Acquisition of theH19Methylation Imprint Occurs Differentially on the Parental Alleles during Spermatogenesis

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The imprinted mouseH19gene is hypomethylated on the expressed maternal allele and hypermethylated on the silent paternal allele. A 2-kb region of differential methylation located from −2 to −4 kb relative to theH19transcriptional start site has been proposed to act as the imprinting mark since hypermethylation in this region is inherited from sperm and retained on the paternal allele throughout development. Here, we describe a temporal analysis of the methylation patterns at theH19locus during postnatal male germ cell development. The 2-kb region is methylated on the paternal allele throughout spermatogenesis, suggesting that methylation is acquired in this region prior to the resumption of mitosis in postnatal male mice. Likewise, more than half of the maternal alleles are hypermethylated prior to the resumption of mitosis. However, the remaining maternal alleles are not hypermethylated until the completion of meiosis I, indicating thatde novomethylation in this region is a continuous process. Sequences proximal to theH19promoter, which are methylated in spermatozoa and on the paternal allele in somatic cells, are differentially methylated in diploid, mitotic spermatogonia. The maternal allele becomes hypermethylated in this region during meiotic prophase. Thus, the parentalH19alleles acquire methylation differentially in the male germline.