Reproduction Abstracts

Reprogramming of DNA methylation patterns in the early embryo is required for normal development. A longstanding paradigm of global loss of 5′-methylcytosine (5meC) over the first few cell-cycles of early development has recently been challenged (Li & O’Neill 2012, Li & O’Neill 2013)^1,2. Here we show that immuno-detectable levels of 5meC and 5′-hydroxymethylcytosine (5hmC) persist throughout zygotic maturation and the first cell-cycles (to the eight-cell stage). The inner cells of the morula showed a marked reduction in global levels of both 5meC and 5hmC and further loss occurred in the pluripotent inner cell mass (ICM) of the blastocyst stage embryo. High levels of both modifications persisted in the outer cells of the morula and the resulting trophectoderm of the blastocyst. This lineage-dependent differential pattern of cytosine modifications was dependent upon the activity of DNA methyltransferases. The low level of both cytosine modifications persist as cells of the ICM differentiate into the epiblast and hypoblast lineages after implantation, while the high levels of 5meC and 5hmC in the trophectoderm persisted as it differentiated into trophoblast and then into trophoblast giant cells. Our results redefines the dynamics of programming covalent modifications to cytosine in early development and shows that the global changes in the pattern of nuclear 5meC and 5hmC are early markers of commitment of cells to the pluripotent lineage in the early embryo.

References: 1. Li Y & O’Neill C (2012) Persistence of cytosine methylation of DNA following fertilisation in the mouse. PLoS ONE 7 (1) e30687.

2. Li Y & O’Neill C (2013) 5′-methylcytosine and 5′-hydroxymethylcytosine each provide epigenetic information to the mouse zygote. PLoS ONE 8 (5) e63689.