As well as genetic information, the egg and sperm also contribute epigenetic annotations that may influence gene activity after fertilisation. These annotations may be direct modifications of the DNA bases or of the proteins around which the DNA is wrapped into chromatin. Our goal is to understand whether, through epigenetics, factors such as a mother’s age or diet have consequences on the health of a child. We examine how epigenetic states are set up in oocytes – or egg cells – and influence gene expression in the embryo. For example, repressive chromatin marks in oocytes lead to long-term silencing of genes inherited from the mother, particularly in cells that will form the placenta. We are also interested in how variations in DNA methylation come about in oocytes and whether we can use this variation as a marker for oocyte quality and embryo potential. To investigate these questions, we develop methods to profile epigenetic information in very small numbers of cells or even in single cells.
Profiling combinations of histone modifications identifies gene regulatory elements in different states and discovers features controlling transcriptional and epigenetic programs. However, efforts to map chromatin states in complex, heterogeneous samples are hindered by the lack of methods that can profile multiple histone modifications together with transcriptomes in individual cells. Here, we describe single-cell multitargets and mRNA sequencing (scMTR-seq), a high-throughput method that enables simultaneous profiling of six histone modifications and transcriptome in single cells. We apply scMTR-seq to uncover dynamic and coordinated changes in chromatin states and transcriptomes during human endoderm differentiation. We also use scMTR-seq to produce lineage-resolved chromatin maps and gene regulatory networks in mouse blastocysts, revealing epigenetic asymmetries at gene regulatory regions between the three embryo lineages and identifying Trps1 as a potential repressor in epiblast cells of trophectoderm-associated enhancer networks and their target genes. Together, scMTR-seq enables investigation of combinatorial chromatin landscapes in a broad range of heterogeneous samples, providing insights into epigenetic regulatory systems.
The advent of single-cell RNA sequencing (scRNA-seq) has revolutionized the study of gene expression in individual cells, providing unprecedented insights into cellular heterogeneity and developmental processes. The application of scRNA-seq to oocyte biology has facilitated the identification of species-specific transcriptional signatures and developmental trajectories, enhancing our understanding of oogenesis. This chapter presents a detailed protocol for scRNA-seq analysis of growing bovine oocytes.
Experimental models and epidemiological data suggest that environmental factors, for example, adverse nutrition prior to conception, can lead to phenotypes in offspring of exposed parents in the absence of continued exposure. As a result these phenotypes have been described as epigentically inherited. The mechanistic basis for such phenomena has not been established in most cases. In this review, we consider possible contributing mechanisms for environmentaly induced epigenetic inheritance, with a focus on maternally transmitted effects and by comparing to paradigms of epigenetic inheritance with a clear mechanistic understanding. Genomic imprinting has provided an important conceptual framework for how the epigenetic states of parental germlines can determine allelic expression in offspring, yet, generally speaking, imprinted genes appear resilient to epigenetic disruption from altered parental environments. Metastable epialleles are environmentally sensitive and variably expressed loci that can impact organism phenotype, but the nature of any epigenetic marker at these loci transferred to offspring is unclear. Studies of examples across these forms of epigenetic inheritance show predominant effects are mediated by oocyte factors involved inreprogramming of the genome post-fertilization, rather than direct effects on gametic DNA methylation, with the exception of genomic imprinting. The potential contribution of additional oocyte chromatin features to the specific liability of phenotypic effector genes and their potential to persist through this reprogramming, however, remains to be investigated.