A subset of genes, known as imprinted genes, is present in the mammalian genome. 
Genomic imprinting governs the monoallelic expression of these genes, depending 
on whether the gene was inherited from the sperm or the egg. This 
parent-of-origin specific gene expression is generally dependent on the 
epigenetic modification, DNA methylation, and the DNA methylation status of CpG 
dinucleotides residing in loci known as differentially methylated regions 
(DMRs). The enzymatic machinery responsible for the addition of methyl (-CH(3)) 
groups to the cytosine residue in the CpG dinucleotides are known as DNA 
methyltransferases (DNMTs). Correct establishment and maintenance of methylation 
patterns at imprinted genes has been associated with placental function and 
regulation of embryonic/fetal development. Much work has been carried out on 
imprinted genes in mouse and human; however, little is known about the 
methylation dynamics in the bovine oocyte. The primary objective of the present 
study was to characterize the establishment of methylation at maternally 
imprinted genes in bovine growing oocytes and to determine if the expression of 
the bovine DNMTs-DNMT3A, DNMT3B, and DNMT3L-was coordinated with DNA methylation 
during oocyte development. To this end, a panel of maternally imprinted genes 
was selected (SNRPN, MEST, IGF2R, PEG10, and PLAGL1) and putative DMRs for MEST, 
IGF2R, PEG10, and PLAGL1 were identified within the 5' regions for each gene; 
the SNRPN DMR has been reported previously. Conventional bisulfite sequencing 
revealed that methylation marks were acquired at all five DMRs investigated in 
an oocyte size-dependent fashion. This was confirmed for a selection of genes 
using pyrosequencing analysis. Furthermore, mRNA expression and protein analysis 
revealed that DNMT3A, DNMT3B, and DNMT3L are also present in the bovine oocyte 
during its growth phase. This study demonstrates for the first time that an 
increase in bovine imprinted gene DMR methylation occurs during oocyte growth, 
as is observed in mouse.