Shin-Ichi Tomizawa

Kyushu University, Hukuoka, Fukuoka, Japan

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Publications (6)79.73 Total impact

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    ABSTRACT: Allele-specific methylation of the endogenous H19 imprinting control region (ICR) is established in sperm. We previously showed that the paternal H19 ICR in yeast artificial chromosome (YAC) transgenic mice (TgM) was preferentially methylated in somatic cells, but not in germ cells, suggesting that differential methylation could be established after fertilization. In this report, we discovered small RNA molecules in growing oocytes, the nucleotide sequences of which mapped to the H19 ICR. To test if these small RNA sequences play a role in the establishment of differential methylation, we deleted the sequences from the H19 ICR DNA and generated YAC TgM. In somatic cells of these mice, methylation imprinting of the transgene was normally established. In addition, the mutant fragment was not methylated in sperm and eggs. These data demonstrate that sequences in the H19 ICR that correspond to the small RNA sequences are dispensable for methylation imprinting in YAC TgM.
    Gene 08/2012; 508(1):26-34. · 2.20 Impact Factor
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    Shin-ichi Tomizawa, Hiroyuki Sasaki
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    ABSTRACT: Genomic imprinting is an epigenetic gene-marking phenomenon that occurs in the germline, whereby genes are expressed from only one of the two parental copies in embryos and adults. Imprinting is essential for normal mammalian development and its disruption can cause various developmental defects and diseases. The process of imprinting in the germline involves DNA methylation of the imprint control regions (ICRs), and resulting parental-specific methylation imprints are maintained in the zygote and act as the marks controlling imprinted gene expression. Recent studies in mice have revealed new factors involved in imprint establishment during gametogenesis and maintenance during early development. Clinical studies have identified cases of imprinting disorders where involvement of factors shared by multiple ICRs for establishment or maintenance is suspected. These include Beckwith-Wiedemann syndrome, transient neonatal diabetes, Silver-Russell syndrome and others. More severe disruptions can lead to recurrent molar pregnancy, miscarriage or infertility. Imprinting defects may also occur during assisted reproductive technology or cell reprogramming. In this review, we summarize our current knowledge on the mechanisms of imprint establishment and maintenance, and discuss the relationship with various human disorders.
    Journal of Human Genetics 02/2012; 57(2):84-91. · 2.37 Impact Factor
  • Shin-Ichi Tomizawa, Joanna Nowacka-Woszuk, Gavin Kelsey
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    ABSTRACT: DNA methylation in the oocyte has a particular significance: it may contribute to gene regulation in the oocyte and marks specific genes for activity in the embryo, as in the case of imprinted genes. Despite the fundamental importance of DNA methylation established in the oocyte, knowledge of the mechanisms by which it is conferred and how much is stably maintained in the embryo has remained very limited. Next generation sequencing approaches have dramatically altered our views on DNA methylation in oocytes. They have revealed that most methylation occurs in gene bodies in the oocyte. This observation ties in with genetic evidence showing that transcription is essential for methylation of imprinted genes, and is consistent with a model in which DNA methyltransferases are recruited by the histone modification patterns laid down by transcription events. These findings lead to a new perspective that transcription events dictate the placing and timing of methylation in specific genes and suggest a mechanism by which methylation could be coordinated by the events and factors regulating oocyte growth. With these new insights into the de novo methylation mechanism and new methods that allow high resolution profiling of DNA methylation in oocytes, we should be in a position to investigate whether and how DNA methylation errors could arise in association with assisted reproduction technologies or in response to exposure to environmental toxins.
    The International journal of developmental biology 01/2012; 56(10-11-12):867-875. · 2.16 Impact Factor
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    ABSTRACT: Elucidating how and to what extent CpG islands (CGIs) are methylated in germ cells is essential to understand genomic imprinting and epigenetic reprogramming. Here we present, to our knowledge, the first integrated epigenomic analysis of mammalian oocytes, identifying over a thousand CGIs methylated in mature oocytes. We show that these CGIs depend on DNMT3A and DNMT3L but are not distinct at the sequence level, including in CpG periodicity. They are preferentially located within active transcription units and are relatively depleted in H3K4me3, supporting a general transcription-dependent mechanism of methylation. Very few methylated CGIs are fully protected from post-fertilization reprogramming but, notably, the majority show incomplete demethylation in embryonic day (E) 3.5 blastocysts. Our study shows that CGI methylation in gametes is not entirely related to genomic imprinting but is a strong factor in determining methylation status in preimplantation embryos, suggesting a need to reassess mechanisms of post-fertilization demethylation.
    Nature Genetics 06/2011; 43(8):811-4. · 35.21 Impact Factor
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    ABSTRACT: Genomic imprinting causes parental origin-specific monoallelic gene expression through differential DNA methylation established in the parental germ line. However, the mechanisms underlying how specific sequences are selectively methylated are not fully understood. We have found that the components of the PIWI-interacting RNA (piRNA) pathway are required for de novo methylation of the differentially methylated region (DMR) of the imprinted mouse Rasgrf1 locus, but not other paternally imprinted loci. A retrotransposon sequence within a noncoding RNA spanning the DMR was targeted by piRNAs generated from a different locus. A direct repeat in the DMR, which is required for the methylation and imprinting of Rasgrf1, served as a promoter for this RNA. We propose a model in which piRNAs and a target RNA direct the sequence-specific methylation of Rasgrf1.
    Science 05/2011; 332(6031):848-52. · 31.20 Impact Factor
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    ABSTRACT: Mammalian imprinted genes are associated with differentially methylated regions (DMRs) that are CpG methylated on one of the two parental chromosomes. In mice, at least 21 DMRs acquire differential methylation in the germline and many of them act as imprint centres. We previously reported the physical extents of differential methylation at 15 DMRs in mouse embryos at 12.5 days postcoitum. To reveal the ontogeny of differential methylation, we determined and compared methylation patterns of the corresponding regions in sperm and oocytes. We found that the extent of the gametic DMRs differs significantly from that of the embryonic DMRs, especially in the case of paternal gametic DMRs. These results suggest that the gametic DMR sequences should be used to extract the features specifying methylation imprint establishment in the germline: from this analysis, we noted that the maternal gametic DMRs appear as unmethylated islands in male germ cells, which suggests a novel component in the mechanism of gamete-specific marking. Analysis of selected DMRs in blastocysts revealed dynamic changes in allelic methylation in early development, indicating that DMRs are not fully protected from the major epigenetic reprogramming events occurring during preimplantation development. Furthermore, we observed non-CpG methylation in oocytes, but not in sperm, which disappeared by the blastocyst stage. Non-CpG methylation was frequently found at maternally methylated DMRs as well as non-DMR regions, suggesting its prevalence in the oocyte genome. These results provide evidence for a unique methylation profile in oocytes and reveal the surprisingly dynamic nature of DMRs in the early embryo.
    Development 03/2011; 138(5):811-20. · 6.60 Impact Factor

Publication Stats

257 Citations
79.73 Total Impact Points

Institutions

  • 2012
    • Kyushu University
      • Division of Epigenomics
      Hukuoka, Fukuoka, Japan
    • Babraham Institute
      Cambridge, England, United Kingdom
  • 2011
    • National Institute of Genetics
      • Division of Human Genetics
      Mishima, Shizuoka-ken, Japan