Correction of Three Prominent Mutations in Mouse and Human Models of Duchenne Muscular Dystrophy by Single-Cut Genome Editing.

Duchenne muscular dystrophy (DMD), one of the most common neuromuscular disorders of children, is caused by the absence of dystrophin protein in striated muscle. Deletions of exons 43, 45, and 52 represent mutational "hotspot" regions in the dystrophin gene. We created three new DMD mouse models harboring deletions of exons 43, 45, and 52 to represent common DMD mutations. To optimize CRISPR-Cas9 genome editing using the single-cut strategy, we identified single guide RNAs (sgRNAs) capable of restoring dystrophin expression by inducing exon skipping and reframing. Intramuscular delivery of AAV9 encoding SpCas9 and selected sgRNAs efficiently restored dystrophin expression in these new mouse models, offering a platform for future studies of dystrophin gene correction therapies. To validate the therapeutic potential of this approach, we identified sgRNAs capable of restoring dystrophin expression by the single-cut strategy in cardiomyocytes derived from human induced pluripotent stem cells (iPSCs) with each of these hotspot deletion mutations. We found that the potential effectiveness of individual sgRNAs in correction of DMD mutations cannot be predicted a priori, highlighting the importance of sgRNA design and testing as a prelude for applying gene editing as a therapeutic strategy for DMD.

A consolidated AAV system for single-cut CRISPR correction of a common Duchenne muscular dystrophy mutation.

Duchenne muscular dystrophy (DMD), caused by mutations in the X-linked dystrophin gene, is a lethal neuromuscular disease. Correction of DMD mutations in animal models has been achieved by CRISPR/Cas9 genome editing using Streptococcus pyogenes Cas9 (SpCas9) delivered by adeno-associated virus (AAV). However, due to the limited viral packaging capacity of AAV, two AAV vectors are required to deliver the SpCas9 nuclease and its single guide RNA (sgRNA), impeding its therapeutic application. We devised an efficient single-cut gene-editing method using a compact Staphylococcus aureus Cas9 (SaCas9) to restore the open reading frame of exon 51, the most commonly affected out-of-frame exon in DMD. Editing of exon 51 in cardiomyocytes derived from human induced pluripotent stem cells revealed a strong preference for exon reframing via a two-nucleotide deletion. We adapted this system to express SaCas9 and sgRNA from a single AAV9 vector. Systemic delivery of this All-In-One AAV9 system restored dystrophin expression and improved muscle contractility in a mouse model of DMD with exon 50 deletion. These findings demonstrate the effectiveness of CRISPR/SaCas9 delivered by a consolidated AAV delivery system in the correction of DMD in vivo, representing a promising therapeutic approach to correct the genetic causes of DMD.

Enhanced CRISPR-Cas9 correction of Duchenne muscular dystrophy in mice by a self-complementary AAV delivery system.

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.

CRISPR-Cas9 corrects Duchenne muscular dystrophy exon 44 deletion mutations in mice and human cells.

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.

In vivo non-invasive monitoring of dystrophin correction in a new Duchenne muscular dystrophy reporter mouse.

Duchenne muscular dystrophy (DMD) is a fatal genetic disorder caused by mutations in the dystrophin gene. To enable the non-invasive analysis of DMD gene correction strategies in vivo, we introduced a luciferase reporter in-frame with the C-terminus of the dystrophin gene in mice. Expression of this reporter mimics endogenous dystrophin expression and DMD mutations that disrupt the dystrophin open reading frame extinguish luciferase expression. We evaluated the correction of the dystrophin reading frame coupled to luciferase in mice lacking exon 50, a common mutational hotspot, after delivery of CRISPR/Cas9 gene editing machinery with adeno-associated virus. Bioluminescence monitoring revealed efficient and rapid restoration of dystrophin protein expression in affected skeletal muscles and the heart. Our results provide a sensitive non-invasive means of monitoring dystrophin correction in mouse models of DMD and offer a platform for testing different strategies for amelioration of DMD pathogenesis.

Single-cut genome editing restores dystrophin expression in a new mouse model of muscular dystrophy.

Duchenne muscular dystrophy (DMD) is a severe, progressive muscle disease caused by mutations in the dystrophin gene. The majority of DMD mutations are deletions that prematurely terminate the dystrophin protein. Deletions of exon 50 of the dystrophin gene are among the most common single exon deletions causing DMD. Such mutations can be corrected by skipping exon 51, thereby restoring the dystrophin reading frame. Using clustered regularly interspaced short palindromic repeats/CRISPR-associated 9 (CRISPR/Cas9), we generated a DMD mouse model by deleting exon 50. These ΔEx50 mice displayed severe muscle dysfunction, which was corrected by systemic delivery of adeno-associated virus encoding CRISPR/Cas9 genome editing components. We optimized the method for dystrophin reading frame correction using a single guide RNA that created reframing mutations and allowed skipping of exon 51. In conjunction with muscle-specific expression of Cas9, this approach restored up to 90% of dystrophin protein expression throughout skeletal muscles and the heart of ΔEx50 mice. This method of permanently bypassing DMD mutations using a single cut in genomic DNA represents a step toward clinical correction of DMD mutations and potentially those of other neuromuscular disorders.

Postnatal genome editing partially restores dystrophin expression in a mouse model of muscular dystrophy.

CRISPR/Cas9-mediated genome editing holds clinical potential for treating genetic diseases, such as Duchenne muscular dystrophy (DMD), which is caused by mutations in the dystrophin gene. To correct DMD by skipping mutant dystrophin exons in postnatal muscle tissue in vivo, we used adeno-associated virus-9 (AAV9) to deliver gene-editing components to postnatal mdx mice, a model of DMD. Different modes of AAV9 delivery were systematically tested, including intraperitoneal at postnatal day 1 (P1), intramuscular at P12, and retro-orbital at P18. Each of these methods restored dystrophin protein expression in cardiac and skeletal muscle to varying degrees, and expression increased from 3 to 12 weeks after injection. Postnatal gene editing also enhanced skeletal muscle function, as measured by grip strength tests 4 weeks after injection. This method provides a potential means of correcting mutations responsible for DMD and other monogenic disorders after birth.

Prevention of muscular dystrophy in mice by CRISPR/Cas9-mediated editing of germline DNA.

Duchenne muscular dystrophy (DMD) is an inherited X-linked disease caused by mutations in the gene encoding dystrophin, a protein required for muscle fiber integrity. DMD is characterized by progressive muscle weakness and a shortened life span, and there is no effective treatment. We used clustered regularly interspaced short palindromic repeat/Cas9 (CRISPR/Cas9)-mediated genome editing to correct the dystrophin gene (Dmd) mutation in the germ line of mdx mice, a model for DMD, and then monitored muscle structure and function. Genome editing produced genetically mosaic animals containing 2 to 100% correction of the Dmd gene. The degree of muscle phenotypic rescue in mosaic mice exceeded the efficiency of gene correction, likely reflecting an advantage of the corrected cells and their contribution to regenerating muscle. With the anticipated technological advances that will facilitate genome editing of postnatal somatic cells, this strategy may one day allow correction of disease-causing mutations in the muscle tissue of patients with DMD.