Current Gene Therapy - Volume 17, Issue 4, 2017

Introduction: Targeted genome editing using the CRISPR/Cas9 technology is becoming a major area of research due to its high potential for the treatment of genetic diseases. Our understanding of this approach has expanded in recent years yet several new challenges have presented themselves as we explore the boundaries of this exciting new technology. Chief among these is improving the efficiency but also the preciseness of genome editing. The efficacy of CRISPR/Cas9 technology relies in part on the use of one of the major DNA repair pathways, Homologous recombination (HR), which is primarily active in S and G2 phases of the cell cycle. Problematically, the HR potential is highly variable from cell type to cell type and most of the cells of interest to be targeted in vivo for precise genome editing are in a quiescent state. Conclusion: In this review, we discuss the recent advancements in improving targeted CRISPR/Cas9 based genome editing and the promising ways of delivering this technology in vivo to the cells of interest.

Introduction: The ability of most laboratories to easily access CRISPR/Cas9 engineering tools has caused a revolution in biology. One of the areas that will continue to be impacted by genome editing is the drug discovery process. Objective: CRISPR/Cas9 will not only serve to accelerate the drug discovery pipeline, but also streamline line it by identifying high-value targets, facilitating the validation of drug: target interactions and mechanisms of action, and stimulating the development of phenotype-based high throughput screens as alternatives to target-based assays. Conclusion: We review the literature and hurdles that have been overcome to develop the current generation of tools being used to enrich the drug discovery paradigm.

Introduction: Genome editing using CRISPR/Cas9 has advanced very rapidly in its scope, versatility and ease of use. Zebrafish (Danio rerio) has been one of the vertebrate model species where CRISPR/Cas9 has been applied very extensively for many different purposes and with great success. In particular, disease modeling in zebrafish is useful for testing specific gene variants for pathogenicity in a preclinical setting. Here we describe multiple advances in diverse species and systems that can improve genome editing in zebrafish. Objective: To achieve temporal and spatial precision of genome editing, many new technologies can be applied in zebrafish such as artificial transcription factors, drug-inducible or optogenetically-driven expression of Cas9, or chemically-inducible activation of Cas9. Moreover, chemically- or optogenetically- inducible reconstitution of dead Cas9 (catalytically inactive, dCas9) can enable spatiotemporal control of gene regulation. In addition to controlling where and when genome editing occurs, using oligonucleotides allows for the introduction (knock-in) of precise modifications of the genome. Conclusion: We review recent trends to improve the precision and efficiency of oligo-based point mutation knock-ins and discuss how these improvements can apply to work in zebrafish. Similarly to how chemical mutagenesis enabled the first genetic screens in zebrafish, multiplexed sgRNA libraries and Cas9 can enable the next revolutionary transition in how genetic screens are performed in this species. We discuss the first examples and prospects of approaches using sgRNAs as specific and effective mutagens. Moreover, we have reviewed methods aimed at measuring the phenotypes of single cells after their mutagenic perturbation with vectors encoding individual sgRNAs. These methods can range from different cell-based reporters to single-cell RNA sequencing and can serve as great tools for high-throughput genetic screens.

Background: Duchenne muscular dystrophy (DMD) is an X-linked neuromuscular disease caused by the lack of dystrophin due to mutations in the DMD gene. Since dystrophin is essential in maintaining the integrity of the sarcolemmal membrane, the absence of the protein leads to muscle damage and DMD disease manifestation. Currently, there is no cure with only symptomatic management available. Objective: The most recent advancements in DMD therapies do not provide a permanent treatment for DMD. CRISPR/Cas technology poses as an attractive platform for DMD gene therapy both dependent and independent of the specific mutation. Method: CRISPR/Cas technology can be utilized independent of the patient mutation by modulating disease modifiers. Regarding DMD duplication mutations, full length dystrophin can be restored using a single sgRNA approach. For DMD deletion and point mutations, the open reading frame (ORF) can be restored by removing or reframing exon(s) to produce a shorter form of dystrophin. The full-length wildtype dystrophin can also be restored using homologous recombination (HR). The CRISPR/Cas components for these strategies were delivered in vivo using the adeno-associated virus (AAV) vector. Results: The upregulation of a dystrophin homologue called utrophin can compensate for the lack of dystrophin protein, and has been successfully demonstrated in patient cells. Full-length dystrophin was restored in patient cells carrying duplication mutations. The shorter form and full-length dystrophin was recovered using CRISPR strategies in vitro and in vivo. Conclusions: Restoration of the wild type and shorter form of dystrophin highlights the therapeutic potential of CRISPR technology for DMD.

A prospective first-in-human Phase 1 CRISPR gene editing trial in the United States for patients with melanoma, synovial sarcoma, and multiple myeloma offers hope that gene editing tools may usefully treat human disease. An overarching ethical challenge with first-in-human Phase 1 clinical trials, however, is knowing when it is ethically acceptable to initiate such trials on the basis of safety and efficacy data obtained from pre-clinical studies. If the pre-clinical studies that inform trial design are themselves poorly designed – as a result of which the quality of pre-clinical evidence is deficient – then the ethical requirement of scientific validity for clinical research may not be satisfied. In turn, this could mean that the Phase 1 clinical trial will be unsafe and that trial participants will be exposed to risk for no potential benefit. To assist sponsors, researchers, clinical investigators and reviewers in deciding when it is ethically acceptable to initiate first-in-human Phase 1 CRISPR gene editing clinical trials, structured processes have been developed to assess and minimize translational distance between pre-clinical and clinical research. These processes draw attention to various features of internal validity, construct validity, and external validity. As well, the credibility of supporting evidence is to be critically assessed with particular attention to optimism bias, financial conflicts of interest and publication bias. We critically examine the pre-clinical evidence used to justify the first-inhuman Phase 1 CRISPR gene editing cancer trial in the United States using these tools. We conclude that the proposed trial cannot satisfy the ethical requirement of scientific validity because the supporting pre-clinical evidence used to inform trial design is deficient.

Introduction: Leber's Optic Hereditary Neuropathy (LHON) is a common cause of teenaged blindness in both eyes for which there is currently no effective treatment. In 1871, the German ophthalmologist Theodor Leber was the first to describe the clinical characteristics of his namesake disease, and through unremitting efforts over the past 100 years, researchers have continued to increase their understanding of LHON. In recent years, using gene therapy, several groups have obtained breakthroughs in the treatment of the disease. Conclusion: In this article, we will review the challenging journey that researchers faced towards our current understanding of LHON, and describe the transition of gene therapy research for LHON from the bench to bedside.