Differential DNA methylation reprogramming of various repetitive sequences in mouse preimplantation embryos Seok-Ho Kim a,b , Yong-Kook Kang a , Deog-Bon Koo a , Man-Jong Kang b , Seung-Ju Moon b , Kyung-Kwang Lee a , Yong-Mahn Han a, * a Laboratory of Development and Differentiation, Korea Research Institute of Bioscience and Biotechnology (KRIBB), P.O. Box 115, Yuseong, Daejeon 305-600, Republic of Korea b Department of Animal Science, Chonnam National University, Gwangju 500-600, Republic of Korea Received 3 June 2004 Available online 21 September 2004 Abstract Genome-wide changes of DNA methylation by active and passive demethylation processes are typical features during preimplan- tation development. Here we provide an insight that epigenetic reprogramming of DNA methylation is regulated in a region-specific manner, not a genome-wide fashion. To address this hypothesis, methylation states of three repetitive genomic regions were mon- itored at various developmental stages in the mouse embryos. Active demethylation was not observed in the IAP sequences whereas methylation reprogramming of the satellite sequences was regulated only by the active mechanism. Etn elements were actively demethylated after fertilization, passively demethylated by the 8-cell stage, and de novo methylated at the morular and blastocyst stages, showing dynamic epigenetic changes. Thus, our findings suggest that the specific genomic regions or sequences may spatially/ temporally have their unique characteristics in the reprogramming of the DNA methylation during preimplantation development. Ó 2004 Elsevier Inc. All rights reserved. Keywords: DNA methylation; Repetitive sequences; Epigenetic reprogramming; Preimplantation embryo; Mouse DNA methylation at CpG dinucleotides in mammals plays important roles in a variety of biological processes during embryogenesis and in adult tissues such as tissue specific gene expression, cell differentiation, genomic imprinting, X-chromosome inactivation, carcinogenesis, and aging [1]. DNA methylation also functions to set up chromatin structure during mouse development [2]. Reprogramming of genome-wide DNA methylation takes place independently twice during preimplantation development and gametogenesis periods in mammals. Dynamic change of DNA methylation is observed dur- ing mouse preimplantation development. Reprogram- ming of DNA methylation occurs in early embryos by both active (DNA replication-independent) and passive (DNA replication-dependent) mechanisms [3,4]. The first genome-wide active demethylation is observed in the paternal genome during pronuclear formation of the zygote before the first cell division [5–7]. However, imprinted genes are not influenced by both active and passive demethylation mechanisms and maintain their methylation states throughout preimplantation develop- ment [3]. After the completion of the first cell cycle, DNA methylation of embryonic genomes is gradually erased during cleavage stage by a passive demethylation which is due to the lack of maintenance DNA methyl- transferase (DNMT1) [8–11]. The oocyte form of DNMT1 (DNMTo) is predominantly localized in the cytoplasm of oocyte and then undergoes a transient nu- clear localization at 8-cell stage [12–14]. In this process, DNMTo induces re-methylation with de novo meth- yltransferases (DNMT3a and 3b) and HDAC in the nucleus [15]. 0006-291X/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.bbrc.2004.09.023 * Corresponding author. Fax: +82 42 860 4608. E-mail address: ymhan@kribb.re.kr (Y.-M. Han). www.elsevier.com/locate/ybbrc Biochemical and Biophysical Research Communications 324 (2004) 58–63 BBRC