Influence of Cryopreservation of Pre-Implantation Embryos on the Epigenome

The cryopreservation of pre-implantation embryos has become routine in medically assisted reproduction (MAR), and the proportion of frozen embryo transfers has steadily increased in recent years. Because cryopreservation through either slow-cooling protocols or ultra-rapid vitrification requires potentially cytotoxic cryoprotective agents to prevent uncontrolled and detrimental ice crystal formation, the safety of these procedures must be carefully considered. Evidence from human epidemiological studies, including retrospective and prospective controlled studies, and data from national patient registries indicate that children born after frozen embryo transfer have a higher birth weight than those born after spontaneous conception and have an increased risk of rare genomic imprinting disorders, such as Beckwith–Wiedemann, Silver–Russell, or Prader–Willi syndrome. Encompassing not only reversible DNA methylation patterns established during gametogenesis, but also the timed abundance and availability of transcripts and proteins required to establish or maintain epigenetic marks throughout development and differentiation, as well as persistent or transient post-translational histone modifications and non-coding RNAs, the epigenome may be particularly sensitive to cryopreservation. Importantly, epigenetic regulation is highly complex. Alterations of the epigenome at any developmental stage are often not monocausal, do not necessarily result in immediate disturbances in the pre-implantation embryo, and are unlikely to operate through simple all-or-nothing mechanisms; however, they may have long-lasting effects at later developmental stages. To make matters even more complex, differences between species in terms of epigenetic regulation or lineage differentiation are well known and translation from animal model systems to humans must be considered with caution. More recently, epigenetic regulation by non-coding RNAs has also come into focus, as these molecules are crucial, either directly or indirectly, for gene expression, translation, and protein biosynthesis during development. Therefore, assessing potential adverse effects of cryopreservation on the entire epigenome remains a major challenge, particularly because little is known about indirect factors, such as post-translational histone modifications and non-coding RNAs. In this review, we focus on the potential influence of the cryopreservation of pre-implantation embryos on the epigenetic profile in humans and animals. Specifically, we consider DNA methylation of imprinted genes and global DNA methylation; post-translational histone modifications; the abundance and availability of transcripts and proteins required to establish, maintain, or protect epigenetic patterns; and the presence of non-coding RNAs involved in epigenetic control.