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Biomedical subjects

Wendy Dean

Publications and source records attributed to Wendy Dean.

30 records · Page 2Linked to original sources
PubMed2003

Epigenetic modifications in an imprinting cluster are controlled by a hierarchy of DMRs suggesting long-range chromatin interactions.

Imprinted genes and their control elements occur in clusters in the mammalian genome and carry epigenetic modifications. Observations from imprinting disorders suggest that epigenetic modifications throughout the clusters could be under regional control. However, neither the elements that are responsible for regional control, nor its developmental timing, particularly whether it occurs in the germline or postzygotically, are known. Here we examine regional control of DNA methylation in the imprinted Igf2-H19 region in the mouse. Paternal germline specific methylation was reprogrammed after fertilization in two differentially methylated regions (DMRs) in Igf2, and was reestablished after implantation. Using a number of knockout strains in the region, we found that the DMRs themselves are involved in regional coordination in a hierarchical fashion. Thus the H19 DMR was needed on the maternal allele to protect the Igf2 DMRs 1 and 2 from methylation, and Igf2 DMR1 was needed to protect DMR2 from methylation. This regional coordination occurred exclusively after fertilization during somatic development, and did not involve linear spreading of DNA methylation, suggesting a model in which long-range chromatin interactions are involved in regional epigenetic coordination. These observations are likely to be relevant to other gene clusters in which epigenetic regulation plays a role, and in pathological situations in which epigenetic regulation is disrupted.

Animals↗
PubMed2003

Regulation of supply and demand for maternal nutrients in mammals by imprinted genes.

The placenta has evolved in eutherian mammals primarily to provide nutrients for the developing fetus. The genetic control of the regulation of supply and demand for maternal nutrients is not understood. In this review we argue that imprinted genes have central roles in controlling both the fetal demand for, and the placental supply of, maternal nutrients. Recent studies on Igf2 (insulin-like growth factor 2) knockout mouse models provide experimental support for this hypothesis. These show effects on placental transport capacity consistent with a role of IGF-II in modulating both the placental supply and fetal demand for nutrients. Imprinting of genes with such functions may have coevolved with the placenta and new evidence suggests that transporter proteins, as well as the regulators themselves, may also be imprinted. These data and hypotheses are important, as deregulation of supply and demand affects fetal growth and has long term consequences for health in mammals both in the neonatal period and, as a result of fetal programming, in adulthood.

Animals↗
PubMed2003

Resistance of IAPs to methylation reprogramming may provide a mechanism for epigenetic inheritance in the mouse.

Genome-wide epigenetic reprogramming by demethylation occurs in early mouse embryos and primordial germ cells. In early embryos many single-copy sequences become demethylated both by active and passive demethylation, whereas imprinted gene methylation remains unaffected. In primordial germ cells single-copy and imprinted sequences are demethylated, presumably by active demethylation. Here we investigated systematically by bisulphite sequencing the methylation profiles of IAP and Line1 repeated sequence families during preimplantation and primordial germ cell development. Whereas Line1 elements were substantially demethylated during both developmental periods, IAP elements were largely resistant to demethylation, particularly during preimplantation development. This may be desirable in order to prevent IAP retrotransposition, which could cause mutations. In turn, this can result in the transgenerational inheritance of epigenetic states of IAPs, which could lead to heritable epimutations of neighbouring genes through influencing their transcriptional states.

Alleles↗
PubMed2003

Epigenetic reprogramming in early mammalian development and following somatic nuclear transfer.

Epigenetic modifications of the genome play a significant role in the elaboration of the genetic code as established at fertilisation. These modifications affect early growth and development through their influence on gene expression especially on imprinted genes. Genome-wide epigenetic reprogramming in germ cells is essential in order to reset the parent-of-origin specific marking of imprinted genes, but may have a more general role in the restoration of totipotency in the early embryo. In a similar way, on somatic nuclear cloning, a differentiated cell must become 'reprogrammed' restoring totipotency in order to undergo development. Here we discuss the dynamic epigenetic reprogramming that takes place during normal development and highlight those areas with relevance to somatic nuclear cloning and the possibility of improving the efficiency of this process. We propose the concept of 'epigenetic checkpoints' for normal progression of development and the loss of totipotency.

Animals↗
PubMed2003

Identification of novel imprinted genes in a genome-wide screen for maternal methylation.

A characteristic of imprinted genes is that the maternal and paternal alleles show differences in methylation. To perform a genome-wide screen for novel imprinted loci, we applied methylation-sensitive representational difference analysis (Me-RDA) to parthenogenetic mouse embryos, to identify differentially methylated regions (DMRs) methylated specifically on the maternal allele. We isolated a total of 26 distinct clones from known and novel DMRs and identified three novel imprinted genes. Nap1l5 is located on proximal chromosome 6 and encodes a protein with homology with nucleosome assembly proteins (NAPs); it has tissue-specific imprinting with expression from the paternal allele. We identified two DMRs on chromosome 15, a chromosome that was not thought to contain imprinted loci, and demonstrated that each is associated with a paternally expressed transcript. Peg13 gives rise to a noncoding RNA that is highly expressed in the brain and imprinted in all tissues examined. A DMR was also identified at the chromosome 15 Slc38a4 gene, which encodes a system A amino acid transporter; we show that Slc38a4 is imprinted in a tissue-specific manner. Interestingly, two of the three novel genes identified in this screen are located within the introns of other genes; their identification indicates that such "microimprinted" domains may be more common than previously thought.

Alleles↗
PubMed2003

Epigenetic properties and identification of an imprint mark in the Nesp-Gnasxl domain of the mouse Gnas imprinted locus.

The Gnas locus in the mouse is imprinted with a complex arrangement of alternative transcripts defined by promoters with different patterns of monoallelic expression. The Gnas transcript is subject to tissue-specific imprinted expression, Nesp is expressed only from the maternal allele, and Gnasxl is expressed only from the paternal allele. The mechanisms controlling these expression patterns are not known. To identify potential imprinting regulatory regions, particularly for the reciprocally expressed Nesp and Gnasxl promoters, we examined epigenetic properties of the locus in gametes, embryonic stem cells, and fetal and adult tissues. The Nesp and Gnasxl promoter regions are contained in extensive CpG islands with methylation of the paternal allele at Nesp and the maternal allele at Gnasxl. Parental allele-specific DNase I-hypersensitive sites were found at these regions, which correlate with hypomethylation rather than actual expression status. A germ line methylation mark was identified covering the promoters for Gnasxl and the antisense transcript Nespas. Prominent DNase I-hypersensitive sites present on paternal alleles in embryonic stem cells are contained within this mark. This is the second gametic mark identified at Gnas and suggests that the Nesp and Gnasxl promoters are under separate control from the Gnas promoter. We propose models to account for the regulation of imprinting at the locus.

Alleles↗
PubMed2002

Placental-specific IGF-II is a major modulator of placental and fetal growth.

Imprinted genes in mammals are expressed from only one of the parental chromosomes, and are crucial for placental development and fetal growth. The insulin-like growth factor II gene (Igf2) is paternally expressed in the fetus and placenta. Here we show that deletion from the Igf2 gene of a transcript (P0) specifically expressed in the labyrinthine trophoblast of the placenta leads to reduced growth of the placenta, followed several days later by fetal growth restriction. The fetal to placental weight ratio is thus increased in the absence of the P0 transcript. We show that passive permeability for nutrients of the mutant placenta is decreased, but that secondary active placental amino acid transport is initially upregulated, compensating for the decrease in passive permeability. Later the compensation fails and fetal growth restriction ensues. Our study provides experimental evidence for imprinted gene action in the placenta that directly controls the supply of maternal nutrients to the fetus, and supports the genetic conflict theory of imprinting. We propose that the Igf2 gene, and perhaps other imprinted genes, control both the placental supply of, and the genetic demand for, maternal nutrients to the mammalian fetus.

Alleles↗
PubMed2002

Dynamic reprogramming of DNA methylation in the early mouse embryo.

Dynamic epigenetic modification of the genome occurs during early development of the mouse. Active demethylation of the paternal genome occurs in the zygote, followed by passive demethylation during cleavage stages, and de novo methylation, which is thought to happen after implantation. We have investigated these processes by using indirect immunofluorescence with an antibody to 5-methyl cytosine. In contrast to previous work, we show that demethylation of the male pronucleus is completed within 4 h of fertilisation. This activity is intricately linked with and not separable from pronucleus formation. In conditions permissive for polyspermy, up to five male pronuclei underwent demethylation in the same oocyte. Paternal demethylation in fertilised oocytes deficient for MBD2, the only candidate demethylase, occurred normally. Passive loss of methylation occurred in a stepwise fashion up to the morulae stage without any evidence of spatial compartmentalisation. De novo methylation was observed specifically in the inner cell mass (ICM) but not in the trophectoderm of the blastocyst and hence may have an important role in early lineage specification. This is the first complete and detailed analysis of the epigenetic reprogramming cycle during preimplantation development. The three phases of methylation reprogramming may have roles in imprinting, the control of gene expression, and the establishment of nuclear totipotency.

Animals↗
PubMed2002

Community mental health care for women with severe mental illness who are parents.

Women with SMI are involved in childbearing and childrearing, but may have problems with parenting related to symptoms, lack of knowledge and skills, or lack of environmental supports. They may need intensive services to manage their mental illness and to parent to the best of their abilities. Preliminary research suggests that mental health interventions can improve family functioning, but that these services are not widely available in community mental health systems. This paper describes services to help clients who are parents, which includes integration of adult's and children's services, long-term, home-based parent training, and linkage with community supports.

Adult↗
PubMed2002

Nucleus replacement in Mammalian oocytes.

Our contribution discusses the potential use of cell therapies (nucleus replacement) in mammalian oocytes. It is assumed that these approaches may be used, for example, for the elimination of mutated maternally transmitted mitochondrial DNA (mtDNA) as well as for the reconstruction of normal oocytes from oocytes that are developmentally compromised. Moreover, it is speculated that the replacement of germinal vesicles by somatic cells may result in cells of the haploid genome: the production of germ cells from somatic cells. The preliminary results obtained in our laboratories are discussed in this article.

Animals↗
PubMed2002

Disruption of mesodermal enhancers for Igf2 in the minute mutant.

The radiation-induced mutation minute (Mnt) in the mouse leads to intrauterine growth retardation with paternal transmission and has been linked to the distal chromosome 7 cluster of imprinted genes. We show that the mutation is an inversion, whose breakpoint distal to H19 disrupts and thus identifies an enhancer for Igf2 expression in skeletal muscle and tongue, and separates the gene from other mesodermal and extra-embryonic enhancers. Paternal transmission of Mnt leads to drastic downregulation of Igf2 transcripts in all mesodermal tissues and the placenta. Maternal transmission leads to methylation of the H19 differentially methylated region (DMR) and silencing of H19, showing that elements 3' of H19 can modify the maternal imprint. Methylation of the maternal DMR leads to biallelic expression of Igf2 in endodermal tissues and foetal overgrowth, demonstrating that methylation in vivo can open the chromatin boundary upstream of H19. Our work shows that most known enhancers for Igf2 are located 3' of H19 and establishes an important genetic paradigm for the inheritance of complex regulatory mutations in imprinted gene clusters.

Animals↗