rahuis a mutant allele ofDnmt3c, encoding a DNA methyltransferase homolog required for meiosis and transposon repression in the mouse male germline

AbstractTranscriptional silencing by heritable cytosine-5 methylation is an ancient strategy to repress transposable elements. It was previously thought that mammals possess four DNA methyltransferase paralogs—Dnmt1,Dnmt3a,Dnmt3bandDnmt3l—that establish and maintain cytosine-5 methylation. Here we identify a fifth paralog,Dnmt3c, that is essential for retrotransposon methylation and repression in the mouse male germline. From a phenotype-based forward genetics screen, we isolated a mutant mouse called ‘rahu’, which displays severe defects in double-strand-break repair and homologous chromosome synapsis during male meiosis, resulting in sterility.rahuis an allele of a transcription unit (Gm14490, renamedDnmt3c) that was previously mis-annotated as aDnmt3-family pseudogene.Dnmt3cencodes a cytosine methyltransferase homolog, andDnmt3crahumutants harbor a non-synonymous mutation of a conserved residue within one of its cytosine methyltransferase motifs, similar to a mutation in human DNMT3B observed in patients with immunodeficiency, centromeric instability and facial anomalies syndrome. Therahumutation lies at a potential dimerization interface and near the potential DNA binding interface, suggesting that it compromises protein-protein and/or protein-DNA interactions required for normal DNMT3C functionin vivo.Dnmt3crahumutant males fail to establish normal methylation within LINE and LTR retrotransposon sequences in the germline and accumulate higher levels of transposon-derived transcripts and proteins, particularly from distinct L1 and ERVK retrotransposon families. Phylogenetic analysis indicates thatDnmt3carose during rodent evolution by tandem duplication ofDnmt3b, after the divergence of the Dipodoidea and Muroidea superfamilies. These findings provide insight into the evolutionary dynamics and functional specialization of the transposon suppression machinery critical for mammalian sexual reproduction and epigenetic regulation.