Methylation Changes in Porcine Primordial Germ Cells

Epigenetic re-programming is an important event in the development of primordial germ cells (PGC) into functional gametes, characterized by genome-wide erasure of DNA methylation and re-establishment of epigenetic marks, a process essential for restoration of the potential for totipotency. In this study changes in the methylation status of centromeric repeats and two IGF2-H19 differentially methylated domain (DMD) sequences were examined in porcine PGC between days 24 and 31 of pregnancy. The methylation levels of centromeric repeats and IGF2-H19 DMD sequences decreased rapidly from days 24 to 28 in both male and female PGC. At days 30 and 31 of pregnancy centromeric repeats and IGF2-H19 DMD sequences acquired new methylation in male PGC, while in female PGC these sequences were completely demethylated by day 30 and remained hypomethylated at Day 31. To characterize methylation changes that PGC undergo in culture, the methylation status of embryonic germ cells (EGCs) derived from PGC at day 26 of pregnancy was examined. Centromeric repeats and IGF2-H19 DMD sequences were similarly methylated in both male and female EGC and hypermethylated in female EGC compared with female PGC at the same embryonic age. Our results show that, similar to murine PGC, porcine PGC undergo genome-wide DNA demethylation shortly after arrival in the genital ridges. When placed in culture porcine PGC terminate their demethylation program and may acquire new DNA methylation marks. To our knowledge, this is the first report regarding epigenetic re-programming of genital ridge PGC in the pig.