title: Accumulation and loss of asymmetric non-CpG methylation during male germ-cell development.
abstract: DNA methylation is a well-characterized epigenetic modification involved in gene regulation and transposon silencing in mammals. It mainly occurs on cytosines at CpG sites but methylation at non-CpG sites is frequently observed in embryonic stem cells, induced pluriotent stem cells, oocytes and the brain. The biological significance of non-CpG methylation is unknown. Here, we show that non-CpG methylation is also present in male germ cells, within and around B1 retrotransposon sequences interspersed in the mouse genome. It accumulates in mitotically arrested fetal prospermatogonia and reaches the highest level by birth in a Dnmt3l-dependent manner. The preferential site of non-CpG methylation is CpA, especially CpApG and CpApC. Although CpApG (and CpTpG) sites contain cytosines at symmetrical positions, hairpin-bisulfite sequencing reveals that they are hemimethylated, suggesting the absence of a template-dependent copying mechanism. Indeed, the level of non-CpG methylation decreases after the resumption of mitosis in the neonatal period, whereas that of CpG methylation does not. The cells eventually lose non-CpG methylation by the time they become spermatogonia. Our results show that non-CpG methylation accumulates in non-replicating, arrested cells but is not maintained in mitotically dividing cells during male germ-cell development.
authors.affiliation: Division of Epigenomics, Medical Institute of Bioregulation, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan.
authors.forename: Tomoko
authors.initials: T
authors.lastname: Ichiyanagi
authors.forename: Kenji
authors.initials: K
authors.lastname: Ichiyanagi
authors.forename: Miho
authors.initials: M
authors.lastname: Miyake
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Sasaki
mesh_terms: D000818:Animals; D001483:Base Sequence; D018899:CpG Islands; D019175:DNA Methylation; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008938:Mitosis / Q000235:genetics; D008969:Molecular Sequence Data; D020086:Short Interspersed Nucleotide Elements; D013093:Spermatogonia / Q000254:growth & development / Q000378:metabolism*