GSE60377
Setdb1 is required for persistence of H3K9me3 and repression of endogenous retroviruses in mouse primordial germ cells
Transcription of endogenous retroviruses (ERVs) is inhibited by de novo DNA methylation during gametogenesis, a process initiated after birth in oocytes and at ~E15.5 in prospermatogonia. Earlier in germline development however, the genome, including most retrotransposons, is progressively demethylated, with young ERVK and ERV1 elements retaining intermediate methylation levels. As DNA methylation reaches a low point in E13.5 primordial germ cells (PGCs) of both sexes, we determined whether retrotransposons are marked by H3K9me3 and H3K27me3 using a recently developed low input ChIP-seq method. Although these repressive histone modifications are predominantly found on distinct genomic regions in E13.5 PGCs, they concurrently mark partially methylated LTRs and LINE1 elements. Germline-specific conditional knock-out (KO) of the H3K9 methyltransferase SETDB1 yields a decrease of both histone marks and DNA methylation at H3K9me3 enriched retrotransposon families. Strikingly, Setdb1-KO E13.5 PGCs show concomitant de-repression of many marked ERVs, including IAP, ETn and ERVK10C elements and ERV-proximal genes, a subset in a sex-dependent manner. Furthermore, Setdb1 deficiency is associated with a reduced number of male PGCs and postnatal hypogonadism in both sexes. Taken together, these observations reveal that SETDB1 is an essential guardian against proviral expression prior to the onset of de novo DNA methylation in the germline. Overall design: H3K9me3, H3K27me3 and expression profiles in Setdb1 WT, Het and KO male and female E13.5 PGCs.