Tissue-specific expression of Cas9 has no impact on whole-body metabolism in four transgenic mouse lines

Temporal restriction of Cas9 expression improves CRISPR-mediated deletion efficacy and fidelity

Clinical application of CRISPR-Cas9 technology for large deletions of somatic mutations is inefficient, and methods to improve utility suffer from our inability to rapidly assess mono- vs. biallelic deletions. Here we establish a model system for investigating allelic heterogeneity at the single-cell level and identify indel scarring from non-simultaneous nuclease activity at gRNA cut sites as a major barrier to CRISPR-del efficacy both in vitro and in vivo. We show that non-simultaneous nuclease activity is partially prevented via restriction of CRISPR-Cas9 expression via inducible adeno-associated viruses (AAVs) or lipid nanoparticles (LNPs). Inducible AAV-based expression of CRISPR-del machinery significantly improved mono- and biallelic deletion frequency in vivo, supporting the use of the Xon cassette over traditional constitutively expressing AAV approaches. These data depicting improvements to deletions and insight into allelic heterogeneity after CRISPR-del will inform therapeutic approaches for phenotypes that require either large mono- or biallelic deletions, such as autosomal recessive diseases or where mutant allele-specific gRNAs are not readily available, or in situations where the targeted sequence for excision is located multiple times in a genome.

Deletion of the muscle enriched lncRNA Oip5os1 induces atrial dysfunction in male mice with diabetes

Long ncRNAs (lncRNAs) have been shown to play a biological and physiological role in various tissues including the heart. We and others have previously established that the lncRNA Oip5os1 (1700020I14Rik, OIP5-AS1, Cyrano) is enriched in striated muscles, and its deletion in mice leads to defects in both skeletal and cardiac muscle function. In the present study, we investigated the impact of global Oip5os1 deletion on cardiac function in the setting of streptozotocin (STZ)-induced diabetes. Specifically, we studied male WT and KO mice with or without diabetes for 24 weeks, and phenotyped animals for metabolic and cardiac endpoints. Independent of genotype, diabetes was associated with left ventricular diastolic dysfunction based on a fall in E'/A' ratio. Deletion of Oip5os1 in a setting of diabetes had no significant impact on ventricular function or ventricular weight, but was associated with left atrial dysfunction (reduced fractional shortening) and myopathy which was associated with anesthesia intolerance and premature death in the majority of KO mice tested during cardiac functional assessment. This atrial phenotype was not observed in WT diabetic mice. The most striking molecular difference was a reduction in the metabolic regulator ERRalpha in the atria of KO mice compared with WT mice. There was also a trend for a reduction in Serca2a. These findings highlight Oip5os1 as a gene of interest in aspects of atrial function in the setting of diabetes, highlighting an additional functional role for this lncRNA in cardiac pathological settings.

Modern Approaches to Mouse Genome Editing Using the CRISPR-Cas Toolbox and Their Applications in Functional Genomics and Translational Research

Mice have been used in biological research for over a century, and their immense contribution to scientific breakthroughs can be seen across all research disciplines, with some of the main beneficiaries being the fields of medicine and life sciences. Genetically engineered mouse models (GEMMs), along with other model organisms, are fundamentally important research tools frequently utilised to enhance our understanding of pathophysiology and biological mechanisms behind disease. In the 1980s, it became possible to precisely edit the mouse genome to create gene knockout and knock-in mice, although with low efficacy. Recent advances utilising CRISPR-Cas technologies have considerably improved our ability to do this with ease and precision, while also allowing the generation of desired genetic variants from single nucleotide substitutions to large insertions/deletions. It is now quick and relatively easy to genetically edit somatic cells which were previously more recalcitrant to traditional approaches. Further refinements have created a 'CRISPR toolkit' that has expanded the use of CRISPR-Cas beyond gene knock-ins and knockouts. In this chapter, we review some of the latest applications of CRISPR-Cas technologies in GEMMs, including nuclease-dead Cas9 systems for activation or repression of gene expression, base editing and prime editing. We also discuss improvements in Cas9 specificity, targeting efficacy and delivery methods in mice. Throughout, we provide examples wherein CRISPR-Cas technologies have been applied to target clinically relevant genes in preclinical GEMMs, both to generate humanised models and for experimental gene therapy research.

Muscle cell-derived cytokines in skeletal muscle regeneration

Regeneration of the mammalian adult skeletal muscle is a well-orchestrated process regulated by multiple proteins and signalling pathways. Cytokines constitute a major class of regulators of skeletal myogenesis. It is well established that infiltrating immune cells at the site of muscle injury secrete cytokines, which play critical roles in the myofibre repair and regeneration process. In the past 10-15 years, skeletal muscle itself has emerged as a prolific producer of cytokines. Much attention in the field has been focused on the endocrine effects of muscle-secreted cytokines (myokines) on metabolic regulation. However, ample evidence suggests that muscle-derived cytokines also regulate myogenic differentiation and muscle regeneration in an autocrine manner. In this review, we survey cytokines that meet two criteria: (a) evidence of expression by muscle cells; (b) evidence demonstrating a myogenic function. Dozens of cytokines representing several major classes make up this group, and together they regulate all steps of the myogenic process. How such a large array of cytokines coordinate their signalling to form a regulatory network is a fascinating, pressing question. Functional studies that can distinguish the source of the cytokines in vivo are also much needed in order to facilitate exploration of their full therapeutic potential.

Applications of and considerations for using CRISPR-Cas9-mediated gene conversion systems in rodents

Genetic elements that are inherited at super-Mendelian frequencies could be used in a 'gene drive' to spread an allele to high prevalence in a population with the goal of eliminating invasive species or disease vectors. We recently demonstrated that the gene conversion mechanism underlying a CRISPR-Cas9-mediated gene drive is feasible in mice. Although substantial technical hurdles remain, overcoming these could lead to strategies that might decrease the spread of rodent-borne Lyme disease or eliminate invasive populations of mice and rats that devastate island ecology. Perhaps more immediately achievable at moderate gene conversion efficiency, applications in a laboratory setting could produce complex genotypes that reduce the time and cost in both dollars and animal lives compared with Mendelian inheritance strategies. Here, we discuss what we have learned from early efforts to achieve CRISPR-Cas9-mediated gene conversion, potential for broader applications in the laboratory, current limitations, and plans for optimizing this potentially powerful technology.