Adenovirus Gene Expression Vector

Overview

The adenoviral vector system is effective in transducing many (but not all) mammalian cell types, where the vector remains as episomal DNA without integration into the host genome. It is the preferred gene delivery system in vivo, often used in gene therapy and vaccination.

Adenoviral vectors are derived from adenovirus, which causes the common cold. Wildtype adenovirus has a double-stranded linear DNA genome.

An adenoviral vector is first constructed as a plasmid in E. coli. It is then transfected into packaging cells, where the region of the vector between the two inverted terminal repeats (ITRs) is packaged into live virus.

When the virus is added to target cells, the DNA cargo is delivered into cells where it enters the nucleus and remains as episomal DNA without integration into the host genome. Any gene(s) that were placed in-between the two ITRs during vector construction are introduced into target cells along with the rest of viral genome.

By design, adenoviral vectors lack the E1A, E1B and E3 genes (delta E1 + delta E3). The first two are required for the production of live virus (these two genes are engineered into the genome of packaging cells). As a result, virus produced from the vectors have the important safety feature of being replication incompetent (meaning that they can transduce target cells but cannot replicate in them).

For further information about this vector system, please refer to the papers below.

References Topic
Proc Natl Acad Sci U S A. 91:8802 (1994) The 2nd generation adenovirus vectors
J Gen Virol. 36:59 (1977) A packaging cell line for adenovirus vectors
J Virol. 79:5437 (2005) Replication-competent adenovirus (RCA) formation in 293 Cells
Gene Ther. 3:75 (1996) A cell line for testing RCA

Highlights

Our vector is derived from the adenovirus serotype 5 (Ad5). It is optimized for high copy number replication in E. coli, high-titer packaging of live virus, efficient transduction of host cells, and high-level transgene expression.

Advantages

Low risk of host genome disruption: Upon transduction into host cells, adenoviral vectors remain as episomal DNA in the nucleus. The lack of integration into the host genome can be a desirable feature for in vivo human applications, as it reduces the risk of host genome disruption that might lead to cancer.

Very high viral titer: After our adenoviral vector is transfected into packaging cells to produce live virus, the virus can be further amplified to very high titer by re-infecting packaging cells. This is unlike lentivirus, MMLV retrovirus, or AAV, which cannot be amplified by re-infection. When adenovirus is obtained through our virus packaging service, titer can reach >109 plaque-forming unit per ml (PFU/ml).

Broad tropism: Cells from commonly used mammalian species such as human, mouse and rat can be transduced with our vector. But some cell types have proven difficult to transduce (see disadvantages below).

Large cargo space: The upper limit size of the adenovirus genome for efficient virus packaging is ~38.7 kb (from 5' ITR to 3' ITR). After excluding the required backbone components for adenovirus gene expression, our vector has about ~7.5 kb of cargo space to accommodate the user's DNA of interest (such as promoter, ORF, and fluorescence marker). This is bigger than the ~6.4 kb cargo space in our lentiviral expression vector, and is sufficient for most applications.

Effectiveness in vitro and in vivo: Our vector is often used to transduce cells in live animals, but it can also be used effectively in vitro.

Safety: The safety of our vector is ensured by the fact that it lacks genes essential for virus production (these genes are engineered into the genome of packaging cells). Virus made from our vector is therefore replication incompetent except when it is used to transduce packaging cells.

Disadvantages

Non-integration of vector DNA: The adenoviral genome does not integrate into the genome of transduced cells. Rather, it exists as episomal DNA, which can be lost over time, especially in dividing cells. 

Difficulty transducing certain cell types: While our adenoviral vectors can transduce many different cell types including non-dividing cells, it is inefficient against certain cell types such as endothelia, smooth muscle, differentiated airway epithelia, peripheral blood cells, neurons, and hematopoietic cells.

Strong immunogenicity: Live virus from adenoviral vectors can elicit strong immune response in animals, thus limiting certain in vivo applications.

Technical complexity: The use of adenoviral vectors requires the production of live virus in packaging cells followed by the measurement of viral titer. These procedures are technical demanding and time consuming.

Key components

5' ITR: 5' inverted terminal repeat. In wild type virus, 5' ITR and 3' ITR are essentially identical in sequence. They reside on two ends of the viral genome pointing in opposite directions, where they serve as the origin of viral genome replication.

Ψ: Adenovirus packaging signal required for the packaging of viral DNA into virus.

Promoter: The promoter that drives your gene of interest is placed here.

Kozak: Kozak consensus sequence. It is placed in front of the start codon of the ORF of interest because it is believed to facilitate translation initiation in eukaryotes.

ORF: The open reading frame of your gene of interest is placed here.

TK pA: Herpes simplex virus thymidine kinase polyadenylation signal. It facilitates transcriptional termination of the upstream ORF.

ΔAd5: Portion of Ad5 genome between the two ITRs minus the E1A, E1B and E3 regions.

3' ITR: 3' inverted terminal repeat.

pBR322 ori: pBR322 origin of replication. Plasmids carrying this origin exist in medium copy numbers in E. coli.

Ampicillin: Ampicillin resistance gene. It allows the plasmid to be maintained by ampicillin selection in E. coli.

PacI: PacI restriction site (PacI is a rare cutter that cuts at TTAATTAA). The two PacI restriction sites on the vector can be used to linearize the vector and remove the vector backbone from the viral sequence, which is necessary for efficient packaging.