Lack of AKR ecotropic provirus amplification in AKR leukemic thymuses

Integration and Expression of Mcf-13 Provirus in Mcf-13-Induced Lymphomas

Polytropic viruses were isolated from 11 MCF-13-induced lymphomas using multiple infectious cycles on the mink lung and SC1 cell lines. The viruses were characterized by host range infectious properties and by biochemical properties using Cleveland's polypeptide mapping of the p30 and gp 70 viral proteins demonstrating identity in their biological properties with the MCF-13 lymphoma-inducing virus. Restriction enzyme analysis of the high molecular weight tumoral DNAs confirmed the presence of proviral sequences homologous to the inducing provirus in the lymphoma genomes indicating that MCF-13 MuLV has integrated in the cellular genome of the induced lymphomas.

Differential cell killing by lymphomagenic murine leukemia viruses occurs independently of p53 activation and mitochondrial damage

Upon inoculation into AKR mice, mink cell focus-forming murine leukemia virus (MCF MLV) accelerates thymic lymphoma formation. During the preleukemic phase of disease, we observed the induction of apoptosis in thymic lymphocytes. A similar induction of apoptosis was observed for cultured mink epithelial cells after MCF13 MLV infection. In this study, the relevance of viral pathogenicity to cell killing was determined by testing the susceptibility of various cell types from different species to lymphomagenic MLVs. We observed that the cytopathic effect of lymphomagenic MLVs was restricted to mink cells. Southern blot analysis of MLV-infected cells revealed an accumulation of the linear form of unintegrated viral DNA, particularly in mink cells after MCF13 MLV infection. Thus, a strong correlation was observed between viral superinfection, which results in the accumulation of high levels of unintegrated viral DNA, and cell killing. Immunoblot analysis for MCF13 MLV-infected mink epithelial cells did not show a significant change in total p53 levels or its phosphorylated form at Ser-15 compared with that in mock-treated cells. Moreover, a time course analysis for mink epithelial cells infected with MCF13 MLV did not reveal mitochondrial depolarization or a significant change in Bax levels. These results demonstrate that MCF13 MLV induces apoptosis preferentially in cells in which superinfection occurs, and the mechanism involved is independent of p53 activation and mitochondrial damage.

Mink cell focus-forming murine leukemia virus killing of mink cells involves apoptosis and superinfection

Induction of apoptosis by different types of pathogenic retroviruses is an important step in disease development. We have observed that infection of thymic lymphocytes by the mink cell focus-forming murine leukemia virus (MCF MLV) during the preleukemic period resulted in an enhancement of apoptosis of these cells. To further study the ability of MCF MLVs to induce apoptosis and the role of this process in viral pathogenesis, we have developed an in vitro system of virus-induced apoptosis. MCF13 MLV infection of mink epithelial cells resulted in the production of cytopathic foci. In contrast, infection of mink cells with the 4070A amphotropic MLV did not produce any cytopathic effects. Staining of MCF13 MLV-infected cells with propidium iodide and annexin V-fluorescein isothiocyanate indicated that virus-induced cell death was due to apoptosis. At 6 days postinfection, the percentage of apoptotic MCF13 MLV-infected cells was 27% compared with 2 to 3% for mock- or amphotropic MLV-infected cells, representing a 9- to 14-fold difference. Assays for caspase-3 activation confirmed the detection by flow cytometry of apoptosis of MCF13 MLV-infected cells. Large amounts of unintegrated linear viral DNA were detectable by Southern blot analysis during the acute phase of infection, which indicated that MCF13 MLV is able to superinfect mink cells. Unintegrated viral DNA of only the linear form was detectable in thymic lymphocytes isolated from MCF13 MLV-inoculated mice during the preleukemic period. These results indicated that the ability of MCF13 MLV to induce apoptosis is correlated with its ability to superinfect cells and that this occurs as an early step in thymic lymphoma development.

The effects of leukemosuppressive immunotherapy on bone marrow infectious cell centers in AKR mice

The bone marrow of AKR mice is the richest source of infectious ecotropic cell centers (ICCs) during the neonatal period. The bone marrow ICCs reside in a low-density population expressing high levels of viral glycoprotein (gp71) and Class I histocompatibility antigens (H-2Kk). In addition, ICCs are enriched in the lymphoid band of Ficoll-Hypaque-fractionated bone marrow, the adherent population of nylon wool separated cells and among the low-density subpopulation of Percoll-fractionated marrow. The observed dichotomy between viral antigen expression and actual virus production suggests that actively cycling cells may be the primary virus producers in the AKR bone marrow. The phenotypic and physical data indicate that bone marrow stem cells and/or prothymocytes may be among the initial virus producing cells in the AKR bone marrow. Leukemosuppressive antiviral immunotherapy delays the appearance of ICCs in the bone marrow but does not exert any major long-term changes on the populations of cells present.

Ecotropic and mink cell focus-forming murine leukemia viruses integrate in mouse T, B, and non-T/non-B cell lymphoma DNA

Structures of somatically acquired murine leukemia virus (MuLV) genomes present in the DNA of a large panel of MuLV-induced C57BL and BALB/c B and non-T/non-B cell lymphomas were compared with those present in MuLV-induced T-cell lymphomas induced in the same low-"spontaneous"-lymphoma-incidence mice. Analyses were performed with probes specific for the gp70, p15E, and U3-long terminal repeat (LTR) regions of ecotropic AKV MuLV and a mink cell focus-forming virus (MCF)-LTR probe annealing with U3-LTR sequences of a unique endogenous xenotropic MuLV, which also hybridizes with U3-LTR sequences of a substantial portion of somatically acquired MCF genomes in spontaneous AKR thymomas. The DNAs of both T- and B-cell tumors induced by neonatal inoculation with the highly oncogenic C57BL-derived MCF 1233 virus predominantly contain integrated MCF proviruses. In contrast, the DNAs of more slowly developing B and non-T/non-B cell lymphomas induced by poorly oncogenic ecotropic or MCF C57BL MuLV isolates mostly contain somatically acquired ecotropic MuLV genomes. Approximately 50% of the spontaneous C57BL lymphoma DNAs contain somatically acquired MuLV genomes. None of the integrated MuLV proviruses annealed with the MCF-LTR probe, which indicates a clear difference in LTR structure with a substantial portion of the somatically acquired MuLV genomes present in the DNA of spontaneous AKR thymomas. This study stresses a dominant role of MuLV with ecotropic gp70 and LTR sequences in the development of slowly arising MuLV-induced B and non-T/non-B cell lymphomas.

SMX-1 virus-induced inhibition of ecotropic and recombinant proviral sequence amplification in thymuses of AKR mice

SMX-1 virus delays the appearance of spontaneous thymomas in AKR mice which have been inoculated as young adults by the intrathymic route. Analyses of high-molecular-weight thymus DNAs from SMX-1 virus-inoculated AKR mice indicated the absence of 3' recombinant proviral-cellular DNA junction fragments generated by EcoRI and PvuII digestion. An average of five recombinant proviral fragments were detected in DNAs from spontaneous thymomas that developed in medium-injected control mice. Preleukemic mice that amplify murine leukemia virus-related antigens on their thymocyte surface contained unintegrated proviruses in their thymus DNAs, and 2.3-kilobase EcoRI and 2.1-kilobase PvuII recombinant DNA fragments were detected.

Ecotropic and dualtropic mink cell focus-inducing murine leukemia viruses can induce a wide spectrum of H-2 controlled lymphoma types

Neonatal infection of C57BL and BALB/c mice by cloned ecotropic and dualtropic mink cell focus-inducing (MCF) murine leukemia viruses (MuLV) induces a wide spectrum of different lymphomas of T, B, and non-T/non-B cell types. Oncogenic dualtropic MCF viruses and poorly oncogenic ecotropic MuLV act synergistically in lymphomagenesis. Within one mouse strain virus-induced T-cell lymphomas arise earlier than B-cell lymphomas after neonatal inoculation of a single-cloned MuLV. The host genetic constitution, notably the H-2 complex has a marked influence on lymphoma type. This H-2 influence can be explained by an H-2-linked difference in penetration of the thymus early in life by oncogenic thymotropic MuLV, which in turn is correlated with, but not necessarily due to the magnitude of the anti-MuLV antibody response.

Murine T lymphomas in which the cellular myc oncogene has been activated by retroviral insertion

The myc oncogene is implicated here in T lymphocyte neoplasia. Cloning revealed a retroviral insert 0.7-1.3 kb 5' to c-myc in two T lymphomas induced by Soule murine leukemia virus and in a spontaneous T lymphoma ( Tikaut ) of an AKR mouse, a strain in which leukemogenesis involves recombinant retroviruses (MCF viruses). The tumor c-myc mRNAs appear normal but their level is approximately 5-fold higher than in most T lymphomas lacking c-myc rearrangement. Since each insert would be transcribed away from c-myc, its activation cannot involve the promoter of the long terminal repeat (LTR) but could reflect an enhancer, like that demonstrated within the Soule LTR. The Tikaut provirus has an MCF-like recombinant env gene and LTR sequence. MCF-like inserts were found near c-myc in seven of 31 other AKR T lymphomas; two lie 3' to c-myc and the five upstream are oriented away from c-myc. We conclude that a quarter of retrovirus-induced T lymphomas involve activation of c-myc, probably via the LTR enhancer.

Specific integration of recombinant proviral sequences in ecotropic Gross virus-accelerated AKR thymomas

Integration and amplification of ecotropic and recombinant proviral sequences in high-molecular-weight cellular DNAs from ecotropic Gross virus-accelerated AKR thymomas were analyzed using an ecotropic-specific probe, p400, and an envelope-specific probe, pAKV-5. New ecotropic proviral sequences were detected at three sites in the DNAs from eight Gross virus-accelerated thymomas following EcoRI restriction endonuclease digestion and at six sites following PvuII restriction endonuclease digestion. The integration of these new ecotropic proviral sequences appeared to be random. Recombinant 3' proviral-cellular DNA junction fragments were detected at 30 sites following digestion with EcoRI. These new recombinant fragments ranged in size from 9.0 to 2.5 kb with 6/8 thymoma DNAs containing a fragment of 2.7 kb. PvuII generated new recombinant 3' proviral-cellular junction fragments that ranged in size from 12.5 to 2.1 kb with 5/8 thymoma DNAs containing a fragment of 2.5 kb. It appears that the leukemia-accelerating ecotropic Gross virus is responsible for the generation of a unique 3' recombinant proviral-cellular junction fragment. This fragment can be detected against a background of randomly integrated ecotropic and recombinant proviruses.

A time-course study of MuLV env gene expression in the AKR thymus: qualitative and quantitative analysis of ecotropic and recombinant virus gene products

Low levels of synthesis of two species of MuLV env gene polyprotein (PrENV protein) in thymocytes of 3-month-old AKR mice were identified. Synthesis of PrENV proteins which comigrate with those of ecotropic and recombinant, dualtropic MuLV represented, respectively, 0.03-0.05 and 0.01-0.03% of total cell protein synthesis in these animals. An increase in the rate of synthesis of both PrENV species was observed in animals at 5-6 months of age; ecotropic virus PrENV represented 0.2-0.6% of total protein synthesis and recombinant virus PrENV, 1-2.5% of total protein synthesis in thymocytes of mice of this age. This same increase in env gene expression of both the ecotropic and recombinant virus was induced in 3-month-old animals by intrathymic injection of recombinant MuLV at 4-6 weeks of age. The level of recombinant virus env gene synthesis in thymomas was similar to that observed in thymocytes of 5- to 6-month-old animals and in experimentally injected animals; elevated synthesis of ecotropic virus PrENV protein was detected in 85% of these leukemias. Partial protease digest mapping of the recombinant virus PrENV protein isolated from 23 primary thymomas revealed that the predominant type of recombinant (18/23) expressed in these cells was that of the MCF 69L1/247 type. A notable finding was the identification of expression of variant ecotropic MuLV in these thymomas. Of ten leukemias studied, eight expressed ecotropic virus PrENV proteins which were distinguishable from that of Akv virus. Four unique ecotropic virus PrENV proteins were observed.

Strong selection for cells containing new ecotropic recombinant murine leukemia virus provirus after propagation of C57BL/6 radiation-induced thymoma cells in vitro or in vivo

Using the Southern procedure, we have studied the presence of ecotropic-specific murine leukemia viral sequences in genomic DNA isolated from primary X-ray-induced thymomas, from lymphoid cell lines established from them, or from secondary tumors passaged in vivo. We found that primary radiation-induced thymomas and infiltrated spleens do not harbor newly acquired ecotropic provirus. However, additional ecotropic proviruses (which appear recombinant in the gagpol region) could be detected in most of the tumorigenic cell lines established in vitro from them and in tumors arising from subcutaneous transplantation of the primary thymomas. These results suggest that primary radiation-induced thymomas may not be clonal. They also indicate a strong correlation between the presence of ecotropic recombinant proviruses in the genome and the growth ability, both in vitro and in vivo, of specific cells within these thymomas, suggesting a possible mitogenic function for murine leukemia virus.

Linkage of the Fv-2 gene to a newly reinserted ecotropic retrovirus in Fv-2 congenic mice

Restriction enzyme and Southern gel analyses were used to determine the number and location of endogenous ecotropic retroviruses in the germ line of several mouse strains congenic at the Fv-2 gene locus. A new endogenous ecotropic provirus was observed in the germ line of B6.S (Fv-2ss) mice, in addition to the resident provirus found in its congenic partner C57BL/6 (Fv-2rr). This new provirus was similar in structure to the C57BL provirus. The SIM strain of mice, the donors of the Fv-2s allele in B6.S mice, does not contain ecotropic proviruses, suggesting that the new provirus in the B6.S mouse strain arose by germ-line reintegration during the construction of this strain. Mendelian segregation analysis indicated that this new provirus was linked to the Fv-2 gene locus on chromosome 9. In three other Fv-2s congenic mouse strains--B10.C (47N), B6.C (H-7b), and C57BL/6J Trfa, Bgsd--no additional ecotropic endogenous viruses were detected, suggesting that the reinsertion event that occurred during the construction of B6.S is not essential for the acquisition of the Fv-2s phenotype in the C57BL genetic background. Although numerous reports of germ-line reinsertions of ecotropic virus in high-virus mouse strains have been received, the present results provide definitive evidence that similar germ-line amplifications of endogenous ecotropic virus can occur in a low-virus mouse strain.

Strong selection for cells containing new ecotropic recombinant murine leukemia virus provirus after propagation of C57BL/6 radiation-induced thymoma cells in vitro or in vivo

Using the Southern procedure, we have studied the presence of ecotropic-specific murine leukemia viral sequences in genomic DNA isolated from primary X-ray-induced thymomas, from lymphoid cell lines established from them, or from secondary tumors passaged in vivo. We found that primary radiation-induced thymomas and infiltrated spleens do not harbor newly acquired ecotropic provirus. However, additional ecotropic proviruses (which appear recombinant in the gagpol region) could be detected in most of the tumorigenic cell lines established in vitro from them and in tumors arising from subcutaneous transplantation of the primary thymomas. These results suggest that primary radiation-induced thymomas may not be clonal. They also indicate a strong correlation between the presence of ecotropic recombinant proviruses in the genome and the growth ability, both in vitro and in vivo, of specific cells within these thymomas, suggesting a possible mitogenic function for murine leukemia virus.

Localization of the leukemogenic determinants of SL3-3, an ecotropic, XC-positive murine leukemia virus of AKR mouse origin

SL3-3 is a potent leukemogenic retrovirus that closely resembles the non-leukemogenic virus Akv. Both viruses were isolated from AKR mice, have ecotropic host ranges, and form plaques in the XC assay. They differ at only 1 to 2% of the nucleotides in the viral genomes but differ markedly in virulence properties. SL3-3 induces leukemia in a high percentage of inoculated AKR, C3H, CBA, and NFS mice, whereas Akv does not induce disease in any of these strains. To determine which region of the genome accounts for the leukemogenic potential of SL3-3, we constructed recombinant genomes between molecular clones of SL3-3 and Akv. Recombinant, viral DNA genomes were cloned and then were transfected onto NIH 3T3 fibroblasts to generate infectious virus. The recombinant viruses were tested for leukemogenicity in AKR/J, CBA/J, and C3Hf/Bi mice. We localized the primary leukemogenic determinant to a 3.8-kilobase fragment of the SL3-3 genome containing the viral long terminal repeat, 5' untranslated sequences, gag gene, and 5', 30% of the pol gene. Reciprocal recombinants containing the equivalent region from Akv, linked to the env gene and the remainder of the pol gene from SL3-3, did not induce leukemia. We conclude that the primary virulence determinant of SL3-3 lies outside the region of the genome that encodes the envelope proteins gp70 and p15E.

Thymic epithelial genotype influences the production of recombinant leukemogenic retroviruses in mice

By using T1 oligonucleotide fingerprinting and mapping techniques, we analyzed the genomic structure of retroviruses produced by thymocytes and splenocytes of reciprocal bone marrow-and thymus-grafted chimeras. We found that the genetic factor(s) derived from NZB mice that suppresses the development of thymic leukemia in (AKR X NZB)F1 mice also prevents the formation of recombinant leukemogenic viruses and the expression of preleukemic changes in the (AKR X NZB)F1 thymocytes. The NZB mouse gene or genes appeared to exert this suppressive effect by acting on the thymic reticuloepithelial cells and not on the thymic lymphocytes of (AKR X NZB)F1 hybrids. Prospective studies with thymic epithelial grafts from young mice showed that the AKR thymic epithelium could mediate the formation and expression of leukemogenic recombinant viruses and preleukemic changes in thymocytes that lead to the development of thymic leukemia, whereas the (AKR X NZB)F1 thymic epithelium was deficient in this regard. Our results also confirmed a previous observation that during in vivo generation of recombinant leukemogenic viruses, the acquisition of polytropic virus-related sequences in the 3' portion of the p15E gene and the U3 region and in the 5' part of the gp70 gene can occur independently.

Somatically acquired recombinant murine leukemia proviruses in thymic leukemias of AKR/J mice

We have probed the structure and arrangement of murine leukemia virus genomes in eight spontaneous AKR thymic leukemias by Southern hybridization with one ecotropic pol and four ecotropic env probes. These probes revealed many (in 2 cases over 15) somatically acquired proviruses that had undergone complex patterns of recombination. The large majority were not deleted and were structurally analogous to the oncogenic mink cell focus-inducing murine leukemia viruses isolated from AKR tumors in that the amino-terminal p15E-coding region derived from ecotropic AKR murine leukemia virus sequences, whereas certain gp70-coding sequences were nonecotropic. Nevertheless, we observed a few proviruses which did not appear to be gp70 recombinants; however, these proviruses were in general clearly recombinant within the p15E-coding sequences. Although the proviral recombination patterns were quite variable, in general the large majority of recombinant proviruses within each tumor appeared structurally identical, indicating that they originate from a common parent. Each tumor contained a unique pattern of provirus integrations; densitometer tracings of the Southern hybridizations indicated that many of the integrated proviruses were present at one copy per cell, suggesting that the tumors derive from a single cell which contained multiple integrated copies of a unique recombinant virus structurally similar to the mink cell focus-inducing viruses.

Isolation and mapping of cDNA hybridization probes specific for ecotropic and nonecotropic murine leukemia proviruses

To study the structure of murine leukemia proviruses in AKR mice by Southern hybridization, we have isolated and mapped ecotropic AKV and nonecotropic MCF-derived cDNA restriction fragments. The ecotropic-specific probes originate from four regions of the AKV genome which include the corresponding recombinant region of two MCF viruses (VI-36 and 247). We also isolated two nonecotropic probes from the recombinant region of MCF V1-36. The probes were characterized by (i) mapping of restriction fragments at the 3' end of AKV and MCF V1-36 by a two-dimensional gel strategy, (ii) hybridization of restriction fragments to the related viral RNA genomes followed by electrophoresis, (iii) two-dimensional fingerprinting of single-stranded restriction fragments, and (iv) DNA sequence analysis of the ecotropic probes. The ecotropic AKV and nonecotropic MCF probes discriminate between two populations of endogenous murine leukemia viruses and show that the MCF viruses are not present in the germ line of AKR mice.

AKR thymic lymphomas involving mink cell focus-inducing murine leukemia viruses have a common region of provirus integration

Newly acquired proviruses related to a mink cell focus-inducing murine leukemia virus were detected in low copy number in restriction endonuclease-digested DNAs from thymic lymphomas of AKR/J mice. These extra proviruses were not present in DNAs of either normal thymus or leukemic brain tissues. Extra tumor-specific DNA fragments generated by restriction endonucleases either were identical in size or fell into similar size classes, suggesting a common site(s) of provirus integration. Characterization of extra EcoRI DNA fragments for mink cell focus-inducing viral sequences revealed that all of them contained large terminal repeat sequences and that a significant number represented proviruses with deletions.

Characterization of target cells for MCF viruses in AKR mice

The recombinant (MCF) class of murine leukemia virus appears to play an important role in lymphomagenesis in AKR and other mice. Although much effort has been extended in characterizing MCF viruses, relatively little is known about the cells they infect. I examined what cells were targets in AKR mice for both lymphomagenic and nonlymphomagenic MCF viruses. Lymphomagenic MCF viruses of thymic origin (AKR-247 and C58L1) were found to infect and replicate selectively in immature lymphocytes only present in thymic cortex, whereas nonlymphomagenic MCF viruses of splenic origin (C58v-1-C77 and C58v-2-C45) selectively infected and replicated in cells that appeared to B lymphocytes. Virus-binding studies suggested that neither T- nor B-lymphocyte tropisms were determined by selective attachment of virus to the respective cells. These findings demonstrate that in contrast with ecotropic viruses, which can infect many types of cells in the mouse, specific cellular tropisms can exist for MCF viruses, and that MCF infection, and therefore oncogenicity, is closely linked to cellular differentiation.

Analysis of the env gene of a molecularly cloned and biologically active Moloney mink cell focus-forming proviral DNA

A biologically active molecular clone of BALB/Moloney mink cell focus-forming (Mo-MCF) proviral DNA has been reconstructed in vitro. It contains the 5' half of BALB/Moloney murine leukemia virus (Mo-MuLV) DNA and the 3' half of BALB/Mo-MCF DNA. The complete nucleotide sequence of the env gene and the 3' long terminal repeat (LTR) of the cloned Mo-MCF DNA has been determined and compared with the sequence of the corresponding region of parental Mo-MuLV DNA. The substitution in the Mo-MCF DNA encompasses 1,159 base pairs, beginning in the carboxyl terminus of the pol gene and extending to the middle of the env gene. The Mo-MCF env gene product is predicted to be 29 amino acids shorter than the parental Mo-MuLV env gene product. The portion of the env gene encoding the p15E peptide is identical in both viral DNAs. There is an additional A residue in the Mo-MCF viral DNA in a region just preceding the 3' LTR. The nucleotide sequence of the 3' LTR of Mo-MCF DNA is similar to that of the 5' LTR of BALB/Mo-MuLV DNA with the exception of two single base substitutions. We conclude that the sequence substitution in the env gene is responsible for the dual-tropic properties of Mo-MCF viruses.

Identification of ecotropic proviral sequences in high- and low-ecotropic-virus-producing mouse strains

The arrangement of endogenous ecotropic retroviruses in selected high- and low-ecotropic-virus-producing mouse strains was examined by Southern blot hybridization analysis, using an ecotropic retrovirus-specific DNA probe. High-ecotropic-virus-producing mouse strains of the AKR family displayed heterogeneity with respect to the number of copies and the sites of insertion of endogenous ecotropic specific DNA. This diversity was seen even among individuals of the same AKR subline. Contrastingly, individuals within the same low-ecotropic-retrovirus-producing mouse strain showed no evidence of variability in their endogenous ecotropic proviral sequences. These results favored the hypothesis that germ line proviral reinsertion was responsible for the proviral sequence heterogeneity observed in high-ecotropic-virus-producing mouse strains.

Molecular cloning of a highly leukemogenic, ecotropic retrovirus from an AKR mouse

SL3-3 is a leukemogenic, ecotropic retrovirus produced by a T-cell line derived from a spontaneous lymphoma of an AKR mouse. We have isolated a molecular clone of its DNA provirus from infected NIH 3T3 fibroblasts. Cloned proviral DNA produced infectious virus upon transfection onto NIH 3T3 cells. Virus derived by transfection induced lymphomas at high frequency in AKR/J, C3H(f)/Bi, CBA/J, and NFS/N mice. Heteroduplex and RNase T1 fingerprinting analyses showed that the genomes of SL3-3 and the non-leukemogenic virus, Akv, contain no major substitutions relative to one another and differ by only a few base changes. These results unambiguously show that SL3-3 is a highly leukemogenic virus and that major rearrangements of the genome relative to Akv are not required for virulence.

Variation in the number of copies and in the genomic organization of ecotropic murine leukemia virus proviral sequences in sublines of AKR mice

DNAs isolated from individual mice of four AKR sublines (AKR/J, AKR/N, AKR/Cum, and AKR/Boy) were examined by hybridization of electrophoretically separated restriction enzyme fragments to a 500-base pair, 32P-labeled probe specific for env sequences of ecotropic murine leukemia virus. Variation in the number of proviral DNA copies and in their genomic organization, as reflected by the location of restriction enzyme sites in flanking cellular sequences, was observed both between and within AKR sublines. Evidence is presented for the continual acquisition of new proviruses in the four sublines studied. The ecotropic proviral DNA copies present in the four AKR sublines can be related to their genealogy; each subline contains two or three copies of proviral DNA in common with other sublines and from one to six unique ecotropic proviruses. Overall, a new copy appears about every 12 generations of inbreeding. Some of the unique proviral DNA copies contain internal alterations, as reflected by restriction enzyme maps that differ from those of prototype ecotropic proviruses.

Novel class of mouse mammary tumor virus-related DNA sequences found in all species of Mus, including mice lacking the virus proviral genome

Mice in breeding colonies of feral Mus musculus brevirostris (Azrou, Morocco), M. m. musculus (Studenec, Czechoslovakia), and M. m. molossinus (Fukuoka, Japan) were found to lack the mouse mammary tumor virus (MMTV-alpha) proviral genome in their germ line. MMTV-alpha proviral genomes have been found in all inbred strains of M. musculus by using high-stringency nucleic acid hybridization conditions. We conclude that feral mice in these colonies are heterozygous for a limited number of MMTV-alpha proviral genomes and that those lacking them arose as a result of random chromosomal segregation. All mice in another breeding colony of feral M. m. musculus (Sladeckovce, Czechoslovakia) lack MMTV proviral genes. By relaxing the conditions of nucleic acid hybridization, MMTV-related sequences (designated MMTV-beta) were detected in restricted cellular DNA from MMTV-negative mice and all other inbred strains and feral species of the genus Mus. The apparent ubiquity of the MMTV-beta DNA sequences in the genus Mus and the lack of variation in the pattern of restriction fragments containing these sequences within a species distinguishes them from MMTV-alpha. These results suggest that the MMTV-beta DNA sequences either are the evolutionary progenitors of the infectious MMTV genome or represent an accumulation of evolutionarily divergent MMTV-alpha insertions into the mouse germ line.

Identification of a DNA fragment from a molecularly cloned mink cell focus-inducing murine leukemia virus specific for xenotropic virus-related sequences

Molecular clones of closed circular DNA molecules of a mink cell focus-inducing murine leukemia virus (MCF-13 MuLV) were generated. Closed circular DNA molecules isolated from a Hirt extraction of recently infected NIH/3T3 cells were inserted at their unique EcoRI site into lambda gtWES.lambda B. Restriction endonuclease analysis of inserts of two clones indicated that they represented intact MCF-13 MuLV genomes. One viral insert contained two large terminal repeat sequences, and the other contained only one. A 300-base-pair DNA fragment located in the envelope region of the MCF-13 MuLV genome was determined to be related to xenotropic MuLV sequences.

Ecotropic murine leukemia virus DNA content of normal and lymphomatous tissues of BXH-2 recombinant inbred mice

BXH-2 recombinant inbred mice spontaneously produce a B-tropic murine leukemia virus (MuLV) beginning early in life and have a high incidence of non-T-cell lymphomas. These traits are not characteristic of the progenitor strains (C57BL/6J and C3H/HeJ) or of 11 other BXH recombinant inbred strains. Since B-tropic virus expression may be causally related to the high incidence of lymphoma in this strain, we have analyzed the ecotropic MuLV DNA content of both normal and lymphomatous tissues of BXH-2 mice. Southern analysis and hybridization with an ecotropic MuLV DNA-specific probe showed that DNA of normal BXH-2 tissues contained both parental N-tropic MuLV proviruses but lacked endogenous B-tropic MuLV DNA sequences. In addition, none of 116 F1 hybrid mice derived from male BXH-2 mice spontaneously produced ecotropic MuLV early in life. These results suggest that the B-tropic virus is horizontally transmitted in BXH-2 mice. Southern analysis of DNA from tumor tissues of 12 BXH-2 mice showed that amplification of ecotropic-specific DNA sequences had occurred in lymphomatous tissues of 3 mice and suggested that these tumors were monoclonal. The number of newly acquired proviruses, which appeared to be structurally nondefective and integrated at different sites, varied from one to three copies. Since lymphomatous tissues from only 3 of 12 mice examined carried additional detectable ecotropic proviruses, these results suggest that amplification of ecotropic MuLV DNA sequences is not required for maintenance of transformation in BXH-2 lymphomas.