Contribution of epigenetic landscapes and transcription factors to X-chromosome reactivation in the inner cell mass

Sex-specific perturbations of neuronal development caused by mutations in the autism risk gene DDX3X

DDX3X is an X-linked RNA helicase that escapes X chromosome inactivation and is expressed at higher levels in female brains. Mutations in DDX3X are associated with intellectual disability (ID) and autism spectrum disorder (ASD) and are predominantly identified in females (DDX3X syndrome). Using cellular and mouse models, we show that Ddx3x mediates sexual dimorphisms in brain development at a molecular, cellular, and behavioral level. During cortical neuronal development, Ddx3x sustains a female-biased signature of enhanced ribosomal biogenesis and mRNA metabolism. Compared to male neurons, female neurons display larger nucleoli, higher expression of a set of ribosomal proteins, and a higher cytoplasm-to-nucleus ratio of ribosomal RNA. All these sex dimorphisms are obliterated by Ddx3x loss. Ddx3x regulates dendritic arborization complexity in a sex- and dose-dependent manner in both female and male neurons. Ddx3x modulates the development of dendritic spines but only in female neurons. Further, ablating Ddx3x conditionally in forebrain neurons is sufficient to yield sex-specific changes in developmental outcomes and motor function. Together, these findings pose Ddx3x as a mediator of sexual differentiation during neurodevelopment and open new avenues to understand sex differences in health and disease.

Temporal and regional X-linked gene reactivation in the mouse germline reveals site-specific retention of epigenetic silencing

Random X-chromosome inactivation is a hallmark of female mammalian somatic cells. This epigenetic mechanism, mediated by the long noncoding RNA Xist, occurs in the early embryo and is stably maintained throughout life, although inactivation is lost during primordial germ cell (PGC) development. Using a combination of single-cell allele-specific RNA sequencing and low-input chromatin profiling on developing mouse PGCs, we provide a detailed map of X-linked gene reactivation. Despite the absence of Xist expression, PGCs still harbor a fully silent X chromosome at embryonic day 9.5 (E9.5). Subsequently, X-linked genes undergo gradual and distinct regional reactivation. At E12.5, a substantial part of the inactive X chromosome resists reactivation, retaining an epigenetic memory of its silencing. Our findings define the orchestration of reactivation of the inactive X chromosome, a key event in female PGC reprogramming with direct implications for reproduction.

[Epigenetic reprogramming, germline and genomic imprinting]

The memory of cellular identity is crucial for the correct development of an individual and is maintained throughout life by the epigenome. Chromatin marks, such as DNA methylation and histone modifications, ensure the stability of gene expression programmes over time and through cell division. Loss of these marks can lead to severe pathologies, including cancer and developmental syndromes. However, reprogramming of cellular identity is also a natural phenomenon that occurs early in mammalian development, particularly in the germ line, which enables the production of mature and functional gametes. The germ line transmits genetic and epigenetic information to the next generation, contributing to the survival of the species. Primordial germ cells (PGCs) undergo extensive chromatin remodelling, including global DNA demethylation and erasure of the parental imprints. This review introduces the concept of epigenetic reprogramming, its discovery and key steps, as well as the transcriptional and chromatin changes that accompany germ cell formation in mice. Finally, we discuss the epigenetic mechanisms of genomic imprinting, its discovery, regulation and relevance to human disease.

Sex-specific perturbations of neuronal development caused by mutations in the autism risk gene DDX3X

DDX3X is an X-linked RNA helicases that escapes X chromosome inactivation and is expressed at higher levels in female brains. Mutations in DDX3X are associated with intellectual disability (ID) and autism spectrum disorder (ASD) and are predominantly identified in females. Using cellular and mouse models, we show that Ddx3x mediates sexual dimorphisms in brain development at a molecular, cellular, and behavioral level. During cortical neuronal development, Ddx3x sustains a female-biased signature of enhanced ribosomal biogenesis and mRNA translation. Female neurons display higher levels of ribosomal proteins and larger nucleoli, and these sex dimorphisms are obliterated by Ddx3x loss. Ddx3x regulates dendritic outgrowth in a sex- and dose-dependent manner in both female and male neurons, and dendritic spine development only in female neurons. Further, ablating Ddx3x conditionally in forebrain neurons is sufficient to yield sex-specific changes in developmental outcomes and motor function. Together, these findings pose Ddx3x as a mediator of sexual differentiation during neurodevelopment and open new avenues to understand sex differences in health and disease.

Lineage-specific dynamics of loss of X upregulation during inactive-X reactivation

In mammals, X chromosome dosage is balanced between sexes through the silencing of one X chromosome in females. Recent single-cell RNA sequencing analysis demonstrated that the inactivation of the X chromosome is accompanied by the upregulation of the active X chromosome (Xa) during mouse embryogenesis. Here, we have investigated if the reactivation of inactive-X (Xi) leads to the loss of Xa upregulation in different cellular or developmental contexts. We find that while Xi reactivation and loss of Xa upregulation are tightly coupled in mouse embryonic epiblast and induced pluripotent stem cells, that is not the case in germ cells. Moreover, we demonstrate that partial reactivation of Xi in mouse extra-embryonic endoderm stem cells and human B cells does not result in the loss of Xa upregulation. Finally, we have established a mathematical model for the transcriptional coordination of two X chromosomes. Together, we conclude that the reactivation of Xi is not always synchronized with the loss of Xa upregulation.

Female-bias in systemic lupus erythematosus: How much is the X chromosome to blame?

Systemic lupus erythematosus (SLE or lupus) is an immune-mediated disease associated with substantial medical burden. Notably, lupus exhibits a striking female bias, with women having significantly higher susceptibility compared to men, up to 14-fold higher in some ethnicities. Supernumerary X chromosome syndromes, like Klinefelter (XXY) and Triple X syndrome (XXX), also present higher SLE prevalence, whereas Turner syndrome (XO) displays lower prevalence. Taken together, SLE prevalence in different X chromosome dosage sceneries denotes a relationship between the number of X chromosomes and the risk of developing lupus. The dosage of X-linked genes, many of which play roles in the immune system, is compensated between males and females through the inactivation of one of the two X chromosomes in female cells. X-chromosome inactivation (XCI) initiates early in development with a random selection of which X chromosome to inactivate, a choice that is then epigenetically maintained in the daughter cells. This process is regulated by the X-Inactive-Specific Transcript (XIST), encoding for a long non-coding RNA, exclusively expressed from the inactive X chromosome (Xi). XIST interacts with various RNA binding proteins and chromatin modifiers to form a ribonucleoprotein (RNP) complex responsible for the transcriptional silencing and heterochromatinization of the Xi. This ensures stable silencing of most genes on the X chromosome, with only a few genes able to escape this process. Recent findings suggest that the molecular components involved in XCI, or their dysregulation, contribute to the pathogenesis of lupus. Indeed, nonrandom XCI, elevated gene escape from XCI, and the autoimmune potential of the XIST RNP complex have been suggested to contribute to auto-immune diseases, such as lupus. This review examines these current hypotheses concerning how this dosage compensation mechanism might impact the development of lupus, shedding light on potential mechanisms underlying the pathogenesis of the disease.

Emerging X-linked genes associated with neurodevelopmental disorders in females

A significant source of risk for neurodevelopmental disorders (NDDs), including intellectual disability (ID) and autism spectrum disorder (ASD), lies in genes located on the X chromosome. Males can be particularly vulnerable to X-linked variation because of hemizygosity, and male-specific segregation in pedigrees has guided earlier gene discovery for X-linked recessive conditions. More recently, X-linked disorders disproportionally affecting females, with complex inheritance patterns and/or presenting with sex differences, have surfaced. Here, we discuss the genetics and neurobiology of X-linked genes that are paradigmatic to understand NDDs in females. Integrating genetic, clinical, and functional data will be key to understand how X-linked variation contributes to the risk architecture of NDDs.

DDX3X syndrome: From clinical phenotypes to biological insights

DDX3X syndrome is a neurodevelopmental disorder accounting for up to 3% of cases of intellectual disability (ID) and affecting primarily females. Individuals diagnosed with DDX3X syndrome can also present with behavioral challenges, motor delays and movement disorders, epilepsy, and congenital malformations. DDX3X syndrome is caused by mutations in the X-linked gene DDX3X, which encodes a DEAD-box RNA helicase with critical roles in RNA metabolism, including mRNA translation. Emerging discoveries from animal models are unveiling a fundamental role of DDX3X in neuronal differentiation and development, especially in the neocortex. Here, we review the current knowledge of genetic and neurobiological mechanisms underlying DDX3X syndrome and their relationship with clinical phenotypes.

The interferon γ pathway enhances pluripotency and X-chromosome reactivation in iPSC reprogramming

Reprogramming somatic cells into induced pluripotent stem cells (iPSCs) requires activation of the pluripotency network and resetting of the epigenome by erasing the epigenetic memory of the somatic state. In female mouse cells, a critical epigenetic reprogramming step is the reactivation of the inactive X chromosome. Despite its importance, a systematic understanding of the regulatory networks linking pluripotency and X-reactivation is missing. Here, we reveal important pathways for pluripotency acquisition and X-reactivation using a genome-wide CRISPR screen during neural precursor to iPSC reprogramming. In particular, we discover that activation of the interferon γ (IFNγ) pathway early during reprogramming accelerates pluripotency acquisition and X-reactivation. IFNγ stimulates STAT3 signaling and the pluripotency network and leads to enhanced TET-mediated DNA demethylation, which consequently boosts X-reactivation. We therefore gain a mechanistic understanding of the role of IFNγ in reprogramming and X-reactivation and provide a comprehensive resource of the molecular networks involved in these processes.

Regulatory principles and mechanisms governing the onset of random X-chromosome inactivation

X-chromosome inactivation (XCI) has evolved in mammals to compensate for the difference in X-chromosomal dosage between the sexes. In placental mammals, XCI is initiated during early embryonic development through upregulation of the long noncoding RNA Xist from one randomly chosen X chromosome in each female cell. The Xist locus must thus integrate both X-linked and developmental trans-regulatory factors in a dosage-dependent manner. Furthermore, the two alleles must coordinate to ensure inactivation of exactly one X chromosome per cell. In this review, we summarize the regulatory principles that govern the onset of XCI. We go on to provide an overview over the factors that have been implicated in Xist regulation and discuss recent advances in our understanding of how Xist's cis-regulatory landscape integrates information in a precise fashion.

The compleX balancing act of controlling X-chromosome dosage and how it impacts mammalian germline development

In female mammals, the two X chromosomes are subject to epigenetic gene regulation in order to balance X-linked gene dosage with autosomes and in relation to males, which have one X and one Y chromosome. This is achieved by an intricate interplay of several processes; X-chromosome inactivation and reactivation elicit global epigenetic regulation of expression from one X chromosome in a stage-specific manner, whilst the process of X-chromosome upregulation responds to this by fine-tuning transcription levels of the second X. The germline is unique in its function of transmitting both the genetic and epigenetic information from one generation to the next, and remodelling of the X chromosome is one of the key steps in setting the stage for successful development. Here, we provide an overview of the complex dynamics of X-chromosome dosage control during embryonic and germ cell development, and aim to decipher its potential role for normal germline competency.

Inhibition of histone lysine demethylase 6A promotes chondrocytic activity and attenuates osteoarthritis development through repressing H3K27me3 enhancement of Wnt10a

Histone hypermethylation represses gene transcription, which affects cartilage homeostasis or joint remodeling. Trimethylation of lysine 27 of histone 3 (H3K27me3) changes epigenome signatures, regulating tissue metabolism. This study aimed to investigate whether loss of H3K27me3 demethylase Kdm6a function affected osteoarthritis development. We revealed that chondrocyte-specific Kdm6a knockout mice developed relatively long femurs and tibiae as compared to wild-type mice. Kdm6a deletion mitigated osteoarthritis symptoms, including articular cartilage loss, osteophyte formation, subchondral trabecular bone loss, and irregular walking patterns of destabilized medial meniscus-injured knees. In vitro, loss of Kdm6a function compromised the loss in expression of key chondrocyte markers Sox9, collagen II, and aggrecan and improved glycosaminoglycan production in inflamed chondrocytes. RNA sequencing showed that Kdm6a loss changed transcriptomic profiles, which contributed to histone signaling, NADPH oxidase, Wnt signaling, extracellular matrix, and cartilage development in articular cartilage. Chromatin immunoprecipitation sequencing uncovered that Kdm6a knockout affected H3K27me3 binding epigenome, repressing Wnt10a and Fzd10 transcription. Wnt10a was, among others, functional molecules regulated by Kdm6a. Forced Wnt10a expression attenuated Kdm6a deletion-induced glycosaminoglycan overproduction. Intra-articular administration with Kdm6a inhibitor GSK-J4 attenuated articular cartilage erosion, synovitis, and osteophyte formation, improving gait profiles of injured joints. In conclusion, Kdm6a loss promoted transcriptomic landscapes contributing to extracellular matrix synthesis and compromised epigenetic H3K27me3-mediated promotion of Wnt10a signaling, preserving chondrocytic activity to attenuate osteoarthritic degeneration. We highlighted the chondroprotective effects of Kdm6a inhibitor for mitigating the development of osteoarthritic disorders.

Cohesin controls X chromosome structure remodeling and X-reactivation during mouse iPSC-reprogramming

Reactivation of the inactive X chromosome is a hallmark epigenetic event during reprogramming of mouse female somatic cells to induced pluripotent stem cells (iPSCs). This involves global structural remodeling from a condensed, heterochromatic into an open, euchromatic state, thereby changing a transcriptionally inactive into an active chromosome. Despite recent advances, very little is currently known about the molecular players mediating this process and how this relates to iPSC-reprogramming in general. To gain more insight, here we perform a RNAi-based knockdown screen during iPSC-reprogramming of mouse fibroblasts. We discover factors important for X chromosome reactivation (XCR) and iPSC-reprogramming. Among those, we identify the cohesin complex member SMC1a as a key molecule with a specific function in XCR, as its knockdown greatly affects XCR without interfering with iPSC-reprogramming. Using super-resolution microscopy, we find SMC1a to be preferentially enriched on the active compared with the inactive X chromosome and that SMC1a is critical for the decompacted state of the active X. Specifically, depletion of SMC1a leads to contraction of the active X both in differentiated and in pluripotent cells, where it normally is in its most open state. In summary, we reveal cohesin as a key factor for remodeling of the X chromosome from an inactive to an active structure and that this is a critical step for XCR during iPSC-reprogramming.

Causality in transcription and genome folding: Insights from X inactivation

The spatial organization of genomes is becoming increasingly understood. In mammals, where it is most investigated, this organization ties in with transcription, so an important research objective is to understand whether gene activity is a cause or a consequence of genome folding in space. In this regard, the phenomena of X-chromosome inactivation and reactivation open a unique window of investigation because of the singularities of the inactive X chromosome. Here we focus on the cause-consequence nexus between genome conformation and transcription and explain how recent results about the structural changes associated with inactivation and reactivation of the X chromosome shed light on this problem.

H3K27me3 at pericentromeric heterochromatin is a defining feature of the early mouse blastocyst

Early mouse development is characterized by structural and epigenetic changes while cells progress towards differentiation. At blastocyst stage, the segregation of the three primordial lineages is accompanied by establishment of differential patterns of DNA methylation and post-translational modifications of histones, such as H3K27me3. Here, we analysed the dynamics of H3K27me3 at pericentromeric heterochromatin (PCH) during early development. We also followed the localization of EZH2 and BEND3, previously shown in ESCs to drive PRC2 to hypomethylated PCH. We show that the location of H3K27me3 at PCH, in addition to H3K9me3, is a defining feature of embryonic cells in vivo. Moreover, it may play an important role in structuring PCH and preserving genomic integrity at a time of globally relaxed chromatin. At peri-implantation stages, while DNA methylation is still low, EZH2 and then H3K27me3, leave PCH in epiblast progenitors at the time of their spatial segregation from primitive endoderm cells, while BEND3 remains there up to implantation. The comparison with stem cells (ESCs and TSCs) reveals that the epigenetic marks (i.e. H3K9me3 and H3K27me3) of PCH are reset during in vitro derivation and only partially restored thereafter. This highlights possible divergences between in vitro and "in embryo" epigenetic regulation regarding constitutive heterochromatin.

Controlled X-chromosome dynamics defines meiotic potential of female mouse in vitro germ cells

The mammalian germline is characterized by extensive epigenetic reprogramming during its development into functional eggs and sperm. Specifically, the epigenome requires resetting before parental marks can be established and transmitted to the next generation. In the female germline, X‐chromosome inactivation and reactivation are among the most prominent epigenetic reprogramming events, yet very little is known about their kinetics and biological function. Here, we investigate X‐inactivation and reactivation dynamics using a tailor‐made in vitro system of primordial germ cell‐like cell (PGCLC) differentiation from mouse embryonic stem cells. We find that X‐inactivation in PGCLCs in vitro and in germ cell‐competent epiblast cells in vivo is moderate compared to somatic cells, and frequently characterized by escaping genes. X‐inactivation is followed by step‐wise X‐reactivation, which is mostly completed during meiotic prophase I. Furthermore, we find that PGCLCs which fail to undergo X‐inactivation or reactivate too rapidly display impaired meiotic potential. Thus, our data reveal fine‐tuned X‐chromosome remodelling as a critical feature of female germ cell development towards meiosis and oogenesis.

Xist-mediated silencing requires additive functions of SPEN and Polycomb together with differentiation-dependent recruitment of SmcHD1

X chromosome inactivation (XCI) is mediated by the non-coding RNA Xist, which directs chromatin modification and gene silencing in cis. The RNA binding protein SPEN and associated corepressors have a central role in Xist-mediated gene silencing. Other silencing factors, notably the Polycomb system, have been reported to function downstream of SPEN. In recent work, we found that SPEN has an additional role in correct localization of Xist RNA in cis, indicating that its contribution to chromatin-mediated gene silencing needs to be reappraised. Making use of a SPEN separation-of-function mutation, we show that SPEN and Polycomb pathways, in fact, function in parallel to establish gene silencing. We also find that differentiation-dependent recruitment of the chromosomal protein SmcHD1 is required for silencing many X-linked genes. Our results provide important insights into the mechanism of X inactivation and the coordination of chromatin-based gene regulation with cellular differentiation and development.

The Molecular and Nuclear Dynamics of X-Chromosome Inactivation

In female eutherian mammals, dosage compensation of X-linked gene expression is achieved during development through transcriptional silencing of one of the two X chromosomes. Following X chromosome inactivation (XCI), the inactive X chromosome remains faithfully silenced throughout somatic cell divisions. XCI is dependent on Xist, a long noncoding RNA that coats and silences the X chromosome from which it is transcribed. Xist coating triggers a cascade of chromosome-wide changes occurring at the levels of transcription, chromatin composition, chromosome structure, and spatial organization within the nucleus. XCI has emerged as a paradigm for the study of such crucial nuclear processes and the dissection of their functional interplay. In the past decade, the advent of tools to characterize and perturb these processes have provided an unprecedented understanding into their roles during XCI. The mechanisms orchestrating the initiation of XCI as well as its maintenance are thus being unraveled, although many questions still remain. Here, we introduce key aspects of the XCI process and review the recent discoveries about its molecular basis.

X-chromosome reactivation: a concise review

Mammalian females (XX) silence transcription on one of the two X chromosomes to compensate the expression dosage with males (XY). This process - named X-chromosome inactivation - entails a variety of epigenetic modifications that act synergistically to maintain silencing and make it heritable through cell divisions. Genes along the inactive X chromosome are, indeed, refractory to reactivation. Nonetheless, X-chromosome reactivation can occur alongside with epigenome reprogramming or by perturbing multiple silencing pathways. Here we review the events associated with X-chromosome reactivation during in vivo and in vitro reprogramming and highlight recent efforts in inducing Xi reactivation by molecular perturbations. This provides us with a first understanding of the mechanisms underlying X-chromosome reactivation, which could be tackled for therapeutic purposes.

Epigenetic imprinting alterations as effective diagnostic biomarkers for early-stage lung cancer and small pulmonary nodules

Background

Early lung cancer detection remains a clinical challenge for standard diagnostic biopsies due to insufficient tumor morphological evidence. As epigenetic alterations precede morphological changes, expression alterations of certain imprinted genes could serve as actionable diagnostic biomarkers for malignant lung lesions.

Results

Using the previously established quantitative chromogenic imprinted gene in situ hybridization (QCIGISH) method, elevated aberrant allelic expression of imprinted genes GNAS, GRB10, SNRPN and HM13 was observed in lung cancers over benign lesions and normal controls, which were pathologically confirmed among histologically stained normal, paracancerous and malignant tissue sections. Based on the differential imprinting signatures, a diagnostic grading model was built on 246 formalin-fixed and paraffin-embedded (FFPE) surgically resected lung tissue specimens, tested against 30 lung cytology and small biopsy specimens, and blindly validated in an independent cohort of 155 patients. The QCIGISH diagnostic model demonstrated 99.1% sensitivity (95% CI 97.5–100.0%) and 92.1% specificity (95% CI 83.5–100.0%) in the blinded validation set. Of particular importance, QCIGISH achieved 97.1% sensitivity (95% CI 91.6–100.0%) for carcinoma in situ to stage IB cancers with 100% sensitivity and 91.7% specificity (95% CI 76.0–100.0%) noted for pulmonary nodules with diameters ≤ 2 cm.

Conclusions

Our findings demonstrated the diagnostic value of epigenetic imprinting alterations as highly accurate translational biomarkers for a more definitive diagnosis of suspicious lung lesions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13148-021-01203-5.

Early reactivation of clustered genes on the inactive X chromosome during somatic cell reprogramming

Reprogramming of murine female somatic cells to induced pluripotent stem cells (iPSCs) is accompanied by X chromosome reactivation (XCR), by which the inactive X chromosome (Xi) in female somatic cells becomes reactivated. However, how Xi initiates reactivation during reprogramming remains poorly defined. Here, we used a Sendai virus-based reprogramming system to generate partially reprogrammed iPSCs that appear to be undergoing the initial phase of XCR. Allele-specific RNA-seq of these iPSCs revealed that XCR initiates at a subset of genes clustered near the centromere region. The initial phase of XCR occurs when the cells transit through mesenchymal-epithelial transition (MET) before complete shutoff of Xist expression. Moreover, regulatory regions of these genes display dynamic changes in lysine-demethylase 1a (KDM1A) occupancy. Our results identified clustered genes on the Xi that show reactivation in the initial phase of XCR during reprogramming and suggest a possible role for histone demethylation in this process.

•

Partially reprogrammed iPSCs enabled analyses of early events in XCR

•

XCR initiates at a subset of genes clustered near the centromere region

•

XCR occurs before complete shutoff of Xist expression during reprogramming

•

KDM1A inhibition appears to directly reactivate transcription from the Xi

Enhanced chromatin accessibility contributes to X chromosome dosage compensation in mammals

BACKGROUND

Precise gene dosage of the X chromosomes is critical for normal development and cellular function. In mice, XX female somatic cells show transcriptional X chromosome upregulation of their single active X chromosome, while the other X chromosome is inactive. Moreover, the inactive X chromosome is reactivated during development in the inner cell mass and in germ cells through X chromosome reactivation, which can be studied in vitro by reprogramming of somatic cells to pluripotency. How chromatin processes and gene regulatory networks evolved to regulate X chromosome dosage in the somatic state and during X chromosome reactivation remains unclear.

RESULTS

Using genome-wide approaches, allele-specific ATAC-seq and single-cell RNA-seq, in female embryonic fibroblasts and during reprogramming to pluripotency, we show that chromatin accessibility on the upregulated mammalian active X chromosome is increased compared to autosomes. We further show that increased accessibility on the active X chromosome is erased by reprogramming, accompanied by erasure of transcriptional X chromosome upregulation and the loss of increased transcriptional burst frequency. In addition, we characterize gene regulatory networks during reprogramming and X chromosome reactivation, revealing changes in regulatory states. Our data show that ZFP42/REX1, a pluripotency-associated gene that evolved specifically in placental mammals, targets multiple X-linked genes, suggesting an evolutionary link between ZFP42/REX1, X chromosome reactivation, and pluripotency.

CONCLUSIONS

Our data reveal the existence of intrinsic compensatory mechanisms that involve modulation of chromatin accessibility to counteract X-to-Autosome gene dosage imbalances caused by evolutionary or in vitro X chromosome loss and X chromosome inactivation in mammalian cells.

Single-cell landscape of nuclear configuration and gene expression during stem cell differentiation and X inactivation

BACKGROUND

Mammalian development is associated with extensive changes in gene expression, chromatin accessibility, and nuclear structure. Here, we follow such changes associated with mouse embryonic stem cell differentiation and X inactivation by integrating, for the first time, allele-specific data from these three modalities obtained by high-throughput single-cell RNA-seq, ATAC-seq, and Hi-C.

RESULTS

Allele-specific contact decay profiles obtained by single-cell Hi-C clearly show that the inactive X chromosome has a unique profile in differentiated cells that have undergone X inactivation. Loss of this inactive X-specific structure at mitosis is followed by its reappearance during the cell cycle, suggesting a "bookmark" mechanism. Differentiation of embryonic stem cells to follow the onset of X inactivation is associated with changes in contact decay profiles that occur in parallel on both the X chromosomes and autosomes. Single-cell RNA-seq and ATAC-seq show evidence of a delay in female versus male cells, due to the presence of two active X chromosomes at early stages of differentiation. The onset of the inactive X-specific structure in single cells occurs later than gene silencing, consistent with the idea that chromatin compaction is a late event of X inactivation. Single-cell Hi-C highlights evidence of discrete changes in nuclear structure characterized by the acquisition of very long-range contacts throughout the nucleus. Novel computational approaches allow for the effective alignment of single-cell gene expression, chromatin accessibility, and 3D chromosome structure.

CONCLUSIONS

Based on trajectory analyses, three distinct nuclear structure states are detected reflecting discrete and profound simultaneous changes not only to the structure of the X chromosomes, but also to that of autosomes during differentiation. Our study reveals that long-range structural changes to chromosomes appear as discrete events, unlike progressive changes in gene expression and chromatin accessibility.

Integrated analysis of Xist upregulation and X-chromosome inactivation with single-cell and single-allele resolution

To ensure dosage compensation between the sexes, one randomly chosen X chromosome is silenced in each female cell in the process of X-chromosome inactivation (XCI). XCI is initiated during early development through upregulation of the long non-coding RNA Xist, which mediates chromosome-wide gene silencing. Cell differentiation, Xist upregulation and gene silencing are thought to be coupled at multiple levels to ensure inactivation of exactly one out of two X chromosomes. Here we perform an integrated analysis of all three processes through allele-specific single-cell RNA-sequencing. Specifically, we assess the onset of random XCI in differentiating mouse embryonic stem cells, and develop dedicated analysis approaches. By exploiting the inter-cellular heterogeneity of XCI onset, we identify putative Xist regulators. Moreover, we show that transient Xist upregulation from both X chromosomes results in biallelic gene silencing right before transitioning to the monoallelic state, confirming a prediction of the stochastic model of XCI. Finally, we show that genetic variation modulates the XCI process at multiple levels, providing a potential explanation for the long-known X-controlling element (Xce) effect, which leads to preferential inactivation of a specific X chromosome in inter-strain crosses. We thus draw a detailed picture of the different levels of regulation that govern the initiation of XCI. The experimental and computational strategies we have developed here will allow us to profile random XCI in more physiological contexts, including primary human cells in vivo.

Chromosome compartments on the inactive X guide TAD formation independently of transcription during X-reactivation

A hallmark of chromosome organization is the partition into transcriptionally active A and repressed B compartments, and into topologically associating domains (TADs). Both structures were regarded to be absent from the inactive mouse X chromosome, but to be re-established with transcriptional reactivation and chromatin opening during X-reactivation. Here, we combine a tailor-made mouse iPSC reprogramming system and high-resolution Hi-C to produce a time course combining gene reactivation, chromatin opening and chromosome topology during X-reactivation. Contrary to previous observations, we observe A/B-like compartments on the inactive X harbouring multiple subcompartments. While partial X-reactivation initiates within a compartment rich in X-inactivation escapees, it then occurs rapidly along the chromosome, concomitant with downregulation of Xist. Importantly, we find that TAD formation precedes transcription and initiates from Xist-poor compartments. Here, we show that TAD formation and transcriptional reactivation are causally independent during X-reactivation while establishing Xist as a common denominator.

Visualization of X chromosome reactivation in mouse primordial germ cells in vivo

X chromosome inactivation (XCI), determined during development, remains stable after embryonic cell divisions. However, primordial germ cells (PGCs) are exceptions in that XCI is reprogrammed and inactivated X chromosomes are reactivated. Although interactions between PGCs and somatic cells are thought to be important for PGC development, little is known about them. Here, we performed imaging of X chromosome reactivation (XCR) using the ‘Momiji’ mouse system, which can monitor the X chromosome's inactive and active states using two color fluorescence reporter genes, and investigated whether interactions would affect XCR in PGCs. Based on their expression levels, we found that XCR of the Pgk1 locus began at embryonic day (E)10.5 and was almost complete by E13.5. During this period, PGCs became distributed uniformly in the genital ridge, proliferated, and formed clusters; XCR progressed accordingly. In addition, XCR of the Pgk1 locus preceded that of the Hprt locus, indicating that the timing of epigenetic memory erasure varied according to the locus of each of these X-linked genes. Our results indicate that XCR proceeds along with the proliferation of PGCs clustered within the genital ridge.

This article has an associated First Person interview with the first author of the paper.

Summary: Imaging of mouse X chromosome activities reveals that they undergo reprogramming and reactivation while the primordial germ cells form clusters and proliferate in the genital ridge.

Investigating the Inner Cell Mass of the Mouse Blastocyst by Combined Immunofluorescence Staining and RNA Fluorescence In Situ Hybridization

Immunofluorescence and RNA fluorescence in situ hybridization (FISH) methods enable the detection of, respectively, proteins and RNA molecules in single cells. Adapted to preimplantation mouse embryos, these techniques allow the investigation of transcriptional dynamics in the first embryonic and extraembryonic lineages and can circumvent the limited amount of starting material. This can as well be coupled to examination of chromatin modification, i.e., histone marks, by immunofluorescence. Here is outlined an immunofluorescence protocol combined to nascent RNA-FISH after immunosurgery of the mouse inner cell mass of the blastocyst to study early changes in transcription and/or histone marks of both primitive endoderm and epiblast cells. The method describes the different steps from coverslips and FISH probe preparation to inner cell mass isolation and immunofluorescence followed by RNA-FISH. Furthermore, this is applicable to earlier developmental stages and other mammalian species provided little technical adjustments.

Specification and epigenomic resetting of the pig germline exhibit conservation with the human lineage

Investigations of the human germline and programming are challenging because of limited access to embryonic material. However, the pig as a model may provide insights into transcriptional network and epigenetic reprogramming applicable to both species. Here we show that, during the pre- and early migratory stages, pig primordial germ cells (PGCs) initiate large-scale epigenomic reprogramming, including DNA demethylation involving TET-mediated hydroxylation and, potentially, base excision repair (BER). There is also macroH2A1 depletion and increased H3K27me3 as well as X chromosome reactivation (XCR) in females. Concomitantly, there is dampening of glycolytic metabolism genes and re-expression of some pluripotency genes like those in preimplantation embryos. We identified evolutionarily young transposable elements and gene coding regions resistant to DNA demethylation in acutely hypomethylated gonadal PGCs, with potential for transgenerational epigenetic inheritance. Detailed insights into the pig germline will likely contribute significantly to advances in human germline biology, including in vitro gametogenesis.

Single-Cell RNA Sequencing Reveals that the Switching of the Transcriptional Profiles of Cysteine-Related Genes Alters the Virulence of Entamoeba histolytica

Studies on the trophozoite of Entamoeba histolytica suggested this organism could accumulate polyploid cells in its proliferative phase and differentiate its cell cycle from that of other eukaryotes. Therefore, a single-cell sequencing technique was used to study the switching of the RNA transcription profiles of single amoebic trophozoites.

Entamoeba histolytica is an intestinal protozoan that causes human amoebic colitis and extraintestinal abscesses. Virulence variation is observed in the pathogenicity of E. histolytica trophozoites, but the detailed mechanism remains unclear. Here, a single trophozoite was cultured alone, and the progeny of the trophozoites of each generation were subjected to single-cell RNA sequencing (scRNA-seq) to study the transcriptional profiles of trophozoites. The scRNA-seq analysis indicated the importance of sulfur metabolism and the proteasome pathway in pathogenicity, whereas the isobaric tags for relative and absolute quantitation (iTRAQ) proteomic analysis did not identify the bulk trophozoites. The trophozoite improved the synthesis of cysteine under cysteine-deficient conditions but downregulated the expression of the intermediate subunit of the lectin of E. histolytica trophozoites and retained the expression of the heavy subunit of lectin, resulting in decreased amoebic phagocytosis and cytotoxicity. The variation in the transmembrane kinase gene family might be critical in regulating the proteasome pathway. Thus, the scRNA-seq technique provided an improved understanding of the biological characteristics and the mechanism of virulence variation of amoebic trophozoites.

IMPORTANCE Studies on the trophozoite of Entamoeba histolytica suggested this organism could accumulate polyploid cells in its proliferative phase and differentiate its cell cycle from that of other eukaryotes. Therefore, a single-cell sequencing technique was used to study the switching of the RNA transcription profiles of single amoebic trophozoites. We separated individual trophozoites from axenic cultured trophozoites, CHO cell-incubated trophozoites, and in vivo trophozoites. We found important changes in the sulfur and cysteine metabolism in pathogenicity. The trophozoites strategically regulated the expression of the cysteine-rich protein-encoding genes under cysteine-deficient conditions, thereby decreasing amoebic phagocytosis and cytotoxicity. The single-cell sequencing technique shows evident advantages in comparison with the isobaric tags for relative and absolute quantitation (iTRAQ) proteomic technology (bulk trophozoite level) and reveals the regulation strategy of trophozoites in the absence of exogenous cysteine. This regulation strategy may be the mechanism of virulence variation of amoebic trophozoites.

New Insights into X-Chromosome Reactivation during Reprogramming to Pluripotency

Dosage compensation between the sexes results in one X chromosome being inactivated during female mammalian development. Chromosome-wide transcriptional silencing from the inactive X chromosome (Xi) in mammalian cells is erased in a process termed X-chromosome reactivation (XCR), which has emerged as a paradigm for studying the reversal of chromatin silencing. XCR is linked with germline development and induction of naive pluripotency in the epiblast, and also takes place upon reprogramming somatic cells to induced pluripotency. XCR depends on silencing of the long non-coding RNA (lncRNA) X inactive specific transcript (Xist) and is linked with the erasure of chromatin silencing. Over the past years, the advent of transcriptomics and epigenomics has provided new insights into the transcriptional and chromatin dynamics with which XCR takes place. However, multiple questions remain unanswered about how chromatin and transcription related processes enable XCR. Here, we review recent work on establishing the transcriptional and chromatin kinetics of XCR, as well as discuss a model by which transcription factors mediate XCR not only via Xist repression, but also by direct targeting of X-linked genes.

Molecular Mechanisms of Facultative Heterochromatin Formation: An X-Chromosome Perspective

Facultative heterochromatin (fHC) concerns the developmentally regulated heterochromatinization of different regions of the genome and, in the case of the mammalian X chromosome and imprinted loci, of only one allele of a homologous pair. The formation of fHC participates in the timely repression of genes, by resisting strong trans activators. In this review, we discuss the molecular mechanisms underlying the establishment and maintenance of fHC in mammals using a mouse model. We focus on X-chromosome inactivation (XCI) as a paradigm for fHC but also relate it to genomic imprinting and homeobox (Hox) gene cluster repression. A vital role for noncoding transcription and/or transcripts emerges as the general principle of triggering XCI and canonical imprinting. However, other types of fHC are established through an unknown mechanism, independent of noncoding transcription (Hox clusters and noncanonical imprinting). We also extensively discuss polycomb-group repressive complexes (PRCs), which frequently play a vital role in fHC maintenance.

Maternal H3K27me3-dependent autosomal and X chromosome imprinting

Genomic imprinting and X-chromosome inactivation (XCI) are classic epigenetic phenomena that involve transcriptional silencing of one parental allele. Germline-derived differential DNA methylation is the best-studied epigenetic mark that initiates imprinting, but evidence indicates that other mechanisms exist. Recent studies have revealed that maternal trimethylation of H3 on lysine 27 (H3K27me3) mediates autosomal maternal allele-specific gene silencing and has an important role in imprinted XCI through repression of maternal Xist. Furthermore, loss of H3K27me3-mediated imprinting contributes to the developmental defects observed in cloned embryos. This novel maternal H3K27me3-mediated non-canonical imprinting mechanism further emphasizes the important role of parental chromatin in development and could provide the basis for improving the efficiency of embryo cloning.

A Conserved Noncoding Locus Regulates Random Monoallelic Xist Expression across a Topological Boundary

cis-Regulatory communication is crucial in mammalian development and is thought to be restricted by the spatial partitioning of the genome in topologically associating domains (TADs). Here, we discovered that the Xist locus is regulated by sequences in the neighboring TAD. In particular, the promoter of the noncoding RNA Linx (LinxP) acts as a long-range silencer and influences the choice of X chromosome to be inactivated. This is independent of Linx transcription and independent of any effect on Tsix, the antisense regulator of Xist that shares the same TAD as Linx. Unlike Tsix, LinxP is well conserved across mammals, suggesting an ancestral mechanism for random monoallelic Xist regulation. When introduced in the same TAD as Xist, LinxP switches from a silencer to an enhancer. Our study uncovers an unsuspected regulatory axis for X chromosome inactivation and a class of cis-regulatory effects that may exploit TAD partitioning to modulate developmental decisions.

No imprinted XIST expression in pigs: biallelic XIST expression in early embryos and random X inactivation in placentas

Dosage compensation, which is achieved by X-chromosome inactivation (XCI) in female mammals, ensures balanced X-linked gene expression levels between the sexes. Although eutherian mammals commonly display random XCI in embryonic and adult tissues, imprinted XCI has also been identified in extraembryonic tissues of mouse, rat, and cow. Little is known about XCI in pigs. Here, we sequenced the porcine XIST gene and identified an insertion/deletion mutation between Asian- and Western-origin pig breeds. Allele-specific analysis revealed biallelic XIST expression in porcine ICSI blastocysts. To investigate the XCI pattern in porcine placentas, we performed allele-specific RNA sequencing analysis on individuals from reciprocal crosses between Duroc and Rongchang pigs. Our results were the first to reveal that random XCI occurs in the placentas of pigs. Next, we investigated the H3K27me3 histone pattern in porcine blastocysts, showing that only 17-31.8% cells have attained XCI. The hypomethylation status of an important XIST DMR (differentially methylated region) in gametes and early embryos demonstrated that no methylation is pre-deposited on XIST in pigs. Our findings reveal that the XCI regulation mechanism in pigs is different from that in mice and highlight the importance of further study of the mechanisms regulating XCI during early porcine embryo development.

Dynamic reversal of random X-Chromosome inactivation during iPSC reprogramming

Induction and reversal of chromatin silencing is critical for successful development, tissue homeostasis, and the derivation of induced pluripotent stem cells (iPSCs). X-Chromosome inactivation (XCI) and reactivation (XCR) in female cells represent chromosome-wide transitions between active and inactive chromatin states. Although XCI has long been studied, providing important insights into gene regulation, the dynamics and mechanisms underlying the reversal of stable chromatin silencing of X-linked genes are much less understood. Here, we use allele-specific transcriptomics to study XCR during mouse iPSC reprogramming in order to elucidate the timing and mechanisms of chromosome-wide reversal of gene silencing. We show that XCR is hierarchical, with subsets of genes reactivating early, late, and very late during reprogramming. Early genes are activated before the onset of late pluripotency genes activation. Early genes are located genomically closer to genes that escape XCI, unlike genes reactivating late. Early genes also show increased pluripotency transcription factor (TF) binding. We also reveal that histone deacetylases (HDACs) restrict XCR in reprogramming intermediates and that the severe hypoacetylation state of the inactive X Chromosome (Xi) persists until late reprogramming stages. Altogether, these results reveal the timing of transcriptional activation of monoallelically repressed genes during iPSC reprogramming, and suggest that allelic activation involves the combined action of chromatin topology, pluripotency TFs, and chromatin regulators. These findings are important for our understanding of gene silencing, maintenance of cell identity, reprogramming, and disease.

Recent Advances in Understanding the Reversal of Gene Silencing During X Chromosome Reactivation

Dosage compensation between XX female and XY male cells is achieved by a process known as X chromosome inactivation (XCI) in mammals. XCI is initiated early during development in female cells and is subsequently stably maintained in most somatic cells. Despite its stability, the robust transcriptional silencing of XCI is reversible, in the embryo and also in a number of reprogramming settings. Although XCI has been intensively studied, the dynamics, factors, and mechanisms of X chromosome reactivation (XCR) remain largely unknown. In this review, we discuss how new sequencing technologies and reprogramming approaches have enabled recent advances that revealed the timing of transcriptional activation during XCR. We also discuss the factors and chromatin features that might be important to understand the dynamics and mechanisms of the erasure of transcriptional gene silencing on the inactive X chromosome (Xi).

Blastocyst activation engenders transcriptome reprogram affecting X-chromosome reactivation and inflammatory trigger of implantation

Implantation of the blastocyst into the uterus is the gateway for further embryonic development in mammals. Programming of blastocyst to an implantation-competent state known as blastocyst activation is the determining factor for implantation into the receptive uterus. However, it remains largely unclear how the blastocyst is globally programmed for implantation. Employing a delayed implantation mouse model, we show here that the blastocyst undergoes extensive programming essential for implantation. By analyzing the transcriptional profile of blastocysts with different implantation competency, we reveal the dynamic change in the biosynthesis, metabolism, and proliferation during blastocyst reactivation from diapause. We also demonstrate that reactivation of the X chromosome, one of the most important events during periimplantation of female embryonic development, is not completed even in blastocysts under conditions of dormancy, despite long term suspension in the uterus. Moreover, the mural trophectoderm (TE), but not the polar TE, differentiates to be more invasive through the weakened cell-cell tight junctions and extracellular matrices (ECMs). By analyzing the differentially expressed profile of secretory proteins, we further demonstrate that the blastocyst functions as a proinflammatory body to secrete proinflammatory signals, such as TNFα and S100A9, thereby triggering embryo-uterine attachment reaction during implantation. Collectively, our data systematically and comprehensively disclose the programming of blastocyst reactivation from diapause for implantation and uncover previously undefined roles of blastocyst during implantation.

PRDM14 controls X-chromosomal and global epigenetic reprogramming of H3K27me3 in migrating mouse primordial germ cells

Background

In order to prepare the genome for gametogenesis, primordial germ cells (PGCs) undergo extensive epigenetic reprogramming during migration toward the gonads in mammalian embryos. This includes changes on a genome-wide scale and additionally in females the remodeling of the inactive X-chromosome to enable X-chromosome reactivation (XCR). However, if global remodeling and X-chromosomal remodeling are related, how they occur in PGCs in vivo in relation to their migration progress and which factors are important are unknown.

Results

Here we identify the germ cell determinant PR-domain containing protein 14 (PRDM14) as the first known factor that is instrumental for both global reprogramming and X-chromosomal reprogramming in migrating mouse PGCs. We find that global upregulation of the repressive histone H3 lysine 27 trimethylation (H3K27me3) mark is PRDM14 dosage dependent in PGCs of both sexes. When focusing on XCR, we observed that PRDM14 is required for removal of H3K27me3 from the inactive X-chromosome, which, in contrast to global upregulation, takes place progressively along the PGC migration path. Furthermore, we show that global and X-chromosomal reprogramming of H3K27me3 are functionally separable, despite their common regulation by PRDM14.

Conclusions

In summary, here we provide new insight and spatiotemporal resolution to the progression and regulation of epigenome remodeling along mouse PGC migration in vivo and link epigenetic reprogramming to its developmental context.

Electronic supplementary material

The online version of this article (10.1186/s13072-019-0284-7) contains supplementary material, which is available to authorized users.

Xist/Tsix expression dynamics during mouse peri-implantation development revealed by whole-mount 3D RNA-FISH

During peri-implantation development in mice, X chromosome inactivation (XCI) status changes dynamically. Here, we examined the expression of Xist and its antisense partner, Tsix, via whole-mount 3D RNA-FISH using strand-specific probes and evaluated XCI status. The results indicate that Xist expression disappears completely by embryonic day (E) 4.5 without Tsix activation in the ICM and that Xist re-expression occurs at E4.75 in some cells, suggesting that random XCI is already initiated in these cells. Intriguingly, epiblast cells exhibiting biallelic Xist expression were observed frequently (~15%) at E5.25 and E5.5. Immunostaining analysis of epigenetic modifications suggests that global change in epigenomic status occurs concomitantly with the transition from imprinted to random XCI. However, global upregulation of H3K27me3 modifications initiated earlier than other modifications, occurring specifically in ICM during progression of Xist erasure. Although both Xist expression and imprinted XCI are thought to be stable in the primitive endoderm/visceral endoderm and trophectoderm/extraembryonic ectoderm lineages, transient loss of Xist clouds was noted only in a subset of extraembryonic ectodermal cells, suggesting distinct features of Xist regulation among the three different embryonic tissue layers. These results will serve as a basis for future functional studies of XCI regulation in vivo.

Maternal Eed knockout causes loss of H3K27me3 imprinting and random X inactivation in the extraembryonic cells

In this study, Inoue et al. investigated the regulatory mechanisms and functions of the maternal H3K27me3 mechanism. They found that maternal Eed, an essential component of the Polycomb group complex 2 (PRC2), is required for establishing H3K27me3 imprinting, and their results also reveal unique XCI dynamics in the absence of Xist imprinting.

Genomic imprinting is essential for mammalian development. Recent studies have revealed that maternal histone H3 Lys27 trimethylation (H3K27me3) can mediate DNA methylation-independent genomic imprinting. However, the regulatory mechanisms and functions of this new imprinting mechanism are largely unknown. Here we demonstrate that maternal Eed, an essential component of the Polycomb group complex 2 (PRC2), is required for establishing H3K27me3 imprinting. We found that all H3K27me3-imprinted genes, including Xist, lose their imprinted expression in Eed maternal knockout (matKO) embryos, resulting in male-biased lethality. Surprisingly, although maternal X-chromosome inactivation (XmCI) occurs in Eed matKO embryos at preimplantation due to loss of Xist imprinting, it is resolved at peri-implantation. Ultimately, both X chromosomes are reactivated in the embryonic cell lineage prior to random XCI, and only a single X chromosome undergoes random XCI in the extraembryonic cell lineage. Thus, our study not only demonstrates an essential role of Eed in H3K27me3 imprinting establishment but also reveals a unique XCI dynamic in the absence of Xist imprinting.

REX1 is the critical target of RNF12 in imprinted X chromosome inactivation in mice

In mice, imprinted X chromosome inactivation (iXCI) of the paternal X in the pre-implantation embryo and extraembryonic tissues is followed by X reactivation in the inner cell mass (ICM) of the blastocyst to facilitate initiation of random XCI (rXCI) in all embryonic tissues. RNF12 is an E3 ubiquitin ligase that plays a key role in XCI. RNF12 targets pluripotency protein REX1 for degradation to initiate rXCI in embryonic stem cells (ESCs) and loss of the maternal copy of Rnf12 leads to embryonic lethality due to iXCI failure. Here, we show that loss of Rex1 rescues the rXCI phenotype observed in Rnf12^−/− ESCs, and that REX1 is the prime target of RNF12 in ESCs. Genetic ablation of Rex1 in Rnf12^−/− mice rescues the Rnf12^−/− iXCI phenotype, and results in viable and fertile Rnf12^−/−:Rex1^−/− female mice displaying normal iXCI and rXCI. Our results show that REX1 is the critical target of RNF12 in XCI.

REX1 has been shown to regulate pluripotency of ESCs, genomic imprinting and preimplantation development in mice. Here the authors provide evidence that REX1 is the prime target of RNF12 E3 ubiquitin ligase and that Rex1 removal rescues the Rnf12 knockout phenotype in imprinted X chromosome inactivation in mice.

X-Chromosome Inactivation: A Crossroads Between Chromosome Architecture and Gene Regulation

In somatic nuclei of female therian mammals, the two X chromosomes display very different chromatin states: One X is typically euchromatic and transcriptionally active, and the other is mostly silent and forms a cytologically detectable heterochromatic structure termed the Barr body. These differences, which arise during female development as a result of X-chromosome inactivation (XCI), have been the focus of research for many decades. Initial approaches to define the structure of the inactive X chromosome (Xi) and its relationship to gene expression mainly involved microscopy-based approaches. More recently, with the advent of genomic techniques such as chromosome conformation capture, molecular details of the structure and expression of the Xi have been revealed. Here, we review our current knowledge of the 3D organization of the mammalian X-chromosome chromatin and discuss its relationship with gene activity in light of the initiation, spreading, and maintenance of XCI, as well as escape from gene silencing.

Role of SmcHD1 in establishment of epigenetic states required for the maintenance of the X-inactivated state in mice

X inactivation in mammals is regulated by epigenetic modifications. Functional deficiency of SmcHD1 has been shown to cause de-repression of X-inactivated genes in post-implantation female mouse embryos, suggesting a role of SmcHD1 in the maintenance of X inactivation. Here, we show that de-repression of X-inactivated genes accompanied a local reduction in the enrichment of H3K27me3 in mouse embryonic fibroblasts deficient for SmcHD1. Furthermore, many of these genes overlapped with those having a significantly lower enrichment of H3K27me3 at the blastocyst stage in wild type. Intriguingly, however, depletion of SmcHD1 did not compromise the X-inactivated state in immortalized female mouse embryonic fibroblasts, in which X inactivation had been established and maintained. Taking all these findings together, we suggest that SmcHD1 facilitates the incorporation of H3K27me3 and perhaps other epigenetic modifications at gene loci that are silenced even with the lower enrichment of H3K27me3 at the early stage of X inactivation. The epigenetic state at these loci would, however, remain as it is at the blastocyst stage in the absence of SmcHD1 after implantation, which would eventually compromise the maintenance of the X-inactivated state at later stages.

Transcriptome Profiling of Single Mouse Oocytes

Single-cell RNA-sequencing (scRNAseq) enables the detection and quantification of mature RNAs in an individual cell. Assessing single cell transcriptomes can circumvent the limited amount of starting material obtained in oocytes or embryos, in particular when working with mutant mice. Here we outline our scRNAseq protocol to study mouse oocyte transcriptomes, derived from Tang et al., Nat Methods 6(5):377-382, 2009 . The method describes the different steps from single cell isolation and cDNA amplification to high-throughput sequencing. The bioinformatics pipeline used to analyze and compare genome-wide gene expression between individual oocytes is then described.

Studying X chromosome inactivation in the single-cell genomic era

Single-cell genomics is set to revolutionise our understanding of how epigenetic silencing works; by studying specific epigenetic marks or chromatin conformations in single cells, it is possible to ask whether they cause transcriptional silencing or are instead a consequence of the silent state. Here, we review what single-cell genomics has revealed about X chromosome inactivation, perhaps the best characterised mammalian epigenetic process, highlighting the novel findings and important differences between mouse and human X inactivation uncovered through these studies. We consider what fundamental questions these techniques are set to answer in coming years and propose that X chromosome inactivation is an ideal model to study gene silencing by single-cell genomics as technical limitations are minimised through the co-analysis of hundreds of genes.