Increased mutation efficiency of CRISPR/Cas9 genome editing in banana by optimized construct

The CRISPR/Cas9-mediated genome editing system has been used extensively to engineer targeted mutations in a wide variety of species. Its application in banana, however, has been hindered because of the species’ triploid nature and low genome editing efficiency. This has delayed the development of a DNA-free genome editing approach. In this study, we reported that the endogenous U6 promoter and banana codon-optimized Cas9 apparently increased mutation frequency in banana, and we generated a method to validate the mutation efficiency of the CRISPR/Cas9-mediated genome editing system based on transient expression in protoplasts. The activity of the MaU6c promoter was approximately four times higher than that of the OsU6a promoter in banana protoplasts. The application of this promoter and banana codon-optimized Cas9 in CRISPR/Cas9 cassette resulted in a fourfold increase in mutation efficiency compared with the previous CRISPR/Cas9 cassette for banana. Our results indicated that the optimized CRISPR/Cas9 system was effective for mutating targeted genes in banana and thus will improve the applications for basic functional genomics. These findings are relevant to future germplasm improvement and provide a foundation for developing DNA-free genome editing technology in banana.

Enhanced Efficiency of flySAM by Optimization of sgRNA Parameters in Drosophila

The flySAM/CRISPRa system has recently emerged as a powerful tool for gain-of-function studies in Drosophila melanogaster. This system includes Gal4/UAS-driven dCas9 activators and U6 promoter-controlled sgRNA. Having established dCas9 activators superior to other combinations, to further enhance the efficiency of the targeting activators we systematically optimized the parameters of the sgRNA. Interestingly, the most efficient sgRNAs were found to accumulate in the region from -150bp to -450bp upstream of the transcription start site (TSS), and the activation efficiency showed a strong positive correlation with the GC content of the sgRNA targeting sequence. In addition, the target region is dominant to the GC content, as sgRNAs targeting areas beyond -600bp from the TSS lose efficiency even when containing 75% GC. Surprisingly, when comparing the activities of sgRNAs targeting to either DNA strand, sgRNAs targeting to the non-template strand outperform those complementary to the template strand, both in cells and in vivo. In summary, we define criteria for sgRNA design which will greatly facilitate the application of CRISPRa in gain-of-function studies.

Assembly of SNAPc, Bdp1, and TBP on the U6 snRNA Gene Promoter in Drosophila melanogaster

ABSTRACT U6 snRNA is transcribed by RNA polymerase III (Pol III) and has an external upstream promoter that consists of a TATA sequence recognized by the TBP subunit of the Pol III basal transcription factor IIIB and a proximal sequence element (PSE) recognized by the small nuclear RNA activating protein complex (SNAPc). Previously, we found that Drosophila melanogaster SNAPc (DmSNAPc) bound to the U6 PSE can recruit the Pol III general transcription factor Bdp1 to form a stable complex with the DNA. Here, we show that DmSNAPc-Bdp1 can recruit TBP to the U6 promoter, and we identify a region of Bdp1 that is sufficient for TBP recruitment. Moreover, we find that this same region of Bdp1 cross-links to nucleotides within the U6 PSE at positions that also cross-link to DmSNAPc. Finally, cross-linking mass spectrometry reveals likely interactions of specific DmSNAPc subunits with Bdp1 and TBP. These data, together with previous findings, have allowed us to build a more comprehensive model of the DmSNAPc-Bdp1-TBP complex on the U6 promoter that includes nearly all of DmSNAPc, a portion of Bdp1, and the conserved region of TBP.

Novel mosaic mice with diverse applications

Gene-deficient mouse models are indispensable for interrogating mammalian gene functions, but the conventional models allow the study of only one or few genes per mouse line, which has been a bottleneck in functional genomics. To confront the challenge, we have combined the CRISPR-Cas and Cre-Lox systems to develop a novel type of mosaic mice termed MARC (Mosaic Animal based on gRNA and Cre) for targeting many genes per mouse but only one gene per cell. This technology employs a transgene comprising a modified U6 promoter upstream of a series of floxed gRNA genes linked together in tandem, with one gRNA expressed per cell following Cre-mediated recombination. At least 61 gRNA genes can be stably maintained in the transgene, and importantly, enables robust proof-of-principle in vivo screens, demonstrating the potential for quickly evaluating the functions of many genes in diverse tissues in a single MARC line. In theory, MARC can also be analyzed by single-cell sequencing, and should enable cost-effective derivation of conventional single-gene-KO lines via simple breeding. Our study establishes MARC as an important addition to the mouse genetics toolbox.

Generation of a CRISPR/Cas9-Based Vector Specific for Gene Manipulation in Leishmania major

Background: Gene manipulation strategies including gene knockout and editing are becoming more sophisticated in terms of mechanism of action, efficacy and ease of use. In classical molecular methods of gene knockout, homologous arms are designed for induction of crossing over event in double strand DNA. Recently, CRISPR/Cas9 system has been emerged as a precise and powerful tool for gene targeting. In this effort, we aimed to generate a CRISPR/Cas9-based vector specific for targeting genes in Leishmania parasites. Methods: U6 and DHFR promoters and neomycin-resistance gene were amplified from genome of L. major (MHRO/IR/75/ER) and pEGFP-N1, respectively. U6 promoter was cloned in pX330 vector which is named as pX330-U6. DHFR promoter and neo resistance gene sequence fragments were fused using a combination of SOE (Splicing by overlap extension)-PCR and T/A cloning techniques. To generate pX-leish, fused fragments su-bcloned into the pX330-U6. Two sgRNAs were designed to target the gp63 gene and cloned in pX-leish. Results: The pX-leish vector was designed for simultaneous expression of cas9 and G418 resistance proteins along with a self-cleaving 2A peptide for efficient separation of the two proteins. In this study pX-leish was designed with 3 features: 1) Compatible promoters with Leishmania parasites. 2) Insertion of antibiotic selection marker 3) Designing an all-in-one vector containing all components required for CRISPR/Cas9 system. Conclusion: This modified system would be valuable in genome manipulation studies in Leishmania for vaccine research in future.

Evaluation of off-target and on-target scoring algorithms and integration into the guide RNA selection tool CRISPOR

Background The success of the CRISPR/Cas9 genome editing technique depends on the choice of the guide RNA sequence, which is facilitated by various websites. Despite the importance and popularity of these algorithms, it is unclear to which extent their predictions are in agreement with actual measurements. Results We conduct the first independent evaluation of CRISPR/Cas9 predictions. To this end, we collect data from eight SpCas9 off-target studies and compare them with the sites predicted by popular algorithms. We identify problems in one implementation but found that sequence-based off-target predictions are very reliable, identifying most off-targets with mutation rates superior to 0.1 %, while the number of false positives can be largely reduced with a cutoff on the off-target score. We also evaluate on-target efficiency prediction algorithms against available datasets. The correlation between the predictions and the guide activity varied considerably, especially for zebrafish. Together with novel data from our labs, we find that the optimal on-target efficiency prediction model strongly depends on whether the guide RNA is expressed from a U6 promoter or transcribed in vitro. We further demonstrate that the best predictions can significantly reduce the time spent on guide screening. Conclusions To make these guidelines easily accessible to anyone planning a CRISPR genome editing experiment, we built a new website (http://crispor.org) that predicts off-targets and helps select and clone efficient guide sequences for more than 120 genomes using different Cas9 proteins and the eight efficiency scoring systems evaluated here.

Optimization of Bcl11a Knockdown By miRNA Scaffold Embedded Shrnas Leading to Enhanced Induction of Fetal Hemoglobin in Erythroid Cells for the Treatment of Beta-Hemoglobinopathies

Repression of the BCL11A protein could represent a therapeutic target for beta-hemoglobinopathies, as its knock-down has been shown to induce the expression of the fetal HBG (y-globin) gene ultimately leading to increased levels of the fetal hemoglobin tetramer (HbF, a2y2). In mice, Bcl11a is a key repressor of the murine HBG homolog Hbb-y. RNA interference (RNAi) technology using short hairpin RNAs (shRNAs) expressed via pol III promoters has been used to modulate gene expression in a variety of mammalian cell types. However, we found a negative impact of Bcl11a knockdown on hematopoietic stem cells (HSCs), limiting the repopulation efficiency and long-term engraftment after genetic modification, which is a major impediment for its translation into human therapeutic applications. To achieve lineage-specific targeting of mRNAs in an attempt to reduce HSC toxicity, expression of shRNAs via pol II promoters is required, necessitating embedding the shRNA in mammalian microRNA (shRNAmir) sequences for expression and processing. To achieve optimal knockdown of the Bcl11a transcription factor in erythroid progenitor and precursor cells, we first compared the efficiency of mRNA modulation via pol III (U6-promoter) vs pol II (SFFV-promoter) based lentiviral vectors. We demonstrate a 100-1000 fold lower Hbb-y induction using shRNAmir vs pol III mediated shRNA vector backbones due to reduced Bcl11a knockdown efficiency. In order to understand the molecular basis for these differences, small RNA sequence analysis was performed on murine erythroleukemia cells (MEL) cells transduced by multiple shRNA–shRNAmir pairs. We show that shRNAs expressed via a U6 promoter yield guide strand sequences which differ by a 2-4 nt shift compared to pol II driven (shRNAmir) mature guide strand sequences. RNA sequencing demonstrated that the stretch of uridines making up part of the pol III termination signal is transcribed and included at the 3’ end of the shRNA. This results in the generation of mature guide strand sequences with an alternative seed sequence compared to the predicted sequence and compared to miRNA embedded shRNAs. The difference in the seed sequences between the two expression systems strongly influences the efficacy of target gene knockdown, leading to reduced knockdown in pol II based vectors. We engineered a 4bp shift into guide strands of shRNAmirs that resulted in a faithfully processed shRNA sequence (a mature guide strand sequence identical to U6-driven sh-RNAs) and improved knock-down efficiency of Bcl11a at the protein level in most cases. The improved knockdown of Bcl11a was associated with a 100-300-fold enhancement of Hbb-y induction in MEL cells. Based on these results, we propose a modified strategy for the prospective design of shRNAmirs derived from shRNA screens in pol III vector backbones to achieve lineage-specific regulation of target genes. Targeted expression of shRNAmiRs to the erythroid compartment driven by a b-globin promoter/LCR element circumvented the detrimental effect on HSC engraftment, while still mediating efficient BCL11A knockdown, leading to high y-globin induction and formation of substantial amounts of fetal hemoglobin in human CD34-derived erythroid cells in vitro. Disclosures No relevant conflicts of interest to declare.