DNA demethylation pattern of in-vitro fertilized and cloned porcine pronuclear stage embryos

Recent studies in mice showed that the Ten-eleven translocation Enzymes (TET) family is involved in the active DNA demethylation. The isotype TET-3 is responsible for the conversion of 5mc (5-methylcytosine) to 5hmc (5-hydroxymethylcytosine) at the pronuclear stages of mouse embryo. This study was performed to investigate the pattern of methylation change and the role of TET family in the demethylation process of porcine in-vitro fertilization (IVF) and somatic cell nuclear transfer (SCNT) derived embryo. Bisulfite-sequencing PCR (BSP) and DNA glucosylation and digestion before quantitative PCR (qGluMS-PCR) were done to evaluate the exact change of methylation during porcine pronuclear stages. The results showed that the amount of 5hmc detected increased whereas 5mc decreased in IVF embryo from pronuclear stage 2 (PN2) to pronuclear stage 5 (PN5). In addition, Immunofluorescent staining showed that the 5hmc signal, also detected in oocytes, significantly increased in both pronucleus from fertilization to PN2. The amount of 5hmc continued to rise in male pronucleus but decreased to a very low level in female pronucleus from PN2 to PN5. The above results indicate that female pronucleus might undergo active demethylation only at early pronuclear stages. On the other hand, male pronucleus might undergo active demethylation throughout all pronuclear stages. The expression of three TET isotypes (TET-1, TET-2, TET-3) were tested and TET-3 was found to be the highest expressed isotype. High TET-3 concentrations observed mainly in male pronucleus using immunofluorescent staining, implying that TET-3 might be the main enzyme which catalyzes the conversion of 5mc to 5hmc. In contrast, no TET-3 signal was detected in female pronucleus through the pronuclear stages. The demethylation pattern of SCNT embryos resembled that of the male pronucleus of IVF embryos, suggesting that active demethylation might happen in porcine cloned embryo.