The initiation of mammalian embryonic transcription: to begin at the beginning
2022; Elsevier BV; Volume: 33; Issue: 5 Linguagem: Inglês
10.1016/j.tcb.2022.08.008
ISSN1879-3088
AutoresAnthony C.F. Perry, Maki Asami, Brian Lam, Giles S.H. Yeo,
Tópico(s)CRISPR and Genetic Engineering
ResumoOur high-resolution single-cell RNA-sequencing revealed that in human embryos, transcription initiates at the one-cell stage, earlier than widely thought.The new findings suggest that fertilization itself triggers transcription [embryonic genome activation (EGA)] by activating maternal transcription factors.EGA mostly utilizes canonical promoters and produces normatively spliced, protein-coding transcripts.The first genes expressed in the embryo predict upregulation by cancer-associated transcription factors, including MYC and MYCN, linking the onset of embryogenesis to cancer. Gamete (sperm and oocyte) genomes are transcriptionally silent until embryonic genome activation (EGA) following fertilization. EGA in humans had been thought to occur around the eight-cell stage, but recent findings suggest that it is triggered in one-cell embryos, by fertilization. Phosphorylation and other post-translational modifications during fertilization may instate transcriptionally favorable chromatin and activate oocyte-derived transcription factors (TFs) to initiate EGA. Expressed genes lay on cancer-associated pathways and their identities predict upregulation by MYC and other cancer-associated TFs. One interpretation of this is that the onset of EGA, and the somatic cell trajectory to cancer, are mechanistically related: cancer initiates epigenetically. We describe how fertilization might be linked to the initiation of EGA and involve distinctive processes recapitulated in cancer. Gamete (sperm and oocyte) genomes are transcriptionally silent until embryonic genome activation (EGA) following fertilization. EGA in humans had been thought to occur around the eight-cell stage, but recent findings suggest that it is triggered in one-cell embryos, by fertilization. Phosphorylation and other post-translational modifications during fertilization may instate transcriptionally favorable chromatin and activate oocyte-derived transcription factors (TFs) to initiate EGA. Expressed genes lay on cancer-associated pathways and their identities predict upregulation by MYC and other cancer-associated TFs. One interpretation of this is that the onset of EGA, and the somatic cell trajectory to cancer, are mechanistically related: cancer initiates epigenetically. We describe how fertilization might be linked to the initiation of EGA and involve distinctive processes recapitulated in cancer. Measuring mammalian embryonic transcriptional initiationThe first embryonic transcription, EGA (see Glossary), is a critical aspect of the establishment of totipotency [1.Condic M.L. Totipotency: what it is and what it is not.Stem Cells Dev. 2014; 23: 796-812Crossref PubMed Scopus (69) Google Scholar], yet the initiation and early profile of EGA are poorly understood in any vertebrate species [2.Jukam D. et al.Zygotic genome activation in vertebrates.Dev. Cell. 2017; 42: 316-332Abstract Full Text Full Text PDF PubMed Scopus (161) Google Scholar]. In fish and amphibian models, the kinetics of EGA are elusively rapid and in mammals, the focus here, embryos have been of indeterminate age, asynchronous, pooled (smoothing expression), or transcript levels obscured by changes in poly(A) tail length [2.Jukam D. et al.Zygotic genome activation in vertebrates.Dev. Cell. 2017; 42: 316-332Abstract Full Text Full Text PDF PubMed Scopus (161) Google Scholar, 3.Blower M.D. et al.Combining different mRNA capture methods to analyze the transcriptome: analysis of the Xenopus laevis transcriptome.PLoS One. 2013; 8e77700Crossref PubMed Scopus (19) Google Scholar, 4.Olsen T.K. Baryawno N. Introduction to single-cell RNA sequencing.Curr. Protoc. Mol. Biol. 2018; 122e57Crossref PubMed Scopus (59) Google Scholar, 5.Temeles G.L. Schultz R.M. Transient polyadenylation of a maternal mRNA following fertilization of mouse eggs.J. Reprod. Fertil. 1997; 109: 223-228Crossref PubMed Scopus (22) Google Scholar]. Mouse embryonic transcription is thought to initiate by the late one-cell stage ('minor' EGA), followed by 'major' EGA in two-cell embryos [6.Xue Z. et al.Genetic programs in human and mouse early embryos revealed by single-cell RNA sequencing.Nature. 2013; 500: 593-597Crossref PubMed Scopus (614) Google Scholar]; EGA in humans has been held to begin at the four-to-eight cell stage [7.Braude P. et al.Human gene expression first occurs between the four- and eight-cell stages of preimplantation development.Nature. 1988; 332: 459-461Crossref PubMed Scopus (1123) Google Scholar], even though the latency period between fertilization and EGA would have to last at least 2 days. Notwithstanding hints that human EGA initiates earlier than the four-to-eight cell stage [6.Xue Z. et al.Genetic programs in human and mouse early embryos revealed by single-cell RNA sequencing.Nature. 2013; 500: 593-597Crossref PubMed Scopus (614) Google Scholar,8.Leng L. et al.Single-cell transcriptome analysis of uniparental embryos reveals parent-of-origin effects on human preimplantation development.Cell Stem Cell. 2019; 25: 697-712Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar,9.Vassena R. et al.Waves of early transcriptional activation and pluripotency program initiation during human preimplantation development.Development. 2011; 138: 3699-3709Crossref PubMed Scopus (186) Google Scholar], no model accounts for how the embryo genome would be sustained in a transcriptionally silent state, embryonic processes regulated, or the nature of the autonomous cue that triggered transcription.Resolving these issues would benefit from a detailed profile of transcriptional initiation to enable a model of the underlying mechanism. Accurate determination of transcription immediately after fertilization is confounded by modulation of transcript polyadenylation in early embryos to control translation, which may distort estimates of relative levels [3.Blower M.D. et al.Combining different mRNA capture methods to analyze the transcriptome: analysis of the Xenopus laevis transcriptome.PLoS One. 2013; 8e77700Crossref PubMed Scopus (19) Google Scholar,5.Temeles G.L. Schultz R.M. Transient polyadenylation of a maternal mRNA following fertilization of mouse eggs.J. Reprod. Fertil. 1997; 109: 223-228Crossref PubMed Scopus (22) Google Scholar]. This was recently addressed by single-cell RNA-sequencing (scRNA-seq) in human embryos [10.Asami M. et al.Human embryonic genome activation initiates at the one-cell stage.Cell Stem Cell. 2022; 29: 209-216Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar]. The method employed a subtractive approach that was independent of transcript polyadenylation status and accommodated the potential for further profile distortion due to maternal RNA inheritance and degradation, in which RNA from the mature, metaphase II (mII) oocyte is transmitted to the embryo during fertilization and subsequently degraded [8.Leng L. et al.Single-cell transcriptome analysis of uniparental embryos reveals parent-of-origin effects on human preimplantation development.Cell Stem Cell. 2019; 25: 697-712Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar,11.Sha Q.Q. et al.Dynamics and clinical relevance of maternal mRNA clearance during the oocyte-to-embryo transition in humans.Nat. Commun. 2020; 11: 4917Crossref PubMed Scopus (36) Google Scholar]. Because scRNA-seq read counts for mII oocytes and embryos required negligible normalization, they were readily comparable, facilitating the detection of EGA [10.Asami M. et al.Human embryonic genome activation initiates at the one-cell stage.Cell Stem Cell. 2022; 29: 209-216Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar]. Upregulation was corroborated orthogonally and by conservation with the onset of EGA in mouse one-cell embryos [12.Asami M. et al.Mouse fertilization triggers a transcription program conserved in human one-cell embryos.bioRxiv. 2020; (Published online September 15, 2020)https://doi.org/10.1101/2020.09.15.298018Google Scholar]. Moreover, maternal transcript degradation in human embryos is a gradual process occurring over several days following fertilization [8.Leng L. et al.Single-cell transcriptome analysis of uniparental embryos reveals parent-of-origin effects on human preimplantation development.Cell Stem Cell. 2019; 25: 697-712Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar,11.Sha Q.Q. et al.Dynamics and clinical relevance of maternal mRNA clearance during the oocyte-to-embryo transition in humans.Nat. Commun. 2020; 11: 4917Crossref PubMed Scopus (36) Google Scholar], so its contribution at the one-cell stage is small. Addressing these challenges by scRNA-seq has shown that EGA initiates at the one-cell stage in humans [9.Vassena R. et al.Waves of early transcriptional activation and pluripotency program initiation during human preimplantation development.Development. 2011; 138: 3699-3709Crossref PubMed Scopus (186) Google Scholar]. A discrete set of genes is expressed, suggesting that EGA is programmed, rather than stochastic. We now speculate on the implications of this finding.EGA onset predicts the involvement of cancer-associated TFsGenes upregulated in EGA [10.Asami M. et al.Human embryonic genome activation initiates at the one-cell stage.Cell Stem Cell. 2022; 29: 209-216Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar] can be used bioinformatically to infer the TFs mediating expression. These include the pleiotropic TF, MYC, which binds to many genes, often in cells with regenerative and proliferative potential and affecting metabolic pathways and differentiation [13.Das S.K. et al.MYC: a complex problem.Trends Cell Biol. 2022; (Published online August 10, 2022. https://doi.org/10.1016/j.tcb.2022.07.006)Abstract Full Text Full Text PDF PubMed Scopus (3) Google Scholar, 14.Eilers M. Eisenman R.N. Myc’s broad reach.Genes Dev. 2008; 22: 2755-2766Crossref PubMed Scopus (733) Google Scholar, 15.Lourenco C. et al.MYC protein interactors in gene transcription and cancer.Nat. Rev. Cancer. 2021; 21: 579-591Crossref PubMed Scopus (39) Google Scholar] (see Table 1). MYC is commonly dysregulated in cancer [16.Dang C.V. MYC on the path to cancer.Cell. 2012; 149: 22-35Abstract Full Text Full Text PDF PubMed Scopus (2016) Google Scholar], present in mouse mII oocytes [17.Suzuki T. et al.Expression of c-MYC in nuclear speckles during mouse oocyte growth and preimplantation development.J. Reprod. Dev. 2009; 55: 491-495Crossref PubMed Scopus (13) Google Scholar], and predicted EGA regulators include other cancer-associated TFs, such as MYCN, FOXM1, and E2F4 [10.Asami M. et al.Human embryonic genome activation initiates at the one-cell stage.Cell Stem Cell. 2022; 29: 209-216Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar]. MYC often induces small changes in target gene expression, as observed at the onset of EGA [10.Asami M. et al.Human embryonic genome activation initiates at the one-cell stage.Cell Stem Cell. 2022; 29: 209-216Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar,12.Asami M. et al.Mouse fertilization triggers a transcription program conserved in human one-cell embryos.bioRxiv. 2020; (Published online September 15, 2020)https://doi.org/10.1101/2020.09.15.298018Google Scholar,18.Baluapuri A. et al.Target gene-independent functions of MYC oncoproteins.Nat. Rev. Mol. Cell Biol. 2020; 21: 255-267Crossref PubMed Scopus (93) Google Scholar]. This low expression amplitude may partly explain why EGA was previously missed in human one-cell embryos and highlights the possibility that cellular potency is commonly modulated by small expression changes [19.Berrozpe G. et al.Polycomb responds to low levels of transcription.Cell Rep. 2017; 20: 785-793Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar,20.Chronis C. et al.Cooperative binding of transcription factors orchestrates reprogramming.Cell. 2017; 168: 442-459Abstract Full Text Full Text PDF PubMed Scopus (271) Google Scholar].Table 1Functional pathways shared by cancer and the onset of human EGAaTop 15 pathways shared between cancer and upregulated genes in human one-cell embryos as determined by Ingenuity Pathway Analysis (IPA), ranked by z-score (highest uppermost).CategoryFunctionP valuez-scoreNo.bNumber of shared genes in the IPA category.ExamplescTop five genes (logFC > 2) in the category.Cancer, organismal injury, and abnormalitiesGrowth of tumor7.91e-064.921105EIF2AK3, PRPS1, NAMPT, SP1, GNA13CancerCell transformation8.15e-064.44758TEAD1, CCNE1, MAP2K3, GNA13, THRACancerTransformation of fibroblast cell lines2.01e-073.67838NFKBIA, SMC3, PTTG1, JAK2, AKAP13Cancer, organismal injury and abnormalitiesInvasive tumor1.43e-062.773131RNA5SP141, MTCO1P12, RIPK4, CDH15, CMTR2Cancer, organismal injury and abnormalitiesMetastasis1.18e-052.675109RNA5SP141, MTCO1P12, RIPK4, CDH15, CMTR2Cancer, organismal injury and abnormalitiesAdvanced malignant tumor1.18e-042.675112RNA5SP141, MTCO1P12, RIPK4, CDH15, CMTR2Cancer, organismal injury and abnormalitiesGenitourinary carcinoma6.23e-381.982734TIGD5, RGS12, ETV1, LGR4, PIP5K1ACancer, organismal injury and abnormalitiesNeoplasia of cells8.51e-231.818559RNA5SP141, MTCO1P12, RIPK4, CDH15, CMTR2Cancer, endocrine system disorders, organismal injury and abnormalitiesNeuroendocrine tumor7.28e-061.516229RNA5SP141, MTCO1P12, RIPK4, CDH15, CMTR2Cancer, organismal injury and abnormalitiesSquamous-cell carcinoma5.83e-131.300370TEAD1, CCNE1, BCL9L, PTTG1, DICER1Cancer, endocrine system disorders, organismal injury and abnormalitiesEndocrine gland tumor2.17e-391.152830NFKBIA, SMC3, PTTG1, JAK2, AKAP13Cancer, organismal injury and abnormalitiesAnogenital cancer1.92e-381.109733RNA5SP141, MTCO1P12, RIPK4, CDH15, CMTR2Cancer, organismal injury, and abnormalities, reproductive system diseaseMammary tumor6.15e-221.030329RNA5SP141, MTCO1P12, RIPK4, CDH15, CMTR2Cancer, organismal injury and abnormalitiesMalignant solid organ tumor4.57e-251.029701RNA5SP141, MTCO1P12, RIPK4, CDH15, CMTR2Cancer, dermatological diseases and conditions, organismal injury and abnormalitiesSkin cancer2.50e-241.029693MAB21L1, ETV1, TFEB, MAU2, LAMC3a Top 15 pathways shared between cancer and upregulated genes in human one-cell embryos as determined by Ingenuity Pathway Analysis (IPA), ranked by z-score (highest uppermost).b Number of shared genes in the IPA category.c Top five genes (logFC > 2) in the category. Open table in a new tab Predicted EGA-initiating TFs are regulated by post-translational modifications [21.Joshi K. et al.MELK-dependent FOXM1 phosphorylation is essential for proliferation of glioma stem cells.Stem Cells. 2013; 31: 1051-1063Crossref PubMed Scopus (149) Google Scholar, 22.Morillo S.M. et al.Nerve growth factor-induced cell cycle reentry in newborn neurons is triggered by p38MAPK-dependent E2F4 phosphorylation.Mol. Cell Biol. 2012; 32: 2722-2737Crossref PubMed Scopus (31) Google Scholar, 23.Sjostrom S.K. et al.The Cdk1 complex plays a prime role in regulating N-Myc phosphorylation and turnover in neural precursors.Dev. Cell. 2005; 9: 327-338Abstract Full Text Full Text PDF PubMed Scopus (106) Google Scholar, 24.Vervoorts J. et al.The ins and outs of MYC regulation by posttranslational mechanisms.J. Biol. Chem. 2006; 281: 34725-34729Abstract Full Text Full Text PDF PubMed Scopus (183) Google Scholar, 25.Wu H. et al.Tumor necrosis factor receptor-associated factor 6 promotes hepatocarcinogenesis by interacting with histone deacetylase 3 to enhance C-Myc gene expression and protein stability.Hepatology. 2020; 71: 148-163Crossref PubMed Scopus (29) Google Scholar]. For example, MYC localization, stability, and activity are sensitive to acetylation [25.Wu H. et al.Tumor necrosis factor receptor-associated factor 6 promotes hepatocarcinogenesis by interacting with histone deacetylase 3 to enhance C-Myc gene expression and protein stability.Hepatology. 2020; 71: 148-163Crossref PubMed Scopus (29) Google Scholar], isomerization [26.Su Y. et al.Post-translational modification localizes MYC to the nuclear pore basket to regulate a subset of target genes involved in cellular responses to environmental signals.Genes Dev. 2018; 32: 1398-1419Crossref PubMed Scopus (32) Google Scholar], and phosphorylation [24.Vervoorts J. et al.The ins and outs of MYC regulation by posttranslational mechanisms.J. Biol. Chem. 2006; 281: 34725-34729Abstract Full Text Full Text PDF PubMed Scopus (183) Google Scholar]: cyclin-dependent and MAP kinases phosphorylate MYC [23.Sjostrom S.K. et al.The Cdk1 complex plays a prime role in regulating N-Myc phosphorylation and turnover in neural precursors.Dev. Cell. 2005; 9: 327-338Abstract Full Text Full Text PDF PubMed Scopus (106) Google Scholar], whose dephosphorylation is mediated by (among others) protein phosphatase 2A [27.Janghorban M. et al.Targeting c-MYC by antagonizing PP2A inhibitors in breast cancer.Proc. Natl. Acad. Sci. USA. 2014; 111: 9157-9162Crossref PubMed Scopus (131) Google Scholar]. Several of these are components of phosphorylation pathways regulating Emi2 and Mos to control meiosis in vertebrate oocytes (Figure 1A ) [28.Perry A.C.F. Verlhac M.-H. Second meiotic arrest and exit in frogs and mice.EMBO Rep. 2008; 9: 246-251Crossref PubMed Scopus (43) Google Scholar]. It is also possible that during meiotic progression, de novo translation of maternal transcripts itself contributes to the onset of EGA, as it apparently does for 'minor' EGA [29.Zhang C. et al.Profiling and functional characterization of maternal mRNA translation during mouse maternal-to-zygotic transition.Sci Adv. 2022; 8: eabj3967Crossref PubMed Scopus (3) Google Scholar].Acetylation can directly enhance MYC target gene activation, or do so indirectly, as when MYC recruits histone acetyltransferase (HAT) or deacetylase (HDAC) activities [30.Zhou Y. et al.Targeting Myc interacting proteins as a winding path in cancer therapy.Front. Pharmacol. 2021; 12748852Google Scholar]. In addition, MYC can recruit the scaffold protein, TRAPP, to stabilize expression-activating HATs, Tip60 and GCN5 [31.Liu X. et al.c-Myc transformation domain recruits the human STAGA complex and requires TRRAP and GCN5 acetylase activity for transcription activation.J. Biol. Chem. 2003; 278: 20405-20412Abstract Full Text Full Text PDF PubMed Scopus (97) Google Scholar], and the cAMP response-element binding coactivator protein, CBP/p300, to modify chromatin and promote MYC acetylation [32.Kwok R.P. et al.Distribution of co-activators CBP and p300 during mouse oocyte and embryo development.Mol. Reprod. Dev. 2006; 73: 885-894Crossref PubMed Scopus (16) Google Scholar,33.Zhang K. et al.Six lysine residues on c-Myc are direct substrates for acetylation by P300.Biochem. Biophys. Res. Commun. 2005; 336: 274-280Crossref PubMed Scopus (38) Google Scholar]. MYC homeostasis also involves the deacetylase, HDAC3 [25.Wu H. et al.Tumor necrosis factor receptor-associated factor 6 promotes hepatocarcinogenesis by interacting with histone deacetylase 3 to enhance C-Myc gene expression and protein stability.Hepatology. 2020; 71: 148-163Crossref PubMed Scopus (29) Google Scholar]. These and other modifiers and associated HAT and HDAC activities are present in mouse mII oocytes and one-cell embryos [32.Kwok R.P. et al.Distribution of co-activators CBP and p300 during mouse oocyte and embryo development.Mol. Reprod. Dev. 2006; 73: 885-894Crossref PubMed Scopus (16) Google Scholar,34.Yoshida N. et al.Epigenetic discrimination by mouse metaphase II oocytes mediates asymmetric chromatin remodeling independently of meiotic exit.Dev. Biol. 2007; 301: 464-477Crossref PubMed Scopus (40) Google Scholar], where they might remodel chromatin after fertilization.Post-translational modifications of candidate EGA regulators other than MYC include MYCN methylation by protein arginine methyltransferase 1 [35.Eberhardt A. et al.Protein arginine methyltransferase 1 is a novel regulator of MYCN in neuroblastoma.Oncotarget. 2016; 7: 63629-63639Crossref PubMed Scopus (14) Google Scholar], phosphorylation of the G1-/S-phase mitotic gene expression activator, FOXM1, by polo-like kinase 1 and CDK4/6 [36.Fu Z. et al.Plk1-dependent phosphorylation of FoxM1 regulates a transcriptional programme required for mitotic progression.Nat. Cell Biol. 2008; 10: 1076-1082Crossref PubMed Scopus (244) Google Scholar], and of E2F4 by aurora kinase to regulate DNA binding and subcellular localization [37.Dhanasekaran K. et al.Unraveling the role of aurora A beyond centrosomes and spindle assembly: implications in muscle differentiation.FASEB J. 2019; 33: 219-230Crossref PubMed Scopus (6) Google Scholar]. We speculate that together with chromatin remodeling, these modulate TF activity in response to fertilization-induced signaling or chromatin (epigenetic) remodeling, or less directly, as can occur in response to cytoskeletal remodeling or reconfigured phase compartmentalization [38.Li J. et al.Control of chromatin organization and chromosome behavior during the cell cycle through phase separation.Int. J. Mol. Sci. 2021; 22: 12271Crossref PubMed Scopus (2) Google Scholar]. In an extension of this model, the onset of embryogenesis is ectopically recapitulated in events that lead to carcinogenesis (Figure 1).How are genes marked for the first embryonic transcription?At least four mutually inclusive possibilities account for how one-cell embryos designate genes for expression.First, by association with transcription-activating chromatin. Topologically associating domains (TADs) occur in mouse early embryos and could coordinate genome activation by spatial proximation of EGA genes [39.Flyamer I.M. et al.Single-nucleus Hi-C reveals unique chromatin reorganization at oocyte-to-zygote transition.Nature. 2017; 544: 110-114Crossref PubMed Scopus (403) Google Scholar,40.Ke Y. et al.3D chromatin structures of mature gametes and structural reprogramming during mammalian embryogenesis.Cell. 2017; 170: 367-381Abstract Full Text Full Text PDF PubMed Scopus (243) Google Scholar]. It is unclear if TADs are preformed in gametes [39.Flyamer I.M. et al.Single-nucleus Hi-C reveals unique chromatin reorganization at oocyte-to-zygote transition.Nature. 2017; 544: 110-114Crossref PubMed Scopus (403) Google Scholar] or assemble after fertilization [40.Ke Y. et al.3D chromatin structures of mature gametes and structural reprogramming during mammalian embryogenesis.Cell. 2017; 170: 367-381Abstract Full Text Full Text PDF PubMed Scopus (243) Google Scholar], whether the onset of EGA initiates within such domains or, indeed, if early activated genes are clustered and chromatin remodeling seeds expression of nearby genes. Alternatively, activation may occur independently at dispersed loci.Secondly, via TF cognate DNA sequences. For example, MYC preferentially recognizes the E-box, CACGTG, in a DNA-methylation-sensitive manner [41.Prendergast G.C. Ziff E.B. Methylation-sensitive sequence-specific DNA binding by the c-Myc basic region.Science. 1991; 251: 186-189Crossref PubMed Scopus (429) Google Scholar], whilst E2F4 has the consensus binding site, TTTCGCGC [42.Kovesdi I. et al.Identification of a cellular transcription factor involved in E1A trans-activation.Cell. 1986; 45: 219-228Abstract Full Text PDF PubMed Scopus (321) Google Scholar]. The default in this second scenario is transcriptional quiescence (the initial state of most genes), with binding of activating TFs to motifs within targeted promoters causing expression of associated genes at the onset of EGA.Thirdly, meiotic exit triggered by fertilization may result in an epigenetic configuration that is more favorable for transcription at genes upregulated in EGA, likely involving the appearance of histone modifications associated with transcriptional activation [12.Asami M. et al.Mouse fertilization triggers a transcription program conserved in human one-cell embryos.bioRxiv. 2020; (Published online September 15, 2020)https://doi.org/10.1101/2020.09.15.298018Google Scholar].Fourthly, parentally inherited, gamete-borne epigenetic marks might instate transcription. Inherited marks characterize parentally biased, imprinted gene expression in mouse blastocysts and most correspond to trimethylated histone 3 lysine 27 (H3K27me3) and some to DNA methylation [43.Santini L. et al.Genomic imprinting in mouse blastocysts is predominantly associated with H3K27me3.Nat. Comm. 2021; 12: 3804Crossref PubMed Scopus (7) Google Scholar]. Parentally inherited marks pass through the one-cell stage, so could influence EGA. Other marks affecting transcription (e.g., H3K4me3) may not correspond to canonical imprints [43.Santini L. et al.Genomic imprinting in mouse blastocysts is predominantly associated with H3K27me3.Nat. Comm. 2021; 12: 3804Crossref PubMed Scopus (7) Google Scholar,44.Samata M. et al.Intergenerationally maintained histone H4 lysine 16 acetylation is instructive for future gene activation.Cell. 2020; 182: 127-144Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar]. It is also possible that parentally acquired (i.e., nonprogrammed) traits are epigenetically transmitted to offspring to promote gene expression at the one-cell stage. This would exemplify epigenetic inheritance [45.Skvortsova K. et al.Functions and mechanisms of epigenetic inheritance in animals.Nat. Rev. Mol. Cell Biol. 2018; 19: 774-790Crossref PubMed Scopus (207) Google Scholar] that may not become apparent until adulthood, as can occur in nuclear transfer cloning [46.Perry A.C.F. Wakayama T. Untimely ends and new beginnings in mouse cloning.Nature Genet. 2002; 30: 243-244Crossref PubMed Scopus (35) Google Scholar].Maternal chromatin transitions from a condensed metaphase configuration during meiotic progression, whereas protamines, which comprise 85% of human sperm nucleoprotein, are exhaustively removed and replaced with histones [47.Zhou L. Dean J. Reprogramming the genome to totipotency in mouse embryos.Trends Cell Biol. 2015; 25: 82-91Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar]. Differences in parental chromatin thus likely produce distinctive parental gene expression profiles at the onset of EGA.Totipotency and the expression trajectory of upregulated transcriptsA complete model of totipotency will accommodate transcription in one-cell embryos: in humans, transcript levels for most of this set of EGA genes decline with the onset of 'major' EGA at the four-to-eight-cell stage [1.Condic M.L. Totipotency: what it is and what it is not.Stem Cells Dev. 2014; 23: 796-812Crossref PubMed Scopus (69) Google Scholar,7.Braude P. et al.Human gene expression first occurs between the four- and eight-cell stages of preimplantation development.Nature. 1988; 332: 459-461Crossref PubMed Scopus (1123) Google Scholar,10.Asami M. et al.Human embryonic genome activation initiates at the one-cell stage.Cell Stem Cell. 2022; 29: 209-216Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar]. Although 1~5% of pluripotent mouse embryonic stem (ES) cells are thought to exhibit features of totipotent two-cell embryo blastomeres [48.Macfarlan T.S. et al.Embryonic stem cell potency fluctuates with endogenous retrovirus activity.Nature. 2012; 487: 57-63Crossref PubMed Scopus (644) Google Scholar], one-cell embryos are transcriptionally distinct. Transcripts for drivers of gene expression in the cleavage-stage embryo, including pluripotency factors DUX4 and LEUTX, are not upregulated in human one-cell embryos [6.Xue Z. et al.Genetic programs in human and mouse early embryos revealed by single-cell RNA sequencing.Nature. 2013; 500: 593-597Crossref PubMed Scopus (614) Google Scholar,8.Leng L. et al.Single-cell transcriptome analysis of uniparental embryos reveals parent-of-origin effects on human preimplantation development.Cell Stem Cell. 2019; 25: 697-712Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar,10.Asami M. et al.Human embryonic genome activation initiates at the one-cell stage.Cell Stem Cell. 2022; 29: 209-216Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar,49.Hendrickson P.G. et al.Conserved roles of mouse Dux and human Dux4 in activating cleavage-stage genes and MERVL/HERVL retro-transposons.Nat. Genet. 2017; 49: 925Crossref PubMed Scopus (309) Google Scholar]. In addition, TFs upregulated from around the eight-cell stage, including OCT4, SOX2, and NANOG, are not predicted to regulate genes expressed at the one-cell stage [10.Asami M. et al.Human embryonic genome activation initiates at the one-cell stage.Cell Stem Cell. 2022; 29: 209-216Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar,50.Niakan K.K. Eggan K. Analysis of human embryos from zygote to blastocyst reveals distinct gene expression patterns relative to the mouse.Dev. Biol. 2013; 375: 54-64Crossref PubMed Scopus (225) Google Scholar], contrasting with the requirement of pluripotency factors to initiate EGA in zebrafish [51.Lee M.T. et al.Nanog, Pou5f1 and SoxB1 activate zygotic gene expression during the maternal-to-zygotic transition.Nature. 2013; 503: 360-364Crossref PubMed Scopus (280) Google Scholar].Little trace of most transcripts upregulated in human one-cell embryos remains at the eight-cell stage (Figure 1). Maternal transcript degradation, which occurs over several days in human embryos [8.Leng L. et al.Single-cell transcriptome analysis of uniparental embryos reveals parent-of-origin effects on human preimplantation development.Cell Stem Cell. 2019; 25: 697-712Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar,11.Sha Q.Q. et al.Dynamics and clinical relevance of maternal mRNA clearance during the oocyte-to-embryo transition in humans.Nat. Commun. 2020; 11: 4917Crossref PubMed Scopus (36) Google Scholar], may also act on transcripts produced by de novo embryonic expression. However, downregulation of expressed genes is precipitous [10.Asami
Referência(s)