CRISPR-Cas9 has emerged as a powerful technology that relies on Cas9/sgRNA ribonucleoprotein complexes (RNPs) to target and edit DNA. However, many therapeutic targets cannot currently be accessed due to the lack of carriers that can deliver RNPs systemically. Here, we report a generalizable methodology that allows engineering of modified lipid nanoparticles to efficiently deliver RNPs into cells and edit tissues including muscle, brain, liver, and lungs. Intravenous injection facilitated tissue-specific, multiplexed editing of six genes in mouse lungs. High carrier potency was leveraged to create organ-specific cancer models in livers and lungs of mice though facile knockout of multiple genes. The developed carriers were also able to deliver RNPs to restore dystrophin expression in DMD mice and significantly decrease serum PCSK9 level in C57BL/6 mice. Application of this generalizable strategy will facilitate broad nanoparticle development for a variety of disease targets amenable to protein delivery and precise gene correction approaches.

Mutations in the dystrophin gene cause Duchenne muscular dystrophy (DMD), which is characterized by lethal degeneration of cardiac and skeletal muscles. Mutations that delete exon 44 of the dystrophin gene represent one of the most common causes of DMD and can be corrected in ~12% of patients by editing surrounding exons, which restores the dystrophin open reading frame. Here, we present a simple and efficient strategy for correction of exon 44 deletion mutations by CRISPR-Cas9 gene editing in cardiomyocytes obtained from patient-derived induced pluripotent stem cells and in a new mouse model harboring the same deletion mutation. Using AAV9 encoding Cas9 and single guide RNAs, we also demonstrate the importance of the dosages of these gene editing components for optimal gene correction in vivo. Our findings represent a significant step toward possible clinical application of gene editing for correction of DMD.

CRISPR-mediated genome editing efficiently corrected cardiac abnormalities associated with DMD in human engineered heart tissue.

The ability to efficiently modify the genome using CRISPR technology has rapidly revolutionized biology and genetics and will soon transform medicine. Duchenne muscular dystrophy (DMD) represents one of the first monogenic disorders that has been investigated with respect to CRISPR-mediated correction of causal genetic mutations. DMD results from mutations in the gene encoding dystrophin, a scaffolding protein that maintains the integrity of striated muscles. Thousands of different dystrophin mutations have been identified in DMD patients, who suffer from a loss of ambulation followed by respiratory insufficiency, heart failure, and death by the third decade of life. Using CRISPR to bypass DMD mutations, dystrophin expression has been efficiently restored in human cells and mouse models of DMD. Here, we review recent progress toward the development of possible CRISPR therapies for DMD and highlight opportunities and potential obstacles in attaining this goal.

The adult mammalian heart has limited capacity for regeneration following injury, whereas the neonatal heart can readily regenerate within a short period after birth. To uncover the molecular mechanisms underlying neonatal heart regeneration, we compared the transcriptomes and epigenomes of regenerative and nonregenerative mouse hearts over a 7-d time period following myocardial infarction injury. By integrating gene expression profiles with histone marks associated with active or repressed chromatin, we identified transcriptional programs underlying neonatal heart regeneration, and the blockade to regeneration in later life. Our results reveal a unique immune response in regenerative hearts and a retained embryonic cardiogenic gene program that is active during neonatal heart regeneration. Among the unique immune factors and embryonic genes associated with cardiac regeneration, we identified Ccl24, which encodes a cytokine, and Igf2bp3, which encodes an RNA-binding protein, as previously unrecognized regulators of cardiomyocyte proliferation. Our data provide insights into the molecular basis of neonatal heart regeneration and identify genes that can be modulated to promote heart regeneration.

Duchenne muscular dystrophy (DMD) is a lethal neuromuscular disease caused by mutations in the dystrophin gene (DMD). Previously, we applied CRISPR-Cas9–mediated “single-cut” genome editing to correct diverse genetic mutations in animal models of DMD. However, high doses of adeno-associated virus (AAV) are required for efficient in vivo genome editing, posing challenges for clinical application. In this study, we packaged Cas9 nuclease in single-stranded AAV (ssAAV) and CRISPR single guide RNAs in self-complementary AAV (scAAV) and delivered this dual AAV system into a mouse model of DMD. The dose of scAAV required for efficient genome editing were at least 20-fold lower than with ssAAV. Mice receiving systemic treatment showed restoration of dystrophin expression and improved muscle contractility. These findings show that the efficiency of CRISPR-Cas9–mediated genome editing can be substantially improved by using the scAAV system. This represents an important advancement toward therapeutic translation of genome editing for DMD.

During skeletal muscle development, myoblasts fuse to form multinucleated myofibers. Myomaker [Transmembrane protein 8c (TMEM8c)] is a muscle-specific protein that is essential for myoblast fusion and sufficient to promote fusion of fibroblasts with muscle cells; however, the structure and biochemical properties of this membrane protein have not been explored. Here, we used CRISPR/Cas9 mutagenesis to disrupt myomaker expression in the C2C12 muscle cell line, which resulted in complete blockade to fusion. To define the functional domains of myomaker required to direct fusion, we established a heterologous cell-cell fusion system, in which fibroblasts expressing mutant versions of myomaker were mixed with WT myoblasts. Our data indicate that the majority of myomaker is embedded in the plasma membrane with seven membrane-spanning regions and a required intracellular C-terminal tail. We show that myomaker function is conserved in other mammalian orthologs; however, related family members (TMEM8a and TMEM8b) do not exhibit fusogenic activity. These findings represent an important step toward deciphering the cellular components and mechanisms that control myoblast fusion and muscle formation.P lasma membrane fusion is a fundamental cellular process required for the conception, development, and physiology of multicellular organisms. Membrane fusion occurs during viral infection of a host cell, between intracellular membranes, and between two plasma membranes to form syncytial tissues (1). Cell-cell fusion is critical for a wide array of cellular processes, including sperm-egg fertilization, macrophage function, bone and placental development, and skeletal muscle formation. This form of fusion must be precisely controlled to prevent inappropriate cellular mixing.The fusion of myoblasts requires cell recognition, migration, adhesion, signaling, and finally, membrane coalescence (2). Much of our knowledge about myoblast fusion has originated from studies performed in Drosophila. In this system, intracellular signaling results in cytoskeletal alterations and actin polymerization, which drive the formation of cellular projections that invade neighboring cells to cause fusion (3). Recent evidence also indicates a critical function for branched actin polymerization during Drosophila indirect flight muscle fusion (4). The essential role of the cytoskeleton in fusion is conserved in mammals, in which the actin regulators Rac1, cdc42, and N-WASp are required for muscle development (5-8). In addition to actin dynamics, numerous proteins that regulate diverse cellular processes have been associated with myoblast fusion (9-13).We recently discovered a muscle-specific membrane protein named myomaker [annotated as Transmembrane protein 8c (TMEM8c)] that is absolutely required for skeletal myocyte fusion in the mouse (14,15). To begin to decipher the mechanisms whereby myomaker controls cell-cell fusion, we designed a heterologous cell-cell fusion assay to monitor the ability of a series of myomaker mutants to direct the fusion of lab...

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