Baculoviral transduction facilitates TALEN-mediated targeted transgene integration and Cre/LoxP cassette exchange in human-induced pluripotent stem cells.

Safety and reliability of transgene integration in human genome continue to pose challenges for stem cell-based gene therapy. Here, we report a baculovirus-transcription activator-like effector nuclease system for AAVS1 locus-directed homologous recombination in human induced pluripotent stem cells (iPSCs). This viral system, when optimized in human U87 cells, provided a targeted integration efficiency of 95.21% in incorporating a Neo-eGFP cassette and was able to mediate integration of DNA insert up to 13.5 kb. In iPSCs, targeted integration with persistent transgene expression was achieved without compromising genomic stability. The modified iPSCs continued to express stem cell pluripotency markers and maintained the ability to differentiate into three germ lineages in derived embryoid bodies. Using a baculovirus-Cre/LoxP system in the iPSCs, the Neo-eGFP cassette at the AAVS1 locus could be replaced by a Hygro-mCherry cassette, demonstrating the feasibility of cassette exchange. Moreover, as assessed by measuring γ-H2AX expression levels, genome toxicity associated with chromosomal double-strand breaks was not detectable after transduction with moderate doses of baculoviral vectors expressing transcription activator-like effector nucleases. Given high targeted integration efficiency, flexibility in transgene exchange and low genome toxicity, our baculoviral transduction-based approach offers great potential and attractive option for precise genetic manipulation in human pluripotent stem cells.

In vivo knockdown of hippocampal miR-132 expression impairs memory acquisition of trace fear conditioning.

MicroRNA-132 (miR-132) has been demonstrated to affect multiple neuronal functions, including dendritic growth and spinogenesis in cultured neurons and brain slices, as well as learning behavior of animals. However, its role in acquisition of temporal-associated memory remains unclear. In this study, we demonstrated that the mature miR-132 level in mouse hippocampus was significantly increased at 30 min after trace fear conditioning, a type of temporal-associated learning, and returned to baseline values in 2 h. We then knocked down miR-132 expression in vivo by infusing a lentivector expressing anti-miR-132 hairpin RNA into the third ventricle near the anterior hippocampi such RNA diffused laterally to both hippocampal formations, later confirmed by histological analysis. This approach successfully reduced hippocampal miR-132 expression in both naïve and trace fear conditioned groups, and impaired acquisition of trace fear memory in mice. To our knowledge, this result is the first demonstration of change in temporal learning behavior by reducing microRNA (miRNA) level specifically in the hippocampal region.

Targeted transgene insertion into the AAVS1 locus driven by baculoviral vector-mediated zinc finger nuclease expression in human-induced pluripotent stem cells.

BACKGROUND: The AAVS1 locus is viewed as a 'safe harbor' for transgene insertion into human genome. In the present study, we report a new method for AAVS1 targeting in human-induced pluripotent stem cells (hiPSCs). METHODS: We have developed two baculoviral transduction systems: one to deliver zinc finger nuclease (ZFN) and a DNA donor template for site-specific gene insertion and another to mediate Cre recombinase-mediated cassette exchange system to replace the inserted transgene with a new transgene. RESULTS: Our ZFN system provided the targeted integration efficiency of a Neo-EGFP cassette of 93.8% in G418-selected, stable hiPSC colonies. Southern blotting analysis of 20 AASV1 targeted colonies revealed no random integration events. Among 24 colonies examined for mono- or biallelic AASV1 targeting, 25% of them were biallelically modified. The selected hiPSCs displayed persistent enhanced green fluorescent protein expression and continued the expression of stem cell pluripotency markers. The hiPSCs maintained the ability to differentiate into three germ lineages in derived embryoid bodies and transgene expression was retained in the differentiated cells. After pre-including the loxP-docking sites into the Neo-EGFP cassette, we demonstrated that a baculovirus-Cre/loxP system could be used to facilitate the replacement of the Neo-EGFP cassette with another transgene cassette at the AAVS1 locus. CONCLUSIONS: Given high targeting efficiency, stability in expression of inserted transgene and flexibility in transgene exchange, the approach reported in the present study holds potential for generating genetically-modified human pluripotent stem cells suitable for developmental biology research, drug development, regenerative medicine and gene therapy.

Genetic rearrangements of variable di-residue (RVD)-containing repeat arrays in a baculoviral TALEN system.

Virus-derived gene transfer vectors have been successfully employed to express the transcription activator-like effector nucleases (TALENs) in mammalian cells. Since the DNA-binding domains of TALENs consist of the variable di-residue (RVD)-containing tandem repeat modules and virus genome with repeated sequences is susceptible to genetic recombination, we investigated several factors that might affect TALEN cleavage efficiency of baculoviral vectors. Using a TALEN system designed to target the AAVS1 locus, we observed increased sequence instability of the TALE repeat arrays when a higher multiplicity of infection (MOI) of recombinant viruses was used to produce the baculoviral vectors. We also detected more deleterious mutations in the TALE DNA-binding domains when both left and right TALEN arms were placed into a single expression cassette as compared to the viruses containing one arm only. The DNA sequence changes in the domains included deletion, addition, substitution, and DNA strand exchange between the left and right TALEN arms. Based on these observations, we have developed a protocol using a low MOI to produce baculoviral vectors expressing TALEN left and right arms separately. Cotransduction of the viruses produced by this optimal protocol provided an improved TALEN cleavage efficiency and enabled effective site-specific transgene integration in human cells.

Zinc finger nuclease-expressing baculoviral vectors mediate targeted genome integration of reprogramming factor genes to facilitate the generation of human induced pluripotent stem cells.

Integrative gene transfer using retroviruses to express reprogramming factors displays high efficiency in generating induced pluripotent stem cells (iPSCs), but the value of the method is limited because of the concern over mutagenesis associated with random insertion of transgenes. Site-specific integration into a preselected locus by engineered zinc-finger nuclease (ZFN) technology provides a potential way to overcome the problem. Here, we report the successful reprogramming of human fibroblasts into a state of pluripotency by baculoviral transduction-mediated, site-specific integration of OKSM (Oct3/4, Klf4, Sox2, and c-myc) transcription factor genes into the AAVS1 locus in human chromosome 19. Two nonintegrative baculoviral vectors were used for cotransduction, one expressing ZFNs and another as a donor vector encoding the four transcription factors. iPSC colonies were obtained at a high efficiency of 12% (the mean value of eight individual experiments). All characterized iPSC clones carried the transgenic cassette only at the ZFN-specified AAVS1 locus. We further demonstrated that when the donor cassette was flanked by heterospecific loxP sequences, the reprogramming genes in iPSCs could be replaced by another transgene using a baculoviral vector-based Cre recombinase-mediated cassette exchange system, thereby producing iPSCs free of exogenous reprogramming factors. Although the use of nonintegrating methods to generate iPSCs is rapidly becoming a standard approach, methods based on site-specific integration of reprogramming factor genes as reported here hold the potential for efficient generation of genetically amenable iPSCs suitable for future gene therapy applications.