Comparative molecular genetic analysis of lymphomas from six inbred mouse strains

Forward and Reverse Genetics of B Cell Malignancies: From Insertional Mutagenesis to CRISPR-Cas

Cancer genome sequencing has identified dozens of mutations with a putative role in lymphomagenesis and leukemogenesis. Validation of driver mutations responsible for B cell neoplasms is complicated by the volume of mutations worthy of investigation and by the complex ways that multiple mutations arising from different stages of B cell development can cooperate. Forward and reverse genetic strategies in mice can provide complementary validation of human driver genes and in some cases comparative genomics of these models with human tumors has directed the identification of new drivers in human malignancies. We review a collection of forward genetic screens performed using insertional mutagenesis, chemical mutagenesis and exome sequencing and discuss how the high coverage of subclonal mutations in insertional mutagenesis screens can identify cooperating mutations at rates not possible using human tumor genomes. We also compare a set of independently conducted screens from Pax5 mutant mice that converge upon a common set of mutations observed in human acute lymphoblastic leukemia (ALL). We also discuss reverse genetic models and screens that use CRISPR-Cas, ORFs and shRNAs to provide high throughput in vivo proof of oncogenic function, with an emphasis on models using adoptive transfer of ex vivo cultured cells. Finally, we summarize mouse models that offer temporal regulation of candidate genes in an in vivo setting to demonstrate the potential of their encoded proteins as therapeutic targets.

Recombinant Origins of Pathogenic and Nonpathogenic Mouse Gammaretroviruses with Polytropic Host Range

Ecotropic, xenotropic, and polytropic mouse leukemia viruses (E-, X-, and P-MLVs) exist in mice as infectious viruses and endogenous retroviruses (ERVs) inserted into mouse chromosomes. All three MLV subgroups are linked to leukemogenesis, which involves generation of recombinants with polytropic host range. Although P-MLVs are deemed to be the proximal agents of disease induction, few biologically characterized infectious P-MLVs have been sequenced for comparative analysis. We analyzed the complete genomes of 16 naturally occurring infectious P-MLVs, 12 of which were typed for pathogenic potential. We sought to identify ERV progenitors, recombinational hot spots, and segments that are always replaced, never replaced, or linked to pathogenesis or host range. Each P-MLV has an E-MLV backbone with P- or X-ERV replacements that together cover 100% of the recombinant genomes, with different substitution patterns for X- and P-ERVs. Two segments are always replaced, both coding for envelope (Env) protein segments: the N terminus of the surface subunit and the cytoplasmic tail R peptide. Viral gag gene replacements are influenced by host restriction genes Fv1 and Apobec3 Pathogenic potential maps to the env transmembrane subunit segment encoding the N-heptad repeat (HR1). Molecular dynamics simulations identified three novel interdomain salt bridges in the lymphomagenic virus HR1 that could affect structural stability, entry or sensitivity to host immune responses. The long terminal repeats of lymphomagenic P-MLVs are differentially altered by recombinations, duplications, or mutations. This analysis of the naturally occurring, sometimes pathogenic P-MLV recombinants defines the limits and extent of intersubgroup recombination and identifies specific sequence changes linked to pathogenesis and host interactions.IMPORTANCE During virus-induced leukemogenesis, ecotropic mouse leukemia viruses (MLVs) recombine with nonecotropic endogenous retroviruses (ERVs) to produce polytropic MLVs (P-MLVs). Analysis of 16 P-MLV genomes identified two segments consistently replaced: one at the envelope N terminus that alters receptor choice and one in the R peptide at the envelope C terminus, which is removed during virus assembly. Genome-wide analysis shows that nonecotropic replacements in the progenitor ecotropic MLV genome are more extensive than previously appreciated, covering 100% of the genome; contributions from xenotropic and polytropic ERVs differentially alter the regions responsible for receptor determination or subject to APOBEC3 and Fv1 restriction. All pathogenic viruses had modifications in the regulatory elements in their long terminal repeats and differed in a helical segment of envelope involved in entry and targeted by the host immune system. Virus-induced leukemogenesis thus involves generation of complex recombinants, and specific replacements are linked to pathogenesis and host restrictions.

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

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

Murine leukemia retrovirus integration induces the formation of transcription factor complexes on palindromic sequences in the signal transducer and activator of transcription factor 5a gene during the development of pre-B lymphomagenesis

Murine leukemia retrovirus (MLV) vectors are highly effective tools for introducing a foreign gene into a target host genome. However, it remains unclear how integrated retroviral promoter activity is influenced by the upstream or downstream sequences and how the host cell phenotype is influenced by the integrated promoter activity. Herein, we analyzed a set of pre-B lymphoma clones in which the MLV genome was integrated into the signal transducer and activator of transcription factor 5a (Stat5a) gene. Among the clones, the lymphoma clones with a provirus integrating into the middle position of the palindromic target sequences showed significantly higher transcription of the Stat5a gene; and p300 and other transcriptional factors formed complexes, with binding to the proviral-host junctional DNA segment. By using a luciferase assay, the upstream and downstream sequences of the provirus contributed to the up-regulation of proviral promoter activity. In concomitance with the higher Stat5a transcription, the immunoglobulin gene recombination was arrested. Antiapoptotic activity was significantly higher, with an increase in Bcl-xL, one of the targets of STAT5A, when IL-7 was supplied. Thus, a minute difference between MLV integration sites can lead to large differences in the host phenotype through the formation of transcription factor complexes on the proviral-host junctional DNA segment, suggesting that caution is necessary in monitoring integration sites when working with MLV vectors.

Resistance of athymic nude mice to experimental cutaneous Bacillus anthracis infection

BACKGROUND

Previous studies in a murine cutaneous anthrax model have demonstrated that hairless and haired HRS/J mice are extremely resistant to Bacillus anthracis. Because these mice are relatively thymus deficient, we used C57BL/6 athymic nude and euthymic mice to evaluate the relationship between T cell deficiency and this heightened resistance.

METHODS

Animals were epicutaneously inoculated with 1 X 10(7) B. anthracis (Sterne) spores onto abraded skin or injected with the spores intradermally or subcutaneously. The mice were then either monitored for survival or killed for quantitative histological experiments.

RESULTS

Athymic mice were found to be markedly resistant to all 3 inoculation routes, compared with euthymic C57BL/6 mice. Athymic mice rendered leukopenic with cyclophosphamide became susceptible. Histological examination demonstrated increased inflammation and absence of organisms in the skin of athymic mice, compared with euthymic ones. The numbers of organisms in the athymic animals increased markedly after cyclophosphamide treatment. Superficial exudate fluids of inoculated skin showed many more neutrophils and ingested bacilli in the athymic mice.

CONCLUSIONS

These experiments demonstrate that athymic nude C57BL/6 mice are markedly resistant to experimental cutaneous anthrax, apparently because of a superficial neutrophilic response that clears the inoculated organisms before they can invade the underlying skin.

Retroviral insertions in the VISION database identify molecular pathways in mouse lymphoid leukemia and lymphoma

AKXD recombinant inbred (RI) strains develop a variety of leukemias and lymphomas due to somatically acquired insertions of retroviral DNA into the genome of hematopoetic cells that can mutate cellular proto-oncogenes and tumor suppressor genes. We generated a new set of tumors from nine AKXD RI strains selected for their propensity to develop B-cell tumors, the most common type of human hematopoietic cancers. We employed a PCR technique called viral insertion site amplification (VISA) to rapidly isolate genomic sequence at the site of provirus insertion. Here we describe 550 VISA sequence tags (VSTs) that identify 74 common insertion sites (CISs), of which 21 have not been identified previously. Several suspected proto-oncogenes and tumor suppressor genes lie near CISs, providing supportive evidence for their roles in cancer. Furthermore, numerous previously uncharacterized genes lie near CISs, providing a pool of candidate disease genes for future research. Pathway analysis of candidate genes identified several signaling pathways as common and powerful routes to blood cancer, including Notch, E-protein, NFκB, and Ras signaling. Misregulation of several Notch signaling genes was confirmed by quantitative RT-PCR. Our data suggest that analyses of insertional mutagenesis on a single genetic background are biased toward the identification of cooperating mutations. This tumor collection represents the most comprehensive study of the genetics of B-cell leukemia and lymphoma development in mice. We have deposited the VST sequences, CISs in a genome viewer, histopathology, and molecular tumor typing data in a public web database called VISION (Viral Insertion Sites Identifying Oncogenes), which is located at http://www.mouse-genome.bcm.tmc.edu/vision.

Experimental cutaneous Bacillus anthracis infections in hairless HRS/J mice

Previous studies of experimental Bacillus anthracis cutaneous infections in mice have implicated hair follicles as a likely entry site. Hairless HRS/J mice were used to investigate this possibility because of their non-functional hair follicles that lack penetrating hair shafts. These mice also have diminished macrophage function, increased susceptibility to Listeria, and enhanced neutrophil responses. HRS/J and Balb/c mice were found to be resistant to epicutaneous inoculation with Bacillus anthracis (Sterne) spores onto abraded skin when compared with DBA/2 mice or leucopenic C57BL/6 mice. The HRS/J mice also resisted spore injections that bypassed hair follicles. Haired HRS/J heterozygote mice demonstrated similar reduced susceptibility to B. anthracis spores. Hairless HRS/J mice that were made leucopenic did become susceptible to the epicutaneous spore inoculations. Histologically, the hairless and haired HRS/J mice showed markedly reduced numbers of organisms in hair follicles and the interfollicular dermis when compared even with the resistant Balb/c mice; inflammatory cell infiltrates in the superficial dermis were increased in the HRS/J mice compared with more sensitive strains. Therefore, resistance in the HRS/J mice was apparent at the initial site of epicutaneous inoculation and seemed related to an accumulation of dermal neutrophils rather than to a lack of functional hair follicles.

Analysis of expansion of myeloid progenitors in mice to identify leukemic susceptibility genes

The myeloid progenitor cell compartment (MPC) exhibits pronounced expansion in human myeloid leukemias. It is becoming more apparent that progression of myelodysplastic syndromes and myeloproliferative diseases to acute myelogenous leukemia is the result of defects in progenitor cell maturation. The MPC of bone marrow was analyzed in mice using a cell culture assay for measuring the relative frequency of proliferative myeloid progenitors. Response to the cytokines SCF, IL-3, and GM-CSF was determined by this assay for the leukemic mouse strain BXH-2 and ten other inbred mouse strains. Significant differences were found to exist among ten inbred mouse strains in the nature of their MPC in bone marrow, indicating the presence of genetic polymorphisms responsible for the divergence. The SWR/J and FVB/J strains show consistently low frequencies of myeloid progenitors, while the DBA/2J and SJL/J inbred strains show consistently high frequencies of myeloid progenitors within the bone marrow compartment. In addition, in silico linkage disequilibrium analysis was conducted to identify possible chromosomal regions responsible for the phenotypic variation. Given the importance of this cell compartment in leukemia progression and the soon to be released genomic sequence of 15 mouse strains, these differences may provide a valuable tool for research into leukemia.

Identification of candidate alkylator-induced cancer susceptibility genes by whole genome scanning in mice

Secondary malignancies are a serious adverse consequence of alkylator chemotherapy. The risk of developing an alkylator-associated malignancy is influenced by genetic background, although the relevant genetic factors are poorly understood. To screen for novel susceptibility factors, we established a mouse model of alkylator-induced malignancy. We exposed mice from 20 inbred strains to the prototypical alkylating agent, N-nitroso-N-ethylurea (ENU). ENU was a potent carcinogen in many of the strains tested, inducing 140 tumors in 240 ENU-treated mice (66% incidence of at least one tumor in evaluable mice), compared with a background incidence of 8% spontaneous tumors in 240 strain-, age-, and sex-matched control mice (relative risk, 8.4; P < 0.0001). A wide variety of tumor histologies were noted, including epithelial carcinomas, soft tissue sarcomas, and hematopoietic tumors. Cancer susceptibility was a heritable trait for the most common tumor types, lung adenocarcinoma (H(2) = 0.25), T cell lymphoma (H(2) = 0.19), and myeloid malignancies (H(2) = 0.10). Quantitative trait locus mapping identified regions on chromosomes 3, 6, 9, and 15 containing candidate genes associated with lung adenoma, lung carcinoma, and lymphoma susceptibility. This novel mouse model recapitulates many features of human alkylator-associated cancer and supports the hypothesis that susceptibility to this syndrome is influenced by inherited polymorphisms that could be used to make informed clinical treatment decisions.

Distinct roles of enhancer nuclear factor 1 (NF1) sites in plasmacytoma and osteopetrosis induction by Akv1-99 murine leukemia virus

Murine leukemia viruses (MLVs) can be lymphomagenic and bone pathogenic. In this work, the possible roles of two distinct proviral enhancer nuclear factor 1 (NF1) binding sites in osteopetrosis and tumor induction by B-lymphomagenic Akv1-99 MLV were investigated. Akv1-99 and mutants either with NF1 site 1, NF1 site 2 or both sites disrupted induced tumors (plasma cell proliferations by histopathology) with remarkably similar incidence and mean latency in inbred NMRI mice. Clonal immunoglobulin gene rearrangement detection, by Southern analysis, confirmed approximately half of the tumors induced by each virus to be plasmacytomas while the remaining lacked detectable clonally rearranged Ig genes and were considered polyclonal; a demonstration that enhancer NF1 sites are dispensable for plasmacytoma induction by Akv1-99. In contrast, X-ray analysis revealed significant differences in osteopetrosis induction by the four viruses strongly indicating that NF1 site 2 is critical for viral bone pathogenicity, whereas NF1 site 1 is neutral or moderately inhibitory. In conclusion, enhancer NF1 sites are major determinants of osteopetrosis induction by Akv1-99 without significant influence on viral oncogenicity.

Evi3, a zinc-finger protein related to EBFAZ, regulates EBF activity in B-cell leukemia

Retroviral insertions that activate proto-oncogenes are a primary cause of tumors in certain strains of mice. The AKXD recombinant inbred mice are predisposed to a variety of leukemias and lymphomas as a result of viral integration. One common insertion site, the ecotropic viral insertion site 3 (Evi3), has been implicated in most B-cell tumors in the AKXD-27 strain. The Evi3 gene encodes a zinc-finger protein with sequence similarity to the Early B-cell Factor-Associated Zinc-finger gene (EBFAZ). We show that the Evi3 gene is overexpressed in several tumors with viral insertions at Evi3, which results in the upregulation of Early B-cell Factor (EBF)-target gene expression, suggesting that Evi3 modulates EBF activity. Reconstitution of primary leukemia cells showed that these tumors express high densities of the B-cell surface proteins CD19 and CD38, which are EBF targets. Using a transactivation assay, we show that the terminal six zinc-fingers of Evi3 are required for modification of EBF activity. This is the first evidence that Evi3 expression in tumors alters the level of EBF target genes, and the first characterization of the Evi3 protein domains required for modulation of EBF activity. Further, these data imply that Evi3 misexpression initiates tumorigenesis by perturbing B-cell development via an interaction with EBF.

Tnfrsf13c (Baffr) is mis-expressed in tumors with murine leukemia virus insertions at Lvis22

In susceptible strains of mice, leukemia is caused by the somatic integration of murine leukemia retroviruses into the host genome. Integration sites that are common to several tumors are likely to affect genes that are important in oncogenesis. Here we present the analysis of a common site of retroviral integration on mouse chromosome 15, which includes the genomic structure of three genes near the integration site. One of the genes misexpressed at the insertion site has recently been characterized as a B-cell receptor, Tnfrsf13c (formerly Baffr), indicating that this approach is useful in defining genes that function in lymphocyte development and tumor progression. Current genome databases provide powerful resources for the rapid identification of genes at common proviral insertion sites. The characterization of these genes in tumor samples will allow a function to be assigned to many novel loci identified by the genome sequencing projects.

TCR v(beta) repertoire restriction and lack of CDR3 conservation implicate TCR-superantigen interactions in promoting the clonal evolution of murine thymic lymphomas

Thymic lymphoma development is a multistage process in which genetic and epigenetic events cooperate in the emergence of a malignant clone. The notion that signaling via TCR-ligand interactions plays a role in promoting the expansion of developing neoplastic clones is a matter of debate. To investigate this issue, we determined the TCR V(beta) repertoire of thymic lymphomas induced in AKR/J mice by either endogenous retroviruses or the carcinogen, N-methyl-N-nitrosourea (MNU). Both spontaneous and MNU-induced lymphomas displayed restricted V(beta) repertoires. However, whereas V(beta)6, V(beta)8 and V(beta)9 were expressed by a greater than expected frequency of MNU-induced lymphomas, V(beta)8, V(beta)7, V(beta)13 and V(beta)14 were over-represented on spontaneous lymphomas. The dissimilar TCR V(beta) profiles indicate that different endogenous ligands promote neoplastic clonal expansion in untreated and MNU-treated mice. Although the nature of these ligands is not clear, the lack of conservation in TCR beta chain CDR3 regions among lymphomas that express the same V(beta) segment suggests that endogenous superantigens (SAG), as opposed to conventional peptide ligands, are likely to be involved in the selection process. The biased representation of lymphomas expressing V(beta)6-, V(beta)7- and V(beta)9-containing TCRs that recognize endogenous SAG is consistent with this hypothesis. The finding that Bcl-2 is expressed at high levels in spontaneous and MNU-induced lymphomas suggests that preneoplastic thymocytes may be resistant to SAG-induced clonal deletion. A working model is presented in which preneoplastic clones expressing TCRs that recognize endogenous SAG are selectively expanded as a consequence of sustained TCR-mediated signaling.

Accelerated appearance of multiple B cell lymphoma types in NFS/N mice congenic for ecotropic murine leukemia viruses

Spontaneous lymphomas occur at high frequency in NFS x V+ mice, strains congenic for ecotropic murine leukemia virus (MuLV) proviral genes and expressing virus at high titer. In the present study, a total of 703 NFS x V+ lymphomas were studied by histopathology, immunophenotypic analysis, immunoglobulin heavy chain or T cell receptor beta chain rearrangements, and somatic ecotropic MuLV integrations; 90% of the lymphomas tested were of B cell lineage. Low-grade tumors included small lymphocytic, follicular, and splenic marginal zone lymphomas, while high-grade tumors comprised diffuse large-cell (centroblastic and immunoblastic types), splenic marginal zone, and lymphoblastic lymphomas. Comparison of mice of similar genetic background except for presence (NFS x V+) or absence (NFS x V-) of functional ecotropic MuLV genomes showed that NFS x V-clonal lymphomas developed at about one-half the rate of those occurring in NFS x V+ mice, and most were low-grade B cell lymphomas with extended latent periods. In NFS x V+ mice, clonal outgrowth, defined by Ig gene rearrangements, was associated with acquisition of somatic ecotropic proviral integrations, suggesting that, although generation of B cell clones can be virus independent, ecotropic virus may act to increase the rate of generation of clones and speed their evolution to lymphoma. The mechanism remains undefined, because only rare rearrangements were detected in several cellular loci previously associated with MuLV insertional mutagenesis.

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.

T lymphomagenesis is determined by a dominant host gene thymic lymphoma susceptible mouse-1 (TLSM-1) in mouse models

Susceptibility to T lymphomas in mice is determined by a number of viral and host genetic factors. We analyzed the types and latent period of lymphomas spontaneously occurring in crosses between AKR/Ms, a T lymphoma-prone mouse strain, and SL/Kh, a pre-B lymphoma-prone strain. The incidence of T lymphomas in the F1 hybrids backcross to SL/Kh as well as F2 generation mice indicated that a dominant host gene thymic lymphoma susceptible mouse-1 (Tlsm-1) of AKR/Ms determined the type of lymphomas to be thymic. Linkage analysis with microsatellite markers assigned Tlsm-1 to the map position 61 cM from centromere of the chromosome 7. Close scrutiny of this region of AKXD recombinant inbred strains for spontaneous T lymphomas revealed the presence of Tlsm-1-like gene most likely between D7MIT71 (map position 62) and D7MIT13 (map position 70). On the other hand, a SL/Kh-derived recessive allele at a major histocompatibility complex (MHC)-linked locus accelerated development of both T and B lymphomas.

Identification of Evi-3, a novel common site of retroviral integration in mouse AKXD B-cell lymphomas

We have identified a novel common site of ecotropic viral integration called ecotropic viral integration site 3 (Evi-3) in B-cell lineage lymphomas of the AKXD recombinant inbred strains of mice. A number of virally induced pre-B-, B-, myeloid, and T-cell lymphomas were screened for viral rearrangements at Evi-3; rearrangements were found in pre-B- and B-cell lymphomas but not in other hematopoietic tumors. Genetic mapping studies localized Evi-3 to mouse chromosome 18, distinct from proto-oncogene and common viral integration loci identified previously in the mouse. Each proviral integration at Evi-3 is contained within a 200-bp region that lies inside a CpG island. All but one of the proviruses have integrated in the same 5'-to-3' transcriptional orientation. Transcripts from Evi-3 are expressed in a developmentally regulated manner in B cells. Taken together, these data suggest that Evi-3 represents a novel proto-oncogene involved in mouse B-cell lymphomas.

Retrovirus-induced lymphoproliferation as a model for developing diagnostic criteria for malignant lymphoma in mice

Several methods for evaluating lymphoproliferative lesions in mice were compared. The model systems included spontaneous lymphomas arising in CWD mice and NFS mice congenic for ecotropic murine leukemia virus (MuLV) induction loci and a series of transplants in mice with severe combined immunodeficiency disease mutation of cells derived from mice infected with LP-BM5 MuLV. Primary lymphomas and donor tissues and transplants were examined using histopathology, flow cytometry, and Southern blot analysis of DNA for rearrangements of immunoglobulin and T-cell receptor genes and for viral integrations. The use of flow cytometric analysis, to establish cell lineage and define population size, and DNA analysis, for cell lineage and clonality determination, allowed the identification of malignant lymphoproliferations. Histologic evaluation did not define clonal populations of particular lineage but did provide other indications of malignancy such as invasiveness and presence of a dominant morphologic cell type. Thus, the precision of diagnosis of mouse lymphomas can be considerably enhanced by augmenting histopathologic examination with antigenic and molecular characterizations that can define malignant populations.

Beta cell expression of endogenous xenotropic retrovirus distinguishes diabetes-susceptible NOD/Lt from resistant NON/Lt mice

Endogeneous retroviral expression in beta cells is a feature of prediabetes in nonobese diabetic (NOD) mice. The purpose of this study was to characterize the class-specific pattern of retroviral gene expression in NOD/Lt beta cells versus a related, but diabetes-resistant strain, NON/Lt. Electron microscopic comparison of beta cells from both strains indicated low constitutive expression of the intracisternal type A (IAP) retroviral class. However, NOD beta cells, in contrast to NON beta cells, expressed an additional intracisternal retroviral form resembling a type C particle. Antibodies against both IAP and type C were detected in NOD, with the humoral response to type C, but not IAP, preceding decline in beta cell function. RNA was extracted from freshly isolated islets from NOD and NON males. Comparative Northern blot analysis of total type C retroviral gene expression using a gag-pol DNA probe corroborated expression of endogenous type C proviruses in both NOD and NON islet cells and thymus. Use of class-specific retroviral probes identified the class of expressed endogenous retrovirus distinguishing the two inbred strains. The single ecotropic provirus present in both the NOD and NON genome (Emv-30) was not expressed in islets or thymus of either strain. Comparison of endogenous xenotropic provirus content by Southern blot analysis revealed two unique xenotropic loci (Xmv-65, -66) in NOD; 8.4 and 3.0 kb xenotropic envelope (env) RNA transcripts were detected in NOD, but not NON islets and thymus. NON contained three xenotropic loci common to other inbred strains (Xmv-21, -25, and -28). Both strains were partially characterized for content of recombinant (polytropic and modified polytropic) proviruses. IAP RNA expression was common to both NOD and NON islets and hence could not be specifically associated with the unique intracisternal type C particle found in NOD, but not NON beta cells. In conclusion, this study shows that expression of xenotropic type C but not IAP distinguishes retroviral activity in NOD/Lt versus NON/Lt beta cells. The potential pathogenic role of retroviral gene expression in NOD beta cells is discussed.

The nonobese diabetic scid mouse: model for spontaneous thymomagenesis associated with immunodeficiency

Homozygosity for the severe combined immunodeficiency (scid) mutation results in a block in T- and B-lymphocyte development. An unusually high incidence of spontaneous thymic lymphoma development was observed after transfer of this mutation from the C.B-17 congenic strain background onto the diabetes-susceptible nonobese diabetic (NOD) background. Thymomagenesis in the NOD-scid/scid mouse was associated with expression of an NOD mouse-unique endogenous ecotropic murine leukemia provirus locus (Emv-30, mapped to proximal region of chromosome 11) not expressed in the standard substrain NOD/Lt thymus. All tumors exhibited insertions of ecotropic proviruses, whereas only a subset also exhibited proviral integrations of mink cell focus-forming retrovirus. Neither class of retrovirus was associated with consistent integration into genes previously associated with activation of oncogenesis. We propose that the unusual features of T-cell ontogeny characteristic of the NOD inbred strain synergize with the scid-imparted block in thymocyte development, leading to activation of the NOD-unique Emv-30 to initiate thymomagenesis.

Activation of Lyt-2 associated with distant upstream insertion of an SL3-3 provirus

Two Lyt-2+ mutants of the T-cell lymphoma SL12.4.10 were selected by fluorescence activated cell sorting. Both mutants expressed Lyt-2 (CD8 alpha-chain) but not Lyt-3 (CD8 beta-chain). Derivatives of one Lyt-2+ mutant that expressed Lyt-3 could be isolated by sorting for Lyt-3+ cells. Southern blotting analysis indicated that both mutants had structural rearrangements within or immediately 3' of the Lyt-3 gene, accompanied by demethylation of at least one Hpa II site within the Lyt-2 gene. Gene cloning analysis of one mutant demonstrated that the structural rearrangement was due to insertion of an SL3-3 provirus 35 kb 5' to the Lyt-2 gene. It is likely that Lyt-2 gene activation is a direct or indirect consequence of proviral insertion at this site.

Influence of murine leukemia proviral integrations on development of N-methyl-N-nitrosourea-induced thymic lymphomas in AKR mice

The AKR mouse strain is characterized by a high incidence of spontaneous thymic lymphoma that appears in older animals (greater than 6 months of age) and is associated with novel provirus integrations of ecotropic and recombinant murine leukemia viruses (MuLVs). Treatment of 4- to 6-week-old AKR/J mice with the carcinogen N-methyl-N-nitrosourea (MNU) results in thymic lymphomas that arise as early as 3 to 4 months of age and contain novel somatically acquired MuLV provirus integrations. The AKR/J strain develops MNU-induced lymphoma with a higher incidence and shorter latency than has been observed for other inbred mouse strains. To determine whether provirus integrations of endogenous MuLV account for the enhanced susceptibility of the AKR strain, the incidence and latency of MNU-induced lymphoma development was compared in AKR/J and AKR.Fv-1b mice. The restrictive b allele of the Fv-1 locus restricts integration and replication of endogenous N-tropic MuLV; therefore, AKR-Fv-1b mice have a very low incidence of spontaneous lymphoma. In contrast, AKR.Fv-1b mice develop MNU-induced lymphomas with an incidence and latency similar to those of the AKR/J strain. Furthermore, thymic lymphomas from both strains express an immature CD4-8+ phenotype, indicating neoplastic transformation of the same thymocyte subset. Southern blot analysis confirmed that lymphoma DNA from AKR.Fv-1b mice did not contain somatically acquired provirus integrations. These results demonstrate that provirus integration does not contribute to the predisposition of AKR mice to develop a high incidence of early MNU-induced lymphomas. Nevertheless, MNU treatment stimulated high-level expression of infectious ecotropic MuLV in AKR.Fv-1b as well as in AKR/J mice, suggesting that viral gene products might enhance lymphoma progression.

Infection by mink cell focus-forming viruses confers interleukin 2 (IL-2) independence to an IL-2-dependent rat T-cell lymphoma line

The development of T-cell lymphomas in rodents infected with type C retroviruses has been linked to the generation of a class of envelope (env) recombinant viruses called mink cell focus-forming viruses (MCF viruses) in the preleukemic thymus. To determine whether infection by MCF viruses altered the growth phenotype of retrovirus-induced T-cell lymphomas, a Moloney murine leukemia virus-induced interleukin-2 (IL-2)-dependent rat T-cell lymphoma line (4437A) was infected with MCF-247, modified MCF-V33 (mMCF-V33), or NZB-xenotropic (NZB-X) virus. The effects of virus infection on the IL-2 dependence of these cells was examined by cultivating them in the absence of IL-2. After IL-2 withdrawal, the uninfected and NZB-X-infected cells went through a crisis period characterized by massive death. All the independently maintained cultures of MCF- and mMCF-V33-infected cells, on the other hand, became IL-2 independent without a crisis. All the polytropic virus-infected IL-2-independent cultures contained a population of cells that was polyclonal with regard to polytropic provirus integration. Over this polyclonal background each culture produced multiple clones of cells that were selected rapidly after IL-2 withdrawal. Furthermore, the resulting MCF- or mMCF-V33-infected IL-2-independent cells retained the expression of IL-2 receptor. These data show that MCF and mMCF-V33 viruses may alter the growth phenotype of a T-cell lymphoma line and suggest that their effect on cell growth may be due to the direct interaction of the MCF envelope glycoprotein with cellular components, perhaps the IL-2 receptor.

Oncogenicity and U3 region sequences of class II recombinant MuLVs of CWD mice

The pathogenic potential of Class II env recombinant murine leukemia viruses (MuLV) found in the high leukemia strain CWD has not been defined. We found that neonatal CWD mice that were injected with the phenotypic mixture of the spontaneous CWD class II env recombinant, CWM-T-15, and the AKR endogenous ecotropic virus, Akv 623, developed non-T-cell lymphomas more rapidly than controls inoculated with either virus alone or with a CWD ecotropic virus. In contrast, CWN-T-25, a class II env MuLV that was recovered from a CWD mouse injected with the AKR ecotropic virus SL3-3, dramatically accelerated the onset of T-cell lymphomas in the same assay. Southern blots of the tumor DNAs from each set of animals revealed the integration of recombinant and ecotropic proviruses. We also found that there were differences in the nucleotide sequences of the viral enhancer elements of the CWD viruses. The results indicate that (1) the two CWD class II env recombinants that were tested contained oncogenic determinants; (2) phenotypic mixing with ecotropic viruses was required for the full expression of the pathogenic potential of the CWM-T-15 recombinant; and (3) the distinct phenotypes of the CWD viruses likely reflected the differences in the origin of the viral enhancer element.

Retrovirally induced murine B-cell tumors rarely show proviral integration in sites common in T-cell tumors

The molecular etiology of retrovirally induced T-cell tumors has been shown in many cases to involve proviral integration near a cellular oncogene, c-myc, N-myc, Pim-1 and pvt-1 being frequent targets for insertional activation. Murine B-cell tumors induced by infection with murine leukemia virus have been studied for rearrangements in these and other loci. In contrast to the T-cell lymphomas, tumors of the B-cell lineage, either early B-cell tumors induced in nude mice or late B-cell tumors in immunocompetent mice, did not show disruption of N-myc or Pim-1 in any of the tumors studied, although those lymphomas had acquired many new proviruses. The loci c-abl, bcl-2, fis-1, c-erbB, c-myb, and neu were likewise not involved. Rearrangement of c-myc was seen in 1 out of 71 and rearrangement of the pvt-1 locus in 4 out of 73 (5%) of the B-cell tumors. Thus it appears that mechanistic differences exist in the development of T-cell tumors and B-cell tumors caused by the same etiological agent.

Pattern of expression of ecotropic murine leukemia virus in gonads of inoculated SWR/J mice

An ecotropic murine leukemia virus (MuLV) isolate has recently been shown to be able to infect the germ line or the early embryo or both when inoculated at birth to SWR/J females (J. J. Panthier, H. Condamine, and F. Jacob, Proc. Natl. Acad. Sci. USA 85:1156-1160, 1988). We have used this isolate to further study this phenomenon. By using the techniques of RNA-RNA in situ hybridization, immunocytochemistry, and transmission electron microscopy, the identities of two important cell types that are infected by ecotropic MuLV in the gonads of inoculated mice were determined. These cells are the thecal cells surrounding the follicles in the ovary and the Leydig cells in the testis. Both types actively synthesize viral RNA and express a viral antigen. Furthermore, we documented the production of viral particles by the thecal cells. The expression of ecotropic MuLV by nonlymphoid cells in vivo may play a key role in the vertical transmission of these viruses by females as well as in their horizontal transmission.

Ecotropic virus involvement in spontaneous B-cell lymphomas of CWD/LeAgl mice

The inbred mouse strain CWD/LeAgl, which has a high incidence of spontaneous B-cell lymphomas, expresses both ecotropic MuLV and MCF viruses. Studies indicated that the MCF viruses expressed in CWD tumors were characteristic of nononcogenic MCF viruses and that ecotropic MuLV may be the etiological agent in spontaneous B-cell lymphomagenesis. Somatically acquired proviruses of approximately the same size were detected in several tumor DNAs suggesting that integration of proviral sequences into specific regions of the mouse genome may be an important step in lymphomagenesis of this strain.

Provirus tagging as an instrument to identify oncogenes and to establish synergism between oncogenes

Insertional mutagenesis is one of the mechanisms by which retroviruses can transform cells. Once a provirus was found in the vicinity of c-myc, with the concomitant activation of this gene, other proto-oncogenes were shown to be activated by proviral insertion in retrovirally-induced tumors. Subsequently, cloning of common proviral insertion sites led to the discovery of a series of new (putative) oncogenes. Some of these genes have been shown to fulfill key roles in growth and development. In this review I shall describe how proviruses can be used to identify proto-oncogenes, and list the loci, identified by this method. Furthermore, I shall illuminate the potential of provirus tagging by showing that it not only can mark new oncogenes, but can also be instrumental in defining sets of (onco)genes that guide a normal cell in a step-by-step fashion to its fully transformed, metatasizing, counterpart.