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

Porcine endogenous retrovirus integration sites in the human genome: features in common with those of murine leukemia virus

Porcine endogenous retroviruses (PERV) are a major concern when porcine tissues and organs are used for xenotransplantation. PERV has been shown to infect human cells in vitro, highlighting a potential zoonotic risk. No pathology is associated with PERV in its natural host, but the pathogenic potential might differ in the case of cross-species transmission and can only be inferred from knowledge of related gammaretroviruses. We therefore investigated the integration features of the PERV DNA in the human genome in vitro in order to further characterize the risk associated with PERV transmission. In this study, we characterized 189 PERV integration site sequences from human HEK-293 cells. Data showed that PERV integration was strongly enhanced at transcriptional start sites and CpG islands and that the frequencies of integration events increased with the expression levels of the genes, except for the genes with the highest levels of expression, which were disfavored for integration. Finally, we extracted genomic sequences directly flanking the integration sites and found an original 8-base statistical palindromic consensus sequence [TG(int)GTACCAGC]. All these results show similarities between PERV and murine leukemia virus integration site selection, suggesting that gammaretroviruses have a common pattern of integration and that the mechanisms of target site selection within a retrovirus genus might be similar.

Identification of a novel Gammaretrovirus in prostate tumors of patients homozygous for R462Q RNASEL variant

Ribonuclease L (RNase L) is an important effector of the innate antiviral response. Mutations or variants that impair function of RNase L, particularly R462Q, have been proposed as susceptibility factors for prostate cancer. Given the role of this gene in viral defense, we sought to explore the possibility that a viral infection might contribute to prostate cancer in individuals harboring the R462Q variant. A viral detection DNA microarray composed of oligonucleotides corresponding to the most conserved sequences of all known viruses identified the presence of gammaretroviral sequences in cDNA samples from seven of 11 R462Q-homozygous (QQ) cases, and in one of eight heterozygous (RQ) and homozygous wild-type (RR) cases. An expanded survey of 86 tumors by specific RT-PCR detected the virus in eight of 20 QQ cases (40%), compared with only one sample (1.5%) among 66 RQ and RR cases. The full-length viral genome was cloned and sequenced independently from three positive QQ cases. The virus, named XMRV, is closely related to xenotropic murine leukemia viruses (MuLVs), but its sequence is clearly distinct from all known members of this group. Comparison of gag and pol sequences from different tumor isolates suggested infection with the same virus in all cases, yet sequence variation was consistent with the infections being independently acquired. Analysis of prostate tissues from XMRV-positive cases by in situ hybridization and immunohistochemistry showed that XMRV nucleic acid and protein can be detected in about 1% of stromal cells, predominantly fibroblasts and hematopoietic elements in regions adjacent to the carcinoma. These data provide to our knowledge the first demonstration that xenotropic MuLV-related viruses can produce an authentic human infection, and strongly implicate RNase L activity in the prevention or clearance of infection in vivo. These findings also raise questions about the possible relationship between exogenous infection and cancer development in genetically susceptible individuals.

Prostate cancer is the most frequent cancer and the second leading cause of cancer deaths in US men over the age of 50. Several genetic factors have been proposed as potential risk factors for the development of prostate cancer, including a viral defense gene called RNASEL. A common genetic variant in this gene, R462Q, was recently implicated in up to 13% of prostate cancer cases. Given the antiviral role of RNASEL, the authors sought to examine if a virus might be present in prostate cancers associated with the R462Q variant. Using a DNA microarray designed to detect all known viral families, the authors identified a novel virus, named XMRV, in a subset of prostate tumor samples. Polymerase chain reaction testing of 86 prostate tumors for the presence of XMRV revealed a strong association between the presence of the virus and being homozygous for the R462Q variant. Cloning and sequencing of the virus showed that XMRV is a close relative of several known xenotropic murine leukemia viruses. This report presents the first documented cases of human infection with a xenotropic retrovirus. Future work will address the potential connection between XMRV infection and the increased prostate cancer risk in patients with the R462Q RNASEL variant.

Virus safety in xenotransplantation: first exploratory in vivo studies in small laboratory animals and non-human primates

For xenotransplantation, the transplantation of animal cells, tissues and organs into human recipients, to date, pigs are favored as potential donors. Beside ethical, immunological, physiological and technical problems, the microbiological safety of the xenograft has to be guaranteed. It will be possible to eliminate all of the known porcine microorgansims in the nearby future by vaccinating or specified pathogen-free breeding. Thus, the main risk will come from the porcine endogenous retroviruses (PERVs) which are present in the pig genome as proviruses of different subtypes. PERVs will therefore be transmitted, with the xenograft, to the human recipient. PERVs can infect numerous different types of human primary cells and cell lines in vitro and were shown to adapt to these cells by serial passaging on uninfected cells. Furthermore, PERVs have high homology to other retroviruses, such as feline leukemia virus (FeLV) or murine leukemia virus (MuLV), which are known to induce tumors or immunodeficiencies in the infected host. To evaluate the potential risk of a trans-species transmission of PERV in vivo, naive and immunosuppressed rats, guinea pigs and minks were inoculated with PERV and screened over a period of 3 months for an antibody reaction against PERV proteins or for the integration of proviral DNA into the genomic DNA of the host's cells. Furthermore, we inoculated three different species of non-human primates, rhesus monkey (Macaca mulatta), pig-tailed monkey (Macaca nemestrina) and baboon (Papio hamadryas) with high titers of a human-adapted PERV. To simulate a situation in xenotransplantation, the animals received a daily triple immunosuppression using cyclosporine A, methylprednisolone and RAD, a rapamycin derivative, presently under development by Novartis. None of the small laboratory animals or the non-human primates showed production of antibodies against PERV or evidence of integration of proviral DNA in blood cells or cells of several organs, 3 months after virus inoculation, despite the observation that cells of the animals used in the experiment were infectible in vitro. This apparent difference in the outcome of the in vitro and the in vivo data might be explained by an efficient elimination of the virus by the innate or adaptive immunity of the animals.

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.

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-2d), B10.Br (H-2k), B10.Q (H-2q), and B10.RIII (H-2r) 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-2b) 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-2b. These results demonstrate that a host gene within the major histocompatibility complex can influence the genetic evolution of pathogenic retroviruses in vivo.

The E47 transcription factor binds to the enhancer sequences of recombinant murine leukemia viruses and influences enhancer function

The genomes of most recombinant murine leukemia viruses (MuLVs) inherit pathogenic U3 region sequences from the endogenous xenotropic provirus Bxv-1. However, the U3 regions of about one-third of recombinant MuLVs from CWD mice, such as CWM-T15, have nonecotropic substitutions that are probably derived from an endogenous polytropic provirus. The CWM-T15 U3 region sequences contain five nucleotide substitutions compared with the less pathogenic sequences of the endogenous ecotropic virus parent, Emv-1. Three of these substitutions are located immediately 3' of the enhancer core, and two form part of an E-box motif that is also found in the Bxv-1 sequence. A series of electromobility shift assays revealed that nuclear extracts from S194 cells and the basic helix-loop-helix transcription factor E47 could distinguish between oligonucleotides that contained the core region sequences of CWM-T15 or Emv-1. The E47 homodimers appeared to bind to the CWM-T15 E-box motif and when expressed at high levels in cells transactivated the CWM-T15 but not the Emv-1 enhancer. Taken together, these results suggest that E47 or related basic helix-loop-helix proteins that are expressed in lymphoid cells bind to and transactivate the CWM-T15 enhancer in vivo. This transactivation may explain why the CWM-T15 and Bxv-1 U3 regions accelerate the onset of lymphoid neoplasms and why related enhancer core region sequences are preferentially incorporated into the genomes of recombinant MuLVs and are found in other leukemogenic mammalian retroviruses.

Origins of enhancer sequences of recombinant murine leukemia viruses from spontaneous B- and T-cell lymphomas of CWD mice

Recombinant murine leukemia viruses from the highly leukemic mouse strains AKR, HRS, and C58 usually acquire pathogenic U3 region sequences fro the endogenous xenotropic virus, Bxv-1. However, the majority of tumors from another highly leukemic strain, CWD, contained recombinant viruses that lacked Bxv-1-specific sequences. The nucleotide sequence of the U3 regions of two such CWD recombinants was nearly identical to that of the endogenous ecotropic virus parent Emv-1, but they shared three nucleotide substitutions immediately 3' of the enhancer core. These substitutions were found in recombinant proviruses from about one-third of spontaneous CWD lymphomas as determined by an oligonucleotide hybridization assay of proviral fragments that had been nucleotide substitutions in the CWD viruses were inherited from an endogenous polytropic provirus that is absent in the other highly leukemic strains. On the basis of the results of these and previous studies, we propose that CWD recombinants acquire pathogenic U3 region sequences through recombination with an endogenous polytropic virus or Bxv-1 and that the pathogenicity of these sequences may be related to a sequence motif that is known to bind members of the basic helix-loop-helix class of transcription factors.

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.

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.