DNA methylation can alter CRISPR/Cas9 editing frequency and DNA repair outcome in a target-specific manner

Profiling Cas9- and Cas12a-induced mutagenesis in Arabidopsis thaliana

With the advancement of CRISPR technologies, a comprehensive understanding of repair mechanisms following double-strand break (DSB) formation is important for improving the precision and efficiency of genetic modifications. In plant genetics, two Cas nucleases are widely used, i.e. Cas9 and Cas12a, which differ with respect to PAM sequence composition, position of the DSB relative to the PAM, and DSB-end configuration (blunt vs. staggered). The latter difference has led to speculations about different options for repair and recombination. Here, we provide detailed repair profiles for LbCas12a in Arabidopsis thaliana, using identical experimental settings previously reported for Cas9-induced DSBs, thus allowing for a quantitative comparison of both nucleases. For both enzymes, non-homologous end-joining (NHEJ) produces 70% of mutations, whereas polymerase theta-mediated end-joining (TMEJ) generates 30%, indicating that DSB-end configuration does not dictate repair pathway choice. Relevant for genome engineering approaches aimed at integrating exogenous DNA, we found that Cas12a similarly stimulates the integration of T-DNA molecules as does Cas9. Long-read sequencing of both Cas9 and Cas12a repair outcomes further revealed a previously underappreciated degree of DNA loss upon TMEJ. The most notable disparity between Cas9 and Cas12a repair profiles is caused by how NHEJ acts on DSB ends with short overhangs: non-symmetric Cas9 cleavage produce 1 bp insertions, which we here show to depend on polymerase Lambda, whereas staggered Cas12a DSBs are not subjected to fill-in synthesis. We conclude that Cas9 and Cas12a are equally effective for genome engineering purposes, offering flexibility in nuclease choice based on the availability of compatible PAM sequences.

Chromatin context-dependent effects of epigenetic drugs on CRISPR-Cas9 editing

The efficiency and outcome of CRISPR/Cas9 editing depends on the chromatin state at the cut site. It has been shown that changing the chromatin state can influence both the efficiency and repair outcome, and epigenetic drugs have been used to improve Cas9 editing. However, because the target proteins of these drugs are not homogeneously distributed across the genome, the efficacy of these drugs may be expected to vary from locus to locus. Here, we systematically analyzed this chromatin context-dependency for 160 epigenetic drugs. We used a human cell line with 19 stably integrated reporters to induce a double-stranded break in different chromatin environments. We then measured Cas9 editing efficiency and repair pathway usage by sequencing the mutational signatures. We identified 58 drugs that modulate Cas9 editing efficiency and/or repair outcome dependent on the local chromatin environment. For example, we find a subset of histone deacetylase inhibitors that improve Cas9 editing efficiency throughout all types of heterochromatin (e.g. PCI-24781), while others were only effective in euchromatin and H3K27me3-marked regions (e.g. apicidin). In summary, this study reveals that most epigenetic drugs alter CRISPR editing in a chromatin-dependent manner, and provides a resource to improve Cas9 editing more selectively at the desired location.

Noodles, the all-in-one system for on-target efficiency analysis of CRISPR guide RNAs

The efficiency of clustered regularly interspaced short palindromic repeats (CRISPR) guide RNA (gRNA) targeting is critical for CRISPR associated protein 9 (Cas9)-dependent genomic modifications. Here, we developed Noodles, an all-in-one system to test the on-target activity of gRNAs easily and efficiently. Single-strand annealing repair mechanism of the split luciferase gene allows a positive selection of gRNAs efficiently driving nuclease activity of Cas9 from Streptococcus pyogenes (SpCas9). Our system can reliably validate in silico-predicted gRNAs before implementing them for in vitro and in vivo applications. Altogether, Noodles might be an asset for researchers and bioengineers, saving their time and efforts, while keeping the screening efficient and sensitive. •All-in-one dual-luciferase system to easily probe on-target activity of gRNAs based on homology-directed repair mechanism.•Easy-to-subclone spCas9-based 2-plasmid system comprising Renilla luciferase for transfection efficiency control.

Deep learning models incorporating endogenous factors beyond DNA sequences improve the prediction accuracy of base editing outcomes

Adenine base editors (ABEs) and cytosine base editors (CBEs) enable the single nucleotide editing of targeted DNA sites avoiding generation of double strand breaks, however, the genomic features that influence the outcomes of base editing in vivo still remain to be characterized. High-throughput datasets from lentiviral integrated libraries were used to investigate the sequence features affecting base editing outcomes, but the effects of endogenous factors beyond the DNA sequences are still largely unknown. Here the base editing outcomes of ABE and CBE were evaluated in mammalian cells for 5012 endogenous genomic sites and 11,868 genome-integrated target sequences, with 4654 genomic sites sharing the same target sequences. The comparative analyses revealed that the editing outcomes of ABE and CBE at endogenous sites were substantially different from those obtained using genome-integrated sequences. We found that the base editing efficiency at endogenous target sites of both ABE and CBE was influenced by endogenous factors, including epigenetic modifications and transcriptional activity. A deep-learning algorithm referred as BE_Endo, was developed based on the endogenous factors and sequence information from our genomic datasets, and it yielded unprecedented accuracy in predicting the base editing outcomes. These findings along with the developed computational algorithms may facilitate future application of BEs for scientific research and clinical gene therapy.

Engineering Cas9: next generation of genomic editors

The Cas9 endonuclease of the CRISPR/Cas type IIA system from Streptococcus pyogenes is the heart of genome editing technology that can be used to treat human genetic and viral diseases. Despite its large size and other drawbacks, S. pyogenes Cas9 remains the most widely used genome editor. A vast amount of research is aimed at improving Cas9 as a promising genetic therapy. Strategies include directed evolution of the Cas9 protein, rational design, and domain swapping. The first generation of Cas9 editors comes directly from the wild-type protein. The next generation is obtained by combining mutations from the first-generation variants, adding new mutations to them, or refining mutations. This review summarizes and discusses recent advances and ways in the creation of next-generation genomic editors derived from S. pyogenes Cas9. KEY POINTS: • The next-generation Cas9-based editors are more active than in the first one. • PAM-relaxed variants of Cas9 are improved by increased specificity and activity. • Less mutagenic and immunogenic variants of Cas9 are created.

Characteristics of subtype III-A CRISPR-Cas system in Mycobacterium tuberculosis: An overview

CRISPR-Cas systems are the only RNA- guided adaptive immunity pathways that trigger the detection and destruction of invasive phages and plasmids in bacteria and archaea. Due to its prevalence and mystery, the Class 1 CRISPR-Cas system has lately been the subject of several studies. This review highlights the specificity of CRISPR-Cas system III-A in Mycobacterium tuberculosis, the tuberculosis-causing pathogen, for over twenty years. We discuss the difference between the several subtypes of Type III and their defence mechanisms. The anti-CRISPRs (Acrs) recently described, the critical role of Reverse transcriptase (RT) and housekeeping nuclease for type III CRISPR-Cas systems, and the use of this cutting-edge technology, its impact on the search for novel anti-tuberculosis drugs.

Genome editing in plants using the compact editor CasΦ

Clustered regularly interspaced short palindromic repeats and CRISPR-associated proteins (CRISPR-Cas) systems have been developed as important tools for plant genome engineering. Here, we demonstrate that the hypercompact CasΦ nuclease is able to generate stably inherited gene edits in Arabidopsis, and that CasΦ guide RNAs can be expressed with either the Pol-III U6 promoter or a Pol-II promoter together with ribozyme mediated RNA processing. Using the Arabidopsis fwa epiallele, we show that CasΦ displays higher editing efficiency when the target locus is not DNA methylated, suggesting that CasΦ is sensitive to chromatin environment. Importantly, two CasΦ protein variants, vCasΦ and nCasΦ, both showed much higher editing efficiency relative to the wild-type CasΦ enzyme. Consistently, vCasΦ and nCasΦ yielded offspring plants with inherited edits at much higher rates compared to WTCasΦ. Extensive genomic analysis of gene edited plants showed no off-target editing, suggesting that CasΦ is highly specific. The hypercompact size, T-rich minimal protospacer adjacent motif (PAM), and wide range of working temperatures make CasΦ an excellent supplement to existing plant genome editing systems.

Epigenetic switch reveals CRISPR/Cas9 response to cytosine methylation in plants

This article is a Commentary on Přibylová et al. (2022) 235: 2285–2299.