Analysis of DNA Methylation Near the Transcription Starting Site of KPNA7 Gene During Bovine Early Embryonic Development

Epigenetic reprogramming is essential for normal early embryonic development. KPNA7, an oocyte-specific maternal effect gene, plays an important role as a nuclear transporter for critical transcription factors which govern key check points during development. In mouse and cattle, KPNA7 is completely depleted in somatic tissues and lack of KPNA7 leads to developmental incompetency of preimplatation embryos. The expression of KPNA7 is high in oocytes and early stage embryos, but drops sharply after embryonic genome activation. To elucidate whether an epigenetic mechanism is involved in the controlled temporal-spatial and tissue specific expression of KPNA7, we examined the dynamics of DNA methylation near its transcription starting site (TSS) including proximal promoter, exon 1 and partial intron 1 in oocytes, early embryos and somatic tissues. Twenty CpG sites were characterized, consisting of 14 CpG sites in the promoter (-210 to -1) and 6 CpG sites in exon 1 (+1 to +116). Results show that 18 CpG sites are hypomethylated (<20%) in 16-cell embryo and blastocyst while 19 sites in somatic tissue are hypermethylated (>80%). Comparing GV oocyte to somatic samples, four differentially demethylated sites (-149, -63, -54, +37) were discovered, which are considered as potential sites in the control of tissue-specific expression. CpG site at -54 is within the binding sequence of transcription factor Sp1, which is known to control tissue-specific expression of other genes and its activity is highly affected by methylation. The study suggests that the tissue-specific expression of KPNA7 might be controlled by both transcription factors and sensitive-site cytosine methylation.