Showing papers on "Genomic imprinting published in 2006"

The epigenetic progenitor origin of human cancer

Abstract: Cancer is widely perceived as a heterogeneous group of disorders with markedly different biological properties, which are caused by a series of clonally selected genetic changes in key tumour-suppressor genes and oncogenes. However, recent data suggest that cancer has a fundamentally common basis that is grounded in a polyclonal epigenetic disruption of stem/progenitor cells, mediated by 'tumour-progenitor genes'. Furthermore, tumour cell heterogeneity is due in part to epigenetic variation in progenitor cells, and epigenetic plasticity together with genetic lesions drives tumour progression. This crucial early role for epigenetic alterations in cancer is in addition to epigenetic alterations that can substitute for genetic variation later in tumour progression. Therefore, non-neoplastic but epigenetically disrupted stem/progenitor cells might be a crucial target for cancer risk assessment and chemoprevention.

Genes in Conflict: The Biology of Selfish Genetic Elements

Abstract: Preface 1. SELFISH GENETIC ELEMENTS Genetic Cooperation and Conflict Three Ways to Achieve "Drive" Within-Individual Kinship Conflicts Rates of Spread Effects on the Host Population The Study of Selfish Genetic Elements Design of This Book 2. AUTOSOMAL KILLERS The t Haplotype Discovery Structure of the t Haplotype History and Distribution Genetics of Drive Importance of Mating System and Gamete Competition Fate of Resistant Alleles Selection for Inversions Recessive Lethals in t Complexes Enhancers and Suppressors t and the Major Histocompatability Complex Heterozygous (+/t) Fitness Effects: Sex Antagonistic? Accounting for t Frequencies in Nature Other Gamete Killers Segregation Distorter in Drosophila Spore Killers in Fungi Incidence of Gamete Killers Maternal-Effect Killers Medea in Flour Beetles HSR, scat+, and OmDDK in Mice The Evolution of Maternal-Effect Killers Gestational Drive? Gametophyte Factors in Plants 3. SELFISH SEX CHROMOSOMES Sex Chromosome Drive in the Diptera Killer X Chromosomes Killer Y Chromosomes Taxonomic Distribution of Killer Sex Chromosomes Evolutionary Cycles of Sex Determination Feminizing X (and Y) Chromosomes in Rodents The Varying Lemming The Wood Lemming Other Murids Other Conflicts: Sex Ratios and Mate Choice 4. GENOMIC IMPRINTING Imprinting and Parental Investment in Mammals Igf2 and Igf2r: Oppositely Imprinted, Oppositely Acting Growth Factors in Mice Growth Effects of Imprinted Genes in Mice and Humans Evolution of the Imprinting Apparatus The Mechanisms of Imprinting Involve Methylation and Are Complex Conflict Between Different Components of the Imprinting Machinery History of Conflict Reflected in the Imprinting Apparatus Evolutionary Turnover of the Imprinting Apparatus Intralocus Interactions, Polar Overdominance, and Paramutation Transmission Ratio Distortion at Imprinted Loci Biparental Imprinting and Other Possibilities Other Traits: Social Interactions after the Period of Parental Investment Maternal Behavior in Mice Inbreeding and Dispersal Kin Recognition Functional Interpretation of Tissue Effects in Chimeric Mice Deceit and Selves-Deception Imprinting and the Sex Chromosomes Genomic Imprinting in Other Taxa Flowering Plants Other Taxa Predicted to Have Imprinting 5. SELFISH MITOCHONDRIAL DNA Mitochondrial Genomics: A Primer Mitochondrial Selection within the Individual "Petite" Mutations in Yeast Within-Individual Selection and the Evolution of Uniparental Inheritance Within-Individual Selection under Uniparental Inheritance DUI: Mother-to-Daughter and Father-to-Son mtDNA Inheritance in Mussels Cytoplasmic Male Sterility Uniparental Inheritance Implies Unisexual Selection Disproportionate Role of mtDNA in Plant Male Sterility Mechanisms of Mitochondrial Action and Nuclear Reaction CMS and Restorers in Natural Populations CMS, Masculinization, and the Evolution of Separate Sexes Pollen Limitation, Frequency Dependence, and Local Extinction Resource Reallocation Versus Inbreeding Avoidance Importance of Mutational Variation CMS and Paternal Transmission Other Traces of Mito-Nuclear Conflict Mitochondria and Apoptosis Mitochondria and Germ Cell Determination Mitochondria and RNA Editing 6. GENE CONVERSION AND HOMING Biased Gene Conversion Molecular Mechanisms Effective Selection Coefficients Due to BGC in Fungi BGC and Genome Evolution BGC and Evolution of the Meiotic Machinery Homing and Retrohoming How HEGs Home HEGs Usually Associated with Self-Splicing Introns or Inteins HEGs and Host Mating System Evolutionary Cycle of Horizontal Transmission, Degeneration, and Loss HEG Domestication and Mating-Type Switching in Yeast Group II Introns Artificial HEGs As Tools for Population Genetic Engineering The Basic Construct Increasing the Load Preventing Natural Resistance and Horizontal Transmission Population Genetic Engineering Other Uses 7. TRANSPOSABLE ELEMENTS Molecular Structure and Mechanisms DNA Transposons LINEs and SINEs LTR Retroelements Population Biology and Natural Selection Transposition Rates Low But Greater Than Excision Rates Natural Selection on the Host Slows the Spread of Transposable Elements Rapid Spread of P Elements in D. melanogaster Net Reproductive Rate a Function of Transposition Rate and Effect on Host Fitness Reducing Harm to the Host Transposition Rate and Copy Number "Regulation" Selection for Self-Recognition Defective and Repressor Elements Extinction of Active Elements in Host Species Horizontal Transmission and Long-Term Persistence Transposable Elements in Inbred and Outcrossed Populations Beneficial Inserts Rates of Fixation Transposable Elements and Host Evolution Transposable Elements and Chromosomal Rearrangements Transposable Elements and Genome Size Co-Option of Transposable Element Functions and Host Defenses Transposable Elements As Parasites, Not Host Adaptations or Mutualists Origins Ancient, Chimeric, and Polyphyletic Origins 8. FEMALE DRIVE Selfish Centromeres and Female Meiosis Abnormal Chromosome 10 of Maize Other Knobs in Maize Deleterious Effects of Knobs in Maize Knobs, Supernumerary Segments, and Neocentromeres in Other Species Meiosis-Specific Centromeres and Holocentric Chromosomes Selfish Centromeres and Meiosis I The Importance of Centromere Number: Robertsonian Translocations in Mammals Sperm-Dependent Female Drive? Female Drive and Karyotype Evolution Polar Bodies Rejoining the Germline 9. B CHROMOSOMES Drive Types of Drive Genetics of A and B Factors Affecting B Drive Transmission Rates inWell-Studied Species Absence of Drive Degree of Outcrossing and Drive Effects on the Phenotype Effects on Genome Size, Cell Size, and Cell Cycle Effects on the External Phenotype Disappearance from Somatic Tissue B Number and the Odd-Even Effect Negative Effects of Bs More Pronounced under Harsher Conditions Is the Sex of Drive Associated with the Sex of Phenotypic Effect? B Effects on Recombination Among the As Pairing of A Chromosomes in Hybrids Neutral and Beneficial Bs Beneficial B Chromosomes B Chromosomes in Eyprepocnemis plorans: A Case of Continuous Neutralization? Structure and Content Size Polymorphism Heterochromatin Genes Tandem Repeats The Origin of Bs A Factors Associated with B Presence Genome Size Chromosome Number Ploidy Shape of A Chromosomes Bs and the Sex Ratio Paternal Sex Ratio (PSR) in Nasonia X-B Associations in Orthoptera Has the Drosophila Y Evolved from a B? Other Effects of Bs on the Sex Ratio Male Sterility in Plantago 10. GENOMIC EXCLUSION Paternal Genome Loss in Males, or Parahaplodiploidy PGL in Mites PGL in Scale Insects PGL in the Coffee Borer Beetle PGL in Springtails? Evolution of PGL PGL and Haplodiploidy Sciarid Chromosome System Notable Features of the Sciarid System An Evolutionary Hypothesis Mechanisms PGL in Gall Midges Hybridogenesis, or Hemiclonal Reproduction The Topminnow Poeciliopsis The Water Frog Rana esculenta The Stick Insect Bacillus rossius-grandii Evolution of Hybridogenesis Androgenesis, or Maternal Genome Loss The Conifer Cupressus dupreziana The Clam Corbicula The Stick Insect Bacillus rossius-grandii 11. SELFISH CELL LINEAGES Mosaics Somatic Cell Lineage Selection: Cancer and the Adaptive Immune System Cell Lineage Selection in the Germline Evolution of the Germline Selfish Genes and Germline-Limited DNA Chimeras Taxonomic Survey of Chimerism Somatic Chimerism and Polar Bodies 12. SUMMARY AND FUTURE DIRECTIONS Logic of Selfish Genetic Elements Molecular Genetics Selfish Genes and Sex Fate of a Selfish Gene within a Species Movement between Species Distribution among Species Role in Host Evolution The HiddenWorld of Selfish Genetic Elements References Glossary Index

CTCF binding at the H19 imprinting control region mediates maternally inherited higher-order chromatin conformation to restrict enhancer access to Igf2

Abstract: It is thought that the H19 imprinting control region (ICR) directs the silencing of the maternally inherited Igf2 allele through a CTCF-dependent chromatin insulator. The ICR has been shown to interact physically with a silencer region in Igf2, differentially methylated region (DMR)1, but the role of CTCF in this chromatin loop and whether it restricts the physical access of distal enhancers to Igf2 is not known. We performed systematic chromosome conformation capture analyses in the Igf2/H19 region over >160 kb, identifying sequences that interact physically with the distal enhancers and the ICR. We found that, on the paternal chromosome, enhancers interact with the Igf2 promoters but that, on the maternal allele, this is prevented by CTCF binding within the H19 ICR. CTCF binding in the maternal ICR regulates its interaction with matrix attachment region (MAR)3 and DMR1 at Igf2, thus forming a tight loop around the maternal Igf2 locus, which may contribute to its silencing. Mutation of CTCF binding sites in the H19 ICR leads to loss of CTCF binding and de novo methylation of a CTCF target site within Igf2 DMR1, showing that CTCF can coordinate regional epigenetic marks. This systematic chromosome conformation capture analysis of an imprinting cluster reveals that CTCF has a critical role in the epigenetic regulation of higher-order chromatin structure and gene silencing over considerable distances in the genome.

Elongation of the Kcnq1ot1 transcript is required for genomic imprinting of neighboring genes

Abstract: The imprinted gene cluster at the telomeric end of mouse chromosome 7 contains a differentially methylated CpG island, KvDMR, that is required for the imprinting of multiple genes, including the genes encoding the maternally expressed placental-specific transcription factor ASCL2, the cyclin-dependent kinase CDKN1C, and the potassium channel KCNQ1. The KvDMR, which maps within intron 10 of Kcnq1, contains the promoter for a paternally expressed, noncoding, antisense transcript, Kcnq1ot1. A 244-base-pair deletion of the promoter on the paternal allele leads to the derepression of all silent genes tested. To distinguish between the loss of silencing as the consequence of the absence of transcription or the transcript itself, we prematurely truncated the Kcnq1ot1 transcript by inserting a transcriptional stop signal downstream of the promoter. We show that the lack of a full-length Kcnq1ot1 transcript on the paternal chromosome leads to the expression of genes that are normally paternally repressed. Finally, we demonstrate that five highly conserved repeats residing at the 5′ end of the Kcnq1ot1 transcript are not required for imprinting at this locus.

The c-Myc Oncogene Directly Induces the H19 Noncoding RNA by Allele-Specific Binding to Potentiate Tumorigenesis

Abstract: The product of the MYC oncogene is widely deregulated in cancer and functions as a regulator of gene transcription. Despite an extensive profile of regulated genes, the transcriptional targets of c-Myc essential for transformation remain unclear. In this study, we show that c-Myc significantly induces the expression of the H19 noncoding RNA in diverse cell types, including breast epithelial, glioblastoma, and fibroblast cells. c-Myc binds to evolutionarily conserved E-boxes near the imprinting control region to facilitate histone acetylation and transcriptional initiation of the H19 promoter. In addition, c-Myc down-regulates the expression of insulin-like growth factor 2 ( IGF2 ), the reciprocally imprinted gene at the H19/IGF2 locus. We show that c-Myc regulates these two genes independently and does not affect H19 imprinting. Indeed, allele-specific chromatin immunoprecipitation and expression analyses indicate that c-Myc binds and drives the expression of only the maternal H19 allele. The role of H19 in transformation is addressed using a knockdown approach and shows that down-regulation of H19 significantly decreases breast and lung cancer cell clonogenicity and anchorage-independent growth. In addition, c-Myc and H19 expression shows strong association in primary breast and lung carcinomas. This work indicates that c-Myc induction of the H19 gene product holds an important role in transformation. (Cancer Res 2006; 66(10): 5330-7)

Deletion of Peg10 , an imprinted gene acquired from a retrotransposon, causes early embryonic lethality

Abstract: By comparing mammalian genomes, we and others have identified actively transcribed Ty3/gypsy retrotransposon-derived genes with highly conserved DNA sequences and insertion sites1,2,3,4,5,6. To elucidate the functions of evolutionarily conserved retrotransposon-derived genes in mammalian development, we produced mice that lack one of these genes, Peg10 (paternally expressed 10)1,2,3,7, which is a paternally expressed imprinted gene on mouse proximal chromosome 6. The Peg10 knockout mice showed early embryonic lethality owing to defects in the placenta. This indicates that Peg10 is critical for mouse parthenogenetic development and provides the first direct evidence of an essential role of an evolutionarily conserved retrotransposon-derived gene in mammalian development.

Epigenetics of autism spectrum disorders

Abstract: The autism spectrum disorders (ASD) comprise a complex group of behaviorally related disorders that are primarily genetic in origin. Involvement of epigenetic regulatory mechanisms in the pathogenesis of ASD has been suggested by the occurrence of ASD in patients with disorders arising from epigenetic mutations (fragile X syndrome) or that involve key epigenetic regulatory factors (Rett syndrome). Moreover, the most common recurrent cytogenetic abnormalities in ASD involve maternally derived duplications of the imprinted domain on chromosome 15q11-13. Thus, parent of origin effects on sharing and linkage to imprinted regions on chromosomes 15q and 7q suggest that these regions warrant specific examination from an epigenetic perspective, particularly because epigenetic modifications do not change the primary genomic sequence, allowing risk epialleles to evade detection using standard screening strategies. This review examines the potential role of epigenetic factors in the etiology of ASD.

Maintenance of DNA Methylation during the Arabidopsis Life Cycle Is Essential for Parental Imprinting

Abstract: Imprinted genes are expressed predominantly from either their paternal or their maternal allele. To date, all imprinted genes identified in plants are expressed in the endosperm. In Arabidopsis thaliana, maternal imprinting has been clearly demonstrated for the Polycomb group gene MEDEA (MEA) and for FWA. Direct repeats upstream of FWA are subject to DNA methylation. However, it is still not clear to what extent similar cis-acting elements may be part of a conserved molecular mechanism controlling maternally imprinted genes. In this work, we show that the Polycomb group gene FERTILIZATION-INDEPENDENT SEED2 (FIS2) is imprinted. Maintenance of FIS2 imprinting depends on DNA methylation, whereas loss of DNA methylation does not affect MEA imprinting. DNA methylation targets a small region upstream of FIS2 distinct from the target of DNA methylation associated with FWA. We show that FWA and FIS2 imprinting requires the maintenance of DNA methylation throughout the plant life cycle, including male gametogenesis and endosperm development. Our data thus demonstrate that parental genomic imprinting in plants depends on diverse cis-elements and mechanisms dependent or independent of DNA methylation. We propose that imprinting has evolved under constraints linked to the evolution of plant reproduction and not by the selection of a specific molecular mechanism.

Imprinted genes, placental development and fetal growth.

Abstract: In mammals, imprinted genes have an important role in feto-placental development. They affect the growth, morphology and nutrient transfer capacity of the placenta and, thereby, control the nutrient s

Oocyte growth-dependent progression of maternal imprinting in mice.

Abstract: In mammals, some genes categorized as imprinted genes are exclusively expressed either from maternal or paternal allele. This parental-origin-specific gene expression is regulated by epigenetic modification of DNA methylation in differentially methylated region (DMR), which is independently imposed during oogenesis and spermatogenesis. It is known that methylation of DMR in the female germ line is established during oocyte growth phase. However, the cause of the progression of methylation on DMR, due to either aging of mice or growth-size of oocyte was unclear up to now. Here, we analyzed the methylation of DMR for each eight imprinted genes (Igf2r, Lit1, Zac1, Snrpn, Peg1/Mest, Impact, Meg1/Grb10, and H19) by bisulfite sequencing methylation assay, using oocytes from 10 dpp (days post partum), 15 dpp, 20 dpp, and adult mice. To find whether the size of oocytes is the cause of methylation, above oocytes were classified into seven groups (each oocyte diameter ranging from 40 to 75 microm with intervals of 5 microm). The results from juvenile mice oocytes showed that DMR methylation progressed according to oocyte growth each imprinted gene. More than 85% of DMR methylation was achieved for both Igf2r, Zac1 & Lit1 with oocyte size of reaching 55 microm and Snrpn, Peg1/Mest, Impact, and Meg1/Grb10 with oocyte size of reaching 60 microm. Preferential methylation of maternal allele was observed in Zac1 and Peg1/Mest of juvenile oocytes and in Snrpn of juvenile and adult oocytes. The oocyte size-dependent-methylation progressed equally for all three different-age juvenile mice. The size-dependent-methylation was also recognized in the growing oocytes collected from adult mice, although the progress is slightly slower than that of juvenile mice. From these results, we concluded that DNA methylation is established with oocyte size dependent manner, not with aging of mice.

Limited evolutionary conservation of imprinting in the human placenta

Abstract: The epigenetic phenomenon of genomic imprinting provides an additional level of gene regulation that is confined to a limited number of genes, frequently, but not exclusively, important for embryonic development. The evolution and maintenance of imprinting has been linked to the balance between the allocation of maternal resources to the developing fetus and the mother's well being. Genes that are imprinted in both the embryo and extraembryonic tissues show extensive conservation between a mouse and a human. Here we examine the human orthologues of mouse genes imprinted only in the placenta, assaying allele-specific expression and epigenetic modifications. The genes from the KCNQ1 domain and the isolated human orthologues of the imprinted genes Gatm and Dcn all are expressed biallelically in the human, from first-trimester trophoblast through to term. This lack of imprinting is independent of promoter CpG methylation and correlates with the absence of the allelic histone modifications dimethylation of lysine-9 residue of H3 (H3K9me2) and trimethylation of lysine-27 residue of H3 (H3K27me3). These specific histone modifications are thought to contribute toward regulation of imprinting in the mouse. Genes from the IGF2R domain show polymorphic concordant expression in the placenta, with imprinting demonstrated in only a minority of samples. Together these findings have important implications for understanding the evolution of mammalian genomic imprinting. Because most human pregnancies are singletons, this absence of competition might explain the comparatively relaxed need in the human for placental-specific imprinting.

Genomic Imprinting in Mammals: Emerging Themes and Established Theories

Abstract: The epigenetic events that occur during the development of the mammalian embryo are essential for correct gene expression and cell-lineage determination. Imprinted genes are expressed from only one parental allele due to differential epigenetic marks that are established during gametogenesis. Several theories have been proposed to explain the role that genomic imprinting has played over the course of mammalian evolution, but at present it is not clear if a single hypothesis can fully account for the diversity of roles that imprinted genes play. In this review, we discuss efforts to define the extent of imprinting in the mouse genome, and suggest that different imprinted loci may have been wrought by distinct evolutionary forces. We focus on a group of small imprinted domains, which consist of paternally expressed genes embedded within introns of multiexonic transcripts, to discuss the evolution of imprinting at these loci.

The epigenetic imprinting defect of patients with Beckwith—Wiedemann syndrome born after assisted reproductive technology is not restricted to the 11p15 region

Abstract: Background: Genomic imprinting refers to an epigenetic marking resulting in monoallelic gene expression and has a critical role in fetal development. Various imprinting diseases have recently been reported in humans and animals born after the use of assisted reproductive technology (ART). All the epimutations implicated involve a loss of methylation of the maternal allele (demethylation of KvDMR1/ KCNQ1OT1 in Beckwith–Wiedemann syndrome (BWS), demethylation of SNRPN in Angelman syndrome and demethylation of DMR2/ IGF2R in large offspring syndrome), suggesting that ART impairs the acquisition or maintenance of methylation marks on maternal imprinted genes. However, it is unknown whether this epigenetic imprinting error is random or restricted to a specific imprinted domain. Aim: To analyse the methylation status of various imprinted genes ( IGF2R gene at 6q26, PEG1/MEST at 7q32, KCNQ1OT1 and H19 at 11p15.5, and SNRPN at 15q11–13) in 40 patients with BWS showing a loss of methylation at KCNQ1OT1 (11 patients with BWS born after the use of ART and 29 patients with BWS conceived naturally). Results: 3 of the 11 (27%) patients conceived using ART and 7 of the 29 (24%) patients conceived normally displayed an abnormal methylation at a locus other than KCNQ1OT1 . Conclusions: Some patients with BWS show abnormal methylation at loci other than the 11p15 region, and the involvement of other loci is not restricted to patients with BWS born after ART was used. Moreover, the mosaic distribution of epimutations suggests that imprinting is lost after fertilisation owing to a failure to maintain methylation marks during pre-implantation development.

The Testis-Specific Factor CTCFL Cooperates with the Protein Methyltransferase PRMT7 in H19 Imprinting Control Region Methylation

Abstract: Expression of imprinted genes is restricted to a single parental allele as a result of epigenetic regulation—DNA methylation and histone modifications. Igf2/H19 is a reciprocally imprinted locus exhibiting paternal Igf2 and maternal H19 expression. Their expression is regulated by a paternally methylated imprinting control region (ICR) located between the two genes. Although the de novo DNA methyltransferases have been shown to be necessary for the establishment of ICR methylation, the mechanism by which they are targeted to the region remains unknown. We demonstrate that CTCFL/BORIS, a paralog of CTCF, is an ICR-binding protein expressed during embryonic male germ cell development, coinciding with the timing of ICR methylation. PRMT7, a protein arginine methyltransferase with which CTCFL interacts, is also expressed during embryonic testis development. Symmetrical dimethyl arginine 3 of histone H4, a modification catalyzed by PRMT7, accumulates in germ cells during this developmental period. This modified histone is also found enriched in both H19 ICR and Gtl2 differentially methylated region (DMR) chromatin of testis by chromatin immunoprecipitation (ChIP) analysis. In vitro studies demonstrate that CTCFL stimulates the histone-methyltransferase activity of PRMT7 via interactions with both histones and PRMT7. Finally, H19 ICR methylation is demonstrated by nuclear co-injection of expression vectors encoding CTCFL, PRMT7, and the de novo DNA methyltransferases, Dnmt3a, -b and -L, in Xenopus oocytes. These results suggest that CTCFL and PRMT7 may play a role in male germline imprinted gene methylation.

A maternal-offspring coadaptation theory for the evolution of genomic imprinting.

Abstract: Imprinted genes are expressed either from the maternally or paternally inherited copy only, and they play a key role in regulating complex biological processes, including offspring development and mother–offspring interactions. There are several competing theories attempting to explain the evolutionary origin of this monoallelic pattern of gene expression, but a prevailing view has emerged that holds that genomic imprinting is a consequence of conflict between maternal and paternal gene copies over maternal investment. However, many imprinting patterns and the apparent overabundance of maternally expressed genes remain unexplained and may be incompatible with current theory. Here we demonstrate that sole expression of maternal gene copies is favored by natural selection because it increases the adaptive integration of offspring and maternal genomes, leading to higher offspring fitness. This novel coadaptation theory for the evolution of genomic imprinting is consistent with results of recent studies on epigenetic effects, and it provides a testable hypothesis for the origin of previously unexplained major imprinting patterns across different taxa. In conjunction with existing hypotheses, our results suggest that imprinting may have evolved due to different selective pressures at different loci.

Identification of an imprinting control region affecting the expression of all transcripts in the Gnas cluster.

Abstract: Genomic imprinting results in allele-specific silencing according to parental origin1. Silencing is brought about by imprinting control regions (ICRs) that are differentially marked in gametogenesis2. The group of imprinted transcripts in the mouse Gnas cluster (Nesp, Nespas, Gnasxl, Exon 1A and Gnas) provides a model for analyzing the mechanisms of imprint regulation. We previously identified an ICR that specifically regulates the tissue-specific imprinted expression of the Gnas gene3. Here we identify a second ICR at the Gnas cluster. We show that a paternally derived targeted deletion of the germline differentially methylated region (DMR) associated with the antisense Nespas transcript unexpectedly affects both the expression of all transcripts in the cluster and methylation of two DMRs. Our results establish that the Nespas DMR is the principal ICR at the Gnas cluster and functions bidirectionally as a switch for modulating expression of the antagonistically acting genes Gnasxl and Gnas. Uniquely, the Nespas DMR acts on the downstream ICR at exon 1A to regulate tissue-specific imprinting of the Gnas gene.

Regulation of growth and metabolism by imprinted genes.

Abstract: A small sub-set of mammalian genes are subject to regulation by genomic imprinting such that only one parental allele is active in at least some sites of expression. Imprinted genes have diverse functions, notably including the regulation of growth. Much attention has been devoted to the insulin-like growth factor signalling pathway that has a major influence on fetal size and contains two components encoded by the oppositely imprinted genes, Igf2 (a growth promoting factor expressed from the paternal allele) and Igf2r (a growth inhibitory factor expressed from the maternal allele). These genes fit the parent-offspring conflict hypothesis for the evolution of genomic imprinting. Accumulated evidence indicates that at least one other fetal growth pathway exists that has also fallen under the influence of imprinting. It is clear that not all components of growth regulatory pathways are encoded by imprinted genes and instead it may be that within a pathway the influence of a single gene by each of the parental genomes may be sufficient for parent-offspring conflict to be enacted. A number of imprinted genes have been found to influence energy homeostasis and some, including Igf2 and Grb10, may coordinate growth with glucose-regulated metabolism. Since perturbation of fetal growth can be correlated with metabolic disorders in adulthood these imprinted genes are considered as candidates for involvement in this phenomenon of fetal programming.

How imprinting centres work

Abstract: Imprinted genes tend to be clustered in the genome. Most of these clusters have been found to be under the control of discrete DNA elements called imprinting centres (ICs) which are normally differentially methylated in the germline. ICs can regulate imprinted expression and epigenetic marks at many genes in the region, even those which lie several megabases away. Some of the molecular and cellular mechanisms by which ICs control other genes and regulatory regions in the cluster are becoming clear. One involves the insulation of genes on one side of the IC from enhancers on the other, mediated by the insulator protein CTCF and higher-order chromatin interactions. Another mechanism may involve non-coding RNAs that originate from the IC, targeting histone modifications to the surrounding genes. Given that several imprinting clusters contain CTCF dependent insulators and/or non-coding RNAs, it is likely that one or both of these two mechanisms regulate imprinting at many loci. Both mechanisms involve a variety of epigenetic marks including DNA methylation and histone modifications but the hierarchy of and interactions between these modifications are not yet understood. The challenge now is to establish a chain of developmental events beginning with differential methylation of an IC in the germline and ending with imprinting of many genes, often in a lineage dependent manner.

A maternal hypomethylation syndrome presenting as transient neonatal diabetes mellitus

Abstract: The expression of imprinted genes is mediated by allele-specific epigenetic modification of genomic DNA and chromatin, including parent of origin-specific DNA methylation. Dysregulation of these genes causes a range of disorders affecting pre- and post-natal growth and neurological function. We investigated a cohort of 12 patients with transient neonatal diabetes whose disease was caused by loss of maternal methylation at the TNDM locus. We found that six of these patients showed a spectrum of methylation loss, mosaic with respect to the extent of the methylation loss, the tissues affected and the genetic loci involved. Five maternally methylated loci were affected, while one maternally methylated and two paternally methylated loci were spared. These patients had higher birth weight and were more phenotypically diverse than other TNDM patients with different aetiologies, presumably reflecting the influence of dysregulation of multiple imprinted genes. We propose the existence of a maternal hypomethylation syndrome, and therefore suggest that any patient with methylation loss at one maternally-methylated locus may also manifest methylation loss at other loci, potentially complicating or even confounding the clinical presentation.

CTCF binding sites promote transcription initiation and prevent DNA methylation on the maternal allele at the imprinted H19/Igf2 locus

Abstract: Imprinting at the H19/Igf2 locus depends on a differentially methylated domain (DMD) acting as a maternal-specific, methylation-sensitive insulator and a paternal-specific locus of hypermethylation. Four repeats in the DMD bind CTCF on the maternal allele and have been proposed to recruit methylation on the paternal allele. We deleted the four repeats and assayed the effects of the mutation at the endogenous locus. The H19DMD-DeltaR allele can successfully acquire methylation during spermatogenesis and silence paternal H19, indicating that these paternal-specific functions are independent of the CTCF binding sites. Maternal inheritance of the mutations leads to biallelic Igf2 expression, consistent with the loss of a functional insulator. Additionally, we uncovered two previously undescribed roles for the CTCF binding sites. On the mutant allele, H19 RNA is barely detectable in 6.5 d.p.c. embryos and 9.5 d.p.c. placenta, for the first time identifying the repeats as the elements responsible for initiating H19 transcription. Furthermore, methylation is abruptly acquired on the mutant maternal allele after implantation, a time when the embryo is undergoing genome-wide de novo methylation. Together, these experiments show that in addition to being essential for a functional insulator, the CTCF repeats facilitate initiation of H19 expression in the early embryo and are required to maintain the hypomethylated state of the entire DMD.

Epigenetic asymmetry of imprinted genes in plant gametes.

Abstract: Plant imprinted genes show parent-of-origin expression in seed endosperm, but little is known about the nature of parental imprints in gametes before fertilization. We show here that single differentially methylated regions (DMRs) correlate with allele-specific expression of two maternally expressed genes in the seed and that one DMR is differentially methylated between gametes. Thus, plants seem to have developed similar strategies as mammals to epigenetically mark imprinted genes.

Dynamics of global transcriptome in bovine matured oocytes and preimplantation embryos

Abstract: Global activation of the embryonic genome is the most critical event in early mammalian development. After fertilization, a rich supply of maternal proteins and RNAs support development whereas a number of zygotic and embryonic genes are expressed in a stage-specific manner leading to embryonic genome activation (EGA). However, the identities of embryonic genes expressed and the mechanism(s) of EGA are poorly defined in the bovine. Using the Affymetrix bovine-specific DNA microarray as the biggest available array at present, we analyzed gene expression at two key stages of bovine development, matured oocytes (MII) and 8-cell-stage embryos, constituting the ultimate reservoir for life and a stage during which EGA takes place, respectively. Key genes in regulation of transcription, chromatin-structure cell adhesion, and signal transduction were up-regulated at the 8-cell stage as compared with 8-cell embryos treated with α-amanitin and MII. Genes controlling DNA methylation and metabolism were up-regulated in MII. These changes in gene expression, related to transcriptional machinery, chromatin structure, and the other cellular functions occurring during several cleavage stages, are expected to result in a unique chromatin structure capable of maintaining totipotency during embryogenesis and leading to differentiation during postimplantation development. Dramatic reprogramming of gene expression at the onset of development also has implications for cell plasticity in somatic cell nuclear transfer, genomic imprinting, and cancer.

Analysis of allelic differential expression in human white blood cells

Abstract: Allelic variation of gene expression is common in humans, and is of interest because of its potential contribution to variation in heritable traits. To identify human genes with allelic expression differences, we genotype DNA and examine mRNA isolated from the white blood cells of 12 unrelated individuals using oligonucleotide arrays containing 8406 exonic SNPs. Of the exonic SNPs, 1983, located in 1389 genes, are both expressed in the white blood cells and heterozygous in at least one of the 12 individuals, and thus can be examined for differential allelic expression. Of the 1389 genes, 731 (53%) show allele expression differences in at least one individual. To gain insight into the regulatory mechanisms governing allelic expression differences, we analyze a set of 60 genes containing exonic SNPs that are heterozygous in three or more samples, and for which all heterozygotes display differential expression. We find three patterns of allelic expression, suggesting different underlying regulatory mechanisms. Exonic SNPs in three of the 60 genes are monoallelically expressed in the human white blood cells, and when examined in families show expression of only the maternal copy, consistent with regulation by imprinting. Approximately one-third of the genes have the same allele expressed more highly in all heterozygotes, suggesting that their regulation is predominantly influenced by cis-elements in strong linkage disequilibrium with the assayed exonic SNP. The remaining two-thirds of the genes have different alleles expressed more highly in different heterozygotes, suggesting that their expression differences are influenced by factors not in strong linkage disequilibrium with the assayed exonic SNP.

Dynamic regulatory interactions of Polycomb group genes: MEDEA autoregulation is required for imprinted gene expression in Arabidopsis.

Abstract: The imprinted Arabidopsis Polycomb group (PcG) gene MEDEA (MEA), which is homologous to Enhancer of Zeste [E(Z)], is maternally required for normal seed development. Here we show that, unlike known mammalian imprinted genes, MEA regulates its own imprinted expression: It down-regulates the maternal allele around fertilization and maintains the paternal allele silent later during seed development. Autorepression of the maternal MEA allele is direct and independent of the MEA-FIE (FERTILIZATION-INDEPENDENT ENDOSPERM) PcG complex, which is similar to the E(Z)-ESC (Extra sex combs) complex of animals, suggesting a novel mechanism. A complex network of cross-regulatory interactions among the other known members of the MEA-FIE PcG complex implies distinct functions that are dynamically regulated during reproduction.