48 EFFECT OF DIMETHYL SULFOXIDE- OR GLYCEROL-BASED VITRIFICATION PROTOCOLS ON THE DNA METHYLATION OF BOVINE CUMULUS-OOCYTE COMPLEXES

Abstract

Although vitrification is becoming increasingly common for oocyte preservation, there has been recent evidence that some cryoprotectants may alter DNA methylation and so result in decreased oocyte developmental competence and abnormal embryonic development post-warming. The objective of this study was to determine the effect of dimethyl sulfoxide (DMSO)- or glycerol-based vitrification protocols on DNA methylation in bovine cumulus-oocyte complexes (COC). The vitrification protocols evaluated included a combination of ethylene glycol (EG) with either DMSO or glycerol (GLY). Cumulus-oocyte complexes were first exposed to equilibration solution (ES) consisting of 7.5% DMSO or GLY and 7.5% EG for 9 min at room temperature (RT) before being transferred to vitrification solution (VS) containing 15% DMSO or GLY, 15% EG, and 0.5 M sucrose. While in VS, 3 to 4 COC were loaded into an open system vitrification device (Cryolock®, BioTech Inc.) and plunged into LN within 1 min. For warming, COC were exposed to dilution solution 1 consisting of 0.5 M sucrose (37°C) for 2.5 min and to dilution solution 2 consisting of 0.25 M sucrose for 2.5 min (RT). Base media for all solution was PBS supplemented with 20% FBS. Cumulus-oocyte complexes (n = 137) were collected from crossbred cows by ovum pick-up on 3 separate days (repetitions), with half of the oocytes each day being matured to metaphase II (MII) stage before vitrification and the other half vitrified as germinal vesicle (GV) COC. The resulting treatments were DMSO GV v. Glycerol GV and DMSO MII v. Glycerol MII. Fresh COC (GV and MII stage, respectively) were utilised as controls. DNA methylation analysis of oocytes was performed through fluorescent staining using a primary antibody anti-5 mC (1:1000) and secondary antibody Alexa Fluor 488 (1:500). Fluorescence intensity of samples was measured using ImageJ software (NIH, Bethesda, MD, USA). Data were analysed through ANOVA with post hoc Tukey’s test. No differences between the vitrification groups or controls were found when analysing DNA methylation of GV stage COC (P = 0.1825). Metaphase II stage COC showed statistical difference between groups (P < 0.001) where glycerol-treated MII oocytes exhibited higher DNA methylation compared with DMSO MII and fresh MII COC. Results of the experiment showed that the glycerol-based vitrification protocol causes alteration of DNA methylation of bovine metaphase II cumulus-oocyte complexes, and as such may not be an optimal choice in mature oocyte vitrification protocols.