Epigenetic Changes in Equine Embryos After Short-term Storage at Different Temperatures

Abstract

Background: In embryos subjected to assisted reproductive techniques, epigenetic modifications may occur that can influence embryonic development and establishment of pregnancy. In horses, the storage temperature during transport of fresh embryos before transfer is a major concern. The aim of this study was to determine the effects of two storage temperatures (5˚C and 20˚C) on equine embryos, collected at day 7 after ovulation and stored for 24 hours, on the following characteristics: (i) morphological and histological development; (ii) expression of candidate genes associated with embryo growth and development, maternal recognition of pregnancy, methylation, and apoptosis; and (iii) gene-specific and global-DNA methylation. Embryos (n=80) were collected from Haflinger mares (n=15) on day 7 (n=60) or day 8 (n=20) after ovulation and assigned to 4 groups: day 7 control (E7F, fresh); day 7, stored for 24h at 5°C (E5C); day 7, stored for 24h at 20°C (E20C); and day 8 control (E8F, fresh 24h time control). The embryos and the storage medium from all groups were analyzed for: (i) medium temperature, pH, and lipid peroxidation (malondialdehyde; MDA), (ii), embryo morphology, mRNA expression, and DNA methylation (immunohistochemistry and gene-specific DNA methylation). Results: Temperature during storage (E5C and E20C) did not affect embryo size (382±47 and 553±99 µm, respectively). There were no changes in pH and MDA accumulation irrespective of group. The relative mRNA abundance of specific genes related to growth and development (POU5F1, SOX2, NANOG), maternal recognition of pregnancy (CYP19A1, PTGES2), DNA methylation (DNMT1, DNMT3A, DNMT3B), and apoptosis (BAX) in the E5C and E20C were either up or downregulated (P<0.05) when compared to controls (E7F and E8F). The global methylation status, even as 5mC and 5hmC immune expression were similar among treatment groups. The specific genes ESR1, NANOG, and DNMT1 were hypomethylated (P<0.001) in E20C embryos when compared to E8F (advanced stage). Conclusions: Therefore, our study demonstrates for the first time the gene-specific and global-DNA methylation status of fresh equine embryos collected on days 7 and 8 after ovulation. In addition, short-term storage, regardless of temperature, modified gene expression and methylation of genes involved in embryo development and may therefore compromise embryo viability and development after transfer.