CRISPR/Cas9-mediated genome editing via postnatal administration of AAV vector cures haemophilia B mice

Split Cas9, Not Hairs - Advancing the Therapeutic Index of CRISPR Technology

The discovery that the bacterial CRISPR/Cas9 system can be translated into mammalian cells continues to have an unprecedented impact on the biomedical research community, as it largely facilitates efforts to experimentally interrogate or therapeutically modify the cellular genome. In particular, CRISPR promises the ability to correct disease-associated genetic defects, or to target and destroy invading foreign DNA, in a simple, efficient, and selective manner directly in affected human cells or tissues. Here, we highlight a set of exciting new strategies that aim at further increasing the therapeutic index of CRISPR technologies, by reducing the size of Cas9 expression cassettes and thus enhancing their compatibility with viral gene delivery vectors. Specifically, we discuss the concept of splitCas9 whereby the Cas9 holo-protein is segregated into two parts that are expressed individually and reunited in the cell by various means, including use of 1) the gRNA as a scaffold for Cas9 assembly; 2) the rapamycin-controlled FKBP/FRB system; 3) the light-regulated Magnet system; or 4) inteins. We describe how these avenues, despite pursuing the identical aim, differ in critical features comprising the extent of spatio-temporal control of CRISPR activity, and discuss additional improvements to their efficiency or specificity that should foster their clinical translation.

Generation of the Adenovirus Vector-Mediated CRISPR/Cpf1 System and the Application for Primary Human Hepatocytes Prepared from Humanized Mice with Chimeric Liver

The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) 9 system is now widely used as a genome editing tool. CRISPR-associated endonuclease in Prevotella and Francisella 1 (Cpf1) is a recently discovered Cas endonuclease that is designable and highly specific with efficiencies comparable to those of Cas9. Here we generated the adenovirus (Ad) vector carrying an Acidaminococcus sp. Cpf1 (AsCpf1) expression cassette (Ad-AsCpf1) for the first time. Ad-AsCpf1 was applied to primary human hepatocytes prepared from humanized mice with chimeric liver in combination with the Ad vector expressing the guide RNA (gRNA) directed to the Adeno-associated virus integration site 1 (AAVS1) region. The mutation rates were estimated by T7 endonuclease I assay around 12% of insertion/deletion (indel). Furthermore, the transduced human hepatocytes were viable (ca. 60%) at two weeks post transduction. These observations suggest that the Ad vector-mediated delivery of the CRISPR/AsCpf1 system provides a useful tool for genome manipulation of human hepatocytes.

In vivo genome editing in animals using AAV-CRISPR system: applications to translational research of human disease

Adeno-associated virus (AAV) has shown promising therapeutic efficacy with a good safety profile in a wide range of animal models and human clinical trials. With the advent of clustered regulatory interspaced short palindromic repeat (CRISPR)-based genome-editing technologies, AAV provides one of the most suitable viral vectors to package, deliver, and express CRISPR components for targeted gene editing. Recent discoveries of smaller Cas9 orthologues have enabled the packaging of Cas9 nuclease and its chimeric guide RNA into a single AAV delivery vehicle for robust in vivo genome editing. Here, we discuss how the combined use of small Cas9 orthologues, tissue-specific minimal promoters, AAV serotypes, and different routes of administration has advanced the development of efficient and precise in vivo genome editing and comprehensively review the various AAV-CRISPR systems that have been effectively used in animals. We then discuss the clinical implications and potential strategies to overcome off-target effects, immunogenicity, and toxicity associated with CRISPR components and AAV delivery vehicles. Finally, we discuss ongoing non-viral-based ex vivo gene therapy clinical trials to underscore the current challenges and future prospects of CRISPR/Cas9 delivery for human therapeutics.