Vitrification of pig oocytes induces changes in histone H4 acetylation and histone H3 lysine 9 methylation (H3K9).

In the present study, acetylation status of histone H4 and methylation status of the lysine 9 residue of histone H3 (H3K9) were assessed by immunofluorescence in order to determine the effect of vitrification on epigenetic status of pig MII oocytes. Hyperacetylation of H4 and dimethylation of H3K9 were assessed in control oocytes, after cryoprotectant treatment and after vitrification at two time points, immediately after warming and after a post-warming incubation for 2 h. While no changes in the immunopositivity for both the epitopes were recorded after cryoprotectants, the percentage of negative oocytes for dimethyl H3K9 was observed to increase immediately after devitrification. The influence of vitrification was more evident after 2 h post-thaw incubation when acetylation status of H4 significantly increased and a rise in the percentages of both oocytes exhibiting strong positivity and negative oocytes for dimethyl H3K9 was observed. In conclusion, acetylation of H4 and methylation of H3K9 are altered by vitrification procedure that may lead to an aberrant epigenetic presentation of female chromatin to the fertilizing event and may be, at least in part, responsible for the reduction of developmental competence of vitrified pig oocytes.