Abstract. Epigenetic systems play crucial roles in the differentiation of a mammalian fertilized egg into hundreds of cell types exhibiting distinct phenotypes, using a set of DNA molecules comprising about 3 billion nucleotides. Genomewide analyses of epigenetic marks have revealed the remarkably well-established and well-maintained structure of the epigenome, consisting of DNA methylation and histone modifications that vary their state in a tissue type-and developmental stage-specific manner at numerous genomic loci. DNA methylation profiles comprising numerous tissuedependent and differentially methylated regions (T-DMRs), found at such loci, are unique to every type of cell and tissue, and illuminate molecular networks that represent their phenotypes. T-DMRs are located in not only genic but also nongenic regions-including transposable genetic elements, such as short interspersed transposable element. Epigenetic studies indicate that the molecules that perform these modifications directly, such as DNA methyltransferases and eukaryotic histone methyltransferases, or indirectly, such as CpG-binding protein and noncoding RNAs-and combinations of these-contribute to the DNA methylation profile. It remains to be addressed how these molecules precisely find their target genomic loci. Key words: CpG island, DNA methylation profile, Methylome, Tissue-dependent and differently methylated region (T-DMR) (J. Reprod. Dev. 58: [265][266][267][268][269][270][271][272][273] 2012) T he mammalian body consists of about 200 types of cells, which can be discriminated from each other by their phenotype. All cells are descendants of a single fertilized egg. During development, cells change their phenotypes and their fates by responding to extrinsic and intrinsic signals using appropriate, but distinct, sets of genes without changing the genome sequence. Sets of transcription factors are the major players that govern the cell type-and tissue-specific use of the genome. Recent studies indicate that expression of these key players is under the control of another mechanism [1, 2]-the epigenetic system-that enables cells to exhibit their stable cell type-specific phenotypes over multiple generations. The epigenetic system, therefore, is considered a memory for genome use that defines cell types and tissues.The epigenome is the genome-wide combination of DNA methylation, histone modifications, and chromatin architecture. Recent advances in technology allow us to analyze the epigenome comprehensively. In this review, we would like to elucidate the mammalian epigenome by profiling T-DMRs, and to address the relationship between the epigenome and the genome.
DNA Methylation is a Major Player in the Epigenetic SystemThe major components of the epigenetic system are DNA methylation and histone modifications. In mammals, DNA methylation occurs at the cytosine residues (C) in CpG dinucleotide sequences, and is involved in imprinting, X-inactivation, immobilization of transposable genomic elements, and the regulation of gene activity [3]. ...