Conventional gene targeting protocols lead to loss of targeted cells when applied to a silent gene locus in primary fibroblasts

Knock-in of large reporter genes in human cells via CRISPR/Cas9-induced homology-dependent and independent DNA repair

CRISPR/Cas9-induced site-specific DNA double-strand breaks (DSBs) can be repaired by homology-directed repair (HDR) or non-homologous end joining (NHEJ) pathways. Extensive efforts have been made to knock-in exogenous DNA to a selected genomic locus in human cells; which, however, has focused on HDR-based strategies and was proven inefficient. Here, we report that NHEJ pathway mediates efficient rejoining of genome and plasmids following CRISPR/Cas9-induced DNA DSBs, and promotes high-efficiency DNA integration in various human cell types. With this homology-independent knock-in strategy, integration of a 4.6 kb promoterless ires-eGFP fragment into the GAPDH locus yielded up to 20% GFP+ cells in somatic LO2 cells, and 1.70% GFP+ cells in human embryonic stem cells (ESCs). Quantitative comparison further demonstrated that the NHEJ-based knock-in is more efficient than HDR-mediated gene targeting in all human cell types examined. These data support that CRISPR/Cas9-induced NHEJ provides a valuable new path for efficient genome editing in human ESCs and somatic cells.

AAV-mediated gene targeting methods for human cells

Gene targeting with adeno-associated virus (AAV) vectors has been demonstrated in multiple human cell types, with targeting frequencies ranging from 10(-5) to 10(-2) per infected cell. These targeting frequencies are 1-4 logs higher than those obtained by conventional transfection or electroporation approaches. A wide variety of different types of mutations can be introduced into chromosomal loci with high fidelity and without genotoxicity. Here we provide a detailed protocol for gene targeting in human cells with AAV vectors. We describe methods for vector design, stock preparation and titration. Optimized transduction protocols are provided for human pluripotent stem cells, mesenchymal stem cells, fibroblasts and transformed cell lines, as well as a method for identifying targeted clones by Southern blots. This protocol (from vector design through a single round of targeting and screening) can be completed in ∼10 weeks; each subsequent round of targeting and screening should take an additional 7 weeks.

[New advances in animal transgenic technology]

Animal transgenic technology is one of the fastest growing biotechnology in the 21st century. It is used to integrate foreign genes into the animal genome by genetic engineering technology so that foreign genes can be expressed and inherited to the offspring. The transgenic efficiency and precise control of gene expression are the key limiting factors on preparation of transgenic animals. A variety of transgenic techniques are available, each of which has its own advantages and disadvantages and still needs further study because of unresolved technical and safety issues. With the in-depth research, the transgenic technology will have broad application prospects in the fields of exploration of gene function, animal genetic improvement, bioreactor, animal disease models, organ transplantation and so on. This article reviews the recently developed animal gene transfer techniques, including germline stem cell mediated method to improve the efficiency, gene targeting to improve the accuracy, RNA interference (RNAi)-mediated gene silencing technology, and the induced pluripotent stem cells (iPS) transgenic technology. The new transgenic techniques can provide a better platform for the study of trans-genic animals and promote the development of medical sciences, livestock production, and other fields.

Gene targeting from laboratory to livestock: current status and emerging concepts

The development of methods for cell-mediated transgenesis, based on somatic cell nuclear transfer, provides a tremendous opportunity to shape the genetic make-up of livestock animals in a much more directed approach than traditional animal breeding and selection schemes. Progress in the site-directed modulation of livestock genomes is currently limited by the low efficiencies of gene targeting imposed by the low frequency of homologous recombination and limited proliferative capacity of primary somatic cells that are used to produce transgenic animals. Here we review the current state of the art in the field, discuss the crucial aspects of the methodology and provide an overview of emerging approaches to increase the efficiency of gene targeting in somatic cells.

Natural allelic variants of bovine ATP-binding cassette transporter ABCG2: increased activity of the Ser581 variant and development of tools for the discovery of new ABCG2 inhibitors

ATP-binding cassette transporter ABCG2 [breast cancer resistance protein (BCRP)] is a member of the ABC transporter superfamily that actively extrudes xenotoxins from cells and is a major determinant of the bioavailability of many compounds. ABCG2 expression is strongly induced during lactation in the mammary gland and is related to the active secretion of drugs into the milk. The presence of drug residues and environmental pollutants in milk is an outstanding problem for human milk consumption and milk industrial processes, involving important risks to public health and the dairy industry. In cows, a single nucleotide polymorphism (SNP) in this protein has been described previously (Tyr581) and is associated with higher fat and protein percentages and lower milk yield. However, whether this amino acid substitution affects ABCG2-mediated drug transport in cows, including milk secretion, required further exploration. We cloned the two variants of bovine ABCG2 and evaluated the effect of this SNP on mitoxantrone accumulation assays performed in ovine primary fibroblasts transiently expressing either of the variants. It is interesting to note that statistically significant differences in activity between both variants were observed, and the Ser581 variant was related with an increased efflux activity. In addition, we demonstrated that genistein is a very good inhibitor of bovine ABCG2 and identified new inhibitors of the transporter, such as the macrocyclic lactones, ivermectin, and selamectin. Moreover, the inhibitory effect of these compounds on human and murine ABCG2 homologs was confirmed using transduced Marbin-Dabin canine kidney II cells. These findings may have important implications regarding the presence of drug residues in milk and drug interactions affecting the pharmacological behavior of ABCG2 substrates.