PiggyBac miR30-Based shRNA Knockdown Vector

The piggyBac miR30-based shRNA knockdown vector system is a simple and efficient method for stably knocking down expression of target gene(s) in mammalian cells. This transposon-based system utilizes plasmid transfection (rather than viral transduction) to permanently integrate a polycistronic expression cassette consisting of one or more miR30-based shRNAs (shRNAmiR) targeting gene(s) of interest and a user-selected ORF into the host cell genome. The shRNAmiR transcript is processed by endogenous, cellular micro-RNA pathways to produce mature shRNAs, which facilitate degradation of target gene mRNAs. The permanent nature of knockdown by the piggyBac system has several major advantages over transient knockdown by synthetic siRNA (see Advantages section below).

The piggyBac miR30-based shRNA knockdown vector system contains two vectors, both engineered as E. coli plasmids. One vector, referred to as the helper plasmid, encodes the transposase. The other vector, referred to as the transposon plasmid, contains two terminal repeats (TRs) bracketing the region to be transposed, which includes the polycistronic shRNAmiR and ORF expression cassette. When the helper and transposon plasmids are co-transfected into target cells, the transposase produced from the helper plasmid recognizes the two TRs on the transposon, and inserts the flanked region including the two TRs into the host genome. Insertion typically occurs at host chromosomal sites that contain the TTAA sequence, which is duplicated on the two flanks of the integrated fragment.

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). 

PiggyBac is a class II transposon, meaning that it moves in a cut-and-paste manner, hopping from place to place without leaving copies behind. (In contrast, class I transposons move in a copy-and-paste manner.) Because the helper plasmid is only transiently transfected into host cells, it will get lost over time. With the loss of the helper plasmid, the integration of the transposon in the genome of host cells becomes permanent. If these cells are transfected with the helper plasmid again, the transposon could get excised from the genome of some cells, footprint free.

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.

Permanent integration and knockdown: Conventional transfection results in almost entirely transient delivery of DNA into host cells due to the loss of DNA over time. This problem is especially prominent in rapidly dividing cells. In contrast, transfection of mammalian cells with the piggyBac transposon plasmid along with the helper plasmid can deliver DNA sequences carried on the transposon permanently into host cells due to the integration of the transposon into the host genome. As a result, the knockdown of the target gene(s) achieved with this vector is stable and permanent. This can be an important advantage for many experimental goals. It allows long-term analysis of the knockdown phenotype. It facilitates the isolation of clones having different levels of knockdown and/or different phenotypes. When the knockdown vector carries a fluorescence marker such as EGFP, it also allows cells with different amounts of transposon integration (and hence potentially different levels of knockdown) to be isolated by flow sorting cells with different fluorescence intensity.

Reversibility: If cells carrying a piggyBac shRNA transposon are transfected with the helper plasmid again, the transposon may be excised from the genome of some cells, footprint free, eliminating expression of the shRNA from those cells. However, this will occur in only a subset of cells.

Technical simplicity: Delivering plasmid vectors into cells by conventional transfection is technically straightforward, and far easier than virus-based vectors which require the packaging of live virus.

Safety: Conventional transfection does not have the safety concerns which are often associated with viral vectors.

Limited cell type range: The delivery of piggyBac vectors into cells relies on transfection. The efficiency of transfection can vary greatly from cell type to cell type. Non-dividing cells are often more difficult to transfect than dividing cells, and primary cells are often harder to transfect than immortalized cell lines. Some important cell types, such as neurons and pancreatic β cells, are notoriously difficult to transfect. Additionally, plasmid transfection is largely limited to in vitro applications and rarely used in vivo. These issues limit the use of the piggyBac system.

Single miR30-shRNA piggyBac shRNA knockdown vector

5' ITR: 5' inverted terminal repeat. When a DNA sequence is flanked by two ITRs, the piggyBac transpose can recognize them, and insert the flanked region including the two ITRs into the host genome.

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.

rBG pA: Rabbit β-globin polyadenylation signal. Facilitates transcription termination and polyadenylation of the upstream ORF and shRNAmiR polycistron.

CMV promoter: Human cytomegalovirus immediate early promoter. It drives the ubiquitous expression of the downstream marker gene.

Marker: A drug selection gene (such as neomycin resistance), a visually detectable gene (such as EGFP), or a dual-reporter gene (such as EGFP/Neo). This allows cells transduced with the vector to be selected and/or visualized.

BGH pA: Bovine growth hormone polyadenylation signal. It facilitates transcriptional termination of the upstream ORF.

3' ITR: 3' inverted terminal repeat.

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

pUC ori: pUC origin of replication. Plasmids carrying this origin exist in high copy numbers in E. coli.

Multiple miR30-shRNA piggyBac shRNA knockdown vector