Leukemogenesis by Gross passage A murine leukemia virus: expression of viruses with recombinant env genes in transformed cells

The mouse "xenotropic" gammaretroviruses and their XPR1 receptor

The xenotropic/polytropic subgroup of mouse leukemia viruses (MLVs) all rely on the XPR1 receptor for entry, but these viruses vary in tropism, distribution among wild and laboratory mice, pathogenicity, strategies used for transmission, and sensitivity to host restriction factors. Most, but not all, isolates have typical xenotropic or polytropic host range, and these two MLV tropism types have now been detected in humans as viral sequences or as infectious virus, termed XMRV, or xenotropic murine leukemia virus-related virus. The mouse xenotropic MLVs (X-MLVs) were originally defined by their inability to infect cells of their natural mouse hosts. It is now clear, however, that X-MLVs actually have the broadest host range of the MLVs. Nearly all nonrodent mammals are susceptible to X-MLVs, and all species of wild mice and several common strains of laboratory mice are X-MLV susceptible. The polytropic MLVs, named for their apparent broad host range, show a more limited host range than the X-MLVs in that they fail to infect cells of many mouse species as well as many nonrodent mammals. The co-evolution of these viruses with their receptor and other host factors that affect their replication has produced a heterogeneous group of viruses capable of inducing various diseases, as well as endogenized viral genomes, some of which have been domesticated by their hosts to serve in antiviral defense.

Genetic determinants of neoplastic diseases induced by a subgroup F avian leukosis virus

Two subgroup F avian leukosis viruses, ring-necked pheasant virus (RPV) and RAV-61, were previously shown to induce a high incidence of a fatal proliferative disorder in the lungs of infected chickens. These lung lesions, termed angiosarcomas, appear rapidly (4 to 5 weeks after infection), show no evidence of proto-oncogene activation by proviral integration, and are not induced by avian leukosis viruses belonging to other subgroups. To identify the viral sequences responsible for induction of these tumors, we constructed recombinant viruses by exchanging genomic segments of molecularly cloned RPV with those of a subgroup A leukosis virus, UR2AV. The ability to induce rapid lung tumors segregated only with the env sequences of RPV; the long terminal repeat of RPV was not required. However, recombinants carrying both env and long terminal repeat sequences of RPV induced lung tumors with a shorter latency. In several cases, recombinant viruses exhibited pathogenic properties differing from those of either parental virus. Recombinants carrying the gag-pol region of RPV and the env gene of UR2AV induced a high incidence of a muscle lesion termed infiltrative intramuscular fibromatosis. One recombinant, EU-8, which carries the gag-pol and LTR sequences of RPV, and the env gene of UR2AV, induced lymphoid leukosis after an unusually short latent period. The median time of death from lymphoid leukosis was 6 to 7 weeks after infection with EU-8 compared with approximately 5 months for UR2AV.

AKR ecotropic murine leukemia virus SL3-3 forms envelope gene recombinants in vivo

The ecotropic AKR virus SL3-3 was injected into neonatal mice of the high-leukemia strains HRS/J and CWD/J and the low-leukemia strains CBA/J, SEA/J, and NIH Swiss. SL3-3 was highly leukemogenic in each strain, and 90% of the inoculated animals died by 6 months of age. T1 oligonucleotide fingerprint analysis of the genomic RNAs of viruses recovered from 9 of 13 leukemic animals revealed the presence of the SL3-3 virus and recombinant viruses with polytropic virus-related envelope gene sequences. Recombinant proviruses were detected by the Southern blot technique in the DNAs of 17 of 18 tumors. The pattern of substitutions within the envelope genes of the SL3-3 recombinant viruses appeared to be dependent on the strain of the animal. These observations indicate that the SL3-3 virus formed envelope gene recombinants in vivo in each of the strains that were studied. However, the role of these recombinants during leukemogenesis remains to be defined.

Association of recombinant murine leukemia viruses of the class II genotype with spontaneous lymphomas in CWD mice

We determined the phenotype and genotype of murine leukemia viruses associated with the development of spontaneous nonthymic lymphomas in the high-leukemia mouse strain CWD/J. By T1 oligonucleotide fingerprint analysis of the viral RNA, the ecotropic viruses recovered from the spleen or thymus of preleukemic CWD/J mice were found to represent the progeny of the two endogenous ecotropic proviruses present in this strain. Polytropic murine leukemia viruses were produced by tissues from one-half of the leukemic mice, and fresh tumor cells from one of the two animals tested expressed recombinant envelope glycoproteins. The genomic structure of the recombinant viruses resembled those of class II polytropic viruses of NFS X Akv mice and differed from those of class I recombinant viruses that are commonly isolated from other high-leukemia strains such as AKR and HRS. Acquired retroviral sequences with the structural features of class II recombinant proviruses were detected in the DNA from each CWD/J tumor by the Southern blot technique. Finally, the injection of a mixture of CWD/J ecotropic and class II recombinant polytropic viruses into neonatal CWD/J mice accelerated the onset of lymphoma, whereas the endogenous ecotropic virus was inactive in these assays.

Cytolytic T lymphocyte-defined retroviral antigens on normal cells: encoding by the Akv-1 proviral locus

We previously described the generation and specificity of H-2-restricted cytolytic lymphocytes (CTL) directed against tumors induced by AKR/Gross murine leukemia viruses (MuLV). Such anti-AKR/Gross virus CTL demonstrated type specificity; only tumors induced by endogenous MuLV that expressed the Gross cell-surface antigen were lysed. These CTL and their precursor also recognized normal spleen cells from AKR-H-2b, but not AKR-H-2b, Fv-1b mice, however, suggesting that N-ecotropic, retrovirus-associated antigens were primarily involved. Here, expression of these CTL-defined retroviral antigens by H-2b-positive AKR X C57L recombinant inbred strains was examined by using normal spleen cells as stimulators in the generation of specific anti-AKR/Gross virus CTL. Analysis of the strain distribution pattern of stimulation indicated that a single proviral locus, Akv-1, was primarily, if not entirely, responsible for CTL-defined retroviral antigen expression. The lack of correlation with two other well-defined proviral loci was interesting. Whereas Akv-3 is known to encode a defective virus, Akv-4 has been shown to code for an infectious virus thought to be very similar or identical to that of Akv-1. Although quantitative differences cannot be formally excluded, dose response experiments argued against this possibility and suggested that Akv-1 and Akv-4 may exhibit qualitative differences germane to antiviral CTL recognition.

The pathophysiology of murine retrovirus-induced leukemias

Murine leukemia viruses (MuLVs) are retroviruses which induce a broad spectrum of hematopoietic malignancies. In contrast to the acutely transforming retroviruses, MuLVs do not contain transduced cellular genes, or oncogenes. Nonetheless, MuLVs can cause leukemias quickly (4 to 6 weeks) and efficiently (up to 100% incidence) in susceptible strains of mice. The molecular basis of MuLV-induced leukemia is not clear. However, the contribution of individual viral genes to leukemogenesis can be assayed by creating novel viruses in vitro using recombinant DNA techniques. These genetically engineered viruses are tested in vivo for their ability to cause leukemia. Leukemogenic MuLVs possess genetic sequences which are not found in nonleukemogenic viruses. These sequences control the histologic type, incidence, and latency of disease induced by individual MuL Vs.

The tandem direct repeats within the long terminal repeat of murine leukemia viruses are the primary determinant of their leukemogenic potential

To map the viral sequences encoding the leukemogenic determinant(s) of nondefective murine leukemia viruses (MuLVs), we constructed chimeric viral genomes in vitro between cloned viral DNAs from the highly leukemogenic Gross passage A (Gross A) MuLV and from the related nonleukemogenic BALB/c N-tropic MuLV. Infectious chimeric MuLVs, recovered from murine cells microinjected with these DNAs, were inoculated into newborn mice to test the leukemogenic potential of these viruses. We found that the U3 long terminal repeat region from Gross A genomes was sufficient to confer an intermediate leukemogenic potential to chimeric MuLVs. Sequencing data indicated that the U3 tandem direct repeat was responsible for this effect. Adding most of the Gross A p15E-coding sequences to the Gross A U3 long terminal repeat enhanced the leukemogenic potential of chimeric viruses significantly. Adding a larger 3'-end env region (all p15E-coding sequences and 345 base pairs of the carboxy terminus of gp70) to the Gross A U3 long terminal repeat restored the full leukemogenic potential of Gross A MuLV. Chimeric viruses harboring only the Gross A 3'-end env region were, however, nonleukemogenic. Similar chimeric MuLVs, constructed with genomes from the parental weakly leukemogenic BALB/c B-tropic MuLVs and nonleukemogenic BALB/c N-tropic MuLVs, were also studied. Our data indicate that the U3 tandem direct repeat sequences appear to be necessary and sufficient to confer some leukemogenic potential to MuLV. However, env 3'-end sequences, mostly the p15E-encoding sequences, are required for the expression of fully leukemic phenotypes.

Role of recombinant ecotropic and polytropic viruses in the development of spontaneous thymic lymphomas in HRS/J mice

The biological and genetic characteristics of murine leukemia viruses (MuLV) derived from leukemic and normal HRS/J mice were studied. T1-oligonucleotide fingerprinting and mapping of viral RNAs from unpassaged isolates revealed the presence of complex mixtures of viral genomes. MuLV that were purified by endpoint dilution were genetically heterogeneous. Thus, endogenous retroviral sequences expressed in the tissues of HRS/J mice readily recombined with one another. Furthermore, the regular recovery of recombinant ecotropic MuLV suggested reciprocal in vivo complementation of a genetic defect(s) in each of the endogenous ecotropic proviruses Emv-1 and Emv-3. Some recombinant ecotropic viruses contained sequences in the p15E-U3 region that were not derived from Emv-1 and Emv-3 but were found in recombinant polytropic HRS/J viruses. Finally, comparison of the genetic structures of leukemogenic and nonleukemogenic MuLV of this strain implied that the oncogenic phenotype of these MuLV is encoded within env or the U3 region of the genome or both. Our results are consistent with a stepwise convergent evolution of recombinant MuLV in vivo in individual HRS/J mice. Ultimately, this process of selection results in formation of leukemogenic polytropic viruses.

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.

The high leukemogenic potential of Gross passage A murine leukemia virus maps in the region of the genome corresponding to the long terminal repeat and to the 3' end of env

The Gross passage A murine leukemia virus (MuLV) is a highly leukemogenic, ecotropic fibrotropic retrovirus. Its genome is similar to that of other nonleukemogenic ecotropic fibrotropic MuLVs but differs at the 3' end and in the long terminal repeat. To determine whether these modifications were related to its leukemogenic potential, we constructed a viral DNA recombinant in vitro with cloned infectious DNA from this highly leukemogenic Gross passage A MuLV and from a weakly leukemogenic endogenous BALB/c B-tropic MuLV. Infectious viruses, recovered after microinjection of murine cells with recombinant DNA, were injected into newborn mice. We show here that the Gross passage A 1.35-kilobase-pair KpnI fragment (harboring part of gp70, all of p15E, and the long terminal repeat) is sufficient to confer a high leukemogenic potential to this recombinant.

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.

Molecular cloning of viral DNA from leukemogenic Gross passage A murine leukemia virus and nucleotide sequence of its long terminal repeat

The viral DNA genome of the leukemogenic Gross passage A virus was cloned in phage Charon 21A as an infectious molecule. The virus recovered by transfection with this infectious DNA was ecotropic, N-tropic, fibrotropic, and XC+. It was leukemogenic when reinjected into newborn SIM mice, indicating that ecotropic murine leukemia virus (MuLV) from an AKR mouse thymoma can harbor leukemogenic sequences. Its restriction map was similar to that of nonleukemogenic AKR MuLV, its putative parent, but differed at the 3' end and in the long terminal repeat (LTR). The nucleotide sequence of the Gross A virus LTR was identical to the AKR MuLV LTR sequence (Van Beveren et al., J. Virol. 41:542-556, 1982) in U5, R, and part of U3. All differences between both LTRs were found in U3. Only one copy of the U3 tandem direct repeat was conserved in the Gross A virus LTR, and it was rearranged by the insertion of a 36-base-pair sequence and by five point mutations. Only one additional point mutation common to several oncogenic MuLVs was present in U3. These structural changes in the U3 LTR and at the 3' end of the genome may be related to the leukemogenicity of this virus.