Publications

Transgenerational epigenetic inheritance in mammals has long been debatable. In this issue of Cell, Takahashi et al. induce DNA methylation at promoter-associated CpG islands (CGIs) of two metabolism-related genes and show that the acquired epigenetic changes and associated metabolic phenotypes are stably propagated across several generations in transgenic mice.

Mammalian parental imprinting represents an exquisite form of epigenetic control regulating the parent-specific monoallelic expression of genes in clusters. While imprinting perturbations are widely associated with developmental abnormalities, the intricate regional interplay between imprinted genes makes interpreting the contribution of gene dosage effects to phenotypes a challenging task. Using mouse models with distinct deletions in an intergenic region controlling imprinting across the Dlk1-Dio3 domain, we link changes in genetic and epigenetic states to allelic-expression and phenotypic outcome in vivo. This determined how hierarchical interactions between regulatory elements orchestrate robust parent-specific expression, with implications for non-imprinted gene regulation. Strikingly, flipping imprinting on the parental chromosomes by crossing genotypes of complete and partial intergenic element deletions rescues the lethality of each deletion on its own. Our work indicates that parental origin of an epigenetic state is irrelevant as long as appropriate balanced gene expression is established and maintained at imprinted loci.

The mammalian body plan is established shortly after the embryo implants into the maternal uterus, and our understanding of post-implantation developmental processes remains limited. Although pre- and peri-implantation mouse embryos are routinely cultured in vitro
^1,2, approaches for the robust culture of post-implantation embryos from egg cylinder stages until advanced organogenesis remain to be established. Here we present highly effective platforms for the ex utero culture of post-implantation mouse embryos, which enable the appropriate development of embryos from before gastrulation (embryonic day (E) 5.5) until the hindlimb formation stage (E11). Late gastrulating embryos (E7.5) are grown in three-dimensional rotating bottles, whereas extended culture from pre-gastrulation stages (E5.5 or E6.5) requires a combination of static and rotating bottle culture platforms. Histological, molecular and single-cell RNA sequencing analyses confirm that the ex utero cultured embryos recapitulate in utero development precisely. This culture system is amenable to the introduction of a variety of embryonic perturbations and micro-manipulations, the results of which can be followed ex utero for up to six days. The establishment of a system for robustly growing normal mouse embryos ex utero from pre-gastrulation to advanced organogenesis represents a valuable tool for investigating embryogenesis, as it eliminates the uterine barrier and allows researchers to mechanistically interrogate post-implantation morphogenesis and artificial embryogenesis in mammals.

The epigenetic dynamics of induced pluripotent stem cell (iPSC) reprogramming in correctly reprogrammed cells at high resolution and throughout the entire process remain largely undefined. Here, we characterize conversion of mouse fibroblasts into iPSCs using Gatad2a-Mbd3/NuRD-depleted and highly efficient reprogramming systems. Unbiased high-resolution profiling of dynamic changes in levels of gene expression, chromatin engagement, DNA accessibility, and DNA methylation were obtained. We identified two distinct and synergistic transcriptional modules that dominate successful reprogramming, which are associated with cell identity and biosynthetic genes. The pluripotency module is governed by dynamic alterations in epigenetic modifications to promoters and binding by Oct4, Sox2, and Klf4, but not Myc. Early DNA demethylation at certain enhancers prospectively marks cells fated to reprogram. Myc activity drives expression of the essential biosynthetic module and is associated with optimized changes in tRNA codon usage. Our functional validations highlight interweaved epigenetic- and Myc-governed essential reconfigurations that rapidly commission and propel deterministic reprogramming toward naive pluripotency.

Parental imprinting results in monoallelic parent-oforigin- dependent gene expression. However, many imprinted genes identified by differential methylation do not exhibit complete monoallelic expression. Previous studies demonstrated complex tissue-dependent expression patterns for some imprinted genes. Still, the complete magnitude of this phenomenon remains largely unknown. By differentiating human parthenogenetic induced pluripotent stem cells into different cell types and combining DNA methylation with a 50 RNA sequencing methodology, we were able to identify tissue- and isoform-dependent imprinted genes in a genome-wide manner.We demonstrate that nearly half of all imprinted genes express both biallelic and monoallelic isoforms that are controlled by tissue-specific alternative promoters. This study provides a global analysis of tissue-specific imprinting in humans and suggests that alternative promoters are central in the regulation of imprinted genes.

Embryonic stem cells (ESCs) of mice and humans have distinctmolecular and biological characteristics, raising thequestionofwhetheranearlier, "naive" state of pluripotency may exist in humans. Here we took a systematic approach to identify small molecules that support self-renewal of naive human ESCs based on maintenance of endogenous OCT4 distal enhancer activity, amolecular signature of ground state pluripotency. Iterative chemical screening identified a combination of five kinase inhibitors that induces and maintains OCT4 distal enhancer activity when applied directly to conventional human ESCs. These inhibitors generate human pluripotent cells in which transcription factors associatedwith the ground state of pluripotency are highly upregulated and bivalent chromatin domains are depleted. Comparison with previously reported naive human ESCs indicates that our conditions capture a distinct pluripotent state in humans that closely resembles that ofmouse ESCs. This study presents a framework for defining the culture requirements of naive human pluripotent cells.

Parental imprinting is an epigenetic phenomenon by which genes are expressed in a monoallelic fashion, according to their parent of origin. DNA methylation is considered the hallmark mechanism regulating parental imprinting. To identify imprinted differentially methylated regions (DMRs), we compared the DNA methylation status between multiple normal and parthenogenetic human pluripotent stem cells (PSCs) by performing reduced representation bisulfite sequencing. Our analysis identified over 20 previously unknown imprinted DMRs in addition to the known DMRs. These include DMRs in loci associated with human disorders, and a class of intergenic DMRs that do not seem to be related to gene expression. Furthermore, the study showed some DMRs to be unstable, liable to differentiation or reprogramming. A comprehensive comparison between mouse and human DMRs identified almost half of the imprinted DMRs to be species specific. Taken together, our data map novel DMRs in the human genome, their evolutionary conservation, and relation to gene expression.

Although the field of induced pluripotent stem (iPS) cells is a very new, hundreds of research papers regarding them have been published over the past three years. This chapter concentrates on the medical relevance of iPS cells and where the research regarding iPS cells has reached in such a short period time. The reprogramming of cells using the “stemness” genes and the resultant populations similarity to human embryonic stem (ES) cells has allowed for another source of pluripotent stem cells to be generated which have fewer ethical ramifications then ES cells. We have compared other forms of reprogramming somatic cells to pluripotent cells and explain that even though generating iPS cell lines using the “stemness” factors is slow and inefficient it is far superior in generating pluripotent stem cells then the other methods. This relatively new technology has enabled pluripotent cell lines to be generated from various animal species such as pig, which as yet has a no counterpart in ES cell lines. One of the biggest advantages to using iPS cells is the ability to generate patient specific cells that can be used to treat patients without the complications of rejection and immunosuppression associated with using allogeneic ES cells. However, the ability to generate the correct cell type appropriate for treating the disease and, in the case of patients with genetic disorders, generating iPS cells that do not contain the mutation, are problems that must be overcome for the technology to be useful. On the other hand, using iPS cells generated from various disease types could help unfold the stages of development of the disease and enable drug testing on the diseased cells, which ultimately could be applied to treat the disease in patients. There are still some hurdles that need to be overcome; the most crucial is the safety issues associated with the generation of iPS cell lines. At the moment somatic cells are reprogrammed with vectors that integrate the DNA into the host genome in a manner not fully controlled, which could result in unfavorable insertion sites. In addition, there is the fear that the transgene might reactivate oncogenes; MYC, for instance, one of the reprogramming factors, is also known to be an oncogenes. Overall, the ability to reprogram somatic cells using stemness genes to generated iPS cells is a breakthrough whose full potential is still hard to estimate.