Comparative dynamics of 5-methylcytosine reprogramming and TET family expression during preimplantation mammalian development in mouse and sheep

Despite previous assumption that paternal active DNA demethylation is an evolutionary conserved phenomenon in mammals, emerging studies in other species, particularly sheep, do not support this issue. Recently, ten eleven translocation (TET) enzymes have been suggested as intermediates in genome-wide DNA demethylation through the iterative conversion of five methylcytosine (5mC) into 5-hydroxymethylcytosine (5hmC)/5-formylcytosine/5-carboxylcytosine (5caC) derivatives. This study investigated whether TET enzymes and 5mC derivatives are also involved in dynamic reprogramming of early sheep embryos derived by fertilization. Mouse zygotes and developing embryos were considered as control. Obtained results reported substantial differences in dynamics of parent-of-origin-specific patterns of 5mC reprogramming and generation/dilution of 5mC derivatives (5hmC and 5caC) between mouse and sheep early zygotes. Sheep zygotes reported a gradual and insignificant decrease pattern of parental pronucleus 5mC, which was notably replication independent, coincided with gradual generation of 5hmC and 5caC. Although the expression profiles of TET family of enzymes (Tet1, Tet2, and Tet3), with the main exception being Tet2 at later developmental stages, were similar between mouse and sheep developing embryos. In addition, although the expression level of Tet3 was higher than Tet1 and Tet2 in MII oocytes and zygotes in both mouse and sheep, the expression of Tet3 in mouse was higher than sheep in both MII oocytes and zygotes. The contrasting dynamics of 5mC reprogramming between these two species may be associated with the particular evolutionary differences that exist between developmental program of rodents and ruminants, particularly during peri-implantation stages.

5-methylcytosine and its derivatives

Epigenetics has undergone an explosion in the past decade. DNA methylation, consisting of the addition of a methyl group at the fifth position of cytosine (5-methylcytosine, 5-mC) in a CpG dinucleotide, is a well-recognized epigenetic mark with important functions in cellular development and pathogenesis. Numerous studies have focused on the characterization of DNA methylation marks associated with disease development as they may serve as useful biomarkers for diagnosis, prognosis, and prediction of response to therapy. Recently, novel cytosine modifications with potential regulatory roles such as 5-hydroxymethylcytosine (5-hmC), 5-formylcytosine (5-foC), and 5-carboxylcytosine (5-caC) have been discovered. Study of the functions of 5-mC and its oxidation derivatives promotes the understanding of the mechanism underlying association of epigenetic modifications with disease biology. In this respect, much has been accomplished in the development of methods for the discovery, detection, and location analysis of 5-mC and its oxidation derivatives. In this review, we focus on the recent advances for the global detection and location study of 5-mC and its oxidation derivatives 5-hmC, 5-foC, and 5-caC.