(91h) Mapping the Methylome in Single Cells Reveals Heterogeneity in DNA Methylation Turnover during Early Mammalian Development

DNA methylation (5-methylcytosine or 5mC) is a key epigenetic modification that is inherited from mother to daughter cells. This property of 5mC plays an important role in facilitating the propagation of cellular identity through cell divisions and restricting the developmental potential of terminally differentiated cells. Consequently, during preimplantation mammalian development, 5mC patterns on the terminally differentiated paternal sperm and maternal egg genomes are erased post-fertilization on a genome-wide scale to revert cellular memory towards an undifferentiated state in the blastocyst. In the first half of this talk, I will describe a new single-cell sequencing method (scMspJI-seq) that enables strand-specific quantification of 5mC, providing insights into the mechanism underlying global DNA demethylation dynamics in preimplantation mouse and human embryos.

While 5mC is generally thought to be stably inherited during cell division, the fidelity with which this mark is faithfully copied remains unclear. To investigate this, in the second half of my talk, I will describe another new single-cell sequencing method that quantifies the methylation status at individual CpG dyads (scDyad-seq). Using scDyad-seq, we find dramatic heterogeneity in the fidelity of copying 5mC during cell division and show that it is strongly dependent on the density of the methylome and the distribution of histone modifications at a given region of the genome. Together, these single-cell technologies provide detailed mechanistic understanding of DNA methylation turnover in mammalian systems.