Dna Methylation Establishment Of Cpg Islands Near Maternally Imprinted Genes On Chromosome 7 During Mouse Oocyte Growth

The genome methylation is globally erased in early fetal germ cells, and it is gradually re-established during gametogenesis. The expression of some imprinted genes is regulated by the methylation status of CpG islands, while the exact time of DNA methylation establishment near maternal imprinted genes during oocyte growth is not well known. Here, growing oocytes were divided into three groups based on follicle diameters including the S-group (60-100 mu m), M-group (100-140 mu m), and L-group (140-180 mu m). The fully grown germinal vesicle (GV)-stage and metaphase II (M2)-stage mature oocytes were also collected. These oocytes were used for single-cell bisulfite sequencing to detect the methylation status of CpG islands near imprinted genes on chromosome 7. The results showed that the CpG islands nearNdn,Magel2,Mkrn3,Peg12, andIgf2were completely unmethylated, but those ofPeg3,Snrpn, andKcnq1ot1were hypermethylated in MII-stage oocytes. The methylation of CpG islands near different maternal imprinted genes occurred asynchronously, being completed in later-stage growing oocytes, fully grown GV oocytes, and mature MII-stage oocytes, respectively. These results show that CpG islands near some maternally imprinted genes are not necessarily methylated, and that the establishment of methylation of other maternally imprinted genes is completed at different stages of oocyte growth, providing a novel understanding of the establishment of maternally imprinted genes in oocytes.