Improvements of TKC Technology Accelerate Isolation of Transgene-Free CRISPR/Cas9-Edited Rice Plants

Elimination of the CRISPR/Cas9 constructs in edited plants is a prerequisite for assessing genetic stability, conducting phenotypic characterization, and applying for commercialization of the plants. However, removal of the CRISPR/Cas9 transgenes by genetic segregation and by backcross is laborious and time consuming. We previously reported the development of the transgene killer CRISPR(TKC) technology that uses a pair of suicide genes to trigger self-elimination of the transgenes without compromising gene editing efficiency. The TKC technology enables isolation of transgene-free CRISPR-edited plants within a single generation, greatly accelerating crop improvements. Here, we presented two new TKC vectors that show great efficiency in both editing the target gene and in undergoing self-elimination of the transgenes. The new vectors replaced the CaMV35 S promoter used in our previous TKC vector with two rice promoters to drive one of the suicide genes, providing advantages over our previous TKC vector under certain conditions. The vectors reported here offered more options and flexibility to conduct gene editing experiments in rice.

A transgene-free method for rapid and efficient generation of precisely edited pigs without monoclonal selection

Gene-edited pigs for agricultural and biomedical applications are typically generated using somatic cell nuclear transfer(SCNT).However, SCNT requires the use of monoclonal cells as donors, and the time-consuming and laborious monoclonal selection process limits the production of large populations of gene-edited animals. Here, we developed a rapid and efficient method named RE-DSRNP(reporter RNA enriched dual-sg RNA/CRISPR-Cas9 ribonucleoproteins) for generating gene-edited donor cells. RE-DSRNP takes advantage of the precise and efficient editing features of dual-sg RNA and the high editing efficiency, low off-target effects, transgene-free nature, and low cytotoxic characteristics of reporter RNA enriched RNPs(CRISPR-Cas9ribonucleoproteins), thus eliminating the need for the selection of monoclonal cells and thereby greatly reducing the generation time of donor cells from 3–4 weeks to 1 week, while also reducing the extent of apoptosis and chromosomal aneuploidy of donor cells. We applied RE-DSRNP to produce cloned pigs bearing a deletion edit of the wild-type p53-induced phosphatase 1(WIP1)gene: among 32 weaned cloned pigs, 31(97%) carried WIP1 edits, and 15(47%) were homozygous for the designed fragment deletion, and no off-target event was detected. The WIP1 knockout(KO) pigs exhibited male reproductive disorders, illustrating the utility of RE-DSRNP for rapidly generating precisely edited animals for functional genomics and disease research. REDSRNP's strong editing performance in a large animal and its marked reduction in the required time for producing SCNT donor cells support its application prospects for rapidly generating populations of transgene-free cloned animals.

Technological breakthroughs in generating transgene-free and genetically stable CRISPR-edited plants

CRISPR/Cas9 gene-editing technologies have been very effective in editing target genes in all major crop plants and offer unprecedented potentials in crop improvement.A major challenge in using CRISPR gene-editing technology for agricultural applications is that the target gene-edited crop plants need to be transgene free to maintain trait stability and to gain regulatory approval for commercial production.In this article,we present various strategies for generating transgene-free and target geneedited crop plants.The CRISPR transgenes can be removed by genetic segregation if the crop plants are reproduced sexually.Marker-assisted tracking and eliminating transgenes greatly decrease the time and labor needed for identifying the ideal transgene-free plants.Transgenes can be programed to undergo self-elimination when CRISPR genes and suicide genes are sequentially activated,greatly accelerating the isolation of transgene-free and target gene-edited plants.Transgene-free plants can also be generated using approaches that are considered non-transgenic such as ribonucleoprotein transfection,transient expression of transgenes without DNA integration,and nano-biotechnology.Here,we discuss the advantages and disadvantages of the various strategies in generating transgene-free plants and provide guidance for adopting the best strategies in editing a crop plant.

Integration-free Methods for Generating Induced Pluripotent Stem Cells

Induced pluripotent stem （iPS） cells by exogenous expression of four factors, Oct4, can be generated from mouse or human fibroblasts Sox2, Klf4 and c-Myc, and hold great potential for transplantation therapies and regenerative medicine. However, use of retroviral vectors during iPS cell generation has limited the techniques clinical application due to the potential risks resulting from genome integration of transgenes, including insertional mutations and altered differentiation potentials of the target cells, which may lead to pathologies such as tumorigenesis. Here we review recent progress in generating safer transgene-free or integration-free iPS cells, including the use of non-integrating vectors, excision of vectors after integration, DNA-free delivery of factors and chemical induction of pluripotency.

Single-nucleotide editing for zebra3 and wsl5 phenotypes in rice using CRISPR/Cas9-mediated adenine base editors

The CRISPR/Cas9-mediated base editing technology can efficiently generate point mutations in the genome without introducing a double-strand break(DSB)or supplying a DNA donor template for homology-directed repair(HDR).In this study,adenine base editors(ABEs)were used for rapid generation of precise point mutations in two distinct genes,OsWSL5,and OsZEBRA3(Z3),in both rice protoplasts and regenerated plants.The precisely engineered point mutations were stably inherited to subsequent generations.These single nucleotide alterations resulted in single amino acid changes and associated wsl5 and z3 phenotypes as evidenced by white stripe leaf and light green/dark green leaf pattern,respectively.Through selfing and genetic segregation,transgene-free,base edited wsl5 and z3 mutants were obtained in a short period of time.We noticed a novel mutation(V540A)in Z3 locus could also mimic the phenotype of Z3 mutation(S542P).Furthermore,we observed unexpected non-A/G or T/C mutations in the ABE editing window in a few of the edited plants.The ABE vectors and the method from this study could be used to simultaneously generate point mutations in multiple target genes in a single transformation and serve as a useful base editing tool for crop improvement as well as basic studies in plant biology.

Optimized protoplast isolation and transfection with a breakpoint:accelerating Cas9/sgRNA cleavage efficiency validation in monocot and dicot

The CRISPR-Cas genome editing tools are revolutionizing agriculture and basic biology with their simplicity and precision ability to modify target genomic loci.Software-predicted guide RNAs(gRNAs)often fail to induce efficient cleavage at target loci.Many target loci are inaccessible due to complex chromatin structure.Currently,there is no suitable tool available to predict the architecture of genomic target sites and their accessibility.Hence,significant time and resources are spent on performing editing experiments with inefficient guides.Although in vitro-cleavage assay could provide a rough assessment of gRNA efficiency,it largely excludes the interference of native genomic context.Transient in-vivo testing gives a proper assessment of the cleavage ability of editing reagents in a native genomic context.Here,we developed a modified protocol that offers highly efficient protoplast isolation from rice,Arabidopsis,and chickpea,using a sucrose gradient,transfection using PEG(polyethylene glycol),and validation of single guide RNAs(sgRNAs)cleavage efficiency of CRISPR-Cas9.We have optimized various parameters for PEG-mediated protoplast transfection and achieved high transfection efficiency using our protocol in both monocots and dicots.We introduced plasmid vectors containing Cas9 and sgRNAs targeting genes in rice,Arabidopsis,and chickpea protoplasts.Using dual sgRNAs,our CRISPRdeletion strategy offers straightforward detection of genome editing success by simple agarose gel electrophoresis.Sanger sequencing of PCR products confirmed the editing efficiency of specific sgRNAs.Notably,we demonstrated that isolated protoplasts can be stored for up to 24/48 h with little loss of viability,allowing a pause between isolation and transfection.This high-efficiency protocol for protoplast isolation and transfection enables rapid(less than 7 days)validation of sgRNA cleavage efficiency before proceeding with stable transformation.The isolation and transfection method can also be utilized for rapid validation of editing strategies,evaluating diverse editing reagents,regenerating plants from transfected protoplasts,gene expression studies,protein localization and functional analysis,and other applications.