INSIGHT REVIEW NATURE Vol 447 24 May 2007 This active demethylation of the paternal genome is followed by pas- regulates the expression of the nearby gene. Because IAPs seem gen ive demethylation of both maternal and paternal genomes, presumably erally resistant to reprogramming during PGC and pre-implantation brought about by exclusion of DNMTlo(the main form of DNMTI development, the state of expression of the genes that are regulated by esent in the oocyte)from the nuclei of pre-implantation embryos". IAP insertion can be inherited across several generations. It is interest Although DNMTIs can maintain the methylation of imprinted-gene ing to note that there is an example of epigenetic inheritance being DMRs during this period, total genome methylation decreases, reaching maternally transmitted but not paternally transmitted (the agouti viable an overall low at the blastocyst stage. The purpose of active and passive yellow epiallele in mice), and the methylation of the IAP in the sperm demethylation during early embryogenesis is unknown. Demethylation is, unusually, erased in the zygote in this case. So epigenetic inherit of the paternal genome has been proposed to account for the paucity ance is'broken' by erasure of methylation of the paternal genome after of paternal imprints or to be a consequence of DNA-repair processes fertilization that are potentially involved in the protamine-to-histone transition. There are other possible spillovers across generations. In Caenon General demethylation during this period could also have a role in habditis elegans, the X chromosomes are epigenetically marked(by turning the gametic genomes to pluripotency. For example, early histone modifications)during gametogenesis, and some of these xpression of genes such as Oct4 or Nanog is required for the establish- marks are maintained for several cell divisions in the new embryo(for ment and maintenance of the inner-cell-mass lineage in the blastocyst". an unknown reason). In mammalian embryos, some of the histone Because the Nanog and Oct4 promoters are methylated in sperm, and modifications acquired during the silencing of X-linked genes in because methylation of these promoters is repressive, they need to be spermatogenesis might be carried over into the zygote, leading to early demethylated for proper expression to occur( Fig 4b) silencing of some genes on the paternal X chromosome without the action of xist Epigenetic spillover across generations One other area that is unique to mammalian biology deserves Many of the epigenetic marks that are inherited and acquired by germ consideration with regard to epigenetic spillovers from the previous cells are therefore erased in PGCs and in early embryos, making way for generation. At present, we have no understanding of how molecular ew generations to develop and grow into adults purely on the basis of decisions are taken to set up the first two cell lineages in the embryo eir genetic make-up. However, it also seems that epigenetic informa- the trophectoderm and the inner cell mass. However, a recent study tion can spill over to the next generation. The ability of somatic cells suggests that differential histone arginine methylation of individual in the offspring to inherit the methylation of imprinted genes from blastomeres, as early as the four-cell stage, could be one of the earliest parental germ cells is a mechanistic example of this( Fig. 4c). Another marks for this lineage commitment. There is much work to be done in important example of spillover is inheritance of the epigenetic states this area, but it is an exciting possibility that the spillover of epigenetic conferred on some genes by adjacent insertion of IAPs. This can alter marks from the gametes of parents might be responsible for setting up the expression of the endogenous genes; however, more importantly, some of the earliest developmental decisions in the newly developing the epigenetic state of the IAP(that is, methylated or unmethylated) embryo a Erasure of methylation in PGCs Later PGC Histone demethylase? DNA demethylase? r DNA replication? b Erasure of methylation at and after fertilization DNA demethylase? A00、D、a DNA replication c Protection against DNA demethylation at fertilization DNA demethylase? osLr I Reprogramming of epigenetic marks in the germ line and the early for the expression of pluripotency-associated genes. Active histone . a, During the development of PGCs, methylation of CpG islands in marks are also likely to be important for the expression of pluripotency- d genes and other genes can be erased. This is a rap c, In mat gametes, some DNA sequence might involve a demethylase or might occur by dNa replication without methylated are protected from demethylation at or after fertilization. These thylated and some transposons. The protein mature gametes become demethylated in the early embryo. Some of this stella has recently been implicated in protection against demethylation at demethylation occurs in the absence of DNA replication and is therefore fertilization H3K9 methylation is shown in green, H3K4 methylation in likely to be mediated by a demethylase. Demethylation might be important blue and DNA methylation in red @2007 Nature Publishing GroupThis active demethylation of the paternal genome is followed by pas￾sive demethylation of both maternal and paternal genomes, presumably brought about by exclusion of DNMT1o the main form of DNMT1 present in the oocyte from the nuclei of pre-implantation embryos27. Although DNMT1s can maintain the methylation of imprinted-gene DMRs during this period, total genome methylation decreases, reaching an overall low at the blastocyst stage. The purpose of active and passive demethylation during early embryogenesis is unknown. Demethylation of the paternal genome has been proposed to account for the paucity of paternal imprints59 or to be a consequence of DNA-repair processes that are potentially involved in the protamine-to-histone transition53. General demethylation during this period could also have a role in returning the gametic genomes to pluripotency. For example, early expression of genes such as Oct4 or Nanog is required for the establish￾ment and maintenance of the inner-cell-mass lineage in the blastocyst60. Because the Nanog and Oct4 promoters are methylated in sperm, and because methylation of these promoters is repressive, they need to be demethylated for proper expression to occur (Fig. 4b). Epigenetic spillover across generations Many of the epigenetic marks that are inherited and acquired by germ cells are therefore erased in PGCs and in early embryos, making way for new generations to develop and grow into adults purely on the basis of their genetic make-up. However, it also seems that epigenetic informa￾tion can spill over to the next generation. The ability of somatic cells in the offspring to inherit the methylation of imprinted genes from parental germ cells is a mechanistic example of this (Fig. 4c). Another important example of spillover is inheritance of the epigenetic states conferred on some genes by adjacent insertion of IAPs. This can alter the expression of the endogenous genes; however, more importantly, the epigenetic state of the IAP (that is, methylated or unmethylated) regulates the expression of the nearby gene61. Because IAPs seem gen￾erally resistant to reprogramming during PGC and pre-implantation development, the state of expression of the genes that are regulated by IAP insertion can be inherited across several generations. It is interest￾ing to note that there is an example of epigenetic inheritance being maternally transmitted but not paternally transmitted (the agouti viable yellow epiallele in mice), and the methylation of the IAP in the sperm is, unusually, erased in the zygote in this case62. So epigenetic inherit￾ance is ‘broken’ by erasure of methylation of the paternal genome after fertilization. There are other possible spillovers across generations. In Caenor￾habditis elegans, the X chromosomes are epigenetically marked (by histone modifications) during gametogenesis, and some of these marks are maintained for several cell divisions in the new embryo (for an unknown reason)63. In mammalian embryos, some of the histone modifications acquired during the silencing of X-linked genes in spermatogenesis might be carried over into the zygote, leading to early silencing of some genes on the paternal X chromosome without the action of Xist64. One other area that is unique to mammalian biology deserves consideration with regard to epigenetic spillovers from the previous generation. At present, we have no understanding of how molecular decisions are taken to set up the first two cell lineages in the embryo: the trophectoderm and the inner cell mass65. However, a recent study suggests that differential histone arginine methylation of individual blastomeres, as early as the four-cell stage, could be one of the earliest marks for this lineage commitment66. There is much work to be done in this area, but it is an exciting possibility that the spillover of epigenetic marks from the gametes of parents might be responsible for setting up some of the earliest developmental decisions in the newly developing embryo. Erasure of methylation in PGCs Erasure of methylation at and after fertilization Protection against DNA demethylation at fertilization a b c Mature gamete Early PGC Later PGC Mature gamete Histone demethylase? DNA demethylase? DNA replication? Pluripotent cell Pluripotent cell DNA replication Histone methyltransferase DNA demethylase? DNA demethylase? Stella Figure 4 | Reprogramming of epigenetic marks in the germ line and the early embryo. a, During the development of PGCs, methylation of CpG islands in imprinted genes and other genes can be erased. This is a rapid process and might involve a demethylase or might occur by DNA replication without methylation being maintained. b, Many gene sequences that are methylated in mature gametes become demethylated in the early embryo. Some of this demethylation occurs in the absence of DNA replication and is therefore likely to be mediated by a demethylase. Demethylation might be important for the expression of pluripotency-associated genes. Active histone marks are also likely to be important for the expression of pluripotency￾associated genes. c, In mature gametes, some DNA sequences that are methylated are protected from demethylation at or after fertilization. These sequences include imprinted genes and some transposons. The protein stella has recently been implicated in protection against demethylation at fertilization. H3K9 methylation is shown in green, H3K4 methylation in blue and DNA methylation in red. 430 INSIGHT REVIEW NATURE|Vol 447|24 May 2007 ￾
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