Methods for Optimizing CRISPR-Cas9 Genome Editing Specificity

CRISPR-Cas9 Structures and Mechanisms

Many bacterial clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated (Cas) systems employ the dual RNA-guided DNA endonuclease Cas9 to defend against invading phages and conjugative plasmids by introducing site-specific double-stranded breaks in target DNA. Target recognition strictly requires the presence of a short protospacer adjacent motif (PAM) flanking the target site, and subsequent R-loop formation and strand scission are driven by complementary base pairing between the guide RNA and target DNA, Cas9-DNA interactions, and associated conformational changes. The use of CRISPR-Cas9 as an RNA-programmable DNA targeting and editing platform is simplified by a synthetic single-guide RNA (sgRNA) mimicking the natural dual trans-activating CRISPR RNA (tracrRNA)-CRISPR RNA (crRNA) structure. This review aims to provide an in-depth mechanistic and structural understanding of Cas9-mediated RNA-guided DNA targeting and cleavage. Molecular insights from biochemical and structural studies provide a framework for rational engineering aimed at altering catalytic function, guide RNA specificity, and PAM requirements and reducing off-target activity for the development of Cas9-based therapies against genetic diseases.

TMEPAI genome editing in triple negative breast cancer cells

Background: Clustered regularly interspaced short palindromic repeats/CRISPR-associated 9 (CRISPR/Cas9) is a powerful genome editing technique. It consists of RNA-guided DNA endonuclease Cas9 and single guide RNA (gRNA). By combining their expressions, high efficiency cleavage of the target gene can be achieved, leading to the formation of DNA double-strand break (DSB) at the genomic locus of interest which will be repaired via NHEJ (non-homologous end joining) or HDR (homology-directed repair) and mediate DNA alteration. We aimed to apply the CRISPR/Cas9 technique to knock-out the transmembrane prostate androgen-induced protein (TMEPAI) gene in the triple negative breast cancer cell line.Methods: Designed gRNA which targets the TMEPAI gene was synthesized, annealed, and cloned into gRNA expression vector. It was co-transfected into the TNBC cell line using polyethylenimine (PEI) together with Cas9-GFP and puromycin resistant gene vector. At 24-hours post-transfection, cells were selected by puromycin for 3 days before they were cloned. Selected knock-out clones were subsequently checked on their protein levels by western blotting.Results: CRISPR/Cas9, a genome engineering technique successfully knocked-out TMEPAI in the Hs578T TNBC cell line. Sequencing shows a frameshift mutation in TMEPAI. Western blot shows the absence of TMEPAI band on Hs578T KO cells.Conclusion: TMEPAI gene was deleted in the TNBC cell line using the genomic editing technique CRISPR/Cas9. The deletion was confirmed by genome and protein analysis.

Generation of chromosomal translocations that lead to conditional fusion protein expression using CRISPR-Cas9 and homology-directed repair

Recurrent chromosomal translocations often lead to expression of fusion proteins associated with oncogenic transformation. To study translocations and downstream events, genome editing techniques have been developed to generate chromosomal translocations through non-homologous end joining of DNA double-strand breaks introduced at the two participating endogenous loci. However, the frequencies at which these events occur is usually too low to efficiently clone cells carrying the translocation. This article provides a detailed method using CRISPR-Cas9 technology and homology-directed repair to efficiently isolate cells harboring a chromosomal translocation. For an additional level of control, the resulting fusion protein is conditionally expressed to allow early events in oncogenic transformation to be studied. We focus on the generation of the EWSR1-WT1 fusion using human mesenchymal cells, which is associated with the translocation found in desmoplastic small round cell tumors.

Functional interrogation of non-coding DNA through CRISPR genome editing

Methodologies to interrogate non-coding regions have lagged behind coding regions despite comprising the vast majority of the genome. However, the rapid evolution of clustered regularly interspaced short palindromic repeats (CRISPR)-based genome editing has provided a multitude of novel techniques for laboratory investigation including significant contributions to the toolbox for studying non-coding DNA. CRISPR-mediated loss-of-function strategies rely on direct disruption of the underlying sequence or repression of transcription without modifying the targeted DNA sequence. CRISPR-mediated gain-of-function approaches similarly benefit from methods to alter the targeted sequence through integration of customized sequence into the genome as well as methods to activate transcription. Here we review CRISPR-based loss- and gain-of-function techniques for the interrogation of non-coding DNA.

GUIDEseq: a bioconductor package to analyze GUIDE-Seq datasets for CRISPR-Cas nucleases

Background

Genome editing technologies developed around the CRISPR-Cas9 nuclease system have facilitated the investigation of a broad range of biological questions. These nucleases also hold tremendous promise for treating a variety of genetic disorders. In the context of their therapeutic application, it is important to identify the spectrum of genomic sequences that are cleaved by a candidate nuclease when programmed with a particular guide RNA, as well as the cleavage efficiency of these sites. Powerful new experimental approaches, such as GUIDE-seq, facilitate the sensitive, unbiased genome-wide detection of nuclease cleavage sites within the genome. Flexible bioinformatics analysis tools for processing GUIDE-seq data are needed.

Results

Here, we describe an open source, open development software suite, GUIDEseq, for GUIDE-seq data analysis and annotation as a Bioconductor package in R. The GUIDEseq package provides a flexible platform with more than 60 adjustable parameters for the analysis of datasets associated with custom nuclease applications. These parameters allow data analysis to be tailored to different nuclease platforms with different length and complexity in their guide and PAM recognition sequences or their DNA cleavage position. They also enable users to customize sequence aggregation criteria, and vary peak calling thresholds that can influence the number of potential off-target sites recovered. GUIDEseq also annotates potential off-target sites that overlap with genes based on genome annotation information, as these may be the most important off-target sites for further characterization. In addition, GUIDEseq enables the comparison and visualization of off-target site overlap between different datasets for a rapid comparison of different nuclease configurations or experimental conditions. For each identified off-target, the GUIDEseq package outputs mapped GUIDE-Seq read count as well as cleavage score from a user specified off-target cleavage score prediction algorithm permitting the identification of genomic sequences with unexpected cleavage activity.

Conclusion

The GUIDEseq package enables analysis of GUIDE-data from various nuclease platforms for any species with a defined genomic sequence. This software package has been used successfully to analyze several GUIDE-seq datasets. The software, source code and documentation are freely available at http://www.bioconductor.org/packages/release/bioc/html/GUIDEseq.html.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3746-y) contains supplementary material, which is available to authorized users.

In Vivo Excision of HIV-1 Provirus by saCas9 and Multiplex Single-Guide RNAs in Animal Models

CRISPR-associated protein 9 (Cas9)-mediated genome editing provides a promising cure for HIV-1/AIDS; however, gene delivery efficiency in vivo remains an obstacle to overcome. Here, we demonstrate the feasibility and efficiency of excising the HIV-1 provirus in three different animal models using an all-in-one adeno-associated virus (AAV) vector to deliver multiplex single-guide RNAs (sgRNAs) plus Staphylococcus aureus Cas9 (saCas9). The quadruplex sgRNAs/saCas9 vector outperformed the duplex vector in excising the integrated HIV-1 genome in cultured neural stem/progenitor cells from HIV-1 Tg26 transgenic mice. Intravenously injected quadruplex sgRNAs/saCas9 AAV-DJ/8 excised HIV-1 proviral DNA and significantly reduced viral RNA expression in several organs/tissues of Tg26 mice. In EcoHIV acutely infected mice, intravenously injected quadruplex sgRNAs/saCas9 AAV-DJ/8 reduced systemic EcoHIV infection, as determined by live bioluminescence imaging. Additionally, this quadruplex vector induced efficient proviral excision, as determined by PCR genotyping in the liver, lungs, brain, and spleen. Finally, in humanized bone marrow/liver/thymus (BLT) mice with chronic HIV-1 infection, successful proviral excision was detected by PCR genotyping in the spleen, lungs, heart, colon, and brain after a single intravenous injection of quadruplex sgRNAs/saCas9 AAV-DJ/8. In conclusion, in vivo excision of HIV-1 proviral DNA by sgRNAs/saCas9 in solid tissues/organs can be achieved via AAV delivery, a significant step toward human clinical trials.

Marker-free coselection for CRISPR-driven genome editing in human cells

Targeted genome editing enables the creation of bona fide cellular models for biological research and may be applied to human cell-based therapies. Therefore, broadly applicable and versatile methods for increasing its efficacy in cell populations are highly desirable. We designed a simple and robust coselection strategy for enrichment of cells with either nuclease-driven nonhomologous end joining (NHEJ) or homology-directed repair (HDR) events by harnessing the multiplexing capabilities of CRISPR-Cas9 and Cpf1 systems. Selection for dominant alleles of the ubiquitous sodium/potassium pump (Na⁺/K⁺ ATPase) that rendered cells resistant to ouabain was used to enrich for custom genetic modifications at another unlinked locus of interest, thereby effectively increasing the recovery of engineered cells. The process is readily adaptable to transformed and primary cells, including hematopoietic stem and progenitor cells. The use of universal CRISPR reagents and a commercially available small-molecule inhibitor streamlines the incorporation of marker-free genetic changes in human cells.

Reprogramming of somatic cells: iPS and iN cells

Limited access to human neurons has posed a significant barrier to progress in biological and preclinical studies of the human nervous system. The advent of cell reprogramming technologies has widely disclosed unprecedented opportunities to generate renewable sources of human neural cells for disease modeling, drug discovery, and cell therapeutics. Both somatic reprogramming into induced pluripotent stem cells (iPSCs) and directly induced Neurons (iNeurons) rely on transcription factor-based cellular conversion processes. Nevertheless, they rely on very distinct mechanisms, biological barriers, technical limitations, different levels of efficiency, and generate neural cells with distinctive properties. Human iPSCs represent a long-term renewable source of neural cells, but over time genomic aberrations might erode the quality of the cultures and the in vitro differentiation process requires extensive time. Conversely, direct neuronal reprogramming ensures a fast and straightforward generation of iNeurons endowed with functional properties. However, in this last case, conversion efficiency is reduced when starting from adult human cells, and the molecular and functional fidelity of iNeurons with respect to their corresponding native neuronal subtype is yet to be fully ascertained in many cases. For any biomedical research application, it should be carefully pondered the reprogramming method to use for generating reprogrammed human neuronal subtypes that best fit with the following analysis considering the existing limitations and gap of knowledge still present in this young field of investigation.

From Reductionism to Holism: Toward a More Complete View of Development Through Genome Engineering

Paradigm shifts in science are often coupled to technological advances. New techniques offer new roads of discovery; but, more than this, they shape the way scientists approach questions. Developmental biology exemplifies this idea both in its past and present. The rise of molecular biology and genetics in the late twentieth century shifted the focus from the anatomical to the molecular, nudging the underlying philosophy from holism to reductionism. Developmental biology is currently experiencing yet another transformation triggered by '-omics' technology and propelled forward by CRISPR genome engineering (GE). Together, these technologies are helping to reawaken a holistic approach to development. Herein, we focus on CRISPR GE and its potential to reveal principles of development at the level of the genome, the epigenome, and the cell. Within each stage we illustrate how GE can move past pure reductionism and embrace holism, ultimately delivering a more complete view of development.

Nucleosomes Selectively Inhibit Cas9 Off-target Activity at a Site Located at the Nucleosome Edge

Nucleosomes affect Cas9 binding and activity at on-target sites, but their impact at off-target sites is unknown. To investigate how nucleosomes affect Cas9 cleavage at off-target sites in vitro, we used a single guide RNA (sgRNA) that has been previously shown to efficiently direct Cas9 cleavage at the edge of the strongly positioned 601 nucleosome. Our data indicate that single mismatches between the sgRNA and DNA target have relatively little effect on Cas9 cleavage of naked DNA substrates, but strongly inhibit cleavage of nucleosome substrates, particularly when the mismatch is in the sgRNA "seed" region. These findings indicate that nucleosomes may enhance Cas9 specificity by inhibiting cleavage of off-target sites at the nucleosome edge.

Genome editing: progress and challenges for medical applications

The development of the CRISPR-Cas platform for genome editing has greatly simplified the process of making targeted genetic modifications. Applications of genome editing are expected to have a substantial impact on human therapies through the development of better animal models, new target discovery, and direct therapeutic intervention.