Production of MSTN knockout porcine cells using adenine base-editing-mediated exon skipping

Gene-knockout pigs have important applications in agriculture and medicine. Compared with CRISPR/Cas9 and cytosine base editing (CBE) technologies, adenine base editing (ABE) shows better safety and accuracy in gene modification. However, because of the characteristics of gene sequences, the ABE system cannot be widely used in gene knockout. Alternative splicing of mRNA is an important biological mechanism in eukaryotes for the formation of proteins with different functional activities. The splicing apparatus recognizes conserved sequences of the 5' end splice donor and 3' end splice acceptor motifs of introns in pre-mRNA that can trigger exon skipping, leading to the production of new functional proteins, or causing gene inactivation through frameshift mutations. This study aimed to construct a MSTN knockout pig by inducing exon skipping with the aid of the ABE system to expand the application of the ABE system for the preparation of knockout pigs. In this study, first, we constructed ABEmaxAW and ABE8eV106W plasmid vectors and found that their editing efficiencies at the targets were at least sixfold and even 260-fold higher than that of ABEmaxAW by contrasting the editing efficiencies at the gene targets of endogenous CD163, IGF2, and MSTN in pigs. Subsequently, we used the ABE8eV106W system to realize adenine base (the base of the antisense strand is thymine) editing of the conserved splice donor sequence (5'-GT) of intron 2 of the porcine MSTN gene. A porcine single-cell clone carrying a homozygous mutation (5'-GC) in the conserved sequence (5'-GT) of the intron 2 splice donor of the MSTN gene was successfully generated after drug selection. Unfortunately, the MSTN gene was not expressed and, therefore, could not be characterized at this level. No detectable genomic off-target edits were identified by Sanger sequencing. In this study, we verified that the ABE8eV106W vector had higher editing efficiency and could expand the editing scope of ABE. Additionally, we successfully achieved the precise modification of the alternative splice acceptor of intron 2 of the porcine MSTN gene, which may provide a new strategy for gene knockout in pigs.

[Overexpression of Keap1 inhibits the cell proliferation and metastasis and overcomes the drug resistance in human lung cancer A549 cells]

OBJECTIVE

To investigate the effect of Keap1-Nrf2 pathway on cell proliferation, metastasis and drug resistance of human lung cancer A549 cell line.

METHODS

A549-Keap1 cell line, constantly expressing wild type Keap1, was established by lentiviral transfection. Real-time RT-PCR and western blot were used to determine the expression of Nrf2 and its target gene in A549 cells. Sulforhodamine B (SRB) assay, flow cytometry, colony formation assay, transwell assay, and cell wound-healing assay were performed to explore the effect of wild type Keap1 expression on the proliferation, invasion, migration and drug resistance of A549 cells.

RESULTS

Over-expressed Keap1 decreased the expression of Nrf2 protein and the mRNA level of its downstream target genes and inhibited the ability of cell proliferation and clone formation of A549 cells. Keap1 overexpression induced G0/G1 phase arrest. The percentage of A549-Keap1 cells in G0/G1 phase was significantly higher than that of A549-GFP cells (80.2±5.9)% vs. (67.1±0.9%)(P<0.05). Compared with the invasive A549-Keap1 cells (156.33±17.37), the number of invasive A549-GFP cells was significantly higher (306.67±22.19) in a high power field. Keap1 overexpression significantly enhanced the sensitivity of A549 cells to carboplatin and gemcitabine (P<0.01). The IC50s of carboplatin in A549-Keap1 and A549-GFP cells were (52.1±3.3) μmol/L and (107.8±12.9) μmol/L, respectively. The IC50s of gemcitabine in A549-Keap1 and A549-GFP cells were (6.8±1.2) μmol/L and (9.9±0.5) μmol/L, respectively.

CONCLUSION

Keap1 overexpression significantly inhibits the expression of Nrf2 and its downstream target genes, suppresses tumor cell proliferation and metastasis, and enhances the sensitivity of A549 cells to anticancer drugs.

Repeated intrathecal administration of plasmid DNA complexed with polyethylene glycol-grafted polyethylenimine led to prolonged transgene expression in the spinal cord

Gene delivery into the spinal cord provides a potential approach to the treatment of spinal cord traumatic injury, amyotrophic lateral sclerosis, and spinal muscular atrophy. These disorders progress over long periods of time, necessitating a stable expression of functional genes at therapeutic levels for months or years. We investigated in this study the feasibility of achieving prolonged transgene expression in the rat spinal cord through repeated intrathecal administration of plasmid DNA complexed with 25 kDa polyethylenimine (PEI) into the lumbar subarachnoid space. With a single injection, DNA/PEI complexes could provide transgene expression in the spinal cord 40-fold higher than naked plasmid DNA. The transgene expression at the initial level persisted for about 5 days, with a low-level expression being detectable for at least 8 weeks. When repeated dosing was tested, a 70% attenuation of gene expression was observed following reinjection at a 2-week interval. This attenuation was associated with apoptotic cell death and detected even using complexes containing a noncoding DNA that did not mediate any gene expression. When each component of the complexes, PEI polymer or naked DNA alone, were tested in the first dosing, no reduction was found. Using polyethylene glycol (PEG)-grafted PEI for DNA complexes, no attenuation of gene expression was detected after repeated intrathecal injections, even in those rats receiving three doses, administered 2 weeks apart. Lumbar puncture is a routine and relatively nontraumatic clinical procedure. Repeated administration of DNA complexed with PEG-grafted PEI through this less invasive route may prolong the time span of transgene expression when needed, providing a viable strategy for the gene therapy of spinal cord disorders.

Polyethylene glycol modified polyethylenimine for improved CNS gene transfer: effects of PEGylation extent

Poor solubility of polycation complexes with DNA is one drawback for their in vivo use as gene delivery systems. PEGylation often can improve the solubility of the complexes, minimize their aggregation and reduce their interaction with proteins in the physiological fluid. We investigated in vivo application of polyethylene glycol (PEG) modified polyethylenimine (PEI) for gene expression in the central nervous system. Varied numbers of linear PEG (2 kDa) were grafted to branched PEI (25 kDa) from the average number of PEG per one PEI macromolecule at 1-14.5. While higher degrees of PEG grafting did not improve gene expression, a PEI conjugate with one segment of PEG was able to mediate transgene expression in the spinal cord up to 11-fold higher than PEI homopolymer after intrathecal administration of its DNA complexes into the lumbar spinal cord subarachnoid space. Improved gene expression with this conjugate was observed as well in the brain after the lumbar injection. As assessed in in vitro studies, the PEI conjugate with a low degree of PEG grafting was able to reduce the size of polymer DNA complexes, prevent the aggregation of complexes, decrease the interactions of the complexes with serum proteins, counter the inhibition of serum to gene transfer, and enhance transfection efficiency, although not significant in affecting complex formation and reducing in vitro cell toxicity of PEI. The study provides the in vivo evidence that an appropriate degree of PEG modification is decisive in improving gene transfer mediated by PEGylated polymers.