Infectivity and methylation of retroviral genomes is correlated with expression in the animal

Endogenous retroviruses mobilized during friend murine leukemia virus infection

We have demonstrated in a mouse model that infection with a retrovirus can lead not only to the generation of recombinants between exogenous and endogenous gammaretrovirus, but also to the mobilization of endogenous proviruses by pseudotyping entire polytropic proviral transcripts and facilitating their infectious spread to new cells. However, the frequency of this occurrence, the kinetics, and the identity of mobilized endogenous proviruses was unclear. Here we find that these mobilized transcripts are detected after only one day of infection. They predominate over recombinant polytropic viruses early in infection, persist throughout the course of disease and are comprised of multiple different polytropic proviruses. Other endogenous retroviral elements such as intracisternal A particles (IAPs) were not detected. The integration of the endogenous transcripts into new cells could result in loss of transcriptional control and elevated expression which may facilitate pathogenesis, perhaps by contributing to the generation of polytropic recombinant viruses.

Monitoring Dynamics of DNA Methylation at Single-Cell Resolution during Development and Disease

DNA methylation is a broadly studied epigenetic modification that is essential for normal mammalian development. Over the years, numerous methodologies were developed trying to cope with the intrinsic challenge of reading the "second dimension" epigenetic code. The recent rapid expansion of sequencing technologies has made it possible to fully chart the methylation landscape of different cell types at single-base resolution. Surprisingly, accumulating data suggest that, in addition to the massive epigenome remodeling during early development, cell type and tissue specification is associated with high levels of DNA methylation dynamics at distal regulatory elements. However, current methods provide only a static "snapshot" of DNA methylation, thus precluding the study of real-time methylation dynamics during cell fate changes. Here we review the principles of a new approach that enables monitoring loci-specific DNA methylation dynamics at single-cell resolution. We also discuss potential applications and promises for implementing this methodology to study DNA methylation changes during development and disease.

Hematopoietic Stem and Immune Cells in Chronic HIV Infection

Hematopoietic stem cell (HSC) belongs to multipotent adult somatic stem cells. A single HSC can reconstitute the entire blood system via self-renewal, differentiation into all lineages of blood cells, and replenishment of cells lost due to attrition or disease in a person's lifetime. Although all blood and immune cells derive from HSC, immune cells, specifically immune memory cells, have the properties of HSC on self-renewal and differentiation into lineage effector cells responding to the invading pathogens. Moreover, the interplay between immune memory cell and viral pathogen determines the course of a viral infection. Here, we state our point of view on the role of blood stem and progenitor cell in chronic HIV infection, with a focus on memory CD4 T-cell in the context of HIV/AIDS eradication and cure.

Epigenetic analysis of HIV-1 proviral genomes from infected individuals: predominance of unmethylated CpG's

Efforts to cure HIV-1 infections aim at eliminating proviral DNA. Integrated DNA from various viruses often becomes methylated de novo and transcriptionally inactivated. We therefore investigated CpG methylation profiles of 55 of 94 CpG's (58.5%) in HIV-1 proviral genomes including ten CpG's in each LTR and additional CpG's in portions of gag, env, nef, rev, and tat genes. We analyzed 33 DNA samples from PBMC's of 23 subjects representing a broad spectrum of HIV-1 disease. In 22 of 23 HIV-1-infected individuals, there were only unmethylated CpG's regardless of infection status. In one long term nonprogressor, however, methylation of proviral DNA varied between 0 and 75% over an 11-year period although the CD4+ counts remained stable. Hence levels of proviral DNA methylation can fluctuate. The preponderance of unmethylated CpG's suggests that proviral methylation is not a major factor in regulating HIV-1 proviral activity in PBMC's. Unmethylated CpG's may play a role in HIV-1 immunopathogenesis.

Eradication of HIV and Cure of AIDS, Now and How?

Recent studies have highlighted the importance of eradication of human immunodeficiency virus (HIV) and cure of acquired immunodeficiency syndrome (AIDS). However, a pivotal point that the patient immunity controls HIV reactivation after highly active anti-retroviral therapy [HAART or combination anti-retroviral therapy (cART)] remains less well addressed. In spite of the fact that both innate and adaptive immunities are indispensable and numerous cells participate in the anti-HIV immunity, memory CD4 T-cells are indisputably the key cells organizing all immune actions against HIV while being the targets of HIV. Here we present a view and multidisciplinary approaches to HIV/AIDS eradication and cure. We aim at memory CD4 T-cells, utilizing the stem cell properties of these cells to reprogram an anti-HIV memory repertoire to eliminate the viral reservoir, toward achieving an AIDS-free world.

A severe de novo methylation of episomal vectors by human ES cells

Episomal vectors can allow efficient genetic modification of cells and have the potential advantage of avoiding chromosomal position of integration effects. Here we explore the use of an Epstein-Barr virus-based episomal vector with human embryonic stem (ES) cells, and find high initial transfection rates, but a rapid loss of reporter gene expression. Similar to mouse ES cells, human ES cells express high levels of the de novo DNA methyltransferases, and we detected dramatic CpG methylation and minor non-CpG methylation on the episomes recovered from the human ES cells 7 days after the transfection, which was not present on the same episome recovered from 293 cells. Interestingly, the oriP region of the episomes was relatively excluded from this methylation. These findings define some of the limitations of using episomal vectors with human ES cells and offer a unique platform for analyzing epigenetic gene silencing in human ES cells.

Dynamic DNA methylation and histone modifications contribute to lentiviral transgene silencing in murine embryonic carcinoma cells

Embryonic stem cells are subjected to a dynamic genome regulation during development. Here we report that the ectopic lentiviral transgenes are quickly silenced in murine embryonic carcinoma P19 cells. The silencing was correlated with CpG hypermethylation in the transgene promoter. Using high-resolution sodium bisulfite genome sequencing, we detected distinct DNA methylation kinetics in different proviral regions. DNase I sensitivity and chromatin immunoprecipitation assays revealed condensed chromatin structure and histone code switch during silencing. Longitudinal analysis of nonsilenced and silenced identical single-cell clones revealed that the silencing was coupled with CpG methylation in the promoter, as well as a global histone H3 deacetylation. Interestingly, the primer binding site and the packaging signal region appeared to serve as a DNA methylation initiation center which was rapidly hypermethylated regardless of transgene silencing and chromatin modifications. Analysis of cellular genes 45 to 50 kbp upstream and downstream of the integration site indicated that transcriptional activities of the flanking host genes were not affected. Genetic modifications of stem cells have great therapeutic potentials and our results picture a dynamic embryonic genome response to ectopic transgene integration that may have important implications in the future safety and efficacy modifications of stem cells.

Hypomethylation: one side of a larger picture

Hypomethylation signifies one end of a spectrum of DNA methylation states. In most cases hypomethylation refers to a relative state that represents a change from the "normal" methylation level. Hypomethylation, when approached from a topographical perspective, has been used to describe either overall decreases in the methylation status of the entire genome (global hypomethylation) or more localized relative demethylation of specific subsets of the genome, such as the promoter regions of protooncogenes or normally highly methylated repetitive sequences. Global hypomethylation accompanied by gene-specific hypermethylation is observed in at least two important settings: cancer and aging. Global hypomethylation is generally reflective of decreased methylation in CpGs dispersed throughout repetitive sequences as well as the bodies of genes. Hypomethylation of repetitive and parasitic DNA sequences correlates with a number of adverse outcomes. For example, decreased methylation of repetitive sequences in the satellite DNA of the pericentric region of chromosomes is associated with increased chromosomal rearrangements, a hallmark of cancer. Decreased methylation of proviral sequences can lead to reactivation and increased infectivity. However, hypomethylation in cancer can also affect the CpGs in the promoters of specific genes-namely, protooncogenes-leading to their overexpression and resulting in the functional outcome of increased cell proliferation. Thus, hypomethylation, in a variety of settings in which it represents a deviation from "normal," appears to correlate with progression to cancer and offers potential mechanisms to explain the carcinogenic process.

Latent HIV-1 reactivation in transgenic mice requires cell cycle -dependent demethylation of CREB/ATF sites in the LTR

OBJECTIVE

We previously produced a line of transgenic mice that carried the HIV-1 genome deficient in the gene. Although the HIV-1 genome in the lymphocytes was dormant under normal physiological conditions, it could be reactivated by lipopolysaccharide (LPS) administration via induction of interleukin-1alpha/beta and tumour necrosis factor-alpha. In this report, we analysed further the reactivation mechanism of the latent HIV-1 using this transgenic mouse model.

DESIGN

and methods: Possible involvement of CpG methylation in HIV-1 latency was examined by treating transgenic lymphocytes with a demethylating agent, 5'-azacytidine. CpG methylation in the HIV-1 long terminal repeat (LTR) was analysed using the bisulfite genomic sequencing method. As previous studies suggested that CpG demethylation depended on the cell cycle progression, we analysed the relation between cell cycle progression and LPS-induced reactivation of HIV-1 by labelling lymphocytes with an intracellular fluorescein, carboxyfluorescein diacetate succinimidyl ester.

RESULTS

We found that 5'-azacytidine enhanced HIV-1 expression ninefold compared to treatment with LPS alone. Furthermore, HIV-1 p24 induction by LPS was observed only in cells that had undergone cell division, while induction was prevented in cells in which cell cycle progression was blocked either by mimosine, aphidicolin, or nocodazole. LPS-induced HIV-1 reactivation was associated with demethylation of two CpG sites located in the CREB/ATF binding sites in the HIV-1 LTR in a cell cycle-dependent manner.

CONCLUSIONS

These observations indicate that cell cycle progression-dependent demethylation of the CREB/ATF sites in the LTR is crucial for the reactivation of latent HIV-1 genome in transgenic mice.

Targeting transgene expression to antigen-presenting cells derived from lentivirus-transduced engrafting human hematopoietic stem/progenitor cells

Hematopoietic stem cells (HSCs) represent an important target for the treatment of various blood disorders. As the source of critical cells within the immune system, genetic modification of HSCs can also be used to modulate immune responses. The effectiveness of HSC-mediated gene therapy largely depends on efficient gene delivery into long-term repopulating progenitors and targeted transgene expression in an appropriate progeny of the transduced pluripotent HSCs. Self-inactivating (SIN) lentiviral vectors have been demonstrated to be capable of transducing mitotically inactive cells, including HSCs, and accommodating a nonviral promoter to control the transgene expression in transduced cells. In this study, we constructed 2 SIN lentiviral vectors, EF.GFP and DR.GFP, to express the green fluorescent protein (GFP) gene controlled solely by the promoter of either a housekeeping gene EF-1alpha or the human HLA-DRalpha gene, which is selectively expressed in antigen-presenting cells (APCs). We demonstrated that both vectors efficiently transduced human pluripotent CD34+ cells capable of engrafting nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice. When the EF.GFP vector was used, constitutive high-level GFP expression was obtained in all the human HSC progeny detectable in NOD/SCID mice and in subsequent in vitro differentiation assays, indicating that engrafting human HSCs have been transduced. In contrast, the DR.GFP vector mediated transgene expression specifically in human HLA-DR+ cells and highly in differentiated dendritic cells (DCs), which are critical in regulating immunity. Furthermore, human DCs derived from transduced and engrafted human cells potently stimulated allogeneic T-cell proliferation. This study demonstrated successful targeting of transgene expression to APCs/DCs after stable gene transduction of pluripotent HSCs.

Retroviruses as tools to study the immune system

Retrovirus-based vectors provide an efficient means to introduce and express genes in cells of the immune system and have become a popular tool to study immune function. They are easy to manipulate and provide stable, long-term gene expression because they integrate into the genome. Current retroviral vectors do have limitations that affect their usefulness in certain applications. However, recent advances suggest a number of ways in which these vectors might be improved to extend their utility in immunological research.

Migration and multipotentiality of PSA-NCAM+ neural precursors transplanted in the developing brain

By optimizing the previously described strategy for obtention of spheres enriched in PSA-NCAM+ precursors, we prepared PSA-NCAM-immunoselected cell populations from cerebral hemispheres of neonatal MBP-LacZ transgenic mice. These cells expressed Nestin, exhibited clonal expansion potential and formed spheres, which were initially enriched in PSA-NCAM+ cells but became enriched in GD3+ oligodendrocyte progenitors after 1 week in B104 contionned medium. One month after their periventricular transplantation into the brain of wild-type and/or shiverer newborn mice, cells from PSA-NCAM+ spheres exhibited a higher rostral migration potential than cells from GD3+ spheres, and clearly contributed to myelination in the olfactory bulb. In shiverer hosts, both sphere populations generated oligodendrocytes with similar myelination potential. In addition PSA-NCAM+ sphere cells generated GFAP+ astrocytes and NeuN+ neurons, depending on their site of insertion. These results evidence the high plasticity of newborn PSA-NCAM+ neural precursors and suggest that they are promising tools for cell therapy of CNS diseases, including myelin disorders.

Bone-directed expression of Col1a1 promoter-driven self-inactivating retroviral vector in bone marrow cells and transgenic mice

Gene therapy of bone would benefit from the availability of vectors that provide stable, osteoblast-specific expression. This would allow bone-specific expression of Col1a1 cDNAs for treatment of osteogenesis imperfecta. In addition, such a vector would restrict expression of secreted therapeutic proteins to the bone-synthesizing regions of the bone marrow after ex vivo transduction of marrow stromal cells and reintroduction of the cells into patients. Retrovirus vectors stably integrate into target cell genomes; however, long-term regulated expression from internal cellular promoters has not been consistently achieved. In some cases this is due to a stem cell-specific mechanism for transcriptional repression of retroviruses. We evaluated the ability of self-inactivating ROSA-derived vectors containing a bone-directed 2.3-kb rat Col1a1 promoter to display osteoblast-specific expression. In vitro expression was examined in bone marrow stromal cell cultures induced to undergo osteoblastic differentiation. In vivo expression was evaluated in chimeric mice derived from transduced embryonic stem cells. The results indicate that self-inactivating retrovirus vectors containing the Col1a1 promoter are not permanently inactivated in embryonic stem cells and are specifically expressed in osteoblasts in vivo and in vitro. Thus these vectors should be useful for bone-directed gene therapy.

Modular retro-vectors for transgenic and therapeutic use

Retroviruses have been used for many years as vectors for human gene therapy as well as for making transgenic animals. However, the efficient insertion of genes by retroviruses is often complicated by transcriptional inactivation of the retroviral long terminal repeats (LTRs) and by the production of replication-competent retroviruses (RCR). Solutions to these and other difficulties are being found in modular vectors, in which the desirable features of different vector systems are combined. Examples of synergistic vectors include virosomes (liposome/virus delivery), adeno-retro vectors, and MLV/VL30 chimeras. As gene delivery systems become increasingly complex, methodology is also needed for precise assembly of modular vectors. Gene self-assembly (GENSA) technology permits seamless vector construction and simultaneous, multifragment assembly.

Long-term silencing of retroviral vectors is resistant to reversal by trichostatin A and 5-azacytidine

One problem limiting the development of long-term gene replacement therapy is gene silencing. A variety of experiments have implicated DNA methylation and histone deacetylation in gene silencing and shown that the agents 5-azacytidine (5-Aza) and trichostatin A (TSA) are able to reverse these effects. To begin to investigate clinically relevant strategies to reverse silencing with these drugs, we transduced the MEL and FDCP-1 hematopoietic cell lines with Moloney murine leukemia virus (MMLV) and Harvey murine sarcoma virus (HMSV)-based retroviral vectors carrying the beta-galactosidase/neomycin resistance fusion gene (beta-geo). Fifty-one clones were isolated under G418 selection over 2 weeks and then allowed to grow without selection as beta-gal activity was monitored over time. More than 80% of these clones showed significant silencing over a period of 70-80 days. The clones were then exposed to a wide range of 5-Aza and TSA concentrations, both alone and in combination, in an effort to reverse silencing. Despite demonstration that the agents were able to decrease DNA methylation and increase histone acetylation, significant reversal of long-term silencing was not seen under any experimental condition. These results suggest that long-term retroviral silencing involves mechanisms in addition to DNA methylation and histone acetylation and that new pharmacologic strategies are needed to overcome the silencing process.

Dynamic analysis of proviral induction and De Novo methylation: implications for a histone deacetylase-independent, methylation density-dependent mechanism of transcriptional repression

Methylation of cytosines in the CpG dinucleotide is generally associated with transcriptional repression in mammalian cells, and recent findings implicate histone deacetylation in methylation-mediated repression. Analyses of histone acetylation in in vitro-methylated transfected plasmids support this model; however, little is known about the relationships among de novo DNA methylation, transcriptional repression, and histone acetylation state. To examine these relationships in vivo, we have developed a novel approach that permits the isolation and expansion of cells harboring expressing or silent retroviruses. MEL cells were infected with a Moloney murine leukemia virus encoding the green fluorescent protein (GFP), and single-copy, silent proviral clones were treated weekly with the histone deacetylase inhibitor trichostatin A or the DNA methylation inhibitor 5-azacytidine. Expression was monitored concurrently by flow cytometry, allowing for repeated phenotypic analysis over time, and proviral methylation was determined by Southern blotting and bisulfite methylation mapping. Shortly after infection, proviral expression was inducible and the reporter gene and proviral enhancer showed a low density of methylation. Over time, the efficacy of drug induction diminished, coincident with the accumulation of methyl-CpGs across the provirus. Bisulfite analysis of cells in which 5-azacytidine treatment induced GFP expression revealed measurable but incomplete demethylation of the provirus. Repression could be overcome in late-passage clones only by pretreatment with 5-azacytidine followed by trichostatin A, suggesting that partial demethylation reestablishes the trichostatin-inducible state. These experiments reveal the presence of a silencing mechanism which acts on densely methylated DNA and appears to function independently of histone deacetylase activity.

5-Azacytidine prevents transgene methylation in vivo

Retroviral sequence can silence transgene expression in vitro and in vivo. We report that this effect can be efficiently prevented by in vivo administration of the demethylating agent 5-azacytidine (aza-C). We engineered the U937 human cell line with a retroviral vector consisting of the thymidine kinase suicide gene (tk), which induces sensitivity to ganciclovir (gcv) and through an IRES sequence, the bacterial beta-galactosidase gene (lacZ) as a marker gene. About 90% of the U937 cells expressed the transgene. By injecting the transduced U937 cells in severe combined immunodeficient disease (SCID) mice, we generated a tumor which, during in vivo treatment with aza-C, maintained the high expression of lacZ and tk genes at the baseline values. LacZ-positive cells in the tumour masses after death was weak (1-2%) in the control group, while in mice treated with aza-C it was maintained at 90%. The delay in tumour onset was significantly longer when animals were treated with both aza-C and gcv (P < 0.0001) compared with animals treated with gcv or with aza-C alone. The prevention of silencing phenomena has important implications for gene therapy, because an efficient transduction associated with appropriate drug therapy, might be a powerful strategy for successful application of gene therapy protocols.

Sp1 binding sites inserted into the rous sarcoma virus long terminal repeat enhance LTR-driven gene expression

Although the Rous sarcoma virus (RSV) long terminal repeat (LTR) is an efficient promoter of transcription, most RSV proviruses are down-regulated upon retroviral integration in non-permissive mammalian cells. Among other mechanisms, DNA methylation has been shown to be involved in proviral silencing. The presence of Sp1 binding sites has been demonstrated to be essential for protection of a CpG island and also non-island DNA regions from de novo methylation. Also, the presence of these sites in the LTRs correlates with the transcriptional activity of certain proviral structures. Using transient and stable transfection assays, we demonstrate that insertion of Sp1 binding sites into the RSV LTR remarkably increases expression of the LTR-driven genes in permissive and non-permissive cells, despite the reported negative effect of insertion of the non-specific DNA into the LTR promoter/enhancer sequences. Particular arrangement of inserted Sp1 sites was effective even in stably transfected reporter gene constructs into non-permissive mammalian cells, where additional factors exert negative effects on expression.

Complementation of methylation deficiency in embryonic stem cells by a DNA methyltransferase minigene

Previous attempts to express functional DNA cytosine methyltransferase (EC 2.1.1.37) in cells transfected with the available Dnmt cDNAs have met with little or no success. We show that the published Dnmt sequence encodes an amino terminal-truncated protein that is tolerated only at very low levels when stably expressed in embryonic stem cells. Normal expression levels were, however, obtained with constructs containing a continuation of an ORF with a coding capacity of up to 171 amino acids upstream of the previously defined start site. The protein encoded by these constructs comigrated in SDS/PAGE with the endogenous enzyme and restored methylation activity in transfected cells. This was shown by functional rescue of Dnmt mutant embryonic stem cells that contain highly demethylated genomic DNA and fail to differentiate normally. When transfected with the minigene construct, the genomic DNA became remethylated and the cells regained the capacity to form teratomas that displayed a wide variety of differentiated cell types. Our results define an amino-terminal domain of the mammalian MTase that is crucial for stable expression and function in vivo.

Transcriptional Silencing of Retroviral Vectors

Although retroviral vector systems have been found to efficiently transduce a variety of cell types in vitro, the use of vectors based on murine leukemia virus in preclinical models of somatic gene therapy has led to the identification of transcriptional silencing in vivo as an important problem. Extinction of long-term vector expression has been observed after implantation of transduced hematopoietic cells as well as fibroblasts, myoblasts and hepatocytes. Here we review the influence of vector structure, integration site and cell type on transcriptional silencing. While down-regulation of proviral transcription is known from a number of cellular and animal models, major insight has been gained from studies in the germ line and embryonal cells of the mouse. Key elements for the transfer and expression of retroviral vectors, such as the viral transcriptional enhancer and the binding site for the tRNA primer for reverse transcription may have a major influence on transcriptional silencing. Alterations of these elements of the vector backbone as well as the use of internal promoter elements from housekeeping genes may contribute to reduce transcriptional silencing. The use of cell culture and animal models in the testing and improvement of vector design is discussed. Copyright 1996 S. Karger AG, Basel

Expression of transduced genes in mice generated by infecting blastocysts with avian leukosis virus-based retroviral vectors

Transgenic mouse lines have been developed that express the tv-a receptor under the control of the chicken beta-actin promoter. These mice express the tv-a receptor in most or all tissues and in the early embryo. An avian leukosis virus (ALV)-based retroviral vector system was used for the efficient delivery of genes into preimplantation mouse embryos from these transgenic lines. Experimental animals could be generated quickly and easily by infecting susceptible blastocysts with ALV-based retroviral vectors. Expression of the delivered genes was controlled by either the constitutive viral promoter contained in the long terminal repeat or an internal nonviral tissue-specific promoter. Mating the infected founder chimeric animals produced animals that carry the ALV provirus as a transgene. A subset of the integrated proviruses expressed the chloramphenicol acetyltransferase reporter gene from either the promoter in the long terminal repeat or an internal promoter, which we believe indicates that many of the sites that are accessible to viral DNA insertion in preimplantation embryos are incompatible with expression in older animals. This approach should prove useful for studies on murine cell lineage and development, providing models for studying oncogenesis, and testing gene therapy strategies.

Germ-line passage is required for establishment of methylation and expression patterns of imprinted but not of nonimprinted genes

Embryonic stem (ES) cells homozygous for a disruption of the DNA (cytosine-5)-methyltransferase gene (Dnmt) proliferate normally with their DNA highly demethylated but die upon differentiation. Expression of the wild-type Dnmt cDNA in mutant male ES cells caused an increase in methylation of bulk DNA and of the Xist and Igf2 genes to normal levels, but did not restore the methylation of the imprinted genes H19 and Igf2r. These cells differentiated normally in vitro and contributed substantially to adult chimeras. While the Xist gene was not expressed in the remethylated male ES cells, no restoration of the normal expression profile was seen for H19, Igf2r, or Igf2. This indicates that ES cells can faithfully reestablish normal methylation and expression patterns of nonimprinted genes but lack the ability to restore those of imprinted genes. Full restoration of monoallelic methylation and expression was imposed on H19, Igf2, and Igf2r upon germ-line transmission. These results are consistent with the presence of distinct de novo DNA methyltransferase activities during oogenesis and spermatogenesis, which specifically recognize imprinted genes but are absent in the postimplantation embryo and in ES cells.

Loss of methylation activates Xist in somatic but not in embryonic cells

The mouse Xist gene, which is expressed only from the inactive X chromosome, is thought to play a role in the initiation of X inactivation. The 5' end of this gene is fully methylated on the active X chromosome and completely demethylated on the inactive X chromosome, suggesting that DNA methylation may be involved in controlling allele-specific transcription of this gene. To directly investigate the importance of DNA methylation in the control of Xist expression, we have examined its methylation patterns and expression in ES cells and embryos that are deficient in DNA methyltransferase activity. We report here that demethylation of the Xist locus in male mutant embryos induces Xist expression, thus establishing a direct link between demethylation and expression of the Xist gene in the postgastrulation embryo. The transcriptional activity of Xist in undifferentiated ES cells, however, appears to be independent of its methylation status. These results suggest that methylation may only become essential for Xist repression after ES cells have differentiated or after the embryo has undergone gastrulation.

Efficient production of transgenic cattle by retroviral infection of early embryos

Production of transgenic cattle by microinjection of DNA has been difficult and costly. To explore an alternative method, one- to four-cell bovine embryos were exposed to a replication-defective retrovirus by microinjection of retrovirus producer cells into the perivitelline space. Embryos were cultured in vitro for 3-4 days, then transferred to recipient cows for further development. Thirteen of 22 embryos recovered at 15 days gestation and each of four fetuses recovered at 90 days gestation were transgenic. Fetuses harbored between 2 and 12 proviruses, and within each fetus, identical patterns of integration were observed in seven tissues tested. Estimates of the number of proviruses per cell suggested that in three of the four fetuses, most, and possibly all, cells were transgenic. This technique should facilitate application of transgenic technology to cattle and other agriculturally important species.

Infection of central nervous system cells by ecotropic murine leukemia virus in C58 and AKR mice and in in utero-infected CE/J mice predisposes mice to paralytic infection by lactate dehydrogenase-elevating virus

Certain mouse strains, such as AKR and C58, which possess N-tropic, ecotropic murine leukemia virus (MuLV) proviruses and are homozygous at the Fv-1n locus are specifically susceptible to paralytic infection (age-dependent poliomyelitis [ADPM]) by lactate dehydrogenase-elevating virus (LDV). Our results provide an explanation for this genetic linkage and directly prove that ecotropic MuLV infection of spinal cord cells is responsible for rendering anterior horn neurons susceptible to cytocidal LDV infection, which is the cause of the paralytic disease. Northern (RNA) blot hybridization of total tissue RNA and in situ hybridization of tissue sections demonstrated that only mice harboring central nervous system (CNS) cells that expressed ecotropic MuLV were susceptible to ADPM. Our evidence indicates that the ecotropic MuLV RNA is transcribed in CNS cells from ecotropic MuLV proviruses that have been acquired by infection with exogenous ecotropic MuLV, probably during embryogenesis, the time when germ line proviruses in AKR and C58 mice first become activated. In young mice, MuLV RNA-containing cells were found exclusively in white-matter tracts and therefore were glial cells. An increase in the ADPM susceptibility of the mice with advancing age correlated with the presence of an increased number of ecotropic MuLV RNA-containing cells in the spinal cords which, in turn, correlated with an increase in the number of unmethylated proviruses in the DNA extracted from spinal cords. Studies with AKXD recombinant inbred strains showed that possession of a single replication-competent ecotropic MuLV provirus (emv-11) by Fv-1n/n mice was sufficient to result in ecotropic MuLV infection of CNS cells and ADPM susceptibility. In contrast, no ecotropic MuLV RNA-positive cells were present in the CNSs of mice carrying defective ecotropic MuLV proviruses (emv-3 or emv-13) or in which ecotropic MuLV replication was blocked by the Fv-1n/b or Fv-1b/b phenotype. Such mice were resistant to paralytic LDV infection. In utero infection of CE/J mice, which are devoid of any endogenous ecotropic MuLVs, with the infectious clone of emv-11 (AKR-623) resulted in the infection of CNS cells, and the mice became ADPM susceptible, whereas littermates that had not become infected with ecotropic MuLV remained ADPM resistant.

Transient expression of a human beta-actin promoter/lacZ gene introduced into mouse embryos correlates with a low degree of methylation

We have analysed the correlation between expression and methylation for the human beta-actin promoter introduced into mouse embryos. The beta-actin promoter was fused to the reporter gene lacZ, and expression was analysed after pronuclear injection into fertilized mouse eggs. We analysed transient expression in in vitro cultured preimplantation embryos and expression after chromosomal integration in 5 independent lines of transgenic mice. The in vitro cultured preimplantation embryos expressed lacZ from the 2-cell to the blastocyst stages, and most abundantly at the morula stage. By increasing the amount of injected DNA, a larger proportion of embryos expressed lacZ. Embryos expressing lacZ in only a subset of the blastomeres were detected at all preimplantation stages. In contrast to the transient expression after injection, we have not detected lacZ expression in any of the 5 analysed lines of transgenic mice carrying the same construct. The lack of expression in transgenic mice correlates with hypermethylation of C residues in the vast majority of CG sequences in the integrated beta-actin/lacZ construct, whereas the injected construct was completely nonmethylated. We discuss methylation and other possible reasons for the observed differences in expression between injected and integrated copies of the beta-actin/lacZ construct and for lacZ expression in only a subset of blastomeres in preimplantation embryos.

Rat cellular mutants for expression of mRNA from the long terminal repeat of murine retrovirus

Previously we isolated revertants from a rat cell line transformed by recombinant murine retrovirus containing the v-src gene. These mutant cell lines, R78 and R107, showed low src-kinase activity, but retained wild-type transforming retrovirus, suggesting that a cellular gene involved in viral gene expression was mutated. Southern and Northern hybridization analyses showed that the expression of viral mRNAs from the integrated proviral DNA was reduced in these mutant cells. DNA transfection experiments with various transforming genes and promoters revealed that the mutant cell lines were resistant to transformation by transforming genes expressed under the long terminal repeat (LTR) of Moloney murine leukemia virus (Mo-MuLV). In contrast, these cell lines could be efficiently transformed by the same transforming genes with human metallothionein promoter, polyomavirus promoter-enhancer, and c-H-ras promoter. Transient expression assays using plasmids containing the CAT gene under the LTR of Mo-MuLV also showed that CAT activity expressed under the LTR in these mutant cells was lower than that in the parental cell line, No. 7. These results suggest that cellular mutations of R78 and R107 cells affect specific transcription from the LTR of Mo-MuLV. Studies using various constructs of the LTR CAT indicated that the region responsible for the repression was located in a fragment (-328 to -150) of the LTR containing the 72-bp repeat enhancer. Somatic cell hybridization experiments showed that the mutant phenotype of these mutant cell lines is dominant to that of the parental cell line.

Targeted mutation of the DNA methyltransferase gene results in embryonic lethality

Gene targeting in embryonic stem (ES) cells has been used to mutate the murine DNA methyltransferase gene. ES cell lines homozygous for the mutation were generated by consecutive targeting of both wild-type alleles; the mutant cells were viable and showed no obvious abnormalities with respect to growth rate or morphology, and had only trace levels of DNA methyltransferase activity. A quantitative end-labeling assay showed that the level of m5C in the DNA of homozygous mutant cells was about one-third that of wild-type cells, and Southern blot analysis after cleavage of the DNA with a methylation-sensitive restriction endonuclease revealed substantial demethylation of endogenous retroviral DNA. The mutation was introduced into the germline of mice and found to cause a recessive lethal phenotype. Homozygous embryos were stunted, delayed in development, and did not survive past mid-gestation. The DNA of homozygous embryos showed a reduction of the level of m5C similar to that of homozygous ES cells. These results indicate that while a 3-fold reduction in levels of genomic m5C has no detectable effect on the viability or proliferation of ES cells in culture, a similar reduction of DNA methylation in embryos causes abnormal development and embryonic lethality.

Methylation of HSV-1 DNA as a mechanism of viral inhibition: studies of an analogue of methyldeoxycytidine: trifluoromethyldeoxycytidine (F3mdCyd)

Although several hypomethylating agents such as 5-azadeoxycytidine and 5-fluorodeoxycytidine have been shown to activate transcription after incorporation into viral or cellular DNA, agents which selectively affect the methylation status of virus-infected cells have not been described. Studies on the antiviral effect of the methyldeoxycytidine (mdCyd) analogue trifluoromethyldeoxycytidine (F3mdCyd) showed significant antiviral activity against herpes simplex virus type 1 (HSV-1). This analogue of both dCyd and dThd is selectively incorporated into the DNA of herpesvirus infected cells due to the unique specificity of the herpesvirus thymidine kinase (TK) because the HSV-1 TK is both a dCyd and dThd kinase. In contrast, the deoxycytidine kinase of uninfected cells preferentially phosphorylates dCyd and has a poor affinity for F3mdCyd. F3mdCyd hemisubstituted M13 DNA displayed the same properties as mdCyd-substituted M13 DNA with respect to cleavage by restriction enzymes, and acted as an efficient template for eukaryotic DNA methyltransferase (S-adenosyl-L-methionine DNA (cytosine-5) methyltransferase: EC 2.1.1.37). Using the persistently infected CEM cell model system, the extent of DNA methylation was shown to increase in a dose-related manner when HSV-1-infected CEM cells were treated with increasing concentrations of F3mdCyd. Higher levels of methylation correlated with significant decreases in HSV-1 titers. Isoschizomer analyses followed by Southern blotting and hybridization with genomic HSV-1 DNA showed that DNA from HSV-1-infected, analogue-treated Vero cells was resistant to cleavage by restriction enzymes at a time when productive virus was not present in culture. We infer from these results that the methylation-like properties of the incorporated F3mdCyd occur concomitantly with, and appear to be involved in, the mechanisms of the analogue's antiviral effect towards HSV-1.

Retrovirus-induced insertional mutagenesis: mechanism of collagen mutation in Mov13 mice

The Mov13 mouse strain carries a mutation in the alpha 1(I) procollagen gene which is due to the insertion of a Moloney murine leukemia provirus into the first intron. This insertion results in the de novo methylation of the provirus and flanking DNA, the alteration of chromatin structure, and the transcriptional inactivity of the collagen promoter. To address the mechanism of mutagenesis, we reintroduced a cloned and therefore demethylated version of the Mov13 mutant allele into mouse fibroblasts. The transfected gene was not transcribed, indicating that the transcriptional defect was not due to the hypermethylation. Rather, this result strongly suggests that the mutation is due to the displacement or disruption of cis-acting regulatory DNA sequences within the first intron. We also constructed a Mov13 variant allele containing a single long terminal repeat instead of the whole provirus. This construct also failed to express mRNA, indicating that the Mov13 mutation does not revert by provirus excision as has been observed for other retrovirus-induced mutations.

Retrovirus-induced insertional mutagenesis: mechanism of collagen mutation in Mov13 mice

The Mov13 mouse strain carries a mutation in the alpha 1(I) procollagen gene which is due to the insertion of a Moloney murine leukemia provirus into the first intron. This insertion results in the de novo methylation of the provirus and flanking DNA, the alteration of chromatin structure, and the transcriptional inactivity of the collagen promoter. To address the mechanism of mutagenesis, we reintroduced a cloned and therefore demethylated version of the Mov13 mutant allele into mouse fibroblasts. The transfected gene was not transcribed, indicating that the transcriptional defect was not due to the hypermethylation. Rather, this result strongly suggests that the mutation is due to the displacement or disruption of cis-acting regulatory DNA sequences within the first intron. We also constructed a Mov13 variant allele containing a single long terminal repeat instead of the whole provirus. This construct also failed to express mRNA, indicating that the Mov13 mutation does not revert by provirus excision as has been observed for other retrovirus-induced mutations.

DNA methylation and gene expression

A large body of evidence demonstrates that DNA methylation plays a role in gene regulation in animal cells. Not only is there a correlation between gene transcription and undermethylation, but also transfection experiments clearly show that the presence of methyl moieties inhibits gene expression in vivo. Furthermore, gene activation can be induced by treatment of cells with 5-azacytidine, a potent demethylating agent. Methylation appears to influence gene expression by affecting the interactions with DNA of both chromatin proteins and specific transcription factors. Although methylation patterns are very stable in somatic cells, the early embryo is characterized by large alterations in DNA modification. New methodologies are now becoming available for studying methylation at this stage and in the germ line. During development, tissue-specific genes undergo demethylation in their tissue of expression. In tissue culture cells this process is highly specific and appears to involve an active mechanism which takes place in the absence of DNA replication. The X chromosome undergoes inactivation during development; this is accompanied by de novo methylation, which appears necessary to stably maintain its silent state. As opposed to the programmed changes in DNA methylation which occur in vivo, immortalized tissue culture cells demonstrate alterations in DNA modification which take place over a long time scale and which appear to be the result of selective pressures present during the growth of these cells in culture.

Retroviral vector-mediated in vivo expression of low-density-lipoprotein receptors in the Watanabe heritable hyperlipidemic rabbit

We have achieved in vivo expression of recombinant low-density-lipoprotein (LDL) receptors in the Watanabe heritable hyperlipidemic (WHHL) rabbit, an animal model for the human disease familial hypercholesterolemia. A retroviral vector was constructed containing the human LDL receptor cDNA and was used to stably transduce primary skin fibroblasts from WHHL rabbits. The integrity and function of the introduced LDL receptor was established by immunoprecipitation, by a fluorescent LDL binding assay, and by the ability of the transduced cells to suppress 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase activity in response to exogenous cholesterol. Autologous transduced fibroblasts were reimplanted into donor rabbits; in vivo LDL receptor expression and the survival of the transduced cells were analyzed by immunohistochemistry and by LDL binding assays performed on cells recovered from the implants. LDL receptor-bearing cells could be identified on tissue sections and recovered from implants for up to four weeks. Total and LDL cholesterol levels decreased significantly after implantation of the transduced cells; however, control experiments indicated that the decreases were not mediated through the recombinant LDL receptor. While in vivo stable expression of recombinant LDL receptors in Watanabe rabbits is possible, consequent changes in lipid levels must be interpreted with caution. This system of site-specific in vivo expression of recombinant LDL receptors permits further evaluation of the role of LDL receptor-gene replacement in the therapy of hypercholesterolemia.

Persistent gene expression after retroviral gene transfer into liver cells in vivo

The development of liver-directed gene therapy protocols depends upon the ability to transfer genes into a large number of liver cells such that the genes are expressed persistently. We used a retroviral vector to transfer the gene for neomycin phosphotransferase (neo) into mouse liver cells in vivo. Direct injection of the retrovirus preparation into mitotically active (regenerating) liver parenchyma resulted in efficient gene transfer, with neo sequences detectable in the livers of every animal tested 10 weeks to 6 months later. The neo gene was expressed for at least 3 months. This methodology may eventually be applicable to the treatment of human disease.

Heritable retroviral transgenes are highly expressed in chickens

This report describes expression of heritable reticuloendotheliosis virus (REV) vector ME111 in 20 independent lines of transgenic chickens. The results are strikingly different from studies of Moloney virus in transgenic mice, where restricted expression of inherited proviruses has led to their use primarily as insertional mutagens rather than general agents for gene transfer. In contrast, the REV ME111 provirus is actively transcribed in a variety of tissues from transgenic chickens, is expressed from transcriptional control elements present in the long terminal repeat of the provirus, and codes for active neomycin phosphotransferase II. The REV vector system as applied to the chicken represents a departure from the long-established paradigm of retroviral transgenes in mice and provides a new approach to the study of avian biology.

Retrovirus-induced interference with collagen I gene expression in Mov13 fibroblasts is maintained in the absence of DNA methylation

We have studied the role of DNA methylation in repression of the murine alpha 1 type I collagen (COL1A1) gene in Mov13 fibroblasts. In Mov13 mice, a retroviral provirus has inserted into the first intron of the COL1A1 gene and blocks its expression at the level of transcriptional initiation. We found that regulatory sequences in the COL1A1 promoter region that are involved in the tissue-specific regulation of the gene are unmethylated in collagen-expressing wild-type fibroblasts and methylated in Mov13 fibroblasts, confirming and extending earlier observations. To directly assess the role of DNA methylation in the repression of COL1A1 gene transcription, we treated Mov13 fibroblasts with the demethylating agent 5-azacytidine. This treatment resulted in a demethylation of the COL1A1 regulatory sequences but failed to activate transcription of the COL1A1 gene. Moreover, the 5-azacytidine treatment induced a transcription-competent chromatin structure in the retroviral sequences but not in the COL1A1 promoter. In DNA transfection and microinjection experiments, we found that the provirus interfered with transcriptional activity of the COL1A1 promoter in Mov13 fibroblasts but not in Xenopus laevis oocytes. In contrast, the wild-type COL1A1 promoter was transcriptionally active in Mov13 fibroblasts. These experiments showed that the COL1A1 promoter is potentially transcriptionally active in the presence of proviral sequences and that Mov13 fibroblasts contain the trans-acting factors required for efficient COL1A1 gene expression. Our results indicate that the provirus insertion in Mov13 can inactivate COL1A1 gene expression at several levels. It prevents the developmentally regulated establishment of a transcription-competent methylation pattern and chromatin structure of the COL1A1 domain and, in the absence of DNA methylation, appears to suppress the COL1A1 promoter in a cell-specific manner, presumably by assuming a dominant chromatin structure that may be incompatible with transcriptional activity of flanking cellular sequences.

Inactivation of the Moloney murine leukemia virus long terminal repeat in murine fibroblast cell lines is associated with methylation and dependent on its chromosomal position

The expression of a retroviral vector with the Moloney murine leukemia virus (Mo-MuLV) long terminal repeat (LTR) promoter after integration into the genome of murine fibroblast cell lines was monitored with the Escherichia coli-derived beta-galactosidase (beta-gal) gene as the reporter. Monoclonal cell lines derived after retroviral infection exhibited a marked heterogeneity in their expression of the reporter gene. We studied two monoclonal cell lines with a single unrearranged copy of the vector provirus integrated into their genome. The first, BB10, expressed the marker enzyme in only 8% of its cell population, whereas in the second, BB16, beta-gal expression could be detected in over 98% of the cells. Treatment of BB10 with the DNA-demethylating agent 5-azacytidine raised the number of beta-gal-positive cells to over 60%. Transfection experiments showed that the Mo-MuLV LTR promoter-enhancer is potentially fully functional in both the BB10 and BB16 cell lines. The inactivated provirus from BB10 cells was cloned and subsequently used to generate retrovirus stocks. The promoter-enhancer activity of its LTR after infection with these BB10-derived viruses showed a variation similar to that of the original virus stocks. Our data showed that (1) inactivation of the Mo-MuLV LTR is a frequent event in murine fibroblast cell lines, (2) inactivation is associated with de novo methylation of cytidine residues, (3) the frequency of inactivation of the provirus must be determined by its chromosomal position, (4) the process of methylation of sequences within the LTR is not necessarily the same as the transcription-repression mechanism that is operating in undifferentiated embryonal carcinoma cells.

Retrovirus-induced interference with collagen I gene expression in Mov13 fibroblasts is maintained in the absence of DNA methylation

We have studied the role of DNA methylation in repression of the murine alpha 1 type I collagen (COL1A1) gene in Mov13 fibroblasts. In Mov13 mice, a retroviral provirus has inserted into the first intron of the COL1A1 gene and blocks its expression at the level of transcriptional initiation. We found that regulatory sequences in the COL1A1 promoter region that are involved in the tissue-specific regulation of the gene are unmethylated in collagen-expressing wild-type fibroblasts and methylated in Mov13 fibroblasts, confirming and extending earlier observations. To directly assess the role of DNA methylation in the repression of COL1A1 gene transcription, we treated Mov13 fibroblasts with the demethylating agent 5-azacytidine. This treatment resulted in a demethylation of the COL1A1 regulatory sequences but failed to activate transcription of the COL1A1 gene. Moreover, the 5-azacytidine treatment induced a transcription-competent chromatin structure in the retroviral sequences but not in the COL1A1 promoter. In DNA transfection and microinjection experiments, we found that the provirus interfered with transcriptional activity of the COL1A1 promoter in Mov13 fibroblasts but not in Xenopus laevis oocytes. In contrast, the wild-type COL1A1 promoter was transcriptionally active in Mov13 fibroblasts. These experiments showed that the COL1A1 promoter is potentially transcriptionally active in the presence of proviral sequences and that Mov13 fibroblasts contain the trans-acting factors required for efficient COL1A1 gene expression. Our results indicate that the provirus insertion in Mov13 can inactivate COL1A1 gene expression at several levels. It prevents the developmentally regulated establishment of a transcription-competent methylation pattern and chromatin structure of the COL1A1 domain and, in the absence of DNA methylation, appears to suppress the COL1A1 promoter in a cell-specific manner, presumably by assuming a dominant chromatin structure that may be incompatible with transcriptional activity of flanking cellular sequences.

Gene expression of an endogenous retrovirus-like element during murine development and aging

We have measured intracisternal A-particle (IAP) RNA levels during development and aging in C57BL/6J mouse tissues to determine possible age-dependent changes in gene expression of these retrovirus-like sequences. Total RNA was isolated from tissues of embryonic and new born mice and mice ranging in age from 2 months to 32 months of age. RNA samples were either slot-blotted directly or fractionated on denaturing agarose gels and transferred to nylon membranes. Hybridization with cloned, 32P-labeled IAP sequences showed that both the mass amounts and the relative proportions of IAP transcripts varied between tissues and as a function of age. IAP gene products were higher in brain and kidney tissues than in liver and heart tissues. The relative proportion of transcripts increased in embryonic tissues until birth and following birth, was highest in neonatal or 2-month-old tissues. The adult levels of IAP-related RNAs did not change significantly from 6 to 24 months of age. However, 32-month-old tissues exhibited the lowest content of IAP transcripts, with the exception of heart tissue which did not change with age. A 5.4-kb RNA was the predominant IAP transcript in most samples, but each tissue had a characteristic size distribution of IAP-related transcripts. These results demonstrate that transcription of IAP genes continues throughout the life span of this mouse strain with tissue-specific and age-dependent regulation of expression.

Differential plasmid rescue from transgenic mouse DNAs into Escherichia coli methylation-restriction mutants

Plasmids comprising transgene insertions in four lines of transgenic mice have been retrieved by plasmid rescue into a set of Escherichia coli strains with mutations in different members of the methylation-dependent restriction system (MDRS). Statistical analysis of plasmid rescue frequencies has revealed that the MDRS loci detect differential modifications of the transgene insertions among mouse lines that show distinctive patterns of transgene expression. Plasmids in mice that express hybrid insulin transgenes during development can be readily cloned into E. coli strains carrying mutations in two of the MDRS loci, mcrA and mcrB. In mice in which transgene expression is inappropriately delayed into adulthood, plasmids can only be cloned into E. coli that carry mutations in all known MDRS activities. Differential cloning frequencies in the presence or absence of the various methylation-dependent restriction genes represent a further way to distinguish regions of mammalian chromosomes. These multiply deficient E. coli strains will also facilitate the molecular cloning of modified chromosomal DNA.

Transcription of HIV1 is inhibited by DNA methylation

A possible role of DNA methylation as a factor in HIV latency was studied by methylating a HIV1-LTR-CAT plasmid in vitro and measuring its expression after transfection on Vero cells. Methylation with a eukaryotic DNA methylase resulted in a 70% inhibition of chloramphenicol acetyltransferase expression, in the absence as well as in the presence of the HIV1 trans-activator protein TAT in the cell. A similar degree of transcription inhibition was obtained by methylation of the only Hpa II site at position-143 in the HIV1-LTR with the bacterial Hpa II methylase. In contrast to the effect by eukaryotic methylation, the inhibition by Hpa II methylation could be partially reversed by cotransfection of the TAT gene. The reason may lie in an about 40% demethylation at the Hpa II site which was concomitantly observed.

Negative regulation of retrovirus expression in embryonal carcinoma cells mediated by an intragenic domain

An intragenic region spanning the tRNA primer binding site of a Moloney murine leukemia virus recombinant retrovirus was found to restrict expression specifically in embryonal carcinoma (EC) cells. When the inhibitory domain was present, the levels of steady-state RNA synthesized from integrated recombinant templates in stable cotransformation assays were reduced 20-fold in EC cells but not in C2 myoblast cells. Transient-cotransfection assays showed that repression of a template containing the EC-specific inhibitory component was relieved by an excess of specific competitor DNA. In addition, repression mediated by the inhibitory component was orientation independent. This evidence demonstrates the presence of a saturable, trans-acting negative regulatory factor(s) in EC cells and suggests that the interaction of the factor(s) with the intragenic inhibitory component occurs at the DNA level.

Effects of cytidine analogs on methylation of DNA and retrovirus induction

Several cytidine analogs with known mutagenic capability were tested for their effects on DNA methylation and on induction of endogenous murine retrovirus. For each of the compounds tested it was found that DNA methylation was inhibited at the same concentrations that were required to induce virus expression. With each compound it was observed that increased dose levels produced an increase in the ability to inhibit methylation and an increase in the ability to induce virus.

Absence of expression of a human endogenous retrovirus is correlated with choriocarcinoma

We examined the RNA expression of a human endogenous provirus, termed ERV3, in 170 human tissue and cell specimens. The highest expression was found in normal placental chorionic villi as mRNAs of 9, 7.3, and 3.5 kb. The 7.3-kb RNA species was found only in the placenta. ERV3 mRNA was expressed in most other normal and malignant tissues at a level which was 2-10% of that seen in placenta. However, several tissues and tumor cell lines had higher transcriptional levels, equal to 10-60% of the placental level. In contrast, an almost complete abrogation of ERV3 mRNA expression was noted in choriocarcinoma cell lines and in an invasive hydatidiform mole tissue biopsy. This abrogation was not linked to deletions or rearrangements of the ERV3 genome. It appeared to be unassociated with methylation because the ERV3 provirus was similarly methylated in the DNA of placental chorionic villi and choriocarcinoma cells, and ERV3 transcription in choriocarcinoma cells was not induced by 5-azacytidine. These results suggest that the loss of ERV3 mRNA expression is associated with susceptibility to choriocarcinoma.

Inoculation of newborn SWR/J females with an ecotropic murine leukemia virus can produce transgenic mice

Endogenous ecotropic murine leukemia proviruses that were not present in the parental stock are acquired by the progeny of some SWR/J X RF/J hybrid females. We have made a stock of an ecotropic murine leukemia virus produced by such a hybrid female and inoculated newborn SWR/J females with it. We show that upon crossing of the inoculated females to SWR/J males, some of their progeny acquire ecotropic proviruses. Although most of these proviruses appear to be distributed in somatic tissues in a mosaic way, some are transmitted through the germ line. Thus an exogenous infection is able to mimic the phenomenon observed in SWR/J X RF/J hybrid mice. Available evidence suggests that this infection occurs during oogenesis in the recipient female. Our results document the conversion of an exogenous infectious ecotropic murine leukemia virus to an endogenous provirus without any manipulation of either eggs or embryos.

Poorly expressed endogenous ecotropic provirus of DBA/2 mice encodes a mutant Pr65gag protein that is not myristylated

DBA/2 mice carry a single endogenous ecotropic murine leukemia provirus designated Emv-3. Although this provirus appears to be nondefective by genomic restriction enzyme mapping, weanling mice do not produce virus and only about one-third of adult mice ever express virus. 5-Iododeoxyuridine and 5-azacytidine, two potent inducers of ecotropic virus expression, are relatively ineffective at inducing Emv-3 expression. However, the chemical carcinogen 7,12-dimethylbenz(a)anthracene can induce ecotropic virus expression in approximately 95% of treated DBA/2 mice. Previous experiments involving DNA transfection and marker rescue analysis of molecularly cloned Emv-3 DNA suggested that Emv-3 carries a small defect(s) in the gag gene, not detectable by restriction enzyme mapping, that inhibits virus expression in vivo and in vitro. Using a combination of approaches, including DNA sequencing, peptide mapping, and metabolic labeling of cells with [3H]myristate, we have demonstrated that the defect in Emv-3 most likely results from a single nucleotide substitution within the gene for p15gag that inhibits myristylation of the Pr65gag N terminus. Myristylation of Pr65gag is thought to be required for this protein to associate with the plasma membrane and is essential for virus particle formation. These results provide a conceptual framework for understanding how Emv-3 expression is regulated during development and after chemical induction.

Introduction and expression of the interleukin 2 receptor (Tac) gene in hematopoietic stem cells with retrovirus vectors

Retrovirus vectors provide an efficient carrier for introducing a gene into hematopoietic stem cells although expression of the inserted gene is not always successful. We constructed and compared three retrovirus vectors which carried cDNA encoding the light chain (Tac) of the interleukin 2 receptor under the control of different promoters; long terminal repeat (LTR) of murine retroviruses, the early promoter of simian virus 40 (SV40) and the promoter of the class I antigen gene of the major histocompatibility complex. We made three constructs containing these promoters. A first construct did not contain any additional promoter but LTR. A second and a third constructs contained the SV40 and the class I antigen gene promoters, respectively, in addition to LTR. The LTR of retrovirus vectors is derived from MoMuLV except that the U3 region of the 3'LTR of the third construct is derived from myeloproliferative sarcoma virus (MPSV). The second and third constructs were used for infection of bone marrow stem cells as the first construct was less efficient in expression of the interleukin 2 receptor in fibroblasts. Hematopoietic stem cells infected with the recombinant viruses were transplanted into lethally irradiated mice, and the expression of the transduced gene in hematopoietic progenitor cells was analyzed. Analysis of RNA isolated from spleen colonies showed that substantial amounts of interleukin 2 receptor mRNA were made by the construct containing the class I gene promoter and MPSV LTR. However, we could not detect any transcripts from the constructs containing MoMuLV LTR and SV40 early region promoter.

A cellular enhancer of retrovirus gene expression in embryonal carcinoma cells

Murine embryonal carcinoma (EC) cells are refractory to infection by retroviruses because retroviral long terminal repeat (LTR) enhancers have little activity in EC cells. A previous report described the isolation of clonal EC cell lines that express the integrated neomycin-resistance gene (neo) linked to the Moloney murine leukemia virus LTR. The expression of the neo gene was explained by a cis-acting mechanism [Taketo, M., Gilboa, E. & Sherman, M. I. (1985) Proc. Natl. Acad. Sci. USA 82, 2422-2426]. From one such EC cell line, we isolated the flanking cellular sequence 5' to the proviral genome, ligated it to various test constructs, and transfected into the parental EC cells. The cellular sequence increased expression of the LTR-linked neo gene significantly, in a manner independent of its orientation and position. The neo mRNA was initiated at the bona fide promoter of the LTR. By deletion analyses, we defined a region of DNA essential for the enhancer activity and determined its sequence. This region contains distinctly characteristic stretches as well as some similarity to various viral and cellular enhancers. Thus the LTR-linked neo gene is expressed because the provirus is integrated in the vicinity of this enhancer that is active in undifferentiated EC cells.