Showing papers on "Genomic imprinting published in 1995"

Disruption of imprinting caused by deletion of the H19 gene region in mice

Abstract: The imprinted H19 gene, which encodes an untranslated RNA, lies at the end of a cluster of imprinted genes in the mouse. Imprinting of the insulin-2 and insulin-like growth factor 2 genes, which lie about 100 kilobases upstream of H19, can be disrupted by maternal inheritance of a targeted deletion of the H19 gene and its flanking sequence. Animals inheriting the H19 mutation from their mothers are 27% heavier than those inheriting it from their fathers. Paternal inheritance of the disruption has no effect, which presumably reflects the normally silent state of the paternal gene. The somatic overgrowth of heterozygotes for the maternal deletion is attributed to a gain of function of insulin-like growth factor 2, rather than a loss of function of H19.

Inherited microdeletions in the Angelman and Prader-Willi syndromes define an imprinting centre on human chromosome 15.

Abstract: A subset of patients with Angelman and Prader-Willi syndrome have apparently normal chromosomes of biparental origin, but abnormal DNA methylation at several loci within chromosome 15q11-13, and probably have a defect in imprinting. Using probes from a newly established 160-kb contig including D15S63 (PW71) and SNRPN, we have identified inherited microdeletions in two AS families and three PWS families. The deletions probably affect a single genetic element that we term the 15q11-13 imprinting centre (IC). In our model, the IC regulates the chromatin structure, DNA methylation and gene expression in cis throughout 15q11-13. Mutations of the imprinting centre can be transmitted silently through the germline of one sex, but appear to block the resetting of the imprint in the germline of the opposite sex.

Gametic imprinting in mammals.

Abstract: Embryonic development in mammals is distinct from that in other vertebrates because it depends on a small number of imprinted genes that are specifically expressed from either the maternal or paternal genome. Why mammals are uniquely dependent on sexual reproduction and how this dependency is dictated at a molecular level are questions that have been intensively investigated during the past 2 years. Gene inactivation experiments have confirmed predictions that imprinted genes regulate embryonic and placental growth and that DNA methylation is part of the imprinting mechanism. Despite these considerable achievements, the reason why imprinted hemizygosity is used as a mechanism to regulate the intrauterine growth of mammalian embryos remains elusive.

A paternal-specific methylation imprint marks the alleles of the mouse H19 gene.

Abstract: Imprinting, the differential expression of the two alleles of a gene based on their parental origin, requires that the alleles be distinguished or marked. A candidate for the differentiating mark is DNA methylation. The maternally expressed H19 gene is hypermethylated on the inactive paternal allele in somatic tissues and sperm, but to serve as the mark that designates the imprint, differential methylation must also be present in the gametes and the pre-implantation embryo. We now show that the pattern of differential methylation in the 5' portion of H19 is established in the gametes and a subset is maintained in the pre-implantation embryo. That subset is sufficient to confer monoallelic expression to the gene in blastocysts. We propose that paternal-specific methylation of the far 5' region is the mark that distinguishes the two alleles of H19.

An enhancer deletion affects both H19 and Igf2 expression.

Abstract: The distal end of mouse Chromosome 7 contains four tightly linked genes whose expression is dependent on their parental inheritance. Mash-2 and H19 are expressed exclusively from the maternal chromosome, whereas Insulin-2 (Ins-2) and Insulin-like growth factor 2 (Igf2) are paternally expressed. The identical expression during development of the 3'-most genes in the cluster, Igf2 and H19, led to the proposal that their imprinting was mechanistically linked through a common set of transcriptional regulatory elements. To test this hypothesis, a targeted deletion of two endoderm-specific enhancers that lie 3' of H19 was generated by homologous recombination in embryonic stem cells. Inheritance of the enhancer deletion through the maternal lineage led to a loss of H19 gene expression in cells of endodermal origin, including cells in the liver, gut, kidney, and lung. Paternal inheritance led to a very similar loss in the expression of Igf2 RNA in the same tissues. These results establish that H19 and Igf2 utilize the same endoderm enhancers, but on different parental chromosomes. Mice inheriting the enhancer deletion from fathers were 80% of normal size, reflecting a partial loss-of-function of Igf2. The reduction was uniformly observed in a number of internal organs, indicating that insulin-like growth factor II (IGFII), the product of Igf2, acts systemically in mice to affect prenatal growth. A modest decline in Ins-2 RNA was observed in the yolk sac. In contrast Mash-2, which is expressed in spongiotrophoblast cells of the placenta, was unaffected by the enhancer deletion.

Genomic imprinting of Mash2, a mouse gene required for trophoblast development

Abstract: The mouse gene Mash2 encodes a transcription factor required for development of trophoblast progenitors. Mash2- homozygous mutant embryos die at 10 days post−coitum from placental failure. Here we show that Mash2 is genomically imprinted. First, Mash2+/- embryos inheriting a wild−type allele from their father die at the same stage as -/- embryos, with a similar placental phenotype. Second, the Mash2 paternal allele is initially expressed by groups of trophoblast cells at 6.5 and 7.5 days post−coitum, but appears almost completely repressed by 8.5 days post−coitum. Finally, we have genetically and physically mapped Mash2 to the distal region of chromosome 7, within a cluster of imprinted genes, including insulin−2, insulin−like growth factor−2 and H19.

Uniparental disomy in humans: development of an imprinting map and its implications for prenatal diagnosis

Abstract: Uniparental disomy (UPD) in humans is caused primarily by meiotic nondisjunction events, followed by trisomy or monosomy 'rescue'. The majority of cases appear to be associated with advanced maternal age, and may be initially detected as mosaic trisomies during routine prenatal diagnosis by chorionic villus sampling or amniocentesis. In addition, structural abnormalities including Robertsonian translocations, reciprocal translocations and supernumerary marker chromosomes appear to be associated with an increased risk of UPD. Predicting the phenotypic effects of UPD is complex, as three independent factors are involved : (i) effects of trisomy on the placenta or the fetus ; (ii) autosomal recessive disease due to reduction to homozygosity ; and (iii) imprinted gene effects for some chromosomes. To date, UPD in humans has been reported for 25 of the 47 possible uniparental types. Imprinting effects have been established with certainty for four human chromosomes that have homology to mouse chromosomes which have been shown to have significant phenotypic effects in uniparental animals. A normal phenotype has been reported for 14 other UPD types. Thus, collection of data on UPD cases in humans is providing an imprinting map analogous to the experimentally derived imprinting map in mouse. This human imprinting map has important clinical implications, particularly in the area of prenatal diagnosis.

Peg1/Mest imprinted gene on chromosome 6 identified by cDNA subtraction hybridization.

Abstract: Parthenogenesis in the mouse is embryonic lethal partly because of imprinted genes that are expressed only from the paternal genome. In a systematic screen using subtraction hybridization between cDNAs from normal and parthenogenetic embryos, we initially identified two apparently novel imprinted genes, Peg1 and Peg3. Peg1 (paternally expressed gene 1) or Mest, the first imprinted gene found on the mouse chromosome 6, may contribute to the lethality of parthenogenones and of embryos with a maternal duplication for the proximal chromosome 6. Peg1/Mest is widely expressed in mesodermal tissues and belongs to the alpha/beta hydrolase fold family. A similar approach with androgenones can be used to identify imprinted genes that are expressed from the maternal genome only.

DNA methylation in early development

Abstract: Several lines of evidence strongly suggest that DNA methylation is involved in embryo development. Perhaps the most direct evidence comes from experiments with methyltransferase deficient mice. Embryos that express low levels of the maintenance methyltransferase do not develop to term and die at the 5 to 20 somite stage corresponding to the level of the enzyme. In the mouse, dramatic methylation changes have been observed during the early steps of embryo development. Most genes are subject to a process of active demethylation starting promptly after fertilization. Except for a small number of methylated CpG sites in imprinted genes the vast majority of the sites are unmethylated by the stage of cavitation (16 cell). Such genome-wide demethylation may signify an erasure of epigenetic information originating in the highly differentiated gametes. This erasure may be essential for the establishment of a pluripotent state, before specific cell lineages can be determined. The process of laying down a new developmental program involves, initially, global de novo methylation at the stage of pregastrulation followed by gene specific demethylations associated with the onset of activity of these genes. CpG islands characteristic of housekeeping genes, appear to be protected from the global de novo methylation. An exception to this rule is the hypermethylation of CpG islands in X-linked housekeeping genes on the inactive X chromosome and of specific differentially methylated CpG sites in imprinted genes. Primordial germ cells escape the global de novo methylation which takes place at the pregastrula stage and undergo a very similar de novo methylation process in the differentiated gonads (15.5-18.5 days post coitum), forming the methylation patterns which are specific to the gametes. Specific demethylations then form a terminal methylation pattern which is then clonaly inherited in the soma and erased after fertilization.

Genomic imprinting of p57KIP2, a cyclin-dependent kinase inhibitor, in mouse

Abstract: p57KIP2 is a potent tight-binding inhibitor of several G1 cyclin/Cdk complexes, and is a negative regulator of cell proliferation. The gene encoding human p57KIP is located on chromosome 11p15.5 (ref. 2), a region implicated in both sporadic cancers and Beckwith-Wiedemann syndrome, a familial cancer syndrome, marking it a tumour suppressor candidate. Several types of childhood tumours including Wilm's tumour, adrenocortical carcinoma and rhabdomyosarcoma display a specific loss of maternal 11p15 alleles, suggesting that genomic imprinting plays an important part. Genetic analysis of the Beckwith-Wiedemann syndrome has indicated maternal carriers as well as suggested a role in genomic imprinting. Here, as a first step towards elucidating the genesis of human cancers in this region, we showed that a mouse homologue of p57KIP2 is genomically imprinted. The paternally inherited allele is transcriptionally repressed and methylated. This murine gene maps to the distal region of chromosome 7, within a cluster of imprinted genes, including insulin-2, insulin-like growth factor-2, H19 and Mash2 (refs 14-18).

Biallelic expression of imprinted genes in the mouse germ line: implications for erasure, establishment, and mechanisms of genomic imprinting.

Abstract: Genomic imprinting in mammals determines parental-specific (monoallelic) expression of a relatively small number of genes during development. Imprinting must logically be imparted in the germ line, where inherited maternal and paternal imprinting is erased and new imprinting established according to the individual's sex. We have assessed the allele-specific expression of four imprinted genes, two of which exhibit maternal-speeitic (H19 and Ig/2r} and two of which exhibit paternal-specific (lg/2 and Snrpn) monoaUelie somatic expression, in the germ line of F1 hybrid mice utilizing quantitative RT-PCR single-nueleotide primer extension assays. The expression of each gene was biaUelic in the female and male germ line from the time that migratory mitotic PGCs entered the embryonic genital ridge and throughout gametogenesis, except that H19 RNA was not detected late in gametogenesis. These findings demonstrate that inherited imprinting is erased, or not recognized, in germ cells by the time of genital ridge colonization; also that new imprinting may not be established until late in gametogenesis, or that it is incomplete or not recognized at this stage. Regardless of imprinting status, a generalized neutralization of imprinting is evident in the germ line, associated with the totipotent state of this unique cell lineage.

Imprinting mutations in the Beckwith—Wiedemann syndrome suggested by an altered imprinting pattern in the IGF2–H19 domain

Abstract: Regional regulation of parental imprinting in the IGF2-H19 domain of imprinted genes was studied in the Beckwith-Wiedemann syndrome (BWS). We identified BWS patients who had inherited a normal biparental chromosome complement of the chromosome 11p15.5 region (where IGF2 and H19 reside), but had an altered pattern of allelic methylation of both genes, with the maternal chromosome carrying a paternal imprinting pattern. In fibroblasts, IGF2 was expressed from both parental alleles and H19 was not expressed, precisely as predicted from the altered pattern of allelic methylation. Interestingly, DNA replication patterns in the 11p15.5 region remained asynchronous as in controls. Out results therefore provide the first example of a dissociation of regional control of DNA replication from regional control of allelic methylation and expression in imprinting. We suggest that the altered pattern of allelic methylation and expression arises in the germline or in the early embryo from defects in resetting or setting of imprinting in the maternal germline. Potential candidate regions for mutations include the previously identified translocation breakpoint clusters and the H19 gene itself. The finding of possible 'imprinting mutations' in BWS raises the prospect of identifying genetic factors that control imprinting in this region.

Frequent loss of imprinting of the H19 gene is often associated with its overexpression in human lung cancers.

Abstract: Accumulating evidence suggests that deregulation of the insulin-like growth factor II (IGF2) and H19 genes at 11p15, due to loss of imprinting (LOI), plays a role in the oncogenesis of Wilms' tumors. We previously reported LOI of IGF2 in carcinomas of the lung, a common cancer of adults. We show here that LOI of H19 is also a frequent event in lung cancer development, and correlated with hypomethylation of the promoter region. Furthermore, the present study also revealed that overexpression of H19 is often associated with LOI of H19 in lung cancers retaining both parental alleles, showing a contrast to LOI in Wilms' tumors. Interestingly, in contrast to frequent LOI of the imprinted genes at 11p15, LOI was not observed for the remaining gene known to be imprinted in man, SNRPN at 15q12.

Allele-specific expression and total expression levels of imprinted genes during early mouse development: implications for imprinting mechanisms.

Abstract: Genomic imprinting determines the monoallelic expression of a small number of genes during at least later stages of development. To obtain information necessary for the elucidation of imprinting mechanisms, we assessed the allele-specific expression and total expression level of four imprinted genes during early stages of development of normal F1 hybrid mice utilizing quantitative allele-specific reverse transcription-PCR (RT-PCR) single-nucleotide primer extension assays. The Igf2r and Snrpn genes were activated by the early 4-cell stage and exhibited biallelic and monoallelic expression, respectively, throughout preimplantation development. Thus, with respect to different imprinted genes, epigenetic systems determining monoallelic expression are not uniform in their time of establishment. Biallelic expression of Igf2r was observed in single blastomeres, discounting the possibility of random allelic inactivation at this stage. The closely linked H19 and Igf2 genes were activated after the blastocyst stage and often exhibited biallelic and monoallelic expression respectively in tissues of pregastrulation postimplantation-stage embryos, rather than reciprocal monoallelic modes as observed at later stages. This raises the possibility that imprinting of H19 is involved only in the maintenance and not in the initiation of monoallelic expression of Igf2. Monoallelic expression of Snrpn was observed in each blastomere at the 4-cell stage, demonstrating that the germ line, which exhibits biallelic expression of imprinted genes, must be derived from cells in which imprinting was once manifest.

Epigenetic programming of differential gene expression in development and evolution

Abstract: This review covers data on changing patterns of DNA methylation and the regulation of gene expression in mouse embryonic development. Global demethylation occurs from the eight-cell stage to the blastocyst stage in preimplantation embryos, and global de novo methylation begins at implantation. We have used X-chromosome inactivation in female embryos as a model system to study specific CpG sites in the X-linked Pgk-1 and G6pd housekeeping genes and in the imprinted regulatory Xist gene to elucidate the role of methylation in the initiation and maintenance of differential gene activity. Methylation of the X-linked housekeeping genes occurs very close in time to their inactivation, thus raising the question as to whether methylation could be causal to inactivation, as well as being involved in its maintenance. A methylation difference between sperm and eggs in the promoter region of the Xist gene, located at the X-chromosome inactivation centre, is correlated with imprinted preferential inactivation of the paternal X chromosome in extra-embryonic tissues. Based on our data, a picture of the inheritance of methylation imprints and speculation on the significance of the Xist imprint in development is presented. On a more general level, an hypothesis of evolution by "adaptive epigenetic/genetic inheritance" is considered. This proposes modification of germ line DNA in response to a change in environment and mutation at the site of modification (e.g., of methylated cytosine to thymine). Epigenetic inheritance could function to shift patterns of gene expression to buffer the evolving system against changes in environment. If the altered patterns of gene activity and inactivity persist, the modifications may become "fixed" as mutations; alternatively, previously silenced gene networks might be recruited into function, thus appearing as if they are "acquired characteristics." An extension of this hypothesis is "foreign gene acquisition and sorting" (selection or silencing of gene function according to use). "Kidnapping" and sorting of foreign genes in this way could explain the observation that increased complexity in evolution is associated with more "junk" DNA. Adaptive epigenetic/genetic inheritance challenges the "central dogma" that information is unidirectional from the DNA to protein and the idea that Darwinian random mutation and selection are the sole mechanisms of evolution.

Methylation of the mouse Xist gene in sperm and eggs correlates with imprinted Xist expression and paternal X-inactivation.

Abstract: Preferential paternal X-inactivation in the extra-embryonic tissues of the female mouse embryo is correlated with imprinted expression of the paternal allele of the Xist gene in pre-implantation development. Here we examine 11 CpG sites in Xist to determine whether differential methylation might be the molecular basis for imprinting. We find that three sites in the promoter region are methylated in eggs but not in sperm and that this differential methylation is maintained to the blastocyst stage when the paternal X-inactivation occurs. This is the first example of a primary gametic methylation imprint governing differential expression of parental alleles in pre-implantation embryos.

Temporal and spatial regulation of H19 imprinting in normal and uniparental mouse embryos

Abstract: The mouse H19 gene is imprinted so that the paternal copy is both methylated and repressed during fetal development. However, the CpG-rich promoter region encompassing the transcription start is not methylated in sperm; this region must therefore become methylated postzygotically. We first examined the timing of DNA methylation of this region and the corresponding expression of H19. Both parental copies are initially undermethylated in blastocysts and the paternal copy then becomes fully methylated in the embryo around implantation; this methylation is more protracted in the extraembryonic lineages, especially in the trophoblast. By contrast to the lineage-dependent methylation, we observed exclusive expression of the maternal copy in preimplantation embryos and in all the lineages of early postimplantation embryos although variability may exist in cultured embryos. This indicates that methylation of the CpG-rich promoter is not a prerequisite for the paternal repression. We then examined whether methylation and expression occurs appropriately in the absence of a maternal or a paternal genome. Both H19 copies in androgenetic embryos are fully methylated while they are unmethylated in parthenogenetic embryos. This correlates with the lack of expression in androgenetic embryos but expression in parthenogenetic embryos. However, the androgenetic trophoblast was exceptional as it shows reduced methylation and expresses H19. These results suggest that promoter methylation is not the primary inactivation mechanism but is a stabilizing factor. Differential methylation in the more upstream region, which is established in the gametes, is a likely candidate for the gametic signal and may directly control H19 activity.

Evidence for two tumour suppressor loci on chromosomal bands 1p35–36 involved in neuroblastoma: one probably imprinted, another associated with N-myc amplification

Abstract: Previous reports on possible genomic imprinting of the neuroblastoma tumour suppressor gene on chromosome 1p36 have been conflicting. Here we report on the parental origin of 1p36 alleles lost in 47 neuroblastomas and on a detailed Southern blot analysis of the extent of the 1p deletions in 38 cases. The results are remarkably different for tumours with and without N-myc amplification. In the N-myc single copy tumours we show that the lost 1p36 alleles are of preferential maternal origin (16 of 17 cases) and that the commonly deleted region maps to 1p36.2-3. In contrast, all N-myc amplified neuroblastomas have larger 1p deletions, extending from the telomere to at least 1p35-36.1. These deletions are of random parental origin (18 of 30 maternal LOH). This strongly suggests that different suppressor genes on 1p are inactivated in these two types of neuroblastoma. Deletion of a more proximal suppressor gene is associated with N-myc amplification, while a distal, probably imprinted, suppressor can be deleted in N-myc single copy cases.

Epigenetic changes encompassing the IGF2/H19 locus associated with relaxation of IGF2 imprinting and silencing of H19 in Wilms tumor.

Abstract: In most tissues IGF2 is expressed from the paternal allele while H19 is expressed from the maternal allele. We have previously shown that in some Wilms tumors the maternal IGF2 imprint is relaxed such that the gene is expressed biallelically. We have now investigated this subset of tumors further and found that biallelic expression of IGF2 was associated with undetectable or very low levels of H19 expression. The relaxation of IGF2 imprinting in Wilms tumors also involved a concomitant reversal in the patterns of DNA methylation of the maternally inherited IGF2 and H19 alleles. Furthermore, the only specific methylation changes that occurred in tumors with relaxation of IGF2 imprinting were solely restricted to the maternal IGF2 and H19 alleles. These data suggest that there has been an acquisition of a paternal epigenotype in these tumors as the result of a pathologic disruption in the normal imprinting of the IGF2 and H19 genes.

Fine mapping of a putatively imprinted gene for familial non-chromaffin paragangliomas to chromosome 11q13.1 : evidence for genetic heterogeneity

Abstract: Autosomal, dominantly inherited, non-chromaffin paragangliomas are tumors of the head and neck region occurring with a frequency of 1∶30 000. Genomic imprinting probably influences the expression of the disorder, because tumor development is limited to individuals who have inherited the trait from their father. By linkage analysis and haplotyping of a single large family in which the pattern of inheritance is consistent with genomic imprinting, we have mapped the gene to a 5 cM region of chromosome 11q13.1 between D11S956 and PYGM. A maximum lod score of 7.62 at Θ = 0.0 was obtained for D11S480. This interval does not overlap with a recently assigned locus for glomus tumors in other families: 11q22.3-q23.3. Furthermore, analysis of a second family showing the imprinting phenomenon resulted in the exclusion of the 5 cM area as the location of the disease gene, whereas an indication for linkage was obtained (Z = +2.65) with markers from the distal locus. These observations argue for the presence of two distinct imprinted genes for glomus tumors on 11q. A model for tumor initiation and progression is presented based on all available information.

Deregulation of both imprinted and expressed alleles of the insulin–like growth factor 2 gene during β–cell tumorigenesis

Abstract: In a mouse model of multistage carcinogenesis elicited by the SV40 large T–antigen (Tag) oncogene in pancreatic β cells, the gene for insulin–like growth factor IGF2 is focally up–regulated and functionally implicated in tumour development. The IGF2 gene is differentially regulated in normal tissues: the paternal allele is transiently expressed during embryogenesis, whereas the maternal allele is genomically imprinted and inactive. Crossbred mice carrying the Tag oncogene and a disruption of either the paternal or maternal allele of IGF2 reveal that both alleles are co–activated early during tumour development, and that each contributes to malignant hyperproliferation and consequent tumour volume.

Molecular analysis of de novo germline mutations in the von Hippel-Lindau disease gene

Abstract: VHL disease is a dominantly inherited familial cancer syndrome with variable expression and age-dependent penetrance. The diagnosis of isolated cases is often delayed compared with familial cases, and estimates of the new mutation rate have varied more than 20-fold. To investigate the frequency and origin of de novo VHL gene mutations we have analysed: (i) families with identical mutations to determine if there is a common haplotype, and (ii) apparent new mutation cases to determine whether the clinical diagnosis of such cases is reliable and to define the parental origin of de novo VHL gene mutations. Haplotyping of 12 VHL mutations occurring in two or more families (total 42 kindreds) revealed that for most mutations there was no evidence of a founder effect. A marked bias for a paternal origin of new mutations has been reported in other familial cancer syndromes such as neurofibromatosis type 1 (NF1), multiple endocrine neoplasia (MEN) 2B and bilateral retinoblastoma, but it is unclear whether this bias results from a greater susceptibility for mutagenesis during male gametogenesis because of the larger number of cell divisions compared with that in oogenesis, or from genomic imprinting effects. Analysis of 13 de novo VHL mutations in which the parent of origin could be established, showed no evidence for a bias for a paternal origin (seven paternal, six maternal), and differed significantly from that reported in NF1, MEN2B and bilateral retinoblastoma. This result demonstrates that an increased susceptibility to paternal allele mutation is not a universal finding in autosomal genetic diseases and that the origin of new mutations may be influenced by both genomic imprinting effects and the increased number of cell divisions in spermatogenesis compared with oogenesis.

Conservation of a maternal-specific methylation signal at the human IGF2R locus

Abstract: The human IGF2R gene has been reported to be either biallelically or very rarely monoallelically expressed, in contrast to the maternally expressed mouse counterpart. We describe here an analysis of the 5' portion of the human IGF2R gene and show that it contains a maternally methylated CpG island in the second intron. A similar maternally methylated intronic element has been proposed to be the imprinting box for the mouse gene and although the relevance of this element has yet to be directly demonstrated, methylation has been reported to be essential to maintain allele-specific expression of imprinted genes. Allelic expression analysis of human IGF2R in 70 lymphoblastoid cell lines identified only one line showing monoallelic expression. Thus, in this tissue monoparental methylation of the IGF2R gene does not correlate with allele-specific expression. We also confirm here that the human IGF2R gene is located in an asynchronously replicating chromosomal region, as are all other imprinted genes so far analyzed. The mouse and human IGF2R intronic CpG islands both contain numerous large direct repeats that are methylated following maternal, but not paternal, transmittance. Thus features that attract maternal-specific methylation are conserved between the mouse and human genes. Since these intronic CpG islands share organizational rather than sequence homology, this suggests that secondary DNA structure may play a role in attracting a maternal methylation imprint.

Gamete-specific methylation correlates with imprinting of the murine Xist gene.

Abstract: We have investigated the potential role of DNA methylation as a regulator of imprinted Xist expression in mouse preimplantation embryos. The active paternal allele was found to be unmodified in sperm at CpG loci near the 5' end of the gene transcription unit. In contrast, on the inactive maternal allele, these same sites are initially methylated in the oocyte and then remain modified in the early embryo. In the male germ line, these methyl moieties are removed during spermatogenesis, and this occurs before the programmed reactivation of Xist in the testis. This represents a clear-cut example of a potential methylation imprinting signal that is reprogrammable and gamete derived.

Establishment of functional imprinting of the H19 gene in human developing placentae

Abstract: We have found that the imprinted H19 gene can be expressed either biallelically or monoallelically in the developing human placentae. H19 biallelic expression is confined to the placenta until 10 weeks of gestation, after which it becomes exclusively maternal, and does not affect allele-specificity or levels of IGF2 expression. The promoter region of H19 is hypomethylated at all stages of placental development, while the 3' portion shows progressive methylation of the paternal allele with gestation. Our observations demonstrate that the establishment of functional H19 imprinting occurs during the early development of the placenta and provide an opportunity to understand the mechanism by which the H19 primary imprint is manifested in somatic cells.