Role of small RNAs in shaping the epigenome of germ cells for genome defense and imprinting Hiroyuki Sasaki Division of Epigenomics, Medical Institute of Bioregulation, Kyushu University During mammalian germ cell development, retrotransposons and many unique sequences including imprint control regions are de novo DNA methylated. This predominantly occurs at the prospermatogonium stage in the fetal testis. The global de novo methylation is important for maintenance of genomic integrity and regulation of developmental genes including imprinted genes in the offspring. De novo DNA methyltransferases Dnmt3a and Dnmt3b, and a related protein Dnmt3L, are responsible for this process, but how specific sequences are selected for methylation is not fully understood. In fission yeast and plants, examples are known where small interfering RNAs serves as a guide to recruit epigenetic modifiers to specific targets. We were therefore interested in looking at the role of small RNAs in shaping the epigenome of germ cells. We found that the imprint control region of the mouse Rasgrf1 locus lost methylation in spermatogonia deficient for MitoPLD, a component of the Piwi-interacting RNA (piRNA) pathway. A retrotransposon sequence in a novel non-coding RNA spanning the imprint control region was targeted by piRNAs generated from a different locus. Furthermore, a tandem repeat region required for methylation of the Rasgrf1 imprint control region was found to be the promoter for this non-coding RNA. We therefore propose a model in which piRNAs and its target non-coding RNA play a critical role in DNA methylation and imprinting of Rasgrf1.

Role of small RNAs in shaping the epigenome of germ cells for genome defense and imprinting

Hiroyuki Sasaki

Division of Epigenomics, Medical Institute of Bioregulation, Kyushu University

During mammalian germ cell development, retrotransposons and many unique sequences including imprint control regions are de novo DNA methylated. This predominantly occurs at the prospermatogonium stage in the fetal testis. The global de novo methylation is important for maintenance of genomic integrity and regulation of developmental genes including imprinted genes in the offspring. De novo DNA methyltransferases Dnmt3a and Dnmt3b, and a related protein Dnmt3L, are responsible for this process, but how specific sequences are selected for methylation is not fully understood. In fission yeast and plants, examples are known where small interfering RNAs serves as a guide to recruit epigenetic modifiers to specific targets. We were therefore interested in looking at the role of small RNAs in shaping the epigenome of germ cells. We found that the imprint control region of the mouse Rasgrf1 locus lost methylation in spermatogonia deficient for MitoPLD, a component of the Piwi-interacting RNA (piRNA) pathway. A retrotransposon sequence in a novel non-coding RNA spanning the imprint control region was targeted by piRNAs generated from a different locus. Furthermore, a tandem repeat region required for methylation of the Rasgrf1 imprint control region was found to be the promoter for this non-coding RNA. We therefore propose a model in which piRNAs and its target non-coding RNA play a critical role in DNA methylation and imprinting of Rasgrf1.