The higher methylation levels of

The higher methylation levels of non-CpG sites in human oocytes is consistent with what has been reported for other cell types that possess high potential for maturation or further differentiation, including mammalian pd0332991 cell types and human pluripotent stem cells (Lister et al., 2013), as well as mouse oocytes (Tomizawa et al., 2011). The biological roles that non-CpG methylation may play in these specific cell types remain to be investigated. Could non-CpG methylation be marking a poised epigenetic state for future differentiation and continued development in these cell types? This question needs to be answered by future mechanistic studies. Future investigation in single-cell transcriptomic changes associated with human oocyte maturation may help to elucidate the physiological roles played by non-CpG methylation during human oocyte maturation. The DNA methylome of human oocytes shares many other similar features with mouse oocytes, such as preferentially acquiring DNA methylation within the gene bodies, especially in those genes that are highly transcribed (Kocabas et al., 2006; Shirane et al., 2013; Smallwood et al., 2011; Veselovska et al., 2015). However, to the best of our knowledge, there has not been a study in mouse or other species that directly examines the non-CpG methylation changes during oocyte maturation. Several studies investigated non-CpG methylation in mouse or human oocytes in one of the maturation stages, but none compared oocytes in two or three different stages of maturation. Specific DNA methyltransferases (DNMTs) involved in genomic non-CpG methylation in mouse oocytes have been examined by Shirane et al. (2013). Using mutant mouse GV oocytes lacking Dnmt1, 3a, 3b, or 3L, Transformation was shown that genomic non-CpG methylation in oocytes depends on the Dnmt3a-Dnmt3L complex, while Dnmt1 and 3b are dispensable. Our study could hold potential clinical significance, because in vitro maturation of oocytes is an important assisted reproductive intervention that has been under intense investigation for many years, and its success will have wide applications in infertility treatments and especially in fertility preservation in childhood cancer patients. By manipulating the non-CpG methylation in immature oocytes such as by upregulating the responsible DNMTs, it may be possible to facilitate or accelerate the maturation process, and subsequently use the resulting mature oocytes for therapeutic purposes.
Experimental Procedures
Author Contributions
Acknowledgments The authors would like to thank Adam Auton, PhD, for guidance in data analysis and statistics and Moonsook Lee for technical guidance in single-cell WGBS library preparation. This work was supported by grants from the Reproductive Scientist Development Program (NIH5K12HD000849), American Society for Reproductive Medicine, Howard and Georgeanna Jones Foundation for Reproductive Medicine, Einstein Nathan Shock Center of Excellence in Basic Biology of Aging (to B.Y.); NIH P01AG017242, the Glenn Foundation, and the SENS Foundation (to J.V.).