Polymorphic functional imprinting of the human IGF2 gene among individuals, in blood cells, is associated with H19 expression

Recent Advances of LncRNA H19 in Diabetes LncRNA H19 in Diabetes

Diabetes mellitus (DM) causes damage to major organs, including the heart, liver, brain, kidneys, eyes, and blood vessels, threatening the health of the individuals. Emerging evidence has demonstrated that lncRNAs has important functions in the pathogenesis of human diseases, such as cancers, neurodegenerative diseases, cardiac fibroblast phenotypes, hypertension, heart failure, atherosclerosis and diabetes. Recently, H19, a lncRNA, has been reported to shown to participate in the regulatory process of muscle differentiation, glucose metabolism, and tumor metastasis, as well as endometrial development. However, the roles of H19 in DM were still not completely understood. This review was conducted to summarize the functions of H19 in diabetes and discuss the challenges and possible strategies of H19 in DM.

Imprinted Gene Expression and Function of the Dopa Decarboxylase Gene in the Developing Heart

Dopa decarboxylase (DDC) synthesizes serotonin in the developing mouse heart where it is encoded by Ddc_exon1a, a tissue-specific paternally expressed imprinted gene. Ddc_exon1a shares an imprinting control region (ICR) with the imprinted, maternally expressed (outside of the central nervous system) Grb10 gene on mouse chromosome 11, but little else is known about the tissue-specific imprinted expression of Ddc_exon1a. Fluorescent immunostaining localizes DDC to the developing myocardium in the pre-natal mouse heart, in a region susceptible to abnormal development and implicated in congenital heart defects in human. Ddc_exon1a and Grb10 are not co-expressed in heart nor in brain where Grb10 is also paternally expressed, despite sharing an ICR, indicating they are mechanistically linked by their shared ICR but not by Grb10 gene expression. Evidence from a Ddc_exon1a gene knockout mouse model suggests that it mediates the growth of the developing myocardium and a thinning of the myocardium is observed in a small number of mutant mice examined, with changes in gene expression detected by microarray analysis. Comparative studies in the human developing heart reveal a paternal expression bias with polymorphic imprinting patterns between individual human hearts at DDC_EXON1a, a finding consistent with other imprinted genes in human.

Insights into imprinting from parent-of-origin phased methylomes and transcriptomes

Imprinting is the preferential expression of one parental allele over the other. It is controlled primarily through differential methylation of cytosine at CpG dinucleotides. Here we combine 285 methylomes and 11,617 transcriptomes from peripheral blood samples with parent-of-origin phased haplotypes, to produce a new map of imprinted methylation and gene expression patterns across the human genome. We demonstrate how imprinted methylation is a continuous rather than a binary characteristic. We describe at high resolution the parent-of-origin methylation pattern at the 15q11.2 Prader-Willi/Angelman syndrome locus, with nearly confluent stochastic paternal methylation punctuated by 'spikes' of maternal methylation. We find examples of polymorphic imprinted methylation unrelated (at VTRNA2-1 and PARD6G) or related (at CHRNE) to nearby SNP genotypes. We observe RNA isoform-specific imprinted expression patterns suggestive of a methylation-sensitive transcriptional elongation block. Finally, we gain new insights into parent-of-origin-specific effects on phenotypes at the DLK1/MEG3 and GNAS loci.

Genomic imprinting effects on complex traits in domesticated animal species

Monoallelically expressed genes that exert their phenotypic effect in a parent-of-origin specific manner are considered to be subject to genomic imprinting, the most well understood form of epigenetic regulation of gene expression in mammals. The observed differences in allele specific gene expression for imprinted genes are not attributable to differences in DNA sequence information, but to specific chemical modifications of DNA and chromatin proteins. Since the discovery of genomic imprinting some three decades ago, over 100 imprinted mammalian genes have been identified and considerable advances have been made in uncovering the molecular mechanisms regulating imprinted gene expression. While most genomic imprinting studies have focused on mouse models and human biomedical disorders, recent work has highlighted the contributions of imprinted genes to complex trait variation in domestic livestock species. Consequently, greater understanding of genomic imprinting and its effect on agriculturally important traits is predicted to have major implications for the future of animal breeding and husbandry. In this review, we discuss genomic imprinting in mammals with particular emphasis on domestic livestock species and consider how this information can be used in animal breeding research and genetic improvement programs.

Imprinted loci in domestic livestock species as epigenomic targets for artificial selection of complex traits

The phenomenon of genomic imprinting, whereby a subset of mammalian genes display parent-of-origin-specific monoallelic expression, is one of the most active areas of epigenetics research. Over the past two decades, more than 100 imprinted mammalian genes have been identified, while considerable advances have been made in elucidating the molecular mechanisms governing imprinting. These studies have helped to unravel the epigenome--a separate layer of regulatory information contained in eukaryotic chromosomes that influences gene expression and phenotypes without involving changes to the underlying DNA sequence. Although most studies of genomic imprinting in mammals have focussed on mouse models or human biomedical disorders, there is burgeoning interest in the phenotypic effects of imprinted genes in domestic livestock species. In particular, research has focused on imprinted genes influencing foetal growth and development, which are associated with economically important production traits in cattle, sheep and pigs. These findings, when coupled with the data emerging from the various different livestock genome projects, have major implications for the future of animal breeding, health and management. Here, we review current scientific knowledge regarding genomic imprinting in livestock species and evaluate how this information can be used in modern livestock improvement programmes.

Functional evaluation of ES-somatic cell hybrids in vitro and in vivo

Embryonic stem cells (ESCs) have previously been reported to reprogram somatic cells following fusion. The resulting ES-somatic cell hybrids have been shown to adopt the transcriptional profile of ESCs, suggesting that the pluripotent program is dominant. ES-somatic cell hybrids have most characteristics of pluripotent cells in vitro; however, it remains unclear whether the somatic genome is an active partner in the hybrid cells or simply retained predominately as silent cargo. Furthermore, the functional properties of ES-somatic cell hybrids in vivo have been limited to studies on their contribution to teratomas and developing embryos/chimeras. The extent of their pluripotency remains largely unclear. Here we determined that the somatic genome is actively transcribed by generating ES-somatic cell hybrids using Rag2-deficient ESCs fused to autologous wild-type somatic cells. Rag2 expression was detected during in vitro differentiation, suggesting that the somatic genome follows the correct temporal cues during differentiation. Furthermore, ES-somatic cell hybrids maintain their tetraploid state following 4 weeks of differentiation in vivo and are immune tolerated when transferred into matched individuals. The ES-somatic cell hybrids can efficiently differentiate into hematopoietic precursors in both myeloid and lymphoid lineages in vitro, suggesting that the somatic genome is actively transcribed following cell fusion based reprogramming. However, the ES-somatic cell hybrids showed an altered hematopoietic potential following in vitro differentiation and were unable to show hematopoietic engraftment in a mouse model.

A novel approach identifies new differentially methylated regions (DMRs) associated with imprinted genes

Imprinted genes are critical for normal human growth and neurodevelopment. They are characterized by differentially methylated regions (DMRs) of DNA that confer parent of origin-specific transcription. We developed a new strategy to identify imprinted gene-associated DMRs. Using genome-wide methylation profiling of sodium bisulfite modified DNA from normal human tissues of biparental origin, candidate DMRs were identified by selecting CpGs with methylation levels consistent with putative allelic differential methylation. In parallel, the methylation profiles of tissues of uniparental origin, i.e., paternally-derived androgenetic complete hydatidiform moles (AnCHMs), and maternally-derived mature cystic ovarian teratoma (MCT), were examined and then used to identify CpGs with parent of origin-specific DNA methylation. With this approach, we found known DMRs associated with imprinted genomic regions as well as new DMRs for known imprinted genes, NAP1L5 and ZNF597, and novel candidate imprinted genes. The paternally methylated DMR for one candidate, AXL, a receptor tyrosine kinase, was also validated in experiments with mouse embryos that demonstrated Axl was expressed preferentially from the maternal allele in a DNA methylation-dependent manner.

Methylation allelic polymorphism (MAP) in chorionic gonadotropin beta5 (CGB5) and its association with pregnancy success

CONTEXT

Increased epigenetic variability in the placenta may have evolved in response to its role in mediating the conflicting demands of the mother and fetus. One essential guardian of early pregnancy maintenance is the placental hormone human chorionic gonadotropin (HCG).

OBJECTIVE

Among the four primate-specific duplicate HCGβ-coding genes, chorionic gonadotropin-β8 (CGB8) and chorionic gonadotropin-β5 (CGB5) jointly contribute 62-82% of the total HCGβ transcript pool. Because these genes share common features with known imprinted placenta-expressed loci, we addressed the role of epigenetic mechanisms affecting their action.

DESIGN AND SUBJECTS

Parental origin of CGB5 and CGB8 transcripts and promoter methylation patterns were addressed in trophoblastic tissues from 23 mother-offspring duos and nine mother-father-offspring trios including the following: 1) third-trimester normal delivery at term (n = 14), 2) first-trimester elective termination of uncomplicated pregnancy (n = 10), and 3) first-trimester recurrent (≥3) miscarriage (n = 8).

RESULTS

A normal uncomplicated pregnancy was characterized by balanced, biallelic expression of CGB5 and CGB8. However, in three (two recurrent miscarriage and one early elective termination of uncomplicated pregnancy) of nine genetically informative cases of CGB5, monoallelic expression of maternal alleles and hemimethylated gene promoters were identified.

CONCLUSION

Our finding may represent a novel methylation allelic polymorphism or gain of imprinting in CGB5 promoter leading to expressional silencing of paternal alleles and increasing susceptibility to pregnancy loss. Aberrant methylation patterns in placenta may result from random reprogramming defects affecting normal implantation process. Alternatively, methylation allelic polymorphism in the placenta favoring the failure of pregnancy may arise as a response to cellular stress caused by, in general, aneuploidy or conditions in placental-maternal interface.

Effect of alcohol consumption on CpG methylation in the differentially methylated regions of H19 and IG-DMR in male gametes: implications for fetal alcohol spectrum disorders

BACKGROUND

Exposure to alcohol in utero is the main attributable cause of fetal alcohol spectrum disorders (FASD) which in its most severe form is characterized by irreversible behavioral and cognitive disability. Paternal preconception drinking is not considered to be a significant risk factor, even though animal studies have demonstrated that chronic paternal alcohol consumption has a detrimental effect on the physical and mental development of offspring even in the absence of in utero alcohol exposure. It has been documented that alcohol can reduce the levels and activity of DNA methyltransferases resulting in DNA hypomethylation and that reduced methyltransferase activity can cause activation of normally silenced genes. The aim of this study was to establish a link between alcohol use in men and hypomethylation of paternally imprinted loci in sperm DNA in genomic regions critical for embryonic development, thus providing a mechanism for paternal effects in the aetiology of FASD.

METHODS

Sperm DNA from male volunteers was bisulfite treated and the methylation patterns of 2 differentially methylated regions (DMRs), H19 and IG-DMR, analyzed following sequencing of individual clones. The methylation patterns were correlated with the alcohol consumption levels of the volunteer males.

RESULTS

There was a pattern of increased demethylation with alcohol consumption at the 2 imprinted loci with a significant difference observed at the IG-DMR between the nondrinking and heavy alcohol consuming groups. Greater inter-individual variation in average methylation was observed at the H19 DMR and individual clones were more extensively demethylated than those of the IG-DMR. CpG site #4 in the IG-DMR was preferentially demethylated among all individuals and along with the H19 DMR CpG site #7 located within the CTCF binding site 6 showed significant demethylation in the alcohol consuming groups compared with the control group.

CONCLUSION

This study demonstrates a correlation between chronic alcohol use and demethylation of normally hypermethylated imprinted regions in sperm DNA. We hypothesize that, should these epigenetic changes in imprinted genes be transmitted through fertilization, they would alter the critical gene expression dosages required for normal prenatal development resulting in offspring with features of FASD.

Sex-specific genetic architecture of human disease

Sexual dimorphism in anatomical, physiological and behavioural traits are characteristics of many vertebrate species. In humans, sexual dimorphism is also observed in the prevalence, course and severity of many common diseases, including cardiovascular diseases, autoimmune diseases and asthma. Although sex differences in the endocrine and immune systems probably contribute to these observations, recent studies suggest that sex-specific genetic architecture also influences human phenotypes, including reproductive, physiological and disease traits. It is likely that an underlying mechanism is differential gene regulation in males and females, particularly in sex steroid-responsive genes. Genetic studies that ignore sex-specific effects in their design and interpretation could fail to identify a significant proportion of the genes that contribute to risk for complex diseases.

Insulin-like growth factor-II: a novel autocrine growth factor modulating the apoptosis and maturation of umbilical cord blood erythroid progenitors

OBJECTIVE

To search a novel function of erythroid progenitor cells circulating as the major nucleated cell population in umbilical cord blood (CB) cells.

MATERIALS AND METHODS

Human CB-derived CD36(+) erythroid progenitors were subjected to cDNA microarray. Gene expression and biological property of CB-erythroid progenitors and adult peripheral blood (PB)-erythroid progenitors were compared by using real-time polymerase chain reaction (PCR) and serum-free culture system with erythropoietin (EPO).

RESULTS

The microarray revealed 124-fold higher levels of insulin-like growth factor-II (IGF-II) gene expression in CB-CD36(+) erythroid progenitors than in stimulated lymphocytes of adult PB. Real-time PCR verified that IGF-II mRNA levels were highest in CB-CD36(+) erythroid progenitors compared to other CB- or adult PB-fractionated cells. When CB-CD36(+) erythroid progenitors were cultured with EPO in serum-free medium, anti-IGF-II-antibody (Ab) reduced the number of erythroid colonies. When CB- and adult PB-derived erythroid colony-forming cells (ECFCs) were cultured with interleukin-3, stem cell factor, and EPO, mRNA levels per cells of IGF-II peaked on day 12, but those of type 1 and type 2 receptors did not increase with ECFCs maturation. The maturation rate by IGF-II was higher in CB-ECFCs than in adult PB-ECFCs. The majority of CB-ECFCs expressed IGF-II protein. Anti-IGF-II-Ab, but not anti-IGF-I-Ab, reduced the number of CB-ECFCs in liquid culture with EPO. Anti-IGF-II-Ab accelerated apoptosis of ECFCs, assessed by dimethylthiazole tetrazolium bromide, bromodeoxyuridine, and flow cytometric analyses. ECFCs failed to attain full maturity in the presence of anti-IGF-II-Ab.

CONCLUSIONS

These results suggest that IGF-II is produced by erythroid progenitors themselves, and has a crucial role in fetal erythropoiesis by modulating apoptosis and maturation in an autocrine fashion.

No loss of genomic imprinting of IGF-II and H19 in placentas of diabetic pregnancies with fetal macrosomia

OBJECTIVES

Fetal macrosomia is a common complication of maternal diabetes mellitus and is associated with substantial morbidity, but the precise cellular and molecular mechanisms that induce fetal macrosomia are not well understood. The imprinted genes IGF-II and H19 are crucial for placental development and fetal growth. The term placentas from diabetic pregnancies express more insulin-like growth factor II (IGF-II) than those from normal pregnancies. Deregulation of their imprinting status is observed in the macrosomia-associated syndrome, the Beckwith-Wiedemann syndrome. The aim of this study was to determine whether loss of imprinting hence biallelic expression was also a hallmark of macrosomia in diabetic pregnancies.

DESIGN AND METHODS

IGF-II and H19 maternal and paternal expressions were studied in placentas from two groups of type 1 diabetic mothers: one with macrosomic babies and the other with babies of normal weight. Maternal or paternal allele specific expressions were defined by using DNA polymorphic markers of the IGF-II and H19 genes. RFLP analysis was performed on PCR products from genomic DNA of the father, the mother and the child, and on RT-PCR products from placental mRNA.

RESULTS

RFLP analysis showed that the IGF-II gene remains paternally expressed and the H19 gene remains maternally expressed in all placentas examined, independently of the birth weight status.

CONCLUSIONS

These results suggest that, in contrast with Beckwith-Wiedemann syndrome-associated macrosomia, loss of imprinting for IGF-II or H19 is not a common feature of diabetic pregnancies associated with macrosomia.

Increased plasticity of genomic imprinting of Dlk1 in brain is due to genetic and epigenetic factors

The expression of six imprinted genes (Dlk1, Gt12, Igf2r, Kcnq1, Nnat, and Peg1) was examined in brains of 21 mice derived from N2 x N2 intercrosses between C57BL/6 and MOLF/Ei strains. Imprinting of Igf2r, Kcnq1, Gt12, and Dlk1 varied among individuals. As three of these genes are implicated in cell-cell signaling or cell-environment interactions, variation in their imprinting may influence a wide range of biological processes from cell differentiation to behavior. To elucidate the mechanisms underlying the interindividual imprinting variation in the brain, we focused our effort on the paternally expressed gene Dlk1. We investigated expression of Dlk1 in the brains of animals from N9 and N10 backcrosses and found that reactivation of the normally silent maternal Dlk1 allele in the N9 and N10 mice occurred less often than in N2 x N2 animals. Our data suggest that trans-acting genetic factors of MOLF/Ei origin facilitate the reactivation of the normally silent maternal allele of Dlk1. We mapped one of these factors to the proximal part of Chr 7. The results of bisulfite sequencing methylation analysis show that reactivation of the maternal allele was also associated with hypermethylation of the intragenic differentially methylated region (IG DMR), which is the imprinting control region for the Dlk1-Gt12 domain. Thus, the imprinting status of Dlk1 in the brain depends upon trans-acting genetic influences and correlates with the methylation status of a specific subregion of the IG DMR.

Intralocus sexual conflict can drive the evolution of genomic imprinting

Genomic imprinting is a phenomenon whereby the expression of an allele differs depending upon its parent of origin. There is an increasing number of examples of this form of epigenetic inheritance across a wide range of taxa, and imprinting errors have also been implicated in several human diseases. Various hypotheses have been put forward to explain the evolution of genomic imprinting, but there is not yet a widely accepted general hypothesis for the variety of imprinting patterns observed. Here a new evolutionary hypothesis, based on intralocus sexual conflict, is proposed. This hypothesis provides a potential explanation for much of the currently available empirical data, and it also makes new predictions about patterns of genomic imprinting that are expected to evolve but that have not, as of yet, been looked for in nature. This theory also provides a potential mechanism for the resolution of intralocus sexual conflict in sexually selected traits and a novel pathway for the evolution of sexual dimorphism.

Epigenetics and assisted reproductive technology: a call for investigation

A surprising set of recent observations suggests a link between assisted reproductive technology (ART) and epigenetic errors--that is, errors involving information other than DNA sequence that is heritable during cell division. An apparent association with ART was found in registries of children with Beckwith-Wiedemann syndrome, Angelman syndrome, and retinoblastoma. Here, we review the epidemiology and molecular biology behind these studies and those of relevant model systems, and we highlight the need for investigation of two major questions: (1) large-scale case-control studies of ART outcomes, including long-term assessment of the incidence of birth defects and cancer, and (2) investigation of the relationship between epigenetic errors in both offspring and parents, the specific methods of ART used, and the underlying infertility diagnoses. In addition, the components of proprietary commercial media used in ART procedures must be fully and publicly disclosed, so that factors such as methionine content can be assessed, given the relationship in animal studies between methionine exposure and epigenetic changes.

The imprinted region on human chromosome 7q32 extends to the carboxypeptidase A gene cluster: an imprinted candidate for Silver-Russell syndrome

Imprinted gene(s) on human chromosome 7q32-qter have been postulated to be involved in intrauterine growth restriction associated with Silver-Russell syndrome (SRS) as 7-10% of patients have mUPD(7). Three imprinted genes, MEST, MESTIT1, and COPG2IT1 on chromosome 7q32, are unlikely to cause SRS since epigenetic and sequence mutation analyses have not shown any changes. One hundred kilobases proximal to MEST lies a group of four carboxypeptidase A (CPA) genes. Since most imprinted genes are found in clusters, this study focuses on analysing these CPAs for imprinting effects based on their proximity to an established imprinted domain. Firstly, a replication timing study across 7q32 showed that an extensive genomic region including the CPAs, MEST, MESTIT1, and COPG2IT1 replicates asynchronously. Subsequently, SNP analysis by sequencing RT-PCR products of CPA1, CPA2, CPA4, and CPA5 indicated preferential expression of CPA4. Pyrosequencing was used as a quantitative approach, which confirmed predominantly preferential expression of the maternal allele and biallelic expression in brain. CPA5 expression levels were too low to allow reliable evaluation of allelic expression, while CPA1 and CPA2 both showed biallelic expression. CPA4 was the only gene from this family in which an imprinting effect was shown despite the location of this family of genes next to an imprinted cluster. As CPA4 has a potential role in cell proliferation and differentiation, two preferentially expressed copies in mUPD patients with SRS syndrome would result in excess expression and could alter the growth profiles of these subjects and give rise to intrauterine growth restriction.

The glial growth factors deficiency and synaptic destabilization hypothesis of schizophrenia

BACKGROUND

A systems approach to understanding the etiology of schizophrenia requires a theory which is able to integrate genetic as well as neurodevelopmental factors.

PRESENTATION OF THE HYPOTHESIS

Based on a co-localization of loci approach and a large amount of circumstantial evidence, we here propose that a functional deficiency of glial growth factors and of growth factors produced by glial cells are among the distal causes in the genotype-to-phenotype chain leading to the development of schizophrenia. These factors include neuregulin, insulin-like growth factor I, insulin, epidermal growth factor, neurotrophic growth factors, erbB receptors, phosphatidylinositol-3 kinase, growth arrest specific genes, neuritin, tumor necrosis factor alpha, glutamate, NMDA and cholinergic receptors. A genetically and epigenetically determined low baseline of glial growth factor signaling and synaptic strength is expected to increase the vulnerability for additional reductions (e.g., by viruses such as HHV-6 and JC virus infecting glial cells). This should lead to a weakening of the positive feedback loop between the presynaptic neuron and its targets, and below a certain threshold to synaptic destabilization and schizophrenia.

TESTING THE HYPOTHESIS

Supported by informed conjectures and empirical facts, the hypothesis makes an attractive case for a large number of further investigations.

IMPLICATIONS OF THE HYPOTHESIS

The hypothesis suggests glial cells as the locus of the genes-environment interactions in schizophrenia, with glial asthenia as an important factor for the genetic liability to the disorder, and an increase of prolactin and/or insulin as possible working mechanisms of traditional and atypical neuroleptic treatments.

The insulin-like growth factor system in hematopoietic cells

The insulin-like growth factor (IGF) system regulates proliferation and differentiation of hematopoietic cells. IGFs exert their effects through specific receptors on growing and differentiating blood cells as they emerge from their small pool of ancestral stem cells. The IGF system is complex as both stimulating and inhibiting effects occur by interaction of IGFs and IGF-binding proteins (IGFBPs). IGFs stimulate erythrocytes and lymphocytes but also promote leukemic hematopoietic cell proliferation. IGF-I appears to be correlated with hemoglobin levels in anemia and could also be of benefit for patients with bone marrow aplasia after transplantation. Hypersensitivity to IGF-I has been implicated as an underlying cause of polycythemia vera. Loss of imprinting of IGF-II is found in acute myeloid leukemia and myelodysplastic syndrome. Apoptosis of hematopoietic cells is significantly reduced by IGF-I involving an intriguing signal transduction pathway. IGFs could therefore, although not classical hematopoietic growth factors, be of benefit for patients with diverse hematopoietic disorders.

The effect of parental imprinting on the INS-IGF2 locus of Korean type I diabetic patients

BACKGROUND

Insulin-dependent diabetes mellitus (IDDM) is caused by the autoimmune destruction of pancreatic beta-cells. Susceptibility to IDDM appears to depend on more than one genetic locus. Evidence of a genetic linkage for IDDM2 was found in male meioses from French and North American populations. It is linked to maternal imprinting (i.e. monoalleleic expression of the insulin gene) that is considered the most likely cause of these gender-related differences. IGF2 is expressed only in the paternal allele and, therefore, is considered a candidate gene for IDDM2 transmission because of its important autocrine/paracrine effects on the thymus, lymphocytes and pancreas. Nevertheless, it remains controversial whether the parental origin of IDDM2 influences IDDM susceptibility.

METHODS

Using PCR and semi-quantitative RT-PCR, we analyzed the INS/Pstl + 1127 and IGF2/Apal polymorphisms and RNA expression level between Pstl (+/-) and Pstl (+/+) to determine genotype and allele-specific expression of the INS and IGF2 genes.

RESULTS

INS/Pstl (+/+) and IGF2/Apal (+/-) were observed in 36 (97.3%) of 37 IDDM patients and in 29 (72.5%) of 40 IDDM patients, respectively. The presence of both IGF2 alleles in RNA was observed in 21 (91.6%) of 24 IDDM patients. Our results show a 3-fold increase in RNA expression from Pstl (+/-) allele over Pstl (+/+) allele.

CONCLUSION

Our conclusion does not entirely exclude IGF2 as the gene involved in IDDM2, even though the parental effect of IDDM2 transmission is not related to IGF2 maternal imprinting. The INS genotype appeared mostly in the Pstl (+/+) homozygote and, therefore, we could not explain the INS imprinting pattern in Korean type 1 diabetic patients. Genetic differences between populations may account for the discrepancy between Korean type I diabetic patients and American or French type I diabetic patients.

Imprinting of insulin-like growth factor 2 is modulated during hematopoiesis

The transcription of insulin-like growth factor 2 (IGF-2) is affected by genomic imprinting, a multistep process through which the parental origin of a gene influences its transcription. The maternal copy of IGF-2 is silenced in most human tissues, but in the choroid plexus and the adult liver both alleles of IGF-2 are expressed. This study shows that though in peripheral blood mononuclear cells IGF-2shows paternal allele-specific expression, in total bone marrow both alleles are transcribed. This modulation of imprinting is not attributable to use of the P1 promoter, because transcription from the P3 promoter occurred from both alleles. These results suggest that transcriptional recognition of the IGF-2 imprint can be modulated during hematopoiesis and may facilitate the development of in vitro model systems to study the transcriptional recognition of a genomic imprint.

Imprinting of insulin-like growth factor 2 is modulated during hematopoiesis

The transcription of insulin-like growth factor 2 (IGF-2) is affected by genomic imprinting, a multistep process through which the parental origin of a gene influences its transcription. The maternal copy of IGF-2 is silenced in most human tissues, but in the choroid plexus and the adult liver both alleles of IGF-2 are expressed. This study shows that though in peripheral blood mononuclear cells IGF-2shows paternal allele-specific expression, in total bone marrow both alleles are transcribed. This modulation of imprinting is not attributable to use of the P1 promoter, because transcription from the P3 promoter occurred from both alleles. These results suggest that transcriptional recognition of the IGF-2 imprint can be modulated during hematopoiesis and may facilitate the development of in vitro model systems to study the transcriptional recognition of a genomic imprint.

H19 sense and antisense transgenes modify insulin-like growth factor-II mRNA levels

The oppositely-imprinted genes insulin-like growth factor-II (IGF2) and H19, a putative tumor suppressor, often show coordinate, reciprocal regulation and are believed to play a role in carcinogenesis. To explore the possible interactions between these genes, we stably transfected diHepG2 cells with a plasmid containing either the sense or the antisense H19 cDNA sequences and verified their expression by Northern analysis and by RNase protection analysis. Levels of H19, IGF2 and gamma-actin mRNA were quantified by competitive RT-PCR analysis. Although H19 sense transgene overexpression (n = 24 clones) did not decrease the low, basal levels of IGF2 mRNA compared to control cells, levels of IGF2 mRNA were positively correlated with the levels of H19 antisense mRNA (P < 0.0001, n = 40 clones). Furthermore, the increase in IGF2 mRNA level was accompanied by an elevation of IGF-II peptide in conditioned media. To see if H19 mRNA had a specific effect on transcription, we also performed transient transfections with reporter gene constructs containing IGF2 promoter 3 in the presence of sense or antisense H19 cDNA sequences under control of a cytomegalovirus promoter. We show a lower reporter gene activity from reporter gene constructs in the presence of sense H19 cDNA than from those with antisense or neomycin. Our results suggest that H19 participates in the repression of IGF2, at least in part through effects on IGF2 transcription, an effect which may contribute to its action as a tumor suppressor.

Expression of the imprinted tumour-suppressor gene H19 is tightly regulated during normal haematopoiesis and is reduced in haematopoietic precursors of patients with the myeloproliferative disease polycythaemia vera

cDNA subtraction was employed to uncover differences in gene expression between myeloproliferative polycythaemia vera (PV) and normal haematopoietic precursors. Following cDNA subtraction using mRNAs isolated from PV and normal CD34+/CD33- bone-marrow cells, expression of the tumour suppressor H19 was found to be low or absent in the PV sample. Low levels of H19 expression in PV patients were confirmed by in situ hybridization. Using semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) to examine expression in the pluripotent haematopoietic cell line FDCP-mix and single bone-marrow precursors, unambiguous IGF2 and H19 expression was demonstrated in normal haematopoietic precursors. Examination of individual bone-marrow precursors revealed that all IGF2-expressing haematopoietic precursors also co-expressed H19, indicating that H19 and IGF2 may be co-ordinately regulated during haematopoiesis. Analysis of FDCP-mix undergoing differentiation and single pluripotent and committed bone-marrow precursors revealed that the pattern of H19 expression coincided with the commitment to a single lineage. Taken together, these observations demonstrate that H19 and IGF2 are specifically expressed during haematopoiesis and that low levels of H19 expression are associated with PV and may contribute to the pathology of the disease.

Polymorphic X-chromosome inactivation of the human TIMP1 gene

X inactivation silences most but not all of the genes on one of the two X chromosomes in mammalian females. The human X chromosome preserves its activation status when isolated in rodent/human somatic-cell hybrids, and hybrids retaining either the active or inactive X chromosome have been used to assess the inactivation status of many X-linked genes. Surprisingly, the X-linked gene for human tissue inhibitor of metalloproteinases (TIMP1) is expressed in some but not all inactive X-containing somatic-cell hybrids, suggesting that this gene is either prone to reactivation or variable in its inactivation. Since many genes that escape X inactivation are clustered, we examined the expression of four genes (ARAF1, ELK1, ZNF41, and ZNF157) within approximately 100 kb of TIMP1. All four genes were expressed only from the active X chromosome, demonstrating that the factors allowing TIMP1 expression from the inactive X chromosome are specific to the TIMP1 gene. To determine if this variable inactivation of TIMP1 is a function of the hybrid-cell environment or also is observed in human cells, we developed an allele-specific assay to assess TIMP1 expression in human females. Expression of two alleles was detected in some female cells with previously demonstrated extreme skewing of X inactivation, indicating TIMP1 expression from the inactive chromosome. However, in other cells, no expression of TIMP1 was observed from the inactive X chromosome, suggesting that TIMP1 inactivation is polymorphic in human females.

Expression of the imprinted H19 oncofetal RNA in epithelial ovarian cancer

STUDY

To examine the expression of the imprinted maternally expressed H19 gene in benign, low malignant potential (borderline) and malignant surface epithelial ovarian tumors.

DESIGN

In situ hybridization for H19 RNA using S-labeled and digoxigenin-labeled probes was performed on paraffin sections of ovarian surface epithelial tumors. The serous tumors included nine section cystadenomas, twelve serous tumors of low malignant potential and twenty serous carcinomas, grade I-IIII (FIGO classification). A smaller group included two mucinous cystadenomas, four mucinous tumors of low malignant potential and two mucinous cystadenocarcinomas.

RESULTS

H19 expression was found to be positive in 6/9 (67%) serous cystadenomas, 9/12 (75%) of serous tumors of low malignant potential and 13/20 (65%) of invasive serous carcinomas. Expression in mucinous tumors was confined to the stroma beneath the epithelial lining.

CONCLUSION

H19 is expressed in the majority of serous epithelial tumors. Taking into consideration the high percentage of H19 expressing serous ovarian neoplasms we suggest that H19 RNA may be used as an adjuvant tumor marker for the diagnosis and mainly for staging and follow-up of patients with serous ovarian carcinoma.

Heterogeneous gene expression from the inactive X chromosome: an X-linked gene that escapes X inactivation in some human cell lines but is inactivated in others

In mammalian females, most genes on one X chromosome are transcriptionally silenced as a result of X chromosome inactivation. Whereas it is well established that some X-linked genes "escape" X inactivation and are expressed from both active (Xa) and inactive (Xi) X chromosomes, most models for the chromosomal control of X-linked gene expression assume that the X inactivation status of a given gene is constant among different females within a population. In this report, we test the expression of human X-linked genes in primary cell lines from females with complete nonrandom X inactivation, by using transcribed polymorphisms to distinguish Xa and Xi expression. Six X-linked genes used to document this assay system showed monoallelic expression in all informative cell lines, consistent with X inactivation. However, a novel pattern of expression was observed for another gene, REP1; monoallelic expression, indicating inactivation, was detected in some lines, whereas biallelic expression, indicating escape from inactivation, was detected in others. Furthermore, levels of Xi expression varied among cell lines that expressed REP1. The cellular basis of Xi expression was examined by expression assays in single cells. These data indicate that REP1 is expressed from the Xi in all cells, but that the level of expression relative to Xa levels is reduced. These findings suggest that Xi gene expression is under a previously unsuspected level of genetic or epigenetic control, likely involving local or regional changes in chromatin organization that determine whether a gene escapes or is subject to X inactivation.

Genomic imprinting: implications for human disease

Genomic imprinting refers to an epigenetic marking of genes that results in monoallelic expression. This parent-of-origin dependent phenomenon is a notable exception to the laws of Mendelian genetics. Imprinted genes are intricately involved in fetal and behavioral development. Consequently, abnormal expression of these genes results in numerous human genetic disorders including carcinogenesis. This paper reviews genomic imprinting and its role in human disease. Additional information about imprinted genes can be found on the Genomic Imprinting Website at http://www.geneimprint.com.

Proinsulin-like growth factor-II overexpression does not alter monoallelic H19 gene expression in transfected human embryonic kidney fibroblasts

Insulin-like growth factor-II (IGF-II) is a potent mitogen for cells in culture. The H19 gene is a developmentally regulated gene with putative tumor suppressor activity, and loss of H19 expression may be involved in tumorigenesis. The H19 gene is closely linked to the human IGF-II gene (IGF2) on chromosome 11p15.5 and these genes are reciprocally imprinted in most fetal tissues. H19 is expressed only from the maternal and IGF2 from the paternal chromosome. We have asked whether overexpression of proIGF-II alters H19 imprinting status and/or expression. Human embryonal kidney fibroblasts (293 cells) were stably transfected with a PCMV5 vector containing the full length human IGF-II cDNA or a control cDNA. Transfectant clones expressed large quantities of IGF-II mRNA and secrete 1-5 ug/ml and 150-230 ng/ml proIGF-II within 24 hours of serum-free culture (transfectant 293-9 and -11 respectively) (1). Cells were genotyped at the exon 5, RsaI restriction fragment length polymorphism (RFLP) and found to be informative (+/-). H19 expression was monoallelic (+) indicating preservation of H19 imprinting in all cell lines. Using quantitative RT-PCR with internal competitors for H19 and for IGF-II cDNA, overexpression of IGF2 in 293-11 and 293-9 cells was confirmed. In contrast, no significant difference with respect to H19 expression was detected between the overexpressing cells and control lines. In conclusion, (1) human embryonal fibroblasts express the H19 gene. (2) H19 imprinting is preserved in these cells. (3) proIGF-II overexpression does not alter H19 expression.

Polymorphic imprinting of the serotonin-2A (5-HT2A) receptor gene in human adult brain

We have examined the imprinting status of the serotonin-2A (5-HT2A) receptor gene (HTR2A) in a series of 41 tissue samples from human adult brain. Using a HpaII RFLP polymorphism in the coding region, we obtained evidence of monoallelic expression of HTR2A in 4 out of 18 informative individuals. However, biallelic expression of HTR2A was observed in other individuals (14/18), suggesting that within the human population HTR2A imprinting in brain is polymorphic.

The INS 5' variable number of tandem repeats is associated with IGF2 expression in humans

The minisatellite DNA polymorphism consisting of a variable number of tandem repeats (VNTR) at the human INS (insulin gene) 5'-flanking region has demonstrated allelic effects on insulin gene transcription in vitro and has been associated with the level of insulin gene expression in vivo. We now show that this VNTR also has effects on the nearby insulin-like growth factor II gene (IGF2) in human placenta in vivo and in the HepG2 hepatoma cell line in vitro. We show that higher steady-state IGF2 mRNA levels are associated with shorter alleles (class I) than the longer class III alleles in term placentae. In vitro, reporter gene activity was greater from reporter gene constructs with IGF2 promoter 3 in the presence of class I alleles than from those with class III. Taken together with the documented transcriptional effects on the insulin gene, we propose that the VNTR may act as a long range control element affecting the expression of both INS and IGF2. The localization of a type 1 diabetes susceptibility locus (IDDM2) to the VNTR itself suggests that either or both of these genes may be involved in the biologic effects of IDDM2.

Imprinting mechanisms in mammals

Imprinting is a genetic mechanism that determines expression or repression of genes according to their parental origin. Some imprinted genes occur in clusters in the genome. Recent work using transgenic mice shows that multiple cis-acting sequences are needed for correct imprinting. Mutation analysis in a normal chromosomal context reveals the importance of imprinting centres for regional establishment or maintenance of imprinting in a cluster. Elements that contribute to the function of imprinting centres and regional propagation of the imprints are CpG-rich differentially methylated regions (that during development retain germline imposed methylation or demethylation), direct repeat clusters, and unusual RNAs (antisense, non-translated etc.). The interaction of these cis elements with transacting factors such as methylase and chromatin factors establishes a hierarchical control system with local and regional effects.