ChIP analysis of H3K9me3 and H3K4me3 for mouse spermatogonia and leptotene/zygotene spermatocytes defficient in Dnmt3l and Pld6.

In the male germline of mammals, retrotransposon expression is restricted by DNA methylation, trimethylation of histone H3 at lysin-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, 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 upregulation 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 likely 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.