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DNA Viruses And
By NIDA REHMAN
• 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
• Viral particles outside a host cell (so called virions) are inert
entities with a genome surrounded by a protective coat.
• The Baltimore classification, developed by David Baltimore, is a
virus classification system that groups viruses into families,
depending on their type of genome
• Viral vector is the most effective means of gene transfer to modify
specific cell type or tissue and can be manipulated to express
• Several virus types are currently being investigated for use to
deliver genes to cells to provide either transient or permanent
• 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 vectors
• 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, 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 Vectors
• Retroviral Vectors
• Lentiviral Vectors
• Adenoviral Vectors
• Adeno-associated viral vectors
• Herpes Simplex Virus Vectors
• Retroviral 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.
• Lentiviruses 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
• Adenoviral 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 vectors
• Similarly 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 Vectors
• This 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