GSE122599
Dynamics of mitochondrial DNA methylation in oocytes, embryos and granulosa cells
Comparison of mitochondrial DNA (mtDNA) methylation patterns in oocytes, blastocysts and ovarian granulosa cells indicates hitherto unsuspected dynamics. Oocytes and blastocysts recovered from animals subjected to ovarian stimulation and from abattoir ovaries were analyzed using bisulphite transformation of DNA followed by whole genome sequencing. The number of mtDNA copies is quite high in oocytes (200–400,000) and early embryos, resulting in very high coverage (> 3000 X) and very low p values for each of 716 cytosine-based nucleosides. Methylation was lowest in oocytes, following by blastocysts then granulosa cells and was not restricted to CpG sites but found also at CHG and CHH sites not distributed randomly. Analysis of the corresponding genomic DNA also revealed numerous methylated CHG and CHH sites, particularly in oocytes. In association with ovarian stimulation, mtDNA methylation was lower in oocytes than in blastocysts, suggesting a developmental function. RNA analysis of mitochondria encoded genes showed a significant inverse correlation between methylation and expression level for all sequences. Methylation was more extensive in mature animals than in pre-pubertal animals and the profile in granulosa cells differed strikingly from those in oocytes and blastocysts. Although preliminary, the results suggest that mtDNA methylation might play a role during gametogenesis and might be subject to regulation according to animal environment and/or age. Overall design: Six samples were analysed in a single replicate with the OPU and Old condidered as good conditions and the abattoir and young as less optimal conditions.