG gene expression was increased in BAC transfer senescent and apoptotic clones. Our results suggest that the disruption of native genomic sequence by SV40 may alter expression of genes involved in senescence and apoptosis by modulating chromatin structure. These studies imply that identification of genes located in the vicinity of viral integration sites in human cancers may be helpful in developing new diagnostic and therapeutic strategies.

MMTV infectious cycle and the contribution of virus-encoded proteins to transformation of mammary tissue

Mouse mammary tumor virus has served as a major model for the study of breast cancer since its discovery 1920's as a milk-transmitted agent. Much is known about in vivo infection by this virus, which initially occurs in lymphocytes that then carry virus to mammary tissue. In addition to the virion proteins, MMTV encodes a number of accessory proteins that facilitate high level in vivo infection. High level infection of lymphoid and mammary epithelial cells ensures efficient passage of virus to the next generation. Since MMTV causes mammary tumors by insertional activation of oncogenes, which is thought to be a stochastic process, mammary epithelial cell transformation is a by-product of the infectious cycle. The envelope protein may also participate in transformation. Although there have been several reports of a similar virus in human breast cancer, the existence of a human MTV has not been definitely established.

ALY is a common coactivator of RUNX1 and c-Myb on the type B leukemogenic virus enhancer

Type B leukemogenic virus (TBLV), a mouse mammary tumor virus (MMTV) variant, often induces T-cell leukemias and lymphomas by c-myc activation following viral DNA integration. Transfection assays using a c-myc reporter plasmid indicated that the TBLV long terminal repeat (LTR) enhancer is necessary for T-cell-specific increases in basal reporter activity. The sequence requirements for this effect were studied using mutations of the 62-bp enhancer region in an MMTV LTR reporter vector. Deletion of a nuclear factor A-binding site dramatically reduced reporter activity in Jurkat T cells. However, a 41-bp enhancer missing the RUNX1 site still retained minimal enhancer function. DNA affinity purification using a TBLV enhancer oligomer containing the RUNX1 binding site followed by mass spectrometry resulted in the identification of ALY. Subsequent experiments focused on the reconstitution of enhancer activity in epithelial cells. ALY overexpression synergized with RUNX1B on TBLV enhancer activity, and synergism required the RUNX1B-binding site. A predicted c-Myb binding site in the enhancer was confirmed after c-myb overexpression elevated TBLV LTR reporter activity, and overexpression of c-Myb and RUNX1B together showed additive effects on reporter gene levels. ALY also synergized with c-Myb, and coimmunoprecipitation experiments demonstrated an interaction between ALY and c-Myb. These experiments suggest a central role for ALY in T-cell enhancer function and oncogene activation.

Conversion of mouse mammary tumor virus to a lymphomagenic virus

Type B leukemogenic virus is a variant of mouse mammary tumor virus (MMTV) that causes thymic lymphomas rather than mammary tumors in mice. We demonstrate that conversion of a mammotropic MMTV to a T-cell-tropic virus requires two alterations in the long terminal repeat: (i) acquisition of a T-cell-specific enhancer and (ii) loss of transcriptional repression through deletion of negative regulatory elements (NREs) or by suppression of NRE activity after appropriate positioning of the enhancer.

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.

Substitution of feline leukemia virus long terminal repeat sequences into murine leukemia virus alters the pattern of insertional activation and identifies new common insertion sites

The recombinant retrovirus, MoFe2-MuLV (MoFe2), was constructed by replacing the U3 region of Moloney murine leukemia virus (M-MuLV) with homologous sequences from the FeLV-945 LTR. NIH/Swiss mice neonatally inoculated with MoFe2 developed T-cell lymphomas of immature thymocyte surface phenotype. MoFe2 integrated infrequently (0 to 9%) near common insertion sites (CISs) previously identified for either parent virus. Using three different strategies, CISs in MoFe2-induced tumors were identified at six loci, none of which had been previously reported as CISs in tumors induced by either parent virus in wild-type animals. Two of the newly identified CISs had not previously been implicated in lymphoma in any retrovirus model. One of these, designated 3-19, encodes the p101 regulatory subunit of phosphoinositide-3-kinase-gamma. The other, designated Rw1, is predicted to encode a protein that functions in the immune response to virus infection. Thus, substitution of FeLV-945 U3 sequences into the M-MuLV long terminal repeat (LTR) did not alter the target tissue for M-MuLV transformation but significantly altered the pattern of CIS utilization in the induction of T-cell lymphoma. These observations support a growing body of evidence that the distinctive sequence and/or structure of the retroviral LTR determines its pattern of insertional activation. The findings also demonstrate the oligoclonal nature of retrovirus-induced lymphomas by demonstrating proviral insertions at CISs in subdominant populations in the tumor mass. Finally, the findings demonstrate the utility of novel recombinant retroviruses such as MoFe2 to contribute new genes potentially relevant to the induction of lymphoid malignancy.

Unique three-repeat sequences containing FVa, LVb/C4, and CORE motifs in LTR-U3 of Friend murine leukemia virus clone A8 accelerate the induction of thymoma in rat

Friend murine leukemia virus (Fr-MLV) clone A8 causes thymoma 7 weeks postinfection in rats with a more rapid progression than clone 57. The U3 region of A8-LTR contains a unique structure of enhancer motifs consisting of three repeats of a 38-bp sequence containing FVa, LVb/C4, and CORE motifs. Replacement or deletion of the 38-bp sequence in the A8-U3 resulted in a marked reduction in tumorigenicity. Furthermore, the virus with 57-U3 gained high tumorigenicity after construction of the three 38-bp repeats in the U3 region. These findings indicated that the repeats of the 38-bp sequence of A8-LTR are essential for the rapid induction of thymoma. Interestingly, the repeat of the 38-bp sequence did not accelerate the amount of integrated viral DNA in the thymus during the early phase of infection, although it contributed to higher production of infectious virus. Thus, it was demonstrated that the ability to induce thymoma, which correlates with virus titer in the thymus, is not determined by the rate of viral DNA integration into the host genome.

Rorgamma (Rorc) is a common integration site in type B leukemogenic virus-induced T-cell lymphomas

The retrovirus type B leukemogenic virus (TBLV) causes T-cell lymphomas in mice. We have identified the Rorgamma locus as an integration site in 19% of TBLV-induced tumors. Overexpression of one or more Rorgamma isoforms in >77% of the tumors tested may complement apoptotic effects of c-myc overexpression.

Cloning and expression of cDNA for a luciferase from the marine copepod Metridia longa. A novel secreted bioluminescent reporter enzyme

Metridia longa is a marine copepod from which a blue bioluminescence originates as a secretion from epidermal glands in response to various stimuli. We demonstrate that Metridia luciferase is specific for coelenterazine to produce blue light (lambda(max) = 480 nm). Using an expression cDNA library and functional screening, we cloned and sequenced the cDNA encoding the Metridia luciferase. The cDNA is an 897-bp fragment with a 656-bp open reading frame, which encodes a 219-amino acid polypeptide with a molecular weight of 23,885. The polypeptide contains an N-terminal signal peptide of 17 amino acid residues for secretion. On expression of the Metridia luciferase gene in mammalian Chinese hamster ovary cells the luciferase is detected in the culture medium confirming the existence of a naturally occurring signal peptide for secretion in the cloned luciferase. The novel secreted luciferase was tested in a practical assay application in which the activity of A2a and NPY2 G-protein-coupled receptors was detected. These results clearly suggest that the secreted Metridia luciferase is well suited as a reporter for monitoring gene expression and, in particular, for the development of novel ultrahigh throughput screening technologies.

Duplication of U3 sequences in the long terminal repeat of mink cell focus-inducing viruses generates redundancies of transcription factor binding sites important for the induction of thymomas

The ability of mink cell focus-inducing (MCF) viruses to induce thymomas is determined, in part, by transcriptional enhancers in the U3 region of their long terminal repeats (LTRs). To elucidate sequence motifs important for enhancer function in vivo, we injected newborn mice with MCF 1dr (supF), a weakly pathogenic, molecularly tagged (supF) MCF virus containing only one copy of a sequence that is present as two copies (known as the directly repeated [DR] sequence) in the U3 region of MCF 247 and analyzed LTRs from supF-tagged proviruses in two resulting thymomas. Tagged proviruses integrated upstream and in the reverse transcriptional orientation relative to c-myc provided the focus of our studies. These proviruses are thought to contribute to thymoma induction by enhancer-mediated deregulation of c-myc expression. The U3 region in a tagged LTR in one thymoma was cloned and sequenced. Relative to MCF 1dr (supF), the cloned U3 region contained an insertion of 140 bp derived predominantly from the DR sequence of the injected virus. The inserted sequence contains predicted binding sites for transcription factors known to regulate the U3 regions of various murine leukemia viruses. Similar constellations of binding sites were duplicated in two proviral LTRs integrated upstream from c-myc in a second thymoma. We replaced the U3 sequences in an infectious molecular clone of MCF 247 with the cloned proviral U3 sequences from the first thymoma and generated an infectious chimeric virus, MCF ProEn. When injected into neonatal AKR mice, MCF ProEn was more pathogenic than the parental virus, MCF 1dr (supF), as evidenced by the more rapid onset and higher incidence of thymomas. Molecular analyses of the resultant thymomas indicated that the U3 region of MCF ProEn was genetically stable. These data suggest that the arrangement and/or redundancy of transcription factor binding sites generated by specific U3 sequence duplications are important to the biological events mediated by MCF proviruses integrated near c-myc that contribute to transformation.

The type B leukemogenic virus truncated superantigen is dispensable for T-cell lymphomagenesis

Type B leukemogenic virus (TBLV) is a variant of mouse mammary tumor virus (MMTV) that causes T-cell lymphomas in mice. We have constructed a TBLV-MMTV hybrid, pHYB-TBLV, in which 756 bp of the C3H MMTV long terminal repeat (LTR) was replaced with 438 bp of the TBLV LTR. Intraperitoneal injection of pHYB-TBLV transfectants consistently resulted in T-cell lymphomas in 50% of injected weanling BALB/c mice with an average latency period of 5.7 (+/- 1.5) months. Transfectants of pHYB-TBLV containing a double-frameshift mutation in the truncated superantigen gene (sag) induced T-cell lymphomas with similar incidences, latency periods, and phenotypes, suggesting that cis-acting elements in the TBLV LTR determine disease specificity.

Tag, you're hit: retroviral insertions identify genes involved in cancer

Retroviral integrations have been used for many years to identify genes involved in cancer. The recently published mouse genome sequence has allowed large-scale identification of potential human cancer genes and their classification into distinct signaling pathways.

What retroviruses teach us about the involvement of c-Myc in leukemias and lymphomas

Overexpression of the cellular oncogene c-Myc frequently occurs during induction of leukemias and lymphomas in many species. Retroviruses have enhanced our understanding of the role of c-Myc in such tumors. Leukemias and lymphomas induced by retroviruses activate c-Myc by: (1) use of virally specified proteins that increase c-Myc transcription, (2) transduction and modification of c-Myc to generate a virally encoded form of the gene, v-Myc, and (3) proviral integration in or near c-Myc. Proviral integrations elevate transcription by insertion of retroviral enhancers found in the long terminal repeat (LTR). Studies of the LTR enhancer elements from these retroviruses have revealed the importance of these elements for c-Mycactivation in several cell types. Retroviruses also have been used to identify genes that collaborate with c-Myc during development and progression of leukemias and lymphomas. In these experiments, animals that are transgenic for c-Mycoverexpression (often in combination with the overexpression or deletion of known proto-oncogenes) have been infected with retroviruses that then insertionally activate novel co-operating cellular genes. The retrovirus then acts as a molecular 'tag' for cloning of these genes. This review covers several aspects of c-Myc involvement in retrovirally induced leukemias and lymphomas.