Recent Advances and Future Perspectives of In Vivo Targeted Delivery of Genome-Editing Reagents to Germ cells, Embryos, and Fetuses in Mice

Abstract

The recently discovered clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein 9 (Cas9) systems that occur in nature as microbial adaptive immune systems are considered an important tool in assessing the function of genes of interest in various biological systems. Thus, development of efficient and simple methods to produce genome-edited (GE) animals would accelerate research in this field. The CRISPR/Cas9 system was initially employed in early embryos, utilizing classical gene delivery methods such as microinjection or electroporation, which required ex vivo handling of zygotes before transfer to recipients. Recently, novel in vivo methods such as genome editing via oviductal nucleic acid delivery (GONAD), improved GONAD (i-GONAD), or transplacental gene delivery for acquiring genome-edited fetuses (TPGD-GEF), which facilitate easy embryo manipulation, have been established. Studies utilizing these techniques employed pregnant female mice for direct introduction of the genome-editing components into the oviduct or were dependent on delivery via tail-vein injection. In mice, embryogenesis occurs within the oviducts and the uterus, which often hampers the genetic manipulation of embryos, especially those at early postimplantation stages (days 6 to 8), owing to a thick surrounding layer of tissue called decidua. In this review, we have surveyed the recent achievements in the production of GE mice and have outlined the advantages and disadvantages of the process. We have also referred to the past achievements in gene delivery to early postimplantation stage embryos and germ cells such as primordial germ cells and spermatogonial stem cells, which will benefit relevant research.

Hydrodynamics-Based Transplacental Delivery as a Useful Noninvasive Tool for Manipulating Fetal Genome

“…Notably, data shown in Table S2 were found to be consistent with those obtained when CRISP-ID analysis was applied using the Sanger sequence chart [37]. In this case, the chromosomal integration of the CRISPR/Cas9 vector is not always required; in other words, transient expression of this vector is sufficient for that purpose, as indicated by Sato et al [8].…”

“…To explore the molecular mechanism involved in early organogenesis and to generate animal models with defects in organogenesis, gene delivery of functional nucleic acids (NAs), such as plasmid DNA, RNA interference (as exemplified by siRNA), and genome editing components, to these fetuses is considered to be an important strategy [ 7 ]. Gene delivery targeted to mid-gestational fetuses is largely divided into two routes: one is in utero gene delivery, based on an injection of NAs into fetuses exposed externally with subsequent electroporation at the injected site, and the other is transplacental delivery of NAs complexed with DNA delivery reagents into pregnant females [ 8 ]. Since the former is performed by a local injection of NAs under a dissecting microscope, sites suitable for transfection appear to be very limited.…”

Current status of the application of gene editing in pigs

“…Delivery of CRISPR/Cas9 system via electroporation during embryogenesis: Electroporation-mediated gene editing is a micromanipulation-free method in which large numbers of gene-edited zygotes/embryos can be prepared by introducing gene editors into zygotes. In mice, electroporation is widely used to introduce gene editors [ 51 ]. Gene editing via electroporation has also been applied to porcine zygotes [ 52 ], with successful gene modification (knockout) [ 52 , 53 , 54 , 55 ].…”

Advances in Genome Editing and Application to the Generation of Genetically Modified Rat Models

“…It was first described to generate NHEJ using Cas9 mRNA (Takahashi et al, 2015;Gurumurthy et al, 2016Gurumurthy et al, , 2019b and then the improved GONAD (iGONAD) was reported by Ohtsuka et al (2018) in mice to efficiently generate indels mutations, large deletions, and ssODN and lsDNA-based KI (up to 1 kb), by replacing Cas9 mRNA by Cas9 RNP. Other groups have demonstrated the efficiency of iGONAD in rats for gene disruption and ssODN-based KI (Kobayashi et al, 2018;Takabayashi et al, 2018) and in mice by substituting Cas9 with AsCpf1 (Ohtsuka et al, 2018) (for review see Sato et al, 2020).…”