Suppression of leukaemia virus pathogenicity by polyoma virus enhancers

Identification of LTR-specific small non-coding RNA in FeLV infected cells

The U3-LTR region of leukemia viruses transactivates cancer-related signaling pathways through the production of a non-coding RNA transcript although the role of this transcript in virus infection remains unknown. In this study we demonstrate for the first time that an long terminal repeat (LTR)-specific small non-coding RNA is produced from a feline leukemia virus (FeLV)-infected feline cell line. RNA cloning identified this as a 104 base transcript that originates from the U3-LTR region. We also demonstrate that in in vitro assays this LTR-RNA transcript activates NF kappaB signaling. Taken together, our findings suggest a possible role for this LTR transcript in FeLV pathogenesis.

Enhanced replication and pathogenesis of Moloney murine leukemia virus in mice defective in the murine APOBEC3 gene

Human APOBEC3G (hA3G), a member of the AID/APOBEC family of deaminases, is a restriction factor for human immunodeficiency virus (HIV). In the absence of the viral Vif protein hA3G is packaged into virions and during reverse transcription in a recipient cell it deaminates cytosines, leading to G-->A hypermutation and inactivation of the viral DNA. Unlike humans, who carry seven APOBEC3 genes, mice only carry one, mA3. Thus the role of mA3 in restriction of retroviral infection could be studied in mA3 -/- knockout mice, where the gene is inactivated. M-MuLV-infected mA3 -/- mice showed substantially higher levels of infection at very early times compared to wild-type mice (ca. 2 logs at 0-10 days), particularly in the bone marrow and spleen. Restriction of M-MuLV infection was studied ex vivo in primary bone marrow-derived dendritic cells (BMDCs) that express or lack mA3, using an M-MuLV-based retroviral vector expressing enhanced jellyfish green fluorescent protein (EGFP). The results indicated that mA3 within the virions as well as mA3 in the recipient cell contribute to resistance to infection in BMDCs. Finally, M-MuLV-infected mA3 +/+ mice developed leukemia more slowly compared to animals lacking one or both copies of mA3 although the resulting disease was similar (T-lymphoma). These studies indicate that mA3 restricts replication and pathogenesis of M-MuLV in vivo.

Insertional gene activation by lentiviral and gammaretroviral vectors

Gammaretroviral and lentiviral vectors are promising tools for gene therapy, but they can be oncogenic. The development of safer vectors depends on a quantitative assay for insertional mutagenesis. Here we report a rapid, inexpensive, and reproducible assay which uses a murine cell line to measure the frequency of interleukin-3 (IL-3)-independent mutants. Lentiviral and gammaretroviral vectors cause insertional mutagenesis at similar frequencies; however, they use different mechanisms. Human immunodeficiency virus (HIV)-based vectors generate mutants by insertion only into the growth hormone receptor (Ghr) locus. The HIV enhancer/promoter is active in the absence of the HIV Tat protein in this locus, and an HIV/Ghr spliced transcript expresses GHR and cells respond to GH. Deletion of the enhancer/promoter in a self-inactivating HIV-based vector prevents this mechanism of insertional mutagenesis. In contrast, gammaretroviral vectors insert into other loci, including IL-3 and genes identified as common insertion sites in the Retroviral Tagged Cancer Gene Database (RTCGD).

Chimeras between SRS and Moloney murine leukemia viruses reveal novel determinants in disease specificity and MCF recombinant formation

SRS 19-6 MuLV is a murine retrovirus originally isolated in mainland China. A noteworthy feature of this virus (referred to as SRS MuLV here) induces tumors of multiple hematopoietic lineages, including myeloid, erythroid, T-lymphoid and B-lymphoid. To identify the determinants of disease specificity, chimeras between SRS and Moloney MuLV (M-MuLV) were generated by molecular cloning, and the pathogenic properties of the chimeras were investigated. The results indicated that, while the M-MuLV LTR can confer lymphoid specificity to SRS MuLV, the SRS LTR by itself was not sufficient to confer multiple lineage tumorigenesis to M-MuLV; additional sequences in gag or pol were also required. Thus, a secondary determinant for myeloid/erythroid leukemia in SRS MuLV is located in gag-pol. In these chimeras, an independent determinant for T-lymphoma was found in M-MuLV gag-pol. It was also interesting that insertion of M-MuLV env into SRS MuLV decreased the rate of leukemogenicity, while insertion of SRS env into M-MuLV (SEM) accelerated leukemogenesis. The enhanced pathogenicity of SEM was found to correlate with earlier formation of MCF recombinants. The basis for the accelerated MCF recombinant formation was investigated. The endogenous polytropic MuLV env sequences contributing to several SEM MCF recombinants were identified, and the cross-over points were identified. While no obvious differences in the relative homologies between SRS MuLV env and polytropic env vs. M-MuLV and polytropic envs suggested a reason for the more rapid MCF recombinant formation, an overlapping but different set of polytropic env proviruses were found to participate in MCF formation for M-MuLV vs. SEM. Thus, the mechanisms for MCF formation appear to differ for M-MuLV and SEM.

A Moloney murine leukemia virus driven by the Jaagsiekte sheep retrovirus enhancers shows enhanced specificity for infectivity in lung epithelial cells

Jaagsiekte sheep retrovirus (JSRV) is the etiologic agent of ovine pulmonary adenocarcinoma (OPA), a transmissible lung cancer in sheep. One of the unique features of this virus is that in infected animals, the only tissues that show expression of the virus are the tumor cells in the lung. We previously showed that the JSRV long terminal repeat (LTR) is preferentially active in murine lung epithelial cell lines (MLE-15 and mtCC1-2). To further explore the tissue specificity, we inserted the JSRV enhancer sequences from the U3 region of the LTR into a Moloney murine leukemia virus (M-MuLV) LTR lacking its own enhancer sequences, to give the chimeric LTR DeltaMo + JS. Transient transfection assays indicated that the DeltaMo + JS LTR is > 5-fold more active in lung epithelial cell lines than in non-lung lines, compared to the wild-type M-MuLV LTR. This was due to preferential activity of the JSRV enhancers in lung epithelial cells. Moreover, M-MuLV driven by the DeltaMo + JS LTR was > 3 logs more infectious in MLE-15 cells compared to non-lung cell lines. This chimeric virus may facilitate investigations of the tissue-specificity of JSRV.

Leukemia virus long terminal repeat activates NFkappaB pathway by a TLR3-dependent mechanism

The long terminal repeat (LTR) region of leukemia viruses plays a critical role in tissue tropism and pathogenic potential of the viruses. We have previously reported that U3-LTR from Moloney murine and feline leukemia viruses (Mo-MuLV and FeLV) upregulates specific cellular genes in trans in an integration-independent way. The U3-LTR region necessary for this action does not encode a protein but instead makes a specific RNA transcript. Because several cellular genes transactivated by the U3-LTR can also be activated by NFkappaB, and because the antiapoptotic and growth promoting activities of NFkappaB have been implicated in leukemogenesis, we investigated whether FeLV U3-LTR can activate NFkappaB signaling. Here, we demonstrate that FeLV U3-LTR indeed upregulates the NFkappaB signaling pathway via activation of Ras-Raf-IkappaB kinase (IKK) and degradation of IkappaB. LTR-mediated transcriptional activation of genes did not require new protein synthesis suggesting an active role of the LTR transcript in the process. Using Toll-like receptor (TLR) deficient HEK293 cells and PKR(-/-) mouse embryo fibroblasts, we further demonstrate that although dsRNA-activated protein kinase R (PKR) is not necessary, TLR3 is required for the activation of NFkappaB by the LTR. Our study thus demonstrates involvement of a TLR3-dependent but PKR-independent dsRNA-mediated signaling pathway for NFkappaB activation and thus provides a new mechanistic explanation of LTR-mediated cellular gene transactivation.

A novel gene transfer strategy that combines promoter and transgene activities for improved tumor cell inhibition

Typically, gene transfer strategies utilize a promoter/transgene arrangement that treat these elements independently and do not offer any interplay between them. Our goal was to establish a promoter/transgene combination that would result in improvement in both expression and therapeutic effect by utilizing the transcriptional properties of p53 to drive its own expression as well as act as a tumor suppressor. The pCL retroviral system was modified in the U3 region of the 3' LTR by the addition of a p53-responsive sequence (the PG element), creating the pCLPG system. Upon reverse transcription, the 5' LTR is converted, as shown here, to a p53-dependent promoter. We also show, using a temperature-sensitive model, that the pCLPG system could be driven by p53 encoded within the virus construct and expression was modulated depending on the p53 phenotype, demonstrating a regulatory feedback loop. Moreover, the pCLPG system was shown to express the transgene at a higher level and to inhibit tumor cell proliferation more robustly than the original pCL system. This novel system employs the transgene to serve two purposes, drive viral expression and inhibit tumor cell proliferation. The pCLPG vectors represent a new gene transfer strategy of synergizing the promoter and transgene activities.

Upstream conserved sequences of mouse leukemia viruses are important for high transgene expression in lymphoid and hematopoietic cells

Highly conserved enhancer sequences located in the upstream part of the long terminal repeat (LTR) of murine leukemia retroviruses (MLV) were reported to compromise viral gene expression in multipotent embryonic cells in vitro and to reduce the likelihood for maintenance of retroviral gene expression in hematopoietic cells in vivo. We show that deletion of these sequences (nucleotides +37 to +95) attenuates rather than increases the transcriptional activity of retroviral vectors in hematopoietic cells almost independently of the developmental lineage (erythroid, myeloid, or lymphoid). Expression rates of modified vectors were reduced by as much as 34-65%, although the strong enhancer array located in the direct repeat of the LTR was preserved. Sequence analysis and electrophoretic mobility shift assays revealed the presence of a highly conserved binding site for NFAT (nuclear factor of activated T cells) proteins that immediately neighbors a known binding site for the transcription factor Yin-Yang1 (YY1) [corrected]. Specific inactivation of the NFAT site reduced transgene expression in all cell types investigated and had a similar effect as the destruction of a neighboring SP1 motif. Combined destruction of individual motifs for NFAT, SP1, and E twenty-six transcription factors (ETS) resulted in a severe attenuation (by 40-60%) of the retroviral enhancer. These results provide novel clues for the manipulation of retrovirus replication and vector tropism.

In vivo analysis of retroviral enhancer mutations in hematopoietic cells: SP1/EGR1 and ETS/GATA motifs contribute to long terminal repeat specificity

The objective of this work was to identify, in the context of chromosomally integrated DNA, the contribution of defined transcription factor binding motifs to the function of a complex retrovirus enhancer in hematopoietic cells in vivo. Repopulating murine hematopoietic cells were transduced with equal gene dosages of replication-incompetent retrovirus vectors encoding enhanced green fluorescent protein. Enhancer sequences were derived from mouse spleen focus-forming virus. Destruction of GC-rich sites representing overlapping targets for SP1 or EGR1 uniformly attenuated gene expression (approximately 25 to 70% of wild-type levels) in all hematopoietic lineages, as shown by multicolor flow cytometry of peripheral blood and bone marrow cells at various time points posttransplantation. In contrast, a point mutation within a dual ETS/GATA motif that abolished transactivation by ETS factors but not by GATA-1 slightly increased activity in erythroid cells and significantly attenuated enhancer function in T lymphocytes. This study shows that controlled gene transfer in transplantable hematopoietic cells allows a functional analysis of distinct cis elements within a complex retrovirus enhancer, as required for the characterization and engineering of various cellular and viral regulatory sequences in basic research and gene therapy.

The helper virus envelope glycoprotein affects the disease specificity of a recombinant murine leukemia virus carrying a v-myc oncogene

Many retroviruses that carry oncogenes (acute transforming viruses) are generally replication-defective and therefore require co-infection with a replication competent 'helper' retrovirus for infectivity. The helper virus provides the retroviral proteins necessary for particle production and infection. These include the envelope glycoproteins that specifically bind to cell surface receptors and mediate viral adsorption and entry. Thus, a particular helper virus may influence the nature of disease induced by an oncogene-containing retrovirus due to tissue tropism of the helper. In a previous study, a replication-defective recombinant Moloney murine leukemia virus containing the v-myc oncogene was generated (M-MuLV(myc); Brightman B.K., Pattengale P.K., and Fan H., J Virol 60: 68-81, 1986). When M-MuLV(myc) was inoculated into mice using the non-pathogenic amphotropic murine leukemia virus (Am-MuLV 4070) as a helper, T- and B-lymphoblastic lymphomas resulted with the following two surface phenotypes, namely, (1) Thy 1.2+, B220- and (2) Thy 1.2-, B220+. Thy 1.2 surface antigen is characteristic of cells of the lymphoid lineage, whereas B220 surface antigen is characteristic of cells of the B-lymphoid lineage. In these experiments, to assess the influence of the helper virus on the disease specificity of M-MuLV(myc), two weakly pathogenic ecotropic helper MuLVs that interact with different cell surface receptors than Am-MuLV (Mo+PyF101 and AKV MuLV) were used to pseudotype M-MuLV(myc). In both cases, when inoculated into mice, these pseudotypes induced only T-lymphoblastic lymphoma. These results indicate that for M-MuLV(myc) the types of the tumors induced are influenced by the helper virus utilized, and they suggest that different lymphoid cells may express different levels of retroviral receptors.

Role of the LTR region between the enhancer and promoter in mink cell focus-forming murine leukemia virus pathogenesis

Long terminal repeat (LTR) sequences are important determinants of mink cell focus-forming (MCF) murine leukemia virus pathogenesis. These sequences include the enhancer and sequences between the enhancer and promoter (DEN). In a previous study we showed that a virus missing the DEN region in its LTR was severely attenuated in its ability to induce thymic lymphoma. In this study we observed that a virus with an LTR consisting of DEN but no enhancer sequences was pathogenic. We compared the pathogenicity of this DEN virus with other LTR mutant MCF13 viruses that contained a single enhancer (1R) or a single enhancer plus DEN (1R + DEN). All LTR mutant viruses generated thymic lymphoma, however, at a much lower incidence and with a longer latency compared with wild-type (WT) MCF13 virus. DEN virus replication in the thymus was the lowest compared with the 1R and 1R + DEN viruses. Viral replication in a different thymic subpopulation could not explain the decreased pathogenicity of the LTR mutant viruses compared with WT virus. However, lower levels of mutant virus replication in the thymus compared with WT during the preleukemic period may contribute to the attenuation of pathogenicity. The phenotype of tumors induced by the mutant viruses was similar and differed from tumors induced by WT virus by the presence of CD3(-)CD4(-)CD8(-) cells. Analysis of LTR sequences of infectious virus rescued from tumors induced by the 1R and 1R + DEN viruses showed that amplification of enhancer sequences had occurred during tumor development. The lack of DEN virus expression by tumor cells led us to propose that DEN sequences may play a role at an early step in tumorigenesis.

Moloney murine leukemia virus-induced tumors show altered levels of proapoptotic and antiapoptotic proteins

Moloney murine leukemia virus (M-MuLV) is a replication-competent, simple retrovirus that induces T-cell lymphomas when inoculated into neonatal mice. The tumor cells are typically derived from immature T cells. During preleukemic times, a marked decrease in thymic size is apparent in M-MuLV-inoculated mice. We previously demonstrated that this thymic regression is correlated with enhanced levels of thymocyte apoptosis (C. Bonzon and H. Fan, J. Virol. 73:2434-2441, 1999). In this study, we investigated the apoptotic state of M-MuLV-induced tumors. M-MuLV-induced tumors were screened for expression of the apoptotic proteins Fas and Bcl-2 by three-color flow cytometric analysis. Single-positive (SP; CD4(+) CD8(-) and CD4(-) CD8(+)) tumor cells generally displayed lower cell surface expression of Fas than SP thymocytes from uninoculated control mice. Double-positive (DP; CD4(+) CD8(+)) M-MuLV-induced tumor cells fell into two categories: those with normal high levels of Fas and those with low levels of Fas. Additionally, the vast majority of DP tumors showed elevated Bcl-2 levels. The DP tumor cells retaining normal/high Fas expression were capable of transducing an apoptotic signal upon anti-Fas engagement. In addition, DP and CD4(+) SP tumor populations displayed higher levels of Fas ligand than normal thymocytes with the same phenotypes. In contrast, CD8(+) SP and CD4(-) CD8(-) tumors did not show elevated Fas ligand expression. There was no significant correlation between Fas and Fas ligand expression in the DP tumors, suggesting that Fas Ligand expression was not the driving force behind Fas down-regulation. These results suggest that both the Fas death receptor and mitochondrial pathways of apoptotic death are active in M-MuLV-induced tumors and that they must be modulated to permit cell survival and tumor outgrowth.

Appearance of mink cell focus-inducing recombinants during in vivo infection by moloney murine leukemia virus (M-MuLV) or the Mo+PyF101 M-MuLV enhancer variant: implications for sites of generation and roles in leukemogenesis

One hallmark of murine leukemia virus (MuLV) leukemogenesis in mice is the appearance of env gene recombinants known as mink cell focus-inducing (MCF) viruses. The site(s) of MCF recombinant generation in the animal during Moloney MuLV (M-MuLV) infection is unknown, and the exact roles of MCF viruses in disease induction remain unclear. Previous comparative studies between M-MuLV and an enhancer variant, Mo+PyF101 MuLV, suggested that MCF generation or early propagation might take place in the bone marrow under conditions of efficient leukemogenesis. Moreover, M-MuLV induces disease efficiently following both intraperitoneal (i.p.) and subcutaneous (s.c.) inoculation but leukemogenicity by Mo+PyF101 M-MuLV is efficient following i.p. inoculation but attenuated upon s. c. inoculation. Time course studies of MCF recombinant appearance in the bone marrow, spleen, and thymus of wild-type and Mo+PyF101 M-MuLV i.p.- and s.c.-inoculated mice were carried out by performing focal immunofluorescence assays. Both the route of inoculation and the presence of the PyF101 enhancer sequences affected the patterns of MCF generation or early propagation. The bone marrow was a likely site of MCF recombinant generation and/or early propagation following i.p. inoculation of M-MuLV. On the other hand, when the same virus was inoculated s.c., the primary site of MCF generation appeared to be the thymus. Also, when Mo+PyF101 M-MuLV was inoculated i.p., MCF generation appeared to occur primarily in the thymus. The time course studies indicated that MCF recombinants are not involved in preleukemic changes such as splenic hyperplasia. On the other hand, MCFs were detected in tumors from Mo+PyF101 M-MuLV s. c.-inoculated mice even though they were largely undetectable at preleukemic times. These results support a role for MCF recombinants late in disease induction.

Transcriptional Targeting of Retroviral Vectors to the Erythroblastic Progeny of Transduced Hematopoietic Stem Cells

Targeted expression to specific tissues or cell lineages is a necessary feature of a gene therapy vector for many clinical applications, such as correction of hemoglobinopathies or thalassemias by transplantation of genetically modified hematopoietic stem cells. We developed retroviral vectors in which the constitutive viral enhancer in the U3 region of the 3′ LTR is replaced by an autoregulatory enhancer of the erythroid-specific GATA-1 transcription factor gene. The replaced enhancer is propagated to the 5′ LTR upon integration into the target cell genome. The modified vectors were used to transduce human hematopoietic cell lines, cord blood-derived CD34+ stem/progenitor cells, and murine bone marrow repopulating stem cells. The expression of appropriate reporter genes (▵LNGFR, EGFP) was analyzed in the differentiated progeny of transduced stem cells in vitro, in liquid culture as well as in clonogenic assay, and in vivo, after bone marrow transplantation in lethally irradiated mice. The GATA-1 autoregulatory enhancer effectively restricts the expression of the LTR-driven proviral transcription unit to the erythroblastic progeny of both human progenitors and mouse-repopulating stem cells. Packaging of viral particles, integration into the target genome, and stability of the integrated provirus are not affected by the LTR modification. Enhancer replacement is therefore an effective strategy to target expression of a retroviral transgene to a specific progeny of transduced hematopoietic stem cells.

Moloney murine leukemia virus-induced preleukemic thymic atrophy and enhanced thymocyte apoptosis correlate with disease pathogenicity

Moloney murine leukemia virus (M-MuLV) is a replication-competent, simple retrovirus that induces T-cell lymphoma with a mean latency of 3 to 4 months. During the preleukemic period (4 to 10 weeks postinoculation) a marked decrease in thymic size is apparent for M-MuLV-inoculated mice in comparison to age-matched uninoculated mice. We were interested in studying whether the thymic regression was due to an increased rate of thymocyte apoptosis in the thymi of M-MuLV-inoculated mice. Neonatal NIH/Swiss mice were inoculated subcutaneously (s.c.) with wild-type M-MuLV (approximately 10(5) XC PFU). Mice were sacrificed at 4 to 11 weeks postinoculation. Thymic single-cell suspensions were prepared and tested for apoptosis by two-parameter flow cytometry. Indications of apoptosis included changes in cell size and staining with 7-aminoactinomycin D or annexin V. The levels of thymocyte apoptosis were significantly higher in M-MuLV-inoculated mice than in uninoculated control animals, and the levels of apoptosis were correlated with thymic atrophy. To test the relevance of enhanced thymocyte apoptosis to leukemogenesis, mice were inoculated with the Mo+PyF101 enhancer variant of M-MuLV. When inoculated intraperitoneally, a route that results in wild-type M-MuLV leukemogenesis, mice displayed levels of enhanced thymocyte apoptosis comparable to those seen with wild-type M-MuLV. However, in mice inoculated s.c., a route that results in attenuated leukemogenesis, significantly lower levels of apoptosis were observed. This supported a role for higher levels of thymocyte apoptosis in M-MuLV leukemogenesis. To examine the possible role of mink cell focus-forming (MCF) recombinant virus in raising levels of thymocyte apoptosis, MCF-specific focal immunofluorescence assays were performed on thymocytes from preleukemic mice inoculated with M-MuLV and Mo+PyF101 M-MuLV. The results indicated that infection of thymocytes by MCF virus recombinants is not required for the increased level of apoptosis and thymic atrophy.

Moloney murine leukemia virus infects cells of the developing hair follicle after neonatal subcutaneous inoculation in mice

The nature of Moloney murine leukemia virus (M-MuLV) infection after a subcutaneous (s.c.) inoculation was studied. We have previously shown that an enhancer variant of M-MuLV, Mo+PyF101 M-MuLV, is poorly leukemogenic when used to inoculate mice s.c., but not when inoculated intraperitoneally. This attenuation of leukemogenesis correlated with an inability of Mo+PyF101 M-MuLV to establish infection in the bone marrow of mice at early times postinfection. These results suggested that a cell type(s) is infected in the skin by wild-type but not Mo+PyF101 M-MuLV after s.c. inoculation and that this infection is important for the delivery of infection to the bone marrow, as well as for efficient leukemogenesis. To determine the nature of the cell types infected by M-MuLV and Mo+PyF101 M-MuLV in the skin after a s.c. inoculation, immunohistochemistry with an anti-M-MuLV CA antibody was performed. Cells of developing hair follicles, specifically cells of the outer root sheath (ORS), were extensively infected by M-MuLV after s.c. inoculation. The Mo+PyF101 M-MuLV variant also infected cells of the ORS but the level of infection was lower. By Western blot analysis, the level of infection in skin by Mo+PyF101 M-MuLV was approximately 4- to 10-fold less than that of wild-type M-MuLV. Similar results were seen when a mouse keratinocyte line was infected in vitro with both viruses. Cells of the ORS are a primary target of infection in vivo, since a replication defective M-MuLV-based vector expressing beta-galactosidase also infected these cells after a s.c. inoculation.

Identification of directly infected cells in the bone marrow of neonatal moloney murine leukemia virus-infected mice by use of a moloney murine leukemia virus-based vector

Early bone marrow infection of Moloney murine leukemia virus (M-MuLV)-infected mice was studied. Previous experiments indicated that early bone marrow infection is essential for the efficient development of T lymphoma. In order to identify the cellular pathway of infection in the bone marrow, infection of mice with a helper-free replication-defective M-MuLV-based retroviral vector was carried out. Such a vector will undergo only one round of infection, without spreading to other cells; thus, cells infected by the initially injected virus (directly infected cells) can be identified. For these experiments, the BAG vector that expresses bacterial beta-galactosidase was employed. Neonatal NIH/Swiss mice were inoculated intraperitoneally with ca. 10(6) infectious units of a BAG vector pseudotyped with M-MuLV proteins, and bone marrow cells were recovered 2 to 12 days postinfection. Single-cell suspensions were tested for infection by staining with X-Gal (5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside) or by immunofluorescence with an anti-beta-galactosidase antibody. Two sizes of infected cells were evident: large multinucleated cells and small nondescript (presumptively hematopoietic) cells. Secondary stains for lineage-specific markers indicated that the large cells were osteoclasts. Some of the small cells expressed nonspecific esterase, which placed them in the myeloid lineage, but they lacked markers for hematopoietic progenitors (mac-1, gr-1, sca-1, and CD34). These results provide evidence for primary M-MuLV infection of osteoclasts or osteoclast progenitors in the bone marrow, and they suggest that known hematopoietic progenitors are not primary targets for infection. However, the subsequent spread of infection to hematopoietic progenitors was indicated, since bone marrow from mice infected in parallel with replication-competent wild-type M-MuLV showed detectable infection in small cells positive for mac-1 or CD34, as well as in osteoclasts.

Second gene expression in bicistronic constructs using short synthetic intercistrons and viral IRES sequences

In this study, we describe the efficiency of second gene translation in bicistronic constructs containing either a short (36bp) synthetic intercistron or known internal ribosomal entry sites (IRES). Experiments were performed using two different gene combinations: Herpes simplex virus-thymidine kinase (HSV-TK) and neomycine (NEO) or human glucocerebrosidase (hGC) and a methotrexate (MTX) resistant mutant dihydrofolate reductase (DHFR). We demonstrate that upon transfection, second gene translation is efficient using either an IRES or a 36-bp intercistron. Infection with retrovirus carrying the TK and NEO genes linked via a 36-bp intercistron resulted in both G418R (NEO expression) and gancyclovir (GCV) sensitivity (TK expression), indicating that both genes were expressed and thus that the genomic DNA and RNA of this bicistronic construct were intact. Likewise, retrovirus carrying the hGC and mutant DHFR gene separated by a short intercistron was harvested from MTXR murine PsiCRE cells. However, infection of PA317 cells with this virus supernatant did not result in the presence of hGC enzyme activity in these murine cells. Proviral DNA and RNA analyses indicated that the hGC coding region was lost from the original construct in the infected PA317 cells. In contrast, retrovirus carrying the hGC and DHFR cDNAs was linked via an IRES functioned as expected. Based on these results, we conclude that the efficiency of second gene translation using short synthetic intercistrons might prove useful in bicistronic constructs, depending on the gene combination used.

Differential behavior of the Mo + PyF101 enhancer variant of Moloney murine leukemia virus in rats and mice

The Mo + PyF101 enhancer variant of Moloney murine leukemia virus (M-MuLV) has been very useful in investigating M-MuLV leukemogenesis. When inoculated subcutaneously (s.c.) into neonatal mice, Mo + PyF101 M-MuLV is attenuated for development of disease. Previous studies in mice infected with wild-type M-MuLV have revealed several important preleukemic events, including development of splenic hyperplasia, defects in bone marrow hematopoiesis, and in vivo generation of MCF viruses that arise by recombination in the uninfected mouse. Mo + PyF101 M-MuLV is defective in inducing these effects after s.c. inoculation. In the experiments reported here, a study of Mo + PyF101 M-MuLV infection in rats was carried out. Wild-type M-MuLV is leukemogenic in rats, but infected rats do not form MCF recombinants since they lack the necessary endogenous polytropic envelope sequences. Since Mo + PyF101 M-MuLV's leukemogenic defect is correlated with a failure to generate MCF recombinants, it seemed possible that wild-type M-MuLV might not have a leukemogenic advantage over Mo + PyF101 M-MuLV in rats, where MCF recombinants cannot form. Neonatal Fisher F344 rats were inoculated s.c. or intraperitoneally by wild-type and Mo + PyF101 M-MuLVs. Surprisingly, Mo + PyF101 M-MuLV was completely deficient in leukemogenesis in rats when inoculated by either route while wild-type M-MuLV induced lymphoma with the predicted time course. The leukemogenic defect for Mo + PyF101 M-MuLV resulted from a pronounced defect for establishing infection in rats. Further studies of wild-type M-MuLV in rats indicated that infection was confined almost exclusively to the thymus at early times. In mice wild-type M-MuLV establishes substantial infection in other hematopoietic organs such as spleen and bone marrow as well. Thymic infection was also correlated with a decrease in thymic cellularity at early times.

Moloney murine leukemia virus long terminal repeat activates monocyte chemotactic protein-1 protein expression and chemotactic activity

Moloney murine leukemia virus (Mo-MuLV) is a thymotropic and leukemogenic retrovirus which causes T lymphomas. Recently, Mo-MuLV has been shown to trans-activate cellular genes. Monocyte chemoattractant protein-1 (MCP-1) is a chemokine which can promote the migration and diapedesis of monocytes and lymphocytes, as well as inducing metastasis of lymphomas. Here we demonstrate that introduction of Mo-MuLV or the MuLV LTR alone, transiently or stably, into Balb/c-3T3 cells or HeLa cells resulted in 9-11 fold increases in MCP-1 transcripts. This trans-activation of the MCP-1 gene by the Mo-MuLV LTR is independent of the physical location of the MCP-1 gene or of the LTR, occurring whether the LTR or the MCP-1 gene is integrated in the genome or transiently expressed. Immunoblot analysis using an anti-MCP-1 polyclonal antibody showed that the expression of the MuLV LTR in HeLa cells also induced the appearance of the MCP-1 protein. Boyden Chamber analysis demonstrated that the MCP-1 chemotactic activity produced by HeLa cells with an integrated MuLV LTR was elevated by 11 fold and that neutralizing antibody to human MCP-1 abrogated monocyte migration in response to MuLV LTR expression. Promoter deletional analysis showed the LTR responsive cis-acting element in the MCP-1 promoter is located between -141 and -88. Deletion of this region abolished the trans-activation of MCP-1 by the LTR. These LTR-mediated activations of a chemotactic and inflammatory cytokine may be relevant as mechanisms whereby retroviruses which do not contain oncogenes can induce neoplasia.

Low-frequency loss of heterozygosity in Moloney murine leukemia virus-induced tumors in BRAKF1/J mice

To identify potential involvement of tumor suppressor gene inactivation during leukemogenesis by Moloney murine leukemia virus (M-MuLV), a genome-wide scan for loss of heterozygosity (LOH) in tumor DNAs was made. To assess LOH, it is best to study mice that are heterozygous at many loci across the genome. Accordingly, we generated a collection of 52 M-MULV-induced tumor DNAs from C57BR/cdJ x AKR/J F1 (BRAKF1) hybrid mice. By using direct hybridization with oligonucleotides specific for three different classes of endogenous MuLV-related proviruses, 48 markers on 16 of 19 autosomes were simultaneously examined for allelic loss. No allelic losses were detected, with the exception of a common loss of markers on chromosome 4 in two tumors. The three autosomes that lacked informative endogenous proviral markers were also analyzed for LOH by PCR with simple-sequence length polymorphisms (SSLPs); one additional tumor showed LOH on chromosome 15. Further screening with chromosome 4 SSLPs identified one additional tumor with LOH on chromosome 4. Therefore, in total, the average fractional allelic loss was quite low (0.002), but the LOH frequency of 6% on chromosome 4 was highly statistically significant (P < 0.0005). Detailed SSLP mapping of the three tumors with LOH on chromosome 4 localized the region of common LOH to the distal 45 centimorgans, a region syntenic with human chromosomes 1 and 9. Candidate tumor suppressor genes, Mts1 (p16INK4a) and Mts2 (p15INK4b), have been mapped to this region, but by Southern blot analysis, no homozygous deletions were detected in either gene. One of three tumors with LOH on chromosome 4 also showed a proviral insertion near the c-myc proto-oncogene. These results suggested that tumor suppressor inactivation is generally infrequent in M-MuLV-induced tumors but that a subset of these tumors may have lost a tumor suppressor gene on chromosome 4.

Leukemogenesis by Moloney murine leukemia virus: a multistep process

Moloney murine leukemia virus is a prototypical simple retrovirus that has been an extremely useful model for leukemogenesis. Important steps in leukemogenesis include proviral activation of cellular proto-oncogenes, generation of mink cell focus-inducing recombinants, and early (preleukemic) virus-induced changes in hematopoiesis.

Gene transfer into hematopoietic stem cells of nonhuman primates

Nonhuman primates provide an appropriate preclinical large-animal model to test the efficacy of bone marrow gene therapy procedures. Successful retroviral vector-mediated gene transfer into monkey pluripotent hematopoietic stem cells (PHSC) has closed the gap between gene transfer experiments in mouse models and clinical application of bone marrow gene therapy. After initial bone marrow transplant failures, ex vivo bone marrow culture conditions were found that sufficiently supported maintenance of the long-term repopulating ability of genetically modified autologous monkey grafts. The efficiency of gene transfer into primate PHSC has, however, remained at least one order of magnitude lower than has been achieved in mice. Similar gene transfer efficiencies have been obtained with total bone marrow grafts, CD34+ bone marrow grafts, and mobilized peripheral blood progenitor cell grafts; however, various attempts to increase the transduction efficiency have been without significant success. Primate PHSC seem to require quite different culture conditions for their maintenance and transduction than mouse PHSC, in particular regarding hematopoietic growth factor addition. In contrast to observations in other species, some form of conditioning appeared essential for engraftment of transduced PHSC in monkeys. Although it has been shown that mouse retroviruses can replicate in monkeys and are capable of inducing neoplasms, experiments in monkeys have sufficiently confirmed the safety of current gene transfer procedures to allow their clinical application.

A transcript from the long terminal repeats of a murine retrovirus associated with trans activation of cellular genes

Infection of human or murine cells with murine leukemia viruses rapidly increases the expression of a number of genes that belong to the immunoglobulin superfamily and are involved in T-lymphocyte activation, including the class I major histocompatibility complex antigens. We have reported recently that the long terminal repeat (LTR) of Moloney murine leukemia virus encodes a trans activator which induces transcription and expression of class I major histocompatibility complex genes and certain cytokine genes. The portion of the LTR responsible for trans activation was mapped by deletions to lie within the U3 region. We demonstrate here that a transcript is initiated within the U3 region and that its presence correlates with the trans-activating activity. Analysis of the LTR region reveals a potential internal promoter element for RNA polymerase III transcription within the U3 region. Studies with polymerase inhibitors suggest that this LTR transcript, designated let (LTR-encoded trans activator), is a product of RNA polymerase III. The mechanisms whereby RNA leukemia viruses cause lymphoid neoplasia after a long latent period have been extensively studied but are only partially understood. The region of the LTR identified here as being important in trans activation has recently been shown to be a critical determinant of the leukemogenicity and latency of Moloney murine leukemia virus. These findings suggest a novel mechanism of retrovirus-induced activation of cellular gene expression, potentially contributing to leukemogenesis.

Gene therapy in peripheral blood lymphocytes and bone marrow for ADA- immunodeficient patients

Adenosine deaminase (ADA) deficiency results in severe combined immunodeficiency, the first genetic disorder treated by gene therapy. Two different retroviral vectors were used to transfer ex vivo the human ADA minigene into bone marrow cells and peripheral blood lymphocytes from two patients undergoing exogenous enzyme replacement therapy. After 2 years of treatment, long-term survival of T and B lymphocytes, marrow cells, and granulocytes expressing the transferred ADA gene was demonstrated and resulted in normalization of the immune repertoire and restoration of cellular and humoral immunity. After discontinuation of treatment, T lymphocytes, derived from transduced peripheral blood lymphocytes, were progressively replaced by marrow-derived T cells in both patients. These results indicate successful gene transfer into long-lasting progenitor cells, producing a functional multilineage progeny.

A retroviral vector containing a muscle-specific enhancer drives gene expression only in differentiated muscle fibers

Genetically modified myogenic cells have a number of potentially relevant applications for gene therapy of genetic defects. Retroviral vectors proved to be a safe and efficient tool to transfer and express genes into satellite cells and their differentiated progeny, although muscle-specific regulation of the transferred gene is very difficult to achieve in a conventional vector framework. We modified a Moloney murine leukemia virus (MoMLV)-derived retroviral vector containing a bacterial beta-galactosidase (beta-Gal) reporter gene by inserting a muscle creatinine kinase (MCK) enhancer element into the U3 region of the viral long terminal repeat (LTR). The resulting vector (mLBSN) was transferred into cells of different histological origin, including undifferentiated murine and human myogenic cells, which were unable to express the transgene at detectable levels. Instead, gene expression from the modified LTR was obtained in a mouse myogenic cell line and in human primary satellite cells upon induction of differentiation into myotubes in culture, and correlated with the activation of the muscle differentiation program. beta-Gal-negative, mLBSN-transduced human satellite cells were also transplanted into the quadricep muscle of immunodeficient mice, where activation of the transgene expression was observed in vivo after differentiation and fusion into muscle fibers. These results show that retroviral vectors carrying LTRs modified in the enhancer sequences may be used to target tissue- and differentiation-specific gene expression into the muscle. For practical purposes, satellite cells engineered by muscle-specific retroviral vectors might represent an effective tool to deliver expression of a given gene product specifically into the muscle tissue, avoiding undesired protein accumulation in mononucleated cells. More generally, this type of vector might be useful whenever regulated expression of a transferred gene is necessary in a target cell or tissue.

Recombinant mink cell focus-inducing virus and long terminal repeat alterations accompany the increased leukemogenicity of the Mo+PyF101 variant of Moloney murine leukemia virus after intraperitoneal inoculation

We recently showed that different routes of inoculation affect the leukemogenicity of the Mo+PyF101 variant of Moloney murine leukemia virus (M-MuLV). Intraperitoneal (i.p.) inoculation of neonatal mice with Mo+PyF101 M-MuLV greatly enhanced its leukemogenicity compared with subcutaneous (s.c.) inoculation. We previously also suggested that the leukemogenicity defect of Mo+PyF101 M-MuLV when inoculated s.c. may result from the inability of this virus to form env gene recombinant (mink cell focus-inducing [MCF]) virus. In this study, virus present in end-stage tumors and in preleukemic animals inoculated i.p. by Mo+PyF101 M-MuLV was characterized. In contrast to s.c. inoculation, all tumors from i.p.-inoculated mice contained high levels of recombinant MCF virus. Furthermore, Southern blot analyses demonstrated that the majority of the tumors contained altered Mo+PyF101 M-MuLV long terminal repeats. The U3 regions from several tumors with altered long terminal repeats were cloned by PCR amplification. Sequence analyses indicated that the M-MuLV 75-bp tandem repeat in the enhancer region was triplicated. This amplification was also previously observed in mice infected s.c. with a pseudotypic mixture of Mo+PyF101 M-MuLV and Mo+PyF101 MCF virus. The enhancer triplication was an early event, and it occurred within 2 weeks postinfection. Recombinant MCF viruses were not detected by Southern blot analyses until 4 weeks postinfection. Thus, the M-MuLV enhancer triplication event was initially important for efficient propagation of ecotropic Mo+PyF101 M-MuLV. The increased leukemogenicity following i.p. inoculation could be explained if the triplication enhances Mo+PyF101 M-MuLV replication in the bone marrow and bone marrow infection is required for recombinant MCF virus formation.

Long terminal repeat enhancer core sequences in proviruses adjacent to c-myc in T-cell lymphomas induced by a murine retrovirus

The transcriptional enhancer in the long terminal repeat (LTR) of the T-lymphomagenic retrovirus SL3-3 differs from that of the nonleukemogenic virus Akv at several sites, including a single base pair difference in an element termed the enhancer core. Mutation of this T-A base pair to the C-G C-G sequence found in Akv significantly attenuated the leukemogenicity of SL3-3. Thus, this difference is important for viral leukemogenicity. Since Akv is an endogenous virus, this suggests that the C-G in its core is an adaptation to being minimally pathogenic. Most tumors that occurred in mice inoculated with the mutant virus, called SAA, contained proviruses with reversion or potential suppressor mutations in the enhancer core. We also found that the 72-bp tandem repeats constituting the viral enhancer could vary in number. Most tumors contained mixtures of proviruses with various numbers of 72-bp units, usually between one and four. Variation in repeat number was most likely due to recombination events involving template misalignment during viral replication. Thus, two processes during viral replication, misincorporation and recombination, combined to alter LTR enhancer structure and generate more pathogenic variants from the mutant virus. In SAA-induced tumors, enhancers of proviruses adjacent to c-myc had the largest number of core reversion or suppressor mutations of all of the viral enhancers in those tumors. This observation was consistent with the hypothesis that one function of the LTR enhancers in leukemogenesis is to activate proto-oncogenes such as c-myc.

The leukemogenic potential of an enhancer variant of Moloney murine leukemia virus varies with the route of inoculation

We previously showed that the Mo+PyF101 variant of Moloney murine leukemia virus (M-MuLV) is poorly leukemogenic when inoculated subcutaneously (s.c.) into neonatal mice. We recently found that intraperitoneal (i.p.) inoculation of neonatal mice with the same virus significantly enhanced its leukemogenicity. In this study, infections of neonatal mice by the two different routes of inoculation were compared. We studied replication of the virus in vivo to identify critical preleukemic events. These would be observed in mice inoculated i.p. by Mo+PyF101 M-MuLV but not when inoculation was s.c. Infectious center assays indicated that regardless of the route of inoculation, Mo+PyF101 M-MuLV showed delayed infection of the thymus compared with wild-type M-MuLV. On the other hand, i.p.-inoculated mice showed more rapid appearance of infectious centers in the bone marrow than did s.c.-inoculated animals. Thus, the enhanced leukemogenicity of i.p. inoculation correlated with efficient early infection of the bone marrow and not with early infection of the thymus. These results suggest a role for bone marrow infection for efficient leukemogenesis in Mo+PyF101 M-MuLV-infected mice. Consistent with this notion, if bone marrow infection was decreased by injecting 10- to 12-day-old animals i.p., leukemogenicity resembled that of s.c. inoculation. Thus, two cell types that are critical for the induction of efficient leukemia were implicated. One cell delivers virus from the site of s.c. inoculation (the skin) to the bone marrow and is apparently restricted for Mo+PyF101 M-MuLV replication. The second cell is in the bone marrow, and its early infection is required for efficient leukemogenesis.

Escape from in vivo restriction of Moloney mink cell focus-inducing viruses driven by the Mo+PyF101 long terminal repeat (LTR) by LTR alterations

Mo+PyF101 M-MuLV is a variant Moloney murine leukemia virus containing polyomavirus F101 enhancers inserted just downstream from the M-MuLV enhancers in the long terminal repeat (LTR). The protein coding sequences for this virus are identical to those of M-MuLV. Mo+PyF101 M-MuLV induces T-cell disease with a much lower incidence and longer latency than wild-type M-MuLV. We have previously shown that Mo+PyF101 M-MuLV is defective in preleukemic events induced by wild-type M-MuLV, including splenic hematopoietic hyperplasia, bone marrow depletion, and generation of recombinant mink cell focus-inducing viruses (MCFs). We also showed that an M-MCF virus driven by the Mo+PyF101 LTR is infectious in vitro but does not propagate in mice. However, in these experiments, when a pseudotypic mixture of Mo+PyF101 M-MuLV and Mo+PyF101 MCF was inoculated into newborn NIH Swiss mice, they died of T-cell leukemia at times almost equivalent to those induced by wild-type M-MuLV. Tumor DNAs from Mo+PyF101 M-MuLV-Mo+PyF101 MCF-inoculated mice were examined by Southern blot analysis. The predominant forms of Mo+PyF101 MCF proviruses in these tumors contained added sequences in the U3 region of the LTR. The U3 regions of representative tumor-derived variant Mo+PyF101 MCFs were cloned by polymerase chain reaction amplification, and sequencing indicated that they had acquired an additional copy of the M-MuLV 75-bp tandem repeat in the enhancer region. NIH 3T3 cell lines infected with altered viruses were obtained from representative Mo+PyF101 M-MuLV-Mo+PyF101 MCF-induced tumors, and mice were inoculated with the recovered viruses. Leukemogenicity was approximately equivalent to that in the original Mo+PyF101 M-MuLV-Mo+PyF101 MCF viral stock. Southern blot analysis on the resulting tumors now predominantly revealed loss of the polyomavirus sequences. These results suggest that the suppressive effects of the PyF101 sequences on M-MuLV-induced disease and potentially on MCF propagation were overcome in two ways: by triplication of the M-MuLV direct repeats and by loss of the polyomavirus sequences.

Infection of human cells with murine amphotropic replication-competent retroviruses

Replication of the murine wild-type 4070A amphotropic retrovirus and a recombinant amphotropic replication-competent retrovirus arising from the PA12 packaging cell line varied considerably among the primate cell types tested. Medium from infected primate fibroblasts and endothelial cells contained the highest viral titers [10(4)-10(5) focus-forming units (ffu)/ml], while most hematopoietic cell lines, such as K562 and MOLT4, were associated with viral titers in the range of 10(3)-10(4) ffu/ml. Interestingly, HTLV-1-transformed T cell lines (TJF-2 and HM) and primary tumor infiltrating lymphocytes (TIL) had very low viral titer (0-10(1) ffu/ml). The low production of virus was not due to low infectivity and, in contrast to the virus, retroviral vectors were expressed without difficulty. Because screening for replication-competent retrovirus (RCR) is an important component of human retroviral-mediated gene therapy clinical protocols, a variety of assays were tested for their ability to detect RCR in virus-exposed cell lines. A biologic assay (3T3 amplification) and polymerase chain reaction (PCR) for the 4070A viral envelope are effective screening methods for RCR, even in cell lines associated with low virus production.

Human immunodeficiency viruses containing heterologous enhancer/promoters are replication competent and exhibit different lymphocyte tropisms

The human immunodeficiency virus (HIV) type 1 long terminal repeat (LTR) contains binding sites for nuclear factor kappa B (NF-kappa B) and the constitutively expressed transcription factor Sp1, both of which are highly conserved in HIV and simian immunodeficiency virus isolates. To delineate the effects of these motifs on the replicative capacity of HIV and to explore the possibility of extending the virus host range, known heterologous enhancer/promoters were inserted into the HIV-1 LTR in place of the NF-kappa B and Sp1 binding sites. The effects of these substitutions on viral replication in transfected HeLa cells and on HIV infection of human peripheral blood lymphocytes or continuous T-leukemia cell lines were evaluated. HIVs in which the NF-kappa B/Sp1 enhancer plus the downstream TATA element were replaced with heterologous enhancer/promoters were also constructed. Viruses containing the human cytomegalovirus immediate-early enhancer exhibited infectious kinetics similar to that of wild-type HIV in activated human peripheral blood lymphocytes and AA2 cells but replicated less efficiently in H9 and CEM cells. These studies indicate that heterologous enhancer elements are capable of restoring Tat responsiveness to the HIV LTR in the context of directing reporter gene expression as well as in the production of infectious progeny virions.

Amphotropic retroviruses with a hybrid long terminal repeat as a tool for gene therapy of cystic fibrosis

We have made two retroviral vectors encoding the bacterial beta-galactosidase (lacZ) as a marker gene and a long terminal repeat (LTR) containing an enhancer of the polyoma F101 virus [symbol: see text]. One vector, [symbol: see text], can be used as a test vector in grafting, lineage analysis and gene therapy studies. The other, [symbol: see text] carries an additional unique cloning site in which a gene of interest can be cloned. Titration experiments showed that in human epithelial cell lines, [symbol: see text] produces a transcriptionally active integration more often than the commonly used BAG vector with the wild type LTR. Human epithelial cells in primary culture could be successfully infected. Our data suggest that gene therapy protocols requiring infection in situ, such as in the case of cystic fibrosis, will be hampered by the relatively low local titres that can be achieved at present.

Genetic correction of cultured T cells from an adenosine deaminase-deficient patient: characteristics of non-transduced and transduced T cells

T lymphocytes derived from peripheral blood of a patient with adenosine deaminase (ADA) deficiency were expanded in vitro. The human ADA (hADA) gene was introduced into these replicating ADA- T cells with the use of an amphotropic recombinant retrovirus carrying the hADA gene. Subsequently, infected T cells were selected on the basis of their ADA expression, by exposure to a combination of the toxic agent xylofuranosyl-adenine and the specific ADA inhibitor 2'-deoxycoformycin. CD4+ and CD8+ T cells could be infected and selected with equal efficiencies. The genetically modified T cells were shown to contain intact copies of the provirus and to express normal levels of hADA. As expected, uninfected T cells from the ADA-deficient patient displayed an increased sensitivity to 2'-deoxyadenosine. Following genetic modification, however, this sensitivity was restored to normal levels in both CD4+ and CD8+ T cells. The introduction of the hADA gene into the genome of the in vitro expanded T cells did not alter their phenotype, proliferative capacity or cytotoxic potential. These characteristics were identical to those of T cells derived from healthy individuals. These findings are of critical importance for the clinical application of hADA gene-transducted T cells.

Acquisition of responsiveness to chemical carcinogens by rodent embryo fibroblasts expressing high levels of the c-myc proto-oncogene

We investigated the ability of overexpression of the c-myc proto-oncogene to potentiate in vitro transformation by model chemical carcinogens. A mouse c-myc gene was introduced to C3H 10T1/2 and Rat 6 embryo fibroblast cell lines via a retroviral vector containing the gene for neomycin resistance. Our present work extends previous findings by showing that individual vectored C3H 10T1/2 clones have enhanced (two-fold to sevenfold) sensitivity to benzo[a]pyrene (BP) and N-methyl-N-nitro-N'-nitrosoguanidine (MNNG). Rat 6 clones acquiring the c-myc gene display various degrees of altered morphology. They form orderly but densely packed cells, grow to higher saturation density, and yield microcolonies in soft agar. The degree of altered growth properties is directly correlated with the level of c-myc expression. Transient exposure of c-myc-expressing clones to BP and MNNG induced the formation of distinct, large colonies in soft agar, whereas the untreated cells formed microcolonies and the parental Rat 6 cells remained single cells in soft agar. We also demonstrated that the degree of responsiveness to chemical carcinogens of the clones correlates with their ability to form microcolonies in soft agar. These cells overexpressing c-myc may be used as a model system to study the interaction between oncogenes and chemical carcinogens in the process of multistage carcinogenesis.

Deletion of a GC-rich region flanking the enhancer element within the long terminal repeat sequences alters the disease specificity of Moloney murine leukemia virus

Moloney murine leukemia virus (M-MuLV) is a replication-competent retrovirus which induces T-lymphoblastic lymphoma 2 to 4 months after inoculation. Enhancer sequences in the U3 region of the M-MuLV long terminal repeat, primarily the 75-bp tandem repeats, strongly influence the disease specificity and latency of M-MuLV. We investigated the role of GC-rich sequences downstream of the tandem repeats in the disease specificity of M-MuLV. A recombinant M-MuLV lacking 23 bases of a GC-rich sequence (-174 to -151), Delta 27A M-MuLV, was tested for pathogenesis in neonatal NIH Swiss mice. Delta 27A M-MuLV induced disease with a longer latency than did M-MuLV (7 versus 3 months) in greater than 85% of inoculated mice. More interestingly, this virus showed an expanded repertoire of hematopoietic diseases. Molecular analyses and histopathologic examinations indicated that while 39% of mice inoculated with Delta 27A M-MuLV developed T-cell lymphoblastic lymphoma typical of wild-type M-MuLV, the majority developed acute myeloid leukemia, erythroleukemia, or B-cell lymphoma. Viral DNA corresponding to Delta 27A M-MuLV was detectable in most of the tumors analyzed. These findings indicate that the GC-rich region significantly influences the disease specificity and latency of M-MuLV.

Bone marrow depletion by 89Sr complements a preleukemic defect in a long terminal repeat variant of Moloney murine leukemia virus

We previously described a preleukemic state induced by Moloney murine leukemia virus (Mo-MuLV) characterized by hematopoietic hyperplasia in the spleen. Further experiments suggested that splenic hyperplasia results from inhibitory effects in the bone marrow, leading to compensatory extramedullary hematopoiesis. An enhancer variant of Mo-MuLV, Mo + PyF101 Mo-MuLV, fails to induce preleukemic hyperplasia and has greatly reduced leukemogenicity, indicating the importance of this state to efficient leukemogenesis. An alternative method for induction of preleukemic hyperplasia was sought. Treatment of mice with 89Sr causes specific ablation of bone marrow hematopoiesis and compensatory extramedullary hematopoiesis in spleen and nodes. NIH Swiss mice were inoculated neonatally with Mo + PyF101 Mo-MuLV and treated with 89Sr at 6 weeks of age. Approximately 85% developed lymphoid leukemia with a time course resembling that caused by wild-type Mo-MuLV. In contrast, very few animals treated with Mo + PyF101 Mo-MuLV or 89Sr alone developed disease. In approximately one-third of cases, the Mo + PyF101 Mo-MuLV proviruses were found at common sites for wild-type Mo-MuLV-induced tumors (c-myc, pvt-1, and pim-1), indicating that this virus is capable of performing insertional activation in T-lymphoid cells. These results support the proposal that splenic hyperplasia results from inhibitory effects in the bone marrow. They also indicate that Mo + PyF101 Mo-MuLV is blocked in early and not late events in leukemogenesis.

Differential disease restriction of Moloney and Friend murine leukemia viruses by the mouse Rmcf gene is governed by the viral long terminal repeat

Neonatal CxD2 (Rmcfr) and Balb/c (Rmcfs) mice inoculated with Moloney murine leukemia virus (M-MuLV) exhibited approximately equivalent time course and pathology for disease. CxD2 mice showed only slightly reduced presence of Moloney mink cell focus-forming virus (M-MCF) provirus as seen by Southern blot analysis compared to Balb/c mice. This lack of restriction for disease and spread of MCF was in sharp contrast to that seen for CxD2 mice inoculated with Friend murine leukemia virus (F-MuLV), where incidence of disease and propagation of MCFs were severely restricted, as previously reported. Inoculation of CxD2 mice with FM-MuLV, a recombinant F-MuLV virus containing M-MuLV LTR sequences (U3 and R), resulted in T cell disease of time course equal to that seen in Balb/c mice; there also was little restriction for propagation of MCFs. This indicated that presence of the M-MuLV long terminal repeat (LTR) was sufficient for propagation of MCFs in CxD2 mice. Differing restriction for F-MuLV vs. M-MuLV in CxD2 mice was explained on the basis of different "MCF propagator cells" for the two viruses. It was suggested that cells propagating F-MCF (e.g., erythroid progenitors) are blocked by endogenous MCF-like gp70env protein, whereas cells propagating M-MCF (e.g., lymphoid) do not express this protein on their surface. F-MuLV disease in CxD2 mice was greatly accelerated when neonates were inoculated with a F-MuLV/F-MCF pseudotypic mixture. However, F-MCF provirus was not detectable or only barely detectable in F-MuLV/F-MCF-induced tumors, suggesting that F-MCF acted indirectly in induction of these tumors.

Polyomavirus DNA replication in the pancreas and in a transformed pancreas cell line has distinct enhancer requirements

The regulatory DNA (enhancer) of polyomavirus (Py) is a major determinant of tissue-specific DNA replication during acute infection of newborn mice. Previously, we reported that the combination of one of the two Py enhancers (A enhancer) and the repeated Moloney murine leukemia virus (Mo-MuLV) enhancer gave a chimeric Py genome (Py-MuLV) that replicates predominantly in the acinar cells of the pancreas, a tissue not permissive for wild-type PyA2 replication (R. Rochford, B. A. Campbell, and L. P. Villareal. Proc. Nat. Acad. Sci. USA 84:449-453,1987). In this report, we further examine the combined enhancer requirements for acinar cell-specific Py replication. We also compare enhancer requirements for Py replication in the acinar cells of the pancreas with those of a transformed acinar cell line (266-6 cells). The deletion of sequences within the A enhancer of Py-MuLV (nucleotides 5098 to 5132) results in a virus with 10-fold-reduced levels of pancreas-specific replication. The deletion, however, of one of the 72-bp repeated Mo-MuLV enhancer sequences from Py-MuLV results in a complete loss of pancreas-specific DNA replication. Thus, the Py A enhancer is required for efficient replication of Py in the pancreas without otherwise altering organ specificity, but both of the repeated copies of the Mo-MuLV enhancer are essential for pancreas-specific Py replication. In contrast to the enhancer requirements for in vivo pancreas replication, in transformed acinar cells (266-6), PyA2 wild-type replicated efficiently and the Py-MuLV recombinant replicated inefficiently. These data suggest that the cell-specific control of DNA replication is different between normal pancreas cells and their transformed cell line counterparts and that this difference is apparent in the enhancer requirement of cell-specific Py DNA replication.

Safety issues related to retroviral-mediated gene transfer in humans

The first three approved human clinical trials utilizing retroviral-mediated gene transfer are now underway. While this technology holds great promise for the study and treatment of human disease, it also poses a number of safety concerns. In evaluating clinical protocols, potential complications and the likelihood of their occurrence are estimated by review committees so that a risk/benefit assessment can be made. Current knowledge, reviewed in this article, suggests that no acute complications secondary to retroviral-mediated gene transfer are likely, but the possibility of long-term or unforeseen sequelae in patients suggests the need for post-treatment monitoring.

An enhancer variant of Moloney murine leukemia virus defective in leukemogenesis does not generate detectable mink cell focus-inducing virus in vivo

Moloney murine leukemia virus (Mo-MuLV) induces T-cell lymphoma when inoculated into neonatal mice. This is a multistep process. Early events observed in infected mice include generalized hematopoietic hyperplasia in the spleen and appearance of mink cell focus-inducing (MCF) recombinants; end-stage tumors are characterized by insertional proviral activation of protooncogenes. We previously showed that an Mo-MuLV enhancer variant, Mo+PyF101 Mo-MuLV, has greatly reduced leukemogenicity and is deficient in induction of preleukemic hyperplasia. In this report, we have examined Mo+PyF101 Mo-MuLV-inoculated mice for the presence of MCF recombinants. In contrast to wild-type Mo-MuLV-inoculated mice, Mo+PyF101 Mo-MuLV-inoculated mice did not generate detectable MCF recombinants. This failure was at least partly due to an inability of the MCF virus to propagate in vivo, since a molecularly cloned infectious Mo+PyF101 MCF virus did not replicate, even when inoculated as a Mo+PyF101 Mo-MuLV pseudotype. These results show that the leukemogenic defect of Mo+PyF101 Mo-MuLV is associated with its inability to generate MCF recombinants capable of replication in vivo. This, in turn, is consistent with the view that MCF recombinants play a significant role in Mo-MuLV-induced disease and, in particular, may play a role early in the disease process.

Substitution of murine transthyretin (prealbumin) regulatory sequences into the Moloney murine leukemia virus long terminal repeat yields infectious virus with altered biological properties

The effects of inserting cellular regulatory sequences from the murine transthyretin (TTR) gene into the Moloney murine leukemia virus (M-MuLV) long terminal repeat (LTR) were investigated. Transthyretin is expressed predominantly in the liver and choroid plexus in adult mice, and TTR upstream regulatory elements were previously shown to potentiate transcription in liver-derived cells. The effects of inserting the TTR distal enhancer and/or promoter-proximal sequences into an M-MuLV LTR lacking its enhancers were measured in three ways. (i) Chimeric LTRs were fused to the bacterial chloramphenicol acetyltransferase gene (cat) and tested for transient gene expression by transfection into liver-derived cells or NIH 3T3 fibroblasts. (ii) Infectious M-MuLV containing an altered LTR [delta Mo + TTR(PD) MuLV) was generated, and infectivity in culture on hepatocyte lines and NIH 3T3 cells was tested. (iii) Infection of delta Mo + TTR(PD) MuLV in vivo was tested by inoculating NFS/N mice and performing in situ hybridization of whole animal sections. Chimeric LTR-cat constructs showed higher levels of cat gene expression in liver-derived cell lines than in NIH 3T3 cells, indicating increased LTR activity in these cells. However, in vitro infection did not show significantly higher infectivity in hepatocytes for delta Mo + TTR(PD) M-MuLV than did wild-type M-MuLV. In vivo, delta Mo + TTR(PD) MuLV showed expression in the same tissues as with wild-type M-MuLV-inoculated mice, i.e., lymphoid organs and the intestines and, additionally, two novel sites not seen in wild-type M-MuLV-inoculated animals. Of 10 mice, 8 showed viral expression in the brain and 3 showed expression in the liver. Thus, insertion of TTR elements into the M-MuLV LTR altered LTR activity both in vitro and in vivo.

Expression of human adenosine deaminase in mice transplanted with hemopoietic stem cells infected with amphotropic retroviruses

Amphotropic recombinant retroviruses were generated carrying sequences encoding human adenosine deaminase (ADA). Transcription of the human ADA gene was under control of a hybrid long terminal repeat in which the enhancer from the Moloney murine leukemia virus was replaced by an enhancer from the F101 host-range mutant of polyoma virus. Hemopoietic stem cells in murine bone marrow were infected with this virus under defined culture conditions. As a result, 59% of day-12 colony forming unit spleen (CFU-S) stem cells became infected without any in vitro selection. Infected CFU-S were shown to express human ADA before transplantation and this expression sustained upon in vivo maturation. Mice transplanted with infected bone marrow exhibited human ADA expression in lymphoid, myeloid, and erythroid cell types. Moreover, human ADA expression persisted in secondary and tertiary transplanted recipients showing that human ADA-expressing cells were derived from pluripotent stem cells. These characteristics of our amphotropic viruses make them promising tools in gene therapy protocols for the treatment of severe combined immunodeficiency caused by ADA deficiency. In this respect it is also relevant that the viral vector that served as backbone for the ADA vector was previously shown to be nonleukemogenic.

Polyomavirus-based shuttle vectors for studying mechanisms of mutagenesis in rodent cells

We have constructed a series of polyomavirus-based shuttle vectors for analyzing mechanisms of mutagenesis in rodent cell systems. These vectors contain the supF suppressor tRNA gene which serves as the mutagenesis target; the pBR327 replication functions and ampr gene for replication and selection in bacteria; and the polyomavirus genome which permits replication in rodent cells. The polyoma genomes used in these vectors vary in their enhancer regions, causing varying efficiencies of replication in different types of rodent cells. One of the vectors (pPySLPT-2) which replicates particularly well in several different rodent cell types (i.e., Chinese hamster ovary, mouse hepatoma and mouse lymphoma) was used to compare mutation induction by UV radiation in UV repair-deficient mouse lymphoma L5178Y-R cells with mutagenesis in the related UV repair-proficient line, L5178Y-S. In both cell types, UV-induced mutants could be recovered at frequencies up to 50-fold higher than that of the spontaneous background. At a given UV fluence the L5178Y-R cells were more highly mutable than the L5178Y-S cells. Our results indicate that these new polyomavirus-based vectors should be useful for analysis of the molecular mechanisms of mutation induction in rodent cell systems, and in particular should allow detailed analysis of mutagenesis in the well characterized rodent somatic cell mutants.