Viral vectors are a tool commonly used by biologists to deliver genetic material into cells inside a living organism or cultured in vitro. Viruses have evolved specialised molecular mechanisms to efficiently transport their genomes inside the cells they infect. Delivery of genes by a virus is termed transduction and the infected cells are described as transduced. Molecular biologists first harnessed this machinery in the 1970s. Paul Berg used a modified SV40 virus containing DNA from the bacteriophage lambda to infect monkey kidney cells maintained in culture.Goff SP and Berg P. (1976) Construction of hybrid viruses containing SV40 and lambda phage DNA segments and their propagation in cultured monkey cells. Cell. 9:695-705.

Key properties of a viral vector

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Viral vectors are tailored to their specific applications but generally share a few key properties.

• Safety. Although viral vectors are occasionally created from pathogenic viruses, they are modified in such a way as to minimize the risk of handling them. This usually involves the deletion of a part of the viral genome critical for viral replication. Such a virus can efficiently infect cells but, once the infection has taken place, requires a helper virus to provide the missing proteins for production of new virions.
• Low toxicity. The viral vector should have a minimal effect on the physiology of the cell it infects.
• Stability. Some viruses are genetically unstable and can rapidly rearrange their genomes. This is detrimental to predictability and reproducibility of the work conducted using a viral vector and is avoided in their design.
• Cell type specificity. Most viral vectors are engineered to infect as wide a range of cell types as possible. However, sometimes the opposite is preferred. The viral receptor can be modified to target the virus to a specific kind of cell.

Applications

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Basic research

Viral vectors were originally developed as an alternative to transfection of naked DNA for molecular genetic experiments. Compared to traditional methods such as calcium phosphate precipitation, transduction can ensure that nearly 100% of cells are infected without severely affecting cell viability. Furthermore, some viruses integrate into the cell genome facilitating stable expression. However, transfection is still the method of choice for many applications as construction of a viral vector is a much more laborious process. Protein coding genes can be expressed using viral vectors, commonly to study the function of the particular protein. Viral vectors, especially retroviruses, stably expressing marker genes such as GFP are widely used to permanently label cells to track them and their progeny, for example in xenotransplantation experiments, when cells infected in vitro are implanted into a host animal. Genes inserted into the vector can encode shRNAs and miRNAs used to efficiently block or silence production of a specific protein. Such knock-down experiments are much quicker and cheaper to carry out than gene knockout. But as the silencing is sometimes non-specific and has off-target effects on other genes, it provides less reliable results.

Gene therapy

In the future gene therapy may provide a way to cure genetic disorders, such as severe combined immunodeficiency or cystic fibrosis. Several gene therapy trials have used viruses to deliver 'good' genes to the cells of the patient's body.

However, several problems of viral gene therapy must be overcome before it gains widespread use. Immune response to viruses not only impedes the delivery of genes to target cells but can cause severe complications for the patient. In one of the early gene therapy trials in 1999 this led to the death of Jesse Gelsinger, who was treated using an adenoviral vector.Beardsley T, February 2000, A tragic death clouds the future of an innovative treatment method. Scientific American

Some viral vectors, for instance lentiviruses, insert their genomes at a random location on one of the host chromosomes, which can disturb the function of cellular genes and lead to cancer. In a severe combined immunodeficiency retroviral gene therapy trial conducted in 2002, two of the patients developed leukemia as a consequence of the treatment. McDowell N, 15 January 2003, New cancer case halts US gene therapy trials. New Scientist Adeno-associated virus-based vectors are much safer in this respect as they always integrate at the same site in the human genome.

Vaccines

Viruses expressing pathogen proteins are currently being developed as vaccines against these pathogens, based on the same rationale as DNA vaccines. T-lymphocytes recognise cells infected with intracellular parasites based on the foreign proteins produced within the cell. T cell immunity is crucial for protection against viral infections and such diseases as malaria. A viral vaccine induces expression of pathogen proteins within host cells similarly to the Sabin Polio vaccine and other attenuated vaccines. However, since viral vaccines contain only a small fraction of pathogen genes, they are much safer and sporadic infection by the pathogen is impossible. Adenoviruses are being actively developed as vaccines.

Types of viral vectors

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Lentiviruses

Lentiviruses are widely adapted as vectors thanks to their ability integrate into the genome of non-dividing as well as dividing cells. The viral genome in the form of RNA is reverse-transcribed when the virus enters the cell to produce DNA, which is then inserted into the genome at a random position by the viral integrase enzyme. The vector, now called a provirus, remains in the genome and is passed on to the progeny of the cell when it divides. The site of integration is unpredictable, which can pose a problem. The provirus can disturb the function of cellular genes and lead to activation of oncogenes promoting the development of cancer, which raises concerns for possible applications of lentiviruses in gene therapy.

Production of lentiviral vectors

For safety reasons lentiviral vectors never carry the genes required for their replication. To produce a lentivirus, several plasmids are transfected into a so-called packaging cell line, commonly HEK 293. One or more plasmids, generally referred to as packaging plasmids, encode the virion proteins, such as the capsid and the reverse transcriptase. Another plasmid contains the genetic material to be delivered by the vector. It is transcribed to produce the single-stranded RNA viral genome and is marked by the presence of the ψ (psi) sequence. This sequence is used to package the genome into the virion.

Adenoviruses

As opposed to lentiviruses, adenoviral DNA does not integrate into the genome and is not replicated during cell division. This limits their use in basic research, although adenoviral vectors are occasionally used in in vitro experiments. Their primary applications are in gene therapy and vaccination. Since humans commonly come in contact with adenoviruses, which cause respiratory, gastrointestinal and eye infections, they trigger a rapid immune response with potentially dangerous consequences. To overcome this problem scientists are currently investigating adenoviruses that humans do not have immunity to.

Adeno-associated viruses

References

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