The location of v-src in a retrovirus vector determines whether the virus is toxic or transforming.

We prepared infectious stocks of an avian retrovirus, a modified spleen necrosis virus, containing the herpes simplex virus type 1 thymidine kinase gene and the avian sarcoma virus v-src gene. Viruses were recovered after cotransfection of chicken cells with DNA of recombinants between cloned spleen necrosis virus thymidine kinase and v-src and with DNA of cloned reticuloendotheliosis virus strain A. When v-src was inserted near the 5'end of the viral genome, only low titers of recombinant virus were recovered. Most of the recovered viruses were smaller than expected and did not transform the morphology of rat or chicken cells. A very small amount of virus of the expected structure was recovered; this virus transformed rat cells and expressed v-src. Cotransfection data indicated that one reason we failed to recover a significant titer of recombinant virus is that efficient expression of v-src is acutely toxic to chicken and dog cells. Insertion of v-src near the 3' end of the viral genome, such that it was expressed at a lower level compared with the 5'-v-src-containing virus, yielded a higher titer of recombinant virus, and this virus was transforming. The differences in the recovery and transforming activity of these viruses indicate that the location of an oncogene in the viral genome is an important factor regulating the level of its expression and whether or not this expression is toxic or transforming to cells.     

Expression of complete chicken thymidine kinase gene inserted in a retrovirus vector.

The chicken thymidine kinase (tk) gene was inserted into spleen necrosis virus. Thymidine kinase activity was expressed even when the promoter and terminator sequences for tk RNA synthesis were retained. When the promoter was present in the same orientation as the promoter in the long terminal repeat of the virus, deletions occurred both in the virus and in the tk gene, and the thymidine kinase-transforming activity of the recovered virus was low. Splicing of apparent intervening sequences in the tk gene was also observed. When the orientation of the tk promoter was opposite to the promoter in the long terminal repeat, virus synthesis was diminished, whereas thymidine kinase activity was expressed at an elevated level compared with virus in which the promoter was in the same orientation. However, when the apparent tk promoter was deleted from virus with the tk gene in the opposite orientation, a high level of virus synthesis was observed, probably as a result of absence of interference of RNA synthesis from converging promoters. The intervening sequences in the virus in which the promoters were in opposite orientation were not spliced.     

Construction of a helper cell line for avian reticuloendotheliosis virus cloning vectors.

We wished to construct cell lines that supply the gene products of gag, pol, and env for the growth of replication-defective reticuloendotheliosis retrovirus vectors without production of the helper virus. To do this, first we located by S1 mapping the donor and acceptor splice sites of reticuloendotheliosis virus strain A. The donor splice site is ca. 850 base pairs from the 5' end of proviral DNA. It is close to or overlaps the encapsidation sequences for viral RNA. The splice acceptor site is ca. 5.6 kilobase pairs from the 5' end of proviral DNA. Therefore, the encapsidation sequences and the donor splice site were removed from viral DNA to give expression of the gag and pol genes without virus production. The promoter in the long terminal repeat was fused to a site near the first ATG codon of the env gene, thereby deleting the encapsidation sequences and the gag and pol genes to give expression of the env gene without virus production. The permissive canine cell line D17 was transfected with the two modified viral DNAs. Two cell clones that contain both modified viral DNAs support the production of replication-defective spleen necrosis virus-thymidine kinase recombinant retrovirus vectors without the production of helper virus. To prevent recombination, the vector contains deletions that overlap with deletions in the integrated helper virus DNAs. This helper cell-vector system will be useful to derive infectious recombinant virus stocks of high titer (over 10(5) thymidine kinase transforming units per ml) which are able to infect avian, rat, and dog cells without the aid of helper virus.     

携带HSV-1TK基因的逆转录病毒载体构建及在HeLa细胞中的表达

目的:构建携带单纯疱疹病毒脱氧胸腺嘧啶激酶基因(herpes simplex virus thymidine kinase,HSV-TK)的逆转录病毒,用于宫颈癌的治疗研究。方法:用限制性内切酶从质粒pcDNA3.1/HA-myc-His(-)Z-TK切下HSV-1TK cDNA序列,亚克隆入逆转录病毒载体pLXSN得到重组质粒pLXSN-TK,鉴定正确的阳性重组质粒经PA317细胞包装,G418筛选,在NIH3T3细胞进行病毒滴度测定。然后用病毒感染人宫颈癌细胞HeLa。PCR、RT-PCR和Western blotting方法检测HSV-1TK基因在HeLa中的整合和表达情况。结果:重组质粒pLXSN-TK经PA317细胞包装后收获病毒上清,感染HeLa细胞,检测发现HSV-1TK基因整合到细胞基因组DNA中,并且能有效的转录和翻译。结论:成功构建了逆转录病毒pLXSN-TK,该病毒能有效感染HeLa细胞,并使携带的治疗基因HSV-1TK在细胞中表达,为今后HSV-1TK基因治疗宫颈癌的研究奠定基础。     

Secretion of enzymatically active human renin from mammalian cells using an avian retroviral vector

Recombinant plasmid-based retroviral expression vectors were constructed using a modified spleen necrosis virus (SNV) containing the Herpes simplex virus thymidine kinase gene promoter controlling the expression of the Tn5 neomycin phosphotransferase II gene (NPTII gene). The human renin (HRn) gene (hrn) was inserted into the 5' end of the SNV sequences such that in concatemeric plasmid DNA its expression was controlled by the strong promoter in the SNV long terminal repeat (LTR). Dog cells transfected with the concatemeric plasmid DNA secreted a small amount of a HRn-like 43-kDa protein. After cotransfection of chicken cells with concatemeric plasmid DNA and proviral DNA of reticuloendotheliosis virus strain A, infectious stocks of viruses were recovered. Cells infected with the virus carrying the viral LTR-hrn gene oriented for expression secreted the 43-kDa HRn-like protein at about 100-fold higher levels than the cells transfected with the plasmid DNAs. Biological activity of secreted HRn was determined by measuring levels of angiotensin I generated by incubating culture media with either a porcine or human angiotensinogen substrate. Infected dog cells produce about 40 ng of enzymatically active HRn per 10(6) cells per 24 h. These data indicate that retroviral expression vectors provide a good system for obtaining the secretion of high levels of enzymatically active heterologous proteins from mammalian cells.     

A generalized technique for deletion of specific genes in large genomes: a gene 22 of herpes simplex virus 1 is not essential for growth

We describe a general method for inactivation and deletion of genes at specific sites in large DNA genomes. In the first step of the procedure, the herpes simplex virus thymidine kinase is inserted into the genome at a specific site. In the second step, the thymidine kinase gene and desired sequences flanking the insertion site are deleted. Both steps involve recombination of the genomes with cloned chimeric fragments and utilize the available selection for or against thymidine kinase to select the desired genomes. We have applied the procedure to inactivate and to delete portions of an a gene of herpes simplex virus 1 specifying protein 22. The recombinant virus carrying the thymidine kinase inserted into the gene 22 and viruses exhibiting 0.1 kb and 0.7 kb deletions in the gene 22 specify new α polypeptides with molecular weights approximately 30% of the wild-type gene 22 product and grown normally in Vero cell cultures.     

Presence of a retroviral encapsidation sequence in nonretroviral RNA increases the efficiency of formation of cDNA genes.

We showed previously that retrovirus vector particles can encapsidate RNAs without retroviral cis-acting sequences, that such RNAs are reverse transcribed in infected target cells, and that the cDNA copies are inserted into the host genome resulting in cDNA genes (R. Dornburg and H. M. Temin, Mol. Cell. Biol. 8:2328-2334, 1988). To provide further evidence that this retrovirus-mediated gene transfer occurred through an RNA intermediate, we constructed retroviral vectors containing an intron from a cellular gene. This intron was lost in a cDNA gene formed after infection with retroviral particles, establishing that an RNA intermediate had existed. Retroviral vectors with additional encapsidation sequences were constructed. The presence of a murine leukemia virus encapsidation sequence in an mRNA transcribed from the hygromycin B phosphotransferase gene increased the efficiency of encapsidation into spleen necrosis virus vector particles and the formation of cDNA genes by approximately 2 orders of magnitude.     

Retrovirus transduction: segregation of the viral transforming function and the herpes simplex virus tk gene in infectious Friend spleen focus-forming virus thymidine kinase vectors.

A series of deletions and insertions utilizing the herpesvirus thymidine kinase gene (tk) were constructed in the murine retrovirus Friend spleen focus-forming virus (SFFV). In all cases, the coding region for the SFFV-specific glycoprotein (gp55), which is implicated in erythroleukemic transformation, was left intact. These SFFV-TK and SFFV deletion vectors were analyzed for expression of tk and gp55 after DNA-mediated gene transfer. In addition, virus rescued by cotransfection of these vectors with Moloney murine leukemia virus was analyzed for infectious TK-transducing virus, gp55 expression, and erythroleukemia-inducing ability. The experiments demonstrated that deletions or insertions within the intron for the gp55 env gene can interfere with expression of gp55 after both DNA-mediated gene transfer and virus infection. In contrast, the gene transfer efficiency of the tk gene was unaffected in the SFFV-TK vectors, and high-titer infectious TK virus could be recovered. Revertant viruses capable of inducing erythroleukemia and expressing gp55 were generated after cotransfection of the SFFV-TK vectors with murine leukemia virus. The revertant viruses lost both tk sequences and the ability to transduce TK- fibroblasts to a TK+ phenotype. These experiments demonstrate that segregation of the TK and erythroleukemia functions can occur in retrovirus vectors which initially carry both markers.     

Replication and virulence of pseudorabies virus mutants lacking glycoprotein gX.

Pseudorabies virus (PRV) glycoprotein gX accumulates in the medium of infected cells. In an attempt to study the function of gX, two viruses were constructed that lacked a functional gX gene. One virus, PRV delta GX1, was derived by insertion of the herpes simplex virus thymidine kinase gene into the gX-coding region. The other virus, PRV delta GXTK-, was derived by subsequent deletion of the inserted herpes simplex virus thymidine kinase gene. Both viruses replicated in cell cultures but produced no gX. Furthermore, PRV delta GX1 was capable of killing mice with a 50% lethal dose of less than 100 PFU.     

New retrovirus helper cells with almost no nucleotide sequence homology to retrovirus vectors.

We prepared retrovirus packaging cell lines containing gag-pol genes from spleen necrosis virus (expressed from a cytomegalovirus promoter and the simian virus 40 (SV40) polyadenylation sequences) and, on a separate vector, either the env gene from spleen necrosis virus (expressed from the Rous sarcoma virus promoter and the SV40 polyadenylation sequences) or the env gene from amphotropic murine leukemia virus (expressed from a cytomegalovirus promoter and the SV40 polyadenylation sequences). The nucleotide sequences in these packaging cell lines have almost no homology to the retrovirus vectors we used. Retrovirus vectors were produced from these new helper cell lines without any genetic interactions between the vectors and sequences in the helper cells and without transfer of the packaging sequences.     

Comparison of promoter suppression in avian and murine retrovirus vectors.

Previously, we described "promoter suppression" in infectious retrovirus vectors with two genes and an internal promoter. Here, we examined several parameters of promoter suppression and found that the amount of suppression in an integrated retrovirus vector was dependent both on whether the vector was derived from spleen necrosis virus or murine leukemia virus and on which internal promoter was present in the vector. Murine leukemia virus vectors showed less suppression than analogous spleen necrosis virus vectors. Furthermore, the amount of suppression was not dependent on either the relative strengths of the promoters nor the distance between the promoters. Moreover, we found that in vectors in which one promoter was suppressed, there was an inverse correlation between the DNaseI sensitivity of the chromatin surrounding a promoter and the suppression of its expression.     

Avian retrovirus integrase-enhanced transgene integration into mammalian cell DNA in vivo

Systems for introducing DNA genes-of-interest into mammalian cellular genomes have ranged from the use of different physical techniques to viruses including retroviruses. We have developed a microinjection method for an efficient and permanent integration of a DNA transgene into the cell genome by use of the retrovirus integrase. A 3.0-kb linear DNA fragment containing an internal herpes simplex virus thymidine kinase gene (tk) with flanking avian retrovirus U5 and U3 terminal attachment sites (U5-pgk/tk-U3) recognized by the integrase was constructed. The other donor, a 3.3-kb linear DNA fragment containing the same gene (pgk/tk) flanked by ApaL1 restriction sites not recognized by integrase, was also produced. After assembly of integrase-transgene complexes on ice, the complexes were microinjected into the nucleus of human fibroblast cells (143Btk) containing a defective thymidine kinase. The number of hypoxanthine/aminopterin/thymidine (HAT)-resistant colonies produced upon microinjection of either naked DNA or the independently assembled integrase-transgene complexes were determined. Our data suggests that enhanced integration of U5-pgk/tk-U3 required the DNA attachment sites and co-delivery of integrase. The data was consistent with a direct role for both of these elements in producing an approximate 4-fold increase in the number of HAT-resistant colonies observed over microinjection of just naked U5-pgk/tk-U3 (P < 0.0001).     

Construction and isolation of a transmissible retrovirus containing the src gene of Harvey murine sarcoma virus and the thymidine kinase gene of herpes simplex virus type 1

We constructed lambda recombinants containing the Harvey murine sarcoma virus genome and the thymidine kinase (tk) gene of herpes simplex virus type 1 linked to each other. The tk gene was located in a position downstream from both the long terminal repeat and the src gene of Harvey murine sarcoma virus. The DNAs of the lambda recombinants were used to transfect NIH3T3 mouse fibroblasts in order to obtain Harvey murine sarcoma virus DNA-induced foci of transformed cells. The transformed foci were superinfected with a helper-independent retrovirus, and new individual retrovirus were isolated from the superinfected foci. The new viruses could induce focus formation on NIH3T3 cells and could convert NIH3T3(TK-) cells into TK+ cells by carrying the herpes simplex virus type 1 tk gene into the TK- cells. From virus-infected cells, we isolated nonproducer foci on NIH3T3 cells and TK+ transformants on NIH3T3(TK-) cells containing one such new viral genome coding for the dual properties. The new retroviral sequence in the nonproducer cells could be rescued into virus particles at high titers by superinfection with a helper-independent retrovirus. A hybridization analysis indicated that the recombinant virus contained both the Harvey murine sarcoma virus src sequence and the tk gene sequence in a single RNA species approximately 4.9 kilobases long. We concluded that retroviruses can be used as true vectors for genes other than genes that lead to oncogenesis.     

Isolation of cis-acting vaccinia virus DNA fragments promoting the expression of herpes simplex virus thymidine kinase by recombinant viruses.

Recombinant TK- vaccinia viruses containing the pBR322 sequence inserted in either orientation within the coding sequence of the viral thymidine kinase gene were constructed. They were characterized by genomic analysis, hybridization studies, reversion to wild-type virus by in vivo recombination, and rescue from their genomes of plasmids which contained all or parts of the pBR322 sequence. TK- cells were infected with one of these recombinant viruses and then transfected with pools of chimeric plasmids composed of a cloned herpes simplex virus thymidine kinase gene which contained upstream inserts of different vaccinia DNA fragments prepared by restriction or sonication. Recombination between homologous pBR322 sequences within infected cells generated selectable recombinant viruses in which expression of the herpes simplex virus thymidine kinase gene was promoted by the upstream vaccinia insert. These viruses were characterized by genomic analysis, hybridization, and in vivo or in vitro phosphorylation of (5-[125I]deoxycytidine as a specific assay for the expressed herpes simplex virus thymidine kinase. Vaccinia DNA inserts were isolated conveniently for transfer to bacteria by rescuing appropriate plasmids from the genome of recombinant viruses. The sequence of 100 nucleotides adjacent to the upstream region of the herpes simplex virus gene was determined in nine different inserts measuring 0.17 to 1.07 kilobase pairs.