Characterization of a molecular clone of RFM/Un mouse chromosomal DNA that contains a full-length endogenous murine leukaemia virus-related proviral genome

Nanog retrotransposed genes with functionally conserved open reading frames

The Nanog gene plays a key role in the pluripotency of early embryonic cells in vitro and in vivo. In this article retrotransposed copies of Nanog, termed NanogPc and NanogPd, are identified on mouse Chromosomes 4 and 7, respectively. In contrast to the two previously characterized mouse Nanog retrogenes that contain multiple frameshifts and point mutations, NanogPc and NanogPd are 98% identical to NANOG within the open reading frame and encode proteins with activity in an embryonic stem cell self-renewal assay. Mutations common to all four retrotransposed genes but distinct from Nanog suggest divergence from a common progenitor that appears likely to be Nanog because transcripts derived from Nanog but not from the retrogenes are detected in germ-line cells. The possibility that expression of Nanog could be erroneously attributed to novel cellular sources is suggested by the high homology among Nanog, NanogPc, and NanogPd. Analysis of distinct Mus species suggests that NanogPc and NanogPd arose between divergence of M. caroli and M. spretus and indicates that Nanog retrotransposition events continue to occur at a high frequency, a property likely to extend to other germ-line transcripts.

Antigenic subclasses of polytropic murine leukemia virus (MLV) isolates reflect three distinct groups of endogenous polytropic MLV-related sequences in NFS/N mice

Polytropic murine leukemia viruses (MLVs) are generated by recombination of ecotropic MLVs with members of a family of endogenous proviruses in mice. Previous studies have indicated that polytropic MLV isolates comprise two mutually exclusive antigenic subclasses, each of which is reactive with one of two monoclonal antibodies termed MAb 516 and Hy 7. A major determinant of the epitopes distinguishing the subclasses mapped to a single amino acid difference in the SU protein. Furthermore, distinctly different populations of the polytropic MLV subclasses are generated upon inoculation of different ecotropic MLVs. Here we have characterized the majority of endogenous polytropic MLV-related proviruses of NFS/N mice. Most of the proviruses contain intact sequences encoding the receptor-binding region of the SU protein and could be distinguished by sequence heterogeneity within that region. We found that the endogenous proviruses comprise two major groups that encode the major determinant for Hy 7 or MAb 516 reactivity. The Hy 7-reactive proviruses correspond to previously identified polytropic proviruses, while the 516-reactive proviruses comprise the modified polytropic proviruses as well as a third group of polytropic MLV-related proviruses that exhibit distinct structural features. Phylogenetic analyses indicate that the latter proviruses reflect features of phylogenetic intermediates linking xenotropic MLVs to the polytropic and modified polytropic proviruses. These studies elucidate the relationships of the antigenic subclasses of polytropic MLVs to their endogenous counterparts, identify a new group of endogenous proviruses, and identify distinguishing characteristics of the proviruses that should facilitate a more precise description of their expression in mice and their participation in recombination to generate recombinant viruses.

Characterization of two porcine endogenous retrovirus integration loci and variability in pigs

The pig (Sus scrofa) is a potential organ donor for man but porcine endogenous retroviruses (PERVs) represent an important concern for patients, and identification or engineering of PERV-free pigs suitable for xenotransplantation is a major undertaking. Consequently, studies of variability in pigs for the presence of PERVs at specific loci are a prerequisite. We identified genomic flanking sequences of two PERVs cloned in bacterial artificial chromosomes, a replication-competent PERV-A at locus 1q2.4 and a defective PERV-B at locus 7p1.1-2. PERV-A is embedded in the second repeat of a tandem of eight 190 bp repeats. A short duplicated 4 bp cellular motif, AGAC, was found at each flank of PERV-A and a degenerate 4 bp motif was found for PERV-B. At each locus, the PERV flanks matched expressed sequence tags available in public databases. Primer pairs were designed to amplify either genomic flanks or PERV-genomic junctions. Polymerase chain reaction screening was performed on pigs from 11 distinct Chinese breeds and from the European Large White breed. PERV-B at locus 7p1.1-2 was detected in all animals whereas the presence of PERV-A at locus 1q2.4 was variable. Our results suggest that a genetic selection can be designed to identify animals lacking a potentially active PERV at a specific locus and that Chinese and European pig breeds represent large biodiversity reservoirs to explore. Our results point also to the existence of PERVs that might be fixed in the pig genome, and that might not be eliminated by classical genetic selection.

Identification of a full-length cDNA for an endogenous retrovirus of miniature swine

Endogenous retroviruses of swine are a concern in the use of pig-derived tissues for xenotransplantation into humans. The nucleotide sequence of porcine endogenous retrovirus taken from lymphocytes of miniature swine (PERV-MSL) has been characterized. PERV-MSL is a type C retrovirus of 8,132 bp with the greatest nucleic acid sequence identity to gibbon ape leukemia virus and murine leukemia virus. Constitutive production of PERV-MSL RNA has been detected in normal leukocytes and in multiple organs of swine. The copy numbers of full-length PERV sequences per genome (approximately 8 to 15) vary among swine strains. The open reading frames for gag, pol, and env in PERV-MSL have over 99% amino acid sequence identity to those of Tsukuba-1 retrovirus and are highly homologous to those of endogenous retrovirus of cell line PK15 (PK15-ERV). Most of the differences in the predicted amino acid sequences of PK15-ERV and PERV-MSL are in the SU (cell attachment) region of env. The existence of these PERV clones will enable studies of infection by endogenous retroviruses in xenotransplantation.

Characterization of the endogenous nonecotropic murine leukemia viruses of NZB/B1NJ and SM/J inbred strains

We characterized 84 endogenous nonecotropic proviruses of NZB/B1NJ and SM/J inbred strains by examining proviral junction fragment segregation in recombinant inbred (RI) and backcross mice. Forty-five proviruses were shared with other laboratory strains, but 28 were unique to NZB/B1NJ or SM/J. Proviral loci were located on 17 of the 19 mouse autosomes and on both sex chromosomes. These markers will facilitate gene mapping in the NXSM RI set and contribute to the pursuit of a more complete map of the mouse genome.

Virological events leading to spontaneous AKR thymomas

The spontaneous leukemias of AKR mice are caused by mink cell focus-forming (MCF) viruses. These viruses are generated by recombination between several endogenous murine retroviruses. The virological events leading to the generation of the leukemogenic agent were investigated by using an oligonucleotide specific for the U3 region of the leukemogenic virus and env-reactive oligonucleotide probes specific for the different classes of endogenous murine leukemia virus. It was shown that (i) the leukemogenic MCF virus is formed by recombination between at least three different endogenous sequences; (ii) the U3 donor for the leukemogenic virus is the inducible xenotropic virus Bxv-1; (iii) all spontaneous tumors contain viruses with duplicated enhancer regions in their long terminal repeats; (iv) enhancer duplication is a somatic event, since Bxv-1 contains only one copy; (v) the first recombinant virus detectable in mass populations of thymocytes by Southern hybridization analysis contains all structural features of the ultimate leukemogenic virus; and (vi) the multiple novel viruses in a given tumor represent progeny of the same unique recombination events. On the basis of these results, an analysis of the virological events leading to AKR thymomas is presented.

Provirus integration at the 3' region of N-myc in cell lines established from thymic lymphomas spontaneously formed in AKR mice and a [(BALB/c x B6)F1----AKR] bone marrow chimera

Among 18 thymic leukemia cell lines which have been established from spontaneous thymic lymphomas in AKR mice as well as in bone marrow chimeras which were constructed by transplanting allogeneic bone marrow cells into irradiated AKR mice, three proviral integration sites were identified; near c-myc, N-myc and pim-1 loci. No integration site specific for chimeric leukemia cell lines was found. In three thymic leukemia cell lines which contained rearranged N-myc genes, insertions of long terminal repeats (LTRs) of murine leukemia viruses were detected at 18 or 20 bp downstream of the translational termination codon. These results demonstrate that the 3' region of the N-myc gene is one of the integration targets for murine leukemia viruses in spontaneous thymic lymphomas. In these three cell lines, N-myc mRNA was stably transcribed and transcription of c-myc mRNA was down-regulated. The integrated murine leukemia viruses in AKR thymic leukemia were most likely AKV, though the DNA sequence of the LTR inserted in the genome of a leukemic cell line from [(BALB/c x B6)F1----AKR], CAK20, was different from LTRs of murine leukemia viruses so far reported.

Truncated gag products encoded by Gv-1-responsive endogenous retrovirus loci

The conversion of endogenous or exogenous murine retroviruses to a leukemogenic phenotype involves recombination with retroviral sequences present in host genomic DNA. In the 129 Gix+ inbred strain, these endogenous sequences are replication defective but still express retroviral proteins under the apparent transcriptional control of the Gv-1 regulatory locus. To study the protein-coding potential of Gv-1-regulated endogenous retroviral loci, we used oligonucleotide probes directed to env deletion breakpoints identified in previously characterized cDNA clones. Four endogenous retroviral loci were isolated from a library of 129 Gix+ genomic DNA with these probes. Three loci cloned with the env deletion probe del env-1 had virtually identical proviral inserts by restriction analysis. A unique locus was identified and cloned with the del env-2 probe, which must therefore represent a Gv-1-responsive element. Restriction enzyme and nucleotide sequence analyses indicated that the del env-1 and del env-2 loci represented members of the polytropic and modified polytropic classes of endogenous retrovirus, respectively. Despite this divergence, members of both classes contained identical deletions of 19 nucleotides within p30gag and of 1,474 nucleotides from p10gag into the reverse transcriptase-coding region of pol, suggesting that a recombination event had occurred between these proviral sequences prior to insertion within the genome. The del env-1 and del env-2 loci retained coding capacity for truncated gag polyproteins, confirmed by in vitro translation and immunoprecipitation of the protein products. Nucleotide sequence comparison of the untranslated leader (L) regions of the del env-1 and del env-2 loci to a replication-competent ecotropic virus indicated regions that might be important to dispersion of these endogenous retroviral elements throughout the host genome.

Negative regulatory element associated with potentially functional promoter and enhancer elements in the long terminal repeats of endogenous murine leukemia virus-related proviral sequences

Three series of recombinant DNA clones were constructed, with the bacterial chloramphenicol acetyltransferase (CAT) gene as a quantitative indicator, to examine the activities of promoter and enhancer sequence elements in the 5' long terminal repeat (LTR) of murine leukemia virus (MuLV)-related proviral sequences isolated from the mouse genome. Transient CAT expression was determined in mouse NIH 3T3, human HT1080, and mink CCL64 cultured cells transfected with the LTR-CAT constructs. The 700-base-pair (bp) LTRs of three polytropic MuLV-related proviral clones and the 750-bp LTRs of four modified polytropic proviral clones, in complete structures either with or without the adjacent downstream sequences, all showed very little or negligible activities for CAT expression, while ecotropic MuLV LTRs were highly active. The MuLV-related LTRs were divided into three portions and examined separately. The 3' portion of the MuLV-related LTRs that contains the CCAAC and TATAA boxes was found to be a functional promoter, being about one-half to one-third as active as the corresponding portion of ecotropic MuLV LTRs. A MboI-Bg/II fragment, representing the distinct 190- to 200-bp inserted segment in the middle, was found to be a potential enhancer, especially when examined in combination with the simian virus 40 promoter in CCL64 cells. A PstI-MboI fragment of the 5' portion, which contains the protein-binding motifs of the enhancer segment as well as the upstream LTR sequences, showed moderate enhancer activities in CCL6 cells but was virtually inactive in NIH 3T3 cells and HT1080 cells; addition of this fragment to the ecotropic LTR-CAT constructs depressed CAT expression. Further analyses using chimeric LTR constructs located the presence of a strong negative regulatory element within the region containing the 5' portion of the enhancer and the immediate upstream sequences in the MuLV-related LTRs.

Specific sequence deletions in two classes of murine leukemia virus-related proviruses in the mouse genome

Characteristic long terminal repeats (LTR) of approximately 700 and 750 bp were found, respectively, in the two classes (polytropic and modified polytropic) of murine leukemia virus (MuLV)-related nonecotropic nonxenotropic proviral sequences in eight individual molecular clones of RFM/Un mouse chromosomal DNA fragments. Three proviral clones, two polytropic and one modified polytropic, contained sequence deletions in the viral structural genes. Nucleotide sequence analysis revealed that 7-bp direct repeats occur at both ends of deleted sequences in intact structures and one of the repeats remains in genomes with the deletion. Specifically, the deleted sequences were a 1487-bp gag-pol sequence with ACTGCCC repeat, a 113-bp mid-pol sequence with CAGGCAA repeat, and a 1811-bp env sequence with GGTCCAG repeat. The same specific sequence deletions were found in both classes of MuLV-related proviral structures. Examination of chromosomal DNA from eight inbred laboratory mouse strains and six wild mouse species showed that a minor population of proviruses with these specific deletions were present in Mus musculus and Mus spretus, all of which contain prominent 700-bp LTR polytropic proviral structures. The 750-bp LTR modified polytropic proviral structures were phylogenetically more restricted, being equally predominant in Mus musculus domesticus mice, but minor to undetectable in Mus spretus subspecies, and absent in other wild mouse populations.

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.

The four classes of endogenous murine leukemia virus: structural relationships and potential for recombination

The process by which leukemogenic viruses are generated during the lifetime of certain strains of mice is poorly understood. We have therefore set out to define all the murine leukemia virus-related endogenous proviruses of HRS/J mice. We have cloned 34 different proviral fragments and their flanking cellular sequences. These have been characterized by restriction enzyme analysis, by fingerprinting in vitro-synthesized RNA, and by DNA sequencing. We conclude that all the proviruses can be assigned into one of four different classes: the previously characterized ecotropic, xenotropic, and polytropic viruses, as well as a new class we have termed modified polytropic viruses. The xenotropic, polytropic, and modified polytropic classes are closely related to one another, but as a group they differ considerably from the ecotropic class. Sequence analyses show that both polytropic and modified polytropic sequences can contribute env sequences to recombinant viruses.