91 DISRUPTION OF TET1 DURING PORCINE EMBRYOGENESIS USING CRISPR/Cas9 SYSTEM

Abstract

Ten-eleven translocation (TET) enzymes catalyse oxidation of 5-methylcytosine to 5-hydroxymethyl cytosine. This TET-mediated conversion of 5-methylcytosine to 5-hydroxymethyl cytosine is implicated in initiating the DNA demethylation process, observed post-fertilization. Three members (TET1–3) of the TET family are differentially expressed during embryo development and appear to have different roles. Previous studies in mice suggest that TET1 is a key regulator in maintaining pluripotency in embryonic stem cells by managing epigenetic marks such as DNA methylation. This would imply that TET1 should be a regulator of epigenetic marks during embryo development, although this has not been demonstrated. Previously, we have cloned porcine TET1 from blastocysts (GenBank accession number KC137683) and demonstrated that the level of TET1 (mRNA and protein) was high in blastocysts. The protein level was greater in the inner cell mass compared with the trophectoderm. In this study, we generated TET1 knockout porcine embryos using CRISPR/Cas9 system to study the role of TET1 in controlling epigenetic marks during porcine embryo development. First, 2 sgRNA, immediately downstream of the presumable translation initiation site, were designed and synthesised; location of the sgRNA were nucleotide position at 2 to 21 bp and 23 to 42 bp, respectively (KC137683). Then, sgRNA (10 ng μL⁻¹ each) and Cas9 mRNA (20 ng μL⁻¹) were injected into the cytoplasm of IVF zygotes, and Day 7 blastocysts were genotyped. All embryos carried mutations on both alleles of TET1 (10/10), one homozygous and 9 biallelic mutations. However, immunocytochemistry analysis of other CRISPR/Cas9 injected embryos revealed that TET1 was not removed (10/10), indicating that the sgRNA may have not introduced a premature stop codon 3′ to the presumable translation initiation site. Therefore, 2 new sgRNA were designed to generate a premature stop codon at the 5′ side of a key functional domain, the 2-oxoglutarate-Fe(II)-dependent oxygenase domain (4690 to 5160 bp); the locations of the 2 sgRNA were 4450 to 4469 bp and 4501 to 4520 bp, respectively. Similarly, all of the embryos carried mutations in TET1 (7/7), 2 homozygous and 5 biallelic mutations. In addition, TET1 proteins were not detected in 11 of 16 blastocysts, confirmed by immunocytochemistry. In this study, we successfully generated embryos lacking TET1 by introducing designed CRISPR/Cas9 system during embryogenesis. Presence of TET1 from the first injection experiment suggests that the presumable translation initiation site is not accurate. Discrepancy between genotyping and immunocytochemistry results from the second injection experiment indicates that embryos possessing TET1 protein probably have mutations in triplets, thus no premature stop codon was synthesised. Further studies will focus on identifying the role of TET1 in maintaining pluripotency and epigenetic modification during pre-implantation stage using these embryos.