Dysregulated H19/Igf2 expression disrupts cardiac-placental axis during development of Silver-Russell syndrome-like mouse models

Dysregulation of the imprinted H19/IGF2 locus can lead to Silver-Russell syndrome (SRS) in humans. However, the mechanism of how abnormal H19/IGF2 expression contributes to various SRS phenotypes remains unclear, largely due to incomplete understanding of the developmental functions of these two genes. We previously generated a mouse model with humanized H19/IGF2 imprinting control region (hIC1) on the paternal allele that exhibited H19/Igf2 dysregulation together with SRS-like growth restriction and perinatal lethality. Here, we dissect the role of H19 and Igf2 in cardiac and placental development utilizing multiple mouse models with varying levels of H19 and Igf2. We report severe cardiac defects such as ventricular septal defects and thinned myocardium, placental anomalies including thrombosis and vascular malformations, together with growth restriction in mouse embryos that correlated with the extent of H19/Igf2 dysregulation. Transcriptomic analysis using cardiac endothelial cells of these mouse models shows that H19/Igf2 dysregulation disrupts pathways related to extracellular matrix and proliferation of endothelial cells. Our work links the heart and placenta through regulation by H19 and Igf2, demonstrating that accurate dosage of both H19 and Igf2 is critical for normal embryonic development, especially related to the cardiac-placental axis.

Tissue-specific Grb10/Ddc insulator drives allelic architecture for cardiac development

Allele-specific expression of imprinted gene clusters is governed by gametic DNA methylation at master regulators called imprinting control regions (ICRs). Non-gametic or secondary differentially methylated regions (DMRs) at promoters and exonic regions reinforce monoallelic expression but do not control an entire cluster. Here, we unveil an unconventional secondary DMR that is indispensable for tissue-specific imprinting of two previously unlinked genes, Grb10 and Ddc. Using polymorphic mice, we mapped an intronic secondary DMR at Grb10 with paternal-specific CTCF binding (CBR2.3) that forms contacts with Ddc. Deletion of paternal CBR2.3 removed a critical insulator, resulting in substantial shifting of chromatin looping and ectopic enhancer-promoter contacts. Destabilized gene architecture precipitated abnormal Grb10-Ddc expression with developmental consequences in the heart and muscle. Thus, we redefine the Grb10-Ddc imprinting domain by uncovering an unconventional intronic secondary DMR that functions as an insulator to instruct the tissue-specific, monoallelic expression of multiple genes-a feature previously ICR exclusive.

Two sides of the Dlk1-Dio3 story in imprinting

Loss of imprinting (LOI) at the Dlk1-Dio3 locus is linked to Kagami-Ogata and Temple syndromes, and to cancer, but molecular mechanisms that prevent LOI are under-studied. In this issue of Developmental Cell, Aronson et al. demarcate the bipartite regulation of the Dlk1-Dio3 imprinting control region (ICR) IG-DMR, which maintains locus imprinting.

Derivation and investigation of the first human cell-based model of Beckwith-Wiedemann syndrome

Genomic imprinting is a rare form of gene expression in mammals in which a small number of genes are expressed in a parent-of-origin-specific manner. The aetiology of human imprinting disorders is diverse and includes chromosomal abnormalities, mutations, and epigenetic dysregulation of imprinted genes. The most common human imprinting disorder is Beckwith-Wiedemann syndrome (BWS), frequently caused by uniparental isodisomy and DNA methylation alterations. Because these lesions cannot be easily engineered, induced pluripotent stem cells (iPSC) are a compelling alternative. Here, we describe the first iPSC model derived from patients with BWS. Due to the mosaic nature of BWS patients, both BWS and non-BWS iPSC lines were derived from the same patient's fibroblasts. Importantly, we determine that DNA methylation and gene expression patterns of the imprinted region in the iPSC lines reflect the parental cells and are stable over time. Additionally, we demonstrate that differential expression in insulin signalling, cell proliferation, and cell cycle pathways was seen in hepatocyte lineages derived from BWS lines compared to controls. Thus, this cell based-model can be used to investigate the role of imprinting in the pathogenesis of BWS in disease-relevant cell types.

H19 is not hypomethylated or upregulated with age or sex in the aortic valves of mice

Epigenetic dysregulation of long noncoding RNA H19 was recently found to be associated with calcific aortic valve disease (CAVD) in humans by repressing NOTCH1 transcription. This finding offers a possible epigenetic explanation for the abundance of cases of CAVD that are not explained by any clear genetic mutation. In this study, we examined the effect of age and sex on epigenetic dysregulation of H19 and subsequent aortic stenosis. Cohorts of littermate, wild-type C57BL/6 mice were studied at developmental ages analogous to human middle age through advanced age. Cardiac and aortic valve function were assessed with M-mode echocardiography and pulsed wave Doppler ultrasound, respectively. Bisulfite sequencing was used to determine methylation-based epigenetic regulation of H19, and RT-PCR was used to determine changes in gene expression profiles. Male mice were found to have higher peak systolic velocities than females, with several of the oldest mice showing signs of early aortic stenosis. The imprinting control region of H19 was not hypomethylated with age, and H19 expression was lower in the aortic valves of older mice than in the youngest group. These results suggest that age-related upregulation of H19 is not observed in murine aortic valves and that other factors may initiate H19-related CAVD in humans.

Tissue-specific and mosaic imprinting defects underlie opposite congenital growth disorders in mice

Differential DNA methylation defects of H19/IGF2 are associated with congenital growth disorders characterized by opposite clinical pictures. Due to structural differences between human and mouse, the mechanisms by which mutations of the H19/IGF2 Imprinting Control region (IC1) result in these diseases are undefined. To address this issue, we previously generated a mouse line carrying a humanized IC1 (hIC1) and now replaced the wildtype with a mutant IC1 identified in the overgrowth-associated Beckwith-Wiedemann syndrome. The new humanized mouse line shows pre/post-natal overgrowth on maternal transmission and pre/post-natal undergrowth on paternal transmission of the mutation. The mutant hIC1 acquires abnormal methylation during development causing opposite H19/Igf2 imprinting defects on maternal and paternal chromosomes. Differential and possibly mosaic Igf2 expression and imprinting is associated with asymmetric growth of bilateral organs. Furthermore, tissue-specific imprinting defects result in deficient liver- and placenta-derived Igf2 on paternal transmission and excessive Igf2 in peripheral tissues on maternal transmission, providing a possible molecular explanation for imprinting-associated and phenotypically contrasting growth disorders.

Abstract 1936: Epigenetic challenges in derivation of the first cell-based model of Beckwith-Wiedemann Syndrome

Beckwith-Wiedemann Syndrome (BWS) is a cancer predisposition syndrome that affects at least 1 in 10,500 children. Up to 25% of children with BWS develop tumors, primarily Wilms tumor and hepatoblastoma. BWS is due to genetic or epigenetic changes that affect imprinted loci on chromosome 11 and these same changes are also found in other types of cancer. There are no cell-based models of BWS and most mouse models do not recapitulate the tumor phenotype. To understand more about the mechanisms leading to tumor formation in BWS, we developed the first human cell-based model of BWS. Human induced pluripotent stem cell (iPSC) models are commonly used to study disease mechanisms in tissue types that are not normally accessible for study. In the case of BWS, we plan to use such models to study how the genetic and epigenetic changes in BWS lead to tumor formation in hepatic and renal cells. Using skin fibroblasts from four BWS patients, we derived the first iPSC models of BWS. Prior to iPSC derivation, we characterized different tissues available from these four patients, and demonstrated genetic mosaicism in different tissue types, including blood, skin, and pancreas. During the derivation process, we demonstrated that both normal and BWS iPSC lines could be derived from the same patient fibroblast sample and the number of clones of each type from each sample approximated the initial level of mosaicism in the original sample. For each patient, these lines are isogenic except for the BWS critical region. The BWS and isogenic normal iPSCs were characterized for pluripotency markers and demonstrated to have normal karyotypes. Following this analysis, BWS and isogenic normal lines were characterized extensively for DNA methylation at specific imprinted loci in both early passage and extended culture. Methylation analysis was performed by both pyrosequencing and COBRA assays. Methylation was maintained at some imprinted loci but not at others in extended culture. Importantly, relatively stable methylation levels were observed at the BWS critical imprinted regions (H19/IGF2 and KvDMR), regardless of methods of reprogramming, indicating a relatively stable state of DNA methylation in this region. Additionally, normal methylation was seen at the SNRPN locus. In contrast, another imprinted locus, IG/MEG3, displayed abnormal hypermethylation in iPSCs. These data indicate that reprogramming and extended culture of iPSCs can affect stability of DNA methylation at certain imprinted loci. Therefore caution should be used in interpreting studies using iPSCs as these aberrant methylation states at imprinted loci can affect the downstream functionality of iPSC models. BWS iPSCs and isogenic controls with normal methylation will be used for further study of the mechanism of tumor formation in BWS.

Note: This abstract was not presented at the meeting.

Citation Format: Stella K. Hur, Joanne L. Thorvaldsen, Suhee Chang, Carolyn Lye, Alice Yu, Monserrat C. Anguera, Marisa S. Bartolomei, Jennifer M. Kalish. Epigenetic challenges in derivation of the first cell-based model of Beckwith-Wiedemann Syndrome [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1936. doi:10.1158/1538-7445.AM2017-1936

Humanized H19/Igf2 locus reveals diverged imprinting mechanism between mouse and human and reflects Silver-Russell syndrome phenotypes

Genomic imprinting affects a subset of genes in mammals, such that they are expressed in a monoallelic, parent-of-origin-specific manner. These genes are regulated by imprinting control regions (ICRs), cis-regulatory elements that exhibit allele-specific differential DNA methylation. Although genomic imprinting is conserved in mammals, ICRs are genetically divergent across species. This raises the fundamental question of whether the ICR plays a species-specific role in regulating imprinting at a given locus. We addressed this question at the H19/insulin-like growth factor 2 (Igf2) imprinted locus, the misregulation of which is associated with the human imprinting disorders Beckwith-Wiedemann syndrome (BWS) and Silver-Russell syndrome (SRS). We generated a knock-in mouse in which the endogenous H19/Igf2 ICR (mIC1) is replaced by the orthologous human ICR (hIC1) sequence, designated H19(hIC1) We show that hIC1 can functionally replace mIC1 on the maternal allele. In contrast, paternally transmitted hIC1 leads to growth restriction, abnormal hIC1 methylation, and loss of H19 and Igf2 imprinted expression. Imprint establishment at hIC1 is impaired in the male germ line, which is associated with an abnormal composition of histone posttranslational modifications compared with mIC1. Overall, this study reveals evolutionarily divergent paternal imprinting at IC1 between mice and humans. The conserved maternal imprinting mechanism and function at IC1 demonstrates the possibility of modeling maternal transmission of hIC1 mutations associated with BWS in mice. In addition, we propose that further analyses in the paternal knock-in H19(+/hIC1) mice will elucidate the molecular mechanisms that may underlie SRS.

Tissue-specific insulator function at H19/Igf2 revealed by deletions at the imprinting control region

Parent-of-origin-specific expression at imprinted genes is regulated by allele-specific DNA methylation at imprinting control regions (ICRs). This mechanism of gene regulation, where one element controls allelic expression of multiple genes, is not fully understood. Furthermore, the mechanism of gene dysregulation through ICR epimutations, such as loss or gain of DNA methylation, remains a mystery. We have used genetic mouse models to dissect ICR-mediated genetic and epigenetic regulation of imprinted gene expression. The H19/insulin-like growth factor 2 (Igf2) ICR has a multifunctional role including insulation, activation and repression. Microdeletions at the human H19/IGF2 ICR (IC1) are proposed to be responsible for IC1 epimutations associated with imprinting disorders such as Beckwith-Wiedemann syndrome (BWS). Here, we have generated and characterized a mouse model that mimics BWS microdeletions to define the role of the deleted sequence in establishing and maintaining epigenetic marks and imprinted expression at the H19/IGF2 locus. These mice carry a 1.3 kb deletion at the H19/Igf2 ICR [Δ2,3] removing two of four CCCTC-binding factor (CTCF) sites and the intervening sequence, ∼75% of the ICR. Surprisingly, the Δ2,3 deletion does not perturb DNA methylation at the ICR; however, it does disrupt imprinted expression. While repressive functions of the ICR are compromised by the deletion regardless of tissue type, insulator function is only disrupted in tissues of mesodermal origin where a significant amount of CTCF is poly(ADP-ribosyl)ated. These findings suggest that insulator activity of the H19/Igf2 ICR varies by cell type and may depend on cell-specific enhancers as well as posttranslational modifications of the insulator protein CTCF.

Maternal imprinting at the H19-Igf2 locus maintains adult haematopoietic stem cell quiescence

The epigenetic regulation of imprinted genes by monoallelic DNA methylation of either maternal or paternal alleles is critical for embryonic growth and development. Imprinted genes were recently shown to be expressed in mammalian adult stem cells to support self-renewal of neural and lung stem cells; however, a role for imprinting per se in adult stem cells remains elusive. Here we show upregulation of growth-restricting imprinted genes, including in the H19-Igf2 locus, in long-term haematopoietic stem cells and their downregulation upon haematopoietic stem cell activation and proliferation. A differentially methylated region upstream of H19 (H19-DMR), serving as the imprinting control region, determines the reciprocal expression of H19 from the maternal allele and Igf2 from the paternal allele. In addition, H19 serves as a source of miR-675, which restricts Igf1r expression. We demonstrate that conditional deletion of the maternal but not the paternal H19-DMR reduces adult haematopoietic stem cell quiescence, a state required for long-term maintenance of haematopoietic stem cells, and compromises haematopoietic stem cell function. Maternal-specific H19-DMR deletion results in activation of the Igf2-Igfr1 pathway, as shown by the translocation of phosphorylated FoxO3 (an inactive form) from nucleus to cytoplasm and the release of FoxO3-mediated cell cycle arrest, thus leading to increased activation, proliferation and eventual exhaustion of haematopoietic stem cells. Mechanistically, maternal-specific H19-DMR deletion leads to Igf2 upregulation and increased translation of Igf1r, which is normally suppressed by H19-derived miR-675. Similarly, genetic inactivation of Igf1r partly rescues the H19-DMR deletion phenotype. Our work establishes a new role for this unique form of epigenetic control at the H19-Igf2 locus in maintaining adult stem cells.

A YY1 bridge for X inactivation

Xist RNA inactivates one mammalian X chromosome (the Xi) by associating with it in cis. The mechanism of this interaction is unresolved. Jeon and Lee (2011) now show that YY1 binds both Xist RNA and DNA, thereby providing a mechanism to anchor Xist to the Xi and facilitate X chromosome inactivation.

Novel cis-regulatory function in ICR-mediated imprinted repression of H19

Expression of coregulated imprinted genes, H19 and Igf2, is monoallelic and parent-of-origin-dependent. Like most imprinted genes, H19 and Igf2 are regulated by a differentially methylated imprinting control region (ICR). CTCF binding sites and DNA methylation at the ICR have previously been identified as key cis-acting elements required for proper H19/Igf2 imprinting. Here, we use mouse models to elucidate further the mechanism of ICR-mediated gene regulation. We specifically address the question of whether sequences outside of CTCF sites at the ICR are required for paternal H19 repression. To this end, we generated two types of mutant ICRs in the mouse: (i) deletion of intervening sequence between CTCF sites (H19(ICR∆IVS)), which changes size and CpG content at the ICR; and (ii) CpG depletion outside of CTCF sites (H19(ICR-8nrCG)), which only changes CpG content at the ICR. Individually, both mutant alleles (H19(ICR∆IVS) and H19(ICR-8nrCG)) show loss of imprinted repression of paternal H19. Interestingly, this loss of repression does not coincide with a detectable change in methylation at the H19 ICR or promoter. Thus, neither intact CTCF sites nor hypermethylation at the ICR is sufficient for maintaining the fully repressed state of the paternal H19 allele. Our findings demonstrate, for the first time in vivo, that sequence outside of CTCF sites at the ICR is required in cis for ICR-mediated imprinted repression at the H19/Igf2 locus. In addition, these results strongly implicate a novel role of ICR size and CpG density in paternal H19 repression.

Ribonuclease protection

The ribonuclease protection assay (RPA) is a sensitive technique for the analysis of total cellular RNA. It involves generating a specific antisense riboprobe, hybridizing the probe to total RNA, removing unprotected RNA by RNases, and finally isolating and analyzing the protected RNA on a denaturing gel. Although the RPA is somewhat more labor-intensive than Northern analysis, it has the advantage of being more sensitive (as little as 0.1 pg of target RNA can be detected with ideal hybridization conditions). RPAs are also more tolerant of partially degraded RNA (provided the area that is protected is intact). Although RPAs are not as sensitive as polymerase chain reaction (PCR)-based RNA analyses, the target RNA is analyzed directly; a reverse transcription step is not required. Finally, the RPA is quantitative as long as the probe is in excess. More important for the study of imprinted genes, the RPA can be designed to detect allele-specific expression of the target gene of interest.

Three-dimensional conformation at the H19/Igf2 locus supports a model of enhancer tracking

Insight into how the mammalian genome is structured in vivo is key to understanding transcriptional regulation. This is especially true in complex domains in which genes are coordinately regulated by long-range interactions between cis-regulatory elements. The regulation of the H19/Igf2 imprinted region depends on the presence of several cis-acting sequences, including a methylation-sensitive insulator between Igf2 and H19 and shared enhancers downstream of H19. Each parental allele has a distinct expression pattern. We used chromosome conformation capture to assay the native three-dimensional organization of the H19/Igf2 locus on each parental copy. Furthermore, we compared wild-type chromosomes to several mutations that affect the insulator. Our results show that promoters and enhancers reproducibly co-localize at transcriptionally active genes, i.e. the endodermal enhancers contact the maternal H19 and the paternal Igf2 genes. The active insulator blocks traffic of the enhancers along the chromosome, restricting them to the H19 promoter. Conversely, the methylated inactive insulator allows the enhancers to contact the upstream regions, including Igf2. Mutations that either remove or inhibit insulator activity allow unrestricted access of the enhancers to the whole region. A mutation that allows establishment of an enhancer-blocker on the normally inactive paternal copy diminishes the contact of the enhancer with the Igf2 gene. Based on our results, we propose that physical proximity of cis-acting DNA elements is vital for their activity in vivo. We suggest that enhancers track along the chromosome until they find a suitable promoter sequence to interact with and that insulator elements block further tracking of enhancers.

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

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.

X-tra! X-tra! News from the mouse X chromosome

X chromosome inactivation (XCI) is the phenomenon through which one of the two X chromosomes in female mammals is silenced to achieve dosage compensation with males. XCI is a highly complex, tightly controlled and developmentally regulated process. The mouse undergoes two forms of XCI: imprinted, which occurs in all cells of the preimplantation embryo and in the extraembryonic lineage, and random, which occurs in somatic cells after implantation. This review presents results and hypotheses that have recently been proposed concerning important aspects of both imprinted and random XCI in mice. We focus on how imprinted XCI occurs during preimplantation development, including a brief discussion of the debate as to when silencing initiates. We also discuss regulation of random XCI, focusing on the requirement for Tsix antisense transcription through the Xist locus, on the regulation of Xist chromatin structure by Tsix and on the effect of Tsix regulatory elements on choice and counting. Finally, we review exciting new data revealing that X chromosomes co-localize during random XCI. To conclude, we highlight other aspects of X-linked gene regulation that make it a suitable model for epigenetics at work.

Developmental profile of H19 differentially methylated domain (DMD) deletion alleles reveals multiple roles of the DMD in regulating allelic expression and DNA methylation at the imprinted H19/Igf2 locus

The differentially methylated domain (DMD) of the mouse H19 gene is a methylation-sensitive insulator that blocks access of the Igf2 gene to shared enhancers on the maternal allele and inactivates H19 expression on the methylated paternal allele. By analyzing H19 DMD deletion alleles H19DeltaDMD and H19Delta3.8kb-5'H19 in pre- and postimplantation embryos, we show that the DMD exhibits positive transcriptional activity and is required for H19 expression in blastocysts and full activation of H19 during subsequent development. We also show that the DMD is required to establish Igf2 imprinting by blocking access to shared enhancers when Igf2 monoallelic expression is initiated in postimplantation embryos and that the single remaining CTCF site of the H19DeltaDMD allele is unable to provide this function. Furthermore, our data demonstrate that sequence outside of the DMD can attract some paternal-allele-specific CpG methylation 5' of H19 in preimplantation embryos, although this methylation is not maintained during postimplantation in the absence of the DMD. Finally, we report a conditional allele floxing the 1.6-kb sequence deleted from the H19DeltaDMD allele and demonstrate that the DMD is required to maintain repression of the maternal Igf2 allele and the full activity of the paternal Igf2 allele in neonatal liver.

An N-ethyl-N-nitrosourea mutagenesis screen for epigenetic mutations in the mouse

The mammalian epigenetic phenomena of X inactivation and genomic imprinting are incompletely understood. X inactivation equalizes X-linked expression between males and females by silencing genes on one X chromosome during female embryogenesis. Genomic imprinting functionally distinguishes the parental genomes, resulting in parent-specific monoallelic expression of particular genes. N-ethyl-N-nitrosourea (ENU) mutagenesis was used in the mouse to screen for mutations in novel factors involved in X inactivation. Previously, we reported mutant pedigrees identified through this screen that segregate aberrant X-inactivation phenotypes and we mapped the mutation in one pedigree to chromosome 15. We now have mapped two additional mutations to the distal chromosome 5 and the proximal chromosome 10 in a second pedigree and show that each of the mutations is sufficient to induce the mutant phenotype. We further show that the roles of these factors are specific to embryonic X inactivation as neither genomic imprinting of multiple genes nor imprinted X inactivation is perturbed. Finally, we used mice bearing selected X-linked alleles that regulate X chromosome choice to demonstrate that the phenotypes of all three mutations are consistent with models in which the mutations have affected molecules involved specifically in the choice or the initiation of X inactivation.

Analysis of sequence upstream of the endogenous H19 gene reveals elements both essential and dispensable for imprinting

Imprinting of the linked and oppositely expressed mouse H19 and Igf2 genes requires a 2-kb differentially methylated domain (DMD) that is located 2 kb upstream of H19. This element is postulated to function as a methylation-sensitive insulator. Here we test whether an additional sequence 5' of H19 is required for H19 and Igf2 imprinting. Because repetitive elements have been suggested to be important for genomic imprinting, the requirement of a G-rich repetitive element that is located immediately 3' to the DMD was first tested in two targeted deletions: a 2.9-kb deletion (Delta D MD Delta G) that removes the DMD and G-rich repeat and a 1.3-kb deletion (Delta G) removing only the latter. There are also four 21-bp GC-rich repetitive elements within the DMD that bind the insulator-associated CTCF (CCCTC-binding factor) protein and are implicated in mediating methylation-sensitive insulator activity. As three of the four repeats of the 2-kb DMD were deleted in the initial 1.6-kb Delta DMD allele, we analyzed a 3.8-kb targeted allele (Delta 3.8kb-5'H19), which deletes the entire DMD, to test the function of the fourth repeat. Comparative analysis of the 5' deletion alleles reveals that (i) the G-rich repeat element is dispensable for imprinting, (ii) the Delta DMD and Delta DMD Delta G alleles exhibit slightly more methylation upon paternal transmission, (iii) removal of the 5' CTCF site does not further perturb H19 and Igf2 imprinting, suggesting that one CTCF-binding site is insufficient to generate insulator activity in vivo, (iv) the DMD sequence is required for full activation of H19 and Igf2, and (v) deletion of the DMD disrupts H19 and Igf2 expression in a tissue-specific manner.

Akt1/PKBalpha is required for normal growth but dispensable for maintenance of glucose homeostasis in mice

The serine-threonine kinase Akt, also known as protein kinase B (PKB), is an important effector for phosphatidylinositol 3-kinase signaling initiated by numerous growth factors and hormones. Akt2/PKBbeta, one of three known mammalian isoforms of Akt/PKB, has been demonstrated recently to be required for at least some of the metabolic actions of insulin (Cho, H., Mu, J., Kim, J. K., Thorvaldsen, J. L., Chu, Q., Crenshaw, E. B., Kaestner, K. H., Bartolomei, M. S., Shulman, G. I., and Birnbaum, M. J. (2001) Science 292, 1728-1731). Here we show that mice deficient in another closely related isoform of the kinase, Akt1/PKBalpha, display a conspicuous impairment in organismal growth. Akt1(-/-) mice demonstrated defects in both fetal and postnatal growth, and these persisted into adulthood. However, in striking contrast to Akt2/PKBbeta null mice, Akt1/PKBalpha-deficient mice are normal with regard to glucose tolerance and insulin-stimulated disposal of blood glucose. Thus, the characterization of the Akt1 knockout mice and its comparison to the previously reported Akt2 deficiency phenotype reveals the non-redundant functions of Akt1 and Akt2 genes with respect to organismal growth and insulin-regulated glucose metabolism.

Insulin resistance and a diabetes mellitus-like syndrome in mice lacking the protein kinase Akt2 (PKB beta)

Glucose homeostasis depends on insulin responsiveness in target tissues, most importantly, muscle and liver. The critical initial steps in insulin action include phosphorylation of scaffolding proteins and activation of phosphatidylinositol 3-kinase. These early events lead to activation of the serine-threonine protein kinase Akt, also known as protein kinase B. We show that mice deficient in Akt2 are impaired in the ability of insulin to lower blood glucose because of defects in the action of the hormone on liver and skeletal muscle. These data establish Akt2 as an essential gene in the maintenance of normal glucose homeostasis.

Molecular biology. Mothers setting boundaries

Certain genes are only expressed at one allele, a phenomenon called imprinting. Although it is well established that one allele of certain imprinted genes is silenced through methylation, this does not appear to be the case for all imprinted genes. In a thoughtful Perspective, Thorvaldsen and Bartolomei discuss new findings showing that insertion of insulator elements (boundary regions) between the promoter of a gene and its enhancer (a sequence that boosts gene expression) may be another way in which genes are silenced during imprinting.

Role of a 461-bp G-rich repetitive element in H19 transgene imprinting

The molecular mechanism leading to the imprinted expression of genes is poorly understood. While no conserved cis-acting elements have been identified within the known loci, many imprinted genes are located near directly repetitive sequence elements, suggesting that such repeats might play a role in imprinted gene expression. The maternally expressed mouse H19 gene is located approximately 1.5 kb downstream from a 461-bp G-rich repetitive element. We have used a transgenic model to investigate whether this element is essential for H19 imprinting. Previous results demonstrated that a transgene, which contains 14 kb of H19 sequence, exhibits parent-of-origin specific expression and methylation analogous to the endogenous H19 imprinting pattern. Here, we have generated transgenes lacking the G-rich repeat. One transgene, containing a deletion of the G-rich repetitive element but which includes an additional 1.7 kb of 5' H19 sequence, is imprinted similarly to the endogenous H19 gene. To determine whether the G-rich repeat is conserved in other imprinted mammalian H19 homologues, additional 5' flanking sequences were cloned from the rat and human. This element is conserved in the rat but not in human DNA. These results suggest that the 461-bp G-rich repetitive element is not essential for H19 imprinting.

Deletion of the H19 differentially methylated domain results in loss of imprinted expression of H19 and Igf2

Differentially methylated sequences associated with imprinted genes are proposed to control genomic imprinting. A 2-kb region located 5' to the imprinted mouse H19 gene is hypermethylated on the inactive paternal allele throughout development. To determine whether this differentially methylated domain (DMD) is required for imprinted expression at the endogenous locus, we have generated mice harboring a 1.6-kb targeted deletion of the DMD and assayed for allelic expression of H19 and the linked, oppositely imprinted Igf2 gene. H19 is activated and Igf2 expression is reduced when the DMD deletion is paternally inherited; conversely, upon maternal transmission of the mutation, H19 expression is reduced and Igf2 is activated. Consistent with the DMD's hypothesized role of setting up the methylation imprint, the mutation also perturbs allele-specific methylation of the remaining H19 sequences. In conclusion, these experiments show that the H19 hypermethylated 5' flanking sequences are required to silence paternally derived H19. Additionally, these experiments demonstrate a novel role for the DMD on the maternal chromosome where it is required for the maximal expression of H19 and the silencing of Igf2. Thus, the H19 differentially methylated sequences are required for both H19 and Igf2 imprinting.

Identification of the Zn(II) site in the copper-responsive yeast transcription factor, AMT1: a conserved Zn module

The N-terminal metal-binding domains of the copper-activated yeast transcription factors, ACE1 and AMT1, bind to specific DNA sequences in a Cu-dependent fashion. Recombinant AMT1 and ACE1 metal-binding domains are isolated as Cu4Zn1-protein complexes. Site-directed mutagenesis of AMT1 was used in this study to map the ligands of the Cu(I) and Zn(II) ions. The results are consistent with the N-terminal halves of AMT1 and ACE1 consisting of two independent submodules, one binding a single Zn(II) ion and the second binding the tetracopper cluster. The basis of this conclusion is, first, that mutations of two cysteinyl codons and a histidyl codon in the first 42 residues of AMT1 do not alter DNA binding. In contrast, serine substitutions at four cysteine positions at codons 43, 61, 90, and 98 abolish DNA binding. We demonstrated previously that population of the Zn(II) site in AMT1 does not alter the ability of the protein to bind DNA but bound Cu(I) ions are essential for DNA binding [Thorvaldsen, J. L., et al. (1994) Biochemistry 33, 9566-9577]. Second, mutations in the N-terminal 42 residue segment reduce the Zn(II) content of purified mutant AMT1 molecules. Third, a synthetic peptide consisting of the N-terminal 42 residues in AMT1 forms a stable Zn(II) complex and substitution with Co(II) reveals an electronic spectrum identical to that of the Co-substituted intact Cu4AMT1 protein. 113Cd(II) NMR studies reveal that the divalent metal site consists of ligands provided by three cysteinyl thiolates and a single histydyl imidazole. The sequence homology between AMT1, ACE1, and MAC1 in the N-terminal 42 residues suggests that ACE1 and MAC1 will, likewise, contain N-terminal Zn modules. A 42-residue ACE1 synthetic peptide gives identical metal binding properties to the corresponding AMT1 synthetic peptide. Thus, AMT1 and likely ACE1 consist of two contiguous modules, residues 1-42 forming an independent Zn(II) module and residues 43-110 enfolding a tetracopper cluster.

Analysis of copper-induced metallothionein expression using autonomously replicating plasmids in Candida glabrata

Candida glabrata strains and a stable plasmid were developed that were suitable for analysis of copper-inducible expression from promoters of the three metallothionein (MT) genes. The two homologous MTII genes, MTIIa and MTIIb, encode the same polypeptide but are differentially induced by copper salts. MTIIb is more highly inducible than MTIIa and cells harboring a single MTIIb exhibit a greater resistance to copper salts compared to cells harboring a single MTIIa. The differential copper inducibility was mapped to sequences between -03 and -292 upstream of the MT coding sequences. Expression of MTI is highly Cu-regulated, but this MT gene confers much less resistance than MTII genes.

Mixed Cu+ and Zn2+ coordination in the DNA-binding domain of the AMT1 transcription factor from Candida glabrata

AMT1 is the transcription factor required for Cu-induced expression of metallothionein genes in the yeast Candida glabrata. The copper-binding, DNA-binding domain of AMT1 has been purified after expression of an AMT1 synthetic gene in bacteria and was confirmed as active in a gel shift assay. The Cu-activated AMT1 was shown to contain a Cu(+)-thiolate tetracopper center and a single Zn2+ site. AMT1 is purified as a Cu-Zn protein from bacterial cultures grown in the presence of CuSO4. Chemical analysis suggested that 4.2 +/- 0.2 and 1.2 +/- 0.2 molar equiv copper and zinc ions bound, respectively. Electrospray mass spectrometry was used to verify that a uniform species was present with 4 Cu+ ions and 1 Zn2+ ion bound per AMT1 molecule. Cu+ binding to form a tetracopper center occurs cooperatively as shown by electrospray MS of apoAMT1 samples reconstituted with increasing equivalency of Cu+. Copper-thiolate coordination was indicated by Cu-S charge-transfer transitions in the ultraviolet, luminescence typical of Cu-thiolate clusters and EXAFS. Analysis of the EXAFS of CuZnAMT1 revealed predominantly trigonal Cu+ coordination and the presence of a polycopper cluster by virtue of a short Cu-Cu distance of 2.7 A. Zn K-edge EXAFS of Cu4Zn1AMT1 and electronic spectroscopy of AMT1 with Co2+ substituted for the single Zn2+ ion are consistent with tetrahedral Zn2+ coordination with thiolate ligands. The Cu-activated AMT1 exhibited a conformation distinct from that of metal-free AMT1 as shown by circular dichroism. DNA binding by AMT1 was dependent on the tetracopper center but was independent of occupancy of the Zn2+ site. This is the first report of a single, uniform tetracopper center in a metal-activated transcription factor.

Heterologous gene expression and protein secretion from Candida glabrata

We have examined heterologous protein secretion from Candida glabrata with the aid of a stable C. glabrata vector and a secretion reporter cassette comprising the Saccharomyces cerevisiae PGK-gene promoter and a Kluveromyces Iactis secretion signal to drive secretion of Escherichia coli beta-lactamase. Abundant secretion of beta-lactamase from C. glabrata indicates that the S. cerevisiae PGK promoter functions in C. glabrata. Furthermore, we show that C. glabrata processes the secreted beta-lactamase in a manner similar to, but not identical with, S. cerevisiae and K. lactis. C. glabrata may be a suitable new host for the expression of foreign genes.

Regulation of metallothionein genes by the ACE1 and AMT1 transcription factors

The AMT1 metalloregulatory trans-acting factor from Candida glabrata was found to functionally mimic the ACE1 metalloregulatory trans-acting factor from Saccharomyces cerevisiae in the copper-induced expression of the chromosomal S. cerevisiae metallothionein gene. Plasmid constructs with promoters of various metal-inducible genes fused to the bacterial beta-galactosidase (lacZ) reporter gene were used in S. cerevisiae to evaluate the roles of ACE1 and AMT1 in mediating metal-stimulated expression. Promoters from the S. cerevisiae CUP1 gene and Cu,Zn-superoxide dismutase (SOD1) and from the C. glabrata MT genes MTI, MTIIa, and MTIIb were used. The ACE1 factor was effective in the metalloregulation of the two S. cerevisiae promoters, CUP1 and SOD1, but of only one C. glabrata promoter, MTI. AMT1 was found to be effective in the metalloregulation of all three C. glabrata MT promoters and the two S. cerevisiae promoters tested. The regulation mediated by both ACE1 and AMT1 was copper-dependent and copper-specific. Episomally expressed SWI5, a distinct trans-acting factor of S. cerevisiae, enhanced only the basal expression from promoters. The SWI5 enhancement was not metal dependent. In conclusion, AMT1 and ACE1 are functionally homologous in metal-specific regulation, AMT1 appears to be more promiscuous than ACE1 in this function.

Disruption analysis of metallothionein-encoding genes in Candida glabrata

Candida glabrata harbors multiple genes encoding metallothionein (MT). We have disrupted MT-IIa, an amplified locus, and MT-IIb, a single-copy gene, to determine the roles of various MT genes in CuSO4 resistance in C. glabrata. The concentration of CuSO4 required to inhibit the growth by 50% (IC50) of a C. glabrata strain harboring an amplified MT-IIa locus and a single-copy MT-IIb and MT-I genes was 7 mM in a synthetic complete medium. The IC50 decreased to approx. 1 mM when the amplified MT-IIa locus was deleted. The disruption of the MT-IIb gene decreased the IC50 further to 0.1 mM. The CuSO4 resistance in a strain lacking both of the MT-II genes was attributable to MT-I; no evidence was found for the production of (gamma EC)nG isopeptides. The comparison of the nucleotide sequence of MT-IIb to that of MT-IIa revealed the same coding sequence with differences in the 5' region. However, substantial differences were found in the 3' region. MT-IIb was expressed since we were able to purify the protein from the strain that had an intact MT-IIb gene, but a deleted MT-IIa gene. In addition, CuSO4 resistance was provided by MT-IIb. Northern analysis of the total RNA from varied C. glabrata strains indicated no significant changes in the expression of MT-I in the presence or absence of the MT-II genes.

Cloning system for Candida glabrata using elements from the metallothionein-IIa-encoding gene that confer autonomous replication

The yeast Candida glabrata harbors two distinct gene families that encode metallothioneins (MTs). One of these loci, the MT-IIa locus, exhibits selective and tandem amplification in many wild type strains of C. glabrata. The present paper demonstrates that the amplified MT-IIa gene contains autonomously replicating sequences (ARS). These ARS elements have been used to construct vectors capable of replicating in C. glabrata. The ARS element(s) in the MT-IIa gene were localized to a 457-bp segment downstream from the MT-IIa coding sequence. Although plasmids containing this fragment transform C. glabrata with high frequency, the stability of the transformants and the copy number of the plasmid improve when the entire 1.25-kb MT-IIa gene is used. Transformation of C. glabrata with plasmids carrying the 2 microns circle ARS of Saccharomyces cerevisiae led to the formation of micro-colonies, indicating that the ARS elements of 2 microns plasmids replicate only to a limited extent in C. glabrata. Conversely, a C. glabrata plasmid carrying three copies of the MT-IIa gene was able to transform S. cerevisiae.