CRISPR-Cas9 is poised to become the gene editing tool of choice in clinical contexts. Thus far, exploration of Cas9-induced genetic alterations has been limited to the immediate vicinity of the target site and distal off-target sequences, leading to the conclusion that CRISPR-Cas9 was reasonably specific. Here we report significant on-target mutagenesis, such as large deletions and more complex genomic rearrangements at the targeted sites in mouse embryonic stem cells, mouse hematopoietic progenitors and a human differentiated cell line. Using long-read sequencing and long-range PCR genotyping, we show that DNA breaks introduced by single-guide RNA/Cas9 frequently resolved into deletions extending over many kilobases. Furthermore, lesions distal to the cut site and crossover events were identified. The observed genomic damage in mitotically active cells caused by CRISPR-Cas9 editing may have pathogenic consequences.

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Repair of double-strand breaks induced by CRISPR–Cas9 leads to large deletions and complex rearrangements

CRISPR–Cas9 has been adopted as a powerful genome-editing technology in various species. By generating libraries of thousands of guide RNAs — which direct the Cas9 nuclease to chosen genomic loci — high-throughput genetic perturbations are now possible. This Review discusses the latest applications of CRISPR–Cas9 in mammalian functional genomics screens. It covers related genome-scale applications of Cas9 for either gene knockout or transcriptional modulation, and provides comparisons with complementary RNA interference (RNAi)-based approaches.

High-throughput functional genomics using CRISPR–Cas9

CRISPR-Cas-mediated genome editing relies on guide RNAs that direct site-specific DNA cleavage facilitated by the Cas endonuclease. Here we report that chemical alterations to synthesized single guide RNAs (sgRNAs) enhance genome editing efficiency in human primary T cells and CD34+ hematopoietic stem and progenitor cells. Co-delivering chemically modified sgRNAs with Cas9 mRNA or protein is an efficient RNA- or ribonucleoprotein (RNP)-based delivery method for the CRISPR-Cas system, without the toxicity associated with DNA delivery. This approach is a simple and effective way to streamline the development of genome editing with the potential to accelerate a wide array of biotechnological and therapeutic applications of the CRISPR-Cas technology.

/pdf/chemically-modified-guide-rnas-enhance-crispr-cas-genome-4eji1fefl2.pdf

Chemically modified guide RNAs enhance CRISPR-Cas genome editing in human primary cells

The Cas9 protein (CRISPR-associated protein 9), derived from type II CRISPR (clustered regularly interspaced short palindromic repeats) bacterial immune systems, is emerging as a powerful tool for engineering the genome in diverse organisms. As an RNA-guided DNA endonuclease, Cas9 can be easily programmed to target new sites by altering its guide RNA sequence, and its development as a tool has made sequence-specific gene editing several magnitudes easier. The nuclease-deactivated form of Cas9 further provides a versatile RNA-guided DNA-targeting platform for regulating and imaging the genome, as well as for rewriting the epigenetic status, all in a sequence-specific manner. With all of these advances, we have just begun to explore the possible applications of Cas9 in biomedical research and therapeutics. In this review, we describe the current models of Cas9 function and the structural and biochemical studies that support it. We focus on the applications of Cas9 for genome editing, regulation, and imaging, discuss other possible applications and some technical considerations, and highlight the many advantages that CRISPR/Cas9 technology offers.

CRISPR/Cas9 in Genome Editing and Beyond

Based on engineered or bacterial nucleases, the development of genome editing technologies has opened up the possibility of directly targeting and modifying genomic sequences in almost all eukaryotic cells. Genome editing has extended our ability to elucidate the contribution of genetics to disease by promoting the creation of more accurate cellular and animal models of pathological processes and has begun to show extraordinary potential in a variety of fields, ranging from basic research to applied biotechnology and biomedical research. Recent progress in developing programmable nucleases, such as zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and clustered regularly interspaced short palindromic repeat (CRISPR)–Cas-associated nucleases, has greatly expedited the progress of gene editing from concept to clinical practice. Here, we review recent advances of the three major genome editing technologies (ZFNs, TALENs, and CRISPR/Cas9) and discuss the applications of their derivative reagents as gene editing tools in various human diseases and potential future therapies, focusing on eukaryotic cells and animal models. Finally, we provide an overview of the clinical trials applying genome editing platforms for disease treatment and some of the challenges in the implementation of this technology.

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Applications of genome editing technology in the targeted therapy of human diseases: mechanisms, advances and prospects

https://www.cell.com/cell-reports/pdf/S2211-1247(14)00154-5.pdf

Quantifying genome-editing outcomes at endogenous loci with SMRT sequencing.

Provided herein are methods for inducing CRISPR/Cas-based gene regulation (e.g., genome editing or gene expression) of a target nucleic acid (e.g., target DNA or target RNA) in a cell. The methods include using modified single guide RNAs (sgRNAs) that enhance gene regulation of the target nucleic acid in a primary cell for use in ex vivo therapy or in a cell in a subject for use in in vivo therapy. Additionally, provided herein are methods for preventing or treating a genetic disease in a subject by administering a sufficient amount of a modified sgRNA to correct a mutation in a target gene associated with the genetic disease.

Chemically modified guide rnas for crispr/cas-mediated gene regulation

CRISPR/Cas9 Genome Engineering in Engraftable Human Brain-Derived Neural Stem Cells.

The invention provides methods for generating a genetically modified human neural stem cell, genetically modified human neural stem cells, and pharmaceutical compositions comprising the genetically modified human neural stem cells. In particular said methods involve the use of a DNA nuclease, such as TALEN, ZFN or CRISPR/Cas mediated genome editing. Also provided are associated kits. The invention also provides methods for preventing or treating a neurodegenerative disease or a neurological injury in a human subject using genetically modified human neural stem cells.

Joseph T. Clark

Papers

Chemically modified guide RNAs enhance CRISPR-Cas genome editing in human primary cells

Quantifying genome-editing outcomes at endogenous loci with SMRT sequencing.

Chemically modified guide rnas for crispr/cas-mediated gene regulation

CRISPR/Cas9 Genome Engineering in Engraftable Human Brain-Derived Neural Stem Cells.

Genome editing of human neural stem cells using nucleases