CRISPR/Cas9 as tool for functional study of genes involved in preimplantation embryo development

The CRISPR/Cas9 system has proven to be an efficient gene-editing tool for genome modification of cells and organisms. However, the applicability and efficiency of this system in pig embryos have not been studied in depth. Here, we aimed to remove porcine OCT4 function as a model case using the CRISPR/Cas9 system. Injection of Cas9 and single-guide RNA (sgRNA) against OCT4 decreased the percentages of OCT4-positive embryos to 37-50% of total embryos, while ~100% of control embryos exhibited clear OCT4 immunostaining. We assessed the mutation status near the guide sequence using polymerase chain reaction (PCR) and DNA sequencing, and a portion of blastocysts (20% in exon 2 and 50% in exon 5) had insertions/deletions near protospacer-adjacent motifs (PAMs). Different target sites had frequent deletions, but different concentrations of sgRNA made no impact. OCT4 mRNA levels dramatically decreased at the 8-cell stage, and they were barely detectable in blastocysts, while mRNA levels of other genes, including NANOG, and CDX2 were not affected. In addition, the combination of two sgRNAs led to large-scale deletion (about 1.8 kb) in the same chromosome. Next, we injected an enhanced green fluorescent protein (eGFP) vector targeting the OCT4 exon with Cas9 and sgRNA to create a knockin. We confirmed eGFP fluorescence in blastocysts in the inner cell mass, and also checked the mutation status using PCR and DNA sequencing. A significant portion of blastocysts had eGFP sequence insertions near PAM sites. The CRISPR/CAS9 system provides a good tool for gene functional studies by deleting target genes in the pig.