Stem Cells, Vol. 19, No. 4, 287-294,
July 2001
© 2001 AlphaMed Press
Imprinting in the Germ Line
Jeffrey R. Mann
Section of Mammalian Development, Division of Biology, Beckman Research Institute of the City of Hope, Duarte, California, USA
Key Words. Imprinting • Germ line • Epigenetics • Methylation • Replication timing • Embryonic stem cells
Jeffrey R. Mann, Ph.D., Division of Biology, Beckman Research Institute, 1450 E. Duarte Rd., Duarte, California 91010-3011, USA. Telephone: 626-301-8813; Fax: 626-358-7703; e-mail: jmann{at}coh.org
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Abstract
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Genomic imprinting is an epigenetic system of gene regulation in mammals. It determines the parent-of-origin-dependent expression of a small number of imprinted genes during development, i.e., the maternal allele is inactive while the paternal is active, or vice versa. Imprinting is imparted in the germ line and involves differential DNA methylation such that particular DNA regions become methylated in one sex of germ line but not in the other. Inheritance of these differential egg and sperm methylation states is then transmitted to somatic cells, where they lead to differential maternal and paternal allelic activity, or monoallelic expression. Increasing evidence indicates that the inherited and stable differential allelic methylation regulates monoallelic expression by influencing the activity of gene regulatory elements—for one allele the element is switched off by methylation, while for the other the element is left potentially active by the lack of methylation. An interesting feature of the germ line is that, despite the presence of genomic imprinting, either as imprints inherited from the zygote or as new imprints imparted according to germ cell sex, imprinted genes are biallelically expressed as if imprints were not present. One explanation for this observation is that imprints have no influence over the germ cell's transcriptional machinery, i.e., imprinting may be neutralized in the germ cell lineage. This phenomenon may have a common basis with other unique features of the germ line, such as totipotency, perhaps in some unique aspect of chromatin structure.
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INTRODUCTION
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Germ cells "are the most fascinating cells there are and, what's more, they are still deeply mysterious" [1]. While for some they may not be the most fascinating, they certainly are the most unique cells there are. They originate outside of the developing embryo, in the extraembryonic component of the conceptus, then migrate to the embryo and colonize the gonad. They exist in two forms, female and male. As a population they are quiescent for a relatively long period of developmental time before they fully mature. They are the only cells that undergo genetic recombination—through meiosis. They are immortal—eggs and sperm live on in their union at fertilization contributing cytoplasm and recombined DNA to the new generation. They are totipotent—their job is to give rise to new individuals. In some vertebrates, totipotency is clearly evident for the female germ line alone: adult parthenogenetic fish, amphibians, reptiles, and birds have been recorded. Parthenogenones are derived from unfertilized eggs when only the egg-derived genomes contribute to development [2]. In mammals, parthenogenetic embryos can occasionally reach advanced stages, and parthenogenetic cells can be defined as totipotent as they readily contribute to many tissue types in chimeras including the germ line [3]. For male germ cells the situation is not as obvious. Spontaneous differentiation of male germ cells into multiple cell types in teratomas has not been reported—mammalian testicular teratomas are thought to be derived from primordial germ cells (PGCs) before sexual differentiation [4]. Nevertheless, clearly the nucleus of the male gamete is prepared to support embryonic development: androgenetic fish, at least, are readily produced when only sperm genomes are allowed to contribute to early cleavage and embryonic development [5-7]. Also, mammalian androgenones undergo preimplantation development at high frequency, although, like mammalian parthenogenones, eventually fail.
The failure of mammalian parthenogenones and androgenones reveals yet another unique feature of the germ line peculiar to mammals: while the female and male zygotic genomes have a common set of functions for development, the large majority of the total necessary functions, there are other functions which are unique to each genome. This latter set of partitioned functions must have an epigenetic basis, as the egg and sperm each provides the same DNA sequences to the zygote. This epigenetic system is termed genomic imprinting [8-11]. The functions that are partitioned include, and are probably limited to, gene expression. Thus, a small subset of imprinted genes are expressed from only one of the two parental alleles, in accordance with parental origin [12, 13].
The monoallelic expression pattern of imprinted genes poses an interesting problem in gene regulation and epigenetics: how are two alleles of the same DNA sequence differentially active in the same cell? Through cell division, heritable cis-acting modifications would seem crucial, as stably expressed and freely diffusible trans-acting factors potentially have access to both alleles [14].
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SOMATIC CELLS AND THE REGULATION OF MONOALLELIC EXPRESSION
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An understanding of the epigenetic basis of monoallelic expression in somatic cells is prerequisite for understanding how these modifications originate in germ cells. The expression state of an allele of an imprinted gene appears to be determined solely by parental origin, or heritable cis-acting epigenetic modifications that have their origin in female or male germ cells of parents. Thus, as distinct from other systems of allelic exclusion, e.g., random X-chromosome inactivation, there is no clear evidence for any role of allelic counting and trans-sensing mechanisms in initiating or maintaining monoallelic expression [15]. In searching for the epigenetic controls of this expression, much effort is placed on investigating maternal and paternal allele-specific DNA methylation patterns. This is for good reason—methylation is a good candidate for a system of cell memory, being clonally heritable in cis, and there is considerable evidence that it can affect gene activity [14, 16, 17].
Differentially Methylated Regions (DMRs)
Imprinted genes are often characterized by differential DNA methylation: in somatic cells, imprinted genes contain one or more DMRs such that for one parental allele CpGs in a region are methylated, while for the other parental allele they are not [18-25]. In the simplest situation there are two types of DMRs: primary DMRs are homologous DNA sequences methylated differently in oocytes and sperm, while secondary DMRs are produced after fertilization, or after the wave of de novo methylation at the early postimplantation stage. Secondary DMRs could result from spreading of methylation from a primary DMR or be in response to gene activity. Often, promoters are methylated on the inactive alleles of imprinted genes, possibly to initiate or aid in the maintenance of the inactive state, e.g., the maternal and paternal promoters of H19 and the insulin-like growth factor-2 receptor (Igf2r) gene, while not markedly differentially methylated in gametes, become significantly differentially methylated during embryonic development [26-31]. This secondary methylation is similar to the methylation of inactive non-imprinted tissue-specific genes. For most imprinted genes examined, DMRs are closely associated with CpG islands and direct repeats. The direct repeats of various imprinted genes have no clear sequence in common, yet their occurrence in conjunction with methylated CpG islands suggests functional significance of the overall structural arrangement for genomic imprinting [32].
Evidence that at least some DMR sequences—not necessarily the methylation contained therein—are critical for monoallelic expression is provided by knockouts of these regions in transgenic mice and cultured cells. For the Igf2 and H19 genes, removal of most of the primary DMR sequence in mice resulted in disruption of monoallelic expression [33]. For the Igf2r gene, appropriate imprinting, i.e., maternal-specific expression, of a YAC transgene was disrupted when the primary DMR sequence was deleted [34]. Also, deletion by homologous recombination in cultured cells of a DMR within an intron of the human potassium voltage-gated channel, subfamily Q, member-1 (KCNQ1) gene resulted in loss of monoallelic expression of this gene [35]. Evidence that the methylation contained within DMR sequences is important for monoallelic expression has been provided by genetic manipulation at the DNA methyltransferase (cytosine-5) (Dnmt) locus in mice. Homozygous mutant or Dnmt-/- embryos have hypomethylated genomes and die early in embryogenesis. They also lack methylation at DMRs and monoallelic expression of imprinted genes is disrupted [36-39]. Furthermore, when a Dnmt cDNA was knocked-in to the mutant locus in hypomethylated Dnmt-/- embryonic stem (ES) cells, derived somatic cells in chimeras had normal levels of genome-wide methylation but still lacked in primary DMR methylation and monoallelic expression, i.e., imprinting remained disrupted despite restoration of genome-wide methylation [40]. These experiments suggest that at least primary DMR methylation is important for maintaining monoallelic expression in somatic cells.
Imprinting Control Regions (ICRs)
A DMR is also termed an ICR if shown to be primarily important for imprinting through experimental analysis, such as by deletion studies in transgenic mice. Thus, as described above, the primary DMRs of Igf2/H19, Igf2r, and possibly mouse Kcnq1, are ICRs. How does an ICR act to regulate monoallelic expression in somatic cells? There are two components in the answer to this question: A) ICRs appear to be gene regulatory elements that can significantly affect the expression of genes in cis (Fig. 1
). The Igf2/H19 ICR is a chromatin insulator that blocks the interaction of enhancers with the Igf2 promoter [41-46], while the Igf2r and Kcnq1 ICRs contain the promoter of an antisense RNA that may cause repression of the sense transcript in cis [23, 47, 48]; B) These ICR regulatory elements can be turned "off" on one parental allele and "on" on the other parental allele by the presence and absence of DNA methylation, respectively, as inherited from the germ line. This brings about allelic activity and inactivity in trans, or monoallelic expression. For the Igf2/H19 insulator, methylation blocks the binding of the protein CTCF that is responsible for insulator activity [41, 42, 44, 46]. For the Igf2r and Kcnq1 antisense promoters, it is suggested that methylation may render them inactive [23, 34, 35, 47, 48], and certainly there is precedence for promoter silencing by methylation [17]. In this second component, gene regulatory element "methylation switching" in the germ line, inherited by and maintained in somatic cells, appears to lie the uniqueness of genomic imprinting as a system of gene regulation.
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GERM CELLS—INHERITED AND NEW IMPRINTS
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PGCs, like somatic cells, presumably also inherit one maternally and paternally imprinted genome (inherited imprints). Then, as they develop and differentiate, certainly beyond the stage of sexual differentiation at 11.5 dpc, both genomes must become maternalized or paternalized according to germ cell sex (new imprints). If allele-specific methylation, and in particular methylation of ICRs, is the key epigenetic component of genomic imprinting, then maternalization and paternalization of germ cells can be considered equivalent to the gaining or losing of ICR methylation. The only information published on this subject pertains to the H19/Igf2 ICR: for the female germ line, both alleles become fully unmethylated or maternalized in fetuses at 16.5 days post-coitum (dpc), while for the male germ line, both alleles become fully methylated or paternalized by 18.5 dpc [49, 50] (Fig. 2). This establishment of ICR methylation in male germ cells corresponds to the time of genome-wide de novo methylation, occurring in germ cells of both sexes from approximately 15.5 dpc to 18.5 dpc when neither germ line is dividing [51, 52]. Therefore it is possible the two processes may be connected. Other studies indicate that imprints can also be laid down at later times during oocyte growth: parthenogenones constructed by combining a nucleus from a non-growing oocyte and a fully grown oocyte fare better developmentally than standard parthenogenones which are made by combining two nuclei from fully grown oocytes [53].
If ICR methylation status is the means by which imprinting information is carried from the gametes to the embryo, then how these states are formed in the germ line is the imprinting "mechanism," and remains entirely undefined. Female and male germ cells have many similarities, but they are also very different. In the simplest terms, there must exist a primary protein difference between them that results in a DMR sequence becoming methylated in one sex of germ line and not in the other. What characteristics might the protein difference have? There are a number of possibilities: A) Differential expression or availability of a trans-acting factor that can influence epigenetic modification, e.g., a DNA methyltransferase enzyme. An isoform of DNMT resulting from differential RNA splicing is present in oocytes and differentially distributed at the subcellular level in respect to the usual form of the protein [54]. Other methyltransferases with significant de novo activity have been identified and are essential for normal embryonic development, DNMTIIIA and DNMTIIIB, and a role for these enzymes in imprinting has been postulated [55]. As another example, if the binding of CTCF, the insulator protein of the Igf2/H19 ICR, also served to inhibit methylation, then its lack of expression in the male germ line would lead to methylation of this region. As suggested previously, this possibility is unlikely as CTCF is probably ubiquitous. However, it is possible there may be differential expression of a protein that modulates CTCF binding, or another protein which can bind to the ICR and influence methylation status [46]. B) Differential expression of chromatin proteins. Different forms or ratios of histone proteins in the two germ lines might induce a differential chromatin structure of DMR sequences which renders them differentially amenable to methylation. C) Differential expression of proteins responsible for erasing inherited ICR methylation. Recent data as discussed for the Igf2-H19 ICR suggest that in male germ cells, methylation of the ICR sequence is never completely erased before de novo methylation begins, while in female germ cells this inherited methylation is fully erased [50]. Thus, in male germ cells only, non-erased or residual methylation might be responsible for the seeding of new methylation. Of course, such a mechanism would require an unlikely transvection component to explain how methylation of the unmethylated maternally inherited DMR is achieved, although it is interesting that this is delayed relative to the paternal homologue [49, 50].
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GERM CELLS AND EXPRESSION OF IMPRINTED GENES
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Irrespective of the presence of inherited or new imprints in germ cells, by the time migrating PGCs have arrived at the genital ridge, imprinted genes are expressed biallelically—like non-imprinted genes [56], and this mode of expression persists throughout the rest of female and male germ cell development [56, 57]. This germ cell-specific biallelic expression does not appear to be related to low-level expression, for at least two imprinted genes, Igf2r and Snrpn, the RNA concentration in PGCs is at least as high as in monoallelically expressing gonadal somatic cells [56]. Two extreme alternative explanations for this phenomenon seem possible: A) The complete array of epigenetic modifications necessary for maternal- or paternal-specific expression is never in place in germ cells. In early PGCs some inherited modifications may already be erased and in later germ cells new modifications may not be complete. The missing modifications may be secondary somatic cell-specific modifications that, in conjunction with primary imprints from germ cells, are required to achieve the final expression state, e.g., promoter inactivation. B) The complete array of epigenetic modifications is in place, but has no influence over the germ cell's transcriptional machinery, i.e., the concept of imprinting "neutralization" [56]. Deciding which of these two possibilities is correct is not possible until there is a better picture of the allele-specific expression and methylation profiles of imprinted genes. Also, the answer may be different depending on the gene. Nevertheless, some level of imprinting neutralization appears to exist in that Igf2 expression persists in female germ cells despite lack of ICR methylation and therefore potential insulator activity of this sequence [56].
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ASYNCHRONOUS DNA REPLICATION
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Replication timing is another mechanism that can explain the maintenance of differential chromatin states through cell division [58], and the activity state of an imprinted allele has been shown to be correlated with the time of DNA replication [59, 60]. However, a lack of correlation has been shown in other studies [61, 62]. The inherited pattern of asynchronous replication of imprinted genes is maintained in germ cells as in somatic cells, and is not erased in both sexes until the onset of meiosis [63]. Thus, in the germ line, asynchronous replication is maintained well beyond the stage that imprinted genes become biallelically expressed. These observations could be interpreted to mean that asynchronous DNA replication plays no role in maintaining monoallelic expression in somatic cells and germ cells and is merely correlative. Alternatively, if replication timing is crucial for monoallelic expression in somatic cells, then in germ cells the coincidence of biallelic expression and asynchronous replication, e.g., in postmigratory PGCs, is a clear example of imprinting neutralization. In terms of a role for replication timing in imprinting establishment, it has been shown that in the male germ line, Igf2 adopts the paternal-specific pattern of early DNA replication as early as the primitive spermatogonial stage [63]. This is just after the developmental phase that the Igf2/H19 ICR has adopted its methylation imprints—at late gestation during the period of mitotic arrest. Therefore it is possible that the early timing of Igf2 replication in primitive spermatogonia is a consequence of imprints imparted during the earlier non-replicative phase, and is therefore secondary to the imprinting process.
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IMPRINTING IN ES CELLS
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ES cells are a cell culture derivative of the blastocyst inner cell mass (ICM), the latter which gives rise to the embryo, the amnion, the yolk sac and the chrorioallantoic portion of the placenta. Blastocyst injection chimera experiments show that ES cells are similar to early-stage ICM cells in that they contribute to the primitive ectoderm and endoderm derivatives [64]. However, it is probably not possible to equate these two cell types as ES cells appear to be produced by the cell culture environment and have no exact counterpart in the blastocyst. Instead, ES cells could be thought of as being ICM cells which, instead of undergoing rapid differentiation as they would in vivo, are abnormally locked into continuing cycles of division in the undifferentiated state by virtue of the action of exogenous factors. In the mouse, leukemia inhibitory factor (LIF) is one such factor [65, 66] and is indispensable for the propagation of ES cells. This is evident in the failed growth of ES cells on LIF-deficient fibroblast feeder layers [67].
ES cells retain imprints as assessed by the developmental potential of chimeras [68-70]. Nevertheless, their derivation and unlimited capacity for division can result in epigenetic change. For example, methylation of the paternal H19 allele in ES cells appears to resemble more the later somatic cell pattern rather than the pattern in the ICM, suggesting that this methylation is more a function of the number of cell cycles in development rather than the stage of differentiation [30, 71-73]. In addition, the allele-specific methylation and expression patterns of imprinted genes in ES cells are unstable with passage [71]. While ES cells retain imprints as ascertained by the developmental potential of chimeras, they also appear to lose some. Parent-specific expression of imprinted genes is destabilized in ES cells and cannot be corrected upon differentiation of the cells in chimeras [71, 74]. Also, this destabilization may contribute to the developmental abnormalities observed [74, 75]. However, despite continued interest in this area, there is no reason to suspect the deregulation of imprinted genes over non-imprinted genes in developmental abnormalities generated by ES cells, or by the use of ES or somatic cell nuclei in animal cloning: as far as we know, imprinted genes are a special case only in that each of the parental alleles is regulated by a differing epigenetic mechanism, while for non-imprinted genes each allele is regulated by the same epigenetic mechanism. Thus, epigenetic deregulation of imprinted and non-imprinted alleles in experimental situations would appear equally likely.
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CONCLUSIONS
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We have addressed two broad aspects of genomic imprinting in respect to the germ line. Due to the relative inaccessibility of this cell lineage and to its "mystery," both aspects remain poorly explored. The first involves when and how genomic imprints are erased and established in germ cells, and could be considered equivalent to the gaining or losing of ICR methylation. An understanding of how differential germ cell ICR methylation is brought about is now one of the most important questions in genomic imprinting as it could be considered the imprinting mechanism. The second is that there is increasing evidence for a lack of regard of the germ cell's transcriptional machinery to the presence of imprints, i.e., imprinting neutralization, and this may prove to be another unique feature of the germ line. Further exploration of this phenomenon should provide a better understanding of how imprints work to regulate the expression of genes in cis. Also, these endeavors may shed light on broader questions of germ cell biology in that imprinting neutralization may share a common mechanism with other unique epigenetic states of germ cells, e.g., global hypomethylation and totipotency, perhaps in some germ cell-specific aspect of chromatin structure.
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Acknowledgements
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I thank my colleagues Piroska Szabó, Michael Reed, and Art Riggs for stimulating discussions. Supported by NIH Grant 1RO1GM58803-01A1.
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REFERENCES
|
|---|
-
McLaren A. Introduction. In: Germline Development. Ciba Foundation Symposium 182. Chichester: John Wiley & Sons, 1994:1.
-
Mittwoch U. Parthenogenesis. J Med Genet 1978;15:165–181.[Medline]
-
Stevens LC. Totipotent cells of parthenogenetic origin in a chimaeric mouse. Nature 1978;276:266–267.[CrossRef][Medline]
-
Stevens LC. Origin of testicular teratomas from primordial germ cells in mice. J Natl Cancer Inst 1967;38:549–552.
-
Corley-Smith GE, Lim CJ, Brandhorst BP. Production of androgenetic zebrafish (Danio rerio). Genetics 1996;142:1265–1276.[Abstract]
-
Parsons JE, Thorgaard GH. Production of androgenetic diploid rainbow trout. J Hered 1985;76:177–181.[Abstract/Free Full Text]
-
Scheerer PD, Thorgaard GH, Allendorf FW. Genetic analysis of androgenetic rainbow trout. J Exp Zool 1991;260:382–390.[CrossRef][Medline]
-
Lyon MF, Glenister PH. Factors affecting the observed number of young resulting from adjacent-2 disjunction in mice carrying a translocation. Genet Res 1977;29:83–92.[Medline]
-
Mann JR, Lovell-Badge RH. Inviability of parthenogenones is determined by pronuclei, not egg cytoplasm. Nature 1984;310:66–67.[CrossRef][Medline]
-
McGrath J, Solter D. Completion of mouse embryogenesis requires both the maternal and paternal genomes. Cell 1984;37:179–183.[CrossRef][Medline]
-
Surani MA, Barton SC, Norris ML. Development of reconstituted mouse eggs suggests imprinting of the genome during gametogenesis. Nature 1984;308:548–550.[CrossRef][Medline]
-
Cattanach BM, Kirk M. Differential activity of maternally and paternally derived chromosome regions in mice. Nature 1985;315:496–498.[CrossRef][Medline]
-
DeChiara TM, Robertson EJ, Efstratiadis A. Parental imprinting of the mouse insulin-like growth factor II gene. Cell 1991;64:849–859.[CrossRef][Medline]
-
Riggs AD, Porter TN. Overview of epigenetic mechanisms. In: Russo VEA, Martienssen RA, Riggs AD, eds. Epigenetic Mechanisms of Gene Regulation. Cold Spring Harbor: Cold Spring Harbor Laboratory Press, 1996:29-45.
-
Szabó PE, Mann JR. Maternal and paternal genomes function independently in mouse ova in establishing expression of the imprinted genes Snrpn and Igf2r: no evidence for allelic trans-sensing and counting mechanisms. EMBO J 1996;15:6018–6025.[Medline]
-
Riggs AD. DNA methylation and cell memory. Cell Biophys 1989;15:1–13.[Medline]
-
Jones PA. The DNA methylation paradox. Trends Genet 1999;15:34–37.[CrossRef][Medline]
-
Shemer R, Razin A. Establishment of imprinted methylation patterns during development. In: Russo VEA, Martienssen RA, Riggs AD, eds. Epigenetic Mechanisms of Gene Regulation. Cold Spring Harbor: Cold Spring Harbor Laboratory Press, 1996:215-229.
-
Constancia M, Pickard B, Kelsey G et al. Imprinting mechanisms. Genome Res 1998;8:881–900.[Abstract/Free Full Text]
-
Gabriel JM, Gray TA, Stubbs L et al. Structure and function correlations at the imprinted mouse Snrpn locus. Mamm Genome 1998;9:788–793.[CrossRef][Medline]
-
Shibata H, Yoda Y, Kato R et al. A methylation imprint mark in the mouse imprinted gene Grf1/Cdc25Mm locus shares a common feature with the U2afbp-rs gene: an association with a short tandem repeat and a hypermethylated region. Genomics 1998;49:30–37.[CrossRef][Medline]
-
Pearsall RS, Plass C, Romano MA et al. A direct repeat sequence at the Rasgrf1 locus and imprinted expression. Genomics 1999;55:194–201.[CrossRef][Medline]
-
Smilinich NJ, Day CD, Fitzpatrick GV et al. A maternally methylated CpG island in KvLQT1 is associated with an antisense paternal transcript and loss of imprinting in Beckwith-Wiedemann syndrome. Proc Natl Acad Sci USA 1999;96:8064–8069.[Abstract/Free Full Text]
-
Peters J, Wroe SF, Wells CA et al. A cluster of oppositely imprinted transcripts at the Gnas locus in the distal imprinting region of mouse chromosome 2. Proc Natl Acad Sci USA 1999;96:3830–3835.[Abstract/Free Full Text]
-
Li LL, Szeto IY, Cattanach BM et al. Organization and parent-of-origin-specific methylation of imprinted Peg3 gene on mouse proximal chromosome 7. Genomics 2000;63:333–340.[CrossRef][Medline]
-
Stoger R, Kubicka P, Liu C-G et al. Maternal-specific methylation of the imprinted mouse Igf2r locus identifies the expressed locus as carrying the imprinting signal. Cell 1993;73:61–71.[CrossRef][Medline]
-
Bartolomei MS, Webber AL, Brunkow ME et al. Epigenetic mechanisms underlying the imprinting of the mouse H19 gene. Genes Dev 1993;7:1663–1673.[Abstract/Free Full Text]
-
Ferguson-Smith AC, Sasaki H, Cattanach BM et al. Parental-origin-specific epigenetic modification of the mouse H19 gene. Nature 1993;362:751–755.[CrossRef][Medline]
-
Feil R, Walter J, Allen ND et al. Developmental control of allelic methylation in the imprinted mouse Igf2 and H19 genes. Development 1994;120:2933–2943.[Abstract]
-
Tremblay KD, Duran KL, Bartolomei MS. A 5' 2-kilobase-pair region of the imprinted mouse H19 gene exhibits exclusive paternal methylation throughout development. Mol Cell Biol 1997;17:4322–4329.[Abstract]
-
Olek A, Walter J. The pre-implantation ontogeny of the H19 methylation imprint. Nat Genet 1997;17:275–276.[CrossRef][Medline]
-
Neumann B, Kubicka P, Barlow DP. Characteristics of imprinted genes. Nat Genet 1995;9:12–13.[CrossRef][Medline]
-
Thorvaldsen JL, Duran KL, Bartolomei MS. Deletion of the H19 differentially methylated domain results in loss of imprinted expression of H19 and Igf2. Genes Dev 1998;12:3693–3702.[Abstract/Free Full Text]
-
Wutz A, Smrzka OW, Schweifer N et al. Imprinted expression of the Igf2r gene depends on an intronic CpG island. Nature 1997;389:745–749.[CrossRef][Medline]
-
Horike S, Mitsuya K, Meguro M et al. Targeted disruption of the human LIT1 locus defines a putative imprinting control element playing an essential role in Beckwith-Wiedemann syndrome. Hum Mol Genet 2000;9:2075–2083.[Abstract/Free Full Text]
-
Li E, Bestor TH, Jaenisch R. Targeted mutation of the DNA methyltransferase gene results in embryonic lethality. Cell 1992;69:915–926.[CrossRef][Medline]
-
Li E, Beard C, Forster AC et al. DNA methylation, genomic imprinting, and mammalian development. Cold Spring Harbor Symposia on Quantitative Biology, vol. LVIII. Cold Spring Harbor: Cold Spring Harbor Laboratory Press, 1993:297-305.
-
Li E, Beard C, Jaenisch R. Role for DNA methylation in genomic imprinting. Nature 1993;366:362–365.[CrossRef][Medline]
-
Caspary T, Cleary MA, Baker CC et al. Multiple mechanisms regulate imprinting of the mouse distal chromosome 7 gene cluster. Mol Cell Biol 1998;18:3466–3474.[Abstract/Free Full Text]
-
Tucker KL, Beard C, Dausmann J et al. Germ-line passage is required for establishment of methylation and expression patterns of imprinted but not of nonimprinted genes. Genes Dev 1996;10:1008–1020.[Abstract/Free Full Text]
-
Bell AC, Felsenfeld G. Methylation of a CTCF-dependent boundary controls imprinted expression of the Igf2 gene. Nature 2000;405:482–485.[CrossRef][Medline]
-
Hark AT, Schoenherr CJ, Katz DJ et al. CTCF mediates methylation-sensitive enhancer-blocking activity at the H19/Igf2 locus. Nature 2000;405:486–489.[CrossRef][Medline]
-
Kaffer CR, Srivastava M, Park KY et al. A transcriptional insulator at the imprinted H19/Igf2 locus. Genes Dev 2000;14:1908–1919.[Abstract/Free Full Text]
-
Kanduri C, Pant V, Loukinov D et al. Functional association of CTCF with the insulator upstream of the H19 gene is parent of origin-specific and methylation-sensitive. Curr Biol 2000;10:853–856.[CrossRef][Medline]
-
Kanduri C, Holmgren C, Pilartz M et al. The 5' flank of mouse H19 in an unusual chromatin conformation unidirectionally blocks enhancer-promoter communication. Curr Biol 2000;10:449–457.[CrossRef][Medline]
-
Szabó PE, Tang S-HE, Rentsendorj A et al. Maternal-specific footprints at putative CTCF sites in the H19 imprinting control region give evidence for insulator function. Curr Biol 2000;10:607–610.[CrossRef][Medline]
-
Mitsuya K, Meguro M, Lee MP et al. LIT1, an imprinted antisense RNA in the human KvLQT1 locus identified by screening for differentially expressed transcripts using monochromosomal hybrids. Hum Mol Genet 1999;8:1209–1217.[Abstract/Free Full Text]
-
Lyle R, Watanabe D, te Vruchte D et al. The imprinted antisense RNA at the Igf2r locus overlaps but does not imprint Mas1. Nat Genet 2000;25:19–21.[CrossRef][Medline]
-
Davis TL, Trasler JM, Moss SB et al. Acquisition of the H19 methylation imprint occurs differentially on the parental alleles during spermatogenesis. Genomics 1999;58:18–28.[CrossRef][Medline]
-
Ueda T, Abe K, Miura A et al. The paternal methylation imprint of the mouse H19 locus is acquired in the gonocyte stage during foetal testis development. Genes Cells 2000;5:649–659.[Abstract]
-
Coffigny H, Bourgeois C, Ricoul M et al. Alterations of DNA methylation patterns in germ cells and Sertoli cells from developing mouse testis. Cytogenet Cell Genet 1999;87:175–181.[CrossRef][Medline]
-
Kafri T, Ariel M, Brandeis M et al. Developmental pattern of gene-specific DNA methylation in the mouse embryo and germ line. Genes Dev 1992;6:705–714.[Abstract/Free Full Text]
-
Kono T. Influence of epigenetic changes during oocyte growth on nuclear reprogramming after nuclear transfer. Reprod Fertil Dev 1998;10:593–598.[CrossRef][Medline]
-
Mertineit C, Yoder JA, Taketo T et al. Sex-specific exons control DNA methyltransferase in mammalian germ cells. Development 1998;125:889–897.[Abstract]
-
Okano M, Bell DW, Haber DA et al. DNA methyltransferases Dnmt3a and Dnmt3b are essential for de novo methylation and mammalian development. Cell 1999;99:247–257.[CrossRef][Medline]
-
Szabó PE, Mann JR. Biallelic expression of imprinted genes in the mouse germ line: implications for erasure, establishment, and mechanisms of genomic imprinting. Genes Dev 1995;9:1857–1868.[Abstract/Free Full Text]
-
Villar AJ, Eddy EM, Pedersen RA. Developmental regulation of genomic imprinting during gametogenesis. Dev Biol 1995;172:264–271.[CrossRef][Medline]
-
Riggs AD, Pfeifer GP. X-chromosome inactivation and cell memory. Trends Genet 1992;8:169–174.[Medline]
-
Izumikawa Y, Naritomi K, Hirayama K. Replication asynchrony between homologs 15q11.2: cytogenetic evidence for genomic imprinting. Hum Genet 1991;87:1–5.[CrossRef][Medline]
-
Kitsberg D, Selig S, Brandeis M et al. Allele-specific replication timing of imprinted gene regions. Nature 1993;364:459–463.[CrossRef][Medline]
-
Kawame H, Gartler SM, Hansen RS. Allele-specific replication timing in imprinted domains: absence of asynchrony at several loci. Hum Mol Genet 1995;4:2287–2293.[Abstract/Free Full Text]
-
Shuster M, Dhar MS, Olins AL et al. Parental alleles of an imprinted mouse transgene replicate synchronously. Dev Genet 1998;23:275–284.[CrossRef][Medline]
-
Simon I, Tenzen T, Reubinoff BE et al. Asynchronous replication of imprinted genes is established in the gametes and maintained during development. Nature 1999;401:929–932.[CrossRef][Medline]
-
Beddington RSP, Robertson EJ. An assessment of the developmental potential of embryonic stem cells in the midgestation mouse embryo. Development 1989;105:733–737.[Abstract/Free Full Text]
-
Smith AG, Heath JK, Donaldson DD et al. Inhibition of pluripotential embryonic stem cell differentiation by purified polypeptides. Nature 1988;336:688–690.[CrossRef][Medline]
-
Williams RL, Hilton DJ, Pease S et al. Myeloid leukemia inhibitory factor maintains the developmental potential of embryonic stem cells. Nature 1988;336:684–687.[CrossRef][Medline]
-
Stewart CL, Kaspar P, Brunet LJ et al. Blastocyst implantation depends on maternal expression of leukemia inhibitory factor. Nature 1992;359:76–79.[CrossRef][Medline]
-
Mann JR, Stewart CL. Development to term of mouse androgenetic aggregation chimeras. Development 1991;113:1325–1333.[Abstract]
-
Mann JR. Properties of androgenetic and parthenogenetic mouse embryonic stem cell lines; are genetic imprints conserved? Semin Dev Biol 1992;3:77–85.
-
Allen ND, Barton SC, Hilton K et al. A functional analysis of imprinting in parthenogenetic embryonic stem cells. Development 1994;120:1473–1482.[Abstract]
-
Szabó P, Mann JR. Expression and methylation of imprinted genes during in vitro differentiation of mouse parthenogenetic and androgenetic embryonic stem cell lines. Development 1994;120:1651–1660.[Abstract]
-
Szabó PE, Pfeifer GP, Mann JR. Characterization of novel parent-specific epigenetic modifications upstream of the imprinted mouse H19 gene. Mol Cell Biol 1998;18:6767–6776.[Abstract/Free Full Text]
-
Warnecke PM, Mann JR, Frommer M et al. Bisulfite sequencing in preimplantation embryos: DNA methylation profile of the upstream region of the mouse imprinted H19 gene. Genomics 1998;51:182–190.[CrossRef][Medline]
-
Dean W, Bowden L, Aitchison A et al. Altered imprinted gene methylation and expression in completely ES cell-derived mouse fetuses: association with aberrant phenotypes. Development 1998;125:2273–2282.[Abstract]
-
Nagy A, Rossant J, Nagy R et al. Derivation of completely cell culture-derived mice from early-passage embryonic stem cells. Proc Natl Acad Sci USA 1993;90:8424–8428.[Abstract/Free Full Text]
-
Srivastava M, Hsieh S, Grinberg A et al. H19 and Igf2 monoallelic expression is regulated in two distinct ways by a shared cis acting regulatory region upstream of H19. Genes Dev 2000;14:1186–1195.[Abstract/Free Full Text]
Received February 16, 2001;
accepted for publication March 28, 2001.
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Abstract
Introduction
) as inherited from the egg, and binds CTCF (
). This imparts insulator or chromatin boundary activity (
) and the Igf2 promoters (TATA) and Igf2/H19 enhancers (
) cannot interact. The maternal Igf2 promoters are therefore silent (
). The H19 promoter, on the same side of the insulator as the enhancers, can interact, therefore transcription proceeds (
). The maternal H19 promoter is occupied by transcription factors (
). Paternal chromosome (Pat); ICR region is methylated (
) as inherited from sperm. CTCF cannot bind hence ICR has no insulator activity. This allows Igf2 promoters and enhancers to interact and allow transcription. Early in postimplantation development, the H19 promoter is inactivated by a spreading of ICR methylation to form a secondary DMR and by packaging into a closed chromatin structure or rotationally positioned nucleosomes (
). This inactivation is ICR-dependent [
), partially methylated (
). H19 promoter methylation: methylated (
), non-methylated (
), partially methylated (
).