Selective loss of imprinting in the placenta following preimplantation development in culture

TLDR

Preimplantation development is marked by dynamic epigenetic remodeling of parental genomes while imprinting marks are maintained to direct appropriate gene expression. The study aimed to define the lability of genomic imprints during preimplantation and to determine whether loss of imprinting persists at later stages by examining imprinted gene expression and methylation after in vitro culture. Imprinted gene expression and methylation were examined after in vitro preimplantation culture. In vitro preimplantation culture in Whitten’s medium caused loss of H19 imprinting, with aberrant paternal allele expression and reduced methylation in ~65 % of blastocysts, a pattern that persisted into mid‑gestation and was evident in placental tissues for multiple genes, whereas embryonic expression remained largely imprinted, indicating trophectoderm tissues lack mechanisms to restore imprints.

Abstract

Preimplantation development is a period of dynamic epigenetic change that begins with remodeling of egg and sperm genomes, and ends with implantation. During this time, parental-specific imprinting marks are maintained to direct appropriate imprinted gene expression. We previously demonstrated that H19 imprinting could be lost during preimplantation development under certain culture conditions. To define the lability of genomic imprints during this dynamic period and to determine whether loss of imprinting continues at later stages of development, imprinted gene expression and methylation were examined after in vitro preimplantation culture. Following culture in Whitten's medium, the normally silent paternal H19 allele was aberrantly expressed and undermethylated. However, only a subset of individual cultured blastocysts (∼65%) exhibited biallelic expression, while others maintained imprinted H19 expression. Loss of H19 imprinting persisted in mid-gestation conceptuses. Placental tissues displayed activation of the normally silent allele for H19, Ascl2, Snrpn, Peg3 and Xist while in the embryo proper imprinted expression for the most part was preserved. Loss of imprinted expression was associated with a decrease in methylation at the H19 and Snrpn imprinting control regions. These results indicate that tissues of trophectoderm origin are unable to restore genomic imprints and suggest that mechanisms that safeguard imprinting might be more robust in the embryo than in the placenta.