Mouse Oocyte Methylomes at Base Resolution Reveal Genome-Wide Accumulation of Non-CpG Methylation and Role of DNA Methyltransferases

Abstract

DNA methylation is an epigenetic modification that plays a crucial role in normal mammalian development, retrotransposon silencing, and cellular reprogramming. Although methylation mainly occurs on the cytosine in a CG site, non-CG methylation is prevalent in pluripotent stem cells, brain, and oocytes. We previously identified non-CG methylation in several CG-rich regions in mouse germinal vesicle oocytes (GVOs), but the overall distribution of non-CG methylation and the enzymes responsible for this modification are unknown. Using amplification-free whole-genome bisulfite sequencing, which can be used with minute amounts of DNA, we constructed the base-resolution methylome maps of GVOs, non-growing oocytes (NGOs), and mutant GVOs lacking the DNA methyltransferase Dnmt1, Dnmt3a, Dnmt3b, or Dnmt3L. We found that nearly two-thirds of all methylcytosines occur in a non-CG context in GVOs. The distribution of non-CG methylation closely resembled that of CG methylation throughout the genome and showed clear enrichment in gene bodies. Compared to NGOs, GVOs were over four times more methylated at non-CG sites, indicating that non-CG methylation accumulates during oocyte growth. Lack of Dnmt3a or Dnmt3L resulted in a global reduction in both CG and non-CG methylation, showing that non-CG methylation depends on the Dnmt3a-Dnmt3L complex. Dnmt3b was dispensable. Of note, lack of Dnmt1 resulted in a slight decrease in CG methylation, suggesting that this maintenance enzyme plays a role in non-dividing oocytes. Dnmt1 may act on CG sites that remain hemimethylated in the de novo methylation process. Our results provide a basis for understanding the mechanisms and significance of non-CG methylation in mammalian oocytes.

Author Summary

Methylation of cytosine bases in DNA is an epigenetic modification crucial for normal development, retrotransposon silencing, and cellular reprogramming. In mammals, the vast majority of 5-methylcytosine occurs at CG dinucleotides, and thus most studies to date have focused on this dinucleotide. However, recent studies have shown that 5-methylcytosine is abundant at non-CG (CA, CT, and CC) sites in certain tissues and certain cell types in human and mouse. We previously identified non-CG methylation in CG-rich sequences, including the imprint control regions in mouse germinal vesicle oocytes, but its global distribution and the enzymes responsible are unknown. Using advanced high-throughput sequencing technology applicable to minute amounts of DNA, we obtained high-resolution methylation maps of newborn non-growing oocytes, adult germinal vesicle oocytes, and mutant germinal vesicle oocytes lacking any of the four DNA methyltransferase family proteins. Our results revealed that non-CG methylation accumulates genome-wide in close proximity to highly methylated CG sites during the oocyte growth stage. We also found that the de novo DNA methyltransferase proteins Dnmt3a and Dnmt3L are responsible for non-CG methylation in oocytes. Unexpectedly, we found that the maintenance methyltransferase Dnmt1 has a role in de novo CG methylation. Our study provides a basis for understanding the mechanisms and significance of non-CG methylation in mammalian oocytes.