Characterization of the IGF2 Imprinted Gene Methylation Status in Bovine Oocytes during Folliculogenesis.

Anelise Dos Santos Mendonça,Margot Alves Nunes Dode,Alexandre Rodrigues Caetano,Ana Luíza Silva Guimarães,Maurício Machaim Franco,Naiara Milagres Augusto Da Silva,Wei Shen

doi:10.1371/journal.pone.0142072

Anelise Dos Santos Mendonça, Margot Alves Nunes Dode + Show 5 more

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https://doi.org/10.1371/journal.pone.0142072

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Abstract

DNA methylation reprogramming occurs during mammalian gametogenesis and embryogenesis. Sex-specific DNA methylation patterns at specific CpG islands controlling imprinted genes are acquired during this window of development. Characterization of the DNA methylation dynamics of imprinted genes acquired by oocytes during folliculogenesis is essential for understanding the physiological and genetic aspects of female gametogenesis and to determine the parameters for oocyte competence. This knowledge can be used to improve in vitro embryo production (IVP), specifically because oocyte competence is one of the most important aspects determining the success of IVP. Imprinted genes, such as IGF2, play important roles in embryo development, placentation and fetal growth. The aim of this study was to characterize the DNA methylation profile of the CpG island located in IGF2 exon 10 in oocytes during bovine folliculogenesis. The methylation percentages in oocytes from primordial follicles, final secondary follicles, small antral follicles, large antral follicles, MII oocytes and spermatozoa were 73.74 ± 2.88%, 58.70 ± 7.46%, 56.00 ± 5.58%, 65.77 ± 5.10%, 56.35 ± 7.45% and 96.04 ± 0.78%, respectively. Oocytes from primordial follicles showed fewer hypomethylated alleles (15.5%) than MII oocytes (34.6%) (p = 0.039); spermatozoa showed only hypermethylated alleles. Moreover, MII oocytes were less methylated than spermatozoa (p<0.001). Our results showed that the methylation pattern of this region behaves differently between mature oocytes and spermatozoa. However, while this region has a classical imprinted pattern in spermatozoa that is fully methylated, it was variable in mature oocytes, showing hypermethylated and hypomethylated alleles. Furthermore, our results suggest that this CpG island may have received precocious reprogramming, considering that the hypermethylated pattern was already found in growing oocytes from primordial follicles. These results may contribute to our understanding of the reprogramming of imprinted genes during bovine oogenesis.

Highlights

DNA methylation, an epigenetic event, regulates important biological processes, such as genomic imprinting, transposon silencing and chromosomal stability, and has an essential role in mammalian gametogenesis and embryogenesis [1,2,3,4].During the mammalian life cycle, two waves of DNA methylation reprogramming occur
Understanding the life cycle of DNA methylation that occurs during gametogenesis may contribute to improving fertility traits in animals and increasing the efficiency of assisted reproductive technologies (ARTs), especially because epigenetic events may be susceptible to environment effects [7]
No significant differences in DNA methylation percentage were found between the oocyte from primordial follicles and MII oocytes (p = 0.088)

Summary

Introduction

During the mammalian life cycle, two waves of DNA methylation reprogramming occur. A de novo methylation process is initiated, at the 8–16 cell and blastocyst stage in bovine and mouse embryos, respectively [6]. From this point, embryonic cells start receiving tissue-specific methylation patterns [3,4,5]. Understanding the life cycle of DNA methylation that occurs during gametogenesis may contribute to improving fertility traits in animals and increasing the efficiency of assisted reproductive technologies (ARTs), especially because epigenetic events may be susceptible to environment effects [7]

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