Adenovirus miR30-Based shRNA Knockdown Vector

The adenovirus miR30-based shRNA knockdown vector system is a highly efficient viral vector for knocking down expression of target gene(s) in a variety of mammalian cell types. It utilizes adenovirus-mediated delivery of a polycistronic expression cassette consisting of one or more miR30-based shRNAs (shRNAmiR) targeting gene(s) of interest and a user-selected ORF, where the vector remains as episomal DNA without integration into the host genome. The shRNAmiR transcript is processed by endogenous, cellular micro-RNA pathways to produce mature shRNAs, which facilitate degradation of target gene mRNAs. It is the preferred gene knockdown system in vivo, often used in gene therapy and vaccination. 

An adenovirus miR30-based shRNA vector is first constructed as a plasmid in E. coli. The polycistronic expression cassette consisting of one or more shRNAmiRs targeting gene(s) of interest and a user-selected ORF is cloned between the two inverted terminal repeats (ITRs). It is then transfected into packaging cells, where the region of the vector between the 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.

Unlike conventional shRNA vectors, which utilize RNA polymerase III promoters such as U6, miRNA-based shRNA systems are placed under the control of standard RNA polymerase II promoters. This allows the use of tissue-specific, inducible, or variable-strength promoters, enabling a variety of experimental applications not possible with constitutive U6 promoters.

The ability of RNA polymerase II promoters to efficiently transcribe long transcripts in the miRNA-based shRNA systems also provides additional advantages relative to other knockdown vector systems. Multiple shRNAmiRs can be transcribed as a single polycistron, which is processed to form mature shRNAs within the cell. This allows knockdown of multiple genes or targeting of multiple regions within the same gene using a single transcript. As a result, this vector is available for expressing either single or multiple shRNAmiRs. Secondly, in this vector system, a user-selected protein coding gene is also positioned within the same polycistron as the shRNAmiRs. The expression of this ORF can be used to directly monitor shRNA transcription (if a marker ORF is used) or can be used for other purposes requiring co-expression of an ORF and shRNA(s).

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.

Promoter choice: Unlike standard shRNA systems, which utilize RNA polymerase III promoters such as U6, miR30-based shRNAs can be transcribed by diverse RNA polymerase II promoters. This also enables the use of tissue-specific or inducible promoters.

Multiple shRNA co-expression: Because RNA polymerase II efficiently transcribes long RNAs, multiple shRNAmiRs can be expressed as a polycistron from a single promoter. Therefore, this vector is available for expressing either single or multiple shRNAmiRs.  

Co-expression of a reporter ORF: A user-selected gene of interest or reporter gene ORF is co-expressed with the shRNAmiRs, as a polycistron. This facilitates direct monitoring of shRNA transcription.

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 >10¹¹ 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).

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.

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 technically demanding and time consuming.

Single miR30-shRNA adenovirus shRNA knockdown vector

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: Drives transcription of the downstream ORF and shRNAmiR polycistron. This is an RNA polymerase II promoter, rather than an RNA polymerase III promoter such as U6.

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 or reporter gene is placed here. This can be used to monitor shRNA expression.

5' miR-30E: An optimized version of the human miR30 5’ context sequence. Facilitates maturation and processing of the shRNA and separation from the tandemly transcribed ORF and other shRNAs.

3' miR-30E: An optimized version of the human miR30 3’ context sequence. Facilitates maturation and processing of the shRNA and separation from the tandemly transcribed ORF and other shRNAs.

miR30-shRNA: This sequence is derived from your target sequence and is transcribed to form the stem portion of the “hairpin” structure of the shRNA.

TK pA: Herpes simplex virus thymidine kinase polyadenylation signal. Facilitates transcription termination and polyadenylation of the upstream ORF and shRNAmiR polycistron.

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.