DNA methylation dictates histone modifications in developing male germ cells in the mouse

In the male germline of mammals, retrotransposon expression is restricted by DNA methylation, trimethylation of histone H3 at lysine 9 (H3K9me3), and PIWI-interacting small RNAs (piRNAs). To elucidate their relative importance in regulating retrotransposons during germ cell development and the relationships between these mechanisms, we performed mRNA sequencing, DNA methylation, and histone methylation analyses using mouse spermatogonia from Dnmt3l and Pld6 mutants deficient in de novo DNA methylation and piRNA production, respectively. The results revealed that loss of DNA methylation resulted in decreased H3K9me3 in young L1 subfamilies and increased H3K4me3 in many retrotransposons, suggesting a pivotal role of DNA methylation in maintaining epigenomic integrity in spermatogonia and later stages of spermatogenesis. The transcriptional up-regulation of retrotransposons by a loss of DNA methylation was more evident during meiosis (spermatocytes) than before meiosis (spermatogonia). These results are aligned with a global reduction of H3K9me3 at retrotransposons in spermatocytes. The piRNA system also regulated H3K9me3 and H3K4me3 at retrotransposons in spermatogonia, probably through the regulation of DNA methylation, since the loss of DNA methylation resulted in decreased H3K9me3 and increased H3K4me3 at the same retrotransposon loci even in the presence of piRNAs.