Epstein-Barr virus based expression vectors

Transcriptional signature and memory retention of human-induced pluripotent stem cells

Genetic reprogramming of somatic cells to a pluripotent state (induced pluripotent stem cells or iPSCs) by over-expression of specific genes has been accomplished using mouse and human cells. However, it is still unclear how similar human iPSCs are to human Embryonic Stem Cells (hESCs). Here, we describe the transcriptional profile of human iPSCs generated without viral vectors or genomic insertions, revealing that these cells are in general similar to hESCs but with significant differences. For the generation of human iPSCs without viral vectors or genomic insertions, pluripotent factors Oct4 and Nanog were cloned in episomal vectors and transfected into human fetal neural progenitor cells. The transient expression of these two factors, or from Oct4 alone, resulted in efficient generation of human iPSCs. The reprogramming strategy described here revealed a potential transcriptional signature for human iPSCs yet retaining the gene expression of donor cells in human reprogrammed cells free of viral and transgene interference. Moreover, the episomal reprogramming strategy represents a safe way to generate human iPSCs for clinical purposes and basic research.

Establishment and Applications of Epstein-Barr Virus-Based Episomal Vectors in Human Embryonic Stem Cells

Human embryonic stem (hES) cells are capable of unlimited cell proliferation yet maintain the potential to differentiate into many cell types. Here we reported an Epstein-Barr virus (EBV)-based vector system used to improve transfection efficiency in hES cells. Plasmids containing oriP, the latent replication origin of EBV, can be propagated stably as episomal DNA in human cells that express the EBV nuclear antigen 1 (EBNA1), which binds to oriP and functions as the trans-acting replication initiator. It was reported that the EBV replicon could harbor a DNA fragment of up to 330 kilobase pairs. Plasmids containing an enhanced green fluorescent protein (EGFP)/puromycin resistance gene cassette along with or without oriP were used to transfect hES cells that stably express EBNA1. The presence of oriP moderately increased the transient transfection efficiency and more importantly it elevated the stable transfection efficiency by approximately 1,000-fold as compared with oriP-minus plasmids. The oriP plasmid as episomal DNA and green fluorescent protein expression in hES cells was maintained for months in the presence of drug selection and gradually lost (2%-4% per cell doubling) in the absence of selection. The presence of EBNA1 did not interfere with the hES cell properties or differentiation we tested and could maintain stable EGFP expression during differentiation. In addition to transgene expression, the EBV vector system could effectively enhance the RNA interference efficiency in hES cells. Thus, the EBV vector system that allows a large DNA insert and sustained expression of transgene or small hairpin RNA will enhance basic and translational research using hES cells.

DNA sequence motifs which direct adeno-associated virus site-specific integration in a model system

The DNA sequence motifs which direct adeno-associated virus type 2 site-specific integration are being investigated using a shuttle vector, propagated as a stable episome in cultured cell lines, as the target for integration. Previously, we reported that the minimum episomal targeting elements comprise a 16-bp binding motif (Rep binding site [RBS]) for a viral regulatory protein (Rep) separated by a short DNA spacer from a sequence (terminal resolution site [TRS]) that can serve as a substrate for Rep-mediated nicking activity (R. M. Linden, P. Ward, C. Giraud, E. Winocour, and K. I. Berns, Proc. Natl. Acad. Sci. USA 93:11288-11294, 1996; R. M. Linden, E. Winocour, and K. I. Berns, Proc. Natl. Acad. Sci. USA 93:7966-7972, 1996). We now report that episomal integration depends upon both the sequence and the position of the spacer DNA separating the RBS and TRS motifs. The spacer thus constitutes a third element required for site-specific episomal integration.

HIV protease as a target for retrovirus vector-mediated gene therapy

The dimeric aspartyl protease of HIV has been the subject of intense research for almost a decade. Knowledge of the substrate specificity and catalytic mechanism of this enzyme initially guided the development of several potent peptidomimetic small molecule inhibitors. More recently, the solution of the HIV protease structure led to the structure-based design of improved peptidomimetic and non-peptidomimetic antiviral compounds. Despite the qualified success of these inhibitors, the high mutation rate associated with RNA viruses continues to hamper the long-term clinical efficacy of HIV protease inhibitors. The dimeric nature of the viral protease has been conducive to the investigation of dominant-negative inhibitors of the enzyme. Some of these inhibitors are defective protease monomers that interact with functional monomers to form inactive protease heterodimers. An advantage of macromolecular inhibitors as compared to small-molecule inhibitors is the increased surface area of interaction between the inhibitor and the target gene product. Point mutations that preserve enzyme activity but confer resistance to small-molecule inhibitors are less likely to have an adverse effect on macromolecular interactions. The use of efficient retrovirus vectors has facilitated the delivery of these macromolecular inhibitors to primary human lymphocytes. The vector-transduced cells were less susceptible to HIV infection in vitro, and showed similar levels of protection compared to other macromolecular inhibitors of HIV replication, such as RevM10. These preliminary results encourage the further development of dominant-negative HIV protease inhibitors as a gene therapy-based antiviral strategy.

Infection control in gene therapy

Gene therapy is now being studied for the treatment of a wide variety of acquired and inherited diseases. Viruses used as vectors for gene transfer include retroviruses, adenoviruses, vaccinia viruses, adeno-associated viruses, and herpesviruses. These vectors, developed in the laboratory and in animal studies, are now being introduced into the clinical arena Infection control practitioners will be involved invariably in reviewing the use of these agents in their clinics and hospitals. This review summarizes key aspects of the more common vectors and makes recommendations for infection control.

The use of Epstein-Barr virus-based shuttle vectors in rat PC12 cells

1. The rat pheochromocytoma PC12 cell line has been a commonly used model for studies of neuronal development, function, and death. Thus the ability to transfect PC12 cells in an efficient manner and to manipulate their gene expression would enhance the usefulness of these cells. 2. We demonstrate that EBV-based vectors provide a useful expression system for gene manipulation in rat PC12 cells. 3. The EBV-based vectors replicate episomally in PC12 cells for at least 2 months, as evidence by their recovery from the transfected cells and by the digestion of the episomal plasmid with the isoschizomer MboI and DpnI restriction enzymes. 3. PC12 cells are efficiently transfected by EBV-based vectors both transiently and stably. 4. Transfection of PC12 cells with an EBV-based vector containing tau cDNA in the antisense orientation resulted in a decrease in the level of tau protein in the transfected cells. 5. The results demonstrate that EBV-based vectors can be a useful expression system for gene manipulation in PC12 cells.

Expression of galectin-3 modulates T-cell growth and apoptosis

Galectin-3 is a member (if a large family of beta-galactoside-binding animal lectins. It has been shown that the expression of galectin-3 is upregulated in proliferating cells, suggesting a possible role for this lectin in regulation of cell growth. Previously, we have shown that T cells infected with human T-cell leukemia virus type I express high levels of galectin-3, in contrast to uninfected cells, which do not express detectable amounts of this protein. In this study, we examined growth properties of human leukemia T cells transfected with galectin-3 cDNA, and thus constitutively overexpressing this lectin. Transfectants expressing galectin-3 displayed higher growth rates than control transfectants, which do not express this lectin. Furthermore, galectin-3 expression in these cells confers resistance to apoptosis induced by anti-Fas antibody and staurosporine. Galectin-3 was found to have significant sequence similarity with Bcl-2, a well-characterized suppressor of apoptosis. In particular, the lectin contains the NWGR motif that is highly conserved among members of the Bcl-2 family and shown to be critical for the apoptosis-suppressing activity. We further demonstrated that galectin-3 interacts with Bc1-2 in a lactose-inhibitable manner. We conclude that galectin-3 is a regulator of cell growth and apoptosis and it may function through a cell death inhibition pathway that involves Bcl-2.

Approaches to gene transfer in keratinocytes

The introduction and expression of exogenous genetic material in cultured cells has provided a powerful tool for studying gene function and regulation. Immortalized cell lines have been useful for establishing gene transfer methodologies that are generally inefficient. For investigators of epidermal and mucosal biology, wishing to make use of the tissue architecture produced by primary keratinocytes in vitro, the limited life span of these cells presents a host of unique problems. Primary cells require the use of gene transfer methods that are highly efficient and will not significantly alter the cell's normal differentiation pathway. The purpose of this review is to evaluate gene transfer technology as it applies to keratinocytes.

Site-specific integration by adeno-associated virus is directed by a cellular DNA sequence

Different regions of an 8.2-kb cloned DNA segment containing the target for adeno-associated virus (AAV) integration in human chromosome 19q13-3-qter (AAVS1 locus) were subcloned in an Epstein-Barr virus-based shuttle vector and propagated as episomes in a derivative of the 293 human embryonic kidney cell line. Preferential recombination with an infecting AAV genome was assessed by measuring the frequency of recombinants among the shuttle vectors recovered in Escherichia coli. The signals which direct recombination with the AAV genome were localized to a 510-nt region at the 5' end of the 8.2-kb AAVS1 DNA. Hence, the results indicate that site-specific integration of AAV is directed by a specific DNA sequence on human chromosome 19. An unusual degree of DNA heterogeneity in the recovered vector was also associated with the 510 nt at the 5' end of AAVS1 DNA, suggesting that the AAV chromosomal integration locus may be involved in genomic instability.