Long-term storage does not affect the DNA methylation profiles of vitrified-warmed human embryos

With the widespread application of embryo cryopreservation in assisted reproductive techniques, it is necessary to assess the safety of long-term cryopreservation of human embryos and it is unclear whether storage time has an impact on the DNA methylation profiles of human embryos. Nine women who received IVF treatment were recruited for this study. The retrieved eight-cell human embryos were classified into three groups including fresh embryos, cryopreserved embryos stored for 3 years, and cryopreserved embryos stored for 8 years. Single-cell whole-genome bisulfite sequencing (scWGBS) was conducted. The genome-wide methylation pattern of the fresh and two cryopreserved groups were similar. In addition, the methylation level in different genomic regions showed comparable patterns and no significant differences were observed in the methylation level of imprinted genes among the three groups. A total of 587 differentially methylated regions (DMRs) in the 3-year group and 540 DMRs in the 8-year group were identified comparing to fresh group. However, they were not enriched in promoters and had a similar genome-wide distributions, suggesting that these DMRs may not contribute to the changes in corresponding gene expressions. Our study illustrated that long-term cryopreservation will not affect the DNA methylation profiles of human eight-cell embryos at single-cell level.