DNA viruses and viral vectors

DNA Viruses And Viral Vectors

By NIDA REHMAN

Introduction Viruses are infectious particles composed of a protein coat and a nucleic acid core.They exist in a huge variety of forms and infect practically all living creatures: animals, plants, insects and bacteria. Viruses can be considered as mobile genetic particles, containing instructions for reproducing themselves using foreign cellular resources. The amount of viruses that exist in the biosphere is enormous, varying in their virion shapes, genomes and lifestyles. Classification of viruses is defined by host preference, viral morphology, genome type and auxiliary structures such as tails or envelopes. Viral particles outside a host cell (so called virions) are inert entities with a genome surrounded by a protective coat.

Baltimore classificationThe Baltimore classification, developed by David Baltimore, is a virus classification system that groups viruses into families, depending on their type of genome

Viral vectors Viral vector is the most effective means of gene transfer to modify specific cell type or tissue and can be manipulated to express therapeutic genes. Several virus types are currently being investigated for use to deliver genes to cells to provide either transient or permanent transgene expression. Viruses have evolved to become highly efficient at nucleic acid delivery to specific cell types while avoiding immunosurveillance by an infected host. These properties make viruses attractive gene-delivery vehicles, or vectors, for gene therapy. Several types of viruses, including retrovirus, adenovirus, adeno-associated virus (AAV), and herpes simplex virus, have been modified in the laboratory for use in gene therapy applications. Because these vector systems have unique advantages and limitations, each has applications for which it is best suited.

Key properties of viral vectorsSafety: 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, allowing the virus to efficiently infect cells and deliver the viral payload, but preventing the production of new virions in the absence of a helper virus that provides the missing critical proteins. However, an ongoing safety concern with the use of viral vectors is insertional mutagenesis, in which the ectopic chromosomal integration of viral DNA either disrupts the expression of a tumor-suppressor gene or activates an oncogene, leading to the malignant transformation of cells (Glover et al., 2005).

Low toxicity: The viral vector should have a minimal effect on the physiology of the cell it infects. This is especially important in studies requiring gene delivery in vivo, because the organism will develop an immune response if the vector is seen as a foreign invader (Nayak and Herzog, 2009).

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. Therefore, unstable vectors are usually avoided.

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. Viruses modified in this manner are said to be pseudotyped.

Selection: Viral vectors should contain selectable markers, such as resistance to a certain antibiotic, so that the cells that have taken up the viral vector can be isolated

Types of Viral VectorsRetroviral VectorsLentiviral VectorsAdenoviral VectorsAdeno-associated viral vectorsHerpes Simplex Virus Vectors

Retroviral VectorsRetroviral vectors are commonly used and known to integrate into the genome of the infected cell in a stable and permanent fashion. Reverse transcriptase in the virus allows integration into the host genome.

There are two types of retroviral vectors: replication-competent and replication defective. Usually replication-defective vectors are preferred in practice as they allow for several rounds of replication due to their coding regions.

Lentiviral VectorsLentiviruses are a type of retrovirus that are able to integrate into non-dividing cells and do not require mitotic cell division in order to function. Instead, the genome enters the cell DNA via reverse transcription and is incorporated in a random position of the cell genome.

Adenoviral VectorsAdenoviral vectors have a wide range of action and are able to deliver nucleic acids to both dividing and non-dividing cells. This can make their use in basic research difficult, but they are sometimes used in vitro. When utilized in vivo, adenoviral vectors often precipitate immune elimination of the cells, which also limits their functionality.

Adenoviruses are often responsible for respiratory, gastrointestinal and eye infection that affect humans. As a result, research is currently being conducted to investigate the use of adenoviral vectors in applications of gene therapy and vaccination.

Adeno-associated viral vectorsSimilarly to adenoviral vectors, adeno-associated viral (AAV) vectors can deliver genetic material to dividing and non-dividing cells. It is a small virus that is known to affect humans with a very mild immune response. As a result AAV vectors have beneficial properties for gene therapy that are effective with limited negative effects. However, the utility of this type of vector is significantly limited by its restricted capacity of DNA.

Herpes Simplex Virus VectorsThis type of viral vector has the ability to deliver large-scale quantities of exogenous DNA. The primary concerns with the use of herpes simplex virus to deliver genetic material are cytotoxicity and the maintenance of transgene expression.

Viral Vector Applications

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