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.     

An increase in disease latency is associated with a host-dependent selection for recombinant murine leukemia viruses with substitutions in the p15E (TM) gene.

The genomes of recombinant murine leukemia viruses recovered from HRS/J (type I env recombinants) and CWD (type II env recombinants) mice have distinct envelope gene structures. To better understand the biologic significance of these differences, we examined the differences in the responses of HRS/J and CWD mice to inoculation with an oncogenic type II env recombinant. The CWD recombinant accelerated the onset of lymphoma in both strains, but the disease latency in the HRS/J mice was about 2 months longer. Analysis of the recombinant viruses in the HRS/J tumors revealed that the injected type II env recombinant had recombined in vivo with the endogenous ecotropic viruses to generate secondary recombinants with type I envelope genes. In another set of experiments, comparison of complete or partial DNA sequences of the envelope genes from six recombinant proviruses confirmed that the origins of the sequences that encode an amino-terminal region of the TM envelope protein, p15E, distinguish type I envelope genes from type II. Taken together with the results of previous studies, these observations suggest that the differences in the responses of HRS/J and CWD mice to the oncogenic type II env recombinant resulted from an interaction between the viral TM protein and a host factor expressed in HRS/J mice.     

Class II polytropic murine leukemia viruses (MuLVs) of AKR/J mice: possible role in the generation of class I oncogenic polytropic MuLVs

We examined the frequency of occurrence of polytropic murine leukemia viruses (MuLVs) in the spleens and thymuses of preleukemic AKR/J mice from 1 week to 6 months of age and analyzed the genomic RNAs of several polytropic isolates by RNase T1 oligonucleotide fingerprinting. Polytropic MuLVs were first detected in the spleens of 3-week-old mice and preceded the appearance of polytropic MuLVs in the thymus by over 1 month. At 4 months of age and older, nearly all mice expressed polytropic MuLVs in both organs. In contrast to previous studies which have identified class I polytropic MuLVs in AKR/J mice, fingerprint analysis of polytropic MuLVs from both young (3- to 4-week-old) and older (5- to 6-month-old) preleukemic mice indicated that a large proportion of viruses at both ages were class II polytropic MuLVs. All polytropic viruses (five isolates) analyzed from 3- to 4-week-old mice were recovered from spleen cells and were class II polytropic MuLVs. In older preleukemic mice, five of seven isolates were class II polytropic MuLVs and two were class I polytropic viruses. Class I and class II polytropic MuLVs were recovered from both the spleens and thymuses of older preleukemic mice. A detailed comparison of the class I and class II polytropic MuLVs from 5- to 6-month-old mice revealed that the nonecotropic gp70 sequences of most of the class I and class II MuLVs were identical, consistent with a common origin for these sequences. In contrast, the nonecotropic p15E sequences of class I MuLVs were clearly derived from different endogenous sequences than the nonecotropic p15E sequences of the class II MuLVs. The in vitro host ranges of class I and class II polytropic viruses were clearly distinguishable. Examination of the in vitro host range of several isolates suggested that the predominant polytropic viruses initially identified in the thymus (2 to 3 months of age) were class II polytropic viruses. The order of appearance of the class I and class II polytropic MuLVs and the identity of the gp70 oligonucleotides of these MuLVs suggested a model for the stepwise generation of class I polytropic MuLVs involving a class II polytropic MuLV intermediate.     

Origin of pathogenic determinants of recombinant murine leukemia viruses: analysis of Bxv-1-related xenotropic viruses from CWD mice.

The acquisition of U3 region sequences derived from the endogenous xenotropic provirus Bxv-1 appears to be an important step in the generation of leukemogenic recombinant viruses in AKR, HRS, C58, and some CWD mice. We report here that each of three CWD lymphomas produced infectious xenotropic murine leukemia virus related to Bxv-1. In Southern blot experiments, these proviruses hybridized to probes that were specific for the xenotropic envelope and Bxv-1 U3 region sequences. Nucleotide sequence analysis of a cloned CWD xenotropic provirus, CWM-S-5X, revealed that the envelope gene was closely related to but distinct from those of other known xenotropic viruses. In addition, the U3 region of CWM-S-5X contained a viral enhancer sequence that was identical to that found in MCF 247, a recombinant AKR virus that is thought to contain the Bxv-1 enhancer. Finally, restriction enzyme sites in the CWM-S-5X provirus were analogous to those reported within Bxv-1. These results establish that the virus progeny of Bxv-1 have the potential to donate pathogenic enhancer sequences to recombinant polytropic murine leukemia viruses. Interestingly, the three CWD polytropic viruses that were isolated from the same tumor cells that produced the Bxv-1-like viruses had not incorporated Bxv-1 sequences into the U3 region.     

Fv-1 N- and B-tropism-specific sequences in murine leukemia virus and related endogenous proviral genomes

Oligonucleotide probes specific for the Fv-1 N- and B-tropic host range determinants of the gag p30-coding sequence were used to analyze DNA clones of various murine leukemia virus (MuLV) and endogenous MuLV-related proviral genomes and chromosomal DNA from four mouse strains. The group of DNA clones consisted of ecotropic MuLVs of known Fv-1 host range, somatically acquired ecotropic MuLV proviruses, xenotropic MuLV isolates, and endogenous nonecotropic MuLV-related proviral sequences from mouse chromosomal DNA. As expected, the prototype N-tropism determinant is carried by N-tropic viruses of several different origins. All seven endogenous nonecotropic MuLV-related proviral sequence clones derived from RFM/Un mouse chromosomal DNA, although not recognized by the N probe, showed positive hybridization with the prototype B-tropism-specific probe. The two xenotropic MuLV clones derived from infectious virus (one of BALB:virus-2 and one of AKR xenotropic virus) failed to hybridize with the N- and B-tropic oligonucleotide probes tested and with one probe specific for NB-tropic Moloney MuLV. One of two endogenous xenotropic class proviruses derived from HRS/J mouse chromosomal DNA (J. P. Stoye and J. M. Coffin, J. Virol. 61:2659-2669, 1987) also failed to hybridize to the N- and B-tropic probes, whereas the other hybridized to the B-tropic probe. In addition, analysis of mouse chromosomal DNA from four strains indicates that hybridization with the N-tropic probe correlates with the presence or absence of endogenous ecotropic MuLV provirus, whereas the B-tropic probe detects abundant copies of endogenous nonecotropic MuLV-related proviral sequences. These results suggest that the B-tropism determinant in B-tropic ecotropic MuLV may arise from recombination between N-tropic ecotropic MuLV and members of the abundant endogenous nonecotropic MuLV-related classes including a subset of endogenous xenotropic proviruses.     

Characterization of endogenous and recombinant proviral elements of a highly tumorigenic AKR cell line

As an approach to evaluating the contribution of classes of endogenous viral sequences to leukemogenesis, a genomic library was prepared from the highly tumorigenic AKR SL12.3 cell line and screened for env-containing proviruses. An extensive battery of virus-derived probes and specific oligonucleotide probes were used to segregate 83 positive clones into related groups. The nonecotropic endogenous retroviruses were identified as members of the polytropic, modified polytropic, or xenotropic groups. At least three unique xenotropic proviruses were detected that differed from the published xenotropic sequence within a variable region of the 5' portion of env. Changes among the xenotropic proviruses included relative insertions and/or deletions that maintain an open reading frame and hence the potential to encode viable envelope gene products. Several recombinant viruses were also detected. Recombination was not random and primarily involved the formation of mink cell focus-inducing class I retroviruses via recombination between polytropic elements and ecotropic virus. One other recombinant was detected which contained ecotropic virus sequences in the 5' region encoding p15 of an otherwise xenotropic provirus. An interesting observation was the finding that certain clones contained more than one provirus within the average 20-kb cloned insert. This would not be expected if integration were totally random. The de novo recombinant proviruses identified here provide a series of potential candidates to be evaluated for their contribution to the tumorigencity of the SL12.3 cell line.     

New class of leukemogenic ecotropic recombinant murine leukemia virus isolated from radiation-induced thymomas of C57BL/6 mice

We previously reported the establishment of several lymphoid cell lines from X-ray-induced thymomas of C57BL/Ka mice, and all, except one, produce retroviruses (P. Sankar-Mistry and P. Jolicoeur, J. Virol.35:270-275, 1980). Biological characterization of five of these new primary radiation leukemia viruses (RadLVs) indicated that they had a B-tropic, fibrotropic, and ecotropic host range and were leukemogenic when reinjected into C57BL/Ka newborn mice. The leukemogenic potential of one isolate (G(6)T(2)) was further assessed and shown to be retained after prolonged passaging on fibroblasts in vitro. Restriction endonuclease analysis of the DNA of four of our new RadLV isolates (G(6)T(2), Ti-7, Ti-8, and Ti-9) revealed that G(6)T(2) and Ti-7 murine leukemia virus (MuLV) genomes had identical restriction maps, whereas Ti-8 and Ti-9 genomes were different from each other and from the G(6)T(2) and Ti-7 genomes. The physical maps of these genomes were similar to that of known ecotropic MuLV genomes (including the C57BL/Ka endogenous ecotropic MuLV) within their long terminal repeats, env, the right portion of pol, and the left portion of gag. However, a region covering the end of gag and the beginning of pol was different and showed several similarities with xenotropic MuLV genomes of BALB/c, AKR, and C58 mice previously mapped. Our results suggest that these primary RadLV genomes are recombinants between the parental ecotropic MuLV genome and a nonecotropic (xenotropic) sequence. This nonecotropic gag-pol region might be important in conferring the leukemogenic potential to these isolates. Therefore, these RadLVs appear to form a new class of leukemogenic recombinant MuLVs recovered from leukemic tissues of mice. They appear to be distinct from the recombinant AKR mink cell focus-inducing MuLVs which have a dual-tropic host range and harbor xenotropic env sequences. To further study the leukemogenic potential of these RadLVs, the genome of one of them (G(6)T(2)) was cloned in Charon 21A as an infectious molecule.     

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.     

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.     

The Mouse H-2A Region Influences the Envelope Gene Structure of Tumor-Associated Murine Leukemia Viruses

C57BL/10 (B10) strains congenic at the mouse major histocompatibility locus (H-2) were injected with a modified ecotropic SL3-3 murine leukemia virus (MuLV) to determine the effect of the H-2 genes on the envelope gene structure of recombinant MuLVs. All tested strains rapidly developed T-cell lymphomas, and recombinant proviruses were detected in the tumor DNAs by Southern blot. The B10.D2 (H-2^d), B10.Br (H-2^k), B10.Q (H-2^q), and B10.RIII (H-2^r) strains exhibited a TI phenotype in which almost all tumors contained type I recombinants. These recombinants characteristically acquire envelope gene sequences from the endogenous polytropic viruses but retain the 5′ p15E (TM) gene sequences from the ecotropic virus. The parental B10 (H-2^b) strain, however, had a novel phenotype that was designated NS for nonselective. Only 30% of the B10 tumors had detectable type I recombinants, whereas a proportion of the others appeared to contain type II recombinants that lacked the type I-specific ecotropic p15E gene sequences. Studies of other B10 congenic strains with hybrid H-2 loci and selected F₁ animals revealed that the NS phenotype was regulated by a dominant gene(s) that mapped to the A region of H-2^b. These results demonstrate that a host gene within the major histocompatibility complex can influence the genetic evolution of pathogenic retroviruses in vivo.     

A locus that enhances the induction of endogenous ecotropic murine leukemia viruses is distinct from genome-length ecotropic proviruses.

The segregation of genes that enhance the induction of ecotropic murine leukemia viruses (In loci) has been compared with the segregation of ecotropic-specific nucleotide sequences in 12 low-leukemic mouse strains and 18 recombinant inbred strains. Endogenous ecotropic viruses of these strains are of genome length and structurally similar to AKR ecotropic proviruses. Low-leukemic strains of related pedigree contain ecotropic proviruses at common integration sites. Loci previously identified which enhance induction of ecotropic viruses (In genes) were correlated with the inheritance of ecotropic viral sequences in inbred low-leukemic mouse strains and in CXB recombinant inbred mouse strains. However, four BXH recombinant inbred strains were observed to possess an In gene(s) yet lack the probed envelope gene region for the corresponding endogenous ecotropic virus. These observations indicate that at least one gene that enhances ecotropic virus expression in vitro is encoded by DNA sequences outside ecotropic proviruses or by subgenomic viral sequences.     

Tissue-specific replication of Friend and Moloney murine leukemia viruses in infected mice.

We have studied the replication of ecotropic murine leukemia viruses (MuLV) in the spleens and thymuses of mice infected with the lymphocytic leukemia-inducing virus Moloney MuLV (M-MuLV), with the erythroleukemia-inducing virus Friend MuLV (F-MuLV), or with in vitro-constructed recombinants between these viruses in which the long terminal repeat (LTR) sequences have been exchanged. At 1 week after infection both the parents and the LTR recombinants replicated predominantly in the spleens with only low levels of replication in the thymus. At 2 weeks after infection, the patterns of replication in the spleens and thymuses were strongly influenced by the type of LTR. Viruses containing the M-MuLV LTR exhibited a remarkable elevation in thymus titers which frequently exceeded the spleen titers, whereas viruses containing the F-MuLV LTR replicated predominantly in the spleen. In older preleukemic mice (5 to 8 weeks of age) the structural genes of M-MuLV or F-MuLV predominantly influenced the patterns of replication. Viruses containing the structural genes of M-MuLV replicated efficiently in both the spleen and thymus, whereas viruses containing the structural genes of F-MuLV replicated predominantly in the spleen. In leukemic mice infected with the recombinant containing F-MuLV structural genes and the M-MuLV LTR, high levels of virus replication were observed in splenic tumors but not in thymic tumors. This phenotypic difference suggested that tumors of the spleen and thymus may have originated by the independent transformation of different cell types. Quantification of polytropic MulVs in late-preleukemic mice infected with each of the ecotropic MuLVs indicated that the level of polytropic MuLV replication closely paralleled the level of replication of the ecotropic MuLVs in all instances. These studies indicated that determinants of tissue tropism are contained in both the LTR and structural gene sequences of F-MuLV and M-MuLV and that high levels of ecotropic or polytropic MuLV replication, per se, are not sufficient for leukemia induction. Our results further suggested that leukemia induction requires a high level of virus replication in the target organ only transiently during an early preleukemic stage of disease.     

Precise identification of endogenous proviruses of NFS/N mice participating in recombination with moloney ecotropic murine leukemia virus (MuLV) to generate polytropic MuLVs

Polytropic murine leukemia viruses (MuLVs) are generated by recombination of ecotropic MuLVs with env genes of a family of endogenous proviruses in mice, resulting in viruses with an expanded host range and greater virulence. Inbred mouse strains contain numerous endogenous proviruses that are potential donors of the env gene sequences of polytropic MuLVs; however, the precise identification of those proviruses that participate in recombination has been elusive. Three different structural groups of proviruses in NFS/N mice have been described and different ecotropic MuLVs preferentially recombine with different groups of proviruses. In contrast to other ecotropic MuLVs such as Friend MuLV or Akv that recombine predominantly with a single group of proviruses, Moloney MuLV (M-MuLV) recombines with at least two distinct groups. In this study, we determined that only three endogenous proviruses, two of one group and one of another group, are major participants in recombination with M-MuLV. Furthermore, the distinction between the polytropic MuLVs generated by M-MuLV and other ecotropic MuLVs is the result of recombination with a single endogenous provirus. This provirus exhibits a frameshift mutation in the 3' region of the surface glycoprotein-encoding sequences that is excluded in recombinants with M-MuLV. The sites of recombination between the env genes of M-MuLV and endogenous proviruses were confined to a short region exhibiting maximum homology between the ecotropic and polytropic env sequences and maximum stability of predicted RNA secondary structure. These observations suggest a possible mechanism for the specificity of recombination observed for different ecotropic MuLVs.     

Free and integrated recombinant murine leukemia virus DNAs appear in preleukemic thymuses of AKR/J mice.

We studied the appearance and structure of murine leukemia viral genomes in preleukemic AKR/J mice by Southern hybridization. Up to an average of one to two copies per thymocyte of unintegrated murine leukemia virus DNA appears in the thymuses of preleukemic mice beginning at 4 to 5 months of age and disappears in leukemic thymuses. The free viral genomes are absent in the spleens, livers, and brains of preleukemic mice. Using a series of ecotropic and nonecotropic murine leukemia virus hybridization probes, we showed that the unintegrated viral genomes are structurally analogous to those of recombinant mink cell focus-forming viruses that appear as proviruses in leukemic AKR thymocytes, suggesting that these free viral DNAs are the direct precursors to the leukemia-specific proviruses. The mosaic of ecotropic and nonecotropic sequences within these unintegrated viral DNAs varies from one preleukemic thymus to another but often appears structurally homogeneous within individual thymuses, indicating that often each thymus was being infected by a unique mink cell focus-forming virus. Analysis of high-molecular-weight DNA shows that recombinant proviruses reside in the chromosomal DNA of thymocytes within the preleukemic thymus, with the number rising to an average of several copies per thymocyte, but we do not detect any preferred integration sites. These results suggest that, in general, before the development of thymic leukemias in AKR mice there is a massive infection by a unique mink cell focus-forming virus which then integrates into many different sites of individual thymocytes, one of which grows out to become a tumor.     

Nucleotide sequence of the 3' end of MCF 247 murine leukemia virus

We isolated DNA clones of MCF 247, a leukemogenic, recombinant type C virus obtained from the thymus of an AKR mouse. We determined the nucleotide sequence of the viral long terminal repeat (LTR) and the 3' end of env, and we compared the sequences to corresponding sequences of the genome of Akv virus, the putative ecotropic parent of MCF 247. By analogy with Moloney leukemia virus, we identified the amino terminus of Prp15E, the C-terminal proteolytic cleavage product of env and precursor to mature virion p15E. In MCF 247 the presumptive Prp15E is encoded by a 603-nucleotide open reading frame. The majority of this sequence is identical to that of Akv. However, a recombination event near the 3' end of the Prp15E-coding region introduces nonecotropic sequences into MCF 247, and these extend to the 3' end through the U3 portion of the LTR. The U3 regions of Akv and MCF 247 are about 83% homologous. The R and U5 regions of the LTR of MCF 247 and Akv are identical. Large RNase T1-resistant oligonucleotides analyzed previously in numerous ecotropic and MCF viral genomes were located within the Akv and MCF 247 DNA sequences. The resulting precise T1 oligonucleotide maps of the 3' ends of MCF viral genomes reveal that the biologically defined, leukemogenic class I MCFs isolated from thymic neoplasms of inbred mice all share the sequence pattern seen in MCF 247, a representative of this group; they possess recombinant Prp15E genes and derive U3 from their nonecotropic parents.     

Comparative molecular genetic analysis of lymphomas from six inbred mouse strains.

Previous studies of 21 highly lymphomatous AKXD recombinant inbred mouse strains demonstrated correlations between lymphoma type, the somatic proviral DNA content of the lymphoma, and the frequency of virally induced rearrangements in eight common sites of viral integration (Myc, Pim-i, Pvt-1, Mlvi-1, Mlvi-2, Fis-1, Myb, and Evi-1). In this study we analyzed lymphomas from six inbred mouse strains, AKR/J, C58/J, HRS/J (hr/hr and hr/+), SJL/J, SEA/GnJ, and CWD/LeAgl, to determine whether these correlations are also evident in these strains. Mice of the AKR/J, C58/J, and HRS/J strains died exclusively of T-cell lymphomas. In contrast to earlier studies which showed a great disparity in the rate and incidence of lymphomas in HRS/J hr/hr and HRS/J hr/+ mice, we found a high incidence of T-cell lymphomas and the same mean age of onset of disease in both strains. SJL/J mice died primarily of pre-B-cell lymphomas, whereas CWD/LeAgl and SEA/GnJ mice died primarily of B-cell lymphomas. Somatically acquired mink cell focus-forming proviruses were detected only in T-cell lymphomas, whereas ecotropic proviruses were found in lymphomas from all hematopoietic cell lineages. No rearrangements were detected in the Fis-1, Mlvi-2, and Myb loci, whereas rearrangements were detected in the Mlvi-1, Myc, Pim-1, Pvt-1, and Evi-1 loci. Most rearrangements were found in T-cell lymphomas, and many were virally induced. These results are similar to those we obtained previously for lymphomas of 21 highly lymphomatous AKXD recombinant inbred mouse strains.     

At least four viral genes contribute to the leukemogenicity of murine retrovirus MCF 247 in AKR mice.

Nucleotide sequences encoding gp70, Prp15E, and the U3 region of the long terminal repeat (LTR) distinguish mink cell focus-forming (MCF) retroviruses that can induce leukemia in AKR mice from closely related MCF and ecotropic murine retroviruses that are nonleukemogenic in all inbred mouse strains tested (Lung et al., Cold Spring Harbor Symp. Quant. Biol. 44:1269-1274, 1979; Lung et al., J. Virol. 45:275-290, 1983). We used a set of recombinants constructed in vitro from molecular clones of leukemogenic MCF 247 and nonleukemogenic ecotropic Akv to separate and thereby directly test the role of these genetic elements in disease induction. Leukemogenicity tests of recombinants in AKR mice show that introduction of fragments containing either an MCF LTR or MCF gp70 coding sequences can confer only a very low incidence of disease induction on Akv virus, whereas an MCF type Prp15E alone is completely ineffective. Recombinants with an MCF 247 LTR in combination with MCF Prp15E are moderately oncogenic, whereas those with an MCF 247 LTR plus MCF gp70 coding segment are quite highly leukemogenic. Mice infected with the latter virus show a substantial increase in latent period of disease induction relative to MCF 247; this delay can be reduced when Prp15E, and hence the entire 3' half of the genome, is from MCF 247. Surprisingly, sequences in the 5' half of the genome can also contribute to disease induction. We found a good correlation between oncogenicity and recovery of MCF viruses from thymocytes of injected mice, with early recovery and high titers of MCF in the thymus being correlated with high oncogenicity. This correlation held for recombinants with either an MCF or ecotropic type gp70. Together, these results (i) demonstrate that at least four genes contribute to the oncogenicity of MCF viruses in AKR mice and (ii) suggest that recombinants with only some of the necessary MCF type genes induce leukemia because they recombine to generate complete MCF genomes. Although neither Akv nor MCF 247 is leukemogenic in NFS mice, recombinant viruses whose gp70 gene was derived from Akv but whose LTRs were derived from MCF 247 induced a low incidence of leukemia in this mouse strain.