Expression of Epstein-Barr virus (EBV) DNA and cloned DNA fragments in human lymphocytes following Sendai virus envelope-mediated gene transfer

Gene therapy in surgical oncology

BACKGROUND

Surgery remains the only potentially curative treatment modality for the majority of patients with solid tumors. Conventional chemotherapy and radiotherapy only have roles as adjuvant or palliative therapies for most common cancers. Two decades of research have led to the first attempts at producing and introducing clinically useful genetically modified cells into humans.

METHODS

Modern molecular methods have been developed that allow the stable transfer of foreign DNA sequences into human and other mammalian somatic cells. At the present time, gene therapy predominantly involves gene insertion either directly into a target cell in situ or into an appropriate cell in vitro that is then introduced to a physiologically relevant site. We present an overview of the potential applications of molecular biology for practicing surgeons, particularly in the field of surgical oncology, to show how genes are harnessed and inserted into target somatic cells.

CONCLUSIONS

Although significant clinical therapies have and will continue to emerge from these initial experiments, only the future will provide evidence of whether the present technical skills are sufficient to have an impact on the long-term benefits for patients with cancer and genetic defects.

Concepts and strategies for human gene therapy

Methods of modern molecular genetics have been developed that allow stable transfer and expression of foreign DNA sequences in human and other mammalian somatic cells. It is therefore no surprise that the methods have been applied in attempts to complement genetic defects and correct disease phenotypes. Two decades of research have now led to the first clinically applicable attempts to introduce genetically modified cells into human beings to cure diseases caused at least partially by genetic defects. We discuss here some of the strategies being followed for both in vitro and in vivo application of therapeutic gene transfer and summarize some of the technical and conceptual difficulties associated with somatic-cell gene therapy.

Human neonatal lymphocytes immortalized after microinjection of Epstein-Barr virus DNA

Epstein-Barr virus (EBV) is a highly efficient acute transforming agent in human cells, provided that the intact virus is used. To investigate the ability of viral DNA alone to transform cells, we introduced the EBV genome into human lymphocytes. After microinjection of EBV DNA into neonatal B lymphocytes, we established a cell line that in early passages contained multiple viral fragments. This cell line retained sequences from the short, unique (Us) region of the EBV genome and sequences from EcoRI-E. The viral sequences were not expressed; however, the cells expressed a 2.3-kilobase polyadenylated message homologous to the c-fgr oncogene, a cellular locus believed to be activated by EBV infection [M. S. C. Cheah, T. J. Ley, S. R. Tronick, and K. C. Robbins, Nature (London) 319:238-240.]. The cell line was monoclonal with rearrangement at the immunoglobulin locus and had a reciprocal translocation t(1;7)(p34;q34) and a deletion of sequences within the locus for the beta chain of the T-cell receptor. The close proximity of the translocation to the chromosomal loci for c-fgr on chromosome 1 and the T-cell receptor beta chain on chromosome 7 suggests that structural alteration of these genes was critical to this transformation event.

Epstein-Barr virus nuclear antigen 2 specifically induces expression of the B-cell activation antigen CD23

Epstein-Barr virus (EBV) infection of EBV-negative Burkitt lymphoma (BL) cells induces some changes similar to those seen in normal B lymphocytes that have been growth transformed by EBV. The role of individual EBV genes in this process was evaluated by introducing each of the viral genes that are normally expressed in EBV growth-transformed and latently infected lymphoblasts into an EBV-negative BL cell line, using recombinant retrovirus-mediated transfer. Clones of cells were derived that stably express the EBV nuclear antigen 1 (EBNA-1), EBNA-2, EBNA-3, EBNA-leader protein, or EBV latent membrane protein (LMP). These were compared with control clones infected with the retrovirus vector. All 10 clones converted to EBNA-2 expression differed from control clones or clones expressing other EBV proteins by growth in tight clumps and by markedly increased expression of one particular surface marker of B-cell activation, CD23. Other activation antigens were unaffected by EBNA-2 expression, as were markers already expressed on the parent BL cell line, including BL markers (cALLA and BLA), proliferation markers (transferrin receptor and BK19.9), and cell adhesion-related molecules (LFA-1 and LFA-3). Increased CD23 expression in cells expressing EBNA-2 was apparent from monoclonal anti-CD23 antibody binding to the cell surface, from immunoprecipitation of the 45-kDa and 90-kDa CD23 proteins with monoclonal antibody, and from RNA blots probed with labeled CD23 DNA. The results indicate that EBNA-2 is a specific direct or indirect trans-activator of CD23. This establishes a link between an EBV gene and cell gene expression. Since CD23 has been implicated in the transduction of B-cell growth signals, its specific induction by EBNA-2 could be important in EBV induction of B-lymphocyte transformation.

Activated v-myc and v-ras oncogenes do not transform normal human lymphocytes

Activated v-myc (pSV v-myc) and v-Ha-ras (GT10) oncogenes were introduced into normal human lymphocytes, NIH 3T3 fibroblasts, B-lymphoblastoid cells, and human epithelial cells, using a reconstituted Sendai virus envelope-mediated gene transfer technique. Efficient transfer of the plasmid in each cell type was demonstrable within 1.5 h of transfection by Southern blotting of extrachromosomal DNA extracts, which unexpectedly revealed that v-myc plasmid DNA was unstable in normal lymphocytes but not in the other cell types. The v-myc plasmid was stabilized when cotransfected into lymphocytes together with v-Ha-ras. The transfected v-Ha-ras plasmid was stable in all the cell types tested. v-myc plasmid expression was clearly detectable by 5 h in all cell types except human lymphocytes. Lymphocytes expressed v-myc when transfected together with v-Ha-ras. Transfected ras oncogene was efficiently expressed in all the cell types tested. Expression of the transfected genes increased at 24 and 48 h after transfection. Even though plasmid stability and expression were achieved in myc-ras-cotransfected lymphocytes, no effects on cellular DNA synthesis or immortalization were observed, in contrast to efficient transformation of NIH 3T3 fibroblasts by the same procedure. Our data suggest that efficient expression of transfected myc and ras oncogenes in normal quiescent human lymphocytes is not sufficient for the induction of cell growth and immortalization.

Frequency of B-lymphocyte transformation by Epstein-Barr virus decreases with entry into the cell cycle

The relationship between in vitro B-cell activation and transformation by Epstein-Barr virus (EBV) was studied. B cells were fractionated using discontinuous Percoll gradients to purify cells with resting morphology. Activation of resting cells for 24 hr with anti-Ig (mu chain specific) or Staphylococcus aureus Cowan I (SAC) resulted in transition of susceptible cells into the G1 phase of the cell cycle as shown by an increase in cell size, an increase in uridine incorporation and an increase in sensitivity to B-cell growth factor (BCGF). Entry into S phase was achieved by extending the period of activation to 48-96 hr with high concentrations of SAC or anti-mu or using BCGF. SAC-activated cells entered S phase on Day 2 and anti-mu treated cells on Day 3. Control (G0) cells and cell activated for varying lengths of time (G0/G1, G1/S) were exposed to EBV and plated in a limiting dilution assay to determine the frequency of EBV-transformable cells. Control cells and cells activated for 24 hr had a transformation frequency of 1-2%. With continued activation with SAC or anti-mu, however, transformation frequency decreased at a rate paralleling the entry of the population into S phase. Treating cells with low concentrations of anti-mu or SAC in combination with BCGF decreased the transformation frequency to levels lower than anti-mu or SAC alone, further suggesting that entry into S phase is accompanied by a reduction in transformability. These results indicate that resting B cells are highly susceptible to transformation, and that with in vitro activation into the cell cycle B cells become resistant to EBV transformation.

Activated v-myc and v-ras oncogenes do not transform normal human lymphocytes

Activated v-myc (pSV v-myc) and v-Ha-ras (GT10) oncogenes were introduced into normal human lymphocytes, NIH 3T3 fibroblasts, B-lymphoblastoid cells, and human epithelial cells, using a reconstituted Sendai virus envelope-mediated gene transfer technique. Efficient transfer of the plasmid in each cell type was demonstrable within 1.5 h of transfection by Southern blotting of extrachromosomal DNA extracts, which unexpectedly revealed that v-myc plasmid DNA was unstable in normal lymphocytes but not in the other cell types. The v-myc plasmid was stabilized when cotransfected into lymphocytes together with v-Ha-ras. The transfected v-Ha-ras plasmid was stable in all the cell types tested. v-myc plasmid expression was clearly detectable by 5 h in all cell types except human lymphocytes. Lymphocytes expressed v-myc when transfected together with v-Ha-ras. Transfected ras oncogene was efficiently expressed in all the cell types tested. Expression of the transfected genes increased at 24 and 48 h after transfection. Even though plasmid stability and expression were achieved in myc-ras-cotransfected lymphocytes, no effects on cellular DNA synthesis or immortalization were observed, in contrast to efficient transformation of NIH 3T3 fibroblasts by the same procedure. Our data suggest that efficient expression of transfected myc and ras oncogenes in normal quiescent human lymphocytes is not sufficient for the induction of cell growth and immortalization.

Transfection of lymphoblastoid cells using DNA-loaded reconstituted Sendai virus envelopes: expression of transfected DNA and selection of transfected cells

The American Burkitt's lymphoma cell line Loukes was cotransfected with cloned BamHI K fragment of EBV DNA and a vector pSV2neo. Reconstituted Sendai virus envelopes (RSVE) loaded with DNA were used for efficient gene transfer. Two cell lines have been obtained following culture in the presence of geneticin sulfate (G-418). Messenger RNA from both transfected DNAs was expressed during the whole period of observation, 42 days after transfection. This method provides a relatively simple and efficient means for selection of lymphoblastoid cells expressing a transfected gene.

Isolation of glycoproteins of parainfluenza I (Sendai) by Helix pomatia Lectin column

Glycoproteins of Sendai virus were successfully isolated on a column of Helix pomatia Lectin-Sepharose 6MB following solubilization with Nonidet P-40. This technique can be used as a preparative step for the study of viral glycoproteins.

U2 region of Epstein-Barr virus DNA may encode Epstein-Barr nuclear antigen 2

Sequence analysis of the U2 regions of the B95-8 and AG876 Epstein-Barr virus (EBV) isolates reveals divergence within a long open reading frame previously identified as encoding 1.5 kilobases of the 3' end of a viral RNA expressed in latently infected, growth-transformed, B-lymphocyte cell lines. Differences among EBV isolates within the U2 open reading frame are shown to correlate with differences in an EBV nuclear antigen, EBNA2. B95-8, W91, Raji, Cherry, and Lamont EBV isolates have similar U2 domains and encode similar-size EBNA2 proteins, while AG876, Jijoye, and P3HR-1 have variant or absent U2 domains and variant or absent EBNA2 proteins. The AG876 U2 open reading frame and EBNA2 protein are both shorter than those of B95-8. These data indicate that the U2 open reading frame encodes EBNA2.