The third and fourth weeks of gestation in primates are marked by several developmental milestones, including gastrulation and the formation of organ primordia. However, our understanding of this period is limited due to restricted access to in vivo embryos. To address this gap, we developed an embedded 3D culture system that allows for the extended ex utero culture of cynomolgus monkey embryos for up to 25 days post-fertilization. Morphological, histological, and single-cell RNA-sequencing analyses demonstrate that ex utero cultured monkey embryos largely recapitulated key events of in vivo development. With this platform, we were able to delineate lineage trajectories and genetic programs involved in neural induction, lateral plate mesoderm differentiation, yolk sac hematopoiesis, primitive gut, and primordial germ-cell-like cell development in monkeys. Our embedded 3D culture system provides a robust and reproducible platform for growing monkey embryos from blastocysts to early organogenesis and studying primate embryogenesis ex utero.

Splice-switching antisense oligonucleotides (ASOs) and engineered U7 small nuclear ribonucleoprotein (U7 Sm OPT) are the most commonly used methods for exon skipping. However, challenges remain, such as limited organ delivery and repeated dosing for ASOs and unknown risks of by-products produced by U7 Sm OPT. Here, we showed that antisense circular RNAs (AS-circRNAs) can effectively mediate exon skipping in both minigene and endogenous transcripts. We also showed a relatively higher exon skipping efficiency at the tested Dmd minigene than U7 Sm OPT. AS-circRNA specifically targets the precursor mRNA splicing without off-target effects. Moreover, AS-circRNAs with adeno-associated virus (AAV) delivery corrected the open reading frame and restored the dystrophin expression in a mouse model of Duchenne muscular dystrophy. In conclusion, we develop an alternative method for regulating RNA splicing, which might be served as a novel tool for genetic disease treatment.

CRISPR/Cas9 is now widely used in biomedical research and has great potential for clinical applications. However, the safety and efficacy of this gene-editing technique are significant issues. Recent reports on mouse models and human cells have raised concerns that off-target mutations could hamper applying the CRISPR technology in patients. The high similarities of nonhuman primates to humans in genome content and organization, genetic diversity, physiology, and cognitive abilities have made these animals ideal experimental models for understanding human diseases and developing therapeutics. Off-target mutations of CRISPR/Cas9 have been analyzed in previous studies of nonhuman primates, but no report has investigated genome-wide off-target effects in living monkeys. Here, we used rhesus monkeys in which a genetic disorder mimicking Duchenne muscular dystrophy had previously been produced with CRISPR/Cas9. Using whole-genome sequencing to comprehensively assess on- and off-target mutations in these animals, we found that CRISPR/Cas9-based gene editing is active on the expected genomic sites without producing off-target modifications in other functional regions of the genome. These findings suggest that the CRISPR/Cas9 technique could be relatively safe and effective in modeling genetic disease in nonhuman primates and in future therapeutic research of human diseases.

Activation of endogenous full-length utrophin, a dystrophin homolog, presents an attractive therapeutic strategy for Duchenne muscular dystrophy (DMD), regardless of mutation types and loci. However, current dCas9-based activators are too large for efficient adeno-associated virus delivery, and the feasibility and durability of such treatments remain unclear. Here, we develop a muscle-targeted utrophin activation system using the compact dCasMINI-VPR system, termed MyoAAV-UA. Systemic administration of MyoAAV-UA in male mdx mice leads to substantial upregulation of utrophin at the sarcolemma, resulting in significant improvements in skeletal muscle function and a slowing of heart function deterioration. These benefits remain observable at six months post-treatment. In male nonhuman primates, systemic administration of MyoAAV-UA increases utrophin expression by twofold in skeletal muscle, with no significant side effects observed. Furthermore, MyoAAV-UA upregulates utrophin and utrophin-glycoprotein complexes in induced pluripotent stem cell-derived myotubes from DMD patients. In conclusion, these findings demonstrate the potential of MyoAAV-UA as a therapeutic approach for DMD.